U.S. patent application number 09/775416 was filed with the patent office on 2001-10-18 for molded article.
Invention is credited to Delusky, Arthur K., Ellison, Thomas M., Lucke, Robert, McCarthy, Stephen P..
Application Number | 20010031349 09/775416 |
Document ID | / |
Family ID | 27055132 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031349 |
Kind Code |
A1 |
Ellison, Thomas M. ; et
al. |
October 18, 2001 |
Molded article
Abstract
A molded plastic article having improved electromagnetic
interference (EMI) shielding properties. The article includes a
plastic layer and an outer film layer over the plastic layer. The
outer film layer has an inner surface adjacent to and bonded to the
plastic layer and an outer surface opposed to the inner surface.
The plastic layer and outer layer are electrically conductive.
Preferably the outer layer has higher conductivity than the plastic
layer.
Inventors: |
Ellison, Thomas M.; (Fort
Mill, SC) ; Delusky, Arthur K.; (Detroit, MI)
; Lucke, Robert; (Cincinnati, OH) ; McCarthy,
Stephen P.; (Tynesboro, MA) |
Correspondence
Address: |
Robert H. Bachman
BACHMAN & LaPOINTE, P.C.
Suite 1201
900 Chapel Street
New Haven
CT
06510-2802
US
|
Family ID: |
27055132 |
Appl. No.: |
09/775416 |
Filed: |
February 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09775416 |
Feb 1, 2001 |
|
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09505406 |
Feb 16, 2000 |
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Current U.S.
Class: |
428/292.1 |
Current CPC
Class: |
Y10T 428/2931 20150115;
Y10T 428/26 20150115; B32B 27/20 20130101; Y10T 428/249924
20150401; Y10T 428/24993 20150401; Y10S 428/925 20130101; Y10S
428/923 20130101; Y10T 428/294 20150115; Y10S 428/931 20130101 |
Class at
Publication: |
428/292.1 |
International
Class: |
D04H 001/00 |
Claims
What is claimed is:
1. A molded plastic article, which comprises: a plastic layer; an
outer film layer over said plastic layer and forming a composite
laminate therewith, said outer film layer having an inner surface
adjacent to and bonded to said plastic layer, and an outer surface
opposed to said inner surface; wherein said plastic layer and said
outer layer are electrically conductive to provide resistance to
electromagnetic interference.
2. Article according to claim 1, wherein the plastic layer has a
higher resistivity than the outer layer.
3. Article according to claim 1, wherein the volume resistivity of
the plastic layer ranges from about 3.times.10.sup.6 to about
3.times.10.sup.2 ohm meter and the surface resistance of the outer
layer is from 10.sup.1 to 10.sup.-3 ohms/square.
4. Article according to claim 3, wherein the barrier resistance
between the outer film and the plastic is on the order of magnitude
of the filled plastic.
5. Article according to claim 3, wherein the plastic layer and
outer film layer include conductive material incorporated
therein.
6. Article according to claim 1, wherein the conductivity of the
plastic layer is provided by incorporating therein at least one of
conductive fibers and conductive particles.
7. Article according to claim 1, wherein the outer film layer
includes at least one of the following conductive materials:
conductive fibers, conductive particles, metal foil, metal mesh,
carbon fiber, metallized film, metallized fiber and woven
conductive fabric.
8. Article according to claim 1, including a third layer bonded to
said plastic layer on the side thereof opposed to said outer film
layer.
9. Article according to claim 8, wherein said third layer is
electrically conductive.
10. Article according to claim 9, wherein the conductivity of the
third layer is provided by incorporating therein at least one of
conductive fibers and conductive particles.
11. Article according to claim 6, wherein plastic layer includes
carbon fibrils as conductive fibers, said fibrils having fiber
lengths of from 1 to 10 microns.
12. Article according to claim 11, wherein the plastic layer
includes 3-5% by weight of said carbon fibrils.
13. Article according to claim 1, wherein said article is
compression molded.
14. Article according to claim 1, including a reinforcing layer
between the outer film layer and plastic layer.
15. Article according to claim 1, wherein said outer film layer is
colored.
16. Article according to claim 1, wherein said outer film layer is
plastic.
17. Article according to claim 1, wherein said outer film layer has
a colored topcoat.
18. Article according to claim 17, wherein said outer film has a
clear coat on top of the colored topcoat.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation-In-Part of U.S. patent
application Ser. No. 09/505,406, filed Feb. 16, 2000.
