Natural Rubber Latex - Origin, Specification and Application

Abstract

The chapter contains information about the origin of natural rubber latex (NRL) (Hevea brasiliensis) and the processing of field latex, considering quality changes occurring during the preparation of raw materials for distribution. The main types of concentrated natural rubber latex are described. A specification of natural rubber latex (NRL) in terms of key parameters tested by manufacturers and customers is presented. Test methods for verifying if the material meets the requirements of ISO 2004 and internal specifications are described based on standards and commonly used techniques. The next subject touched in the chapter is prevulcanization as the processing of concentrated latex with a change of its properties. One of the main industrial applications of NRL as prevulcanized latex (PV) is the production of dipped goods like gloves or balloons. Currently, some trends and challenges relate to sustainability issues are presented (carbon footprint, FSC).

1. Introduction

Natural latex is made from the milk sap of rubber plants. The first historical references to the use of the rubber from the resin of these trees can be found in the relations of Christopher Columbus’s expeditions (1492–1496) from Haiti and Fernando Cortez relations from Montezuma’s court in Mexico (1519). However, more than 200 years had to pass before Charles Marie de la Condamine presented to the Royal Academy of Sciences in Paris a well-founded description of rubber, the way it was produced, and suggestions for its use with information from C. Franêois Fresneau. Fresneau was probably the first European who described the most productive rubber tree Hevea brasiliensis (from Hheve—in the language of the local Indians) [1, 2].

Systematizing the nomenclature: natural rubber (NR) refers to dry rubber, e.g. in sheet, block, or crepe form, while natural rubber latex (NRL) is a suitably stabilized, usually concentrated, liquid form of rubber solution. However, due to the dominance of dry rubber in the market, the term natural rubber (NR) often covers both types of raw materials. Over the years, latex has grown in importance and its production has moved to other parts of the world. Global production of natural rubber (NR and NRL) in year 2018 was 13,8 million of tones, and more than 90% of this production comes from South and South-East Asia [3]. Production of NRL (1,6 million of tones in 2018 [3]) represents approximately 12% of whole natural rubber market. Thailand is the world leader in natural rubber market as a producer of 37% of total rubber and 69% of NRL in 2018 [3].

Unique properties of natural latex as an elastomer makes it applicable in many industries, like the manufacture of dipped products, such as gloves, balloons, condoms, catheters, and other medical products. Foamed latex is used to produce mattresses, pillows, or in footwear industry. Latex is used in adhesive-binders industry or in production of carpets as backing.

Natural rubber latex is a colloidal mixture which means it contains water-based serum phase and the solid rubber phase. The rubber obtained from H. brasiliensis is a polymer of cis-1, 4-polyisoprene. Polymer chains have a length from 1000 to 3000 of isoprene units [4].

Latex has very specific organization of the rubber molecules in the rubber particles. Rubber particles are described as spherical or pear shape core-shell structures with polymer core (inside) surrounded by a thin “shell” built of phospholipids and protein. In the model presented by D. C. Blackley [5], there are two layers: inner shell of phospholipids and outer shell of proteins. However, it was also mentioned that proteins and phospholipids are more likely mixed. Mixed structure of the surface has been confirmed and more precisely presented in further works [6]. Each polymer chain has two unique terminal groups: α and ω. α-terminal linked with phosphate groups associated with phospholipids and ω-terminal linked with protein, which play a vital role in forming the unique structures of NR particle [7, 8].

Rubber particles have bimodal diameter size distribution, and rubber particles can be classified based on the size as small rubber particles (SRP) with an average particle size of 10–250 nm and large rubber particles (LRP) with an average particle size of 250–3000 nm [4, 5, 9, 10]. That bimodal size distribution looks different in different types of latices (fresh latex, concentrated latex, and prevulcanized latex (PV) latex) and changes during processing—for example during centrifuging or during clarification [9, 10].

1.1 Stability of latex mixture

Stability of the mixture is the crucial issue in colloidal solution. In case of natural rubber latex, colloidal stability is needed for storage, transport, and processing. Decreasing of this stability brings irreversible aggregation as micro-coagulation and coagulation, but on the other hand, over stabilized latex can be difficult in processing.

From physical point of view, the optimal stability occurs due to the proper interparticle repulsive forces. On the one hand, destabilizing Van der Waals attractive forces act, and on the other hand particles are stabilized by repulsive electrostatic and steric forces. Electrostatic stabilization of latex is consequence of the negative charge of the rubber particle surface with proteins and phospholipids. Around the rubber particle, ions present in the serum solution are adsorbed and create conditions that prevent aggregation. Additional level of protection is a steric stability which is an effect of the presence of adsorbed long-chain molecules at the surface of rubber particles. This kind of barrier can be formed naturally by fatty acids derived from the hydrolysis of fresh latex phospholipids; however to increase stability, additionally commonly used are soaps added at the different stages of latex processing (see 2.3).

