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Description  |
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FIELD OF THE INVENTION
The present invention relates to powder coatings and more particularly to opaquely pigmented or thick filmed ultraviolet (UV) radiation curable powder coatings that can be cured not only at the surface, but also down through the coating to the
substrate. Full cure can be obtained despite the presence of opaque pigments or thick films that normally impede the penetration of radiation in the coating and, consequently, inhibit cure below the surface. This is accomplished by incorporating a
thermal initiator in the UV curable powder coatings together with the usual UV initiator. Surprisingly, the presence of a thermal initiator does not detract from the exceptional smoothness of the hardened film finishes. The dual thermal and UV curable
powder coatings of the present invention are especially suited for coating over heat sensitive substrates, such as wood and plastic, since these coatings cure at faster speeds and/or lower temperatures, reducing the potentially damaging heat load on the
substrate.
BACKGROUND OF THE INVENTION
Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings for a number of reasons. For one, powder coatings are user and
environmentally friendly materials, since they are virtually free of harmful fugitive organic solvent carriers that are normally present in liquid coatings. Powder coatings, therefore, give off little, if any, volatile materials to the environment when
cured. This eliminates the solvent emission problems associated with liquid coatings, such as air pollution and dangers to the health of workers employed in coating operations.
Powder coatings are also clean and convenient to use. They are applied in a clean manner over the substrate, since they are in dry, solid form. The powders are easily swept up in the event of a spill and do not require special cleaning and
spill containment supplies, as do liquid coatings. Working hygiene is, thus, improved. No messy liquids are used that adhere to worker's clothes and to the coating equipment, which leads to increased machine downtime and clean up costs.
Powder coatings are essentially 100% recyclable. Over sprayed powders can be fully reclaimed and recombined with the powder feed. This provides very high coating efficiencies and also substantially reduces the amount of waste generated.
Recycling of liquid coatings during application is not done, which leads to increased waste and hazardous waste disposal costs.
Despite their many advantages, powder coatings are generally not employed in coating heat sensitive substrates, such as wood and plastic. Heat sensitive substrates demand lower cure temperatures, preferably below 250.degree. F., to avoid
significant substrate degradation and/or deformation. Lower cure temperatures are not possible with traditional heat curable powders. Unsuccessful attempts have been made to coat heat sensitive substrates with traditional powders.
For instance, when wood composites, e.g., particle board, fiber board, and other substrates that contain a significant amount of wood, are heated to the high cure temperatures required for traditional powders, the residual moisture and resinous
binders present in the wood composites for substrate integrity are caused to invariably evolve from the substrate. Outgassing of the volatiles during curing results in severe blisters, craters, pinholes, and other surface defects in the hardened film
finish. Furthermore, overheating causes the wood composites to become brittle, friable, charred, and otherwise worsened in physical and chemical properties. This is unacceptable from both a film quality and product quality viewpoint.
Low temperature UV curable powders have recently been proposed for coating heat sensitive substrates. UV powders still require exposure to heat, which is above either the glass transition temperature (T.sub.g) or melt temperature (T.sub.m), to
sufficiently melt and flow out the powders into a continuous, smooth, molten film over the substrate prior to radiation curing. However, the heat load on the substrate is significantly reduced, since UV powders are formulated to melt and flow out at
much lower temperatures than traditional powder coatings, typically on the order of about 200.degree. F. Therefore, UV powders only need to be exposed to enough low temperature heat required to flow out the powders into a smooth molten film.
Curing or hardening of UV powders is accomplished by exposing the molten film to light from a UV source, such as a mercury UV lamp, which rapidly cures the film. Since the crosslinking reactions are triggered with UV radiation rather than heat,
this procedure allows the powder coatings to be cured more quickly and at much lower temperatures than traditional heat curable powders.
Another significant advantage of UV curable powders is that the heated flow out step is divorced from the UV cure step. This enables the UV powders to completely outgas substrate volatiles during flow out and produce exceptionally smooth films
prior to the initiation of any curing reactions. Accordingly, the film finishes created with UV powders are known to have extraordinary smoothness.
One drawback is that opaque pigmentation of UV curable powders is known to be problematic. Opaque pigments inherently absorb, reflect, or otherwise interfere with the transmittance of UV light through the pigmented coating, and, consequently,
impede the penetration of UV light into the lower layers of the pigmented film during curing. Pigmented UV powders, when cured, can still provide exceptionally smooth film finishes with good surface cure properties, including good solvent resistance.
However, pigmented UV powders are not able to be adequately cured down through the film to the underlying substrate. As a result, pigmented UV powder coatings exhibit poor through cure properties, including poor pencil hardness, poor adhesion, and poor
flexibility. Clear UV powder formulations which are applied as thicker films greater than about 2 mils present similar curing problems. Most UV curable powder coatings produced today are formulated as thin clear coats for wood and metals without
pigmentation.
