Note: Descriptions are shown in the official language in which they were submitted.
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CONDUCTIVE EXTERIOR/ANTI-STATIC INTRRIOR CONTAINER
~ FIELD O~ THE INVENTION
The field of the invention encompasses portable containers for the
electrostatic discharge protection of sensitive electronic circuits or solid
state devices during transportation and storage. More particularly, the
present invention includes containers which, while closed, protect the
contents of the container from external electrical and physical forces.
BACKGROUND OF THE INVENTION
When two bodies, particularly of unlike materials, are brought
together into intimate contact, a redistribution of electrons across the
interface is likely to occur. An attractive force is established as equi-
librium is achieved. Work must be done in opposition to these attractive
forces if and when the bodies are separated. The energy so expended mani-
fests it~elf as an increase in electrical tension or voltage between the
respective surfaces, which become electrically charged with respect to each
other. If a conductive path is available, the charges thus separated will
reunite immediately. If no such path is available, as in the case of non-
conductors, the potential increase with separation may reach values of
several thousand volts. The generation of such electrical forces by contact
is known as triboelectricity.
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The charge on a charged object will be located on the exterior
surface thereof, and these forces have a strong influence on nearby objects.
If a neighboring object is a conductor it will experience a separation of
charges by induction. Its repelled charge is free to give or receive
S electrons as the case may be; if another conductor is broug~t near, the
transfer may occur through the agency of the spark, very often an energetic
spark.
Triboelectrically generated charges may adversely affect or even
electronically destroy a number of electronic circults or solid state de-
vices sensitive to sudden or stray electric charges or static electricity.Micro-circuit devices such as integrated circuit chips may be destroyed or
weakened by electrostatic discharge prior to their incorporation into the
electrical or electronic equipment for which they were designed. Damage
Erom electrostatic discharge may make such devices prone to latent or
catastrophic failure during use.
To prevent electrostatic breakdown, containers in which such
devices are stored and transported have been provided with means for short
circuiting the device terminals or pins. This short circuiting serves to
prevent the accumulation of potentially damaging static charges on the
device.
U. S. Patent No. 4,171,049 discusses the utilization of a series
of conductive slots or grooves in which solid state devices may be inserted
and later dispensed to manufacturing equipment.
Other containers have been developed for portable use as for
example in the device replacement market. These are typically small box
like containers that house conductive sponge or foam sheets into which the
device terminals are temporarily imbedded. An example is found in U. S.
Patent No. 4,333,565.
In addition to the requirements for inhibiting electrostatic
charge build up and shielding solid state devices from electrical fields, a
container useful for transporting such devices should also provide pro-
tection from mechanical shock and vibration. In addition, such a container
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should be of lightweight construction and convenient to use when gaining
access to the equipment to be repaired. The container should also be
adaptable to the storage of different si~es and shapes of solid state de-
vices without change in the size and shape of the container itself. Re-
useability, tamper security, and cost economy are also importantconsiderations.
Many of the containers referred to above are designed for online
in-factory production use and do not meet many of the requirements desirable
in a portable container. For example, many of the prior art containers are
not adaptable to solid state devices of different shapes and sizes.
Another means well known in the art for providing protection of
solid state devices is the use of carbon black or carbon powder in varying
amounts in the material forming the container for the solid state devices.
U. S. Patent No. 4,494,651 issued to Malcolm discloses a portable
work station in which electrically conductive material, such as carbon black
particles, aluminum particles, and metal filings, is blended with
~` thermoplastic material to make an electrically conductive case.
Electrically conductive containers are often provided with an
electrically conductive or anti-static foam pad which is used to line the
bottom interior surface of the container. The foam pad has hygroscopic
wetting or other anti-static agents incorporated therein or thereon.
The presence of the wetting agents, which attract moisture to the
; surface of the pad, substantially diminishes the possibility of static
;~ electricity being generated in the interior of the container as the result
of friction between the solid state devices and the foam pads supporting the
devices. Moreover, since the foam pad has been rendered electrically
conductive and is in electrically conductive communicatîon with the
container, any static charge which may be internally generated will harm-
` lessly float to the surface of the enclosure means.
