Note: Descriptions are shown in the official language in which they were submitted.
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Method for producing a sandwich structure, sandwich structure
produced thereby and use thereof
The invention relates to a method for producing a sandwich structure on the
basis of at least one layer on the basis of plastics with at least one layer
on
the basis of metallic material, for example sheet steel, wherein a special
adhesion-promoting and conversion-protecting coating is undertaken on at
least one surface, the sandwich structure produced in this manner and the
use of the sandwich structure produced in this manner.
1.0 In automobile production and aircraft construction, but also in many
other
construction types, metallic components are nowadays used, in particular, as
supporting parts. However, the weight of such components is large and is to
be further reduced in a number of uses.
One route to weight reduction is the use of sandwich structures in which a
part of the usually heavy but typically mechanically very stable metallic
material is replaced with organic material and with which a well-adhering
composite in a stable construction can be used.
Particularly preferred sandwich structures are, for example, those on the
basis of metal-plastics, metal-plastics-metal or metal-plastics-metal-plastics-
metal.
The coating of metallic surfaces for reasons of corrosion protection and/or
paint adhesion is basically known from many publications. For this purpose,
above all, conversion coatings are used. Herein, it is above all decided
whether passivations (= treatments) which are intended to protect primarily a
paint layer against corrosion in the long term or whether pre-treatments
should be used before a further coating, for example, a paint or adhesive.
EP 1 651 432 B1 teaches a metal laminate which comprises between two
outer metal sheets an adhesive polymer layer comprising polyamide, a
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copolymer of ethylene and an unsaturated carboxylic acid and/or a derivative
thereof and a reactive copolymer which comprises a styrene maleic
anhydride copolymer, which has a molecular weight from 1400 to 10000.
DE 102007046226 Al relates to a composite component consisting of a first
and second metal sheet with at least one polymer foam layer arranged
between them.
DE 2627681 Al teaches phosphating methods on the basis of acidic metal
phosphate solution in the presence of a water-soluble polymer with monomer
units of (methyl)acrylic acid and/or (meth)acrylamide.
US 3,721,597 relates to production methods of laminates on the basis of
aluminum panels, ethylene-acrylic acid copolymer and HD-PE as the inner
layer.
As compared with single or multi-layered paint coatings which often have a
thickness of the individual paint layer on metal substrates in the region of 1
to
50 pm, one or more layers of organic polymer (= polymer layer) which are
used for a sandwich structure and are glued on as a layer and/or are pressed
onto one another, require a significantly greater thickness than a paint
layer,
for example, a thickness in the region of 0.1 to 5 mm.
A high degree of adhesion between the individual layers of a sandwich
structure is necessarily required for the production, in particular, of a
stiffness-optimized sandwich structure, for example, a metal-plastics-metal
sandwich structure. Previously, it has often been realized by means of a wet
chemical application of a thin surface-activating and/or adhesion-promoting
intermediate layer on the metallic surface of a metallic layer and the melting
on, in particular, of thermoplastic components of a polymer layer. In order to
apply the surface-activating and/or adhesion-promoting intermediate layer
onto the metallic surface of a metallic layer, separate plant passes for pre-
treatment, for example, of a metal sheet in a coil coating plant are required.
For example, galvanized steel sheets, depending on their production method,
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for example, hot-dip galvanized steel sheet Z or electrolytically galvanized
steel sheet ZE, require different process control, which entails additional
effort. Other metal substrates can be created with the conventional method
sometimes only with insufficient adhesion of the layers of the sandwich
structure. Whereas on similar metallic surfaces as on ZE, a certain degree of
adhesion has been achieved by means of an acidic conversion composition
to form a conversion layer with approximately 30 mg Ti per m2, on Z, an
insufficient adhesion has been achieved. A sufficiently high level of adhesion
could only be achieved on Z when by additional process steps with a first
1.0 alkaline conversion composition, furthermore with additional rinsing
with
water and then with a second acidic conversion composition, a second
conversion layer having approximately 30 mg Ti per m2 was formed. When
the manner of zinc coating was changed from time to time, this was an
unacceptable disadvantage.
Furthermore, the conventionally used technique had the disadvantage, in
particular, for hot-dip galvanized steel sheets, that the metal-plastics
composite had only a limited adhesion in a peel strength test.
For this reason, further wet-chemical conversion compositions should be
investigated. It would be advantageous if the conventional wet-chemical
coating method for surface activation and/or adhesion-promotion could be
more effectively adapted to the plastics used. It was an aim to ascertain
alternative wet-chemical conversion compositions for all common metal
substrates and, in particular, for metallic-coated steel substrates. It was
also
an aim to provide metallic layers for a sandwich structure with the aid of a
thin adhesion-promoting conversion coating which serves as an intermediate
layer of a sandwich structure and hereby to achieve a sandwich composite
adhesion of at least 300N/4cm at 4 cm sample width, which results in
75N/cm in the peel strength test according to ISO 11339, 2010.
It was an object to improve the adhesion between the surface of a layer on
the basis of organic polymers, for example, a KTL temperature-stable
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polymer layer and the surface of a metallic layer, for example, as a cover
panel, in order to produce a sandwich structure, in particular on the basis of
metal-plastics, metal-plastics-metal, plastics-metal-plastics, metal-plastics-
metal-plastics or metal-plastics-metal-plastics-metal. Sandwich structures, in
particular, stiffness-optimized metal-plastics-metal sandwich structures, as
planned under the trademark Litecore, are particularly preferred.
Furthermore, it would be advantageous if the process complexity of the
existing method for manufacturing sandwich structures could be improved by
reducing the number of plant passes and if the bath stability for preparing
the
io metallic surface and/or for adhesion-promotion could be increased and if
the
number of treatment steps necessary for particular metallic surfaces could
also be reduced.
It has surprisingly been determined that with a special conversion
composition, a single-step and, in particular, economical conversion coating
is can be carried out for all the technically relevant metallic surfaces,
which
leads to a sufficient, or even to an unexpectedly high level of adhesion of
the
metal-plastics composite.
