JPS5828093B2 - Netsukaso Seibutsu Shitsudekizaiohifukusuruhouhououoyobisonosouchi - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for coating substrates, particularly sheets of paper or metal foil-backed paper, with thermoplastics.

In the present invention, a thermoplastic material is extruded into a thin film, brought into contact with a substrate, cooled to a temperature below its softening point, and pressed onto the substrate.

For example, in the production of coated products for packaging, a thermoplastic material (e.g. polyethylene) is extruded into a thin film and then bonded to a substrate such as cardboard or metal foil while in a molten or semi-molten state. It often becomes necessary to provide a layer of thermoplastic material on the material.

To obtain good adhesion between the extruded thermoplastic coating and the substrate, the thermoplastic layer is brought into good contact with the substrate by pressing them together between cooled pressure rollers. At the same time, it may be stabilized by cooling.

However, during such a coating process, adhesion of the thermoplastic resin material to the cooling roll often occurs, in which case the coated product is subjected to forces primarily at right angles to the coated surface.

This force causes delamination of the coated product, which delamination is not limited to the adhesive surface of the supplied resin layer, but may extend to the substrate if the substrate is paper or other fibrous material.

To overcome these difficulties, we wet the surface of the cooling roll with water or use polytetrafluoroethylene (trade name:
Attempts have been made to coat the rolls with Teflon (Teflon), and the best results have been obtained with Teflon-coated rolls, but even in this case delamination cannot be completely avoided.

Moreover, Teflon cannot withstand long-term mechanical and thermal stresses occurring on the outer surface of the cooling roller.

The present invention provides the following method for coating a substrate with a thermoplastic material.

That is, by extruding a thermoplastic material into a thin film, bringing it into contact with a substrate while still hot, and passing it through a set of rolls together with the substrate while cooling to a temperature below the softening point of the thermoplastic material. Crimp.

At least one of the string rolls that can come into contact with the thermoplastic coating has a critical surface tension of 73 dyne/C.
The surface is moistened with a liquid with a boiling point of 100°C or less, and the liquid must be supplied so that a liquid layer is continuously formed on the entire surface of the roll that comes into contact with the thermoplastic coating. is necessary.

A suitable substrate is paper or paper lined with metal foil.

In the following, the invention will be explained by way of illustrations and photographs.

FIG. 1 is a schematic diagram of an example of a coating apparatus according to the present invention; FIG. 2 is a graph showing how the adhesion between a chill roll and a thermoplastic varies depending on the critical surface tension of the chill roll; FIG. The figure is an enlarged photograph of a cross-section of a coated product with cracked layers.
The figure is an enlarged photograph of a cross section of a coated product manufactured according to the present invention, and FIG. 5 is an enlarged photograph of the outer surface layer of a thermoplastic coating of a coated product manufactured by a conventional coating method using a steel cooling roll. FIG. 6 is an enlarged photograph of the outer surface layer of the thermoplastic coating of a coated product made in accordance with the present invention.

The critical surface tension of a substance is a measure of wettability, and it is said that the lower the critical surface tension, the better the wettability.

The physical unit of critical surface tension is dyne/Cm, and its magnitude can be determined experimentally by many methods well known in the art.

The critical surface tension (measured in vacuum) for pure metals is very high, approximately 1,000-4,000 dyne/Cm
However, under the conditions of coating substrates such as paper with extruded plastic films, the critical surface tension is quite low even for polished metal rollers, around 100
dyne/cm.

By moistening a cleanly polished cooling roller with water, the critical surface tension of the roller can be changed to the critical surface tension of water,
That is, it can be reduced to about 73 dyne 7 cm.

This is because the extruded plastic coating comes into contact only with the film of water covering the roller and not with the metal roller.

When polytetrafluoroethylene (trade name "Teflon") is supplied to a pressure roller, the critical surface tension is approximately 17 dy.
ne 7 cm, ie the critical surface tension of Teflon.

The plastic material used as coating material is usually about 3
For example, polyethylene has a critical surface tension of 29 to 32 d under prevailing humidity, temperature, and other conditions during extrusion.
It has a critical surface tension of yne /CrrL.

By adding a surface tension reducing agent (for example, ethyl alcohol, acetic acid, acetone, etc.) to water, the surface tension can be reduced as shown in the tables below (Tables 1 to 3).

That is, a desired critical surface tension can be obtained by appropriately adding these substances.

In the apparatus shown schematically in FIG. 1, 1 designates a roll 1 which supplies a sheet 2 of paper or paper coated with aluminum foil to be provided with a plastic coating.

A plastic coating material 3 (for example polyethylene) is fed in molten state to the sheet 2 by an extrusion press 4.

In the press 4 the plastic material is heated until melted and extruded through a long slit-like bulb 5 to form a thin film 3.

The sheet 2 covered with a plastic film 3 is fed between a pair of pressure rolls 6 and 7, and the roll 6
is cooled by a coolant, preferably water, circulating therethrough.