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A preferred material for the pad is known in the art as "pink
poly". This material is a polyethylene, such as low-density polyethylene,
which has been impregnated with an organic liquid which can act as a
hygroscopic wetting agent to attract and hold moisture onto the surface of
the pad to render the pad electrically conductive. The pad may also be
formed in an electrically conductive loaded plastic foam such as carbon or
aluminum loaded polyurethane foam. This second material differs from the
first material in that the electrical conductivity is present throughout the
foam pad as opposed to the surface only of the pad. Still another type of
conductive pad is a polyethylene foam which has been sprayed with a
conductive carbon or similar conductive solution to affect a conductive
surface layer.
These pads are typically preshaped to the configuration of the
solid state device to accommodate the shape of the container, and are
designed to direct static charges generated by frictional movement or o~her
slight vlbrations o~ the solid state device against the pad to the surface
of the container. Typical hygroscopic wetting agents used in coniunction
with the internal foam pads are well known to those skilled in the art and
disclosed for example in U.S. Patent ~o. 3,355,313 Eastes and Canadian
Patent No. 810,595 also issued to Eastes.
One negative feature associated with the use of such internal
pads, such as "pink poly" pads is that the anti-blocking or wetting agent
used to provide the anti-static effect and to render the pad electrically
conductive, may also deteriorate a solid state device.
It has also been found that at elevated temperatures volatiles can
be produced in the pink polyethylene foam pad which can corrode a solid
state device.
Of course, an additional consideration is the cost of installing
these pads in each container.
It is, therefore, an object of the present invention to provide a
means for protecting solid state devices within an electrically conductive
container without the need for internal polyethylene foam pads or the like.
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64536-613
SUMMARY OE THE_I VENTION
bl aw ~ vnol~el
In one aspect of the presen~ invention, a~container
comprises electrically conductive body and cover portions formed
from a multilayer parison comprising an outer layer of a first
blend of a first polymeric resin and electrically conductive
particles, said blend extending circumferentially around
substantially half of the parison, and a second blend of a second
polymeric resin and an antistatic agent, said second blend
extending circumferentially around substantially half of the
parison; and an inner layer of a polymeric resin.
In another aspect of the present invention, a process
bl~o~
for makiny an electrically conductivelcontailler comprises blencling
a first polyolefln with electr.ically conductive particles;
blending a second polyolefin with an antistatic material;
coextruding a multilayer parison so that the two blends each forms
substantially half, circumferentially, of an outer layer, and
; having an inner layer of a polyolefin; and blow molding the
parison to provide an electrically conductive body and cover
portion having an interior surface comprisincJ a polyolefin treated
. 20 such that the resin provides antistatic characteristics.
The term '`electrically conductive", and similar terms,
are used harein to describe a material having a surface
resistivity of less than 105 ohms/square ~Department of Defense
Standards).
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The term "solid state device" is used herein to describe devices
such as electronic devices whose utility may be impaired or destroyed by
charge transfer actions, static electricity, or electrostatic discharge.
The term "antistatic" as used herein describes material having a
5surface resistivity in the range of about 109 to 1014 ohms/square
~Department of Defense Standard) and/or a material which can disipate 99% of
an applied static charge of ~ 5000 volts direct current in a short amount of
time, preferably less than 20 seconds, more preferably less than 5 seconds,
most preferably less than 2 seconds (~ederal Test Method Standard lOlC,
10Method 4046.1, "Electrostatic Properties of Materials"), and/or a material
having a surface resistivity in the range of about 105 to 10 3 ohms/square
(Electronics Industry Association Standard).
The term "parison" is used herein to refer to a hollow tube or
other preformed shape of a thermoplastic material or blend or aggregation of
thermoplastic materials which is inflated inside a mold in the blow- molding
process.
"Polymeric resin" is used herein generally to include for example
homopolymers, copolymers, terpolymers etc., and blends and modifications
thereof.
