During the processing of plastic profiles, if the polymer is subjected to high temperature, strong shearing force, etc., under these strong effects, the polymer chain will be broken, and the molecular weight will decrease, 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, the degradation of polymers is usually carried out randomly. This is because the energy of all chemical bonds in the polymer are very close. Under the action of these physical factors, the degradation mechanism is also very similar, usually through the intermediate steps of forming free radicals according to the chain reaction mechanism, including the production of active centers, chain transfer and chain scission, and chain termination.
2. Gradual degradation: This degradation is mainly the selective degradation of the polymer containing trace moisture, acid or alkali, etc. under the high temperature of processing. The degradation generally occurs in the carbon-heterochain (such as CN, CO, CS, C -SI, etc.), this is because the carbon-heterochain bond energy is small and the stability is poor. Degradation has the characteristics of a gradual reaction. Each step is independent. The intermediate product is stable. The chance of chain scission increases with the increase of molecular weight. Therefore, as the degradation reaction progresses gradually, the molecular weight of the polymer gradually decreases. The 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 the temperature, the faster the degradation; the longer the residence time at high temperature, the more severe the degradation. Oxygen is often present in the processing process. Oxygen can oxidize the polymer to form a peroxide structure at high temperature. The peroxide is easily decomposed to produce free radicals, thereby triggering a chain degradation reaction, which is called thermal oxygen degradation, which is the main cause of polymer degradation. course.
During the processing, the polymer is repeatedly subjected to stress (mainly shear stress). When the energy of the shear stress exceeds the bond energy, it will cause the break of the chemical bond and cause degradation. The shearing effect and the heat effect together have a strong degradation promotion effect on the degradation of the polymer.
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 procedure.
2. Solution
After the polymer is degraded during processing, the appearance of the product deteriorates, the internal quality is reduced, and the service life is shortened. Therefore, in most cases, the processing should try to minimize and avoid polymer degradation. Therefore, the following measures can usually be adopted:
1. Strictly control the technical indicators of raw materials, try to remove the catalyst residues in the polymer and other magazines.
2. Strictly dry the polymer before use, especially the storage process of polyester, polyether, polyamide and other polymers is easy to absorb moisture from the air, and the moisture content should usually be reduced to 0.01-0.05% before use
3. Determine reasonable processing technology and processing conditions, so that the polymer can be processed and formed under conditions that are not prone to degradation.
4. Processing equipment and molds should have a good structure. Mainly should eliminate the dead angle or gap that may exist in the part of the equipment that is in contact with the polymer, reduce the excessively long flow channel, improve the heating device, increase the sensitivity of the temperature display device and the cooling efficiency of the cooling system.
5. Consider using antioxidants, stabilizers, etc. in the formulation to enhance the polymer's resistance to degradation.
In some cases, the degradation effect of the polymer during the processing can be used, such as the rubber refining (plastic refining) to reduce the molecular weight and improve the processability; polymer blends, the use of free radicals generated by the shearing effect, but two One or more polymers produce grafting, copolymerization and other reactions to improve the performance of the blend
