Water-blown Foams Research Articles

Polyurethane foams blown with various types of environmentally friendly blowing agents

Three different types of environmently friendly blowing agents (HFC 365mfc, HFC 245fa and water) have been used to fabricate rigid polyurethane foams from crude methylene diphenyl diisocyanate and polypropylene glycols. Water induced the earliest bubble, temperature and pressure rises, whereas HFC 365mfc gave the slowest reactivity. Water blown foams showed the largest cell and the lowest density and compression strength, together with the highest glass transition temperature and the greatest dimensional stalibility at high temperature. With the combined effects of large cell and large thermal conductivity of CO2, water blown foam gave the highest conductivity of the foam. On the other hand, HFC 365mfc gave the smallest cell size, the greatest foam density and compression strength, whereas HFC 245fa gave the intermediate values, except the lowest thermal conductivity of the foam.

Spray Foam Beyond HCFC-141b

Sprayed rigid foam is a significant segment of the polyurethane industry, and as in other rigid foam segments, spray foam formulators need to find ways to meet current product requirements when the use of HCFC-141b is phased out in January 2003. Due to some of the unique characteristics of the spray foam industry, though, the choice of a replacement blowing agent is perhaps more complex than in any other rigid foam sector. The alternatives receiving the most consideration today are HFC-245fa, hydrocarbons, and water. All of these potential blowing agents have significant hurdles to overcome before they could serve as satisfactory replacements for HCFC-141b in the spray foam market. As of May 1999, a firm commitment to commercialize HFC-245fa has not been made. The use of hydrocarbons in spray would require significant improvements in safe handling procedures and modifications to spray equipment due to flammability issues. And with water-blown systems, dimensional stability and adhesion are two technical hurdles that must be overcome. It is conceded as well that insulation performance (as measured by initial k-factor) will not be as good for a water-blown foam as with HCFC-141b. It was because of the lack of an obvious HCFC-141b replacement for the spray foam industry that Huntsman Polyurethanes decided to investigate possible solutions. Any alternative would need to meet the processing and handling requirements that are currently met with HCFC-141b systems. The objective of this work is to develop zero-ODP (ozone depletion potential) technology that meets the performance characteristics that are currently achieved with HCFC-141b spray foam roofing systems. The focus of this paper is on the development of water-blown technology for roofing systems. Foams were processed via high-pressure spray equipment in a controlled environment. The study looked at the effect of polyol, catalyst, blowing agent, and processing variables on foam quality.

Water-Blown Rigid Foams

With the scheduled phase out of CFCs near at hand, many companies have been investigating completely water-blown (CO2) rigid foams. Several problems surfaced immediately such as a substantial increase in k-factor, a reluctance for the foaming mass to flow and fill molds properly, and a tendency for the foam to be dimensionally unstable in the density range commonly considered. Several applications do not require that k-factor be the primary concern. For instance, the insulation provided by some units containing rigid foam is affected by the tightness of the lid to the case, and insulation is required for only a limited, not an extended period of time, as for example, a picnic cooler. In such applications, a water-blown foam provides sufficient insulation and rigidity in the cooler walls to provide a satisfactory result. However, the foam in such cases must be dimensionally stable, i.e., over a period of time in which the cooler may be subjected to a number of heating and cooling cycles, the foam must not shrink, expand, or distort. One method to accomplish this is to use open-celled foam, since the cells contain no trapped gas to exert internal pressure on heating or cause a semi-vacuum on cooling. At BASF we have uncovered a family of additives which act as cell openers when added to resin formulations. Using standard rigid foam polyols, we have prepared foams with densities as low as 0.90 pcf which show less than 5% dimensional changes when kept at 158°F and 100% R.H. for 28 days. The measured closed cell content can be as low as 3-5%, although foams in the 1.1-1.4 pcf density range were found to be dimensionally stable at closed cell contents as high as 88%.

Understanding the Fundamentals of Forced Cooling in the Production of Blowing Agent Free Flexible Slabstock Polyurethane Foams

A rapidly developing method for blowing agent elimination in the production of flexible slabstock polyurethane foam is the use of forced cooling. This process overcomes many of the common problems related to discoloration and physical property loss in traditional low density water blown foams. Additional benefits of forced cooling include uniformity of certain physical properties throughout the foam bun and the elimination of storage space for bun curing. Understanding the relationship between the development and potential degradation of physical properties versus time in the production of blowing agent free flexible polyurethane foam is critical to the foam producers' successful implementation of these novel technologies. The prospect of quickly cooling internal bun temperatures to quench the reaction and reduce the resulting exotherm can be influenced by several important parameters. These parameters consist of: (a)novel polyol and modifier compositions for improved foam physical properties,(b) a thorough understanding of amine and tin catalysis, with the possibility of utilizing novel amine and tin catalysts, (c) understanding the time versus temperature relationship, and how this information can be used to ones advantage, and (d) understanding the equipment and process specific conditions involved in the cooling process and deciding what final physical properties are feasible and acceptable. Experiments and data pertaining to these important parameters will be presented and discussed with respect to the current scenarios confronting today's flexible slabstock polyurethane foam manufacturers.