2. Processing: from fresh, field latex to concentrated latex

Fresh latex is obtained from a rubber tree by tapping (cutting) a bark, and it is collected into a special cup installed on a tree (Figure 1). Making correct incising requires knowledge, precision, and experience. It has to be done carefully and not too deep to avoid damage of a tree. Vessels with latex run spirally in clockwise direction up the tree. Optimal way of tap is from high left- hand to low right hand, which maximizes the number of the opened vessels [5, 11]. These actions have to be carried out early in the morning, when tree “gives” more latex sap due to the greatest turgor of cells during relatively lower transpiration (high humidity and low temperature). On the other hand, in morning conditions, evaporation is lower and latex can stay longer as a stable liquid.

Figure 1.

Tapped Hevea brasiliensis rubber tree and fresh latex collecting.

Fresh field latex is a sensitive material, and it coagulates within a few hours after leaving the tree. Fresh latex has different composition than on the next steps of processing (Table 1). First of all, it has lower concentration of the rubber material: depend on the clone, it is 30–35% [5]. It has also high variety of non-rubber substances. Characteristics for fresh latex only are, e.g., lutoids. These are vacuolar structures surrounded by a phospholipid membrane containing, among others, significant amounts of proteins [12]. Neutral pH 6–7 and relatively high number of organic compounds, like carbohydrate, make fresh latex sensitive to bacterial growth and putrefaction. Therefore, it is crucial to protect it by the proper preservation.

Constituents	Fresh latex [%]	Centrifuged latex [%]
Water	59,5–66	38,5–39
Rubber	30–35	min. 60
Non-rubber substances (total)	4–6,5	max. 1,7
Proteinaceous substances	1–1,5
Resinous substances	1–2
Carbohydrates	1
Mineral matter	1

Table 1.

Composition of fresh natural rubber latex (NRL) and centrifuged latex.

2.1 Preservation and role of ammonia in latex

The most common preservative for latex is ammonia. Theoretically, addition of 0,2% m/m of ammonia is a minimal dosage, which is good enough for short time protection of collected latex; however, 0,35–0,7% [m/m] is safe regarding the further processing [5]. In practice, fresh latex is sieved to the bulking tanks and then ammonia gas is bubbled through the latex to get a minimum concentration of 1% by weight [13]. The early addition of ammonia is crucial to maintain an optimal property of the latex or rubber material in next stages of processing [14, 15].

Ammonia not only acts as a biocide but also as alkali enhances the negative charge on the rubber particle and improves colloidal stability, works as a buffer, rises the pH, reduces viscosity, and neutralizes free acids formed in the latex.

Additionally, ammonia deactivates some of the metal ions present in the latex (Mg²⁺, Cu²⁺), which can have negative effect on the properties and stability in further processing. Copper ions are deactivated by complex formation. Magnesium can be precipitated as magnesium ammonium phosphate. The phosphate comes from the hydrolysis of the phospholipids from natural membrane structures, like rubber particles and lutoids. However, diammonium hydrogen orthophosphate (DAHP) may be added for an excess of magnesium ions over phosphates of natural origin. Phosphates settle down and are left at the bottom of the tank as sludge. It is important to maintain a good magnesium-phosphate balance, as excess phosphate can also bring unexpected difficulties like thickening in further processing.

Because of the odor nuisance at higher concentrations of ammonia, it can be reduced to 0,2% with addition of secondary preservative. The most common and effective is mixture of TMTD and zinc oxide (TMTD/ZnO). Both substances are added to the latex as a well-prepared water dispersion. Small 0,025% dosage of TMTD/ZnO preserves the latex properly, which can be indicated as a lack of volatile fatty acids (VFAs) increase (see 3.9). However, the usage of TMTD requires special care. TMTD is classified as hazardous substance which can be problematic in usage it as a raw material. Additionally, TMTD can be a significant source of limited N-nitrosamines and nitrosatable substances in final dipped products. That is the reason for different secondary preservatives development, such as ZDEC (zinc diethyldithiocarbamate) or TBzTD (tetrabenzyl thiuram disulfide). Latex without TMTD is labeled as “TMTD free” and can be sold finally as a premium latex for safe production (for example medical use).

2.2 Concentration

From the practical and commercial point of view, latex for onward transport to downstream users has to be concentrated. Concentration is a process of increasing the amount of rubber in the latex. According to the accepted standards, the concentrated latex should contain minimum 60% of rubber.

One of the most popular and effective concentrating methods is centrifuging (Figure 2). The method uses the difference in density (specific gravity) of the rubber compared to the density of the serum (water phase). The weight of serum (1,02 g/ml) with non-rubber substances is higher than the weight of a rubber (0,92 g/ml). Latex in a centrifugal separator is subjected to high centrifugal force on plates and separated in two fractions: concentrated latex and skim latex. During the process, these two materials go different ways inside the machine to two different outlets. The efficiency of centrifuging process is 85–90% [13], which means skim latex is also a usable material with 2,5–10% of rubber content. Skim fraction can be coagulated and used as a source of dry rubber (skim rubber in blocks or crepe).

Figure 2.

Centrifuges used in processing of natural rubber latex.

