FI58182C - PROCEDURE FOR THE CONDITIONING OF THERMAL PLASTICS - Google Patents

Description

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Switzerland-Schwei z (CH) 100

_ (71) Cordotex SA, 2, rue de la Paix, 1003 Lausanne, Switzerland-Schweiz (CH) (72) Erwin Wilhelm Wartenberg, Stuttgart, Federal Republic of Germany-Förbunds-rebubliken lyskland (DE) (7 ^) Oy Kolster Ab ( The present invention relates to a method for coating a substrate, e.g. is warm, contacted with the substrate and compressed together with the substrate while cooling to a temperature below the softening temperature of the plastic material, the compression taking place by means of two or more cooperating rollers between which a polyethylene coated substrate is fed, at least one of the rolls are so arranged that they can be brought into contact with a plastic layer, is moistened with a liquid having a critical surface tension of less than 73 x 10 ~ N / cm and a boiling point of less than 100 ° C, ensuring that a uniform and uninterrupted liquid layer is applied over the entire contact surface of the roll or rolls. The invention also relates to an apparatus for carrying out the method.

When preparing layer materials intended for packaging purposes, for example, it often happens that the layer of thermoplastic material, e.g.

2,518,182 polyethylene, applied to a substrate, the thermoplastic material being extruded into a thin film which, while still in the molten or semi-molten state, is combined with a substrate, which may be, for example, a paperboard web or a metal foil.

In order to achieve a good adhesion force, then the substrate and the thermoplastic layer are pressed together between the extruded thermoplastic layer and the substrate between the cooled pressure rollers, whereby the thermoplastic layer comes into good contact with the substrate and at the same time stabilizes during cooling. When carrying out said coating process, it often happens that the plastic material adheres to the cooling roll and that the layer material in this case becomes exposed - with a force directed substantially perpendicular to the surface of the layer material. This force can easily cause delamination of the material layers, which does not have to be limited to the boundary between the plastic layer and the substrate, but the delamination can occur in the substrate itself, for example if it is paper or other cellulose.

Attempts have been made to avoid the above-mentioned drawbacks by wetting the cooling roll with water and applying a polytetrafluoroethylene coating sold under the name "TEFLON" to the outer surface of the roll. The best results are obtained with a roll coated with "TEFLON *", but delamination can also occur with such a cooling roll, and it has been shown that "TEFLON" is not able to withstand the mechanical and thermal stresses to which it is subjected on the outer surface of the cooling roll.

A better method, which also has the advantage that the surface of the plastic layer of the layer material becomes very smooth, is a method according to the invention, characterized in that said liquid consists of water to which alcohol, preferably ethyl alcohol, has been added.

The invention will now be described, by way of example, with reference to the accompanying schematic drawing and photographs.

Figure 1 shows a schematic view of a coating device.

Figure 2 shows diagrammatically how the adhesive force between the cooling roll and the thermoplastic layer varies according to the critical surface tension of the cooling roll.

Figure 3 shows a photographic magnification of the cross-section of the layer material with the layers detached.

Figure 4 shows a photographic enlargement of a cross-section of a layer material made according to the invention.

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Figure 5 shows a photographic enlargement of the surface of the outer plastic layer of the layer material, the material of which the coating procedure has been carried out conventionally by means of a steel cooling roll.

Figure 6 shows a photographic enlargement of the surface of the outer plastic layer of the layer material made according to the invention.

For the sake of clarity, we must first talk more closely about the concept of critical surface tension.

It can be said that the critical surface tension of a material is a measure of the wetting tendency, whereby the wetting tendency is higher at low critical surface tension than at high. The physical measure of the critical surface tension is N / cm, and the magnitude of the critical surface tension can be experimentally determined in several ways, which can be easily stated from basic physics textbooks.

It has been found that the critical surface tension (in vacuum) of pure metals is quite high, about 1 ... 4 x 10 N / cm. However, under the conditions prevailing, for example, when coating a paper substrate with an extruded plastic film, it has been found that the critical surface tension of a polished metal roll is considerably lower, namely about 10 "* / N / cm. , which corresponds to about 73 x 10 ”^ N / cm, because the extruded plastic film does not come into contact with the metal roll, but only with the water film covering the roll.

If a polytetrafluoroethylene layer is applied to the printing roll, i.e. the material sold under the name "TEFLON ', then the critical surface tension of the roll w becomes about 17x ~ ^ N / cm, i.e. it will decrease to the critical surface tension of" TEFLON ".

The critical surface tension of the plastic material belonging to the layer material is generally about 30x10 ~ ^ N / cm. (The critical surface tension of polyethylene is 29 ... 32x10 ”^ N / cm, depending on the humidity, temperature and other conditions of extrusion.)