20The term "outer layer" is used herein to refer to a layer of a
multi-layer parisor which comprises an outer surface thereof.
The term "inner layer" is used herein to refer to a layer of a
multi-layer parison which comprises an inner surface thereof.
The term "intermediate layer" is used herein to refer to a layer
of a multi-layer parison positioned between an outer layer and an inner
layer.
The term "polyolefin" is used herein to refer to a polymer of
olefins such as ethylene, propylene, etc.~ and copolymers and modifications
thereof, such as linear low density polyethylene (LLDPE) and ethylene vinyl
acetate copolymer.
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The term "regrind" is used herein to refer to waste material such
as excess parison material from blow-molding operations, which is reclaimed
by for example shredding or granulating of the excess material. This
regrind material is generally mixed with other unprocessed material at some
predetermined percentage.
BRIEF DESCRIPTION OE THE DRAWINGS
Figure 1 is a sectional view of a coextruded multi-layer film in
; accordance with the present invention.
Figure 2 is a schematic side view of a blow-molded container
utilizing the multi-layer film.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, a multi-layer film 10 is coextruded prior
to forming into a parison. Film 10 includes an outer layer 12 comprising
two different blends, 12a and 12b, each blend extending circumferentially
around substantially half of the parison.
The first blend 12a preferably comprises a blend of a polymeric
resin and electrically conducti~e particles. The polymerlc resin is more
preferably a polyolefin and even more preferably a high density poly-
ethylene. The electrically conductive particles may be for example a form
of carbon such as carbon black, or for example carbon fibers.
The second blend 12b is a blend of a polymeric resin, preferably
different from the first polymeric resin, and an antistatic agent to pro-
vide antistatic characteristics to the interior surface of the container
formed from the parison.
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Antistatic agents suitable for use in the second blend 12b may be
for example an organic liquid which acts as a wetting agent, or for example
ionic surface-active agents which will migrate to the surface of the blend
layer 12b to provide the antistatic characteristics necessary in the in-
terior surface of the container.
Antistatic agents useful in the present invention may also be
those which are substantially non-hygroscopic and substantially non-
migratable, and such antistatic agents may be selected from the aromatic
sulfonamides. The aromatic sulfonamides may be ortho, meta, or para sub-
stituted on the benzene ring thereof, or may be N-substituted on the amide
group thereof. Representative examples of aromatic sulfonamides include,
but are not limited to, o,p-toluenesulfonamide, N-ethyl-o,p-toluene-
sulfonamide, N-butyl benzene sulfonamide, or benzenesulfonamide.
Various companies supply aromatic sulfonamides. For lnstance a
mixture of ortho and para toluenesulfonamide was formerly supplied by
Monsanto Company, St. Louis, Missouri, under the name Santicizer (l~) 9, and
currently by Akzo Chemie America, Chicago, Illinois, under the name Ketjen-
flex (TM) 9. N-ethyl-ortho,para-toluenesulfonamide was formerly supplied by
Monsanto Company under the name Santicizer (TM) 8, and currently by Akzo
Chemie America under the name Ketjenflex (TM) 8. N-butyl benzene sul-
fonamide is supplied by Unitex Chemical Corporation, Greensboro, North
Carolina, under the name Uniplex (TM) 214.
In a preferred embodiment, the antistatic polymeric composition
which may be employed in the outer layer of the parison is obtained from
N-butyl benzene sulfonamide in an amou~t of about 15% to about 30% by weight
of the total which has been mixed in a blender with nylon 12, and the
resultant granulation pelletized. Preferably, a nylon 6/12, in a weight
amount less than the nylon 12, is included in the blending. Also nylon 12
- containing about 30% by weight N-butyl benzenesulfonamide and about 15% by
30 weight nylon 6/12 can be purchased as Grilamid (TM) L25150 and nylon 12
containing about 15% by weight N-butyl benzenesulfonamide and about 15% by
weight nylon 6/12 can be purchased as Grilamid (TM) L25W40, both of which
are marketed by Emser Werke AG9 Zurich, Switzerland.
