MICROSTRUCTURED POLYMERIC SUBSTRATE

Abstract: A method of preparing a polymeric substrate having a surface which includes a plurality of microaberrations (5a, 5b) is provided. A polymeric material (1) in a flowable state is contacted with a resilient surface (9) to form microaberrations (5a, 5b) which are inverted replicas of microaberrations (4a, 4b) present on the resilient surface. A cleaning article and a slip control material which include the microstructured polymeric substrate are also provided.

Description: MICROSTRUCTURED POLYMERIC SUBSTRATE Background ofthe Invention Polymeric substrates with a microstructured pattern on their surface have a wide variety of potential applications. Microstructured polymeric films may be applied to a surface in order to improve its slip control properties, e.g., to improve the grip on tools or athletic equipment. Other surfaces which may benefit from the application of materials providing enhanced slip control include traffic bearing surfaces, such as stairs, floors and ramps. Polymeric substrates with a suitably textured surface may also be employed in cleaning articles. For example, non-abrasive polymeric scouring pads may be used for cleaning dishes or pots or for removing insects and other debris from windows. Polymeric materials have desirable physical properties for fabricating cleaning pads while offering the potential for very economical production costs. A variety of methods for producing polymeric films having a textured surface are known. For example, an embossed thermoplastic film having a plurality of micro depressions imparted by placing a coarse sandblast pattern on a metal embossing roll is disclosed in U.S. Patent No. 5,229,186 (Tribble et al ). Similarly, U.S. Patent No. 4,861,635 (Carpenter et al.) describes the replication of negative images of microtopographical features from a metal chill roll or a Teflon™ covered metal roll onto a polypropylene film. U.S. Patent No. 4,463,045 (Ahr et al.) discloses the preparation of a plastic web having a microscopic pattern of surface aberrations by contacting a heated thermoplastic film with a photoetched metallic laminae which has a microscopic pattern of surface aberrations on the uppermost lamina. The formation of a thermoplastic sheet having a plurality of "nubbles" is disclosed in U.S. Patent Nos. 4,239,286 (Louis et al.) and 4,327,730 (Sorenson). The nubbles are formed by contacting a ribbon of thermoplastic film with a texturing cylinder coated with a mixture of particles (e.g., aluminum oxide particles) and cured epoxy. An article with a microstructure-bearing surface is described in U.S. Patent No. 4,576,850 (Martens). The article is formed by filling a mold master, which bears a microstructure, with a radiation curable oligomeric composition. U.S. Patent Nos.4,772,444 (Curro et al.) and 5,221,276 (Battrell) disclose a microbubbled polymeric web. The microbubbles are produced by subjecting a flat polymeric film supported on a fine woven wire mesh to a high pressure liquid jet. In order for the articles containing microstructured polymeric materials to realize their full potential, versatile, inexpensive methods of fabricating such polymeric materials must be available. Current methods do not readily allow controlled variations in the shape, orientation and positioning of a microscale texturing pattern on a polymer surface. In addition, current methodology typically only permits the generation of polymeric substrates in which the microstructure is limited to either the bottom of depressions or the tops of projections; it does not provide microstructure on the sides of macroscopic features such as depressions or projections nor does it permit a choice as to the type of microstructure generated. A need, therefore, continues to exist for improved methods of producing polymeric substrates having a surface textured with a defined microscopic pattern. Optimally, such a method would also permit the introduction of macroscopic structural features (e.g., via embossing) and/or would allow the choice of generating a microscopic pattern on either all or a portion ofthe surface. Summary ofthe Invention The present invention provides a method of preparing a polymeric substrate having a surface which includes a plurality of microaberrations. The method includes contacting a flowable polymeric material with a resilient surface to form the microaberrations. The microaberrations produced on the polymeric surface are typically inverted replicas of microaberrations on the resilient surface, i.e., if the resilient surface includes a plurality of microdepressions, the polymer substrate which is generated will include a plurality of microprotrusions. The present method also allows the formation of macroscopic features on the microstructured polymeric substrate. By incorporating embossing techniques, the present method permits the formation of polymeric substrates which include a plurality of projecting elements having microaberrations on their outer surface. In particular, the method allows the formation of microaberrations on the side * walls of the projecting elements. If desired, the present method permits the introduction of microaberrations onto the entire surface ofthe polymeric substrate, i.e., on the portions ofthe surface in between the projecting elements (the "land area") as well as on the projecting elements themselves. The method also allows microaberrations to be generated only on the outer surface ofthe projecting elements and not on the land area. In addition, the present method allows the production of a microstructured polymeric substrate having microaberrations with a variety of controlled shapes and sizes. The method may be employed with a wide range of polymeric materials capable of existing in a state which is sufficiently flowable to allow the polymer to conform to the resilient surface and are capable of being at least partially solidified under the processing conditions. The present method allows the economical production of a wide variety of polymeric articles having a microstructured surface which may optionally be simultaneously embossed to introduce macroscopic features. The present method also provides a polymeric substrate having a plurality of projecting elements. The outer surface ofthe projecting elements has sidewalls which include a plurality of microprotrusions. The microprotrusions typically have a width and heighth of about lOμm to about 400μm. In another embodiment, the present method provides a unitary polymeric substrate which includes a plurality of undercut-shaped solid microprotrusions. The polymeric substrate may also include a plurality of projecting elements having the microprotrusions on their outer surface. Yet another embodiment ofthe present invention is directed to a dual structured polymeric substrate which includes a plurality of projecting elements. The outer surfaces ofthe projecting elements have sidewalls which include a plurality of microaberrations. The present invention is also directed to cleaning articles and slip control materials which include the microstructured polymeric substrate. The microstructured polymeric substrate may be formed by a process which includes contacting a flowable polymeric material with a resilient surface