Radio hardenable powder paints

Details Radio hardenable powder paints Radio hardenable powder paints

2002-10-22

2004-09-07

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...

C522S107000, C427S407200, C427S408000, C427S409000, C427S487000, C427S496000, C427S500000

Reexamination Certificate

active

06787581

ABSTRACT:

The invention relates to powder coating materials curable with high-energy radiation and to a method of coating substrate surfaces with such a powder coating material.
The coating of any substrates with radiation-curable powder coating materials, preferably UV-curable powder coating materials, is gaining increasingly in interest. Advantages are expected from the fact that, in contrast to systems which cure by heat alone, there is a theoretical separation of the melting process and the curing reaction.
In the practical development of such UV powder coating materials, however, a series of problems have arisen. With the majority of polymers, these problems result substantially from the impossibility of unifying the desired blocking resistance of the powders, on the one hand, and the required elasticity of the cured coating films, on the other. When the polymers which are the basis of the respective UV powder coating material are formulated to be so hard that the resulting powders are resistant to blocking, the coating films which result after curing are brittle.
The majority of known UV powder coating materials are based on polymer systems containing acrylic and/or vinylic unsaturation. A great technical problem of these systems results from the risk of premature thermally activated polymerization of the individual components during their preparation and during the process of compounding to ready-to-use coating materials, which generally takes place in a melt extruder. A particularly critical operation is the melting of the applied coatings prior to radiation crosslinking. During this operation, the desire is for temperatures as high as possible in order to achieve melt viscosities which are as low as possible. In the course of trials with known acrylically unsaturated systems it was found that, owing to this premature thermally activated polymerization, the hoped-for effect of better leveling could not be achieved.
DE-A-31 07 450 disclosed unsaturated polyesters containing oligomers of cyclopentadiene as end groups, which in the form of solutions in ethylenically unsaturated monomers can be used to produce moldings and coatings. As such ethylenically unsaturated monomers it specifies the customary copolymerizable vinyl monomers or monomer mixtures, such as styrene, vinyltoluene, divinylbenzene, diallyl phthalate, and methyl methacrylate, for example.
EP-A-0 101 585 discloses unsaturated polyester resins which are modified by adding cyclopentadiene onto the double bond of the polyester and are then dissolved in vinyl monomers.
In EP-A-0 585 742, unsaturated crystalline polyesters are combined with acrylically unsaturated polyurethane acrylates in order to increase the blocking resistance.
EP-A-0 636 669 describes powder coating mixtures comprising unsaturated polyesters or acrylically unsaturated polyacrylates with crosslinkers, especially polyurethane crosslinkers, that are functionalized with vinyl ethers, vinyl esters or (meth)acrylic esters. The examples of this document reveal only a mixture of a polyester with a vinyl ether urethane.
As a further document, WO 99/14254 describes combinations of unsaturated polyesters or unsaturated polyacrylates with crosslinkers, preferably polyurethanes, that are functionalized with a (poly)isocyanate and vinyl ethers or unsaturated alcohols.
On the basis of this prior art, an object of the present invention was to provide powder coating materials curable with high-energy radiation, preferably UV light, which have sufficient thermal stability during application, i.e., during the melting process, in combination with a sufficient blocking-resistant hardness, but which still give elastic coating films after curing.
This object has been achieved by means of powder coating materials comprising an unsaturated polyester resin (A) and a polymeric crosslinker (B) containing, based on the polymer main chain, terminal and/or pendant propenyl, butenyl and/or isoprenyl groups, said unsaturated polyester resin (A) and/or said polymeric crosslinker (B) comprising structural units of the general formula I and/or II.
The novel powder coating materials of the invention are curable with high-energy radiation, preferably UV light, and are notable in particular for a very good viscosity stability during the preparation, in the melt, of the resins and the compounding of the coating materials, for outstanding coating properties, and for a low oxygen sensitivity on curing, which takes place preferably from the melt. The films resulting after the cure, moreover, exhibit outstanding elasticity. In principle, the powder coating materials of the invention may also be cured by heat with initiators which in response to heat provide free radicals, such as peroxides, azo initiators or C—C labile compounds. Particularly worthy of mention in this context is a combination of the two cited curing methods, which has also become known by the term “dual curing”. In the case of dual cure, curing is first of all carried out to a so-called B stage, i.e., a partly cured state, at which point curing is interrupted and is started again at a later point in time by another mechanism.
The novel powder coating materials of the invention are notable for improved leveling on application to the substrate that is coated. The dicyclopentadiene content of the powder coating materials results in extremely high UV reactivities. Moreover, the dicyclopentadiene-modified polyesters are formulated for a sufficient blocking-resistant hardness.
The polyester resins (A) of the powder coating materials of the invention consist of unsaturated polyester resins, known per se, containing structural units of the general formula I and/or II which are derived from dicyclopentadiene (DCPD). The polyester resins are synthesized by methods already known in the prior art, generally by polycondensation of polyfunctional hydroxyl compounds with polyfunctional acids and/or their anhydrides at relatively high temperatures.
Moreover, it is often advantageous to start from the esters of such compounds and to obtain the polyesters by transesterification at relatively high temperatures, since such transesterifications may proceed more readily and more rapidly than the direct esterification. The unsaturated nature of the polyesters comes about through the (additional) use of unsaturated compounds in the acid component and/or unsaturated alcohol components, such as alkenediols and/or oxalkylated alkenediols, for example. Preferably, unsaturated polyester resins are obtained with maleic acid and/or maleic anhydride and/or fumaric acid, since these compounds are available industrially and are inexpensive. Furthermore, polyesters containing amide structures may be obtained by the (additional) use of polyfunctional amines. The additional use of monofunctional starting materials is also possible in order, for example, to regulate the molecular weight. Below, examples are given of compounds suitable for synthesizing the polyester resins.
Examples of suitable compounds of this kind are: adipic acid, suberic acid, phthalic acid isomers, tetrahydrophthalic acid, endomethylenetetrahydrophthalic acid, hexahydrophthalic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, trimellitic acid, pyromellitic acid, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycols, butanediol isomers, hexanediol, neopentyl glycol, trimethylolpropane, glycerol, pentaerythritol, bisphenol A, hydrogenated bisphenol A, OH-polyfunctional polymers, such as hydroxyl-modified polybutadienes or hydroxyl-bearing polyurethane prepolymers and epoxy resins, polyfunctional natural substances or derivatives thereof, such as linseed oil fatty acid, dimeric and polymeric linseed oil fatty acid, castor oil, and castor oil fatty acid. In addition to the compounds exemplified, the hydroxyl compounds specified later on below in the context of the polymeric crosslinker (B) are also suitable for synthesizing the polyesters. The introduction of amide and imide structures into polyester resins is known, for example, from DE-A-15 700 273 and D

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