Polyurethane And Other Polymeric Foams

Polymeric foams as a class of materials is of particular interest in the present discussion not only because they are an important component of the waste stream but also because ozone depleting substances (ODS) such as Freon were once widely used as auxiliary blowing agents in their manufacture. However, in the United States and in western Europe, Freon is no longer used for the purpose and the practice is being phased out in many other parts of the world. The Montreal Protocol (along with its subsequent associated improvements) has been singularly successful in eliminating the use of major ODSs in polymeric foams and is expected to ensure the phase out of even the low-ODS agents presently allowed for the purpose, in the near future. This section reviews the chemistry of polymeric foams, the role played by Freon, and the substitutes available today for the purpose.

Foam is a polymeric material where gas-filled cells that are either open to or are isolated from each other fill the bulk of the matrix.

2.8.1. Polyurethane Foam

Polyurethane (PUR) foam is the primary insulating material used in refrigerators (residential and industrial) and freezers. It is an ideal material for the application because the rigid foam used provided some mechanical support, and the gas trapped within the cells afforded excellent insulation. It is an insulation material that is convenient to generate within door panels and difficult to access locations in the appliances by pumping the liquid prepolymer with other chemicals for reaction in situ. The liquid reactants might also be sprayed using an appropriate gun to obtain sprayed-on insulation. Rigid polyurethane is also cast as slabstock for use in insulation of large freezers, refrigerated transportation containers, and storage tanks. Rigid polyurethane insulation is used for conservation of heat (as with hot-water heaters) and in a number of consumer products such as coolers. Rigid foams are also finding increasing use as floatation materials in the hull of recreational watercraft.

The appliance market for polyurethane foam was recently estimated15 at 291 million pounds (North America in 2000) or about 5% of the total annual demand for polyurethanes in this region. As the volume of foam used in appliances as well as in building is tied to the economy, the demand is expected to slowly increase in the long-term as the gross domestic product (GDP) in the region increases. Increasingly stringent energy standards that appliance manufacturers (and possibly builders in the future) will have to comply with may also increase the demand for insulation materials.

In building applications rigid polyurethane foam is used laminated to board stock (or to other substrates such as foil or paper), discontinuous or continuous sandwich panels, slabstock, spray, and as pipe insulation material. Some of these sandwich structures are used in building (and transportation) applications that do not exploit their insulation characteristics. The largest volume building application is in roofing insulation [where both PUR and polyisocyanurate (PIR) foams are used].

Flexible PUR slabstock enjoys a large market in seat cushions, upholstery, and carpet backings. Over 2 billion pounds of the flexible foam is annually produced in the United States alone. Selecting the appropriate density of foam is important to ensure the durability of the product (such as a cushion) as well as its comfort level. The latter is quantified in the industry by measures such as the compression modulus, flex fatigue, resilience, indentation force, and support factor. Polyurethane foam slabstock is manufactured by continuously dispensing accurately metered reactants through a mixing head onto a lined trough on a

15 Data was reported in Appliance Engineer (June 2000, p. 68). The survey was sponsored by the Alliance for Polyurethane Industry.

moving belt. The prepolymer reacts exothermically expanding into a foam as it moves forward on the conveyer belt through fittings designed to give it a flat upper surface. The foam can also be made in the batch mode, using a box mold with a floating lid, into which the mixed chemicals are poured and allowed to expand. A variety of organic auxiliary blowing agents (such as pentane, methylene chloride, acetone, and liquid carbon dioxide) are available to replace any ODS blowing agents in use.

Integral skin PUR foams are made by injecting the reactants into a closed (vented) or open mold. Reaction under these conditions produces a foam with a high-density outside "skin" layer and a soft foam interior. Common products such as steering wheels, footwear components, and computer housings generally employ integral skin PUR foams.

Chemistry Polyurethane is produced by the reaction of a polyol with an diiso-cyanate (or in some instances a polyisocyanate) in the presence of catalysts. The polyols of choice are poly(propylene glycol), block copolymers of ethylene oxide (10-15%) with propylene oxide, or the newer polymer polyols (based on polymers such as polystyrene or styrene-acrylonitrile copolymer). Polyester diols such as polycaprolactone diol can be used in place of the polyether polyol in this reaction. The isocyanate of choice is a mixture of the 2,4 and 2,6 isomers of tolylene di-isocyanate in the ratio of 80:20, generally referred to as 80:20TDI. Other isocyanates such as diphenylmethane di-isocyanate (MDI), hexamethylene di-isocyanate (HMDI), and isophorone di-isocyanate (IPDI) are also used. A tin-based or amine catalyst is used to promote the reaction. Given the wide choice of reactants available, the reaction can yield foams with a range of different mechanical and thermal characteristics.

The basic reactions involved are shown below. The foaming is due to the generation of carbon dioxide from the reaction of water with excess isocyanate present in the system. While the foam density can be controlled to some extent by changing the amount of water, excess water can encourage the formation of relatively stiffer urea bonds, affecting the quality of the foam. The auxiliary blowing agents (ABA) are therefore used to increase foaming to obtain lower density foams that have good insulation properties, thanks to the thermal characteristics of the ABA. ABAs in the foam slowly diffused out during use and were released at the end of their useful life.

O=C=N—R1—N=C-O + HO-(-R2-O)-H Diisocyanate Polyol

Polyurethane

The polyol is shown above as a small molecule for convenience of illustration. In practice, they are low-molecular-weight polymers with a number-average molecular weight of a few thousand grams per mole.

Abbreviation Chemical name

Structure

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