Generally speaking, most of the polyols used in rigid foams are polyether (or polyester) polyols with many functional groups, high hydroxyl values, and low molecular weights. After reacting with polyisocyanates, they have a network structure in the molecule. More (that is, there are many cross-linking points and dense), the foam obtained has large hardness, high compressive strength, good dimensional stability and temperature resistance.
On the other hand, if the flexible foam plastics need better softness and elasticity, polyols with fewer functional groups, lower hydroxyl value and higher molecular weight are often used. 聚酯多元醇 等，这样所形成的氨基甲酸酯高聚物分子中交联点之间的平均分子量在1000左右。 For example, a difunctional group with a molecular weight of 2,000 or a trifunctional polyether or polyester polyol with a molecular weight of 3000, etc., the average molecular weight between the crosslinking points in the urethane polymer molecule thus formed is about 1,000. The rigid foam requires a hydroxyl functionality of the polyol of 3 to 8, an average molecular weight of about 400 to 800, and a hydroxyl value equivalent of about 100. The distance between the crosslinking points in the urethane polymer molecules thus formed is on average 100 to 150. The polyether or polyester polyol used in the semi-rigid foam has a functional group number and molecular weight between the two or a mixture of the two different polyols with different hydroxyl values. The molecular structure of the flexible foam and the rigid foam is different.