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Description  |
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BACKGROUND OF THE INVENTION
Polyolefin plastic materials have been used to form plastic articles,
especially polyolefin materials of the polyethylene type, that is, high
density, medium density and low density materials. One of the difficulties
which has been encountered with polyethylene plastic parts, especially
those of complex shapes, is the relatively low flexural strength of
polyethylene.
As is known, polyethylene is a partially crystalline and partially
amorphous material, the side chain branching of the molecule being the
factor which controls the degree of crystallinity. High density
polyethylene has fewer side chains than low density polyethylene and
accordingly a higher degree of crystallinity may be obtained. In general,
increasing the crystallinity increases rigidity, tensile strength and
hardness, and high molecular weight polyethylenes (those of low melt
index) generally have better physical properties than the low molecular
weight counterparts. Typical of the better physical properties are those
such as impact resistance and stress crack resistance. However, the higher
melt viscosity and the low melt index of the high molecular weight
polyethylenes render them more difficult to process.
Low density polyethylenes generally are considered to have a density in the
range of 0.90 to 0.925, while the high density material is generally
regarded as having a density in the range of 0.941 to 0.965. For medium
density polyethylene, the range is 0.926 to 0.940.
A particular material in the polyolefin family which deserves special
comment is a material known as a cross-linked polyethylene which is
thermosetting in its nature. In processing this type of material, a
peroxide type cross-linking agent is generally used to affect the three
dimensional branching network characteristic of cross-linked polyethylene.
In the case of large complex shapes used as structural members, it is
frequently not possible to obtain all of the desirable physical
characteristics by the use of a single polyolefin material. The difficulty
which is encountered is that the use of two different types of
polyethylene materials may provide the desired physical characteristics
but there is considerable difficulty in providing a structural member
formed of two separate plastic materials which has sufficient integrity to
provide the overall desired physical characteristics, or in the
alternative, the processing thereof becomes quite complicated and
expensive.
By way of example, a cross-linked polyethylene material has a relatively
low flexural modulus, about 100,000 psi. Normally, the processing includes
starting with a high density material which, when cross-linked lowers the
density of the material. For example one may start with a polyethylene
having a density of 0.955 and when cross-linked the density is
approximately 0.94 which is in the range of the lower end of the high
density materials or the higher end of the medium density materials, or
what is sometimes referred to as a medium density material.
Where high flexural strengths are needed, for example in structural
components for the recreational market such as recreational vehicles and
boats, the use of a cross-linked polyethylene material does not provide
sufficient flexural strength.
It is also known that high density polyethylene foams are quite rigid,
having a flexural modulus of between 200,000 and 300,000 psi. These
particular materials, however, have a relatively low impact resistance in
that a foamed product may be easily fractured. Thus, in those instances in
which the plastic part is to be subjected to impact, for example,
automotive doors and tops, camper tops, boats, carboys and containers, the
use of a polyethylene foam which has an appreciable flexural modulus
presents practical problems.
It is possible, separately to form two components and join them together by
a bonding procedure. By way of example, polyethylene and polypropylene may
be heat sealed, but generally require melting. Where a particular part is
relatively large in size, such a boat, or a camper top or a top for a
recreational vehicle, joining together two separately formed plastic
elements by an adhesive or by any of the conventional bonding methods used
in the plastics industry is not acceptable from the standpoint of the
result produced and the substantial expense necessary to handle large
bulky items of complex shapes.
Nonetheless, the potential strengths obtained from polyolefin plastic
materials, and perhaps other thermal plastic materials as well as
thermosetting materials renders them attractive candidates for the
formation of structural parts provided the structural part has sufficient
integrity to remain tightly adhered such that the strengths of the
resulting product are sufficiently high for the intended use. Accordingly,
it is desirable to provide a plastic article having the desirable features
of rigidity, thermal and sound insulation, flotation, impact resistance,
relative lightweight, and weatherability. Moreover, it is desirable to be
able to fabricate complex shapes so as to provide a bond between the
respective components of the plastic article which assures the maximum
utilization of the strengths of the individual components making up the
plastic article. Particularly advantageous is a method by which an article
may be formed in one operation such that separate processing of separate
parts followed by a step of adhering the two together is eliminated.
