Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2000-10-26
2003-04-29
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S480000, C525S438000, C528S272000
Reexamination Certificate
active
06555226
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to a carboxyl group-containing polyester resin for use in a powder coating and more particularly concerns an aforesaid polyester having suitable properties for use in a powder coating for wood 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-containing 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. 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.
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 beings 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.
The viscosity of the epoxy-polyester powder-coating resins presents an area for improvement. The polyester resins have a relatively high viscosity (generally greater than 20 poise in a polyester designed for use in a mixture containing 70 weight percent polyester) under curing conditions. (Viscosity is measured using an ICI cone and plate viscometer at 200° C.) 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.
It is therefore a general object of the present invention to provide an improved 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 group-containing polyester resin having a viscosity of less than 1500 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.
It is a related object of the present invention to provide an improved powder coating based on carboxyl group-containing polyester resins for use on temperature sensitive nonmetallic substrates.
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 a carboxyl group-containing polyester resin for use in a powder coating and comprising (a) from about 45 to 55 mole percent of an aromatic diacid component of which at least 90 mole percent is isophthalic acid, (b) from about 40 to about 50 mole percent of a diol component, (c) from about zero to about 7 mole percent of an aliphatic diacid containing from 4 to 13 carbon atoms, and (d) from about zero to about 3 mole percent of a branching agent having at least 3 alcohol or acid groups or both, such that the 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, and a viscosity of less than 1500 centipoise when measured at 200° C. using ICI cone and plate viscometer. The present invention is also a powder coating comprising the aforesaid polyester resin.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Polyester resins find use in polyester-isocyanate, polyester-epoxy hybrid, and polyester-triglycidylisocyanurate systems. The stability of the finished polyester resin-based powder coating material during storage, the flow properties of the enamel during the cure cycle, and the ultimate performance of the cross-linked coating are all dependent upon the composition of the polyester binder resin.
The carboxyl group-containing polyesters of the present invention are prepared by well known polycondensation methods, for example, by esterification or interesterification, optionally in the presence of typical catalysts such as dibutyl tin or tetrabutyl titanate whereby, through a suitable choice of reaction conditions, polyester resins having the desired properties are obtained.
After being formed, the polyester resins of the present invention have 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 of less than 10 milligrams of potassium hydroxide per gram of resin, and a viscosity of less than 1500 centipoise.
It has been found that this particular combination of glass transition temperature, acid number, hydroxyl number and viscosity render a polyester resin of the present invention especially useful in powder coatings for use on temperature sensitive metallic substrates. The glass transition temperature must exceed the storage temperature to which the formulated powder will be exposed. Furthermore, in order to form a smooth finish, the powder coatings must melt and have a sufficiently low viscosity within a particular temperature range to permit timely and ample flow of the polyester resin prior to crosslinking.
However, the viscosity of the polyester resin is influenced by a number of conflicting factors. For example, the viscosity of a resin is lower if its molecular weight is lower. However, the acid number of the resin is too high if the molecular weight of the resin is too low. Similarly, the viscosity of a resin is lower if its branching is eliminated or reduced. However, increased levels of
Kulzick Matthew A.
Pretzer Wayne R.
BP Corporation North America Inc.
Dawson Robert
Feely Michael J
Kanady Mary Jo
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