The object is achieved with a method for producing a sandwich structure
from at least one layer of metallic material and at least one layer of organic
20 polymer, characterized in that at least one surface on at least one
metallic
layer which is to be combined with at least one layer of organic polymer is
brought into contact with an aqueous conversion composition which contains:
0.5 to 20 g/I zinc,
0.01 to 10 g/I manganese, 0.01 to 10 g/I aluminum, 0.01 to 1 g/I
25 chromium(III), 0.01 to 5 g/I iron(II), 0.01 to 5 g/I iron(III)
and/or 0.01 to 5 g/I magnesium,
0 or 0.01 to 5 g/I of the total as nickel and/or cobalt,
0 or 0.01 to 5 g/I of the total as molybdenum, tantalum, vanadium
and/or tungsten,
30 2 to 100 g/I P205, which corresponds to 2.68 to 133.8 g/I PO4,
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0.1 to 10 g/I polyacrylic acid, but not more than 25 `)/0 of the content of
the composition of P205 in g/I, and
0 or 0.01 to 3 g/I silane, but not more than 25% of the content of the
composition of P205 in g/I,
in that the liquid film produced therewith is dried on, in that the metallic
layer
coated in this manner is cut, if required - in particular, into panels - and
in that
the thus cut metallic layer is brought into contact with at least one layer on
the basis of organic polymer and is combined into a sandwich structure by
means of compaction under pressure and/or temperature.
Preferably, the organic polymer is a thermoplastic polymer and/or a
thermoplastic copolymer.
Before the coating of surfaces of the metallic layers, the surfaces to be
coated are typically cleaned and then rinsed with water. If metallic layers
are
put into intermediate storage, they are usually oiled. If oiled surfaces are
to
be treated or if the plant is at least somewhat contaminated with oil, it is
to be
recommended particularly to clean the surfaces with an alkaline cleaner
before the coating and then to rinse them with water in order to remove
adhering cleaning solution from the surface. A surface that is not completely
water-wettable can result in an uneven no-rinse coating.
Before the application of the inventive aqueous conversion composition, no
activation is used, since it is desirable to generate an extremely fine-
grained
and, at the same time, largely or completely amorphous phosphate coating
and as far as possible no crystalline phosphate coating. The resulting
conversion layer is preferably completely X-ray amorphous or is X-ray
amorphous with slight crystalline fractions, which is unusual for a zinc
phosphate layer. It is important that the surface of the metallic layer is
wetted
as completely and evenly as possible with the no rinse conversion
composition. On application with the aid of rollers as coater rollers and/or
as
squeezing rollers, the condition of the application roller is very important
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since it is intended to create an even wet film preferably in the range from
0.5
to 10 ml/m2.
The inventive conversion coating method therefore takes place without an
activation step with a colloidal titanium phosphate or with a surface
conditioner on the basis of phosphate particles, which is unusual for a
typical
zinc phosphating. This is also a sign that the inventive conversion
composition and its coating method are no typical zinc phosphating.
Preferably, the aqueous conversion composition has a total content of
manganese, aluminum, chromium(III), iron(II), iron(III) and/or magnesium in
the region of 0.01 to 8 g/I, particularly preferably in the range from 0.1 to
6 g/I,
from 0.2 to 5 g/I, from 0.3 to 4 g/I, from 0.4 to 3 g/I, from 0.5 to 2 g/I or
from
0.6 to 1 g/I.
Preferably, the aqueous conversion composition has the following
composition:
1 tO 10 g/lzinc,
0.5 to 6 g/I manganese, 0.01 to 0.5 g/I aluminum, 0.01 to 0.8 g/I
chromium(III), 0.01 to 1 g/I iron(II), 0.01 to 1 g/I iron(III)
and/or 0.01 to 1.5 g/I magnesium, wherein the total of these
elements/cations without zinc preferably lies in the range
from 0.01 to 8 g/I,
0 or 0.01 to 2.5 g/I nickel, 0 or 0.01 to 2.5 g/I cobalt, wherein the total
of nickel and cobalt is 0 or lies in the range from 0.01 to 4 g/1,
0 or 0.01 to 5 g/I of the total of molybdenum and/or vanadium,
8 to 60 g/I P205, which corresponds to 10.72 to 80.28 g/I PO4,
0.5 to 5 g/I polyacrylic acid, but not more than 25 % of the P205
content of the composition in g/I, and
0 or 0.01 to 3 g/I silane, but not more than 25 % of the P205 content
of the composition in g/I, and also no content of a complex
fluoride of titanium or zirconium.
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Particularly preferably, the aqueous conversion composition has the following
composition:
2 to 8 g/I zinc,
1 to 5 g/I manganese, 0.01 to 0.2 g/I aluminum, 0.01 to 0.5 g/I
chromium(III), 0.01 to 0.5 g/I iron(II), 0.01 to 0.5 g/I iron(III)
and/or 0.01 to 0.8 g/I magnesium, wherein the total of these
elements/cations without zinc preferably lies in the range
from 0.01 to 8 g/I,
0 or 0.01 to 2 g/I nickel,
0 or 0.01 to 2 g/I cobalt, wherein the total of nickel and cobalt is not
more than 2.5 g/I,
0 or 0.01 to 5 g/I of the total of molybdenum and/or vanadium,
9.5 to 50 g/I P205, which corresponds to 12.73 to 66.9 g/I PO4,
0.5 to 3 g/I polyacrylic acid, but not more than 25 % of the P205
content of the composition in g/I, and
0 or 0.01 to 2 g/I silane, but not more than 25 % of the P205 content
of the composition in g/I, and also no content of a complex
fluoride of titanium or zirconium.
Preferable herein is an initial aqueous conversion composition in which the
ratio by weight of zinc: PO4 lies in the range from 0.02: 1 to 0.6: 1, from
0.05 : 1 to 0.5 : 1, from 0.08 : 1 to 0.4 : 1 or from 0.1 : 1 to 0.2 : 1
relative to
the initial composition before an uptake of steeped zinc which can become
enriched in the acidic solution over time.
Preferable herein is an aqueous conversion composition in which the ratio by
weight of zinc: PO4 lies in the range from 0.02 : 1 to 8 : 1, from 0.05 : 1 to
4 :
1, from 0.08: 1 to 1.5: 1 or from 0.1 : 1 to 0.8: 1, related to the
composition
which has additionally already taken up etched out zinc when used, for
example, on galvanized surfaces.
The ratio by weight of the aqueous conversion composition and/or the
coating produced therefrom of the total (Ni and Co) : Zn preferably lies in
the
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range from 0.0001 : 1 to 1 : 1, from 0.002: 1 to 0.7: 1, from 0.01 : 1 to 0.4:
1
or from 0.04 : 1 to 0.2 : 1. Depending on the content of nickel and/or of
cobalt, the corrosion-resistance of the conversion coating created therewith
can be influenced.
