COMPOSITES

Field of Invention

The present invention relates to pressure-sensitive adhesives products which are initially repositionable and upon application of pressure display more aggressive adhesion to a variety of substrates. More particularly, the invention is directed to repositionable pressure- sensitive adhesive composites which includes a substrate or face stock material supporting a pressure-sensitive adhesive layer comprising a blend of infusible, inherently tacky, elastomeric pressure-sensitive adhesive microspheres in a more aggressive pressure-sensitive adhesive matrix.

Background of the Invention

Removable and repositionable note papers and tapes were introduced in the 1970 ^, s and experience broad acceptance in the marketplace. The adhesives used in many of such products were infusible, inherently tacky, elastomeric, pressure-sensitive adhesive microspheres prepared by an aqueous suspension polymerization process. Aqueous suspension polymerization processes are described, for example, in U.S. Patent 3,691,140 to Silver; U.S.

Patent 4,166,152 to Baker et al; and U.S. Patents 4,495,318, and 4,598,212, to Howard each incorporated herein by reference.

To overcome the problems associated with adhesive microspheres produced by an aqueous suspension polymerization process, such as the need to recover the microspheres from the medium in which they are prepared and then redispersing them in an organic solvent for application to paper or other substrate, another process was developed which involves suspension polymerization in an organic medium. The organic medium is one in which the microspheres are prepared using shear to control microsphere diameter. Polymerization of one or more monomers preferably occurs in the presence of an initiator which is soluble in the monomers and substantially insoluble in the organic medium and in the presence of a surfactant and one or more suspension stabilizers, which may be nonionic, anionic, cationic or amphoteric in nature. Such process is disclosed in U.S. Patent 4,810,763, to Mallya et al, the disclosure of which is also incorporated herein by reference.

Repositionable pressure-sensitive adhesive articles have been described in various patents. U.S. Patent 3,314,838 to Erwin describes a pressure-sensitive adhesive containing from about 1 to 40 percent by volume of thin walled hollow non-tacky spheroidal particles having a particle size ranging from about 20 to 500 microns. These hollow spheres are described as either organic or inorganic in nature and they may comprise clay, glass, urea or phenol-formaldehyde resins, etc. By using spheroids, the pressure-sensitive adhesive is initially repositionable. However, on application of pressure, the glass spheres break increasing contact of the adhesive with a substrate to the advantage of the permanence of the

adhesive. U.S. Patent 3,331,729 to Berg et al, relates to a similar product.

A pressure-sensitive adhesive product which exhibits no adhesive tack, except upon application of threshold pressure, is described in U.S. Patent 4,376,151 to Parrotta. In this patent, microspheres of particle size from about 20 to 80 microns at about 15 - 25 percent by weight of the adhesive are used and the microspheres form a top layer completely covering the intermediate pressure- sensitive adhesive layer. On application of pressure, the top layer of microspheres are displaced by the pressure- sensitive adhesive layer.

A two step operation, wherein the adhesive is first coated followed by deposition of non-adhesive particles on the surface, is disclosed in U.S. Patent 4,556,595 to Ochi. In U.S. Patents 4,248,762 and 4,248,917 both to Hornibrook et al, the layer of pressure-sensitive adhesive contains non-leafing metallic flakes. Such a product may be repositioned when first applied without sufficient pressure and after application of pressure, exhibits more permanent adhesion.

U.S. Patent 4,151,319 to Sackoff et al describes intimately mixing a polysiloxane with a pressure-sensitive adhesive to reduce the zero minute peel value to give repositionability. Adhesion grows as the polysiloxane migrates into the adhesive with time. U.S. Patent 4,693,935 to Mazurek also describes a pressure-sensitive adhesive compositions comprising a copolymer having a vinyl polymer backing with polysiloxane moieties grafted thereto.

European Patent Publication 257984 describes tape products obtained by applying to a face stock, a layer of normally tacky pressure-sensitive adhesive containing and completely surrounding, resilient non-tacky hollow or

solid microspheres. A repositionable product may be initially formed where the microspheres are hollow. On crushing of the microspheres adhesive permanence is realized. Otherpatents whichdescriberepositionablepressure- sensitive adhesive products include U.S. Patent 3,857,731 to Courtney et al, U.S. Patent 4,149,483 to Scott, U.S. Patent 4,624,893 to Kimura et al and U.S. Patent 4,636,432 also to Kimura et al. Repeatedly useful pressure-sensitive adhesive products are described in U.S. Patents 3,857,731 to Merrill et al, U.S. Patent 4,624,893 and 4,636,432 to Shibano et al and 4,049,483 to Loder. The patent to Merrill et al is representative disclosing the adhesive microspheres which are partially embeded in and partially protruding from a binder. The '483 patent described to combining adhesive microspheres in an adhesive which binds to a substrate on activation by application of heat.

