Open access peer-reviewed chapter
Abstract
Since the early 1800s, siloxane has been an industrial staple due to its remarkable structure, but even though there are many benefits for using siloxanes, there are significant environmental implications, one of which being the lack of recyclability. As the first step to polymerization or the depolymerization of polymers, the scission of the silicone bond is essential. While condition-specific reactions investigating what triggers polymerization have been extensively studied, traditional synthesis methods are unfortunately not ideal due to their high cost and detrimental release of greenhouse gases. Since the 1950s, several studies have related to rupturing the siloxane bond, including hydrolysis, catalytic depolymerization, thermal depolymerization, and radical extractions. This work has resulted in new polymers, cyclics, and monomeric silanes. However, only a few studies have focused on how to build new silicone-based materials from the primary siloxane cyclic forms. Thus, more investigation into better methods for recycling siloxanes is needed. This chapter summarizes and categorizes the published data on the degradation and depolymerization of polysiloxanes based on their reaction temperature up to July 2021.
Keywords
- siloxane degradation
- silicones
- polymer recycling
- polymer depolymerization
- PDMS
1. Introduction
There is evidence that silicone-based materials have been used for tool making since the Stone Age. During the 27 BC and 14 AD, from Sidon and Babylon to the Roman Empire Ages, ancient people used silicone-based materials for glass manufacturing [1]. After the discovery of the silicon element in the early 1800s by J.J. von Berzelius [2], silicone chemistry began being explored due to the findings of James Crafts and Charles Friedel with the synthesis of the first silicon-organic compound in 1863 and Ladenburg’s development of the first polymeric siloxane in 1872 [3, 4]. During the early 1900’s Fredric Kipping’s work on the synthesis of organosilicon, Eugene Rochow’s and James Hyde’s work on the direct synthesis of silicon-based materials expanded silicone-based materials into commercial materials [5, 6].
Today, siloxane polymers or silicone-based materials are an important industrial commodity based on the structure –(R2-Si-O)–. Figure 1 demonstrates how R can be a series of hydride, alkyl, vinyl, and aryl functionalities [7], and the IUPAC named the repeating unit “siloxane” [8]. Polydimethylsiloxane (PDMS) silicones, mainly linear polymeric liquids even for larger n-values, are the most common example. Figure 2 highlights how a silicon atom connected to one atom is known as M, and structures D, T, and Q are similarly defined. The combination of these units creates different valuable materials such as resins, oils, and elastomers [9].
Figure 1.
Trimethylsilyl-terminated polydimethylsiloxane.
Figure 2.
Siloxane-bonding motifs.
The hybrid nature of siloxanes with organic and inorganic components leads to unique properties not possible with other materials [10]. For example, their ability to modify their structure allows them to morph into different chemical properties such as hydrophobicity. Additionally, their properties allow for easy alteration of their melting and refractive index, boiling points, viscosity, and density [11]. Likewise through different R-groups, processing conditions, and polymer lengths, the thermal stability properties of these materials can be modified [12] and their non-toxic nature enables a variety of biological applications [13]. As a result, polysiloxanes have become widely used exceptional polymers due to their excellent physical [10], chemical [14], and mechanical properties [15], which are important in medical, cosmetic, aerospace, and other high-tech industries [10]. To combine these superior properties into an advanced system, commercial performance silicone materials are formulated as complex multicomponent systems leading to elastomers and resins (Figure 3) [17, 18].
Figure 3.
