Note: Descriptions are shown in the official language in which they were submitted.
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Title: A Composite Material of Continuous Fiber and Ultra High
Molecular Weight Polyethylene
Inventor: David B. Park
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. 119 from
provisional application serial no. 60/578,420, filed June 9,
2004, and entitled "A Composite Material of Continuous Fiber and
Ultra High Molecular Weight Polyethylene".
FIELD OF THE INVENTION
This invention relates generally to the art of composite
materials, more particularly to composite materials comprising
thermoplastics and high strength fibers.
BACKGROUND OF THE INVENTION
Thermoplastics are plastics that are made by heating resin
pellets or powders (the raw material) until they melt, molding
the material to a desired shape, and then resolidifying the
material through a cooling process. Because the chemistry of
the plastic does not change, this process can be performed
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multiple times. In other words, thermoplastics can be
reprocessed or recycled with additional heating and cooling.
Additionally, the manufacturing process for thermoplastic
products can be as quick as the heating, molding, and cooling
processes can occur (e.g. seconds). Hence thermoplastics have
the manufacturing advantages of rapid processing and multi-step
manufacturing, both of which can translate and have translated
to significant benefits and cost savings.
Some common thermoplastics are polyethylene "PE",
polypropylene "PP", polyamide "PA"(more commonly known as nylon)
and polyethylene terepthalate "PET"(more commonly known as
polyester). Products made from such thermoplastics are very
diverse, for example, toys, rope, clothing and bottles. Melt
temperatures for most thermoplastics fall between 125 C and
300 C. In their softened or partially melted state
thermoplastics flow under pressure and thus can be shaped by
processes such as injection molding, blow molding and extrusion.
Thermoplastics have many excellent qualities. Metals,
however, generally have much higher strength and stiffness than
thermoplastics. To increase the strength and/or stiffness of
thermoplastics it has long been known that mixing thermoplastic
resins with short high strength fibers results in a product with
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substantially increased strength and/or stiffness. Often higher
temperature properties are also improved. This mixing is done
either before melting the thermoplastic resin or as the
thermoplastic resin is melted but in either case before forming
the product by extrusion or injection molding, as disclosed for
example in US Patent 3,577,378. Types of fiber commonly used
for this purpose may be glass, carbon, aramid or other materials
which have suitably high strength and/or stiffness and melt
temperatures above the thermoplastic of choice.
These materials are sometimes referred to as short fiber
composites or more commonly fiber reinforced plastics "FRP".
Such FRP materials typically comprise 10 to 30 percent fiber by
weight with the balance being the thermoplastic of choice. The
fibers are typically .1 to 5 mm in length. The use of higher
percentages of fiber and/or longer fibers is typically avoided
because of difficulties achieving good flow of the mixture and
good uniform dispersion and distribution of the fibers,
unacceptable accumulation of the fibers in the nozzle of the
injection molding equipment, and unacceptable reduction in
toughness and impact strength in the product.
Typically with the short fibers from .1 to 5 mm in length,
FRP thermoplastics have tensile and/or flexural strength and
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stiffness up to 5 times greater than the same thermoplastic
without the reinforcing fibers.
U.S. Patent 3,577,378 teaches that the added tensile and
flexural strength and stiffness is due to the high strength and
stiffness of the fibers coupled with the excellent mixing and
distribution of the fibers in the polymer and the ease with
which the molten FRP material flows in the extruder and
injection mold. There is possibly, however, a characteristic
loss of impact strength in the short fiber FRP product compared
with the unreinforced thermoplastic product as reported with
respect to polycarbonate in U.S. Patent 3,577,378.
A more current example of the enhanced strength and
stiffness properties of FRP comes from Victrex plc,
headquartered at Hillhouse International in Lancashire, UK.
Victrex is the world's largest producer of certain high
performance high temperature engineered thermoplastics, in
particular polyetheretherketone (PEEK). Published Victrex data
for unreinforced "150G" PEEK and 30% short carbon fiber
reinforced "150CA30" PEEK is indicative of the difference in
properties that successful introduction of short fibers can
produce, as illustrated in Figure 1. This data is excerpted
from Victrex USA Inc. publication 1100/2.5m titled PEEK Material
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Properties Guide. Both materials are suitable for extrusion and
injection molding.
Engineers and designers, however, continue to seek
materials which have strength and stiffness that exceed
5 conventional FRP short fiber thermoplastics and that also have
strength to weight and stiffness to weight ratios that are
higher than those of metals. In the past twenty five years
several processes have been invented wherein thermoplastic
composites have been produced with continuous fibers and with
fiber content as high as 70%. These materials generally have
much higher strength and/or stiffness than short fiber FRP
materials. In fact these composites exhibit strength and/or
stiffness comparable and in many cases superior to metals,
including steel. Also, generally, the density of these
materials is much lower than metals. Hence their strength to
weight and stiffness to weight ratios are much higher than
metals, including steel. US Patent 4,680,224 discloses a
preferred method for producing such continuous fiber
thermoplastic composites.