EP Publication No. 0 636 669 A2 to DSM, N.V. dated Feb. 1, 1995 discloses UV or electron beam radiation curable powder coatings which can be applied to heat sensitive substrates, such as wood, e.g., medium density fiber board, and plastic. The
UV powders of EP 0 636 669 A2 contain: a) an unsaturated resin from the group of (semi)crystalline or amorphous unsaturated polyesters, unsaturated polyacrylates, and mixtures thereof, with unsaturated polyesters derived from maleic acid and fumaric acid
being especially preferred; b) a crosslinking agent selected from an oligomer or polymer having vinyl ether, vinyl ester or (meth)acrylate functional groups, with vinyl ether functional oligomers being especially preferred, such as divinyl ether
functionalized urethanes; and, c) a photoinitiator for UV or electron beam radiation cure, in which the the equivalent ratio of polymer unsaturation to crosslinker unsaturation is preferably 1:1. However, the UV powders of EP 0 636 669 A2 are
practically limited to being formulated as unpigmented, i.e., clear, coatings, as demonstrated in Example 1. A similar clear coat UV powder based on an unsaturated polyester, an allyl ether ester crosslinker, and a hydroxyketone photoinitiator is
disclosed in Example 2 of International Publication No. WO 93/19132 to DSM, N.V. dated Sep. 30, 1993.
K. M. Biller and B. MacFadden (Herberts Powder Coatings), UV-Curable Powders: A Marriage Of Compliant Coatings, Industrial Paint & Powder, pp. 22-25 (July 1996) and K. M. Biller and B. MacFadden (Herberts Powder Coatings) UV-Curable Powder
Coatings: The Perfect Marriage of Compliant Coatings, Radtech North America 1996 Conference, pp.437-445 (Apr. 28-May 2, 1996), suggest the incorporation of special solid UV initiators which are designed to activate despite the presence of various
pigmentations. Presumably, these special UV initiators have some ability to absorb UV light at wavelengths above the reflectance of the pigments.
F. M. Witte and E. S. de Jong (DSM, N.V.), Powder Coatings On Heat Sensitive Substrates, presented at the DSM, N.V. seminar held in Amsterdam (Mar. 27-28, 1996), disclose that some success has been achieved in the laboratory with pigmented UV
curable powder coating formulations based on a binder which comprises a blend of: a) an unsaturated polyester resin, e.g., an unsaturated polyester derived from maleic acid; and, b) a vinyl ether functionalized polyurethane crosslinking agent. This
binder is similar to that described in the aforementioned EP 0 636 669 A2. In these formulations, a special class of UV photoinitiators especially suited for pigmented UV coatings are used, e.g., bis-acylphosphineoxides and a 75/25 blend of a
hydroxyketone (Irgacure 184) and bis-acylphosphineoxide, which is now available as Irgacure 1800 from Ciba-Geigy Corporation. Pigmented formulations with varying amounts of pigments between 5 and 20 wt. % are applied electrostatically to medium density
fiber board at a layer thickness of 100 microns. Yet, although sufficient hiding is achieved with pigment loadings at 15 wt. % and 20 wt. % pigment, the pendulum hardness and, consequently, the through cure is not fully developed shortly after UV
curing, which is undesirable.
The state of the powder coating art, therefore, is that opaquely pigmented or thick filmed UV curable powder coatings cannot be fully cured with UV light.
U.S. Pat. No. 4,753,817 to Meixner et al. discloses opaquely pigmented liquid UV radiation curable coatings for application to wood, wood-like materials and films of plastic. Such pigmented liquid UV paints are made by incorporating
hydroperoxide thermal initiators alongside the UV photoinitiators in liquid resins derived from copolymerizable monomer-free air-drying unsaturated polyesters. The inclusion of the hydroperoxides in the liquid UV formulation is said to improve the cure
at the lower layers of the coating that can not be penetrated by UV radiation due to the opacity of the pigments. The liquid paints also contain other essential ingredients not found in powder coatings, including plasticized colloidal cotton, e.g.,
nitrocellulose, siccatives to promote air-drying, and also volatile organic solvents to adjust the processing viscosity for liquid applications, e.g., toluene, xylene, isopropanol, and butyl acetate. These pigmented UV liquids are first applied to the
substrate in liquid form, next pregelled at temperatures of 120.degree. F. to 175.degree. F., and then hardened under UV radiation. There is nothing in Meixner et al. that provides any indication that solid compositions could be made that are suitable
as powder coatings.
