Note: Descriptions are shown in the official language in which they were submitted.
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'W'Q 99/48686 pCT/FP99/01924
Ii-EFi1e:77442/ISC
Metallic Comuasite Material and a Method for the Production Thereof
The present invention relates to a metallic composite material comprising a
metallic band
(strip) and plastic layers applied thereon by extrusion and co-extrusion,
respectively, as
well as a method for the production thereof.
It is known to use metallic composite materials comprising metallic band, film
or sheet
and plastic layers applied on one side or both sides thereof for the
production of hollow
~ bodies, e.g. containers, by a deep-drawing or ironing method.
GB-A-2 003 415 discloses a method of deep-drawing and ironing thin-walled,
deep
containers of a composite material which consists of a sheet metal, e.g.
aluminium, which
at least on one side is laminated with a plastic layer on an adhesive resin
layer. This is to
prevent that while processed into containers the plastic layer is destroyed
and the sheet
metal is exposed.
EP-A-O 407 313 describes a method of producing a multi-layer material which
can be
used for the production of containers deeper than those described in GB-A-2
003 415 by
deep-drawing or ironing. The multi-layer material comprises a surface-treated
substrate of
aluminium alloy, which is coated with a plastic material on one side and with
a varaish
layer on its other side. In order to be able to produce the hollow bodies with
a greater
~ height/diameter ratio without destroying the coating, the varnish layer
additionally
contains a solid, particulate lubricant which is harder than the varnish
itself but softer than
the tool used for ironing.
DE-A-195 29 583 and EP-13-0 690 185 disclose methods of coating metal webs or
sheets
by laminating a plastic sheet onto the metal web and heating it together with
the metal
web in a subsequent step to improve adhesion, so that the plastic sheet
becomes plastically
deformable.
Hollow bodies of deep-drawa mctallic composite materials for use as containers
can be
sterilized after filling by heating them to temperatures of about 120-135 C
for about 30-34
minutes. In contrast to the conventional composite materials produced by
laminating and
coating, respectively. the adhesion of the plastic layers to the metal, in
particular to non-
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pretreated metal, is not sufficient when using an extrusion coating - even if
employing
additional adhesives - to render the resulting composite material both deep-
drawable and
resistant to serilization.
Therefore, one problem of the present invention consists in providing a method
for the
production of a metallic composite material by extrusion coating, the
resulting metallic
composite material being able to be deep-drawn and sterilized without
separation of the
composite occurring.
It has been found surprisingly that this problem can be solved by a method in
which a
metallic band is coated by extrusion on one side or both sides with a modified
plastic
layer on the basis of modified polypropylene and the resulting metallic
composite
material is then heated for a period of 1 to 10 seconds, more preferably 1 to
3 seconds,
and most preferably 1 to 2 seconds, to a temperature above the melting point
of the
modified plastic layer.
The improved adhesion between metallic band and coating during the use of the
combination of extrusion coating and heat treatment according to the
invention, is inter
alia surprising because in the extrusion stage the extruded material is
applied in its molten
state onto the substrate. Therefore, improvement of adhesion to the substrate
by a
subsequent heat treatment in which the coating is again converted into its
molten state
was not expected on the basis of the prior art.
The metallic band can optionally be heated prior to the extrusion coating, but
preferably
not above 100 C, in particular not above the melting point of the material to
be extruded.
However, the extrusion coating is preferably applied to a metallic band with
approximately ambient temperature, e.g. room temperature. In this way, the
extruded
material temporarily changes into its solid state after the coating and before
the
subsequent heat treatment step. The heating in the heat treatment step can be
carried out
by conventional methods, e.g. using hot air, infrared radiation or by
inductive heating of
the metallic band, the composite material including the modified plastic layer
molten in
this step failing to contact the rollers (e.g. by what is called a suspension
drier in which
the band is guided on an air cushion).
The cooling and the cooling rate after the heat treatment step are not subject
to special
restrictions. In particular, they can be forced by a special apparatus so as
to achieve rapid
cooling within the range of 100 to 200 C/s or more. Cooling is preferably
effected by the
ambient atmosphere alone without another apparatus being required, and cooling
rates of
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less than 100 C/s, e.g. less than 50 C/s, less than 30 C/s and less than 10
C/s generally
suffice to achieve the effects of the method according to the invention.
