Polyurethane foam (PU foam) is primarily composed of polyurethane, with the main raw materials being polyisocyanates and polyols. Various additives are also incorporated, with the most important being foaming agents that play a critical role in the foaming process. These additives cause the formation of foam within the reaction product, resulting in the creation of polyurethane foam. This article offers a brief overview of the key raw materials used in the production of PU foam, as well as the role of foaming agents.
The most commonly used polyisocyanates in the industrial production of polyurethane foams include toluene diisocyanate (TDI), polymethylene polyphenyl isocyanate (PAPI), diphenylmethane diisocyanate (MDI), and liquid MDI (L-MDI).
TDI is primarily used in the production of flexible polyurethane foams. It is less reactive than MDI but more volatile. Some modified forms of MDI can substitute TDI in producing flexible foams, including high-density polyurethane foams and semi-rigid or microcellular polyurethane elastomers.
MDI, with higher reactivity than TDI and lower volatility, is commonly used in rigid polyurethane foams, and some of its modified forms can also serve as substitutes for TDI in flexible foam applications.
PAPI, also known as crude MDI or polymerized MDI, typically has an average molecular weight ranging from 30 to 400, with an NCO content of 31% to 32%. PAPI and modified versions are mainly used to produce various types of rigid polyurethane foams, but can also be employed in the production of high-rebound flexible foams, integral skin foams, and semi-rigid foams. Additionally, PAPI can be blended with TDI to create cold-cure, high-rebound foam plastics.
Polyether polyols used in the production of polyurethane flexible foams are typically long-chain, low-functionality polyethers. For flexible foam formulations, the functionality of polyether polyols generally ranges from 2 to 3, with an average molecular weight between 2000 and 6500. Polyether triols, often initiated with glycerol (propane-1,2,3-triol), are most commonly used in flexible foams. These polyols are produced through ring-opening polymerization with 1,2-epoxy propane or copolymerization with a small amount of ethylene oxide. Their molecular weight typically falls between 3000 and 7000.
Polyether Polyols for Flexible Foams
High-activity polyether polyols are primarily used in the production of high-rebound flexible foams and can also be applied in semi-rigid foam formulations and other foam types. Some polyether diols are used as auxiliary materials in flexible foam formulations, often mixed with polyether triols. Low unsaturation and high molecular weight polyether polyols are used for soft foam production, as they reduce the amount of TDI required.
Polyether Polyols for Rigid Foams
For rigid foam formulations, polyether polyols are generally high-functionality, high hydroxyl value types, which ensure adequate cross-linking and rigidity. The hydroxyl value of polyether polyols for rigid foam formulations typically ranges from 350 to 650 mg KOH/g, with an average functionality of 3 or higher. Rigid foam formulations often combine two types of polyether polyols, with an average hydroxyl value around 4000 mg KOH/g.
Polyether Polyols for Semi-Rigid Foams
Semi-rigid foam formulations often incorporate high molecular weight polyether polyols, particularly high-activity polyether triols, along with high-functionality, low molecular weight polyether polyols derived from rigid foam formulations.
Low-viscosity aliphatic polyester polyols, such as hexanediol adipate diols with a hydroxyl value of approximately 56 mg KOH/g or slightly branched polyester polyols, can be used to produce polyester-based polyurethane flexible foams. Polyester polyols have high reactivity. Currently, block polyurethane foam made from polyester is primarily used in niche applications, such as auxiliary materials for clothing.
Aromatic Polyester Polyols
Aromatic polyester polyols, synthesized from dicarboxylic acids (such as phthalic anhydride, terephthalic acid) and small-molecule diols (such as ethylene glycol) or polyols, are used to produce polyurethane rigid foams and polyisocyanurate rigid foams. Lower hydroxyl value polyester polyols derived from phthalic anhydride can also be used in high-rebound flexible foams, integral skin foams, semi-rigid foams, and non-foam polyurethane materials.
Polymer polyols, including rigid styrene, acrylonitrile homopolymers, copolymers, and grafted polymers, serve as organic "fillers" to enhance load-bearing performance. These polyols are used in the production of high-hardness flexible block foams, high-rebound foams, thermoplastic flexible foams, semi-rigid foams, self-skinning foams, and reaction injection molded (RIM) products. Polymer polyols can reduce product thickness, lower foam density (thereby reducing costs), increase foam cell opening, and impart flame-retardant properties to the products.
Polyurea Polyols
Polyurea polyols (PHD dispersions) are a special class of polymer-modified polyols used in high-rebound flexible foams, semi-rigid foams, and soft foams, although their presence in the market is limited.
There are also special polyols used in polyurethane foam production, such as vegetable oil-based polyols, rosin-based polyester polyols, and polymer polyesters, which are not covered in detail in this article.
