A non-exhaustive list of allergens identified in soya.
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More than half of the publishers listed alongside IntechOpen (18 out of 30) are Social Science and Humanities publishers. IntechOpen is an exception to this as a leader in not only Open Access content but Open Access content across all scientific disciplines, including Physical Sciences, Engineering and Technology, Health Sciences, Life Science, and Social Sciences and Humanities.
\\n\\nOur breakdown of titles published demonstrates this with 47% PET, 31% HS, 18% LS, and 4% SSH books published.
\\n\\n“Even though ItechOpen has shown the potential of sci-tech books using an OA approach,” other publishers “have shown little interest in OA books.”
\\n\\nAdditionally, each book published by IntechOpen contains original content and research findings.
\\n\\nWe are honored to be among such prestigious publishers and we hope to continue to spearhead that growth in our quest to promote Open Access as a true pioneer in OA book publishing.
\\n\\n\\n\\n
\\n"}]',published:!0,mainMedia:null},components:[{type:"htmlEditorComponent",content:'
Simba Information has released its Open Access Book Publishing 2020 - 2024 report and has again identified IntechOpen as the world’s largest Open Access book publisher by title count.
\n\nSimba Information is a leading provider for market intelligence and forecasts in the media and publishing industry. The report, published every year, provides an overview and financial outlook for the global professional e-book publishing market.
\n\nIntechOpen, De Gruyter, and Frontiers are the largest OA book publishers by title count, with IntechOpen coming in at first place with 5,101 OA books published, a good 1,782 titles ahead of the nearest competitor.
\n\nSince the first Open Access Book Publishing report published in 2016, IntechOpen has held the top stop each year.
\n\n\n\nMore than half of the publishers listed alongside IntechOpen (18 out of 30) are Social Science and Humanities publishers. IntechOpen is an exception to this as a leader in not only Open Access content but Open Access content across all scientific disciplines, including Physical Sciences, Engineering and Technology, Health Sciences, Life Science, and Social Sciences and Humanities.
\n\nOur breakdown of titles published demonstrates this with 47% PET, 31% HS, 18% LS, and 4% SSH books published.
\n\n“Even though ItechOpen has shown the potential of sci-tech books using an OA approach,” other publishers “have shown little interest in OA books.”
\n\nAdditionally, each book published by IntechOpen contains original content and research findings.
\n\nWe are honored to be among such prestigious publishers and we hope to continue to spearhead that growth in our quest to promote Open Access as a true pioneer in OA book publishing.
\n\n\n\n
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She received her BS and MS in Chemistry from Lanzhou University, China and her PhD in Neuroscience from the Catholic University of Leuven, Belgium. Dr. Qu has spent part of her career at the National Institutes of Health, USA, studying depression mechanisms underlying serotonin post-receptor regulated signaling transduction. She is also involved in a drug discovery program at Johnson and Johnson in the USA developing novel dual-acting antidepressants with selective serotonin reuptake inhibitors. In 2002, she received a Sevier Young Investigator Award from the Serotonin Club at the International Union of Basic and Clinical Pharmacology (IUPHAR) Satellite Meeting on Serotonin. 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Emulsion polymerization (EP) is a complex heterophase process which involves many components and also runs sequentially and in parallel physicochemical processes that determine the composition of the emulsion, the formation mechanism of polymer-monomeric particles (PMP), conditions for the formation of interfacial layer, etc. There are a number of factors that collectively affect the kinetics of emulsion polymerization, among them:
initiator decomposition,
dispersion of the monomer,
emulsifier redistribution between phases,
microemulsification,
formation of interfacial adsorption layer,
initiation of polymerization,
PMPs formation,
diffusion of monomer into PMPs.
The initial stage of emulsion polymerization defines the basic parameters of the process: the rate of polymerization and the formation of interfacial layer, the number of particles, their size distribution, molecular masses of polymers and molecular mass distribution. The mechanism of formation of PMPs has been widely discussed in the literature and the researchers can not possibly come to a consensus.
This can be explained by the fact that in the original system, different types of particles may be present depending on the nature of system components and their concentrations:
individual molecules of surfactant
micelles of surfactant
monomer swollen surfactant micelles
surface-active oligomers formed during the initiation of polymerization in the aqueous phase
mixed micelles (surfactant + oligomers)
macro- and microdroplets of the monomer.
In addition to these particles, the emulsion of the monomer may contain specially added low-molecular-weight substances necessary for the preparation of polymeric suspensions with desired properties.
In the literature there are various hypotheses about the mechanism of particle formation in emulsion polymerization, the main ones assumethe formation of PMPs from:
micelles of emulsifier,
macromolecules of the polymer dropped out in water (the mechanism of homogeneous nucleation),
by the mechanism of nucleation aggregative which combines elements of different modes of particle formation
microdroplets of monomer.
None of the hypotheses proposed to date is confirmed by reliable experimental data.Hereinafter we briefly recall these hypotheses.
Micellar mechanism of particle formation is based on a qualitative model proposed simultaneously and independently by Harkins [1,2] and Yurzhenko [3,4]. The basic points of such mechanism are as follows:
Emulsion polymerization is a usual process of a radical polymerization, its features are explained by the fact that the main site of reactions - micelles and PMP - have discrete volumes.
At the initial stage of the polymerization the reaction system consists of emulsifier micelles, PMP and large monomer droplets.
Polymerization starts only in the emulsifier micelles, which when attacked by a radical convert to PMP, monomer droplets being only the source of monomer. Diffusion of monomer from droplets through the aqueous phase and intothe PMPs does not limit the process that leads to the establishment of equilibrium monomer concentration PMP, which persists as long as monomer droplets are present in the system.
According to these ideas, in the presence of surfactants, the initial emulsion consists of two kinds of particles of different sizes: the monomer droplets with diameters of 5-20 microns and colloidal degree of dispersion and monomer-swollen surfactant micelles (5-10 nm).
The mechanism of Harkins-Yurzhenko lied in the base of Smith and Ewart quantitative theory [5-7], with further refinements in the works of other authors [8-70]. The main aspect of this theory is that radicals formed in the aqueous phase are trapped by the monomer swollen surfactant micelles and turn to PMP. It is assumed that only one out of every 100-1000 micelles captures a radical and becomes PMP, and the rest of the micelles are spent tostabilize the growing PMPs.Polymer-monomer particles formation ends with the disappearance of micelles of the emulsifier in the aqueous phase, after which the number of particles remains constant.
Initial system contains monomer droplets with a diameter of 5-15 microns, their concentration being 1012-1014 droplets/l, the monomer-swollen micelles with diameters of 5-10 nm and the number of micelles 1018-1021 l-1, and a water-soluble initiator (usually potassium persulfate) at a concentration of 1% per monomer [8]. Only a small fraction of the molecules of the monomer is located in the interior of the micelles (1-2%) and is dissolved in the aqueous phase (0.03% for styrene). In the aqueous phase, 1014-1016 PMP / l are formed with a diameter in the range of 20-200 nm. Monomer droplets due to their relatively small surface area hardly compete with micelles in capturing radicals.
In the polymerization process PMPs increase their size due to the diffusion of the monomer from droplets and monomer-swollen micelles which contains no radicals [1-3, 5, 9-15].
Three limiting cases were considered:
The number of free radicals in the particles is small compared with the total number of radicals, thus the average number of radicals per particle is much less that unity.
The average number of radicals per particle is equal to 0.5. This case is realized under following conditions: the activity of the radical in the particle persists as long as the second radical enter the particle, and the time of chain-breaking is small as compared with the average time interval of successive absorption of radicals by the particle.
The authors believe that case 2 corresponds to the emulsion polymerization of styrene in the presence of potassium persulfate [5]. The rate of formation of primary radicals is equal to 1013 radicals• ml-1s-1. The average number of polymer particles is of the order of 1014-1016 in 1 ml of the system. If all the radicals formed by the decay of the initiator enter the polymer particles, the average frequency at which a radical enters a particle is once per 10 - 100 sec.
At any time, the particle contains either one radical or does not contain radicals at all, since it is assumed that chain termination occurs immediately in contact with the second radical in the particle. The particle is inactive until the next radical does not enter the particle, i.e. by 10-100 sec.Consequently, half of time each particle contains a radical and another half of time does not contain radicals, i.e. one half of the particles are active and each of them contains one polymer radical. Thus, the polymerization rate, referred to 1 ml of latex, is expressed by the equation:
where Nis thenumber of PMPs in 1 ml of latex, kp is the propagation rate constant,[M] is the concentration of monomer in the PMP.
The rate of polymerization is determined by the number of particles and it does not depend on particle size if it isnot very large.
If the particle size is large, they may contain several radicals at the same time, as in the case of suspension polymerization. This condition corresponds to the third case: the number of free radicals in the polymer particle is large, each particle has a certain steady-state concentration of radicals.
After completion of the formation of PMPs their concentration in water remains relatively constant until the end of polymerization. The size of particles becomes larger due to the diffusion of monomers from monomer droplets which serve as reservoirs for the growing particles. Most of the monomer is consumed in the growth stage of particles (approximately 10 to 60% monomer conversion). The stage of particle growth (interval II) ends with the disappearance of monomer droplets in the system.Forcase 2 thefollowingassumptionswereimplanted:
The number of particles per unit volume of water remains constant throughout the polymerization;
The particle size distribution is relatively narrow;
No desorption of free radicals from the particles;
Bimolecular termination of polymer radicals, located in a particle diffusing with a radical from the aqueous phase is instantaneous.
Figure 1 illustrates the evolution of the rate of emulsion polymerization. The kinetic curve contains three phases: a relatively short stage I, characterized by the growing polymerization rate, stage II of a constant rate of the process, and stage III, where the polymerization rate decreases.
Stage I refers to a micellar stage of emulsion polymerization. It describes the formation of PMP and the increase in their number.In stage II, the formation of new particles does not occur, and the main process parameters: growth rate constant, kр, the number of radicals in the particle, n; number of particles, N, and monomer concentration [M], respectively, and the constant changes in the rate of polymerization not observed.StageIIIbeginsafterthedisappearanceofmonomerdroplets. In this case, all the monomer is contained in the volume of PMP and the rate of polymerization of the consumption of monomer decreases.
Shortly after the publication of the Smith-Ewart theory, many other interpretations that discuss deviations from the theory were issued [8-33]. There have been many recent indications of change in the number of particles in the emulsion polymerization. In stage II, the Smith-Ewart assumption on the discreteness of the latex particles is not supported.
Nevertheless, the consideration of the number of particles in the emulsion system as the main kinetic factor remains the basic idea of the Smith-Ewart theory.
Three intervals of the dependence of polymerization rate on monomer conversion (from Prog. Polym. Sci. 26, 2001, p. 2094) [16]
Today it is obvious that the observed phenomena in emulsion polymerization are extremely diverse and can not yet be explained by a single theory.
Kinetic description of emulsion polymerization is complicated due to the difficulty of establishing a placewhere elementary reactions proceed, and concentrations of reagents in these places.
The classical theory of emulsion polymerization of the Smith-Ewart, establishing a link between the synthesis conditions and the basic parameters of the process, proved to be applicable only to a limited number of objects. It describes EP of nonpolar monomers in the presence of surfactants insoluble in monomer.
Later on, these ideas have been modified by Gardon[17-23], Harada [24], Stokmayer [25], O\'Toole [26], Ugelstad [27-29], Kuchanov [30-32], etc.
A more general review of topochemistry and kinetics of polymerization of the latex were presented by Medvedev et al. [8-11, 14]. Agreeing with Harkins-Yurzhenko\'s ideas concerning particles formation, Medvedev, nevertheless, has proposed that micelles have properties of swarms, and an exchange of molecules and radicals between (a) micelles themselves, and (b) micelles and polymer particles takes place. This assumption considers the processes occurring with the participation of micelles as homogeneous ones averaged over the main parameter - the concentration of emulsifier.
