Polyurethane is a polymer material mainly polymerized from diisocyanate, chain extender, and oligomer polyol as basic raw materials. It has the comprehensive properties of rubber and plastic. It has excellent mechanical properties, wear resistance, oil resistance, tear resistance, chemical corrosion resistance, radiation resistance, good adhesion and other excellent properties, but its use temperature generally does not exceed 80 ℃, and materials above 100 ℃ will soften and deform, mechanical The performance is obviously weakened, and the short-term use temperature does not exceed 120 °C, which seriously limits its application in high temperature fields.
Today, Xiaobian reviewed the factors affecting the heat resistance of elastomers from the aspects of oligomer polyols, isocyanates, chain extenders, catalysts, polymerization process conditions, introduction of intramolecular groups, addition of fillers, and composites with nanomaterials.
1. The influence of raw materials on the heat resistance of polyurethane elastomers
Polyurethane elastomer consists of soft segment (oligomer polyol, mainly divided into polyester type, polyether type and polyolefin type polyol, etc.) and hard segment (diisocyanate and chain extender). The relative molecular weight of oligomer polyols is polydispersed, while polyisocyanates are often a mixture of various isomers. The existence of isomers will destroy the regularity of hard segments and reduce the heat resistance of elastomers. Strictly controlling the purity of raw materials and reducing the molar fraction of groups with poor thermal stability such as biuret and allophanate can improve the heat resistance of elastomers.
A. Oligomer polyol
The thermal decomposition temperature of urethanes formed by the reaction of oligomeric polyols with different structures and the same isocyanate is very different, the primary alcohol is the highest, and the tertiary alcohol is the lowest. This is because the bonds close to the tertiary and quaternary carbon atoms are the easiest. due to breakage. Since the thermal stability of the ester group is relatively good, and the hydrogen on the carbon atom of the ether group is easily oxidized, the heat resistance of polyester polyurethane is better than that of polyether polyurethane. Polyurethanes made from polyesters have little effect on thermal properties depending on the type of polyester.
For polyether polyurethane, the type of polyether has a certain influence on its heat resistance, such as toluene diisocyanate (TDI), 3,3'-dichloro-4,4'-diphenylmethanediamine (MOCA) ) and the polyurethane prepared by polyoxypropylene diol and polytetrahydrofuran ether diol (PTMG), respectively, after being aged at 121 ° C for 7 days, there is a significant difference in the tensile strength of the two. The tensile strength retention rate of the former is at room temperature. 44%, while the latter has a retention rate of 60%. The relative molecular mass or molecular chain length of oligomer polyols has no obvious effect on the characteristic decomposition temperature of thermal degradation of polyurethane. Liu Liangbing studied the degradation mechanism of polyester and polyether polyurethane, and analyzed the factors affecting its thermal resistance. , it is concluded that the heat resistance of polyester polyurethane elastomer is better than that of polyether type.
B. Isocyanates
The hard segment is the main structural factor that affects the heat resistance of polyurethane elastomers. The better the rigidity, regularity and symmetry of the hard segment, the higher the thermal stability of the elastomer. The mass fraction of hard segment increases, forming more ordered structure and subcrystalline structure of hard segment, so that the two phases are reversed, the hard segment phase becomes a continuous phase, and the soft segment is dispersed in the hard segment phase, thereby improving the tensile strength of the elastomer at high temperature strength and heat resistance. In terms of molecular structure, diphenylmethane diisocyanate (MDl) is similar to TDI in molecular structure, both containing NCO group and benzene ring structure, but due to its structural simplicity, rigidity, regularity and symmetry, its elastomer is weak. The degree of microphase separation is insufficient, and the thermal stability of the obtained elastomers is average. In general, the higher the purity of isocyanate, the less isomers, the higher the regularity and symmetry of the resulting polyurethane elastomer, and the better the heat resistance. The hard segments formed by isocyanates with regular structure are easy to aggregate, which improves the degree of microphase separation. The polar groups between the hard segments generate hydrogen bonds to form the crystalline region of the hard segment phase, so that the whole structure has a higher melting point.
For example, 1,5-naphthalene diisocyanate (NDl) has an aromatic naphthalene ring structure and a highly regular molecular chain, and the synthesized elastomer has excellent properties. Zhen Jianjun et al. synthesized polyurethane elastomers with NDI and TDI and polyethylene adipate diol (PEPA), respectively, and found that the thermal decomposition temperature of NDI-type polyurethane elastomers was higher than that of TDI-type polyurethane elastomers by thermogravimetric analysis. In addition, the comparison of high temperature retention rate of mechanical properties of elastomers at different temperatures shows that the heat resistance of NDI-type polyurethane elastomers is better than that of TDI-type polyurethane elastomers.
The PPDI-type elastomer prepared from p-phenylene diisocyanate (PPD1) has several times better heat resistance than MDI and TDI-type elastomers due to the regularity of the structure of PPDI. And 1,4-cyclohexanediisocyanate (CHDl) is also due to its simple molecular structure, high symmetry and regularity, strong crystallinity, and the resulting elastomer has an excellent degree of phase separation. Li Fen, etc. compared the main physical properties of CHDI-type polyurethane elastomer with MDI, PPDI, methylene dicyclohexyl-4,4',-diisocyanate (HMD1). The results show that CHDI-type polyurethane elastomer has higher hardness at lower hard segment content, and has better high-temperature mechanical properties than MDI-type, HMDI-type and even PPDI-type elastomers.
