Method for forming a crosslinkable pressure-sensitive adhesive construction containing an aminoxysilicon compound and an amidosilicon compound

Abstract: The invention relates to a method of contacting two components to form a pressure-sensitive adhesive (PSA) construction, wherein the first component comprises a conventional PSA and an aminoxysilicon compound and the second component comprises the same PSA and an amidosilicon compound. The method provides improved adhesive bond strength over an unmodified PSA construction.

Description: The present invention relates to an improved method for forming a pressure-sensitive adhesive construction. More particularly, the present invention relates to a method of contacting two distinct pressure-sensitive adhesive components to form the construction, wherein an aminoxysilicon compound is incorporated in the first component and an amidosilicon compound is incorporated in the second component. It is well known in the adhesives art that significant improvement in adhesion to various substrates can be obtained by incorporating various adhesion promoters into adhesive compositions or employing such adhesion promoters in primer compositions for coating the substrate. Development of a wide-ranging adhesives art has benefited from this method of adhesion improvement and various organosilanes have been typically employed as the adhesion promoter. Organosilanes which are useful in these applications generally contain hydrolyzable groups (e.g., halogen, alkoxy) attached to the silicon atom thereof which generate silanol groups upon contact with ambient moisture and thus readily form chemical and/or physical bonds with mineral and metal surfaces. Also attached to the silicon of the organosilane adhesion promoter is an organic moiety which is reactive with or at least shows some affinity towards, one of the components of the adhesive (usually the polymer). This latter requirement usually limits the practical utility of a specific organosilane to particular adhesive-substrate combinations. In this way, a chemical or physical "molecular bridge" is believed to be formed between the adhesive and the substrate which results in the observed macroscopic improvement in adhesion. A distinct category of adhesives which comprises natural or synthetic rubbers and resin tackifiers is known in the art as "pressure-sensitive adhesives" (PSAs). PSAs uniquely maintain a permanent "tack" which provides a bond of measurable strength immediately upon contact with another surface. Again, adhesion to a substrate, as well as the cohesive strength of the PSA composition itself, can be improved by curing the PSA. Aside from silicone-based PSAs, the use of silanes as adhesion promoters or compositions wherein reactive silicon groups are chemically attached to organic molecules to promote cure therebetween, silicones are generally known in the art to be adhesive materials (i.e. they impart good release of adhesives) and have, indeed, been widely employed in release liners for pressure-sensitive tape. Methods have been proposed for contacting two components to form a pressure-sensitive adhesive (PSA) construction, wherein the first component comprises a conventional PSA and a reactive organopolysiloxane, respectively an alkoxy-functional silicon compound and the second component comprises the PSA and a cure agent for the organopolysiloxane, respectively for the alkoxy-functional silicon compound (EP-A-0 327 235 and EP-A-0 350 181 which are both earlier European applications). The method provides improved adhesion, which generally increases with the passage of time, over the unmodified PSA. It has now been further discovered that certain aminoxysilicon compounds and amidosilanes lead to still further improvements. The present invention, therefore, relates to a method for forming a crosslinkable pressure-sensitive adhesive construction comprising: contacting a first component comprising: (I) a pressure-sensitive adhesive (PSA) natural rubbers and/or one or more synthetic rubbers selected from butyl, silicone, acrylic, styrene-butadiene, acrylonitrile-butadiene, polyisobutylene, isoprene, isoprene-isobutylene, chloroprene, polyvinylether, chlorinated and polyurethane rubbers, and (II) an aminoxysilicon compound having the general formula: in which Me denotes a methyl radical, R'' is independently selected from the group consisting of methyl and ethyl radicals m ranges from 1 to 5,000 and n ranges from 2 to 50, wherein the aminoxysilicon compound (II) constitutes