WO1997012703A1 - Method for making longer-lasting composite metal/plastic packagings - Google Patents

Abstract

A method for making metal containers by supplying a strip of multilayer material including a metal layer coated on one side with a thermoplastic film, shaping the strip, and heat treating the resulting shaped container, wherein (a) a film with an outer polyolefin layer is selected as the thermoplastic film, and (b) said container is heat treated at a temperature and for a time that are determined by any method enabling damage to said polyolefin layer to be determined.

Description

METHOD FOR MANUFACTURING METALLOPLASTIC PACKAGES WITH INCREASED LIFE

FIELD OF THE INVENTION

The invention relates to the manufacture of metalloplastic packaging and more particularly to packaging internally coated with a polyolefin layer and to methods for improving the service life of metalloplastic packaging.

STATE OF THE ART

European Patent No. 0 404 420 B1 describes a process for the manufacture of packages which are deep-drawn containers internally covered with a protective thermoplastic film, a process which aims in particular to improve the resistance to corrosion.

The means described to obtain this improvement consist of a heat treatment of deep drawn containers. This heat treatment takes place at a temperature higher than that of the glass transition of said film, but at a temperature below the melting temperature of said film.

Furthermore, this European patent mentions a prior art showing a heat treatment of containers at a temperature close to the melting temperature of said film.

PROBLEM

The Applicant has studied deep-drawn metalloplastic packaging in the particular case where the thermoplastic film is a polyolefin, typically PP. There is an ever increasing demand for packaging with improved performance with regard to corrosion resistance, that is to say ultimately the service life of the product packaged in this packaging, or the possibility of conditioning aggressive products, that is to say relatively rich in salt and / or acid.

The Applicant having observed that the solutions known from the state of the art did not provide a completely satisfactory solution as regards resistance to corrosion, whether this is expressed in the form of a lifetime or of 'a limited salt and / or acid content, sought a method to achieve this objective.

DESCRIPTION OF THE INVENTION

According to the invention, the method of manufacturing metal containers comprises supplying a strip of multilayer material comprising a metal layer coated at least on one face with a thermoplastic film corresponding to the interior surface of said container, shaping of said strip by at least one stamping step, the heat treatment of the container obtained after shaping, and is characterized in that, a) a film comprising externally a layer of polyolefin is chosen as the thermoplastic film, b) it is treated thermally said container at a temperature and for a time which are determined by any method making it possible to assess the damage to said polyolefin layer.

One possible method for evaluating said damage is impedance measurement.

This method is described in Matériaux & Techniques N ^β l-2 1994, page 59 ("Use of impedancemetry in the packaging industry - by MM. Gimenez, Frichet and Le

Talludec). It allows a rapid evaluation of the corrosion of metallic packaging in the presence of their content. According to a person skilled in the art, this corrosion is directly linked to the damage to the protective layers (films, varnishes) of the metal layer.

Another method for evaluating said damage consists in using X-rays, in particular by scattering X-rays at small angles. This method makes it possible to highlight the presence of microvides in the film of thermoplastic material following said shaping, and to follow the evolution of these microvides during a heat treatment.

Preferably, the temperature T ( ^β C) corresponding to the melting peak of the polyolefin is determined by differential scanning calorimetry (Differential Scanning Calorimetry≈DSC), and said container is heat treated at a temperature between T-5 and T + 10, so as to reduce the damage suffered by said film during said shaping.

The temperature limits according to the invention (between T-5 and T + 10 ^β C) result from the study of the restoration of the protective layer as a function of the temperature. Preferably, the heat treatment takes place between T and T + 10 ^β C, so as to reduce the duration of the heat treatment.

For a temperature below T-5 ° C, the heat treatment is long and ineffective. Beyond T + 10 ^β C, the packaging is unnecessarily heated, with all the risks of degradation that may occur.

Thus, the invention provides a solution, both general, rigorous and precise to all the problems of damage to films or varnishes which protect the metal layer packaging, since it offers both the means to act to, if not restore the initial protective layer, at least reduce its damage, by carrying out a heat treatment in a very precise temperature range, and also a means quick to assess the effects of said heat treatment.

DESCRIPTION OF THE FIGURES

Figure 1 shows a typical curve of a PP film in DSC. The temperature is plotted on the abscissa and the enthalpy variation on the ordinate.

The abscissa corresponding to the optimum of the curve determines the temperature T.

FIGS. 2a and 2b represent X-ray scattering diagrams at small angles, with the angles on the abscissa and a function of the intensity scattered on the ordinate. Diagram 2a shows a curve A relating to the thermoplastic film after said shaping, and before said heat treatment, and a curve B relating to the same film but soaked in an oil having the density of PP and blocking the microvides formed during said shaping in shape. Diagram 2b reproduces curve A and shows a curve C relating to the thermoplastic film after heat treatment. It also shows a curve D relative to the initial thermoplastic film.

FIG. 3 is an impedancemetry diagram giving the evolution of Log RBF ( ^{on the} ordinate) as a function of time (in days, on the abscissa) for two tests: A according to the invention and B as a control test.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, after said heat treatment, said the container can preferably be cooled to room temperature by quenching in cold water.

This provides a constant and reproducible heat treatment.

Said polyolefin can be chosen from PE, PP, PE-PP copolymers, polyolefins comprising polybutene.

Said thermoplastic film may be a bilayer film constituted by said polyolefin layer, and an adhesive layer based on polyolefin provided with acid groups allowing the adhesion of said polyolefin layer to said metallic layer.