Alternative and less popular methods of latex concentration are creaming, evaporation, or electrodecantation. Creaming is a process of separation of rubber and serum based on the difference in weight of these phases, like this occurring during centrifuging. However, during creaming, lighter rubber particles tend to cream up in heavier serum. The velocity of the separation process can be increased by the addition of creaming agent solutions, like sodium alginate or ammonium alginate. Preserved latex mixed well with creaming agents stays undisturbed in vertical creaming tank for approximately 48 hours. After that time, the bottom fraction of skim latex is drained off through the valve, and latex is mixed again. It is worth to mention that this process is used rather for well-ammoniated aged latex (3 weeks after tapping), because the creaming efficiency of fresh latex is lower. Creamed latex, due to the nature of the process it undergoes, has a higher solid content and rubber content but also a higher non-rubber content than centrifuged latex.

Concentrated latex is tested and adjusted to desired specification. Regarding alkalinity (see 3.4), latex can be classified into low ammonia (LA; <0,29% NH₃) and high ammonia (HA; >0,6% NH₃). Currently, the most common commercially used type of LA is LATZ (where “TZ” means TMTD/ZnO preserved). Due to the toxicity of TMTD, new types of LA, “TMTD free,” are developed, but in case of HA with content of ammonia above 0,6%, usage of TMTD can be significantly reduced or eliminated. On the other hand, higher concentration of ammonia can be a problematic issue in the facility of final user, especially in countries with colder climate. Medium ammonia (0,3–0,6% NH₃) is much less popular type of latex.

2.3 Maturation of latex

The period between tapping and packing of the latex is called maturation. In this period of 3–5 weeks, latex enhances its colloidal stability and changes quality parameters. Main and widely used measurement of stability is mechanical stability time (MST) (see 3.5). Mechanical stability of the latex during the first days of processing is relatively low, less than 100 s, which makes it too sensitive for use. Enhancement of MST is an effect of aging, natural processes, and additives used during maturation [16, 17, 18]. The natural process of hydrolysis of phospholipids is a source of fatty acids. The soaps they form play an important role in stabilizing the latex. Some amount of soaps is added additionally, which has to be mentioned in protocol from processing. Commonly used ones are ammonium laurate and oleate. Properly adjusted and stored latex increase its MST above 700 s after 3 weeks. The good practice of latex producers is preparing “the MST graph.” In this report, producers submit monitored changes of MST result during maturation time with all notes about dosages of additives like soaps or ammonium.

To optimize the maturation process, use of the enzymes is showing good results. The use of lipase, the enzyme responsible for the hydrolysis of lipids and phospholipids, reduces the maturation time for 10–14 days. Additional advantage of enzymatic approach can be the reduction of additives, like soaps, ammonia, or TMTD during processing the latex [18].

3. Natural rubber latex analyses of parameters

The basic specification of concentrated latex is included in norm ISO 2004 [19]. However, in some cases, a more completed characterization is needed. Table 2 shows all these parameters. The scope of testing and importance of parameters depend on the application of the latex: dipped products, foams, coating, etc.

Property	Centrifuged		Creamed		Method
	HA latex	LA latex	HA latex	LA latex
Total solid content, min. % (by mass)	61,0 or as agreed between the two parties		65,0		ISO 124 [20], ASTM D1076 [21]
Dry rubber content, min. % (by mass)	60,0		64,0		ISO 126 [22]; ASTM D1076 [21]
Non-rubber solids max. % (by mass)	1,7		1,7		—
Alkalinity (as NH3), calculated with respect to the latex concentrate, % (by mass)	0,60 min.	0,29 max.	0,55 min	0,35 max.	ISO 125 [23]; ASTM D1076 [21]
Mechanical stability, min, seconds	650		650		ISO 35 [24]; ASTM D1076 [21]
Coagulum content, max., % (by mass)	0,03		0,03		ISO 706 [25]; ASTM D1076 [21]
Copper content, max., mg/kg of total solid	8		8		ISO 8053 [26]; ASTM D1278 [27]
Manganese content, max., mg/kg of total solid	8		8		ISO 7780 [28]; ASTM D1278 [27]
Sludge content max., % (by mass)	0,10		0,10		ISO 2005 [29]; ASTM D1076 [21]
Volatile fatty acid (VFA) number, max.	0,06		0,06		ISO 506 [30]; ASTM D1076 [21]
KOH number, max	0,70		0,70		ISO 127 [31]; ASTM D1076 [21]
Parameters out of ISO 2004:
Magnesium content, max, % (by mass)	0,004 max [32]				ISO 17403 [33], TIS 980–2552 [32]
Phosphate content, max, % (by mass)	—				ISO 19043 [34]
Zinc oxide heat stability (ZHST), min., seconds	300				Test Methods – Revertex Malaysia [35]
Zinc oxide stability (ZST), min., seconds	100
Zinc oxide viscosity (ZOV5 and ZOV60) min., %	25

Table 2.

Requirements of ISO 2004 for centrifuged and creamed natural rubber latex concentrates and additional parameters out of ISO 2004.

3.1 Total solid content (TSC)

Total solid content (TSC) of latex is tested by the method described in ISO 124 [20] or ASTM D 1076 [21]. The latex sample is weighed and then dried in an oven (100°C or 70°C) until it completely evaporates, i.e. the mass of sample becomes stable. The result is presented as a percentage base on below equation (Eq. (1)), where m₀ is a weight of latex sample and m_dry is a weight of dried sample.

$$TSC\left[\%\right]=\frac{m_{\mathit{dry}}}{m_0}\times 100$$E1

The result of TSC is needed for subsequent analyses: MST, KOH number, or VFA number. In laboratory practice, a faster but less accurate test can be performed parallel by using a moisture analyzer.

