COATING ADDITIVES FOR WATER-BASED ADHESIVE FORMULATIONS

Abstract: A water-based pressure sensitive adhesive formulation is provided with one or more high molecular weight linear aliphatic polymers. The resulting composition can be used to form stable filaments or delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation without polymer. Low levels of polymer can provide dramatic improvement in the viscosity of such compositions, without detrimental effect on the adhesive and other desirable qualities of the composition.

Description: COATING ADDITIVES FOR WATER-BASED ADHESIVE FORMULATIONS TECHNICAL FIELD The present invention relates to the use of rheology modifiers for adjusting the viscosity of water-based formulations. In a further aspect, the invention relates to modifiers such as poly(ethylene oxide) resins for adjusting the viscosity of aqueous systems. In another aspect, the present invention relates to premetered coating techniques for coating substrates with thin films of compositions such as latex adhesives. BACKGROUND OF THE INVENTION Rheology Modifiers Rheology modifiers are useful for adjusting liquid shear viscosity as a function of shear rate. Latex paints, for example, must have suitable viscosity at high shear rates to give correct coverage and brush drag. They must also, however, have suitable viscosity at low shear rates so as not to sag on vertical surfaces while permitting leveling of coating defects. The adjustment of viscosity in water-borne latex paints is often accomplished by the addition of such materials as alkali swellable thickeners, lower molecular weight associative thickeners, water soluble polymers, and the like. See also Soules et al. "Dynamic Uniaxial Extensional Viscosity", Chapter 20, pp. 322-332 in Polymers as Rheology Modifiers. D.N. Schulz and J.E. Glass eds., in American Chemical Society Symposium Series #462 (1991), which describes the use of additives to affect various rheological properties of water-borne latex paint coatings, and Soules et al. J. Rheology 32(2):181-198 (1988) which describes the use of fillers, including poly(oxyethylene), to affect the extensional viscosity of formulations for spray applications. Two types of water-based rheology modifiers are commonly used to increase shear viscosity, and are often referred to as "thickeners". The first type includes alkali-activated copolymers of acrylic or methacrylic acid monomer and alkyl (meth)acrylates or butyl acrylate monomer. Thickening is achieved when initially acidic solutions are neutralized to pH greater than about 6.5 and the acid groups become soluble, causing swelling and interaction of the polymer chains. The second type of thickeners, referred to as associative thickeners, are becoming increasingly popular in the paint industry for providing desired "brush drag" and leveling properties. These thickeners are similar to the alkali-activated copolymers, but include a grafted nonionic surfactant to provide physical intermolecular crosslinks. Commercial HMW PEO resins are said to be extremely effective thickening agents in both fresh and salt water. The aqueous solutions are described as pseudoplastic (i.e., "shear thinning"). (See, e.g., "POLYOX Water Soluble Resins", Union Carbide Product literature, 12 pp., 1991, 1992). Glass, "Dynamics of Roll Spatter and Tracking", J. Coatings Tech. 50(641):56-71 (1978) describes the addition of water soluble polymers, including high molecular weight poly(ethylene oxide) polymers, to latex paint in order to study their effect on such properties as spatter and tracking patterns. Others have described the use of high or low molecular weight epoxide polymers in water-based systems. See, for example, U.S. Patent No. 3,944,100 (Brockway et al.) which describes a "containment" coating composition for glass containers, the composition including the optional use of 100-800 ppm PEO, which can be added for shear enhancement. U.S. Patent No. 4,507,413 (Thoma et al.) describes a spreadable polyurethane reactive coating composition for coating textile sheet products and the like. The patent describes the optional use of substances for thickening the spread coating viscosity of an aqueous dispersion of blocked isocyanate prepolymer. Included in the group of thickening substances are water soluble polymers, including long chain polyethylene oxides. Schuppiser et al., in U.S. Patent No. 4,542,182 describe the use of ethylene glycol polymer having low molecular weights (MW of between 1,000 and 50,000) for preparing lattices of particles of vinyl acetate olefin copolymers. The lattices, in turn, are applied as binders in the manufacture of paints, and other materials. U.S. Patent No. 5,108,782 to Reed describes a silicone release coating emulsion that includes the use of a water soluble or water dispersible polymeric thickening agent such as high molecular weight PEO. The silicone emulsion is said to exhibit improved holdout of the silicone from the substrate. U.S. Patent No. 5,342,883 (Jenkins et al.) discloses the use of water soluble polymers formed from alpha, beta-monoethylenically unsaturated monomers. The polymers are either soluble in or swelled by an aqueous alkaline medium. When added to latex paints and neutralized, both the viscosity of the paint and brush drag are said to be increased. High molecular weight polymers such as poly(ethylene oxides) have not previously been used or described as useful for improving the rheological properties of materials such as water-based pressure sensitive adhesives. Adhesive Coating Techniques Premetered liquid coaters include those in which the flow rate of the coating material is controlled before its application to a substrate. Such coaters, which include slot coaters, slide coaters, and curtain coaters, can be used to deliver thin sheets of materials such as adhesives