The Effect of Polyol Functionality on Water Blown Rigid Foams
Authors: Tabor Rick , Lepovitz Jan , Potts Willis , Latham Dwight , Latham Laura 
Published: July 2017

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.