3.2 Dry rubber content (DRC)

According to ASTM D 1076 [21] or ISO 126 [22] standards, the latex sample is diluted to 30% TSC and then coagulated with a weak acid (acetic acid 5% or formic acid 2%). After coagulation by the acid, the sample is additionally placed in a hot water bath for 15 minutes. The coagulate is squeezed, washed, and then dried in an oven (70°C or 105°C). Drying the coagulate sample in an oven is time-consuming, so alternatively a quick result can be obtained using microwaves with subsequent verification of the result, using a reference method. Result of dry rubber content (DRC) is presented as percentage base on below equation (Eq. (2)), where m₀ is a mass of liquid latex sample and m_dry is a mass of dried coagulum obtained from the sample;

$$DRC\left[\%\right]=\frac{m_{\mathit{dry}}}{m_0}\times 100$$E2

DRC value is needed to calculate further parameters like VFA number. The DRC value strict refers to the amount of rubber in latex and therefore has an importance from a commercial point of view. Transactions of latex are based on the rubber content, not on the wet material. DRC should be used in calculation of ingredients added into latex as active material quantities (phr) during compounding of the latex (e.g. during prevulcanization).

3.3 Non-rubber solid (NRS)

Non-rubber solid (NRS) is a parameter calculated indirectly as the difference between TSC and DRC. NRS indicates non-rubber components, like minerals or salt content in an aqueous serum phase of latex and residue of chemicals used during processing of field latex. Many non-rubber substances are hydrophilic and may have a negative effect on the conductivity increase of the dipped products, e.g. protective gloves.

3.4 Alkalinity (as NH₃)

The concentration of ammonia is determined by titration of a diluted latex sample with acid (HCl 0,1 N). The method is described in the norm ISO 125 [23]. The result may be expressed as total or as concentration of ammonia in the water phase.

The concentration of ammonia is crucial, both in terms of adequate protection of the latex against microorganisms and in terms of maintaining the colloidal stability of the solution. An irreversible deterioration of latex can be observed when the ammonia concentration drops below 0.2% NH₃. There is an increased risk of coagulation as well as an uncontrolled increase of viscosity during processing. At the same time, the quality, mechanical properties, and the shelf life of the final products are reduced.

3.5 Mechanical stability time (MST)

Mechanical stability time (MST) refers to the resistance of the latex liquid material to the shock caused by the mechanical stress, like mixing or pumping. This stress is imitated by standardized high-speed stirrer (14,000 rpm) of MST machine (Figure 3). The test can be done in accordance with the ISO 35 [24] or ASTM 1076. Standard mixers are compatible with both methods.

Figure 3.

MST Klaxon machine.

Sample of latex is diluted to 55% of TSC with ammonia solution (0,6% or 1,6% depend on the alkalinity of the sample) and mixed in MST machine in standard cup until it coagulates. The result is expressed as time (seconds) of mixing the latex till the first visible sign of coagulation occurs. The coagulation process can be observed, for example, by spreading latex on the glass plate during the test. In accordance with the norms, MST should be longer than 650 seconds. On the one hand, too short MST increases the risk of coagulum forming and thickening during processing of the latex. On the other hand, too long MST (>30 minutes) can be also problematic. Mechanical stability of concentrated latex comes mostly from the soaps naturally formed and soaps added during maturation process (ammonium laurate or potassium oleate). Overstabilization of the latex by the soaps can be the reason of difficulties in dipping process as not effective deaeration and too intensive foaming or less effective gelling on the former.

3.6 Coagulum content

Formation of coagulum in latex is an irreversible process. It is the result of destabilization factors, such as overheating, poor latex stability, and excessive agitation. Method of testing is described in ISO 706 [25] or ASTM D1076. Coagulum content is defined as the percentage mass (on total solids) of the materials, comprising pieces of coagulated rubber, latex skin, and coarse foreign matter retained under the condition of test on a stainless-steel sieve. The latex sample mixed with soap solution (50/50) is poured through a sieve with a mesh size of 180 micrometers (Figure 4). The weight of the coagulum is determined from the weight of the sieve dried in the oven.

Figure 4.

Sieve with a mesh size of 180 µm after testing the coagulum of latex sample.

3.7 Copper and manganese content

Copper and Manganese are prooxidants and accelerate the thermal degradation of raw or vulcanized rubber. Hence, lattices, if contaminated with copper or manganese ions, will have poor aging characteristics.

There are several methods to determinate these metals (Table 2) [26, 27, 28]. In these methods, dry film needs to be prepared and incinerated. The ash is introduced into solution, and metals are determined spectrophotometrically based on the prepared calibration curve.

3.8 Sludge content

Sludge content refers to the non-polymeric impurities in the latex which tend to sediment under the influence of gravity. In the case of concentrated latex, the sludge is mainly magnesium ammonium phosphate, which remains after the precipitation of magnesium ions with phosphate ions. Magnesium ammonium phosphate content in latex can be kept low by proper desludging of field latex with diammonium hydrogen phosphate (DAHP) before centrifuging (see 2.1).