to a substrate. In each situation, a fluid is fed to a coating die, and in turn is applied to a substrate in the form of a stable sheet (also known as a "bead" in the context of slot coating). With other (e.g., postmetered) coating techniques, such as dip coaters and roll coaters, the amount of fluid applied to the substrate depends on the fluid properties themselves, such as fluid viscosity and surface tension, as well as on the operating conditions such as substrate speed and coating gap. Typically a premetered coater provides a coating die for the delivery of material, and provides for a physical gap between the die and substrate. With slot and slide coaters, the gap is generally more than 100 microns (about 4 mils) in distance, while with curtain coaters the gap can be on the order of centimeters. The combination of flow rate per unit width and substrate speed together determine the thickness of the delivered sheet, and in turn of the coating itself. In order to obtain thinner coatings, it is often more desirable to decrease the flow rate rather than increase the substrate speed, since the latter is generally more prone to problems with flow stability and drying limitations. The ability to decrease flow rate, however, is generally limited by the rheological properties of the coating composition itself. Lowering a flow rate too much will result in the application of non-uniform or unstable sheets. Since coating thickness is also known to correlate with the size of the coating gap between the die and substrate, there is a strong tendency in the extrusion coating industry to use very tight gaps to produce very thin coatings. However gaps under about 100 microns frequently result in operating difficulties, since particulate matter that accumulates in the coating gap can cause streaks. Tight gaps also make it easy for large particles or taped splices in the substrate to tear the substrate (e.g., web). As a result, most conventional slot coating processes continue to rely on gaps greater than about 50 microns. Pressure-sensitive adhesive latexes are typically thickened for best handling on coating equipment. The shear thickeners typically used have included cellulosic thickeners, poly(vinyl alcohol) and aqueous solutions of alkali soluble acrylic emulsions. (See, e.g., D. Satas, Chapter 13, Handbook of Pressure-Sensitive Adhesive Technology. VanNostrand Reinhold (1982). It is clear that the industry has a need for methods for improving the coatability of pressure-sensitive adhesive formulations, and in particular, for decreasing the flow rate, or increasing the coating gap in the course of premetered coating. SUMMARY OF THE INVENTION The present invention provides a coating composition comprising a water- based pressure sensitive adhesive formulation, and one or more high molecular weight polymers. The polymer is present at a total concentration suitable to allow the composition to be delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation without polymer. Applicant has found that surprisingly low levels of polymer can provide dramatic improvement in the viscosity of such compositions, without detrimental effect on the adhesive and other desirable qualities of the composition. In one embodiment, the composition of the present invention permits the use of a minimum flow rate that is less than about 70%, and more preferably less than about 50% of the minimum flow rate necessary to maintain a stable sheet of the formulation without added polymer, under comparable operating conditions. In addition to affecting the flow rate, the present invention permits the coating gap distances typically used to be increased by two or more times. The ability to use a significantly wider gap to provide an equally thin coating carries significant advantages in the processing of such materials. In one embodiment, the coated material is a pressure-sensitive adhesive ("PSA") formulation such as a latex, acrylate-based PSA. Also, the high molecular weight polymer can be a linear aliphatic polymer, such as a high molecular weight ("HMW") poly(ethylene oxide) ("PEO") polymers. In another embodiment the composition comprises a water-based latex pressure sensitive adhesive and between about 1 ppm and about 50 ppm of a high molecular weight PEO. In another aspect, the invention provides a method of preparing a coating composition. The method comprises the steps of forming a water-based pressure sensitive adhesive formulation, and adding, during or after formation of the formulation, a high molecular weight polymer. The polymer is added to a final concentration suitable to allow the composition to be delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation itself. In one embodiment, the polymer is added to achieve a final concentration of between about 1 ppm and about 50 ppm, based on the weight of the composition. In yet other aspects, the invention provides a coating composition prepared by the method of the invention, as well as a coating composition preparable by the method of the invention. In further aspects, the invention provides a coating delivered using a composition of the invention, and a coated product comprising such a coating. DETAILED DESCRIPTION Unless otherwise apparent, the following words and phrases shall have the following meanings. "Formulation" will refer to a water-based material, e.g., pressure-sensitive adhesive, capable of being provided in coated form on a desired substrate, as by conventional liquid coating techniques. "Composition" will refer to a formulation in combination with one or more high molecular weight polymers, according to the teaching of the present invention. "Water-based" will refer to a formulation prepared in aqueous form and including a suspension of pressure-sensitive adhesive. The adhesive suspension can be in any suitable