WO1997021536A1

WO1997021536A1 - Sheet material incorporating particulate matter

Info

Publication number: WO1997021536A1
Application number: PCT/US1996/018748
Authority: WO
Inventors: John E. Krech
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-12-08
Filing date: 1996-11-22
Publication date: 1997-06-19
Also published as: AU1406697A; ZA9610115B; EP0868292A1

Abstract

Sheet material is formed by mixing polymer and particulate together in an extruder (2) and passing the mixture through a slot die (1); an electrode assembly (8) applies an electrostatic charge to the extruded sheet from the die (1) so that, after the sheet is passed around a casting roller (6), the polymer (which is still soft) will be attracted towards the surface of the roller, and the surface of the sheet nearest to the roller thus becomes smoother than the other surface, from which the particulate continues to project.

Description

SHEET MATERIAL INCORPORATING PARTICULATE MATTER

Field of the Invention

The present invention relates to a method and apparatus for producing sheet material which incoφorates particulate matter. In one aspect, the invention relates to a method and apparatus for producing abrasive sheet material.

Background of the Invention

There are uses in many fields for sheet material with particular properties which result from the presence of particulate matter in and, specifically, on the surface ofthe sheet material. In particular, sheet materials with abrasive particles at the surface are available in the form of rolls or pre-cut sheets of sandpaper or abrasive discs and belts, and can also be used to provide anti-slip surfaces for walkways and similar locations. In addition, sheet materials with magnetizable particles at the surface are used as magnetic recording media and sheet materials with reflective particles at the surface are used for marking purposes and signs. Abrasive sheet materials, for example, are conventionally produced by a comparatively complex coating process which typically involves the steps of applying an adhesive to a substrate; depositing abrasive grit particles on the adhesive (optionally using an electrostatic coater to orient the particles); drying and/or curing the adhesive; and, optionally, applying a size coating to anchor the grit particles more firmly in place. The coating process is comparatively slow because it involves multiple coating steps together with long drying and/or curing times, and careful control ofthe coating conditions is required to ensure the continuous output of an acceptable product. The process may involve the use of organic solvents, the disposal of which should be carefully controlled to reduce the risk of damage to the environment. Moreover, the process does not readily accommodate changes in the various coating materials when it is required to produce a different grade of product. 1

2

As an alternative to that process, it has been proposed to produce abrasive products by embedding abrasive grit in the softened surface of a substrate U.S Patent No 2,712,987 describes a process for making an abrasive belt, in which abrasive particles are distributed over the surface of a nylon substrate softened with a suitable solvent so that the particles become embedded, following which the solvent is dried U.S Patent No. 2,899,288 describes a process for making an abrasive product, in which the upper surface of a thermoplastic backing sheet is softened by heating before abrasive material is spread over the surface and pressed into the sheet between rollers U S Patent No 3,813,231 describes a process in which abrasive grit is distributed over the surface of a copolymer film which is then heated in a platen press to bond the gπt to the film U S Patent No 4,240,807 describes a process in which the substrate is paper coated with a heat-activatable binder which is softened by heating; abrasive grit is then distributed over, and allowed to sink into, the softened binder Polymer-based materials in the form of sheets are also known and can be produced simply and economically by extrusion through a slot die Slot dies can be used to produce comparatively thick cast film and also to coat a film onto a substrate Alternatively, the extrudate from a slot die may be stretched in the longitudinal and transverse directions (tentered) to produce a film which is biaxially oriented

It is also known, when using a slot die, to co-extrude two or more polymeric materials simultaneously in a single die to form a multilayer sheet (see the "Encyclopaedia of Polymer Science and Technology", Volume 7, page 106 et seq., published in 1987 by John Wiley and Sons) Sheets produced in that way are typically used as packaging materials The use of a co-extrusion process to produce a film base which is then coated with magnetizable particles to form a magnetic recording medium is described in U.S Patent No. 4,603,073 In that process, one ofthe extruded layers can include carbon black beads so that the film base will be opaque It is also known to add other materials to polymeric material that is to be extruded, depending on the desired properties ofthe extrudate for example, GB-A- 2 268 748 describes a process which involves the extrusion of a polyurethane material, an epoxy resin and an aggregate, to produce an anti-slip tread.

