Within only a few years, foamed plastics materials have managed to grow into an integral, and important, phase of the plastics industry -- and the end is still not yet in sight.
Urethane foam, as only one example, was only introduced commercially in this country in 1955.
Yet last year's volume probably topped 100 million lb. and expectations are for a market of 275 million lb. by 1964.
Many of the other foamed plastics, particularly the styrenes, show similar growth potential.
And there are even newer foamed plastics that are yet to be evaluated.
As this issue goes to press, for example, one manufacturer has announced an epoxy foam with outstanding buoyancy and impact strength;
another reports that a cellular polypropylene, primarily for use in wire coating applications, is being investigated.

On the following pages, each of the major commercial foamed plastics is described in detail, as to properties, applications, and methods of processing.

It might be well to point out, however, some of the newer developments that have taken place within the past few months which might have a bearing on the future of the various foamed plastics involved.
In urethane foams, for example, there has been a definite trend toward the polyether-type materials (which are now available in two-component rigid foam systems) and the emphasis is definitely on one-shot molding.
Most manufacturers also seem to be concentrating on formulating fire-resistant or self-extinguishing grades of urethane foam that are aimed specifically at the burgeoning building markets.
Urethane foam as an insulator is also coming in for a good deal of attention.
In one outstanding example, Whirlpool Corp. found that by switching to urethane foam insulation, they could increase the storage capacity of gas refrigerators to make them competitive with electric models.
Much interest has also been expressed in new techniques for processing the urethane foams, including spraying, frothing, and molding (see article, p. 391 for details).
And in meeting the demands for urethane foam as a garment interlining, new adhesives and new methods of laminating foam to a substrate have been developed.

New techniques for automatic molding of expandable styrene beads have helped boost that particular material into a number of new consumer applications, including picnic chests, beverage coolers, flower pots, and flotation-type swimming toys.
Two other end-use areas which contributed to expandable styrene's growth during the year were packaging (molded inserts replacing complicated cardboard units) and foamed-core building panels.
Extruded expandable styrene film or sheet -- claimed to be competitive price-wise with paper -- also showed much potential, particularly for packaging.
Sandwich panels for building utility shelters that consist of kraft paper skins and rigid styrene foam cores also aroused interest in the construction field.

In vinyl foam, the big news was the development of techniques for coating fabrics with the material (for details, see P. 395).
Better "hand", a more luxurious feel, and better insulating properties were claimed to be the result.
Several companies also saw possibilities in using the technique for extruding or molding vinyl products with a slight cellular core that would reduce costs yet would not affect physical properties of the end product to any great extent.

Readers interested in additional information on foams are referred to the Foamed Plastics Chart appearing in the Technical Data section and to the list of references which appears below.

Urethane foams

Since the mid 1950s, when urethane foam first made its appearance in the American market, growth has been little short of fantastic.
Present estimates are that production topped the 100-million-lb. mark in 1960 (85 to 90 million lb. for flexible, 10 or 11 million lb. for rigid);
by 1965, production may range from 200 to 350 million lb. for flexible and from 115 to 150 million lb. for rigid.
The markets that have started to open up for the foam in the past year or so seem to justify the expectations.
Furniture upholstery, as just one example, can easily take millions of pounds;
foamed refrigerator insulation is under intensive evaluation by every major manufacturer;
and use of the foam for garment interlining is only now getting off the ground, with volume potential in the offing.
Basic chemistry

Urethane foams are, basically, reaction products of hydroxyl-rich materials and polyisocyanates (usually tolylene diisocyanate).
Blowing can be either one of two types -- carbon dioxide gas generated by the reaction of water on the polyisocyanate or mechanical blowing through the use of a low-boiling liquid such as a fluorinated hydrocarbon.

The most important factor in determining what properties the end-product will have is quite naturally the type of hydroxyl-rich compound that is used in its production.
Originally, the main types used were various compositions of polyesters.
These are still in wide use today, particularly in semi-rigid formulations, for such applications as cores for sandwich-type structural panels, foamed-in-place insulation, automotive safety padding, arm rests, etc.
More recently, polyethers -- again in varied compositions, molecular weights, and branching -- have come into use at first for the flexible foams, just lately for the rigids.
The polyether glycols are claimed to give flexible urethanes a spring-back action which is much desired in cushioning.

Although the first polyether foams on the market had to be produced by the two-step prepolymer method, today, thanks to new catalysts, they can be produced by a one-shot technique.
It is possible that the polyether foams may soon be molded on a production basis in low-cost molds with more intricate contours and with superior properties to latex foam.

