There are many varieties of polyurethane products , different shapes, affecting various polyurethanesadhesivesThere are many factors that determine product performance, and there is a certain relationship between these factors. For polyurethane elastomer materials and foams, the determinants of performance vary, but there are some commonalities.
1, Cohesive energy of the group
Polyurethane materials are mostly made by reacting long-chain polyols such as polyester and polyether with polyisocyanates, chain extenders or cross-linking agents. The properties of polyurethane are related to its molecular structure, and groups are the basic components of the molecule. Generally, various properties of polymers, such as mechanical strength, crystallinity, etc., are related to the cohesive energy of the groups. In addition to urethane groups, polyurethane molecules also contain ester groups, ether groups, urea groups, allophanate groups, biurets, aromatic rings and aliphatic chains in different polyurethane products. of one or more.
The cohesive energy of the ester group is higher than that of the aliphatic hydrocarbon and ether groups; the cohesive energy of the urea group and the urethane group is high and the polarity is strong . Therefore, the strength of polyester polyurethane is higher than that of polyether and polyolefin types, and the cohesion, adhesion and softening point of polyurethaneurea are higher than those of polyurethane. The crystallinity and phase separation degree of polyurethane materials are related to the cohesive energy between macromolecules and within molecules. These are related to the types of soft and hard segments that make up polyurethane, that is, to the type and density of groups.
2, Hydrogen bond
Hydrogen bonds exist in groups containing highly electronegativeN atoms and oxygen atoms and in groups containing The groups of H atoms are related to the cohesion energy of the group. The urethane or urea group of the hard segment is highly polar, and hydrogen bonds mostly exist between the hard segments. It is reported that most of the imino groups (NH) of various groups in polyurethane can form hydrogen bonds, and most of them are formed by NH and the carbonyl group in the hard segment, and a small part with the ether oxygen group or ester in the soft segment. formed between carbonyl groups. Compared with the bonding force of chemical bonds within a molecule, hydrogen bonding is a physical attraction. It is much smaller than the bonding force between atoms, but the existence of a large number of hydrogen bonds is an important factor affecting the performance of polar polymers. one. Hydrogen bonds are reversible. At lower temperatures, the close arrangement of polar chain segments promotes the formation of hydrogen bonds; at higher temperatures, the chain segments accept energy and move, and the hydrogen bonds disappear. Hydrogen bonds play the role of physical cross-linking, which can make polyurethane elastomers have higher strength and wear resistance. The more hydrogen bonds there are, the stronger the intermolecular forces and the stronger the material.
3, Crystalline
Linear polyurethane with regular structure, many polar and rigid groups, many intermolecular hydrogen bonds, and crystallization of the material The degree is high, which affects certain properties of polyurethane, such as strength and solvent resistance. The strength, hardness and softening point of polyurethane materials increase as the degree of crystallization increases, while the elongation and solubility decrease. For some applications, such as one-component thermoplastic polyurethane adhesives, fast crystallization is required to obtain initial tack. Some thermoplastic polyurethane elastomers release quickly due to their high crystallinity. Crystalline polymers are often opaque due to anisotropy of refracted light.
If a small amount of branches or side groups are introduced into crystalline linear polyurethane, the crystallinity of the material will decrease, the cross-linking density will increase to a certain extent, and the soft segments will lose crystallinity. , the entire polyurethane elastomer can change from a harder crystalline state to an amorphous state with better elasticity. When the material is stretched, the tensile stress increases the regularity of the soft segment molecular groups and increases the crystallinity, which will increase the strength of the material. The stronger the polarity of the hard segment, the more conducive it is to the crystallization of the material.
4, Cross-linking degree
Moderate intramolecular cross-linking can increase the hardness, softening temperature and elastic modulus of polyurethane materials, while reducing elongation at break, permanent deformation and swelling in solvents. For polyurethane 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 cross-linking is excessive, properties such as tensile strength and elongation may be reduced.
Polyurethane chemical cross-linking is generally made of polyols (occasionally polyamines or other multi-functional raw materials) raw materials or cross-linking bonds (urea) formed by high temperature and excess isocyanate methyl formate and biuret, etc.). Compared with physical cross-linking caused by hydrogen bonding, chemical cross-linking has better thermal stability.
Polyurethane foam is a cross-linked polymer, in which flexible foam is composed of long-chain polyether (or polyester) diol and triol and diisocyanate and expanded Made of chain cross-linking agent, it has good elasticity and softness; rigid foam is made of high functionality, low molecular weight polyether polyol and polyisocyanate (PAPI), etc., due to its high The degree of cross-linking and the presence of more rigid benzene rings make the material more brittle.
5, Molecular weight
The molecular weight of linear polyurethane (elastomer) has a greater impact on the mechanical properties to a certain extent. As the molecular weight increases, The tensile strength, elongation and hardness of the polyurethane material increase, while the solubility in organic solvents decreases. For polyurethane materials with high cross-linking degree, such as foam plastics, coatings, etc., molecular weight is not the main factor affecting their performance.
6 , Temperature
Temperature affects the morphology and structure of polyurethane molecules and affects the performance of the material. The initial reaction temperature of polyurethane can affect the regularity of the molecular structure; thermal aging not only allows the reactive groups to react completely, but also gives the groups and chain links the opportunity to be arranged in an orderly manner; higher temperature reaction can cause the linear molecular chain to form a small number of branches. and cross-linking; curing at room temperature or placing at low temperature can form hydrogen bonds between polymer molecular chains and produce moderate phase separation, which is beneficial to the improvement of performance.
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