John M. Kinyanjui

FTIR Analysis of Chemical Gradients in Thermally Processed Molded Polyurethane Foam

Thermally processed PU foams are examined as a function of processing temperatures (25, 45, 65, and 85°C) at the side, middle, and center of a simple cylindrical mold. The PU foams show both chemical and morphological differences as a function of the processing temperature and radial position within the mold. Thermal degradation of uretoneimine structures, the emergence of carbodiimide structures, and extent of reaction of isocyanate groups are measured using photoacoustic FTIR spectroscopy. Chemical gradients and morphology differences between the side, middle, and center of the molded foam are observed for all processing temperatures. The data indicate that thermal activation at the center of the mold is important for samples regardless of processing temperature. Furthermore, in spite of thermal processing at temperatures well above the decomposition of uretoneimine structures (40°C), chemical gradients remain within the simple molded foams.

Thermally Induced Changes in the Chemical and Mechanical Properties of Epoxy Foam

The influence of thermal treatment on the chemistry, physical, and mechanical properties of epoxy foam was evaluated. The foam was subjected to repeated thermal cycles (25—95°C), encompassing the temperature regime in which the forward and the retro Diels—Alder reaction occurs. Changes in the chemistry of the foam were evaluated using in situ FTIR spectroscopy during thermal exposure. In addition, the structural siloxane units within the epoxy foam were identified and evaluated using FTIR analysis. Thermal analysis was used to evaluate expansion, degradation, and mass loss during thermal exposure. Finally, the physical and mechanical properties were evaluated to determine how thermal cycling influences the density and modulus of the epoxy foam. Thermal exposure below the temperature required for the breakage of conjugated double bonds via Diels—Alder mechanism increases thermal expansion influencing the structural integrity and packing of siloxane chains. The data indicates that the chemical changes and the thermal expansion of the foam are irreversible. The combination of thermal expansion and the change in chemistry for the system strongly diminished the structural rigidity of the foam lowering the density and modulus of the material.

Effects of Processing Temperature on ReCrete Polyurethane Foam

Research is conducted to determine the effect of processing temperature on some of the physical and mechanical properties of a polyurethane foam called ReCrete. The polyurethane foaming process is manipulated to change the foams density, chemistry, and mechanical properties. There is a 30-min period after ReCrete components are mixed when the materials are still undergoing significant chemical reaction. Researchers manipulate these chemical reactions by changing the environmental temperature during this process. This study investigates the effect of processing temperature on the chemistry and the resulting mechanical properties for a polyurethane foam system molded in aluminum cylinders and boxes. Processing temperature is varied from 25°C to 85°C. Researchers show that the processing temperature has a significant effect on ReCrete chemistry and density. The average density decreases by 19% over this temperature range. The chemistry, in turn, affects the static and dynamic mechanical properties. The axial compressive modulus and strength decrease by 24 and 16%, respectively. The chemistry changes that results from higher processing temperatures produce foam that is less rigid in compression, but tougher and more flexible. The dynamic flexural failure strength increases by 38% when the processing temperature is increased from 25°C to 85°C. Foam processed at 85°C has significantly greater resistance to brittle failure under impact.