Polyurethane elastomer, also known as polyurethane elastomer, is a polymer synthetic material containing more urethane groups on the main chain, generally made of polyesters, polyethers and polyolefins and other oligomeric polyols with polyisocyanate and diols or diamine chain extenders gradually add up to poly and become. It is a kind of elastic material between general rubber and plastic, that is, with the high elasticity of rubber, but also has the high strength of plastic. It has a large elongation and a wide range of hardness; its abrasion resistance, biocompatibility and blood compatibility are particularly outstanding. It also has excellent oil resistance, impact resistance, low temperature resistance, radiation resistance and weight-bearing, heat insulation and insulation properties. Therefore, the application field of polyurethane elastomer is very wide. It has become an indispensable and valuable material in the national economy and people’s life.
The wide range of polyurethane elastomer properties is closely related to its structure, which depends on many factors such as reactants, reaction time, reaction temperature, etc. Even small changes in water content can cause great differences in the mechanical properties of polyurethane elastomers.
Keywords: polyurethane, elastomer, structure and properties, applications
1 Overview of polyurethane elastomers
Polyurethane elastomers, also known as polyurethane rubber, are special synthetic rubbers, a class of elastic polymers containing more carbamate groups (-NHCOO-) in the molecular backbone, and are typical multi-block copolymer materials. Polyurethane elastomers are usually produced by polymer polyol, isocyanate, chain extender, cross-linking agent and a few additives as raw materials for the polymerization reaction. In terms of molecular structure, polyurethane elastomer (PUE) is a block polymer, whose molecular chain generally consists of two parts, one in the highly elastic state at room temperature, called the soft segment, and the other in the glassy or crystalline state, called the hard segment. Generally, the long flexible chain of polymer polyol constitutes the soft segment, and the hard segment is composed of isocyanate and chain extender, and the soft and hard segments are arranged alternately, thus forming repeated structural units. In addition to urethane groups, the main chain of polyurethane molecule also contains polar groups such as ether, ester or and urea groups. Due to the presence of a large number of these polar groups, hydrogen bonds can be formed within and between polyurethane molecules, soft and hard segments can be induced to form hard and soft segment micro-regions and produce micro-phase separation structures due to thermodynamic incompatibility, and even linear polyurethanes can be physically cross-linked through hydrogen bonds. These structural characteristics make polyurethane elastomers have excellent wear resistance and toughness, and are known as “wear resistant rubber” [1], and because there are many varieties of polyurethane raw materials, the variety and ratio of raw materials can be adjusted to synthesize products with different performance characteristics, making polyurethane elastomers are widely used in the national economy. Although the production of polyurethane elastomers does not account for a large proportion of polyurethane products, its variety and wide range of applications are incomparable to those of other materials. Polyurethane elastomer has excellent overall performance, and its modulus is between general rubber and plastic. It has the following characteristics: ① high strength and elasticity, can be in a wide range of hardness (Shore A10 – Shore D75) to maintain high elasticity; ② in the same hardness, than other elastomers high load-bearing capacity; ③ excellent wear resistance, its wear resistance is natural rubber 2-10 times; ④ fatigue resistance and good resistance to vibration, suitable for high-frequency flexing applications; ⑤ high impact resistance; ⑥ aromatic polyurethane radiation resistance, oxygen resistance and ozone resistance. (6) Excellent radiation, oxygen and ozone resistance; (7) Excellent resistance to grease and chemicals; (8) Generally, the required low hardness can be achieved without plasticizers, so there are no problems caused by plasticizer migration; (9) Low molding and processing costs; (10) Ordinary polyurethane cannot be used above 100℃, but the formulation can withstand 140℃. In general, compared with metal materials, polyurethane elastomer products have the advantages of light weight, loss resistance, low sound, low processing cost and corrosion resistance; compared with rubber, polyurethane elastomer has the advantages of wear resistance, cut resistance, tear resistance, high load-bearing, castable, potable, transparent or translucent, ozone resistance, hardness range, etc.; compared with plastic, polyurethane elastomer has the advantages of no brittleness, elastic memory, wear resistance and other advantages. There are various processing methods for polyurethane elastomers, new technologies and varieties are emerging, and the application prospects will be very broad [2].
