GB1601498A - Adhesive skin film - Google Patents

Description

(54) ADHESIVE SKIN FILM (71) We, GRANGES WEDA AB, of 194 OL Upplands Vasby, Sweden, a Swedish Company, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The resent invention relates to a new material for packaging purposes according to the so-called skin pack method, the material consisting of a co-extrudate of two or more different plastics materials.

The skin pack method involves packaging an object with a plastics film, the film being heated by radiant heat and, in a heated condition, sucked by vacuum onto a gaspermeable substrate and bonded to said substrate, said object thus being enclosed by a transparent plastics film on a supporting substrate, usually of cardboard and provided with pores or holes allowing the passage of gases.

Up to now various films have been used, such as polyvinyl chloride, ionomeric polymers, ethylene ethylacrylate copolymers, and coextrudates of such plastics. If polyvinylchloride film is used, the supporting cardboard must be coated with a plastics or size on top of any possible printing to obtain sufficient adhesion.

Although this procedure means that the substrate can be provided with printing, the necessary coating of the cardboard with plastics or size then required before skin packing is an extra step creating extra costs and to some extent a deterioration in appearance of the finished package.

In the use of commercially available films for skin packaging with colour-printed cardboard, it has been found to be a difficult problem that only letterpress printed cardboard using water-based glycol diluted pigments can be satisfactorily coated with skin film, while for cardboard printed by other methods it has been necessary to coat the printed surface with an adhesive film or the like. Dirt and grease spots have also caused difficulties in the skin pack method.

The present invention provides a skin film for packaging purposes, which on heating can adhere to printed surfaces while air is extracted from between the film and the surface to which it is being applied, the film comprising a 5-300 ,um thick coextruded.

material in two layers, of which a first layer is a base layer of a polyolefin or polyvinylchloride constituting 10-50% of the total thickness of the material, the second layer constituting the other 5090% of the thickness of the material and comprising a copolymerisate of ethylene with 7-20 wt. % of vinyl acetate, acrylate acid or an acrylic acid ester, more than 90% of the exposed surface of the second layer being treated by a surface oxidation to increase the polar character of the surface and impart a surface tension greater than 40 kN/m.

The coextruded film thus consists of a base layer of polyolefin or polyvinylchloride, the polyolefin suitably consisting of polyethylene or polypropylene. A polyolefin with a density of over 925 kg/m3, preferably over 930 kg/m3 is especially suitable. The coextrudate is preferably cooled in such a way that low crystallinity is obtained in the material produced. If another polyolefin is used, the stated characteristics of the polyethylene can serve as guidance for one skilled in the art when selecting a polyolefin with suitable characteristics. When using polyvinylchloride, one skilled in the art can by selecting the type and amount of plasticizer, obtain suitable characteristics in a similar way with guidance from the known characteristics of the preferred polyethene.

On top of the base layer, there is a film comprisin a copolymer of ethylene and 7-20 wot00 of one or more of the monomers vinylacetate, acrylic acid or an acrylic acid ester, ethylacrylate or butylacrylate, the use of about 17% vinylacetate being preferred here.

Finally, the copolymerized film is surface treated over more than 90% of the film surface to obtain a surface tension corresponding to a value greater than 40 kN/m, preferably at least 42 kN/m and more especially 47 kN/m or more. This can be done by corona treatment or by other methods involving surface oxidation to obtain a more polar surface character. This can be done physically as well as chemically.

Coextrusion of the film can take place in ways known per se. Thus the film can be produced by blown film extrusion or chill roll extrusion. The technique of coextrusion is well known to one skilled in the art and with ordinary experience he can provide a co-extruded film according to the definition set forth in the following claims.

Another large advantage with the new material in the film is that the film can then be rolled up and unrolled from storage rolls without undesirable adhesion between the layers in the roll. Furthermore, the film has the exceptionally advantageous property of good absorption of long-wave radiation.

This results in quick heating in the skin packing machine and thus to an increase in production for a given installation.

The new film has very good optical properties and provides clearly transparent packages. The film has excellent shrinking properties in skinpacking and in the packages obtained the film has good elasticity and strength, even for low temperatures. Its stiffness makes it suitable for good handling in skin packing machines.

Example A film with a thickness of 100 ,um was produced and consisted of a polyethene having a density of 934 kg/m3 with a thickness of about 30,am and a copolymerisate of ethylene with 17 wt% vinyl acetate and having a density of 937 kg/m3.

The copolymerisate surface was corona treated to a surface tension of 47 kN/m. for 97% of the film surface, an area along the edges being left untreated. Exceptionally good adhesion on cardboard printed in colour with standard offset pigments was obtained with the film produced in this way.