Summary of the Invention According to the invention, a method of producing sheet material comprises: (a) forming a mixture of polymer and particulate; (b) supplying the mixture to an extrusion die; (c) extruding the mixture through the die to produce an extruded sheet of polymer having particulate therein; (d) passing the extruded sheet over a casting surface while the polymer is still in a softened form and, at the same time; (e) causing polymer in the sheet to move towards the casting surface. During step (d) of the method in accordance with the invention, the extruded sheet has a first side that is closest to the casting surface and a second side that is furthest away from the casting surface. Step (e) ofthe method may cause the particulate to be concentrated towards the second side ofthe sheet and/or may cause the particulate to project from the second side ofthe sheet. The term "project" as used herein means that the particulate protrudes above a first plane formed by the essentially flat surface ofthe sheet, although most, and possibly all, of the particulate may still be covered by the polymer.

The invention also provides apparatus for producing sheet material, the apparatus comprising a source of a material to be extruded, the material comprising a mixture of polymer and particulate; an extrusion die connected to receive the mixture and operable to produce on extruded sheet of polymer having particulate therein; a casting surface located downstream ofthe extrusion die so that the extruded sheet will pass over the casting surface while the polymer is still in a softened form; and means operable, while the sheet is passing over the casting surface, to cause polymer in the sheet to move towards the casting surface.

Brief Description of the Drawings

By way of example only, processes and apparatus in accordance with the invention will be described with reference to the accompanying drawings, in which: Fig. 1 is a diagram of one form of apparatus for producing sheet material in accordance with the invention;

Fig. 2 is a diagram illustrating schematically the production of sheet material by a process in accordance with the present invention; Fig. 3 is a diagram of another form of apparatus for producing sheet material, in accordance with the present invention;

Fig. 4 is a cross-sectional view showing the construction of one form of sheet material produced using a process and apparatus in accordance with the present invention, and Figs. 5 and 6 are cross-sectional views showing the construction of other forms of sheet material produced using a process and apparatus in accordance with the present invention.

Detailed Description of the Invention The apparatus shown in Fig. 1 comprises a slot die 1 supplied with material by a screw extruder 2. The extruder 2, in turn, is supplied with material from two hoppers 3, 4 one of which (hopper 3) contains solid polymeric material and the other of which (hopper 4) contains particulate material. During passage through the extruder 2, polymer from the hopper 3 becomes molten as a result of heat generated by the action of compression forces and shear forces within the extruder and is also mixed thoroughly with particulate from the hopper 4. The mixture of polymer and particulate is then delivered through the die 1, the extrudate being in the form of a continuous web of sheet material 5 which is cooled by being passed around a casting roller 6 before being wound onto a roll 7. While the web 5 is passing around the casting roller 6, it is subjected to conditions which, as described below, cause molten polymer to move towards the casting roller whereby the side ofthe web closest to the casting roller becomes substantially smoother than the side ofthe web furthest away from the casting roller. Those conditions may comprise a field of force, (for example a magnetic force, an electrostatic force (possibly in combination with heat), and combinations thereof) applied to the web to pull the web towards the casting roller A preferred technique for causing molten polymer in the web 5 to move towards the casting roller 6 is to generate an electrostatic field between the web and the casting roller, for example by applying an electrostatic charge to the web so that it will be attracted to the roller This technique is illustrated by the apparatus of Fig 1 An alternative technique (which can be used if the polymer in the web contains a magnetic filler and non-magnetic particulate) is to generate a magnetic field which will pull the polymer towards the casting roller Examples of magnetic fillers are particles of iron and cobalt oxides, which would typically have a smaller particle size than the particulate in the hopper 4

A further alternative, when an electrostatic charge is used to cause polymer in the web to flow towards the casting roller 6, is to apply heat to the web 5 or to the casting roller 6 The heat may be applied in the form of microwave or infrared radiation, and should be applied and removed relative quickly The technique of using heat in addition to an electrostatic charge may allow for faster production speed/greater productivity since the time until solidification ofthe polymer is extended The heat applied is based on the melting point ofthe polymer used The heating temperature, for either direct application to the web 5 or for the casting roller 6, should not exceed that melting point Typically, temperatures can range from about 38°C ( 100°F) to about 316°C (400°F) In the case in which the casting roller contains a fluid (e g water), heating can be accomplished by heating the fluid Movement ofthe polymer towards the casting roller 6, as the web 5 moves around the roller, causes the particulate in the web to be concentrated towards the side ofthe web furthest away from the roller, and/or to project from that side ofthe web

