The original research on polyurethane polymers started as far back as 1937 in Germany. Initially, work concentrated on the production of fibers and flexible foams. It was not until 1954 that commercial production of flexible polyurethane foam began.
Polyurethane foam (‘PU foam’) is usually made by adding small amounts of volatile materials called ‘blowing agents’ to the mix of base chemicals which are reacting to produce the polyurethane. These ‘blowing agents’ add important performance characteristics including density reduction, cushioning/energy absorption and thermal insulation. They act in the same way that baking soda produces the aeration of soda bread.
Technically, polyurethane foam is a thermosetting polymer that generates gas when an isocyanate is mixed with a polyolefin. It can be produced in batches (‘bun casting’) or a continuous foaming process. The resulting sheets or blocks may then be cut to size as required. The liquid mix may also be pumped or sprayed before it foams.
Is there a health risk?
One drawback is that the closed foam cells contain the gas and over time this could escape (‘outgasses’) producing unpleasant odors. Environmental protection agencies (e.g. the EPA in the US) generally consider the foams to be safe; however for sprayed application (for example insulating a loft) regulations require that spray operators wear suitable protection. There is anecdotal evidence that people can become sensitized to the isocyantes and experience symptoms of asthma when exposed to low levels of the gas, even years after installation. Generally though, health concerns are very low.
Most building and furnishings regulations require that the PU foam contains a fire retardant. The fumes produced by burning PU are highly toxic.
Uses as a structural core
Honeycomb cores have a higher strength-to-weight ratio than foam, but foam wins out in several other ways for composite sheet construction. A foam core is in contact with almost 100 percent of the skin area, which spreads impact loads over a larger area than honeycomb. Also, the compression strength of a foam core prevents thin surface skin from failure due to buckling (given the good adhesion).
Further, the high shear strength inhibits the surface sheets from sliding independently of one other when subjected to bending – again, the adhesive is a critical component.
Foam cores are widely used in boat and marine construction, though the particular materials must be selected with care as some foam formulations are dissolved by polyester and vinyl ester resins. For example, EPS foam dissolves with styrene, a major component of polyester and vinyl ester resins. However, PU foam is not effected by styrene.
Uses as insulation
Because the foam has a high R-Value (high resistance to conducting heat), it is widely used for insulation purposes. It is used in sheet form in new-build construction to provide building insulation, and may be pumped into wall cavities in older buildings – retrofitting the insulation. It may also be sprayed on to surfaces (such as loft walls).
Sprayed polyurethane foam is also widely used in the construction of steel and aluminium boats and ships where it provides both insulation and inhibits corrosion by preventing condensation in voids and water traps.
‘Builders Foam’ is widely available in DIY depots. These use one part PU formulations which are cured by moisture.
PU foam may also be molded to produce shaped insulation panels though the pressure produced during the expansion stage can damage the mold. Pressure relief holes and other techniques are used to prevent such expansion damage occurring.
Density, flexibility and hardness can be fine-tuned by today’s chemical engineers to suit a huge range of applications, including:
- Seating in trucks, buses, aircraft, trains, commercial and household furniture
- Construction: moldings such as door frames and columns
- Resin transfer molding cores
- Paint and print rollers
- Footwear liners
Engineers have uncovered numerous advantages including ease of use, versatility and good insulation properties, with few disadvantages given appropriate material combinations. Given this strong balance of advantages, the future of PU foam is assured.
‘Green’ issues such as using only ozone-friendly foaming agents are being resolved by the industrial chemists. The already very wide spectrum of foam properties is being expanded further, which opens up new uses for the material. For example, soy is now being used by many foam manufacturers.
Forecasts predict that the annual worldwide production of polyurethane will reach 10 million tons soon after 2015. 75% of production (currently) is used for PU foam.