Key topochemical and kinetic features of emulsion polymerization according to Medvedev may be formulated as follows:
I. With water-soluble peroxide compounds which form initiating radicals in aqueous solution polymerization, depending on the nature of monomers, starts in water or micelles. When using oil-soluble polymerization initiators, regardless of the nature of the monomer, it begins in micelles since they contain both monomer and initiator.
The process proceeds further in 10-100 nm PMP, in which monomer concentration during the reaction remains constant (for polymers which are soluble in their monomers, this concentration is 40-50%).
It is essential that the polymerization does not start simultaneously in all the micelles, but only in a small part of them. This is because the concentration of initiating radicals is usually much lower than the number of micelles. The emulsifier contained in the polymer-free micelles is used to stabilize the increase in volume of PMP. Since initiating radicals are formed in the molecular aqueous solution or in adsorption layers of the emulsifier (which means in the micelles or at the surface of polymer particles), the polymer radicals occur near these adsorbed layers. Because of the low rate of diffusion of polymer radicals in viscous phase of PMP the reaction growth and chain termination take place not in the whole volume of the particles (especially in their relatively large volume), but in some zone near the surface. The volume of this zone is determined by the concentration of emulsifier in the process, and in some cases remains approximately constant.
II. The total rate of emulsion polymerization is 102-103 higher than the polymerization rate in homogeneous systems with the same initiators (peroxides, azo compounds, radiation). The average molecular mass of the polymers is also much higher in emulsion polymerization.
Therefore increasing the total rate of the process due to the decrease of the reaction rate of chain termination, leads to an increase in both rate and average length of the polymer molecular chains.
Reducing rate of chain termination reactions may be due to two reasons: 1) a low rate of diffusion of polymer radicals in a viscous medium, as in emulsion polymerization, even in early stages, the process takes place in concentrated solutions of polymer in monomer, 2) the radical separation to individual particles. Qualitatively, these two assumptions explain the simultaneous increase in the speed of the process and the molecular weight of polymer in latex polymerization.
Medvedev theory satisfactorily describes the quantitative relationships emulsion polymerization of many monomers.
Ivanchev and Pavlyuchenko [33, 34] attempted to study the elementary reactions of emulsion polymerization and use the results for the synthesis of polymers with properties fundamentally different from those obtained in homogeneous systems. In their study of initiation reaction in emulsion polymerization of styrene, it was suggested that the adsorption layer of PMP has concentrating and orienting effect on the initiator molecule.
According to the hypothesis of the formation mechanism of PMP for homogeneous nucleation [27, 35-42], the formation of particles or macromolecules derived from the radicals who have reached the critical length (jс), where they lose solubility and precipitate in the aqueous phase. Further growth of the polymer particles dropped in the water is treated differently. Some authors believe that these particles grow by diffusion of monomer from the monomer droplets, i.e. as well as in the case of micellar mechanism, other authors suggest that there exist their limited flocculation and, consequently, the formation of PMPs.
First who suggested the formation of the PMP precipitated from an aqueous solution of polymer molecules was Khomikovsky in 1948 in his study of emulsion polymerization of methyl methacrylate and vinylcyanide [43]. He found that the dependence of the rate of polymerization of the two monomers on the emulsifier concentration is different: the rate of vinylcyanide polymerization decreases with increasing concentration of emulsifier and the rate of MMA polymerization increases, the concentration of emulsifier in these experiments being above the critical micelle concentration (CMC). Khomikovsky explained the results in such a way that vinylcyanide polymerization, initiated by potassium persulfate, begins in water.
Upon reaching the length of polymer chain when oligomers became insoluble in water they precipitate in the aqueous phase to form particles that are stabilized by the adsorption emulsifier molecules on their surface. Initiation of polymerization transforms them to PMP. Here, the polymerization proceeds similarly to that in PMP formed from micelles during polymerization of monomers poor soluble in water.
Later on, these ideas found their development in the investigations of Priest [35], Roe [36], Fitch and Tsai [39, 40], Yeliseyeva [37, 38], Christiansen [44], Ugelstad [27], Pepard [45] Wilkinson [46-48], Oganesyan [49-51], Tauer [52-57], etc.
A quantitative description of the processes of polymerization of the monomers, partially soluble in water, was given by Fitch and Barrett [58].
Fitch proposed the theory called the theory of homogeneous nucleation. Quantitative analysis of this theory was based on the process of self-exhaustion of oligomeric radicals that have reached a critical degree of polymerization.
Considering the theory of Fitch, Barrett has identified two possible distribution of radicals between the aqueous phase and particles:
The equilibrium distribution of radicals between the particle and the volume of the aqueous phase, the radical capture rate here is proportionalto the volume of the particle.
The equilibrium distribution of radicals between the surface of the particle and the aqueous phase, the radical capture rate in this case is proportionalto the square of the particle surface.
He showed that the second case is realized more likely, explanation being that small particles are more effective for the capture of oligomeric radicals than larger particles.
The number of particles, according to the theory of Barrett, depends mainly on the distribution coefficient of oligomeric radicals between the surface and volume of the particles and the aqueous phase.
As the number of particles increases during polymerization, the rate of radical absorption by particles increases, as long as all the newly formed radicals will not be absorbed by the particles without having to reach a critical length.
Two variants of the flow of polymerization in the absence of coagulation of particles were considered:
a short nucleation time (due to a high rate of initiator decomposition), here all the particles start to grow almost simultaneously at the same average rate, and reach finally approximately the same size. The result of this reaction is the large number of particles of small diameter and narrow particle size distribution.
a long time of nucleation (due to a slow initiation), the particles are formed longer, which leads to a broader particle size distribution.
The rate of flocculation of the particles can be controlled by varying the concentration of emulsifier, which is adsorbed on the surface of primary particles and prevents flocculation due to the formation of the electric double layer or due to steric stabilization factor.
In the absence of emulsifier or at its very low concentration and a high rate of initiation,a rapid increase in the number of particles due to the high rate of decomposition of the initiator and the formation of free radicals is noticed during the first stage of polymerization.
Further, due to flocculation, the number of particles drops sharply at the end of the nucleation, particles reach a certain stability by increasing the density of the surface electric charge. If the period of the particles nucleation is short, then, the polymer suspension is characterized by a narrow particle size distribution, on the contrary, the distribution is broad if this period is long.
Ugelstadt and Hansen [27] consider that EP model with partially water soluble monomers should take into account the different stages of the diffusion of oligomeric radicals, as well as the reaction of oligomeric radicals with monomer in the particles.
Using the basic assumption of Fitch that the oligomeric radicals have to reach a certain critical degree of polymerization at which they spontaneously precipitate from the aqueous phase, Ugelstadt and Hansen examined the effect of diffusion of oligomeric radicals in the formation of PMP on the number of particles formed by the mechanism of homogeneous nucleation.
The main provisions of the model are similar to those observed in the Fitch theory and are as follows:
Oligomeric radicals spontaneously precipitate when a critical degree of polymerization is reached;
Absorption of oligomeric radicals by particles depends on the molecular weight of the radicals;
There is a desorption of monomeric radicals from particles;
The role of the emulsifier (at a concentration below CMC) is to stabilize the primary particles.
According to this model, in the absence of emulsifier radicals which are formed by the decay of the initiator can:
add monomer dissolved in the aqueous phase;
be captured by existing polymer-monomeric particles or adsorbed on the surface of the PMP;
recombine in the aqueous phase with the other radicals;
associates to form micelles with other types of radicals dissolved;
precipitate when the oligomeric radicals critical chain length (jc).
The tendency of dissolved oligomeric radicals to the association is small because of their low concentrations (<10-7 mol/l).
Termination of oligomeric radicals in the aqueous phase may, or may not lead to the formation of new particles. Loss due to recombination leads to a doubling of the chain length of oligomeric radicals, but in this case oligomeric product contains two ionic end groups, resulting in a critical chain length required will increase. Therefore it is not clear whether the loss be oligomeric product from the solution.
According to Fitch model, Ugelstadt determined thenumber of PMPs in the reaction system is determined by a limited coagulation process of the primary particles.
To explain the influence of emulsifier on the nature of the stabilization process of PMPs Yeliseyeva et al. [12, 13] proposedto use the emulsifier adsorption isotherms at PMPs as a main characteristic.
These authors believe that in EP the rate of emulsifier adsorption on the surface of PMP is a function of adsorption energy, which, in turn, depends on the nature of the emulsifier, and the nature of the surface of the PMP. Later, in the publications of Yeliseyeva et al. adsorption of emulsifier on the particle surface was considered as the main factor determining the kinetics of EP of polar monomers.
Because of the high flow rate of the polymerization process it is difficult experimentally to determine the number of particles during their formation.
Published data show that existing models of EP, based on ideas about the formation mechanism of PMPs homogeneous nucleation satisfactorily describe only the polymerization with a low content of monomer in the initial system (5-10% wt.) in the absence of an emulsifier, or at very low (below the CMC ) concentrations.
Along with the micellar and homogeneous mechanisms, aggregative nucleation of particles in EP is widely recognized. It is carefully designed by Lichti [59-61] and Feeney [62-64] on the basis of the study of the size distribution of polystyrene latex particles stabilized with sodium dodecyl sulfate. According to them, precursor particles are formed as a result of the growth of oligomeric radicals in the surfactant micelles. Precursor particles are highly unstable and aggregate, reducing the interfacial tension at the oil / water boundary. Thus, there are germs of the particles.
The rate of nucleation time is extremely high, the authors explain this fact as a result of coagulative (aggregative) nucleation. To describe aggregate nucleation a mathematical model has been used which is a combination of the kinetic theory of coagulation, and Muller-Smoluchowski theory of DLVO (Derjaguin, Landau, Verwey and Overbeek) [65], which determines the strength of the electrostatic repulsion opposing the dispersion forces of attraction as the main factor responsible for the stability of the colloidal particles.
A further developmentof the mechanism of nucleation of aggregate particles is reflected in the publications of Tauer, Kuhn et al. [52-57, 66]. This theory is indeed a combination of ideas proposed by Fitch et al. [39-41] and Oganesyan [51] with the difference that the authors consider the surface energy of the particles as an adjustable parameter of the model.
The basic assumption of aggregative mechanism is the formation of water-soluble oligomers of PMP with a certain chain length. The proposed mechanism can be regarded as a refinement of the mechanism of homogeneous nucleation of particles and act as an independent theory. This mechanism is similar to the mechanism of homogeneous nucleation, but in this case the particles are not dropped from the aqueous phase oligomeric radicals that have reached a critical chain length, and from particles formed by aggregation of several water-soluble oligomeric radicals which are called clusters [66].
This theory is based on the results of a study of the polymerization of styrene, methyl methacrylate, and vinyl acetate in the absence of surfactant [58]. Selected monomers suitable for this model as well studied in the literature are presented all the necessary constants and experimental data on the critical value (jc) the chain length of oligomeric radicals, in which they precipitate in the aqueous phase, in addition, these monomers differ significantly in their solubility in water [22, 67-70].
As part of this mechanism depends significantly on the over-saturation of the oligomeric chain length (j), which in turn depends on the polymerization conditions. The higher values of jc, the lower the solubility of oligomers and the lower concentration required to achieve saturation and the phase formation.
For different monomers nucleation occurs at different times and for different values of j: the higher solubility of the monomer in water, the higher the jc and the longer time required for the appearance of the particles. For the three monomers selected above the calculated jcagree well with the experimental values.
The theory of Hansen and Ugelstad of homogeneous nucleation of particles is based on several assumptions whose validity has not yet been proved. As a main place of particle formation the authors consider a saturated aqueous solution of monomer and initiator and do not take into account the processes occurring at the monomer/water interface.
The formation of the polystyrene suspension during emulsifier-free polymerization initiated by potassium persulfate and the detection of styrene oligomers with the 8-9 degree of polymerization in the aqueous phase are considered as an experimental proof of this theory [27].