This APC-2 thermoplastic composite manufactured by Cytec
Engineering Materials, a division of Cytec, Inc. It is made
with continuous unidirectional AS4 carbon fiber and PEEK
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thermoplastic resin. For illustration of the enhanced strength
of these continuous fiber composite thermoplastics, Figure 2
depicts selected mechanical properties at room temperature of
APC-2. The fiber content in this composite is 68% by weight.
Note the enhanced properties in the fiber direction as compared
to the unreinforced PEEK and the FRP short fiber PEEK properties
listed in Figure 2. The strength in the fiber direction is 9
times greater than the 150CA30 short fiber PEEK FRP and more
than 20 time greater than the unreinforced 150 PEEK. The
strength and stiffness perpendicular to the fibers is however,
much lower. The tensile strength perpendicular to the fibers is
approximately the same as the unreinforced resin while the
stiffness perpendicular to the fibers is about double the
unreinforced resin.
These values clearly demonstrate both the tremendous
strength and stiffness of continuous fiber thermoplastic
composites and the influence of fiber direction at the same
time. This directional or non-isotropic nature of continuous
fiber composites allows the designer much greater freedom to
create cptimized designs when compared to most conventional
plastics and metals which have more or less isotropic or non
directional properties. If the designer is desirous of more
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isotropic properties in the thermoplastic composite, any number
of layers of this material having fibers in various directions
may be combined under heat and pressure to create a more
isotropic material. Most often multiples of 4 layers are used
with -45 , 0 , 45 , 90 fiber orientations to produce what are
termed "quasi-isotropic" material properties.
Advanced Materials engineers finally discovered how to
manufacture more complex thermoplastic composites with difficult
plastics. The initial process consisted of adding chemical
solvents to the heated thermoplastic resin, to make the resin
sufficient watery to saturate a mass of fibers. Although this
solvent had to be removed later in the process, composite
manufacturers could now produce a thermoplastic composite with a
relatively even distribution of plastic matrix to hold the
fibers in place.
Such methods of producing short fiber reinforced
thermoplastics as described above are widely practiced today
with virtually all thermoplastics, but not with UHMW PE.
However, is UHMW PE has a linear polyethylene structure just
like high density polyethylene but its molecules are
exceptionally large, having a molecular weight of from 2 to 6
million daltons. This high molecular weight provides UHMW PE
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with exceptional impact strength and abrasion resistance;
however, it also results in special well known processing
characteristics which preclude use of standard extrusion and
molding techniques. In short, UHMW PE does not "flow" at high
temperature as do virtually all other thermoplastics. In fact,
when attempting to injection mold UHMW PE, the extreme pressures
result in shear-degradation of the polymer.
Because of UHMW PE's unusual behavior, the processes used
to make UHMW PE stock materials (plate, sheet and bar being the
most common forms of UHMW PE) are most akin to compression
molding or sintering. The typical procedure is to fill a form
or mold with UHMW PE in powder form and then apply heat and
pressure to remove the air between the particles and to force
the particles to soften and deform and fuse into a single mass.
This ideally results in an essentially void free solid although
microscopic porosity will still exist. In some special cases a
lower pressure is used specifically to produce a porous
structure. This porous solid form of UHMW PE is sometimes used
for filtration. These processes do not require the UHMW PE to
flow.
Desiring reinforced products of UHMW PE, many individuals
have sought ways to successfully add short fibers to UHMW PE.
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U.S. Patents 4,055,862, 5,622,767 and 5,620,770 disclose
successful methods for making such "FRP" materials with UHMW PE
and carbon fibers without requiring the UHMW PE or the fibers to
flow wherein the carbon fibers may be up to 8 mm long. These
patents disclose methods that are essentially the same as the
method described above for making unreinforced UHMW PE
materials. In these "compression molding like" or "sintering
like" methods the short carbon fibers are mechanically mixed
with the particles of UHMW PE powder prior to introducing the
mixture to the form or mold, and prior to melting the UHMW PE.
SUMMARY OF THE INVENTION
In one embodiment of the invention, the invention relates
to a composite material of continuous fibers and ultra high
molecular weight polyethylene, the composite material comprising
ultra high molecular weight polyethylene, and continuous high
strength fibers, wherein the ultra high molecular weight
polyethylene forms a continuous matrix among and surrounding the
fibers.
In another embodiment of the invention, the invention
relates to a composite material of continuous fibers and ultra
high molecular weight polyethylene, the composite material
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comprising ultra high molecular weight polyethylene; continuous
high strength fibers, wherein the ultra high molecular weight
polyethylene forms a continuous matrix among and surrounding the
fibers; and at least one additive or filler.
5 The invention also relates to a method of manufacturing an
ultra high molecular weight polyethylene composite, the method
comprising selecting unidirectional and continuous high strength
fibers; impregnating the fibers with ultra high molecular weight
polyethylene in a fine powder to form a composite; optionally
10 adding additives or fibers to the composite; and forming a
continuous matrix of the ultra high molecular weight
polyethylene surrounding the fibers.
The techniques described above, however, until the present
invention, did not particularly work well with UHMW plastics.
