Noise and vibration management is a significant issue for vehicle manufacturers, as cabin noise is a major factor in the comfort experience of automotive passengers. Therefore, noise and vibration abatement measures are routinely incorporated into motor vehicles. These abatement measures often utilize flexible polyurethane foams. However, such foams typically are called upon to perform one or more functional purpose that can not be compromised at the expense of noise and vibration absorption.
Flexible polyurethane foam is based on the polymerization of polyether and/or polyester polyols with isocyanates in the presence of water acting as blowing agent. These systems generally contain additional components such as cross-linkers, chain extenders, surfactants, cell regulators, stabilizers, antioxidants, flame retardant additives, fillers, and typically catalysts such as tertiary amines and organometallic salts. Levels of the catalysts in the polyurethane formulation are adjusted during the foam manufacturing process to optimize processing as well as final foam properties such as cell structure, density, hardness, resiliency, airflow, elongation, tear resistance, aging, and emission characteristics.
In addition to performance properties, cost is another consideration as to material selection as manufacturers are always trying to reduce their production costs. One way to do with this with foam products is to reduce their density, thus reducing the amount of raw materials needed to prepare a part with a given volume. Some manufacturers now desire to reduce the foam density of these products by about 10%, from the range of about 44-50 kilogram per cubic meter (kg/m3) down to about 36-42 kg/m3. The simplest and most economical approach to accomplish this is to increase the amount of water in the formulation. Water reacts with isocyanate groups to release carbon dioxide, which acts as the blowing gas. By increasing the amount of water in the formulation, more carbon dioxide can be formed, provided that there are enough isocyanate groups to react with the additional water.
The reaction between water and isocyanate groups also extends the growing polymer chains by creating urea linkages between polyisocyanate molecules. The water-polyisocyanate reaction by itself forms a very rigid and brittle polymer. To overcome this brittleness and produce a flexible and resilient material, high equivalent weight polyols are added to the foam formulation. Hydroxyl groups on the polyols react with isocyanate groups to form urethane linkages. The urethane-forming reaction that takes place between the polyol and the isocyanate groups therefore is in competition with the water-isocyanate reaction. These reactions must be balanced so the construction of high molecular weight polymer chains and the generation of carbon dioxide proceed in the proper sequence. As the amount of water is increased, relative to the amount of polyols, the balance between these reactions becomes difficult to maintain. High water systems tend to become sensitive to small variations in processing conditions, such as, for example, small variations in the amount of catalysts or component or mold temperatures. It therefore becomes increasingly difficult to produce good quality foam consistently, in a manufacturing setting, with these high water formulations. The foams tend to have large voids, and areas of incomplete mold filling, especially at the end of the shot. In addition, there is often a large variation in foam quality from part to part, which again indicates the instability of the processing. At higher densities, these problems can be overcome to some extent by overpacking the mold (i.e., injecting more of the foam formulation than is needed to barely fill the mold). But as the foam density is reduced, little or no overpacking can take place as the molded foam density more closely approaches the so-called minimum fill density of the foam formulation.
Another way to reduce the cost of a foamed article is by improving (i.e., reducing) the cycle time for its production. The shorter the cycle time, the lower the cost of the foam article. One approach to accomplish shorter cycle times is to reduce the cure time, sometimes referred to as de-mold time, for the foam. To reach faster foam cure time high reactivity profile is needed by increasing the concentration of the catalysts. However, the reactive type of catalysts necessary for the low emission technology are mainly mono-functional in nature; which affect the chain growth during the polymerization process as they may act as chain inhibitors stopping the poly-addition of reactive species. Thus, the resulted polymeric network contains high concentration of short chain polymers and/or oligomers adversely affecting the main required characteristics needed for material's application such as the processing behavior or the targeted mechanical properties. In addition, the high concentration of catalysts may affect negatively the emissions results of the molded foam.
It would be desirable to provide a composition for producing resilient, flexible polyurethane foam which have a low density (at most 42 kg/m3), good sound absorption properties, low organic compound emissions, process easily, and de-mold quickly.