WO2003099586A1

WO2003099586A1 - A process for the preparation of partially metallised and printed plastic films

Info

Publication number: WO2003099586A1
Application number: PCT/EP2003/005553
Authority: WO
Inventors: Roberto Scrima
Original assignee: Mipa S.P.A.
Priority date: 2002-05-28
Filing date: 2003-05-27
Publication date: 2003-12-04
Also published as: ITMI20021150A0; ITMI20021150A1; AU2003242582A1; EP1509406A1

Abstract

The present invention relates to a process for the preparation of partially metallised and printed films and to the films thus obtained, said process comprising the application of a hydrosoluble varnish over the whole surface of a plastic film, the metallisation of the film, the printing of the film with water-resistant inks and the washing of the film with water with the removal of the hydrosoluble varnish and of the metal deposited on the varnish itself.

Description

A PROCESS FOR THE PREPARATION OF PARTIALLY METALLISED AND PRINTED PLASTIC FILMS

PRIOR ART

The deposition of a thin layer of aluminium onto plastic films (metallisation) is a known process and is used both to improve the barrier properties of the plastic films and to confer a particular optical appearance to the films and to the printed wrappers made with the above mentioned films.

In certain cases it is desirable to limit the metallisation to particular areas of the film and, in such a case, the metallisation process can be carried out after having applied on specific areas a detaching resin which, at the end of the process, is removed thus obtaining the partially metallised film (JP 59109529; AN 1984-

192378(31 )WPINDEX).

Another common method consists in the use of shaped templates which, by blocking the passage of the metal during the process of metallisation under vacuum, limit the deposition of the metal onto the areas of interest.

In certain cases it is necessary to produce metallised and printed wrappers with printing only on the metallic parts of the film in which the printing covers only certain, well defined areas of the film, for example when one wishes to leave a transparent window on the wrapper to allow the viewing of the contents. A system used to date for this aim consists in the zonal printing of a metallised film with materials resistant to chemical attack, and in the successive removal of the metal from the non protected areas by treatment with aqueous solutions containing acids or alkalis.

Examples of this process are found in patents CA 385735, US 4552614, BR 8800032.

This process has however certain disadvantages which notably limit its applications.

For example, the process, when carried out at high speed, does not guarantee the homogeneity of the product; in addition the process requires the heating of the washing solution to a temperature of 60-100°C in order to allow to reduce the concentration of chemical agents (for example NaOH) and, at the same time, to maintain the same solubilising capacity. The process provides the use of inks resistant to the aggressive chemicals, in particular inks containing pigments with such characteristics; the use of these inks consequently limits graphic possibilities and the achievable tonalities. Of no less importance is the fact that the process requires particular attention during the washing step to protect both the various parts of the machine from the chemical agents and the individuals carrying out the process; in addition careful disposal of the chemical agents is required after neutralisation . Finally, due to the aggressivity of the chemical agents both on the inks and on the plastic film, the process produces wrappers of insufficient qualitative level from the point of view of both the final print quality and the transparency of the non printed area. In particular, the process is not applicable onto lacquered films due to their sensitivity to chemical agents. SUMMARY It has now been found that when using a hydrosoluble varnish applied over the whole surface of plastic films prior to metallisation, by metallising, printing with inks resistant to water and later washing with water, it is possible to obtain partially metallised and printed films having an extremely high graphical quality and an exceptional transparency of the plastic film, which are not achievable with the methods known to date. Said process is in addition easy and safe to apply and can be carried out at high speed using water at room temperature. DESCRIPTION OF THE INVENTION

The invention relates to a process for the preparation of partially metallised and printed plastic film and to the monofilm and coupled wrappers obtained from it. Examples of plastic films usable in the present invention comprise polypropylene, bioriented, co-extruded or lacquered polypropylene, polyesters and polyamides, preferably polypropylene and more preferably lacquered polypropylene. The process comprises the following steps:

A) a hydrosoluble varnish is applied over the whole surface of a plastic film using a rotogravure printing machine;

B) the film is metallised under vacuum, depositing a thin layer of aluminium over the whole surface of the varnished side of the film; C) the metallised film is printed with water-resistant inks using the rotogravure machine;

D) the film thus obtained is then washed with water so as to remove the hydrosoluble varnish and, with it, the metal deposited onto it, from the areas not protected by the inks.

The process of the present invention allows to obtain printed wrappers having the characteristic of bearing a metallic layer under the inks and, at the same time, transparent areas which allow the viewing of the contents of the package.

The printed areas have particularly brilliant colours and exceptional metallic appearance, producing therefore graphics of a qualitative level never achieved with similar techniques known to date.

In addition, the transparent areas have a light transmission level never achieved with the processes known to date, particularly in the case of lacquered plastic films. The hydrosoluble varnish used in step A) is preferably a varnish containing polyvinylpyrrolidone in an alcoholic solvent, having a dry residue comprised between 32 and 36 % and a viscosity comprised between 52" and 58" (#4 Ford cup).