Typical of the prior art patents are U.S. Pat. Nos. 3,649,407; 3,748,214;
3,655,497; 3,673,033; 3,705,071; 3,458,380; 3,472,715; 3,715,256;
2,341,260; 3,709,966; 3,607,600; 3,193,437; 3,228,819; and 3,709,967.
SUMMARY OF THE INVENTION
This invention relates to an improved plastic structure and a method of
forming the same, and more particularly to an improved plastic structure
having significant strength characteristics and formed of an outer skin
and a rigid foam core wherein the foam and skin are formed in one molding
or forming operation followed by fusion of the two in a single cycle to
provide a tightly adherent laminate which has significant and unexpected
strength properties.
In accordance with the present invention, a plastic article of relatively
complex shape is provided of an intimately secured laminate including an
outer skin surface and a rigid foam core. The outer skin surface is
tightly secured to the foam along the entire interface, to provide a
laminate having substantial rigidity and strength, particularly flexural
strength. Typical of the materials which may be used to form the outer
skin surface are polyolefin materials especially polyethylene materials of
the low, medium density and high density types and cross-linked types. The
foam core, is preferably a closed cell foam having a density of between 8
and 20 pounds per cubic foot, with a maximum of the cells in the foam
being closed, for example 90 percent to 97 percent. This type foam offers
the advantages of buoyancy, and especially good thermal and acoustic
insulation properties as well as rigidity.
Optionally, a second skin member may be used with the laminate described in
order to increase substantially the structural strength characteristics of
the product.
In a preferred form, the foam core is a linear high density or medium
density polyethylene foam, the polyethylene material having a density of
at least about 0.960. The skin surface is preferably a cross-linked
polyethylene material initially having a density of 0.95 or higher, and
cross-linked to provide a resultant medium density outer skin having a
density of approximately 0.94. Where a cross-linked outer skin is used in
combination with a high density linear foam, it has been observed that the
resultant product has a flexural strength of approximately 500,000 psi
whereas the skin alone has a flexural strength of 100,000 psi while the
foam alone has a flexural strength in the range of 200,000 to 300,000 psi.
Thus, by fabricating the structure in the manner hereinafter described,
the flexural strength of the resultant product is greater than the sum of
the individual flexural strengths of the components making up the
laminate, an entirely unexpected but desirable feature.
One of the principal features of the structural plastic article of the
present invention is the nature of the bond between the outer skin surface
and the foam core. Where a cross-linked material is used as one of the
skin members in contact with the foam, the bond between the foam and the
cross-linked material, at the interface between the two, is believed to be
a cross-linked interface in which the plastic material of the foam is
cross-linked into the cross-linked skin so as to form an intimately
secured laminate of one plastic material in contact with another. It is in
part, by virtue of this tightly adherent bond at the interface that the
structural strengths herein described are achieved.
In its broader aspects, the present invention also involves an improved
procedure for forming complex shapes of considerable structural strength
by a rotational molding procedure in which at least two plastic materials
are deposited on the mold wall and simultaneously fused together in one
heating cycle so as to form a secure bond between the two plastic
materials making up the laminate without the use of any separate adhesive,
or any of the conventional separate adhering operations heretofore used in
the prior art. Thus, in one molding cycle, at least two plastic materials
are formed into a single integrated laminated structure having a tightly
secured and adherent interface which prevents delamination of the
component plastic parts making up the laminate.
Typical materials which may be used for the foam core are materials such as
high density, medium density and low density polyethylene, or
polycarbonate materials. The skin members may be made of high density,
medium density, low density, and cross-linked polyethylene or materials
such as acrylic modified vinyl chloride polymers.
It will be apparent to those skilled in the art that the plastic article
and method of the present invention are not considered to be limited to
the specific constructions and methods as herein described and
illustrated. Rather, the specific articles and methods shown and described
herein are illustrative of a preferred form of the present invention, and
other modifications and embodiments will become apparent to those skilled
in the art upon reading the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view in perspective of a plastic article constructed in
accordance with the present invention;
FIG. 2 is an enlarged fragmentary section taken along the lines 2--2 of
FIG. 1 showing the laminated plastic member and the bond therebetween;
FIG. 3 is a sectional view of a laminate including a foam core with skin
members on each surface thereof in accordance with the present invention;
FIG. 4 is a diagrammatic view of a mold assembly for use in rotational
molding in accordance with the present invention; and
FIG. 5a--5d inclusive are diagrammatic views illustrating the condition of
the plastic material within the mold at various stages during the
rotational molding procedure in accordance with the present invention.