Identifying the formation of particles according to the homogeneous nucleation process that takes place in a perfectly pure supersaturated solutions, these authors did not find the original monomer/water system emulsifier micelles or microdroplets of monomer.
In these studies it is assumed that ion-radicals falls out of solution as a new phase at attaching a certain number of monomer molecules (up to 10). However, the size of oligomeric radicals is widely debated by many authors.
Detailed studies of physico-chemical processes occurring at the interface as well as in separate bulk phases in monomer-water solution of potassium persulfate static systems were performed and described by Oganesyan et al. [49-51].
The authors studied the polymerization of styrene in static conditions in the absence of an emulsifier, using potassium persulfate as initiator at 50°C in temperature-controlled reactors. The system was kept in the oven for 2 hrs where decomposition of initiator and initiation of polymerization took place, the aqueous phase became turbid. After standing for more than two days, the aqueous phase transformed into stable latex. These studies were carried out at a volume ratio styrene/water equal to 1:7.
The authors explained these results via the formation of particles at the droplets of the monomer / water interface.
To confirm this conclusion, the authors have found conditions which allow to extend the residence time of particles at the interface and to enable them to build-out there. They increased the density of the aqueous phase by increasing the concentration of initiator. The experiments require a high purity and it was necessary to exclude the influence of possible impurities on the stability of the system under study and eventual variations in temperature. Experiments were carried out in batch reactors, which are attached to the cover of the crystallization apparatus designed for growing crystals under isothermal conditions and having an electronic device to provide a constant temperature with high accuracy.
Crystals of potassium persulfate are grown up in the crystallization apparatus. They are used to initiate polymerization at concentrations in the range of 0.5 - 3% by weight. Aqueous solutions of potassium persulfate and styrene separately thermostated at a temperature of 50°C, and carefully layered on the aqueous phase styrene. Painting turbidity of the aqueous phase was dependent on initiator concentration (>2%), turbidity appeared at high concentrations of initiator in the narrow boundary layer from the aqueous phase, and then distributed throughout the volume. For the remaining initiator concentrations area of the initial turbidity of water phase was extended.
Authors believe that the formation of monomer droplets in static conditions is necessary to find the source of energy to perform work on the dispersion of the monomer, which is determined by temperature and chemical potential of the contacting phases. It is believed that if the heat of polymerization is able to transfer a certain amount of monomer in the aqueous phase, then the polymerization reaction at the interface can deform the surface of the interface and disperse system. Further, they assume that at an initial equilibrium state of the system the transfer of certain amount of monomer in the monomer-rich aqueous phase is equivalent to supersaturation of water molecules of the monomer, andnucleation of monomer droplets in the aqueous phase near the interface can be expected.
According to the representations of micellar [1-7] and homogeneous [27, 35-40] theories of particles nucleation, the monomer droplets play the role of a reservoir from which the monomer in the polymerization process goes into growing PMP by the diffusion through the aqueous phase. Subsequent studies performed by Pravednikov, Gritskova, Taubman, Nikitina, Ugelstad and co-workers have shown that the monomer droplets may be involved in the formation of PMPs if their size may be reduced to the size of the order of 1 micron or less [28, 49-51, 71-111].
Droplet size of emulsions obtained by emulsification of the monomer in the aqueous solution with stirring may be evaluated by the equation:
where σ12 is interfacial tension, dwpis the density of the dispersion medium, and E isthe energy expended to move a unit mass of the medium [80].
The dispersion of the monomer depends on the power consumed for mixing of the reactor design, the type of the mixer and mixing rate. The presence of the surfactantin the system has a significant influence on the dispersion of the monomer emulsion : adsorption of surfactant at the interface alters the interfacial tension, which facilitates the process of fragmentation and prevents the coalescence of droplets of the monomer when they collide. On the other hand, the irregular distribution of surfactant between monomer and aqueous phases in the initial system provokes transfer of surfactant across the interface, which can lead to the destruction of the border and microemulsification. Thus, in addition to fragmentation and coalescence processes that determine the size of the emulsion droplets in the presence of surfactants, microemulsification may take place. Assignment of microemulsification as a separate factor influencing the formation and composition of the initial emulsion system relates to the fact that the size of droplets produced here is significantly less than those determined by the direct effect of mechanical agitation (less than 0.2 microns). Therefore, the investigation of microemulsification goes beyond traditional ideas about the formation of disperse systems under mixing.
A significant attention in the literature was devoted to the investigation of the properties of microemulsions (ME) and conditions of their formation [81-94], however, the thermodynamic equilibrium of lyophilic systems [81, 82] was mainly considered.
Rebinder was one of the pioneers who studiedand later developed the process ofspontaneous microemulsification [89, 90]. According to his ideas, this process results in the formation of lyophilic colloidal phase corresponding to such a state when an increase of free energy in the formation of the colloidal phase is close to the energy of thermal motion. This process is accompanied by an increase in entropy of the system due to the formation of a large number of colloidal particles. Entropy factor will compensate for the increase in free energy associated with an increase in the interface area.
In contrary to the real (true) spontaneous emulsification when dispersion occurs in the total volume of the dispersed phase with the formation of thermodynamically stable lyophilic system [79, 95-100], in this case the dispersion may occur in one or both phases simultaneously with interfacial surface tension, far from the critical (σmin), as a result of hydrodynamic instability of the interfacial layer, and leading to mass transfer of one phase to another. As a result, only part of the dispersed phase the in the layer adjacent to the interface can be involved in emulsification [91-94].
The mechanism of interphase mass transfer of surfactant with microemulsification remains open. Various hypotheses are put forward, including those which are based on interfacial instability in the development of interfacial turbulence caused by Marangoni effect. Besides, local fluctuations of the interfacial tension, which lead to the movement of the surface layers of liquid, which, in turn, can increase the interfacial tension gradients in the presence of phase transfer surfactant may be also took into consideration [101].
Thermodynamically stable ME are translucent, lyophilic systems containing spherical aggregates whose size is in the range of 10 to 20 nm. The formation and the type of microemulsion depend on the ratio of the components in the system and the interaction between the surfactant molecules, hydrocarbon and water, the length of the alkyl chain in the molecules of surfactant and co-surfactant (if the later exists), as well as on the nature of the hydrocarbon [111-116].
It should be noted that microemulsions formed as a result of surfactant phase transfer are only kinetically stable and significantly different from those traditionally considered as thermodynamically stable ME. Again we note that the size of monomer microdroplets exceeds the characteristic size of the emulsifier micelles, but is much smaller than droplet size which can be obtained with the corresponding value of interfacial tension by mechanical fragmentation of the monomer.
Mass transfer at the interface caused by the diffusion of surfactant, soluble in both phases, provided greater solubility of the surfactant in one phase, without stirring, bringing with him through the phase boundary, and enjoys a certain amount of the solvent (monomer) that is emulsified in the other phase.
The intensity of microemulsification is slowed over time and can be completely terminated with prolonged contact of phases. This is explained by the fact that the interface formed a dense layer ofME, retarding the process of mass transfer. Microemulsification observed in systems with sufficiently high values of interfacial tension σ12 (about 1-10 mN / m). The increase in interfacial surface at high values of σ12 can not be explained by thermodynamic factors, because the corresponding increase in free energy can not be offset by an increase in entropy of microdroplets. This means that the stability of such a dispersed system has a kinetic rather than thermodynamic in nature.
In this case, when the surface energy of the droplets is much higher than KT, the formation of droplets can not be explained only by thermal fluctuations. Note that the ratio of the surface energy of a KT scan, depending on the size of droplets can be large, even for low values of σ12. So for a drop having a diameter D = 100 nm and σ12 = 10-2 mN / m, for this ratio, we obtain
This process is a kind of "quasi-spontaneous" emulsification, and once again we recall that in contrast to the spontaneous emulsification of the true droplets can form various degrees of dispersity, from colloidal particles to the droplets, significantly exceeding the size, for example, microscopic investigation of emulsion obtained in a glass capillary at the interface of 10% aqueous solution of sodium butylnaphthalenesulfonate / xylene, clearly showed that the formation of direct and inverse emulsions. Unstable inverse emulsion of water in coarse xylene - the diameter of water droplets is in the range 1-5 microns. The emulsion is xylene in the aqueous phase (direct emulsion) is visually observable in the form of a milky-white layer below the interface and is extremely stable and formed droplets whose size is beyond the interval of an optical microscope. Therefore, the determination of the dispersion ultra ME is carried out by electron microscopy [79]. The diameters of the particles according to electron microscopy were in 20-40 nm intervals.
There is a proportional relationship which was noticed between the resulting number ofME microdropletsand the quantity of surfactant have passed through the interface. The effectiveness of microemulsification is associated not only with surfactant phase transfer caused by initial non-equilibrium distribution of surfactants between the phases. Parameters determining the condition of interfacial instabilities are also: a sign of the derivative dσ12/dG (G - surface concentration of surfactant), the relationship of diffusion coefficients D1/D2 and coefficients of the kinetic viscosity γ1/γ2 in different phases [77]. It should be noted that the surface concentration of surfactant uniquely determines the surface tension only in the absence of local gradients of G, leading to a surface instability. Dynamic surface tension measured in the presence of interfacial instability, depends on the distribution of local values of G, and may be substantially lower than the equilibrium value. Numerous experimental determination of σ12 under non-equilibrium conditions confirms this fact.
Interphacial transfer causing the destruction of the interface can also lead to the fragmentation of large monomer droplets, whose dimensions are determined mainly by conditions of mechanical stirring. By analogy with the effect of interphase mass transfer direction that depends on the surfactant distribution coefficient on the rate of microemulsification, we can assume that the fragmentation of large droplets of the monomer will depend on the way the surfactant is introduced to the system:
surfactant is dissolved initially in aqueous phase (traditional way);
surfactant is dissolved initially in monomer;
surfactant is formed on the interface layer (when acid and alkaline components of surfactant are dissolved in monomer and aqueous phase respectively)
Investigation of polymerization of vinyl and diene monomers in the presence of a nonionic emulsifier, performed in [79, 95, 100] led the authors to conclude that monomer microemulsification is the first stage of PMP formation in EP.
When free radicals are injected into the microdroplets of monomer, they become PMP. Polymer-monomer particles are microdroplets of polymer solution in monomer, on the surface of which the polymer sedimentate, forming a polymer film. This film is the site of fixation of polymer radicals and, therefore, it is the place of formation of high molecular mass polymer.
Monomer to the reaction site (surface layer) of a particle diffuses as from inside this particle as from the outside monomer droplets, if later are present in the system and if the monomer concentration in microdroplets reached critical values lower than monomer/polymer equilibrium concentration. The authors believe that, along with the diffusion of monomer through the water, the transfer of the monomer can be carried out through direct contact of PMP with a monomer droplets.
Thermodynamic substantiation of formation of monomer microdroplets was investigated by Oganesyan [49, 51, 109]. In his study of the static monomer/water system the author tried to find the source of energy needed to perform work on the formation of droplets. His analysis came from the following considerations: the minimum work necessary to create a unit of the interface (the specific surface free energy) is the interfacial tension. New surface can be created by elastic deformation of the interface, the transfer of some amount of a substance from one phase to another and the creation of the surface bumps or depressions, as well as the division of each of the phases in small particles. If both phases are liquid, the minimum work to create a unit surface area for all modes is the same because it is defined only by temperature and chemical potential of the contacting phases [79]. If the released heat of polymerization at the interface is able to transfer a certain amount of monomer in the aqueous phase, it can be assumed that the polymerization reaction can also deform the interface and disperse system. If we start from an initial equilibrium state of the system, the transfer of certain amount of monomer in the monomer-rich aqueous phase of water is equivalent to supersaturation with respect to monomer and nucleation can be expected in the monomer droplets in the aqueous phase in the vicinity of the interface.
