Long Term Performance of Rigid Foams Blown with Non-CFC Blowing Agents

Abstract

Since the phaseout of CFC blowing agents, a variety of novel blowing agents have been used in the polyurethanes industry. These range from HCFCs to HFC and hydrocarbons. Solutions adopted by end users have depended very much on the geographical region and end application considered. Two questions now need to be addressed objectively: what is the long term performance of these systems and are existing test methods adequate?

Right at the start of CFC replacement, ICI Polyurethanes identified the need for new ways of testing. The current paper describes the integrated approach ICI Polyurethanes has adopted to testing and prediction of the long-term performance of rigid foams. Special emphasis is placed on hydrocarbon-and hydrofluorocarbon-blown foams.

A number of tests have been developed by ICI Polyurethanes to look into the specific problem of long term dimensional stability:

Quantifying the effect of temperature changes on polymer strength—such a test in combination with mathematical calculations gives strong indications of the most critical operation temperatures. It thus helps to focus further testing.

Determining the effect on blowing agent diffusion and cell gas composition changes—the key feature of ICI Polyurethanes' "Dim Vac" accelerated test is to obtain a low internal cell gas pressure in less than 15 days and to quantify this effect on dimensional distortions. Examples will be given for all new blowing agents, including HFCs, and a correlation is provided with traditional dimensional stability testing.

Evaluating the problem of matrix plasticization by the blowing agents—this involves saturating the polyurethane matrix with blowing agents in a few days. The saturation process may take months to years in service conditions. The effect of the blowing agent dissolved into the polyurethane matrix on the compression strength is quantified. This latter test gives a good indication of the safety margin which needs to be built into the foam.

All these tests have been verified and their findings compared with measurements on construction insulation foams blown with CFC and HCFC which have been in service for a number of years. This has resulted in the information necessary to develop adequate foam formulations in terms of their long term dimensional stability.

Dimensional stability is not the only key criterion for selecting polyurethane insulation. Thermal performance is equally important. So ICI Polyurethanes has also developed a number of test methods in this field to quantify the various processes occurring with time.

Gas chromatographic cell gas analysis is used to measure the composition of gases at any time and place in the foam. A modification of this technique allows measurement of the amount of blowing agent present in the polyurethane matrix. A special sampling and measuring protocol gives information about the diffusion process of the physical blowing agent. A second technique measures the rate of diffusion for the natural occurring gases in the foam. State-of-the-art FTIR and SEM image analysis techniques are used to quantify the radiative and solid conductivity of the foam.

All the data generated by these tools are brought together in a mathematical model which enables prediction of the long term thermal performance for a number of given boundary conditions.