Polymers tend to degrade under the influence of heat, force, oxygen, water, light, ultrasound, and nuclear radiation, thereby deteriorating their performance. The essence of degradation is: 1 broken chain; 2 cross-linked; 3 changes in the molecular chain structure; 4 changes in the side of the base; In many of these roles, free radicals are often a lively intermediate product. The result of the action is a change in the molecular structure of the polymer. In the case of molding, thermal degradation is a major factor in the normal operation. Degradation caused by force, oxygen, and water is secondary, while degradation of light, ultrasound, and nuclear radiation is rare.
First, thermal degradation
Thermal degradation is related to the molecular structure and presence or absence of impurities that promote degradation. The thermal degradation of the polymer begins with the weakest chemical bond in the molecule. Regarding the order of the strength of chemical bonds, it is agreed that:
C â€” F> Câ€”H (alkenes and alkanes)> Câ€”C0 aliphatic chains> Câ€”Cl
In carbon: Stability... Câ€”Câ€”C...> Tertiary carbon atoms> Quaternary carbon atoms
Polymers that are prone to thermal degradation include PVC, PVDC, POM, and the like.
Effective prevention method: Add heat stabilizers to minimize the time for high-temperature residence.
Second, the force degradation
In the molding process, the polymer is often degraded by shearing and tensile stress due to crushing, grinding, high-speed stirring, kneading, and extrusion. The factors that affect the force degradation are as follows: (1) The greater the relative molecular mass of the polymer, the more likely it is to undergo force degradation. (2) The greater the applied stress, the greater the rate of degradation and the shorter the resulting segment of broken molecules. (3) A certain amount of stress can only break the molecule to a certain length. When all the molecular chains have been broken to a length that the applied stress can degrade, the force degradation will not continue. (4) The tendency of force to degrade when the polymer is warmed and plasticizers are added.
Third, oxidative degradation
At room temperature, most of the polymers can react with oxygen very slowly. Only under the combined effects of heat and ultraviolet radiation, the oxidation effect is significant. The degradation process is very complex, and the nature of the reaction varies with the type of polymer. However, in most cases, the oxidation is carried out in a chain reaction. The polymer first forms free radicals by the initiation of heat or other energy sources. Free radicals then combine with oxygen to form peroxy radicals, which in turn react with the polymer to form peroxohydrides and another free radical. These two steps are the chain transmission.
Effective prevention methods: Add light stabilizers and antioxidants to prevent high temperature and oxygen from contacting.
Polymers containing amide, ester, etc. groups may undergo hydrolysis. The method of prevention is drying.
Source: Light Industry Engineering Department of Zhengzhou Light Industry Vocational College
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