Oganesyan et al. [49-51] believe that the formation of droplets of the monomer due to polymerization at the interface monomer - water, can also be explained on the basis of their dependence on the specific surface energy, γ, and temperature.
where n is a constant depending on the nature of the substance (for organic liquids it is equal to 11/9 [51]). Tc is the critical temperature (at T = Tc there is a mixture of phases). From equation (4) it follows that with increasing T, γstrongly decreases. Thus, for every act of the reaction heat release in certain areas of the interface performs a partial mixing of fluids, and if stabilizing agents, such as surfactants or oligomeric growing radicals, are present in the system this process will lead to the formation of microdroplets of the monomer.
In [34, 35, 117, 118] Hansen and Ugelstad proposed to consider all three mechanisms of nucleation of PMP (micellar, homogeneous, and from microdroplets) when investigating EP at a concentration of emulsifier above CMC, assuming that the micelles containing solubilized monomer and monomer microdroplets compete with one another in the capture of oligomeric radicals from the aqueous medium. This causes that the rate of formation is the sum of the rate of the particle nucleation by all three mechanisms.
where Pmand Pd- the probability of absorption of oligomeric radicals by solubilized micelles and microdroplets of monomer respectively, and Ph is the probability of homogeneous nucleation in the aqueous phase. The sum of all probabilities is equal to unity:
where Pp1 and Pp0are the probability of capture of oligomeric radicals by PMP containing one and zero free radical.
On the basis of this model, several extreme cases have been investigated, such as
homogeneous nucleation of particles, taking into account the possible absorption of oligomeric radicals by particles and nuclei of small flocculation [34];
the limited flocculation of particles and nuclei, and small micellar nucleation PMP microdrop [34, 35];
competing homogeneous and micellar nucleation, with the possibility of desorption and reabsorption of free radicals with little flocculation of particles [117];
homogeneous nucleation of microdrop and low flocculation of particles [118].
Song and Poeleyn suggested a scheme (Figure 2) of particles nucleation at the initiation of EP monomer and potassium persulfate developed general kinetic model that takes into account all the mentioned above three possible mechanisms of particle nucleation [119-120].
Paths for the formation of particle nuclei starting from persulfate initiator radicals generated in the continuous aqueous phase. The symbols M and S represent monomer and surfactant species, respectively (from Principles and applications of emulsion polymerization / by Chorng-Shyan Chern. John Wiley & Sons, Inc., Hoboken, New Jersey 2008) [121].
These published data suggest that current theoretical ideas on the mechanism of PMP varied. This complicates the establishment of methods of synthesis of polymeric suspensions with regulated properties and makes it relevant and important research aimed at solving this problem.
The models of latex particle nucleation described in the literature were generalized in [122]. These models are the “collision process” [123, 124], “diffusion process” [27, 29], “diffusion/propagational” model (Maxwell et al. [125]), “collision/empirical” model (Dougherty [126] and Penlidis et al. [127]), “surface coverage” model (Yeliseyeva and Zuikov [128]), and “colloidal” model (Penboss et al. [129]).
The authors of [74] assume that the original monomer emulsion contains, in addition to surfactant micelles, monomer microdroplets with a size of 50–150 nm that are formed owing to fragmentation of monomer droplets in the initiation of polymerization and owing to the mass transfer of the emulsifier at the monomer/water interface. It was shown that the ratio of the number of micelles and microemulsion droplets in the system depends on both the type of surfactant and the method of introducing it into the system [101]. The number of microdroplets affects the pattern of the particle size distribution and the molecular mass distribution of the polymer. According to the authors of [101], PMPs are formed from both micelles and microemulsion droplets.
The simulation of emulsion polymerization of styrene is the subject of many publications, in which the authors discuss the dependence of the kinetics of the process on the emulsifier and initiator concentration [130], the particle size distribution [127, 131-134], and the molecular mass distribution [135-139] and perform a complete simulation of the kinetic characteristics of the process [24, 38, 130, 140-145]. However, all these studies are based on the Harkins–Yurzhenko qualitative theory and disregard the participation of microdroplets in the formation of PMPs.
In this study, we attempt to develop a mathematical model of emulsion polymerization for a partially water-soluble monomer (styrene) that takes into consideration that the original emulsion contains microdroplets that participate in the formation of PMPs.
The calculations are performed with disregard for homogeneous nucleation, polymerization in emulsion macrodroplets, and radical desorption from PMPs. In our opinion, these assumptions are fully justified for the polymerization of partiallywater-soluble monomers, such as styrene, and in the presence of a fairly high amount of emulsifier.
Let us introduce the following notations: The concentration of PMPs with volume ν with volume fraction of the monomer in them containing i growing radicals at time t is fi(ν, φ, t), the total concentration of PMPs in the system is expressed through fi(ν, φ, t) as, and their average diameter
To determine the effect of the dispersion state of the original emulsion system on the characteristics of emulsion polymerization, we performed a model calculation of the polymerization rate and the PMP size distribution in systems with different initial states.
Consider two limiting cases:
A system that contains, in addition to the aqueous phase and monomer emulsion droplets, microdroplets of the same size (here micelles can be regarded also as a limiting case of microdroplets). With this aim in view, we calculate the dependence of the polymerization rate and the PMP size distribution on the diameter of microdroplets.
A system that contains the aqueous phase, monomer emulsion droplets, and micelles and microdroplets of the same size. In this case, we estimate the effect of the ratio between the number and size of micelles and microemulsion droplets on the polymerization rate and the PMP size distribution.
Consider an emulsion system that contains emulsion droplets dispersed in the aqueous phase and microdroplets of diameter D0 (accordingly, of volume V0), the concentration of which is M0 particles per cubic centimeter. Monomer microdroplets will be present in the system if
If the emulsifier concentration in the system is high, that is, much higher than the critical concentration of micelle formation (it is this case that will be discussed below), we can disregard the fraction of a surfactant adsorbed on droplets. In this case, M0 and D0 are related as follows:
where SEis the emulsifier concentration in the system (mol•cm–3), amis the area occupied by one surfactant molecule on the microdroplet surface, and NAis Avogadro’s number.
In the absence of PMP coalescence and radical desorption from particles, the system of equations for the PMP distribution function fi(ν) for volume ν; the volume fraction of the monomer in them,
In Eq. (8), the first term in the right-hand side describes the arrival of a radical to a particle from the aqueous phase, the second term defines the change in the number of radicals during termination, and the last term describes the formation of new particles after the entry of a radical into a microdroplet. In the left-hand side of Eq. (8), θν and
From the balance for the emulsifier, we derive the time dependence of the microdroplet concentration:
Depending on the time of PMP formation, the volume fraction of the monomer,
The total concentration of PMPs in the system is
The polymerization rate W is expressed in terms of the functions Xi and Yi according to the equation
Equations (12)–(14) completely describe the first and second stages of emulsion polymerization. To calculate the distribution function fi, we introduce dimensionless variables and parameters:
In the approximation of fast termination we obtain the following system
where κ is the dimensionless parameter in equations
The criterion of a small size of microdroplets is short time tM of settling of the equilibrium volume fraction of the
Monomer
Figure 3 shows the PMP size distribution by the end of the first stage and the time dependence of microdroplet concentration in the system. It is evident that the PMP concentration increases almost linearly with time nearly to the end the first stage (t <t1), and the microdroplet concentration decreases also almost linearly; only near the end of the first stage, a sharp decrease in the microdroplet concentration down to zero occurs.
These results are derived under the assumption that the time of settling of the equilibrium volume fraction of the monomer,
or, since
Thus, if inequality (13) is fulfilled (the system consists of small microdroplets), then the PMP concentration will depend only on the initiator and emulsifier concentrations and will not depend on the size of microdroplets.
a) Time dependences of the concentrations of (1) microdroplets and (2) PMPs and (b) PMP size distributionby the end of the first stage:Dmax=D0(1+θt1/V0), and t1 is the time of exhaustion of microdroplets.
If the original emulsion system contains microdroplets with a large diameter (κ ≤ 1), throughout the first stage (at certain sizes, including a part or the entire second stage), the volume fraction of the monomer in PMPs formed from these microdroplets will be higher than equilibrium,
In this system, the concentration of PMPs with
Let us use
For the time dependence of conversion, from the condition of balance for the monomer, we have
Figure 4 shows the time dependence of conversion for the original system of large microdroplets, which is calculated through formula (15), and, for comparison, the time dependence of conversion for the original system of small microdroplets. It is evident that, in the approximation of fast termination, the polymerization occurs faster in the system with small microdroplets.
Time dependence of monomer conversion in systems with microdroplets of different sizes: D0 = (1) 150 and (2) 10 nm, am= 0.4 nm2, SE = 2 •10–4 mol cm–3, and ρ = 1012 cm–3 s–1.
Figure 5 shows the calculated dependence of the final concentration of PMPs in the system on the size of microdroplets for the case in which the rate of radical entry into particles is proportional to their surface. The characteristic size of microdroplets, DC, starting from which the PMP concentration decreases, is determined by the value of parameter κ = 1; therefore, it depends not only on D0 but also on the initiation rate and the emulsifier concentration. For typical values of concentrations of the components, this size is DC ≈ 40–60 nm.
To determine the effect of microdroplet size on the PMP size distribution through solution of system of equations (12)–(13), we calculated also the PMP size distribution by the end of the first stage for different values of D0 and changes in the distributions during polymerization. Figure 6 represents the PMP diameter distribution by the end of the first stage; Fig. 6 depicts a change in the distribution during polymerization, it shows that the PMP size distribution becomes narrower as the size of microdropletsincreases.
Dependence of PMP concentration in the system on the initial size of microdroplets: ρ = 1011\n\t\t\t\t\t\t\t\t\t(1), 1012\n\t\t\t\t\t\t\t\t\t(2) and 5х1012cm–3 s–1 (3), am= 0.4 nm2, SE = 2 •10–4 mol cm–3.
Change in the PMP diameter distribution during polymerization at conversions Р= (1) 5, (2) 15, and(3) 50%. Initial microdroplet size of (a) 10 and (b) 50 nm,am= 0.4 nm2, SE = 2 •10–4 mol cm–3, and ρ = 1012 cm–3 s–1.
To determine the mechanisms of polymerization in the case of comparable rates of PMP formation from micelles and microemulsion droplets, we study a bidisperse system that contains М0 microemulsion droplets with diameter D0 and μ0 micelles with diameter d0 and calculate the polymerization process in this system.
Depending on the method of introducing the emulsifier into the system, the formation of new droplets of the microemulsion can occur during polymerization; however, owing to the absence of experimental data on the rate of microemulsification in these systems and its dependence on the concentration of the free emulsifier and its distribution between phases, we restrict ourselves to the discussion of a system with a fixed number of microemulsion droplets. Consideration for the finite rate of formation of the microemulsion will only lead to a change in the duration of the stage of PMP formation and slightly affect other characteristics of the process.
To describe the rate of polymerization and the concentration of PMPs in the system under discussion, we introduce parameter Γ that characterizes the fraction of the surfactant adsorbed on the microemulsion surface:
where М0, D0, μ0, and d0 are related as follows
We shall divide all PMPs in the system into two types: PMPs formed from micelles (with subscript μ) and PMPs formed from the microemulsion droplets (with subscript М):
Here, we take into account that, in PMPs formed from micelles, the equilibrium volume fraction of the monomer,
.
the total number of PMPs formed from the microemulsion droplets, NM, is equal to the initial number of droplets, М0, and the number of PMPs formed from micelles, Nµ, is
where
is the total number of PMPs formed in the system if it contained only micelles. Since for large particles, for which the above formulas are valid, M0θ/V0ρ< 1, then the total number of PMPs is less than
The PMP size distribution is the sum of two distributions: the distributioncorresponding to PMPs formed from micelles and the distribution corresponding to PMPs formed from microemulsion droplets. The index of diameter polydispersity by the end of the first stage is derived through calculation of the average diameter and the square of the average diameter of PMPs:
Figure 7 shows the dependence of KD on Γ for κ = 104. It is evident that index KD by the end of the first stage depends on the d0/D0 ratio and has maximum.
