Cellular polymers such as, for example, polyethylene, polystyrene, or polyurethane are of value in many application areas including notably thermal insulation. In this case it is particularly advantageous that such foam exhibits attractive dimensional stability and a relatively stable thermal insulation performance. Both of these traits are largely determined by the cellular structure of the foam and by the composition of the gas(es) within the cell cavity. A finer cell structure generally confers better insulation properties to a foam. However, as cells become finer, that is of smaller diameter, the compressive strength and dimensional stability of the foam frequently becomes inferior. Susceptibility to poor dimensional stability is greatest when the cell cavity contains a gas which is likely to condense or alternatively diffuse out of the cell. Either phenomenon leads to a loss of partial gas pressure within the cell, resulting in poor dimensional stability of the foam. Exemplary of a cell gas noted to diffuse out of a cell cavity, especially of polyurethane, is carbon dioxide. The use of carbon dioxide to prepare cellular polymers and especially polyurethane is presently highly favored as a substitute for many of the commonly used physical blowing agents considered harmful to the ozone layer. In support of the favored use of carbon dioxide, there exists a need to develop an improved process for the preparation of cellular polymers which exhibit attractive dimensional stability and thermal insulation properties. For rigid polyurethane foam, there exists a need for a process which permits the use of carbon dioxide as blowing agent and yet provides for a foam having acceptable dimensional stability and thermal insulation performance.
A potential solution to the dimensional stability problem is to provide a foam with an open-celled structure, that is, a structure where one or more of the cell windows are completely open and not sealed by a membrane, permitting the free passage of air or other gases. Expanded polymer having an open celled structure may be prepared by crushing the expanded polymer after its preparation to break cell windows. The latter means is only available for elastomeric polymers since crushing of rigid cellular polymers would lead to permanent deformation and damage. To provide for cell opening in a rigid polymer, use of a cell opening agent during the process of its manufacture is required. Typically such cell opening agents are high boiling liquids such as, for example, high molecular weight poly(oxyalkylene) adducts. Such methods of preparing open celled rigid polyurethane are disclosed in various patent publications including, for example, U.S. Pat. Nos. 5,284,882; 5,350,777; 5,318,997; 5,248,704; 3,694,385; G.B. 1102,391; G.B. 1,065,590; EP-622,388-A; EP-610,734-A; EP-547,515-A; and EP-A-188,806. However, a disadvantage associated with the use of such cell opening agents is that they generally promote the formation of polymer containing a coarse cell structure and consequently unattractive physical properties including thermal insulation. Thus, there is a need to provide an alternative process for the preparation of rigid open-celled foam which can permit the formation of a foam having a fine cell structure.