Complex multicomponent silicone elastomer [
By using different types of additives, catalysts, and filler materials, it is possible to create a multi-component 3D network system, but it is important to note that the initial polymer structure and functionality are essential for final properties. For example, the formation of the cyclic distribution after depolymerization would significantly differ if the same PDMS system is different in the network system [19]. Likewise, if the network system consists of a copolymer of PDMS and polyphenylmethylsiloxanes (PMPS) or PDMS and nanocomposites, the decomposition pattern will differ based on their network structure [20]. Also, based on the addition of reinforcing agents, the strength or the mechanical properties and the onset temperature of degradation of the silicone matrix are changed [21]. There are different reinforcing agents used in PDMS systems. For instance, silicon metal oxides [15], carbon black [22], montmorillonite nanocomposite [23], silicates [24], fume silica [25], nano-silica [26], and carbon & SiC-based fibers [27]. Sometimes, filler is added to the system to affect other properties. For example, carbon black is added to improve electrical conductivity, titanium dioxide improves dielectric constant, and barium sulfate improves radiopacity. Because of these advantages, polysiloxanes are widely used in an immense number of industrial applications [28] varying from simple baking and measuring cups [29] to surface modifiers (elastomers/sealants, antifoaming agents, personal care products, surfactants, coatings, insulations) [30], biomedical [31], microporous organic and inorganic materials [32], and aerospace and other high-tech industries [33].
The majority of the technological products are made from PDMS and to a smaller extent PMPS. As a result, they are considered as the classical products of the silicones. Due to high- and low-temperature resistance, suitable siloxanes can be heated up to 200°C for a year without degradation and even for short terms at 450°C [34]. This, as well as a high degree of chemical inertness [35], makes them especially difficult to degrade and recycle controllably due to the strength of the Si-O bond, which is ~450 kJ/mol [36]. Another disadvantage of this material is synthesizing cost. Synthesis of the Silicon metal involves exclusive process that relies on carbothermal reduction by heating the silica with carbon at >2000°C. In addition to the high cost, this process also unfortunately releases a mol of CO2 for each mol of silicon metal produced [37] and needs to further functionalize to form different monomeric units for advanced polymerization [9]. Due to the environmental and financial costs, methods to break down silicon-based materials have been explored extensively through hundreds of papers; however, so far only a few studies have considered recycling.
1.1 Polysiloxanes, -Si-0- bond breaking approaches
The deconstruction of polymeric siloxane materials depends on the structure of the Si-O bond and the combination of the M, D, T, and Q units. Notably, this deconstruction often makes product identification challenging to determine. As the makeup of the siloxane bond (Si-O) is unique, it formed σ-bond using s and p electrons of the silicon and p electrons of oxygen and π-bond using unshared p electrons from the oxygen and 3d orbitals from silicon. According to the the Schomaker-Stevenson rule, siloxane bond length 1.64 Å, is lower than its theoretical length of 1.77 Å, due to partial π- and σ-bond character [38]. Additionally, the higher Si-P bond angle compared to the C-P bond angle is unusual, as seen in in the bond angle of dimethyl ether at 111.5° [39] when it increases to 144.1° in disiloxane, partially due to the larger radii of Si and partial delocalization of the electron lone pairs of oxygen [40]. These significant features can be explained by the (p-d) σ-and π-bonding nature and the slightly higher electronegativity of the silicon atom compared with the carbon [41]. While the formation of four tetrahedral σ-bonds is similar to carbon, free 3d orbitals corresponding to the M shell are allowed since silicone is in the third period, resulting in six as the maximum coordination of silicon [42]. The silicon atom can form a wide variety of σ and π bonds with other elements due to its angular functions of the d orbitals. These unoccupied 3d orbitals can act as an acceptor in the donor-acceptor (p-d) π bond. Also, the concept of “one angle nonbonded radii” explains the large bond angles and short Si-O bond [43, 44].
The remarkable properties of silicone materials are due to these unique properties and inorganic hybrid nature [10]. The methods discussed below [41] highlight how even though siloxanes are chemically and thermally stable polymers up to 350°C [45], the presence of a strong acid or a base with moisture can easily break the siloxane bond.
