Degradation: During the processing process, polymers are subjected to high temperature, strong shear force, etc., under these strong actions, the polymer chain will produce a broken chain, so the molecular weight decreases, which is degradation.

1. Classification of degradation

1. Free radical chain degradation reaction
Degradation caused by physical factors such as heat and stress. Under the influence of heat or shear forces, the degradation of polymers is usually carried out by irregular selection. This is because the energies of all chemical bonds in polymers are very close.

Under the action of these physical factors, the degradation mechanism is also very similar, usually through the intermediate steps of free radical formation according to the chain reaction mechanism, including the generation of active centers, chain transfer and chain breakage, and chain termination.

2. Gradual degradation

This degradation is mainly at high temperature processing, polymers contain trace amounts of water, acids or alkalis, etc. for selective degradation, and degradation generally occurs at carbon-heterochains (such as C-N, C-O, C-S, C-Si, etc.), because the carbon-heterochain bond energy is small and the stability is poor.

Degradation has the characteristics of gradual reaction, each step is independent, the intermediate product is stable, and the chance of chain breakage increases with the increase of molecular weight, so with the gradual progress of the degradation reaction, the molecular weight of the polymer gradually decreases, and its molecular weight dispersion also gradually decreases. Polymers containing amides, esters, and acetal are prone to hydrolysis, esterization, acid hydrolysis, and amine hydrolysis at high temperatures.

The higher the temperature, the faster the degradation; The longer it stays at high temperatures, the more degradation will be. Oxygen can oxidize the polymer to form a peroxide structure at high temperature, and the peroxide is easy to decompose to produce free radicals, thus triggering a chain degradation reaction, called thermal oxygen degradation, which is the main process of polymer degradation.

During the processing process, the polymer is repeatedly subjected to stress (mainly shear stress), and when the energy of the shear stress exceeds the bond energy, it will cause the breakage of chemical bonds and degradation. Shear action and heat action together play a strong role in promoting the degradation of polymers.

Trace moisture is the main factor in the degradation of some polymers, such as PC, nylon (PA), ABS, polyester, etc. Therefore, drying before processing is a necessary process.

II. Solutions

1. Strictly control the technical specifications of raw materials and try to remove impurities such as residual catalysts in the polymers.

2. Conduct rigorous drying of polymers before use. Especially for polymers like polyesters, polyethers and polyamides, which tend to absorb moisture from the air during storage, the moisture content should generally be reduced to below 0.01-0.05% prior to application.

3. Determine reasonable processing technologies and conditions to enable the molding of polymers under circumstances that are not prone to degradation.

4. Ensure that processing equipment and molds have sound structural designs. The main focus is to eliminate potential dead corners or gaps in the parts of the equipment that come into contact with polymers, shorten excessively long runners, improve heating devices, and enhance the sensitivity of temperature display devices as well as the cooling efficiency of cooling systems.

5. Consider adding antioxidants, stabilizers and other additives in the formulation to strengthen the polymers’ resistance to degradation.

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