In addition, adding a trimerization catalyst or post-vulcanization under the premise of excessive isocyanate can form stable isocyanate crosslinks in the elastomer, thereby improving the heat resistance of the elastomer.
C. Catalyst
Alicyclic isocyanates have low reactivity, and a catalyst must be added to the reaction system to promote the reaction to proceed in the desired direction and speed. The most practical catalysts are organometallic compounds. Polymeric organic carboxylic acids and tertiary amine compounds also have a very good role in promoting the chemical reaction of isocyanates.
Zhang Xiaohua, et al. synthesized transparent polyurethane elastomers with PTMG, isophorone diisocyanate (1PDl), 1,4-butanediol (BDO) and different catalysts such as stannous isooctoate, dibutyltin dilaurate and cocatalyst K. The effect of catalyst species on the mechanical properties, optical transparency, degree of reaction and thermal stability of the elastomer was investigated. The results show that the composite catalyst stannous isooctanoate and its co-catalyst K are used, because the co-catalyst K can absorb the CO2 released by the reaction of the NCO group with water and is conducive to the formation of cross-linking bonds, so the prepared polyurethane elastomer has a good comprehensive performance. Mechanical properties and excellent thermal stability.
D. Cross-linking agent
The excellent properties of polyurethane elastomers are closely related to their physical crosslinking and chemical crosslinking structures. Physical cross-linking refers to the hydrogen bonding between hard segments and between hard and soft segments; chemical cross-linking refers to the covalent cross-linking bonds between molecules formed by the cross-linking agent.
The generation of chemical cross-linking hinders the mobility of the soft segment. In this way, the spatial freedom of the lattice lattice is reduced, which is not conducive to the crystallization of the soft segment, and prevents the hard segments from moving closer to each other. The degree of microphase separation is reduced. Zhang Xiaohua, et al. used a one-step method to synthesize a transparent polyurethane elastomer with isophorone diisocyanate, polyoxytetramethylene glycol, 1,4-butanediol and polyoxypropylene triol (N3010) as raw materials. The effects of physical and chemical cross-linking on the mechanical properties, optical transparency and thermal stability of polyurethane elastomers were studied by FT-IR, TG and other methods. The results show that the addition of the cross-linking agent triol N3010, the polyurethane elastomer forms cross-links between the hard segments, and the light transmittance, thermal stability and mechanical properties are significantly improved compared with the polyurethane elastomer without cross-linking agent.
E. Chain extender
The effect of chain extenders on heat resistance is related to its rigidity. In general, the higher the rigid segment content, the better the heat resistance of the elastomer. Huang Zhixiong, etc. used 4,4'-diphenylmethane-5-maleimide and 3,3'-dichloro-4,4'-diphenylmethanediamine (BMI-MOCA) chain extender to avoid The high activity of MOCA provides favorable conditions for casting large-scale products, and it is also easy to synthesize polyurethane elastomers with high hardness. Due to the introduction of the BMI aromatic ring structure, the relative increase of the rigid segment can significantly improve the thermal stability of the polyurethane elastomer.
In addition, the chain extender hydroquinone bishydroxyethyl ether (HQEE) is a new type of non-toxic chain extender, which can replace MOCA. It has many advantages and is widely used in polyurethane elastomers, which can improve the heat resistance and tear resistance of polyurethane. crack strength and compound storage stability.
2. The effect of polymerization process conditions on the heat resistance of elastomers
The thermal stability of urea group and urethane group is greater than that of allophanate and biuret, which indicates that increasing the mole fraction of urea group and urethane group in the elastomer molecule reduces allophanate The mole fraction of ester group and biuret group can improve the thermal stability of the elastomer, that is, strictly control the process conditions, especially the amount and purity of the reactants, so that the reaction can generate as many urea groups and carbamates as possible. It is of great significance to improve the heat resistance of elastomers. The heat resistance of polyurethane elastomers can be effectively improved by using diamine chain extension vulcanization to generate urea groups, controlling the reaction between NCO groups and urea groups to generate biurets, and using aromatic diisocyanates. The reaction of polyurethane generally includes one-step method, prepolymerization method and semi-prepolymerization method. The one-step method is relatively simple, but the molecular structure of the product is often irregular and the performance is poor. The prepolymerization method and the semi-prepolymerization method are better.
The German patent reports that a semi-prepolymerization method is used to obtain a polyurethane elastomer with a softening temperature of 147°C. In addition, the post-vulcanization conditions of more than 4 hours at a temperature of about 120 ° C can also improve the heat resistance deformation performance of the polyurethane elastomer casting compound.
3. Effect of modification on heat resistance of polyurethane elastomer
A. The effect of silicone modification on the heat resistance of elastomers
Silicone has a unique structure and excellent high and low temperature resistance and oxidation resistance, excellent electrical insulation and thermal stability, excellent air permeability and biocompatibility, etc. Heat resistance, its heat distortion temperature can reach 190 ℃.