Test made by ISO 2005 [29] or ASTM D1076 involves centrifugation of the latex followed by repeated washing of the resultant sludge with ammonia-alcohol solution. Finally, residue of the sample is weighted after drying as a mass of the sludge.

3.9 Volatile fatty acid (VFA) number

Volatile fatty acid (VFA) number of latex is defined as the number of grams of potassium hydroxide equivalent to the volatile fatty acid in the latex containing 100 g of total solids.

VFA number is one of the most important parameters for natural latex. It indicates the degree of proper protection of latex against biodegradation. Latex serum contains some carbohydrates (e.g. glucose). Therefore, if latex is not well preserved, bacterial growth occurs. A trace of the presence of microorganisms in latex and their metabolism is an increase in volatile fatty acid concentration—mainly acetic acid but also formic acid and propionic acid. Proper preservation of fresh latex, cleanliness during tapping, and collection are the bases to obtain a low VFA number at subsequent stages.

Method of testing is described in norms ISO 506 [30] and ASTM D1076. The latex is coagulated with ammonium sulfate, and the resultant serum is separated and acidified with sulfuric acid. The serum is steam-distilled on Markham still (Figure 5), and the volatile acids (mainly formic acid) present in the latex are determined by acidimetric titration of the distillate. VFA number is calculated from the quantity of barium hydroxide used for titration.

Figure 5.

VFA distillation set with Markham still.

The currently accepted limit for the VFA number according to ISO 2004 is 0,06. However, in previous editions of the standard, with the same method of determination, the limit was higher and was 0,2.

An increase in the concentration of volatile acids goes hand in hand with latex deterioration. The degree of putrefaction can be determined arbitrarily by smelling the latex. The strong smell of ammonia makes it difficult to identify the odor of latex deterioration. The ammonia from the latex sample can be neutralized by a solution of boric acid (5%) before olfactory identification.

3.10 KOH number

The KOH number of latex is defined as the number of grams of potassium hydroxide equivalent to the acid radicals combined with ammonia in latex containing 100 g of total solids. ISO 127 [31] or ASTM D1076 method is based on the titration of the ammonium acid radicals in latex, which has been partially deammoniated with formaldehyde and diluted to 30% TSC. The titration is carried out using 0.5 N KOH solution from about 9.5 to about 12 pH (Figure 6). KOH number is calculated based on the inflection point from the titration curve from the equation below (Eq. (3)) where: V is a volume of KOH solution used during titration to obtain inflection point, TSC is a total solid content, M is mole of standard KOH solution and m is a mass of the sample.

Figure 6.

Example of KOH number titration curve with marked point of inflection.

$$KOH\mathit{number}=\frac{V\times M\times 561}{TSC\times m}$$E3

In latex, there are present different types of anions. Some of them play a role of stabilizers, like higher fatty acids or protein anions, but the others like volatile fatty acids, phosphate, carbonate, citrate, or sulfate can decrease the stability. That is a reason why KOH number result should be related to other parameters as well, to build wider interpretation. Generally, KOH number rises in time, and it can be an indicator of age of latex.

3.11 Viscosity of latex

Viscosity of the latex is presented as kinematic viscosity with unit of millipascals·second (mPa·s) which is equivalent to centipoises (cP) (out of SI system). The most common method of measuring latex viscosity is using a Brookfield viscometer (Figure 7). In accordance with ISO 1652 [36] or ASTM D1076, filtered sample of latex, free of air bubbles, is maintained to desired temperature and tested with viscometer. The viscometer is equipped with a set of spindles for measuring different ranges of viscosity. A spindle must be immersed in the tested sample to a designated depth. The apparatus measures viscosity, while the spindle is rotating at a preset speed (rpm). For a more complete comparability of results, additional information about temperature, spindle type, and rotation speed need to be recorded.

Figure 7.

Brookfield viscometer with spindle number 62. Apparatus in that configuration and rotation speed of 60 rpm has measuring rage 50 – 500 cP.

Viscosity of latex has great impact on the processability; however, it is not included or specified in ISO 2004. Due to varying customer preferences, limits are usually agreed individually with the latex supplier.

3.12 Magnesium

The presence of magnesium has a markedly negative effect on the quality of latex by contributing to its destabilization. This is due to several issues. Firstly, magnesium reacts with carboxylate ions, forms insoluble soaps with fatty acid, and deactivates their stabilization function. Magnesium also precipitates as magnesium hydroxide. Additionally, it can form primary valence linkages between the surfaces of two adjacent latex particles instead of reacting with two free carboxylate ions which can be a reason of flocculation [37]. In practice, these factors can lead to an uncontrolled increase of viscosity or coagulation. As it was mentioned already (see 2.1), to reduce magnesium content, precipitation with phosphates is required (natural or added as DAHP).