from 1-20 percent of the total weight of the pressure sensitive adhesive (PSA)(I) and the aminoxysilicon (II), on a solids basis with a second component comprising: (III) a further portion of said pressure-sensitive adhesive (PSA); and (IV) an amidosilane having the general formula: wherein R''' represents an organic radical selected from the group consisting of methyl, ethyl and phenyl, R'''' is an organic radical selected from the group consisting of methyl, vinyl and phenyl, and Me denotes a methyl radical and the amidosilane (IV) constitutes from 0.1 to 10 percent by weight based on the total weight of (IV) plus (III), on a solids basis. This invention further relates to the crosslinkable, pressure-sensitive adhesive compositions formed by the above described method. In the method of the present invention, a first component comprising (1) a pressure-sensitive adhesive (PSA) as defined above and (II) an aminoxysilicon compound as defined above is contacted with a second component comprising (III) the same pressure-sensitive adhesive (PSA) and (IV) an amidosilicon compound as defined above to form a PSA construction. While not wishing to be bound by any particular mechanism, it is believed that, after the PSA construction is formed, the aminoxysilicon compound (II) and the amidosilicon compound (IV) interact with atmospheric moisture to provide a "permanent set" for the adhesive system. Permanent set, as used herein, refers to the irreversible increase with time of the adhesive bond between the first and second components. This interaction can, of course, be hastened at elevated temperatures. The individual components, however, may generally be stored for prolonged periods under ambient conditions (e.g., room temperature and less than 50% relative humidity) as long as they are not in contact with each other. The pressure-sensitive adhesive (I) employed in the method of the present invention must have a reasonable degree of compatibility with the aminoxysilicon compound (II) and the amidosilicon compound (IV), each described infra. By compatibility is meant that the PSA components are not subject to phase separation upon standing and remain as homogeneous dispersions or solutions. These PSAs are based on a natural or synthetic rubber which has been mixed with a tackifier resin and, optionally, with various fillers, plasticizers, extender oils, catalysts, preservatives, antioxidants, stabilizers or other ingredients normally employed in the preparation of PSAs. The PSAs suitable for use in the method of the present invention are based on natural rubbers and/or one or more synthetic rubbers selected from butyl, silicone, acrylic, styrene-butadiene, acrylonitrile-butadiene, polyisobutylene, isoprene, isoprene-isobutylene, chloroprene, polyvinylether, chlorinated and polyurethane rubbers. Typical resin tackifiers used in conjunction with these rubbers include rosin, hydrocarbon resins, terpene resin, siloxane resin and the like. Particularly preferred PSAs include those based on styrene-butadiene and acrylic polymers. The pressure-sensitive adhesives of ingredient (I) are well known in the art and further description thereof is considered unnecessary. Many of these materials are available commercially. Ingredient (II) of the present invention is selected from copolymers of polydimethylsiloxane having the general structure: wherein Me denotes a methyl radical and R'' is independently selected from methyl and ethyl radicals. In the above formula, the value of m may range from 1 to 5,000, preferably from 1 to 100, and n from 2 to 50, preferably from 2 to 10. A highly preferred aminoxysilicon compound has the average structure: in which Et denotes an ethyl radical. The aminoxysilicon compound of the present invention are known in the art and can be prepared by the method shown in United States Patent No. 3441583. Ingredient (III) of the present invention is a further portion of pressure sensitive adhesive (PSA) selected from the same PSAs as ingredient (I). The amidosilicon compound (IV) is selected from amidosilanes having the general formula: wherein R''' represents an organic radical selected from the group consisting of methyl, ethyl and phenyl, R'''' represents an organic radical selected from the group consisting of methyl, vinyl and phenyl and Me denotes a methyl radical. The amidosilanes (IV) include, for