CA1038246A - Process and equipment for coating a base with a thermoplastic - Google Patents

Abstract

A B S T R A C T

A process for the coating of a substrate, consisting of paper or paper lined with a metal foil, with polyethylene in which the polyethylene is extruded to a thin film which, while it is still warm, is brought into contact with the sub-strate and, with simultaneous cooling to a temperature below the softening temperature of the polyethylene, is pressed together with the substrate by passing the substrate and the extruded polyethylene film through co-operating rolls. At least one roll which is brought into contact with the polyethy-lene coating is moistened on its rolling surface with a liquid having a critical surface tension below 73 dyne/cm and a boiling point below 100°C, and consisting of a mixture of water and between about 10% and about 50% ethyl alcohol, whereby an unin-terrupted liquid layer is applied to the whole contact surface of the roll or rolls with the polyethylene coating.

Description

The present invention is concerned with a process and an equipment for coating a base, especially a strip of paper or paper lined with metal foil, with a thcrmoplastic, in which the thermoplastic material is extruded to a thin film which, while it i9 still warm, is brought into contact with the base and, with simultaneous cooling to a temperature below the softening .
termperature of the plastic material, is pressed to the base.
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In the production, of for example, coating material for - packing purposes, it often happens that a layer of thermoplastic - 10 material, for example polyethylene, is applied to a base where the thermoplastic material is extruded to a thin film which, while .
it is stiil in the molten or semi-molten state, is united with the , base which may, for example, consist of a strip of cardboard or a metal foil. - ~

~ In order to achieve good adhesion between the extruded - - - 15 thermoplastic coating and tne base, the base and thermoplastic coatir~ are pressed together between cooled pressure rollers, -~ whereby the thermoplastic layer is brought into good contact with - the base and at the same time stabilised by cooling~ When carrying out this coating process it often happens that the plastic material - 20 adheres to the cooling roll and that the coating material is ir. this - ~ - case subjected to a force that is mainly at right angles to the coating surface. ~his force can easily cause delamination of the . . .
~ aterial that need not be confined to the boundary zone of the . ~, . ... .
applied plastic layer but the delamination can be localised in the :, . - .
- 25 base, for example if this consists of paper or other cellular - material. i-~-- . . ~k '' ' ` -; - 2 -, 103t~Z46 ~ ttelnpts have been made to avold the above mentloned difficulty by wetting the cooling roll with water as well as by applying a coating of polytetrafluoroethylene, sold under the trade mark "Teflon", to the outer surface of the roll. The best result is achieved with the "Teflon" coated roll but delamination can occur even with such a cooling roll; moreover it has been found that the Teflon material cannot withstand for sufficiently long the mechanical and thermal stresses which arise at the outer sur-face of the cooling roller.
According to the present Invention, there is provided a process for the coating of a substrate, consisting of paper or paper lined with a metal foil, with polyethylene in which the poly-- ethylene is extruded to a thin film which, while it is still warm, is brought into contact with the substrate and, with simultaneous cooling to a temperature below the softening temperature of the polyethylene, is pressed together with the substrate by passing the substrate and the extruded polyethylene film through cooperating rolls wherein at least one roll which is brought into contact with the polyethylene coating is moistened on its rolling surface with a liquid having a critical surface tension below 73 dyne/cm and a boiling point below 100C, and consisting of a mixture of water and between about 10% and about 50% ethyl alcohol, whereby an uninter-~ rupted liquid layer is applied to the whole contact surface of the -- roll or rolls with the polyethylene coating.
The invention is described below by way of example with reference to the enclosed schematic drawing and the photographic representation.
Fig. 1 is a schematic sketch of a coating apparatus.

- ~ie~ 2 is a di~gram showine how the adhesion between cooling roller and thermcplastic coating varies according to the critical surface tension of the coolin~ roller. I

Fig. 3 i8 a photographic enlargement of a cross-section through a coating material which has delaminated.

- Fig 4 is a photo~aphic enlargement of a cross-section through a coating produced in accordance with the invention.