which has a plurality of first microaberrations. A microstructured polymeric substrate formed in this manner includes a plurality of second microaberrations which are inverted replicas ofthe first microaberrations. Brief Description ofthe Drawings Fig. 1 depicts a simplified schematic illustration of one embodiment of a method for producing a microstructured polymeric film according to the present invention. Fig. 2 depicts a simplified schematic illustration of another embodiment of a method for producing a microstructured polymeric film according to the present invention. Fig. 3 depicts a simplified schematic illustration of a third embodiment of a method for producing a microstructured polymeric film according to the present invention. Fig. 4 depicts a simplified schematic illustration of a fourth embodiment of a method for producing a microstructured polymeric film according to the present invention. Fig. 5 depicts a cross sectional view of a portion of a polymeric substrate of the present invention. Fig. 6 shows an electron micrograph (12 X magnification) of a surface of a polymeric substrate ofthe present invention. Fig. 7 shows an electron micrograph of one projecting element from the polymeric substrate depicted in Fig. 6 shown at a higher magnification (60 X magnification). Fig. 8 shows an electron micrograph (15 X magnification) of a surface of a polymeric substrate ofthe present invention. The surface includes a plurality of projecting elements overlayed with a secondary structure of a plurality of raised ribs. Fig. 9 shows an electron micrograph (50 X magnification) of a projecting element from a polymeric substrate ofthe present invention. The outer surface of the projecting element includes a plurality of cube corner out-shaped microprotrusions. Fig. 10 shows an electron micrograph (50 X magnification) of a projecting element from a polymeric substrate ofthe present invention. The outer surface of the projecting element includes a plurality of cube corner in-shaped microdepressions. Fig. 11 shows an electron micrograph (95 X magnification) of a projecting element from a polymeric substrate ofthe present invention. The outer surface of the projecting element includes a plurality of partially spherical microdepressions. Fig. 12 shows an electron micrograph (60 X magnification) of a projecting element from a polymeric substrate ofthe present invention. The outer surface of the projecting element includes a plurality of undercut-shaped microprotrusions. Fig. 13 shows an electron micrograph (18 X magnification) of a projecting element from a polymeric substrate ofthe present invention. The outer surface of the projecting element includes a plurality of microprotrusions. Fig. 14 shows an electron micrograph (13 X magnification) of a surface of a polymeric substrate ofthe present invention. The surface includes a plurality of discontinuous projecting elements in the form ridges including a plurality of undercut-shaped microprotrusions. The substrate surface also includes a plurality of microprotrusions. Fig. 15 shows an electron micrograph (13 X magnification) of a surface of a polymeric substrate ofthe present invention. The surface includes a plurality of undercut-shaped microprotrusions. Fig. 16 depicts a cross sectional view of a portion of a master used to prepare a microstructured polymeric substrate ofthe present invention. Detailed Description ofthe Invention Fig. 1 depicts a schematic illustration of one embodiment ofthe present invention. A polymeric material 1 in a flowable state is brought into contact with a resilient surface, such as the circumferential surface of resilient roll 2. Resilient roll 2 includes an outer layer of resilient material 9 covering a cylindrical core 11. The core 11 is typically formed from a substantially non-resilient material, such as a metal or a high durometer rubber. In zone A, the polymeric material is in a flowable state and has relatively smooth major surfaces 7 and 8. The polymeric material may enter zone A already in flowable state, e.g., after exiting the die of an extruder. Alternatively, the polymer may be treated in zone A, such as by the application of heat, to transform the polymer into a flowable state. During processing, the flowable polymeric material comes into contact with the surface 3 of resilient roll 2 in zone B. Sufficient pressure is exerted in the nip on the flowable material by smooth roll 10 and resilient roll 2 to force the polymeric material to conform to the contours of surface 3. The flowable polymer is forced into any recesses or crevices defined by the microdepressions 4 present in surface 3. This results in the generation of microscopic projections 5a, 5b ("microprotrusions") on the polymeric surface 7 which comes into contact with resilient surface 3. The microprotrusions 5a, 5b formed on the polymeric surface 7 are inverted replicas ofthe corresponding microdepressions 4a, 4b in resilient surface 3. While still in contact with the resilient surface 3, the polymeric material solidifies to a sufficient degree to allow the microprotrusions 5a, 5b to retain their shape (see zone C) as the microstructured polymeric film 6 is pulled away from the surface 3 and the microprotrusions 5a, 5b are pulled out of microdepressions 4a, 4b. This may be accomplished, for example, where the polymeric material is formed from a thermoplastic polymer by maintaining the temperature ofthe resilient roll 2 below the softening point ofthe polymeric material. Alternatively, where the polymeric material has thermoset properties, the solidification may be achieved by applying additional heat to the polymeric material while the material is in zone B, e.g., by maintaining smooth roll 10 at an elevated temperature. The material which forms the surface 3 of resilient roll 2 typically permits the microstructured film 6 to be separated from resilient roll 2 without substantially distorting or destroying the microaberrations 5. This requires that the forming microstructured film 6 does not adhere to resilient surface 3. The outer surface of resilient roll 2 may also be formed from a number of resilient materials which permit the processed polymer to be removed without problems of adhesion. In a preferred embodiment ofthe invention, resilient surface 3 is formed from a silicone rubber. Resilient materials formed from polyurethane or silicone permit the present method to be carried out under a wide range of processing conditions, e.g., temperatures from about 0°C to about 400°C or even higher. The interaction between microaberrations 5, which are at least partially solidified, and the resilient roll surface 3 is such that the microaberrations 5 substantially retain their shape as the microstructured polymeric film 6 is pulled away from the resilient roll 2. To some extent this may be due to some resiliency on the part ofthe microaberrations 5 themselves, as where the solidifying polymeric material exhibits some degree of elasticity. More typically, the resilient interaction is achieved by the resiliency ofthe resilient roll surface material 9. The resilient roll 2 may be covered with an outer layer of a porous resilient material, such as a polymeric foam. Examples of suitable foams for the resilient surface include polyurethane foams and