example, methylvinyldi-(N-methylacetamido) silane, dimethyldi(N-methylacetamido) silane, methylphenyldi-(N-methylacetamido)silane, methylvinyldi-(N-ethylacetamido)silane, dimethyldi(N-ethylacetamido)silane, methylphenyldi-(N-ethylacetamido)silane, methylvinyldi(N-phenylacetamido)silane, dimethyldi(N-phenylacetamido) silane and methylphenyldi(N-phenylacetamido)silane. A preferred amidosilicon compound of the present invention is methylvinyldi-(N-methylacetamido) silane. The amidosilanes are well known in the art and can be prepared by reacting a cholorosilane with an alkali metal salt of an appropriate N-organoacetamide. The polymers containing the amido groups may, in turn, be prepared from the amidosilane by reacting an excess of this compound with a silanol-functional siloxane polymer, the polymer being added to the amidosilane to produce the product. Although the above described aminoxysilicon compounds and amidosilanes may generally be employed in the method of the present invention, it has been found that certain PSAs are themselves incompatible with some of these compounds. The skilled artisan will readily determine which of these compounds are compatible with a given PSA. The compositions of this invention can further comprise typical adjuvants employed in the preparation of pressure sensitive adhesives, such as catalysts, fillers, pigments, dyes, fungicides, bacteriocides, stabilizers, antioxidants and cure-control additives, if desired. In order to practise the present invention, the first component of this invention is prepared by thoroughly mixing the aminoxysilicon compound (II) with the PSA (I) such that the aminoxysilicon compound constitutes from 1 to 20 percent of the total weight of (I) plus (II), on a solids basis. Preferably, the aminoxysilicon compound constitutes from 1 to 10 percent by weight of (I) plus (II). In a similar manner, the second component of this invention is prepared by thoroughly mixing a curing amount of the amidosilicon compound (IV) with the PSA (III). By "curing amount", is meant at least an amount sufficient to improve the adhesion between the two components as demonstrated by an increase in bond strength and/or increased cohesive failure versus the unmodified PSA. Thus, the amount of amidosilicon compound employed will vary according to the nature of the PSA, amount of the aminoxysilicon compound incorporated therein and the particular amidosilicon compound selected. Typically, this will vary from 0.1 to 10 percent by weight of the amidosilane based on the total weight of the amidosilicon compound (IV) plus the PSA (III), on a solids basis. The compositions in which the aminoxysilicon compound (II) is incorporated in a PSA (I) and in which the amidosilicon compound (IV) is incorporated in a PSA (III) are thus employed as intermediates in the method of the present invention. Those skilled in the art will readily arrive at optimum formulations by routine experimentation. For example, when the PSA is selected from the preferred systems based on acrylic or styrene-butadiene rubber, from 1 to 10 percent by weight of the preferred aminoxysilicon compound of formula (ii), above, are effectively employed in the first PSA component and from 0.1 to 10 percent by weight of methylvinyldi-(N-methylacetamido) silane are employed in the second PSA component. As mentioned above, however, the maximum aminoxysilicon compound content and amidosilicon compound content in the two components is limited by compatibility considerations, and these ingredients must not separate from the PSA matrix to form their own macroscopic phases. It is, of course, recognized that separate phases may exist at a microscopic level. As will be readily recognized by those skilled in the art, the PSAs of the present invention may be supplied as a solution or dispersion in an inert organic solvent. By inert, is meant that the solvent does not react with any of the other ingredients of the present invention. The particular solvent selected will generally depend upon the PSA employed. Thus, for a non-polar PSA such as a silicone or a butyl rubber, a non-polar solvent such as toluene, hexane, cyclohexane, mineral spirits or xylene would be appropriate. For a polar PSA such as an urethane or acrylonitrile, a polar solvent such as acetone, ethyl acetate, tetrahydrofuran or dimethylformamide might be used. In general, the PSAs contemplated in the present invention are prepared and marketed in such solvent form and are well known in the art. Any conventional mixing method may be used to mix the ingredients of the above described components. For example, when the PSA is supplied in solution, relatively low shear mixers