Mechanical properties and structure of polyurethane elastomer

 Mechanical property

The structure and length of soft segment and hard segment in polyurethane elastomer, and the force between soft segment and hard segment directly affect the mechanical properties of elastomer. In addition, whether microphase separation can occur, the degree of microphase separation and the uniformity of hard segment distribution in soft segment are also important factors affecting the mechanical properties of polyurethane elastomer.

Effect of soft segment on mechanical properties of polyurethane elastomer

* Influence of polyol type

The flexible segment of polyurethane elastomer is mainly composed of oligomer polyols, which can be divided into polyester polyols, polyether polyols and polyolefin polyols, etc. Their main chain structure has great influence on the tensile strength, tear strength and other properties of elastomer. There are two kinds of polyols commonly used: polyester and polyether. Because polyester polyol contains more polar ester groups in its molecule, it has higher cohesive energy and can form stronger intramolecular hydrogen bonds, so the elastomer polymerized from polyester polyol has higher tensile strength, tear strength, better wear resistance and oil resistance. In the polyurethane elastomer made of polyether polyol, the cohesive energy of ether bond is low, and adjacent methylene groups are separated by oxygen atoms of ether bond, and the hydrogen atoms on the separated methylene groups are also separated far away, which weakens the mutual repulsion between hydrogen atoms of methylene groups. Therefore, the polyurethane elastomer polymerized from polyether polyol has low glass transition temperature, high weather resistance, hydrolytic stability and mold resistance, but poor mechanical properties.

* Influence of molecular weight of polyol

Generally speaking, the mechanical properties of polyether polyurethane elastomer decrease with the increase of polyol molecular weight. This is because with the increase of molecular weight of polyether polyol, the number of ether bonds increases, which improves the flexibility of polyurethane elastomer segment, so the mechanical properties such as tensile strength of elastomer decrease. Polyester polyurethane elastomer is the opposite, because with the increase of the molecular weight of polyester polyol, the methylene number and ester number increase, the intermolecular force and hydrogen bond crosslinking increase, the crystallinity of the chain segment increases, and the hardness, tensile strength, tear strength and tensile stress of the product are improved.

* Effect of hard segment on mechanical properties of polyurethane elastomer

In polyurethane elastomer, only when the cohesive energy of rigid segments is large enough can they be associated with each other to form a hard segment phase. The factors affecting the cohesive energy of rigid segments are the type and dosage of diisocyanate and the type of chain extender.

* Effect of isocyanate

The types of isocyanates have a great influence on the mechanical properties of polyurethane elastomers. When using isocyanate with symmetrical structure to prepare elastomers, the elastic segments contain symmetrical structures, the attraction between molecular segments increases, the crystallinity is enhanced, and various mechanical properties of elastomers are improved. Polyurethane elastomer prepared from aromatic isocyanate has a rigid benzene ring structure, so the cohesive energy of hard segments is increased, and hard phase micro-regions are easily formed between soft segments, which leads to microphase separation of the elastomer and improved mechanical properties, so its tensile strength, modulus stress and tear strength are higher than those of aliphatic isocyanate elastomer. With the increase of isocyanate content, the hard segment content, cohesive energy, crystallinity, flexibility and elongation of polyurethane increased, while the tensile strength, tear strength and modulus stress increased.

* The influence of chain extender

For the same prepolymer made from the same polyol and the same diisocyanate, the mechanical properties of the elastomers obtained are different with different chain extenders. Because the polarity of urea group generated by the reaction of diamine chain extender and diisocyanate is stronger than that of urethane bond obtained by diol chain extension, and the corresponding rigid segments are easy to gather together to form hard segments, so there are more hydrogen bonds, so the strength of polyurethane elastomer obtained by diamine chain extension is greater than that obtained by diol chain extension.

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