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The antistatic polymeric composition of the second blend 12b of
the outer layer of the parison may alternatively comprise a blend of a
polyolefin with a small amount of one or more of the following antistatic
agents. Suitable antistatic agents may be selected from (a) fatty acid
esters of polyhydroxy alcohols; (b) polyalkoxylated compounds (i.e. poly-
ethers) such as polyethylene oxides, polypropylene oxides, polybutylene
oxides, polytetramethylene oxides, the reaction products of polyalkoxylates
with fatty acids, the reaction products of polyalkoxylates with fatty
alcohols, the reaction products of polyalkoxylates with fatty acid esters of
poly-hydroxyl alcohols (for instance polyalkoxylate reaction products of
fatty acids, of fatty glycols, of fatty sorbitols, of fatty sorbitans, and
of fatty alcohols), or a mixture thereof, or a mixture of (a) and (b).
Suitable fatty group chains in either (a) or (b) are desirably from about C8
to about C20. The polyether chains in the suitable polyalkoxylated
compounds are of the formula ~-OCXH2x-)n wherein x is from 2 to about 8,
wherein the a:Lkyl group is straight or branched, and wherein n is from 2 to
about 1000. Each agent will work by itself in a polymeric compositlon, as
such anti-static compositions exhibit excellent static decay times; however,
the combination of agents (a) and (b) in a polymeric composition is more de~
sirable as these antistatic compositions display even shorter static decay
times. Desirable fatty acid ester substituted polyhydroxy alcohols include,
but are not limited to, the polyhydroxy alcohols selected from the C2 to C~
alcohols, such as ethylene glycol, 1,3-propylene glycol, 1,2-propylene
glycol, 1,2-butanediol, meso2,3~butanediol, 1,4-butanediol, pinacol,
25 pentaerythritol, 1,2,3,4,5-pentanepentol, sorbitan~ or sorbitol, which
polyhydroxy alcohol has been substituted with one or more fatty acid ester
groups. A very desirable fatty acid ester substituted polyhydroxy alcohol
is glycerol monostearate. A desirable polyether is polyethylene oxide, such
as that sold by Union Carbide under the trade name Polyox, or is poly-
tetramethylene oxide, such as that sold by du Pont under the trade name
Terathane. A very desirable polyalkoxylate of a fatty alcohol is a poly-
ethoxylated cetyl alcohol, as represented by the formula
C16H33-0(-C2H4-O-)nH wherein n is from 2 to about 50.
404/860128/1/9
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A more preferred blend comprises a blend of linear low density
polyethylene with a small amount of the polyethoxylated cetyl alcohol and/or
glycerol monostearate. An even more preferred blend includes about 98.5%
LLDPE, about 1% of the polyethoxylated cetyl alcohol, and about 0.5%
glycerol monostearate. All percentages are by weight of the total blend. A
small percentage 9 preferably about 5%, of an antiblocking agent, may be
added to this blend prior to blow-molding. A suitable anti~locking agent is
micron-sized silica available from Teknor Apex.
An inner layer 14 of the multi-layer film for forming the parison
is a polymeric resln such as a polyolefin, and more preferably a high
density polyethylene.
An optional intermediate layer 16 may be coextruded simultaneously
with outer layer 12 and inner layer 14. Intermediate layer 16 is preferably
made of a regrind material for example a regrind high density polyethylene
or other polyethylene or polyolefin materials. When the optional interme-
diate layer 16 is used, the inner layer 14 and intermediate layer 16 should
in combination provide sufficient strength to prevent the parison from
breaking during the subsequent blow-molding process. The use of regrind
material provides cost savings in the production of electrically conductive
containers, without jeopardizing the effectiveness of the container for the
storage and transport of electrically sensitive devices.
In one embodiment, the outer layer 12 comprises about 10% of the
total weight of the multi-layer parison, and the inner layer 14 and interme-
diate layer 16 each comprise about 40% and 50% respectively of the total
25 thickness of the parison. Within the outer layer 12, blends 12a and 12b
will each comprise about 50% of the total weight of the outer layer.
