Note: Descriptions are shown in the official language in which they were submitted.
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STRUCTURAL LAMINATE AND
METHOD OF MAKING THE SAME
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates generally to a structural laminate and a
method of making the same and, more particularly, the invention relates to a
metal-paper-metal structural laminate useful in vehicular trailer bodies and
the
like.
Brief Description of Related Technology
Currently, aluminum sheeting is used in the vehicular trailer
market to form structural laminates. For example, a structural laminate is
described in International Publication No. WO 98/35114 dated August 13,
1998 (hereinafter the "' 114 publication"), the entire disclosure of which is
incorporated herein by reference. The ' 114 publication teaches that a
structural
panel can be prepared by laminating skins of sheet metal to a paper core. The
metal skins having a thickness of 5 mils to 12 mils are thicker than metal
foils.
(1 mil equals a unit of length equal to 0.001 inch.) To increase the stiffness
of
the composite sheet without increasing the thickness of the metal sheet, the
' 114 publication suggests laminating successive sheets of low-cost kraft
paper
between layers of adhesive and exposed sheets of metal, such as aluminum,
thereby increasing the moment of inertia and stiffness of the composite sheet.
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According to the ' 114 publication, one laminate includes
sequential layers of a first outer skin of sheet steel, a first adhesive
layer, a kraft
paperboard core, a second adhesive layer, and a second outer skin of sheet
steel. The kraft paperboard core is perforated and has a matrix of circular
openings punched from the paper. The openings define paths for adhesively
bridging the outer metal skins to each other, where the adhesive bridges form
columns of adhesive material oriented transversely to the adhesive layers and
integrally connected to the adhesive layers. Without the bridging, the
integrity
of the laminate is susceptible to failure, in particular from a defect known
as
buckling, where the fibers in the paper core become separated and there is a
local delamination of the paper.
The '114 publication further teaches the importance of the
orientation of the paper in the laminates. Specifically, the laminates exhibit
better performance (i.e., better stiffness and strength) characteristics when
the
machine direction of the kraft paper is aligned parallel to a neutral axis of
bending than when the machine direction of the kraft paper is perpendicular to
a neutral axis of bending.
Alternatively, the ' 114 publication discloses optionally
impregnating a kraft paperboard core with a phenolic resin to improve
performance. When the core is impregnated with the resin, the resin content of
the core is in a range of 40 wt.% to 50 wt.%.
The present invention relates to an outdoor application wherein
the product is riveted in place; hence, there are numerous sites for
undesirable
water penetration which can lead to corrosion and delamination. Therefore, it
would be desirable to provide a structural laminate which, when exposed to
various environmental conditions, does not corrode or delaminate.
Furthermore, it would be desirable to provide a structural laminate with a
high
internal bond strength so that adhesive bridges are not needed. Additionally,
it
would be desirable to provide a laminate that is stronger and lighter than
current aluminum products.
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SUMMARY OF THE INVENTION
It is an objective of the invention to overcome one or more of
the problems described above.
Accordingly, the invention provides a laminate formed of first
and second metal skins with a resin-impregnated paper prepreg layer disposed
between the metal skins. The resin content of the prepreg layer is an
effective
amount of less than about 40 wt.%.
Alternatively, the invention provides a laminate formed of first
and second outer metal skins, a middle structural layer disposed between the
metal skins, and at least two resin-impregnated paper prepreg layers, one
layer
disposed between the middle structural layer and each metal skin. The resin
content of the prepreg layer is an effective amount of less than about 40
wt.%.
A method of preparing the structural laminate is also provided,
including the steps of impregnating a paper with a resin to prepare a prepreg
layer, partially drying the prepreg layer to partially cure the resin,
disposing the
prepreg layer between first and second metal skins to form a structural
laminate, and applying sufficient pressure and heat to the laminate to fully
cure
the resin.