silicone foams. The foam may be a closed cell polyurethane foam such as LSI 525 polyurethane foam (available from EAR™ Specialty Composites Corporation, Indianapolis, IN) or PORON™ polyurethane foam (available from Rogers Corporation, East Woodstock, CT). The closed cell polyurethane foams disclosed in U.S. Patents 3,772,224 and 3,849, 156, may also be employed for the resilient surface. Another example of a suitable polymeric foam is a closed cell silicone foam such as Bisco BF-1000 foam (available from Bisco Products, Elk Grove, IL). The resilient polymeric material may also include an open cell polymeric foam. Altematively, the resilient surface may be covered with a layer of a nonporous flexible material, e.g., a flexible polymeric material such as a silicone rubber. For example, the resilient surface may include Silastic™ brand J- RTV silicone rubber (commercially available from Dow Co ing Corp., Midland, MI). Resilient surfaces having defined microstructural features preferably include a silicone rubber. Alternatively, where the resilient surface has a foam microstructure, the resilient material preferably includes a closed cell polyurethane foam, such as described in U.S. Patents 3,772,224 and 3,849,156. The resilient material which forms resilient surface 3 may inherently include microaberrations 4, e.g., the pocket-like depressions present in the surface ' of a polymeric foam. Where the resilient surface includes a polymeric foam material, the resilient surface may also include a thin outer layer of a nonporous flexible material covering the foam. For example, the resilient surface may include a foam layer covered by a thin layer (e.g., about 0.5 mm to about 1.0 mm) of silicone rubber. The thin silicone rubber layer may include microaberrations on its surface. The resilient surface may also be formed by wrapping the roll with a nonporous flexible microstructured material. For example, a variety of substantially rigid microstructured surfaces may be coated with a thin layer of material (e.g., liquid silicone rubber) which is then cured into a flexible material reflecting the microstructure ofthe underlying rigid structure. A desired pattem and shape of microprotrusions in a flexible material may also be generated by embedding a plurality of microscopic particles in the surface of a resilient material, such as by embedding inorganic particles (e.g., glass beads) in a silicone rubber layer. Microdepressions may be formed in silicone rubber layer (or other nonporous flexible material) by removing microparticles embedded in the silicone rubber to leave a plurality of microdepressions in the rubber surface. A wide variety of polymers may be processed according to the present method into a polymeric substrate having a microstructured surface. Polymeric materials capable of being sufficiently flowable to allow the polymer to conform to the microscopic features ofthe resilient surface and capable of being solidified sufficiently to generate microscopic features on the polymer surface are suitable for use in the present invention. Typically, the polymeric material includes a thermoplastic polymer such as a polyolefin, although other polymeric materials capable of being processed in a flowable state, such as a B-staged thermoset polymer or a plastisol, may also be employed. Typically, the polymeric material includes a thermoplastic polymer having a melt temperature above about 50°C. However, polymeric materials which exist in a flowable state at a considerably higher temperature may also be employed. The physical properties ofthe resilient surface and the polymeric material must be matched such that the microstructural features ofthe resilient surface are stable and resilient under conditions which permit the flowable polymer to conform to the resilient surface and then at least partially solidify. Preferably, thermoplastic materials which can be passed through an embossing nip at or slightly above their glass transition temperature are employed, as such materials may be processed with short cycle times. Examples of suitable thermoplastic polymeric materials which may be employed in the present process include polyolefins such as polypropylene, polyethylene, and polypropylene/polyethylene copolymers. Blends of polypropylene and/or polyethylene, such as a high/low molecular weight polyethylene blend (e.g., Hostalloy™ 731; Hoechst Celanese, Somerville, N.J.), are also suitable for use in the present invention. Other suitable thermoplastic polymers include polyvinyl chloride (PVC), polyamides such as a nylon (e.g., nylon 6, nylon 6,6, or nylon 6,9), and polyesters. Olefin copolymers such as ethylene/vinyl acetate copolymers or copolymers of an olefin and an a,b-unsaturated acid (e.g., an ethylene/methacrylic acid copolymer reacted with metal salts to confer ionic character; available from E.I. du Pont de Nemours & Co., Inc. as Surlyn™ 8527) may also be employed in the present invention. Preferably, the polymeric material includes a polyolefin. In another embodiment ofthe invention, the polymeric material may include a plastisol. The plastisol includes a dispersion of thermoplastic resin particles (e.g., polyvinyl chloride resin particles) in a plasticizer and may also include a volatile organic solvent. Examples of suitable plastisols which may be used in the present method include the vinyl plastisols such as #D 1902-50 Black and #D 1902-78 White available from Plast-O-Meric, Inc. (Waukesha, WI). The microaberrations generated on the polymeric substrate by the present method may have a wide variety of shapes and orientations. The microaberrations include microprotrusions and/or microdepressions. These may be in a random and/or ordered patte . The microaberrations may include irregular shapes and/or regular geometric shapes, e.g., a plurality of discrete mounds, posts, cones, truncated cones, pyramids, or truncated pyramids. The microaberrations may include discontinuous shapes, i.e., a plurality of discrete projections or depressions. Alternatively, the microaberrations may include continuous forms such as ridges and grooves. The dimensions ofthe microaberrations, however, are typically small enough to leave the form of any macroscopic features on the polymeric substrate substantially unaltered. Typically, discontinuous microaberrations have a maximum dimension (height or width) of no more than about 400μm. Preferably, discontinuous microaberrations have a maximum height of no more than about 300μm and a maximum width of no more than about 300μm. More preferably, discontinuous microaberrations have a maximum height of no more than about 200μm and a maximum width of no more than about 200μm. Similarly, where the microaberrations are continuous in nature, the height and width ofthe microaberrations are typically no more than about 400μm, preferably of no more than about 300μm, and more preferably of no more than about 200μm. The microaberrations preferably have an average minimum height and width of at least about lOμm and more preferably at least about 25 μm. One distinct advantage ofthe present method is that it allows the generation of undercut-shaped microaberrations on the surface of a polymeric substrate. As used herein, the term "undercut-shaped" is defined as a shape having a cross- sectional