Where no intermediate layer 16 ls present, the inner layer 14
comprises about 90% of the total thickness of the parison.
404/860128/1/10
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It should be noted that when carbon fibers or other conductive
fibers are used in a blow-molding process, these fibers resist the requisite
stretching during the molding process and, if used throughou~ the multi-
layer parison, would resul~ in breaking of the parison during the blow-
molding operation. These fibers are therefore confined to the outer layerof the multi-layer parison to impart the required electrical conductivity to
the resulting blow-molded container but without affecting the blow-molding
operation. It should be clear to someone of ordinary skill in the art that
a reduction in the preferred percentage of carbon fibers or other conductive
fibers in the outer blend layer may permit, to some extent, a relative
thickening of the outer layer in relation to the total multi-layer parison
thickness without seriously affecting the blow-molding operation. However,
alteration of the concentration of carbon fibers in the outer layer may also
effect the degree of electrical conductivity of the case, particularly in
the surface portions of the container.
Conversely, an lncrease in the percentage of carbon flbers or
other flbers ln the outer blend layer may permit~ to some extent, a relatlve
thinning of the outer layer in relation to the total multi-layer parison
thickness, without losing the desired electrical conductivity of the case.
A preferred method of manufacturing the container is blow-molding.
Blow-molding is generally well known to those of ordinary skill in the art,
as illustrated by U. S. Patent Nos. 3,452,125 and 3,317,955 disclosing blow
molding techniques and methods. In practice, a first blend of a polymeric
resin such as a polyolefin, more preferably polyethylene, and most
preferably hlgh density polyethylene, is blended with carbon black or
conductive fibers such as carbon fibers in a relatively small percentage,
for example about 10% of the carbon or fibers in relation to the total
weight of the blend layer. A second blend of a polymeric resin such as a
polyolefin, more preferably polyethylene, and most preferably linear low
density polyethylene, is blended with any suitable antistatic agent. These
blends are then coextruded with an inner layer of a polymeric resin such as
polyethylene, and optionally with an intermediate layer of a regrind
material. The materials are heated to a temperature whereby the thermo-
plastic materials may be extruded as a parison and blow-molded into the
desired shape or configuration as is well known in the art.
404/860128/1/11
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Figure 2 shows a resulting blow-molded container 18 wherein the
outer layer 12 encloses each of the cover portion 20 and body portion 22 of
the container. It can be seen from the drawing figure that when the con-
tainer 18 is closed, the first blend 12a of outer layer 12 forms the outside
surface of the container, and the second blend 12b of outer layer 12 forms
the interior surface of the container. Thus an electrically conductive
surface is provided on the outside of container 189 as well as an antistatic
layer on the interior surface of the container. This invention therefore
removes the need for a separate or discrete antistatic pad such as the "pink
poly" antistatic pads conventionally used, and with this avoids the cost of
such pads and the cost of properly installing such pads in ESD device
containers, as well as the deteriorating effect of pink poly on ESD
sensitive devices stored in such containers.
The preferred container, illustrated in Fig~lre 2, is of "double
wall" construction, wel:L known :In the art for provided enhanced protection
against mechanical shock and abuse. Cen~.ral region 24 of cover portion 20
and body portion 22 represents the space between the double walls 14.
Several additional advantages are offered by the practice of the
invention. For example, with the use of conventional antistatic foam pads,
there is always a possibility of the pad dislodging from the container, and
thereby raising the risk of electrostatic damage to the FSD sensitive
contents of the container, or requiring the replacement of the pads.
Another advantage is the additional space available within the container
which would otherwise be occupied with the antistatic pads.
Although the present invention has been described in connection
with preferred embodiments, one skilled in the art will understand that
modifications may be made without departing from the scope of the invention.
For example, instead of the first blend as described above a single
polymeric resin may be used without the addition in blending of electrically
conductive particles therein. This combination may be suitable where a
conductive outer surface is not required.
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