Compositions: coating or plastic – Coating or plastic compositions – Carbohydrate or derivative containing
Reexamination Certificate
2001-07-20
2004-08-17
Brunsman, David (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Carbohydrate or derivative containing
C106S197010
Reexamination Certificate
active
06776831
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a release coating composition that may be applied to a film that may then be used as a substrate useful for applications requiring release for a broad range of temperatures and high humidity conditions, which temperatures may range from about 20° C. to about 210° C. These applications include release substrate used in the manufacture of calendared cured sheet rubber and molding paste composites, such as sheet molding compound (SMC), thick molding compound (TMC), bulk molding compound (BMC) and fiberglass composites.
BACKGROUND OF THE INVENTION
In the rubber industry, sheets of cured rubber compound are prepared by a calendaring process. Typically these sheets are from about 100 to about 400 feet in length. The uncured rubber sheet is laid onto a supporting interleaf film or sheet and then the two sheets are wound onto a mandrel. The interleaf is usually cellophane or silicon coated paper. The interleaf does not melt at the curing temperature and prevents the sheets from fusing with each other during the curing process. Sometimes talc or zinc stearate is applied to the interleaf to enhance release of rubber sheets from the interleaf after curing. Subsequently, the roll of rubber and interleaf can be over wound and held under tension using an over-wrap, which can be any film or cloth having good tensile properties that tends to shrink at oven curing temperatures. The cured sheet rubber may be used as components for aircraft engines and gaskets for rubber roofing membranes. Teflon® sheets, talc dust, and cloth are commonly used as interleaves in the rubber industry.
SMC is a composite material and usually comprises crosslinkable polymeric resin, most often unsaturated polyester resin; styrene monomer, plus catalyst; particulate filler, such as calcium carbonate; chopped glass fiber reinforcement; and various other additives in minor amounts, such as pigments and other modifiers.
The manufacture of SMC begins by laying the paste comprising all ingredients except the glass fibers, on a bottom carrier or release sheet, i.e., a film. The glass fibers are poured on top of the resin. More paste is poured over the glass fibers. A top carrier release sheet is laid down, and the edges of the top and bottom sheets are folded over to form a sandwich. The film and hence the composite is then kneaded to mix the glass fibers and the paste. The sandwich is then festooned (folded back and forth in a continuous fashion) into a bin and stored for up to about 14 days to cure or mature. Satisfactory results may be obtainable after as little as 2.5 days, but often more time is required. During this time the viscosity of the composite increases significantly (approximately ten-fold).
At the end of the curing period, the carrier release films, top and bottom are stripped away, the solidified SMC is cut and put into a heated press. In roughly one minute or less, out comes a semi-finished product, such as an auto part, for example, an automobile hood.
TMC is produced by a different machine and a process different from those used for producing SMC. Although TMC is prepared as a continuous length of material, it is cut into slabs for curing and storage because it is thicker than SMC. SMC is usually 1″ thick, but may range from ¼″ to 3″ in thickness. TMC may range from ½″ to 4″ in thickness. TMC is stronger because some of its fiberglass fibers may be positioned vertically, and more filler may be added. A most significant difference between SMC and TMC is that in making TMC, the glass fibers are mixed with the paste prior to being deposited on the carrier or release film, and thus no kneading of the composite sandwich is necessary when TMC is made into slabs. This therefore places different requirements on the carrier or release film as tear strength may not be as critical for carrier release film used to make TMC.
BMC is also a composite material of resins, fillers and reinforcements. Typically, it comprises 30% resins, 50% fillers and additives and 20% reinforcement, such as glass fiber. It may also contain catalysts. The high filler loadings can provide improved stiffness and fire retardence. BMC is manufactured by preparing a putty-like molding compound comprising the above-noted components in a “ready to mold” form. Molding pressures usually range from about 350 to 2000 psi at temperatures of between 250 and 350° F. BMC can be made into precise shapes with various types of inserts, and therefore the moldings can be extremely complex. One limitation of BMC is the loss of strength caused by degradation of glass fiber reinforcements during energy-intensive mixing.
BMC is primarily used as a replacement for cast metals. The actual physical characteristics of BMC are determined primarily by the choice of resin and desired end use. Possible end uses include electrical grade; low shrink/general purpose; appliance/structural; low profile; automotive grade; and corrosion resistant. Major applications of BMC include air conditioner components; pump housings; circuit breakers; computer and business equipment components; garbage disposal housings; motor parts; power tools; gear cases; electrical insulators; and circuit covers.
In selecting a carrier release film there are some basic requirements or properties that are preferably met for the film to be suitable. While styrene barrier, moisture barrier, and mechanical strength are relevant, most important are release from the paste composite, be it SMC, BMC, or TMC, and the cost of the release film.
Nylon films represent a potential replacement for silicon-coated paper and cellophane as interleaves in the rubber calendaring industry, because of their high tensile strength. However, the tendency of currently manufactured nylon films to stick to rubber compounds both cured and uncured limits their use in a rubber release application. Apart from sticking to the sheets of rubber, the latter film sometimes causes wrinkles on the surface of the cured rubber. It is speculated that gases emanated during curing of rubber cause such wrinkles.
Cellulose ethers are water-soluble polymers derived from cellulose. A commercially available cellulose ether is available under the Methocel® brand from The Dow Chemical Company. These products are available in various viscosity grades, ranging from 3 to over 200,000 mPa's. Generally, these viscosities refer to the viscosity of a 2% Methocel® solution in water at 25° C. The methylcellulose products include hydroxypropyl substituted cellulose ethers. Such products are also available from other sources such as China Yixing Kaili Chemical Pharmaceutical Factory of Yixing city, Jiangsu, China; Carbomer Inc of Westborough, Mass.; and Penta Mnfg. Co. of Livingston, N.J. Methocel® products are used as mold-release agents, stabilizers, and thickeners in rubber latexes, where they contribute also to more uniform drying and less pinholing (see Dow METHOCEL® Cellulose Ethers Technical Handbook available from The Dow Chemical Company Website, July 2000).
BACKGROUND ART
Various attempts have been made to make and coat non-stick coatings to film or film structures used for high temperature applications. Some of the prior art patents pertaining to release coatings are summarized hereafter:
U.S. Pat. No. 5,139,835 to Kitamura et al discloses a synthetic resin laminated paper which makes it possible to recover paper (or laminated film) materials easily and rationally. The adhesion-release control agent layer interposed between the polyethylene film and paper layer can be polyvinyl alcohol, silicone based compound, or a reaction product of an organopolysiloxane compound having at least one double bond which has reacted with said hydrogen atom.
U.S. Pat. No. 3,503,773 to Bisschops et al discloses a process for forming films or foils using a high-gloss-surface or the “casting layer”. The film-forming polymer solution is applied to the casting layer and at the end of the process the polymer film is stripped off the casting layer. The casting layer is a m
Chopra Divya
Lang Theodore John
Smillie Benjamin Andrew
Brunsman David
DuPont Canada Inc.
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