PU elastomer, also known as polyurethane elastomer, is a polymer synthetic material containing more urethane groups on the main chain. PU elastomers have a wide range of properties, which is closely related to its structure, and its structure depends on many factors such as reactants, reaction time, reaction temperature, and even small changes in water content can cause PU elastomers Huge difference in mechanical properties. Next, the PU pigment manufacturer will introduce the structure and performance of PU elastomer for you.
(1) Microphase separation structure of PU elastomer
The performance of PU is mainly affected by the morphological structure of the macromolecular chain. The unique flexibility and excellent physical properties of PU can be explained by the two-phase morphology. The degree of microphase separation and the two-phase structure of soft and hard segments in PU elastomers are critical to their performance. Moderate phase separation is beneficial to improve the properties of the polymer. The separation process of microphase separation is that the difference in polarity between the hard segment and the soft segment and the crystallinity of the hard segment itself lead to their thermodynamic incompatibility (immiscibility) and a tendency to spontaneous phase separation, so the hard segment is easy to Aggregate together to form domains, which are dispersed in the continuous phase formed by the soft segments. The process of microphase separation is actually the process of separation and aggregation or crystallization of the hard segment in the elastomer from the copolymer system.
The phenomenon of PU micro-phase separation was first proposed by American scholar Cooper. After that, a lot of research work was done on the structure of polyurethane. The research on the PU aggregate structure has also made progress, forming a relatively complete micro-phase. Structural theory system: In the block PU system, the micro-phase separation of hard and soft segments is induced by thermodynamic incompatibility between segments and soft segments. The attractive force of the segments between the hard segments is much greater than that of the segments between the soft segments. The hard segments are insoluble in the soft segment phase, but are distributed in it, forming a discontinuous microphase structure (sea-island structure). It plays a physical linking and reinforcing role in the soft segment. In the process of microphase separation, the increased interaction between hard segments will facilitate the separation of hard segments from the system and aggregate or crystallize, promoting microphase separation. Of course, there is a certain compatibility between the plastic phase and the rubber phase, and the phases between the plastic micro-domains and the rubber micro-domains are mixed to form a flow-through phase. At the same time, other models related to microphase separation have also been proposed, such as the hard segment and soft segment enrichment regions proposed by Seymour et al. Paik Sung and Schneide proposed a more realistic structural model of microphase separation: the degree of microphase separation in urethane is imperfect, not entirely microphase coexistence, but includes mixed soft segment units. There is mixing between segments in the micro-domain, which has a certain degree of influence on the morphology and mechanical properties of the material. The soft segment contains hard segments, which can lead to a change in the glass transition temperature of the soft segment. Brightly improved, narrowing the range of materials used in low temperature environments. The inclusion of soft segments in the hard segment domains can lower the glass transition temperature of the hard segment domains, thereby reducing the heat resistance of the material.
(2) Hydrogen bonding behavior of PU elastomers
Hydrogen bonds exist between groups containing nitrogen atoms and oxygen atoms with strong electronegativity and groups containing hydrogen atoms. The cohesive energy of the groups is related to the size of the cohesive energy of the groups. Strong, hydrogen bonds mostly exist between segments. According to reports, most of the imine groups in the various groups in PU macromolecules can form hydrogen bonds, and most of them are formed by the imine groups and the carbonyl groups in the hard segment, and a small part is formed with the ether oxygen in the soft segment. group or ester carbonyl formed. Compared with the bonding force of intramolecular chemical bonds, the hydrogen bonding force is much smaller. However, the existence of a large number of hydrogen bonds in polar polymers is also one of the important factors affecting the performance. Hydrogen bonds are reversible. At lower temperatures, the close arrangement of the sexual segments promotes the formation of hydrogen bonds: at higher temperatures, the segments receive energy and undergo thermal motion, the distance between segments and molecules increases, and the hydrogen bonds are weakened or even disappear. Hydrogen bonds play the role of physical cross-linking, which can make the PU body have higher strength, abrasion resistance, solvent resistance and smaller tensile permanent deformation. The more hydrogen bonds, the stronger the intermolecular forces and the higher the strength of the material. The amount of hydrogen bonds directly affects the degree of microphase differentiation of the system.
