Properties of polyurethane elastomers

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1. Properties of polyurethane elastomers: Polyurethane elastomers possess outstanding comprehensive properties, unmatched by any other rubber or plastic. Moreover, polyurethane elastomers can be processed according to the requirements of shaping; they can be manufactured using almost any conventional process for polymer materials, such as mixing and molding, liquid casting, melt injection, extrusion, calendering, blow molding, adhesive coating, spinning, and mechanical processing. Polyurethane elastomers have a wide range of applications, with their products found in almost all fields of use. Polyurethane elastomers exhibit excellent comprehensive properties, primarily because they possess many valuable characteristics that range from those of rubber to those of plastics. (1) Wide hardness range. Moreover, it retains good rubber elasticity and elongation even at high hardness. (2) High strength. At rubber hardness, their tensile strength and tear strength are much higher than those of general-purpose rubbers ； At plastic hardness, their impact strength and bending strength are much higher than those of plastics. (3) The range of adjustable performance is large. Various physical and mechanical performance indicators can be varied within a certain range by selecting appropriate raw materials and adjusting the formulation, thereby meeting users’ different requirements regarding the product’s performance (4) Wear resistance. It has the reputation of being \"wear-resistant rubber\". Especially under working conditions with wetting media such as water and oil, its wear resistance is often several times to dozens of times that of ordinary rubber materials. Metal materials such as steel are very hard, but they are not necessarily wear-resistant. For example, in the irrigation areas of the Yellow River, the metal rings and protective sleeves used in large water pumps suffer severe wear and start leaking after just a few hundred hours of use due to exposure to large amounts of sediment; in contrast, those rings and sleeves covered with polyurethane elastomers remain intact even after 1,800 hours of continuous operation. Other applications for polyurethane elastomers include the rubber rollers used in rice milling machines, the sieve plates used in coal sorting, the running tracks in sports fields, the dynamic oil seals used in cranes and forklifts, as well as elevator wheels and roller skates. It should be noted here that in order to increase the friction coefficient of medium- and low-hardness polyurethane elastomer components and improve their wear resistance under load, small amounts of lubricants such as aluminum disulfide, graphite, or silicone oil can be added to these polyurethane elastomers. (5) Oil-resistant. The oil resistance of polyester-based polyurethane elastomers is not lower than that of nitrile rubber, and is comparable to that of polysulfide rubber. (6) Excellent ozone resistance. (7) Good shock absorption, radiation resistance, and air permeability resistance. (8) It offers a variety of processing methods, ensuring wide applicability. Polyurethane elastomers can be shaped using processes such as plasticization, mixing, and vulcanization, just like conventional rubbers (referring to MPU) ； It can also be made into liquid rubber, which is then molded by casting or spraying, potting, or centrifugal molding (referring to CPUs) ； It can also be made into granules, and like ordinary plastics, it can be molded using processes such as injection molding, extrusion, calendering, and blow molding (referring to CPUs). Molded or injection-molded parts can also undergo mechanical processing such as cutting, grinding, and drilling, within a certain range of hardness. The variety of processing methods makes polyurethane elastomers highly versatile, with their application areas continuously expanding. It is these advantages that enable polyurethane elastomers to be widely used in fields such as military industry, aerospace, acoustics, and biology. Disadvantages of polyurethane elastomers: (1) High internal heat generation, average high-temperature resistance, especially poor resistance to humid heat. The normal operating temperature range is from -40 to 120°C. If long-term operation under high-frequency oscillation conditions or high-temperature conditions is required, corresponding modifications must be made to the structural design or formulation. (2) It is not resistant to highly polar solvents and strong acid-base media. At certain temperatures, alcohols and acids can cause polyurethane elastomers to swell and degrade; solvents such as chloroform, dichloromethane, dimethylformamide, and trichloroethylene can cause polyurethane elastomers to swell at room temperature. The main properties of polyurethane elastomers are described in detail below. 