Note: Descriptions are shown in the official language in which they were submitted.
WO 94/09972 PCr/US93/10313
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COMPOSITES AND METHODS OF MANUFACTURING THE SAME
BACKGROUND OF THE lNv~hllON
The present invention relates to the field of composite
materials and processes for making the same.
Various materials are incorporated in thermoplastics to
modify the properties of the base the ~lastic material. Thus,
elongated fibers of reinforcing materials such as glass, metal,
th~ -setting polymers or high strength thermoplastic materials often
are incorporated in a base ~h~ ~lastic material to form a composite
having higher strength than the base the :plastic. So called "con-
tinuous fiber~' compo~ites have reinforcing fibers which are relatively
long in comparison to the overall dimensions of the composite article.
Each fiber may have a length which is many thousands of times its
diameter. Continuous fiber composites can be made by various
processes such as hand layup or coextrusion, in which the fibers are
positioned in predet- ine~ locations within the base or matrix
material. Processes for making continuous fiber composites are
expensive and limited with resoect to the shapes of the article which
can be produced and the orientation of the fibers within the article.
Moreover, continuous fiber composites incorporating very long fibers
of reinforcing materials having high elastic modulus and low toughness
may be relatively brittle. In such composites, any elongation of the
composite results in an elongation of the fibers which is equal to or
nearly equal to the elongation of the composite itself. Therefore,
the fabrics will break upon relatively small elongation of the compos-
ite.
Considerable effort has been devoted towards development of
so-called discontinuous fiber composites. In discontinuous fiber
composites, each fiber has a length substantially smaller than the
dimension of the composite article in the direction of the fiber
length. Loads applied to discontinuous fiber composite~ are shared
beL~._cn the matrix and the fibers. Discontinuous fiber composites
therefore can provide u~eful combinations of ~,o~e Lies, such as
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comtinations of relatively high strength and elongation. The orienta-
tion of the fibers in a composite strongly influences the structural
~L~peLLies of the compo~ite. So-called "unidirectional~ discontinuous
composites have substantially all of the fibers in the entire com-
po~ite, or in a substantial region of the composite, exten~ing gener-
ally parallel to one another in a pre~elected fiber direction, whereas
so-called "random" discontinuous compo~ites have fibers ext~n~ing in
substantially random directions. Random composites have substantially
isotropic properties in two or in three directions. That is, the
phy~ical propertie~ of the composite are substantially the same in two
or in three directions. By contrast, unidirectional composites
generally have ani~otropic physical ~upe~Lies. Their strength and
ela~tic modulu~ generally are greater with re~pect to loads in the
fiber direction than with respect to loads in directions tran~ver~e to
the fiber direction. Generally, the physical properties of unidirec-
tional composites in the fiber direction are superior to those of
random compo~ites. Unidirectional discontinuous composites therefore
are particularly useful in ~tructural elements intended to resist
load~ in a particular direction. A unidirectional discontinuous
composite article typically is fabricated so that the fiber direction
is parallel to the direction in which the greatest tensile loads will
be applied.
The processes utili7e~ heretofore for manufacturing discon-
tinuous composites, and particularly those used for manufacture of
unidirectional discontinuous composites, have suffered from serious
dr~wb~c~q. Discontinuous composites incorporating th~ ~plastic-based
resins have been fabricated by forming a mass of molten thermoplastic
with fibers dispersed therein. For example, a ma~s of thermoplastic
material can be subjected to an injection molding process wherein the
molten mass is forced into a mold under pressure. The flow of the
~h~ ~lastic tends to orient the fibers to some degree. However, it
i8 difficult to obtain complete orientation of the fibers in the
desired direction. Also, presence of the fihers in the molten ma~s
materially i ~--1es flow of the ~he plastic which in turn ;mroses
significant restrictions on the design of the mold, on the design of
the article and on the process.
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One approach to fabrication of discontinuous fiber composites
is described in European Patent 0.062,142. The European '142 patent
uggests forming a ~u~pension of "short cut" th~ pla~tic fibers and
short cut reinfo.c. ~~~ fibers in a "fluid carrier medium" and then
causing that suspension to flow over a porous wall such as a porous
foil or fleece. The fluid carrier -'i is forced through the porouQ
wall, whereas the fibers are retained on the porous wall. In this
process, the fibers are qaid to be oriented ~ubtantially in ali; - t
with one another on the porous wall. This layer of oriented fibers is
then "cleansed of adhering residues of the carrier ~~ ~" and removed
from the porous wall. The cleansed layer is then subjected to heat
~ufficient to melt the th~ ~pla~tic, thereby fusing the thermoplaqtic
material into a coherent ma~ with the fibers : ~ed~ed therein.