DETAILED DESCRIPTION
Referring to the drawings which illustrate an exemplary form of a preferred
embodiment of the present invention, FIG. 1 shows a plastic article
generally designated 10 of a relatively complex shape. The particular
structure 10 illustrated in FIG. 1 is a camper top, however it is
understood that other complex shapes may be formed, as heretofore
described.
The plastic articles of this invention are characterized by highly
desirable properties which enable the plastic articles to be used as
structural components. For example, thermal and sound insulation,
rigidity, flotation, impact resistance, relative lightweight, and
weatherability are properties which are achieved by virtue of the article
constructed in accordance with the present invention.
Again referring to FIG. 1, the plastic article includes an outer skin
member 12 which forms an exposed outer surface of the article 10. The
outer surface skin member is fluid impermeable and is preferably formed of
a polyolefin plastic material. In a preferred form, as illustrated in FIG.
1, the outer skin member 12 is preferably a cross-linked polyethylene
polymer in which the polymer before cross-linking is a high density
material, having a density of at least 0.955. When a high density
polyethylene material of this type is cross-linked, the result is a medium
density cross-linked polyethylene material which possesses thermosetting
qualities.
Typically, a cross-linked high density polyethylene material has a flexural
modulus of approximately 100,000 psi, a tensile strength of between 2500
and 2700 psi, and a density of between 0.930 and 0.945. It is the
relatively low flexural modulus which creates difficulty in providing
plastic parts having sufficient rigidity and structural integrity to be
used as structural plastic parts.
In the form illustrated in FIG. 1, the cross-sectional dimension of the
skin member 12 may vary from a few mils thickness to 3/8 of an inch or
more, depending upon the structural loads to be imposed on the laminate.
To provide the rigidity, thermal and sound insulation qualities, flotation
and to increase the flexural strength and the impact resistance, the
plastic structural member 10 includes a foam core 14. The foam core is
preferably a foamed polyolefin such as high density, medium density or low
density polyethylene. In the preferred form illustrated in FIG. 1, the
foam member is a closed cell foam of a linear high density polyethylene
wherein the polyethylene has a density of at least 0.960, the foam being
of a density of between 12 to 20 pounds per cubic foot. The foam itself
contains a substantially significant percentage of closed cells, that is,
at least 90 percent of the cells are closed, and preferably at least 97
percent of the cells are closed.
The foam core 14, if it did not include the outer skin member 12, would be
a rigid product of relatively high flexural modulus, for example between
200,000 and 300,000 psi. However, the impact resistance of such a foam
core, per se, is relatively low.
By forming both the skin and core members 12 and 14, respectively, in the
assemblage described and by the procedures hereinafter described, the skin
member 12 operates to increase substantially the impact resistance of the
foam core while the latter operates to increase substantially the flexural
modulus of the laminated plastic member 10. By way of example, the
flexural modulus of the structure described in connection with FIG. 1 is
approximately 500,000 psi, a flexural modulus which is greater than the
sum of the individual moduli of the respective separate parts forming the
laminate.
Referring to FIG. 2, wherein like reference numerals have been used where
applicable, it will be seen that an interface 15 exists between the outer
skin member 12 and the foam core 14. This interface forms an unexpectedly
strong bond between the outer skin member and the foam core member. Where
a cross-linked polyethylene is used as the outer skin member, and a high
density polyethylene is used to form the foam core, the interface is
believed to be composed of a cross-linked interface in the sense that a
portion of the cross-linking agent used to cure the outer skin member 12
also affects cross-linking of a portion of the foam core to the outer skin
member 12 thereby providing a bond between the two materials which is of
unexpectedly high strength.
The strength of this bond may be demonstrated by the fact that when
produced as hereinafter described, the core 14 is tightly adhered over its
entire outer exposed surface to the facing skin member 12. Due to the
tight bond between the two components of the plastic structure, impacts
from the exposed inner surface of the foam do not result in destruction of
the laminate, or puncture or fracture thereof. On the other hand, the
outer skin member 12 has considerable impact resistance because of the
structural strength of the polyethylene plastic, even though the
polyethylene skin per se does not have a high flexural modulus. The
synergism resulting from the assemblage of the two provides the optimum
qualities of both impact resistance and high flexural modulus, qualities
which are present in one or the other of the components making up the
laminate, but not to the extent necessary to provide the high strength of
the laminated product.