None of the prior art described above teaches or suggests the use of inherently tacky, elastomeric, pressure-sensitive adhesive microspheres for achieving a repositionable pressure-sensitive adhesive product that has high adhesion upon application of pressure, as set forth in the present invention.

Summary of the Invention

According to the present invention, there is provided a repositionable pressure-sensitive adhesive composite comprising a substrate or face stock having on at least one surface thereof, a pressure-sensitive adhesive layer comprising a pressure-sensitive adhesive matrix providing at least at the surface thereof infusible, inherently tacky, elastomeric, pressure-sensitive adhesive microspheres of an adhesivity lower than the adhesivity of the pressure-sensitive adhesive matrix and in which the microspheres extend in whole or in part from a segment surface of the pressure-sensitive adhesive matrix. The extending microsphere may be deposited in or embedded in the pressure-sensitive adhesive matrix. This provides an initially repositionable product that relies on the infusible, inherently tacky, elastomeric pressure- sensitive adhesive microspheres to allow removability and repositionability when applied to another substrate or receiving surface. They deform and/or move into the pressure-sensitive adhesive matrix under pressure to cause the more aggressive pressure-sensitive adhesive of the matrix to bond to the substrate. The level of adhesion may be a function of the pressure applied to the composite so that the composite may be adhered to a receiving surface by an adhesion ranging from removable to permanent. This enables the composite to be repositioned for initial precise placement and the bond made permanent once sufficient pressure is applied to adhere the composite to a receiving surface. The repositionable adhesive combination used in accordance with the invention provides very low tack and peel characteristics at low laminating pressure and exhibits high peel forces on application of higher pressures. Such an adhesive may be comprised of a

permanent solution pressure-sensitive adhesive as the matrix, blended with tacky, pressure-sensitive microspheres prepared in or transferred to a compatible organic medium. For emulsion adhesives the microspheres may be conveniently prepared in an aqueous medium.

Unlike a pressure-sensitive adhesive product containing hollow glass spheres, wherein once the glass spheres are broken the adhesive becomes permanent, the pressure-sensitive adhesive composite in accordance with the invention has varying peel characteristics depending on applied pressure and since the permanently tacky adhesive microspheres are able to migrate within the pressure-sensitive adhesive under applied pressure, the adhesive can make intimate contact even with rough or uneven surfaces. In addition, by using tacky, pressure- sensitive adhesive microspheres of lesser adhesivity in a more aggressive, pressure-sensitive adhesive matrix, the initial peel values may be controlled by the particle size and quantity of microspheres employed. The flexibility of use of inherently tacky elastomer polymeric microspheres used offer greater economy of manufacture with the ability of forming a more uniform product of readily controllable initial peel and tack values and final adhesion values. In the preferred construction the pressure-sensitive adhesive microspheres are of a diameter sufficient to extend at least 5 microns above the surface of the matrix. The preferred pressure-sensitive adhesive matrix is a permanent adhesive. Permanent and other adhesives may exhibit some degree of limited removability, however, the construction of the invention is not repeatedly repositionable for once sufficient pressure is applied to cause the surface of the adhesive matrix to contact the

substrates to which the product is applied, the ability to reposition the product becomes lost.

Brief Description of the Drawing

FIG. 1 is a graph illustrating the relationship of adhesion as a function of laminating force for a commercialpressure-sensitive adhesive at different levels of pressure-sensitive adhesive microspheres loadings (Examples 1 and 2 of this invention) in comparison with the adhesive alone (Control A) and a competitive product made by 3M (Control B) .

Detailed Description

The present invention is directed to pressure- sensitive adhesive products which are initially removable and repositionable as a result of using infusible, inherentlytacky, elastomeric, pressure-sensitive adhesive microspheres as part of a more aggressive adhesive matrix. The total adhesive is applied to one or more surfaces of a substrate or face stock to provide products including single- and double-coated tapes, sheet stock, labels and the like.

The infusible, inherently tacky, elastomeric pressure-sensitive adhesive microspheres used in the present invention are made by suspension polymerization. They are provided as such or in a liquid media comparable with the polymer with which the microspheres are combined to form a stable blend which does not separate with time and is easily coated onto a suitable face stock material.

The product provided in accordance with the invention can be readily made by blending a dispersion of infusible, inherently tacky, elastomeric pressure-sensitive adhesive microspheres with a more permanent and more aggressive pressure-sensitive adhesive and the mix coated onto a face stock to yield as coated or upon evaporation of a carrier liquid or solvent a product having initial properties of removability and repositionability. The microspheres are internally crosslinked making them infusible and therefore insoluble in common solvents used for solution polymers and elastomeric enabling deformation to allow the matrix to bond to the receiving substrate or surface upon application of suitable pressure.

As indicated, the adhesive matrix may be based on solution adhesive, emulsion adhesive and bulk adhesive made from a variety of polymeric materials including

acrylic and elastomeric (diene) based pressure-sensitive adhesives.