1.1 Hardness: The hardness range of ordinary rubber is from Shore A 20 to Shore A 90, while that of plastics is approximately from Shore A 95 to Shore D 100. The hardness range of polyurethane elastomers extends from Shore A 10 to Shore D 80, and no filler is required for this purpose. Particularly valuable is the fact that elastomers retain good rubber elasticity and elongation even at plastic hardness levels; in contrast, ordinary rubbers can only achieve higher hardness by adding large amounts of fillers, at the cost of a significant reduction in elasticity and elongation. It is reported that when the hardness exceeds 75D, its elasticity is severely reduced, and when the hardness exceeds 85D, it ceases to be an elastic material. 1.2 Mechanical Strength Polyurethane elastomers possess high mechanical strength, as evidenced by their Young’s modulus, tear strength, and load-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. Like other elastomers, polyurethane elastomers obey Hooke’s law only at low strains (about 2.5%). However, its Young’s modulus is much higher than that of other elastomers. Moreover, the Young’s modulus of polyurethane elastomers spans from that of rubber to that of plastics, offering a range as wide as no other material can match. 1.2.2 Tear strength Polyurethane elastomers have very high tear strength, especially the polyester type, which is more than twice that of natural rubber. 1.2.3 Load-bearing capacity: Although the compressive strength of polyurethane elastomers is not high at low hardness levels, these elastomers can increase their hardness while retaining their rubbery elasticity, thereby achieving a high load-bearing capacity. The hardness of other rubbers is highly limited, so their load-bearing capacity cannot be significantly increased. 1.3 Wear resistance: Polyurethane elastomers exhibit excellent wear resistance; the test results generally fall within the range of 0.03–0.20 mm3/m, which is about 3–5 times that of natural rubber. In actual use, due to factors such as lubricants, its performance is often better. Wear resistance is closely related to factors such as the material’s tear strength and surface condition. The tear strength of polyurethane elastomers is much higher than that of other rubbers, but their friction coefficient is not low, generally above 0.5. Therefore, in practical applications it is necessary to use oil-based lubricants, or add small amounts of molybdenum disulfide, graphite, silicone oil, tetrafluoroethylene powder, etc., in order to reduce the friction coefficient and minimize heat generation due to friction. Furthermore, the friction coefficient is also related to factors such as material hardness and surface temperature. In all cases, the friction coefficient increases as hardness decreases and rises as surface temperature increases. It reaches its maximum at around 60°C. 1.4 Oil and chemical resistance Polyurethane elastomers, particularly polyester-based polyurethane elastomers, are highly polar polymer materials. It has low affinity for non-polar mineral oils, and is hardly eroded by fuel oils (such as kerosene and gasoline) and machine oils (such as hydraulic oil, engine oil, lubricants, etc.). It performs much better than conventional rubber and can be compared to nitrile rubber. However, it swells significantly in alcohols, esters, homologs, and aromatics, and gradually deteriorates at high temperatures. Swelling is significant in halohydrocarbons, and degradation sometimes occurs as well. When a polyurethane elastomer is immersed in an inorganic solution, it behaves similarly to when it is immersed in water, in the absence of a catalyst. Degradation occurs more rapidly in solutions of weak acids and weak bases than in water, and strong acids and strong bases have a greater erosive effect on polyurethanes. The operating temperature of polyurethane elastomers in oil is below 110°C, which is higher than the operating temperature in air. However, in multi-project applications, oil is always contaminated by water. Tests have shown that as long as the oil contains 0.02% water, almost all of the water can be transferred into the elastomer, and at this point there is a significant difference in performance. 1.5 Water resistance: Polyurethane elastomers have good water resistance at room temperature, and no significant hydrolysis occurs within one to two years, especially those of the polybutadiene, polyether, and polycarbonate types. By strengthening the water resistance test and using extrapolation, it was determined that in normal-temperature water at 25°C, the time required for the tensile strength to be reduced by half is 10 years for polyester-based elastomers (polyethylene glycol propylene glycol terephthalate-TDI-MOCA) and 50 years for polyether-based elastomers (PTMG-TDI-MOCA); in other words, the polyether type takes 5 times as long as the polyester type. 1.6 Heat and oxidation resistance: Polyurethane elastomers exhibit good heat resistance in inert gases such as nitrogen, and they also have good oxygen and ozone resistance at room temperature, especially those of the polyester type. However, the combined effect of high temperature and oxygen accelerates the aging process of polyurethanes. The upper temperature limit for the continuous use of ordinary polyurethane elastomers in air is 80–90°C; short-term use can reach 120°C, while the temperature at which thermal oxidation begins to have a significant impact is around 130°C. In terms of type, polyester types have better heat and oxidation resistance than polyether types. Among polyester types, the hexahydroxyhexanedioic acid polyester type is superior to ordinary polyester types. Among polyether types, PTMG performs better than PPG, and both improve as the hardness of the elastomer increases. Furthermore, conventional polyurethane elastomers experience a significant decrease in strength in high-temperature environments. 