Miles, Inc.

The basic morphological unit of a closed cell rigid foam is the gas-filled cavity surrounded by cell walls and struts. Typically, these cavities (cells) are pentagonal dodecahedral or tetrakaidecahedral. Their average dimensions and dispersity are known to influence foam physical properties and foam performance. We have devised a reliable method for the measurement of cell-size and cell-size distribution which exploits both geometry and statistics, and which has allowed the realization of useful correlations and trends in our development of rigid foam formulations and additives. A thin slice of foam (100-300 microns) is subjected to optical microscopy and image analysis. Measurements of the areas of cell windows and of the average ferets of these windows are made independently. Windows are usually pentagonal, hexagonal or square and are variously distorted. Cells are assumed to be dodecahedral and cell diameter ( d) is calculated by the formulae: d = 2.1354 (area)1/2 d = 1.7013 (feret) Thereby, two independent determinations of cell diameter are obtained from each window viewed. Values are typically within 2% of each other. Cell-size distribution is quantified by measuring a large number (~ 500-1000) of windows and calculating the simple and weighted averages. This paper will present the mathematical basis for the method, illustrate the excellent agreement between the window area and the feret cell-size determinations and demonstrate the utility of cell-size and cell-size distribution data in physical property correlations, for example for k-factor.

The various topics discussed by the Committee on Pesticide and Disinfectant Formulations on Sept. 17, 1995 during the AOAC International Annual Meeting are discussed.

Efforts are continually being made to improve the insulation efficiency of appliance foam to reduce dependency on fossil fuels and to meet federally mandated product energy use requirements. These efforts have become even more important due to impending elimination of chlorofluorocarbons (CFCs) as the blowing agent for polyurethane and polyisocyanurate foams. Conversion to hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) generally leads to decreased insulation value of foams and therefore, decreased overall energy efficiency for the appliances. The effects of incorporating carbon black to reduce thermal conductivity of HCFC-141b blown appliance foams are discussed. The work was jointly carried out by Miles Inc. and the Center for Applied Engineering. Studies previously conducted at the Center had demonstrated that carbon black is well suited for use in rigid insulating foams, particularly in urethane-modified isocyanurates. The Celotex Corporation has now commercialized this technology in the building insulation market. Improvements of 6-9% in initial k-factors were observed when carbon black dispersed in polyols or isocyanates was used either in TDI- or PMDI-based appliance systems. The use of carbon black in HCFC-141b blown systems has the potential to produce foams with insulation values equivalent to CFC-11 blown production foams. The very fine particle size and inert character of commercially available carbon blacks allow loadings up to 12% by weight in foam polymer. Since the dispersions can result in excessively high viscosities, efforts were also directed towards modifying formulations to improve flow characteristics. Various approaches were taken to optimize carbon black loading in reactants as well as to facilitate processing in the foam process equipment.