1. Field Of The Invention
The present invention relates to fume ducts and more particularly to novolac epoxy resin and hardener compositions which enable an improved method for assembling fume duct sections wherein a composition is used as a sealant in constructing a circumferential joint bond between pairs of adjoining duct section ends.
2. Description Of The Related Art
Ductwork for corrosive vapor exhaust systems is used extensively in many diverse industries which utilize hazardous chemicals to process raw materials or perform manufacturing procedures, such as the semiconductor industry, the plating industry, and the pharmaceutical industry. Such ductwork also is required in the many research and development laboratories which use highly reactive, toxic or otherwise hazardous chemicals in conducting experiments. Use of such chemicals not only can put personnel in the work environment at risk to hazardous fumes, but also are potential sources of contamination of industrial processes or laboratory experiments. Consequently, vapors from such chemicals must be exhausted through leak-proof air ducts to
PATENT safely remove them from work areas. Duct installations can be very large, consisting of many thousands of feet of ductwork which may be manifolded and connected to multiple exhaust fans. Because of the wide diversity of chemicals used in industrial and research applications, it is extremely difficult to provide a single material for fabricating ductwork which can withstand all the chemicals to which duct interiors may be exposed. Materials which have been used heretofore to fabricate fume exhaust ductwork include black steel, galvanized steel and stainless steel, as well as plastic materials such as polyvinylchloride, polypropylene, coated materials, and fiberglass reinforced plastics (FRP's). Over the past forty years the trend in materials has been away from metals and coated metals and toward the use of plastics, particularly FRP's.
Various types of resins have been used in manufacturing FRP's including bisphenol fumarates, epoxies, chlorendic anhydrides, isopthalic or orthopthalic resins, and vinylester resins. Certain classes of resins are resistant to certain families of chemicals, but no single resin can resist all the chemicals used in industries such as semiconductor manufacturing. For example, polyesters generally have good resistance to acids and, to some degree, to caustics. However, they generally do not have good
PATENT resistance to solvents, particularly halocarbons. Epoxies generally exhibit good resistance to caustics and solvents, but do not have good resistance to strong mineral acids. Various combinations of phenol/aldehyde resins have good resistance to most acids, but not to highly reactive combinations such as concentrated sulfuric acid and an oxidizer such as hydrogen peroxide. These resin systems also have poor resistance to liquid caustics. Thus, FRP resin compositions generally are poor choices for fume exhaust systems handling such types of materials.
In addition to the problem of providing a duct material capable of resisting broad classes of chemical vapors, there is also the problem of providing adequate resistance to fire. Unlike metallic ducts, plastic ducts exhausting chemicals which can react exothermically with themselves or with duct surfaces pose the risk of being set on fire. A problem common to all plastics has been flammability. Plastics can burn rapidly and produce much smoke, creating hazards of their own. The plastics industry often refers to certain classes of materials as "fire-retardant." Typically such materials incorporate heat absorbent fillers, heat sinks such as aluminum trihydrate, and most commonly, halogenated resin systems that include antimony or boron compounds which interfere with combustion at the interface between a plastic surface and ambient air by functioning as a free radical trap depriving the surface fuel of oxygen.
Resin systems and plastic fume exhaust ducts are described in U.S. Pat. Nos. 4,053,447; 4,076,873; and 4,107,127. In general, phenols and similar ring-structured molecules have excellent fire resistance characteristics and also generate low quantities of smoke. Fabrication costs are high for ducts made from phenolic resins because their curing generally requires heat and/or pressure. However, use of the phenol resorcinol in resin compositions can reduce or eliminate the necessity for using heat and pressure; some formulations can be cured at ambient (room) temperature.
As described generally in the above-cited references, various types of aldehydes when used in conjunction with resorcinol or phenol/resorcinol (PRF) combinations enable curing of FRP resins. An excess of aldehydes to the hydroxyl radicals contained within the mix is necessary. Paraformaldehyde, furfuraldehyde or other aldehydes can be used alone or in combination with various types of phenol/resorcinol mixes.
U.S. Pat. No. 5,298,299, which is incorporated in its entirety herein by reference, is directed to a composite fume duct having both good chemical resistance and good fire resistance properties. Ducts made in accordance with the invention described in that patent are generally tubular with a diametral size in the range of 2 inches to 84 inches, and have an inner laminate portion of chemically resistant material covered by and integral with an outer laminate portion of fire retardant material. The inner laminate is comprised of material such as fiberglass which is saturated with a chemically resistant resin such as a halogenated vinyl ester. The outer laminate which covers the inner laminate is similarly comprised of fabric or fiberglass material which is combined with a resorcinol or phenol/resorcinol type fire-retardant resin.
The dual-laminate duct is formed by first coating a mylar wrapped mandrel tool with the chemically resistant resin and then wrapping the mandrel with successive layers of fabric material saturated with the resin. The outer fire-retardant laminate is then formed directly over the inner laminate by applying successive layers of a suitable fabric material saturated with the fire-retardant resin. The composite duct structure is then allowed to cure and harden before being removed from the mandrel.
Ducts are fabricated as sections of standard length(s) which are transported to a job site and assembled there. Since a leak-proof joint is required between each pair of contiguous duct sections, even the smallest installation will have a considerable number of such joints. Therefore, reducing the time needed to assemble multi-section ductwork is significant to improving the profitability of businesses which install fume ducts. Because mechanical interfacing of section ends cannot by itself prevent leakage, a sealant must be applied circumferentially to each interface. Heretofore, the most time-consuming step in joining dual-laminate sections has been preparing the resin-impregnated surfaces to which the sealant must bond in order to effect a leak-proof seal. Specifically, these are surface areas near the ends of each duct section including: the opposed end portions of the inner laminate surface which is typically fiberglass saturated with a halogenated vinyl ester resin; the opposed end portions of the outer laminate surface which is typically fiberglass saturated with a phenol/resorcinol resin; and the exterior surfaces of a "slip" collar interposed internally between a pair of end sections, the collar surfaces typically being fiberglass saturated with a vinyl ester resin. Unless surfaces to be mated were first sanded, the interposing sealant layer would not adhere to the surfaces uniformly, resulting in porosities in the hardened sealant through which fumes could leak.
Each joint must not only prevent fumes from escaping during day-to-day operation, but also must maintain integrity after prolonged exposure to corrosive or otherwise reactive chemicals. Also, a joint must not fail catastrophically in the event a flame propagates through the ductwork or, if exposed directly to heat, become a source of smoke particulates or other contaminants.
Consequently, providing a sealant composition which facilitates joint assembly, withstands exposure to chemicals, and/or withstands exposure to flames is important to improving the state of the art of assembling and maintaining fume duct installations.