The modified plastic layers have, independent of one another, preferably a
weight per unit
area of 3 to 40 g/m2, more preferably one of 7 to 10 g/m2. The modified
plastic layers
consist preferably of a maleic acid anhydride-modified polypropylene. In this
case, the
temperature in the heat treatment step is about 180 C, depending on the
modification
degree of the polypropylene.
The thickness of the metallic band is preferably 40 to 200 m, more preferably
70 to 1 S0
}tm, and most preferably 90 m. The metallic band may be surface-treated, e.g.
by
chromating. However, this is not absolutely required for a good adhesion of
the modified
plastic layer to the metallic band, and for reasons of costs it is preferred
to use a metallic
~ band which is not surface-treated. The metallic band also consists
preferably of
aluminium or aluminium allay.
In a preferred embodiment, the metallic band is coated in the coating step
additionally on
one side or both sides with a polypropylene-based plastic layer by means of co-
extrusion.
In this embodiment, the modified plastic layer serves as a coupling agent
between the
metallic band and the plastic layer, and the weight per unit area of the
modified plastic
layers amounts, indcpendent of one another, in this case preferably to 3 to 10
g/m2, more
preferably to 5 to 7 g/m2.
In another preferred embodiment, the weight per unit area of the plastic
layers amounts,
independent of one another, to 3 to 40 g/m2, and more preferably to 7 to 10
g/m2. The
~ plastic layers consist preferably of polypropylene or polypropylene
copolymer, e.g.
polypropylene/polyethylene copolymer, or polypropylene/polyethylene mixtures.
An
example is a niixture of polypropylene having 5 to 30 % by weight LDPE. In
addition, the
plastic layers can contain, independent of one another, one or several
antiseize agents or
lubricants, e.g. oleic acid amide or erucic acid amide, which enable
deformation of the
metallic composite material by deep-drawing without further lubrication of the
material or
the tools. If desired, one of the plastic layers can be made peelable.
The plastic layers and modified plastic layers can optionally be dyed, e.g.
using dyes or
pigments, it being particularly preferred when using a combination of modified
plastic
layer and plastic layer to dye immediately both the plastic layer and the
modified plastic
layer, one lying on top of the other. The advantage of this is that variations
in the layer
thickness of the individual plastic layer and modified plastic layer,
respectively, do not
change the shade as long as the entire layer thickness remains constant.
Furthermore.
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irregularities of color and striations or streak formations are compensated
for, which may
occur because of the different solubility and/or miscibility of the colors or
pigments when
using copolymers and compounds from various polypropylene kinds and
polypropylene/polyethylene mixtures. Due to the good adhesive properties of
the coating
to the metallic band, the metallic composite material which can be produced by
the
method according to the invention can be used for the production of
sterilizable hollow
bodies, in particular containers.
Figure 1 shows a section through a preferred embodiment of the metallic
composite
material which can be produced by the method according to the invention. The
metallic
composite material comprises a metallic band 1 and the, applied by co-
extrusion,
modified plastic layers 2 and plastic layers 3.
As compared to the known methods, the method according to the invention
comprises -
in addition to the excellent adhesive properties between metallic band and
coating - the
advantages that it only requires one operating cycle, i.e. extrusion and co-
extrusion,
respectively, and no use of solvents as called for in coating methods.
Pretreatment of the
surface of the metallic band, e.g. by chromating as in the case of baked
strips, is not
necessary either. This results in markedly reduced production costs
accompanied by
equal or improved adhesive properties between the individual layers of the
metallic
composite material. Finally, it is possible to produce an easily recyclable
metallic
composite material from only two components, e.g. aluminium and modified
polypropylene.
The present invention therefore provides a method for the production of a deep-
drawable
and sterilizable metallic composite material, comprising: (a) coating of a
metallic strip on
one side or both sides with a modified plastic layer based on modified
polypropylene by
extrusion, wherein the extrusion coating takes place on a metal strip of room
temperature,
such that the plastics layer passes temporarily into the solid state; and (b)
heating of the
resulting metallic composite material to a temperature above the melting point
of the
modified plastic layer for a period from 1 to 10 seconds in order to convert
the plastics
layer again into the melted state.
