Polyurethane PU is a versatile polymer with a wide range of applications due to its tunable properties. These properties can be optimized by varying the diisocyanate and polyol components in the synthesis process. Here are some ways to optimize the properties of polyurethane:1. Selection of diisocyanate: The choice of diisocyanate can significantly impact the final properties of the polyurethane. Common diisocyanates include toluene diisocyanate TDI , methylene diphenyl diisocyanate MDI , and hexamethylene diisocyanate HDI . TDI-based polyurethanes generally have higher reactivity, lower cost, and better low-temperature flexibility. MDI-based polyurethanes have better mechanical properties, chemical resistance, and lower toxicity. HDI-based polyurethanes are known for their excellent weather resistance and low yellowing.2. Selection of polyol: The choice of polyol can also significantly impact the final properties of the polyurethane. Polyols can be classified into two main categories: polyester polyols and polyether polyols. Polyester polyols generally provide better mechanical properties, chemical resistance, and heat resistance, while polyether polyols offer better hydrolytic stability, low-temperature flexibility, and better resistance to oxidative degradation.3. Molecular weight of polyol: The molecular weight of the polyol can influence the mechanical properties, flexibility, and hardness of the polyurethane. Higher molecular weight polyols result in softer, more flexible polyurethanes with lower crosslink density, while lower molecular weight polyols produce harder, more rigid polyurethanes with higher crosslink density.4. Hydroxyl number of polyol: The hydroxyl number of the polyol is a measure of the number of hydroxyl groups present in the polyol. Higher hydroxyl numbers result in higher crosslink density, leading to harder, more rigid polyurethanes with better mechanical properties. Lower hydroxyl numbers produce softer, more flexible polyurethanes with lower crosslink density.5. Diisocyanate-to-polyol ratio NCO/OH ratio : The ratio of diisocyanate to polyol in the reaction mixture can be adjusted to control the crosslink density and final properties of the polyurethane. A higher NCO/OH ratio results in a higher crosslink density, leading to harder, more rigid polyurethanes with better mechanical properties. A lower NCO/OH ratio produces softer, more flexible polyurethanes with lower crosslink density.6. Chain extenders and crosslinkers: The addition of chain extenders and crosslinkers can further modify the properties of the polyurethane. Chain extenders, such as short-chain diols or diamines, can increase the crosslink density and improve the mechanical properties of the polyurethane. Crosslinkers, such as triols or triamines, can also be used to increase the crosslink density and improve the mechanical properties, chemical resistance, and heat resistance of the polyurethane.By carefully selecting and adjusting the diisocyanate, polyol, molecular weight, hydroxyl number, NCO/OH ratio, and the use of chain extenders and crosslinkers, the properties of polyurethane can be optimized for specific applications, such as flexible foams, rigid foams, elastomers, coatings, adhesives, and sealants.