The alcoholic solvent can be methanol, ethanol or isopropanol, preferably ethanol. The varnish is adjusted to the viscosity necessary for the printing process by dilution with a low molecular weight alcohol such as methanol, ethanol or isopropanol, preferably ethanol.

The viscosity of the diluted product can vary between 14" and 22" (#4 Ford cup), preferably between 16" and 20", in particular between 17" and 19". The varnish is applied at a machine speed comprised between 100 and 150 metres/minute and with a drying temperature comprised between 80 and 110°C;

The application takes place, preferably, at a speed comprised between 120 and

130 metres/minute with a drying temperature comprised of between 95 and 105°C.

The amount of varnish deposited onto the film can vary between 0.5 and 2.0 g/m², preferably between 0.5 and 1.4 g/m², more preferably between 0.5 and 1.1 g/m².

Operating under the above indicated conditions it is possible, in a particular way, to apply the varnish directly even onto lacquered polypropylene film with thermowelding acrylic lacquers, avoiding the well known phenomenon of the attacking of the lacquer by the alcoholic solvents.

Such attacking, in addition to compromising the welding property of the lacquer, compromises the optical property of the film, reducing its transparency. A particularly preferred subject of the present invention is hence the process according to steps A), B),C) and D) applied onto lacquered polypropylene film as well as the simple or coupled film thus obtained.

The metallising process of step B) can be carried out according to the known plastic film metallising processes so as to obtain a metallic layer having optical density comprised between 1.2 and 2.0, preferably between 1.4 and 1.8 (Theimer Type DDM 4).

The inks used in step C) are nitrocellulosic type, inks for rotoflexographic use suited to the printing of metallised films without the use of primers and having high resistance to water; the inks can be added to by antiscouring or antiscratch additives to improve their resistance during the washing process.

The inks have a dry residue varying between 30 and 60 % according to the pigment bases used and are dilutable with ethyl acetate or mixtures of ethyl acetate and ethyl alcohol. The amount of ink applied onto the metallised film can vary from 1.5 to 4.5 g/m² depending on the graphic to be realised.

The application of said inks is carried out according to known printing- technologies.

The washing with water of step D) is carried out in a special machine which allows the removal of the hydrosoluble varnish and the metal deposited on it from the areas not protected by the inks; the washing process can be carried out at a speed comprised between 80 and 150 metres/minute and the drying of the washed film at a temperature comprised between 80 and 120 °C.

The washing is preferably carried out at a speed comprised between 100 and 130 metres/minute and with a drying temperature comprised between 90 and 110 °C. The film thus obtained can be used as it is as wrapper for containers, for example such as external wrapper for food containers, or as it is as wrapper in direct contact with food; or can be used following coupling with one or more plastic films. The coupling can be carried out both through the known technologies using solvent based adhesives and through the known technologies based on solvent free adhesives.

The single film or the coupled wrapper can then be subjected to further treatments used in the packaging sector, such as, for example, the application of welding lacquers or "release" varnishes.

Examples of plastic films which can be used for coupling with partially metallised and printed films are polypropylene, bioriented, co-extruded or lacquered polypropylene, polyester and polyethylene, preferably co-extruded or lacquered polypropylene and polyethylene.

Some examples of the process according to the invention are reported as follows.

Such examples have only the aim of illustration and do not imply any limitation.

EXAMPLE 1

Preparation of a partially metallised and printed lacquered polypropylene film A) A polyvinylpyrrolidone based hydrosoluble varnish is applied over the whole surface of a polypropylene film lacquered with acrylic lacquer having a thickness of 32 micrometers.

The varnish is applied after dilution with ethyl alcohol so as to obtain a viscosity of 18" (#4 Ford cup).

The varnish is applied with a drying temperature varying between 95 and 105 °C so as to deposit onto the film a quantity of 1.0 gram per square meter of surface area.

B) The film thus obtained is metallised under vacuum with aluminium in such a quantity as to obtain an optical density value equal to 1.8

(Theimer Type DDM 4).

C) The film is then printed with nitrocellulosic type water-resistant inks so as to obtain a design such as to cover approx. 30% of the surface of the film; 7 different colours of ink are used with an overall quantity equal to 4.0 g per square meter of printed surface.

E) The film is then washed in a water bath in a machine fitted with rollers for the passage of the film at a speed of 120 m/min and with brushes to favour the complete removal of the hydrosoluble varnish and of the metal deposited onto it from the area not protected by the inks.

F) The film is then dried at a temperature varying between 90 and 110 °C.

The plastic film thus obtained has an area printed with a design covering approx. 30% of the surface of the film. In this area the inks are particularly brilliant and have a pronounced metallic appearance due to the presence of the underlying thin layer of aluminium.

