Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Compositions to be polymerized by wave energy wherein said...
Reexamination Certificate
1998-07-17
2001-05-15
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Compositions to be polymerized by wave energy wherein said...
C528S176000
Reexamination Certificate
active
06232365
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods of polymerizing monomers and/or oligomers using accelerated electrons and articles made thereby. The invention particularly relates to the electron beam polymerization of acrylate pressure-sensitive adhesives.
BACKGROUND OF THE INVENTION
Electron beam (e-beam) irradiation is widely used to crosslink a variety of different polymers for purposes of improving various properties such as hardness, tensile strength and shear strength. E-beam irradiation is also used to polymerize multifunctional oligomers and/or multifunctional monomers to make hard, scratch resistant coatings. However, due to the tendency of e-beam irradiation to effect substantial crosslinking, chain scission and grafting within treated polymers, e-beam irradiation is not generally considered an effective option for forming polymers having a high molecular weight distribution between crosslinks. Hence, other forms of irradiation, such as gamma ray and ultraviolet light, are commonly used to make polymers requiring a high molecular weight distribution between crosslinks, such as polymers having a low Tg and pressure-sensitive adhesives requiring high shear strength and high peel adhesion.
Polymers in general, and pressure-sensitive adhesives in particular, are increasingly being asked to meet demanding application requirements within ever tighter performance limitations and manufacturing specifications (e.g., polymerization with minimal use of environmentally damaging solvents).
For certain applications, such as pressure-sensitive adhesive applications, polymers need to exhibit the high tensile strength and elongation properties attainable only with high molecular weight lengths between crosslinks. Polymers having high molecular weight lengths between crosslinks also provide the necessary balancing of viscous and elastic properties required for a pressure-sensitive adhesive. In general, the adhesion, cohesion, stretchiness, and elasticity of a pressure-sensitive adhesive is dictated by the attributes of the polymer used in the composition.
Gamma irradiation is useful for sterilizing products as well as polymerizing polymers in bulk and batch processes, but is generally too slow for use in making adhesive tape in a continuous, on-web process.
Ultraviolet light is useful for polymerizing various polymers, including pressure-sensitive adhesives, without the use of a solvent. However, ultraviolet polymerization has several significant limitations. First, ultraviolet light polymerization occurs only in connection with monomeric species that absorb ultraviolet light. Since most monomers do not absorb sufficient ultraviolet light to initiate polymerization, photoinitiators are usually added to the monomers. Photoinitiators are materials that decompose into free-radicals or cations upon exposure to ultraviolet radiation. These free-radicals and/or charged molecules then initiate polymerization of the monomers. Unfortunately, photoinitiators can contribute various undesirable attributes to the resultant polymer, including yellowing of the polymer, contamination of the polymeric material with unreacted initiator and initiator fragments added to the polymer, and higher overall material costs. These contaminants can be undesirable because they can form color centers, tend to migrate to the adhesive bond surface where they inhibit performance, and are capable of initiating subsequent reactions upon exposure to ultraviolet light. The presence of such contaminants are particularly troublesome in the food and medical areas where the presence of photoinitiators is simply unacceptable due to the potential for skin irritation and toxicity.
Second, only monomeric compositions which are transparent to ultraviolet light can be effectively polymerized with ultraviolet light. Compositions that are translucent or opaque to ultraviolet light will only polymerize on the surface, since ultraviolet light cannot penetrate into the bulk of the composition. Thus, the use of many desired modifiers that absorb ultraviolet light (such as carbon black, glass beads, ultraviolet light stabilizers and colorants) and various physical structures (such as foamed adhesives and suspended particles) is limited when the monomeric composition is to be polymerized by ultraviolet light.
E-beam polymerization overcomes a number of the limitations inherent with gamma ray and ultraviolet light polymerization. However, the use of e-beam polymerization has been limited due to the inherent tendency of e-beam radiation to produce short-chain, branched, highly crosslinked polymeric structures. This phenomenon is exhibited by the tendency for e-beam polymerized pressure-sensitive adhesives to exhibit pop-off failures, frequently accompanied by low peel strength. A second limitation observed with typical e-beam polymerization techniques is a substantial concentration of residuals (e.g., unreacted monomers) remaining in the resultant polymerized product (i.e., low conversion).
SUMMARY OF THE INVENTION
A continuing need exists for a solventless and chemical initiator-free method of effecting the high-conversion radiation polymerization of polymers, particularly those intended for use as pressure-sensitive adhesives, which is effective for producing long-chain polymers with limited crosslinking over a broad range of coated thicknesses and with high conversion, regardless of whether the composition is transparent, translucent or opaque.
The present invention provides a method for polymerizing a free-radical initiated adhesive syrup of a C
8-13
alkyl acrylate monomer, optionally including one or more copolymerizable monomers, after coating of the syrup onto a surface, by irradiating the adhesive syrup with accelerated electrons at a temperature below 20° C. Preferred C
8-13
alkyl acrylate monomers include isooctyl acrylate and 2-ethylhexyl acrylate. The optional copolymerizable monomers are preferably polar monomers selected so as to form a pressure-sensitive adhesive when polymerized with the C
8-13
alkyl acrylate monomer. The invention also encompasses adhesive compositions produced by this process.
A preferred embodiment of the invention involves irradiation of the adhesive syrup at a temperature below 20° C. with about 20 to 100 kilogray (kGy) of accelerated electrons over a residence time of greater than about 1 second, preferably greater than about 5 seconds, to form a pressure-sensitive adhesive. The preferred embodiment is generally effective for making an acrylate pressure-sensitive adhesive having a peel adhesion to glass of greater than about 25 Newton/decimeter (N/dm) and a shear strength of greater than about 300 minutes, with a conversion of greater than about 90 wt %.
A further preferred embodiment of the invention involves irradiation of the adhesive copolymerizable syrup at a temperature below 20° C. with about 20 to 100 kGy of accelerated electrons over a residence time of greater than about 1 second, preferably greater than about 5 seconds, to form a pressure-sensitive adhesive. The further preferred embodiment is generally effective for making an acrylate pressure-sensitive adhesive having a peel adhesion to glass of greater than about 55 N/dm and a shear strength of at least 10,000 minutes, with a conversion of greater than about 97 wt %.
The invention is effective for quickly and efficiently producing polymers having a crosslink density suitable for use in a pressure-sensitive adhesive composition requiring superior peel adhesion and superior shear strength, without the use of solvents or chemical initiators and with high conversion.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
As utilized herein, including the claims, the phrase “syrup” is utilized in accordance with its conventional definition to reference compositions of one or more polymerizable monomers, oligomers and/or polymers that have coatable viscosities and do not exhibit any appreciable pressure-sensitive adhesive characteristics until cured. Such syrups typically achieve a coatable viscosity through partial polymerization or th
Sventek Bruce A.
Tran Thu-Van T.
Weiss Douglas E.
3M Innovative Properties Company
Boykin Terressa M.
Gover Melanie G.
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