In the apparatus shown in Figure 1, as already mentioned, an electrostatic charge is applied to the web 5 to increase the degree of contact between the web and the casting roller 6 The electrostatic charge is applied by on electrode assembly 8, located immediately downstream ofthe die 1, which will be described below The slot die 1 of Fig 1 may be any suitable die, for example a film die with a slot which is 30 mil (0 76 mm) wide, and is preferably constructed from wear- resistant materials. The die gap typically ranges from two to six times (preferably three times) average particle size ofthe material in hopper 4 It is not, however, essential for a slot die to be used The die opening could have any suitable shape (it could, for example, be square) and may have a contoured edge so that the side of the web 5 furthest from the casting roller 6 is patterned or textured.

The extruder 2 of Fig 1 may be any suitable twin screw extruder, preferably one having interchangeable wear-resistant screw elements and wear-resistant barrel elements The extruder 2 may, alternatively, be a single screw extruder A suitable barrel diameter of an extruder depends on the desired production output but may vary within the range from about 25 mm to about 300 mm A suitable length/diameter ratio for the screw(s) of an extruder depends on the polymer used and may typically range from 24/1 to 48/1 The extruder generally is run at a screw speed within the range of from 5 φ to 550 φm (typically 30 m to 200 φm) One suitable twin screw extruder, having a barrel diameter of 40 mm, is available from Berstorff Coφoration of Charlotte, North Carolina, U S A In an extruder of that particular type, the screws typically have a length/diameter ratio of 30/1 When used in the apparatus as shown in Fig 1, the extruder would typically be run at a screw speed of 30 φ and at a temperature within the range of from 95.5 to 260°C (200 to 500°F), preferably 193 to 204 5°C (380 to 400°F) The temperature depends mainly on the polymer used to form the web 5

The electrode assembly 8 shown in Fig 1 comprises a conventional wire 9 which extends across the width ofthe web 5 and is connected to a high voltage D C power supply 10 The power supply is preferably capable of delivering a voltage of at least 5 0 kV The power supply may, for example, be a variable power supply adjusted to deliver a voltage of 10 0 kV The power supply 10 is connected to put a positive charge on the wire 9 although that is not essential Preferably, the speed ofthe web 5 is such that any portion ofthe web is within the electric field of the wire 9 for a period of about 0 25 seconds Alternatively, the electrode assembly 8 may be operated to generate an A C. electric field, or a combination A.C. D.C. electric field, although this would generally be more complex and expensive.

The extruded web 5, as it leaves the die 1, is the mixture of molten polymer and particulate with the particulate being dispersed throughout the polymer. Generally, the mixing action ofthe extruder 2 will result in the particulate being uniformly dispersed throughout the polymer and, typically, protruding to some extent from both surfaces ofthe web (although covered by a thin layer of polymer).

In the absence of an applied electrostatic charge, it is the protruding particulate 11 (rather than the polymer 12) that contacts the surface ofthe casting roller 6 as the web 5 moves over the latter. Consequently, the web 5 will cool substantially in the condition in which it leaves the die 1 , that is, with the particulate 11 protruding to some extent from both surfaces ofthe web The effect of using the electrode assembly 8 to apply an electrostatic charge to the web 5 is that the whole web (including the polymer 12) will be attracted towards the surface ofthe casting roller 6. As a result, the polymer 12 (which is still molten or flowable) will actually move into contact with the casting roller 6, thereby smoothing out the surface ofthe web that lies adjacent the roller. That result is illustrated in Fig. 2, in which the smooth surface is indicated by the reference 13. At the same time, the extent to which the particulate 11 protrudes from the other surface ofthe web 5 will be increased. In Fig. 2, that other surface is indicated by the reference 14. In addition, the number of particles that protrude from the surface 14 ofthe web may also be increased although that is not illustrated in Fig. 2. Consequently, the web will now cool with one surface 13 which is comparatively smooth while the other surface 14 contains particles which protrude to a greater extent, and possibly in greater number, than when the web left the die 1. The terms "comparatively smooth" or "substantially smooth" as used herein with reference to the surface 13 mean that the surface mirrors the surface ofthe casting roller and does not contain protruding particulate. In contrast, the opposite surface 14 has not had the benefit ofthe casting roller and contains protruding particulate. 21536

