Flame-Retardant Flexible Polyurethane Foam

Common polyurethane foam is a combustible polymer. Flexible polyurethane foam, in particular, has a high open-cell rate (over 90%), low density, and a large surface area exposed to air, making it more flammable than other non-foam polyurethane materials. This presents certain limitations in its use for industrial and consumer products. In recent years, stricter fire safety regulations worldwide have driven the need for increased production of flame-retardant flexible polyurethane foam.

Flame-retardant flexible foams are generally classified into two types:

1. Additive Type: Flame-retardant substances, such as compounds containing phosphorus, chlorine, bromine, antimony, or boron, are added to the foaming formulation. The synergistic effects of combinations of these substances can further enhance flame retardancy. Other additives include aluminum hydroxide with crystalline water and various salts.

2. Reactive Type: Flame-retardant elements like phosphorus, chlorine, bromine, antimony, boron, or nitrogen are chemically introduced into polyether (or polyester) polyols or polyisocyanates to achieve flame resistance. Reactive flame-retardant foams generally perform better than additive types. A combination of both methods can also improve flame retardant properties.

Currently, a wide variety of flame retardants are used in flexible polyurethane foam, including tris(2-chloroethyl) phosphate (TCEP), tris(2-chloropropyl) phosphate (3PX), tris(dichloropropyl) phosphate, and tris(dibromopropyl) phosphate.

A major drawback of flame-retardant foams containing elements such as phosphorus and chlorine is the production of dense black smoke and toxic gases when burned. In recent years, researchers globally have focused on developing polyurethane foams with high flame retardancy that produce little to no black smoke or harmful gases.

One reported approach involves using partially reacted polyvinyl alcohol with ammonium phosphate (PV-PNH4) as a reactive flame retardant. When added in amounts ranging from 0.1% to 30%, PV-PNH4 significantly enhances flame resistance. For instance, with a PV-PNH4 content below 10% of the foam's weight, a test sample (20 cm × 2 cm × 0.5 cm) burned at a 45° incline with a wood lamp flame for 20 seconds shows minimal flame spread, with a burn length of less than 2 cm. If the PV-PNH4 content exceeds 10%, the foam self-extinguishes once the flame source is removed.

Another approach introduces isocyanurate rings into the foam structure during synthesis to impart flame retardancy. Isocyanurate compounds are formed through trimerization catalyzed by substances like 2,4,6-tris(dimethylaminomethyl)phenol, potassium octoate, potassium acetate, and N,N',N''-tris(dimethylaminopropyl)hexahydrotriazine. Adding small amounts of conventional phosphorus-halogen flame retardants to isocyanurate-containing foams significantly improves their flame resistance. During combustion, these foams form a phosphorus-rich surface layer, which is unique among flame-retardant foams.

As national economies rapidly develop, the demand for flame-retardant foam products with low smoke emissions, non-toxicity, and low VOC levels is growing. This is particularly critical in industries such as aviation, automotive (passenger cars and buses), and public facilities like hotels and conference centers, where stringent fire safety requirements are mandated.