The Effect of Polyol Functionality on Water Blown Rigid Foams

Abstract

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.

Properties of water blown rigid polyurethane foams with different functionality

“…6 -f u n c t i o n a l f o a m w a s w o r s e t h a n t h a t o f 4.8-functional foam. Similar result also appeared in Tabor's study [7] , but no appropriate explanation was given. This is due to the fact that mixing of the mixture became more difficult and caused blowing efficiency to decrease at the same stirring rate when polyether polyol with higher viscosity such as 5.6-functional polyol was used to prepare rigid foams.…”

“…Since water is a nonozone depleting, non-toxic and cheap blowing agent, some researches on water blown rigid foams and their applications have been performed [4][5][6] . However, compared to the CFCs blown rigid foams, water blown rigid foams encounter some problems such as dimensional stability, foam flow-ability, formulation viscosity, friability, substrate adhesion, insulation performance, K-factor aging and blowing agent solubility [7,8] . A broad study of different effects of polyol, blowing agent, MDI and catalyst on foam properties is very necessary to address these issues.…”

Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent

“…Cell size affects the mechanical properties by spreading the compressive/tensile stresses to the more numerous structures available in small cell size foams to avoid concentrating the stresses onto fewer larger cell structures. So, the mechanical properties are expected to improve with the decrease of cell size [7] . In addition, according to heat transfer theory of rigid foams, about 10%-15% of the heat transfer of rigid foam can be directly attributed to the radiation, which can be minimized by reducing the cell size of the foam [14] .…”

“…Tabor et al [7] investigated the effect of polyol functionality on water blown rigid foams. In their work, it was shown that polyol functionality played an important role in determining the adhesion to aluminum, cell size, K-factor, dimensional stability and glass transition temperature of foam.…”

Fire Retarded Insulating Sheets from Recycled Materials

“…For example the heat insulating polystyrene foam is undesirable because of its high flammability. [1] The heat and sound insulating polyurethane foams tend to degrade strongly in the presence of humidity by losing their use-value. [2] The fire insulating asbestos sheet is banned because of its carcinogenic effect.…”