Foamed Plastics
(Cellular Polymers and Expanded Plastics)
Foamed plastics, also called cellular polymers or expanded plastics, can be made from almost any type of polymer. The choice of polymer mainly depends on the performance requirements, the economics and the required material throughput. The majority of all produced polymeric foams are based on polyurethane (PUR), polystyrene (PS), polyvinylchloride (PVC), polyethylene (PE) and a number of elastomers such as ABS, natural rubber and silicone. The type of polymer determines whether the resulting foam will be hard and rigid or soft and flexible. In general, elastomers yield flexible foams, whereas rigid (glassy) polymers yield rigid foams. The properties of a foam are also affected by its porosity (density) and cell structure. In general, foams are divided into high-, medium- and low-density foams, when considering their density. High density foams have a density between 0.5 g/cm³ and 1 g/cm³, medium density between 0.1 g/cm³ and 0.5 g/cm³ and low density lower than 0.1 g/cm³. Low density foams are mainly used in insulation applications whereas medium density foams find many uses in the packaging, building and construction industry. High density foams have noticeably higher strength and modulus, and thus, can often replace regular plastics in applications where lower electrical/thermal conductivity, weight per volume, dielectric constant, compression modulus as well as greater flexibility and damping is needed or required. Equally important is the cell structure of a foam. The two extreme cases are all open-cell foam and all closed-cell foam. In the case of a closed-cell foam, the majority of the cells are not connected together by passageways and do not share any of their structure with other cells. This type of foam has a high plastic content and very low gas and vapor permeability. It is also stronger and more rigid than open-cell foam. Open-cell foam, on the other hand, consists mainly of interconnected cells which share some of their structure with other cells. This type of foam is softer and more flexible than closed-cell foam, allows gas and vapor to move freely through the cells, and absorbs liquid when immersed in it. Both open and closed cell foams have much better thermal and acoustical insulation properties than non-porous plastics.
Polyurethane (PU) Foams
Polyurethane foams are the largest class of polymeric foams accounting for more than 50% of the worldwide usage of plastic foam.1 The properties of urethane foam can be tailored over a wide range for a large number of applications. They can be thermosetting or thermoplastic, rigid and hard or flexible and soft. They are formed from the reaction of an organic diisocyanate with a polyol which leads to urethane linkages in the backbone (-NH-C(=O)-O-). The polyol compound is typically a polyester or polyether, but can be any other resin having hydroxyl groups. This compound is typically the flexible portion of the urethane polymer and, thus, determines how rigid or flexible the foam will be. To produce foamed urethanes a small amount of water and an excess of isocyanate is added to the polyol. The water reacts with some of the isocyanate groups to form amines and carbon dioxide gas which forms the cellular structure of the foam. As the polymer hardens, the bubbles of carbon dioxide are trapped in the urethane. These bubbles give the polyurethane its cellular texture. Generally, during the early part of the polymerization process, the mixture is vigorously stirred to produce smaller bubbles. Depending on the process conditions and the amount of water added, the density of these materials can range from as low as 0.015g/ml or lower to as high as 0.95 g/ml. Polyurethanes can also be foamed using low-boiling, inert liquids such as fluorocarbons which are mixed with the polyol portion. During the polymerization process, the released heat causes the liquid to volatilize and to act as a blowing agent.
Polyurethanes can be converted into a wide range of materials including soft and flexible foams as well as tough and rigid foams.2 Flexible foams have typically a low density and semi-rigid foams have a medium density. Due to the versatile chemical characteristics and low price, polyurethane foams find a multitude of applications. Major applications for open-cell urethane foam include carpet backing as well as cushioning products for furniture and bedding. Rigid urethane foams are used in many light-weight (structural) parts including automobile bumpers, dashboards, and furniture. The demand of foamed urethane is expected to continuously grow based on the ever increasing demand for foamed plastics.
Expanded Polystyrene (EPS) Foam
Polystyrene foam, also known as Styrofoam (Dow Chemical), is the second most important plastic foam. It amounts to about 28% of the worldwide plastic foam consumption.1 Foamed styrene is typically made from expandable polystyrene (EPS) which is produced in the form of free-flowing pellets or beads impregnated with a low-boiling-point aliphatic hydrocarbon blowing agent such as pentane or hexane. The EPS beads have a closed-cell structure with low thermal conductivity and moisture absorption which can be stored for several month at room temperature in closed containers.
Foamed styrene products are typically produced by a two-step process. In the first step, the expandable beads are preexpanded or preformed by heat and then stored in a storage tank for several hours to allow them to equilibrate. In a second step, the beads are injected into a mold and further expanded to the final dimensions and, as the temperature exceeds the glass transition temperature, fused to the ultimate shape. The typical heat source is steam which is injected through perforations or tubes in the mold.
Polystyrene foam is used mainly as a thermal insulation material in the building & construction and packaging industries. Important products include insulation boards, drinking cups, egg cartons, and various other food containers. One of the major limitations of polystyrene foam is its rather low maximum service temperature of about 80°C (175°F).
Vinyl (Plastisol, PVC) Foams
Vinyl foams are the third largest class of polymeric foams accounting for about 6% of the worldwide usage of plastic foam.1 They can be flexible or rigid depending on the type and amount of
plasticizer added. They are often the preferred material for applications where low-flammability at a low cost is required.
PVC is typically foamed with organic nitrogen compounds such as diazoaminobenzene (DAB)
or azobisbutyronitrile (AIBN). The gas evolution occurs typically over a narrow temperature range depending
on the type of blowing agent. Often metal organic activators are added to lower the decomposition temperature and to increase the gas evolution. In a first step, the powdered blowing agent is dispersed
in the plasticizer which is then added to the PVC. The plastisol blend is then injected into a heated mold or extruded through a heated die. The heat causes the plasticizer to dissolve in the PVC resin
which dramatically increases the viscosity of the plastisol by solvation of the
PVC. To prevent collapse of the gas filled cells at too low temperatures or the formation of large pores and
cracks at too high temperatures, the decomposition temperature of the blowing agent should be very close to the gelation temperature of the plastisol. If the decomposition of the blowing agent occurs
in a closed mold, the internal pressure increases to very high levels which causes the gas to dissolve in the resin system which produces closed-cell plastisol with microscopically small bubbles. The
product is then cooled and solidified in the mold before ejected. Open-cell vinyl foam with high porosity can be produced by either mechanically dispersing air in the plastisol near the gelation
temperature or by a chemical blowing process with subsequent unrestricted expansion of the vinyl. The foamed plastisol is either cast onto a belt or
onto a fabric or sheet, knifed to the chosen thickness, and then
solidified.
Important applications of open-cell vinyl foams include upholstery, garment insulation, flooring underlays, carpet backing, and wall coverings. Rigid close-cell vinyl is often used in composite application where the low-density PVC foam is part of a multi-layer (sandwich) structure. Other applications include life jackets, buoys, and floats.
1Fredonia Industry Study #1436,
Foamed Plastics, June 2001
2Semi-rigid polyurethane foams are typically produced by a reaction-injection molding process, also called RIM.