Polystyrene depolymerization

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Depolymerization trends are illustrated in Table 1.8. The side groups on the a-carbon atom are decisive for the tendency of plastics to depolymerize. Thus, polystyrene depolymerization increases significantly when a methyl group (poly-a-methylstyrene) is present at the a-C atom instead of a hydrogen atom. During depolymerization, monomer cleavage may cause strong odors [20]. 

Hie hydrolytic depolymerization of nylon-6 was followed by gel permeation chromatography (GPC), viscometry, and gravimetry. GPC determinations were performed on a Waters 150C chromatography system using benzyl alcohol as die eluant, two Plgel 10-p.m crosslinked polystyrene columns, and a differential refractometer detector. The flow rate was 1 mL/min. The concentration of the polymer solutions was 0.5 wt% and dissolution was accomplished at 130°C. 

The two main brominated flame retardants used commercially in PET are PyroChek 68PB (see Figure 14.18) and Saytex HP-7010 (Albemarle). Both of these flame retardants are based on brominated polystyrene. While there are similarities between these flame retardants, they are not equivalents. There are quality and performance differences between these two products as they use different raw materials (i.e. polystyrenes) and the process for bromination is different. Saytex HP-7010 has better thermal stability and colour control than does PyroCheck 68 PB. However, if higher flow characteristics are a necessary property of the FR-PET, then Pyrocheck 68 PB would be the product of choice. Sodium antimonate is the appropriate synergist in PET since it is more stable at the higher processing temperatures required of PET and does not cause depolymerization of this polyesters. 

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

Photolytic. Irradiation of styrene in solution forms polystyrene. In a benzene solution, irradiation of polystyrene will result in depolymerization to presumably styrene (Calvert and Pitts, 1966). 

Cross-linked polystyrene can be directly brominated in carbon tetrachloride using bromine in the presence of Lewis acids (Experimental Procedure 6.2 [55-58]). Thal-lium(III) acetate is a particularly suitable catalyst for this reaction [59]. Harsher bro-mination conditions should be avoided, because these can lead to decomposition of the polymer. Considering that isopropylbenzene is dealkylated when treated with bromine to yield hexabromobenzene [60], the expected products of the extensive bromi-nation of cross-linked polystyrene would be soluble poly(vinyl bromide) and hexabromobenzene. In fact, if the bromination of cross-linked polystyrene is attempted using bromine in acetic acid, the polymer dissolves and apparently depolymerizes [61]. 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

Unsaturated polyesters can undergo degradation by oxidation of their aliphatic segments, decarboxylation of esters and partial depolymerization of polystyrene chains. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

A number of chemical analyses require prior depolymerization of the original sample e.g. starch, lignin, chitosan, carbohydrates). This process is occasionally very slow and has scarcely been subjected to US, despite the proven accelerating effect of this form of energy on these polymers [133,134] and others such as polystyrene [135] and poly(ethylene oxide) [136], or even on pharmaceutical precursors [137]. 

In cases where no additional oxygen is present, polystyrene can undergo nearly pure thermal degradation. The two prevalent mechanisms are sequential elimination of monomer units, which is called unzipping or depolymerization. In this case, styrene monomer is formed. Random chain scission can also occur. It is sometimes combined with unzipping at the reactive broken chain ends. At temperatures approaching 300 °C, up to 40 % of a polystyrene molecule can be converted to styrene monomer. 

As mentioned previously, when polystyrene is subjected to the temperatures of a flame it pyrolyzes by a depolymerization mechanism to give monomer and oligomers [14]. The combustion of these volatile products in the vapor phase above the sample supplies heat back to the solid sample (Figure 29.3). If the energy supplied by combustion is sufficient to maintain the pyrolysis process, the flame is self-sustaining even after the test flame has been removed. In order to make polystyrene more flame retardant, the cycle of pyrolysis and combustion must be broken. Flame retardants may act in either the vapor or solid (condensed) phase. 

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

Neat polystyrene feedstocks will depolymerize in a pyrolysis process to give predominantly styrene monomer-a liquid fuel with good energy content. 

Thus polystyrene is not a depolymerizing monomer due to the more favourable reaction of abstraction of the hydrogen at the tertiary carbon site. This is in contrast to poly(a-methyl styrene), which has this position blocked by the methyl group, so unzipping occurs. 

Fig. 24. Application of Gordon s theory (continuous line) to Grassie and Kerr s experimental points for the molecular weight changes during depolymerization of polystyrene [69].

Poly-a,P,P trifluorostyrene has slightly lower stability than polystyrene (Table 12). The relatively large amount of monomer formed suggests that chain scission is followed by depolymerization. Poly-2,3,4,5,6 penta-fluoropolystyrene is much more stable than polystyrene (Table 12). Wall et al. [269] attributed this increased stability to the loss of resonance interaction between the phenyl group and the chain because of the presence of the fluorine atoms. Perfluorostyrene polymerized by 7-irradiation has, however, been found recently to have a stability close to that of polystyrene a rate of weight loss of 0.4% per min was found at 335°C and complete decomposition of the polymer is observed at 432°C [270],... 

The degradation of poly-a-methylstyrene is unaffected by the presence of polystyrene, but depolymerization of the latter polymer can be brought about at temperatures below 300°C by heating in the presence of poly-a-methylstyrene [320], The rate of polystyrene volatilization then varies as an inverse function of the molecular weight of poly-a-methylstyrene. The system is heterogeneous, consisting of micelles of poly-a-methylstyrene embedded in a polystyrene matrix. It has been suggested that the poly-a-methylstyrene chain unzips completely to a monomer radical which diffuses into the polystyrene matrix and attacks a polystyrene molecule. 

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