It is an object of the invention to provide a composite material
of continuous fiber and ultra high molecular weight polyethylene
(UHMW PE) exhibits many improved physical properties in
comparison to both unreinforced and short fiber reinforced UHMW
PE. Another object of the invention is to improve strength and
stiffness in the fiber direction as well as thermal and
electrical conductivity when a highly conductive fiber is
employed. It is yet another object of the invention to provide
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a composite that exhibits many of the outstanding
characteristics of unreinforced UHMW PE, such as excellent
abrasion resistance, low coefficient of friction and high PV.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a data table listing the properties of the prior art
composites, as presented by Victrex USA Inc.
Figure 2 is a data table listing the properties of the prior art
unreinforced and reinforced composites, as manufactured by Cytec
Engineering Materials.
Figure 3 is a data table listing the properties of the prior art
unreinforced composite, as manufactured by Ticona Engineering
Polymers.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention,
which may be embodied in various forms. Therefore, specific
structural and functional details disclosed herein are not to be
interpreted as limiting, but merely as a basis for the claims
and as a representative basis for teaching one skilled in the
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art to variously employ the present invention in virtually any
appropriately detailed structure.
Production of a continuous fiber UHMW PE composite is
expected to exhibit very high strength and stiffness, excellent
toughness, exceptional abrasion resistance, excellent chemical
resistance., a low coefficient of friction and high bearing (PV)
capacity. Continuous fibers are well known in the art as fibers
extending to a length of several feet or meters that maintain
its structural strength continuously throughout the fiber when
the fiber is stretched. Depending on the fiber chosen, the
composite could also be highly conductive or highly resistive,
both electrically and thermally, or able to absorb great amounts
of energy at high speed impact. To be successful in fashioning
such a composite, one would have to overcome the same peculiar
properties of UHMW PE that render the extrusion and injection
molding of both unreinforced and short fiber reinforced UHMW PE
problematic.
In particular, all known methods of making continuous fiber
thermoplastics require flow under pressure such as through film
stacking methods as practiced by Ten Cate and Bond Laminates or
pultrusion type methods practiced by others, including the
pultrusion type method disclosed in U.S. Patent 4,680,224.
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In this invention, a slurry of UHMW PE powder, such as GUR
2126 made by Ticona Engineering Polymers, a division of the
Celanese Corporation, having a molecular weight of approximately
2,500,000 daltons, and water is impregnated into a web of
unidirectional fibers, such as AS4C carbon fibers made by
Hexcel. Figure 3 depicts selected physical properties of
unreinforced GUR 2126 UHMW PE, as published by Ticona. The web
impregnated with UHMW PE powder and water is then passed through
a dryer to remove the water. The web impregnated with loose dry
UHMW PE powder is heated and tensioned in such a way as to cause
the UHMW PE to melt and fuse into a solid band under essentially
no external pressure greater than atmospheric pressure. The
solid band then is then pulled through a heated die to reform it
into the desired cross sectional shape. This method requires no
flow of the UHMW PE under high pressure and therefore the UHMW
PE is not degraded by shearing.
The resultant continuous fiber UHMW PE composite produced
using this method may be made in almost any cross sectional
shape such as flat ribbon, sheet, flat bar, round, square,
triangle, channel, angle, I-beam, etc. The composite of the
chosen cross sectional shape may then be cut into lengths or
coiled for shipment and/or further fabrication depending on its
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shape, thickness, stiffness, customer preference or other
considerations.
In the process described above, the impregnation of the web
of fibers with a slurry of UHMW PE powder and water may be
replaced by impregnation of the web with dry UHMW PE powder in a
fluidized bed. Using the fluidized bed, the dryer is
unnecessary. The fluid in the fluidized bed may be any suitable
gas, such as air.
Fabrication techniques well known to those schooled in the
art of continuous fiber thermoplastic composites may be used to
make subsequent forms of the materials, parts or structures,
such as slitting, pelletizing, weaving, laminating, tape
placement, thermoforming, table rolling, compression molding,
bladder molding, machining, ultrasonic welding, etc. for any
number of end use products.
In other embodiments of the invention, the continuous
unidirectional fibers may be replaced by woven continuous fibers
or randomly oriented continuous fibers, for example a felt or
matted fabric of random fibers.
It should be noted that the strength and stiffness of the
reinforced UHMW PE in the fiber direction are dramatically
higher than the unreinforced UHMW PE, as are the thermal and
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electrical conductivity. The PV (sliding bearing capacity) of
the reinforced UHMW PE is also significantly improved with the
continuous fiber as is thermal expansion. The composite
maintains the low coefficient of friction and the exceptional
5 abrasion resistance of UHMW PE. The only loss of property of
the composite relative to the unreinforced UHMW PE is in impact
strength. This is typical of virtually all fiber reinforced
thermoplastics; however the impact strength of reinforced UHMW
PE is significantly higher than most other fiber reinforced
10 thermoplastics.
It will be readily apparent to those skilled in the art
that various changes and modifications of an obvious nature may
be made, and all such changes and modifications are considered
to fall within the scope of the appended claims. Other
15 embodiments of the invention will be apparent to those skilled
in the art from consideration of the specification and practice
of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with
a true scope and spirit of the invention being indicated by the
following claims and their equivalents.