2 Processing of polyurethane elastomers
In the laboratory, polyurethane elastomers are generally synthesized by hand-cast prepolymer method, including one-step method, prepolymer method and semi-prepolymer method.
The one-step method is a method in which diisocyanate, polyol, catalyst and other additives in the formulation are added at once, mixed at high speed and poured into a mold to make polyurethane elastomer products. Although the one-step method produces products with poor performance uniformity and repeatability, and can introduce a large number of bubbles into the reaction system, making a large number of products, the method is simple, energy-saving and cost-reducing, so this method is mainly used in the foaming industry, but rarely used in the production of cast polyurethane elastomers [3]. Currently, with the emergence of some new molding arts such as reactive injection molding (RIM) technology, the one-step method has also gained more rapid development.
The prepolymerization method of polyurethane elastomers is a two-step process, so it is also called the two-step method. The oligomeric alcohol is first reacted with an excess of polyisocyanate to produce a prepolymer with an NCO group at the end, and then the polymer is reacted with a chain extender during casting to produce a polyurethane elastomer. This method is mostly used in the production of polyurethane elastomers. The disadvantages of this method are that the prepolymer is more sensitive to temperature, high equipment requirements during casting, and a longer process. The difference between the semi-prepolymer method and the prepolymer method is that part of the polyester polyol or polyether polyol is added to the prepolymer in the form of a mixture with chain extenders and agents. In other words, the oligomer polyol in the formulation is divided into parts, one part is reacted with excess diisocyanate to synthesize the prepolymer, and the other part is mixed with the chain extender and added at the time of injection. Generated prepolymer in the free NCO mass fraction is high, generally 0.12 ~ 0.15 (12% ~%), so often called this prepolymer “semi-prepolymer (quasi-prepolymer)”. The characteristics of the semi-prepolymer method: ① pre-polymer component viscosity is low, can be adjusted to the viscosity of the mixed components with the curing agent; ② ratio is also close (i.e., mixed mass ratio can be 1:1). This not only improves the homogeneity of the mixture, but also improves certain properties of the elasticity. The method is easy to achieve industrialization: Among the three methods mentioned above, generally speaking, the polyurethane elastomer made by the prepolymer method has good performance and the one-step method has poor performance. This is because in the one-step method, polymerization and chain expansion reactions are carried out simultaneously, and the reaction is incomplete at a later stage because the viscosity of the system increases sharply and the activities of molecular chains are controlled by diffusion reactions, resulting in a relatively small molecular weight and a non-uniform structure of the polyurethane elastomer, which affects the performance of the polyurethane elastomer. In the prepolymerization process, the reaction of polyurethane prepolymer and the reaction of polyurethane prepolymer and chain extender are carried out step by step, and the reaction is controlled, so the reaction is more complete, and the molecular weight of the polyurethane elastomer obtained is larger and the structure is more uniform, which is conducive to the formation of hydrogen bonds between large molecules, thus improving the performance of polyurethane elastomer. The properties of polyurethane elastomers made by the semi-prepolymer method are between those of the prepolymer method and the one-step method, and the reaction temperature is lower, which is suitable for industrial production. This thesis discusses the relationship between structure and properties of polyurethane elastomers, all of which were synthesized using the prepolymer method.
3 Structure and properties of polyurethane elastomers
The mechanical properties of polyurethane elastomers are directly related to the internal structure of polyurethane elastomers, and their microstructure and morphology are strongly influenced by the interaction between polar groups. For example, the type, structure and morphology of soft and hard segments affect the mechanical properties, heat resistance, etc. of polyurethane elastomers. In recent years, the relationship between the mechanical properties of polyurethane elastomers and their aggregate state structure and microstructure has been investigated to address these issues.