Said metallic layer can be chosen from aluminum and aluminum alloys, iron alloys.

Said metal layer can be coated on one side with a varnish or with a thermoplastic film corresponding to the outer surface of said container,

Another object of the invention is the application of the method according to the invention for the manufacture of bodies of metal boxes or lids coated at least on one face with a film of polyolefin-based material.

EXAMPLES OF REALIZATION

Aluminum bodies of medium 1/2 format (height ≈ 85.6 mm, diameter = 83 mm) were made of aluminum alloy

5052 (standardized designation of the Aluminum Association) of

0.23 mm thick.

The interior coating consisted of a two-layer film of

20 μm of PP and 5 μm of OREVAC C (acid copolymer of PP from the company ATOCHEM) serving as adhesive for PP on the aluminum strip. The external covering was constituted by a standard varnish.

These boxes were obtained by stamping corresponding multilayer formats, themselves formed by heat-bonding the PP / OREVAC bilayer film on the Al strip externally varnished.

The boxes directly obtained by stamping constitute the control test (B).

For the implementation of the method according to the invention, the melting temperature of the PP layer was determined by DSC.

A temperature T of 160 ^β C, corresponding to the optimum of the DSC curve was found.

Different heat treatment tests were carried out at different temperatures (T-20, T, T + 20) and for different times (5 s, 5 min) by circulating the boxes in an induction furnace, these times being those during which the boxes are kept at the chosen temperature.

The damage was followed by impedancemetry and by X-ray scattering at small angles.

I - Results by impedance measurement:

In impedance measurement, the low frequency resistance RBF ^is inversely proportional to the corrosion rate and the degraded surface. The control content was chosen from among the most aggressive products to be packaged: vinegar carrots. Figure 3 shows a typical evolution of RBF over a month.

It seems, according to the applicant's studies, that RβF-1, value of R _BF after 1 day should be considered if we consider the microporosity of the films, corresponding to 1 damage resulting in particular from the shaping, while 1 evolution over time incorporates other elements such as the permeability of the film, or possible decohesion of the film and the aluminum strip.

Results obtained for Ro, .r, —X ... _•

* Tests A with heat treatment:

Duration: t = 5 s 5 min Temperature = 160 ^β C 8 7.6 140 ^β C 6.1 6.5

180 ^β C 7.2 7

* Test B without heat treatment: 5.6

II - Results obtained by X-ray scattering

To examine samples of thermolastic film at the X-rays, portions of boxes were demetallized and portions of corresponding films obtained.

FIG. 2a illustrates the diffusion value in the case of a damaged film, before heat treatment (curve A), and in the case of the same film after filling of the microvides with an oil of the same density (curve B). Curve B therefore serves as a baseline and curve A with a value equal to 100 (taken from the maximum of the curve).

After a heat treatment, the experimental curve (curve C in FIG. 2b) makes it possible to define a "damage value" between 0 (absence of microvides) or 100 (no reduction in microvides). Values obtained:

Duration: t ≈ 5 s 5 min Temperature = 160 ^β C 2-3 1-2 140 ° C 65 32 180 ^β C 3-5 4-6 ADVANTAGES OF THE INVENTION

The invention provides an extremely valuable technical lesson to reduce the damage suffered by the thermoplastic film during shaping.

The invention has on the one hand shown the advantage of a specific heat treatment after the packaging (boxes or lids) has been shaped, but above all, on the other hand, has defined the technical tools which allow rapid correlation of choice of parameters of the heat treatment, with the reduction of the damage, and this, in general, especially for all polyolefin films.

Claims

1 - Method for manufacturing metal containers comprising supplying a strip of multilayer material comprising a metal layer coated at least on one face with a thermoplastic film corresponding to the interior surface of said container, shaping said strip by at least one stamping step, the heat treatment of the container obtained after shaping, characterized in that, a) a film comprising externally a layer of polyolefin is chosen as the thermoplastic film, b) said container is heat treated at a temperature and for a time which are determined by any method making it possible to assess the damage to said polyolefin layer. 2 - Method according to claim 1 wherein said damage is evaluated by impedancemetry. 3 - Method according to claim 1 wherein said damage is evaluated by X-rays. 4 - Method according to any one of claims 1 to 3 wherein it is determined by differential scanning calorimetry (Differential Scanning Calorimetry * DSC) the temperature T ( ^β C) corresponding to the melting peak of the polyolefin, and a heat treatment temperature between T-5 and T + 10 is chosen, so as to reduce the damage suffered by said film during said shaping. 5 - Process according to any one of claims 1 to 4 wherein said container is cooled to room temperature by quenching in cold water. 6 - Process according to any one of claims 1 to 5 wherein said polyolefin is chosen from PE, PP, PE-PP copolymers, polyolefins comprising polybutene. 7 - Process according to any one of claims 1 to 6 wherein said thermoplastic film is a bilayer film constituted by said polyolefin layer, and an adhesive layer based on polyolefin provided with acid groups allowing the adhesion of said polyolefin layer to said metallic layer. 8 - Process according to any one of claims 1 to 7 wherein the metal constituting said metal layer is chosen from aluminum and aluminum alloys, iron alloys. 9 - Method according to any one of claims 1 to 8 wherein said metal layer is coated on one side with a varnish or with a thermoplastic film corresponding to the outer surface of said container, 10 - Application of the method according to any one of claims 1 to 9 for the manufacture of bodies of boxes or metal covers coated at least on one side with a film of polyolefin-based material.