In addition, the liquid UV coatings of Meixner et al. have a number of other shortcomings. For example, the UV liquids contain organic solvents, which generate physiologically and environmentally harmful solvent emissions during drying. As
previously mentioned, powder coatings are essentially solventless and nonpolluting substances. The UV liquids also have pot lives only up to 5 hours, which is rather short. This would not be a suitable system for a powder coating which must not advance
for several weeks at room temperature, especially in a reclaim powder coating booth. Also, cure for the Meixner et al. system occurs between 120 and 175.degree. F. At such temperatures, significant pregelation or even curing during extrusion would
result with a powder coating, which, in turn, would cause processing difficulties as the powder would take a set in the extruder, leading to excessive extruder downtime and expensive clean out costs. The UV liquids are based on lower molecular weight
polyester resins, which are highly reactive liquids at room temperature that cure without crosslinkers. Typical polyester powder coatings require resins with higher molecular weights or higher T.sub.g 's for the material to remain solid at room
temperature, which tends to reduce the reactivity of the resin and require a crosslinker. Also, the UV liquids containing lower molecular weight species would be expected to transport more readily into biological tissues. If improperly handled, the
highly reactive UV liquids could be transported through the skin of an individual and poisonously affect physiological functions. The use higher molecular weights species in powder coatings reduces handling problems and reduces the risk of invasion into
biological tissues.
Moreover, the liquid UV coatings of Meixner et al. do not require an initial melt and flow out step in order to form a smooth film over the substrate, as required with UV powders, since these coatings are liquids at room temperature and
inherently flow out as smooth films when poured. Therefore, incorporation of a thermally activated peroxide cure component in a liquid UV coating is not problematic from a film quality standpoint. However, the inclusion of a thermal peroxide cure
component in a UV powder coating would be expected to produce coatings having poor flow out behavior and, consequently, poor film qualities. Conventional wisdom would lead one to expect that the addition of a thermal cure component to a UV curable
powder would cause pregelation during the heated flow out step and cause undesirable surface roughness, such as orange peel or low gloss, in the hardened film finish, detrimentally affecting the film quality and aesthetic appearance of the coating.
Blisters, craters, pinholes, and other surface defects would also be expected to be visually evident on the surface of the hardened film. Additionally, significant pregelation in the extruder during melt blending the powder ingredients would be expected
when using a thermal initiator, causing processing difficulties as the powder blend would begin to cure and take a set in the extruder, which would lead to considerable extruder down time and increased clean out costs.
What is needed is an opaquely pigmented and/or thick filmed UV curable powder coating composition that is suitable for coating heat sensitive substrates, especially wood, wood composites, and plastic, and that can be fully cured through the
incorporation of a thermal initiator alongside the usual UV initiator without detracting from the exceptional smoothness of the hardened film finish.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide an opaquely pigmented or thick filmed powder coatings that can be fully cured upon exposure to UV radiation despite the presence of opaque pigments and/or thick films, both of which
normally inhibit cure in the coating, through the incorporation of a "dual" cure system, i.e., combination thermal initiator and UV photoinitiator, in the powders, wherein the UV initiator cures the surface, while the thermal initiator cures the lower
layers near the substrate, yet without detracting from the exceptional smoothness of the hardened film finish obtained on the surface.
It is another object of this invention to provide dual thermal and UV curable powder coatings that have exceptional smoothness comparable to traditional UV powder coatings.
It is still another object of this invention to provide dual thermal and UV curable powder coatings that have improved through cure properties, including improved adhesion to the substrate, compared to traditional UV curable powder coatings.
It is yet another object of this invention to provide dual thermal and UV curable powder coatings that have relatively low cure temperatures and/or rapid cure speeds for safe curing on heat sensitive substrates without damaging or worsening the
substrate.
The present inventors have discovered that the use of dual cure initiators, i.e., a UV initiator along side a thermal initiator, in the powder coatings will give not only excellent surface cure to the film finish, but also superior through cure
in the film down to the underlying substrate, while still obtaining exceptional smoothness and desired glossiness in the surface finish. It is quite surprising that the cured powder coating of this invention remains so smooth. Conventional wisdom would
lead one to expect that the addition of a thermal cure component to UV curable powders would cause pregelation during the melt and flow out step and, in turn, cause roughness on the surface, such as orange peel or low gloss. Surprisingly, the hardened
film finishes produced from powder coatings of this invention are at least as exceptionally smooth as those produced from traditional UV curable powders, while also being fully cured throughout.
The invention resides in a dual thermal and ultraviolet curable powder coating composition, which is a composition in solid particulate form that is a blend of: a) an unsaturated resin selected from unsaturated polyesters, unsaturated
polyacrylates, unsaturated polymethacrylates, and mixtures thereof; b) optional second co-polymerizable resin crosslinker having a functional group selected from vinyl ether, acrylate, methacrylate, and allyl ester groups, and mixtures thereof; c) a
photoinitiator selected from photolytically activated free radical generating compounds; d) a thermal initiator selected from thermally activated free radical generating compound, such as peroxides, azo compounds, and mixtures thereof; e) an opacifier
selected from pigments, fillers, and mixtures thereof; and, f) optional catalyst, wherein the composition can be fully cured, both on the surface and throughout, on a heat sensitive substrate upon exposure of sufficient heat to melt and flow out the
powder into a smooth molten film and to activate the thermal component of the cure, followed by exposure of the molten film to sufficient UV radiation to activate the ultraviolet component of the cure and to form a fully cured smooth hardened film.
