Dwight D. Latham

The Effect of Polyol Functionality on Water Blown Rigid Foams

Rigid foam processing and performance issues have all presented themselves as problems to be overcome as the polyurethane industry replaces chlorofluorocarbon (CFC) blowing agents with alternatives such as hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFC), isomers of pentane and water. These problems include dimensional stability, foam flowability, formulation viscosity, friability, substrate adhesion, cycle times, cost, temperature resistance, insulation performance, k-factor aging, blowing agent solubility and flammability. Water blown rigid foams lack performance in many of these areas compared to the CFC blown foams of the past. Since much of the water blown rigid foam work of the past has been narrowly focused on individual applications and formulations, a broad study of the effect of polyol functionality on foam performance is necessary to address these issues. A series of five polyols, each having an equivalent weight of 110 glequiv, and functionalities ranging from two to six were prepared and characterized alone, in thermoset films and in water blown rigid foam formulations. Properties such as dimensional stability, cell size, k-factor, adhesion to aluminum and polystyrene, glass transition temperature, film permeability, relative chemical conversions by photoacoustic FTIR, and solvent swelling of thin sliced foams were characterized. These results are broadly applicable to the development and commercialization of water blown rigid foam polyols and formulations. Dimensional stability of the foams was found to worsen with increasing water level, or decreasing density. Additionally, the density was found to trend higher (at a given water level) with increasing functionality, indicating that blowing becomes less efficient. At all water levels studied, increasing functionality was found to improve dimensional stability, and the effect was most pronounced at the highest water level examined of 8 pph. At constant density, the cell size was found to be dependent on the polyol functionality, decreasing with increasing functionality. This is most likely the result of more numerous bubbles being produced (hence smaller cells) in the case of the higher viscosity formulations (higher functionality polyols) during mixing. The cell size of the foams influenced the initial k-factor of the foams, with small cell sizes yielding lower k-factors. Adhesion to both aluminum and polystyrene film decreased with increasing functionality, a result of a more brittle foam interface. The brittle interface produced in the case of the higher functionality samples was a consequence from reduced isocyanate conversion. The glass transition temperature of the water blown foams, the polyols and compression molded films increased linearly with increasing functionality as a result of the reduced modes of thermal relaxation due to increased crosslink density. The foam glass transitions ranged from 143°C to 228°C, the film glass transitions ranged from 101°C to 234°C and the polyol glass transitions from -72°C to -25°C for the 2-functional to 6-functional polyols, respectively.

Relationship of k-factor versus Density for Various Appliance Foam Formulations Containing Next Generation Blowing Agents

With the eminent elimination of CFC-11 (trichlorofluoromethane) as a blowing agent in rigid polyurethane foams, a greater understanding of the effect of the new alternative blowing agents on foam physical properties is needed. This is especially true in the appliance industry, where k-factor is of supreme importance to the overall energy consumption. Stringent energy requirements, along with the market pressures to continually lower the density of the foam, gives added challenges to new system development. This paper addresses the critical balance between k-factor and density, or cell gas content. Since the thermal conductivity of the gas plays the major role in the overall k-factor equation, it could be theorized that the addition of more blowing agent should lower the overall k-factor of the foam. However, a point exists at which the addition of blowing agent is actually detrimental to the overall k-factor, and lower gas levels can actually improve the insulation properties. This point is largely controlled by the amount of gas trapped within the cell, which directly affects the density and cell size of the foam. When too much gas is trapped within the cells during their formation, the cells will expand due to higher internal pressures, thus increasing the cell size and radiative contribution to k-factor. There will be an optimal level of gas contained within the cells to achieve the lowest k-factor for any given foam formulation. To address this issue, several foam formulations containing various alternate blowing agents were studied. Systems studied include TDI and MDI based formulations, blown with HCFC-141b (1,1-dichloro-1-fluoroethane) along with two MDI based systems; one blown with HFC-134a (1,1,1,2-tetrafluoroethane) and the other with cyclopentane. All foams were co-blown with various amounts of carbon dioxide generated from the water/isocyanate reaction. After the basic components for the foam formulations were determined for each of the blowing agents studied, a systematic approach was used to vary the cell gas level in each formulation. A graphical representation of k-factor versus cell gas level was generated for each system along with other physical properties. Based on this information, a better understanding of the effects of density on k-factor for the next generation of blowing agents can be obtained.