polyurethane

(1) Introduction to polyurethane
Polyurethane is a class of polymer containing urethane-type repeating structural units in the macromolecular chain, which is called polyurethane, abbreviated as PU or PUR in English. PU is a product of the reaction of polyisocyanates with polyether or polyester polyols in a certain ratio, and was first synthesized by a German company in 1937. PU can be divided into two categories: elastomers and foams. In the past, foam plastics have been the mainstay, but at present, elastomers are developing rapidly and are becoming more and more widely used.
(2) Polyurethane elastomers
Elastomer is a kind of PU dense product, its performance is between rubber and plastic, with high resilience, shock absorption, wear resistance, oil resistance, tear resistance, chemical resistance and radiation resistance. Due to its simpler and more simple processing methods and more and more extensive applications, it has developed into the leading product of PU.
PU elastomers can be divided into three types: mixing type, casting type and thermoplastic type, and the current application ratio is: 10% for mixing type, 65% for casting type and 25% for thermoplastic type.
1. Performance of PU elastomers
The properties of PU elastomers are intermediate between plastic and rubber. The properties of PU elastomers synthesized from different types of polyols are slightly different: polyester PU has high mechanical properties and good oil resistance, but poor water resistance, while polyether PU has better low temperature resistance and hydrolysis resistance than polyester type, but oil resistance and mechanical properties are slightly worse.
(1) The mechanical properties of PU The hardness of PU elastomers varies from Shore A10~D80,
The elongation at break is as high as 600%~800%. The highest Shore hardness of natural rubber is only A70, and the elongation at break is 550%.
The tear strength of PU elastomer is high, 2~10 times larger than that of natural rubber, high resilience, good shock absorption effect, good wear resistance, generally 3~10 times that of natural rubber, and its friction coefficient varies in the range of 0.2~3, which is suitable for sole materials.
The strain of PU elastomers lags behind the stress, which turns the lost energy into heat. The large amount of internal heat limits the use of PU elastomers in high-speed sports applications.
(2) Thermal performance of PU PU elastomer has poor heat resistance, and can generally only be used below 80 °C, and the temperature above 80 °C will lead to a decrease in performance. However, PU elastomers have good low temperature resistance, and the polyether type is the best, which can be used at low temperatures of -62°C~-70°C.
(3) The environmental performance of PU PU elastomer has general water resistance, and soaking in water or exposure to moisture will cause a decrease in mechanical properties. The hydrolysis resistance of polyether PU is 5~10 times better than that of polyester type. It has good oil resistance, non-polar and weak polarity solvent resistance, among which the polyether type is better than the polyester type. However, it is not resistant to strong acids, strong alkalis and polar solvents. It has good resistance to ozone and oxidation, ultraviolet light and radiation, and can be used for outdoor use for a long time.
PU elastomers have good physiological compatibility and good anti-thrombotic properties, which can be used in the medical field.
(4) The electrical properties of PU PU elastomers have better electrical properties. The greater its hardness, the better its electrical properties. However, the electrical performance of PU decreases after water absorption, which limits its use in high insulation occasions.
2. Processing of PU elastomers
(1) Cast PU elastomer (CPUR)
CPUR molding is extremely convenient, the liquid reactant is injected into the mold, and the heating can be cured to form a complex shape of the product, especially suitable for the manufacture of large products.
CPUR has a wide range of applications and can be used for synthetic leather, pavement materials, sports tracks, building waterproofing materials, etc.
(2) Thermoplastic PU elastomer (TPUR)
TPUR is very different from CPUR in that it is first synthesized into a linear polymer or a partially cross-linked polymer, which is granular. The polymer is thermoplastic and can be processed by thermoplastic methods such as injection molding, extrusion, blow molding, and calendering, or it can be solvent-melted and coated into leather products.
TPUR can be extruded to produce profiles, tubes, cable jackets, blown films and sheets.