Even though degradation of polysiloxane primarily usually occurs along the Si-O backbone, it can also take place on the substituent atoms or groups (i.e., Si-C bonds), leading to reduced molecular weights. Figure 4 summarizes the four general processes of siloxane breakdown based on their reaction temperature. Catalytic or non-catalytically driven, depolymerization can be from strong acids or bases (25–110°C), nucleophiles or electrophiles use with catalysts (110–350°C), thermal depolymerization (over 350–500°C), and radical scission & calcination (over 500°C) [46]. Lewis or Bronsted acids and bases [12], organo-metallic catalysts [47], alcohols [48], and radiation [49] are the main catalysts or catalytic activity involved in the depolymerization of siloxanes. As the summary of methods in Figure 4 shows [18], the siloxane degradation mainly depends on temperature, oxidative or reductive atmospheres, sample mass, and geometry, and the network architecture [50].
Figure 4.
Mechanisms for the breakdown of siloxanes by various processes.
Even though the disruption of the Si-O bond from one location is enough to initiate degradation in linear siloxane systems, when the siloxane polymers have a 3D network structure, for degradation, it is imperative to rupture from two or more points [51]. Backbiting reaction mechanisms can explain hydrolysis and catalytic depolymerization reactions [52] and these are preferable in the range of 110–250°C and, in some instances, at room temperature [53]. Additionally, backbiting reactions, radical scission, and calcination reactions can explain thermal depolymerizations. Cyclic, hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are the prevailing thermodynamic products of siloxane degradation, and enrichment in certain produces is possible depending on the reaction environment and degradation process. For example, the D3 cyclic form is prominent in the thermal degradation process because products have more variability in lower temperature breakdowns, and the oxidative atmospheric environment is better than the inert atmospheric environment for siloxanes’ degradation process because it helps the radical cleavage of the siloxane bond above 250°C [54]. Moreover, siloxane mass [55], geometry [56], and active chain ends [19] contributes to the depolymerization of siloxane.
2. Low-temperature depolymerization, (25 to 110°C)
2.1 Chemical methods for the depolymerization
Particularly in dimethyl derivatives, siloxane bonds are sterically unhindered and extremely polar due to their π-bond and σ-bond character [44]. Consequently, protons can easily protonate the Si-O bond when there is a strong electrophile or nucleophile present. Accordingly, if water or the nucleophile does a nucleophilic attack on the silicon atom, it will form two silanols that can be further rearranged from backbiting reactions to form cyclic derivatives or free silanols [46]. It is noteworthy that either direct substitution or the d-orbitals can cause this depolymerization, and comprehensive studies of these processes are still needed. Common electrophiles for the depolymerization of polysiloxanes are sulfuric acid, triflic acid, and hydrofluoric acid (H.F.] [57]. Some groups have tried strong nucleophiles such as amines, organotin carboxylates, octanoate catalysts, alcohols, and halides [58]. These studies showed that strong electrophile or nucleophiles can breakdown the siloxane bond and can form series of cyclic forms under lower temperature [59].
2.2 Irradiation methods
Apart from the chemically driven bond breaking strategies, there are a limited number of studies that have focused on the irradiation methods to break the -Si-O- bond. Some studies found that PDMS can be degraded at ambient temperature ~ 30°C, using γ-irradiation under the dose rate of 0.35 Mrads/hr. of 6oCo can form D4, D5, and a little amount of D6 as their major products. Furthermore, irradiation results were compared with the thermal decomposition for the same material, and it was observed strained bonds are not favorable to form highly excited siliconium ion from the γ-irradiation. As a result, D4 was observed as the major cyclic form, about ~50%, D5 ~ 45% by weight and no D3 form from γ-irradiation. On the other hand in thermal decomposition, D3 was observed as the major cyclic form about, ~ 60%, and D4 about ~30% by weight [49]. Based on the dose range, time of the exposure and exposure temperature determined the effectiveness of the radiation depolymerization [60]. Moreover, Xiao et al. studied the degradation of siloxane polymers under direct proton exposure resulting in D3 structure formation [61]. Additionally, a few studies have used ultrasonic methods to instill depolymerization. The recycling of silica-filled rubber was used to depolymerize and repolymerize it back to polymers using ultrasonic energy at 170°C [62].