Figure 8 shows the dependence of the finite number of PMPs formed in the system on parameter Γ at different values of d0/D0. It is evident that, during an insignificant difference between the sizes of the two fractions, the PMP concentration hardly changes at all; however, when the size of large microdroplets is five (or more) times larger than the size of small microdroplets (solubilized micelles), the PMP concentration sharply decreases with an increase in Γ. Figure 9 depicts the size distributions of PMPs by the end of the first stage for various values of Γ and d0/D0. It is evident that the distribution is bimodal. Figure 10 shows that the PMP size distribution by the end of the first stage is bimodal in the general case. The peak relating to a smaller diameter corresponds to PMPs formed from micelles; that relating to a larger diameter corresponds to PMPs formed from microemulsion droplets. The transition from the bimodal size distribution of PMPs to the unimodal size distribution of PMPs by the end of the first stage occurs at a ratio of the initial sizes of micelles and microemulsion droplets of d0/D0 ≈ κ–1/5.
Dependence of the polydispersity index of the PMP size distribution on the amount of the surfactant adsorbed on the microemulsion surface (parameter Γ): am= 0.4 nm2, SE = 2 •10–4 mol cm–3, and ρ = 1012 cm–3 s–1. d0 = 10 nm; D0 = (1) 80, (2) 100, and (3) 150 nm.
Dependence of PMP concentration on parameter Γ: d0 = 10 nm; D0 = 20 (1), 50 (2), 80 (3), 100 (4) and(5) 150 nm
PMP size distribution in systems with simultaneous PMP formation from micelles and microemulsiondroplets. The end of the first stage. Here, d0 = 10 nm, D0 = (1, 2) 100 and (3) 50 nm, and Γ = (1) 0.25 and (2, 3) 0.5.
Change in the PMP diameter distribution during polymerization: d0 = 10 nm, D0 = 50 nm, and Γ = 0.5. Monomer conversion: (1) 3, (2) 15, and (3) 50%.
So a mathematical model of emulsion polymerization of poor soluble monomers has been developed. It takes into account the basic physio-chemical characteristics of the system and enables simultaneous calculating important parameters of the system such as polymerization rate, the time dependence of conversion, the concentration and size distribution of PMPs and molecular mass distribution of polymer [105]. The models predicts that with increasing microdroplet size D0 above the critical value DC~ 40–60 nm, the PMP concentration and the polymerization rate will decrease; at D0< DC, both the polymerization rate and the PMP concentration do not depend on D0. The PMP distribution becomes narrower with an increase in D0. In the case of simultaneous formation of PMPs from micelles and monomer microdroplets, the PMP size distribution is broad or, under certain conditions, even bimodal.
Analysis of experimental data shows that monomer microdroplets play an important role which should not be ignored neither in studying mechanism of polymer-monomer particles nucleation in emulsion polymerization nor in modeling this complicated process.
Soybean (Glycine max (L.) Merr.) has become one of the most important, versatile globally traded commodities, being a widely used source of protein, oil, and biofuel. Its uses include as a source of protein and fibre for livestock and an alternative to meat and dairy products in humans. Soya products are also increasingly used widely in the food industry, in particular as texturisers, emulsifiers, and protein fillers; soya flour is often added to bakery products, such as bread, biscuits, pastry, etc. Soybean oil is the second largest source of vegetable oil globally and is also used in products such as biodiesel and detergents.
Soybeans are crushed to form meal, typically used in animal feed, and oil. The hull or husk of the soybean is a by-product of soybean oil and meal production where the beans are de-hulled prior to crushing. Soya hull is also internationally traded as an animal feedstuff, providing a good source of digestible fibre, albeit of lower protein content of soya meal.
The EU imported about 18 million metric tons of soya in 2018 [1]. Approximately 90% of these imports are used to feed livestock and reflect about 28% of global soya imports. China imports approximately 88 million metric tons. Although the USA remains the largest exporter of soya, projected export growth is concentrated in South America, particularly Brazil, Argentina, Paraguay, and Bolivia. The UK imports some 3 million tons annually with more than 70% directly from Argentina and Brazil. There is also an inter-trade within Europe, with the Netherlands being an important hub. The UK imports approximately two thirds as soya meal/hulls and one third as soybeans [2]. The UK only imports a relatively small quantity of soya oil, approximately 200,000 tons. The UK does not produce biofuels to any extent from imported soya.
Therefore, there is considerable bulk transportation by sea, involving handling at ports equipped to handle bulk grains and foodstuffs. Thereafter there is onward transportation for use in the animal feed industry, further processing, and the human food sector.
However, soya is not without associated risks to health. Soya products are recognised as one of the EU’s 14 major food allergens and listed in Annex II of the EU Regulation 1169/2011 on labelling of foods and UK equivalent domestic legislation [3]. It is also listed as a major food allergen by the FDA (USA) labelling regulations. As soybean and its products are used in many processed foods, it is difficult for the allergic consumer to avoid and is often classified as a “hidden allergen”. Additionally, evidence from a number of sources identify proteins found in soybean and its products as respiratory allergens capable of producing a range of ocular and upper and lower respiratory symptoms, including asthma.
This chapter focuses on both published evidence and our own studies related to the respiratory risk from airborne dusts related to soya.
Soybean (Glycine max (L.) Merr.) or soya bean is the edible seed of an annual legume of the pea family (Fabaceae). The hull or husk of the mature bean is hard and water-resistant and protects the cotyledon of the seed from damage.
The major forms of soya usually encountered in end-user countries in the EU are:
Soybean, after removal of hull covering the bean, containing about 40% protein and 20% fat/lipid.
Soya meal (see Figure 1). This may be of two forms: pure meal produced after de-hulling and possibly extraction of oil or with subsequent added hull to extend the product. Soybean meal made from de-hulled beans has a total protein content of approximately 40–49% and 3% fibre.
Soybean hull, these are often pelletised as a commercial product to make a more handleable, less dusty product (see Figure 1). The protein content of hull is around 9–19%, with a fibre content of 53–74%. The proteins in hull tend to be of lower molecular weight than those in pure soya meal (Figure 2).
Soya oil is produced by crushing and/or chemical extraction. Soya oil, particularly the more highly purified, is considered less allergenic due to the low concentration of soya proteins within it [4, 5]. It is used widely in food processing.
Soya flour—milled in a similar way to cereal flour (e.g. wheat, rye). Flours from various cereals have the propensity to be “dusty”, and the control of their handling is necessary to prevent airborne exposure to flour dust and consequent health effects [6, 7]. Soya flour has become increasingly used in food processing. Allergens in soya flour have been identified and characterised [8].
The left hand image shows an image of a soya meal imported in the UK. The right-hand image shows a sample of soya hull imported into the UK. The pelletised hull material shows some evidence of breakdown, probably due to compaction in the hold of the ship.
Left-hand gel shows a Coomassie blue-stained reducing gel of an extracted soya hull material, lane labelled 1. Right-hand gel shows a similarly stained gel of an extract of soya meal, lane labelled 2. Molecular weight marker bands for lanes marked as M have molecular weights from the top of 200, 150, 100, 75, 50, 37, 25, 20, 15, and 10 kDa.
Figure 2 shows electrophoresis gels of extracts of a soya hull and soya meal, respectively, after extraction at 10% w/v using 0.1% Tween 20 in phosphate buffered saline. These gels separate proteins on the basis of their molecular weights. The patterns of proteins in soya hull show considerable differences to soya meal. There is a predominance of high molecular weight proteins in meal in comparison with hull where the majority of proteins appear to be less than 23 kDa. For some soya meal products, hull is reintroduced to adjust the overall protein content, so differing soya meal imports may contain differing levels of hull proteins.
However not all proteins are allergenic, in terms of sensitising an individual’s immune system to provoke an exaggerated IgE-mediated immune response on subsequent exposure to the same protein, i.e. a type 1 allergic response. Allergenic proteins appear to be restricted to classes or families of proteins based on their structural and functional properties [9, 10, 11, 12].
However, besides intrinsic, specific allergenic proteins, covered in the next section, there are other “contaminants” or extrinsic material that may be associated with soya products and possibly lead to respiratory illnesses or symptoms, if inhaled. These include:
Endotoxin is a pyrogenic lipopolysaccharide and a component of the exterior cell wall of gram-negative bacteria, like E. coli. High concentrations of airborne endotoxin can cause respiratory inflammation, symptoms, and lung function decline [13, 14, 15, 16]. The Netherlands has set a suggested health-based exposure limit for airborne endotoxin [17]. Endotoxin has been found to be extractable from soya meal and husk and becomes airborne when handling bulk [18, 19, 20, 21].
β-Glucans are naturally occurring polysaccharides, being constituents of the cell wall of certain pathogenic bacteria and almost all fungi. Their measurement in airborne samples has been used as an indicator of total fungal exposure. β-Glucans have been linked to activating macrophages, neutrophils, monocytes, and NK cells, thus involving the innate and adaptive immune systems. Biological activity seems related to their degree of branching and molecular weight; greater branching gives rise to greater biological activity, with the (1 → 3) chain essential in the induction of immune responses [22].
Fungi such as Aspergillus spp., particularly the A. glaucus group, and Penicillium spp. are known as storage moulds. Contamination of batches of soya with uncontrolled fungal growth, particularly Aspergillus spp. and Penicillium spp., leads to spoilage. Certain toxicogenic Aspergillus species under the right conditions of moisture and temperature can lead to the production of mycotoxins and carcinogenic aflatoxins [23, 24]. In addition, Aspergillus and Penicillium species are allergenic and can also cause hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA). Aspergillus fumigatus can produce significant numbers of conidia (spores) containing allergenic proteins, e.g. Asp f 1, and in immune-compromised humans is the most common life-threatening, opportunistic fungal pathogen. Nonetheless, several strains of Aspergillus are used in the controlled fermentation of soya to produce soy sauce, including A. oryzae. Alpha amylase from this fungal source is used as an additive improver in cereal flour and associated with significant sensitisation in bakers [25, 26].
Organic dust with no identifiable toxic properties can cause irritation and inflammatory responses in the lungs if the particles are small enough. Larger dust particles will lodge in the nasal passage or the throat and be cleared from the body. Particles of less than 10 μm aerodynamic diameter can enter the lungs past the bronchus, and particles less than 4 μm can reach the alveoli deep in the lungs, producing significant lower respiratory tract symptoms. Limited evidence suggests that this mechanism may be relevant for soya dust [27]. Organic dust toxic syndrome (ODTS) and EAA are distinct pathological entities associated with smaller particles below 5 μm [28]. Asthmatic reactions are generally provoked by particle sizes of 5–10 μm [29].
Of these intrinsic and extrinsic factors potentially associated with soya, it is the health effects from exposure to intrinsic soya allergens that are underpinned by significant scientific evidence. This will be the major thrust of the remainder of this chapter.
A number of allergens have been identified and characterised in soya and its products. A 2012 OECD document on soybean allergens lists 15 proteins designated as allergens, largely derived from one literature review [12]. However there has been a criticism about the lack of evidence for some of these “putative” allergens [30]. A non-exhaustive list of allergens is shown in the Table 1 below. Many of the allergens were identified from a food perspective, with subsequent work to produce “hypoallergenic” cultivars [12]. While there has been considerable research on genetically modified soya with lower levels of endogenous major food allergens, a large natural variation (9–15-fold) in the levels of Gly m 4, Gly m 5, Gly m 6, Gly m Bd 28 k, and Gly m Bd 30 k has also been identified [31].