Preferably, the zinc content of the aqueous conversion composition is 1.5 to
g/I, 2 to 8 g/I or 2.5 to 4.5 g/I. Preferably, the manganese content of the
aqueous conversion composition is 0.5 to 6 or 1 to 5 g/I. Preferably, the
nickel content of the aqueous conversion composition is not more than 1.5
g/I, not more than 1 g/I, not more than 0.5 g/I or not more than 0.01 g/I. The
10 Zn : Mn ratio by weight of the aqueous conversion composition can vary
within wide limits and preferably lies in the range from 0.1 : 1 to 5 : 1 or
from
0.8: 'I to 3: 1.
The aqueous conversion composition can possibly also contain 0.001 to 1 g/I
NO2, 0.01 to 1 g/I NO3 and/or 0.001 to 2 g/I H202 as accelerant.
An addition of polyacrylic acid to the aqueous conversion composition has
proved to be useful since therewith significantly better adhesion results are
achieved, which had not previously succeeded without polyacrylic acid. An
addition of polyacrylic acid provides the conversion coating with a greater
adhesive strength, which has a positive effect, in particular with materials
on
the basis of polyethylene and/or polyamide. It is herein advisable to add a
water-soluble or water-dispersible polyacrylic acid to the conversion
composition. The polyacrylic acid preferably has a molecular weight in the
range from 5,000 to 120,000, from 30,000 to 90,000 or from 50,000 to
70,000. The conversion composition preferably has a content of at least one
water-soluble or water-dispersible polyacrylic acid in the region of 0.2 to 8
g/I,
from 0.3 to 5 g/I or from 0.5 to 3 g/I. The addition of polyacrylic acid has
had
positive effects with all the different metallic surfaces, on the adhesion
firstly
to the metallic surface, and secondly to the surface wetted with organic,
particularly thermoplastic, plastics. Preferably, the content of polyacrylic
acid
and/or its reaction products in the dried-on and/or dried-on and, during
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compaction, thermally loaded conversion coating is 0.05 to 15% by weight of
the conversion coating.
If additionally at least one silane is added to the conversion composition,
the
adhesion between adjacent layers of a sandwich structure, measured here
as the peel strength, can be further improved by up to approximately 25%.
If the content in the aqueous conversion composition of polyacrylic acid is
successively increased and finally exceeds approximately 25% of the
P205 content in g/I, it can arise that no further increase in the peel
strength of
the sandwich structure than at lower content levels, or even a somewhat
lower peel strength, is achieved.
If the content of polyacrylic acid in the aqueous conversion composition falls
below approximately 2% of the P205 content in g/I, it can occur that a
significant worsening in the peel strength of the sandwich structure as
compared with higher levels of polyacrylic acid, results.
For the sake of simplicity, silane, silanol and siloxane are often referred to
below simply as silane. The reason is that silane/silanol/siloxane if often
also
subject to a rapid sequence of reactions, including in water, so that due to
the
changes and due to the great effort for a suitable demonstration of the state,
a determination and a more precise specification are not worthwhile.
As silanes, fundamentally many types are possible individually or in
combination with one another. They are preferably put into a water-soluble or
water-dissolved state. Particularly preferably, they are partially or entirely
hydrolyzed.
Particularly preferable are conversion compositions with a content of at least
one aminosilane with at least one amino group and of at least one
aminosilane different therefrom with at least two amino groups, at least one
aminosilane, and at least one epoxysilane with a content of at least one
aminosilane and at least one bis-silyl-silane with a content of at least one
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ureidosilane and at least one bis-silyl-silane or with a content of at least
one
epoxysilane and at least one bis-silyl-silane.
Particularly preferred are silanes with respectively at least one amino,
epoxy,
glycidoxy, imino and/or ureido group. Particularly preferred are mono-
trialkoxysilanes, bis-trialkoxysilanes, mono-silyl-silanes and/or bis-silyl-
silanes, wherein these particularly preferably have at least one amino, epoxy,
glycidoxy, imino and/or ureido group.
The conversion composition advantageously has a total content of at least
one silane in the range of 0.01 to 4, from 0.1 to 3, 0.3 to 2.5 or from 0.6 to
2
g/I.
If the content of silane(s) in g/I in the aqueous conversion composition
exceeds approximately 25% of the content of P205 in g/I, it can occur that the
adhesion worsens. If the content of silane(s) in g/I in the aqueous conversion
composition falls below approximately 2% of the content of P205 in g/I, it can
occur that the adhesion worsens.
If the polyacrylic acid content in g/I in the aqueous conversion composition
exceeds approximately 25% of the of P205 content in WI, it can occur that the
adhesion worsens.
If the of polyacrylic acid content in g/I in the aqueous conversion
composition
falls below approximately 2% of the P205 content in g/I, it can occur that the
adhesion worsens.
Preferably, the content of at least one silane and/or its/their reaction
products
in the dried-on and/or dried-on and, during compaction, thermally loaded,
conversion coating is 0.01 to 15% by weight of the conversion coating.
However, in some experiments, it has been found that the polyacrylic acid
content in the aqueous conversion composition should preferably be not
more than 25, not more than 20, not more than 15 or not more than 10 % of
the P205 content in g/I of the composition. Under some circumstances, no
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further improvement of the peel strength can be achieved as compared with
compositions having less polyacrylic acid.
However, in some experiments, it has been found that the content of at least
one silane in the aqueous conversion composition should preferably be not
more than 25, not more than 20, not more than 15 or not more than 10 % of
the P205 content in g/I of the composition. Under some circumstances, no
further improvements of the peel strength can be achieved as compared with
compositions having less silane.
The aqueous conversion composition can preferably also contain 0.001 to 20
g/I or 0.2 to 10 g/I of at least one further water-soluble and/or water-
dispersible organic polymer/copolymer apart from polyacrylic acid.
If, furthermore, at least one further organic polymer/copolymer is added to
the aqueous conversion composition, this is preferably selected from acid-
resistant polymers and/or copolymers, which are specifically stabilized, if
relevant. These include, in particular, acid-resistant polymers and/or
copolymers selected from those on the basis of poly(meth)acrylate,
polyacrylamide, polycarbonate, polyepoxide, polyester, polyether,
polyethylene, polystyrene, polyurethane, polyvinyl, polyvinylpyrrolidone and
modification(s) thereof, for example cationic polyurethane resin, modified
anionic acrylate/polyacrylate, epoxy resin with amino groups and, if relevant,
also with phosphate groups and/or cationic copolymer on the basis of
polyester-polyurethane, polyester-polyurethane-poly(meth)acrylate,
polycarbonate-polyurethane and/or polycarbonate-polyurethane-
poly(meth)acrylate.
