Photoreactor composition and applications therefor

Details Photoreactor composition and applications therefor Photoreactor composition and applications therefor

1998-04-29

2001-06-05

Berman, Susan W. (Department: 1711)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S511000, C427S519000, C428S378000, C430S281100, C430S284100, C442S149000, C442S164000, C522S034000, C522S002000, C522S042000, C522S055000, C522S049000, C522S096000, C522S173000, C522S071000, C522S075000, C522S081000, C522S036000, C522S038000, C522S040000, C522S041000, C522S043000, C522S044000, C522S045000, C522S050000, C522S057000, C522S064000, C523S160000, C156S275500

Reexamination Certificate

active

06242057

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a composition and method for generating a reactive species. The present invention more particularly relates to a composition and method for generating reactive species which can be used to polymerize or photocure polymerizable unsaturated material. In particular, the present invention provides a photoreactor that absorbs radiation at a wavelength that is particularly suited to exposure to lamps that emit at substantially the same wavelength.
BACKGROUND OF THE INVENTION
The present invention relates to a method of generating a reactive species. The present invention also relates to radiation-initiated polymerization and curing processes. For convenience, much of the discussion which follows centers on free radicals as a particularly desirable reactive species. Such discussion, however, is not to be construed as limiting either the spirit or scope of the present invention.
Polymers have served essential needs in society. For many years, these needs were filled by natural polymers. More recently, synthetic polymers have played an increasingly greater role, particularly since the beginning of the 20th century. Especially useful polymers are those prepared by an addition polymerization mechanism, i.e., free radical chain polymerization of unsaturated monomers, and include, by way of example only, coatings and adhesives. In fact, the majority of commercially significant processes is based on free-radical chemistry. That is, chain polymerization is initiated by a reactive species which often is a free radical. The source of the free radicals is termed an initiator or photoinitiator.
Improvements in free radical chain polymerization have focused both on the polymer being produced and the photoinitiator. Whether a particular unsaturated monomer can be converted to a polymer requires structural, thermodynamic, and kinetic feasibility. Even when all three exist, kinetic feasibility is achieved in many cases only with a specific type of photoinitiator. Moreover, the photoinitiator can have a significant effect on reaction rate which, in turn, may determine the commercial success or failure of a particular polymerization process or product.
A free radical-generating photoinitiator may generate free radicals in several different ways. For example, the thermal, homolytic dissociation of an initiator typically directly yields two free radicals per initiator molecule. A photoinitiator, i.e., an initiator which absorbs light energy, may produce free radicals by either of two pathways:
(1) the photoinitiator undergoes excitation by energy absorption with subsequent decomposition into one or more radicals; or
(2) the photoinitiator undergoes excitation and the excited species interacts with a second compound (by either energy transfer or a redox reaction) to form free radicals from the latter and/or former compound(s).
While any free radical chain polymerization process should avoid the presence of species which may prematurely terminate the polymerization reaction, prior photoinitiators present special problems. For example, absorption of the light by the reaction medium may limit the amount of energy available for absorption by the photoinitiator. Also, the often competitive and complex kinetics involved may have an adverse effect on the reaction rate. Moreover, commercially available radiation sources, such as medium and high pressure mercury and xenon lamps, emit over a wide wavelength range, thus producing individual emission bands of relatively low intensity. Most photoinitiators only absorb over a small portion of the emission spectra and, as a consequence, most of the lamps' radiation remains unused. In addition, most known photoinitiators have only moderate quantum yields (generally less than 0.4) at these wavelengths, indicating that the conversion of light radiation to radical formation can be more efficient.
Thus, there are continuing opportunities for improvements in free radical polymerization photoinitiators.
SUMMARY OF THE INVENTION
The present invention addresses some of the difficulties and problems discussed above by the discovery of an efficient composition and method for utilizing radiation. Hence, the present invention includes a composition and methods for generating a reactive species which includes providing one or more wavelength-specific sensitizers in association with one or more reactive species-generating photoinitiators and irradiating the resulting wavelength specific photoreactor composition. One of the main advantages of the wavelength specific photoreactor composition of the present invention is that it can be used to efficiently generate reactive species under extremely low energy lamps, such as excimer lamps, as compared to prior art lamps.
The association of one or more wavelength-specific sensitizers with one or more reactive species-generating photoinitiators results in a structure that is a wavelength specific photoreactor composition, that is referred to herein for convenience as a photoreactor. The present invention includes arylketoalkene wavelength-specific sensitizers. One major advantage of the wavelength specific photoreactor compositions is the use of arylketoalkene wavelength-specific sensitizers. The wavelength specific photoreactor compositions that contain the arylketoalkene wavelength-specific sensitizers efficiently absorb radiation at wavelengths between approximately 250 nm and 350 nm. Even more advantageous are the arylketoalkene sensitizers wherein the alkene group is located between the aryl and keto groups.
Another major advantage of the photoreactor compositions of the present invention is that, when combined with polymerizable material, they cause rapid curing times in comparison to the curing times of the prior art with relatively low output lamps. Yet another advantage of the present invention is that the multi-photoinitiator photoreactors of the present invention or the multi-wavelength specific sensitizer photoreactors have even faster curing times in comparison to the single-photoinitiator photoreactors of the present invention and/or are more sensitive than the single sensitizer photoreactors of the present invention.
The photoreactor compositions of the present invention also differ from the prior art in that the prior art sensitizers absorb a band width of radiation, whereas the sensitizer of the present invention absorbs a substantially single wavelength. The use of a wavelength specific photoreactor composition capable of absorbing a substantially single wavelength of radiation results in an efficient photoreactor upon exposure to a very narrow bandwidth of radiation or upon exposure to a single wavelength of radiation. The present method involves effectively tuning the energy-absorbing entity, referred to herein as a wavelength specific photoreactor composition, or photoreactor for convenience, to efficiently utilize an emitted band of radiation. The wavelength-specific sensitizer effectively absorbs photons and efficiently transfers the absorbed energy to a photoinitiator which, in turn, generates a reactive species. The wavelength-specific sensitizer is adapted to have an absorption peak generally corresponding to a maximum emission band of the radiation source.
The present invention includes various combinations of wavelength-specific sensitizers and photoinitiators. By varying the combination, one can effectively increase the number of photoinitiators per wavelength specific photoreactor composition or effectively increase the number of wavelength-specific sensitizers per photoreactive composition.
The present invention also includes a method of polymerizing an unsaturated monomer by exposing the unsaturated monomer to radiation in the presence of the efficacious wavelength specific photoreactor composition described above. When an unsaturated oligomer/monomer mixture is employed in place of the unsaturated monomer, curing is accomplished.
The present invention further includes a film and a method for producing a film, by drawing an admixture of unsaturated polymeri

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