Note: Descriptions are shown in the official language in which they were submitted.
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LOW DENSITY STRUCTURAL LAMINATE
FIELD OF THE INVENTION
[0001] The present invention relates to a structural laminate and more
particularly to a
low density structural laminate. The present invention further relates to a
method for producing
a low density structural laminate.
BACKGROUND OF THE INVENTION
[0002] Sheet steel is used extensively to form panels. The required
structural
characteristics, such as stiffness, vary depending upon the specific
application. When higher
stiffness values are required, the steel thickness is typically increased.
Increasing sheet steel
thickness, however, produces a panel that is not only heavier, but also more
expensive.
[0003] A number of approaches have been taken in the past to provide
improved
structural characteristics of panels, without substantially increasing weight
or material cost. For
example, composites of steel sheets having a solid polymer core have been used
in applications
where sound deadening and vibration dampers are required. The specific
stiffness of polymer
core products, however, is less than desirable.
[0004] United States Patent 5,985,457 [Clifford (Clifford #1)] teaches a
structural panel
which comprises a metal and paper composite. The paper core is a web which is
adhesively
bonded to the metal skins and which may have openings to create paths for
adhesive bridges
between the metal skins to minimize failure caused by buckling.
[0005] United States Patent 6,171,705 [Clifford (Clifford #2)] teaches a
structural
laminate having first and second skins of sheet metal. A fibrous core layer is
provided between
the sheet metal skins and is bonded to the skins. In one aspect, the fibrous
core layer is
impregnated with an adhesive resin which bonds the core layer directly to the
skins. In another
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aspect, layers of adhesive are placed between the core material and the metal
skins that bond the
core to the skins.
[0006] While the paper core and fibrous core laminates of Clifford #1 and
Clifford #2
represent a significant improvement in the art, there remains room for
improvement.
[0007] There is a continual need to produce a panel having the required
structural
properties discussed above and also having a lower density and a lower cost
compared with
traditional panels.
SUMMARY OF THE INVENTION
[0008] In one aspect, the present invention provides a structural
laminate comprising a
core layer disposed between and bonded to each of a first metal skin layer and
a second metal
skin layer, the core layer comprising a low density composite layer.
[0009] In an alternative embodiment the present invention provides a
structural laminate
comprising a composite layer disposed between and bonded to each of a first
metal skin layer
and a second metal skin layer, the composite layer comprising a mixture of
thermoplastic resin
and natural fiber.
[0010] In a further embodiment the present invention provides a
structural laminate
comprising a composite layer disposed between and bonded to each of a first
metal skin layer
and a second metal skin layer, the composite layer comprising a mixture of
thermoplastic resin,
natural fiber and at least one foaming agent.
[0011] In another aspect, the present invention provides a process for
producing a low
density structural laminate comprising the steps of: forming a composite layer
comprising
thermoplastic resin and natural fiber; placing an adhesive layer on each
surface of the composite
layer; disposing the composite layer between a first metal skin layer and a
second metal skin
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layer to define an interim laminate; and pressing and optionally heating the
interim laminate at a
first pressure to produce the structural laminate.
[0012] In an alternate embodiment, the present invention provides a
process as described
above with the additional step of surface treating the composite layer prior
to application of the
adhesive layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention will be described in further detail with
reference to the
accompanying drawings in which:
[0014] Figure 1 illustrates a sectional side view of one embodiment of
the low density
panel of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The present invention provides a low density structural laminate,
indicated
generally at numeral 10 in Figure 1.
[0016] The low density panel 10 includes a first metal skin layer 12 and
a second metal
skin layer 14. Interposed between the first and second metal skin layer 12, 14
is a low density
composite layer 16.
[0017] Disposed between the first metal skin layer 12 and the low density
composite
layer 16 is a first adhesive layer 18. A second adhesive layer 20 is disposed
between the second
metal skin layer 14 and the low density composite layer 16.