It can herein be advantageous if the conversion composition also has a
content of at least one further organic polymer/copolymer and at least one
silane/silanol/siloxane. These additives can contribute, for example, to
increasing the adhesive strength.
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Preferably the aqueous conversion composition and, if relevant, also the
corresponding conversion coating has, in many embodiments, no content of
boron, chromium(VI), hafnium, titanium, zirconium and/or niobium or no
content of an intentionally added compound of boron, chromium(VI), hafnium,
titanium, zirconium, niobium or of aluminum, boron, chromium(VI), hafnium,
titanium, zirconium, niobium, tantalum, vanadium and/or tungsten or no
content of an intentionally added compound of boron, chromium(VI), hafnium,
titanium, zirconium, niobium, tantalum and/or tungsten. In particular, it is
preferred that it has no content of a complex fluoride, in particular of
aluminum, boron, silicon, hafnium, titanium and/or zirconium. In many
variants, the inventive aqueous conversion composition has no content of
other organic polymers and copolymers except from polyacrylic acid, a cross-
linking agent, a photoinitiator and/or a chromium(III) compound. Preferably,
the inventive conversion composition has no content of metallic and/or
inorganic particles, in particular such particles larger than 0.1pm.
In the conversion coating method, the pH value of the conversion
composition can lie in the range from 1 to 5, preferably in the range from 2.2
to 4.5, particularly preferably in the range from 2.8 to 4.
Particularly preferably, in the inventive method, as the inorganic aqueous
basic composition, one such is selected which enables a "microphosphating"
to be carried out with the aqueous conversion composition. Microphosphating
is herein referred to when the layer weight of the conversion coating is less
than or equal to 0.4 g/m2 or less than or equal to 0.3 g/m2. Therefore, in the
inventive method, a conversion coating with a layer weight of up to 0.4
g/m2 is preferably formed. Herein, the zinc phosphate is preferably present at
least as partially X-ray amorphous, since no typical zinc phosphating is
carried out.
The inventive phosphate-rich conversion coating can be largely or entirely
amorphously configured in many embodiments, and far finer than a typical
finely crystalline zinc phosphate layer. In the inventive conversion coating,
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neither particles nor the finest hollow spaces can be discerned with the
naked eye, so that this coating makes a far more even and unified
impression compared with a typical "normal" zinc phosphate layer. This often
succeeds only if no activation and no surface conditioning has been used
before the conversion coating, if the wet film of the aqueous conversion
composition has been homogeneously formed on the surface of the metallic
layer and if the contact times with the aqueous conversion composition until
complete drying out were comparatively short, in particular, shorter than 1
minute. The activation or surface conditioning before a phosphate-rich
conversion composition serves to coat the surfaces to be coated with seed
crystals for forming phosphate crystals.
A typical zinc phosphating is introduced with an activation or surface
conditioning of the metallic surface and causes a longer contact time of the
zinc phosphate solution with the metallic surface, specifically typically
depending on the application type by spraying and/or dipping and depending
on the substrate type as a band or as parts, of between 3 s and
approximately 5 min. Herein, typically bath temperatures in the range from 50
to 70 C and substrate temperatures in the range from 15 to 40 C are used.
For typical zinc phosphating, squeezing is not used except with band. For
typical zinc phosphating, drying out is not used, although rinsing is. Due to
all
these measures, the phosphate layer of a typical zinc phosphating is
unusually formed in crystals of approximately 5 to 40 pm. Their layer weight
is herein typically in the range from 1 to 25 g/m2.
In the inventive conversion coating method, it is preferred to bring the
inventive aqueous conversion composition into contact with the metallic
surface, in particular, of a band for 0.1 to 30 s until water-free during
drying-
on, wherein the temperatures of the metallic surface and of the band
preferably lie in the region of 15 to 40 C. Due to the omission of an
activation
or surface conditioning and due to the shorter contact times and the possibly
also lower temperatures, the crystallization of the zinc phosphate is largely
or
entirely suppressed so that it is predominantly or entirely present
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amorphously and much finer than in a crystalline zinc phosphate layer. These
properties also characterize the microphosphating.
Furthermore, it is preferred in many embodiments that the liquid film applied
is dried on without being rinsed therein or thereafter with aqueous liquid.
Alternatively, rinsing can be performed with an aqueous solution, for
example, of a salt and/or another compound, for example, in order to improve
the adhesion still further.
In the inventive conversion coating method, the surfaces of the metallic layer
can preferably be coated at a peak-metal temperature, PMT, in the range
from 5 to 50 C, particularly at 15 to 40 C. The aqueous conversion
composition can have a temperature at the time point of the application in the
range from 15 to 50 C or from 20 to 40 C, wherein at temperatures of above
40 C, it must be noted whether precipitations possibly arise in the conversion
composition.
In the inventive conversion coating method, the contact times until water-
freedom when drying onto surfaces of the metallic layer, particularly in the
case of metallic band or metallic coil are, in particular, 0.2 to 30 seconds.
In
the case of rolling and/or spraying onto band, the time of contacting until
water-freedom can, under some circumstances, be reduced to less than 1
second or to approximately 0.2 seconds. In particular, on a conversion
coated metallic band or coil, the drying can take place, for example, in a
heated air stream, by induction and/or by IR and/or NIR heat radiation. The
conversion coating of the metallic layer takes place, above all, before the
bringing together of this layer with at least one polymer layer for
compressing
to a sandwich structure.
The polymer layers can have thickness, for example, in the range from 0.1 to
5 mm, from 0.2 to 4 mm, from 0.4 to 3 mm, from 0.6 to 2 mm, from 0.7 to 1.2
mm or of approximately 0.8 mm per length. They preferably consist of
compact plastics and not of plastics foam.
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For example, a plastics coil can be used as the polymer layer. In moisture-
sensitive plastics, this is possibly contained vacuum-welded into film. This
film is to be removed from the polymer layer for compression. Many
polypropylene-based plastics have no moisture-sensitivity. Many types of
polymer layer and also the metallic layers are usually not to be further pre-
treated. The polymer layers and the metallic layers can often be compressed
after the bringing together and placing on one another in advantageous
manner without prior heating.
Two embodiments as to how lamination layers can be built up and how they
can function are described, by way of example, in WO 2009/043777 A2.