Fig. 5 is a photographic enlargement of the surface layer of the outer plastic coat of a coating material the coating process of which has been carried out in the usual way using a steel cooling - rol 1 .
t ~ig. 6 is a photographic enlargement of the surface layer of an - outer plastic layer of a coating material produced in accordance with ~', - the invention.

In order to clarify the matter, the concept of critical surface tensions will be discussed in more detail.
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It can be said that the critical surface tension of a material . .
- is a measure of the tendency to wetting, where the wetting tendency is grea~er at a lower critical surface tension than at a higher. The physical measure of critical surface tension is expressed in dyne/cm and the size of the critical surface tension can be determined .. .
-l experimentally in several ways well known to those skilled in the art.
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It has been found that the critical surface tension for pure - metals (measured in a vacum) is very high, about 1,000 - 4,000 dyne/cm.

Under the conditions ruling in the coating of a base, e.g. paper, with an extruded p]astic film it has however been found that the . - ~ . - .
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103~Z46 critical surface tension is considerably lower in the case of a polished metal roller, namely about 100 dyne/cm. By wetting a brightly polished cooling roller with water the critical surface tension of the roller can be reduced to the critical surface tension of water which is about 73 dyne/cm, because the extruded plastic coating does not come into contact with the metal roller ;~ but only with the water film covering the roller.
If a polytetrafluoroethylene coat, i.e. the material sold under the trade name "Teflon", is applied to the pressing roller, then the critical surface tension of the roller is re-duced to about 17 dyne/cm, i.e. the critical surface tension of the Teflon material.
The plastic material used as a coating usually has a cri-tical sur-Eace tension of about 30 dyne/cm (polyethylene has a cri-tical tension between 29 and 32 dyne/cm depending on moisture, temperature and other conditions prevailing during the extrusion).

` By adding ethyl alcohol as a surface-tension reducing ~r agent to water, the surface tension can be reduced as shown by `~ the table below, which means that the critical surface tension can be adjusted as desired by adding the substances mentioned, a fact which - as shown by the description below - is very ~ important.
- The arrangement shown schematically in Fig. 1 has a supply roll 1 with a strip 2 of paper or paper coated with aluminium foil, - to which a plastic coating is to be applied. The plastic coat 3, which consists of polyethylene, is - while still in the ~' .
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103~246 ~olten stnte - a}-~lie~ to the strip ~ by means of an e-xtrusion press 4 in which the plastic ~terial is heated to melting and extruded throu~h a longish, cleft-like valve 5 to form a thin film 3. The strip 2 coated with the p~astic film 3 is fed in between two cooperating pressing rolls 6 and 7, of which roll 6 is cooled by a cooling agent, preferably water, which is caused to circulate within the roll. By means of a spraying arrange-ment 8, a llquid in finely divided form is sprayed on to the surface of the roll 6 care being taken that the whole surface of the roll is covered by a liquid layer preferably of uniform thickness. The composition of the said liquid is discussed in detail below and the liquid need not necessarily be applied by spraying but good results can also be obtained by introducing the - liquid in other ways, for example by dipping the roller or by -- 15 applying with a moistened guiding roller. The finished coating -~ - material is indicated in ~ig. 1 by 11.
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When the plastic coating 3, stabilised by cooling, has passed - - the cooling roll 6, the contact between the surface of the cooling ~ ~ .
- roll and the plastic coat 3 must be interrupted and, because a certain amount of sticking of the plastic coat to the cooling roll always occurs, a tension arises at right angles to the surface of - the coating material 11. Since the coating material if it consists, for example, of fibrous material or of fibrous material covered with aluminium foil, has its lowest tensile strength at right angles to 25 - the surface of the material (the so-callec Z-direction~, tears in - the material, i.e. so-called cleavage phenomena (delamination)9 are . liable to occur when the coating material has to be pulled off from the cooling roll. In general, clea~age does not occur in the boundary layer between plastic coat and paper, but this cleavage occurs most fre~uently in the paFer layer close to the plastic coat as shown by ~ig. 3 which is a 200-fold enlargement of a cross-.