8

The process described with reference to Fig 1 is a comparatively easy and economical way of manufacturing sheet material which contains particulate and is substantially smoother on one side than on the other The particulate that is used can be selected from a wide range of materials, provided that it will mix with the polymer, as described, and pass through the slot die 1

The electrode assembly 8 in the apparatus of Fig. 1 need not comprise simply a single wire 9 as illustrated but could alternatively comprise a plurality of such wires if required to achieve a more uniform charge on the extrudate. Alternatively, any other suitable arrangement could be used to place an electrostatic charge on the extrudate It is also not essential for the electrode assembly to be located immediately downstream ofthe die 1 as illustrated in Fig 1 the electrode assembly can be located anywhere between the die 1 and the casting roller 6, or can be located within the die itself or even within (but not rotating with) the casting roller as indicated in phantom in Fig 1 It is necessary only that the electrode assembly 8 should be positioned to place an appropriate electrostatic charge on the extrudate 5 before it passes around the casting roller 6 The electrostatic charge depends on the polymer chosen and on the thickness ofthe web and, of course, on the voltage drop across the electrode assembly The electrostatic charge is chosen to achieve a substantially smooth surface 13 as shown in Fig. 2 As mentioned above, particulate used in the apparatus of Fig 1 can be selected from a wide range of materials including, but not limited to, abrasive particles, frictional particles, such as to ceramic beads, glass beads, and glass bubbles, magnetic particles such as magnetic ferrite particles, and/or decorative particles, such as copper, aluminum, and particles available from Minnesota Mining and Manufacturing Company, St Paul, MN, under the trade designation

"Colorquartz" Depending on the nature ofthe particulate material, the product produced can have widely different properties and, consequently, widely different potential uses, for example, magnetic recording media, a reflecting sheet material, a non-slip flooring material, or a decorative packaging material By using abrasive grains as the particulate in the hopper 4 it is possible to form sheet material which has abrasive properties and can be used for certain abrasive products or as a non-slip flooring material. The quality ofthe abrasive sheet material that is produced will be determined by the amount of abrasive grains, the nature and size ofthe abrasive grains, and the nature ofthe polymer. In addition, by using a die slot which has a contoured edge as described above, it is possible to produce a material having a patterned or textured abrasive surface, for example a surface with channels for the removal of debris generated during abrading. An abrasive product formed from the sheet material may further comprise a backing material which may be sealed or porous and may, for example, be cloth, polymeric film, fibre, metallic foil, a metal plate, paper, or a non-woven web. The backing material would improve the strength or rigidity ofthe abrasive product, or provide a means of attaching the abrasive product to support means such as a back- up pad. The backing may, for example, comprise part of a hook and loop type attachment system to secure the abrasive product to a back-up pad. The loop fabric may be on the back side ofthe abrasive with hooks on the back-up pad. Alternatively, the hooks may be on the back side ofthe abrasive with the loops on the back-up pad. Hook and loop type attachment systems are described in U.S. Patent Nos. 4,609,581 and 5,254, 194, and in U.S. Patent No. 5,505,747, all of which are incoφorated herein by reference. The hook/loop attachment sheet may be laminated to the back side ofthe abrasive sheet material. Alternatively, the particulate/polymer web 5 of Fig. 1 can be extruded onto the hook/loop attachment sheet. Another form of backing is a non-woven or open- weave scrim which can be laminated to an abrasive product to import strength and/or rigidity. In addition, the abrasive sheet material may be extruded or laminated onto an adhesive layer, e.g., a pressure-sensitive adhesive layer. Lamination can take place during or after production of the abrasive product.

It is also possible to apply a coating to one side of an abrasive product formed from the sheet material. The coating can be applied for various reasons; for example, a carbon black coating can be applied to the backing side of a product to reduce static charge generated dunng grinding, or a supersize coat, having a gπnding aid such as KBF₄ can be applied to the abrasive side to increase cutting performance The particle size ofthe abrasive grains in the hopper 4 of Fig 1 can range from about 0 1 to 1500 micrometers, typically from about 1 to 150 micrometers