Further objectives and advantages of the invention may become
apparent to those skilled in the art from a review of the following detailed
description, taking in conjunction with the drawing figures and the appended
claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
For a complete understanding of the invention reference should
be made to the following detailed description and accompanying drawings
wherein:
Figure 1 is a schematic view of a structural laminate according
to a preferred embodiment of the invention;
Figure 2 is a schematic view of a structural laminate according
to another embodiment of the invention; and,
Figure 3 is a schematic depiction of a manufacturing process
employed to prepare a structural laminate of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to a structural laminate comprising
first and second metal skins, each having a thickness of preferably at least
about
5 mils, and more preferably in a range of about 7 mils to about 8 mils. For
example, in one embodiment the metal skins can each have a thickness of about
7.5 mils, and in another embodiment the metal skins can each have a thickness
of about 20 mils. The metal skins need not be the same thickness. Suitable
metal skins include, but are not limited to, galvanized steel, standard steel,
and
aluminum. A. thermosetting resin-impregnated paper prepreg layer is disposed
between the first and second metal skins. As used herein, reference to a
prepreg layer includes, but is not limited to, at least one sheet of
thermosetting
resin-impregnated paper.
In another embodiment, the laminate is formed of first and
second metal skins, a middle structural layer, and at least two prepreg
layers,
one layer disposed between the middle structural layer and each metal skin.
The resin is present in the prepreg layer in an effective amount less than
about
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40 wt.%, preferably about 35 wt.%, based on the total weight of the prepreg
layer. Suitable resins include, but are not limited to, thermosetting resins
selected from the group consisting of epoxy resins, phenolic resins, polyester
resins, vinyl ester resins, and combinations thereof.
A method of preparing the structural laminate is also provided,
including the steps of impregnating a paper with a thermosetting resin to
prepare a prepreg layer, partially drying the prepreg layer to partially cure
the
resin, disposing the prepreg layer between first and second metal skins to
form
a structural laminate, and applying sufficient pressure and heat to the
laminate
to fully cure the resin.
Referring to FIG. 1, the invention is directed to a structural
laminate, generally designated 10, formed of first and second metal skins 20
and 22, respectively, each skin having a thickness of at least about 5 mils
(e.g.,
a thickness of about 7 mils to about 8 mils), and a thermosetting resin-
impregnated paper prepreg layer 24 disposed between the first and second
metal skins 20 and 22, respectively. The prepreg layer can be of any suitable
overall thickness, for example about 10 mils to about 210 mils. The prepreg
layer may be made of a plurality of paper sheets each having a thickness of
about 10 mils to about 12 mils, for example.
The thermosetting resin-impregnated paper prepreg layer 24
illustratively includes two plies of 156 pound basis weight kraft paper
obtained
from Westvaco Corporation. The paper preferably is illustratively immersed in
an epoxy resin to apply a resin content in an effective amount less than about
40 wt.% based on the total weight of the prepreg layer. Preferably, the resin
content is at least about 30 wt.%, more preferably in a range of about 32 wt.%
to about 38 wt.%, and most preferably about 35 wt.%. Once the paper is
immersed in the resin, the paper is dried in an oven at varied temperatures
depending on the type of oven used. Preferably the paper is dried at
temperatures ranging from about 200°F to about 365 °F, and more
preferably
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about 315°F to about 365°F. The resin is only partially cured
during this
process, to help consolidate the laminate during the final press operation.
One epoxy resin that may be used to prepare the prepreg layer
24 is formed from bisphenol A-epichlorohydrin (a resin), dicyandiamide (a
promoter), 2-methylimidazole (a catalyst), and methyl Cellosolve (a trademark
of Union Carbide for an ethylene glycol monomethyl ether solvent). A
preferred bisphenol A-epichlorohydrin based epoxy resin solution can be
obtained from Shell under the tradename Epon~. Dicyandiamide is a
commercial product name for cyanoguanidine that can be obtained from SKW
Trostberg AG, Germany. 2-methylimidazole is a product name for 2-methyl-
1H-imidazole that can be obtained from BASF Corporation in Parsippany, NJ.
The epoxy resin is preferably prepared by combining about 50
pounds of dicyandiamide, about 4000 grams of 2-methylimidazole, and about
650 pounds of methyl Cellosolve in a kettle. The mixture then is stirred under
ambient conditions with a high speed sheer for about 20 minutes or until all
of
the dicyandiamide has dissolved. Next, about 1200 pounds of Epon~ resin
solution are added to the mixture and the resultant mixture is stirred under
ambient conditions with a high speed sheer for about 20 minutes. Preferably
the viscosity of the resulting epoxy resin, as measured by a Brookfield
tester, is
in a range of about 100 cps to about 300 cps, and more preferably about 150
cps to about 250 cps. Preferably the specific gravity is in a range of about
1.04
to about 1.07, and a stroke cure is between about 50 seconds and about 90
seconds.
