Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
1995-08-14
2002-04-09
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C522S038000, C522S039000, C522S046000, C522S063000, C522S064000, C522S074000, C522S081000, C522S082000, C522S083000, C522S033000, C522S126000, C522S127000, C522S130000, C522S134000, C522S146000, C522S148000
Reexamination Certificate
active
06369123
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel radiation-activatable photocrosslinking agents. This invention also relates to radiation-crosslinkable elastomers. This invention further relates to radiation-crosslinked elastomners.
BACKGROUND OF THE ART
It is known that crosslinking of polymers produces polymer networks which have quite different mechanical and physical properties compared to their uncrosslinked linear or branched counterparts. For example, polymer networks can show such unique and highly desirable properties as solvent resistance, high cohesive strength, and elastomeric character.
Crosslinked polymers can be made in situ during formation of the desired polymer product, however, since further processing of the polymer product is often necessary, it is more typical to start from the linear or branched polymer which in the final processing step is cured to a crosslinked material. The curing or crosslinking step is typically activated by moisture, thermal energy, or radiation. The latter has found widespread applications, particularly in the use of ultraviolet light as the radiation source.
In the past, a variety of different materials have been used as crosslinking agents, e.g., polyfunctional acrylates, acetophenones, benzophenones, and triazines. The foregoing crosslinking agents, however, possess certain drawbacks which include one or more of the following: high volatility; incompatibility with certain polymer systems; generation of corrosive or toxic by-products; generation of undesirable color; requirement of a separate photoactive compound to initiate the crosslinking reaction; and high sensitivity to oxygen.
Certain polyfunctional benzophenones have been investigated as photocrosslinking agents and/or photosensitizers in various photopolymerizable systems.
JP 54/057560 discloses the use of (bis)benzophenone compounds to photocrosslink non-elastomeric materials in particular, polyester compositions. When incorporated into polyesters, they impart improved tensile strength and elongation to biaxially stretched films of crosslinked poly(ethylene terephthalate). These films also exhibit enhanced weather, heat, and chemical resistance and improved dimensional stability.
U.S. Pat. No. 4,602,097 (Curtis) discloses the use of (bis)benzophenones as photoinitiators and/or photosensitizers in radiation-cured coatings. The poly(ethylene oxide) moiety which separates the terminal benzophenone groups allows the claimed compositions to be more soluble than unsubstituted-benzophenones in waterborne coating compositions. The (bis)benzophenone compounds, however, contain hydrogen donating groups, such as the methylenes adjacent to the oxygen atoms of the ether functionalities. These hydrogen donating groups undergo an intramolecular hydrogen abstraction by the photochemically excited (bis)benzophenone structure to provide a lower energy radical which is effective as an initiator, but unsuitable as a photocrosslinker.
PCT Patent Appln. WO 93/16131 and U.S. Pat. No. 5,407,971 (Everaerts et al.) describes a radiation-crosslinkable elastomeric composition containing: (a) an elastomeric polymer containing abstractable hydrogen atoms in an amount sufficient to enable the elastomeric polymer to undergo crosslinking in the presence of a suitable radiation-activatable crosslinking agent; and (b) a radiation-activatable polyfunctional acetophenone or benzophenone crosslinking agent. According to Formula (1) of this application, if substituent “W” is present (i.e., the aceto- or benzophenone moieties of these crosslinkers have an ether, thioether or amino linkage), then an internal ketone, ester or amide functionality (i.e., substituent “Y”) must also be present. From a synthetic standpoint, such crosslinkers are prepared in a reaction sequence involving at least two steps. The first step involves preparation of an acetophenone- or benzophenone-functional alkyl ester derivative. The second step involves the reaction of this of this alkyl ester with either short chain or higher molecular weight nucleophiles. Additional reaction steps may also be required if other functionalities, such as urethane groups, are desired in spacer “Z”.
U.S. Pat. No. 4,379,201 (Heilmann et al.) is an example of a class of polyacrylic-functional crosslinkers used in the photocuring of (meth)acrylate copolymers. U.S. Pat. No. 4,391,678 (Vesley) and U.S. Pat. No. 4,330,590 (Vesley) describe a class of fast curing triazine photocrosslinkers which, when mixed with an acrylic monomer and, optionally, a monoethylenically unsaturated monomer, and exposed to UV radiation, forms a crosslinked polyacrylate. The crosslinks formed by both the (meth)acrylates and the triazines in these copolymerizations prevent any further processing, such as hot melt coating, reactive extrusion, or solution coating processes, following the initial photopolymerization.
U.S. Pat. No. 4,737,559 (Kellen et al.) discloses acrylate-functional aromatic ketones (in particular, 4-acryloxybenzophenone “ABP”) which are incorporated with other (meth)acrylate monomers to form pressure-sensitive adhesive copolymers containing pendant benzophenone groups. These benzophenone functional pressure-sensitive adhesive copolymers undergo efficient crosslinking upon exposure to UV light, especially when compared to the use of conventional benzophenones as a photocrosslinker. This patent also specifically states that the disclosed compounds must be free of hydroxy groups in a position ortho to the carbonyl functionality. These hydroxy substituents inhibit free-radical formation and hydrogen abstraction from the acrylate copolymer backbone. However, since these acrylate-functional aromatic ketones are monomers to be copolymerized primarily with other acrylic monomers, they are not usefull as a post-polymerization photocrosslinker which may be compounded with previously prepared elastomeric polymers of varying chemical character.
There is a strong desire to be able to crosslink adhesive systems after all processing requirements have been accomplished. As the industry moves towards the use of hot-melt adhesives and away from solvent-based coatings, this requirement becomes even more important. Many approaches and polymer types have been studied to obtain the desired properties. E-beam and UV radiation curing have been leading the way with respect to post-radiation curing. There are problems associated with both routes and no universal solution is currently apparent.
It was against the foregoing background that a search for improved radiation-crosslinkable materials and radiation-activatable crosslinking agents was conducted.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, there is provided a radiation-crosslinkable composition comprising: (a) an elastomeric polymer containing abstractable hydrogen atoms in an amount sufficient to enable the elastomeric polymer to undergo crosslinking in the presence of a suitable radiation-activatable crosslinking agent; and (b) a radiation-activatable crosslinking agent of the formula:
wherein:
X represents CH
3
—; phenyl; or substituted-phenyl with the proviso that any substituents on the substituted-phenyl do not interfere with the light-absorbing capacity of the radiation-activatable crosslinking agent and do not promote intramolecular hydrogen abstraction of the radiation activatable crosslinking agent;
W represents —O—,—NH—, or —S—;
Z represents an organic spacer selected from the group consisting of aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic groups free of esters, amides, ketones, and urethanes, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms intramolecularly accessible to the carbonyl group in formula (I); and
n represents an integer of 2 or greater; preferably 2-6.
It is also within the spirit and scope of the present invention that the phenylene ring of formula (I) linking a carbonyl group and “W” can also contain one or more substituents which do not interfere with the light-absorbing capacity of the crosslinking
Everaerts Albert I.
Stark Peter A.
Stewart Edward G.
3M Innovative Properties Company
Berman Susan W.
Dahl Philip Y.
Evearitt Gregory A.
Skolnick Steven E.
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