surface area which increases and then typically decreases along a perpendicular vector away from the polymer surface. In other words, the cross- sectional surface area is measured in a plane parrallel to the major surface ofthe polymeric substrate with respect to which the undercut-shaped microaberration in question is positioned. The microstructured polymeric film 6 formed by the process illustrated in Fig. 1 includes a number of undercut-shaped microprotrusions 5a. The undercut- shaped microprotrusions are generated when the flowable polymeric material is forced into an undercut-shaped microdepression 4a in resilient surface 3. The parameters ofthe process (e.g., temperature of the thermoplastic polymeric material in Zone A, temperature ofthe resilient surface 3, the compositions ofthe polymeric material and resilient surface 3, the speed at which the polymeric film is processed and pressure exerted on the forming microstructured polymeric film) may be adjusted such that an undercut-shaped microprotrusion 5a on the polymeric film may be pulled out of an undercut-shaped microdepression 4a without substantially altering or distorting the shape ofthe microprotrusion. In addition to facilitating the generation of a microstructured surface on a polymeric substrate, the use of a resilient surface having an outer resilient layer of foam or silicone rubber has another advantage. The polymeric substrate may also be simultaneously shaped or formed to introduce macroscopic features, e.g., by embossing, as part ofthe present method. Figure 2 shows a second embodiment of the present method. The resilience of outer layer 32 of chill roll 27 permits the film to be embossed without requiring that each projecting embossing element 21 be precisely positioned with respect to a corresponding recess in the opposing roll. In the embodiment ofthe present method illustrated in Fig. 2, a flowable polymeric material 25, such as a softened thermoplastic polymeric film, is passed through a nip which includes embossing roll 20 and chill roll 27. Chill roll 27 is covered by a outer layer 32 of resilient material which has a plurality of microdepressions 28 in its outer surface 30 (e.g., the microdepressions present in the surface of a polymeric foam). When the softened thermoplastic polymeric film 25 is contacted with embossing roll 20 and chill roll 27, posts 21 are forced into the film and projecting elements 22 are formed. As this happens, the softened thermoplastic polymeric film 25 is forced into intimate contact with the surface 30 of chill roll 27. As with the method illustrated by Fig. 1, this results in the formation of microprotrusions 23 on the portions ofthe polymeric material in contact with surface 30. The microprotrusions 23 are inverted replicas ofthe corresponding microdepressions 28 in the outer surface 30 of chill roll 27. The resulting polymeric film 26 has a plurality of projecting elements 22 having a hollow core 29. The outer surface 24 of projecting elements 22 includes a plurality ofthe microprotrusions 23. In the process illustrated in Fig. 2, the microprotrusions 23 are generated on the top 24 and side 33 surfaces of projecting elements 22, but not on the land area 31 between adjacent projecting elements 22. By controlling the nip pressure, the present process may be modified to produce microaberrations on either the outer surface ofthe projecting elements 22 or on the entire surface ofthe polymeric film (i.e., on the land area 31 between the -l l- projecting elements 22 as well as on the outer surfaces 24, 33). The nip pressure may be increased by a variety of techniques, including but not limited to positioning embossing roll 20 and chill roll 27 closer together, increasing the height of embossing elements 21 or decreasing the density of embossing elements 21 (i.e., increasing the spacing between adjacent embossing elements 21 and decreasing the surface area of embossing roll 20). Fig. 3 illustrates a third embodiment ofthe present method. Softened thermoplastic polymeric film 46 is passed through a nip which includes embossing roll 40 and chill roll 41. Chill roll 41 is covered with two layers of resilient material, an inner layer of polymeric foam 43 and an outer layer 44 of nonporous flexible material, such as silicone rubber. The surface 47 of layer 44 has a microstructure which includes a plurality of microprotrusions 45. The nip pressure forces the softened polymeric film 46 into contact with embossing roll 40 and chill roll 41. As this occurs, posts 42 are pressed into the film 46 to form projecting elements 48. During this process, the softened film 46 is forced into intimate contact with the microstructured surface 47 of chill roll 41. As with the methods illustrated by Figs. 1 and 2, microscopic features (microdepressions 49) are generated in the portions ofthe film 46 in contact with surface 47. The microstructured polymeric film 50 produced by the method illustrated in Fig. 3 has a plurality of projecting elements 48 having a hollow core 51. The projecting elements 48 include a plurality ofthe microdepressions 49, which are inverted replicas of corresponding microprotrusions 45 in the outer surface 47 of chill roll 41. In the process illustrated in Fig. 3, the microdepressions 49 are generated on the land area 53 as well as on the top 52 and sidewall 54 surfaces of projecting elements 48. By controlling the nip pressure, it is also possible to cause the microscopic features to be generated only on the outer surfaces of the projecting elements (as illustrated in Fig.2). The microprotrusions 45 depicted in Fig. 3 may be formed from a flexible material, such as silicone rubber. The present method may also be carried out with a resilient surface which includes a plurality of nondeforming microscopic regions surrounded by resilient material, e.g., a plurality of rigid microscopic particles partially embedded in the surface of a resilient material. For example, the resilient surface may be formed from a layer of silicone rubber which has a plurality of microscopic inorganic particles (e.g., glass beads having a diameter of about lOμm to about 250μm) partially embedded in surface ofthe rubber layer. Because ofthe flexibility ofthe underlying silicone rubber matrix, it is possible to generate microdepressions in the surface of a polymeric substrate which are substantially undistorted inverted replicas ofthe shape ofthe inorganic particles protmding from the silicone rubber surface. In other words, if the inorganic particles are spherical microscopic glass beads which partially protrude from the silicone rubber, a plurality of microdepressions, which are shaped like a portion of a sphere, may be formed in the polymer substrate surface. In addition, due to the flexibility ofthe resilient surface, the present method permits the generation of embossed polymeric substrates having microdepressions and/or microprotrusions on the sidewalls of macroscopic structural features formed via embossing. Fig. 5 depicts a portion of a polymeric film 80 which has been embossed according to the present method to generate a hollow projecting element 82. The projecting element 82 includes microdepressions on both its top outer surface 83 and its outer sidewalls 84. By employing the present