If a number of layers of metal and plastics are used for the production of a
sandwich structure, it is preferred that, firstly, at least two, at least
three or at
least four metallic layers of metal and/or metallic alloy and/or metallic
layers
with at least one additional metal or alloy layer, for example, galvanized
steel
and/or, secondly, at least two or at least three polymer layers (= polymer
layers), which can possibly also be coated on at least one side and/or
specially treated, for example, polarized by flame-scorching, UV treatment,
corona treatment or plasma treatment, are used. Herein, it is particularly
preferable that each metallic layer alternates with a polymer layer.
Herein, for example, the following sequences of layers can occur, wherein if
two polymer layers adjoin one another, both similar and different plastics or
polymer layers can be used with similar or different dimensions and/or
properties independently of one another: metal-plastics, metal-plastics-metal,
metal-plastics-plastics, metal-plastics-plastics-metal, metal-plastics-
plastics-
plastics-metal, metal-plastics-metal-plastics-plastics, plastics-plastics-
metal-
plastics, plastics-plastics-metal-plastics-plastics, metal-plastics-plastics-
metal-plastics, metal-plastics-plastics-metal-plastics-plastics, plastics-
metal-
plastics-plastics-metal-plastics, plastics-metal-plastics-plastics-metal-
plastics-
plastics, plastics-plastics-metal-plastics-plastics-metal-plastics-plastics,
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metal-plastics-plastics-metal-plastics-metal or metal-plastics-plastics-metal-
plastics-plastics-metal.
Herein, more complex structures, structures with inlays and/or structures
without continuous layers and/or with hollow spaces are also producible,
wherein these peculiarities can each relate to one or more metallic layers
and/or respectively one or more polymer layers. It is particularly preferred,
however, that the polymer layers and/or the metallic layers have no hollow
spaces and/or as far as possible no pores.
For the production of sandwich structures of one or more metallic layers, for
example, of metallic coil or sheet metal sections and one or more polymer
layers of organic, in particular thermoplastic, polymer, for example, plastics
coil or polymer sections, gluing and/or compaction under pressure and/or
temperature are particularly suitable as joining methods.
Many types of organic polymers are suitable as polymer layers, in particular
also as polymer core layers between metallic layers, wherein in particular,
temperature-resistant and/or mechanically loadable layers are preferred.
Polymer layers of an organic composite material (= compound) with
proportions of different plastics are herein preferred. In particular, layers
of
thermoplastics, plastics with a proportion of thermoplastics and/or fiber-
reinforced thermoplastics can be used as layers of organic polymer. If at
least three polymer layers are used in contact with one another, the at least
one middle layer can alternatively also consist of a plastics with a
proportion
of thermoplastics of less than 50% by weight or even of thermosetting
plastics, possibly respectively also fiber-reinforced.
Plastics or thermoplastics that are suitable are, in particular, those on the
basis of polyamide, polyethylene and/or polypropylene, which can possibly
be fiber-reinforced, in particular with aramide, glass, carbon and/or graphite
fibers. Preferably, the layer of organic polymer is a layer on the basis of at
least one thermoplastics and, in particular, on the basis of plastics on the
basis of polyamide, polyethylene and/or polypropylene, which can possibly
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also be fiber-reinforced. Particularly preferred are possibly fiber-reinforced
thermoplastics on the basis of polyamide, polyamide and polyethylene or
polyamide and polypropylene. In particular, polymer layers of a composite
material (compound) with a proportion of PA 6 and/or PE can be used, which
means a polyamide on the basis of c-caprolactam or w-aminohexanoic acid
and/or of polyethylene, which can possibly also be fiber-reinforced.
Particularly preferred is an organic composite material of PA 6 with a
proportion of PE, that is, a polyamide on the basis of c-caprolactam and/or w-
aminohexanoic acid and of polyethylene.
These organic materials are particularly preferred since they are
temperature-resistant and/or mechanically loadable. These composite
materials can preferably contain additives, for example, stabilizers, adhesion-
promoters and/or compounds with adhesive groups. These organic
composite materials can preferably be made so that the organic composite
materials or layers of these organic composite materials are preferably
entirely pore-free and therefore are not foam and/or are deep-drawing
capable at a temperature of up to approximately 100 C and/or are ductile at
room temperature or possibly up to 220 C.
Herein, for large-scale manufacturing, it suggests itself to use band as the
metallic starting material, particularly as a coil or as cut sheet, which is
or has
been coated in a coil coating process and which is joined to at least one
layer
of a plastics material.
Particularly preferred are metallic substrates as coil, band and/or sheet,
which often have a thickness of the layer in the range, in particular, from
0.1
to 2 mm and, if required, can be coated with a metallic protective layer
and/or
with at least one protective layer, for example, on the basis of an adhesion
promoter, passivator or pre-treatment layer. A good compromise between the
weight and properties of the sandwich structure has proved to be the use of a
band/coil/sheet with a thickness in the range from 0.05 to 1 or from 0.1 to
0.5
mm, preferably from 0.2 to 0.3 mm. Metallic coatings which are used in place
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of a metallic layer or on a metallic layer can herein have a thickness in the
range from 1 to 80 pm.
As metallic surfaces, in particular, those of aluminum, iron, magnesium,
titanium, zinc, tin and alloys thereof come into question. As metallic
substrates, in particular, those based on aluminum, iron/steel/high-grade
steel, magnesium, titanium, zinc, tin and alloys thereof can be used. As the
coating of a metallic layer, for example metal such as e.g. zinc, an alloy
containing aluminum, magnesium and/or zinc, for example, a ZnMg alloy or a
chrome plating, in particular, on steel can be used.
As metallic bands, coils and/or sheets and in particular for use as cover
sheets, above all metallic layers can be used which possibly can be coated
with a metallic layer and/or with at least one protective layer, for example,
on
the basis of an adhesion-promoter, passivation or pre-treatment layer and
which are made of one of the following materials: electrolytically galvanized
is steel sheet ZE, for example, electrolytically with a magnesium and/or
zinc-
containing alloy, for example, steel sheet coated with ZnNi, ZnCo, ZnMg, hot-
dip galvanized steel sheet Z, by hot-dipping in a molten aluminum-containing
and/or zinc-containing alloy, for example, a steel sheet coated with an Al,
ZnAl, ZnMg and/or ZnAlMg alloy, chrome-plated sheet, for example, chrome-
plated steel sheet, sheet metal made of aluminum, aluminum alloy,
magnesium, magnesium alloy and/or high-grade steel. If a metallic layer is
coated, it can be coated on one side or both sides. These coatings,
depending on the side, can be different or the same in their thickness and/or
characteristics and/or composition and can additionally have, for example, at
least one conversion layer and/or a passivation layer on a metallic coating.
