How to avoid degradation of the polymer during processing?

First, the classification of degradation

1. Free radical chain reaction

Degradation caused by physical factors such as heat and stress. Under the influence of heat or shear, the degradation of the polymer is usually carried out in an irregular manner. This is because the energy of all chemical bonds in the polymer is very close. Under 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 active center production, chain transfer and chain fragmentation, chain termination.

2. Progressive degradation

This degradation is mainly at the high temperature of processing, the polymer contains trace moisture, acid or alkali, etc. for selective degradation, and the degradation generally occurs at carbon-heterochains (such as CN, CO, CS, C-Si, etc.). This is because the carbon-heterochain bond is small and the stability is poor. Degradation has the characteristics of stepwise reaction, each step has independence, the intermediate product is stable, and 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 while The molecular weight dispersibility is also gradually reduced. Polymers containing amides, esters, and acetals are susceptible to degradation reactions such as hydrolysis, esterification, acid hydrolysis, and amine hydrolysis at high temperatures.

The higher the temperature, the faster the degradation; the longer the residence time at high temperatures, the more severe the degradation. Oxygen is often present in the process. Oxygen can oxidize the polymer to form a peroxide structure at high temperatures. The peroxide is easily decomposed to generate free radicals, which initiates a chain degradation reaction, called thermal oxygen degradation, which causes the degradation of the polymer. 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 causes the chemical bond to break and cause degradation. The shearing action together with the action of heat exerts a strong degradation promoting effect on the degradation of the polymer.

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

Second, the solution

After the polymer is degraded during processing, the appearance of the product is deteriorated, the inherent quality is lowered, 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 taken:

1. Strictly control the technical indicators of raw materials and try to remove the catalyst residues in the polymer.

2. The polymer is strictly dried before use. Especially the polyester, polyether, polyamide and other polymers are easy to adsorb moisture from the air. The moisture content should be reduced to 0.01-0.05% or less before use.

3. Determine reasonable processing techniques and processing conditions so that the polymer can be processed under conditions that are less prone to degradation.

4. Processing equipment and molds should have a good structure. It is mainly necessary to eliminate possible dead angles or gaps in the parts of the device that are in contact with the polymer, to reduce excessively long flow paths, to improve the heating device, to increase the sensitivity of the temperature display device, and to cool the cooling system.

5. Consider the use of antioxidants, stabilizers, etc. in the formulation to enhance the resistance of the polymer to degradation.

In some cases, the degradation effects of the polymer during processing, such as rubber opening (mastication), lowering the molecular weight and improving the processability; polymer blends, free radicals generated by shearing effects, can be utilized. Two or more polymers produce a reaction such as grafting, copolymerization, etc., thereby improving the properties of the blend.