The various objects, features and advantages of this invention will become more apparent from the following description and appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
Throughout this specification, all parts and percentages specified herein are by weight unless otherwise stated.
Base Resins
The dual, thermal and UV, curable powder coatings of this invention are primarily based on unsaturated film-forming polymer resins, such as unsaturated polyester resins, unsaturated polyacrylate or polymethacrylate resins, and/or mixtures
thereof.
The unsaturated polyester resins useful herein are ethylenically unsaturated condensation reaction products of one or more aliphatic or cycloaliphatic di- or polyfunctional alcohols, or mixtures thereof, and one or more aliphatic, cycloaliphatic,
or aromatic di- or polyfunctional carboxylic acids, or mixtures thereof. The carboxylic acids can also be used in their corresponding anhydride form. Corresponding lower alkanol esters are also useful for esterification. Small amounts of
monofunctional alcohols and monofunctional carboxylic acids or esters thereof may be present for polyester chain termination purposes. Although unsaturation may be supplied by the alcohol, typically the acid is unsaturated and the alcohol is saturated.
Saturated acids can also be present to reduce the density of ethylenic unsaturation in the polyester.
Examples of suitable unsaturated di- and polyfunctional carboxylic acids that are useful herein include maleic anhydride, fumaric acid, citraconic anhydride, itaconic acid, endo-cis-bicylco[2,2,1]-5-heptene-2,3-dicarboxylic acid,
1,4,5,6,7,7-hexachlorobicyclo[2,2,1]-5-heptene-2,3-dicarboxylic acid (chlorenedic acid), mesaconic acid, dimeric methacrylic acid, and methylbicyclo[2,2,1]-heptene-2,3-dicarboxylic acid. Maleic anhydride, fumaric acid or mixtures thereof are most
preferred. It should be understood that whether acids, anhydrides, or lower alkanol esters are listed here, any of these forms are contemplated for use herein.
Examples of suitable saturated diacids or polyacids that are useful herein in combination with a substantial proportion of an unsaturated diacid, include tetrachlorophthalic acid, tetrabromophthalic acid, phthalic anhydride, adipic acid,
tetrahydrophthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, azeleic acid, sebacic acid, dimethylterephthalate, dimethylisophthalate, succinic acid, dodecanedicarboxylic acid, hexahydrophthalic acid,
hexaclorooctahydromethanonaphthalene dicarboxylic acid, malonic acid, glutaric acid, oxalic acid, pimelic acid, suberic acid, and pyromellitic anhydride.
Examples of suitable monoacids that can be used herein to terminate the polyester chain, include linoleic acid, linolenic acid, geranic acid, dehydrogeranic acid, sorbic acid, heptatri-1,3,5-ene-1-carboxylic acid,
nonatetra-1,3,5,7-ene-1-carboxylic acid, other fatty acids of vegetable oils, abietic acid, methacrylic acid, and, benzoic acid.
Examples of suitable diols useful herein include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-dimethoxy cylcohexane, 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol,
1,2-cyclopentanediol, 1,3-cyclopentanediol, 1,4-cyclopentanediol, 1,2-cyclohexanediol, 1,3-cyclohexanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 4,4'-methylene-bis(cyclohexanol), 4,4'-isopropylidene-bis(cyclohexanol),
1,3-bis(hydroxymethyl)cyclohexane, 1,3-bis(hydroxyethyl) cyclohexane, 1,3-bis(hydroxypropyl) cyclohexane, 1,3-bis(hydroxyisopropyl) cyclohexane, xylene glycol, bisphenol A, hydrogenated bisphenol A, bisphenol A/propylene oxide adducts,
hydroquinone/propylene oxide adducts, hydroquinone/ethylene oxide adducts, neopentyl glycol, 1,6-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,3-isobutanediol, 1,2-isobutanediol, 2,3-butanediol, and 2-butenediol(1,4).
Examples of polyols having 3 or more hydroxyl functional groups that are useful herein in small amounts to form branched polyesters, include glycerol, trimethylolpropane, pentaerythritol, allyl ether polyols, polyalkylene glycol ethers,
1,1,1-trimethylol ethane, sorbitol, mannitol, diglycerol, and dipentaerythritol.
Instead of or in addition to the alcohol, epoxy compounds, such as ethylene oxide and propylene oxide, can be used.
Preparation of the unsaturated polyester can be carried out with standard techniques well known in the art. For instance, a two step process may be employed. In the first step, saturated or unsaturated glycols and acids are heated in the
presence of an esterification catalyst, such as a tin catalyst, e.g., monobutyl tin oxide, stannous octoate, and monobutyl tin dilaurate, or acid catalyst, e.g., p-toluene sulfonic acid, methane sulfonic acid, and sulfuric acid, at about 400.degree. F.
to 480.degree. F. for about 2 to 24 hours under nitrogen sparge and reacted to a given acid number or hydroxyl number while collecting water formed by esterification. The resultant esterified prepolymer is cooled to about 320.degree. F. to 390.degree. F. Glycol loss is measured by refractive index and the lost glycol is added, if needed. Then, in the second step, unsaturated or saturated glycols and acids are charged to the reaction vessel again under nitrogen sparge. The reaction mixture is heated
to about 350.degree. F. to 450.degree. F. for about 2 to 8 hours and reacted to a given acid number, viscosity and amount of water, if appropriate. The resultant resin is then inhibited with hydroquinone or other substituted phenolic derivative
inhibitor.
