Note: Descriptions are shown in the official language in which they were submitted.
2g~7~
A MULTI-LAYER SHEET STRUCTURE FOR THE REfNFORCEMENT OF PANELS,
A METHOD FOR TIIE REINFORCEMENr OF PANELS
AND A REINFORCED PANEL
.
FIEL~ OF T~IE INVENTION
The present invention refers to a multi-layer sheet struc-
ture for the reinforcement of panels, particularly of panels
made of steel or aluminium sheet material or plastic material,
said sheet structure including a supporting layer and an adhe-
sive layer applied on said supporting layer on the side Eacing
the panel to be reinforced. Further, the invention refers to a
method for the reinforcement of panels, particularly of panels
made of steel or aluminium sheet material or plastic material
by using the multi-layer sheet structure of the invention.
Still further, the invention refers to a panel, preferably made
of ste~l or aluminium sheet material or of plastic material,
reinforced with the multi-layer sheet structure of the inven-
tion.
BACKGROUND OF THE INVENTION
According to the status of the art, special multi-layer
sheet structures or laminates are used for reinforcing, parti-
cularly for stiffening sheet metal parts, molded plastic arti-
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cles, and the like, for example parts of car bodies. For exam-
ple, such multi-layer sheet structures may be composed of a
first supporting layer, that is to say the actual rein~orcing
or stiffening layer, for example a glass cloth ~onded by a
heat-curable plastic resin, the surface of which is provided
with a second layer of heat-curable stic~y plastic resin. Such
a composite may be used for reinforcing ~lat and disiled sheet
metal by depositing a piece of the composi-te onto the sheet
metal, the sticky surface facing the latter, an(l by thermally
treating said composite. By this, on the one hand, bonding of
the composi~e and, on the other hand, curing of the resin
layers is effected, thereby creating a joint between the sheet
metal and the fiber-reinforced layer which is stif~ against
shearing. In this way, a considerable reinforcement and stiff-
ening of the sheet metal section involved can be effected in a
simple manner and without substantial additional weight.
In order to simplify the application of the reinforcing
structure, the second resin layer in general is made to have a
sticky surface so that the multi-layer sheet structure, once
deposited on the sheet metal, is adhering there until thermally
treated.
It is known from previous experience that, when locally
reinforcing, for example, sheet metal of car bodies by means of
such mu]ti-layer sheet structures, al-though invisibly applied
to the back of the sheet metal part, more or less visible de-
formations oE the sheet metal section involved are produced.
This is of course most unwelcome in the manufacturing oE auto~
mobiles, since either these deformations injure the aesthetic
D5735
to an intolerable extent, or time and work consuming reworking
of the visible sheet metal surface will be necessary.
The reason for said deformations are stresses which are
created during c~ring of the multi-layer sheet structure which,
in general, is composed of pLastic material. It is to be empha-
sized that, on the one hand, a certain volume contraction takes
place, and, on ths other hand, when heat-curable plastic mate-
rials are cooled aEter thermal curing, the panel to be rein-
forced and the multi-layer sheet struc~ure have different co-
efficients of thermal expansion.
As disclosed in EP-Bl-0,053,361, these problems are said to
be solved by means of an adhesive film composed of a first
layer o~ an epoxy resin composition and a second layer of an
epoxy resin composition laminated to said first layer, said
layers having the following characteristic properties:
(a) The first layer, that is to say the actual reinforcing
layer (or "supporting layer" in the terminology of the
present invention) is to have a comparatively high modulus
of elasticity of between 30 and 500 kg/mm2, after appli-
cation on-to the me-tal plate and curing.
It is clearly disclosed in said reference, that this layer
takes care of the very task of stiffening~
(b) The second layer, that is to say the adhesive layer for
fixing -the actual reinforcing layer on the panel to be re-
inforced, is to have a comparatively low modulus of elasti-
2~ 735
city of between O.l and 22 kg/mm .
In said reference, it is emphasized expressis verbis thatthe modulus of elasticity oE this adhesive layer is to be
so low that it is insufficient for stiffening the panel.
In this way, it is possible to avoid a visible deformation
of the panel to be reinforced, for example of a sheet metal
part of a car, such as a door, a deck lid~ et cetera. Moreover,
the outer first layer, which has a comparatively high modulus
of elasticity, brings about the actual reinforcing effect. The
second layer facing the panel to be reinforced, that is to say
the adhesive layer, has such a low modulus of elasticity that
it cannot deform the panel -to be reinorced.
The main disadvantage of this arrangement is that the rein-
forcing effect is rather poor because of -the comparatively low
modulus of elasticity of the adhesive layer. A joint between
the actual reinforcing layer and the panel to be reinforced,
which is stiff against shearing) cannot be made in this way.
