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
2000-08-21
2002-06-18
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S524000, C525S438000, C427S195000, C428S411100, C428S480000, C428S413000, C524S497000, C526S273000
Reexamination Certificate
active
06407181
ABSTRACT:
The present invention is directed to glycidyl (meth)acrylate copolymer/carboxylic acid functional polyester coating powders which cure rapidly at low temperatures. Such powder coatings are ideal for coating heat-sensitive substrates such as wood, paper, various plastics and other materials which cannot withstand the excessive heat/time conditions necessary to cure traditional coatings. The inventive powder coatings produce a finish which exhibit reduced gloss, in the range of low to medium, and which exhibit excellent weatherability.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,436,311 discloses a glycidyl (meth)acyrlate copolymer/carboxylic acid functional polyester coating powder which produces a weatherable, matte finish. This coating is described as being especially suitable for coating aluminum and steel because of the high temperatures needed for curing, specifically in the range of from 160° C. to 210° C., for long periods of time in an oven, such as up to 30 minutes. Such time/temperature parameters are unsuitable for heat-sensitive substrates, such as wood, which generally must be cured at temperatures of 170° C. or less for periods of no more than 5 minutes. To produce the matte finish described in the 311 patent, the epoxy content in the gylcidyl (meth)acrylate copolymer must be limited to between 0.3 and 2.2 milliequivalents of epoxy per gram of polymer. These values correspond to an epoxy equivalent weight of at least 450. This patent teaches that if the glycidyl (meth)acrylate copolymer has a higher epoxy content, then a gloss finish is obtained. However, the relatively high epoxy equivalent weight of the copolymer requires comparatively higher cure temperatures and longer cure oven residence times.
Recently, there has been a good deal of effort in the coatings industry directed to finding coatings for heat-sensitive substrates, such as wood, paper and plastics. A significant portion of the commercial market for coating powders is wooden or fiberboard cabinet doors such as those commonly found in kitchens. The coatings for this market must be both extremely durable because of heavy usage and weatherable because these surfaces are exposed to UV light which tends to cause the finish to yellow. Further, for aesthetic purposes, a reduced gloss (60° gloss in the range of 5 to 50: ASTM D523) finish is highly desired.
Conventional powder coatings require high temperatures to cure the resin(s), thus making them unacceptable for coating heat sensitive substrates. Compositions that are useful for heat sensitive substrates and which provide a smooth low gloss finish are based on bisphenol A epoxies. However, these compositions do not provide the UV stability that is required for certain applications, such as, for example, kitchen cabinets having a white surface finish. Over time, with exposure to sunlight, the surface finishes made from bisphenol A epoxies will fade, develop chalking or “yellow” out.
Accordingly, it is an object of the present invention to provide a low temperature curable powder coating which, once cured, exhibits a reduced gloss, weatherable coating.
STATEMENT OF THE INVENTION
In accordance with the present invention there is provided a low temperature curing coating powder which produces a reduced gloss, weatherable (i.e., UV-light resistant to eliminate fading, chalking and/or yellowing) coating on heat sensitive substrates such as, for example, wood, fiberboard, paper and certain plastics. The coating powder is a glycidyl (meth)acrylate/carboxylic acid functional polyester powder which incorporates a substantial amount of a glycidyl (meth)acrylate copolymer (A1) having a low epoxy equivalent weight, i.e., from 250 to 450, preferably from 250 to 400. It is surprising that despite the low epoxy equivalent weight of the copolymer, a low to medium gloss coating is obtained.
The present invention provides a coating powder, the composition of which contains an epoxy component and a carboxylic acid component. The epoxy component consists of a glycidyl (meth)acrylate copolymer having an epoxy equivalent weight of between 150 and 450, and is present at between 2 and 100 percent of the epoxy component. Further, the epoxy component may contain from 0 to 50 weight percent of a glycidyl (meth)acrylate copolymer having an epoxy equivalent weight of between 500 and 800. In addition, the epoxy component may contain from 0 to 20 weight percent of a crystalline aromatic epoxy having a melting point of between 80° and 150° C. The carboxylic acid component contains from 65 to 100 percent by weight of a carboxylic acid functional polyester having an acid number between 30 and 60 and a viscosity of 2500 cps or less at 200° C. Further, the carboxylic acid component may contain from 0 to 20 weight percent of a crystalline polycarboxylic acid or polyanhydride having an acid number between 150 and 400. In addition, an acrylic polymer having an acid number of between 100 and 400 may be present in the carboxylic acid component in an amount of between 0 and 15 weight percent. The stoichiometry of the carboxylic acid component to the epoxy component is between 0.7 and 2.7.
The carboxylic acid functional polyester B1 is synthesized or selected to have a low viscosity, i.e., from 2500 cps or less at 200° C. as measured on an ICI cone plate viscometer. Preferably, the viscosity is 2000 cps or less, more preferably 1500 cps or less, down to a viscosity which is too low to measure on an ICI cone plate viscometer. The low viscosity of the carboxylic functional polyester B1 in combination with the high epoxy functionality of at least part of the glycidyl (meth)acrylate copolymer of the epoxy functional component A provides for rapid fusing and curing of the coating powder, thus enabling application onto heat sensitive substrates.
In a first aspect of the invention, reduced gloss is achieved by employing, as the epoxy functional component A, a mixture of glycidyl (meth)acrylate copolymers having an epoxy equivalent weight between 250 and 450, preferably between 250 and 400, and between 10 and 50 weight percent of a glycidyl (meth)acrylate copolymer A2 having an epoxy equivalent weight of between 500 and 800. The presence of copolymer A1, having a low epoxy equivalent weight, provides for rapid curing at low temperatures, thus making the powder coating ideal for application onto heat sensitive substrates.
In a second aspect of the invention, at least 50 weight percent up to 100 weight percent of the epoxy component is a glycidyl (meth)acrylate copolymer A1 having an epoxy equivalent weight of between 250 and 450, preferably between 250 and 400. To reduce gloss, between 1 and 20 weight percent of the composition is a crystalline material which may be either part of the epoxy component or part of the carboxylic acid component B. Herein, “crystalline” is defined as being a component which exhibits a single sharp melting point on a differential scanning calorimeter (DSC) in the range of 80-150° C. The crystalline material is preferably a polycarboxylic acid B2 having a carboxylic acid group functionality of 2 to 4, and an acid number between 150 and 400, particularly a dicarboxylic acid such as sebacic acid. Alternatively, the crystalline material may be a crystalline aromatic epoxy (A3) such as bisphenyl epoxy, e.g., RSS 1407, sold by Shell.
The coating powder formulation may incorporate both a mixture of lower and higher epoxy equivalent weight glycidyl (methacrylate) copolymers A1 and A2 and an additional crystalline component A3 or B2.
Another means of reducing gloss is to incorporate into the carboxylic acid component B between 2 and 15 weight percent of an acrylic resin (B3) having an acid number between 100 and 400.
The proper selection of components will result in a medium to low gloss finish in a smooth cured coating, i.e., 60° gloss levels (per ASTM D523) of 50 or less, down to as low as 5.
DETAILED DESCRIPTION
The rapid fusion and curing of the coating powders according to the present invention is achieved by the high epoxy functionality of at least a substantial portion of the glyci
Daly Andrew T.
Haley Richard P.
Kozlowski Joseph J.
Muthiah Jeno
Nicholl Edward G.
Dawson Robert
Peng Kuo-Liang
Rohm and Haas Company
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