form, including a colloidal emulsion (i.e., "latex") or a suspension of adhesive microspheres. "Pressure sensitive adhesive" will refer to a viscoelastic material which in solvent-free form remains permanently tacky. Generally, such a material will adhere instantly to most solid surfaces with the application of slight pressure. "Coating" will be used interchangeably with "coatable" and other inflections of the word to refer to the ability of a composition to be delivered in the form of a thin film to a substrate (e.g., a web). "Viscosity" will refer to both elongational (also known as extensional) viscosity and shear viscosity. Shear viscosity involves a fluid flow determination in which the velocity component varies only in a plane normal to its direction. Elongational viscosity involves a fluid flow determination in which the velocity component varies only in its own coordinate direction. "Spinnability" will refer to the ability to pull a liquid formulation or composition into the form of a stable filament. "Stable sheet" will refer to a flat liquid film capable of being formed and delivered from a coating die in a continuous form to a substrate. "Flow rate" will refer to the volumetric flow of liquid per unit time and width, as expressed in terms of cc/cm-sec. S96/05399 The present invention provides water-based coating compositions that include the addition of high molecular weight polymers to stabilize liquid sheets or films in coating applications. The use of such polymers significantly increases the useful range of stable operating parameters such as substrate speed, coating gap, and liquid flow rate. Applicant has discovered these and other benefits of the use of HMW polymer additives in curtain and slot coating applications. The relationship between the coating thickness, T, the substrate speed, S, and the flow rate per unit width, Q, is given by the formula: Q = T x S For liquids typically used in curtain coating, the minimum flow rate to maintain a stable curtain is generally on the order of about 0.5 cc/cm-sec. (See, e.g., E.B. Gutoff, Chapter 4, "Premetered Coating", pp. 117-167, E.D. Cohen and E.B. Gutoff, eds. Modern Coating and Drying Technology (1992). Above this rate, voids that might form in the curtain propagate upstream with a velocity lower than that of the falling liquid, and the defect is swept away. For a 25 mm coating thickness, S=200 cm/sec (120 m/min) for liquids commonly used in curtain coating, which in some cases is too fast given drying limitations. Reduction of the minimum flow rate with the addition of HMW polymer allows lower coating speeds and longer oven residence times. Applicant has found PEO to be an effective additive for curtain and slot coating latex adhesives, particularly when used with formulations having a pH of about 7 or less. Using 8,000,000 MW PEO, for instance, improved coatability was observed at levels from 2 to 20 parts per million. The effect was shown to disappear after several days, but could be re-established by the addition of more PEO. Off-line, stable sheets can be achieved using PEO-thickened latex at flow rates on the order often times lower than for unthickened PSA formulations. Such a large effect makes it possible to tailor water-based formulations for curtain coating at speeds of 10-300 m/min and thicknesses as low as 10 microns. Likewise, slot coating of water-based liquids is made possible at process conditions previously unattainable. Coating Formulations Descriptions and examples of acrylic adhesives suitable for use in the present invention are provided in Chapter 13 "Acrylic Adhesives", Donald Satas, in Handbook of Pressure-Sensitive Adhesive Technology. Van Nostrand Reinhold Company, the disclosure of which is incorporated herein by reference. Such adhesives can be prepared from monomers that include acrylate, diacrylate, triacrylate, methacrylate, dimethacrylate, trimethacrylate, tetramethacrylate, acrylamide, and the like. In one embodiment, the composition of the invention includes a pressure- sensitive adhesive, particularly polyvinyl PSA's of the type described in U.S. Patent No. Re 24,996. Included in these PSA's are the polymers of the acrylate esters of acrylic acid with non-tertiary alkyl alcohols such as n-butanol, n-pentanol, isopentanol, 2-methyl butanol, 1 -methyl butanol, 2-methyl pentanol, 3 -methyl pentanol, 2-ethyl butanol, pentanol-3, 3,5,5-trimethylhexanol, heptanol-3, isooctanol, n-decanol, dodecanol, and the like, as well as various copolymers. In one embodiment, preferred PSA formulations generally comprise a blend of materials that include an acrylate (or methacrylate) copolymer having an inherent viscosity greater than about 0.2 dl/g. The copolymer comprises polymerized primary monomers and optionally polar comonomer. The primary monomer component comprises at least one free radically polymerizable vinyl monomer which is an acrylic or methacrylic acid ester of a non-tertiary alcohol having from 1 to 18 carbon atoms with the average number of carbon atoms being about 4-12. The polar monomer component is copolymerizable with the primary monomer and the amount by weight of polar monomer is up to about 30% of the total weight of all the monomers in the copolymer. Suitable formulations of this type are described, for instance, in U.S. Patent No. 4,737,577 (Brown), the disclosure of which is incorporated herein by reference. Other monomers may also be included in the formulation, including for instance, up to about 150 parts by weight of a compatible tackifying resin per 100 parts by weight copolymer, and up to about 50 parts by weight of a compatible plasticizer per 100 parts by weight of copolymer. Suitable latex PSA formulations for use in the present invention typically include from about 30% to about 60% solids, and preferably about 40% to about 