OBJECTS OE THE INVENTION
It is the object oE the present invention to provide a mul-
ti-layer sheet structure for the reinforcement of panels made
e.g. of sheet metal or plastic Inaterial which multi layer sheet
structure, on the one hand, provides an improved reinEorcing
and stiffening of said panel and, on the other hand, excludes
any deformation of said panel after curing said multi-layer sheet
structure.
S~MMARY OF THE INVENTION
The invention provides a multi-layer sheet structure
for the reinforcement of panels. The sheet structure includes
a supporting layer and an adhesive layer applied on said
supporting layer on the side facing the panel to be reinforced.
The supporting layer comprises a curable plastic material and a
reinforcing material combined with or embedded in said curable
plastic material. The supporting layer is determined, before
curing, to have, after curing, a coefficient of thermal expansion
which is essentially the same as the coefficient of thermal
expansion of the panel to be reinforced. The adhesive layer
comprises a curable plastic material optionally provided with
fillers or other additives, and said adhesive layer is
determined, before curing, to have, after curing, a modulus of
elasticity which is higher than the modulus of elasticity of th~
cured plastic material of said supporting layer.
The invention further provides a method for the
reinforcement of panels by means of a multi-layer sheet
structure. Thereby, in a first step, the coefficient of thermal
expansion of the panel to be reinforced is determined~ In a
second step, a supporting layer comprising a curable plastic
material and a reinforcing material combined with or embedded in
said curable plastic material is prepared, the ratio of the
plastic material to the reinforcing material being chosen in a
manner that the ~
2 ~ 3 ~
supporting layer, after curing, has a coefficient of thermal
expansion ~hich is essentially the same as the coef~icient of
thermal expansion of the panel to be rein~orced.
In a third step, an adhesive layer comprising a curable
plastic material is applied onto said supporting layer, said
adhesive layer, after curing of said plastic material, having a
modulus of elasticity which is higher than the modulus of elas-
ticity of the cured plastic material of said supporting layer.
In a four-th step, the multi-layer sheet structure obtained
by the second and third steps is fixed onto said panel to be
reinforced, with the side of said adhesive layer facing said
panel to be reinforced. In a fif~h, final step, the fixed mul-
ti-layer sheet structure is treated so as to cure the plastic
materials of said supporting layer and said adhesive layer and
to firmly bond said multi-layer sheet structure with said panel
to be reinforced.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sectional view of a multi-layer sheet structure
of the invention in its unused state.
Fig. 2 to ~ are sectional views of three composites com-
prising multi-layer sheet structures oE the invention and show-
ing nega-tive, zero and positive deElection, respectively.
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DETAILED DESCRIPTION OF THE INVENTION
The ~erm "modulus of elasticity" signifies, throughout the
description and the claims, the modulus of elasticity deter-
mined by the tensile test.
The tests made by the Inventors clearly show that deforma-
tion of, for example, a sheet metal is in the first place caus-
ed by the different coefficients of thermal expansion of the
two materials - in the case of said example of the metal panel
to be reinforced and the plastic material used for stiffening -
when the arrangement is cooled after heat-curing. Obviously,
volume contraction is of secondary importance, since it takes
place above the glass transition region where forces can scar-
cely be transmitted.
In this manner, a multi-layer sheet structure having, for
example, two layers can be built up so that after its applica-
tion onto the sheet metal $o be reinforced and its curing a
composite of shee-t metal/adhesive layer/reinforcing layer is
obtained which is symmetrical with respect to its thermal ex-
pansion. It is a matter of course that the reinforcing layer is
to have the same coefficient of thermal expansion as the slleet
metal to be reinforced. Under these preconditions, the sheet
metal on the one hand, and the reinforcing layer on the other
hand will symmetrically contract during the cooling process so
that no deformations of the sheet metal can occur. The contrac-
tion of the adhesive layer, because of the symmetry present, is
compensated in itself and does not have any detrimental effect
to the shape or the appearance of -the sheet metal.
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The coeEficient of thermal expansion of the supporting
layer can be adjusted within a wide range to that of the panel
to be rein~orced, due to the feature -that the adhesive layer,
after curing the resin material contained in said supporting
layer, has a higher modulus of elas-ticity than the cured resin
material of said supporting layer, and due to the fact that the
resin material used for said supporting layer can freely be
combined with reinforcing materials. On the other hand, the
colnbination "resin material o~ the supporting layer + reinforc-
ing material" has a higher modulus of elasticity than the ad-
hesive layer. By this, it is ensured, particularly if the dif-
ferences with respect to the moduli of elasticity of said ad-
hesive layer on the one hand and oE said supporting layer on
the other hand are not too important, that a link is created
between said supporting layer and said panel to be stiffened
which is stiff against shearing. This also ensures a pronounced
stiffening and reinforcing effect onto the panel.