Manife~tly, the need to ~eparate the oriented fibers from the fluid
carrier and cleanse the oriented fiber~ of the carrier residue imposes
undesirable process conqtraints. Also, the requirement to keep the
th~ pla~tic fiber~ and the reinforcing fibers uniformly mixed with
one another in a suspension will impose additional procesfi con-
~traints, particularly where the th~ pla~tic fiberq differ signifi-
cantly in specific gravity from the reinforcing fibers, as is often
the case.
Accordingly, there have been ~ubstantial, unmet needs hereto-
fore for i p oved proces~es for fabricating discontinuous fiber
composites inco~oLating ~h~ ~lastic resins and additional
materials, such as reinforcing materials, in fiber form.
SUMMARY OF THE lNv~h.lON
The present invention addresses these needs.
One a~pect of the present invention provides a method of
making a unidirectional discontinuous composite including a th~ ~
plastic material and an additional material in fiber form. A method
according to this aspect of the invention includes the step of mixing
discontinuous fibers of the ~h~- ~lastic material and discontinuous
fiber~ of the additional material to form a mixture and then carding
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the mixture so as to orient the fibers in the mixture substantially
codirectionally with one another. The carded, generally unidirection-
al fiber mixture is used a~ a preform. The preform is subjected to a
fusion step in which the th~_ plastic fibers are fused to form a
substantially continuous thermoplastic phase surrounding the discon-
tinuous fiber~ of the additional material.
The additional material preferably is a reinforcing material
having a higher elastic modulus than the thr- plastic material. For
example, the reinforcing material may be selected from the group
con~isting of glass, ceramics, metals, carbon, nonthermoplastic
polymers and th~ ~lastic polymers having a heat distortion tempera-
ture higher than the thermoplastic material of the fibers used to form
the continuous phase.
The step of forming the preform may also include the step of
forminy the carded fibers into an elongated inte~ te preform, such
a~ a rope-like ~liver ~o that the codirectionally exte~i ng fiber~
extend generally parallel to the direction of elongation of the
int~ te preform. The preform preparation ~tep may further
include the step of juxtaposing a plurality of lengths of the inte~-
mediate preform or sliver with one another so that these lengths
extend generally codirection~lly with one another. The fusing step
may include the step of subjecting the preform to heat so as to bring
the the plastic material in the thr~ ~lastic fibers to a flowable
condition and compacting the heated preform while maint~; n; ng the
th- ~la~tic material in a flowable condition. For example, the
preform may be squeezed between a pair of opposed ` 215~ such as the
opposed portions of a compression mold. A wide variety of the
plastic materials can be used. However, polyolefins are particularly
preferred. The carding operation produces a high degree of orienta-
tion, which is retained throughout the sub~equent steps of the
process. The carding process is enviLo --tally safe and does not
contaminate the materials with a carrier fluid. Articles of substan-
tially any desired dimensions can be produced readily and economical-
ly, with good quality.
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A further aspect of the invention provides another process
for making unidirectional discontinuous composites incorporating a
th~ plastic material and an additional material in fiber form. A
process according to this aspect of the invention includes the step of
extruding the th~ ~plastic material on substantially continuous
fibers of the additional material to form an extrudate with the
continuous fibers extending substantially in a machine direction,
i.e., the direction of extrusion. The extrudate is then severed along
cutting plane~ tran~ver~e to the ochin~ direction to form a multi-
plicity of pieces, each including relatively short fibers of the
additional material together with the the_ plastic material. These
pieces are then juxt~pose~ with one another so that the short fibers
in the pieces extend ~ubstantially codirectionally with one another in
a preselected fiber direction. The thr plastic material in the
juxtaposed pieces is fused to form a unitary mass including the
thr plastic material together with the fibers, the fibers still
exten~ing substantially in the fiber direction. The unitary mass is
subjected to shear in a direction parallel to the fiber direction
while maint~ining the th~ plastic material in the mass in a flowable
condition.
The shear serves to redistribute the fibers in the fiber
direction. Thu~, although the fibers in the ma~ tely after
fusion may be in sub~tantially end to end disposition at locations
correspon~ing to the original severing or cutting planes and the ends
of the individual piece~, they are redistributed to side by side,
overlapping and interleaved di~position by the applied shear. This
materially ~nh~nces the physical properties of the compo~ite.