Magnesium content is not included in ISO 2004; however, it is a good practice to add the information about the result of free magnesium test in the certificate of analysis for the latex batch. Although there is no international official limit, there are some references. Base on Thai Industrial Standard TIS 980–2552 from 2009, magnesium content should not exceed 40 ppm [32]. This value is similar to the previous literature data where on the studies of Karunanayake L. and Perera G. M. P. [20], 30 ppm of magnesium was acceptable with no significant impact on latex stability. It may be mentioned that a small amount of magnesium can be a desirable evidence of a proper balance being kept when adding phosphates. In other words, a lack of magnesium in the sample may be related to an undesired excessive amount of phosphate used in the earlier stages of latex processing.

According to ISO 17403 [33], the determination of magnesium is performed by complexometric titration with EDTA. In case of concentrated latex, the determination is carried out in a serum sample (after coagulation). Field latex may be analyzed without separation (coagulation is not needed).

Additionally, to improve the correct magnesium-phosphate balance in field latex, quick test kits may be useful. In case of this solution, result of magnesium content can be obtained within a few minutes [38].

3.13 Phosphates in latex

The phosphate issue has been already described in previous parts (see 2.1; 3.12). On the one hand, it is largely linked with the reduction of magnesium; but on the other hand, too high excess of phosphates can have a negative impact on latex stability and cause problems of thickening during further processing [38]. As there is no officially defined limit value for phosphate content, the best approach is to determine it individually for the particular process and latex application. Some manufacturers of dipped products in Thailand consider that the best properties of the latex film are achieved when the amount of phosphates do not exceed 100 ppm [39]. However, in the study of Karunanayake L. and Perera G. M. P., the sample containing small excess of 30 ppm phosphate relative to magnesium showed the highest mechanical stability and best properties after prevulcanization. Moreover, further samples with higher amounts of phosphate showed increasingly poorer stability.

The conventional determination of phosphates in latex is carried out using the spectrophotometric method described in ISO 19043 [34]. The latex sample is coagulated to allow separation of the serum. Sample of filtered serum with developed color is tested on spectrophotometer. The determination is carried out based on a previously prepared calibration curve.

3.14 Zinc oxide stability test (ZOV, ZST & ZHST)

Chemical stress caused during processing by the addition of compounding mixtures should not be a reason for the significant destabilization of latex. Zinc oxide stability tests are a group of tests which are indicators of the latex resistance to this kind of stress. The method is not standardized in ISO but can be helpful as an additional indicator during latex quality control. The basis of the method is an excessive addition of zinc oxide water dispersion (ZnO 40%) in excess, which cause changes in the latex properties (1 phr). Destabilized sample is tested for heat resistance and for changes in viscosity and mechanical stability. Practically, one sample with ZnO (40%) addition is prepared and finally split for all the tests.

Zinc Oxide Heat Stability Test (ZHST) theoretically is a measure of latex resistance to heat. The heat can be an issue, for example, during warm compounding and warm prevulcanization (more details on point 4). In this test, 1 hour after 1 phr ZnO addition to the latex, the sample of 50 g is stirred in warm condition of water bath at 90°C. Result of a test is presented as a time of stirring till final coagulation occurs. Recommended ZHST should exceed 300 seconds [35].

Zinc Oxide Stability Test (ZST) can be some indication of proper resistance of the latex to decrease mechanical stability after compounding and during further processing. Latex sample, 1 hour after 1 phr ZnO addition, is tested on MST (see 3.5). Result of the test is the time of mixing with MST machine till final coalescence (coagulation). Recommended ZST should exceed 100 seconds [35].

Zinc Oxide Viscosity (ZOV) is a measure of latex resistance to drastic changes in viscosity after compounding and during further processing. In case of ZOV, addition of ZnO dispersion changes the viscosity immediately. The measurements of ZOV are percent changes of Brookfield viscosity 5 min (ZOV₅) and 1 hour after 1 phr ZnO addition (ZOV₆₀). It is recommended that increase on viscosity should not exceed 25% [35].

4. Compounding and prevulcanization of NRL

Latex in the non-vulcanized state is not suitable for manufacturing latex goods, as it does not have required flexibility, resistance, and tensile properties. Those properties are obtained during vulcanization process. The term and principle of this process were introduced by Charles Goodyer, who discovered that using sulfur and temperature changes properties of natural rubber. The term itself comes from the Roman god of fire—Vulcan [40]. In a simple way, vulcanization means creation of crosslinking between free chains of rubber into network, by adding special composition of chemicals and heat. Addition of chemicals into the latex is called compounding [11]. This is one of the most important processes in the preparation of latex to be ready for use. Quality of used chemicals, which are used in a form of water-based dispersions, as well as the proper handling, like agitation, time, and temperature, have big impact on the final properties of ready-for-use latex. The process and composition of vulcanizing agent are strictly confidential know-how of each company. Latex after compounding can be used immediately, or can be left for a period, that the crosslinks are created. The first case requires post-vulcanization with use of high temperature. The second case is called prevulcanization and can be conducted as hot prevulcanization in elevated temperature (up to 80°C) for couple of hours or cold prevulcanization in ambient temperature for couple of days. Lower prevulcanization temperatures give generally better physical properties of finished products [11]; however, in the end, the choice of system depends on many factors and is compromised between ability for storage capacity, timing, and heating costs during prevulcanization, during curing and desired final properties.

