Polyurethane is a polymer containing a urethane group (-NHCOO) in the main chain of the polymer, known as polyurethane (PU). Polyurethane is an emerging organic polymer material, known as the “fifth major plastic”.
The main raw materials for polyurethane synthesis are organic polyisocyanates and end-hydroxy compounds. Depending on the end-hydroxyl compounds used, polyurethanes can be divided into two categories: polyester and polyether. Polyester type is the polyurethane prepared from diisocyanate and end-hydroxy polyester. Polyether type is polyurethane prepared from diisocyanate and end-hydroxy polyether.
Using different raw materials, polyurethane products with different properties can be prepared. The raw materials for synthetic polyurethanes mainly include polyisocyanates, polyols, chain extenders and auxiliary materials, etc.
Polyisocyanates
Substances containing two or more NCO groups in the structure are collectively referred to as polyisocyanates. According to the number of NCO groups, they are generally divided into three categories: diisocyanates, ternary isocyanates and polymeric isocyanates. According to the molecular structure, it can be divided into aliphatic, alicyclic and aromatic isocyanates.
1、Toluene diisocyanate (TDI)
Toluene diisocyanate (TDI) is the one with more applications. Toluene diisocyanate has two isomers at 2,4 and 2,6 positions, and is divided into three commodity specifications according to the content of the isomers.
A. TDI I 100: containing 100% of 2,4 a toluene diisocyanate.
B. TDI I 80: containing 80% of 2,4 a toluene diisocyanate and 20% of 2,6 a toluene diisocyanate.
C. TDI I 65: containing 65% of 2,4 a toluene diisocyanate and 35% of 2,6 a toluene diisocyanate.
TDI is often used as a curing agent for polyurethane soft foam, potting and casting elastomer, coatings and adhesives.
2、Diphenylmethane diisocyanate (MDI)
The main structure of diphenylmethane diisocyanate (MDI) includes three isomers: 4,4-MDI, 2,4-MDI and 2,2-MDI. The common products are pure MDI and polymerized MDI.
Pure MDI or MDI-100, which is MDI containing more than 99% of 4,4-diphenylmethane diisocyanate, is used in the polyurethane industry to synthesize fibers (spandex) and also as prepolymer (elastomer).
Polymerized MDI (also called polyurethane “black material”) is a mixture of pure MDI and multi-phenyl multi-methylene polyisocyanate containing a certain proportion. It is mostly used to make polyurethane rigid foam, which is widely used in refrigerator and freezer, reefer container, building materials insulation and automobile field.
3、Hexamethylene diisocyanate (HDI)
Hexamethylene diisocyanate (HDI) is an aliphatic isocyanate with good resistance to yellowing and flexibility because it does not contain benzene ring. HDI has small reactivity, volatility and toxicity, and is generally used in the reaction of HDI with water to make diurea diisocyanate. Mainly used as a synthetic intermediate, its trimers and condensates are commonly used as curing agents.
4、Isoflurone diisocyanate (IPDI)
Isoflurone diisocyanate (IPDI) is an alicyclic isocyanate, the product is a mixture of 75% cis-isomer and 25% trans-isomer. polyurethane resin made of IPDI has excellent light stability and chemical resistance. It is generally used to prepare high-grade product intermediates, such as light-curing polyurethane resins, and its trimer and prepolymer can be used as coating curing agents.
In recent years, isocyanate products have been vigorously developed, and the variety of isocyanates has greatly increased and their performance has been improved, gradually changing from basic raw materials to high-end raw materials.
Polyol
Polyol in polyurethane synthesis mainly includes polyester polyol and polyether polyol. It constitutes the soft segment of polyurethane. It is generally believed that polyurethane reacted with polyester polyol has higher strength and hardness, while polyether polyol reacted with polyurethane has better flexibility and is not easy to be hydrolyzed.