55% solids, by weight, based on the weight of the formulation. Coating formulations can be provided in other forms as well, including in the form of adhesive microspheres. Suitable microspheres include those described in co-pending PCT application WO 94/13751, ("Tacky Microspheres Having Pendant Hydrophilic Polymeric or Oligomeric Moieties", filed 9 December 1993 as International application no. PCT/US93/11967), the disclosure of which is incorporated herein by reference. High Molecular Weight Polymers Suitable high molecular weight polymers for use in forming compositions of the present invention include ethylene oxide polymers and copolymers as described, for instance in "1,2 Epoxide Polymers", pp. 337-343 in Concise Encyclopedia of Polymer Science and Engineering. J. Kroschwitz, ed. J. Wiley & Sons, 1990. Preferred HMW polymers provide an optimal combination of such properties as flexibility, low glass transition temperature ("Tg"), water solubility. Preferred HMW polymers also have a molecular weight of at least about 100,000 g/mole, preferably at least about 1 ,000,000 g/mole, and most preferably at least about 5,000,000 g/mole. Examples of preferred HMW polymers include those known as poly(ethylene oxide) polymers having the repeating unit -(CH2 - CH2 - O)n - and available commercially under the trademark POLYOX. The commercial product is a free-flowing white powder in molecular weight grades ranging from about 1 x 10(5) to 5 x 10(6). Such polymers are nonionic, high molecular weight (from 100,000 to about 8,000,000) water-soluble poly(ethylene oxide) polymers. HMW PEO is preferred since it provides an inexpensive additive for modifying the rheology of latex adhesives for curtain and extrusion coating. Applicant has found that it is effective at extremely low levels, provides improved spinnability ("higher cohesive strength") in the latex, has relatively low glass transition temperature (Tg), and provides reasonable adhesive properties. Preparation of Coating Compositions Formulations of the present invention can be made using conventional mixing techniques. Since PEO polymers are generally provided in the form of a powder, various manufacturers suggest that the powder first be prepared in the form of a concentrated stock solution. A suitable aliquot can be withdrawn from a stock solution and added to a PSA formulation in order to quickly, reproducibly, and effectively form a composition of the invention. The stock solution, in turn, provides greater long term stability than the final composition. In view of the present teaching those skilled in the art will appreciate the manner in which various parameters can be modified within the scope of the present invention. Such parameters include the relative and final latex and polymer concentrations, the polymer molecular weight, formulation pH, and shelf stability. As described above, PSA concentrations will typically be used at a solids concentration of about 30% to about 60% solids, by weight of the formulation. At these preferred PSA concentrations, high molecular weight polymer can be incorporated into a formulation in any suitable manner, e.g., during or after the preparation of the PSA formulation. For use in slot coating applications, one or more high molecular weight polymers are preferably present in a total concentration of between about 1 ppm to about 50 ppm, and preferably between about 2 ppm and about 20 ppm, based on the weight of the final composition. For slot coating microsphere PSA compositions, the high molecular weight polymer is present in a total concentration of between about 10 ppm and about 1000 ppm, and preferably between about 100 ppm and about 500 ppm. For use in curtain coating applications, the high molecular weight polymer is preferably present at a concentration of between about 10 ppm and about 100 ppm, more preferably between about 20 ppm and about 70 ppm, and most preferably between about 30 ppm and about 60 ppm. Generally, lower concentrations of PSA solids are associated with higher HMW polymer concentrations. It appears that the pH of the PSA formulation is an important factor in assuring the effectiveness of polymer. Generally, the pH should be neutral to slightly acidic. pH values of on the order of pH 2 to pH 7 are suitable, with pH values of between about pH 4 and pH 6 being preferred. It also appears that the desired effects of polymer on the PSA formulation are transient in nature, generally decreasing over the course of storage over a week. As described in the EXAMPLES, the effect can be re-established in the same manner that is was initially established, i.e., by the addition of polymer in the manner described herein. Use of Coating Compositions The invention provides a water-based coating composition that includes the use of high molecular weight polymer to provide stabilization of liquid sheets or filaments in coating applications. Compositions of the present invention can be used in a variety of coating techniques, including curtain coating and slot (i.e., extrusion) coating, of latex and microsphere-PSA materials. See, "Premetered Coating", Chapter 4, pp. 117-167 in Modern Coating and Drying Technology. E. Cohen and E. Gutoff, eds. A composition of the present invention can be delivered from the coating die of a premetered coater in the form of a stable sheet or filament at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation itself. The flow rate of a PSA formulation can be determined using the same coater and techniques as that of the comparable PSA/polymer composition. Under these conditions, the minimum flow rate at which the PSA can be delivered in the form of a stable sheet or filament will be compared with the minimum flow rate of the composition capable of reproducibly producing a stable sheet or filament. Curtain and slot coating The use of a