The Main Properties of Polyurethane Elastomer
1.1 Hardness
The hardness range of ordinary rubber is Shore A20 to Shore A90, the hardness of plastic is about Shore A95 to Shore D100, and the hardness of polyurethane elastomer is as low as Shore A10 and as high as Shore D80, and does not require the help of fillers. What is particularly valuable is that the elastomer still has good rubber elasticity and elongation under the plastic hardness, while ordinary rubber can only obtain higher hardness by adding a large amount of filler and at the expense of greatly reducing the elasticity and elongation. It is reported that when the hardness is higher than 75D, its elasticity will be seriously lost, and when the hardness is higher than 85D, it is not an elastic material.
1.2 Mechanical strength
Polyurethane elastomers have high mechanical strength, manifested in Young's modulus, tear strength and bearing capacity.
1.2.1 Young's modulus and tensile strength Within the elastic limit, the ratio of tensile stress to deformation is called Young's modulus (E) or elastic modulus.
Polyurethane elastomers, like other elastomers, obey Hooke's theorem only at low elongation (about 2.5%). But its Young's modulus is much higher than other elastomers. Moreover, the Young's modulus of polyurethane elastomers covers rubber and plastics, and the range is wide, unmatched by other materials.
1.2.2 Tear strength
The tear strength of polyurethane elastomer is very high, especially polyester type, which is more than twice that of natural rubber.
1.2.3 Carrying capacity
Although the compressive strength of polyurethane elastomers is not high at low hardness, polyurethane elastomers can increase the hardness on the premise of maintaining rubber elasticity, thereby achieving high load-bearing capacity. The hardness of other rubbers is greatly limited, so the bearing capacity cannot be greatly improved.
1.3 Wear resistance
The wear resistance of polyurethane elastomers is very outstanding, and the test results are generally in the range of 0.03 to 0.20 mm3/m, which is about 3 to 5 times that of natural rubber. In actual use, due to the influence of factors such as lubricants, the effect is often better. Wear resistance is closely related to the tear strength and surface condition of the material. The tear strength of polyurethane elastomer is much higher than other rubbers, but its own coefficient of friction is not low, generally above 0.5, which requires adding oil lubricants, or adding a small amount of molybdenum disulfide or Graphite, silicone oil, tetrafluoroethylene powder, etc., to reduce friction coefficient and reduce frictional heat generation. In addition, the coefficient of friction is also related to factors such as material hardness and surface temperature. In all cases, the coefficient of friction increases with decreasing hardness and increases with increasing surface temperature. A maximum is reached at about 60°C.
1.4 Oil and chemical resistance properties
Polyurethane elastomer, especially polyester polyurethane elastomer, is a kind of strong polar polymer material. It has little affinity with non-polar mineral oil, and is hardly eroded in fuel oil (such as kerosene, gasoline) and mechanical oil (such as hydraulic oil, engine oil, lubricating oil, etc.), much better than general rubber, and can be combined with Comparable to nitrile rubber. However, it swells greatly in alcohols, esters, ketones and aromatic hydrocarbons, and is gradually destroyed at high temperature. Significant swelling and sometimes degradation in halogenated hydrocarbons. Polyurethane elastomer immersed in inorganic solution, if there is no catalyst, is similar to immersion in water. It degrades faster in weak acid and weak alkali solution than in water, and strong acid and strong alkali have a greater corrosive effect on polyurethane.
The use temperature of polyurethane elastomer in oil is below 110°C, which is higher than that in air. However, in multi-engineering applications, the oil is always contaminated with water. Tests have shown that as long as the oil contains 0.02% water, almost all of the water can be transferred to the elastomer. At this time, the use effect will be significantly different.
1.5 Water resistance
The water resistance of polyurethane elastomers at room temperature is good, and no obvious hydrolysis will occur within one or two years, especially for polybutadiene, polyether and polycarbonate types. Through the enhanced water resistance test, the extrapolation method shows that the time required for the loss of half of the tensile strength in water at room temperature at 25 °C, the polyester elastomer (polyethylene adipate-TDI-MOCA) is 10 years, polyether elastomer (PTMG-TDI-MOCA) is 50 years, that is, polyether type is 5 times that of polyester type.