1.7 Low-temperature performance: Polyurethane elastomers exhibit good low-temperature performance, as evidenced by their generally low brittleness temperatures (–50 to –70°C); some formulations (such as PCL-TDI-MOCA) do not become brittle even at even lower temperatures. At the same time, the low-temperature elasticity of the minor variant types (such as PTMG-TDI-MOCA) is also very good. -At 45°C, the compression cold resistance coefficient can reach levels of 0.2–0.5. However, most varieties, especially those that are widely used such as ordinary polyester-based elastomers, tend to crystallize at low temperatures, resulting in poor elasticity. When used as seals, they are prone to oil leakage at temperatures below -20°C. As the temperature decreases, the hardness, tensile strength, tear strength, and torsional rigidity of polyurethane elastomers increase significantly, while resilience and elongation decrease. 1.8 Vibration absorption performance: Polyurethane elastomers exhibit a significant hysteresis effect in response to alternating stresses. During this process, part of the energy from the external force is dissipated as internal friction within the molecules of the elastomer, turning into heat energy. This property is known as the vibration absorption capacity of a material, also referred to as its energy absorption capacity or damping capacity. Vibration damping performance is usually expressed by the attenuation coefficient. The attenuation coefficient represents the percentage of energy applied to a deforming material that it can absorb. Apart from being related 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, the greater the absorbed energy. The maximum amount of energy is absorbed when the frequency is close to the relaxation time of the macromolecule. Polyurethane elastomers at room temperature can absorb 10%-20% of vibrational energy, which is better than nitrile rubber. It is suitable for absorbing large impact forces when the degree of deformation is small, and small impact forces when the degree of deformation is large. Furthermore, the lagging phenomenon generates internal heat, raising the temperature of the elastomer. As the temperature of the elastomer rises, its resilience increases while its damping performance decreases; therefore, when designing damping components, it is essential to consider the balance among these various properties. 1.9 Electrical Properties The electrical insulation properties of polyurethane elastomers are quite good at normal operating temperatures, roughly on par with those of neoprene and phenolic resins. Since it can be molded by casting as well as by thermoplastic molding, it is commonly used as a material for encapsulating electrical components and as cable sheaths. Due to their high molecular polarity, polyurethane elastomers have an affinity for water; as a result, their electrical properties change significantly with ambient temperature, and they are also not suitable for use in high-frequency electrical components. Furthermore, the electrical properties of polyurethane elastomers decrease as temperature rises, and increase as the hardness of the material increases. 1.10 Radiation resistance Among synthetic polymer materials, polyurethane elastomers exhibit excellent resistance to high-energy radiation. It maintains satisfactory performance even at a radiation dose of 105-106 Gy. However, light-colored or transparent elastomers undergo discoloration under the action of radiation, a phenomenon similar to that observed during heat air or atmospheric aging tests. 1.11 Mold resistance: The mold resistance of polyether-based polyurethane is fairly good; the test rating is 0–1, meaning that it hardly develops any mold at all. However, polyester-based polyurethanes are not resistant to mold; test results showed severe mold growth, making them unsuitable for use in tropical and subtropical outdoor areas as well as for storage in humid and hot conditions. For polyester-based polyurethane elastomers used in outdoor and humid environments, antifungal agents (such as copper octylquinoline, BCM, etc., typically used at a concentration of 0.1%–0.5%) must be added to the formulation to improve their resistance to mold. 1.12 Biomedical properties: Polyurethane materials exhibit excellent biocompatibility. Acute and chronic toxicity tests as well as animal experiments have confirmed that medical-grade polyurethane materials are non-toxic, devoid of teratogenic effects, do not cause allergic reactions, have no local irritant effects, and are non-pyrogenic. They are thus one of the most valuable synthetic polymeric materials for medical applications.