Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LAMINATED MILD STEEL STRIP
This invention relates to laminated mild steel strip for use especially, but
not exclusively, in the packaging industry and to methods of manufacturing
such strip. More especially, the invention relates to a method of chemically
treating mild steel strip prior to lamination with a thermoplastic material.
Organic-coated metal substrates, for example thermoplastic resin-coated
tinplate or blackplate, are used, inter alia, in the production of packaging
materials, for example, food and beverage cans. As a result, organic coatings
so used, are required to conform with strict performance criteria. To maintain
the integrity of a can as well as to ensure that its contents are maintained
in a
suitable condition over a storage period which may span months or even longer,
the coating must exhibit good stain resistance, corrosion resistance and
resistance to delamination.
GB-A-2329608A discloses a process for producing thermoplastic resin-
coated aluminium aAoy plate in which the plate is treated sequentially with
alkali
and acid solutions to put the plate surfaces in such a condition that the
increase
rate of the specific surface area is 3 to 309. The treated plate is then
subjected to an anodic oxidation treatment prior to lamination with
thermoplastic resin.
Organic coatings have traditionally comprised solvent or water-based
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lacquers. Recently however, the use of laminated polymer films and coatings,
such as thermoplastic resins, has been recognised as a viable alternative.
In practice, organic coatings are not applied directly onto mild steel
(otherwise known as blackplate), because for packaging applications the metal
surface is too reactive and underfilm corrosion can spread rapidly. Instead,
the
can-making industry uses metallic-coated mild steels, such as tinplate or
electrolytically chromium-coated steel (ECCS) as substrates for organic
coatings.
Currently, a surface-treated mild steel strip may comprise a chrome/CrOX
or tin layer electrochemically deposited so that the final substrate has
either a
metallic chromium layer of typically from 50 to 150 mg/m2 and a chromium
oxide/hydroxide layer of typically from 10 to 30 mg/m2, or a layer of metallic
tin of typically between 5 and 10 g/m2. in many applications it is preferred
that
tinplate is additionally subjected to chromate solution treatment, the amount
of
oxidisable chromium being between 1 and 10 mg/m2.
Unfortunately, electro-plating pre-treatment is a costly and time
consuming process. Not only are materials expensive, but the electro-plating
process itself consumes large quantities of energy. In addition, this
conventional pre-treatment adds an additional production step in the process
line, which adds costs in terms of line-time, manpower and through yield.
It has been shown that for some applications, the degree of protection
afforded by the ECCS or tin pre-treatment exceeds the performance
requirements of the can. For this reason and the disadvantages associated with
electroplating discussed above, there is an increasing desire to develop an
alternative metal strip pre-treatment which avoids these problems but
maintains
the performance requirements of certain classes of food, beverage or aerosol
cans. Preferably, any such pre-treatment should be capable of application
under
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the present day metal strip coating and lamination fine conditions.
In the past, there has been a general understanding in the industry that
alternatives to electro-deposited tin and/or chrome would afford significantly
less substrate protection. However, if a suitable alternative pre-treatment
could
be found, an electroplating process step would be unnecessary with consequent
increases in yield, savings in energy and reductions in the overall production
costs of laminated metal strip.
It is an object of the present invention to provide a suitable alternative to
conventional electroplating of metal strip prior to coating with an organic
resin,
which provides adequate corrosion protection of the organically coated metal
strip and provides and maintains good adhesion to such organic resin coatings.
According to the present invention in one aspect, there is provided a
process for manufacturing laminated mild steel strip, the process comprising
the
steps of,
(a) cleaning the strip;
(b) chemically pre-treating the cleaned strip to form on one or each of its
surfaces a non-metallic chemical coating of an oxyanion to resist
corrosion of the underlying mild steel substrate and to promote adhesion
to a subsequently applied layer; and,
(c) applying to the chemically-treated strip a coating of thermoplastic resin
to form a protective layer on at least one surface thereof.
The term "non-metallic coatings" as used herein refers to coatings which
despite optionally including metal ions, differ from what is conventionally
described as a metallic layer in that there is no native metal. Unlike a metal
layer wherein metal atoms, through metallic bonding, solely form a crystalline
structure, in the non-metallic coatings of the present invention, both
metallic
and non-metallic ions are distributed within an amorphous network.
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The strip may be cold-rolled mild steel strip. Mild steel strip is
conventionally referred to as blackplate.
Typically, the strip has a gauge of between 0.08 and 0.50mm. A
preferred gauge is 0.18mm.
