Coated Abrasive Sheet Material With Improved Backing

Technical Field

The present invention relates to an improved backing for coated abrasive products.

Background Art

Coated abrasive products are characterized among the general field of abrasive products as having a backing sheet with a layer of abrasive material made up of abrasive granules and binder adhered to at least one side. Backings for coated abrasives have been made from a variety of materials, depending upon the product requirements. Products which generally are not used under high stress situations and may require easy tearability to make smaller size abrasive sheets are generally made from weaker backing materials such as paper and nonwoven fabric. Where stronger backings are needed, stronger backing materials such as woven cloth, leather, plastic film and metal sheets have been employed. Strong, stretch-resistant backings are typically made of vulcanized fiber, woven cloth and laminates containing these materials.

An area of considerable importance which requires strong, stretch-resistant coated abrasives is the production of abrasive belts which are continuous loops of coated abrasive material typically made by joining the ends of an elongate strip of the appropriate size. The use of woven cloth to impart strength and stretch-resistance to coated abrasive backings useful for belts has its disadvantages, however. Such woven backings are expensive because the weaving operation requires the use of weaving apparatus and considerable man hours. Additionally, woven fabrics are not completely stretch resistant due to the typical sine wave deployment of the threads or yarns by virtue of the weaving operation which, when stretched, tend to reduce the

amplitude of the sine wave and permit a limited amount of extension. Additionally, the crossing points where the warp fibers go over the weft fibers result in a multitude of protuberances in the surface of the fabric which often are apparent in the surface of the workpiece being finished with the abrasive belt, typically being manifested by minute but discernible elongate scratches below the desired abraded surface.

Disclosure of Invention The present invention provides a strong, stretch- resistant improved coated abrasive sheet material which is ^* relatively easy to produce yet has the requisite strength to make it useful as a coated abrasive belt.

The improved coated abrasive sheet material of the present invention includes a flexible backing having adhesively bonded to at least one major surface thereof an abrasive layer comprising binder and abrasive grains, the backing comprising:

(a) a flexible sheet; and (b) a multiplicity of weft-free, closely spaced, stretch-resistant, longitudinally aligned, coplanar, continuous-filament reinforcing yarns, bonded to one surface of the flexible sheet.

The term "weft-free" means without woof or filling thread or yarns which are conventional in woven fabrics. The term "weft-free" is intended to denote that the yarns are unattached to one another as in a fabric but instead are separate individual yarns although adjacent yarns may be, at least partially, in contact with one another. The yarns may be adhered to either side of the flexible sheet. That is, the abrasive layer may be adhesively bonded to the backing on its major surface which includes the reinforcing yarns or on the opposite major surface. The preferred coated abrasive sheet material includes an abrasive layer bonded to the major surface of the flexible sheet opposite the surface which includes the reinforcing

yarns and further includes a nonwoven web bonded to the surface of the flexible sheet over the reinforcing yarns. This preferred embodiment avoids the surface protuberances typically attendant with coated abrasives made employing backings of woven fabric material.

Another preferred embodiment is provided by replacing the nonwoven web with a plastic film which is bonded to the surface of the layer of thermoplastic adhesive material over the yarns. The preferred flexible sheet comprises a coextruded film having a first layer of a first polymeric thermoplastic material, preferably polyethylene terephthalate, and a second layer of a second polymeric thermoplastic material, preferably a copolyester of ethylene terephthalate and ethylene isophthalate, most preferably in an 80 to 20 ratio, having a thermoplastic softening temperature lower than that of the .first polymeric thermoplastic material.

The yarns preferably comprise continuous filaments made of a filament-forming material selected from the group consisting of glass and high modulus organic thermoplastic materials such as polyethylene terephthalate, polyvinyl alcohol, rayon, nylon and various other high modulus thermo¬ plastic materials.

Brief Description of Drawings The invention is further illustrated by reference to the accompanying drawings, wherein like reference numerals refer to like parts, and:

FIG. 1 is a greatly enlarged cross-sectional repre¬ sentation of a flexible sheet material which is suitable for use in the backing of the coated abrasive sheet material of the present invention;

FIG. 2 is a greatly enlarged cross-sectional view of a coated abrasive sheet material according to the present invention and the continuous filament reinforcing yarns on the opposite major surface of the coated abrasive sheet material as the abrasive coating;

FIG. 3 is a greatly enlarged cross-sectional view of another embodiment of the coated abrasive sheet material of the present invention wherein the continuous filament reinforcing yarns are on the same major surface of the flexible sheet material as the abrasive coating;

FIG. 4 is a greatly enlarged cross-sectional view of yet another embodiment of a coated abrasive sheet material according to the present invention like that depicted in FIG. 2 but includes a nonwoven web over the surface of the backing opposite the abrasive surface; and

FIG. 5 is a schematic view of a method of producing the coated abrasive sheet material of the present invention.

