Degradation: Polymer is subjected to high temperature, strong shearing force, etc. in process of processing. Under these strong effects, polymer chain will be broken, and molecular weight will decrease, which is degradation.

 

Degradation caused by physical factors such as heat and stress. Under influence of heat or shear, degradation of polymers is usually carried out randomly. This is because energy of all chemical bonds in polymer are very close.
Under action of these physical factors, degradation mechanism is also very similar. It is usually carried out according to chain reaction mechanism through intermediate steps of formation of free radicals, including production of active centers, chain transfer and chain scission, and chain termination.

This kind of degradation is mainly due to selective degradation of polymer containing trace moisture, acid or alkali and other impurities under high processing temperature. Degradation generally occurs at carbon-hetero chain (such as CN, CO, CS, C-Si, etc.) because carbon-heterochain bond energy is small and stability is poor.
Degradation has characteristics of a gradual reaction. Each step is independent. Intermediate product is stable. Chance of chain scission increases with increase of molecular weight. Therefore, with gradual progress of degradation reaction, while molecular weight of polymer gradually decreases, its molecular weight dispersion also gradually decreases. Polymers containing amides, esters, and acetals are prone to degradation reactions such as hydrolysis, esterolysis, acidolysis, and amineolysis at high temperatures.
The higher temperature, the faster degradation; the longer residence time at high temperature, the more severe degradation. Oxygen is often present in processing process. Oxygen can oxidize polymer to form a peroxide structure at high temperature. Peroxide is easy to decompose to produce free radicals, thereby triggering a chain degradation reaction, which is called thermal oxygen degradation, which is main process of polymer degradation.
During processing, polymer is repeatedly subjected to stress (mainly shear stress). When energy of shear stress exceeds bond energy, it will cause break of chemical bond and cause degradation. Shearing effect and heat effect together have a strong degradation promotion effect on degradation of polymer.
Trace moisture is main factor in degradation of some polymers, such as PC, nylon (PA), ABS, polyester, etc. Therefore, drying before processing is a necessary procedure.

 

After polymer is degraded during processing, appearance of product deteriorates, internal quality is reduced, and service life is shortened. Therefore, in most cases, processing process should try to minimize and avoid polymer degradation. Therefore, following measures can usually be adopted:
Strictly control technical indicators of raw materials and try to remove impurities such as catalyst residues in polymer.
Strictly dry polymer before use. In particular, storage process of polyester, polyether, polyamide and other polymers is easy to absorb moisture from air, moisture content should usually be reduced to 0.01-0.05% before use.
Determine reasonable processing technology and processing conditions so that polymer can be processed and formed under conditions that are not prone to degradation.
Processing equipment and molds should have a good structure. It should mainly eliminate dead angle or gap that may exist in part of equipment that is in contact with polymer, reduce excessively long flow channel, improve heating device, increase sensitivity of temperature display device and cooling efficiency of cooling system.
Consider use of antioxidants, stabilizers, etc. in formulation to enhance polymer's resistance to degradation.
In some cases, degradation effect of polymer during processing can be used, such as rubber refining (plastic refining) to reduce molecular weight and improve processability; polymer blends use free radicals generated by shearing effect, but two or more polymers can undergo grafting, copolymerization and other reactions, thereby improving performance of blend.