The extruding step may include the step of coextruding the
~h~ ~plastic material with one or more strands, each including a
multiplicity of fibers. This coextrusion may involve pultrusion,
i.e., a process in which the fibers or strands are pulled through a
die by forces supplied to the extrudate downstream of the extrusion
die. The unitary mass may be subjected to shear by engaging the mass
between confronting surfaces of a pair of opposed I '- 8 while moving
one of the surfaces relative to the other one of the surfaces
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~ub~tantially in the fiber direction. For example, the ma~s may be
pa~ed through a nip defined between a pair of opposed rollers while
rotating the rollers at unequal ~urface velocitie~. The ~hearing step
and the fusing qtep may occur concomitantly with one another. Eor
example, the mas may be fused and qheared in a ~ingle pass through a
roll mill. Sub~tantially the same wide variety of materials can be
used in this proce~s as in the afoL. - tioned cardiny and fusLng
proces~. Thi~ process provide3 a simple and effective way to form
di~continuous unidirectional composites.
These and other objects, features and advantage~ of the
present invention will be more readily apparent from the detailed
description set forth below taken in conjunction with the accompanying
drawingq .
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the subject matter of the
pre~ent invention and the various advantages thereof can be realized
by reference to the following detailed de~cription, in which reference
is made to the ac- ~nying drawings in which:
Figure 1 is a diagrammatic view ~howing the process in
accordance with one ~ L of the present invention; and
Figure 2 is a diagrammatic view ~howing the process in
accordance with another : ~ of the pre~ent invention.
DETAILED DESCRIPTION OF THE Pk~Kk~ EM80DIMENTS
One process according to the present invention utilizes
textured strandq formed from a the ~lastic material. Generally,
such ~h~ plastic materialq may comprise any organic polymer which
will generally retain its shape at room t~ ~ ature, but which can be
deformed at elevated temperatures. Polyolefin~ are particularly
preferred materials in that regard.
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Referring to Figure 1, such thermoplastic materiaLs are
formed into continuous fibers 10 in accordance with generally known
technique~. These fiber~ are then collected into strands 20 consist-
ing of a multiplicity of fibers bundled together. The number of
fibers forming a strand will depend upon the particular the plastic
material employed. For instance, when the the_ ~plastic material
comprise~ poly~opy-lene~ there may be approximately 70-150 of such
fibers in a strand. The strands 20 then undergo a texturing process
30 which may compri~e conventional method~ of crimping the strands,
such as by heating the strand~ above their heat distortion temperature
and then rolling a gear along the length of the strand, of by well-
known stuffer box ~echnique~. Alternatively, texturing may be
effected by heating the strands above their heat distortion tempera-
ture and then directing a jet of air at the strand~ to deform same.
Subsequently, the ~trands are cooled to retain the deformed shape.
After texturing, the continuous bundle~ or strands 20 of
fiber are cut into a plurality of di~continuous fiber bundles 40.
Preferably, the length of these discontinuou~ fiber bundles i5 between
about 0.5 inches and 2.5 inches, and more preferably between about
1.50 inche~ and about 2.0 inche~.
The di~continuou~ fiber bundles 40 are mixed with discon-
tinuous fibers of a reinforcing material to form a mixture. The
reinforcing materials are preferably materials having a higher elastic
modulus than the ~he ~lastic material. Particularly preferred
reinforcing material~ are fibers ~elected from the group comprising
glass, ceramics, metals, carbon, non~h~ ~lastic polymers and thermo-
plastic polymers having a heat distortion temperature higher than that
of the ~h~ ~plastic material from which the fiber bundles 40 are
formed. continuous fibers 50 of these reinforcing materials, formed
in accordance with conven~ional techniques~ are collected into strands
or bundles 60, each of which may include a plurality of fibers. The
number of fibers in each strand will depend, to a large extent, upon
the specific reinforcing material being used, and may include anywhere
from two fibers to tens of thousands of fibers. In the case of
fiberglass reinforcing materials, these strands will typically include
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between about 2,000 and about 4,000 gla~s fibers. The continuou~strandn 60 of these reinforcing material fibers are then cut into a
plurality of di~continuous fiber bundles 70. The length of these
discontinuous f$ber bundles is preferably between about 0.50 inches
and about 2.5 inches, and more preferably between about 1.50 and 2.0
inches. In order to facilitate the mixing and ~ubsequent proce~sing
steps, the bundle~ 70 of the reinforcing material preferably have a
length which i~ similar to the length of the ~he ~lastic material
hUn~l~s 40.
Predetermined amounts of the di~continuous thermoplastic
fiber h~n~leR 40 and the di~continuous reinforcing material fiber
bundle~ 70 are then introduced into a conventional precarding
apparatu~ 80 in which the bundles~ 40 and 70 are at least partially
unbundled 80 that the individual fiber therein become ~eparated and
intimately mixed with one another in a three-dimensional fashion to
form a h -~eneou~ mixture 90. In one form of precarding apparatu~
80, a bed of cour~e ~e~le8 protrude from each one of a pair of
confronting conveyor belts arranged to move in opposite directions.