Detailed Description

Referring to the drawings, there are depicted in FIGS. 2-4 various embodiments of the coated abrasive sheet material of the present invention. Referring to FIG. 2, there is shown a coated abrasive sheet material 20 having a backing 21 comprising a flexible sheet 22 having a surface layer 23 of thermoplastic adhesive material at least on one major surface thereof and including a multiplicity of weft-free, closely spaced, stretch-resistant, longitudinally aligned, coplanar, continuous-filament reinforcing yarns 24 and an abrasive layer 25 comprising abrasive granules 28 adherently bonded therein by adhesive, preferably including a conventional maker adhesive 26 and sand size adhesive 27. The backing may also be primed with a layer 29 of primer to promote improved adhesion.

FIG. 3 shows an alternative embodiment of the coated abrasive sheet material of the present invention identified by reference numeral 30 which includes abrasive layer 25 and a backing 31 which comprises a flexible sheet 32 having a surface layer 33 of thermoplastic adhesive material on at least one major surface thereof and filament yarns 24 partially embedded in and bonded to layer 33 of thermo- plastic material between the abrasive layer 25 and the flexible sheet 32.

FIG. 4 shows a preferred coated abrasive sheet material 40 in accordance with the present invention which is similar to that depicted in FIG. 2 but including a layer 41 of nonwoven fabric which is bonded to the surface of layer 23 of thermoplastic adhesive material over yarns 24.

FIG. 1 shows a flexible sheet material 10 including a thermoplastic layer 23 on one major surface of flexible sheet 22 which may be used in the improved coated abrasive sheet materials of claims 2-4. Layers 22 and 23 are preferably formed by coextrusion.

FIG. 5 shows a preferred method of making the coated abrasive sheet material depicted in FIG. 4 in accordance with the present invention. Flexible sheet material 10 of the type depicted in FIG. 1 is unwound from roll 50 (with thermoplastic layer 23 innermost on roll 50) over idler roll 52 and directed between heated nip rolls 53 and 54 where it merges at the nip to form a top layer with a nonwoven fabric 61 (which forms a bottom layer) supplied from roll 60 over idler roll 62 and reinforcing yarns 71 (disposed between the top and bottom layers) supplied from yarn spools 70, the yarns being aligned with comb 72, to produce composite sheet 55. Sheet 55 is then passed beneath coating station 56 where it is preferably extrusion coated with primer to provide primed composite sheet 57 which is passed through primer curing station 58 over idler roll 59 to invert the sheet. The primed composite sheet is then passed through roll coater 63 to apply maker adhesive, over electrostatic abrasive granule coater station 64 which supplies a uniform coating of abrasive granules 65 and thereafter passed through festoon drying arrangement 66. The granule-coated dried sheet then travels through sand size adhesive coating station 67, through festoon drying arrangement 68, to provide finished product 40 which can be rolled for storage as roll 69 or cut to size for use. The reinforcing yarn suitable for use in the coated abrasive sheet materials of the present invention may be any conventional stretch-resistant substantially continuous

filament yarn which possesses adequate tensile strength to provide an acceptable degree of tensile strength in the backing material. Since backing strength requirements may vary, there will be some variation in the yarn strength requirements. Generally stronger yarns will be required for applications which require high tensile strength backings. The reinforcing yarns tend to flatten somewhat in the backing construction described without loss of necessary strength to provide stretch-resistance in the resultant product. Such flattening is desirable to maximize the surface smoothness of the backing.

Typical useful yarns will consist of bundles of continuous filaments having on the order of 50 to 200 filaments per yarn strand. Continuous filament yarn has most of its individual filaments (i.e., in excess of about 90%) extending substantially throughout the length of the yarn strand. Materials which form stretch-resistant high modulus strong yarns are generally preferred. Suitable yarn filaments having the requisite stretch resistance and strength may be formed of organic polymeric materials such as polyester (e.g., polyethylene terephthalate), polyamide (e.g., nylon such as nylon 6, and aromatic polyamides), rayons, polyvinyl alcohol and the like. Various inorganic materials may also provide useful yarns. A preferred yarn is formed of glass fibers. Such a yarn is available from the Owens Corning-Co. under the trademark "Fiberglas". Glass fiber yarns may include a chemical treatment called sizing which is applied to the glass filaments to coat and lubricate them against abrasive contact to reduce filament breakage during processing. The filaments of the yarns may include twisting or plying to achieve a more uniform product which processes more smoothly than an untwisted yarn. Yarn fibers are twisted to a pre-established number of turns per unit of length and the direction of twist is typically identified as "S" or "Z" designation. Preferred glass yarns weigh on the order of 2 to 9 kg/km.

