What are the main reasons that affect the performance of polyurethane?
1. The influence of the percentage content of the prepomer isocyanate group (NCO): As the NCO increases, the hardness, tear resistance, fixed tensile strength, and tensile strength increase, and the viscosity of the polymer decreases, which is easy to defoam and mix, and the elongation decreases; if the NCO is too high, the curing speed of the rubber is too fast, and the life in the kettle becomes shorter, which is extremely unfavorable to the pouring process.Therefore, NCO% is generally appropriate at 2.4-6.5%.
2. Influence of chain expansion coefficient：
The equivalent ratio (NH2/NCO) of amine curing agent (MOCA) to NCO in the prepomer has a significant effect on the physical properties of the rubber, generally 0.85-0.95% is appropriate.Polyurethane should generally maintain a slight excess of NCO for MOCA crosslinking systems, which can make elastomer products have a suitable degree of crosslinking. When the chain expansion coefficient is 0.88, the chemical crosslinking and physical crosslinking reach a good balance, and the overall performance of the elastomer is the best; : When it reaches 1 or greater, due to the plasticizing effect of MOCA, and the weakening of the chemical crosslinking and hydrogen chain, the strength is significantly reduced, and the permanent deformation is large.(Therefore, if practicality is insurance, the chain expansion system often takes 0.85).
3. The influence of mixing temperature: Due to the different types of prepromers and chain extenders, the process conditions (mixing temperature) are also different. The mixing temperature increases the life of the kettle and the gel time is shortened. The mixing temperature is too low. The solid chain extender is not allowed. Although it is conducive to prolonging the life of the kettle and improving physical properties, it is not easy to defoam and evenly mix (due to increased viscosity), it is appropriate that the chain extender does not precipitate in the prepomer, such as MOCA, 110-120℃ is appropriate.
4. The main factors of the molecular weight of polyols on the performance: the size of the molecular weight determines the density of the characteristic groups in the polyurethane elastomer: the polyester type increases with the molecular weight, its physical properties can be improved; while the polyether type increases with the molecular weight, its compliance is improved, and the tensile strength and modulus will decrease.Generally, the molecular weight of polyols should be controlled between 1100-2200.