What Are the Factors of Plastic Degradation

01. Definition of plastic degradation

Plastic degradation refers to decomposition. In polymer chemistry, it usually refers to the process of decreasing polymerization degree of polymer molecules under chemical or physical action. The polymer will be degraded under the action of heat, force, oxygen, water and light radiation. The degradation process is essentially a result of structural changes of a large number of molecular chains, such as disappearance of elasticity, decrease of strength, decrease or increase of viscosity, etc.

02. Degradation process

Force, water and oxygen play an important role in the degradation of polymer during injection molding. Oxygen and water can decompose the polymer at high temperature. The shear force reduced the viscosity of the polymer. Thermal degradation refers to the phenomenon of yellowing, degradation and decomposition of some polymers at high temperature for too long.

Whether the polymer is easy to degrade is related to the internal and external structure of the molecule, and to the presence of decomposed impurities. The impurities that can cause the degradation of polymers are usually catalysts for thermal degradation, such as hydrogen chloride for the decomposition of polyvinyl chloride (PVC) and formaldehyde for the decomposition of polyformaldehyde (POM), which can aggravate the degradation of polymers.

03. Temperature and degradation

The degradation of polymers is related not only to the temperature level and range, but also to the repeated processing times experienced in the temperature field. The thermal degradation and melt flow rate (MFR) of different polymers are different after repeated processing. Under normal temperature, polystyrene (PS), polycarbonate (PC), polypropylene (PP), has a tendency to increase after several times of processing. But polyethylene (PE), anti-impact PS and so on has the phenomenon of decline.

04. Shear and degradation

The experimental results show that the higher the shear stress and the higher the shear rate, the faster the molecular degradation rate and the shorter the broken chain, and when the heating temperature or the content of plasticizer is increased, the degradation effect of the force will decrease.

05. Moisture and degradation

Hydrolysis of some plastics in injection molding occurs frequently because of the existence of hydrolyzable chemical groups such as amide, ester, nitrile and so on in the polymer, or the formation of hydrolyzable groups under the action of oxidation. If these groups are on the main chain of the molecule, the hydrolysis will break the chain and degrade. Due to the hydrolysis of some polymers, attention should be paid to the hygroscopicity of these plastics.

Some plastics tend to absorb moisture or agglutinate water because they contain polar hydrophilic groups, such as ABS, PMMA, PA, PC, PPE, etc. need drying treatment in injection molding to prevent hydrolysis.