Unsaturated polyesters resins can also be prepared in a single step by heating saturated and unsaturated polycarboxylic acids with polyols and esterification catalyst, such as stannous oxide, under nitrogen sparge to about 320.degree. F. to
480.degree. F. for about 1 to 24 hours. The water of esterification is collected to measure the reaction. The glycol loss is again measured and glycol is added, if needed. The reaction is run to the appropriate acid or hydroxyl number and viscosity.
The unsaturated polyester resins can be crystalline, (semi)crystalline, or amorphous. Crystalline and (semi)crystalline unsaturated polyesters are generally preferred over amorphous unsaturated polyesters, since stable powder coatings with lower
melt viscosity and better flow can be prepared more easily therefrom.
It is known in the art that certain monomers used in the polycondensation reactions impart crystallinity to the unsaturated polyesters. For example, dihydric alcohol monomers that are known to promote crystallinity include ethylene glycol,
1,4-butanediol, neopentyl glycol, and cyclohexanedimethanol. Dicarboxylic acid monomers that are known to promote crystallization include terephthalic acid and cyclohexane dicarboxylic acid.
The preferred unsaturated polyesters useful herein have a long shelf life without cold flow at temperatures substantially above room temperature up to about 120.degree. F. and have a glass transition temperature (T.sub.g) or melt temperature
(T.sub.m) below the desired flow temperature required for preservation of heat sensitive substrates, preferably between about 160.degree. F. and about 250.degree. F.
The preferred unsaturated polyesters have a molecular weight ranging between about 400 and about 10,000, more typically about 800 and about 6,800, and, preferably, between about 2,000 and about 4,500.
The unsaturated polyesters preferably have a degree of unsaturation between about 2 and about 20 wt. % unsaturation, and, preferably, between about 4 and about 10 wt. % unsaturation.
Whether the polyester is carboxylic acid-functionalized or hydroxyl-functionalized depends upon the --COOH/--OH molar ratio of the monomer mix. If the unsaturated polyester is hydroxyl-functionalized, then the hydroxyl number of the polyester is
generally between about 5 and about 100, and, preferably, between about 9 and about 50. If the unsaturated polyester is acid-functionalized, then the acid number of the polyester is generally between about 1 and about 80, and, preferably, between about
9 and about 50.
Preferably, the unsaturated polyesters are solid resins at room temperature or above, so that they can be easily formulated into nonsintering powders. If the resins are liquids, they should be converted to powder form and, thus, be counted as a
solid by absorption onto inert silica-type filler materials, such as fumed silica, before use, as is well known in the art.
Exemplary unsaturated polyester formulations useful herein are further specified in the working examples.
The unsaturated acrylate and methacrylate resins useful herein include polymers containing unreacted acrylate or methacrylate groups in the main chain or side chain. The unsaturated acrylate or methacrylate polymers are the reaction products of
one or more solid functional polymers having reactive functional groups with one or more co-polymerizable monomers having co-reactive functional groups capable of reacting with the functional groups of the polymer, with at least one of the functional
polymer or co-polymerizable monomer also containing an acrylate or methacrylate group available for final curing of the resin. The unsaturated acrylate or methacrylate polymer obtained in the aforesaid reaction may be an acrylated epoxy, acrylated
urethane, acrylated polyether, or acrylated polyester resin, or their corresponding methacrylates.
The polymers containing unreacted acrylate or methacrylate groups in the side chains can prepared by standard techniques well known in the art. For instance, a two step process may be employed. In the first step, a solid functional polymer is
formed using standard polymerization techniques. For example, the polymer formed in step one can be a functionalized acrylate or methacrylate polymer.
Suitable monomers which are commonly used to form the backbone of such functionalized acrylate and methacrylate polymers, include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate,
isobutyl acrylate, isobutyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, and the like. In addition, suitable amounts of functional monomers are co-polymerized during polymerization to obtain the functionalized polymer.
Acid-functional acrylate or methacrylate polymers can be formed from acid-functional monomers, such as acrylic acid and methacrylic acid. Hydroxyl-functional acrylate or methacrylate polymers can be formed from hydroxyl-functional monomers, such as
2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 3,4-dihydroxybutyl methacrylate. Epoxy-functional acrylate or methacrylate polymers can be formed from epoxy-functional monomers, such as glycidyl methacrylate,
2,3-epoxybutyl methacrylate, 3,4-epoxybutyl methacrylate, 2,3-epoxycyclohexyl methacrylate, and 10,11-epoxyundecyl methacrylate. Isocyanate-functional polymers can also be formed from isocyanate functional monomers, such as
meta-isopropenyl-.alpha.,.alpha.-dimethylbenzylisocyanate (TMI).