BACKGROUND OF THE INVENTION
[0002] The use of plastic housings for electronic equipment and
components is widespread in the automotive and electronic equipment
fields as well as in other areas. However, plastic materials
generally suffer from the disadvantage of being transparent or
permeable to Electromagnetic Interference, commonly known as and
referred to herein as "EMI". This disadvantage is of considerable
concern in view of the susceptibility of electronic equipment to
the adverse effects of EMI, and to the large and growing number of
consumer products which produce EMI, as well as the increasing
regulatory controls exercised over such electromagnetic
pollution.
[0003] Present enclosure design approaches are reaching their
electromagnetic interference shielding limits. The need to contain
the extraordinary high-frequency EMI found in today's electronic
equipment, coupled with the higher sensitivity of the equipment to
EMI, has made enclosure design a more demanding task. For example,
due to their smaller geometry, devices operating at higher
frequencies are much more sensitive to EMI.
[0004] Plastic materials are highly desirable as materials of
construction for cabinets used to house electronic equipment. They
can be readily shaped and styled to make space saving, attractive
enclosures. However, plastics are extremely poor electrical
conductors and provide no EMI shielding unless highly modified.
[0005] One method of modifying the plastic is to paint its surface
with a paint formulated with conductive material. However, such
paints are labor intensive due to masking requirements and are
prone to chipping and peeling and do not contribute to conductivity
in the bulk of the plastic.
[0006] Alternatively, conductive filler such as carbon fiber or
metal particles may be compounded into the plastic to be molded.
These additives are expensive when used in sufficient quantity to
achieve the desired volume resistivity (ohm-meter) necessary for
EMI applications. In addition, the physical robustness of the
structural plastic is significantly degraded at higher levels of
conductive filler loading. Another EMI problem is potential loss of
shielding effectiveness. This occurs as a consequence of RF leakage
into or out of an enclosure along the interfacing surfaces between
its outer low surface resistance film and its high volume
resistivity inner plastic structure. The same effect can occur
along the interfacing surfaces between the various conductive
filler loaded molded parts of a multi-part EMI enclosure. The RF
leakage is a consequence of the meniscus wetting characteristics of
plastic that creates a thin insulating film along interface
surfaces.
[0007] Accordingly, it is a principal object of the present
invention to overcome the foregoing disadvantages and to provide a
molded plastic article which has improved EMI shielding properties,
e.g., no loss of physical robustness and enhanced RF shielding.
[0008] It is a further object of the present invention to provide a
molded plastic article as aforesaid which is inexpensive and which
may be simply, conveniently and expeditiously prepared.
[0009] Further objects and advantages of the present invention will
appear hereinbelow.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, the foregoing
objects and advantages are readily obtained.
[0011] The molded plastic article of the present invention
comprises: a plastic layer; an outer film layer over the plastic
layer and forming a composite laminate therewith, the outer film
layer having an inner surface adjacent to and bonded to the plastic
layer, and an outer surface opposed to the inner surface; wherein
the plastic layer and the outer film layer are both electrically
conductive and reflect and/or adsorb electromagnetic interference.
Preferably, the outer film layer will have lower surface resistance
when compared to the plastic layer. In the preferred embodiment,
the volume resistivity of the plastic layer ranges from about
3.times.10.sup.6 to about 3.times.10.sup.2 ohmmeter and the surface
resistance of the outer film layer will be significantly less than
the plastic layer and in the range from 10.sup.1 to 10.sup.-3
ohms/square.
[0012] The conductivity of the plastic layer is provided by
incorporating either conductive fibers or conductive particles, and
the outer film layer may also include conductive fibers or
conductive particles, and may in addition include at least one of
the following conductive materials: metal foil, metal mesh, carbon
fiber, metallized film, metallized fiber and woven conductive
fabric.
[0013] The molded plastic article of the present invention may
include a third layer bonded to the plastic layer on the side
thereof opposed to the outer film layer, wherein the third layer is
desirably electrically conductive.
[0014] Further features of the present invention will appear
hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will be more readily understandable
from a consideration of the accompanying drawings, wherein:
[0016] FIG. 1 is a partly schematic view showing one embodiment of
the preparation of the molded article of the present invention;
[0017] FIG. 2 is a sectional view of the article prepared in FIG.
1; and
[0018] FIG. 3 shows a further embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] EMI shielding is of growing importance in industry. EMI
shielding is required to contain EMI emanating from sources such as
electronic devices in the engine compartment of automobiles, or
from high frequency devices used in today's information technology
equipment. Conversely, shielding is also required to prevent
extraneous EMI from adversely affecting the same sensitive
electronic equipment and systems. The problem of EMI shielding is
exacerbated by the growth in plastics used in automotive body parts
and as enclosure material for electronic devices since plastics are
in general insulators.