103~Z4~;
sectioll of a delan~ina~cd coating ~tcria].. In Fie. 3 the plastic coat i5 shown by 12, the fi.brous material by 14 and the delaminated zone by 13. In order to avoid delamination it is necessary thus ; eithcr to use paper material with such a high Z-strength (tensile streneth in the Z,direction) that, in spite of the stresses that arise in the separation of the coating 11 from the cooling roll 6, , no delamination occurs, or to ensure that the adhesion between the . . . cooling roll 6 and the coating material 11 is as small as possible.
, As-it lS usually difficult to determine the quality and the Z,strength of the paper, the adhesion between the plastic material and the cooling roll 6 must be reduced, which can be done in the way shown by the invention.

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In Fig. 2 a diagram is reproduced in which the horizontal .:
.: ~axis shows on a linear scale the adhesion between cooling roll and :~ ". 15 plastic coat and the vertical axis, expressed in dyne/cm, the - . - critical surface tension. In addition, the dlagram shows a.Curve . indicated by the Pe which illustrates the adhesion between a - - ; -polyethylene coating and a cooling roll with cooling roll coatings . ~ , . . .
having various critical surface tensions.

. 20 The horizontal axis has no subdivisions in absolute numbers ; - . because the adhesion depends on several factors, for example the diameter of the roll and the width of the strip, but the subdivision .
~ . is in relative numbers 0-10.

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. - ~ As can be`seer.from Fig. 2, the adhesion is greatest (10) when the cooling roll consists of a dry, polished steel roll with a ; critical surface tension that should here be 100 dyne/cm. If the : roll is wetted with water, then the critical surface tension . becom0s about 73 dyne/cm and the a &esion ;s reduced to 7. ~y the .-.

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103~46 use of a "Teflon" coated cooling roll the critical surface tension can be still further reduced even to about 17 dyne/cm and, as shown by the diagram, the adhesion becomes about 5, which must mean that only a small reduction of the adhesion has occurred although the critical surface tension has been reduced from 73 to 17. By looking at the experimentally produced diagram we see that the smallest possible adhesion, about 3.5, is obtained if the surface of the cooling roll has a critical surface tension of about 30 dyne/cm, i.e. the same critical surface tension as the poly-ethylene material.
As already mentioned, the cooling roll is wetted with a liquid the critical surface tension of which is regulated by mixing ethyl alcohol in suitable proportions with water. Since the coating material is to be used for the packing of foodstuffs, - ethyl alcohol is a satisfactory additive and from the table below it can be seen that an admixture of about 35% ethylalcohol with - water gives a surface tension of about 30 dyne/cm.
Water Ethyl alcohol (b.p. 78.5) Critical Surface Tension 100% 0% 72 dyne/cm 2090% 10% 46 "
- 75% 25~ 34 "
5~% 50% 27 "
0~ 100~ 21 "

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lU;~Z46 If ~i miYturc if fcd to the cooling roll 6 in a uniform nnd un;n~crrupt~d laycr consisting Or 6~ % ~ater and ~5 % ethyl alcohol, the adhcsion between the cooling roll 6 and the plastic coat of the coating ~terial 11 becomes as small as possible (3.5 according to diagram in Fig. 2), and a delamination can be ..
prevented. A greatly enlarged (100-fold) cross-section of a coating material that had been treated in accordance with the above 1nstructions, is shown in Fig. 4 where the plastic coat is indicated by 12 and the fibrous layer by 13 and it oan be seen from the figure that no delamination has occured.
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- '- ' ' - A further advantage of the method of the invention is that the plastic coat compared with the corresponding surface structure of a plastic of coat a coating material produced in the traditional ~- - way, has a very uniform surface structure.
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- 15 ~Thus in Fig. 5 a greatly enlarged picture of the surface layer " . , . , . -, - .. , . :... ," - . . . , . ...... ~ . . . - -of a plastic coat that has been treated with a dry,.brightly polished - - cooling roll is reproduced. Astne Figure shows, the surface is full o~ p1tS which, when they become deep enough, produce holes in the plastic coat (so-called pin holes). In order to avoid the formation ` 20 of "pinholes", which i~ very important if for example the coating . . .
~ - --material is to be used for the packing of sterilised goods, the . ~.................... . - ~
thickness of the plastic coat must be great enough to ensure that the depth of the pits formed in the surface coat do not exceed the '7 ' ' thickness of the plastic layer. A dimensioning of the plastic ~ .
~ 25 coating without having to take into account the pits formed should .~ - . .
~ therefore result in considerable commercial gains.
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As shown by Fig. 6, which shows exactly the same material that was treated in accordance with the invention, the surface structure of the plastic coat which has been treated with a cooling .