It is also possible in accordance with the present invention to utilize a
polycarbonate resin as the foam core and a high density, low density,
medium or cross-linked density polyethylene as the outer skin, since
polycarbonate plastic is compatable with polyethylene resin and a tight
bond may be obtained at the interface when the parts are processed as
herein described. It is also possible to use a low density, medium
density, or high density or cross-linked polyethylene for the outer skin,
or an acrylic modified vinyl chloride polymer, the latter sometimes being
compatable with the plastic material used to form the foam core.
In certain structural members, it is desirable to protect both the inner
and outer exposed surfaces of the foam core. Referring to FIG. 3, a
plastic article 16 is shown which includes an outer skin member 12 of the
type described, an inner foam core 14 of the type described, and an inner
skin member 18. The inner skin member may be of high density, medium
density, low density, or cross-linked polyethylene, or acrylic modified
vinyl chloride co-polymer as mentioned, or other thermoplastic materials.
The advantage of a structure as illustrated in FIG. 3 is that the foam
core 14 is protected on each side from impact. It will be understood, that
skins 12 and 18 may be of the same or different materials, and may be the
same or different dimensions depending upon the structural characteristics
desired in the final product.
Of significance is the fact that whether a dual skin arrangement is
provided, or a single skin arrangement, the plastic article includes a
rigid foam core of the type described which provides the thermal and sound
insulation, rigidity and flotation.
By way of example, a kayak type of canoe was fabricated from the plastic
product of the present invention, in accordance with the preferred form,
and navigated in white water which includes a substantial number of
partially submerged rocks and boulders. The performance of the kayak was
exceptional in that none of the repeated hits against the submerged rocks
caused damage to the outer skin surface. A comparable test with a kayak
made of fiberglass reinforced plastic resulted in several punctures which
required repair. Also by way of example, a row boat was fabricated in
accordance with the preferred form and dropped in a free fall from a
distance of 50 feet vertically to a concrete pavement. The free fall drop
produced no noticeable damage with respect to the craft.
One of the features of the present invention relates to the process of
fabrication of the plastic article above described, a procedure which is
believed, in part, attributable to the substantial and unexpected strength
characteristics of the resulting product.
Referring to FIG. 4, a rotational molding assembly 20 is shown in
diagrammatic form. The assembly 20 includes a mold element 21 having a
predetermined internal configuration which corresponds to the
configuration of the article desired. The mold assembly 20 is rotated on
two axes, indicated by the arrows, at predetermined speed ratios. The
general art of rotational molding is well known to those skilled in the
art.
Mounted on the mold is a dispenser assembly 23 divided into multiple
compartments 24, 25, and 26, although two compartments may be used if only
two different plastic materials are to be formed into a laminate. The
dispenser 23 is completely insulated thermally by insulation material 28.
The dispenser 23 is mounted above an opening 30 in the mold 21.
During a rotational molding operation, the entire mold assembly 20 is
preferably introduced into an oven while it is being rotated in the two
axes at predetermined speed ratios. In a typical example, the oven is
operated at a temperature of approximately 575.degree.F .+-. 200.degree.F.
Prior to introducing the mold assembly 20 into the oven, an appropriate
charge of plastic material is introduced into the mold cavity and an
appropriate charge is introduced into the dispenser. The amount of plastic
material placed in the mold and the dispenser is sufficient to provide a
desired cross-sectional dimension of the various portions of the plastic
laminate.
After introduction of the mold assembly into the heated oven, the latter is
rotated about the two axes while the mold assembly is heated. The relative
predetermined speed ratios may be varied in accordance with the particular
mold design to assure the proper rotational motion and plastic
distribution as is well known in the art.