A~ the bundle 40 and 70 are fed into the apparatu~ 80, the nee~es
separate the bundles from one another and pull the individual fibers
in the bundles at lea~t partially apart ~o that the fibers of the
reinforcing material can become ~ -~h~ and intimately mixed with the
fibers of the th~ ~lastic material. As they exit precarding
apparatus 80, the fibers in mixture 90 exhibit no preferred orienta-
tion, each of the thermoplatic material fibers and reinforcing
material fibers being randomly arranged with respect to one another.
The reinforcing material de~irably constitutes between about lO
weight % and about 70 weight %, and more desirably, between about 20
weight % and about 60 weight % of the mixture.
From the precarding apparatus 80, mixture 90 i5 fed into a
carding -~hi n~ 100 in which the mixture is ch~ni c~l ly ~eparated
into individual fibers and formed into a cohesive web 120. The
carding -chi ne 100 includes a rotating cylinder 102 covered with a
wire clothing having many fine wire5 protruding therefrom. A ~imilar
wire clothing covers rotatable cylinders 104 and 106. Cylinder 102
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rotates in the ~ame direction but at a faster ~peed than cylinder~ 104
and 106. As the mixture 90 passe~ through the nip 103 formed between
cylinder 102 and 104 and the nip lOS formed beLwccn cylinder 102 and
cylinder 106, the relative v~ - t of the wires on one cylinder with
re~pect to the wire~ on the adjacent cylinder pull and tease the
fibers apart. As a result of this pulling and teasing proce~, the
individual fibers will become substantially aligned codirectionally
with one another in the -ch i n~ direction, i.e., tran~verse to the
axis of rotation of the cylinders. As they are pulled apart, the
fibers be- - entwined with one another to form a continuouq thin veil
or web 120 ~everal fiber diameters in thickn~e~. The width of web 120
will depend upon the size of carding schine 100, but will typically
be on the order of about 1.0 meters. The texture of the ~h~ plastic
fibers helps hold this web together. Carded webs of this ~ort
typically have a bulk density which is beL.~ee:n about 0.2% and about
0.7% of the true density of the mixture, lep~n~;nq on the materialq
carded, their relative ~.-,~o Lions and their fiber length~. For webs
including about 70 wt% poly~opylene strands and about 30 wt% fiber-
glas~ strands, web densities of between about 0.003 gm/cm3 and about
0.01 gm/cm3 are obtained, as compound to about 1.4 gm/cm3 for a fully
den~e composite of thi~ compositior..
After exiting carding -t~hi nc~ 100~ web 120 is fed through a
die 130 having an orifice 132. As it passes through orifice 132, web
120 is collected into a ~liver 140 which will typically have a
diameter of beL~.Len about 2.0 cm and about 5.0 cm, and preferably will
be about 4.0 cm. Thu~, in a typical process, each linear meter of the
one meter wide web will be collected into a linear meter of a 0.04
meter diameter sliver. The bulk den~ity of the resultant qliver will,
of course, depend upon the densities of the ~he ~plastic and rein-
forcing fiber~ th~ ~elves, as well as the proportion of each in the
mixture 90. However, slivers con~i~ting of about 70 wt% poly~.o~ylene
and about 30 wt% fiberglass will typically have a density of about
0.005 gm/cm3. The codirectional orientation of the fibe-s in the web
120 will be sub3tantially unaffected as web 120 i~ formed into qliver
140. Consequently, the fibers in ~liver 140 will extend ~ub~tantially
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codirectionally in a direction parallel to the elongation direction of
the ~liver.
The ~liver 140 may serve as a preform for subsequent process-
ing step~. Thu~, the ~liver may be cut into discrete lengths 150, a
plurality of which may be arranged adjacent to one another to form a
layer 160 in which the length~ 150 all extend generally codirec-
tionally with one another. Additional layers 170 may be formed in
~ubstantially the ~ame fashion and superposed upon layer 160 to form
an uncon~olidated a~embly 180 in which the ~liver lengths 150 in all
of the layers extend in generally the ~ame direction. The number of
~liver lengths in each layer, the number of layers and the length
dimen~ion of the sliver length~ 150 will dictate the ultimate ~ize of
the ma~ produced after consolidation.