The coefficient of thermal expansion can very easily be
controlled by means of the volu.me ratio of resin material to
reinforcing material.
Glass cloth is particularly sui-table as reinforcing mate-
rial. It turned out that in the case of stiffening a steel
sheet (which probably is the most frequent case in practice) a
ratio of glass cloth to resin material of, roughly spoken,
50:50 by weight, and preEerably of about 48:52 by volume, is to
be aimed at.
IJI practice, for stiffening a panel in accordance with the
present invention, one proceeds as follows:
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First, th~ coefficient of thermal expansion o~ the panel to
be reinforced is determined.
Then, a supporting layer comprising a curable resin materi-
al and a reinforcing material combined therewith or embedded
therein is produced, selecting the ratio of resin material to
reinforcing material in such a manner that said supporting lay-
er, after curing, has essentially the same coefficient o~ ther-
mal expansion as the panel to be reinforced.
Thereafter, an adhesive layer is deposited onto said sup-
porting layer, said adhesive layer comprising a curable resin
material and having, after curing of said resin material, a
total modulus of elasticity which is higher than that of the
cured resin material o~ the supporting layer.
The multi-layer sheet structure prepared as described above
is fixed onto the panel to be reinforced, the adhesive layer
facing the panel to be reinforced, and said multi-layer sheet
structure is treated, for example heat-treated, in order to
cure the resin materials of said supporting layer and of said
adhesive layer and to firmly combine said multi-layer sheet
structure with the panel to be reinforced.
DESCRIPTION O~ PREFERRED EMBODIMENTS
The mul-ti-layer sheet structure shown in sec~ion in Fig. 1
was manufactured as explained below. The multi-layer sheet
structure, as it is present before its application, is general-
ly marked by the reference numeral "1" and has the following
;
.
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general structure:
The supporting layer 2 is made of a glass cloth 3, for ex-
ample of a glass-fiber roving cloth having a speciEic gravity
o~ 620 g/m . Said glass cloth 3 is embedded in a resin layer
4 consisting of a heat-curable resin. The following materials
are most suitable:
- Compound a)
90 parts liquid epoxy resin (for example "Epikote 828"
of Shell)
10 parts dicyandiamide
- Compound b)
90 parts diluted epoxy resin (for example "Epikote 215"
of Shell)
10 parts dicyandiamide
~ ~
60 parts solid epoxy resin (Eor example "Epikote 1001"
of Shell)
5 parts dicyandiamide
35 parts dimethylketone (serving as a solvent for the
impregnation and being afterwards evaporated)
The modulus of elasticity of the cured resin is adjusted to
a value of 2100 N/mm2 by suitable selection of the resin ma-
terial and the fillers. The percentage of the glass in layer 2
was varied as explained hereafter. The thickness of layer 2 was
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about 0.6 mm.
An adhesive layer 5, which essentially consisted of a heat-
curable self-adhering resin, was applied to said layer 2. The
following materials are most suitable:
- Compound a)
30 parts liquid epoxy resin (for example "Epikote 828"
of Shell)
20 parts solid epoxy resin (for example "Epikote 1001"
of Shell)
4 par~s dicyandiamide
20 parts talcum
26 parts fly ash ("Fillite")
35 parts liquid epoxy resin (for example "Epiko-te 828"
of Shell)
25 parts solid epoxy resin ~for example "Epikote 1001"
of Shell)
5 parts dicyandiamide
5 parts mica
30 parts talcum
The modulus o~ elasticity o~ the cured resin was adjusted to
a value of 3700 N/mm2 by suitable selection of the resin ma-
terial and the fillers. This value was clearly higher than the
modulus of elasticity of the resin used for layer 2. The thick-
ness of layer 5 was about 0.6 mm.
~ ~ O ~ 3 5
On the other hand, the modulus of elasticity of the cured
layer 2, that is to say of the combination of resin and glass
cloth, varied depending on the percentage of glass, but was at
least 5000 N/mm . Particularly, the coefficient of thermal
expansion can by varied by varying the ratio by volume of resin
to glass cloth.
The back side of layer 2 was covered by an aluminium sheet 6
having a thickness of 20 um, whereas the front side of layer 5,
when the multi-layer sheet structure l was unused, was covered
by a removable protecting paper 7.
