Silicone Surfactants for Pentane Blown Rigid Foam (1995)

Auxiliary blowing agents are utilized in most polyurethane applications. However, the impact of these blowing agents on final foam properties is most critical in the rigid foam area. In most rigid applications the blowing agent serves a dual role in both generating the foam volume, and, in addition, the residual gas remaining within the cells functions to reduce the rate of heat transfer and improve the insulation properties of the foam. Traditionally CFCs have been used as the preferred blowing agent within the polyurethane industry due to their excellent volatility, low flammability, and thermal characteristics. Within the last few years the use of many grades of CFCs have come under restriction due to their implication in depleting atmospheric ozone levels, and these products will ultimately be banned according to the Montreal Protocol. Other types of halogen containing replacements such as HCFCs and HFAs are being examined as potential replacements for the CFCs. However, the future viability of these types of materials is in question, as their toxicological, environmental and economic impact are still under examination. One of the most viable alternatives to replace CFCs as a blowing agent in rigid foam formulations is n-pentane or one of the isomers of pentane. These materials are considered to be excellent replacement candidates due to their availability, low pricing, and most importantly, zero ozone depleting potential(ODP). The use of pentane in rigid continuous laminate boardstock foam has already been initiated, with commercial utilization anticipated in other rigid applications within the next 12 to 18 months. Pentane is the most non polar of the auxiliary blowing agents used in polyurethane foam applications. As a result of the chemical and physical characteristics of pentane, the solubility and compatibility with the polyurethane raw materials and developing foam mixture are completely different compared to all other blowing technologies. This low solubility of pentane in the final polyurethane foam matrix results in very low diffusion rates of pentane. Therefore, initially obtained thermal insulation values are maintained with minimal drift. Due to the inflammable nature of pentane, new processing and handling procedures are required in addition to modifications to the foam formulations. Considering the potential flammability and explosion hazards, minimizing pentane emissions during production is an absolute requirement to safely utilize this new technology. The current and proposed environmental legislations on organic emissions is another reason to ensure minimal levels of VOCs when using pentane as a blowing agent in the production of rigid foams. One of the primary applications for panels produced by the continuous lamination process is in the construction industry, where the flammability rating is one of the most important properties of these materials. The use of an inflammable liquid as a blowing agent, where most of the residual material is retained within the foam cells, requires further formulation modifications to maintain the flame retardancy characteristic of the foams. System modifications, such as incorporating flame retardant additives and water as a co-blowing agent, further increases the polarity of the resultant polyol preblend. The increased polarity of this preblend accompanied by the very low polarity of the pentane puts further limitations on the total system solubility. To improve the solubilization/compatibilization of pentane into these systems, new surfactant technology is required. The primary role of a silicone surfactant in a rigid polyurethane system is to emulsify the incompatible reactants of the preblend and phases that occur during the reaction. In order to achieve similar processing performance and final foam physical properties with these modified formulations containing pentane, new specially designed silicone surfactant molecules are required. In order to investigate the silicone surfactant structure/foam performance relationship, a design study was initiated in a variety of formulations. In addition to the standard performance evaluations, special techniques were developed and effected to understand the solubility, emulsification and compatibility characteristics of pentane during the production and aging of the foam. This paper will discuss the influence of different silicone surfactants on the emulsification and stabilization of pentane in polyol premixes, reaction mixtures and during foam formation. The influence of the silicone surfactants on the physical properties of the final foam will also be discussed.