Patent Application
20090320324
The subject invention is directed to cushioning devices comprising a pressure sensitive adhesive. In particular, the invention concerns the use of pressure sensitive adhesive materials to improve the hold of soft gel cushioning devices. One of the key attributes of the footwear cushions is to prevent the footwear from rubbing sensitive areas of skin, in particular in the heel area. However, in order to achieve all the desired attributes of the cushion, the cushion needs to stay in place firmly during movement associated with normal use of footwear. Cushioning devices made of soft gels are known in the art and are desirable for their comfort, flexibility, durability and look. The assignee of this invention, Schering-Plough Healthcare Products, Inc., markets a cushioning device under the trade name Dr. Scholl's® Gel Heel Liner, which comprises a thermoplastic polyurethane (TPU) film bonded to a polyurethane (PU) gel, where the PU gel is produced in a way to have a sticky surface. However, no additional adhesive is used in this product. The Heel Liner is packaged with a release film to preserve the tackiness of the PU gel sticky surface, which is then applied to the shoe to hold the liner in place. The TPU film provides a nonstick surface opposite the sticky surface, which nonstick contacts the wearers foot, sock, or hosiery.
It has now been determined, however, that the sticky PU gel does not hold the liner in place for extended periods of use as may be desired for a cushioning element. Pressure sensitive adhesive materials have been used in the art. However, direct modification of a sticky PU gel with a pressure sensitive adhesive did not produce a cushioning element with a stronger hold. Thus, a cushioning device that provides the benefit of a gel and provides for long term hold in the shoe where placed would be desirable. Further, a method of manufacturing a gel cushioning device with a pressure sensitive adhesive would also be desirable. These and other advantages follow from the invention described and claimed here.
The subject invention provides a cushioning device comprising a layer of polyurethane gel material comprising top and bottom surfaces, at least one layer of thermoplastic polyurethane film material bonded to the one or both of said top and bottom surfaces thereof, and a layer of pressure sensitive adhesive bonded to one of said at least one thermoplastic polyurethane film material, wherein the pressure sensitive adhesive is located on a surface of the thermoplastic polyurethane film opposite to the surface contacting said bottom surface of said polyurethane gel material.
The subject invention also provides a gel cushioning device comprising a sticky surface, the device comprising a gel material bound to a thermoplastic film material having good compatibility with the gel material, wherein the thermoplastic film material further comprises a pressure sensitive adhesive located on a surface of the film material opposite to the surface that is bonded to the gel material.
The subject invention further provides gel cushioning device comprising a sticky surface, the device comprising a layer of gel material having an upper and lower surface and two layers of thermoplastic film material, each film material comprising upper and lower surfaces, wherein the upper surface of the gel material is bonded with the lower surface of one of said layers of film material and the lower surface of the gel material is bonded with the upper surface of the other of said layers of film material, wherein at least one of the layers of film material comprises a pressure sensitive adhesive on a surface opposite the surface bonded to the gel material thereby providing the sticky surface of the device.
The subject invention also provides a method of constructing a gel cushioning device comprising a sticky surface, which comprises loading gel material into a mold cavity comprising at least one layer of thermoplastic film material, said at least one layer of thermoplastic film material comprising upper and lower surfaces, and curing the gel material to form a solid gel, such that the cured gel material is bonded with the film material and wherein said at least one of the layer of thermoplastic film material comprises a pressure sensitive adhesive on a surface opposite the surface bonded to the gel material thereby providing the sticky surface of the device.