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1038;~4~;
roll to ~liC~I a liquid laycr consis~illg of ~/ater and ethyl alcohol has becn applied, is considerably more uniform and the pits that can be seen in Fiz. 5 hardly occur at all. ~ne reason that the surface structure has been so considerably improved is not entirely clear but everythine indicates that the ethyl alcohol, which has a boiling point of 78.5C, is brought to boiling by the hot plastic coating the surface temperature of which is in general above 100C when it is brought into contact with the cooling roll.
Cwing to the fact that the ethyl alcohol is brought to boiling a thin skin of vapour ls formed around the coollng roll, which contribut-s to the uniform surface structure.

As can be seen from the above, several important advantages are obtained by means of the method of treatment of the invention in comparison with the previously known technique because in the ~_- i15- - - first place a reduction of the tension in the Z,direction of the - coating materlal is achieved and hence a diminished risk of delamination ànd, ln the second place, a very uniform surface structure of the plastic coating from the coating material is achieved which frequently means that the thickness of the plastic - 20 coat can be reduced because, with-the previous dimensioning of the plastic coating the risk of so-called pin-hole formation had to be taken into account.
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The wetting liquid gives rise, of course, in comparision with previously known techniques, additional costs but it has proved - Z5- possible to collect the ~ eater part of the ethyl alcohol for reuse.
- ~ence the costs of the wetting liquid in comparison with the advantanges achieved are not important.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the coating of a substrate, consisting of paper or paper lined with a metal foil, with polyethylene in which the polyethylene is extruded to a thin film which, while it is still warm, is brought into contact with the sub-strate and, with simultaneous cooling to a temperature below the softening temperature of the polyethylene, is pressed together with the substrate by passing the substrate and the extruded polyethylene film through cooperating rolls wherein at least one roll which is brought into contact with the polyethylene coating is moistened on its rolling surface with a liquid having a critical surface tension below 73 dyne/cm and a boiling point below 100°C, and con-sisting of a mixture of water and between about 10% and about 50% ethyl alcohol, whereby an uninterrupted liquid layer is applied to the whole contact surface of the roll or rolls with the polyethylene coating.

2. A process according to claim 1, wherein the amount of ethyl alcohol in the liquid is between about 10% and about 35%.

3. A process according to claim 1, wherein the amount of ethyl alcohol in the liquid is about 25%.

4. A process according to claim 1, wherein the liquid is applied to the roll or rolls by a spraying process.

5. A process according to claim 1, wherein the liquid is fed to said roll by dipping parts of the roll into a vessel below the roll, which vessel contains the liquid.

6. A process according to claim 1, wherein the appli-cation of said liquid to the said roll is made by means of a coating device that is contacted with the roll.

7. A process according to claim 1, wherein the critic-al surface tension of said liquid is adjusted by admixture of sufficient ethyl alcohol to reduce the surface tension to a value approximately corresponding to the critical sur-face tension of the polyethylene thermoplastic material.

8. A coated substrate made by the process of claim 1.

9. A coated substrate made by the process of claim 2.

10. A coated substrate made by the process of claim 3.

11. Arrangement for carrying out the process according to claim 1, characterized in that at least two cooperating rolls are present in an arrangement such that a strip of material, one side of which has a thermoplastic coating, is taken up between them and is pressed during the passage between the rolls, where the roll or rolls that is or are facing the thermoplastic layer of the strip is or are cooled and devices for the application of the liquid are associated with the said rolls, which devices are arranged so that liquid is fed to at least those parts of the rolls which are brought into contact with the thermoplastic coating and that means are provided for feeding the liquid continuously in such a way that a complete wetting of the roll surface or roll surfaces is achieved.