How to avoid polymer degradation during processing?

Time:2021-10-19 09:15:49 / Popularity: / Source:

Degradation: In process of processing, polymers are subject to high temperature, strong shearing force, etc. Under these strong effects, polymer chain will be broken and 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 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, usually through intermediate steps of forming free radicals according to chain reaction mechanism, including production of active centers, chain transfer, chain scission, and chain termination.
2. Gradual degradation
This degradation is mainly due to selective degradation of polymers containing trace amounts of moisture, acids or alkalis at high temperatures during processing. Degradation generally occurs at carbon-heterochain (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, as degradation reaction progresses gradually, molecular weight of polymer gradually decreases at the same time, 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 easily decomposed 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.
polymer degradation 

2. Solution

After polymer is degraded during processing, appearance of product deteriorates, internal quality is reduced, and service life is shortened. Therefore, in most cases, processing should try to minimize and avoid polymer degradation. Therefore, following measures can usually be adopted:
1. Strictly control technical indicators of raw materials, try to remove catalyst residues in polymer and other magazines.
2. Strictly dry polymer before use, especially storage process of polyester, polyether, polyamide and other polymers are easy to absorb moisture from air. Usually, moisture content should be reduced to less than 0.01-0.05% before use.
3. Determine reasonable processing technology and processing conditions, so that polymer can be processed and formed under conditions that are not prone to degradation.
4. Processing equipment and molds should have a good structure. It should mainly eliminate dead angle or gap that may exist in the 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.
5. Consider use of antioxidants, stabilizers, etc. in the formulation to strengthen polymer's resistance to degradation.
In some cases, degradation effect of polymer in 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.

PC polycarbonate resin performance and application overview

PC polycarbonate plastic is an almost colorless glassy amorphous polymer with good optical properties. PC high molecular weight resin has high toughness, Izod notch impact strength is 600~900J/m, heat distortion temperature of unfilled grades is about 130℃. Glass fiber can increase this value by ℃. Flexural modulus of PC can reach more than 2400MPa, and resin can be processed into large rigid products.
Below 100℃, creep rate under load is very low. PC has poor hydrolysis resistance and cannot be used for products that are repeatedly subjected to high-pressure steam. PC material has flame retardancy, wear resistance and oxidation resistance. Polycarbonate is colorless and transparent, heat-resistant, impact-resistant, flame-retardant BI grade, and has good mechanical properties at ordinary temperatures.
Compared with performance close to polymethyl methacrylate, polycarbonate has good impact resistance, high refractive index, and good processing performance. It has UL94V-0 flame retardant performance without additives. However, price of polymethyl methacrylate is lower than that of polycarbonate, and large-scale devices can be produced by bulk polymerization.

1. Main performance

a. Mechanical properties: high strength, fatigue resistance, dimensional stability, and small creep (very few changes under high temperature conditions);
b. Heat aging resistance: enhanced UL temperature index reaches 120~140℃ (long-term outdoor aging is also very good);
c. Solvent resistance: no stress cracking;
d. Water stability: easy to decompose in contact with water (use caution in high temperature and high humidity environments);
e. Electrical performance:
(1) Insulation performance: excellent (it can maintain stable electrical performance even under humidity and high temperature, it is an ideal material for manufacturing electronic and electrical parts);
(2) Dielectric coefficient: 3.0-3.2;
(3) Arc resistance: 120s
f. Molding processability: injection molding or extrusion molding by ordinary equipment. Due to fast crystallization speed and good fluidity, mold temperature is also lower than that of other engineering plastics. When processing thin-walled parts, it only takes a few seconds, and it only takes 40-60s for large parts.

2. Application of polycarbonate

Application and development of PC polycarbonate is developing in direction of high composite, high function, specialization and serialization. It has launched its own dedicated grades for various products such as optical discs, automobiles, office equipment, boxes, packaging, medicine, lighting, and films.
Optical lighting: used to manufacture large-scale lampshades, protective glass, left and right eyepiece tubes of optical instruments, etc., and can also be widely used in transparent materials on airplanes.
Electronic and electrical appliances: Polycarbonate is an excellent E (120℃) grade insulating material, which is used to manufacture insulating connectors, coil frames, tube sockets, insulating sleeves, telephone shells and parts, and battery shells for miner’s lamps. It can also be used to make parts with high dimensional accuracy, such as compact discs, telephones, electronic computers, video recorders, telephone switches, signal relays and other communication equipment. Polycarbonate film is also widely used as capacitors, insulating bags, audio tapes, color video tapes, etc.
Mechanical equipment: used to manufacture various gears, racks, worm gears, worms, bearings, cams, bolts, levers, crankshafts, ratchets, as well as some mechanical equipment housings, covers, frames and other parts.
Medical equipment: cups, tubes, bottles, dental equipment, medicine containers and surgical equipment that can be used for medical purposes, even artificial organs such as artificial kidneys and artificial lungs.
Other aspects: used in construction as hollow rib double-wall panels, greenhouse glass, etc.; in textile industry as textile bobbins, textile machine bearings, etc.; in daily use as milk bottles, tableware, toys, models, LED lamp housings and mobile phone housings, etc. .

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