Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2001-01-16
2002-08-20
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C522S004000, C427S059000
Reexamination Certificate
active
06436532
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the production of adhesives and more particularly, it relates to a multi-stage irradiation process for the production of acrylic-based adhesives, especially pressure-sensitive adhesive tapes. It also relates to acrylic-based adhesives and tapes made by the novel processes of this invention.
BACKGROUND OF THE INVENTION
It is known in the literature and in the industry that there are at least four different feasible methods for the production of acrylic-based pressure-sensitive adhesive (hereinafter “psa”) tapes. These known methods include solution polymerization, emulsion polymerization, irradiation by high energy particulate matter (e.g., electron beams or gamma rays), and ultraviolet light (hereinafter “UV”) photopolymerization. As explained below, however, there are disadvantages and/or limitations incurred with the use of each known process.
When utilizing psa's made by solution polymerization, elaborate drying ovens with massive exhaust ducts and high temperatures are required to carry away the volatile solvents after coating. Furthermore, to prevent the solvents from being vented to the atmosphere (with resulting pollution and solvent loss), expensive solvent recovery equipment has been necessary. Safety hazards in such operations are also severe, as the solvents are extremely flammable and precautions must be taken to avoid explosive mixtures in the oven and exhaust systems. A further limitation of the solvent based systems is the limit on the thickness of the coatings which can be deposited in one trip or pass through the coater. Thus, with coatings above about 5 mils, multiple coating layers must be deposited in successive trips through the coater to avoid blistering of the coating due to solvent evaporation.
While emulsion polymerization has eliminated the problems associated with the handling and evaporation of flammable solvents, the heat of vaporization must be supplied to remove the water from the coating and essentially the same equipment must be employed. Though higher solids coatings are possible, the higher heat of vaporization of water as compared to organic solvents offsets this benefit and about the same total energy for drying is required. Drying times are relatively long, thus limiting production. One of the most serious limitations of the emulsion polymerization process is the water sensitivity of the resulting polymers (caused by the emulsifying agent which is carried along in the process and becomes part of the final polymer). A further limitation of this process is that highly polar monomers, which are water miscible, are difficult to incorporate into the copolymer during polymerization and considerable homopolymerization of such monomers can occur in the aqueous phase.
Various attempts have been made to avoid the difficulties of the solution and emulsion polymerization processes. However, those have thus far resulted either in processing difficulties of their own or have produced polymers in which a proper balance between compliance and cohesive strength is very difficult to control.
More recently, development work has been done with polymerization processes which employ either ultraviolet light or electron beams. One patent which stresses electron beam curing is U.S. Pat. No. 3,897,295, in which the composition subject to the electron beam includes an acrylate monomer selected from a particular specified group, and a homopolymer or copolymer of a substance or substances selected from the same group. The polymer is dissolved in the monomer and the monomer is ultimately polymerized to bind the adhesive together.
The disadvantage of utilizing polymerization processes involving an electron beam, though, is that, generally, it is a rather indiscriminate polymerization process. In polymerization processes utilizing an electron beam, the particulate bombardment of the free-radically polymerizable monomers cannot be precisely controlled, with the result being chain scission of the developing polymer and an inability to control its molecular weight and crosslink density to the most desired range.
In order to avoid the above-discussed disadvantages incurred with the use of an electron beam, some have chosen to utilize a one step or stage low-intensity (e.g., 0.1 to 7 mW/cm
2
) UV photopolymerization process. See, for example, U.S. Pat. No. 4,181,752. Whereas the use of relatively low intensity UV light is very desirable for building higher molecular weight acrylic psa's with good performance properties, an increase in the speed of the photopolymerization process would be desirable. However, if one attempts to increase the speed of the low intensity UV light-based process by increasing the amount of the photoinitiator employed (e.g., benzoin ethers, benzil ketals, etc.), then undesirable lower molecular weight polymers will be obtained. Furthermore, for thick adhesives an uneven polymerization from the front surface to the back surface of an irradiated adhesive composition occurs due to the uneven light absorption by the photopolymerization initiator resulting in a differential performance of the final psa product.
In view of the foregoing discussed disadvantages and limitations that exist with the use of conventional polymerization processes, improvements are continuously desired and sought by those within the industry. It was against this background that an improved polymerization or irradiation process for producing acrylic-based adhesives, and in particular acrylic-based psa tapes was sought.
SUMMARY OF THE INVENTION
By the present invention, we have now discovered that an increase in the speed of the photopolymerization process for the production of acrylic-based adhesives and acrylic-based psa tapes with acceptable product properties can be achieved by utilizing a multi-stage (as opposed to the conventional one-step) irradiation process, the first stage employing electromagnetic radiation at a relatively low intensity and the subsequent stage employing electromagnetic radiation at a relatively higher intensity.
Accordingly, the inventive multi-stage irradiation process for the production of an acrylic-based adhesive comprises the sequential steps of:
(a) forming a monomeric mixture or partially prepolymerized syrup comprising:
(i) 50-100 parts by weight of at least one acrylic acid ester of an alkyl alcohol, the alcohol containing from 1 to 14 carbon atoms;
(ii) 0-50 parts by weight of at least one copolymerizable monomer; and
(iii) a photoinitiator;
(b) irradiating the resulting monomeric mixture or syrup with electromagnetic radiation of from 280 to 500 nanometer wavelength and from 0.01 to 20 mW/cm
2
average light intensity to effect conversion of from about 5-95 wt % of the monomeric mixture or partially prepolymerized syrup to an acrylic copolymer; and
(c) thereafter, irradiating the acrylic copolymer resulting from step (b)) with electromagnetic radiation of from 280 to 500 nm wavelength and having an average light intensity of greater than 20 mW/cm
2
to at least substantially complete the photopolymerization reaction of the acrylic copolymer.
In another embodiment of the present invention, pressure-sensitive adhesive tapes are produced by coating the monomeric mixture or prepolymerized syrup onto a substrate and thereafter, irradiating the coated substrate as per steps (b) and (c) of the foregoing disclosed process.
In still other embodiments of the present invention, acrylic-based adhesives and psa tapes are provided which are produced by the foregoing disclosed, inventive processes.
The inventive multi-stage irradiation process does not have the limitations and drawbacks discussed earlier herein associated with conventional polymerization processes. The fact that the inventive process employs sequential irradiation steps or stages, one involving exposure to electromagnetic radiation at low average intensity and a subsequent stage or step involving exposure to electromagnetic radiation at a higher average intensity, is significant because the process is not only highly e
Anderson Robert D.
Edberg Mark S.
Mehner Robert C.
Moon John D.
Vesley George F.
3M Innovative Properties Company
Dawson Robert
Keehan Christopher M.
Pastirik Daniel R.
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