The reason for its good heat resistance is that on the one hand, the thermal stability of the SiO2 bond is good, and on the other hand, the soft segment with siloxane as the main body has good flexibility, which is beneficial to microphase separation. Stanciu A et al. prepared cross-linked polyols with poly-L-alcohol adipate diol (PEGA), hydroxyl-terminated polydimethylsiloxane (PDMS-OH), MDI, and diglyceride maleate polyols. Polyester-polysiloxane-polyurethane elastomer, performance tests show that PDMS-OH has little effect on the mechanical properties of the final material, but has improved stability and elasticity at low temperatures, and better thermal stability.
Wen Sheng, et al. synthesized a series of siloxane-containing polyurethane elastomers using polydimethylsiloxane (PDMS) with a hydroxyl end group and polytetrahydrofuran ether diol as mixed soft segments. Thermogravimetric analysis (TGA) showed that , the introduction of PDMS improves the thermal stability of traditional polyurethane elastomers.
B. Influence of the introduction of intramolecular groups on the heat resistance of elastomers
The thermal decomposition temperature of polyurethane elastomer mainly depends on the heat resistance of various groups in the macromolecular structure. If there is a double bond in the soft segment, it will reduce the heat resistance of the elastomer, while the introduction of isocyanurate rings and inorganic elements can improve the heat resistance of the polyurethane elastomer. The introduction of a thermally stable heterocycle (such as an isocyanurate ring, a polyimide ring, an oxazolidinone ring, etc.) into the main chain of the PU molecule can significantly improve the heat resistance of the polyurethane elastomer.
The trimer of aliphatic or aromatic polyisocyanate contains an isocyanurate ring, which has excellent heat resistance and dimensional stability, and its products can be used for a long time at 150°C. The polyimide produced by the reaction of dicarboxylic anhydride and diisocyanate has the characteristics of insolubility and high temperature resistance. The introduction of polyimide ring into PU can improve the heat resistance and mechanical stability of polyurethane elastomer. The oxazolidinone compound formed by the reaction of epoxy group and isocyanate in the presence of a catalyst has good thermal stability, the thermal decomposition temperature exceeds 300 °C, and the glass transition temperature is above 150 °C, which is significantly higher than that of ordinary polyurethane elastomers. .
C. The effect of compounding with nanoparticles and fillers on the heat resistance of elastomers
Nanomaterials are "the most promising materials in the 21st century", and polymer-based nanocomposites refer to the size of the dispersed phase in at least one dimension in the nanoscale range. Due to its unique properties, nanoparticles are compounded with polyurethane elastomers to significantly improve their mechanical properties, and can increase the functional properties of elastomers such as heat resistance and anti-aging. The composite of nanoparticles and elastomer is a new type of composite material system worthy of research and development.
Gilman, JW, et al. showed through the X-ray diffraction results of polyurethane-montmorillonite nanocomposites that the montmorillonite was dispersed in the polyurethane matrix with a broad distribution with an average interlayer spacing of not less than 415 nm, and the silicate in the montmorillonite played a role in thermal insulation. It can effectively improve the heat resistance of composite materials. ZhuY et al. used the excellent comprehensive properties of polyurethane elastomers and inorganic particles-nano-SiO2 to prepare SiO2 polyurethane elastomer nanocomposites by sol-gel method. The experimental results show that the addition of nano-SiO2 can significantly improve the mechanical properties of the polyurethane elastomer matrix, and also have a certain improvement in its heat resistance.
Fillers such as calcium carbonate, carbon black, quartz stone, carbon fiber, glass fiber, nylon, and cured resin particles can also improve the thermal deformation resistance of polyurethane elastomers. Du Hui, et al. studied the effects of different inorganic fillers on the mechanical properties and heat resistance of polyurethane elastomers. The results show that the mechanical properties and heat resistance of polyurethane elastomers modified with micron-scale inorganic fillers are significantly better than ordinary polyurethane elastomers. .
4, Formula design application
There are various methods to improve the thermal deformation performance of polyurethane elastomers. In practical applications, a reasonable selection should be made according to product performance indicators and process requirements, and a feasible process route should be determined. Although improving the heat resistance of polyurethane elastomers has always been a very active topic in the field of polyurethane elastomers, and a lot of research has been carried out, there are still few polyurethane elastomers with excellent comprehensive properties such as heat resistance and mechanical properties, and the overall level is still low. in the laboratory development stage. Developing new modification systems and strengthening the industrialization of results are still the main research topics in the polyurethane field in the near future.
Good heat resistance, PPDI, NDI, TODI and CHDI, if you want to make a prepolymer, the NDI activity is too high, which is not realistic at present (it is said that the Prepolymer Research Institute of Burley Bayer has successfully synthesized a good storage stability. NDI prepolymer), the rest is ok. Generally speaking, for those requiring thermal stability and yellowing, CHDI is better, and PPDI that requires heat resistance and dynamic mechanical properties is better. If TODI is extended with amines, the performance is very close to NDI.
