What factors contribute to the perceived firmness of polyurethane foam?

couch cushion foam

The apparent hardness of polyurethane foam is predominantly shaped by the following elements:

1.Isocyanate Type:

The choice of isocyanate plays a crucial role. Commonly used isocyanates include TDI (toluene diisocyanate) with two isomers and MDI (methylene diphenyl diisocyanate) with three isomers. MDI has an advantage due to its two benzene rings, providing more stability in micro-molecular rotation. Polyurethane chains formed with symmetric isocyanates exhibit stronger hydrogen bonding effects, resulting in greater rigidity (hardness). Consequently, foam produced with MDI tends to be harder than that made with TDI. Among MDI isomers, the 4’4 structure outperforms the others, and among TDI isomers, the 2’6 isomer is superior to the 2’4 isomer.

2.Polyether Polyols:

Polyether polyols contribute to the flexible segments of the foam. Generally, higher functionality leads to a more three-dimensional polyurethane structure, which demands increased foam hardness. Structurally regular polyether polyols, like PTMEG, yield superior polyurethane products in terms of hardness.

3.Polyester Polyols:

Commercially produced polyester polyols are typically created by grafting polyether triols with styrene and acrylonitrile. The solid content of these polyols affects the foam’s hardness as it influences the presence of “organic fillers.”

4.Isocyanate Index:

Whether using TDI or MDI, increasing the isocyanate index within a certain range results in increased hardness. This involves two reactions: excessive isocyanate can react with urea and carbamate during post-curing, forming cross-links and introducing more rigid segments to the foam.

5.Chain Extenders:

Chain extenders, as the name suggests, raise the polymer’s molecular weight. The market offers various types of chain extenders, including aliphatic diols, aliphatic diamines, and aromatic diamines. Foam hardness achieved with aromatic diamine chain extenders often surpasses that achieved with aliphatic diols and aliphatic diamines.


This process transforms linear molecular chains into a network structure, resulting in increased rigidity.

Picture of Sally Zhang

Sally Zhang

CEO of Foshan Alforu Technology Co.,Ltd