composition of the present invention significantly increases the useful range of premetered coater operating parameters such as substrate speed, coating gap, and liquid flow rate. Curtain coating offers several advantages over other die coating methods including: easy splice passage, ability to use nonuniform die-to-substrate gaps, and an exposed slot for easy cleaning. However, one major drawback has been the relatively high flow rate required to maintain a stable curtain using many coating liquids. When used in curtain coating processes, for instance, the compositions of the present invention provide significantly improved "spinnability." See, generally, C. Petrie and M. Denn, "Instabilities in Polymer Processing", AIChE Journal 22(2):209-235 (1976), the disclosure of which is incorporated herein by reference. Improved spinnability of curtain coated compositions, in turn, permits desirable changes to be made in processing parameters. Such changes include, for instance, the ability to use a lower flow rate and a lower final coating weight for a given substrate speed, or a lower substrate speed for a given final coating weight. Such spinnable liquids can also maintain stable curtains at flow rates as low as about 0.05 cc/sec-cm of curtain width, as compared to flow rates of on the order of 0.5 to 1 cc/sec-cm for PSA formulations without PEO. Significant effect can in some cases be achieved by as little as 5 parts per million. Such low levels produce little effect on the adhesion properties of the final dried film, and little effect on low shear rate (i.e. leveling) characteristics of the liquid. When coated in a conventional slot coating process, conventional latex adhesive formulations typically require a coating gap of on the order of 75 microns to 100 microns to achieve a 50 micron coating thickness. By comparison, compositions of the present invention can achieve a similar coating thickness using a coating gap of on the order of 250 micron to 500 micron or higher. The method and composition of the present invention can also be used to improve the coatability of compositions containing adhesive microspheres. Such microspheres are typically formed of materials intended for use as pressure sensitive adhesives, and include the materials described in co-pending application WO 94/13751, ("Tacky Microspheres Having Pendant Hydrophilic Polymeric or Oligomeric Moieties", filed 9 December 1993 as International application no. PCT/US93/11967), the disclosure of which is incorporated herein by reference. Filament formation Such compositions can also be used to form PSA filaments, which can be used to form and coat webs. See, for instance, "Fibers, Manufacture", pp. 389-392 in Concise Encyclopedia of Polymer Science and Engineering. J. Kroschwitz, ed. J. Wiley & Sons, 1990, the disclosure of which is incorporated herein by reference. Generally, filaments can be prepared using spinning processes such as dry spinning techniques in which a polymer solution is extruded from a spinneret through a zone in which the solvent is rapidly evaporated, leaving filaments that are wound up at speeds up to about 1000 m/min. Wet spinning techniques are useful as well, wherein a polymer solution is extruded from a spinneret into a nonsolvent that coagulates (i.e., precipitates) the polymer to form filaments. These and other examples of the invention are illustrated by the following examples which should not be viewed as limiting in scope. EXAMPLES TEST METHODS Peel adhesion test The peel adhesion properties of coated materials was determined according to American Society for Testing and Materials Test Method ASTM D3330-90 using glass as the test substrate. Diacetone alcohol and 2-propanol were used to clean the glass plate. A small test piece (1/2 inch by 6-8 inches) was prepared and applied. The coating was then rolled down with a 4.5 pound roller, and evaluated immediately at either 31 cm/minute or 230 cm/minute. Shear test Shear was determined using an adhesive piece (approx. 1.3 cm x 1.3 cm), hung with a 1000 gram load from a stainless steel plate. A piece was applied to a cleaned stainless steel plate, rolled down with a 4.5 pound roller (six passes) and evaluated in a conventional shear station. Shear tests were generally performed at both room temperature and 70 degrees C. Results are reported in minutes, determined at the point the adhesive separated completely from the plate. Probe tack The tack of sheets coated with a composition or formulation described herein was measured with a Polyken Probe Tack tester (available from Kendall Company) according to American Society for Testing and Materials Test Method ASTM D2979-88. Compositions were coated onto 10 mil paper, yielding the dried coating thickness described in the EXAMPLES. After cleaning the probe with ethyl acetate using a lint-free cloth, a 2 cm x 2 cm sample of the coated sheet was placed on the annular ring weight of the apparatus. For latex formulations/compositions, the tack was then measured and recorded using a 10 mm stainless steel probe having an area of 1 square centimeter with a speed of 1 cm/sec and a dwell time of 1 second. EXAMPLE 1 Latex PSA "A" - Effect of PEO (25 ppm. 52 ppm) Latex A was obtained in the form of a polyacrylic latex PSA. A stock solution of 4000 parts per million ("ppm") PEO (based on the weight of the solution) was prepared using 8,000,000 molecular weight poly(ethylene oxide), obtained from Aldrich Chemical Co. (catalogue number 37,283-8). The stock solution was prepared by slowing adding PEO powder to water while stirring with a magnetic stir bar. Following complete addition of the PEO, the composition was continuously stirred until the PEO had completely dissolved, generally within one to four hours. Aliquots of the stock PEO solution were then added to the latex formulations, during