Prior Art WO, A1, 97/21536 shows a method of producing a sheet material, an ap- paratus for producing the material as well as an abrasive product produced in ac- cordance with the method. The method comprises the steps to form a mixture of polymer and particles, to lead the mixture to an extrusion die, to extrude the mix- ture through the die in order to produce an extruded sheet of polymer comprising particles, to bring the extruded sheet to pass over a casting surface while the poly- mer is still in a softened form and at the same time to cause the polymer in the sheet to move towards the casting surface. Owing to the polymer being concen- trated closest to the casting surface particles will be concentrated further from the casting surface in the extruded sheet. The respective concentration is achieved by applying an electrostatic charge to the extruded sheet before the sheet gets in contact with the casting surface in the form of a casting cylinder. The particles may be constituted by abrasing particles and an abrasive material is thus achieved.

Summary of the invention One purpose of the present invention is to create a method by which a film material in general and an abrasive material in particular may be produced without the absolute need for any casting surface. The method of production is thus also to be useful for so called blowing of film. A further purpose of the present invention is to provide an abrasive material which is produced in accordance with the method and thus especially a rationally and economically produced emery or abrasive cloth for dry and wet grinding.

The invention thus comprises a method of producing a film material, the method comprising the steps of creating at least a first, a second and a third mix- ture comprising at least a first, a second and a third polymer respectively, leading each one of the mixtures to a co-extrusion die and co-extruding the mixtures through the die in order to produce a co-extruded film comprising at least a first, a

second and a third layer respectively. At least the mentioned second mixture com- prises a blowing agent at the same time as the mentioned second mixture creating the mentioned second layer in the co-extruded film, the mentioned second layer being surrounded by at least the mentioned first layer on one side and at least the mentioned third layer on the other side. The mentioned first mixture comprises particles. Each one of the mentioned particles is in every direction of a thickness which is larger than the thickness for the mentioned first layer without particles leading to the fact that the particles, through the expansion of the blowing agent in the mentioned second layer, are pressed out upon the surface of the mentioned first layer and create unevennesses in the surface layer there.

The mentioned blowing agent may expand when it leaves the mentioned die. The mentioned blowing agent may be constituted by a chemical blowing agent of endo-or exothermal type.

The mentioned particles may be of a size of 5-500 um. The mentioned particles may be constituted by aluminium oxide, silicon carbide, zirconium alu- mina, grind glass, sapphire, garnet, diamond and/or magnetic powder such as iron powder or iron filings.

The mentioned third layer may be stiffer than the mentioned first layer.

The mentioned third layer may be thicker than mentioned first layer. The men- tioned third polymer may be of a viscosity, which is higher than the viscosity of the mentioned first polymer.

The mentioned co-extrusion die may be constituted by a die ring being part of an equipment for blowing of a film. The mentioned co-extrusion die may be constituted by a slit die being part of an equipment for casting film.

The invention thus also comprises an abrasive material for cutting working and which is produced in accordance with the method above. The abrasive mate- rial shows at least one surface layer with unevennesses, the mentioned uneven- nesses being constituted of particles in a first layer in a co-extruded film compris- ing at least three layers. The mentioned first layer with particles may, when the abrasive material is being used, be somewhat resilient towards a second porous layer situated between the mentioned first layer and a third layer in the mentioned co-extruded film.

List of drawings Figure 1 shows, in cross section, a co-extrusion die during planar extrusion according to the prior art.

Figure 2 shows, in cross section, a co-extrusion die according to figure 1 dur- ing planar extrusion according to the invention.

Figure 3 shows, partly in cross section, an equipment for blowing of film ac- cording to the prior art.

Figure 4 shows, in cross section, a part of the equipment for blowing of film according to figure 3 during blowing according to the invention.

Description of modes of execution In figure 1 a previously known co-extrusion die 1 is shown during planar extrusion according to the prior art of film. In three channels 2,3 and 4 three more or less different but known compounded materials are flowing, each one compris- ing a polymer, and are co-extruded through the slit die to a laminate comprising three layers 5,6 and 7. Co-extrusion dies exist for more as well as fewer layers.