Allergen | Description | Comments |
---|---|---|
Gly m 1 | Hydrophobic soybean protein. MW 7–8 kDa with two isoforms | Abundant in soybean dust. Husk and pods are a rich source. Implicated in epidemic asthma outbreaks in harbour cities caused by soy dust [32, 33] |
Gly m 2 | MW 8 kDa protein with a pI 6. A member of the defensin family | Gly m 2 is abundant in soya husk and implicated in epidemic asthma outbreaks in Spanish dock cities [34]. Shows some homology with a storage protein in the cotyledon of cowpea and green pea |
Gly m 3 | MW 12–15 kDa protein | A profilin type of allergen. Shows some cross-reactivity with birch profilin [11, 35] |
Gly m 4 | MW 17 kDa. Homolog of Bet v 1, a birch allergen | Implicated as the major allergen where patients are allergic to birch pollen and have soy allergy [36] |
Gly m 8 | MW 28 kDa. 2S albumin | 2S albumins [37]. Some homology with Ara h 2, a peanut allergen. Identified as a food allergen |
Gly m 39kD | MW 39 kDa | P39 protein was detectable only in the fully mature dry seed distributed in the matrix of the protein storage vacuoles [38] |
Gly m Bd28K | MW 28 kDa. A vicilin-like glycoprotein | A major food allergen [39] |
Gly m Bd30K | MW 30–34 kDa protein, a thiol protease of the papain superfamily | A soybean oil body-associated glycoprotein, shows 30% sequence homology with Der p 1, a major allergen of house dust mite. An important dietary allergen, widely known as P34 [40] |
Gly m Bd 60 K | MW 63–67 kDa protein | An alpha subunit of beta-conglycinin well-known as a major soybean storage protein. Major food allergen |
Gly m TI | MW 20 kDa, a trypsin inhibitor | Has been implicated as a workplace inhalant allergen in bakers [41]. Found in the seed and soya flour |
Gly m 5 | β-Conglycinin, three isoallergens | Seed storage protein. Sensitisation to Gly m 5 is potentially indicative for severe allergic reactions to soy [42] |
Gly m 6 | Glycinin, five isoallergens | Sensitisation to Gly m 6 is potentially indicative for severe allergic reactions to soy [42] |
Gly m 7 | MW 76 kDa Seed biotinylated protein (SBP) | SBP may represent a class of biologically active legume allergens with structural resilience to many food-manufacturing processes [43] |
A non-exhaustive list of allergens identified in soya.
MW refers to molecular weight.
However, a much smaller number of the allergens in Table 1 have been implicated in terms of airborne exposure during occupational practices and associated health effects.
Further airborne exposure to an allergen in an individual already sensitised can cause a range of symptoms affecting the eyes, nose, and upper and lower respiratory systems, including the development of occupational asthma (OA). OA is a disease characterised by variable airflow limitation and airway hyperresponsiveness due to a particular occupational environment. Two main types of OA are identified [44]. Immunological OA develops after a latent period of exposure during which the worker acquires sensitisation to the causal agent, typically involving IgE-mediated immunological sensitisation to allergenic proteins. Non-immunologic OA is usually due to irritant mechanisms associated with the cumulative effects of exposure to a workplace dust or chemicals. Both forms of OA can be serious enough to prevent an individual’s continued employment in that workplace and even cause permanent disability.
The first study describing soya allergy related to dust from a soybean mill was published in 1934 [45]. In 1977 a study was published of immediate and late-onset OA in a previously non-allergic subject exposed to soya flour in the manufacture of food supplements [46]. Exposure to soya dust and soya flour has been implicated in causing OA or other respiratory health symptoms in persons working in a variety of occupations, such as farmers, millers, soybean processors, and bakers [8, 26, 27, 41, 45, 47, 48].
In the 1990s, a number of scientific papers were published that investigated “asthma epidemics” in harbour cities, the cases of asthma being found in the general population. Investigation discovered these asthma cases were related to the loading or unloading of soya products. Reports were related to New Orleans, the USA [49], Cartagena, Spain [50, 51], Tarragon, Spain [52], Saint-Nazaire, France [53], Naples, Italy [54], Valencia, Coruna, Spain [55], and Barcelona, Spain [56].
The original outbreaks of asthma epidemics occurred in New Orleans, starting in 1953 and continuing for almost 20 years. Sometimes more than 200 people sought treatment in a single day at a hospital serving a largely black, poor population [57]. Initial investigations associated the outbreaks with low wind speeds but from a specific direction and together with particular climatic conditions. However, it was only in 1997, and after the investigations concerning Barcelona, that these community asthma outbreaks were specifically linked to the loading of soya (but not wheat or corn) into ships using an elevator system [49], suggesting that soy dust may be particularly asthmagenic compared with some grain dusts.
The asthma epidemics that occurred in Barcelona have been the best documented, and a considerable amount of research was expended in linking soya unloading at the docks with the asthma epidemics in the city, rather than other possible precipitating factors, such as traffic pollution, moulds, etc. [24, 34, 58, 59, 60]. From 1981 to 1987, 26 outbreaks of asthma occurred in the city of Barcelona, affecting a total of 687 subjects and causing 958 emergency room admissions and 20 deaths. Further outbreaks occurred in 1994 and 1996. The initial asthma events coincided with the unloading of soya into silos without a filter, climatic conditions of high-pressure areas, and the wind direction from the harbour to the city [61].
While it might be that very specific geo-climatic conditions were the drivers for the Barcelona and other asthma epidemics, a number of important factors emerged of wider significance. Some of which were confirmed from other studies of asthma epidemics, and some of which suggested the need for further work as follows:
The latency period from initial unloading of soya in Barcelona to asthma outbreaks appears consistent with that of occupational asthma. Children were rarely affected in these asthma epidemics, and age appeared a risk factor [62].
The primacy of implementing exposure control measures on the occupational processes to control dust emissions and prevent further asthma outbreaks [49, 63].
While climatic conditions may have been important, these phenomena suggest that some soya dusts generated are of a small aerodynamic diameter with high buoyancy to travel relatively large distances and penetrate deep in the lungs.
The allergenic material identified in Barcelona implicated glycoproteins with molecular weights lower than 14 kDa, with the major allergen identified as Gly m 1 [33, 64], localised in soybean hulls/husks (see Table 1). Gly m 2 was also implicated [34]. Ninety-two percent of patients in the Tarragona epidemics were sensitised to soybean hull extracts [52].
In response to the Barcelona episodes, significant effort was put into developing immunoassays capable of quantifying the putative allergen(s) with the necessary sensitivity to measure airborne levels. As with many other aeroallergen immunoassays, they progressed from initial competitive immunoassays utilising pools of serum from sensitised individuals [65] to non-competitive, sandwich assays based on polyclonal [66] or monoclonal antibodies [67]. As found for other aeroallergens, the inhibition assays are less sensitive and give considerably higher results when compared with non-competitive sandwich immunoassays [66].
Airborne Gly m 1 levels were measured by monoclonal sandwich immunoassay, at progressive distances from Ancona’s (Italy) port, where soya is unloaded [68]. Allergen concentrations were less than 171 ng m−3, whereas HSP levels (highly homologous with Gly m 1 [66]) measured by sandwich immunoassay during dockside activities in Barcelona and the UK were considerably higher [19]. Decreases in allergen away from the unloading area in Ancona were detected. Airborne Gly m 1 was not coupled with the presence of soya-carrying ships in the port, but significant relationships between allergen and meteorological parameters were found, suggesting that Gly m 1 appeared part of Ancona’s atmospheric dust. The authors suggest these allergen levels seem consistent with the absence of asthma epidemic outbreaks in Ancona.
There is evidence of genetic factors, atopy, and smoking status modifying the response to exposure to soybean dust [62, 69]. Atopy and smoking have been identified as risk factors for sensitisation and work-related respiratory symptoms with a number of other occupational allergens, e.g. bakers [70], laboratory animals workers [71], and seafood processors [72, 73].
There is evidence of co-exposure and sensitisation to some fungi and moulds, but it does not appear to have been causative of the symptoms/illnesses. Specific IgEs in a small group of asthma epidemic (AE) patients were compared with asthmatic non-epidemic patients and non-allergic controls [24]. The AE group showed low levels of specific IgE to A. flavus, A. fumigatus, A. glaucus, Penicillium notatum, and P. chrysogenum but significantly lower than IgE levels against soybean hull. All the AE group were sensitised to soya hull but between 8 and 92% against the moulds (A. flavus, A. nidulans, A. glaucus, and P. notatum being predominant).
Alvarez [74] showed in a small-scale study that 25% of bakers were sensitised to soybean. A review of cross-sectional studies employing skin prick tests in bakers showed that 5–77% were sensitised to soybean flour [75]. A relatively recent UK study suggested a prevalence of 21% sensitisation using similar methodology [76]. Baur [77] found 21% serological sensitisation to soybean flour in 140 bakers who had a history of greater than 6 months of employment and work-related asthma, rhinitis, and/or conjunctivitis. Two workers were shown to be sensitised to soybean lecithin, although the lecithin was possibly contaminated with low levels of soya proteins [78]. Baur [41] studied a relatively small group of bakers both sensitised to soybean and suffering workplace symptoms. Twelve were also sensitised to wheat, ten to rye, and five to alpha amylase from A. oryzae (FAA). The latter being an enzyme often added to flour in small quantities, but it is now regarded as a potent allergen [25]. Baur identified soya trypsin inhibitor (STI) or Gly m T1 as a major allergen, being recognised by IgE antibodies in the sera of 86% of the examined sensitised bakers. This research was one of the drivers for the Health and Safety Executive (HSE) to develop an immunoassay sensitive enough to detect airborne levels of STI from the use of soya flour and possibly other soya products [18, 19].
In a laboratory study of components of flour improvers, a representative soya flour was neither more inherently “dusty” nor showed a shift to smaller particle sizes than three different wheat flours [79]. However, although the improvers contained a higher percentage of wheat flour than soya flour, there was roughly 10-fold more extractable STI in comparison to wheat alpha amylase inhibitor (WAAI) per unit weight of improver. WAAI is a major allergen and sensitiser in bakers, with a subunit size of around 14–16 kDa and is restricted to the seed storage tissue (endosperm) [80, 81].
Quirce [82] examined four bakers or confectioners who were sensitised to both soya and wheat using skin prick tests. A positive response to STI and FAA was noted in 2/4 cases. IgE-binding bands against soya flour showed bands at molecular weights between 25 and 55 kDa and also high molecular weight IgE-binding bands against hull extract. A case study [8] of a sensitised individual presenting with asthma after 6 years of using soya flour in food processing (not a bakery) showed immunoreactivity against nine soya proteins in the molecular weight range of 15–55 kDa. Interestingly, cross-reactivity studies with other legumes demonstrated apparent immunologic identity between a component in green pea extract and a soybean protein with a molecular weight of 17 kDa [8].
Overall these data confirm that the allergens caused by soya flour are predominantly higher molecular weight proteins, whereas the asthma epidemics in harbour cities were caused by low molecular weight proteins, specifically the allergenic proteins, Gly m1 and Gly m 2.
Early investigations in Yugoslavian soya processors by Zuskin [27, 83] studied dust inhalation and respiratory symptoms after the oil had been extracted. Exposed workers showed a considerable increase in respiratory symptoms over controls, e.g. cough, nasal symptoms, and wheezing being reported by 56, 41, and 30% of workers, respectively. Most workers were smokers, and inhalable dust levels were considerable, with a mean (range) of 29.5 (7.7–59.9) mg m−3. Decreases in lung function were noted over the working week and pre-shift Monday testing suggesting evidence of chronic impairment [83]. Sixteen percent showed serological evidence of specific anti soya IgE, although 68% were positive against house dust mite. Zuskin appeared to be suggesting an irritant rather than immunologic mechanism for the airways disease.