continuous mixing, in amounts sufficient to provide 25 and 52 ppm PEO, respectively, based on total composition weight. Mixing was continued up to 10 minutes following the last addition of PEO. In order to determine the shear viscosity of a sample, a twelve milliliter sample was loaded into the cup of a rotational rheometer (Bohlin VOR) and shear viscosity data were taken at increasing shear rate and 25 degrees centigrade for the latex with and without addition of PEO. The results are provided in TABLE 1. A 200 milliliter sample was loaded into the breaker of an opposed nozzle device (Rheometics RFX) and apparent elongational viscosity data were taken at increasing elongation rate and 20 to 24 degrees centigrade for latex with and without 52 ppm PEO. TABLE 1 Strain Rate Latex A 55% Latex A 55% 52 ppm 8xl0 6 52 ppm 8xl0 6 (sec 1 ) PSA -shear PSA - PEO -shear PEO- viscosity elongational elongational viscosity 1.2 1000 48000 820 16600 1.8 730 600 2.9 530 28500 440 11300 4.6 400 330 7.3 300 20000 250 10100 12 230 210 19 170 10900 150 10600 29 130 150 46 110 5040 780 16600 73 92 660 120 78 2300 520 21800 190 67 430 290 59 1100 370 14100 460 53 350 730 48 920 44 Both the shear and elongational viscosities for the compositions show sudden and significant increase after 46 sec-1 strain rate. In stark contrast, the latex formulation without PEO shows only decreasing viscosities with increasing stain rate. The strain rate at which the viscosity increased and the magnitude of increase correlated directly with the degree of spinnability observed, i.e. more spinnable compositions show larger viscosity increase at a lower strain rate. TABLE 2 shows adhesion data for the latex PSA with and without addition of 25 ppm PEO. Coatings were applied by hand by pulling a polyester substrate under a fixed rigid knife with 50 micron gap at slow and moderate speed of roughly 2 and 10 cm/sec after 0 and 2 days from preparation of the composition. The coatings produced from the latex PSA/PEO compositions produced at 2 cm/sec were more uniform in thickness than those produced at 10 cm/sec. TABLE 2 Coating Time from PEO Peel Adhesion Shears Probe Tack speed mix to coat (ppm) 31 cm/mi 230 cm/min RT (mins) (g) (cm/sec) (days) (g/cm) (g/cm) avg 10 0 0 320 530 68/64/68 73 672 2 0 25 300 460 79/107/79 88 619 10 0 25 240 470 67/39/45 50 505 10 2 0 260 450 104/61/63 76 680 2 2 25 280 470 109/120/69 99 573 10 2 25 260 460 78/75/143 99 443 It can be seen that in each case there was little or no difference in overall performance between compositions with PEO and the original latex formulations. In other words, the benefits of the present invention, in terms of coating operating conditions, can be achieved without detrimental effect on the performance of the final coated composition. EXAMPLE 2 Latex "B" - Effect of PEO (20 ppm) A PSA copolymer identified herein as "Latex B" was prepared using the general method described for the preparation of the control copolymer in Example 41 of U.S. Patent No. 4,737,577, the disclosure of which is incorporated herein by reference. In particular, the principal copolymer was isooctylacrylate (IOA)/acrylic acid (AA) polymerized in a waterborne (emulsion) system. A 5000 ppm PEO stock solution was prepared in the manner described in EXAMPLE 1, and diluted to 500 ppm PEO with water. An aliquot of the 500 ppm solution was then added to the latex formulation using the method described in EXAMPLE 1 in an amount sufficient to provide a final concentration of 20 ppm PEO. Shear and elongational viscosities versus strain rate were determined for the latex with and without addition of PEO. Increased viscosities were observed at 29 sec-1 strain rate for the composition wherein PSA formulations showed only decreasing viscosities with increasing strain rate. EXAMPLE 3 PSA Microspheres - Effect of PEO Latex microspheres were prepared according to the method described in co- pending application WO 94/13751, ("Tacky Microspheres Having Pendant Hydrophilic Polymeric or Oligomeric Moieties", filed 9 December 1 93 as International application no. PCT/US93/11967) the disclosure of which is incorporated herein by reference. A 4000 ppm PEO stock solution was prepared as described in EXAMPLE 1 , and was added to the microsphere PSA suspension using the method described in EXAMPLE 1 in an amount sufficient to give 650 ppm PEO. Shear viscosity data for microsphere compositions and formulations showed that the shear viscosity for the composition with PEO was larger than the formulation without PEO at all shear rates. For the composition, viscosity increased with shear rate past 2 sec-1. TABLE 3 gives adhesion data for a microsphere suspensions with 300 and 500 ppm PEO slot coated to give 40 micron dry coating. TABLE 3 Adhesion properties of PEO thickened microsphere PSA PEO concentration Probe Tack Peel Test Shear Test (ppm) (230 cm/min) (2.54 cm (1 in.) wide, 1 kg) (g/cm) 0 400-600 190 - 225 >7000 min 300 870 183 >7000 min 500 927 177 >7000 min As with the other latex systems, the data in TABLE 3 demonstrates that there is little or no detrimental effect, and at times some apparent improvement of the overall functional properties of the coatings that result from compositions of the invention. EXAMPLE 4 Effect of PEO Concentration Compositions of latex A and 8,000,000 MW PEO were prepared as described in EXAMPLE 1 in concentrations of 0, 5, 12, 25 and 52 ppm PEO by weight. TABLE 4 shows shear viscosities for each composition, it can be seen that increasing the concentration of PEO surprisingly resulted in a sharp increase in viscosity, and also surprisingly, that the sharp increase occurred at a lower shear rate with increasing concentration. TABLE 4 Strain Rate Latex A 52 ppm 25 ppm 12 ppm 5 ppm (sec 1 ) 55% PSA 8xl0 6 PEO- 8x10 6 PEO 8xl0 6 PEO 8x10* -shear shear -shear -shear