The remaining non-printed area of the film is perfectly transparent without any trace of the metal previously deposited. The light transmission of the film (Haze 1.2% - ASTM D 1003) is substantially equivalent to the light transmission of the film prior to the process (Haze 1.0 % - ASTM D1003). This indicates that the process, despite the use of ethyl alcohol, is carried out in such a manner so as not to damage the lacquered surface of the original film.

The analyses reported below refer to the technical parameters of the film obtained, which establish the machinability on packing machines or the organolectic properties, and are all in agreement with the required standards.

Film/film slippage (Coefficient of friction) = 0.25 (ASTM D 1894-e)

Weldability (seal strength) = 550 g/25mm (130°C; 2.5 bar; 0.8 sec)

Solvent retention = 7.0 g/m² (UNI U 59.0B.162.O) EXAMPLE 2

Preparation of a partially metallised and printed coextruded polypropylene film

A coextruded polypropylene film with a thickness of 30 microns is subjected to the process as described in Example 1.

A printed film is obtained with a metallic effect of the inks and with the following characteristics:

Haze (transparent area) 1.3% (ASTM D1003)

Film/film slippage (COF) 0.32 (ASTM D 1894-e)

Weldability (seal strength) 455 g/25mm (130°C; 2.5 bar; 0.8 sec) -

Solvent retention 8.7 mg/m² (UNI U59.0B.162.0)

Claims

1. A process for the preparation of partially metallised and printed plastic films comprising the following steps:

B) the film is metallised under vacuum, "depositing a thin film of aluminium over the whole surface of the varnished side of the film;

C) the metallised film is printed with water resistant inks using a rotogravure machine; D) the film thus obtained is then washed with water so as to remove the hydrosoluble varnish and, with it, the metal deposited onto it, from the area not protected by the inks.

2. The process according to claim 1, characterised by the fact that the hydrosoluble varnish used in step A) is a varnish containing polyvinylpyrrolidone in an alcoholic solvent, having viscosity comprised of between 14" and 22" (#4 Ford cup).

3. The process according to claim 2, characterised by the fact that said alcoholic solvent is selected from the group comprising methanol, ethanol or isopropanol.

4. The process according to claim 1, characterised by the fact that said plastic films are selected from the group comprising polypropylene, bioriented,

^" coextruded or lacquered polypropylene, polyester and polyamide.

5. The process according to claim 4, characterised by the fact that said plastic film is lacquered polypropylene.

6. The process according to claim 1, characterised by the fact that the application of the varnish in step A) is carried out at a speed comprised between 100 and

150 metres/minute and with a drying temperature comprised between 80 and 110°C.

7. The process according to claim 6, characterised by the fact that the application of the varnish of step A) is carried out at a speed comprised between 120 and 130 metres/minute and with a drying temperature comprised between 95 and 105°C.

8. The process according to claim 1 , characterised by the fact that the application of the varnish of step A) is carried out with a quantity of varnish comprised between 0.5 and 2.0 g/m².

9. The process according to claim 8, characterised by the fact that the application of the varnish of step A) is carried out with a quantity of varnish comprised between 0.5 and 1.1 g/m².

10. The process according to claim 1 , characterised by the fact that the metallisation of step B) is carried out so as to obtain a metallic layer having an optical density comprised between 1.2 and 2.0 (Theimer Type DDM 4)

11. The process according to claim 10, characterised by the fact that the metallisation of step B) is carried out so as to obtain a metallic layer having an optical.density comprised between 1.4 and 1.8 (Theimer Type DDM4)

12. The process according to claim 1 , characterised by the fact that the inks used in step C) are nitrocellulosic type inks for rotoflexographic use, suited for the printing of metallised films without the use of primers and having a high resistance to water.

13. The process according to claim 1 , characterised by the fact that the quantity of ink applied to the metallised film is comprised between 1.5 and 4.5 g/m².

14. The process according to claim 1 , characterised by the fact that the washing of step D) is carried out in a machine at a speed comprised between 80 and 150 metres per minute and with the drying of the washed film at a temperature comprised between 80 and 120°C.

15. The process according to claim 14, characterised by the fact that the washing of step D) is carried out in a machine at a speed comprised between 100 and 130 metres per minute and with the drying of the washed film at a temperature comprised between 90 and 110°C.

16. Partially metallised and printed plastic films, characterised by the fact that the print is obtained with water resistant inks applied onto a metallised layer and on a hydrosoluble varnish in direct contact with the plastic film.

17. The partially metallised and printed plastic films obtained by the process of claim 1.

18. The use of the plastic films according to claim 16, optionally coupled to other plastic films, for the preparation of wrappers for containers or of wrappers in direct contact with food.

19. The use according to claim 18 characterised by the fact that said other plastic films for coupling are selected from the group comprising polypropylene, bioriented, co-extruded or lacquered polypropylene, polyester and polyethylene.