While positioned within the oven and exposed to the heated air of the oven,
and while the mold assembly is rotated as described, the following takes
place. As is known in the art, the plastic materials in the dispenser
normally are particulate, in the sense that it is a powdery material or,
in the alternative as is well known in the art a plastisol type material
may be used, which for the purposes of this invention is likewise
considered to be a particulate material. During the rotation of the mold
assembly, with the first charge of plastic material in the interior of the
mold, the particles initially adhere to the heated mold surface, and
thereafter coalesce into a continuous skin element which follows the
contour of the interior of the mold. Thereafter, and in sequence and while
the mold assembly is still within the heated oven the second charge of
plastic is introduced into the interior of the mold, and the rotation
continued until the second material coalesces over the first. In the event
that a third charge is used, the third charge is thereafter sequentially
released to form the inner skin element.
Following the coalescence of the third charge, the processing is complete
in the sense that the mold is then withdrawn from the oven, allowed to
cool and the formed product is thereafter removed from the mold.
To understand precisely what occurs within the mold so that the nature of
the bond between the foam core and the outer skin may be understood more
clearly, reference is made to FIGS. 5a through 5d.
Referring specifically to FIG. 5a, a portion of the mold wall 30 is
illustrated having deposited thereon particles 32 of the first resin or
plastic placed in the interior of the mold. Since the source of heat is
external to the mold wall, the heat gradually transfers through the mold
wall to the interior surface 33 thereof. As the interior surface reaches a
temperature which is sufficient to heat and soften the plastic particles
being tumbled throughout the interior of the mold, the plastic particles
initially attach themselves to the mold wall in a substantially uniform
manner throughout the entire interior surface portion of the mold. As
sufficient heat is transferred to the individual particles, the particles
32 tend to become attached to the mold wall 33 and gradually coalesce into
a continuous skin, the cross-sectional dimension of the skin being
determined by the amount of particulate plastic material initially placed
in the interior of the mold.
For purposes of explanation, and referring to FIG. 5b, a point is reached
during the processing of the first introduced plastic material in which a
continuous skin 36 is formed on the interior wall 33 of the mold, and an
assemblage of particles 32 is adhered to the continuous skin 36 but not
yet coalesced since the heat transfer through the mold wall and the skin
is a progressive transmission of heat. The inside surface 33 of the mold
may reach a temperature of anywhere from 275.degree.F to 750.degree.F
depending upon the transfer of heat through the mold wall, and the type of
plastic material being used, the former being a function of the
cross-sectional dimension of the mold wall and the temperature of the
environment surrounding the mold, all factors which are well known to
those skilled in the art.
In a preferred form of the present invention, the material used for the
outer skin 12, the plastic first placed in the interior of the mold is a
cross-linked high density polyethylene in which a cross-linking agent such
as dicumyl peroxide is used as the cross-linking agent. This plastic
material is preferably precompounded before introduced into the mold 30.
By way of example, the basic plastic may be compounded with anti-oxidants,
a cross-linking agent, colorants and whatever other additives are needed
to provide weatherability and the like, as is known in the art.
Once placed in the mold, while the latter is being exposed to heat and
rotated, the plastic is evenly distributed throughout the interior mold
surface and progressively formed into a skin of predetermined
cross-sectional dimension depending upon the amount of material which is
introduced into the mold and the surface area of the mold.
In the case of a cross-linkable polyethylene, a preferred material for the
outer skin 12, in accordance with the present invention, the cross-linking
begins to start at approximately 300.degree.F. Thus, in order to achieve
the strong structural bond with the foam core, the second plastic material
is introduced into the interior of the mold prior to the completion of the
cross-linking of the uppermost strata of the first deposited plastic
material. Accordingly, at an appropriate moment prior to the complete
cross-linking of the first plastic material, but after the start of the
cross-linking process, the second charge of plastic material is dispensed
into the mold while the latter still remains within the heated environment
and while the mold is being rotated as described.
Referring to FIG. 5c, and by way of explanation, the interior surface 33 of
the mold wall 30 is now substantially completely covered with a
substantially uniform skin 36 of cross-linkable polyethylene. As a result
of dispensing the second charge of plastic, particles 38 of the second
charge are now evenly distributed over the coated mold surface and come
into contact with the exposed partially cross-linked polyethylene, and are
in turn heated and become attached to the cross-linkable skin 36. As
sufficient heat is transferred through the mold wall, through the
cross-linkable media 36 to the interior exposed surface, the second charge
progressively adheres to the inner exposed material, and as the
temperature thereof increases, the second charge likewise coalesces to
form a substantially uniform film on the entire inner surface of the first
formed plastic skin.
