Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
2002-10-04
2004-09-14
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S111000, C522S112000, C427S508000, C427S512000, C428S480000, C428S482000, C428S500000
Reexamination Certificate
active
06790876
ABSTRACT:
The present invention concerns powder compositions hardenable by radiation usable for preparing paints or varnishes, specific amorphous polyesters which are comprised within these powder compositions, the use of the paints or varnishes comprising the powder compositions for coating an article as well as articles coated with the hardened paints or varnishes of the present invention. The powder compositions of the present invention are especially suited for coating over metal and heat-sensitive substrates and combine, upon melting at low temperatures and curing by radiation, a series of properties such as good flow and film hardness along with an outstanding solvent resistance.
Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings. Despite their many advantages, nowadays thermosetting powder coatings generally are cured at temperatures of at least 150° C. Below this recommended temperature the coatings have poor appearance as well as poor physical and chemical properties. In consequence of this restriction, powder coatings are generally not employed in coating heat-sensitive substrates such as wood and plastic or assembled metallic parts containing heat-sensitive components. Heat-sensitive substrates or components both demand low curing temperatures, preferably below 140° C., to avoid significant degradation and/or deformation.
Low temperature UV-curable powders have recently been proposed as a solution to this problem.
The use of unsaturated resins, eventually in combination with unsaturated oligomers, as a binder for radiation curable powder coatings already is disclosed in the prior art. Specifically UV curable powder coating compositions derived from ethylenically unsaturated group containing polyesters, polyesterurethanes or epoxy resins, among others, have been illustrated.
WO 98/18862 is concerned with powder compositions hardenable by radiation usable as paint or varnish comprising a mixture of at least a semi-crystalline polyester containing methacryloyl groups and of at least an amorphous polyester containing methacryloyl groups, comprising the reaction products of glycidyl methacrylate and a semi-crystalline or amorphous polyester containing carboxyl groups.
EP-A-0 702 040 discloses a binder for powder coatings comprising solid unsaturated polyesterurethaneacrylates obtainable by reacting a di-isocyanate with an hydroxyalkyl (meth)acrylate and a hydroxyl group containing polyester.
In U.S. Pat. No. 5,565,246 a method of forming heat-resistant raised print on a substrate using a thermographic radiation-curable powder is disclosed. The radiation-curable powder comprises an acrylated epoxide preparable by the reaction of acrylic or methacrylic acids with an epoxy resin such as bisphenol A-epichlorohydrin epoxy polymer. In addition, the composition may include up to 20% acrylated urethanes prepared by the reaction of toluene diisocyanate with polyols, acrylic acid or hydroxyethyl methacrylic acid.
Radiation curable powder coatings for use as e.g. glavano resists are disclosed in EP-A-0 286 594. The synthesis of an acrylated epoxy resin derived from a bisphenol A based epoxy resin and acrylic acid is exemplified in example 4 of this document.
U.S. Pat. No. 4,129,488 discloses powder paint coatings suitable for ultraviolet curing and comprising specific spatial arrangements of ethylenically unsaturated polymers. The unsaturated polymer is a specific epoxy-polyester polymer having a molecular weight of at least about 1000 providing a suitable crystallinity to the free flowing powder. By way of comparison a powder based on a blend of an acrylated epoxy resin and a semi-crystalline acrylated polyester resin having a melting point of 120° C. is illustrated in example 4. The blends exhibited a poor surface film effect which had poor flow out and orange peel. Known radiation curable powder coatings, especially developed for those applications where an outstanding flexibility is needed, do meet the requirements for solvent resistance, as measured in the methyl ethyl ketone-rub test, yet fall short due to gloss decrease and blistering, for those tests as described below, where solvent resistance is evaluated for a paint film being saturated with solvents. These short comings exclude the nowaday UV curable coatings from being used in the furniture industry.
A typical example of such a test is the “MEK impregnation test”. In this test a 10×10×5 mm felt pad is fully soaked in methyl ethyl ketone (MEK) and placed on the paint surface with a film thickness between 50 and 60 micrometers. Covered by, but not in contact with a watch glass or small Petri dish, the felt pad is kept for 1 hour in contact with the paint film. After 1 hour, the coating is evaluated by comparing the visual assessment and gloss, measured according to ASTM D523, with the initial values.
Another test enabling to quantify the solvent resistance of a solvent-saturated paint film consists in placing a 55 mm diameter absorbent cotton, fully soaked with acetone, on the paint surface and covering it with a watch glass or a small Petri dish. After a contact time of 20 seconds, the cotton is removed and the panel is allowed to dry in an air ventilated oven standing at 50° C. for 30 seconds. Thereupon the pencil hardness accordingly ASTM D3363-92A is measured and compared to the initial value (further called “the acetone test”).
It is accordingly an object of the present invention to provide a powder coating composition, capable of being cured by radiation upon melting, which does not result in a coating exhibiting the above described problems. Specifically the powder coating composition should provide a paint film upon curing which exhibits an excellent solvent resistance even if saturated with solvents. Moreover, the film upon curing the powder coating composition should have an excellent combination of physical properties such as smoothness, flexibility, hardeness and resistance to yellowing.
It now has been surprisingly found that radiation curable powder coating compositions based on a binder comprising a particular mixture of at least one particular unsaturated amorphous polyester and at least one particular unsaturated polyphenoxy resin exhibit upon curing an excellent combination of physical properties such as smoothness, flexibility, hardness, resistance to yellowing, and above all an outstanding durability for the MEK impregnation test as well as the acetone test.
Thus, the present invention provides a radiation curable powder composition which comprises:
a) 10 to 90 weight percentage of at least one (meth)acryloyl group containing amorphous polyester;
b) 10 to 60 weight percentage of at least one (meth)acryloyl group containing polyphenoxy resin; and
c) 0 to 30 weight percentage of an ethylenically unsaturated oligomer and/or at least one (meth)acryloyl group containing semi-crystalline polyester;
each based on the total weight of the components a), b) and c).
The (meth)acryloyl group containing amorphous polyester a) in the powder composition of the present invention is e.g. obtainable from the reaction of a di-isocyanate with an hydroxyalkyl(meth)acrylate and a hydroxyl group containing polyester or from the reaction of glycidyl(meth)acrylate with a polyester containing carboxyl groups and is preferably composed of a polyacid constituent which contains at least 40 mole percentage of terephthalic acid or isophtalic acid, alone or in admixture, and of a polyol constituent which contains at least 20 mole percentage of neopentyl glycol.
The hydroxyalkyl(meth)acrylate used for reaction with the di-isocyanate in the above reaction is preferably selected from hydroxyethyl(meth)acrylate, 2- or 3-hydroxypropyl(meth)acry-late, 2-, 3- and 4-hydroxybutyl(meth)acrylate, etc. The di-isocyanate used for the reaction with the hydroxyalkyl(meth)acrylate and the hydroxyl group containing polyester in the above reaction is preferably selected from 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cycloh
Buysens Kris
Knoops Nele
Maetens Daniel
Moens Luc
Nutter Nathan M.
UCB S.A.
Wenderoth Lind & Ponack LLP
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