Layers of different materials can also be combined with one another, for
example, an upper layer of a high-grade steel sheet with a lower layer of
galvanized steel sheet or, for example, in a combination of metal-plastics-
metal-plastics-metal, on the outside, high-grade steel sheets and as at least
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one middle layer, a galvanized steel sheet and/or two different plastics types
and/or polymer layers with different properties.
The inventive method is particularly advantageous if at this point in the
method, following the bringing together and laying on one another of at least
one metallic layer with at least one polymer layer for compression to a
sandwich structure, only the compression is still required. Preferably, panels
or coils are used as polymer layers and as metallic layers for compression,
which influences the type, design and size of the plant. If coils of plastics
and
of metallic material are used, the compression can preferably take place
continuously and on compression of panels, often discontinuously, for
example, in a double band press. Particularly in continuous methods, pre-
heating of the individual or combined layers can possibly be helpful. For a
continuous compression, preferably, a plant can be used in which the
pressing plates of the pressing tool remain static for the pressing time or,
if
the transport speed is low, move along with the band plant.
The compression of the metallic layers and polymer layers placed against
one another preferably takes place at temperatures of the pressing tool and
particularly of the pressing plates in the range from 100 to 260 C, in
particular from 120 to 240 C. It is herein preferred that the temperature of
the
pressing plates is kept as constant as possible. In a particularly preferred
method variant, during compression, the heated pressing plates which are
configured planar and arranged mutually parallel can be pressed for a
particular duration onto the planar structure of the layers lying on one
another, without the pressings plates being lifted. The pressing duration is
preferably 5 s to 20 min, 10 s to 8 min or 30 s to 8 min, wherein on
compression of overall more than two layers, a longer pressing duration
and/or a slightly raised temperature is/are rather used. During compression,
a sufficiently long action of the heat at or above the melting temperature of
at
least one organic, in particular, thermoplastic component of a compound or of
the, in particular, thermoplastic plastics of the polymer layer is to be
ensured.
Preferably, the melt temperature of at least one component of the compound
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or of the, in particular, thermoplastic plastics of the polymer layer is
reached
or exceeded over at least 5 s and/or up to 10 min. The melt temperature of at
least one component of the compound or of the plastics of the polymer layer
often lies in the range from 150 to 260 C or from 200 to 250 C. The plastics
of the polymer layer herein melts at least partially, attaches firmly to the
conversion-coated surface of the metallic layer and becomes adhesively
connected thereto. The pressing pressure during compression is preferably
in the range from 0 to 10 bar or from 2 to 8 bar. In the inventive method, it
is
necessary that the at least one metallic layer has a coating of the inventive
conversion coating on the surface that is to be joined to plastics, said
coating
acting there as an adhesion promoter and as a corrosion protection at the
future cut edges. This is the case because it has been found that the edge
protection is generally better if the adhesion of the inventive coating is
increased.
If the pressing plates are lifted at the end of the compression process, the
sandwich structure can be cooled by natural cooling and/or by forced cooling,
for example, with cool air. Until the beginning of the winding up of a
laminated-together coil or until the stapling of laminated-together panels,
the
temperature should typically be below 90 C.
When the metallic coils (bands) are coated with the inventive conversion
composition and dried they can be cut, if necessary, into panels which can be
compressed with the polymer layers at a later time point, or the coils (bands)
are continuously compressed - in particular, in a laminating plant on passing
through with the polymer layer. It should herein be ensured that until the
joining with the polymer layer, the conversion coating is not chemically or
mechanically loaded, so as not to damage the conversion coating
therethrough. The inventive conversion coating on the surface of the metallic
layer which is preferably directly joined to the polymer layer, provides the
necessary adhesion for a lasting bond between the solidified polymer layer
and the metallic layer.
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In particular when such sandwich structures are further coated as
components or used as one of a plurality of components and coated
thereafter, a coating with respectively at least a liquid paint, an
electrocoat
paint, for example, a cathodic KTL electrocoating and/or a powder coating is
fundamentally possible. Coating with an electrocoat and/or a powder coating
is particularly advantageous herein since even hard-to-access and hidden
sites can be coated. Particularly preferred are polymer layers which can be
used as temperature-resistant polymer layers or as temperature-resistant
polymer core layers. Electrocoatings of this type are often applied at
temperatures in the range from 15 to 40 C and mostly at temperatures in the
range from 25 to 30 C under the action of current. These electrocoatings are
subsequently baked in an oven, often at temperatures of approximately 160
to 180 C. Furthermore, within the most varied of paint coatings, paints such
as for example a finishing paint, a filler, a clear lacquer or a powder
coating
are used which can be baked at temperatures approximately in the range
from 150 to 240 C. Thus, the expression "temperature-resistant" for a
polymer layer of the present invention means that it withstands the baking
temperatures in the baking oven without any disadvantageous influence on
their properties. Therefore, overall, at least one electrocoating, finishing
paint,
filler, clear lacquer and/or powder coating can be applied onto at least one
side of the composite or laminate.
The inventive sandwich structure can, if required, be at least once
respectively coated, deformed, glued, compressed and/or otherwise joined.
From sandwich structures of this type, for example, bodywork parts can be
produced, for example, by cutting, deforming, painting and/or joining, wherein
the sequence of these steps can be varied.
Unexpected effects:
It was surprising that through the addition of polyacrylic acid in the range
from 1 to 3 g/I, significantly greater peel strength values could be achieved
than with the previously tested and/or used conversion compositions.
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It was also surprising that the conversion composition generated a sufficient
adhesion between the metallic surface and the polymer layer on all such
different metallic substrates such as, for example, CRS, Z, ZE, ZnMg,
ZnAlMg, high-grade steel and Al.
It was further surprising that the conversion coating also did not disrupt a
subsequent cathodic electrocoating (KETL), even on the side of the metal
sheet not joined to a polymer layer.
It was advantageous that two successively performed conversion coatings
could be reduced to a single coating, such that two to four process steps can
1.0 be spared. Apart from this simplification of the production process, a
significant increase in the peel strength could also be achieved.
The inventive production and the inventive sandwich structures can be used
in motor vehicle construction, aircraft construction, space travel, apparatus
construction, in building construction, furniture production or as structural
elements.
