Foaming agents are crucial in the manufacturing of polyurethane foam plastics, including catalysts, foam stabilizers (uniformizers), and blowing agents. Optional additives used as needed include flame retardants, chain extenders/crosslinkers, antioxidants, light stabilizers, foam softeners, open-cell agents, fillers, colorants, antistatic agents, hydrolysis stabilizers, and foam composite material storage stabilizers.
Blowing Agents
Water serves as a primary blowing agent in polyurethane production, classified as a chemical blowing agent. It reacts with isocyanates to produce carbon dioxide gas, which expands, foams, and solidifies the viscoelastic foam material into various polyurethane foams. Given the high thermal conductivity and permeability of carbon dioxide, physical blowing agents are necessary for rigid polyurethane foam formulations requiring high insulation performance. These agents absorb some of the heat generated during initial material mixing and expand the foam through gasification.
In the production of flexible polyurethane foam, maintaining low-density soft foam without stiffness due to excessive water is crucial. This balance is achieved by controlling water usage and adding appropriate amounts of physical blowing agents as auxiliary agents.
CFC-11 (trichlorofluoromethane) became industrially produced in the late 1920s, widely favored for its non-flammability, suitable boiling point, ease of vaporization, low thermal conductivity, low toxicity, compatibility with polyurethane raw materials, non-corrosiveness, low cost, and simple foaming process. From the 1960s to the early 1990s, CFC-11 was extensively used as a blowing agent. However, the discovery in the 1970s of CFC-11’s ozone-depleting effects led to its global environmental scrutiny. Current alternatives to CFC-11 include HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons), HCs (hydrocarbons), liquid CO2, and water.
Foam Stabilizers
Foam stabilizers (uniformizers) are indispensable in polyurethane foam production. They enhance the miscibility of components, emulsify foam materials, stabilize foam, and regulate pore formation. Foam stabilizers are surfactants and are classified into non-silicone compounds and organosilicon compounds. Polyoxyalkylene-polydimethylsiloxane copolymers, or polyether-modified organosilicone surfactants, commonly referred to as “silicone oil,” are widely used due to their broad structural variations and effective performance. These surfactants have become the standard in the polyurethane foam plastics industry.
Open-Cell Agents
To achieve open-cell polyurethane foam, suitable catalysts are used to balance gelation and foaming reactions, leading to bubble wall rupture at the foam’s peak height. This results in an open-cell foam structure. Appropriate polyether polyols can also create open-cell foam. When catalysts and main ingredients are insufficient, small amounts of open-cell agents can disperse urea formed during water foaming, resulting in open-cell foam. Open-cell agents, a special class of surfactants containing hydrophobic and hydrophilic segments, reduce foam surface tension, promote pore rupture, increase the open-cell ratio, and improve issues related to closed-cell soft, semi-rigid, and rigid foam products.
Polyurethane rigid foam typically has a closed-cell structure due to high cross-linking density and strong bubble wall strength during foaming. Adding open-cell agents can produce open-cell rigid polyurethane foam for applications like sound absorption and filtration.
Early materials like liquid paraffin, polybutadiene, and dimethyl polysiloxane served as foam stabilizers and open-cell agents. Today, special chemical compositions like polyoxypropylene-polyoxyethylene copolymers and polyolefin-silicone copolymers are commonly used as open-cell agents.
Softeners
In high-water-content polyurethane flexible foam production, softeners suppress the stiffness caused by excessive urethane groups. Foam softening modifiers reduce isocyanate usage, lowering foam hardness for soft polyurethane foam production. Commercial softeners typically contain special polyethers, special polyols, water, and other components.