[0018] The first adhesive layer 18 serves to bond the low density
composite layer 16 to
the first metal skin 12. Likewise, the second adhesive layer 20 serves to bond
the second metal
skin 14 to the low density composite layer 16.
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[0019] The first and second adhesive layers 18, 20 may be the same or
different, although
preferably the same. Suitable adhesives that may be used include adhesives
that are compatible
with the composite layer and the metal skins to which the adhesive will be
applied. Suitable
quantities of adhesive will depend on the properties of the adhesive used, and
the choice of
adhesive quantity will be within the purview of persons skilled in the art.
Examples of adhesives
that may be used include, but are not limited to, thermoplastic adhesives,
thermoset adhesives or
combination adhesives such as reactive hot melt polyurethane (PUR). The
adhesive may be
applied to the metal skin layer or the composite layer. Examples of suitable
adhesives that may
be used include, but are not limited to Rohm and Haas 1223 PE resin or 5003
PUR resin. When
this resin is used, first adhesive layer 18 and second adhesive layer 20 can
suitably each be
applied in a layer between about 0.0005 inches and about 0.010 inches in
thickness and more
preferably between about 0.001 inches and 0.005 inches in thickness.
[0020] The particular choice of metal for metal skin layers 12 and 14
used in structural
laminate 10 is not particularly restricted. First metal skin layer 12 and
second metal skin layer
14 may be the same or different. Non-limiting examples of suitable metal skin
layers for use in
the present invention include aluminum, cold rolled steel, galvanized steel,
tin-coated steel, zinc
coated steel, low carbon micro-alloyed high-strength steel and stainless
steel. In a preferred
embodiment of the present structural laminate, one or both of first metal skin
layer 12 and
second metal skin layer 14 comprise steel. In a particularly preferred
embodiment of the present
structural laminate, one or both of first metal skin layer 12 and second metal
skin layer 14
comprise pre-painted zinc-coated steel.
[0021] Preferably, first metal skin layer 12 and second metal skin layer
14 have the same
or different thicknesses and the thickness of each is at least 0.005 inches.
More preferably, first
metal skin layer 12 and second metal skin layer 14 have the same or different
thicknesses and the
thickness of each is in the range of from about 0.005 inches to about 0.030
inches. Most
preferably, first metal skin layer 12 and second metal skin layer 14 have the
same or different
thicknesses and the thickness of each is about 0.019 inches.
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[0022] The low density composite layer 16 is a low density natural-fiber
plastic
composite. The low density composite layer 16 is made from a material
including a mixture of
thermoplastic resin and natural fiber. Preferably the low density layer is
formed from uniformly
distributed thermoplastic resin and natural fiber that are extruded together
to form a thin flat
board of uniform thickness. Preferably a foaming agent is incorporated into
the composite layer
which will enable a composite layer to be produced that has a reduced weight.
An example of a
suitable foaming agent that may be used includes the commercially available
product
Expancele, manufactured by Akzo Nobel. Other foaming agents known to a person
skilled in
the art may also be used. The foaming agent may be incorporated in the range
of between about
1% and about 5% and preferably in the range of about 2% and about 3%.
[0023] The thermoplastic resin that is used in the low density core may
be selected from
any thermoplastic resin material. The thermoplastic resin may also be a mix of
more than one
type of thermoplastic resin. Preferably the thermoplastic resin is
polypropylene or polyethylene.
[0024] The natural fiber that is used in the low density composite layer
may be any
natural fiber. Examples of the type of natural fiber that may be used include
wood fiber, for
example oak flour, and rice husks. Preferably the natural fiber is rice husks.
[0025] The low density composite layer includes a mixture of the
thermoplastic resin and
the natural fiber. Preferably the low density composite layer includes between
about 50% and
about 70% of thermoplastic resin and between about 30% and about 50% of
natural fiber. More
preferably, in order to reduce cost and to improve the mechanical properties
of the composite
layer, the low density composite layer includes a 50:50 mix of thermoplastic
resin and natural
fiber. Preferably, the low density composite layer has a thickness of between
about 0.075 inches
and about 0.5 inches
[0026] In one embodiment the low density layer is formed by combining
thermoplastic
pellets with the natural fiber and at least one foaming agent and extruding
the composite layer.