1、Polyether polyol (PPG)
Polyether polyol is a zwitterion containing an ether bond (-R-O-R-) in the molecule with an OH group in the end group. Polyurethane soft foam and hard foam have different requirements for the relative molecular mass and hydroxyl value of polyether.
Polyether polyol for soft foam is generally long-chain, low-functional, with average relative molecular weight around 2000~6500. Polyether polyol for hard foam (commonly known as “white material”) usually has a relative molecular weight of 300~400 and a hydroxyl value of 350~600mgKOH/g.
2、Polyester polyol
Polyester polyol main chain contains ester group (-COO-) or carbonate group (-OCOO-), with strong bond polarity, high cohesive energy and good adhesive performance, which can improve the oil resistance and temperature resistance of polyurethane adhesives, but the ester bond is easily hydrolyzed, which reduces the water resistance of polyurethane. water resistance.
Aliphatic polyester polyols are mostly used in polyurethane inks, polyurethane elastomers, microporous polyurethane soles, polyurethane adhesives, cast polyurethane elastomers, PU leather resins, fabric coatings, etc.
Aromatic polyester polyol, due to the molecule contains benzene ring rigid group, can increase the strength of products, heat resistance, flame retardancy and other characteristics. Generally used in the manufacture of rigid polyurethane foam, also used in non-foam polyurethane, such as: polyurethane coatings, adhesives, elastomers, etc.
Polyurethane material is an important application of polyol. By developing more performance polyol, we can improve the performance of polyurethane, process and develop a wider range of uses.
3、Auxiliary materials
The raw materials of polyurethane usually also use chain extenders, catalysts, surfactants, cross-linking agents, foaming agents and other auxiliary materials.
Application of polyurethane
Polyurethane is widely used in many fields of national economy because of its excellent performance, and its products are used in light industry, chemical industry, electronics, textile, medical, construction, building materials, automobile, national defense, aerospace and aviation, etc. The application of polyurethane is divided into three main areas: polyurethane soft foam, polyurethane rigid foam and polyurethane elastomer.
1、Polyurethane soft foam.
Cushioning material: such as seats, sofas, mattresses, etc., polyurethane soft foam is a very ideal cushioning material, cushioning material is also a large amount of soft foam applications.
Sound-absorbing materials: open-cell polyurethane soft foam has good sound-absorbing and shock-absorbing functions and can be used as indoor sound-absorbing materials.
Fabric composites: shoulder pads, bra sponges, cosmetic sponges.
2、Polyurethane hard foam.
Home appliances and food and other industries freezing and refrigerating equipment: polyurethane rigid foam is the ideal insulation material for freezing and refrigerating equipment, which can achieve a more desirable insulation efficiency. Can be used as refrigerators, freezers, insulation boxes, heaters and other electrical appliances insulation layer.
Industrial equipment and pipeline insulation: polyurethane rigid foam can be used as insulation material for storage tanks and pipelines, which is widely used in petroleum, chemical, natural gas and other industries.
Building insulation materials.
Automobile parts: PU rigid foam can be used for automobile roof, interior parts and many kinds of automobile parts, such as door panels, luggage compartment covers, engine covers, etc.
Composite materials: high density (density of 300~700kg/m3) PU hard foam or glass fiber reinforced hard foam is structural foam, also known as wood imitation material. It can be used in profiles, plates, sporting goods, decorative materials, etc.
3、Polyurethane elastomer.
Polyurethane elastomer includes: cast polyurethane elastomer (CPU), thermoplastic polyurethane elastomer (TPU) and blended polyurethane elastomer (MPU). Urethane elastomers are used in machinery parts, rollers, conveyor belts, seals, rollers and tires. They are also used in the manufacture of elastic fibers, synthetic leather resins, adhesives and coatings.
In recent years, polyurethane materials have been vigorously developed, and with the continuous introduction of new raw materials and the gradual maturation and perfection of application technology, polyurethane is being used more and more in industry and life, and is becoming more and more important.