By mixing surface tension reducing agents, for example ethyl alcohol, acetic acid or acetone, the surface tension can be reduced according to the table below, which means that the critical surface tension can be adjusted by mixing said substances as desired, which is quite important, as will be seen in the following description.

The device schematically shown in Fig. 1 comprises a storage roll 1 with a paper web 2 or paper coated with an aluminum foil 4 58182, on which a plastic layer is to be fitted. The plastic layer 3 », which may be, for example, polyethylene, is applied - still in the molten state - to the web 2 by means of an extruder 4, where the plastic material is heated to melt and extruded through an elongate, slotted nozzle 5 to form a thin film 3. The web 2 coated with the plastic film 3 is guided between two cooperating press rolls 6, 7, of which the roll 6 is cooled in such a way that a coolant, preferably water, is circulated inside the roll. With the spraying device 8, a liquid is sprayed on the surface of the roll 6 in a finely divided form, whereby it is ensured that the entire surface 10 of the roll is preferably covered with a liquid layer of the same strength. The composition of said liquid is discussed in more detail below and it is not necessary to spray the liquid, but good results can also be obtained by introducing the liquid onto the roll in another way, for example by immersing the roll or transferring the liquid to the roll by means of a moistened guide roller. The finished layer material is indicated in Figure 1 by reference numeral 11.

When the cooling-stabilized plastic layer 3 has passed the cooling roll 6, the contact between the cooling roll surface and the plastic layer 3 must be interrupted, and since a certain adhesion between the plastic layer and the cooling roll still occurs, tensile stress is generated at right angles to the layer 11 surface.

Since the layer material, if it is, for example, a fibrous material or a fibrous material coated with an aluminum foil, has the worst tensile strength properties at right angles to the surface of the material (in the so-called Z-direction), the material easily cracks. distribution phenomenon (delamination) when the bed material is withdrawn from the cooling roll. In general, the distribution does not take place in the boundary layer between the plastic layer and the paper layer, but such distribution most often occurs in the paper layer, - close to the plastic layer, as shown in Figure 3, which shows a 200-fold magnification of the distributed layer material. In Fig. 3, the plastic layer is denoted by reference numeral 12, the fibrous material by reference numeral 14 · and the divided zone by reference numeral 13. To avoid division, either a paper material with high tensile strength in the Z direction must be used so that the layers do not divide despite the stresses 6, or care must be taken that the adhesive force between the cooling roll 6 and the layer material 11 is as small as possible. Since it is generally difficult to determine the quality and Z-strength of the paper material, the adhesive force between the plastic material and the cooling roll 6 must be reduced, which can take place according to the invention.

Fig. 2 is a diagram in which the horizontal axis linearly indicates the adhesion force Between the Cooling Roll And the plastic layer, and the vertical axis as the quality of the critical surface tension N / cm. Further, the diagram shows a curve denoted by the letters Pe, which illustrates the adhesion force between the polyethylene layer and the chill roll under different critical surface stresses in the chill roll coating.

The horizontal axis does not have a weapon distribution in absolute numbers because the gripping force depends on several factors, for example the diameter of the roll And the width of the web, but the distribution has taken place in relative numbers 0 ... 10.

^ As can be seen from Fig. 2, the adhesive force is at its maximum (10) when the cooling roll is a dry, polished steel roll, the critical surface tension of which must then be 10 ”^ N / cm. If the roll is moistened with water, the critical surface tension is about 73x10 "^ N / cm. And the adhesion force decreases to 7 · becomes about 5 »which means that the adhesive force decreases only slightly, although the critical surface tension decreases from 73x10"% / cm to 17x10 "^ N / cm. Examining the experimentally presented diagram, it is noticed that the small possible adhesive force, approx. 3» 5 x occurs when the critical surface tension of the surface of the chill roll is about 30x10 ”% / cm, i.e.

^ the same critical surface tension, which is with a polyethylene material.

As already mentioned, the cooling roller can be moistened with a liquid, the critical surface tension of which can be adjusted by mixing, for example, ethyl alcohol, acetic acid or acetone (other substances can also be used) in a suitable ratio with water. When a layer material is to be used for food packaging, ethyl alcohol must be considered as the most appropriate blending agent, And it can be seen from the table below that the mixture,

Containing about 35% ethyl alcohol mixed with water results in a surface tension of about 30x10 ~ N / cm.