Two related studies [21, 26] investigated sensitisation, symptoms, and exposure measurements in three South African soya processing plants. These plants were producing soya flour, based on de-hulling, cooking, and finally milling. Median (range) of inhalable dust levels were 2.58 (0.24–35.02) mg m−3; STI allergen levels gave a median (range) of 70 (50–2580) ng m−3 and were higher in the later parts of the process. There was no significant correlation between dust levels and allergen levels. Thirty-one percent of workers were current smokers, much lower than found in Zuskin’s study. There were significant associations between worked-related chest tightness, nasal symptoms, and cough/chest tightness after handling soya and sensitisation to soybean. Thirty-three percent of the workers were atopic, and 14% were sensitised to soybean not containing hull allergens. Atopy but not smoking was associated with sensitisation to soybeans, confirming the association between atopy and sensitisation to occupational allergens (Section 2.4.1).
Interestingly, Harris-Roberts reported that those transferring soybeans from farms into the processing plants’ silos, where soya hull would be present, had an excess of “flu-like “symptoms of fever, aching, and tiredness [26]. Such work-related, flu-like symptoms unrelated to soya sIgE levels were also noted by Cummings, but in processors not exposed to hull [84]. The biological reason for the “flu-like” symptoms is unclear. Harris-Roberts [26] hypothesised that these symptoms may suggest organic dust toxic syndrome (ODTS) in which inhaled endotoxin has been implicated [85]. Higher levels of endotoxin are found in hull rather than soybean or soya meal [18, 19, 26], but unfortunately airborne endotoxin levels were not measured in the Harris-Roberts study. Hypersensitivity pneumonitis (HP) also called extrinsic allergic alveolitis (EAA) has been reported in a single case while handling soybean as an animal feed [86]. Both ODTS and HP can give rise to similar “flu-like” symptoms some 4–12 hours after exposure. Whatever the cause or pathology, it raises the possibility of other health problems in soya-exposed workers besides those caused by IgE-mediated sensitisation.
A study was carried out in 2007 at a US soya processing plant receiving de-oiled, de-hulled, and crushed soya flakes for further processing. Concerns had been raised about asthma and other respiratory symptoms [48, 87]. Serum IgE immunoblotting studies showed multiple soya antigens, with 48, 54, and 62 kDa being most prominent, including storage proteins Gly m 5 and Gly m 6. As possibly expected, no sIgE to Gly m 1 or Gly m 2 was detected in this de-hulled material. The prevalence of soya specific IgE was 21% (versus 4% in controls), albeit only 7% gave a positive skin prick test for soya. Ten percent showed specific IgE towards storage mites. Those participants with soya-specific IgE had a threefold risk of current asthma or asthma-like symptoms and a six fold risk of work-related asthma symptoms. Thus asthmas and symptoms of asthma were associated with immunogenic nature of this de-hulled soya material. Work-related sinusitis, nasal allergies, and rash were also associated with reported mould exposure.
A single case study was reported from an animal feed factory, where for 5 years a man had been separating the soybean from hull before grinding [88]. He was atopic, although negative to storage mite. He showed a strong bronchial response to a challenge by soya hull but negative to soya flour. Unfortunately this short report is not clearer on the specific tasks being undertaken.
Heederik [89] studied sensitisation and respiratory effects in atopics and asthmatics (cases) living close to a Dutch soya oil producing factory. Soybeans and the oil product were transported by ship. Soya waste, aſter oil extraction, was removed by truck and noted to be “very dusty”. Soybean unloading was carried out without any emission controls and caused visible dust clouds. Loading trucks with waste also caused dust clouds around the factory area, with spillages in transit. Only 11% of the cases were sensitised to soya by skin prick test, the same as in matched controls. Soya-sensitised individuals living in proximity to the factory reported more respiratory symptoms, used bronchodilators more often, and had poorer lung function after having been downwind of the factory. Airborne soya allergen, measured by competitive immunoassay, was found more frequently surrounding the factory with levels higher than in the control area but much lower than found on the factory premises. Periodic, high endotoxin concentrations close to the factory exceeded the suggested Dutch threshold level of 90 EU m−3 [17]. Interestingly only 14% of workers, although more highly exposed than the cases, were sensitised to soya, with 31% being atopic.
A study in Argentina, which is an important producer of soybeans and its products, looked at 365 cases of asthma or allergic rhinitis and 50 healthy controls. Both groups were classified as to whether they had occupational exposure to soya, were in proximity to soybean fields or grain elevators, or lived in an urban environment without obvious exposure to soybean dust [90] . The overall prevalences of sensitisation by skin prick test to soya hull in cases and controls were 15 and 0%, respectively. In the cases subdivided by exposure classification, these sensitisation prevalences were 39% (occupationally), 20% (proximity), and 8% (urban). Positive skin prick tests were higher for mites (mainly storage mites), pollen, and moulds in those positive to soya hull extract. Serological sensitisation (sIgE) to soya hull was 39 and 10% in cases and controls, respectively. The data suggest that atopic status and inhalation of soybean dust are necessary for sensitisation to soya hull. The authors opine that sensitisation to moulds could be related to contaminated soya and noted that no near-fatal or fatal asthma had occurred, unlike the situation in epidemic asthma outbreaks involving sudden exposures to soya dust. The authors suggested that their data indicates that an immunologic mechanism rather than irritancy is responsible for soybean-induced asthma in those repeatedly exposed.
Three studies were undertaken by the HSE during 2012–2017. Two studies involved occupational hygiene monitoring at different UK ports handling soya. The third study was laboratory-based, investigating inherent “dustiness” in seven imported bulk soya products. Two established allergen assays were employed: a polyclonal sandwich assay for hydrophobic seed protein (HSP) established by our collaborators in Barcelona—HSP is highly homologous with the two Gly m 1 isoallergens [66, 91]—and soya trypsin inhibitor (STI) that has been implicated as a major allergen in bakers handling soya flour [41]. Endotoxin measurements were employed to establish the extent of endotoxin contamination of soya products and the levels of airborne endotoxin that workers may inhale.
One of these studies was in response to a complaint of respiratory symptoms in a workplace situated some 300 m from a dock in the South of England. This dock is used for the unloading of soya from bulk cargo ships, its storage, and onward transport to end-users [19, 92]. Essentially, this was an occupational hygiene study but also measured the levels of soya allergen at the perimeter of the dockside operation and slightly beyond. The dock is situated to the west of a city centre of some 250,000 individuals. Containers ships are emptied by dockside grab cranes into hoppers for loading of either heavy goods vehicles for onward transportation or a conveyor belt whereby the soya was transferred to storage warehouses on-site. Concerns had been raised by the stevedores and harbour managers about the unloading of a particularly dusty batch of finely ground soya meal. But generally soya dust was visually noticeable during any unloading activity of soya meal or hull.
Samples of four different soya bulks unloaded during the study were collected. One bulk had evidence of areas of gross fungal contamination, which was identified as Aspergillus glaucus with moderate amounts of Aspergillus fumigatus. Both these fungi are common on vegetation and stored agricultural material and with sufficient available water can allow for potential heavy growth. Inhalation of these fungi is also implicated as causing hypersensitivity pneumonitis. Hull was unloaded on day 1 and meal on days 2 and 3. The hull product was a pelletised material, showing evidence of breakdown (see Figure 1).
The hull sample had considerably more endotoxin than meal samples (Table 2). While the hull sample had 15-fold more HSP than the meal, the difference in STI levels between the hull and meal was much lower. Table 3 shows the results from static air monitoring at or outside the perimeter of the dock operation. Amounts of allergen were measurable at these peripheral sampling sites. On the day of hull unloading, significant levels of the allergen associated with asthma epidemics were measured some 150–200 m in the prevailing wind direction from where soya dust clouds were being generated.
Bulk material | Endotoxin (EU g−1) | HSP (μg g−1) | STI (μg g−1) | Asp f 1 (μg g−1) | Der p 2 (μg g−1) |
---|---|---|---|---|---|
Pelletised hull | 80,364 | 2824 | 798 | Trace | Trace |
Meal | 4630 | 196 | 270 | ND | ND |
Meal, GM-free | 1309 | 178 | 233 | ND | ND |
Amounts of allergens and endotoxin extracted from the three bulk samples not showing evidence of gross fungal contamination.
ND is non-detected.
Site | Position | STI (ng m−3) | HSP (ng m−3) |
---|---|---|---|
Cruise ship customers’ car park | 170 m from nearest source (conveyor or hopper). In prevailing wind direction | 13 (ND-40) | 26 (11–125) |
East end of building off-site | 150–200 m from open sources. In prevailing wind direction | ND (ND-4) | 87 (30–1300)a |
Road entrance | 100–150 m from sources of conveyor or hopper | 19 (2–24) | 54 (26–85) |
Steps at boundary wall | 150 m from hopper. In prevailing wind direction | 56 (7–80) | 339 (27–898)a |
Allergen levels at the perimeter of an UK dock operation and, beyond, unloading soya.
High value associated with day of hull unloading.
Visual dust clouds outside were noted during various activities: (a) loading of hoppers; (b) loading of lorries from the hopper, see Figure 3; (c) the moving conveyor, which was subject to spillages; and (d) from craneage of soya out of the ship’s holds, see Figure 4. Visual clouds of dust were also produced within the storage facility as the unloaded soya was formed into piles by a pusher loader or loaded into vehicles for onward transport. Respiratory protective equipment (FFP2 respirators) was worn by workers in the ship’s hold, but was not uniformly worn elsewhere.
The median (range) of personal atmospheric monitoring sampling in workers over the 3 days of study were 130 (33–3071) ng m−3 and 583 (170–12,629) ng m−3 for STI and HSP, respectively. High allergen values were found when moving soya within the enclosed storage warehouses and within the ship’s holds. Inhalable dust exposures (personal samples), expressed as 8 hour TWA, ranged from 1.2 to 4.5 mg m−3; the current UK workplace exposure limit (WEL) for flour dust and grain dust is 10 mg m−3. Interestingly on the day that hull was unloaded, high levels of HSP (2925 ng m−3) were sampled within the crane’s cab although some 50 m above the dockside.
The visual dust cloud from a lorry loading soya from the hopper and the crane grab depositing soya into the hopper.
The crane is being used to move soya from the ship’s hold to the dockside hoppers. Spillages and dust clouds from the crane’s grab are noticeable.
The second HSE study addressed issues concerning the likely differences in dustiness of various bulk soya products and the categorisation of the particle size of dusts generated [18]. A rotating drum testing method has been established that can investigate the generated levels of a dust under standardised conditions that are associated with the defined inhalable, thoracic, and respirable particle size fractions [93, 94, 95]. Inhalable particles of an aerodynamic diameter (AD) ≤ 100 μm can enter the respiratory tract via the nose and mouth. Thoracic sized particles (AD < 30 μm) are defined as those small enough to penetrate past the larynx as far as the trachea and bronchial areas of the lung. Respirable particles (AD < 10 μm) can enter the deeper part of the lungs.
Essentially, a fixed amount of bulk material is rotated at a set speed and time-period in a drum with vanes that lift and drop the bulk material during rotation. A constant airflow through the drum entrains any airborne dust that is collected on an in-line series of two metal foams with different pore densities and finally a glass microfibre filter. Three replicate runs with gravimetric analysis and extraction of allergens of the foams and filter are used to calculate an average dustiness in the inhalable, thoracic, and respirable sized fractions. This technique was used to compare the intrinsic dustiness in seven different bulk soya consignments recently imported into the UK and Ireland and extended to include the two major soya respiratory allergens (HSP and STI) in the generated dust fractions during dustiness testing. However, care has to be taken about not over-interpreting such results as defining actual worker exposure [96] but rather an indication of the relevant propensity of different bulks to generate dust and allergen aerosols of certain defined sizes.
The seven bulks tests included two pelletised hull and five meal bulks. None of them showed any visual fungal contamination. The mean concentration of allergens and endotoxin for meal and hull samples is shown in Table 4. Whereas the amount of extractable low molecular weight HSP in hull is 23-fold that in meal, there is also on average 4-fold more of 20 kDa STI in the hull product than the meal. Very low levels of the Aspergillus fumigatus allergen were found in all of the bulks. As reported previously [26], higher endotoxin levels tend to be found in hull than meal samples and may represent a potential additional respiratory risk [28, 85, 97].