2.3 Recycling methods to form siloxane polymers at lower temperatures
It is noteworthy that the above-mentioned studies all focus on the breakage of the -Si-O- bond at lower temperatures. However, they did not focus on repolymerization of the monomeric units that they made due to the separation issues. The main reason for this is that cyclic forms are kinetically stable products. As a result, with time, those different cyclic forms together convert to oligomers to form the thermodynamically stable products.
Recently, Rupasinghe et al. explored an efficient room temperature and low mol% (0.5) catalytic fluoride (Bu4N+F−) technique to depolymerize rapidly nearly any silicone-based polymers, elastomers, and resins in the presence of high-swell organic solvents such as Tetrahydrofuran THF (Figure 5) [63]. The main products observed were an equilibrium mixture of D4, D5, and D6 verified by 29Si NMR and Gas chromatography–mass spectrometry (GCMS). Notably, if D5 was used as the starting material, the same products were observed. Furthermore, while complex systems show intricate products alongside distinct cyclics, they observed that silicone-rich systems display the best conversions and the best quantity of distinguishable cyclic forms. By quenching the fluoride ions, they could lock the depolymerization products and enable the collection of the cyclics for repolymerization. They found that if the catalyst is not quenched, cyclics forms can further polymerize to form series of oligomers. This process has strong potential for large-scale industrial processing because it requires minimum energy and lower chemicals. Also, this can depolymerize any commercial silicone-based polymer regardless of additives.
Figure 5.
Depolymerization of Wacker Elastosil to cyclic siloxanes in a tetrabutylammonium fluoride THF bath [
3. Mid-temperature depolymerization, 110 to 350°C
Si-O bond scissions arising along the main chain propel catalytic thermal depolymerization mechanisms [64]. Si-C (78 kcal/mol) bond dissociation energy is lower compared to Si-O bond dissociation energy (108 kcal/mol) [65]. However, Si-O bond scission occurred over Si-C due to the empty silicon and oxygen d-orbitals, which also contributes to the chain flexibility.
The compulsory temperature is lower than the thermal process since this process includes the use of catalysts, and products are most often formed by backbiting reactions (Figure 4). Trimethylaluminum and AlCl3, are common electrophiles for depolymerization of siloxanes by catalytic thermal methods, and common nucleophiles for the depolymerization of siloxanes are alkali metal hydroxides; ammonium or phosphonium bases; alkali metal hydroxides; alkali metal halides; and alcohols. It was noted that in order to break the silicone polymer and make cyclic forms, these reactants need heating support. Also, the dominant product is D4 from the cyclic forms formed ranging from D3 to D8 [48, 66].
3.1 Recycling methods to form siloxane polymers at mid-temperatures
Enthaler et al. studied the depolymerization of polysiloxanes using fatty acid anhydride in the presence of catalytic amount of iron salts. They found out when the optimum conditions for silicone oil M100 with fatty acid anhydride (1:3 mol ratio), 5 mol% iron (III) chloride were heated for 24 hours at 200 ̊C, they would receive four different anhydrated depolymerized products. Afterward, they added water and heated for 24 h at 100°C and observed the cyclic formation of D4, D5 and hydrolysis of the acid anhydride to the acid. Then, they mentioned that they can use the cyclic forms to build long chain silicones. However, their study showed that the depolymerization was difficult when they used complex or higher-molecular-weight silicone polymers [67].
4. High-temperature thermal degradation, over 350°C
As the primary source for prediction of thermal stability of polysiloxanes, high-temperature thermal degradation has been the most studied depolymerization method. Thermally propelled intramolecular exchange processes can decompose polysiloxanes and favor cyclic siloxane forms [68]. Thermal degradation can be described by two opposing mechanisms. First, the catalytic depolymerization method can be explained from the molecular mechanism, with the difference being that the thermal process does not utilize additional catalysts. Second, the homolytic cleavage of Si-C bonds causes the radical mechanism [69]. As it evolves methane through hydrogen abstraction, these mechanisms are theoretically and experimentally observable in higher temperatures [70]. To decrease polymer flexibility and extinguish the formation of cyclic depolymerization derivatives, crosslinking of macromolecular radicals can be utilized [71].