Examples of conventional abrasive particles include silicon carbide, alumina zirconia, and aluminum oxide (including sintered, sol gel, fused, and heat-treated aluminium oxide) The abrasive product may contain a mixture of two or more different abrasive particles This mixture may for example compπse a mixture of hard abrasive particles (I e having a Mohs hardness equal to, or greater than 8) and soft abrasive particles (I e having a Mohs hardness less than 8) or a mixture of two soft, or two hard, abrasive particles The abrasive particles may have specific shapes associated with them

Examples of such shapes include rods, tπangles, pyramids, cones, solid spheres and hollow spheres Alternatively, the abrasive particles may be randomly shaped

The abrasive particles can be treated to provide a surface coating thereon A surface coating can be used to improve the adhesion between the abrasive particle and the polymer in the abrasive product Alternatively, or in addition, a surface coating can alter and improve the abrading charactenstics ofthe abrasive particles The abrasive particle may have multiple coatings

The abrasive particles may also be abrasive agglomerates An abrasive agglomerate compnses a plurality of individual abrasive particles bonded together to form a shaped mass The abrasive agglomerates may be irregularly or randomly shaped or have a predetermined shape such as rods, triangles, pyramids, cones, and solid spheres The abrasive agglomerate may utilize an organic binder or an inorganic binder to bond the abrasive particles together Examples of abrasive agglomerates are descnbed in U S Patent Nos 4,652,275, 4,799,939, 5,500,273, and 5,549,962 incoφorated herein by reference The polymer used in the apparatus of Fig. 1 is preferably a thermoplastic material but could, alternatively, be a thermosetting material. Suitable thermoplastic materials include polyethylenes, polyesters, polystyrenes, polycarbonates, polypropylene, polyamides, polyurethanes and mixtures thereof. The polymer selected should have properties appropriate to the intended use ofthe sheet material. For example, if an abrasive product is desired, the polymer should have a relatively-high melting temperature, heat and water resistance, and a degree of toughness appropriate to such use.

Various materials can be added to the polymer loaded into the hopper 3 of Fig. 1, depending on the desired properties ofthe sheet material 5. Those additives include, for example, pigments, dyes, reinforcing materials, toughening agents, anti¬ static compounds, anti-oxidants, polymer processing additives, plasticizers, fillers (including grinding aids) stabilizers, expanding agents, fibers, initiators, suspending agents, photosensitizers, lubricants, wetting agents, surfactants, and fire retardants. The amounts of these additives are selected to provide the properties desired.

In general, in the case of an abrasive sheet material, the addition of a plasticizer to the polymer will increase the erodibility ofthe material. Examples of plasticizers include polyvinyl chloride, dibutyl phthalate, alkyl benzyl phthalate, polyvinyl acetate, polyvinyl alcohol, cellulose esters, phthalate, silicone oils, adipate and sebacate esters, polyols, polyol derivatives, tricresyl phosphate and castor oil.

A filler generally has an average particle size ranging from 0.1 to 50 micrometers, typically from 1 to 30 micrometers. Examples of useful fillers for the polymer, in the case of an abrasive sheet material, include calcium carbonate and silicate A grinding aid is defined as a material, the addition of which to an abrasive article has a significant effect on the chemical and physical processes of abrading and results in improved performance. It is believed in the art that a grinding aid will either 1) decrease the friction between the abrasive particles and the workpiece being abraded, 2) prevent the abrasive particle from "capping", i.e. prevent metal particles from becoming welded to the tops ofthe abrasive particles, 3) decrease the interface temperature between the abrasive particles the workpiece or 4) decrease the grinding forces In general, the addition of a gπnding aid increases the useful life of an abrasive product Grinding aids encompass a wide variety of different materials and can be inorganic or organic based Examples of grinding aids include cryolite and potassium tetrafluoroborate A combination of different grinding aids may be used and, in some instances, this may produce a synergistic effect

The addition of filler and/or grinding aids in some instances increases the erodibility of the resulting abrasive material, which can be desirable

If the particulate used in the apparatus of Fig 1 is comparatively fine (for example has a particle size less than 25 micrometers) the smoothing effect ofthe casting roller 6 on the adjacent surface ofthe extruded sheet material 5 may be less apparent and, in some cases, may be absent (although the sheet material produced may still be useful) A person skilled in the art could readily determine the particle size below which the casting roller 6 will not have a smoothing effect on the extruded sheet material