The presently preferred yarns are made from high modulus polyester such as polyethylene terephthalate,

preferably of 100 to 2,000 denier. Such yarns are commer¬ cially available.

The flexible sheet material may be any flexible material such as a plastic film, non-woven web (e.g., paper) or the like, preferably having a smooth surface and sufficient strength and integrity to withstand the process¬ ing conditions. The preferred flexible sheet material is a polymeric film. Examples of such preferred polymeric films include films made of polyester such as oriented (preferably biaxially oriented, heat-set) polyethylene terephthalate, nylon and oriented polyolefin such as polyethylene, poly¬ propylene and polybutylene. Other film-forming polymers may also be employed.

The preferred flexible sheet is made of oriented polyethylene terephthalate films such as available under the trade designation "Mylar" or a polyethylene terephthalate film available under the trade designation "Scotch Par". Such films are typically extruded and then drawn or stretched to produce a crystalline type structure resulting from the orientation of the polymer molecules. The drawn film is firm and has a higher and narrower softening temperature range than the intermediate amorphous film. Other useful films include polyester films having polyester linkages in the polymeric backbone chain, partic- ularly those of polybasic aromatic acids and polybasic ali¬ phatic alcohols, including cycloaliphatic alcohols. One such equivalent polyester film is formed of a polymer of tere- phthalic acid and 1,4-bis(hydroxymethylene)cyclohexane. Oriented linear polyamide films are also quite satisfactory. The thickness of the film will generally be in the range of 0.025 to 0.175 mm depending upon the strength of the film and processing conditions. Polyethylene tere¬ phthalate films having a thickness of 0.025 to 0.25 mm are generally sufficiently strong. The flexible sheet preferably includes a layer of thermoplastic polymeric material which softens with heating at a temperature lower than that of the flexible sheet

covering at least one major surface thereof. Useful thermo¬ plastic polymeric materials include olefin-acrylate copolymers, e.g., ethylene acrylic acid copolymer. A preferred thermoplastic material is a_ copolyester comprised of 80 parts polyethylene terephthalate and 20 parts of polyethylene isophthalate. The thermoplastic layer will typically be on the order of 0.001 to 0.5 mm in thickness. A preferred flexible sheet is obtained by coextruding polyethylene terephthalate with the above-described copoly- ester to produce a composite coextruded film having an integral thermoplastic surface layer of the copolyester on a polyethylene terephthalate base layer. Such a flexible sheet is typically oriented and heat set to obtain the desired strength. The reinforcing yarns and flexible sheet preferably are selected to have similar shrinkage and expansion rates in response to temperature or humidity changes, or in response to use or for other reasons, to avoid wrinkling or other distortion caused by differential shrinkage or expansion. A nonwoven web is preferably included in the coated abrasive sheet material of the present invention to protect the yarns in use and to provide a frictional surface on the side opposite the abrasive side, if needed, when the coated abrasive sheet material is used as an abrasive belt to provide a driving surface which has minimal slippage. Any nonwoven web which provides a suitable coefficient of friction to the construction and adheres well to the yarns may be employed. Typical nonwoven webs are formed of short staple fibers of polymeric materials, most typically up to about 5 cm in length, preferably 0.5 to 10 cm in length.

Useful fibers are relatively fine, up to 50 denier, prefer¬ ably about 1 to 20 denier. Preferred fibers have good thermal stability, i.e., they are stable up to about 200°C to permit fusion bonding in the construction. While nonwoven webs of short fibers are preferred, other web constructions may also be employed such as webs of spun bonded nonwoven fibers and the like.

Suitable polymers for forming the fibers of the nonwoven webs include polyester, e.g., polyethylene tere¬ phthalate, polyalkylene (e.g., polyethylene, polypropylene and the like), polyamide (e.g., nylon), rayon, cotton and other similar fibers. Copolymers and mixtures of fibers may also be used.

The presently preferred nonwoven web is formed of staple fibers on the order of 4 denier and 3 cm in length that have a polyethylene terephthalate core with a lower melting copolymer sheath.

The thermoplastic layer may be applied by a separate coating technique employing liquid coating methods or by application of particulate thermoplastic binder material which softens when heated and heat calendered. The particulate thermoplastic binder material may be applied to the flexible sheet or directly to the yarns and under sufficient heat and pressure softens to wet the yarn surfaces and part or all of the adjacent surface of the flexible sheet, forming an adherent bond between the yarns and the surface of the sheet on cooling. Liquid binder materials which are useful include latexes such as those of acrylics or styrene-butadiene resins.