The polyester urethane foam is generally produced with adipic acid polyesters;
the polyether group generally consists of foams produced with polypropylene glycol or polypropylene glycol modified with a triol.
One shot vs. prepolymer

In the prepolymer system, the isocyanate and resin are mixed anhydrously and no foaming occurs.
The foaming can be accomplished at some future time at a different location by the addition of the correct proportion of catalyst in solution.
In one-shot, the isocyanate, polyester or polyether resin, catalyst, and other additives are mixed directly and a foam is produced immediately.
Basically, this means that simpler processing equipment (the mixture has good flowing characteristics) and less external heat (the foaming reaction is exothermic and develops internal heat) are required in one-shot foaming, although, at the same time, the problems of controlling the conditions of one-shot foaming are critical ones.
Properties

Most commercial uses of urethane foams require densities between 2 and 30 lb. / cu. ft. for rigid foams, between 1 and 3 lb. / cu. ft. for flexible foams.
This latter figure compares with latex foam rubber at an average of 5.5 lb. / cu. ft. in commercial grades.
Compression strength:

Graph in Fig. 1, p. 392, indicates how the ratio of compressive strength to density varies as the latter is increased or decreased.
The single curve line represents a specific formulation in a test example.
By varying the formula, this curve may be moved forward or backward along the coordinates to produce any desired compression strength / density ratio.
Thermal conductivity and temperature resistance:

In flexible urethane foams, we are referring to the range between the highest and lowest temperatures under which the materials' primary performance remains functionally useful.
In temperature resistance, this quality is usually related to specific properties, e.g., flexural, tensile strengths, etc.
Thermal conductivity is directly traceable to the material's porous, air-cell construction which effectively traps air or a gas in the maze of minute bubbles which form its composition.
These air or gas bubbles make highly functional thermal barriers.
The K factor, a term used to denote the rate of heat transmission through a material (B.t.u./sq. ft. of material/hr./*0F./in. of thickness) ranges from 0.24 to 0.28 for flexible urethane foams and from 0.12 to 0.16 for rigid urethane foams, depending upon the formulation, density, cell size, and nature of blowing agents used.
Table 1,, p. 394, shows a comparison of K factor ratings of a number of commercial insulating materials in common use, including two different types of rigid urethane foam.
Flexural strength:

This term refers to the ability of a material to resist bending stress and is determined by measuring the load required to cause failure by bending.
The higher-density urethane semi-rigid foams usually have stronger flex fatigue resistance, i.e., the 12 lb. / cu. ft. foam has 8 times the flexural strength of the 3 lb. / cu. ft. density.
Note that flexural strength is not always improved by simply increasing the density, nor is the change always proportional from one formulation to another.
Where flexural strength is an important factor, be sure that your urethane foam processor is aware of it.
Tensile strength:

This property refers to the greatest longitudinal stress or tension a material can endure without tearing apart.
(like compression strength of urethane foams, it has a direct relationship to formulation.
) Exceptional tensile strength is another of urethane foam's strong features.
Figure 2, above, shows the aging properties of urethane foams as determined by the percent of change in tensile strength during exposure to ultra-violet light.
Processing urethanes

There are many ways of producing a foamed urethane product.
The foam can be made into slab stock and cut to shape, it can be molded, it can be poured-in-place, it can be applied by spray guns, etc.

Slab stock is still one of the most important forms of urethane end-product in use today.
Basically, the foam machines that produce such stock consist of two or more pumping units, a variable mixer, a nozzle carriage assembly, and, in many cases, a conveyor belt to transport and contain the liquid during the reaction process and until it solidifies into foam.
The ingredients are fed from tanks through a hose and into the mixer at a predetermined rate.
The mixing head moves back and forth slowly across the width of the receptacle.
It only takes a few minutes for the foaming action to be completed and after a short cure, the material can be cut into lengths as desired.

Much has been done in the way of ingenious slitters to fabricate the slab stock into finished products.
Profile cutting machines are available which can split foam to any desired thickness and produce sine, triangle, trapezoid, and other profiles in variable heights, dimensions, etc.
The convoluted sheets can be combined to attain certain cushioning effects mechanically rather than chemically.
Also available is a slitter which "peels" the inside of a folded block of foam and can be used to slit continuous sheets up to 300 yd. in length, down to 1/16 in. thick.

The low cost and ease of fabrication of the dies for three-dimensional foam cutting plus the wide variety of shapes, dimensions, and contours that can be tailor-made to customer requirements has made the technique useful for producing case liners, materials handling containers, packaging and cushioning devices, and such novelties as soap dishes, toys, head rests, arch supports, and gas pedal covers.
Molding

Although slab stock appeared first, it soon became apparent that for the production of cushions with irregular shapes, crowned contours, or rounded edges, the cutting of slab stock is a wasteful and uneconomical process.
Only by resorting to molding techniques can the cushion manufacturer hope to compete satisfactorily in the established cushion market.

The closed molding of flexible urethane foams has been a problem ever since the introduction of the material (molding in open molds was more feasible).
Satisfactory methods for polyester foams and even prepolymer polyether foams were never fully achieved.
Closed molding generally resulted in parts weighing more (because of higher density) than parts fabricated from free-blown foams.
This counteracted the gain from having no scrap loss.
In addition, there were difficulties with the flow and spreading of the foam mixture over the mold surface, trouble with lack of gel strength in the rising foam, and problems of splits.
The introduction of one-shot polyether foam systems, aided by the development of new catalysts, helped to alleviate some of the problems of closed molding.
While there are still many bugs to be ironed out, the technique is fast developing.
Other techniques

Simple systems are available that make it possible for urethane foam components to be poured, pumped, etc., into a void where they foam up to fill the void.
In a typical application -- the making of rigid urethane foam sandwich panels -- an amount of foam mixture calculated to expand 10 to 20% more than the volume of the panel is poured into the panel void and the top of the panel is locked in place by a jig.