Basic Properties Of Ultra-high Molecular Weight Polyethylene (UHMWPE)

Ultra-high molecular weight Polyethylene (UHMWPE) generally refers to polyethylene with a relative molecular mass of 150×104 or more, and is a new type of engineering plastics. The density of UHMWPE with an average relative molecular mass of 200×104 is only 0.935g/cm3, which is lower than that of all other engineering plastics, generally more than 50% lower than that of PTFE, and more than 30% lower than that of paraformaldehyde, so its products are characterized by light weight. UHMWPE has a variety of excellent mechanical properties, physical properties, chemical properties, very wear-resistant, corrosion-resistant, impact-resistant, self-lubricating, low friction factor, low water absorption, not easy to adhere to foreign objects, hygiene and non-toxicity, recycling and low temperature resistance, etc., in particular, its abrasion resistance is particularly outstanding. Its shortcomings are poor heat resistance, low hardness, low tensile strength, poor flame retardant properties.

1. Performance data of UHMWPE

2. Mechanical properties

(1) Impact resistance

The impact strength of UHMWPE is among the best in the whole engineering plastics. UHMWPE can maintain excellent impact resistance even at low temperature, and even in liquid nitrogen (-196℃), UHMWPE also has good impact strength, which is a characteristic that other plastics do not have. Its impact strength begins to increase with the increase in relative molecular mass, the relative molecular mass of 150 × 104 or so to reach its maximum value, and then with the relative molecular mass of the rise and gradually decline. the impact strength of UHMWPE is about 2 times as much as the polycarbonate, ABS 5 times, PA, paraformaldehyde and PBT more than 10 times.

(2) Abrasion resistance

The abrasion resistance of UHMWPE tops the list of plastics, several times higher than that of carbon steel and brass, and its abrasion resistance can be further improved with the increase of relative molecular mass. Figure 1-1 shows the comparison of wear resistance between UHMWPE and other materials. The test conditions are as follows: the mortar is composed of 2 mass parts of water and 3 mass parts of sand; the rotational speed of the test piece is 900r/min; the running time is 7h.

Comparison of abrasion resistance of UHMWPE with other materials

(3) Self-lubrication

UHMWPE has excellent self-lubrication and a low dynamic friction factor comparable to that of PTFE. Table 1-2 lists the comparison of dynamic friction factor between UHMWPE and other engineering plastics. It can be seen that the kinetic friction factor of UHMWPE under water lubrication is 1/2 of that of Pa66 and POM, and under non-lubricated conditions it is second only to PTFE, which has the best self-lubricating property among plastics. when it works in sliding or rotating form, it has better sliding performance than steel and brass with the addition of lubricating oil. Moreover, the price of UHMWPE is low, so in the field of tribology, it is regarded as a friction material with a very good cost-performance ratio.

Comparison of dynamic friction factor between UHMWPE and other engineering plastics

(4) Tensile strength

The tensile yield strength of UHMWPE is related to the relative molecular mass and density, with the increase of relative molecular mass and density decrease, the tensile yield strength also decreases; however, when the density is 0.94g/cm3 or the relative molecular mass is more than 150×104, the change of the tensile yield strength is relatively small, and basically tends to be a fixed value. The tensile breaking strength is only related to the relative molecular mass, and the effect of density on it is basically negligible. Contrary to the tensile yield strength, with the increase of relative molecular mass, the tensile breaking strength is increased accordingly.

3. Electrical properties

Same as ordinary polyethylene, the molecular chain of UHMWPE is only composed of carbon and hydrogen, the electrical properties of UHMWPE are independent of the size of relative molecular mass, so it has excellent electrical insulation properties. Its volume resistivity is as high as 1016~1018Ω-cm, and the dielectric loss angle tangent is (2~3)×10-4. 4.

4. Thermal properties

The heat resistance of UHMWPE is not high, the use temperature is generally below 100℃. However, because of its relative molecular mass is very large, so its heat distortion temperature and Vicat softening point are higher than that of ordinary hdpe, the melting point of UHMWPE (136℃) is more or less the same as that of ordinary HDPE, however, UHMWPE has very good low temperature resistance, the best among all plastics, the brittleness temperature is below -70℃, even if the liquid helium temperature (-3℃) is below -70℃, and the temperature of the liquid helium temperature (-4℃) is below -70℃, and the temperature of the liquid helium temperature is below -70℃. The brittleness temperature is below -70°C, and even at liquid helium temperatures (-269°C) it still has some impact strength and abrasion resistance. Even at liquid helium temperature (-269℃), it still has certain impact strength and abrasion resistance. The working temperature range is from -265 to 100℃, and it can still maintain good toughness and strength without brittleness when the temperature is as low as -195℃, so it can be used in low temperature parts, pipelines, and very low temperature situations such as nuclear industry.

5. Chemical resistance

UHMWPE has excellent chemical resistance. Diethane, ethylene glycol, gasoline, heptane, hexane, cyclohexane, carbon disulfide, isopentanol, ethyl ester, methyl isobutyl ketone, benzene methanol, etc. More than 80 kinds of organic solvents such as any one of the solvents impregnated for 30d, the appearance of the appearance of no anomalous phenomena, the performance of the various kinds of virtually no change. This is due to the fact that ultra-high molecular weight polyethylene has no functional group in the molecular structure, and there is almost no branched chain and double bond as well as high crystallinity. However, it is not stable in concentrated sulfuric acid, concentrated hydrochloric acid, concentrated nitric acid, halogenated hydrocarbons and aromatic hydrocarbons and other solvents, and the oxidation speed increases with the temperature rise.

6. Water absorption

The water absorption rate of UHMWPE is the smallest among engineering plastics (see Table 1-3), which is due to the fact that the molecular chain of UHMWPE consists of carbon and hydrogen elements only, and there is no polar group in the molecule, so the water absorption rate is extremely low. Therefore, the products will not change in size due to water absorption even in a humid environment, and at the same time, it will not affect the mechanical properties of the products, such as precision and abrasion resistance, and the raw materials do not need to be dried before molding and processing.

Water absorption of several common engineering plastics

7. Ultra-high molecular weight polyethylene applications

With the best overall performance, UHMWPE has been widely used in the fields of machinery, electrical appliances, packaging containers, chemical equipment, transportation, material storage and transportation, conveying pipelines, medical and sports equipment, etc. The main products are: sheets, pipes, rods and finished products. The main products are: plates, pipes, rods and finished products, which include gears, bearings, bushings, rollers, guide rails, sliders, liners and so on. From the processing technology of the products can be divided into molding, machining, extrusion, injection molding, stamping, blow molding, roll molding, thermoplastic processing molding and wire drawing molding.