Preferably, the strip is cleaned to remove all traces of contamination
which may be present as a result of previous cold rolling and annealing
processes. Typically, the strip is cleaned electrolytically using a caustic-
based
solution, although the nature of the cleaner does not influence the subsequent
chemical treatment. After cleaning, the strip may be rinsed with water to
remove all traces of the cleaning solution.
The chemical coating may be applied to the strip using a conventional
application method such as immersion, spraying, roller coating, or a
combination
thereof.
Typically, the chemical coating is applied by immersing the cleaned strip
in chemical contained in one or more treatment vessels. In one embodiment,
the strip is chemically treated for less than 60 seconds; in other
embodiments,
the chemical treatment times are less than 30 seconds or less than 15 seconds.
Preferably, the strip is chemically treated for less than 10 seconds;
typically, 5
seconds.
Typically, the strip is chemically treated at a temperature of less than
100°C, most preferably at less than 30°C.
In one aspect of the invention, the strip is chemically treated to form a
chemical coating which prevents subsequent underfilm corrosion of the strip
and promotes adhesion between the strip and thermoplastic resin. The
chemical coating may be referred to as a coupling agent since it forms a
strong
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and durable chemical bridge at the interface between the metal substrate and
the final organic resin coating. The chemical bridge has a dual role; it
interacts
with receptive inorganic surfaces to form tenacious chemical bonds at the
interface with the metal substrate and at the interface with the organic resin
coating.
The oxyanion coating may comprise a phosphate, a chromate, an oxalate
or an arsenate. Additionally, the coating may comprise yttrium, elements in
the
lanthanum series of the periodic table, silanes or azoles.
When metal substrates are exposed to the atmosphere, the surface of the
metal develops a naturally occurring surface oxide layer. Typically, the oxide
layer on blackplate at ambient temperature will have an average thickness of
between 2 and 20 mm. Thus, in one embodiment, the chemical coating may
be applied to the metal oxide layer on the surface of the metal substrate.
The chemical treatment _may comprise, for example zinc
orthophosphates, manganese phosphates or iron phosphates, thereby
producing crystalline phosphate coatings on the strip.
In a preferred embodiment of the invention the strip is chemically coated
with a composition comprising less than 5% atomic chromium.
After chemical treatment, the strip may be rinsed and/or dried, for
example with hot air, prior to treatment with organic resin.
One or more layers of thermoplastic resin may be applied to one or both
sides of the chemically-treated strip. The layer or layers of thermoplastic
resin
may be melted and rapidly quenched to attain the required degree of
crystalline
structure.
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Typically, a film of thermoplastic resin may be co-extruded with the
chemically-treated strip to form laminated strip. The film of thermoplastic
resin
may be bonded to chemically-treated strip under conditions of elevated
temperature and pressure.
The chemically-treated strip may be coated with a thermoplastic resin
together with a bonding layer. The bonding layer may comprise a polyester, or
an acid or acid-anhydride polyolefin resin containing carboxyl or anhydride
groups. Typically, the bonding layer is between 1 and 10~cm thick.
Alternatively, the chemically-treated strip may be extrusion coated with
at least one thermoplastic resin.
Preferred thermoplastic resins comprise polypropylene (PP),
polyethyleneteraphthalate (PET) or a combination thereof.
Typically, the thickness of the layer, or layers, of thermoplastic resin are
between 3 and 50~cm.
The chemical treatment has two functions; firstly it provides corrosion
protection and inhibits underfilm corrosion, and secondly, it promotes good
adhesion between the organic resin coating and the strip. These properties
combined with the barrier properties of the organic coating provide a
laminated
metal strip product which can be formed into components for a range of
applications whilst maintaining adequate performance criteria with regard to
corrosion resistance and inter layer adhesion during the lifetime of the
products.
Therefore, in another aspect, the invention provides a laminated strip
produced by a process which comprises the steps of chemically treating the
strip to form on at least one of its surfaces a non-metallic coating, and
applying
to that coated surface a coating of a thermoplastic resin to form a layer
thereon.
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The invention will now be described by way of example only with
reference to the accompanying diagrammatic drawings and tables in which:-
Figure 1 is a histogram showing the performance rating of food-filled
cans made from PET-laminated and chemically treated blackplate;
Figure 2 is a histogram showing the performance rating of food-filled
cans made from PP-laminated and chemically treated blackplate; and,
Table 1 tabulates the conditions, concentrations and dipping times of
exemplary chemical treatments.
