Thermal degradation of PLA

Polymeric materials are commonly used above room temperature. Existing commodity polymers, such as polyethylene, polypropylene, polystyrene, polycarbonate, etc., are frequently 

PLA-V = unprocessed raw material PLA-I = injected PLA-EI = extruded and injected PLA-IA = injected and annealed PLA-EIA = extruded, injected and annealed. 

A photosensitizer can be added to enhance the photodegradation of PLA. The purpose of this is to increase the rate of degradation when accelerated PLA waste treatment is required. Tsuji et al. (2005) has studied the effect of N,N,N, N -tetramethyl-l,4-phenylenediamine (TMPD) on amorphous and crystalline PLA 

TMPD enhances the photodegradation irrespective of the crystallinity of PLA. This indicates that the formation of radicals are involved in a free reaction with the backbone, whereas hydrolytic degradation requires water molecules to be in contact with the amorphous structure for chain cleavage to take place (Tsuji et al., 2005). 

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It has been reported that the thermal degradation of PLA predominantly consists of random main-chain scission and unzipping depolymerization reactions. The random degradation reaction involves hydrolysis, oxidative degradation, c/s-elimination, and intramolecular and intermolecular transesterification. Almost all the active chain-end groups, residual catalysts, residual monomers, and other impurities enhance the thermal degradation of PLA. As a consequence... 

The thermal degradation of PLA has been claimed to mainly occur via random scission based on a linear relationship between inverse of the number-average degree of polymerization P and time as shown in Equation 23.2 [28]. Recently, Aoyagi et al. [9] and Abe et al. [29] suggested that the isothermal degradation of PLLA at 220, 290, and 330°C proceeded not only via simple random scission, but also via an unzipping depolymerization of the polymer chain based on the nonlinear relationships of l/P and P with time. 

PLA is well known as a degradable material at higher temperatures. However, the mechanisms of the thermal degradation are complex. To clarify and control the thermal degradation of PLA, many efforts have been made. As a result, the effects of some important factors, such as polymerization catalyst residues, chain-end structures, depolymerization catalysts, stereocomplex structure, racemization, blends with other polymers, and so on, have been clarified. Highly active and selective depolymerization cat-... 

The heat provides the required energy for promoting the oxidation of the carbon in the polymer backbone of molecules constituting the plastics. Generally, thermal degradation of PLA occurs at 159- 178°C depending on its molecular weight and its crystallinity. 

Figures 12.7 and 12.8 show that the thermal degradation of PLA is more complex than the simple reaction that gives L, L lactide.

Although the thermal degradation of PLA proceeds mainly hy non-radical mechanisms, previous studies hy McNeill and Leiper [19, 25], have suggested that degradation in the presence of a free radical inhibitor proceeds much more slowly than in the case of a pure pol3mier. Tsuji et al. [31] also reported that homolysis of PLA occurs and increases with temperature, particularly at temperatures in excess of 270°C. 

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