such as low speed blade mixers or Pfaudlers may be used. When an all-solids PSA is to be used according to the methods of the present invention, a high intensity mixer, such as a Banbury or roll mill, should be used to ensure good dispersion of the ingredients. So far as is known, the order of mixing is not critical to the operability of this invention as long as the aminoxysilicon compound and the amidosilicon compound are thoroughly dispersed in the respective PSA. After the above described first and second components are prepared, said components are contacted in the method of this invention to form a pressure-sensitive adhesive construction. Ordinarily, each component is coated onto a substrate before the contacting step is executed. Thus, this well known method in the art can be practised using the components disclosed herein by first coating a backer, such as a plastics or metallic film or tape, with the first component of this invention, and contacting the resulting composite with the surface of a solid substrate, such as a metal or plastics substrate, was previously coated with the second component of the present invention. It is also contemplated that a free film of the first and second components of the present invention may be prepared by coating each component onto a release liner as is commonly practised in the art in certain bonding operations. To form such coatings on substrates, solvent-based compositions of this invention can be diluted with a sufficient quantity of a good solvent for the respective components in order to provide a desired coating consistency. As described above, such solvents as hexane, heptane, toluene, xylene, mineral spirits or ketones are suitable for this purpose, the exact choice being based on the nature of the PSAs involved, as is commonly practised in the art. Coatings of such solvent-based first and second components may be applied to substrates by any of the methods commonly practised in the coating arts. For example, they may be dipped, brushed, flow coated, blade coated or gravure coated onto a substrate. After coating the backer or solid substrate with the PSA composition, solvent is generally removed by evaporation at ambient temperature or by heating, as commonly practised in the art. Alternatively, 100% solids coatings are also contemplated herein. These may be applied to a substrate by, e.g., a calendaring operation or one of the above-mentioned techniques if the viscosity is low enough. Those skilled in the art will recognize that both the aminoxysilicon compound (II) and the amidosilicon compound (IV) react with water. It is therefore preferred to store the PSA components containing these ingredients in a dry (e.g., hermetically sealed) condition till they are to be contacted by the method of the present invention. The constructions of the present invention may be used in many of the applications currently being served by the PSAs described above (i.e., ingredient I of the present invention). With the constructions of the present invention, however, the adhesion is improved and, with the passage of time, the adhesive bond formed between the first and second components will generally increase in strength. This permanent set aspect of the present invention is, of course, an advantage in many applications wherein a strong bond, rather than reversibility thereof, is of primary importance. Such applications include bonding of automotive and decorative trim, permanent fastening devices such as picture hooks, contact paper and labels, electronic component assembly, substrate lamination (e.g., metals, paper, plastic, floor tiles, carpeting) and the like. The following examples are presented to further illustrate the compositions and methods of this invention, but are not to be construed as limiting the invention, which is delineated in the appended claims. All parts and percentages in the examples are on a weight basis and all are reported on a solids basis unless indicated to the contrary. The following materials were used to prepare the constructions of the present invention as well as comparative constructions: CROSSLINKER A is an aminoxy-functional silicone consisting essentially of an N,N-diethylaminoxypolysiloxane having the formula wherein Me and Et denote methyl and ethyl radicals, respectively. SILANE B is a silane consisting essentially of methylvinyldi-N-methylacetamidosilane having the formula MTO is a methyl trioximosilane having the structure PDMS is a hydroxyl endblocked polydimethylsiloxane fluid having a viscosity of about 40 x 10⁻⁶ m²/s (40 cS) at 25°C. DBTDA is dibutyltin diacetate. Examples 1 -2 An acrylic pressure-sensitive adhesive (PSA) solution, 76 RES 9105, was modified to form the constructions of the present invention. 