Journal of Cellular Plastics

Epoxy and polyurethane foams with different content of voids were tested under compression.

Thereby, the epoxy foam showed a simple correlation between the fraction of the foaming agent and the density and void fraction. In contrast to this foam, the density and void fraction of the polyurethane system is additionally dependent on the position in the body of the foam.

The empirical classic correlation and the mechanical modified Gent-Thomas model as models for the calculation of the compression properties of foams showed good agreement between the measured and the calculated values. The empirical model with its materials parameters showed a bigger flexibility.

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.

Conventional rigid polyisocyanurate (PUIR) foams blown with HCFC-141b often suffer from poorer compressive strengths, dimensional stability and inferior flammability properties when compared to foams blown with CFC-11. It is often hypothesized that these properties can be improved by increasing the isocyanurate (or "trimer") conversion by means of catalyst optimization and increased isocyanate index. A convenient, yet reliable method for the determination of the amount of isocyanurate in a PUIR foam has been missing in this industry for a long time, though several attempts have been documented in the literature. The purpose of this paper is to introduce an improved isocyanurate conversion test by photoacoustic Fourier Transform Infra Red (FTIR) technique. This involves creating a baseline through three anchor points at approximately 1637 cm⁻¹, 1469 cm⁻¹, and 1349 cm⁻¹. The absorbance of the isocyanurate peak at 1410 cm⁻¹ is taken relative to the absorbance of the phenyl peak at 1602 cm⁻¹. The phenyl peak is preferred to the urethane peak because the absorbance of phenyl groups in a foam is inherent to the amounts of polyols and isocyanates used in the foaming reaction, whereas the urethane linkages in the foam are created by a reaction between the two and are, therefore, variable depending on the extent of the reaction in the presence of catalysts and possibly water. In addition, the relative ratios of the absorbances of isocyanate end groups (at 2277 cm⁻¹) and carbodiimide groups (at 2136 cm⁻¹) to the phenyl groups can also be determined by generating a baseline through the anchor points at approximately 2470 cm⁻¹, 2207 cm⁻¹ and 2000 cm⁻¹. This allows one to gain a better assessment of the overall kinetics of the isocyanate reactions and creates opportunities to improve the isocyanurate conversion through formulation optimization. The method is not limited to polyisocyanurate foams, as isocyanate conversion is an important parameter to follow in polyurethane foams as well, especially in all carbon dioxide blown foams. The method was found to be quite reproducible, and further statistical analysis to ensure the validity of this technique is under way.

The mandated switch from CFC-11 (trichlorofluoromethane) to HCFC-141b (1,1-dichloro-1-fluoroethane) as a blowing agent for polyisocyanurate insulation foams has required producers of these foams to make numerous formulation changes. The changes have been warranted due to the differing physical properties of HCFC-141b versus CFC-11. These differences include higher boiling point, higher latent heat of vaporization, increased polymer solubility and decreased viscosity of the polyol and isocyanate blends which contain HCFC-141b compared to CFC-11. These differences have led to concerns with the ultimate physical properties of polyisocyanurate foams blown with HCFC-141b as compared to those with CFC-11 blown foams. In particular, the increased solubility of HCFC-141b in the foam's polymer matrix has led to a reduction in compressive strength and problems with dimensional stability. This paper deals with Dow's efforts at improving foam processing and dimensional stability of these foams with the use of high functional, low equivalent weight polyols and increased viscosity and higher functionality Polymeric MDI (PMDI).

There are critical processing and testing parameters which have major impacts upon the apparent firmness of HR molded polyurethane foam. Inadequate control of these parameters hinders the ability of the molder to meet narrow IFD specifications. This paper addresses five of these parameters and defines model equations which are useful in predicting load response to specific control parameters. In addition, the data are presented in a format that should be useful to the molder in better understanding the criticality of each parameter, singularly and in unison. The study was made possible by utilizing specialized equipment which allows accurate determination of IFD measurements under controlled and variable environments.

It is well known that the load bearing responses of HR polyurethane foam are influenced by ambient temperature and humidity. These load bearing responses are generally reversible, and are not the permanent effects often seen as a function of environmental conditions at time of pour and post mold cure of the polyurethane. The idealized cushioning material would be unaffected by either temperature or humidity. Subsequent experimental designs exploring variables in polyol structures were conducted which have resulted in the definition of new high performance polyol systems that demonstrate less sensitivity to the reversible effects of temperature and humidity.