Additionally, other thermosetting resins such as phenolic resins,
polyester resins, and vinyl ester resins can be used to impregnate one or more
kraft paper sheets forming a part of the prepreg layer. Generally, the resin
content when using a phenolic resin may be reduced from that preferred for
other, e.g., epoxy, resins. For example, the phenolic resin content is
preferably
at least about 25 wt.%, based on the total weight of the prepreg layer. If a
phenolic resin is used to impregnate the kraft paper, a glue line or another
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epoxy prepreg layer can be added adjacent to the metal skins to assist in
bonding the metal skins to the prepreg layer(s). A phenolic resin-impregnated
paper prepreg layer cannot be placed between the metal skins without an
adhesive layer because it will not bond to the metal skins. Preferably, the
resin
may consist essentially of, or consist of, an epoxy resin because it provides
improved moisture resistance, bond strength, and consolidation of the
laminate.
The prepreg layer 24, once partially cured, is then prebuilt with
the metal skins 20 and 22, respectively. The metal skins 20 and 22,
respectively, preferably are formed of a metal selected from the group
consisting of galvanized steel, standard steel, and aluminum. Preferably,
galvanized steel skins are used, each having a thickness of at least about 5
mils.
The resulting thickness of the laminate 10 can be varied by using different
numbers of prepreg layers 24. The resulting total thickness of the laminate 10
can be up to about 2 inches, for example.
Preferably, the metal skins each have a thickness of about 7.5
mils, so that the resulting thickness of the laminate 10 will be about 35
mils.
The prepreg layer 24 and metal skins 20 and 22 are then pressed at about 600
psi and heated to about 165 °C for about 65 minutes to form the
laminate 10.
The laminate 10 is then cooled for about 30 minutes.
In a preferred embodiment to the structural laminate of the
invention, the metal skins 20 and 22 each having a thickness of about 19 mils
are disposed on either side of the prepreg layer 24 to form the laminate 10.
The resulting thickness of the laminate 10 can be varied by using different
numbers of prepreg layers 24. Preferably, the resulting thickness of the
laminate 10 using metal skins 20 and 22 having thicknesses of about 19 mils
will be about 250 mils.
Alternatively, as shown in FIG. 2, a middle structural layer 30 is
disposed between the metal skins 26, and at least two thermosetting resin-
impregnated paper prepreg layers 28 are disposed between the middle
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structural layer 30 and the metal skins 26 to form the laminate 12. The middle
structural layer 30 can be formed of a woven glass cloth, which would improve
the impact and puncture resistance of the laminate 12. The resulting thickness
of the laminate 12 can be varied by using different numbers of prepreg layers
28. Preferred thicknesses of the metal skins 26 and the middle structural
layer
30 are described above.
Referring now to FIG. 3, a manufacturing process for the
structural laminate will be described. An epoxy resin is obtained from a
chemical source represented by an element 32 and then placed in a treating
apparatus 34. Rolls of kraft paper 36 are immersed in an epoxy resin bath, the
resin content is metered using squeeze rolls 37, and then the paper is
partially
dried in one continuous process in an apparatus represented by element 38 to
prepare prepreg layers 39. The prepreg layers 39 are then cut to a desired
length at 40 and prebuilt at 42 with metal skins 43. The prepreg layers 39 and
metal skins 43 are subjected to pressure and heat as shown by a molding
process 44. After the molding process 44 a structural laminate 46 is formed.
The structural laminate 46 then undergoes shearing 48, inspection 50, and
packaging 52. This manufacturing process is by way of illustration only and is
not intended to be limited to only those manufacturing steps disclosed.
The resulting laminate of the invention has superior water
resistance and good adhesion to the galvanized steel. A surprising result in
the
development is the good adhesion of the galvanized steel to the epoxy/paper
prepreg in light of the inherent difficulty when laminating to a galvanized
coating. Although both standard steel and aluminum could be used to form the
laminate, the laminate with the galvanized steel is stronger, lighter, and
lower in
cost.
The foregoing description is given for clearness of
understanding only, and no unnecessary limitations should be understood
therefrom, as modifications within the scope of the invention may be apparent
to those skilled in the art.