PEO- viscosity shear 1.2 1000 820 880 930 850 1.8 730 600 630 670 620 2.9 530 440 460 500 450 4.6 400 330 350 380 340 7.3 300 250 260 280 250 12 230 210 200 220 220 19 170 150 140 140 150 29 130 150 120 120 120 46 110 780 99 100 100 73 92 660 83 86 86 120 78 520 77 72 73 190 67 430 180 62 62 290 59 370 160 67 55 460 53 350 140 74 53 730 48 110 66 51 920 44 49 EXAMPLE 5 Effect of PEO Molecular Weight Compositions of latex A and 52 ppm PEO were prepared as described in EXAMPLE 1 using 8,000,000 and 1,000,000 MW PEO. TABLE 5 shows shear viscosities for each composition and for latex without PEO wherein the 8,000,000 MW PEO composition shows a larger increase in viscosity at 46 sec-1, and the 1,000,000 MW PEO composition shows a smaller increase in viscosity at 460 sec-1. Spinnability was also observed to increase with increased PEO molecular weight. TABLE 5 Strain LatexA 55% 52 ppm 8xl0 6 52 ppm lxlO 6 Rate (sec 1 ) PSA - shear PEO - shear PEO - shear viscosity 1.2 1000 820 760 1.8 730 600 550 2.9 530 440 400 4.6 400 330 300 7.3 300 250 230 12 230 210 190 19 170 150 140 29 130 150 110 46 110 780 97 73 92 660 81 120 78 520 68 190 67 430 58 290 59 370 51 460 53 350 85 730 48 95 920 44 91 EXAMPLE 6 Effect of PSA Particle Weight Compositions of 25 ppm 8,000,000 MW PEO and latex A with particle weight fractions of 50 and 55 percent were prepared as described in EXAMPLE 1. TABLE 6 shows shear viscosities for these compositions and for corresponding latexes without PEO wherein sudden increase in shear viscosity is observed at 190 sec-1 for the 55 percent solids latex at 290 sec-1 for the 50 percent solids latex. Spinnability was also observed to increase with increased latex particle weight fraction. TABLE 6 Strain Rate LatexA 55% 25 ppm 50% PSA - 50% PSA + 25 (sec 1 ) PSA - shear 8xl0 6 -shear shear ppm 8X10 6 PEO - viscosity shear 1.2 1000 880 1.8 730 630 2.9 530 460 4.6 400 350 7.3 300 260 12 230 200 19 170 140 34 42 29 130 120 33 39 46 110 99 30 35 73 92 83 27 30 120 78 77 24 26 190 67 180 22 89 290 59 160 19 100 460 53 140 18 83 730 48 110 17 72 920 44 16 EXAMPLE 7 Effect of pH The latex A formulation described in EXAMPLE 1 was brought to pH 8.5 by the slow addition of 10 percent NH4OH solution. Upon addition of 100 ppm 8,000,000 MW PEO using the method described in EXAMPLE 1, the composition showed no increase in spinnability. Furthermore, a 100 ppm 8,000,000 MW PEO composition prepared as described in EXAMPLE 1 at pH 5.5 losses its spinnability upon addition of NH4OH to pH 8.5, but regained its spinnability upon addition of dilute HCl sufficient to bring the pH below 7. EXAMPLE 8 Effect of Time on Spinnability A composition of latex A and 50 ppm 8,000,000 MW PEO was prepared as described in EXAMPLE 1, and shear viscosity was determined at times of 10 minutes, 5 hours, 20 hours, 48 hours and 72 hours as shown in TABLE 7. The shear rate for onset of viscosity increase is shown to increase with time. Spinnability was observed to decrease in time after 5 hours. TABLE 7 Strain 50 ppm PEO 50 ppm PEO 50 ppm PEO 50 ppm PEO 72 Rate (sec 1 ) 10 min. from 5 hrs. from 20 hrs. from hrs. from mix- mix-shear mix-shear mix-shear shear 1.2 1080 1170 1120 910 1.8 690 740 780 680 2.9 490 520 560 510 4.6 360 380 410 370 7.3 270 280 310 270 12 220 230 250 220 19 160 170 180 160 29 140 150 150 120 46 630 710 120 100 73 670 700 100 83 120 520 550 440 69 190 410 440 390 58 290 340 370 340 67 460 310 320 280 150 730 240 140 920 EXAMPLE 9 Replenishing Aged Compositions The latex/PEO composition aged 72 hours as described in EXAMPLE 8 was combined with a stock solution of 4000 ppm 8,000,000 MW PEO in water as described in EXAMPLE 1 in amount sufficient to bring the total PEO concentration to 80 ppm. Viscosity was measured 10 minutes after the latter addition of the PEO and is shown in TABLE 8 along with viscosities for the initial and aged compositions. The onset value for increase in shear viscosity returns to the pre- aged value, and spinnability was observed return to its pre-aged magnitude upon addition of the PEO to the aged composition. TABLE 8 Strain Rate 50 ppm PEO 50 ppm PEO 72 Rethickened to 80 ppm (sec 1 ) 10 min. from hrs. from mix- PEO from 50 ppm PEO mix-shear shear after72 hours 1.2 1080 910 1180 1.8 690 680 830 2.9 490 510 600 4.6 360 370 450 7.3 270 270 340 12 220 220 270 19 160 160 210 29 140 120 220 46 630 100 900 73 670 83 840 120 520 69 730 190 410 58 620 290 340 67 520 460 310 150 730 140 920 EXAMPLE 10 Curtain Coating Compositions Compositions of latex PSA and 8,000,000 MW PEO at concentrations of 0, 25, 50 and 70 ppm were prepared as described in EXAMPLE 1, and were fed to a curtain coating die with tangential approach of liquid to substrate. For each composition, the liquid flow rate was initially set at a value sufficient to form a continuous liquid sheet between the die on top, the substrate on the bottom, and edge guides on the sides which were separated by 5 cm at the top and 3 cm at the bottom. The liquid flow rate was then slowly decreased until the sheet broke forming discontinuous droplets or filaments. TABLE 9 shows the minimum flow rate that could be maintained for each liquid before the sheet broke. Addition of 25 ppm PEO to the latex allowed a four fold decrease in minimum flow rate; addition of 50 ppm PEO allowed a six fold decrease in minimum flow rate; and addition of 70 ppm PEO allowed an eight fold decrease in minimum flow rate. For the compositions of latex and PEO, TABLE 9 also shows the maximum substrate speed attainable at the minimum liquid flow rate wherein the maximum substrate speed increases from 10 to 20 to 30 meters per minute for 25, 50 and 70 ppm PEO, respectively. TABLE 9 Minimum flow rates and maximum coating speeds for curtain coating PEO concentration Minimum Flow Rate Maximum Coating Speed (ppm) (cc/cm-sec) (m/min) 0 0.67 -not tested- 25 0.16 10 50 0.11 20 70 0.08 30 EXAMPLE 11 Filament Forming - Web Coating A composition of latex B and 10 ppm 8,000,000 MW PEO was prepared as described in EXAMPLE 2, and this composition was pumped to a benchtop filament-forming apparatus wherein liquid was fed to three blunt end syringe needles with 0.5 mm inside diameter from a cylindrical 1 cm diameter manifold. The liquid filament from each needle was drawn down in diameter by application of a high velocity air jet at or below the needle exit, and the filament was captured on a moving substrate 25 cm below the needle exits, and dried in an oven. This process produced a discontinuous but connected coating of PSA filaments that contacted the substrate in random orientation. By increasing the liquid flow rate at fixed substrate speed, or by decreasing the substrate speed at fixed liquid flow rate, the fraction of surface covered by the mat of PSA filaments was increased. In contrast, a coating of discrete droplets was found to result from the use of the filament forming apparatus with a latex B formulation having no PEO. EXAMPLE 12 Filament Formation A composition of latex B and PEO prepared as described in EXAMPLE 11 was pumped to the apparatus described in EXAMPLE 11 except that the needle exits were in close proximity to a moving substrate. The liquid filaments exiting each needle were drawn down in diameter by tension imparted on the filament by the substrate which moved with larger velocity than the mean velocity of liquid at the needle exit, thus producing a discontinuous PSA coating of co-linear stripes which was then dried in an oven. EXAMPLE 13 Slot Coating Latex B prepared as described in EXAMPLE 2 was mixed using the method described in EXAMPLE 1 with a 200 ppm 8,000,000 MW PEO stock solution diluted from a 5000 ppm stock solution prepared as described in EXAMPLE 1 in an amount sufficient to give 2 ppm PEO in the composition. Coating experiments were performed using this composition as well as latex B without PEO in which liquid was pumped at 0.21 cc/cm-sec to a standard slot coating die with slot dimensions 250 microns high by 13 cm wide. Vacuum was applied to the upstream side of the die by means of a vacuum box, and the coating gap was adjusted from large to small until continuous coating was observed. TABLE 10 provides results for each liquid wherein relative to the latex formulation (i.e., without PEO), continuous coating of the latex/PEO composition was achieved at higher coating speeds and wider coating gap. TABLE 10 Observed coatings for slot coating latex PSA with and without PEO PEO concentration Coating Gap Coating Speed Observed Coating (ppm) (micron) (m min) 0 250 12 continuous 0 250 18 rivulets 0 75 30 rivulets 2 305 37 continuous EXAMPLE 14 Microsphere Preparation A composition of microsphere PSA suspension and 500 ppm 8,000,000 MW PEO was prepared as described in EXAMPLE 3, and was used in slot coating experiments with slot dimensions 250 microns high by 3 cm wide, no vacuum applied to the upstream side, and a die to substrate gap of 500 microns. Continuous coating was observed for a liquid flow rate of 0.11 cc/cm-sec and substrate speed from 6 to 31 m/min. For the PSA microsphere suspension at 0 ppm PEO using the same apparatus and coating gap, with a substrate speed of 6 m/min, a liquid flow rate of 0.33 cc/cm-sec was required to achieve a continuous coating.

Claims: What is claimed is: 1. A coating composition comprising a water-based pressure-sensitive adhesive formulation, and one or more high molecular weight polymers, wherein polymer is present at a concentration suitable to allow the composition to be delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation itself 2. A composition according to claim 1 wherein the composition permits the use of a minimum flow rate that is less than 70% of the minimum flow rate necessary to maintain a stable curtain in a curtain coater of the formulation without added polymer. 3. A composition according to claim 1 wherein the composition permits gap distances of on the order of twice the maximum gap distance useful for providing the corresponding formulation in the form of a stable sheet in a slot coater. 4. A composition according to claim 1 wherein the adhesive formulation comprises a latex, acrylate-based PSA and wherein the high molecular weight polymer comprises poly(ethylene oxide) polymers. 5. A composition according to claim 1 wherein the adhesive formulation comprises a water-based latex pressure sensitive adhesive and the polymer comprises between about 1 ppm and about 50 ppm high molecular weight PEO. 6. A method of preparing a coating composition, the method comprising the steps of: (a) forming a water-based pressure-sensitive adhesive formulation, and (b) adding, during or after formation of the formulation, a high molecular weight polymer to yield a final concentration suitable to allow the composition to be delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation itself. 7. A method according to claim 6 wherein the polymer is added to a concentration of between about 1 ppm and about 50 ppm. 8. A coating composition prepared by the method of claim 6. 9. A coated product comprising a composition of claim 1. 10. A method of preparing a coating composition, the method comprising the step of adding to a water-based pressure-sensitive adhesive formulation a high molecular weight polymer to yield a final concentration suitable to allow the composition to be delivered from the coating die of a premetered coater in the form of a stable sheet at a flow rate substantially less than the minimum delivery flow rate achievable using the formulation itself.

Inventors: NORTHEY PAUL J, NORTHEY, PAUL, J.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: C09J7
IPC codes: C08J5, C08L71, C09J7, C09J133, C09J201