In the preferred form of the present invention, the second plastic material
is precompounded from a high density linear polyethylene which includes
between approximately 1/2 and 1 and 1/2 percent by weight of a celogen
i.e. a blowing agent capable of providing a cellular foam structure having
a closed cell arrangement.
Initially, the particles 38 of the second dispensed plastic material are
coalesced to form a continuous layer, and thereafter, as the temperature
is increased to approximately 350.degree.F, the blowing agent is actuated
to provide the closed cell structure described.
As will be apparent, the density of the foam may be varied by varying the
amount of blowing agent, and the time and temperatures used during the
processing during the rotational molding operation. Prior to degradation
of the foamed plastic, the mold and its contents are removed from the
oven, and the interior thereof is coated with a first layer 36
corresponding to the outer skin 12 and an interior foam core 40
corresponding to the foam component 14 previously described. As will be
apparent from the foregoing description, the interface 15 is formed by the
sequential dispensing of the second charge into the interior of the mold
prior to the complete cross-linking of the first charge.
Im similar fashion, if a second skin member or a second foam layer is
described, a third charge may be introduced into the interior of the mold,
and the mold continued to be rotated and exposed to heat to form a skin
member or a second foam layer on the exposed inner surface of the foam
member.
It will be apparent from the foregoing description that the rotational
molding procedure described involves sequentially depositing preformulated
plastic materials at selected time intervals into a mold which is being
rotated and heated. Thus, the bond formed between the two separate plastic
charges is formed during a single processing operation, that is, the
successive layers of different plastic materials are cured and fused in a
single cycle. The result is a substantially stronger bond and a more
uniform bond than may be accomplished, the case of complex shapes, by
separately forming the individual components and thereafter heating the
same to form a bond therebetween, or by applying an adhesive therebetween.
As previously noted, where a cross-linkable material is used, the bond at
the interface between the foam and the cross-linkable material, be there
one skin or two skins, is a cross-linked bond in which there is a
cross-link interface forming the boundary between the separate plastic
materials. In accordance with this invention, a cross-linked interface is
a preferred form because of the substantial and unexpected strength of the
bond and the resultant structure.
It is to be understood, however, that there are singular advantages in
processing a plurality of plastic materials in a single cycle in that the
processing of compatable plastics results in a bond between the two, along
the interface, which is of substantial strength even though the materials
may not be cross-linked materials. By way of example, the skin member 12
may be a medium density polyethylene, the foam may be polycarbonate, in
which event a fusion bond exists at the interface, the fusion bond being
formed during the single cycle processing. Thus, if a second skin member
is to be formed, for example of low density polyethylene on the interior
surface of a polycarbonate foam, the bond at this interface is likewise a
fusion type bond formed during a single processing cycle. Thus, in one
cycle, a multiple layered laminate may be formed to a precise
configuration as determined by the predetermined contour of the mold, the
resulting article having a fusion bond or a cross-link bond formed at one
or more of the interfaces which has substantial strength and inhibits
delamination of the separate plastic parts making up the laminated
composite plastic structure.
As will be apparent, the second skin or second foam member 18 may be a
high, medium or low density polyethylene, while the foam core 14 is the
linear high density polyethylene material described and the outer skin
member 12 being the cross-linked material described. Such a structure has
substantial strength, and offers singular advantages with respect to those
plastic products which must be resistant to impact, fairly rigid, bouyant,
thermally and acoustically insulating, lightweight and weatherable. This
combination of properties and the resulting product is produced by an
improved and simple method which provides a strong bond between the
plastic materials forming the laminate.
It is within the scope of the present invention to use a remote control
system for controlling the dispensing of charges into the mold for
formation of skin and foam layers. For example, a multichannel digital or
analog proportional radio control system may be used. Such a system offers
the advantage of minimum wiring and accurate control of the sequence of
operations since, for example, a seven channel set could provide as many
as 14 functions.
Accordingly, the method and article herein described offer advantages over
those methods and articles known in the prior art.
While the above description and accompanying drawings illustrate an
exemplary embodiment of the preferred form of this invention, it will be
understood by those skilled in the art that changes and modifications may
be made to the article and methods herein described and illustrated
without departing from the scope of the invention as set forth in the
appended claims.
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