The layers according to the figure all have the same thickness but this may also vary. From the figure is also evident that each one of the layers is mainly of the same thickness nearby the die as well as a bit away from the same which charac- terise planar extrusion according to the prior art.

In figure 2 the known co-extrusion die 1 is shown during planar extrusion according to the invention. With the extrusion an abrasive material is produced. In the channel 2 a compounded material flows which is based upon a thermoplastic polymer containing 1-80% abrasive material in the form of particles 8 of an alloy or a mineral. Suitable thermoplastic polymers are polyolefins, polyester, polystyrene, polycarbonate, polyamide, polyurethane, EVA, EMA, EBA, thermoplastic elasto- meres such as SBS as well as EPDM and the like. Suitable materials for the parti- cles 8 are aluminium oxide, silicon carbide, zirconium alumina, grind glass, sap- phire, garnet, diamond and other hard materials according to the Mohr scale. The size of the particles may be 5-500 p. m.

In order for the particles to be able to be coupled chemically to one or several thermoplastic polymers the particles are first surface treated with silanes, titanates or peroxides in a high speed mixer of for example any of the makes Henchel or PMI. After mixture of particles and polymer the material is compounded

in a single or double screw extruder of for example any of the makes Berstorff or Leistritz together with different types of additives and processural agents. The types of polymer, additives and processural agents vary and are combined from case to case. The compoundation means that the particles are dispersed to pri- mary particles and distributed in the polymer material comprising the mentioned additives and processural agents and that the mixture is extruded, cooled and cut to normal size of granulate, a form which is then easy to use in the final production of the grinding material. Suitable additives are pigment, reinforcement, UV-stabi- lisers, antioxidants, impact modifiers, flame retardants, softening agents, antistatic agents and the like. Suitable processural agents are lubricative agents, surface ac- tive agents, fluorelastomers and the like.

In the channel 3 a mixture flows which is based upon a thermoplastic polymer with a considering the polymer types in the channels 2 and 4 balanced melting viscosity. The thermoplastic polymer contains a balanced amount of a chemical blowing agent of endo-or exothermal type. The blowing agent as well as the additives and processural agents are put to the polymer material in the form of a concentrate, a so called master batch.

In the channel 4 a mixture flows which is based upon a thermoplastic polymer which may be filled with filling material in the form of dolomite, calcium carbonate and/or talc. The mixture also contains additives and processural agents.

All or parts of the filling material may, if desired, be replaced for iron powder which has been magnetised in order to give the end product the advantage of easily be- ing able to be mounted upon an abrasive tool which has been adapted for such a magnetic mounting.

When the mixtures flowing in the channels 2,3 and 4 leave the extrusion die 1 they form a laminate film where the layer 5 with the particles 8 after a while form the real abrasive layer in the desired abrasive material. In order for the parti- cles 8 really to give an abrasive effect in the layer 5 in the finished abrasive prod- uct it is needed that the particles 8 are moved out upon the surface of the layer 5 and create unevennesses in the surface layer. This movement is achieved by the blowing agent in the layer 6, as soon as it leaves the extrusion die 1, expanding and thereby pressing the particles 8, which from the beginning nearby the die 1 partly reach into the middle layer 6, completely out of the middle layer 6. Each one of the abrasive particles 8 is large enough to be able to extend from the bottom

level of the layer 5 to and past the top layer 5 irrespective of how the particles are turned and twisted. The particles will thus create the desired unevennesses in the surface layer and these unevennesses are only covered by an extremely thin layer of polymer which mainly follows the forms of the unevennesses and which is re- lated to the above mentioned chemical coupling between particles and polymer.