(1) Microphase separation structure of polyurethane elastomers
The performance of polyurethane is mainly influenced by the structure of macromolecular chain morphology. The unique flexibility and excellent physical properties of polyurethanes can be explained by the two-phase morphology. The degree of microphase separation between the soft and hard segments of polyurethane elastomers and the two-phase structure are critical to their properties. A moderate degree of phase separation facilitates improved polymer properties. The process of microphase separation is a result of the difference in polarity between the hard and soft segments and the crystallinity of the hard segments themselves, which leads to their thermodynamic incompatibility (immiscibility) and a tendency to spontaneous phase separation, so that the hard segments tend to aggregate together to form a microzone and disperse in the continuous phase formed by the soft segments. The process of micro-phase separation is actually the process of separation and aggregation or crystallization of hard segments in elastomers from the copolymer system.
The phenomenon of polyurethane microphase separation was first proposed by Cooper, an American scholar, after which a lot of research work has been done on polyurethane structure [4], and progress has been made in the study of polyurethane aggregate structure, forming a relatively complete microphase structure theory [5] system: in the block polyurethane system, microphase separation occurs in the hard and soft segment microregions induced by the thermodynamic incompatibility between the segments and the soft segments. The attraction of chain segments between hard segments is much greater than the attraction of chain segments between soft segments, and the hard segments are not soluble in the soft segment phase, but distributed in it, forming a discontinuous microphase structure (island structure), which acts as a physical linkage and enhancement in the soft segments at room temperature. In the process of micro-phase separation, the interaction between hard segments increases will facilitate the separation of hard from the system and aggregation or crystallization, promoting micro-phase separation. Of course, there is a certain compatibility between the plastic phase and rubber phase, and the phase mixing between the plastic micro-zone and the rubber micro-zone forms the overflow phase. Paik Sung and Schneide [7] proposed a more realistic model of microphase separation structure: the degree of microphase separation in urethane is imperfect, not entirely microphase coexistence, but includes mixed soft segment units. There is mixing between the chain segments in the micro-zone, which has a definite effect on the morphology and mechanical properties of the material, and there are hard chain segments in the soft chain segment phase zone, which can lead to an explicit increase in the glass transition temperature of the soft segments, narrowing the range of use of the material at low temperatures. The inclusion of soft chain segments in the hard chain segment microregion can lower the glass transition temperature of the hard segment microregion, thus reducing the heat resistance of the material.
(2) Hydrogen bonding behavior of polyurethane elastomers
Hydrogen bonding exists between groups containing strong electronegative nitrogen and oxygen atoms and groups containing hydrogen atoms, and the size of the cohesion energy of the groups is related to the polarity of carbamate and urea groups in hard segments, and hydrogen bonding mostly exists between the segments. It has been reported that most of the imine groups in a variety of groups in polyurethane macromolecules can form hydrogen bonds while most of them are formed between imine groups and carbonyl groups in hard segments, and a small portion is formed with ether oxy or ester carbonyls in soft segments. The hydrogen bonding force is much smaller compared to the bonding force of intramolecular chemical bonds. However, the presence of a large number of hydrogen bonds, in polar polymers, is also one of the important factors affecting the properties. Hydrogen bonding is reversible. At lower temperatures, the close alignment of sexual chain segments drives hydrogen bond formation: at higher temperatures, chain segments receive energy and undergo thermal motion, segments and intermolecular distances increase, and hydrogen bonds weaken or even disappear. Hydrogen bonding plays a physical cross-linking role, which can make polyurethane sexual body have higher strength, wear resistance, solvent resistance and smaller tensile permanent deformation. The more hydrogen bonds there are, the stronger the inter-subunit force is and the stronger the material is. The amount of hydrogen bonding content directly affects the degree of microphase separation of the system [8].
