Introduction to Polyurethane
Polyurethane is a class of polymers containing urethane-type repeating structural units in the macromolecular chain. The full name is polyurethane, and the full English name is Polyurethane , PU or PUR for short. PU is a reaction product of polyisocyanate and polyether or polyester polyol in a certain proportion. It was first synthesized by a German company in 1937 . It does not have a very clear structure like PE and PP , but usually refers to a class of polymers containing specific groups. Due to the different types and compositions of the two synthetic monomers, they can be divided into two types: linear thermoplastic PU and body-shaped thermosetting PU . PU can be divided into two categories: elastomers and foamed plastics. In the past, foamed plastics were the main products. At present, elastomers are developing rapidly and their uses are becoming more and more extensive.
Synthetic raw materials and methods of polyurethane
1. Raw materials for PU synthesis
(1) The main varieties of isocyanate are: toluene diisocyanate (TDl) , divided into 2 , 4 and 2 , 6 isomers, the mixing ratio is
80/20 (TDI80) and 65/35 (TDI65) , which can be used for soft to rigid foam products; diphenylmethane diisocyanate (MDl) , used for semi Rigid and rigid foam products; Polymethylene p-phenyl polyisocyanate (PAPI) , which contains trifunctionality, can be used for thermosetting rigid foam, mixing and casting PU products.
(2) Polyol Generally, it does not refer to the direct use of polyols, but oligomers containing hydroxyl groups at the end, including polyether polyols and polyester polyols. Polyether polyols are formed by ring-opening polymerization of polyols, polyamines or other organic compounds containing active hydrogen and oxyalkylenes. It has a viscosity
Low, high elasticity and other advantages, often used in soft PU . Polyester polyol is formed by polycondensation reaction of organic polybasic acid and polyol, linear polyester polyol synthesized from dibasic acid and diol is mainly used for
Soft PU , dibasic acid and trihydric alcohol synthesized branched polyester polyol are mainly used for hard PU . Polyester polyols have high viscosity and are not as widely used as polyether types. They are often used in PU products with insulation, oil resistance, heat resistance, dimensional stability and high mechanical properties.
(3) Additives
A. The role of the catalyst is to accelerate the polymerization reaction. There are two types of amines and tin; amines such as triethylenediamine, N -alkyl morphine, etc., organic tin Classes such as dibutyltin dilaurate; generally the two are added synergistically.
B. Foaming agents are used for foaming products, specifically water, liquid carbon dioxide, chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, pentane, and cyclopentane alkanes etc.
C. Foam stabilizers are used in foam products, which can reduce surface tension, control cell size and cell wall strength, etc. Water-soluble polyether silicon is commonly used oxane.
D. Glycerin, trimethylolpropane and pentaerythritol are commonly used as crosslinking agents and chain extenders.
2. PU synthesis method
(1) The prepolymer method is also called the two-step method. First, a low-molecular prepolymer with isocyanate at the end is generated from isocyanate and polyol, and then added Add other additives and further react into the final product. This method is often used in polyether foam products.
(2) The semi-prepolymer method reacts isocyanate with some polyols to form a low-molecular prepolymer with isocyanate at the end, and then adds another Some polyols and other additives are further reacted into final products. This method is commonly used in rigid and semi-rigid foam products.
(3) In one-step method, all the monomers and additives participating in the reaction are added together, and one reaction is completed. It is popular because of its simple process and low investment use.
polyurethane elastomer
PU elastomer is a dense product of PU , its performance is between rubber and plastic, with high resilience, shock absorption, wear resistance, Oil, tear resistance, chemical corrosion resistance and radiation resistance. Because its processing methods are becoming simpler and more widely used, it has developed into the leading product of PU .
PU elastomer can be divided into three types: mixing type, casting type and thermoplastic type. The current application ratio is 10% mixing type, 65% casting type , and thermoplastic Type 25% .
1. The structure of PU elastomer
PU elastomers, and the chemical structure is also very complex, but they are all formed by the reaction of polyester or polyether diols with diisocyanates. A block polymer composed of soft segments and hard segments formed by the reaction of low molecular weight diols and diisocyanates. The difference between different PU elastomers lies in the ratio, connection and arrangement of the flexible segment to the rigid segment, which leads to differences in overall performance.