The functional polymers formed in step one that will be reacted with bi-functional acrylate or methacrylate monomers can also be other types of solid resins, other than acrylates or methacrylates, having acid, hydroxyl, epoxy or isocyanate
functional groups, for example, epoxidized bisphenol A resins or acid, hydroxyl, or isocyanate functionalized polyester resins.
In the second step, a reaction is carried out between the unreacted functional groups of the solid functional polymer formed in step one with a co-polymerizable monomer having both a co-reactive functional group capable of reacting with the
reactive functional groups of the functional polymer and an unsaturated acrylate or methacrylate group available for final curing of the resin. Any one of the functionalized acrylate or methacrylate monomers listed above can serve as this bi-functional
co-polymerizable monomer.
The reaction is carried out by dissolving the above formed solid functional polymer in an appropriate solvent for the polymer, such as butyl acetate, and then adding the bi-functional co-polymerizable monomer having pendent acrylate or
methacrylate unsaturated groups in a stepwise manner at elevated temperatures between, for example about 150.degree. F. and 300.degree. F., until substantial completion of the reaction. This reaction can also be done without solvent by heating the
reactants above their melting points. For example, in the second step an acid-functional acrylate or methacrylate polymer can be reacted with a compound having an epoxy group, such as an epoxy-containing acrylate or methacrylate monomer, e.g., glycidyl
methacrylate, to form a methacrylated polyester. An isocyanate-functional polymer, e.g., TMI, can likewise be reacted with a hydroxy-functional acrylate or methacrylate monomer, such as 2-hydroxyethyl methacrylate, to form an acrylated urethane.
Resins containing acrylate or methacrylate groups in the main chain can be prepared by reacting epoxy-, carboxyl-, hydroxyl-, or isocyante-containing resins with bi-functional acrylate or methacrylate monomers having functional groups co-reactive
with the aforesaid functionalities. For example, an epoxy-functional polymer, such as polyglycidyl methacrylate or epoxidized bisphenol A resin, can be reacted with a monomer having acid group, such as acrylic acid or methacrylic acid. In this
reaction, the heat must be closely monitored to assure that the acrylic acid does not polymerize. In addition, an acid-functional polyester, such as one prepared from neopentyl glycol, ethylene glycol, adipic acid and isophthalic acid, can be reacted
with a bi-functional acrylate or methacrylate compound having epoxy groups, such as glycidyl methacrylate. Also of note is the reaction of acrylic or methacrylic acid with an alcohol or hydroxyl functional polyester at elevated temperatures to form a
polyester with acrylate functionality.
The unsaturated group left in the polymer for final curing of the powder coating need not be an acrylate or methacrylate group, although an unsaturated acrylate or methacrylate unsaturation is most preferred. It is also possible to form other
solid resins having allyl, vinyl, vinyl ether, and styryl functionalities. For example, a hydroxyl functional polyester resin, such as one made with neopentyl glycol, 1,4-cyclohexane dimethanol, terephthalic acid and adipic acid, can be reacted with
TMI, to form a styryl-functional resin.
The unsaturated acrylated or methacrylated polymer would be used similarly to the unsaturated polyester resin in this portion of the formulation. The T.sub.g, molecular weight, and % unsaturation range would be similar to that used for the
unsaturated polyesters. The % unsaturation in this case will be governed by the amount of acrylate or methacrylate in the polymer. Whereas the % unsaturation in the polyester is governed by the maleic or fumaric content of polyester.
The acrylate or methacrylate polymer resins are capable of crosslinking without an additional crosslinking agent, although crosslinkers may be used with such formulations.
Preferably, the unsaturated polyacrylate and polymethacrylate resins are solids at room temperature or above, so that they can be easily formulated into nonsintering powders. If the resins are liquids, they should be converted to powder form
and, thus, be counted as a solid, by absorption onto inert silica-type filler materials, such as fumed silica, before use, as is well known in the art.
Exemplary unsaturated acrylate or methacrylate polymer formulations useful herein are further specified in the working examples.
Crosslinkers
The unsaturated polyester resins useful herein work best in combination with co-polymerizable second resins having ethylenic unsaturation and preferably having two sites of unsaturation per molecule. These second resins are used in the
composition as crosslinkers for the base resin. Most preferred is a predominance of monomers or prepolymers that are solid at room temperature or above, so that they can be easily formulated into nonsintering powders.
The co-polymerizable second resins or crosslinkers useful herein are preferably oligomers or polymers having vinyl ether, acrylate or methacrylate, or allyl ester groups, with an oligomer or polymer having vinyl ether groups being most preferred.