method, it is possible to create microdepressions on the sidewalls of a projecting element such that the microdepressions are substantially undistorted inverted replicas of rigid microprotrusions on the resilient surface used to form the microstructured embossed polymeric substrate. Microdepressions having a partially spherical shape (see, e.g., microdepressions 81 in Fig. 5) may be formed by employing a resilient surface which includes rigid partially spherical shaped microprotrusions, such as glass beads partially embedded in a resilient material. Fig. 4 illustrates yet another embodiment ofthe present method. Resilient roll 60 includes an outer layer of resilient silicone rubber material 70. The surface 62 ofthe resilient material 70 includes a number of macroscopic depressions 63. In addition, the entire resilient surface 62 includes a plurality of microdepressions 64. As the flowable polymeric material 69 is passed through the nip of resilient roll 60 and smooth roll 61, surface 68 of polymeric material 69 is forced into intimate contact with resilient surface 62. This causes the polymeric material to fill macroscopic depressions 63, thereby generating solid projecting elements 65 extending from dual structured film 67. In the process illustrated in Fig. 4, the polymeric material is simultaneously forced into microdepressions 64, thereby generating a plurality of solid microprotrusions 66 on the entire surface of textured polymeric substrate 67 (i.e., on the land 72 as well as on the outer surface 71 of projecting elements 65). The projecting elements formed by the present method may have a variety of heights. For example, the method allows the production of microstructured projecting elements having a height within the range of about 0.375mm to about 2.5mm while avoiding substantial distortion of microaberrations generated on the outer sidewalls ofthe projecting elements. Moreover, the present method allows undistorted microaberrations to be formed in the sidewalls of projecting elements which have a very steep draft. For example, substantially undistorted microaberrations may be formed on the sidewalls of projecting elements having a draft of less than about 30° (with respect to a vector perpendicular to the major surface ofthe polymeric substrate). Substantially undistorted microaberrations may be generated on sidewalls which are quite close to being vertical (i.e., have a draft of less than about 10°) or even on sidewalls which are substantially vertical (i.e., have a draft of less than about 5°). Using the present method, substantially undistorted microprotrusions have been generated on the sidewalls of vertical (i.e., 0° draft) 1.5mm high hollow posts. This unitary polymeric structure was generated by embossing a polypropylene film into a resilient roll covered with a layer of a closed cell polyurethane foam. The microstructured polymeric substrates ofthe present invention may be used to form a slip control material. For example, a unitary polymeric sheet having microaberrations on one face may be afixed to traffic bearing surfaces, such as stairs, floors and ramps. Polymeric materials formed by the present method may also used to form the grip on tools or athletic equipment thereby providing enhanced slip control. In yet another embodiment ofthe invention, the microstructured polymeric substrates ofthe present invention may be used to form a cleaning article. In particular, flexible polymeric sheets which have been embossed to form a unitary polymeric stmcture having a plurality of microtextured projecting elements are useful in cleaning applications. Such a flexible sheet of polymeric material may be textured using the present method. Flexible sheets of this type may be utilized as a cleaning rag or may be afixed to a foam backing to form a cleaning pad. The invention is further characterized by the following examples. These examples are not meant to limit the scope ofthe invention as described in the foregoing description. Variations within the concepts ofthe invention will be apparent to those skilled in the art. Process Parameters Embossed polypropylene films were prepared by extmding polypropylene resin (DS7C50, available from Shell Chemical Co., Houston, TX) from a single screw extruder (Model DS15H, available from Davis Standard, Stamford, CT) equipped with a 3.8 cm diameter cylinder, into the nip of a two roll embossing apparatus. The extruder, which was operated at 254°C, delivered a 22.9 cm wide sheet of molten polypropylene through a 30.5 cm die having a 0.25 mm die gap at a rate of 55 g/m 2 , vertically downward into the nip ofthe embossing apparatus which was positioned about 7.6 cm below the die. The embossing apparatus utilized two 24.5 cm diameter by 30.5 cm long steel rolls having independent temperature controls. One ofthe rolls (the embossing roll) was heated to 49°C and carried an embossing pattern while the other roll (the chill roll) was cooled to 7°C and was covered with a resilient material. The polypropylene was embossed at a nip pressure of 138 kPa and line speed of 1.5 m/min. Embossing Apparatus/Process II An embossed polyethylene film was prepared by extmding polyethylene (DOWEX™ 2047 A, available firom Dow Chemical Co., Midland, MI) from a single screw extruder (Model 1.75 TMC30; HPM Corporation, Mount Gilead, OH) equipped with a 4.4 cm diameter cylinder into the nip of a two roll embossing apparatus. The extmder, which was operated at 249°C and a screw rotational speed of 20 rpm, delivered a sheet of molten polyethylene through a 30.5 cm die having a 0.25 mm die gap horizontally into the nip of a two roll embossing apparatus which was positioned such that the width ofthe polyethylene film entering the nip was approximately 15 cm. The embossing apparatus utilized a 25.4 cm diameter by 35.6 cm long steel roll (the embossing roll) which carried an embossing pattem and a similarly sized 75 durometer mbber roll which served as the chill roll. The steel roll was heated to 15.6°C, while the mbber roll, which was covered with a resilient material, was cooled to 0°C. The polyethylene was embossed at a nip pressure of 55 kPa and line speed of 3 m/min. Embossing Roll Surfaces The embossing roll surfaces were generated by conventional photoengraving techniques on magnesium or a magnesium alloy. Specific embossing pattems used in the following examples included: EB-1 A hexagonal close packed array of circular posts (1.0 mm in diameter, 0.5 mm in height) which were spaced 1.9 mm on center. EB-2 A square lattice array of square posts (1.0 mm on a side, 1.0 mm in height) which were spaced 2.5 mm on center. EB-3 A square lattice array of circular posts (1.0 mm in diameter, 1.0 mm in height) which were spaced 1.9 mm on center. EB-4 A herringbone pattem consisting of discontinuous ridges 1.5 mm in height and having top rectangular surfaces 1.0 mm in length and 0.13 mm in width. The rectangular base dimensions ofthe discontinuous ridges were larger than the top surface dimensions as the sidewall portions were inclined at an approximately 25-30° angle to a vector peφendicular to the face ofthe embossing roll. The center-to- center spacing ofthe ridges was 4.4 mm in the down web direction and 3.5 mm in the cross web direction. The ridges were positioned at a 30° angle