The backing of the present invention is preferably made by feeding a preformed nonwoven web between the nip of heated calender rolls together with the reinforcing yarns and the flexible film. In such a method it is preferred to use a nonwoven web formed of fibers which soften when heated in the nip of the calendering rolls to become sufficiently tacky in the softened state to act as the adhesive for the construction.

The preferred nonwoven webs may be formed for example on a web-forming device such as a "Rando-Webber" machine using staple fibers. Webs of such nonwoven webs weighing 0.06 to 0.6 kg per square meter have been found to be useful.

In the preferred method, the calendering rolls are heated to a temperature which softens the heat-softenable

fibers of the nonwoven web, typically on the order of 100 to 300°C, preferably about 130 to 160°C. Pressure is applied between the calendering rolls sufficient to obtain adequate formation of the product, typically on the order of 2 to 50, (preferably 2 to 6) kg per cm at a typical throughput speed on the order of 1.5 to 90 meters per minute, preferably 1.5 to 6 meters per minute.

The reinforcing yarns are preferably spaced from one another on the order of 1 to 25 yarns per cm width, depending upon their denier and flattened by contact with the nip roll to increase coverage and decrease surface irregularities. A greater number of smaller denier yarns are typically utilized per unit width than larger denier yarns. Typically, on the order of 2 to 25 yarns per cm width are used for yarns on the order of 100 denier, while 1 to 5 yarns per cm width are typically utilized for yarns of about 2000 denier.

The flexible film employed in the backing of the present invention is typically primed, if necessary, to improve adhesion of the coating resins by conventional methods, particularly where hard non-adherent polymeric materials such as polyethylene terephthalate are used. Various priming techniques are well known in the art. The abrasive layer of the coated abrasive sheet material of the present invention is applied by conventional coating techniques utilized in the manufacture of conventional coated abrasive sheet materials. In such methods any of a variety of abrasive granules are utilized with any variety of conventional binders typically used for coated abrasive products. Abrasive granules which may be employed and are useful include aluminum oxide, silicon carbide, garnet, zirconia, diamond, synthetic abrasive materials such as those formed by fusion or sol-gel tech¬ niques, mixtures of these, mixtures of these with non- abrasive materials, and the like. Useful conventional abrasive binders include phenolic resins, urea-formaldehyde resins, hide glue, and the like.

EXAMPLES The invention is further illustrated by reference to the following examples wherein all parts are by weight, unless otherwise specified. Example 1

A backing for coated abrasive materials was prepared by simultaneously feeding into a nip of two smooth steel calendering rolls heated a 138 C at a nip pressure of 5 kg per lineal cm and a throughput speed of 3 meters per minute, the following:

(a) A nonwoven web of 4 denier, 3.8 cm heat softenable polyester staple fibers (available under the trade designation "Melty" fibers) having a web weight of 30 grams per square meter and being prepared on a "Rando-Webber" web-forming machine.

(b) Five hundred denier polyester reinforcing filament yarns (available under the trade designation "Celanese" type 787) supplied from a beam and introduced into the nip with the yarns spaced at 10 ends per 2.54 cm of width; and (c) Primed polyethylene terephthalate film having a thickness of 0.12 mm and being primed with 0.02 mm of ethylene acrylic acid primer (applied as 100% solids by extrusion and adhesion to the polyethylene terephthalate film primed by treatment with ultraviolet light) with the primed side down and away from the yarns and the yarns between the nonwoven web and the polyethylene terephthalate film.

The backing was coated on its primed side with a conventional abrasive coating composition consisting of abrasive granules and binder to produce a coated abrasive sheet material which was formed into an endless belt and found to have an F-3 value (force at 3% stretch) of 75 lb./inch (13.4 kg per cm) using an "Instron" tensile testing device and a final break tensile strength of 160 lbs. per lineal inch (28.6 kg per cm). The abrasive belt was used successfully to abrade wooden panels and found to provide acceptable cutting performance without significant

stretch or slippage. This test was carried out on an "Oakley" stroke sander using a 15.2 cm by 762 cm belt running at 1380 surface meters per minute. A flat pine board (38 cm by 61 cm) was sanded by applying pressure with a graphite covered backup pad using a hand lever. Tension on the belt was supplied by 27.3 kg of line air pressure.