Most commonly used vulcanization system is sulfur vulcanization, with the use of accelerators from group of dithiocarbamates (zinc diethyldithiocarbamate, zinc dibenzyldithiocarbamate, and zinc diisononyldithiocarbamate), thiurams (tetra-benzylthiuram disulphide and tetradimethylthuiram disulphide), thiazoles (zinc 2-mercamtobenzothiazole), and zinc oxide as activator. Additionally, stabilization system is required to avoid losing of colloidal stability of compounded latex. For this reason, KOH is widely used, or specific stabilizers commercially offered by specialized companies. Use of specific prevulcanization systems ensures that final product obtains required mechanical and physical properties, as well as functional properties.

Mechanism of crosslinking formation is not fully known yet; nevertheless, the basic of this complexed reaction can be explained in several steps. First step is the creation of active sulfurating agent from sulfur, accelerator, and zinc oxide. Next step is the reaction of sulfurating agent with the rubber molecule and creating rubber-bound intermediates, which react further with other rubber molecules, that is, creation of initial crosslinking network. In further stage, the network matures, as sulfur crosslinks exchange. In the end, matured crosslink network is formed [11, 40].

It is worth to mention that most of the accelerators are precursors of N-nitrosamines, which can be cancerogenic as presented above specific limit values, depending also on the exposure scenario for certain products. The industry must follow up with recent research studies and legislation changes and adjust the technology accordingly [41, 42, 43, 44].

In latex compounding, the quantity of all ingredients is added in units called phr—parts per hundred parts of rubber. This gives clear indication on amount of added active substance into latex of known rubber content.

4.1 Monitoring of prevulcanization process

Prevulcanization process is a kind of chemical stress for latex. That might influence the colloidal properties of latex, as increase of viscosity and decrease of mechanical stability time and appearance of coagulum. It is important to monitor the latex properties during prevulcanization, as well as the prevulcanization stage [11, 13, 45].

There are several methods for the determination of crosslinking formation. Most popular ones are chloroform number, toluene swell, and modulus as part of tensile testing.

4.1.1 Chloroform number

Chloroform number is the easiest and fastest testing method. Equal amount of latex and chloroform are mixed, and during stirring the latex coagulates. Created coagulum has different appearance, from soft and tacky mass to almost dry, crumbled particles, depending on the prevulcanization stage (Figure 8). The chloroform number is described as number from 1 to 4:

1. soft, tacky, and very stretchy mass

2. soft and bit tacky mass, but breaks when stretched

3. not tacky clump, not very stretchy, easily breaks pulled apart

4. dry, crumbled particles

Figure 8.

Sample of unvulcanized LATZ (a.) and vulcanized PV (b.) treated with chloroform during chloroform number testing.

The weak point of this method is subjectiveness, and not exact and standardized numbers. The performers often use additionally half numbers, for example 2,5 or 3,5 [11, 46].

4.1.2 Swelling test

Swelling test, with use of toluene as most suitable solvent, is another fast-testing method for determination of crosslink density. Small quantity of latex is poured on glass plate and with the use of steel bar spreaded equally. After drying, the sheet of latex prior dusting is removed from the plate, and with use of die cutter a circle sample is cut. This sample is immersed in toluene and absorbs solvent. The less the crosslinks in the latex, the more solvent the sample can swell. After certain time, usually 20–45 minutes, diameter or weight of swollen sample is measured and compared to non-swelled sample. Stage of prevulcanization can be determined based on equations below (Eqs. (4, 5)), where Q is a swell ratio calculated from the change of the sample weight (W1 – initial weight; W2 – swollen weight) and L is a swell ratio calculated from the change of the sample diameter (l1 – initial diameter; l2 – swollen diameter) [46]:

$$Q=\frac{W_2–W_1}{W_1}$$E4

$$L=\frac{l_2}{l_1}$$E5

The interpretation of the numerical value of the calculated ratio is shown in the Table 3.

Stage of prevulcanization	Q	L
Unvulcanized	>15	>2,6
Lightly Vulcanized	7-15	2,0-2,6
Moderately Vulcanized	5-7	1,8-2,0
Fully Vulcanized	<5	<1,75

Table 3.

Interpretation of the swelling index results.

4.1.3 Modulus

Modulus is the most accurate parameter, however requires specific instrument, which is a tensile machine. It can be conducted as extensive test of relaxed modulus or tensile modulus. For testing relaxed modulus, latex ring is formed by dipping a tube in latex and then, after rapid drying, modulus at 100% extension is checked. Tensile modulus requires forming latex film and then cutting the sample with dumbbell cutter and checking the modulus at 300%, 500% or 700% extension [11, 46].

4.2 PV grades based on tensile properties

Different applications require latex of different prevulcanization stage or level. The prevulcanized latex is offered also commercially by specialized producers. While during manufacturing of latex goods, the producers use PV specified with chloroform number, the commercially offered prevulcanized latexes are specified or described with modulus (modulus at 700%) tested on tensile machine (Figure 9) [47]. The commercially available grades of PV are as follows:

Figure 9.

Tensile machine during test of rubber sample.

Low modulus (8,0–10,5 MPa)—suitable for latex balloons, easy to inflate.