1.6 Heat and oxidation resistance
The heat resistance of polyurethane elastomers in inert gases (such as nitrogen) is still good, and the resistance to oxygen and ozone at room temperature is also very good, especially polyester. However, the simultaneous action of high temperature and oxygen will accelerate the aging process of polyurethane. The upper temperature limit of general polyurethane elastomers in the air for long-term continuous use is 80-90 °C, and it can reach 120 °C in short-term use. The temperature that has a significant impact on thermal oxidation realization is about 130 °C. In terms of varieties, the thermal oxidation resistance of polyester type is better than that of polyether type. Among the polyester types, the polyethylene adipate type is better than the general polyester type. In polyether type, PTMG is better than PPG type, and both improve with the increase of elastomer hardness. In addition, the strength of general polyurethane elastomers decreases significantly in high temperature environments.
1.7 Low temperature performance
Polyurethane elastomers have good low temperature properties, mainly in the fact that the brittleness temperature is generally low (-50 ~ -70℃), and some formulations (such as PCL-TDI-MOCA) are not brittle even at lower temperatures. At the same time, the low temperature elasticity of decimal varieties (such as PTMG-TDI-MOCA) is also very good. The compression cold resistance coefficient at -45°C can reach the level of 0.2-0.5, but most varieties, especially some bulk varieties, such as general polyester elastomers, have a relatively large tendency to crystallize at low temperature and poor low temperature elasticity, so they are used as seals. It is easy to leak oil in the initial phase at -20℃.
With decreasing temperature, the hardness, tensile strength, tear strength and torsional rigidity of polyurethane elastomers increased significantly, while rebound and elongation decreased.
1.8 Vibration absorption performance
The effect of polyurethane elastomer on alternating stress showed obvious hysteresis. In this process, a part of the energy of the external force is consumed by the internal friction of the molecules of the elastomer and converted into heat energy. This property is called the vibration absorbing performance of the material, also known as energy absorbing performance or damping performance. Vibration absorption performance is usually expressed by attenuation coefficient. The attenuation coefficient expresses the percentage of the energy applied to it that can be absorbed by the deformed material. In addition to the properties of the material, it is also related to the ambient temperature and vibration frequency. The higher the temperature, the lower the attenuation coefficient, the higher the vibration frequency, and the greater the absorbed energy. When the frequency is close to the relaxation time of the macromolecule, the absorbed energy is maximum. Polyurethane elastomers at room temperature can absorb 10%-20% of the vibrational energy, better than nitrile rubber. It is suitable for absorbing large impact force when the deformation amplitude is small, and absorbing small impact force when the deformation amplitude is large.
In addition, hysteresis generates endogenous heat that increases the temperature of the elastomer. As the temperature of the elastomer increases, its resilience increases and the damping performance decreases. Therefore, the balance of various properties must be considered when designing the damping parts.
1.9 Electrical properties
The electrical insulation properties of polyurethane elastomers are relatively good at general working temperatures, roughly equivalent to the levels of neoprene and phenolic resins. Because it can be cast and molded, it is often used as a material for electrical component potting and cable sheathing. Due to its relatively large molecular polarity and affinity for water, the electrical properties of polyurethane elastomers vary greatly with ambient temperature, and are not suitable for high-frequency electrical materials. In addition, the electrical properties of polyurethane elastomers decrease with increasing temperature and increase with increasing hardness of the material.
1.10 Radiation resistance
Among synthetic polymer materials, polyurethane elastomers have good resistance to high-energy rays. It still has satisfactory performance under 105-106Gy radiation dose. However, for light-colored or transparent elastomers, discoloration may occur under the action of radiation, similar to that observed in hot air or atmospheric aging tests.
1.11 Mold resistance
The mold resistance of polyether polyurethane is good, and the test level is 0-1, that is, basically no mold grows. However, polyester polyurethane is not resistant to mildew, and the test result is severe mildew, which is not suitable for tropical and subtropical field use and storage in hot and humid conditions. Polyester polyurethane elastomers used in the field and in hot and humid environments should be added with antifungal agents (such as copper octahydroxyquinoline, BCM, etc., the general dosage is 0.1%-0.5%) to improve its mold resistance. .
1.12 Biomedical properties
Polyurethane materials have excellent biocompatibility. Acute and chronic toxicological tests and animal tests have confirmed that medical polyurethane materials are non-toxic, non-distorting, non-allergic, non-locally irritating, and ignorant of pyrogen, and are the most valuable. One of the synthetic medical polymer materials.