76 RES 9105 (Unocal Chemicals Division, Schamburg, IL) is described as an high performance "self-crosslinking" pressure sensitive adhesive which is a 45% solids solution of an acrylic resin polymer in an oxygenated hydrocarbon solvent blend. A first component was prepared by thoroughly mixing the indicated amount of CROSSLINKER A with this PSA (see Table 1). A second component was prepared by thoroughly mixing SILANE B with another portion of the PSA solution, also indicated in Table 1. Each of these components was then applied to 0.125 mm (5 mil) aluminium foil, dried to form a 0.05 mm (2 mil)-thick film thereon and the coated foil was cut into 25 mm (1 inch)-wide strips. The foil strips bearing the first and second PSA adhesive films were pressed together using a 4.54 kg (10 pound) steel roller at room temperature to form a lap joint having a 12.5 mm (1/2 inch) overlap [i.e., a total of 312.5 mm² (1/2 square inch) contact surface]. The same procedure was followed in the preparation of (Comparative) Example 1, which served as a control and contained no additives in either PSA layer. The lap joints were aged at room temperature for various periods and then tested [at room temperature using a pull rate of 12.5 mm (1/2 inch)/ minute] to determine shear adhesive strength. Results are presented in Table 1, wherein the shear strength is reported in (kiloPascals (kPa) [pounds per square inch (psi)]. (Comparative) Example 1 Example 2 % CROSSLINKER A in first component (solids basis) -- 5.7 % SILANE B in second component (solids basis) -- 3.2 Shear Adhesion [kPa (psi)] After: 1 hour 455 (66) 407 (59) 1 day 414 (60) 593 (86) 2 days 434 (63) 503 (73) 7 days 441 (64) 580 (84 ) 28 days 476 (69) 642 (93) Examples 3 - 4 A styrene-butadiene rubber based PSA solution, DURO-TAK 36-6045, was modified and tested according to the procedures of Example 2, as shown in Table 2. DURO-TAK 36-6045 (National Starch and Chemical Corp., Bridgewater, NJ) is described as a 48% solids solution in toluene, heptane and isopropyl alcohol having a viscosity of about 5.5 Pas (5,500 cP) at 25°C. Again, a control construction (Comparative Example 3) was prepared to illustrate the unmodified styrene-butadiene PSA. Shear adhesion results are shown in Table 2, wherein a twenty-five millimetres (one inch) overlap was used in the case of the control construction. (Comparative) Example 3 Example 4 % CROSSLINKER A in first component (solids basis) -- 5.7 % SILANE B in second component (solid basis) -- 3.2 Shear Adhesion [kPa (psi)] After: 1 hour 48 (7) 676 (98 ) 1 day 69 (10) 600 987 ) 2 days 76 (11) 642 (93 ) 7 days 62 (9) 614 (89 ) 28 days 62 (9) 614 (89 ) Examples 5 - 9 The 76 RES 9105 acrylic pressure-sensitive adhesive solution, described above, was modified to form the PSA construction shown in Table 3. These constructions were tested as described in Examples 1 -2 with the exception that 0.125 mm (5 mil) thick stainless steel foil was used instead of the aluminium foil. (Comparative) Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 % CROSSLINKER A first component (solids basis) 20 20 10 1.0 1.0 % SILANE B second component (solids basis) 0.1 10 1.0 10 0.1 Shear Adhesion [kPa (Psi)] After: 1 hour 166 (24) 435 (63) 262 (38) 1249 (181) 973 (141) 1 day 248 (36) 697 (101) 380 (55) 1649 (239) 1021 (148) 2 days 297 (43) 580 (84) 331 (48) 738 (107) 911 (132) 7 days 407 (59) 780 (113) 649 (94) 1076 (156) 807 (117) 28 days 393 (57) 635 (92) 794 (115) 773 (112) 938 (136) (Comparative) Example 5 in Table 3 illustrates the reduced adhesive strength obtained when there was an improper balance between CROSSLINKER A and SILANE B for this particular acrylic PSA. Example 10 The above described DURO-TAK 36-6045 styrene-butadiene rubber PSA was modified as shown in Table 4, wherein shear adhesion test results using the 0.125 mm (5 mil) thick stainless steel foil as thesubstrate are also reported. Example 10 % CROSSLINKER A in first component (solids basis) 6.7 % SILANE B in second component (solids basis) 4.2 Shear Adhesion [kPa (Psi)] After: 1 hour 524 (76) 1 day 1132 (164) 2 days 1339 (194) 7 days 863 (125) 28 days -- It is seen from the above Tables that the PSA constructions of the present invention provide improved shear adhesive strength versus unmodified acrylic and styrene-butadiene