Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2002-08-16
2004-09-07
Berman, Susan (Department: 1711)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S354000, C428S343000, C428S484100, C428S412000, C428S480000, C428S500000, C428S515000
Reexamination Certificate
active
06787225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to radiation curable coatings, especially UV curable coatings, comprising:
(i) at least one radiation polymerizable reactant, preferably one or more monomers;
(ii) at least one initiator; and
(iii) at least one wax, preferably polytetrafluoroethylene wax.
These coatings are characterized in that they can produce a 60° gloss of less than 40 when the coating is cured by exposure to radiation in the presence of oxygen to provide a dry film thickness of 1.0±0.1 mil.
One useful application of the radiation curable coatings is obtained by applying the coating to a substrate followed by exposure to radiation in the presence of oxygen until cured. One preferred application involves the use of a flexible sheet as the substrate and an especially preferred application involves a flexible sheet having a first side and a second side opposite to the first side, wherein the second side, either before or after the application and/or cure of the radiation polymerization coating, is coated with an adhesive material, and wherein the radiation curable coating is cured by exposing the applied coating to radiation in the presence of oxygen. Once the radiation polymerizable coating has been cured, the adhesive side can then be adhesively applied to any rigid or flexible substrate.
The cured coatings of this invention are especially useful in applications where an abrasion resistant, smooth, cured surface is desirable such as in the preparation of “white boards” which can erasibly accept markings from a dry mark or dry erase writing implement.
2. Description of the Prior Art
Some methods of producing low gloss radiation curable coatings have been known in the art. For example, U.S. Pat. Nos. 3,918,393 and 4,048,036 teach radiation polymerizable coatings having relatively low gloss. Additionally, preparation of flexible sheets which are coated on one side with a polymerizable coating and on the other side with an adhesive have also been taught as set forth, for example, in U.S. Pat. No. 5,387,304.
BRIEF SUMMARY OF THE INVENTION
This invention relates to radiation curable liquid coating compositions comprising:
(i) at least one radiation polymerizable reactant;
(ii) at least one initiator; and
(iii) at least one wax.
The coatings can be applied to any substrate and then cured by exposure to radiation thereby providing smooth, abrasion-resistant cured coatings which can erasibly accept markings from a dry erase writing implement.
The coatings can be cured by any suitable radiation such as electron beam or visible light or, especially preferred, ultraviolet radiation. Although it is not our intent to be bound by theory, it is believed that when the coatings of this invention are cured by exposure to radiation in the presence of oxygen, the oxygen inhibits, but does not totally prevent the cure of the coating at its surface. As a result, the rate of cure is fastest at the “bottom” of the liquid coating and slowest at the surface. It is believed that as a result of this bottom-up curing mechanism that the wax is forced toward the surface of the curing film as the cure takes place. By selecting a sufficient amount of an appropriate wax, the cured coating can provide a hard, abrasion resistant, smooth finish with a high concentration of wax at or near the surface. The wax, apparently as a result of this curing mechanism, appears to provide the dual benefit of a reduction of gloss in the coating, and a top surface of the cured film having a smooth finish which erasibly accepts the markings of dry erase writing implements. Sufficient cure inhibition at the surface can be provided at levels of oxygen in excess of 1,000 parts oxygen per 1,000,000 parts atmosphere. Higher levels of oxygen are also practical, and it is especially convenient to utilize regular atmospheric air.
In one preferred application, the coatings of this invention are applied to a planar flexible sheet having a first side and a second side opposite to the first side, wherein the second side is coated with an adhesive material, and wherein said first side is coated with a cured film of the coating of this invention which is exposed to polymerizing radiation in the presence of oxygen until cured.
Accordingly, one object of this invention is to provide improved radiation curable liquid coating compositions. Another object is to provide radiation curable compositions having low gloss and a smooth, abrasion-resistant surface which can erasibly accept markings from a dry erase writing implement. A further object of this invention is to provide flexible substrates wherein at least one side of the flexible substrate is coated with the coating of this invention and, optionally, the second side is coated with an adhesive material. These and other objects of the invention will become apparent from the following discussions.
DETAILED DESCRIPTION OF THE INVENTION
The radiation polymerizable reactants suitable for use in this invention can be any compound having polymerizable ethylenic unsaturation such as unsaturated polymers, oligomers, monomers or combinations thereof. Representative polymeric or oligomeric polyunsaturated compounds are well known in the art and can include, for example, unsaturated polyesters obtained by the reaction of polyols and maleic or fumaric acid, reaction products of polyacids or polyisocyanates with unsaturated alcohols, reaction products of polyepoxides and unsaturated acids such as acrylic or methacrylic acids, reaction products of polyols and unsaturated acids or esters, and other methods well known in the art.
For many applications, it is especially preferred to utilize unsaturated monomers and/or oligomers in order to minimize the viscosity and provide the highest application solids of the curable coatings. Especially useful monomers include vinyl and allyl compounds such as styrene, vinyl acetate, vinylidene chloride, and (meth)acrylates of monohydric alcohols, acrylamides and similar (meth)acrylate acid derivatives, such as methylmethacrylate, hydroxyethyl acrylate, acrylonitrile, and acrylamide. Especially preferred in the practice of this invention are the alkyl di-, tri-, and poly-acrylates such as, for example, ethylene glycol diacrylate, 1,3-propylene glycol diacrylate, 1,4-butanediol diacrylate,1,4-cyclohexane diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, 1,5-pentanediol diacrylate, 1,8-octanediol diacrylate, trimethylol propane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, and dipentaerythritol pentaacrylate. For some applications it is also preferred to include unsaturated fluoronated materials such as fluoroalkyl(meth)acrylates as part of the curable composition.
It is especially preferred in the practice of this invention to utilize monomers which exhibit a significant amount of shrinkage, or change in density, as a result of their polymerization from monomers to homopolymers. Although it is not our intent to be bound by theory, it is believed that this rapid change in density upon cure, in combination with the oxygen inhibition of cure at the exposed surface of the coating, combine to force the wax to the surface of the cured polymer in order to provide the low gloss and smooth abrasion-resistant surface of the cured polymer. It is especially preferred in the practice of this invention to select monomers which will provide a film shrinkage of the radiation polymerizable reactants of at least 10% upon cure. Percent shrinkage is defined as follows:
(
d2
-
d1
)
d1
×
100
=
Percent
Shrinkage
,
wherein d1 is the density of the liquid coating composition and d2 is the density of the cured coating composition.
Monomers which provide a shrinkage of at least 10% or more upon curing from monomer to homopolymer are especially preferred in the practice of this invention. Especially preferred monomers in the practice of this invention include hexanediol diacrylate, trimethylolpropane triacrylate and tripopyleneglycol diacrylate.
The waxes which can be utilized in the
Dawson William R.
Liang Tai M.
Miller Joseph E.
Berman Susan
McDonald Robert E.
The Sherwin-Williams Company
Tirey Arthi K.
Tsang Vivien Y.
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