The co-polymerizable second resins having vinyl ether groups are preferably composed of vinyl ether functionalized urethanes, for example, a divinyl ether urethane based on the reaction product of a diisocyanate, such as hexamethylene
diisocyanate, and a hydroxyl-functional vinyl ether, such as hydroxybutyl vinyl ether. Other suitable hydroxyl-functional vinyl ethers include hydroxyethyl vinyl ether and trimethylene glycol monovinyl ether. Other suitable diisocyanates include
isophorone diisocyanate, methylene diisocyanate, methylenebiscyclohexylisocyanate, trimethylhexamethylene diisocyanate, hexane diisocyanate, hexamethyl diisocyanate, hexamethylamine diisocyanate, methylenebiscyclohexyl isocyanate, toluene diisocyanate,
1,2-diphenylethane diisocyanate, 1,3-diphenylpropane diisocyanate, diphenylmethane diisocyante, dicyclohexylmethyl diiscoyanate, and other isocyanate prepolymers and terpolymers. Functional prepolymers derived from these diisocyanates, such as urethane
trimers, uretdiones, isocyanurates, and biurets can also be used.
The functionalized vinyl ethers can be obtained in a conventional manner. For example, vinyl ether urethanes are usually prepared by reacting a hydroxyl-functional vinyl ether with a multi-functional isocyanate-containing monomer or polymer in
solvent, such as methylene chloride, under a nitrogen atmosphere, at temperatures between about ambient and 125.degree. C. Additional reference can be made to U.S. Pat. No. 4,751,273 to Lapin et al. for the preparation of vinyl ether functionalized
urethanes, which disclosure is incorporated by reference herein in its entirety. Examples of vinyl ether urethanes are sold under the trade names Uralac resins by DSM Resins and Vectomer Oligomers and Diluents by Allied Signal.
Preferably, vinyl ether urethane prepolymers that are non-crystalline solids at room temperature or higher are used so that they can easily be formulated into non-sintering powders. These non-crystalline solid materials can be obtained by
reacting a non-crystallizing diisocyanate monomer, such as isophorone diisocyanate with a crystallizing or non-crystallizing polyol, such a neopentyl glycol, and then reacting the isocyanate urethane adduct obtained with a hydroxy vinyl ether, such as
4-hydroxybutyl vinyl ether, or otherwise by reacting a non-crystallizing aliphatic polyisocyanate, such as isophorone diisocyanate trimer, with a hydroxy vinyl ether, such as 4-hydroxybutyl vinyl ether. The reactions are typically carried under a
nitrogen atmosphere, in the presence of tin catalyst, such as dibutyltin dilaurate, at temperatures not above about 100-110.degree. C. Due to the polymeric nature of the so prepared vinyl ether urethanes, powder coatings based on the same exhibit
improved flexibility and adhesion to the substrate after curing. Also, such higher molecular weight materials are relatively safer to handle and have lower toxicities than traditional vinyl ether urethane curing agents. For a further description of
such non-crystalline solid vinyl ether curing agents, reference can be made to the U.S. patent application Ser. No. 08/991,125 of Navin B. Shah and Andrew T. Daly entitled Solid Vinyl Ether Terminated Urethane Curing Agent, filed this same day, which
disclosure is incorporated by reference herein in its entirety.
The co-polymerizable second resins having acrylate or methacrylate groups are preferably composed of dimethacrylate functionalized urethanes, for example, based on diisocyanate, such as hexanediisocyanate, and a hydroxyl-functional methacrylate,
such as hydroxyethyl methacrylate. Other isocyantes can be used as well such as those listed above. In addition, other suitable hydroxyl-functional methacrylates include hydoxypropyl methacrylate and other hydroxy alkyl methacrylates. This material
can be reacted the same as the vinyl ethers above.
The co-polymerizable second resins having allyl ester groups are preferably composed of hydroxy functional allyl esters, for example, based on the esterification reaction product of allyl alcohol and phthalic anhydride, which forms diallyl
phthalate. Examples of suitable allyl functionalized oligomers, include diallyl phthalate prepolymers, iso-diallyl phthalate prepolymers, p-diallyl phthalate prepolymers, diallyl maleate, triallyl cyanurate, diallyl chlorendate methacrylamide, and
triallylisocyanurate.
If the co-polymerizable second resin is a liquid or a sticky powder and is used in sufficient quantities that the resultant melt blended powder coating composition is not adequately free flowing, then this coreactant can be absorbed on an inert
filler, such as fumed silica, and thus be counted as a solid within the preferred scope of this invention. Except in small quantities up to about 5 wt. % of the powder coating composition, these liquids species are much less preferred than solid
co-reactants due to sintering problems.
