Plastic and nonmetallic article shaping or treating: processes – With step of cleaning – polishing – or preconditioning...
Reexamination Certificate
1999-06-14
2002-04-16
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of cleaning, polishing, or preconditioning...
C264S051000, C264S238000, C264S338000, C106S038220, C106S287250, C249S115000, C427S133000, C427S235000, C427S236000, C427S353000, C427S387000, C427S421100
Reexamination Certificate
active
06372160
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to mold release compounds, and more specifically to such compounds which are aqueous and suitable for use with molds for foamed polymeric products.
Release agents are materials that are applied to the surface of a mold in order to increase the removability of the molded products from the mold surface. It is usually desirable to deposit a very thin coating of the release agent on the mold surface. To facilitate the application of such a thin film, the release agent is commonly dispersed in a volatile carrier. After the release agent and carrier have been applied to the mold surface, the carrier evaporates, leaving the desired thin film of release agent.
The term “release agent” as used herein shall refer to the material that affects the release of the molded part. The release agent is the functional component that remains on the mold surface after the carrier has evaporated. The term “release compound” as used herein shall refer to the combined release agent and its carrier system.
Carriers commonly used in release compounds can be divided into two categories: hydrocarbon solvent carriers and aqueous carriers. The term “solvent-based” as used herein shall refer to release compounds that use hydrocarbon solvents as the carrier. The terms “aqueous” or “water-based” as used herein shall refer to release compounds that use water as the carrier.
There are a number of disadvantages to the use of solvent-based release compounds. The evaporation of hydrocarbon solvents has a negative effect on the environment, and, in many areas, government restrictions limit the quantity of hydrocarbon emissions. Handling hydrocarbon solvents also poses health risks to personnel. Further, most hydrocarbon solvents are flammable, thus a fire risk is associated with their use. Also, hydrocarbon solvents are more expensive than water. Therefore, water-based release compounds are a highly desirable alternative to solvent-based release compounds.
Various substances have been used as release agents. Soaps and organo-metal salts for which solubility may range from water-soluble (hydrophilic) to water-insoluble (hydrophobic) have been used as release agents. Various organic oils, waxes and pastes have also served as release agents. Due to their low surface energy, silicones are frequently used as release agents. Silicones may take the form of oils, waxes or resins which range from the extremely hydrophobic species which contain only methyl functionality on a polysiloxane structure, to hydrophilic species such as methyl-polyol functional polysiloxane and polysiloxane with organo-salt functionality. There exist applications where each of these release agents is said to perform well. However, some of the most effective release agents are quite hydrophobic. Organo-functional silicones (including dimethyl silicones) and saturated hydrocarbons with little or no oxygen content are examples of hydrophobic materials that are very effective in many release applications.
In order to formulate a water-based release compound of hydrophobic release agents, it is necessary to use rather hydrophilic emulsifiers or surfactants. If a sufficient quantity of emulsifier is mixed with a hydrophobic release agent, a stable emulsion or dispersion can be formed by gently mixing the ingredients with water. However, a quantity of emulsifier sufficient to provide a stable emulsion with gentle mixing generally interferes with the release effectiveness of the release agent once the water carrier has evaporated. When a quantity of emulsifier low enough to avoid substantially hindering the release effectiveness of the release agent is used, intense mixing is required during the emulsification process. Emulsions that require intense mixing in order to form a stable emulsion are referred to as high-energy emulsions.
The fact that high-energy emulsions can deliver highly effective release agents with minimal emulsifier to a mold surface while using water as the carrier makes them an important class of release compounds. High-energy emulsions find use in die casting, plastic molding, natural rubber latex foam molding, polyvinylchloride (PVC) foam molding, and other release applications.
A class of molding operations that is particularly difficult to release is reactive molding. Reactive molding operations involve mixing chemicals (usually liquids) that react within the mold to form a solid part. The reactive chemicals being molded are usually quite adhesive. Additionally, the chemical reaction mechanism of the material being molded is often sensitive to water or emulsifiers. When using a water-based release compound, a reaction may occur between residual water or emulsifiers (or both) and the chemicals being molded. Such competing reactions usually cause imperfections in the molded part. Thus, the use of water-based release compounds in reactive molding operations presents problems.
The process of molding foamed plastic or rubber incorporates a method of trapping a gas within the curing part. The gas may be generated as a by-product of a chemical reaction, by boiling a volatile component or simply by mechanical gas entrainment just prior to filling the mold. The gas ideally forms foam that is uniform throughout the part being molded. It is necessary for the foam formed within the mold to be stable until the part cures so that the physical properties of the molded part are uniform. Often, residual water and emulsifiers from water-based release compounds left on the mold surface, working individually or in combination, act to destabilize the foam within the mold. This results in surface imperfections, voids and non-uniform foam density throughout the molded part, as discussed further below.
The process of molding polyurethane parts is an application for which the use of water-based release compounds has proven particularly problematic. The reactive materials used in polyurethane processes can chemically react with water and with the hydroxyl groups that are often found in emulsifiers. Additionally, the foam within foamed polyurethane parts is sensitive to residual water and/or emulsifiers on the mold surface. For example, water remaining in the mold release composition prior to pouring results in competitive reactions between the water/isocyanate and the polyol/isocyanate. This reaction of water with isocyanate results in carbon dioxide generation which leads to surface and subsurface defects in the foam product including bubbles, blisters, voids and, if severe enough, foam collapse.
In U.S. Pat. No. 4,473,403 issued to Wesala, a water based release compound is disclosed for use in polyurethane foam molding applications. In the Wesala invention, the complete evaporation of water is assured by the inclusion of alcohol, which acts as an evaporation accelerator in the release compound formulation. However, this inclusion of volatile organic alcohol as an evaporation accelerator is counterproductive to the goal of eliminating organic emissions (a primary motive for using water-based release compounds).
In U.S. Pat. No. 4,609,511 issued to Fischer and Krug, a method of suppressing the competing reaction between water and isocyanate in polyurethane molding operations is proposed. This method involves masking of the water molecules through the effect of lyotropic mesomorphism. It is explained that lyotropic mesomorphism is a state where substances simultaneously have properties of liquids and crystalline bodies. The process of the '511 invention is accomplished by adding masking agents to release effective substances (release agents). Suitable masking agents are considered to be conventional surfactants (emulsifiers) which include fatty alcohol polyglycol ethers, particularly ethoxylated fatty alcohols with an HLB-value of 8 to 15. Some release effective substances that are considered suitable for use in the '511 invention include waxes, greases, fats, and silicone compounds.
However, in sharp contrast to the '511 invention, in U.S. Pat. No. 4,783,296 issued to Fischer,
Genesee Polymers Corporation
McDowell Suzanne E.
Young & Basile P.C.
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