systems. (Comparative) Example 11 A first component of the DURO-TAK 36-6045 styrene-butadiene rubber PSA was modified with 5.0 percent of MTO and a second component of this PSA was modified with a mixture of 9.0 percent of polydimethylsiloxane and 1.0 percent of DBTDA according to the above procedures. Shear adhesion testing results, using the 0.125 mm ( 5 mil) thick aluminum foil as the substrate, are present in Table 5. Shear Adhesion [kPa (psi)] After: 1 hour 117 (17) 1 day 124 (18) 2 days 138 (20) 7 days 152 (22) 28 days 117 (17) It is known in the silicone sealants art that compositions containing the above ingredients (i.e., the MTO, PDMS and DBTDA) will cure at room temperature. Nevertheless, the modification of the above PSA with these materials did not improve the shear strength as much as did the modifications according to the present invention. Similar attempts to modify an acrylic PSA, with MTO in the first component and the PDMS and DBTDA in the second component, were also ineffective. Example 12 The above described styrene-butadiene rubber PSA was modified by incorporating 6.7% of CROSSLINKER A in the first component and 4.2% of SILANE B in the second component, as described in Example 10. The modified PSA components were coated onto 0.125 mm (5 mil) thick stainless steel foil. The effect of relative humidity on adhesion performance was evaluated by exposing the coated steel foils to (a) 100% relative humidity (R.H.) at 60°C: (b) essentially 0% R.H. at room temperature; and (c) ambient conditions. After storage for the times indicated in Table 6, the components were pressed together for one hour and shear tested using a twenty-five millimetres (one inch) overlap. Storage Condition Storage Time Shear Strength [kPa(Psi)] 100% R.H. 1 day 1063 (154) do 7 days 1014 (147) do 14 days 725 (105) do 28 days 794 (115) 0% R.H. 1 day 1739 (252) do 7 days 1594 (231) do 14 days 1822 (264) do 28 days 1939 (281) Ambient 1 hour 524 (76) It is seen from Table 6 that exposure of the coated PSA foils of the present invention to high humidity reduces ultimate shear adhesion, but the bond strength is still quite high. Furthermore, the coated foils retained their tack after storage at the high humidity.

Claims: A method for forming a crosslinkable pressure-sensitive adhesive construction comprising: contacting a first component comprising: (I) a pressure-sensitive adhesive (PSA) natural rubbers and/or one or more synthetic rubbers selected from butyl, silicone, acrylic, styrene-butadiene, acrylonitrile-butadiene, polyisobutylene, isoprene, isoprene-isobutylene, cloroprene, polyvinylether, chlorinated and polyurethane rubbers; and (II) an aminoxysilicon compound having the general formula: in which Me denotes a methyl radical, R'' is independently selected from the group consisting of methyl and ethyl radicals m ranges from 1 to 5,000 and n ranges from 2 to 50 wherein the aminoxysilicon compound (II) constitutes from 1 to 20 percent of the total weight of the pressure sensitive adhesive (PSA)(I) and the aminoxysilicon (II) on a solids basis, with a second component comprising: (III) a further portion of said pressure-sensitive adhesive (PSA); and (IV) an amidosilane having the general formula: wherein R''' represents an organic radical selected from the group consisting of methyl, ethyl and phenyl, R'''' is an organic radical selected from the group consisting of methyl, vinyl and phenyl and Me denotes a methyl radical and the amidosilane (IV) constitutes from 0.1 to 10 percent by weight based on the total weight of amidosilicon compound (IV) plus the pressure sensitive adhesive (III), on a solids basis. The method of Claim 1, wherein R''' in said amidosilane (IV) is a methyl radical. The method of Claim 2, wherein m in said aminoxysilicon compound (II) ranges from 1 to 100 and n in said aminoxysilicon compound (II) ranges from 2 to 10. The method of Claim 3, wherein said pressure-sensitive adhesive [(I), (III)] is based on a synthetic rubber selected from the group consisting of acrylic and styrene-butadiene rubbers. The method of Claim 4, wherein said aminoxysilicon compound (II) has the structure: in which Me and Et denote a methyl and an ethyl radical, respectively, and said amidosilane (IV) is methylvinyldi(N-methylacetamido) silane.

Inventors: BLIZZARD JOHN DONALD, SWIHART TERENCE JOHN, BLIZZARD, JOHN DONALD, SWIHART, TERENCE JOHN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1990
Main classification: C09J201
IPC codes: C09J183, C09J201, C09J121, C09J11