Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-07-28
2002-08-13
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S934000, C525S375000
Reexamination Certificate
active
06433099
ABSTRACT:
The present invention relates to powder coating compositions which may be applied to heat sensitive substrates, such as wood, fibreboard or the like. These compositions produce a fine, uniformly distributed textured finish on these heat sensitive substrates.
BACKGROUND OF THE INVENTION
Powder coatings are dry, finely divided, free flowing solid materials at room temperature. They have gained considerable popularity in the surface coatings industry for numerous reasons. For one, since they are virtually free of the harmful fugitive organic solvents which are normally present in liquid coatings, they are considered safer to handle and apply. Further, their use results in less damage to the environment caused by the release of potentially harmful solvents.
Powder coatings are very convenient to use in that they may be easily swept up in the event of a spill. No special containment devices or procedures are needed as would be required for handling liquid coating formulations. Further, powder coatings are essentially 100% recyclable. Over-sprayed powders can be fully reclaimed and recombined with the powder feedstock. This factor provides for a more efficient industrial process and substantially reduces the amount of waste generated. In contrast, oversprayed liquid coatings are not recycled which results in an overall increase in the amount of waste generated. This adds significant costs to the coating process and further burdens the environment in general by increasing the amount of hazardous waste being generated.
The furniture making industry has long desired a coating for heat-sensitive substrates which, when cured, provides a uniformly distributed, fine textured finish. Thermofused vinyl laminates have traditionally provided very fine textured finishes. However, the process of applying vinyl laminates to wood-like substrates is difficult to control and the uniform quality of the surface finish is often inconsistent, especially around the corners and edges of the substrate. Attempts to solve these various problems with powder coatings have, heretofore been unsuccessful.
Historically, powder coatings have been utilized with metallic substrates which can withstand the high temperatures required to cure the coating. Recently, however, coatings have been developed which permit curing at lower temperatures, thus substantially reducing both the chance of charring and the excessive outgassing of moisture from the substrate. A controlled amount of moisture in the wood substrate is essential to the formation of a uniformly bonded coating. U.S. Pat. No. 5,721,052 discloses an epoxy based powder coating system which is able to be cured at lower temperatures. However, in order to give the cured coating a finely textured finish, conventional texturizing agents are employed. Examples of such texturizing agents are PTFE, various PTFE/wax mixtures, organophilic clays and modified rubber particles. These materials, however, produce textures which are too bold when compared to vinyl laminates and often look mottled or blotchy when applied over a large surface such as a cabinet door or counter top. U.S. Pat. No. 5,212,263 discloses a fine texture finish without the use of conventional texturizing agents, but its system employs a mixture of an epoxy resin, methylene disalicylic acid and isopropyl imidazole Bis-A epoxy resin adduct that must be cured at 375° F. Because of the high cure temperature, metal is disclosed as the substrate of choice.
Another problem encountered when searching for a powder coating for wood substrates is the relatively narrow temperature differential between the extrusion process, which is required to uniformly mix the various coating ingredients prior to creating the powder, and the cure temperature. For example, extrusion temperatures may reach 250° F. while the desired cure temperature may only be 250-275° F. Careful control of the extrusion and cure temperatures is essential.
STATEMENT OF INVENTION
It is therefore an object of the present invention to provide a powder coating suitable for application onto heat-sensitive substrates which, when cured, exhibits a uniformly distributed fine textured finish.
It is another object of the present invention to provide a method of coating fine textured finish onto heat sensitive substrates, particularly wood substrates, at cure temperatures of about 300° F. or lower for acceptable curing oven dwell times by use of the inventive powder coating having rapid cure and/or low temperature cure properties without damaging or adversely affecting the physical or physiochemical properties of the substrate.
The present invention provides a powder coating consisting of a glycidyl methacrylic (GMA) resin which is cured with either difunctional or trifunctional carboxylic acids, and 1,3,5-tris (2-carboxyethyl)isocyanurate at low temperatures in the presence of a catalyst. This powder coating may be applied to the surfaces of wood substrates, without damage thereto, to provide a uniform fine textured finish without the need to add a texturizing agent.
DETAILED DESCRIPTION
The powder coating of this invention is intended for use on heat sensitive substrates such as, for example, wood and wood-like materials. For the purposes of this invention, wood may be defined as any lignocellulosic material whether it comes from trees or other plants and whether it be in its natural forms, shaped in a saw mill, separated into sheets and made into plywood, chipped and made into particle board or had its fibers separated, felted and compressed.
The glycidyl methacrylate (GMA) resin is in the form of a copolymer which may be produced by copolymerizing between 20 and 100 wt % gylcidyl acrylate or glycidyl methacrylate and between 0 and 80 wt % other alpha, beta ethylenically unsaturated monomers, such as methyl methacrylate, butyl methacrylate and styrene. Such resin typically has a weight average molecular weight of from about 3,000 to about 20,000, and preferably from about 3,000 to about 20,000, as determined by gel permeation chromatography. The glass transition temperature Tg) of the GMA is preferably between about 40° and 70° C. Its viscosity is referably in the range of between about 10 and 500 poise, and most preferably between about 30 and 300 poise at 150° C., as determined by an ICI Cone and Plate Viscometer.
The GMA can be prepared under traditional reaction conditions known in the art. For instance, the monomers can be added to an organic solvent such as xylene and the reaction conducted at reflux in the presence of an initiator such as azobisisobutyronitrile or benzoyl peroxide. An exemplary reaction may be found in U.S. Pat. No. 5,407,706. In addition, such resins are commercially available under the trademark “ALMATEX” from Anderson Development Company of Adrian, Mich. The GMA resin is present in the powder coating composition in an amount ranging from about 20 to 100 phr (parts per hundred parts resin plus curing agent).
The choice of the curing agents is critical to achieve the desired end product manufactured via the narrow process parameters required by heat sensitive substrates. The 1,3,5-tris-(2-carboxyethyl)isocyanurate (TCI) can be prepared by the reaction of cyanuric acid and acrylonitrile as set forth, for example, in U.S. Pat. No. 3,485,833. In the alternative, TCI may be acquired commercially from Cytec Industries, Inc. of Stamford, Conn. It may be added to the powder coating composition in an amount ranging from 1 to 20 phr, preferably 12 to 18.
A second curing agent selected from the group consisting of difunctional or trifunctional carboxylic acids and polyanhydrides of aliphatic dicarboxylic acids may also be utilized. The functionality number relates to the number of —COOH moieties on the molecule. Preferred are the difunctional carboxylic acids, and sebacic acid and polyanhydrides of aliphatic carboxylic acids are the most preferred. These products are well known curing agents which came readily commercially available, While the second curing agent is a desired component of the inventive formulation, it has been found that the
Nicholl Edward G.
Ottinger Jill A.
Rohm & Haas Company
Wilson Donald R.
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