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As used herein polyurethane material will be understood to encompass polymers that contain a plurality of urethane linkages and comprised of either aliphatic or aromatic isocyanate prepolymers, and combinations thereof, both with either di or trifunctional polyol polymers or prepolymers. The polyurethane material may optionally also contain other kinds of chemical linkages formed from the reactions of polyisocyanates, including but not limited to urea linkages, isocyanurate linkages, oxazolidone linkages, biuret linkages, allophanate linkages, combinations of these, and the like. Polyurethane gel materials are typically formed by reacting polyisocyanates with polyols. Examples of poly isocyanates include those formed from aliphatic or aromatic isocyanate prepolymers or combinations thereof. Examples of polyols include di or trifunctional polyol polymers or prepolymers. Examples of polyols used include polyether polyols such as poly(ethylene oxide) and poly(propylene oxide), modified polyether polyols, polytetramethylene glycol, condensation polyester polyols produced by reacting dicarboxylic acids with diols, lactone-type polyester polyols produced by ring opening polymerization of epsilon-caprolactone or the like, and polycarbonate polyols, vinyl polymers into which hydroxyl groups are introduced such as polyols having polyisobutylene as the main chain.
The gel layer can also be made from a non-foam elastomer such as the class of materials known as viscoelastic polymers or silicone gels, which show high levels of damping when tested by dynamic mechanical analysis performed in the range of −50 degree Celsius to 100 degrees Celsius. Gels material can include, but not limited to, the Kraton family of styrene-olefin-rubber block copolymers, thermoplastic polyurethanes, thermoset polyurethanes, thermoplastic poly olefins, polyamides, polyureas, polyesters and other polymer materials that reversibly soften as a function of temperature. The preferred elastomers are a Kraton block copolymer of styrene/ethylene-co-butylene/styrene (SEBS) or styrene/butadiene/styrene (SBS) with mineral oil incorporated into the matrix as a plasticizer.
As used herein, pressure sensitive adhesives (also known as contact adhesives) are those that form viscoelastic bonds that are aggressively and permanently tacky, adhere without the need of more than slight pressure, and require no activation by water, solvent or heat. Pressure sensitive adhesives are typically available in solvent and latex or water based forms and are often based on non-crosslinked rubber adhesives, acrylics or polyurethanes.
In certain embodiments of the invention, the pressure sensitive adhesives may comprise acrylic and methacrylate adhesives, rubber-based pressure sensitive adhesives, styrene copolymers such as styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) copolymers with at least one tackifier, and silicones. Acrylic adhesives often use an acrylate system such as ethylene ethyl acrylate (EEA) or ethylene methyl acrylate (EMA) copolymers, which are used to form hot melt PSA adhesives. Natural rubber, synthetic rubber or elastomer adhesives may typically comprise a variety of materials such as silicone, polyurethane, chloroprene, butyl, polybutadiene, isoprene or neoprene. Rubber and elastomers are characterized by their high degree of flexibility and elasticity.
In certain embodiments, the pressure sensitive adhesive material may comprise acrylate-based adhesive materials. The thickness of the layer of the pressure sensitive adhesive material may be varied by those of ordinary skill in the art according to the adhesive material used and the desired tack. In the practice of certain example embodiments of the invention, the pressure sensitive adhesive may have a thickness of between about 0.0125 mm and about 0.25 mm.
An additional aspect of the invention comprises a method of manufacturing a cushioning element comprising a polyurethane gel material, which method comprises the step of bonding a polyurethane gel and a thermoplastic film material, wherein the thermoplastic film material comprises a pressure sensitive adhesive. In preparation of one embodiment of the invention it was discovered that the pressure sensitive adhesive layer cannot bond directly to the soft polyurethane gel during the molding process. To overcome this limitation, a layer of thermoplastic film material comprising the pressure sensitive adhesive layer was added to the mold and bonded to the polyurethane gel.