[0020] Plastic conductivity can be increased in several ways. For
example, by compounding in conductive fillers, as carbon powder or
stainless steel fiber. Alternatively, by the application of
conductive coatings as layers to one or both surfaces, such as
vacuum deposited metal, carbon fiber matte or metal foil or mesh. A
still further alternative is to make the plastic out of polymers
that are inherently conductive, such as polyaniline.
[0021] While each of these methods improve conductivity, they are
either very costly or in practice detract from the desirable
performance properties of the plastic. For example, impact
properties of plastics are reduced when conductive fillers, e.g.,
carbon powder or conductive fibers, are used in sufficient
quantities to achieve effective shielding. Conductive surface
layers are effective, but problems can occur with one side coating
due to charge dissipation on the non-coated side or capacitance on
two side coatings. Inherently, conductive polymers are expensive
and do not always have the physical properties desired for a
particular application.
[0022] The present invention overcomes these disadvantages and
addresses the problems of cost increases and property degradation
due to higher conductive filler loading while achieving levels of
conductivity required for effective shielding.
[0023] Thus, in accordance with the present invention, conductive
material is concentrated in at least one outer surface of the
molded article while providing conductivity through the bulk of the
article, with desirably less conductivity provided through the bulk
than at the outer surface(s). Although the molded article may be
obtained by any convenient method, in a preferred embodiment, a
conductive film for the outer surface of the molded article is
placed over a mold cavity, molten plastic containing a limited
amount of conductive material is applied on the film, and the
resultant composite is molded into the desired final shape by a
mold core into a mold cavity. A typical and preferred procedure is
found in copending U.S. patent application Ser. No. SN 09/130,864,
filed Aug. 7, 1998 for PROCESS AND APPARATUS FOR PREPARING A MOLDED
ARTICLE, By Valyi et al., the disclosure of which is incorporated
herein by reference.
[0024] A conductive backing layer is optionally and preferably
applied to the opposing side of the molten plastic to form a
three-layer conductive laminate. Alternatively, the film finish or
conductive outer film may be replaced with or combined with, for
example, metal foil, perforated metal or woven metal screen or
woven or non-woven carbon fiber sheet, with the metal or other
sheet materials having sufficient formability for a given
application. The same foil or other conductive material may if
desired be used on the second or back surface of the molded
part.
[0025] The conductive material is incorporated into the plastic
layer or the outer film surfaces may be metal, metallic particles,
plastic or fibrous particles coated with metal as by vacuum
deposition, or conductive carbonaceous material, as fibers,
graphite particles, etc. A particularly desirable conductive
material is a small carbon fiber having a very high length to
thickness ratio. High conductivity is possible with this material
at relatively low loadings. For appearance purposes, the conductive
material may be incorporated into the outer films and/or into the
plastic layer along with pigments and/or dyes to yield a desired
finished appearance for the final molded article.
[0026] As a further alternative, a durable outer film layer such as
polyvinyl chloride (PVC), polyvinylidiene fluoride (PVDF),
polycarbonate (PC), polyester polymers and copolymers (PET/PBT),
polyvinyl fluoride (PVF), acrylic, olefin polymers such as
polypropylene (PP), polyethylene (PE) or their copolymers,
acrylonitrile-butadiene-styrene (ABS) or other similar polymers and
appropriate blends and copolymers of those named may be formulated
with a high loading of conductive material as for example 5% to 30%
by volume. Alternatively, the outer film may be a laminate of a
polymer filled with conductive additive and a metal mesh or
conductive fiber mat or woven conductive fiber. In a further
embodiment, the outer film may be metal mesh or conductive fibrous
mat or conductive woven fiber. This outer film may have a thin
coating, in the range of 0.0001" to about 0.05", applied to its
outer surface to provide appearance and/or
durability/weatherability.
[0027] The molding resin for the plastic layer is selected for its
structural properties desired for the end use. It is compounded
with the desired level of conductive material to provide the
desired level of volume resistivity, generally about 10.sup.6 to
10.sup.2 ohm-meter and preferably with a higher resistivity than
the outer layer(s). Thus, for example, the outer layer(s), if
plastic, should desirably have from about 5% to 30% by volume of
conductive material incorporated therein and a surface resistance
of 10.sup.1 to 10.sup.-3 ohms per square (typically equivalent to a
volume resistivity of 3.times.10.sup.-2 to 3.times.10.sup.-6).