WHAT WE CLAIM IS: 1. A skin film for packaging purposes, which on heating can adhere to printed surfaces while air is extracted from between the film and the surface to which it is being applied, the film comprising a 50300,um thick coextruded material in two layers, of which a first layer is a base layer of a polyolefin or polyvinylchloride constituting l0-50V0 of the total thickness of the material, the second layer constituting the other 5090% of the thickness of the material and comprising a copolymerisate of ethylene with 7-20 wt. % of vinyl acetate, acrylic acid or an acrylic acid ester, more than 90% of the exposed surface of the second layer being treated by a surface oxidation to increase the polar character of the surface and impart a surface tension greater than 40 kN/m.

2. A skin film according to Claim I wherein the said acrylic acid ester is ethyl acrylate or butyl acrylate.

3. A skin film according to Claim 1 or Claim 2 wherein the base layer comprises polyethylene with a density greater than 925 kg/m3 4. A skin film according to Claim 3 wherein the density of the said polyethylene is greater than 934 kg/m3.

5. A skin film according to any preceding claim wherein the surface of the copolymerisate is corona treated to a surface tension of at least 42 kN/m.

6. A skin film according to Claim 5 wherein the surface of the copolymerisate is corona treated to a surface tension of at least 47 kN/m.

7. A skin film according to any preceding claim which is produced by blown film extrusion.

8. A skin film according to any one of Claims 1 to 6 which is produced by chill roll extrusion.

9. A skin film according to Claim I substantially as herein described and exemplified.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. corresponding to a value greater than 40 kN/m, preferably at least 42 kN/m and more especially 47 kN/m or more. This can be done by corona treatment or by other methods involving surface oxidation to obtain a more polar surface character. This can be done physically as well as chemically. Coextrusion of the film can take place in ways known per se. Thus the film can be produced by blown film extrusion or chill roll extrusion. The technique of coextrusion is well known to one skilled in the art and with ordinary experience he can provide a co-extruded film according to the definition set forth in the following claims. Another large advantage with the new material in the film is that the film can then be rolled up and unrolled from storage rolls without undesirable adhesion between the layers in the roll. Furthermore, the film has the exceptionally advantageous property of good absorption of long-wave radiation. This results in quick heating in the skin packing machine and thus to an increase in production for a given installation. The new film has very good optical properties and provides clearly transparent packages. The film has excellent shrinking properties in skinpacking and in the packages obtained the film has good elasticity and strength, even for low temperatures. Its stiffness makes it suitable for good handling in skin packing machines. Example A film with a thickness of 100 ,um was produced and consisted of a polyethene having a density of 934 kg/m3 with a thickness of about 30,am and a copolymerisate of ethylene with 17 wt% vinyl acetate and having a density of 937 kg/m3. The copolymerisate surface was corona treated to a surface tension of 47 kN/m. for 97% of the film surface, an area along the edges being left untreated. Exceptionally good adhesion on cardboard printed in colour with standard offset pigments was obtained with the film produced in this way. WHAT WE CLAIM IS:

1. A skin film for packaging purposes, which on heating can adhere to printed surfaces while air is extracted from between the film and the surface to which it is being applied, the film comprising a 50300,um thick coextruded material in two layers, of which a first layer is a base layer of a polyolefin or polyvinylchloride constituting l0-50V0 of the total thickness of the material, the second layer constituting the other 5090% of the thickness of the material and comprising a copolymerisate of ethylene with 7-20 wt. % of vinyl acetate, acrylic acid or an acrylic acid ester, more than 90% of the exposed surface of the second layer being treated by a surface oxidation to increase the polar character of the surface and impart a surface tension greater than 40 kN/m.

2. A skin film according to Claim I wherein the said acrylic acid ester is ethyl acrylate or butyl acrylate.

3. A skin film according to Claim 1 or Claim 2 wherein the base layer comprises polyethylene with a density greater than 925 kg/m3

4. A skin film according to Claim 3 wherein the density of the said polyethylene is greater than 934 kg/m3.

5. A skin film according to any preceding claim wherein the surface of the copolymerisate is corona treated to a surface tension of at least 42 kN/m.

6. A skin film according to Claim 5 wherein the surface of the copolymerisate is corona treated to a surface tension of at least 47 kN/m.

7. A skin film according to any preceding claim which is produced by blown film extrusion.

8. A skin film according to any one of Claims 1 to 6 which is produced by chill roll extrusion.

9. A skin film according to Claim I substantially as herein described and exemplified.