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An example of the type of extruder that may be used to mix and extrude the
composite layer is a
melt screw extruder. The extruded product will be a flattened composite layer.
[0027] The low density composite layer provides a solid board that may be
used as a core
layer in a structural laminate allowing for easy manufacturing while providing
the structural
properties required in a panel. The foamed solid board provides a light weight
core that reduces
the overall weight of the panel.
[0028] The low density composite layer also provides an improved impact
resistance
compared with some of the conventional panels. The use of a pre-formed solid
board as the core
reduces issues with defective cores since the core is pre-fabricated.
[0029] To form a low density composite laminate or panel initially the
composite layer is
manufactured, as described above. A panel is then formed by securing the
composite layer
between first and second metal skins. A person skilled in the art will know
several ways of
producing the panel that are known in the art. The following methods provide
examples of
different ways of forming the panel but are not meant to be limiting.
[0030] The composite panel may be formed using a batch press which places
the
composite layer between two metal skins including an adhesive layer between
the composite
layer and each metal skin. The batch press will apply both pressure and
temperature to the panel
to form the panel and adhere the composite layer to the skins. The amount of
pressure that may
be applied using this method is in the range of between about 50 psi and about
150 psi. The batch
press may be used at a temperature in the range of about 250 F to about 400 F
. More preferably
the batch press method is conducted at a temperature about 300 F. It will be
understood that if a
thermoplastic adhesive is used, the panel must be cooled to below about 200 F
to solidify the
adhesive layer before removing pressure from the panel.
[0031] The composite panel may be formed using a continuous laminator
which places
the composite layer between two metal skins, including an adhesive layer
between the composite
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layer and each metal skin, between two moving belt presses. The continuous
laminator will apply
both pressure and temperature to the panel to form the panel and adhere the
composite layer to
the skins. The amount of pressure, temperature and cooling that may be applied
are similar to the
ranges discussed above in relation to the batch press method.
[0032] The composite panel may also be formed using a roll coater which
places a liquid
adhesive between the composite layer and each of the metal skins and allows
the liquid adhesive
to cure and secure the composite layer in place. This process uses a batch
press, continuous
laminator, nip roller or multiple nip rollers to apply a low pressure to
provide good contact
between the composite layer and each of the metal skins in order to form the
panel. For example,
the applied pressure may be in the range of about 25 to about 50 psi.
[0033] In an alternative embodiment, the structural laminate is formed by
extruding the
composite layer between a first and second metal skin without the requirement
of an adhesive
layer.
[0034] In an alternative embodiment, the composite layer may be surface
treated prior to
being placed in the structural laminate. The surface treatment may include the
use of flame,
plasma or corona treating. The use of the surface treatment provides a more
reactive surface on
the composite layer allowing the adhesive to bond more readily to the
composite layer.
[0035] Examples of the type of applications for the low density
structural laminate of the
present invention include, but are not limited to the following: side and/or
wall panels in truck
trailers, interior liner panels in truck trailers, architectural and/or
decorative panels and
automotive applications.
[0036] The following panel was made according to the present invention.
The structural
panel included two 0.018 inch HSLA skins and a composite layer placed
therebetween in
accordance with the description provided above. The total thickness of the
panel was 0.240
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inches and the panel had a flexural stiffness of 1250 lbs/inch (based on a 1
inch x 6 inch sample)
with a nominal weight of 2.351bs/ft2.
[0037] While
this invention has been described with reference to illustrative
embodiments and examples, the description is not intended to be construed in a
limiting sense.
Thus, various modifications of the illustrative embodiments, as well as other
embodiments of the
invention, will be apparent to persons skilled in the art upon reference to
this description.
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