to the left or right ofthe down web direction, i.e., the rows of discontinuous ridges altemated between 30° left and right of the down web direction. Chill Roll Covers Chill rolls were covered with one or more layers of foam, a resilient pattemed silicone mbber sheet or a laminate constmction of a foam and a resilient pattemed silicone mbber sheet. Specific foam materials that were used in the following examples included: RC-1 A 3.2 mm thick layer of polyurethane foam prepared as generally described in U.S. Pat. Nos. 3,772,224 (Marlin et. al.) and 3,849,156 (Marlin et. al). The foam was prepared from a four part mix (A-D), the composition of which were: Part A - 100 parts of a polyol mixture of consisting of Niax 24-32 (97.77 parts) and Niax E-434 (2.23 parts), polyether polyols (available from Arco Chemical Co., Newton Square, PA) dipropylene glycol (9.18 parts per hundred parts polyol (php); fragrance grade), Niax LC-5615 (3.74 php, a nickel catalyst composition available from OSI Specialities, Lisle IL), aluminum trihydrate filler (54.59 php, Aloca C-331, available from Aluminum Company of America, Bauxite, AR), and Hostaflam AP 442 flame retardant (16.38 php, available from Hoechst Celanese Corp., Charlotte, NC); Part B - 37.39 php of an isocyanate mixture consisting of 4,4'- diphenylmethane diisocyanate and a modified 4,4'-diphenylmethane diisocyanate (Rubinate 1920 available from ICI, Rubicon Chemicals, Geismer, LA); Part C - 4.77 php of a 70.9% (w/w) solution of a silicone surfactant (L-5614, available from OSI Specialities) in a polyether glycol (Niax E-351, available from Arco Chemical Co.); and Part D - 6.71 php of an approximately 8% solids (w/w) dispersion of carbon black (Product No. 1607029, available from Spectmm Colors, Minneapolis, MN) in polyether glycol (Niax E-351). Separate feed streams ofthe four parts were pumped into a 90 mm dual head Oakes Frother (available from ET Oakes Corp., Hauppauge, NY) through an entrance manifold attached to the frother. The mixture was frothed by injecting high purity nitrogen through a capillary tube located at the entrance to the frother. The frothed mixture was processed through the frother at a mixing speed of 800 φm and a discharge pressure of about 0.55 MPa and dispensed from an approximately 2.6 m x 1.3 cm hose onto a polyester film and spread over the film using a knife coater (2.4 mm gap). The foam was cured by passage through a 3 chambered 13.7 m forced air oven at a line speed of 1.5-1.8 m/minute. The first chamber was maintained at 135°C. The second and third chambers were maintained at 154°C. RC-2 A 6.3 mm thick layer closed cell silicone foam (Bisco BF-1000, available from Bisco Products, Elk Grove, IL). Pattemed silicone mbber sheets used alone or in silicone mbber sheet/foam laminate chill roll cover constmctions were prepared by applying an approximately 0.8 mm thick coating of uncured silicone mbber (Silastic™ brand J-RTV silicone mbber, available from Dow Coming Corporation, Midland, MI) over a stmctured surface, curing the silicone mbber at 66°C for one hour, and removing the cured pattemed silicone mbber sheet to provide a resilient replica ofthe stmctured surface. Specific silicone mbber sheet pattems and silicone mbber sheet/foam laminate constmctions used in the following examples included: RC-3 A silicone mbber sheet having a pattem of parallel raised ribs 0.18 mm in height and spaced 0.36 mm apart, the sides of which intersect at a 90° angle, which was replicated from the surface of Optical Lighting Film (3M, St. Paul, MN). This silicone mbber sheet was laminated to a layer ofthe closed cell polyurethane foam RC-1, described above, to form a chill roll cover material. RC-4 A silicone mbber sheet having a pattem of cube co er recesses 0.18 mm in depth with a base triangle having angles of 55°, 55°, and 70°, replicated from the surface of a cube comer reflective polycarbonate film as described in U.S. Pat. No. 4,588,258 (Hoopman). RC-5 A silicone mbber sheet having a patte of cube comer protmsions 0.09 mm in height arrayed in a patte similar to that of RC-4. RC-6 A silicone mbber sheet having a random pattern of spherical projections (ranging from approximately 10-25% of a total sphere). The silicone mbber sheet was prepared by (1) randomly capturing glass beads, about 55 mm in diameter, in a 5 mm (dry thickness) adhesive (F-10, an acrylic base adhesive containing a solvent vehicle, available from Rohm & Haas, Philadelphia, PA) coated on polyethylene coated Kraft paper; (2) coating silicone mbber (Silastic™ brand J-RTV silicone mbber) over the glass beads using a knife coater with a coating gap of 0.5 mm; (3) placing the uncured coating in a partial vacuum to remove entrapped air; (5) curing the degassed coating at 70°C for two hours, and (6) removing the cured silicone mbber from the adhesive coated Kraft paper. RC-7 A silicone mbber sheet having a random pattem of spherical depressions (ranging from approximately 75-90% of a total sphere) prepared by removing the glass beads from roll cover RC-6. The glass beads were removed by applying Scotch™ 810 Magic™ Mending Tape (available from 3M, St. Paul, MN) to the glass bead containing surface and flexing the silicone film as the adhesive tape was removed. Example 1 An embossed polypropylene film was prepared using Embossing Apparatus/Process 1, incoφorating an embossing roll having a hexagonal array of circular posts (EB-1) and a closed cell polyurethane foam chill roll cover (RC-1). Electron photomicrographs ofthe face ofthe embossed film that had been in contact with the chill roll cover (see Figs. 6 and 7) show projecting elements consisting of a hexagonal close packed array of hollow circular posts covered with microaberations consisting of a plurality of microprotrusions corresponding to inverted replicas of surface pores in the foam chill roll cover, many of which had an undercut stmcture. The microprotmsions, which covered the top and side surfaces ofthe posts, were not present on the land area or surface area between the projecting elements. Example 2 An embossed polypropylene film was prepared according to the procedure of Example 1 except that the embossing roll had a square lattice array of square posts (EB-2) and the chill roll cover was a silicone mbber sheet having a ribbed patte which was laminated to a layer of polyurethane foam (RC-3). An electron photomicrograph (see Fig. 8) ofthe face ofthe embossed polypropylene film, which had been in contact with the chill roll cover, shows projecting elements consisting of a square lattice array of hollow square posts. The surfaces ofthe square posts were covered with an array of protmding parallel ribs. The ribs were observed on the top and side surfaces ofthe posts as well as the land area between the posts. This experiment demonstrates that in generating microaberations in a polymer film which are inverted replicas of microaberations on the surface of a resilient chill roll, the shape ofthe microaberations need not be limited to pores in the surface of a foam. Example 3 An embossed polypropylene film was prepared according to the procedure of Example 1 except that the embossing roll had a square lattice array of circular posts (EB-3) and the chill roll cover was a silicone mbber sheet having a cube comer