The atomized liquid is sprayed by the sprayer 8 so that the entire surface of the roll 6 is preferably covered with a liquid layer of a constant thickness.

Although the components of the liquid are described in detail below, the liquid need not necessarily be supplied by spraying, but by other suitable means,
For example, it may be supplied by dipping a roll or using a moistened guide roller.

The coated product obtained is designated 11.

When the plastic coating material 3 stabilized by cooling passes through the cooling roll 6, the contact between the surface of the cooling roll and the plastic coating material 3 is broken, but there is some adhesion of the plastic coating material to the cooling roll. always occurs, thus creating a tension force perpendicular to the surface of the coated product 11.

When a coated product is composed of, for example, a fibrous material or a fibrous material covered with aluminum foil, it has the lowest tensile strength in the direction perpendicular to its surface (the so-called Z-force direction) and therefore cannot be peeled off from the cooling roll. When coated, the coated product tends to tear, a so-called cleavage phenomenon (layer cracking).

Cleavage generally does not occur at the interface between the plastic coating and the paper, but often occurs within the paper layer close to the plastic coating, as shown in FIG.

FIG. 3 is a 200 times enlarged photograph of the cross section of the coated product with layer cracks, and clear layer cracks can be seen.

In order to avoid delamination, the coated product 11 must be placed on a cooling roll 6.
High 2-strength (tensile strength in the Z-force direction) that does not cause layer cracking despite the stress of removing insects from the surface.
It is necessary to use paper with a high temperature or to minimize the adhesion that occurs between the cooling roll 6 and the coated product 11, but since it is generally difficult to limit the quality and Z-strength of the paper, it is necessary to use paper with a plastic material and a cooling roll. It would be advantageous if the adhesion between 6 and 6 could be reduced, and this can be achieved by the method of the invention.

In the graph of FIG. 2, the horizontal axis shows the adhesive force between the cooling roll and the plastic film in linear notation, and the vertical axis shows the critical surface tension in dyne/cm.

This graph includes a curve labeled Pe that describes the adhesion between a cooling roll coated with coatings of various critical surface tensions and a polyethylene coating.

On the horizontal axis, since the adhesive strength depends on various conditions, such as the diameter of the roll and the width of the sheet, it is not divided by absolute numbers, but by relative numbers from 0 to 10.

As is clear from Figure 2, the adhesive strength of the cooling roll is 10.
It is greatest when made from a dry polished steel roll with a critical surface tension of 0 dyne 76 m (10
).

When this roll is wetted with water, the critical surface tension is approximately 73 dy.
ne /an, and the adhesive strength decreases to 7.

When a Teflon-coated roll is used, the critical surface tension is further reduced to about 17 dyne/Cm, and the adhesive strength is about 5.
becomes.

This shows that even though the critical surface tension decreased from 73 to 17, the adhesion decreased only slightly.

Looking at this experimentally obtained graph, it can be seen that if the surface of the chill roll has a critical surface tension of about 30 dyn6 /CrrL, i.e. comparable to that of polyethylene material, then the minimum adhesion (about 3. 5) can be obtained.

The chill roll is wetted with a liquid whose critical surface tension is suitably adjusted, for example by mixing ethyl alcohol, acetic acid or acetone (other reagents may be used) with water in appropriate proportions.

Since the resulting coated product is used for food packaging, ethyl alcohol is the most suitable additive possible.

From Table 1, the mixture of ethyl alcohol and water of about 35 parts is about 3. It can be seen that Odyne gives a surface tension of 7 cm.

If a mixture of 65% water and 35% ethyl alcohol is fed to the cooling roll 6 in uniform and continuous layers, the adhesion between the cooling roll 6 and the plastic coating of the coated product 11 will be as small as possible (the 3.5) According to the graph in Figure 2, layer cracking can be prevented.

Enlarged cross-section of the coated product treated according to the above findings (
100 times) and shown in FIG.

It can be seen from this drawing that no layer cracking occurred.

Another advantage of the present invention is that the plastic coatings of the present invention have a very uniform surface structure compared to the surface structure of the plastic coatings of coated products produced by conventional methods.

FIG. 5 is an enlarged view of the surface layer of a plastic coating treated with a clean, dry chill roll.

As shown in this drawing, the surface has many depressions, and when the depressions become deep, holes are formed in the plastic coating (so-called pinholes).

Avoiding the formation of pinholes is very important, for example when the coated product is used for packaging sterilized items, and to avoid this the thickness of the plastic coating must be adjusted to fit the depressions formed on the surface of the coating. It shall be sufficient to ensure that the depth does not exceed the thickness of the plastic layer.

Therefore, it goes without saying that a method of forming a plastic film that does not require consideration of the depressions formed is economically very advantageous.

FIG. 6 shows the surface of a plastic coated product produced by the method of the invention.

