Hydrolysis and thermal decomposition are the great enemies of plastics.
There are many substances that are subject to hydrolysis: acid anhydrides. Acid chlorides, methyl halides, and organometallic compounds are easily hydrolyzed. Polycarbonates and polyamides require caution.
In addition, when plastics are heated, they are not merely deformed, but the chemical bonds between each atom that make up the polymer are broken by oxidative or thermal decomposition without causing any apparent deformation, and the chemical structure begins to change.
Degradation leads to a decrease in molecular weight, which is directly related to a decrease in strength. Figure 1 shows the relationship of molecular weight to melt temperature and time, and with and without drying of polycarbonate.
It shows that dried material is stable for long periods of time at 300°C, while unseasoned, moisture-absorbed material at temperatures above this level shows a very significant decrease in molecular weight with time.
Figure 2, also an example of polycarbonate, shows the relationship between molecular weight and impact strength at different environmental temperatures.
Polycarbonate is characterized by its outstanding impact strength compared to other plastics, but when the molecular weight falls below 20,000, it becomes extremely weak against impact and cannot exhibit any of the characteristics of polycarbonate.
There are two types of degradation behavior of resins: random degradation and depolymerization. In the former, the molecular chains are broken and the molecular weight gradually decreases, such as in PE, PP, ABS, PA, PC, and PPE.
In the latter, cleavage begins at the end of the molecule and progresses rapidly; PMMA, PS, and POM are examples.
Heat stabilizers are used to prevent this progression, and care must be taken because certain colorants may promote degradation.
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