While the thicker inner plastic layer may have the same low volume
resistivity range as the outer layer, to maintain structural
robustness, it preferably has a much higher volume resistivity (in
the range of from about 10.sup.6 to 10.sup.2 ohm-meter). Thus, the
level of conductive material loading is such that the desirable
structural properties of the inner plastic layer are maintained
while allowing a low level of conductivity throughout the plastic
and/or plastic/film intra-structure to minimize electrostatic
build-up and consequential Electrostatic Discharge (ESD). For
example, ABS and an ABS/PC blends are good candidates for
electrical housings. Candidates for the outer layer include
polyvinyl chloride (PVC), polyvinylidiene fluoride (PVDF),
polycarbonate (PC), polyester polymers and copolymers (PET/PBT),
polyvinyl fluoride (PVF), acrylic, olefin polymers such as
polypropylene (PP), polyethylene (PE) or their copolymers.
Acrylonitrile-butadiene-styrene (ABS) or other polymers such as
olefins or polyesters and appropriate blends and copolymers of
those named are other candidates for the molding resin. Naturally,
other suitable or desirable polymers may readily be employed.
[0028] If desired, a backing conductive layer may be used for the
outer layer. If it is a continuous sheet, it may be perforated to
facilitate air removal in the molding process. The backing sheet
may also be a conductive scrim such as a non-woven carbon scrim or
it may be a woven conductive fabric. Alternatively, the backing
layer may be a metal foil, perforated metal or woven metal.
[0029] The conductive material for the outer layers and the plastic
layer may be the same in each layer or they may be different in
each layer.
[0030] In accordance with the illustrative embodiment of FIG. 1,
film or blank 10 is held over mold 12 by clamping frame 14, with
optional reinforcing material 16 adhered to or at least partly
embedded in the inner face 18 of film 10. Outer face 20 of film 10
is opposed to the inner face and forms the external surface of the
molded material in the embodiment of FIG. 1. Platen 22 is shown
with a forming mandrel 24, which is desirably a solid metal
mandrel, but which may also for example be an elastomeric mandrel,
and which may contain air slots and pressure control means
connected thereto (not shown). Mold 12 includes mold cavity 30
therein which forms the shape of the desired molded article 32
shown in FIG. 2. Naturally, any desired mold cavity shape may be
used. Hot plastic 34 is injected or deposited on the film 10
reinforcing material 16 combination by any desired means, as from
an extruder, to form the layered structure shown in FIG. 1.
[0031] In operation, the mandrel 24 enters the clamping frame 14.
Thereby, pressure is applied to the deposited plastic 34 and the
film 10--reinforcing material combination. Pressure may also be
present in mold cavity to support the layers over the mold before
molding, as by air slots in mold 12 (not shown), which should be
controllably released at the same time as the layers are formed
into the mold cavity. This will result in forming the layers, in
part by air pressure, into the molded article shown in FIG. 2. The
mandrel 24 moves simultaneously downward into the mold cavity 30 in
the direction of arrow 38. As the plastic and film reach the mold
surface, the mandrel continues and conforms to the surface of the
plastic away from the film 10--reinforcing material layers, and
continued pressure by the platen results in forming of the final
desired article, as shown in FIG. 2. The mandrel and mold are
preferably cooled. If a solid mandrel is used, it should conform to
the desired shape of the final article. If desired, the
film-reinforcing material combination may first be conformed to the
shape of mold cavity 30, mandrel 24 seated in mold 12 and plastic
material 34 injected against the film-reinforcing material to form
the final article.
[0032] The resultant molded article 32 includes the outer film
layer 10 with conductive particles 11 dispersed therein, inner
reinforcing layer 16 which may also contain conductive particles
and which is adhered to and/or at least partly embedded in the
outer film layer, and the inner plastic layer 34 with conductive
particles 35 dispersed therein. The resultant molded article 32 has
the desired shaped configuration determined by the shape of the
mold cavity.
[0033] The film layer or blank 10 is desirably colored in order to
form a color coated article in a simple and convenient manner.
[0034] Desirably, a second outer film layer may be used over
plastic layer 34 on the side opposed to the first outer film layer
10. This is shown in FIG. 3 without the optional reinforcing
material. FIG. 3 shows first outer film layer 10 with conductive
particles 11 dispersed therein, plastic layer 34 adhered thereto
with conductive particles 34 dispersed therein and second outer
film layer 36 with conductive particles 37 dispersed therein
adhered to the plastic layer 34 on the side opposed to the first
outer film layer. Naturally, a reinforcing layer may be used
adjacent either or both of the outer film layers.