A process line for producing laminated blackplate comprises a plurality
of guide rollers for transporting a strip of blackplate continuously from a
coiled
roll to an exit coil via a multiplicity of vertical tanks. These tanks include
a
cleaning tank, rinsing tanks and a chemical treatment tank. The line speed is
typically 10 to 100 metres per minute with a treatment dwell time of between
1 to 10 seconds. After hot-air drying, the chemically treated strip is
laminated
with organic polymeric resin e.g. a thermoplastic resin such as PET at
elevated
temperature and pressure. The laminated strip is then rapidly quenched to
produce an essentially amorphous organic outer coating.
By way of example, the performances of two commercially available
chemical treatments (referred to below as chemicals A and B) were evaluated
as potential alternatives to the conventional electroplating step in the
production
of an organically coated mild steel strip.
Chemical A comprised a commercially available chemical treatment
comprising chromium, silicon and organic active species. Chemical B comprised
a commercially available chemical treatment comprising a two component
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organic polymer i.e. an acrylic polymer and (NH3)CrZ06.
In the evaluation, blackplate of 0.08 to 0.50mm thickness was subjected
to an electrolytic cleaning process using a commercial cleaning solution at a
temperature not exceeding 100°C, by passing a current of 20A for 5
seconds.
This treatment is considered to return current densities to approximately 10
Adm-2. The nature of the cleaner employed on the blackplate does not influence
any subsequent chemical treatment. It is important that the strip is clean and
free of contamination from prior processing. Before dipping in the chemical
treatment vessels, the samples were washed in two ambient water rinse tanks.
The concentrations of the cleaner and chemical treatments were those
recommended by the respective suppliers. A batch of samples exposed only to
electrolytic cleaning were also prepared as a control sample group, identified
in
Figures 1 and 2 as B-plate.
As well as "cleaned only" samples, an ECCS control sample group was
also laminated. Samples of both 15 ~cm PET andlor 40 ~cm PP were laminated
at elevated temperature and pressure. The hot samples were plunged into cold
water just as the current was switched off. Instant quenching of this nature
has the effect of retaining the amorphous nature of the thermoplastic coating
at ambient temperature. Table 1 illustrates the concentrations, dip times and
treatment section temperatures for evaluated chemicals A and B.
Samples of each variable were subjected to a wedge bend test. Both
treatments A and B performed equally well; no delamination or cracking of the
polymer was observed. A standard Erichsen and cross scored Erichsen were
also performed. The samples were evaluated for signs of blisters and/or
delamination. Again, both A and B performed welt with little to distinguish
between them.
About 350, 73mm diameter classic can ends were produced on a
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conventional MB20 can end press. Approximately 20 samples of each
treatment with both PET and PP were produced. A standard lining compound
was applied to each end. Half the ends were lightly scored prior to filling
with
foodstuff to create a standard defect and potentially allow a greater degree
of
differentiation of the chemical treatments on opening.
8oz cans (73 x 63mm) were filled with either rabbit cat food or cut green
beans in salt brine under standard filling conditions. The cans were stored on
their sides at an elevated temperature (37°C). Cans with scored ends
were
stored with the score running vertically so that it entered the head space
area.
Four cans of each variable were opened after 2, 5, and 15 weeks. Opened cans
were evaluated for sulphide staining, delamination and corrosion (on and off
the
score line).
The can end performance was judged on three main criteria (sulphide
staining, delamination and corrosion (on and off the score line)) using a
points
system. Three points were awarded if the defect was obviously present and
two points if the defect was only minor. No points were allocated if the
defect
was absent. All points were totalled for each category of defect over the
three
openings, for both polymer film types and for each chemical pre-treatment. The
results are illustrated in Figure 1 and Figure 2.
It should be noted that the performance rating system used here gives
equal weighting to each of the attributable defects. Arguably, sulphide
staining
could be regarded as a less serious defect than delamination as it is only
aesthetic and does not directly reflect can performance. Nevertheless, the
approach highlights the chemical treatments which perform relatively
adequately
for use in can-making applications.
In summary, the trials show that chemical pre-treatment in accordance
with the invention provides an effective alternative to metallic electroplated
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coatings prior to coating of strip with organic resins.
It may be envisaged that in another embodiment of the invention,
blackplate can undergo chemical pre-treatment "off-line" with transfer to the
lamination line post treatment. However, this is less cost effective due to
the
necessity for a separate coating facility and any associated transportation or
storage costs.
It will be appreciated that the foregoing is merely exemplary of
treatments in accordance with the invention and that modifications can readily
be made thereto without departing from the true scope of the invention.
TABLE 1
Chemical Working Dip time (seconds)Temperature of
Treatment concentration dip ~C
3% ~ 25
as supplied 3 < 30
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