Fig 3 illustrates a modified form ofthe apparatus shown in Fig 1 In the apparatus shown in Fig 3, the die 1 is replaced by a co-extrusion die 20 which receives material (for example, a pressure sensitive adhesive) from a second extruder 21 in addition to the mixture of polymer and particulate from the extruder 2 The extruder 21 is supplied with material from a single supply hopper 22 and feeds the material to the die 20 so that it is co-extruded with the mixture from the extruder 2 and forms a backing sheet to the web 5 of extruded mixture In addition, the apparatus shown in Fig 3 includes a path for applying a further, ready-formed layer of material 23 (for example, a release liner) to the extrudate from the die 20, enabling a layer of material 23 to be laminated onto the web 5, if required In Fig. 3, the additional layer of material 23 is laminated onto the web 5 just before the latter reaches the casting roller 6 so that the additional layer will actually e between the casting roller and the web As the laminated material then passes around the casting roller 6, the smoothing effect described above will still occur in the web 5 so that the side 13 ofthe web adjacent to the material 23 will be smoothed and move into better contact with the material 23 while the particulate in the web will become more prominent on the other side 14

The apparatus of Fig 3 can produce a sheet material having the structure shown in Fig 4, namely an upper layer 41 comprising polymer 43 with particles 42 protruding from it and, on the rear surface ofthe layer 41, a pressure-sensitive adhesive 44 backed by a release liner 45

It will be appreciated that, through suitable combinations of co-extrusion and lamination steps, sheet materials having various constructions can be produced in a similar way to the material shown in Fig 4 Fig 5 shows, as an example, another form of material which has an upper layer 51 comprising polymer 53 with particles 52 protruding from it and, on the rear surface ofthe layer 51, a cloth backing 56 on the rear surface of which is an extruded layer 54 of pressure-sensitive adhesive backed by a release liner 55 The upper layer 51 is formed in a similar manner to the layer in Fig 4, with the cloth backing 56 being laminated onto the layer 51 before the extrudate is passed around the casting roller 6 The layer 54 of pressure-sensitive adhesive can then be extruded onto the cloth backing 56 downstream ofthe casting roller 6, followed by lamination ofthe release liner 55 Fig. 6 shows another material which can be produced in the same way as the material shown in Fig 5 The material shown in Fig 6 has an upper layer 61 comprising polymer 63 with spherical particles 62 protruding from it and, on the rear surface ofthe layer 61, a cloth backing 56 on the rear surface of which is an extruded layer 64 of pressure-sensitive adhesive backed by a release liner 65

In the material shown in Figs 4, 5 and 6, the particulate 42, 52 and 62, respectively, protrudes above a first plane formed by the essentially flat surface 40 ofthe material, although polymer (43, 53, 63) remains over at least some ofthe particulate

Representative examples of pressure sensitive adhesives suitable for the sheet mateπals of Figs 4, 5 and 6 include latex crepe, rosin, acrylic polymers and copolymers, e g., polybutylacrylate, polyacrylate ester, vinyl ethers, e g polyvinyl n- butyl ether, alkyd adhesives, rubber adhesives, e g , natural rubber, synthetic rubber, chlonnated rubber, and mixtures thereof One preferred pressure sensitive adhesive is an isooctylacrylate acrylic acid copolymer

In the apparatus of Fig. 3, the electrode assembly 8 is located within the casting roller 6 although it could, as in Fig 1, be located immediately downstream ofthe extrusion die 20 or at any location between the die and the casting roller.

In both Fig 1 and Fig 3, the casting roller 6 could be replaced by any other suitable form of casting surface The roller 6, or other casting surface, may be smooth or embossed, and may be heated or unheated

Examples

The following, non-limiting, examples will further illustrate the invention

EXAMPLE 1

Using apparatus ofthe type shown in Fig 1, a pelletized thermoplastic elastomer, commercially available under the trade designation "Hytrel" from E I DuPont de Nemours of Wilmington, Delaware, U S A , was loaded into one hopper of a twin screw extruder similar to the extruder 2 A 20/40 ceramic bead, available under the trade designation "Carbo Prop" from Carbo Ceramics of Iberia, Louisiana, U S A , was loaded into the other hopper ofthe extruder The ceramic bead comprised a blend of (by weight) 70 to 76% aluminium oxide, 1 1 to 15% silicon oxide (SiO₂), 3 2 to 4 5% titanium dioxide, and 8 9 to 10 5% iron oxide (Fe₂O₃) The extruder was operated to mix the elastomer and ceramic beads together thoroughly, in amounts of 70% by weight of elastomer and 30% by weight of beads, and the mixture was then fed through a film extrusion die The die gap was 30 mil (0 76 mm) and was adjusted to obtain optimum flow Immediately adjacent the outlet ofthe die, the extrudate was passed through an electrostatic field applied by a wire at a positive potential of 10 kV and was then solidified on a casting roller The resulting sheet had one smooth surface and ceramic beads projecting from the other surface, and was approximately 635 μm thick EXAMPLE 2