As shown in the drawings, the structure of the plastic coating treated by the cooling roll supplied with a liquid layer of water and ethyl alcohol is very uniform and has almost no depressions as shown in FIG.

The reason for this improvement in surface structure is not entirely clear, but it is generally known that thermoplastic coating materials with a surface temperature of 1000°C or higher have a boiling point of 78°C when in contact with a chill roll.
It is clear that 56C ethyl alcohol boils.

It is presumed that boiling of the ethyl alcohol forms a thin layer of vapor around the cooling roll, which is one reason for the formation of a uniform surface structure.

As can be seen from the foregoing description, the processing method of the present invention provides a number of important benefits over previously known techniques.

First of all is the reduction of the tension in the Z direction of the coated product, which reduces the risk of delamination.

Secondly, the surface of the plastic coating of the coated product becomes very uniform, which can reduce the thickness of the plastic coating.

This is because the thickness of conventional plastic coatings had to take into account the risk of so-called pinholes.

Of course, the dampening fluid adds additional cost compared to the prior art, but it has proven possible to recover a large portion of the ethyl alcohol for reuse.

Therefore, the price of the dampening fluid is not important compared to the benefits obtained.

Hereinafter, specific embodiments falling within the technical scope of the present invention will be described.

1. A thermoplastic material is extruded into a thin film, brought into contact with a substrate while still hot, and compressed through a set of rolls together with the substrate while cooling below its softening point temperature. Below, boiling point 10
Thermal treatment, characterized in that the entire surface in contact with the film layer of thermoplastic material of at least one roll capable of contacting the plastic coating with a liquid below 0'C is wetted in such a way that a continuous liquid layer is formed. A method of coating a substrate with a plastic material.

2. The method according to 1, wherein the substrate is paper or a sheet of paper lined with metal foil.

3. 1° or 2. where the liquid consists of water to which alcohol has been added. Method described.

4. The method according to 3., wherein the alcohol is ethyl alcohol.

5. The liquid consists of water to which acetone or acetic acid has been added 1.
and 2. Method described.

6. The method according to 1 to 5, wherein the liquid is supplied to the roll by spraying.

7. Installing a tank under the roll, filling the tank with liquid and immersing a part of the roll in the tank to supply the liquid to the roll 1 to 5. Method described.

8. 1 to 5, in which the supply of liquid to the roll is produced by a coating device which can be brought into contact with the roll;
．． Method described.

9. The method described in 1 to 8, wherein the critical surface tension of the liquid is adjusted to or near the critical surface tension of the thermoplastic material by mixing a surface tension reducing agent, such as an alcohol.

10. 1 to 9 in which the thermoplastic substance is made of polyethylene
, the method described.

11. A set of rolls arranged such that a sheet having a thermoplastic layer on one side is obtained therebetween, and the thermoplastic layer of the sheet and the base material are pressed together and cooled at the edge of the rollers passing between them. and a liquid supply device for continuously supplying liquid to the contact surface of the roll with the thermoplastic material layer so that the contact surface is completely wetted. equipment for

12. Apparatus for carrying out the method according to 1, which corresponds substantially to the above description with respect to the drawings.

13. A method for coating a substrate according to item 1, which corresponds substantially to the detailed description above.

14, 10. From 13. A coated substrate obtained by a method or apparatus.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the cladding. In the figure, 1 is a sheet supply roll, 2 is a sheet, 3 is a coating material, 4 is an extrusion press, 5 is a valve, 6 and 7 are pressure rolls, 8 is a sprayer, and 11 is a coating product. FIG. 2 is a graph showing the relationship between the adhesive force between the cooling roller and the plastic film and the critical surface tension. Figure 3 is an enlarged photograph of a cross section of a coated product with cracks, Figure 4 is an enlarged photograph of a cross section of a coated product obtained by the method of the present invention, and Figure 5 is an enlarged photograph of a cross section of a coated product obtained by the conventional method using a steel cooling roll. Enlarged photograph of the outer surface of the plastic coating of the coated product, No. 6
The figure is an enlarged photograph of the outer surface of the plastic coating of the coated product obtained by the present invention.

Claims (1)

[Scope of Claims] 1. When a thermoplastic film is heated, it is passed between rolls together with a base material to be compressed while cooling, in contact with the thermoplastic film having a critical surface tension of 30 dyne, which means a surface temperature of 100°C or higher. The roll surface has substantially the same critical surface tension as the thermoplastic film and a boiling point of 100'C.
A method of coating a substrate with a thermoplastic material, characterized in that it is wetted with a liquid: 2. A sheet having a thermoplastic layer on at least one side is obtained therebetween, and when passing therebetween, the thermoplastic layer of the sheet and the substrate come into contact with the set of rolls and at least the thermoplastic layer of the roll. A method for coating a substrate with a thermoplastic material according to claim 1, comprising disposing a liquid supply device that continuously supplies liquid to the interface so that the contact surface is completely wetted. Equipment for implementation.