The inventive sandwich structures can be used, in particular, as parts of
motor vehicles, trailers, mobile homes or aerodynamic vehicles, as parts of
bodywork, as elements of doors, rear hatches or engine hoods, as fenders,
as crash barriers, for electrotechnical equipment, for domestic appliances,
external claddings, facade elements, roof claddings, garage door elements,
fence elements, in interior fittings, as radiators, as lamps, for furniture
items
or as furniture elements, for wardrobe elements or shelving, as bands, as
metal sheets, formed parts, coverings, paneling, screens, frames, isolating
elements, safety elements, supports or as housings.
Examples and comparative examples:
The subject matter of the invention will now be described in greater detail by
reference to exemplary embodiments:
The examples below are based on the following substrates or method steps:
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The following sheet metals were tested:
A) Aluminum alloy AlMg3 5754 W19,
B) Cold rolled continuous annealed steel sheet (CRS) from unalloyed steel
DCO4B,
C) Light gage electrolytically galvanized sheet steel (ZE) in automobile
quality, grade DX54 DZ100,
D) hot-dip galvanized rerolled sheet (Z) from mild unalloyed steel, grade
DX53 with at least 100 g/m2 zinc deposit,
E) High-grade steel (StS), grade 1.4301,
F) Magnesium alloy AZ31, and
G) Magnesium alloy AM50,
each with a thickness of approximately 0.2 to 1 mm, depending on the
metallic material and test.
1. The substrate surfaces of the sheets were cleaned in a 2% aqueous
solution of a strongly alkaline cleaner over 10 to 20 s at 55 to 60 C and
thoroughly degreased in the process.
2. This was followed by rinsing with mains water for 0.5 minutes at room
temperature.
3. Different aqueous conversion compositions were prepared according to
Table 1, all except VB37 having good bath stability. The salts used herein are
given on the second and third pages of the tables, although always only one
of a plurality of the zinc compounds given was added. As polyacrylic acid, an
adhesion-promoting formulation dissolved in water with polyacrylic acid
polymer having a mean molecular weight in the range from 50,000 to 70,000
was used. As epoxysilane 1, 3-glycidoxypropyltrimethoxysilane in the pre-
hydrolyzed state was used. As aminosilane 1, N-(2-(aminoethyl)-3-
aminopropyltrimethoxysilane in the prehydrolyzed state was used. The
conversion compositions showed a pH value in the range from 2 to 3.
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4. In many tests, a plurality of different metallic substrates were treated
one
after another and otherwise treated in the same manner and grouped
together under a number of an example or comparative example for greater
clarity in Table 1.
5. Thereafter, the surfaces of the different types of metal sheet given in
Table
1 under substrates were coated at room temperature with a laboratory
coater, wherein a wet film of approximately 3.5 ml/m2 was applied.
6. Then the coated substrates were dried in a drying oven at 180 C for 20 to
30 s at a peak metal temperature PMT of 60 C, wherein the wet film was
dried on without prior rinsing.
7. On the dried conversion-coated substrates, the layer weight for the
chemical element phosphorus was determined with an X-ray fluorescence
analysis apparatus (RFA) to discover the P205 content. In examples B1 to
B6, layer weights for the conversion coating in the range from 20 to 60
mg/2 P205 were tested, whereas in the further examples and comparative
examples, a layer weight in the range from 10 to 150 mg/m2 P205 was used.
Table 1 shows extracts therefrom.
8. As polymer layers, KTL temperature-resistant layers made of a composite
material (compound) of polyamide PA6 with a proportion of PE, which means
a polyamide on the basis of c-caprolactam or w-aminohexanoic acid and/or
of polyethylene having a thickness in the range from 0.05 to 1.00 mm.
9. In each case, a polymer layer was pressed in a panel press as an
intermediate layer between two identical metallic layers conversion-coated
according to the invention without targeted pre-warming, under loading at a
temperature possibly of up to 240 C without pressure for 60 s and thereby
combined to a durable sandwich structure. Herein, at least a part of the, in
particular, thermoplastic material melted and, on cooling, formed a firm bond
by means of the inventive conversion coating.
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10. Following cooling, peel strength values to ISO 11339:2010 were
determined, wherein triple measurements per sandwich structure were made
on relevant samples. The portions used from the sandwich structure were 4
cm wide and 13 cm long and were expected to produce peel strength results
of at least 300N/4cm, i.e. at least 75N/cm in the peel strength test to ISO
11339:2010.
Table 1: Composition and properties of the aqueous conversion
compositions, the conversion coatings and the sandwich structures produced
therewith.
-
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B1 132 B3 B4 B5 B6 B7 B8
B9 810 811 B12
Substrate Z/ZE CRS/StS Z/ZE/CRS/AUStS
Z/ZE
Bath composition in g/I; remainder: water
Zn 3.33 3.33 , 3.33 6.86 6.86 6.86
8.76 8.76 8.76 1.72 1.72 1.72 P
Mn 2.02 2.02 2.02 4.03
4.03 4.03 5.15 5.15 5.15 1.01 1.01 . 1.01 .
Ni 0.69 0.69 0.69 1.37 1.37 1.37
1.75 1.75 1.75 0.34 0.34 0.34 .
,-,
Mo 0.10 0.10 0.10 0.10 0.10 0.10
0.10 0.10 0.10 0.10 0.10 0.10 ...,
P205
19.15 19.15 19.15 38.30 38.30 38.30 48.86 48.86 48.86 9.57
9.57 9.57 "
,D
,-,
PO4 25.62 25.62 25.62 51.24 . 51.24 51.24
65.39 65.39 65.39 12.81 12.81 . 12.81 .
,
,-,
Zn : PO4 0.13 0.13 0.13 0.130.13 0.13
0.13 0.13 0.13 0.13 0.13 . 0.13
,
,-,
Polyacrylic acid 1.28 2.56 1.59 1.28 , . 2.56
1.59 1.28 2.56 1.59 1.28 2.56 1.59
Epoxysilane 1 1.56 1.56
1.56 1.56
Layer weight P205 mg/m2 20-60 40-120 50-150
10-30
Peel strength N/4cm 500- 500- 400- 300- 300- 350- 300-
300- 350- 200- 200- 400-
700 750 500 400 400 650 400 400
650 350 350 500
,
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B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23
Substrate CRS/StS Z/ZE CRS/StS Z/ZE
CRS/StS
added as Bath composition in g/I;
remainder: water
P
Zn as ZnO+ZnCO3 3.33 3.33 3.33 3.33 6.86 3.33
3.33 3.33 6.86 6.86 6.86 2
Mn as Mn3(PO4)2 2.02 2.02 2.02 2.02 4.03 2.02
2.02 2.02 4.03 4.03 4.03 .
