Dual thermal and ultraviolet curable powder coatings

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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 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),