In certain embodiments of the invention, the cushioning element is prepared by a direct molding process where the gel material is applied directly to the mold, for example by injecting the gel prepolymer into the mold cavity. During a direct molding process, before the gel material is injected, the film material layer comprising the pressure sensitive adhesive is applied to the mold cavity with the adhesive on the side of the film material closest to the mold and then the PU gel material is injected into the mold over the film material. The mold can be treated to resist bonding of the pressure sensitive adhesive or a release liner material can be applied in the mold first before the film material is applied to the mold. In certain embodiments of the invention, the mold may be preheated to the desired curing temperature prior to injecting the gel material or heated after all materials are loaded into the mold. For example, when using polyurethane gel materials according to the invention the mold temperature is typically between 85-95 C and the cycle time is approximately 8 min. The pressure is typically low and not critical for this type of operation provided that the pressure is high enough to fill the cavity-mold with polyurethane prepolymer.
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The products of the invention can be formed into any shape suitable for use as a cushioning device, for example for insertion into footwear to prevent rubbing between sensitive skin and hard or exposed surfaces of the footwear. The cushioning elements can also be applied to other garments e.g., hats, helmets, protective sporting gear such as elbow and knee pads and other garments requiring extra padding for comfort, sizing and/or safety. Cushioning elements according to the invention may also be applied to surfaces which come in contact with body parts to provide protection from injury such as, e.g., chairs, desks, cabinets, tables, doors and door frames, among others. Depending on the size and shape of the object to which the cushioning element is applied, certain shapes will better protect different areas of the body from contact with the object. In certain example embodiments the products are molded in the shape of, e.g., circles, parabolas, stars, squares, rectangles, rounded rectangles, half-moon, among others. The appropriate shape of the cushioning element can be determined based on the size and surface of the object to which the cushioning element will be applied. In alternative example embodiments, the cushioning element can be provided as a sheet of material that the user can cut to a desired shape. The sheet of material can also be provided with preformed shapes that can be removed as desired for the appropriate protection from contact with the object.
A human wear test was conducted to compare the unmodified polyurethane gel cushion containing a sticky gel surface with a modified polyurethane gel cushion of the present invention containing a pressure sensitive adhesive. The test was run to determine if a cushioning device according to the invention would provide appropriate adhesion performance when compared to the unmodified polyurethane gel heel liner. Prior to the wear test, a probe tack test was conducted on both devices according to ASTM D 2979-01 to measure the force required to separate an adhesive and adherend shortly after they have been contacted under a defined load of known duration and at a specified temperature. The devices were also tested for shear hold according to PSTC-107 A, which measures the adhesive material's ability to remain adhered under a constant load applied parallel to the surface of the material and substrate, in particular measuring the shear adhesion when applied to a vertical standard steel panel. The two devices were determined to have the following adhesion tack and shear hold:
Based on the results of the probe tack test and the shear hold test, those of ordinary skill in the art would recognize that the two heel liner devices have essentially the same adhesion tack and shear hold, and would be expected to have the same adhesion performance in use, i.e., the ability to stay in place in a shoe under normal wear.
The adhesion performance of heel liner cushions formed according to the invention was then evaluated in a wear test by volunteers who inserted the devices into their shoes and wore them for three days of normal use. The test subjects were divided such that 23 subjects evaluated the unmodified heel liners while 22 subjects evaluated the heel liner modified with PSA. Both the unmodified heel liners and the modified heel liners were similar in appearance so the test subjects did not know which device they were wearing. After three days, the test subjects were asked to rate their agreement or disagreement with whether the cushion stayed in place during normal use, rating according to a seven point scale:
The sum of the top three rating answers indicated a positive feedback of the performance of the heel liner cushion adhesion and the sum of the bottom three rating answers indicated the negative feedback to the performance of the heel liner cushion The results were as follows:
As shown, the modified heel liners produced according to the subject invention containing the PSA produced an overwhelmingly positive feed back from users. Conversely, the unmodified heel liners showed equivocal results of positive and negative feed back. The statistical analysis (p of 0.0041) of the wear test results indicated that the liners modified with PSA according to the invention provide a better adhesion performance during wear than the unmodified version, which result was unexpected based on the probe tack and shear hold tests on the cushions.
Several embodiments of the present invention are specifically described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.