recess pattem (RC-4). An electron photomicrograph (see Fig. 9) ofthe face ofthe embossed polypropylene film that had been in contact with the chill roll cover shows projecting elements consisting of a square lattice array of hollow circular posts. The top and side surfaces ofthe posts were covered with a pattern of protmding cube comers. The land area between the posts was smooth. Example 4 An embossed polypropylene film was prepared according to the procedure of Example 2 except that the chill roll cover was a silicone mbber sheet having cube comer projections (RC-5). An electron photomicrograph (see Fig. 10) ofthe face ofthe embossed polypropylene film which had been in contact with the chill roll cover showed projecting elements consisting of a square lattice array of hollow square posts. The top and side surfaces ofthe posts were covered by an array of recessed cube comers ("microdepressions"). The land area between the posts was also covered the same array of cube comer microdepressions. Example 5 A 5 cm square piece of flat tooling with a square lattice array of square posts (1.0 mm on a side, 1.0 mm in height) spaced 2.0 mm on center was heated to 193 ° C on a hot plate. A 0.26 mm thick piece of polypropylene (DS7C50) film was placed on the pattemed side ofthe tooling and heated until the polypropylene film softened (approximately 1 minute). Once softened, the polypropylene film was covered with a silicone mbber sheet embedded with glass beads (RC-6) such that the bead containing surface contacted the softened polypropylene. Two layers of the closed cell polyurethane foam chill roll cover (RC-1) were placed on top ofthe silicone mbber sheet, the assembly removed from the hot plate, and pressure applied to the assembly by means of a hand held platen until the polypropylene solidified. An electron photomicrograph ofthe face ofthe embossed polypropylene film that had been in contact with the silicone mbber sheet showed a square lattice array of projecting hollow square posts (see Fig. 11) covered with microaberations. The microaberations consisted of a random array of spherical microdepressions (ranging from approximately 10-25% of a total sphere). The microdepression pattem was observed on the top and side surfaces ofthe posts as well as the land area between the posts. Example 6 An embossed polypropylene film was prepared according to the procedure of Example 5 except that the silicone mbber sheet had a random pattem of spherical microdepressions (RC-7). A number ofthe spherical microdepressions had an undercut shape. An electron photomicrograph ofthe face ofthe embossed polypropylene film that had been in contact with the silicone mbber sheet showed a unitary stmcture which included a square lattice array of projecting hollow square posts (see Fig. 12) covered with a pattern of microaberations. The microaberations consisted of a random array of spherically shaped polypropylene microprotmsions. The microprotmsion patte was observed on the top and side surfaces ofthe posts as well as the land area between the posts. Since the polypropylene microprotmsions were inverted replicas ofthe microdepressions in the silicone mbber sheet, a number ofthe microprotmsions had a spherical undercut shape. Example 7 An embossed polyethylene film was prepared using Embossing Apparatus/Process II, incorporating an embossing roll having a square lattice array of square posts (EB-2) and a closed cell silicone foam chill roll cover (RC-2). An electron photomicrograph (see Fig. 13) ofthe face ofthe embossed polyethylene that had been in contact with the chill roll cover shows a primary stmcture consisting of a square lattice array of projecting hollow square posts covered with microaberations. The microaberations consisted of a random array of microprotmsions corresponding to inverted replicas ofthe surface pores in the foam chill roll cover. The microprotmsions were observed on the side surfaces as well as on the tops ofthe posts. Example 8 An embossed film was prepared using a procedure similar to the procedure of Example 7. An ethylene/propylene copolymer (KS-057, available from Himont USA, Inc., Wilmington, DE) was substituted for the polyethylene resin. The embossing roll had a herringbone pattem of discontinuous ridges (EB-4). The chill roll was covered with a layer of a closed cell polyurethane foam (RC-1). The ethylene/polypropylene copolymer was processed at a nip pressure of 6.9 kPa and line speed of 1.5 m/min. The temperatures ofthe inlet fluids to the embossing roll and chill roll were 15.6°C and 0°C, respectively. An electron photomicrograph (see Fig. 14) ofthe face ofthe embossed film that had been in contact with the chill roll cover showed of a herringbone patte of discontinuous ridges covered with a plurality of microprotmsions. The microprotmsions corresponded to inverted replicas ofthe surface pores in the foam chill roll cover. Example 9 An embossed polypropylene film was prepared using Embossing Apparatus Process I except that a smooth steel roll, which was maintained at 54°C, was substituted for the embossing roll. The extmder was operated at 266°C, and the polypropylene was delivered at a rate of 159 g/m 2 . The chill roll was covered with a closed cell urethane foam (RC-1). An electron photomicrograph of a cross- section ofthe resulting unitary microstmctured film (Fig. 15) showed a plurality of microprotmsions on the face ofthe embossed film which had been in contact with the chill roll. The microprotmsions corresponded to inverted replicas ofthe surface pores ofthe foam chill roll cover. Many ofthe microprotmsions showed an undercut-shaped stmcture. Examples 10-13 A series of embossed films were prepared using Embossing Apparatus/Process I except that a variety of thermoplastic polymers were substituted for the polypropylene and embossing conditions were correspondingly changed to allow processing ofthe polymers. The polymers, processing conditions, and the roll covers used to prepared the embossed films are summarized in Table 1. All films were prepared using an embossing roll having a square lattice array of square posts (EB-2). Table 1 Thermoplastic Polymer/Operating Conditions Expl Polymer Chill Extmder Chill Embossing Line Roll Operating Roll Roll Speed Cover Temp(°C) Temp(°C) Temp(°C) (m/min) 10 Surlyn™ RC-1 225 7 54 1.5 8527 11 Nylon RC-4 254 18 38 1.5 6,9 ] 12 Hostalloy™ RC-1 305 18 38 0.6 731 13 PVC 2 RC-4 170 16 38 0.6 1. Vydyne , available from Monsanto, St. Louis, MO. 2. Apex Telcar ™ , available from Teknor Apex Co., Pawtucket, RI. In Examples 10 and 12, the face ofthe film in contact with the chill roll bore microaberations consisting of a plurality of microprotmsions corresponding to inverted replicas ofthe surface pores ofthe foam chill roll cover. In Examples 11 and 13, the face ofthe film in contact with the chill roll bore microaberations consisting of a pattern of protmding cube corners. Example 14 A microstructured polymeric substrate was prepared by the following procedure. One end of a piece of 3MO Optical Lighting Film #2300 Acrylic (available from 3M, St. Paul, MN), approximately 90 cm X 10 cm, was secured, grooved face down, to a bench top with masking tape. The 3MO Optical Lighting Film has a plurality of substantially 90° included angle ridges. The