Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2000-10-26
2002-08-20
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S176000, C528S190000, C528S193000, C528S194000, C528S271000
Reexamination Certificate
active
06437087
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an unsaturated carboxyl group-terminated polyester resin for use in powder coating and more particularly concerns an aforesaid unsaturated polyester having suitable properties for use in a powder coating for wood, plastic, and other temperature-sensitive substrates.
Powder coatings offer the advantages of high coating efficiency, excellent mechanical properties and zero emission of volatile organic compounds. Carboxyl group terminated polyester resins for use in powder coatings are well known in the art, and in practice these polyesters are processed with epoxy resins to form binding agents for powder coatings. Unsaturated polyester resins are also finding use in powder coating where they are cured, through the unsaturation along their backbone, by UV or other irradiation. Carboxy terminated unsaturated polyester resins could be cured by both irradiation and reaction with epoxy compounds. These coatings are particularly useful since they can be cured at much lower temperature than conventional powder coatings. This saves in energy costs, reduces coating times, and allows powder coatings to be used on heat sensitive substrates such as wood and engineered wood, plastics, and prefabricated articles containing heat sensitive components such as motors and radiators. Curing by both epoxy reaction and irradiation will allow powder coatings to be formed which have much higher hardness than can be achieved by either method alone. This is particularly useful in applications subject to significant surface wear such as counter tops. Powder coatings based on polyester resins are particularly desirable in some applications because of their excellent appearance, resistance to overbake, mechanical properties and weatherability. They have found application in coating a wide range of metal substrates, and it would be highly desirable to be able to use powder coatings based on polyester resins on nonmetallic temperature sensitive substrates such as wood, engineered wood products and plastics. Many such temperature sensitive substrates are exposed directly or indirectly to sunlight or heat. Because of their superior weatherability and color stability, powder coatings based on polyester resins would be particularly effective for use with temperature sensitive substrates, for example, engineered wood products such as medium density fiber board. UV cured powder coatings based on unsaturated carboxy functional powder coatings would have the additional advantage of much better scratch resistance and durability.
Excellent surface smoothness and hardness and a low curing temperature are properties that a powder coating must have for use with temperature sensitive substrates. Existing powder coatings based on polyester resins generally possess the requisite properties of surface hardness and low curing temperature but do not form surfaces with the smoothness comparable to other finishing methods. The inability of powder coatings based on polyester resins to provide the requisite surface smoothness is generally the result of the high viscosities of the polyester resin bases. Achieving a surface smoothness that is equivalent to that achieved with other finishing systems such as liquid paints and laminates is essential in order for powder coatings based on polyester resins to be used effectively with temperature-sensitive substrates such as wood, engineered wood products and plastics.
In order to form a smooth finish, the powder coating must melt within a particular temperature range to permit timely and ample flow of the polymeric material prior to crosslinking. It is also essential that the formulated coating powders remain in a free-flowing, finely divided state for a reasonable period of time after they are manufactured and packaged. Most polyester resins that are used as binders for powder coatings are amorphous solids. The use of multiple components to produce resins having specialized properties generally reduces their potential for crystallinity. The resulting glass transition temperature (T
g
) of these resins must exceed the storage temperature to which the formulated powder will be exposed. When storage temperatures reach or exceed the T
g
, the amorphous resin begins to “cold flow”, resulting in agglomeration of the finely divided particles, thereby rendering the powder unsuitable for application. Therefore, the T
g
and melt viscosity are important considerations for powder coating resins.
Unsaturated polyesters must contain sufficient double bonds to achieve required property improvements most noticeably hardness. It is known that the hardness of fully cured unsaturated resins is a function of the number of double bonds found along the backbone of a particular polymer. For powder coating applications a high number of unsaturations, 4-6 per chain is desirable. Upon irradiation this will form a very highly crosslinked network and give very high surface hardness.
The viscosity and glass transition temperature of unsaturated polyester resins are critical properties for the ultimate performance of the formulated powder coating. Resins with a lower melt viscosity are desirable because they flow together and fuse better to provide a smoother and a more even appearance, and they are easier to process into a powder. It would be desirable to develop a powder-coating composition with substantially the same or better flow and/or appearance and lower viscosity under ordinary use conditions. This cannot be done at the expense of the glass transition temperature of the resin. A glass transition temperature of 45° C. must be maintained and a glass transition temperature of over 50° C. is preferred if the formulated coating is to remaining free flowing during storage.
It is therefore a general object of the present invention to provide an improved unsaturated polyester resin that is suitable as a base material for a powder coating for a temperature sensitive substrate.
More particularly, it is an object of the present invention to provide an improved carboxyl terminated polyester resin having a viscosity of less than 2000 centipoise as well as a sufficient glass transition temperature, acid number, and hydroxyl number to be useful in a powder coating for a temperature sensitive substrate.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims.
SUMMARY OF THE INVENTION
The present invention is an unsaturated carboxyl group terminated polyester resin for use in a powder coating and comprising (a) from about 20 to about 35 mole percent of an aromatic diacid component of which at least 75 mole percent is isophthalic acid, (b) from about 15 to about 30 mole percent of an unsaturated diacid component which is preferably fumaric acid, maleic acid, or maleic anhydride, (c) from about 25 to about 45 mole percent of a hindered diol component containing from 4 to 13 carbon atoms which is preferably neopentyl glycol, 2-methyl-1,3-propanediol, 2,2-butylethyl-1,3-propanediol, or combinations of the aforementioned, (d) from about 0 to about 15 mole percent of a 1,2 dihydroxyl component preferably ethylene glycol or 1,2-propanediol, and (e) from about 0 to about 20 mole percent of a cyclic diol component taken from the group cyclohexanedimethanol, 2,2-bis-[4-(2-hydroxylethoxy)phenyl]-propane, (ethylene oxide adduct of bis-phenol A), 2-[bis-(4-hydroxycyclohexyl)]-propane (hydrogenated bis-phenol A), and ethylene or propylene oxide adducts of bis-phenols, or other hydrogenated bis-phenols, preferably 2,2-bis-[4-(2-hydroxylethoxy)phenyl]-propane, and/or 2-[bis-(4-hydroxycyclohexyl)]-propane such that the resulting unsaturated carboxyl group terminated polyester resin has a glass transition temperature of 44° to 55° C., an acid number of 35-55 milligrams of potassium hydroxide per gram of resin, a hydroxyl number less than 10 milligrams of potassium per gram of resin, an average of 4-6 unsaturations per polymer chain, and a viscosity of les
Kulzick Matthew A.
Pretzer Wayne R.
Boykin Terressa M.
BP Corporation North America Inc.
Henes James R.
Kanady Mary Jo
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