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Water Ethyl (boiling point Critical alcohol point 78.5 ° C) surface tension 100% 0% 72 x 10_5N / cm 90% 10% 46 "75% 25% 34 50% 50% 27" 0% 100% 21 "90% 10% 55 "70% 30% 44" 50 50 $ 38 "0% 100% 26" -

Water Acetone (boiling point- Critical point 56.5 ° C) surface tension 90% 10% 49 x 10 “5N / cm 75% 25% 38 50% 50% 27

When a mixture containing 65% of water and 35% ethyl alcohol is introduced into the cooling roll 6 as a uniform and unbroken layer, the adhesive force between the cooling roll 6 and the plastic layer of the layer material 11 becomes as small as possible (3 → 5 according to the diagram in Fig. 2). A strongly enlarged (100-fold) cross-section of the layer material treated according to the above instructions is shown in Fig. 4, in which the plastic layer is indicated by reference numeral 12 and the fibrous layer by reference numeral 13 », and the figure shows that no peeling has occurred.

A further advantage of the treatment according to the invention is that the plastic layer has a very uniform surface structure compared to the corresponding surface structure of the plastic layer of the layer material prepared in a conventional manner.

Figure 5 shows a greatly enlarged view of the surface layer of a plastic layer treated with a dry, smooth-polished cooling roller. As can be seen from the figure, the surface is full of pits which, when they become deep enough, form holes (so-called pinholes) in the plastic layer. To prevent the formation of these holes, which is very important if the layer material is to be used, for example, for packaging a sterilized filler, the plastic material must be so thick that the depth of the pits formed in the surface layer does not exceed the strength of the plastic layer. Dimensioning the plastic layer without having to take into account the resulting bumps would therefore lead to a significant financial gain.

As can be seen from Figure 6, which shows exactly the same material treated in accordance with the invention, the surface structure of the plastic layer treated with a cooling roll fitted with a liquid layer of water and ethyl alcohol is much smoother and there are almost no pits as seen in Figure 5. The reason for the improved surface structure has not been fully elucidated, but all indications are that a hot plastic layer with a surface temperature generally above 100 ° C can boil ethyl alcohol with a boiling point of 78.5 ° C when the plastic layer comes into contact. with the cooling roller. By bringing the ethyl alcohol to a boil, a thin layer of steam is formed around the cooling roll, which contributes to the formation of a uniform surface structure.

As can be seen from the above, the procedure according to the invention provides several important advantages over the prior art, firstly the tensile stress in the Z-direction of the layer material is reduced and the risk of layering is reduced and secondly the plastic layer of the layer material has a very smooth surface structure. can be reduced because the previous dimensioning of the plastic layer had to take into account the risk of so-called Pinholes formations.

The wetting liquid naturally incurs additional costs compared to the prior art, but it has nevertheless proved possible to collect most of the ethyl alcohol for reuse. Thus, the cost of the wetting fluid is insignificant compared to the benefits achieved.

Claims (6)

8 58182 A method for coating a substrate, e.g. a web of material on paper or a metal film-coated paper, with polyethylene, wherein the thermoplastic material is extruded into a thin film (3) which, while still warm, is brought into contact with the substrate and compressed with the substrate while cooling to is below the softening temperature of the plastic material, whereby the compression takes place by means of two or more co-operating rollers (6, j) between which a polyethylene-coated substrate (2) is fed, whereby at least one of the rollers (6) arranged to be brought into contact with a plastic layer (3), moistened with a liquid having a critical surface tension of less than 73 X 10 'N / eui and a boiling point of less than 100 ° C on its roll surface (10), ensuring that the entire contact surface of the roll or rolls is uniformly and continuously liquid layer, characterized in that said The liquid consists of water to which alcohol, preferably ethyl alcohol, has been added. A method according to claim 1, characterized in that the liquid is applied to the surface of said roll (6) or said rolls by spraying. A method according to claim 1, characterized in that a liquid is introduced into said roll (6) or said rolls by immersing parts of the roll in a container arranged under the roll containing said liquid. Method according to claim 1, characterized in that the liquid is applied to the surface of said roll (6) or said rolls by brushing the liquid by means of a special brushing device which can be brought into contact with the roll or rolls. A method according to claim 1, characterized in that the surface tension of said liquid is adjusted to it by mixing ethyl alcohol to a value which corresponds to or slightly deviates from the critical surface tension of the polyethylene material. Device for carrying out the method according to claim 1, characterized by at least two co-operating rolls (6, 7) arranged so that a web of material (2), one side of which is coated with polyethylene (3), is fed between the rolls and pressed together, the roll (6) or those rolls arranged towards the plastic coating (3) of the web being cooled and in connection with said rolls means (8) for applying a liquid to the surface of the rolls, which devices are arranged so that at least the rollers (6) applying liquid to those parts which come into contact with the thermoplastic coating and that the liquid is continuously applied to the rollers in such a way as to achieve complete sealing of the roll surface (10) or roll surfaces.