The expansion of the blowing agent in the layer 6 may be directed mainly towards the layer 5 by making the second outer layer 7 stiffer and/or thicker than the layer 5 or by monitoring the viscosities of the materials in the melts. The blowing agent expands more or less continuously to the point where the material has solidi- fied/crystallised, the so called frost line 9. It is important that the abrasive particles 8 get a possibility of moving freely to and through the top level of the layer 5 with- out any cooling cylinder hindering the process.

In figure 3 an equipment for blowing of film according to the prior art is shown. A ring formed co-extrusion die 10 is here aimed upwards. The die 10 blows a balloon-like body 11 the wall of which is all the layers being parts of the laminate. The laminate is collected between nip rolls 12, corona treated at 13-i. e. treated with strong UV-light in order to break the surface down-and col- lected upon windup rolls. The film may also be antistatically treated and/or pressed (with paint or the like) before the winding up.

In figure 4 the known equipment for blowing of film is shown under produc- tion of an abrasive material according to the invention. Three layers are used in the laminate in accordance with what has been earlier described in connection with planar extrusion. Accordingly three extruders (not shown) feed the co-extru- sion die 10 via three channels (not shown) and what has been mentioned above regarding materials and procedure in connection with planar extrusion may also be true in the case of blowing of film. Then it should be noted that the layer 5, com- prising particles and constituting the abrasive layer in the finished product, may constitute the outside of the balloon-like body 11, like in the figure 4, or the inside of it. If the layer 5 constitutes the inside of the balloon-like body 11 the wearing upon the nip rolls 12 which are most often made of a rubber material will become less. It may also happen that it is decided to corona treat the backside of the lami- nate in order to facilitate a following paint pressing and/or gluing of the laminate and also in this case it is an advantage if the layer 5 with the abrasive particles 8 is laid as an inner layer at blowing. The choice whether the layer 5 with the abrasive

particles 8 is to lay in an outer or inner layer is often in practice decided by which machine equipment one has to one's disposal. If it is chosen to place the layer 5 with the abrasive particles upon the inside at blowing this is most easily achieved by leading the material which gives the layer 5 to that extruder which gives the in- ner layer according to figure 4 at the same time as the material which gives the layer 7 is lead to that extruder which gives the outer layer in the same figure, i. e. the layers 5 and 7 change places in relation to what is evident from figure 4.

In the following an executed example is given in connection with produc- tion of a blown film.

To an extruder for the layer 5 a compounded composition was led containing: 20% aluminium oxide (particle size 150 m) 1 % coupling agent (silan) 1 % processural agent (for example metal stearates) 78% polymer with a melt of a low viscosity (soft) To the extruder for the layer 6 a composition was led comprising : 5 % blowing agent concentrate in the form of a master batch (modified azodi- carbonamide) 95% polymer with a melt of a high viscosity (hard) To the extruder for the layer 7 a compounded composition was led containing: 50% talc and dolomite 50% polymer with a melt of a high viscosity (hard) After the ring formed co-extrusion die 10 and the rest of the processural steps described in connection with figure 3 a blown film with a thickness of 250 p. m was achieved of which the layer 5 (the abrasive layer in the finished product) showed a thickness of 70 pm, the middle layer 6 a thickness of 110 lm and the layer 7 finally a thickness of 70 pm.

The number of layers in the laminate may be varied within rather wide lim- its as long as the abrasive layer is situated farthest out at any side and at least one layer comprising blowing agent constitutes any one of the middle layers. The number of layers that are recommended however is 3-7 whereby a blown film for the described aim may have a total thickness of 60-500, um.

Due to the fact that the usage of the described blowing agent is relatively simple the production of a film/abrasive material in the described way becomes ra-

tional and very economical. Irrespective of the abrasive material being produced with planar extrusion or by blowing the particles in the abrasive layer 5 and the whole abrasive layer 5 become somewhat resilient when the abrasive material is used since the underlying middle layer 6 is somewhat porous depending upon the effect of the blowing agent within this layer. Due to this, the effectivity is increased when the abrasive material is used compared to a material with unresilient base for the abrasive layer.

The invention is not limited by the modes of execution shown here but may be varied in accordance with the following patent claims.