(3) Compounding PU elastomer (MPUR)
MPUR is mainly based on the one-step method, all the reaction components are first made into viscous fluidic rubber compound, sent to the oven to cure to obtain raw rubber, and then added to the kneader for mixing, and the mixed rubber material can be injected into the mold for cross-linking molding.
3. Application of PU elastomer
PU elastomers can be used as an alternative to natural rubber, nitrile and neoprene, as well as general plastics and metals in many applications.
(1) Automobile industry Thermoplastic elastomers for automobiles are mainly polyester type, which is generally commonly used RIMPUR, and is reinforced with 6%~8% glass fiber or glass beads. Specific products include: bumpers, mud plates, steering wheels, baffles, trunk lids, door handles, armrests, instrument panels and snow chains.
PU elastomer has the advantages of high elasticity, large bearing capacity, good wear resistance, cutting resistance, etc., and the tires made of PU elastomer can be used for low-speed vehicles, including solid tires, carriage tires, forklift tires, flat tires and bicycle tires.
(2) Building materials are mainly used for sports fields artificial runways, underground pipe seals, waterproof materials, concrete walls and ceiling relief templates, etc.
(3) Footwear industry The advantages of PU elastomer such as abrasion resistance, high elasticity, high bending strength and good low temperature resistance are widely used in high-end shoe sole materials such as sports shoes, skates and climbing shoes.
(4) Synthetic leather The synthetic leather material made of PU has the performance closest to natural leather, with good hand feel, high air permeability, moderate softness, and is widely used in clothing, leather shoes, furniture, bags and vehicle seats.
(5) Medical equipment Using the advantages of physiological compatibility and anti-thrombosis of PU elastomer, it can be used to make bandages, cardiac boosters, blood pumps, artificial blood vessels, artificial kidneys and artificial ventricles.
(6) Other aspects of casting PU elastomer to make rolling, can be used in high load-bearing and high wear in the steel and paper industry, PU elastomer can also be used in oilfield, mining and metallurgical industry in high wear resistance and high strength structural materials, specifically oilfield rotary desander, media screen, flotation machine, spiral concentrator, ore conveyor pipe, conveyor belt, etc.
(3) Polyurethane foam
Foam plastic is the most important variety of PU, accounting for more than half of the entire PU. According to the performance of the product, PU foam can be divided into three types: soft, semi-hard and hard.
1. Soft PU foam plastic
Soft PU foam is commonly known as "sponge", with an opening rate of 95% and more than 60% of the entire foam.
Flexible PU foam has a mild cross-linked structure, its relative density is 0.02~0.04, and the resilience is high. It is mainly used in three fields: furniture (such as mattresses, seat cushions and sofas), sports shockproof products and shockproof packaging materials.
2. Semi-rigid PU foam
Semi-rigid PU foam is used in smaller quantities than flexible foam. Similar to flexible PU foam, it has a 90% open cell, a higher degree of crosslinking than soft PU, and has a higher compressive strength.
Semi-rigid PU foam is mainly used for shockproof cushioning materials and packaging materials, such as car headrests and bumpers.
3. Rigid PU foam plastic
Also known as "black material", the amount accounts for about 40% of the total. Rigid PU foam is a highly cross-linked structure, which is basically closed-cell (the opening rate is 5~15%). Its thermal conductivity is 0.008~0.025W/(m•K), which is a high-quality thermal insulation material. It can be used in the temperature range of -200°C~150°C. Good chemical resistance, but not resistant to strong acids and alkalis.
The impact strength of rigid PU foam is low, and polyurethane + epoxy resin + organic fiber system is commonly used for improvement.
The applications of rigid PU foam are as follows:
(1) Thermal insulation materials are used for refrigerators, refrigerators, cold storage, etc., as well as thermal insulation materials for transporting cold and hot medium pipelines.
(2) Building materials are mainly used for roof thermal insulation.
(3) The structural material has the characteristics that the wood can be sawn, planed and nailed, and is called polyurethane synthetic wood. It is mainly used for chair frames, tables, door and window frames, etc.