Processing of acrylic hotmelts by reactive extrusion

Details Processing of acrylic hotmelts by reactive extrusion Processing of acrylic hotmelts by reactive extrusion

2002-06-12

2004-06-22

Buttner, David J. (Department: 1712)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C525S285000, C525S301000, C525S303000, C525S187000, C525S124000, C525S131000

Reexamination Certificate

active

06753079

ABSTRACT:

The invention relates to a process for preparing and for functionalizing polyacrylic hotmelt pressure sensitive adhesives and also to the use of such hotmelt pressure sensitive adhesives for adhesive tapes.
BACKGROUND OF THE INVENTION
Hotmelt pressure sensitive adhesives (hotmelt PSAs) are compounds which combine the properties of hotmelt adhesives with the properties of pressure sensitive adhesives. Hotmelt PSAs melt at elevated temperatures and cool to form a permanently tacky film which flows out adhesively on contact with a substrate. In combination with different substrates, such as paper, fabric, metal, and polymer films, for example, a large number of different products can be produced, particularly PSA tapes and also labels. These PSA products have a wide field of application in the automobile industry, for fastening or for sealing, for example, or in the pharmaceutical industry, for active substance patches, for example.
Processes for preparing such hotmelt PSAs (hotmelt processes) are therefore of growing importance industrially. Generally, environmental regulations and increasing costs are driving this development process. In addition to SIS (styrene/isoprene/styrene copolymer) systems, acrylic polymers are increasingly being applied from the melt as a polymer film to backing materials. For specialty applications, moreover, PSA tapes with very low outgassing characteristics are required. This can be ensured only by means of hotmelt processes, since conventional coatings applied from solution always still contain small fractions of residual solvent.
The typical coating temperature for hotmelt PSAs lies between 80 and 180° C. In order to minimize coating and processing temperatures, the molecular weight of the hotmelt PSA to be applied should be as low as possible. On the other hand, the PSA is also required to have a certain cohesion, so that the PSA tape will not slip from the substrate in use. In order to increase the cohesion, therefore, a high molecular weight is essential.
The properties of the parent PSAs can be modified by virtue of their chemical character. Accordingly, the properties can be influenced by changing the chemical structure of the polymer chains or else by admixing other components.
For the functionalization of PSAs, the prior art has a series of processes available.
For example, PSAs have been developed which possess a relatively low molecular weight but contain double bonds along the sidechains. These polymers, such as polyester acrylates or polyurethane acrylates, for example, can be efficiently crosslinked via the double bonds, using UV or ionizing radiation, and yet have only limited adhesive properties.
In the case of acrylic PSAs, polyfunctional acrylates or methacrylates are added beforehand in order to promote crosslinking; they increase the crosslinking reactivity and thus also raise the cohesion, but react only via a two-stage mechanism during irradiation (attachment to the polymer and then crosslinking via the remaining free acrylate double bond) and therefore exhibit poor crosslinking efficiency.
The principle of functionalizing double bonds by copolymerization cannot be applied to acrylic PSAs, since in this case the corresponding polyacrylates are prepared by free radical polymerization. All of the double bonds are reacted in the polymerization process, or instances of gelling occur during polymerization. One example of this was described by Pastor [U.S. Pat. No. 4,234,662 A], who used allyl acrylate or allyl methacrylate for the polymerization. A key problem resides, however, in the copolymerization of these compounds, which generally gel during the radical polymerization process. Moreover, owing to the relatively low reactivity of the allyl groups in respect of a crosslinking reaction, drastic experimental conditions are necessary: in particular, high temperatures or a long period of irradiation. For use as crosslinked PSAs, therefore, the allyl-modified acrylic polymers are not very suitable.
Another possibility of functionalization with double bonds is presented by polymer-analogous reactions. Generally speaking, polymer-analogous reactions can be conducted in solution or from the melt. EP 0 608 981 B1 likewise refers to the gelling problems with double bonds. This is supported by various further polymer-analogous reactions. For instance, polyacrylates containing carboxylic acid, hydroxyl, epoxy, and amine groups can be reacted polymer-anlogously with compounds containing double bonds; cf. U.S. Pat. No. 4,665,106 A. Owing to the low thermal stability of the components involved, however, this reaction could not be applied to hotmelts. Moreover, unfavorable process conditions resulted from the fact that large amounts of regulator had to be added to the polyacrylate in order to prevent gelling.
Consequently, for acrylic hotmelts, U.S. Pat. No. 5,536,759 A described the reaction of hydroxyl- or carboxyl-containing polyacrylates with 1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl) benzene (m-TMI) in solution with subsequent hotmelt workup. As a result of the functionalization with the double bonds, the polyacrylates described can be crosslinked very efficiently with UV light and so produce PSAs featuring high cohesion. Disadvantages of this process are the high toxicity of the isocyanates used and the complex two-stage process, since the polymerization is carried out in a first step and the polymer-analogous reaction has to be carried out in a second step in a reactor. Moreover, polyacrylates with double-bond functionalization lack thermal stability and are sensitive to shearing.
In the U.S. Pat. No. 5,741,543, polyacrylates functionalized with double bonds, prepared by a polymer-analogous reaction, are UV crosslinked. The polymers were prepared by the UV prepolymerization technique, which exhibits the known disadvantages such as slow process rates and the free traffic of monomers.
In the U.S. Pat. Nos. 3,786,116, 3,832,423, and 3,862,267, polyvinyl chloride or methacrylate esters are functionalized with polystyrene; however, these patents do not describe PSAs. In the U.S. Pat. No. 4,693,776, acrylate- or methacrylate-functionalized polystyrene was copolymerized with acrylic monomers and, in the form of PSA, were used in particular for bonds on human skin. These functionalized PSAs are, however, very difficult to prepare, since these macromonomers possess only low reactivity and can therefore be polymerized only to relatively low conversions. Likewise, these PSAs are difficult to concentrate, since unreacted residual monomers are removed again in the concentration process and thus sensitively disrupt the hotmelt process.
The U.S. Pat. No. 4,994,322 describes repositionable acrylic PSAs prepared again by copolymerization via acrylate- or methacrylate-functionalized polystyrene. These PSAs further comprise microspherical particles. The disadvantages attending U.S. Pat. No. 4,693,776 are present here as well.
In EP 0 707 604 A1, polyethylene/butene macromonomers are used for copolymerization with acrylates. This supports phases with a low glass transition temperature, which in turn allow the adhesives to flow on apolar surfaces, and thus ensure high bond strengths on PE and PP. A disadvantage is the poor conversion of the polymerization process described. Patents U.S. Pat. Nos. 5,614,586 and 5,674,275 describe tacky hydrogels which can be prepared from ethoxylated comonomers. The materials produced are repositionable but are not PSAs.
In contrast, the pros and cons of the individual processes are described [Chemie Ingenieur Technik (70), 1998, pp. 560-566]: “Polymer-analogous reactions in the melt make possible two processes which otherwise take place separately from one another. First of all, the reaction takes place; since the reaction medium is the melt, shaping by extrusion is able to be commenced even during the reaction. In this way, no additional reaction vessel nor any workup whatsoever are required. The absence of the solvent does, however, complicate the course of the reaction in a multiplicity of resp

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