Example 2 A backing for a coated abrasive sheet material was prepared by employing the nonwoven web and polyester reinforcing yarn described in Example 1 laminated together with the filament yarns employed at 4 ends per cm and the resultant laminate was hot calendered to the copolyester side of a 0.12 mm coextruded film having a 0.12 mm thick layer of polyethylene terephthalate and a 0.02 mm thick layer of 80:20 ethylene terephthalate:ethylene isophthalate copolymer which had been first primed on its polyethylene terephthalate surface with 0.02 mm ethylene acrylic acid primer. The resultant backing was coated on its primed surface with a conventional abrasive composition to produce a coated abrasive product which was tested with successful results as an endless belt on a belt sander.

Example 3 A coated abrasive backing was prepared by laminating the nonwoven web and the reinforcing yarns described in Example 1 to form a laminate which was further laminated to a 0.12 mm polyethylene terephthalate film which had been first primed with 0.02 mm on both sides with ethylene acrylic acid primer. The exposed primed polyethylene terephthalate film surface was then coated with an abrasive coating composition consisting of binder and abrasive granules to produce a coated abrasive sheet product which was cut and formed into an abrasive belt. Testing of the belt was successful.

Example 4 A backing material for coated abrasive sheet products was prepared by the procedure described in Example 1, except substituting for the polyester yarns, single strand,

non-plied glass filament yarns sold by the Owens Corning Company under the trade designation DE No. 100 "Fiberglas" filament yarns, type 1/0 having 0.3 twist per cm. The backing was tested on an "Instron" tensile testing device and found to have a tensile strength of 18 kg per cm at 2% stretch and a tensile strength at break of about 40 kg per cm.

Example 5 A backing according to the present invention was prepared by employing "A" weight paper as the flexible sheet, Celanese style 787 polyethylene terephthalate low shrink, high modulus, 500 denies from a beam of 10 yarns ends per 2.54 cm and a thermoplastic film of ethylene acrylic acid (available from the Dow Chemical Co. under the trade designation "Dow" DAF 899 EAA) . The assembled parts with the paper on top, yarns in the middle and adhesive film on the bottom were passed through the nip of calendering rolls heated at about 128 C under pressure of 5 kg/cm, causing the components to adhere together. The resultant backing was coated by applying a make coat of 70% solids

2 phenolic resin at a rate of 26 to 30 g/m , using a coating bar to apply the coating to the paper side of the composite, then sprinkling grade 150 aluminum oxide abrasive at 80 to

100 g/m uniformly on the coated surface, precuring at 88 C for 1 hour. A size coating of 60% solids phenolic resin was brushed on the abrasive grain coated surface at a rate of 50 to 60 g/m and the coated sheet cured at 110 C for 3 hours. The resultant coated abrasive product was utilized to abrade a wood panel. Example 6

The method described in Example 5 was repeated, except substituting for the "A" weight paper, "A" weight paper saturated with a rubber latex saturant (available from the Munsing Paper Division of the Kimberly-Clark Co. under the trade designation "Kimberly-Clark" C-51886). The backing was prepared in the same manner as described in Example 5 and then coated in the same manner to provide a coated abrasive product.

Example 7 A web made of 3.81 cm, 1.2 denier polyethylene terephthalate fibers and having a weight of 3.4 g/m (available from Celanese Corporation as Type 510) was dusted with powdered polyester adhesive available under the trade designation Eastman "Eastobond" FA-252 from Eastman Kodak Company to provide an add-on of about 0.05 to about 0.5 weight percent. The dusted web was placed onto a 0.089 mm (3.5 mil) polyethylene terephthalate film, upon which had been previously placed at a spacing of 5.9 ends per cm, 500 denier polyethylene terephthalate yarns available from Celanese Corporation as yarn style 787. The layers were pressed together for 12 seconds by using a platen press having a 20 cm by 20 cm platen heated at 149 C, resulting in a well adhered laminate which was useful as a backing for a coated abrasive.

Example 8 Using the procedure of Example 7, except using particulate epoxy resin powder available under the trademark Scotchkote 213 from Minnesota Mining and Manufacturing Company in place of the polyester powder, the web was pressed over the yarns onto the polyethylene terephthalate film to provide a laminate according to the invention which was useful as a backing for coated abrasive. Example 9

Upon a 0.089 mm (3.5 mil) polyethylene terephthalate film were placed six 500 denier polyethylene terephthalate yarns (Style 787, available from Celanese Corporation) which had been previously dusted on both sides with Eastman "Eastobond" FA-252 powder polyester adhesive. The yarns were spaced about 6.35 mm apart and held in place with adhesive— coated tape before being placed on the film surface. This construction was placed between two sheets of paper, each being previously coated with a release agent, and then heated to 149°C (300°F) while pressing with a platen press for 15 seconds, producing a laminate which was useful as a backing for coated abrasive.