Polyurethane foam(PU foam) primarily consist of polyurethane as their main component. The raw materials primarily include polyisocyanates and polyols, with the addition of various additives, the most crucial of which are a series of foaming agents related to the foaming process. These additives lead to the production of a significant amount of foam within the reaction product, resulting in polyurethane foam products. This article provides a brief overview of the raw materials used in producing PU foam and the foaming agents.
1.Polyisocyanates
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 mainly used in the production of polyurethane flexible foams. MDI has higher reactivity than TDI, lower volatility, and some modified forms of MDI can be used as substitutes for TDI in the production of polyurethane flexible foams, including high-density polyurethane foam and the manufacturing of semi-rigid or microcellular polyurethane elastomers.
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%. In the field of foam plastics, PAPI and modified PAPI are primarily used to produce various polyurethane rigid foams, with some also used in the production of high-rebound flexible foams, integral skin foams, and semi-rigid foams. PAPI can be mixed with TDI to manufacture cold-cure, high-rebound foam plastics.
2.Polyether and Polyester Polyols
2.1Polyether Polyols
Polyether polyols used for producing polyurethane flexible foams are generally long-chain, low-functionality polyethers. In the formulation of flexible foams, the functionality of polyether polyols is usually between 2 and 3, with an average molecular weight ranging from 2000 to 6500. Polyether triols are most commonly used in flexible foams, typically initiated with glycerol (propane-1,2,3-triol) and obtained through ring-opening polymerization with 1,2-epoxy propane or copolymerization with a small amount of ethylene oxide, with a molecular weight generally falling within the range of 3000 to 7000.
High-activity polyether polyols are mainly used for high-rebound flexible foams and can be used in the production of semi-rigid foams and other foam products. Some polyether diols can be used as auxiliary materials, mixed with polyether triols in flexible foam formulations. Low unsaturation and high molecular weight polyether polyols are used for the production of soft foams, reducing the amount of TDI required.
Polyether polyols used in rigid foam formulations are generally high-functionality, high hydroxyl value polyether polyols to achieve sufficient cross-linking and rigidity. The hydroxyl value of polyether polyols for rigid foam formulations is typically in the range of 350 to 650 mg KOH/g, with an average functionality of 3 or higher. Rigid foam formulations often use a combination of two types of polyether polyols, with an average hydroxyl value of around 4000 mg KOH/g.
Semi-rigid foam formulations often use some high molecular weight polyethers, especially high-activity polyether triols, and some high-functionality, low molecular weight polyether polyols from rigid foam formulations.
2.2Polyester Polyols
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 for producing polyester-based polyurethane flexible foams. Polyester polyols have high reactivity. Currently, block polyurethane foam made from polyester is only used in a few fields such as auxiliary materials for clothing.
Aromatic polyester polyols, synthesized from dicarboxylic acids (such as phthalic anhydride, terephthalic acid, etc.) and small-molecule diols (such as ethylene glycol, etc.) 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 for high-rebound flexible foams, integral skin foams, semi-rigid foams, and non-foam polyurethane materials.
2.3Polymer Polyols
Polymer polyols, including rigid styrene, acrylonitrile homopolymers, copolymers, and grafted polymers, act as organic “fillers” to enhance load-bearing performance. Polymer 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. They can reduce product thickness, lower foam density to reduce costs, increase foam plastic cell opening, and impart flame retardant properties to the products.
Polyurea polyols (PHD dispersions) are a special class of polymer-modified polyols used in high-rebound flexible foams, semi-rigid foams, and soft foams, but their presence in the market is limited.
There are also some special polyols used for the production of polyurethane foams, such as vegetable oil-based polyols, rosin-based polyester polyols, and polymer polyesters. These are not described in detail in this article.
