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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.