PU elastomer has a special effect on the modulus, hardness and tear strength, while the soft segment mainly affects the elasticity and low temperature properties of the product. can; that is to say, the soft and hard segments in the PU endow it with different performances.
PU elastomers also have different degrees of cross-linking structures, and their cross-linking includes primary and secondary structures. The primary cross-linked structure is polyisocyanate, The cross-linking formed by chemical bonds such as carbamate and urea carbamate formed by the reaction of polyhydroxyl compounds is stable and irreversible, and it is formed during the specific molding process, mainly in mixing and casting PU . The secondary cross-linking structure is weaker than the primary cross-linking structure, and it has intermolecular hydrogen bonds to form cross-linking, which makes it have high modulus and mechanical properties; the secondary cross-linking occurs in PU thermoplastic elastomers, The greater the degree of crosslinking, the greater the density, strength and rigidity of the product.
PU thermoplastic elastomer. The hard segments are arranged regularly and closely together to form a crystalline region and endow elasticity The body has the properties of high strength, rigidity and high melting point; the soft segment is arranged in random curls to form an amorphous region, which endows the elastomer with flexibility, elasticity, hygroscopicity and low temperature resistance.
2. Properties of PU elastomer
PU elastomer is between plastic and rubber. The properties of PU elastomers synthesized by different types of polyols are slightly different. Polyester PU has high mechanical properties and good oil resistance, but poor water resistance; polyether PU has better low temperature resistance and hydrolysis resistance than polyester. type, but oil resistance, The mechanical properties are slightly worse.
(l) Mechanical properties of PU The hardness of PU elastomers varies widely, ranging from Shore A10 to D80 , and the elongation at break is as high as 600% to 800% . The highest Shore hardness of natural rubber is only A70 , and the fracture length is 550% .
PU elastomer is higher, which is 2 to 10 times greater than that of natural rubber. PU elastomer has high resilience and good shock absorption effect.
PU elastomer lags behind the stress, so that the lost energy becomes heat ; a large amount of internal heat makes PU elastomer in high-speed motion occasions Use is limited.
PU elastomer has good wear resistance, and the specific wear resistance is shown in Table 52 . Generally, it is 3 to 10 times that of natural rubber ; its friction coefficient varies in the range of 0.2 to 3 , which is suitable for shoe sole materials.
different PU plastics
(2) Thermal properties of PU PU elastomers have poor heat resistance and can only be used below 80 °C. Temperatures exceeding 80 °C will lead to poor performance decline. However, PU elastomers have better low temperature resistance, and the polyether type has the best strength, and can be used in the temperature range of 62-70 ° C.
(3) Environmental performance of PU The water resistance of PU elastomers is general, and immersion in water or exposure to moisture will cause mechanical properties to decline ; but The hydrolysis resistance of polyether PU is 5 to 10 times better than that of polyester.
PU elastomer has good oil resistance, resistance to non-polar and weak polar solvents, among which polyether type is better than polyester type. But it is not resistant to strong acid, strong The role of alkali and polar solvents. It can be used outdoors for a long time.
PU elastomers have good physiological compatibility and good antithrombotic properties, and can be used in medicine.
(4) Electrical properties of PU The electrical properties of PU elastomers are better. The greater the hardness, the better the electrical properties. However, the electrical performance of PIJ decreases after absorbing water. It limits the application in high insulation occasions.
3. Processing of PU elastomer
(1) Casting PU elastomer (CPUR) The molding of CPUR is very convenient. The liquid reactant is injected into the mold, and it can be solidified by heating. Form complex products, especially suitable for the manufacture of large PU products.
CPUR can be processed in one step, prepolymer and reaction injection molding (RIM) . The one-step method is to fully mix polyester diol, diisocyanate and chain extender, etc., and then pour them into the mold. After the size is stable, post-transfer
Combined, the conditions are l00 ℃, 3 ~ 24h . The performance of one-step CPUR products is general, only in the control base number of polyvinyl alcohol and polyisocyanate the number of NCO is greater than 2 , the one-step method is most suitable.
The prepolymer method is suitable for use when the hydroxyl number of the polyester polyol and the -NCO of the polyisocyanate are equal to 2 . The specific method is as follows : the synthesis of the prepolymer, the sufficient mixing of the prepolymer and the crosslinking agent, pouring into the mold and raising the temperature for crosslinking. The cross-linking agents are polyamines and polyols, and the combination of the two works well.