(3) Crystallinity
Linear PU with regular structure, more polar and rigid groups, more intermolecular hydrogen bonds, and good crystalline properties, some properties of PU materials have been improved, such as strength, solvent resistance, etc. The hardness, strength and softening point of PU materials increase with the increase of crystallinity, while elongation and solubility decrease accordingly. For some applications, such as one-component thermoplastic PU adhesives, fast crystallization is required to obtain initial tack. Some thermoplastic PU elastomers release faster due to their high crystallinity. Crystalline polymers often become opaque due to the anisotropy of refracted light. If a small amount of branched or pendant groups are introduced into the crystalline linear PU macromolecule, the crystallinity of the material decreases. When the crosslinking density increases to a certain extent, the soft segment loses its crystallinity. When the material is stretched, the tensile stress makes the molecular chain of the soft segment oriented and the regularity is improved, the crystallinity of the PU elastomer is improved, and the strength of the material is correspondingly improved. The stronger the polarity of the hard segment, the more conducive to the improvement of the lattice energy of the PU material after crystallization. For polyether PU, with the increase of the hard segment content, the polar groups increase, the intermolecular force of the hard segment increases, the degree of microphase separation increases, the hard segment microdomain gradually forms crystals, and the crystallinity increases with the hard segment content. Gradually increase the strength of the material.
(4) Influence of soft segment structure on the performance of PU elastomer
Oligomeric polyols such as polyethers and polyesters make up the soft segments. The soft segment accounts for most of the PU, and the properties of PU prepared from different oligomer polyols and diisocyanates are different. The flexible (soft) segment of PU elastomers mainly affects the elastic properties of the material and contributes significantly to its low temperature and tensile properties. Therefore, the Tg parameter of the soft segment is extremely important, and secondly, the crystallinity, melting point and strain-induced crystallization are also factors that affect its ultimate mechanical properties. The PU elastomer and foam made of polyester with strong polarity as soft segment have better mechanical properties. Because the PU made of polyester polyol contains a large polar ester group, this PU material can not only form hydrogen bonds between the hard segments, but also the polar groups on the soft segment can partially interact with the hard segments. The polar groups form hydrogen bonds, so that the hard segment phase can be more uniformly distributed in the soft segment phase, which acts as an elastic cross-linking point. Some polyester polyols can form soft segment crystals at room temperature, which affects the performance of PU. The strength, oil resistance and thermal oxidative aging of polyester PU material are higher than those of PPG polyether PU material, but the hydrolysis resistance is worse than that of polyether type. Polytetrahydrofuran (PTMG) PU is easy to form crystals due to its regular molecular chain structure, and its strength is comparable to that of polyester PU. Generally speaking, the ether group of the soft segment of polyether PU is easier to rotate internally, has good flexibility, and has excellent low temperature performance, and there is no ester group that is relatively easy to hydrolyze in the polyether polyol chain, which is resistant to hydrolysis. Better than polyester PU. The α carbon of the ether bond of the polyether soft segment is easily oxidized to form peroxide radicals, resulting in a series of oxidative degradation reactions. PU with polybutadiene molecular chain as soft segment has weak polarity, poor compatibility between soft and hard segments, and poor elastomer strength. The soft segment containing the side chain, due to steric hindrance, has weak hydrogen bonds and poor crystallinity, and its strength is worse than that of the same soft segment main chain without side group PU. The molecular weight of the soft segment has an impact on the mechanical properties of PU. Generally speaking, assuming the same molecular weight of PU, the strength of the PU material decreases with the increase of the molecular weight of the soft segment; if the soft segment is a polyester chain, the strength of the polymer material decreases slowly with the increase of the molecular weight of the polyester diol; If the soft segment is a polyether chain, the strength of the polymer material decreases with the increase of the molecular weight of the polyether glycol, but the elongation increases. This is due to the high polarity of the ester soft segment and the large intermolecular force, which can partially offset the decrease in the strength of the PU material due to the increase in the molecular weight and the increase in the soft segment content. However, the polarity of the soft segment of polyether is weak. If the molecular weight increases, the content of the hard segment in the corresponding PU decreases, resulting in a decrease in the strength of the material. The compatibility of PU copolymers is related to the chain structure of macromolecules, and the presence of graft chains has a significant effect on the compatibility and damping properties of polyurethane block copolymers. Generally, the effect of soft segment molecular weight on the resistance and thermal aging properties of PU elastomers is not significant. The crystallinity of the soft segment has a great contribution to the crystallinity of the linear PU. Generally speaking, crystallinity is beneficial to improve the strength of PU. But sometimes crystallization reduces the low temperature flexibility of the material, and crystalline polymers are often opaque. In order to avoid crystallization, the integrity of the molecule can be reduced, such as using copolyester or copolyether polyol, or mixed polyol, mixed chain extender, etc.