Example 1 was repeated except that ANSI Grade 50 aluminum oxide abrasive particles were used instead ofthe ceramic beads The resulting sheet had one smooth surface and aluminum oxide particles projecting from the other surface, and was approximately 635 μm thick.

EXAMPLE 3

Example 1 was repeated except that ANSI Grade 60 quartz particles were used, instead ofthe ceramic beads The resulting product had one smooth surface and quartz particles projecting from the other surface and was approximately 635 μm thick

COMPARATIVE EXAMPLE A

A skid resistant material in the form of 30 grit ceramic beads was coated onto a 2 9 mil (74 μm) thick polyethylene terephthalate film, produced by a conventional process comprising the steps of coating a urethane make coat onto the film with a roll coater, drop coating the ceramic beads; and coating with a second urethane size coat The resulting product was approximately 550 μm thick

COMPARATIVE EXAMPLE B

A skid resistant material was prepared in the same manner as Comparative Example A except that 60 grit aluminum oxide abrasive particles were used Such a skid resistant material is commercially available from Minnesota Mining and Manufacturing Company of St Paul, Minnesota, U S A under the trade designation "General Puφose Safety Walk"

COMPARATIVE EXAMPLE C

Comparative Example C was a ceramic flooring tile, such as those used in residential and commercial buildings COMPARATIVE EXAMPLE D

A skid resistant material was prepared in the same manner as Comparative Example A except that 60 grit quartz particles were used. Such a product is commercially available from Minnesota Mining and Manufacturing Company under the trade designation "General Puφose Safety Walk"

The products of Examples 1 to 3 were tested, with Comparative Examples A to D, using the following Test Procedures as designated:

Test Procedure I Test Procedure I was designed to measure the surface frictional properties of the sample Λ portable "British Pendulum Skid Resistance" tester, commercially available from Stanley/London was used according to ASTM E303 and ASTM E303 Annex

Test Procedure II

Test Procedure II was designed to measure the ability ofthe sample to resist abrasion. The sample was placed on a Schiefer Cut Tester. The test was run under a water flood The wear ofthe sample was measured after 500 and 1000 cycles, and converted to percentage of sample lost, by the calculation (weight lost/initial weight) x 100%

Test Procedure III

Test Procedure III was designed to measure the tensile strength of the samples The sample was converted into a 2.5 cm x 7 5 cm (1 inch x 3 inch) strip. A tensile testing machine, available from Instron Coφ., Model QCII, was used.

The sample was clamped into the jaws ofthe machine, and the jaws were separated at a speed of 5 crn/min (2 inches/min). The load needed to break the sample was recorded, as was the percentage of sample elongation at that peak load.

Table 1 below shows the British Pendulum Numbers (BPN) obtained by carrying out Test Procedure I on samples of Comparative Examples A, B and C, and Examples 1 and 2. In each case, the sample was measured in three different environments (dry, wet, and oily). The numbers (BPN) indicate the functional properties of the surface of the sample that is being tested: a surface that offers greater frictional resistance has a higher BPN.

Table 1 Test Procedure I

Environment

Example Dry Wet on

Example 1 80 70 60

Example 2 65 65 60

Comparative Example A 55 50 45

Comparative Example B 100 90 85

Comparative Example C 50 35 25

Table 2 below shows results from testing samples of Comparative Examples A, B and D, and Examples 1, 2 and 3 using Test Procedure II.

Table 2 Test Procedure II

Example % wear (500 cycles) % wear (1000 cycles)

Example 1 not detected not detected

Example 2 not detected 0.3

Example 3 1.9 2.9

Comparative Example A not detected not detected

Comparative Example B not detected 1.5

Comparative Example D 7.1 11.1

Table 3 below shows results from testing samples of Example 1 and

Comparative Example B using Test Procedure III.

