Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1998-06-11
2001-05-01
Seidleck, James J. (Department: 1711)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S510000, C427S256000, C522S002000, C522S046000, C522S039000, C522S099000
Reexamination Certificate
active
06224949
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a process for polymerizing a radiation polymerizable composition and more particularly to a polymerization process using a monochromatic radiation source. An efficient method for crosslinking a radiation crosslinkable polymeric coating also is described.
BACKGROUND OF THE INVENTION
Radiation-induced free radical polymerization of ethylenically unsaturated monomers is known with both batch and continuous polymerizations having been achieved by this technique. While a desire to build high molecular weight product tends to make batch polymerization methods relatively slow, it generally is desirable to achieve final product in a much shorter time when polymerizing coatings applied to a substrate. Radiation-induced free radical polymerization of coatings currently is accomplished by exposing a polymerizable coating to the output of a high intensity source of radiation, typically a medium pressure mercury lamp or a doped mercury lamp. Photoinitiator(s) present in the coatings absorb certain regions of the spectral output of the lamp generating free radicals and initiating the polymerization process. There is, however, a need to perform radiation-induced free radical polymerization of coatings with greater energy efficiency and with short residence times, particularly on substrates that are adversely affected by heat generated from the radiation sources. Similar considerations are associated with crosslinking of polymeric coatings containing radiation activatable crosslinking agents.
The radiation-induced free radical polymerization of coatings comprising ethylenically unsaturated monomers or oligomers is accomplished by exposing a coating to the spectral output from a high intensity radiation source comprising one or more bulbs that typically have an input power per bulb of about 40 W/cm or greater. These same sources commonly are used for crosslinking polymeric coatings containing radiation activatable crosslinking agents. When a source contains mercury at these power levels, its spectral output occurs over a large range of wavelengths, including the ultraviolet, the visible, and the infrared region of the electromagnetic spectrum. The presence of a radiation activatable species (a photoinitiator for a polymerizable coating or a photocrosslinker for a polymeric coating) generally is required for these systems to work. Photoinitiators and photocrosslinking agents do not, in general, absorb appreciably in either the visible or the infrared regions of the spectrum and therefore only a small percentage of the total spectral output from these sources is used to induce polymerization of a polymerizable coating or to crosslink a polymeric coating. Further, differential absorption by the radiation activatable species within a coating causes large cure gradients to form between its top and bottom surfaces. Infrared radiation and radiant heat often also are undesirable themselves as their presence causes low molecular weight species to volatilize from coatings, distorts heat sensitive backings, and requires shutters and/or safety interlocks to minimize the potential for fire.
Processes that employ energy more efficiently recently have been introduced. Stark and Wright in WO 97/39837 describe use of a monochromatic light source, preferably a xenon chloride excimer lamp having a peak wavelength of 308 nm, to crosslink a hot melt coatable adhesive composition. In these compositions, the absorptivity of the photoactivatable crosslinking agent at the wavelength of the excimer source is low allowing for the penetration of the light through the entire coating thickness causing a minimal cure gradient to form.
Wright in WO 97/40090 describes use of a monochromatic light source, preferably a krypton chloride excimer lamp having a peak wavelength of 222 nm, to cure free radically polymerizable coatings without the necessity of a photoinitiator. When photoinitiators are present, the light is not used efficiently due to competing absorption by the polymerizable monomers and oligomers.
Nohr and MacDonald in WO 96/00740 describe a general method for generating a reactive species by providing a polymolecular photoreactor comprising a wavelength specific sensitizer in association with a reactive species generating photoinitiator and irradiating the sensitizer with actinic radiation. In one embodiment, an excimer lamp is employed as the radiation source.
There remains a need to perform radiation-induced polymerization of free radically polymerizable coatings containing a photoinitiator and a need to perform radiation induced crosslinking of polymeric coatings containing a photoactivatable crosslinking agent with greater energy efficiency and with shorter exposure times.
SUMMARY OF THE INVENTION
Briefly, in one aspect, the present invention provides a polymerization method comprising the steps of:
(a) providing a substrate coated on at least a portion thereof with a radiation polymerizable composition comprising: (i) an ethylenically unsaturated free radically polymerizable monomer, oligomer, or blend thereof, and (ii) a free radical photoinitiator;
(b) providing a source of essentially monochromatic radiation having a wavelength effective to activate said photoinitiator; and
(c) exposing the radiation polymerizable composition to the radiation emitted by the radiation source for a time sufficient to polymerize the composition;
wherein the photoinitiator and the wavelength of the radiation source are selected such that the extinction coefficient of the photoinitiator at the peak wavelength of the source is greater than about 1000 M
−1
cm
−1
and wherein the photoinitiator absorbs at least two percent of the actinic radiation emitted by the source incident on the coating.
In another aspect, this invention provides a polymerization method comprising the steps of:
(a) providing a moving substrate coated on at least a portion thereof with a radiation polymerizable composition comprising: (i) an ethylenically unsaturated free radically polymerizable monomer, oligomer, or blend thereof, and (ii) a free radical photoinitiator;
(b) providing a source of essentially monochromatic radiation comprising one or more lamps wherein each lamp has an input power of less than about 10 W/cm and a wavelength effective to activate said photoinitiator; and
(c) exposing the radiation polymerizable composition to the radiation emitted by the radiation source for a time sufficient to polymerize the composition.
In a further aspect, this invention provides a method for crosslinking a polymeric coating comprising the steps of:
(a) providing a moving substrate coated on at least a portion thereof with a crosslinkable composition comprising: (i) a radiation crosslinkable polymer containing abstractable hydrogen atoms, and (ii) a radiation activatable crosslinking agent;
(b) providing a source of essentially monochromatic radiation comprising one or more lamps wherein each lamp has an input power of less than about 10 W/cm and a wavelength sufficient to activate the crosslinking agent and to crosslink the polymer; and
(c) exposing the radiation crosslinkable composition to the radiation emitted by the monochromatic source for a time sufficient to crosslink the polymer.
Articles made from the above methods, including polymer films having release coatings, adhesive coatings, hard coatings and the like thereon, also are provided. The methods provided by the invention allow for the preparation of polymer coatings by appropriate selection of the wavelength and/or power of the monochromatic radiation source and the photoinitiator or photocrosslinker. Such methods allow for precise manipulation of the cure rate and cure depth such that a rapid and efficient cure may be used. Highly energy efficient radiation-induced polymerization and crosslinking methods also are provided.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In accordance with one aspect of the present invention, a polymerizable composition containing an ethylenically unsaturated free radically polymerizable mono
Vesley George F.
Wright Robin E.
3M Innovative Properties Company
Burtis John A.
Gover Melanie
McClendon Sanza
Seidleck James J.
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