A backbiting mechanism drives the uncatalyzed degradation of PDMS and PMPS and form cyclic forms at 350–400°C. The main thermodynamic product of this high-temperature process is cyclic D3, yielding 30–80% with a tailing distribution of cyclic structures from D4 to D24 [72]. These reactions are the thermodynamically necessary reverse of the ring-opening polymerization reactions. In other words, unmodified linear PDMS in the absence of catalyst residues or impurities is stable up to 350°C [73]. Thermodynamically, polymerization of cyclic silicone to form linear polysiloxane occurs at ~110°C [74].
5. Discussion and conclusion
As highlighted in this chapter, the degradation of siloxane-based materials has been extensively studied. Since the 1950s, there have been several publications related to rupturing the siloxane bond, including hydrolysis, catalytic depolymerization, thermal depolymerization, and radical extractions. This work has resulted in new polymers, cyclics, and monomeric silanes. It is noted that siloxane polymer degradation depends on various factors. First, structural architecture (filler type/level, crosslink density, chemical substitution of the polymer backbone, inter-crosslink chain length, degree of free chain ends, and the active chain ends) and the degradation method (oxidative or reductive atmospheres) mainly determine the thermal degradation behavior of the siloxane polymer. Second, the depolymerization source (chemical or physical) has its own important parameters to increase the efficiency. Also, for the polysiloxanes degradation, the most common and deeply analyzed mechanism is thermal degradation.
In the chemical approaches, various types of nucleophiles and electrophiles have been analyzed for the depolymerization efficiency throughout the years. Based on their strength, the depolymerization effectiveness varies. For instance, the strong nucleophile can break the -Si-O- bond at room temperature where the weaker nucleophiles can break with heat. Likewise, F− ions and the amines behave better with the depolymerization of polysiloxanes.
It is notable that the mechanism for the irradiation process has not yet been researched extensively. Free radical processes are achievable if the ionic character is low in the siloxane skeleton because of the irradiation. More significant, however, is that kinetic factors illustrate only a slight role in several heteroatom equilibrations as thermodynamic factors become more crucial with higher temperatures. Consequently, statistical considerations explain the formation of the cyclic products due to the ring-chain equilibration, including the concentration of any cyclic species in equilibrium. With its open-chain homolog, cyclic form is evaluated by considering the probability of ring closure, or equilibrations are considered as scrambling viewpoints. High energy could be another potential explanation, as H can be initiated by methyl groups forming H and Si-CH3 radicals. Additionally, C-Si bonds can be broken, resulting in the formation of methyl and main chain radicals.
Most studies were focused on the degradation or depolymerization of the polysiloxanes. However, as economic and environmental concerns increase, more focus on the development of recycling and reuse methods is needed. Recently, Enthaler [48] Laine [75] and Rupasinghe & Furgal [53] have focused on developing recycling methods. Their studies have explored the reuse of siloxanes either directly or by using recasting methods with the reformation of starting materials to repolymerize. The possibility of reusing siloxanes is promising, and we suspect the field will transit in this direction so that the energy-intensive processes used to create siloxanes are not wasted. As promising as these initial studies have been, however, further development is needed to “green up” the catalytic processes. Specifically, the use of solvent-free or more earth-friendly solvents can be explored as potential drivers of catalytic processes. Also, the use of other alternative energy sources, such as light, to cause on-demand depolymerization could also trigger the catalytic breakdown of siloxane elastomers and resins in commercial use.
Acknowledgments
The authors thank the Bowling Green State University Building Strength Program and the Faculty Startup Program for Funding.
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