Reaction injection molding is also called liquid injection molding or high-pressure impact hybrid molding, and its essence is the deformation of one-step or semi-prepolymer reaction. The difference is that the reaction is carried out under high pressure and full mixing, which greatly improves the strength and bonding performance of the product.
CPUR has a wide range of applications and can be used for synthetic leather, paving materials, sports tracks, building waterproof materials, etc.
(2) Thermoplastic PU elastomer (TPUR) TPUR is very different from CPUR . It is first synthesized into a linear polymer or a partially cross-linked polymer. Compound, commercially available in granular form. This polymer is thermoplastic and can be processed by thermoplastic methods, such as injection molding, extrusion, blow molding and calendering, etc. It can also be processed into leather products after being solvated and coated.
TPUR can be polyphthalein or polyether, the isocyanate is MDI , and the crosslinking agent is 1 , 4 butanediol or ethylene glycol. When TPUR is a linear molecule The two ends are curved HDH , and the partially cross-linked type is added with polyols such as glycerin, which makes it have the advantages of oil resistance, wear resistance and small permanent deformation.
TPUR needs to be dried before processing to keep its moisture below 0.1% . In specific processing, when TPUR is subjected to strong shear, the internal It is easy to generate heat and degrade, so the screw speed should not be too fast. The flow characteristic of TPUR is a Newtonian fluid, and its viscosity is sensitive to temperature ; a small temperature change can cause a sharp change in viscosity, so the processing temperature range is relatively narrow. TPUR has a strong tendency to adhere to metals, and it needs to be improved by adding lubricants or blending with other materials.
TPUR can choose a general-purpose injection molding machine for injection molding. Due to the low viscosity of the melt, the flow channel can be long, and it needs to be dried before processing. Dry conditions 93 ~ 110 ℃, 1 ~ 2h ; the processing barrel temperature is 170 ~ 220 ℃, the mold temperature is 10 ~ 50 ℃ , the injection pressure is low, generally 14 ~ 18MPa .
TPUR can be extruded to produce profiles, pipes, cable sheaths, blown films and sheets, etc., just use a conventional extruder.
(3) Mixed PU elastomer (MPUR ) MPUR is generally added with a cross-linking agent at a ratio of 1 to 1.02 of NCO/ -OH into, the specific formula is as follows:
MPUR is mainly based on one-step method. All the reaction components first form a viscous and fluid rubber material, which is sent to an oven for curing to obtain raw rubber; then added to the mixer for mixing Kneading, mixing temperature 40 ~ 60 ℃, time 15 ~ 25min ; the mixed rubber material is injected into the mold and cross-linked to form.
4. Application of PU elastomer
PU elastomer can replace natural rubber, butadiene rubber and neoprene, and can also replace plastic and metal in many aspects.
(1) The thermoplastic elastomer used in automobile industry is mainly polyester type, generally RIMPUR is commonly used, and 6% to 8% glass is added to it Fiber or glass microsphere reinforcement. Specific products include: bumpers, fenders, steering wheels, spoilers, trunk lids, door handles, armrests, instrument panels and anti-skid chains, etc.
Due to the advantages of high elasticity, large bearing capacity, good wear resistance, and cutting resistance, PU elastomers can be used for low-speed vehicles. Specifically, there are solid core tires, carriage tires, forklift tires, Xiaoping tires and bicycle tires.
(2) Building materials are mainly used for artificial runways in sports fields, underground pipe seals, waterproof materials, architectural concrete walls and ceiling reliefs Templates etc.
(3) The shoemaking industry utilizes the advantages of PU elastomers such as wear resistance, high elasticity, high bending strength, and good low temperature resistance, and is widely used in high-end shoes Bottom materials such as sports shoes, skates and hiking shoes.
(4) Synthetic leather The synthetic leather material made of PU has the performance closest to natural leather, with good hand feeling, high air permeability, moderate softness, and widely used. Used in clothing, leather shoes, furniture, luggage and vehicle seats, etc.
(5) Medical equipment utilizes the advantages of physiological compatibility and antithrombotic properties of PU elastomers, which can be used in bandages, heart aids, blood pumps, human Angiogenesis, artificial kidney and artificial ventricle, etc.
(6) In other aspects, casting PU elastomers can be used to manufacture rolls, which can be used in high load-bearing and high wear-resistant steel and paper industries ; PU elastomers.
