In the manufacture of refrigeration cabinets, picnic coolers, doors, and other insulated containers, polyurethane foam is poured in placed between two substrates defining a cavity. In the method of preparing refrigeration or cooler containers, an inner plastic liner is placed into an outer optionally metal cabinet in a fixed, nested, spaced relationship, forming a cavity into which polyurethane foam is poured. Once the polyurethane foam is poured into the cavity, the container is held in its fixed position until the foam cures to prevent delamination of the foam from the sides of the container.
There are several requirements that a polyurethane foam should meet in pour in place foaming applications. One requirement is that an alternative blowing agent to ozone depleting CFCs must be found. A second requirement is that the polyurethane foam should flow well so that the entire cavity is filled with the foam. If the foam prematurely gels, voids will form behind the prematurely gelled foam where the foaming mass could not reach. A third requirement is to use the least amount of raw foaming material to fill a particular cavity to save on raw material costs. To adequately fill all portions of the cavity and prevent the presence of voids, it is often necessary to overpack the cavity. The less overpacking that is necessary to completely fill the mold, however, the greater the savings in raw material costs. Thus, it is desired to form a polyurethane-filled container having the lowest density possible.
Many polyurethane foam manufacturers are now turning to water as the sole source of blowing agent instead of CFCs or HCFCs. In the field of cooling containers where the foam is poured in place, water-blown rigid polyurethane foams present a unique problem. Rigid polyurethane foams blown with water tend to be closed-celled foams which shrink and pucker almost immediately after foaming and during cure. This is partly due to the migration of carbon dioxide gas, produced by the water reaction with polyisocyanate, out of the closed cells and leaving behind a vacuum which then tightens and shrinks the foamed mass. A foam which shrinks in foamed-in-place applications will either pull away from the surface of the substrates, reducing adhesion and resulting in blistering, or continue to adhere to the inner surface of the substrates causing saviness and surface deformities on the substrate. The problem of foam shrinkage in CFC-blown foams was not as acute since CFC gases tended to migrate out of the closed cells very slowly over a period of months or years, if at all, which enabled ambient gases to diffuse inward and equalize the pressure.
The problem of foam shrinkage or dimensional stability is more severe in picnic cooler applications where the coolers are often subject to wide temperature variations, from indoor 70.degree.-80.degree. F. temperatures to beach temperatures in direct sun which may climb to 110.degree.-120.degree. F., causing the gas in the cells to further expand and diffuse out.
It is also desired to produce a foam having a lower density yet which fully fills the cavity and is dimensionally stable to lower raw material costs. Lowering the density, however, especially in water-blown foam already having a tendency to shrink has the attendant disadvantage of further exacerbating the dimensional instability of the foam.