Films and foils of plastic material, especially polyolefins such as polyethylene, have been widely used in many fields. One of the most important fields is the. packing industry. These films and foils have several suitable properties, among others, that they are stretch- able but have a good strength, that they are insensitive to the influence of water and other liquids, and that they can readily be heat welded.

In conventional manufacture of plastic film or foil by extrusion, the finished foil or film will mostly have a very smooth and glossy surface. For most plastic .mate¬ rials, especially polyolefins, this means that the sur¬ face of the foil has a very low friction, particularly when two foils of the same kind are rubbed against each other. For certain applications, e.g. in automatic packing machinery, this can be an advantage, but in many other cases the low friction is a serious disadvantage. For example, when filled sacks of polyethylene foil are stacked, there is a great risk that the stack may fall, as the fricition between the sacks is low. This involves risks of serious accidents.

Attempts have already been made to master this problem by laminating a plastic foil with one or more other foils of such a material or of such a design that the surface friction is increased. Thus, it is known,

for example, from US patent 4 045 270 to blow extrude a laminated tube, where the main foil of polyethylene is provided on one or both sides with a layer of a plas-tic material which has been made porous by means of an added 5 blowing agent. At the blowing and stretching of the tube after the extrusion, the porous plastic layer will be stretched so that the cells in the material will break.. In this way, a coarse, net-like surface will be obtained h?ving a considerably increased friction.

10 Although the plastic foil produced in this way has a suitably increased surface friction, it has the dis¬ advantage, however, that it requires an addition of blowing agent. Such blowing agents are not always phy¬ siologically acceptable and may make the use of the foil

15 impossible in the food industry, where there must be a complete safety against foreign and injurious materials migrating from the packing material to the .-packed food¬ stuff.

The disadvantage indicated above is eliminated by

20 the present invention and a plastic foil with increased surface friction is obtained which does not contain any foreign additives.

According to the present invention, this object is achieved by preparing a plastic foil or film by extrusion,

25 lamination and stretching of two or more foils or films of thermoplastic material, the dosage of plastic material to the foil or foils forming the outer layer or layers being controlled in the extrusion operation in such aι ay that the added amount of plastic material is insuffici-

30 ent to form a coherent layer after the stretching.

The extrusion is preferably carried out as extru- ^ sion blowing of a laminated tube through two or more concentric slit nozzles and a subsequent blowing and stretching of the hose.

35 Due to this, the outer layer will crack at the

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^{? r} st ^" retchi-ng and form a rough surface so that the surface ^{'J Z} -friction is increased in the finished foil. ^DSΓ L" ^' : ^'■ A. _n example of how the invention is carried out is ^s '- ^:r:: shown in the drawing. Here a sectional view of an extru- sion head 1 with two annular, concentric slit dies 2 and ^{• '} 3 is sho'wn schematically. A plastic compound 4, which is -° ^" plasticized by heat, is led from a conventional extruder (not shown) through the inner die 3 and is extruded to the tube foil 5. A plastic compound 6 is led from an extruder with a controlled dosage (not shown) to the slit die 2. As the added amount of the plastic compound 6 is too small to form a coherent layer when the extruded tube is blown out and stretched by the air bubble indicated by _. 8, the outer layer will crack and give local deposits 7 of the plastic material. In this way, a rough surface will be obtained.

After the inflation and ^' stretching operation, the produced laminated foil is further treated in a comp¬ letely conventional way. Thus, it can be rolled up us a tube and be transported^as a roll to its final use.

Of course it is also possible to use three con¬ centric slit dies and to control the supply of plastic compound to the innermost and outermost die in the way indicated above. The result of this will be an increased surface friction on both sides of the foil.

The controlling of the supply of plastic material to the extruder die or dies forming the layer which in¬ creases the friction can be carried out in such a way that the added amount is too small to obtain a coherent ^{" '} - ^" layer after the stretching so that the layer thus breaks.

It is also possible to control the supply in such a way that it is made intermittent. This will provide a more markedly rough surface of the foil. Moreover, fillers or fibers giving this plastic material inferior flowing properties and consequently increasing the surface rough-

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ness of the finished foil can also be added to the star¬ ting material of the foil or foils forming the outer layers. Such fillers are known to those skilled in the art. 5 As a plastic material for the foil any thermoplastic material can be used which is conventionally usable for ■ extrusion of foils. Polyolefins are especially suitable and primarily the use of polyethylene is contemplated. The plastic material or materials forming the friction

10 increasing surface layer can often be the same as the plastic material forming the main foil. However, this is not absolutely necessary, but other plastic materials can also be used, provided that they can be laminated with the main foil material.

15 . The extruders are of the type known to those skilled in the art and the adjustment of the extruder providing a controlled dosage can be easily established on the ba¬ sis of simple preliminary tests. The extrusion is carried out in conventional manner also in other respects.

20 The process of the invention has been described in the foregoing with reference to extrusion blowing of.a tube, which is a preferred embodiment. However, it is to be noted that the advantages of the invention can also be obtained in the extrusion of foils through straight

25 "slit dies and a subsequent lamination and stretching. By a controlled dosage to the extrusion nozzle providing the surface layer, it is also here achieved that the surface layer will contain an insufficient quantity of material to give a coherent layer after stretching, and

30 will instead crack and give a rough surface. Asperities may also here be formed on both sides of the foil.

A foil or film of plastic material showing a high surface friction without the necessity of adding any foreign, expensiv and toxic materials is produced accor-

55 ding to the present invention. The prepared foil material

can therefore be used for packing of foodstuffs and other sensitive materials without any risks. This means con¬ siderable technical advantages and also an increased safety.