(3) Crystallinity
Linear polyurethanes with regular structure and containing many polar and rigid groups have more intermolecular hydrogen bonds and good crystallization properties, which improve some properties of polyurethane materials, such as strength, solvent resistance, etc. The hardness, strength and softening point of polyurethane materials increase with the degree of crystallization, while the elongation and solubility decrease accordingly. For some applications, such as one-component thermoplastic polyurethane adhesives, fast crystallization is required to obtain initial adhesion. Some thermoplastic polyurethane elastomers are faster to release due to high crystallinity. Crystalline polymers often become opaque due to the anisotropy of refracted light. If a small number of branched chains or side groups are introduced into the crystalline linear polyurethane macromolecule, the crystallinity of the material decreases. The cross-link density increases to a point where the soft segments lose their crystallinity. When the material is stretched, the tensile stress causes the soft segment molecular chains to orient and become more regular, the crystallinity of the polyurethane elastomer increases, and the strength of the material increases accordingly. The stronger the polarity of the hard segment, the better the lattice energy of the polyurethane material after crystallization. For polyether polyurethane, as the content of hard segment increases, the polar group increases, the intermolecular force of hard segment increases, the degree of microphase separation increases, the crystallization of hard segment microregion gradually forms, and the crystallinity gradually increases with the increase of hard segment content, and the strength of the material is enhanced.
(4) Effect of soft segment structure on the performance of polyurethane elastomers
Oligomeric polyols such as polyether and polyester form soft segments. The soft segment accounts for most of the polyurethane, and different polyurethanes prepared from different oligomeric polyols and diisocyanates have different properties. The flexible (soft) chain segments of polyurethane elastomers mainly affect the elastic properties of the material and contribute significantly to its low temperature and tensile properties. Therefore, the soft chain segment Tg parameter is extremely important, followed by crystallinity, melting point and strain-induced crystallization, which are also factors that affect its ultimate mechanical properties. The mechanical properties of polyurethane elastomers and foams made of polar polyesters as soft segments are better. Because the polyurethane made of polyester polyol contains polar ester groups, this polyurethane material can not only form hydrogen bonds between the hard segments internally, but also the polar groups on the soft segments can partially form hydrogen bonds with the polar groups on the hard segments, so that the hard segment phase can be more uniformly distributed in the soft segment phase, playing the role of elastic cross-linking point. Some polyester polyols can form soft segment crystals at room temperature, which affect the performance of polyurethane. The strength, oil resistance and thermal and oxygen aging properties of polyester polyurethane materials are higher than those of PPG polyether polyurethane materials, but the hydrolysis resistance is worse than that of polyether. Polytetrahydrofuran (PTMG)-type polyurethane is not comparable to polyester-type polyurethane in strength due to its regular molecular chain structure and easy formation of crystals. Generally speaking, the ether group of the soft segment of polyether type polyurethane is easier to rotate internally, has better flexibility, has excellent low temperature performance, and there is no ester group in the polyether polyol chain that is relatively easy to hydrolyze, so its hydrolysis resistance is better than that of polyester type polyurethane. The α carbon of the ether bond in the soft segment of polyether is easily oxidized, forming peroxide radicals and producing a series of oxidative degradation reactions. Polyurethanes with polybutadiene molecular chains as soft segments have poor elastomeric strength due to weak polarity and poor compatibility between soft and hard segments. Soft segments containing side chains have poorer strength than polyurethanes without side groups with the same soft segment main chain due to site-blocking, weak hydrogen bonding and poor crystallinity. The molecular weight of the soft segment has an effect on the mechanical properties of polyurethane. Generally speaking, assuming the same molecular weight of polyurethane, the strength of polyurethane material decreases as the molecular weight of soft segment increases; if the soft segment is polyester chain, the strength of poly material decreases slowly as the molecular weight of polyester diol increases; if the soft segment is polyether chain, the strength of poly material decreases as the molecular weight of polyether diol increases, but the elongation increases. This is due to the higher polarity of the ester soft segment and the higher intermolecular force, which can partially offset the effect of decreasing the strength of polyurethane material due to the increase of molecular weight and the increase of soft segment content. In contrast, the polyether soft segment is less polar, and if the molecular weight increases, the content of the hard segment in the corresponding polyurethane decreases, leading to a decrease in the strength of the material. Zhu Jinhua et al [9] synthesized a series of polyurethane block copolymers and graft copolymers containing different soft segments, and tested their dynamic