[0035] As indicated hereinabove, the conductive material is
incorporated in the outer layer(s) and in the plastic layer to
provide the conductive, multi-layer composite of the present
invention.
[0036] The outer layers 10 and 36 are desirably plastic, and any
desired plastic material may be used for the outer layers or molten
plastic material 34, for example, polyolefins, polyurethanes,
acrylonitrile butadiene styrene (ABS), polyvinyl chloride,
polystyrene, polycarbonates, fluoropolymers, acrylic polymers,
polyesters, blends of the foregoing, etc., and desirably blends of
the foregoing. Layers 10, 34 and 36 may be the same or different
materials. If the same materials are used, one can obtain melt
bonding between the materials. If different materials are used, one
can achieve a mechanical bond or an adhesive bond or through the
use of an adhesive layer. The blank 10 may be cut or stamped from a
web and a supply of blanks having the size and shape to fit over or
into the mold cavity maintained adjacent the mold for transfer to
the mold as needed.
[0037] If desired, a reinforcing material 16 may be adhered to
and/or at least partly embedded in the inner surface of the film or
blank in order to eliminate or significantly minimize the color
thinning. Desirably, the reinforcing material is a textile
material, synthetic or natural. However, one could readily use a
fiberglass mat or scrim or a random fiberglass material, or metal
or additional plastic. The metal should be a metal mesh or metal
which will elongate or form under pressure. The fiberglass mat or
scrim is desirably knitted. This will effectively reinforce the
film without jeopardizing the color tones of the color-coated film
and without interfering with the forming operation. The reinforcing
material serves to distribute the elongating forces produced by the
engagement with the most protruding areas of the mold core. Thus,
the entire film 10 is caused to elongate rather than the immediate
area of contact. Accordingly, film thickness remains relatively
uniform after the part is formed.
[0038] The blank may be applied to the mold with robot means or
removably adhered to a carrier film strip. The carrier film strip
may be provided with means to register the position of the blanks
relative to the mold half onto which the blanks are to be placed,
e.g., edge perforations. The carrier, with the blanks attached, may
then be supplied from a roll. Once the blank and mold are
juxtaposed, suction is applied to the edge of the blank by the
mold, as through channels, sufficient to separate the blank from
the carrier strip. Naturally, other transfer means may be readily
be used.
[0039] Naturally, any other convenient or desirable plastic
delivery means or molding procedure may conveniently be used while
still retaining the advantages of the present invention, as for
example alternative compression molding techniques, injection
molding or injection blow molding.
[0040] Similarly, multiple plastic layers may be deposited on the
film or the film-reinforcing material combination. Alternatively,
one polymer could be deposited in a designed pattern, and a second
or a plurality of second polymers deposited in a designed pattern.
This could be done with one or more extruders feeding for example
separate channels to deposit a predesigned pattern of multiple
resins. As a further alternative, one could sequentially feed
polymers of different characteristics to provide designed
properties in the finished product.
[0041] Thus, in accordance with the present invention, a molded
plastic article or panel is provided having a thermoplastic zone, a
static dissipation zone (SDZ), formulated with conductive material
or fillers, such as graphite fibrils, carbon fiber, stainless steel
fibers or carbon powder. The amount of filler is minimized to limit
the effect of filler on the physical properties of the plastic
layer, to reduce cost in the case of carbon fiber and sufficient to
provide for static dissipation but still well below that required
for EMI and radio frequency (RF) shielding. The surface resistance
of this zone is between about 10.sup.9 to about 10.sup.5 ohms per
square. The SDZ may be molded into the article by for example
compression, injection or extrusion molding.
[0042] A conductive surface zone (CSZ) on one or two sides is
combined with the SDZ in forming the article. The CSZ may be
plastic, metal foil, metal mesh, woven metal, carbon fiber mat, a
blend of carbon or metal fibers with polymer fibers or polymer
fibers having a vacuum deposited metal surface or combinations of
these. Naturally, if a plastic is used, it should include the
desired amount of conductive filler. The surface resistance
(ohm/square) of the CSZ is desirably from 10.sup.1 to 10.sup.-3
ohms/square.
[0043] The present invention will be more readily understandable
from a consideration of the following illustrative examples.
[0044] It is to be understood that the invention is not limited to
the illustrations described and shown herein, which are deemed to
be merely illustrative of the best modes of carrying out the
invention, and which are susceptible of modification of form, size,
arrangement of parts and details of operation. The invention rather
is intended to encompass all such modifications which are within
its spirit and scope as defined by the claims.
* * * * *