The relative amounts of unsaturated polymer base resin to unsaturated copolymerizable second resin provided in the powder coating compositions of this invention depend in part on the choice of materials. For instance, when the resin is an
unsaturated polyester and the second resin crosslinker is a vinyl ether functionalized compound, the equivalent ratio of polyester unsaturation to vinyl ether unsaturation is between about 90:10 and about 10:90, and, preferably about 50:50. When the
resin is an unsaturated polyester and the second resin crosslinker is an allyl ester functionalized compound, the equivalent ratio of polyester unsaturation to allyl ester unsaturation is between about 99:1 and about 1:99, and, preferably, between about
70:30 and about 95:5. When the resin is a acrylate or methacrylate polymer, it is preferred to not use a crosslinker.
Exemplary crosslinker formulations useful herein are further specified in the working examples.
UV Photoinitiators
UV photoinitiators that are incorporated in the powder coating compositions to impart a radiation activated, rapid, and low temperature cure to the powder are well known in the art. Examples of suitable photoinitiators, which are known as alpha
cleavage free radical photoinitiators, include benzoin and its derivatives, for example, benzoin ethers, such as isobutyl benzoin ether and benzyl ketals, such as benzyl dimethyl ketal, 2-hydroxy-2-methyl-1-phenylpropan-1-one and 4-(2-hydroxyethoxy)
phenyl-2-hydroxy-2-propyl ketone. Others include acyl phosphines, such as 2,4,6-trimethylbenzoyl diphenylphosphine oxide. Aryl ketones can also be used, such as 1-hydroxycyclohexyl phenyl ketone,
2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2,2-dimethoxy-2-phenylaceto-phenone, mixture of benzophenone and 1-hydroxycyclohexyl phenyl ketone, perfluorinated diphenyl titanocene, and
2-methyl-1-(4-(methylthiophenyl)-2-(4-morpholinyl))-1-propanone.
Hydrogen abstraction free radical type photoinitiators can be used in combination with the above or alone such as Michler's ketone (4,4'-bisdimethylamino benzophenone), Michler's ethyl ketone (4,4'-bisdiethylamino benzophenone ethyl ketone),
benzophenone, thioxanthone, anthroquinone, d,l-camphorquinone, ethyl d,l-camphorquinone, ketocoulmarin, anthracene, or derivatives thereof, and the like.
Cationic polymerization, especially with vinyl ether containing crosslinkers, can proceed via cationic cure using cationic photoinitiators. Major classes of cationic photoinitiators are diaryliodonium salts and copper synergists, such as
diphenyl iodonium hexafluorophosphate, dibenzyl iodonium hexaflouroarsinate and copper acetate, triarylsulfonium salts, such as triphenyl sulphonium hexafluorophosphate, triphenyl sulphonium tertafluoroborate. Dialkylphenacyl- sulfonium salts,
ferrocenium salts, such as cyclopentadienyl iron(II) hexafluorophosphate, alpha-sulfonyloxy ketone, and silyl benzyl ethers can be used as well.
Preferably, the photoinitiators used herein are solids. If liquid initiators are used, however, preferably they are absorbed on solid carriers, such as fumed silica, prior to incorporation in the powder coating compositions of this invention.
In general, the amount of photoinitiator used in the powder coating composition of the present invention ranges between about 0.1 and 10 parts per hundred resin (phr), and preferably between about 1 and 5 phr.
Exemplary photoinitiator formulations useful herein are further specified in the working example.
Reference can also be made to EP 0 636 669 A2 to DSM, N.V. for further examples of the aforementioned base resins, crosslinkers, and photoinititiators, which disclosure is incorporated by reference herein in its entirety.
Thermal Initiators
The thermal initiators useful in the powder coating compositions of this invention are free radical generating compounds, preferably peroxides and azo initiators. Examples of suitable peroxide initiators, include diacyl peroxides, such as
2-4-diclorobenzyl peroxide, diisononanoyl peroxide, decanoyl peroxide, lauroyl peroxide, succinic acid peroxide, acetyl peroxide, benzoyl peroxide, and diisobutyryl peroxide, acetyl alkylsulfonyl peroxides, such as acetyl cyclohexylsulfonyl peroxide,
dialkyl peroxydicarbonates, such as di(n-propyl)peroxy dicarbonate, di(sec-butyl)peroxy dicarbonate, di(2-ethylhexyl)peroxy dicarbonate, diisopropylperoxy dicarbonate, and dicyclohexylperoxy dicarbonate, peroxy esters, such as alpha-cumylperoxy
neodecanoate, alpha-cumylperoxy pivalate, t-amyl neodecanoate, t-amylperoxy neodecanoate, t-butylperoxy neodecanoate, t-amylperoxy pivalate, t-butylperoxy pivalate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, t-amylperoxy-2-ethyl hexanoate,
t-butylperoxy-2-ethyl hexanoate, and t-butylperoxy isobutyrate, azobis, (alkyl nitrile) peroxy compounds, such as 2,2'-azobis-(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis-(2-methylbutyronitrile); t-butyl-peroxymaleic acid,
1,1'-azobis-(1-cyclohexanecarbonitrile).
Other thermal initiators, include peroxy ketals, such as 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, peroxy esters, such as | | |