COMPARATIVE TESTS
The following tests were made with this multi-layer sheet
structure:
TEST l
A flat strip of steel sheet 8 having a length of 400 mm, a
width of 25 mm and a thickness of 0.75 mm was provided wi-th a
multi-layer sheet structure 1 in the manner described above and
shown in Fig. l. The total thickness of said multi-layer sheet
structure l was l.6 mm. The percentage by volume of glass cloth
3 in the layer 2 of multi-layer sheet structure l was 51 %.
After application, the composite of steel sheet 8 and multi-
layer sheet structure l was heat-treated at 180C for 30 min-
utes. The resulting deflection dl of the composite towards
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the sheet metal was 1.8 mm (shown very exaggeratedly in Fig.
2). This is due to the fact that the coe~ficient of thermal
expansion of the reinforcing layer 2 of ~he mul-ti-layer sheet
structure 1 was lower than that of the steel sheet strip 8.
TEST 2
The same experimental setup as in Tes-t 1 was used with the
difference that the percentage by volume of glass cloth 3 in
layer 2 of the multi-layer sheet structure 1 was 48 %. As shown
in Fig. 3, the composite of steel sheet 8 and multi-layer sheet
structure 1 did not distort9 that is to say that any eventual
deformation was below the accuracy of measurement of 0.1 mm.
TEST 3
The same experimental setup and the same conditions as in
Test 1 were used with the difference that the percentage by
volume of glass cloth 3 in layer 2 of the multi-layer sheet
structure 1 was 44 %. The resulting deflection d2 of the com-
posite of steel sheet 8 and multi-layer sheet structure 1 to-
wards the multi-layer sheet structure was 2.6 mm (shown very
exaggeratedly in Fig. 4). This is due to the fact that the co-
efficient of thermal expansion of the reinforcing layer 2 of
multi-layer sh0et structure 1 was higher than that of the steel
sheet strip 8.
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TEST 4
A sample of the multi-layer sheet structure of Test 2 having
the dimensions of 80 x 80 mm was applied onto a steel sheet
having the dimensions of 200 x 200 x 0.75 mm. No de1ection
could be detected wi-th the naked eye after heat-treatment at
180C for 30 minutes.
TEST 5
A steel sheet strip without an applied multi-layer shee~
structure 1 having a length of 150 mm, a width of 25 mm and a
thickness of 0.75 mm was symmetrically supported at a distance
of 100 mm. Then, the force Fo, acting onto the center of the
sample, which is necessary for deflecting said steel sheet
strip by 10 mm, was determined. AEterwards, the test was re-
peated by using a steel sheet strip of the same dimensions but
provided with a multi-layer sheet structure according to Test
1. Th0 force necessary for deflecting said re.inforced steel
sheet strip by 10 mm was Fv. The reinforcing factor, that is
to say FV/Fo, was 9 4.
TEST 6
.
A steel sheet strip without a multi-layer sheet structure 1
applied onto it, having a length of 150 mm, a width of 25 mm
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and a thickness of 0.75 mm was symmetrically supported at a
distance of 100 mm. Then, the force Fo, acting onto the cen-
ter oE the sample, which is necessary for deflecting said steel
sheet strip by 10 mm was de-termined. Afterwards, -the test was
repeated by using a steel sheet strip of the same dimensions
but provided with a multi-layer sheet structllre according to
Test 2. The force necessary for deflecting said reinforced
steel sheet s-trip by 10 mm was Fv. The reinforcing factor,
that is to say FV/Fo, was 9.1.
TEST 7
A steel sheet strip w;thou-t a multi-layer sheet structur~ 1
applied onto it, having a length of 150 mm, a width of 25 mm
and a thickness of 0.75 mm was symmetrically supported at a
distance of 100 mm. Then, the force Fo, acting onto the cen-
ter of the sample, which is necessary for deflecting said steel
sheet strip by lO mm was determined. Afterwards, the test was
repeated by using a steel sheet strip of the same dimensions
but provided with a multi-layer sheet structure according to
Test 3. The force necessary for deflecting said reinforced
steel sheet strip by 10 mm was Fv. The reinforcing factor,
that is to say FV/Fo, was 9.l.
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CONCLUSIONS
The conclusions which can be drawn Erom these tests may be
summarized as follows:
1. All tests (1 to 3 and 5 to 7) fundamentally showed a dis-
tinctly better reinforcing effect onto the steel sheet sam-
ple than reported by the prior art (for example in said
EP-Bl-0,053,361).
2. No deformation of the steel sheet sample could be observed
if the supporting layer and adhesive layer were designed in
accordance with the present invention, as evidenced by Tests
2 and 6.
; 3. lf the supporting layer and adhesive layer were designed in
accordance with the present invention, only an insignificant
decrease of the reinforcing factor could be observed (9.1
against 9.4 in Tests 5 and 6).