Medium modulus (11,0–13,5 MPa)—suitable for latex gloves.

High modulus (14,0–17,5 MPa)—suitable for condoms and catheters.

5. Dipping process as production technology of latex goods

Dipping technology is one of the most popular methods of producing thin-walled latex products. Products such as latex gloves, balloons, condoms, and catheters are produced with this technology. The process might be conducted as batch dipping process or chain (continuous) dipping process, with the use of formers made of different materials—they can be ceramic, plastic, metal, or glass. Batch dipping is mainly used for irregular shapes or thicker products, like industrial or household gloves and gives lower productivity, while chain dipping is used for mass production volumes of less demanding shapes [13]. Applying latex to the former can be done with different ways, from which the most popular are coagulant dipping (gloves and balloons) and straight dipping (condoms) [48, 49].

In coagulant dipping, at the first stage, the cleaned formers are dipped in coagulant solution, consisting mainly of water, calcium nitrate, and powder, which can be calcium carbonate, talc, and diatomaceous earth. Coagulant creates thin layer on the former, which is then dipped in liquid latex. Calcium ions from coagulant influence liquid latex colloidal stability, causing gelling of latex, which forms thin latex film covering the former. Powder in coagulant prevents from sticking of the film to the former and in the end enables removal of ready products from the formers in the last production stage.

Next step is leaching of latex film with water. In this step, residue of chemicals from latex and coagulant are leached out, as well as proteins, naturally occurring in latex. This step is called also wet gel leaching.

After leaching in water, products are dipped in anti-tack agent, which prevents from sticking outside. For this purpose, composites of soaps or dispersions of powders are used. Then the edges of dipped film are formed by rolling brushes, creating beads. Bead enables good grip and inflation of a balloon or good donning of a glove.

Finally, formers with latex film go for about 1 hour into drying oven, where temperature gradually increases to approx. 90°C. Latex film needs to be finally cured and dried, as it contains about 60% of water. After passing the oven, latex products are stripped, manually or automatically, with the use of compressed air. Water stripping is also used by some producers. After this stage, balloons or gloves are still slightly humid and require final drying in the tumble dryers [50]. The scheme of dipping process flow is presented in Figure 10.

Figure 10.

The scheme of dipping process flow.

6. Trends and challenges in terms of sustainability and environment

Couple of years ago, the main challenge for the industry manufacturing latex goods was that they were required to be resistant to aging and had very long shelf life. In recent times, the trends are going into another direction. Products and the manufacturing process aim to have reduced impact on the environment.

6.1 Classification of modified latex–Single Use Plastics Directive

The production and distribution of latex balloons, as well as other natural latex products, can be limited in the coming years due to environmental impact requirements for latex waste. Latex balloons have already been included in the scope of Directive (EU) 2019/904 of the European Parliament and of the Council on reducing the environmental impact of certain plastic products [51]. Market trends are moving toward a pro-environmental direction and the action of the European Union and individual Member States toward tightening regulations and increasing producer responsibility. Additionally, environmentally friendly products are more often chosen by consumers and end users [50].

6.2 Carbon footprint

In the view of the challenge mentioned in point 6.1, it is worth to highlight the natural plant origin of latex, as many of the end users are not aware of it. Natural rubber latex polymerization occurs in trees, which is different from the synthetic polymerization. The rubber trees during their life cycle absorb carbon dioxide from the environment. On the other hand, cultivation of rubber trees on the plantations and further processing of field latex create emission of carbon dioxide. This data can be used for the calculation of carbon footprint of latex or latex products, which is total emission of greenhouse gas (GHG) emissions expressed as equivalent of carbon dioxide CO₂.

Latex producers and suppliers are aiming for carbon neutral, often with successful outcome. Concerning goods produced from latex, studies on the carbon footprint are conducted, focusing on carbon dioxide emission during processing and manufacturing, following Life Cycle Assessment (LCA) and Green Houses Gases Protocol (GHG Protocol) [52, 53, 54].

6.3 Sustainable sources

Global demand for natural rubber created expansion of rubber plantations, which has been a driver of deforestation in the past. Currently, there are organizations and standards, which encourage the producers to proceed in responsible way. One of them is Forest Stewardship Council—FSC. The core of FSC certification is to ensure and confirm that products are deforestation-free and socially responsible, and that there is full transparency and traceability from the plantation to the final product and end user [55]. In 2017, 4% of total rubber plantation areas were FSC-certified [56].

Other recognizable association that focuses on social part of sustainable approach for the improvement of the working and living conditions of the primary producers of natural latex is the Fair Rubber Association. Similar to FSC, products like mattresses, condoms, balloons, and rubber boots are also certified according to the designated standards [57].

7. Conclusion

The processing of latex due to its natural origin is an extremely challenging issue. Users in the whole supply chain need to consider many factors in a holistic view, from the early stages of latex production on the plantation till the final use. Understanding the latex properties and their influence on further processing is important for obtaining stable process and good quality of final product.

Despite of demanding nature of this raw material, requiring experience in handling, it is worth to gain an insight into latex application. Plant origin of natural rubber latex gives an opportunity for sustainable approach and solutions described in the chapter.