Type | STI (μg g−1) | HSP (μg g−1) | Asp f 1 (μg g−1) | Endotoxin (EU g−1) | Moisture (%) |
---|---|---|---|---|---|
Meal | 127 (28–270) | 122 (54–196) | 17 × 10−3 (5–33) × 10−3 | 12,922a (1309–51,455) | 8.3 (5.2–13.9) |
Hull | 528 (258–798) | 2862 (2824–2900) | 13 × 10−3 (8–19) × 10−3 | 66,577 (52,769–80,364) | 7.4 (6.1–8.7) |
Mean (range) of allergens and endotoxin extractable from the bulks and their moisture content.
Mean inflated by one high bulk value.
Of the seven bulk samples, one sample showed “high” dustiness (gravimetric results) in both the thoracic and respiratory fractions compared with the other samples. This may suggest that this particular material may be the sort of bulk that produces small, buoyant dust particles that could travel further with prevailing winds and penetrate deep into the lungs to cause symptoms of irritation. Interestingly allergen concentrations in the smaller particles of this specific material did not parallel the gravimetric results. The levels of allergen in the three fractions sizes largely depend upon both the dust levels in those fractions and the amount of allergen that was readily extractable from the bulk material. So the highest concentration of HSP in small respirable particles was one of the two hull samples but generally of lower “dustiness”. What is clear is that all the small respirable fractions of the seven generated dusts contained measureable but highly variable levels of allergen, and in 5/7 samples the HSP content was significantly greater than STI, even in meal samples. These data are consistent with asthma epidemics where there was distance between the point source and causation of asthma, the putative allergens being Gly m1 and Gly m 2, measured by the HSP immunoassay.
The data from the drum dustiness testing are compatible with a health risk for lung irritancy or allergic responses depending on the nature of the specific bulk material. The pelletised hull material (both of the tested hull products, Figure 1) seems to be largely assumed by harbour managers and importers to be a “low dust” product. However, it does show some evidence of breakdown after transportation and unloading and has higher content of low molecular weight allergens and endotoxins.
The third HSE study was an occupational hygiene survey in a different dock unloading soya, but the focus on this study was investigating the levels of airborne endotoxin, as well as the allergens STI and HSP (paper in preparation). The levels of these analytes in the unloaded bulks (hull and meal) were also measured. During both days a meal bulk was unloaded, while on the second day, a pelletised hull product was also handled that included manual cleaning or “trimming” of one of the ship’s hold.
The dockside operation was very similar to the previous UK study. Bulk cargo ships were emptied using a dockside crane into a hopper, which was then used to load trailers and transported to the storage facility via a weighbridge. Inside the storage warehouse, the soya was tipped from the trailers and formed into piles, using a pusher vehicle loaded into lorries as required for onward transportation. Some lorries were also directly loaded from the hopper. In emptying the ship’s hold, an excavator and Bobcat shovel loader were lowered into the hold allowing the grab crane to access material efficiently. Final “trimming” of the ship’s hold was done by workers manually scraping and shovelling from the hold’s sides. Spillages on the dockside were cleaned up manually and by the use of a shovel loader. Respiratory protective equipment was available to all staff and invariably worn by those working in the hold or as the hatch man, but not necessarily at other times. The excavator, crane, tractors, loading shovel, and pusher were all fitted with cab filtration.
As previously reported, higher levels of endotoxin and HSP were found in the hull bulk compared to the meal. Airborne sampling results showed geometric means (ranges) of airborne levels of dust, endotoxin, STI, and HSP during unloading of 1.6 (<1–62) mg m−3, 34 (5–2450) EU m−3, 146 (1–122,462) ng m−3, and 608 (2–243,654) ng m−3, respectively. Expressed as 8-hour TWAs, 29% of all personal samples and 100% of those involved in cleaning within the ship’s hold had endotoxin levels greater than 90 EU m−3, the limit for endotoxin proposed by the Netherlands [17]. All workers involved in trimming activity within the hold as part of their working day (both manually trimming and operating the excavator and Bobcat) had estimated endotoxin 8-hour TWAs of endotoxin, between 175 and 888 EU m−3. Personal samples and static samples within the two vehicles involved in trimming activities suggested hold atmospheric levels of endotoxin between 275 and 2450 EU m−3. Two other workers’ exposure to endotoxin, when expressed as 8-hour TWAs and unrelated to trimming, breached the Dutch endotoxin guidance value. This happened on the second day, when hull was being unloaded, and is related to moving the bulk material within the storage warehouse. On day 2 when hull was being handled, there was twice as much endotoxin associated with the airborne dust collected compared with day 1.
So soya hull has higher levels of endotoxin associated with it, and considerable levels of airborne endotoxin are produced when it is handled and moved. A review of dust and endotoxin exposure in livestock farming suggested full-shift, average levels of inhalable dust and endotoxin between 0.8–10.8 mg m−3 and 300–6600 EU m−3, respectively [16], and a review of grain dust exposure in the UK reported a geometric mean exposure levels for endotoxin and dust of 1150 EU m−3 and 4.4 mg m−3, respectively. Trimming activities in the ship’s hold appear consistent with this level of endotoxin exposure. Swan [98], in sampling cereal grain dust exposure on the ships during unloading at two UK docks, measured endotoxin levels between 59 and 190,000 EU m−3 for personal samples and 74,000–7.7 x 106 EU m−3 for static samples. Swan’s study found an even more highly significant association between inhalable dust and endotoxin levels than what we found. These data may suggest higher endotoxin exposure from handling cereal grain dust in these circumstances.
Table 5 compares the airborne levels of dust (gravimetric), STI and HSP, in the two occupational hygiene studies that HSE has undertaken. The static samples at the periphery of the dockside operation, and beyond, in the first study have been removed to allow better comparison. The obvious high values in the upper ranges of the second dock study likely reflect that monitoring of trimming in the hold was monitored, an obviously dusty activity.
Gravimetric dust (mg m−3) | STI (ng m−3) | HSP (ng m−3) | |
---|---|---|---|
First UK dock study [19] | |||
Personal samples | 2.0 (1.2–4.5) | 130 (33–3071) | 583 (170–12,629) |
Workplace static samples | 0.7 (0.1–5.2) | 216 (11–845) | 1970 (40–7438) |
Second UK dock study | |||
Personal samples | 1.8 (0.04–62.3) | 178 (5–122,463) | 763 (15–243,654) |
Workplace static samples | 1.1 (0.2–35.6) | 85 (1–69,956) | 318 (2–139,390) |
Geometric means (range) of airborne concentrations of gravimetric dust and soya allergens compared in the two UK dock studies. Samples are categorised as personal samples or static/background samples.
The evidence from the Barcelona clearly shows the value of controlling emissions of soya dust during bulk soya unloading. Such measures decreased both the measured levels of airborne soya substantially and finally eliminated outbreaks of asthma epidemic that has been serious enough to cause fatalities [63]. However, the initial implementation of control measures in 1987 still led to further outbreaks in 1994 and 1996, and in 1998 the storage silos were fitted with even greater particle retaining filters. The value of airborne monitoring of soya aeroallergens, which started in Barcelona in 1986, was also shown [56].
A number of international authoritative and regulatory bodies have recognised the health hazards from grain dust, and, while soya is not a cereal, some have explicitly encompassed soya within their definition of grain dust [99] or highlighted the similarities in the hazards (intrinsic and extrinsic) posed by dust from grain and soya [100]. A number of regulatory and authoritative bodies in the USA, Canada, and Europe have set occupational exposure limits for grain dust ranging between 1.5 and 10 mg m−3 8 hour. TWA. While Great Britain has an exposure limit for grain dust of 10 mg m−3 (gravimetric measurement), this is augmented by the need under the Control of Substances Hazardous to Health (COSHH) Regulations, given soya is a respiratory sensitiser, to undertake risk assessments, control soya exposures to as low as reasonable practicable, and implement appropriate health surveillance.
Monitoring by gravimetric dust is not necessarily a good surrogate of the extent of exposure to soya allergens or endotoxin. Our two dock studies [19] [paper in preparation] identified that gravimetric measurements only explained 50–70% of the variation in the airborne levels of the two allergens measured, even in a relatively constrained number of bulks. The lack of a good relationship found between gravimetric dust and allergen (HSP and STI) levels in the respiratory-sized fraction generated by drum dustiness tests confirms this [18]. HSE’s second dock study suggested that gravimetric dust measurements only explained about 29–57% of the variation in endotoxin levels over the 2 days of sampling.
Issues about health risks in bakeries from exposure to cereal flour dust have been extensively investigated [6, 70, 81, 101, 102, 103], with many solutions identified for reducing bakers’ exposure in terms of engineering control, local exhaust ventilation, work activity modifications, and training. Such measures to reduce airborne levels of flour should also reduce exposure to soya flour dust. Interestingly one suggestion for reducing the dustiness of certain flours, such as in improver mixes, has been the addition of soya oil [79].
The importance of soya in the global nutrition of animals and humans is well recognised. Largely cultivated in the USA or South American countries, it involves large-scale handling, processing, transportation, and finally use by a wide variety of end-users. Health problems from exposure to soya dust have been found in those directly occupationally exposed and those in the general population, indirectly exposed from occupational/agricultural activities. The major health problem seems to relate to type I, sIgE-mediated allergic reactions. There appears to be a genetic component to sensitisation; atopy status and exposure to soya dust are both significant risk factors, as well as smoking. Interestingly the reports of “flu-like” symptoms, similar to ODTS or HP, in two studies of soya processing suggest that an additional pathological mechanism can occur.
The UK largely imports soya meal and soya hull; some meal products also have an amount of hull deliberately added. The protein and allergen profiles of the two pure products are very different. Hull, which is used solely as an animal feedstuff, has a particular low molecular weight protein signature, including the two allergens Gly m 1 and Gly m 2 identified as causative in harbour city asthma epidemics. Generally, energetic handling of hull at ports can lead to high airborne concentrations of dust containing these allergens and that can travel distances up to at least 200 m in the direction of the prevailing wind. On a smaller scale, energetic handling of hull-based animal feed may produce considerable airborne levels of allergen and endotoxin. Limited data suggest that soya products can be very different in their propensity to be dusty, the particle sizes generated, and their allergen content.
Methods for monitoring airborne levels of relevant soya allergens are available and can be used to good effect in monitoring the efficacy of control measures.
Hull products appear to have a higher endotoxin load that can be become airborne: endotoxin posing its own respiratory risks. Also poor storage conditions can lead to significant growth of fungal contamination, some of the fungal species also being associated with respiratory ill health.
Those employed and living near large-scale operations of agriculture growing soya, storing, processing, and transportation may be exposed. Occupations such as stevedores, farmers, millers, bakers, and food processors may be exposed to soya dust. Bakers, which have had significant problems with occupational asthma and allergic symptoms from cereal flour, are likely to benefit from the measures enacted to control exposure to cereal flour dust, in reducing soya exposure to soya.
Regulatory regimes that tackle issues of respiratory problems from exposure to grain dust appear to either directly or indirectly encompass soya dust. Such measures may involve setting gravimetric workplace exposure limits, although the relationship between airborne dust levels and their allergen content is not necessarily simple. Great Britain has the further regulation of COSHH for asthmagens such as soya. This mandates employers to undertake risk assessments, keep exposure to as low as reasonable practicable, and utilise appropriate health surveillance. However, the efficacy of such a regulatory framework obviously depends on its implementation where soya is encountered.
This publication and some of the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusion expressed, are those of the author alone and do not necessarily reflect HSE policy. My thanks to Andrew Simpson and Peter Baldwin for supplying photos of soya unloading in the UK.
The author declares no “conflict of interest”.
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