depressions in the surface ofthe film are about 0.18 mm in depth. The unsecured end ofthe film was grasped in a manner such that a tear could be initiated in a grove approximately 2 cm from the edge ofthe film and the film torn along its length. The film was torn at an angle of between approximately 15-30° from the horizontal to produce a clean, straight edge having a substantially constant acute angle along one piece ofthe film and a complementary obtuse angle on the edge ofthe adjacent piece of film. Strips produced in this manner were cut into approximately 6.5 cm lengths (herein after referred to as "master elements") and used to assemble a master 112, a portion of the cross-section of which is illustrated in Fig. 16, in the following manner: The grooved faces 105, 106 of two master elements were nested together so that the acute tear angle edge 100 of one element (B in Fig. 16) extended beyond the obtuse tear angle edge 101 ofthe second element (A in Fig. 16) by four ridges. The smooth face 102 of a third master element (C in Fig. 16) was then placed against the smooth face 103 ofthe second master element (B) with the acute tear angle edges 100, 104 aligned. The grooved face 108 of a fourth master element (D in Fig. 16) was then nested with the grooved face 107 ofthe third master element (C) such that the obtuse angle tear edge 109 was indexed four groves below the acute tear angle edge 104 ofthe third element (C) and aligned with the obtuse tear angle edge 101 ofthe first master element (A). The stacking pattem (ABCDABCD ) was repeated until a master, approximately 1.2 cm X 6.5 cm was assembled. The total assembly was clamped together using a number of binder clips (5/8 inch capacity binder clip No. 100500 distributed by IDL Corp., Carlstadt, NJ). A master mold ofthe master 112 was prepared according to the following steps. a) Forcing a vinyl siloxane dental impression material (3M Expresso, available from 3M, St. Paul, MN) into the contoured edge ofthe master with the manufacture's supplied application device, being careful to avoid entrainment of air bubbles between the impression material and the master; b) Containing a pool ofthe dental impression material on a glass plate between two aluminum spacer bars (approximately 0.32 cm X 1.25 cm X 2 cm) positioned approximately 2.5 cm apart; c) Centering the master over the spacer bars such that each end ofthe master overlapped each spacer bar approximately 0.5 cm and forcing the impression filled face ofthe master into the pool of impression material until the impression filled edge ofthe master contacted the spacer bars; d) Positioning two additional aluminum bars (approximately 0.6 cm X 0.6 cm X 10 cm) in the impression material pool such that these bars contacted the ends ofthe spacer bars, thereby forming a containment well for the impression material; e) Allowing the impression material to cure; f) Removing the master from the cured impression material and trimming the ends ofthe cured mold to provide an approximately 2.5 cm long mold ofthe master; and g) Securing end dams to the trimmed mold to provide a well for subsequent molding of polymeric materials in the mold. Microstmctured polymeric substrates were prepared by filing the master mold with a vinyl plastisol (#D1902-50 Black, available from Plast-O-Meric, Inc., Waukesha, WI) and curing the plastisol in a circulating air oven at 204°C (400°F) for 15 minutes. The cured plastisol was demolded to produce a flexible, compliant polymeric substrate (50 durometer) having a plurality of grooved ridges. The grooves ("microaberations") in the ridges were replicas ofthe grooves 111 in the sidewalls ofthe grooved ridges 110 of master 112 (and inverted replicas ofthe of the continuous protmsions on the sides ofthe depressions in the master mold). Example 15 Microstmctured polymeric substrates having a plurality of grooved ridges were prepared from the master mold of Example 14, substantially following the procedure of Example 14, except that a firmer vinyl plastisol (#D 1902-78 White, available from Plast-O-Meric, Inc.) was used to form the polymeric substrates (78 durometer). All publications and patent applications in this specification are indicative of the level of ordinary skill in the art to which this invention pertains. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope ofthe invention.

Claims: WHAT IS CLAIMED IS: 1) A method of producing a polymeric substrate comprising: contacting a polymeric material in a flowable state with a resilient surface and an embossing surface to produce a microstmctured polymeric substrate; the resilient surface comprising a plurality of first microaberrations and the embossing surface comprising a plurality of embossing elements projecting therefrom; wherein the microstmctured polymeric substrate comprises a plurality of hollow projecting elements comprising an outer surface, the outer surface comprising a plurality of second microaberrations which are inverted replicas ofthe first microaberrations. 2) The method of claim 1 wherein the polymeric material comprises a thermoplastic polymer which comprises a polyolefin, a polyamide, polyvinyl chloride, a polyester or an olefin copolymer. 3) The method of claim 1 wherein the resilient surface comprises a polymeric foam or a nonporous flexible polymeric material. 4) The method of claim 3 wherein the resilient surface comprises a nonporous flexible polymeric material comprising a plurality of rigid microparticles embedded therein. 5) A method of producing a polymeric substrate comprising: contacting a polymeric material in a flowable state with a resilient surface to produce a microstmctured polymeric substrate; the resilient surface comprising a plurality of undercut-shaped microdepressions; wherein the microstmctured polymeric substrate comprises a plurality of undercut- shaped microprotmsions which are inverted replicas ofthe microdepressions. 6) A polymeric substrate formed by a process comprising: contacting a polymeric material in a flowable state with a resilient surface, the resilient surface comprising a plurality of first microaberrations; wherein a microstmctured polymeric substrate is formed, the microstmctured polymeric substrate comprising a plurality of second microaberrations which are inverted replicas ofthe first microaberrations. 7) A polymeric substrate of unitary constmction comprising a plurality of undercut-shaped solid microprotmsions. 8) The polymeric substrate of claim 7 further comprising a plurality of projecting elements comprising an outer surface, the outer surface comprising a plurality ofthe microprotmsions. 9) A polymeric substrate comprising a plurality of projecting elements comprising an outer surface which comprises sidewalls, said sidewalls comprising a plurality ofsolid microprotmsions, said microprotmsions having a width of about lOμ to about 400μm and a height of about lOμm to about 400μm. 10) The polymeric substrate of claim 9 wherein the projecting elements comprise a sidewall having a draft of no more than about 30°.

Inventors: AAMODT JENNIFER M, BERG JAMES G, CALHOUN CLYDE D, KRINKE HARLAN L, AAMODT, JENNIFER, M., BERG, JAMES, G., CALHOUN, CLYDE, D., KRINKE, HARLAN, L.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: B29C59
IPC codes: B29C59