(5) Influence of hard segment on the performance of PU elastomer
The hard segment structure is one of the main factors affecting the heat resistance of PU elastomers. The structure of the diisocyanate and chain extender that make up the PU elastomer segment is different, which also affects the heat resistance. The hard segment of PU material is composed of polyisocyanate and chain extender. It contains strong polar groups such as urethane group, aryl group and substituted urea group. Usually, the rigid segment formed by aromatic isocyanate is not easy to change, and stretches at room temperature. rod-shaped. Hard segments usually affect the high temperature properties of PU, such as softening and melting temperature. Commonly used diisocyanates are TDI, MDI, IPDI, PPDI, NDI, etc., commonly used alcohols are ethylene glycol, -butanediol, hexanediol, etc., and commonly used amines are MOCA, EDA, DETDA, etc. The type of hard segment is selected according to the desired mechanical properties of the polymer, such as maximum use temperature, weather resistance, solubility, etc., and its economy should also be considered. Different diisocyanate structures can affect the regularity of the hard segment and the formation of hydrogen bonds, thus having a greater impact on the strength of the elastomer. Generally speaking, the isocyanate containing aromatic ring makes the hard segment have greater rigidity and cohesive energy, which generally increases the strength of the elastomer.
The rigid segment containing urea group composed of diisocyanate and diamine chain extender, because the cohesion of urea group is very large, it is easy to form plastic micro-domain, and the PU composed of this rigid segment is very prone to microphase separation. Generally speaking, the higher the rigidity of the rigid segment constituting PU, the more likely to cause microphase separation. In PU, the higher the content of the rigid segment, the more likely to cause microphase separation.
The chain extender is related to the hard segment structure of the PU elastomer and has a great influence on the performance of the elastomer. Compared with the chain-extended PU of aliphatic diols, the chain-extended PU containing aromatic ring diamine has higher strength, because the amine chain extender can form a urea bond, and the polarity of the urea bond is higher than that of the urethane bond. Moreover, the difference in solubility parameters between the hard segment of urea bond and the soft segment of polyether is large, so the hard segment of polyurea and the soft segment of polyether have greater thermodynamic incompatibility, which makes PU urea have better microphase separation. Therefore, the diamine chain-extended PU has higher mechanical strength, modulus, viscoelasticity, and heat resistance than the diol chain-extended PU, and also has better low-temperature performance. Casting PU elastomers mostly use aromatic diamines as chain extenders because the PU elastomers prepared therefrom have good comprehensive properties. By reacting maleic anhydride and polyol to form carboxyl ester polyol, and then reacting with other monomers such as TDI-80, crosslinking agent and chain extender, the carboxyl-containing PU prepolymer was prepared, which was dispersed in three In the aqueous solution of ethanolamine, water-based PU was made, and the influence of the type and amount of chain extender on the properties of the resin was studied. Using bisphenol A as a chain extender can not only improve the mechanical properties of the resin, but also increase the glass transition temperature of the resin, broaden the width of the internal friction peak, and improve the temperature range of the resin in leather state [12]. The structure of the diamine chain extender used in PU urea directly affects the hydrogen bonding, crystallization, and microphase structure separation in the material, and largely determines the performance of the material [13]. With the increase of hard segment content, the tensile strength and hardness of PU material gradually increased, and the elongation at break decreased. This is because there is microphase separation between the phase with a certain degree of crystallinity formed by the hard segment and the amorphous phase formed by the soft segment, and the crystalline region of the hard segment acts as an effective cross-linking point. It also plays a role similar to filler reinforcement for the amorphous region of the soft segment. When the content increases, the reinforcement effect and effective crosslinking effect of the hard segment in the soft segment are enhanced, which promotes the increase of material strength.
(6) Influence of cross-linking on the properties of PU elastomers
Moderate intramolecular crosslinking can increase the hardness, softening temperature and elastic modulus of PU materials, and reduce elongation at break, permanent deformation and swelling in solvents. For PU elastomers, proper cross-linking can produce materials with excellent mechanical strength, high hardness, elasticity, and excellent wear resistance, oil resistance, ozone resistance and heat resistance. However, if the crosslinking is excessive, the properties such as tensile strength and elongation can be reduced. In block PU elastomers, chemical cross-linking can be divided into two categories: (1) using trifunctional chain extenders (such as TMP) to form a cross-linking structure; (2) using excess isocyanate to react to form dicondensate Urea (via urea groups) or allophanate (via urethane groups) crosslinking. Crosslinking has a significant effect on the degree of hydrogen bonding, and the formation of crosslinks greatly reduces the degree of hydrogen bonding of the material, but chemical crosslinking has better thermal stability than physical crosslinking caused by hydrogen bonding. When the effects of chemical cross-linking network on the morphology, mechanical properties and thermal properties of PU urea elastomers were studied by means of FT-IR and DSC, it was found that PU urea elastomers with different cross-linking networks had different morphologies. As the density increases, the degree of microphase mixing of the elastomer increases, the glass transition temperature of the soft segment increases significantly, and the 300% tensile strength of the elastomer gradually increases, while the elongation at break decreases gradually. When , the mechanical properties (tensile strength and tear strength) of the elastomer reach the highest.