Ni as NiCO3 0.69 0.69 0.69 0.69
0.69 0.69 1.37 1.37 1.37
..,
AI as AlPO4 0.18 0.18
0.18
Cr(III) as Cr(NO3)3
0.2 0.4 0.2
,
mo [(NH4)6mo7024.4H2o] 0.10 0.10 0.10 0.10 0.10
,-,
,
,-,
NO3 as NaNO3 0.60
0.60
P205 19.15 19.15 19.15 19.15 38.30
19.15 19.15 19.15 38.30 38.30 38.30
PO4
25.62 25.62 25.62 25.62 51.24 25.62 25.62 25.62 51.24 51.24
51.24
Zn : PO4 0.13 0.13 0.13 0.13 0.13 0.13
0.13 0.13 0.13 0.13 0.13
Polyacrylic acid 1.28 2.56 1.59 2.56 2.56 1.28
2.56 1.59 1.28 2.56 1.59
Epoxysilane 1 , 1.56 1.56
1.56
Aminosilane 1
0.20
Layer weight P205 mg/m2 20-60 20-60 20-60 20-60
40-120
Peel strength N/4cm 400- 400- 400-
500-750 300-400 500-750 - 400-500
-- -
500 500 500
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B24 325 B26 \ B27 328 B29 B30 B31
B32 B33 B34
Substrate Z/ZE/CRS/AUStS Z/ZE
CRS/StS Z/ZE
added as Bath composition in WI;
remainder: water
P
Zn as ZnO+ZnCO3 8.76 8.76 8.76 1.72 1.72 1.72
3.33 3.33 3.33 3.33 6.86 .
Mn as Mn3(PO4)2 5.15 5.15 5.15 1.01 1.01 1.01
2.02 2.02 2.02 2.02 4.03
-
Ni as NiCO3 1.75 1.75 1.75 0.34 0.69
0.69 0.69 ,
..,
.
.
Co as Co(NO3)2.4H20 0.34 0.34
_
.
Fe(111)[Fe(NO3)2.9H20] 0.20
0.20 0.40 ,
,
Mg as Mg3(PO4)2 0.30 0.60
,
,
,
Mo RNH4)6M0702,41-i20] 0.10 0.10
,
V as NaV03- W as Na2W04 0.20W 0.10 V
H202
0.0002 0.0004
NO2 as NaNO2 0.002
NO3 as NaNO3 0.60
0.60 0.60
P205 48.86 48.86 48.86 9.57 9.57 9.57
19.15 19.15 19.15 19.15 38.30
PO4 65.39 , 65.39 65.39 12.81 12.81 12.81
25.62 25.62 25.62 25.62 51.24
Zn : PO4 0.13 0.13 0.13 0.13 0.13 0.13
0.13 0.13 0.13 0.13 0.13
Polyacrylic acid 1.28 2.56 1.59 1.28 2.56 1.59
1.28 2.56 1.59 2.56 2.56 _
Epoxysilane 1 1.56 1.56
1.56
Layer weight P205 mg/m2 50-150 10-30
20-60 40-120
Peel strength N/4cm 300- 350- 200- 200- 400- 200-
200- 200- 300- 300-
_
400 650 i 350 350 , 500 320 320
320 420 400
,
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B35 B36 VB37 B38 B39 B40 B41 B42 B43 VB44 VB45 VB46 B47 VB48 VB49
Substrate Z/ZE
Bath composition in g/I; remainder: water
Zn 1.28 6.41 12.81 3.33 3.33 3.33 3.33
3.33 3.33 3.33 3.33 3.33 3.33 3.33 3.33 P
Mn 2.02 2.02 2.02 2.02 2.02 2.02 2.02
2.02 2.02 2.02 2.02 2.02 2.02 2.02 2.02 ^,
u,
MO 0.10 0.10 0.10 0.10 0.10 0.10
0.10 0.10 0.10 0.10 0.10 0.10 0.10 .
0
,
...]
P205 19.15 19.15 19.15 19.15 19.15 19.15 19.15
19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 m
PO4 25.62 25.62 25.62 25.62 25.62 25.62 25.62
25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 N,
0
,
..,
Zn : PO4 0.05 0.25 0.50 0.13 0.13 0.13 0.13
0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13 1
,
Polyacrylic acid 1.59 1.59 1.59 0.30 1.56 3.00 0.30
1.56 3.00 5.50 7.50 10.00 1.59 1.59 ,
1
,
Epoxysilane 1 0.20 0.20 0.20 3.00
3.00 3.00 3.50 6.00 u,
Layer weight P205 20-60
mg/m2
Peel strength N/4cm 300- 300-250- 400- 400- 250-
400- 300- 250- 200- 150- 400- 250- 150-
_
650 650 400 800 800 400 800
600 400 300 250 800 400 250
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Table 1 illustrates that on all the very different metallic substrate
materials, with the
inventive aqueous conversion composition, sandwich structures with excellent
or
good values of peel strength were achieved and that even for nickel-free
conversion
compositions, very good values were achieved.
The examples B1 to B6 show excellent results, examples B7 to B17 show very
good results. The examples B18 to B43 illustrate good results with a peel
strength
of more than 300 N/4cm, so that overall good sandwich structures are
producible in
a broad chemical field of the aqueous conversion compositions. The comparative
examples VB44 to VB46 and VB48 show less good results since clearly, for these
aqueous compositions, excessively high levels of polyacrylic acid or of silane
were
added.
Lower layer weights herein typically produced a better peel strength than
higher
layer weights. The inventive sandwich structures showed an outstanding peel
strength. They also met the very high standards of the automotive industry
with
regard to joining behavior, corrosion-resistance, formability and coatability,
as
determined in further tests (not shown here).
First tests with the inventive method, revealed that, under the same
conditions and
without an additional second conversion coating, the galvanized steel sheets Z
and
ZE can now both be successfully coated and further processed to sandwich
structures. The process stability of the preparation of the metallic surfaces,
of the
aqueous conversion composition and of the conversion coating process was also
greater than in the earlier tests, as was revealed by high peel strength
values
overall. There was found to be a greater bath stability of the conversion
composition, which had the effect of more even coatings, which therefore also
showed better property values.