The solution polymer pressure-sensitive adhesive systems that can be used include those described in U.S. Patent No. 4,812,541 to Mallya et al incorporated herein by reference. The infusible, inherently tacky, elastomeric, pressure-sensitive adhesive microspheres useful in the present invention may, for such solution polymers, be prepared in an organic media compatible with solvent of the solution polymer by suspension polymerization as disclosed in U.S. Patent 4,810,763 to Mallya et al. Because the ethylene glycol and glycerol used as the suspension polymerization medium have low vapor pressures, it may expedient to transfer then to a more volatile organic medium, which is compatible with the solvent for the more aggressive pressure-sensitive adhesive. Alteratively, the adhesive particles can be allowed to separate from the medium, the medium drained off and replaced by the more volatile solvent for addition to the solvent based pressure-sensitive adhesive polymer. If the pressure-sensitive adhesive polymer system is an emulsion as described in U.S. Patent 4,994,538 to Lee and U.S. Patent Application Serial Number 393,970 filed August 14, 1989 each incorporated herein by reference, the microspheres can conveniently be prepared in an aqueous media such as described in U.S. Patent No. 3,691,140 to Silver and transferred to the emulsion for coating onto the face stock.

If the adhesive is bulk polymerized it is advisable to recover the microspheres from the media in which they are formed and the microspheres combined with a melt of bulk polymer for application to the face stock.

Although the invention requires use of infusible, inherently tacky, elastomeric pressure-sensitive adhesive

microspheres, it is understood that the product can be made by inclusion of other deformable or rigid microspheres which serve as fillers. They can be added, for instance, to the aggressive adhesive products which contain non adhesive microspheres as described in U.S. Patent No. 4,906,421 to Plamthottam et al., incorporated herein by reference.

A combination of normally tacky and non-tacky microspheres may also be used to achieve slideability on receiving substrates.

The base polymer of the adhesive matrix may be inherently tacky or rendered tacky by addition of a tackifier.

The properties of the adhesive matrix may be advantageously modified by thermal cure and free radical cure using actinic and electron beam radiation. Cure may be enhanced by inclusion of crosslinking agents such as multifunctional acrylates and polythiols. Since the microspheres are separately crosslinked they will undergo little change if the matrix properties are modified after formation of the blend.

Products produced in accordance with the present invention have a discontinuous pattern of the infusible, inherently tacky, elastomeric pressure-sensitive adhesive microspheres with at least segments of microspheres extending from the surface of an adhesive matrix of greater aggression on at least one side of the carrier substrate or face stock material. The microspheres preferably extend to a level of at least from about 5 microns, preferably at least 15 microns or more, above the matrix surface to limit the adhesion to the receiving surface. Given the range of useful coat weights for blends of microspheres with a matrix, the microspheres can have a diameter of at least 5 microns greater than the

thickness of the adhesive layer to assure protrusion from the surface of the adhesive matrix to provide a removable and repositionable product and one in which by application of pressure will cause the microspheres to be compressed into the body of the matrix and supplanted by the adhesive of the matrix.

The microspheres can also be deposited on the surface of the matrix as by spray coating or transfer coating. In this instance the microspheres can be as small as 5 microns in diameter. For broad based application microsphere diameter can range from about 5 to 200 microns preferably from about 20 to 150 microns. Examples l to 6 and Controls A and B

To 100 grams of Polytex 7000 a solution (34% solids) acrylic based pressure-sensitive adhesive manufactured by Avery Chemical, Division of Avery Dennison Incorporated there was added 1.05 grams of a 10% solution of aluminum acetylacetonate and 20.85 grams of an 8.6% solids dispersion of inherently tacky microspheres of 27 micron diameter in heptane. This results in a loading of five parts of dry microspheres per hundred parts of dry pressure sensitive adhesive polymer. After mixing it at high speed, the resulting slurry was directly coated on 2 mil mylar at about 50 grams/sq. m dry coat weight. The same procedure was followed for making other blends at various loadings and with different particle size microspheres at coat weights ranging from 50 to 60 g/m ² . The results are summarized in the following Table along with the results for the solution adhesive alone (Control A) and 3M's Controltac product on vinyl (Control B) . The results clearly indicate that in case of the inherently tacky adhesive microspheres, there is an optimum particle size at a given coat weight to get repositionability. The degree of repositionability as

seen by low loop tack values and low peel values under no applied pressure can be varied by changing the loading. Once squeezed, the product is not repeatedly reusable. If applied on a paper substrate, it would tear paper and would no longer be tacky. The tack values can be further suppressed by using nontacky microspheres.

The basic method of making a repositionable product involves direct mixing and directly coating on a face stock. When transfer coated, however, the microspheres may have a layer of the permanent adhesive on them, thus rendering a nonrepositionable product. This can be overcome by a two transfer coating operation; one in with the pressure-sensitive adhesive matrix is first applied to the face stock by transfer coating and in a second step where the microspheres are applied to the matrix by spraying, transfer coating and the like.

TABLE

SS = Slip Stick

ST = Slight Transfer