Table 3 Test Procedure III

Example Peak strength (psi) Elongation (%)

Example 1 133.9 7.9

Comparative Ex. B 126.6 10.5 Table 4 shows the British Pendulum Numbers (BPN) obtained by carrying out Test Procedure I on samples of Examples 1, 2 and 3 and Comparative Examples A and B. This test was run after each sample had been subjected to 1000 cycles of Test Procedure II.

Table 4 Test Procedure I

Example Dry Environment

Example 1 70

Example 2 65

Example 3 40

Comparative Example A 60

Comparative Example B 40

Overall, the results summarized in Tables 1 to 4 show that the articles prepared in accordance with the present invention exhibit frictional properties, wear performance and tensile strength which are acceptable compared with conventional products.

Claims

IN THE CLAIMS

1 A method of producing sheet material, comprising (a) forming a mixture of polymer and particulate; (b) supplying the mixture to an extrusion die,

(c) extruding the mixture through the die to produce an extruded sheet of polymer having particulate therein,

(d) passing the extruded sheet over a casting surface while the polymer is still in a softened form and, at the same time; (e) causing polymer in the sheet to move towards the casting surface

2 A method as claimed in claim 1, in which step (e) comprises applying a field of force to the sheet to pull the sheet towards the casting surface

3 A method as claimed in claim 2, in which the field of force is an electrostatic field between the sheet and the casting surface.

4 A method as claimed in claim 3, in which the electrostatic field is formed by applying an electrostatic charge to the extruded sheet.

5 A method as claimed in claim 4, in which the electrostatic charge is applied to the extruded sheet before the sheet contacts the casting surface

6 A method as claimed in claim 1, including the step of applying at least one additional layer of material to the extruded sheet

7 A method as claimed in claim 6, in which the additional layer is applied before the extruded sheet reaches the casting surface, and is applied to the side of the sheet nearest to the casting surface.

8 A method as claimed in claim 6, in which the additional layer is co-extruded with the mixture

9 A method as claimed in claim 8, in which the additional layer comprises polymer material.

10 A method as claimed in claim 6, in which at least one ofthe additional layers is a sheet material which is laminated in position on the extruded sheet

1 1 A method as claimed in claim 1, in which the step of passing the extruded sheet over a casting surface comprises passing the sheet around a casting roll

12 A method as claimed in claim 1 , including the step of stretching the extruded sheet as it leaves the die

13 A method as claimed in claim 1, in which the polymer is a thermoplastic material

14 A method as claimed in claim 1, in which the particulate comprises abrasive grains

15 Apparatus for producing sheet material, comprising a source of a material to be extruded, the material comprising a mixture of polymer and particulate, an extrusion die connected to receive the mixture and operable to produce on extruded sheet of polymer having particulate therein; a casting surface located downstream of the extrusion die so that the extruded sheet will pass over the casting surface while the polymer is still in a softened form; and means operable, while the sheet is passing over the casting surface, to cause polymer in the sheet to move towards the casting surface

16 Apparatus as claimed in claim 15, in which the means operable to cause polymer to move towards the casting surface comprises means operable to apply a field of force to the sheet to pull the sheet towards the casting surface.

17 Apparatus as claimed in claim 16, in which the said means is operable to produce an electrostatic field between the sheet and the casting surface

18 Apparatus as claimed in claim 17, in which the means operable to produce an electrostatic field is located upstream ofthe casting surface

19 Apparatus as claimed in claim 15, in which the extrusion die is a co¬ extrusion die and wherein at least one further source of extrudable material is connected to supply material to the co-extrusion die, to be co-extruded with the mixture and form a layer of material on one side of the extruded sheet

20 Apparatus as claimed in claim 15, including a laminator aπanged to apply a sheet material on one side ofthe extruded sheet

21 Apparatus as claimed in claim 15, in which the casting surface comprises a casting roller arranged to receive the extruded sheet, the casting roller being so positioned that the extruded sheet passes around the roller

22 Apparatus as claimed in claim 15, including means arranged to cooperate with the extruded sheet to stretch the sheet as it leaves the die.

23 Apparatus as claimed in claim 15, in which the polymer is a thermoplastic material

24. Apparatus as claimed in claim 15, in which the particulate comprises abrasive grains

25. Apparatus as claimed in claim 15, in which the extrusion die is a slot die.

26. An abrasive product made by a method as claimed in claim 24.