The as~embly 180 is then subjected to a preliminary heating
~tep to ~often the th~ plastic material in the sliver~. The time
and t -_aLure at which thi~ heating step is conducted will depend to
a large extent upon the particular th~ ~la~tic material employed and
the 3ize of the a~sembly 180. The a~sembly 180 may be heated in an
oven or other suitable apparatus to an oven temperature which, for
polypropylene materials, is between about 200C and about 260C, and
preferably between about 215C and about 250C. The duration of the
heating cycle will preferably be at least about two minutes to as~ure
that the th~ plastic material in assembly 180 is heated throughout.
Heating cycles of between about four minutes and about 5iX minutes are
most preferred. For proce~se~ in which the reinforcing material is
also a ~he ~lastic material, the heating step should be carefully
controlled to a~sure that the thermoplastic material having the lower
heat distortion t ~_ature softens, but that the reinforcing material
having the higher heat distortion temperature does not.
In order to facilitate its ..,~v~ --t into the oven and out
therefrom for further proces~ing, assembly 180 desirably will be
placed beL.ce~ webs (not shown) of a material which will remain
~h~ -lly stable and not deform during the heating cycle. More
de~irably, the web material will not strongly adhere thereto after
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further proceqsing of the as~embly 180. Particularly preferred
materials having the~e characteristic~ are Teflon coated fabrics.
When the assembly 180 has been heated sufficiently to soften
the th~ plastic material therein to a flowable condition, the
asqembly 180 in compacted to form a solid, unitary mass. In a typical
compacting proce~, the assembly 180 will be subjected to a compreq-
sive load under which the fibers of the thermoplastic material will
flow together and fu~e with one another to form a compo~ite article
having a ~ubstantially continuou~ ~h~ plastLc phase surrounding the
di3continuous fibers of the reinforcing material. The load is gener-
ally applied to a~sembly 180 in a direction transverse to the fiber
direction ~o that the unidLrectional orientation of the fibers therein
~ ~ i n~ gub~tantially intact, and is then maintained for a length of
time sufficient for the ~hr_ pla~tic material to cool to a non-
flowing state. At that point, the compoQite article will not distort
upon removal of the compressive load. The compacting process is
preferably conducted at a pre~ure of be~een about 150 atm and about
250 atm, and more preferably between about 180 atm and about 220 atm.
De~irably, the compressive load is applied for between about 15 -
secon~ and about 1.5 minutes, to as~ure that the thermoplastic
material has completely fused together.
In preferred compacting processes, the load-applying members
are shaped to produce the desired shape in the compacted article. In
an example of one such compacting procesC~ the heated asqembly 180 i5
placed in a compres~ion mold 190 having opposed ~ 192 and 194
which define substantially the final shape of the article to be
formed. These oppo~ed `- ~ are at a substantially cooler tempera-
ture than the temperature of the assembly 180. The compressive load
applied to assembly 180 as the opposed members converge toward one
another cauges the th~ ~lastic material to flow together and fuse
into a substantially continuous phase. The compressive load is
maintained for a sufficient period of time for the thermoplastic
material to cool to a non-flowing condLtion after which the opposed
member~ are opened to yield a composite article 198 of the desired
shape.
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By subgtantially following the process a~ described above, a
unidirectional discontinuous composite is formed. The composite
denirably includes a continuous phase of a th~ ~plastic material
surro~n~1ng a plurality of discontinuous fibern of a reinforcing
material oriented codirectionally with one another.
Composites formed by the above-described process have
ani~otropic phy~ical properties. Thus, these composites have
strength~ and ela~tic moduli which generally are greater with respect
to loads in the fiber direction than with respect to loads (in the
plane of the composite) in directions transverse to the fiber direc-
tion. The magnitude of these ~ u~e.Lies will depend upon the particu-
lar thl? plastic and reinforcing materials from which the composites
are fabricated. Preferred composite materials comprising discon-
tinuous ~trands of fibergla~ surrounded by a continuous phase of
poly~.o~ylene in a ratio of about 30 wt~ fiberglas~ and about 70 wt%
polypropylene typically exhibit room t -~ature ten~ile strengths in
the fiber direction which are about 2-5 times the room temperature
tennile strength~ in direction3 tran~verse to the fiber direction.
Room temperature flexural strength values which are at least l.5-3
times greater in the fiber direction than in directions transverse to
the fiber direction are typically obtainable with preferred compos-
ite~. Further, the flexural modulus of preferred composites i8
generally at least l.5-4 times greater in the fiber direction than in
transverse directions, as measured at room t- - ature.
Additionally, preferred polypropylene/fiberglass composites
according to the above-described proces~ exhibit toughneYs properties
which are anisitropic. The typical toughness values for these
composite~ as measured by the Charpy Impact test are also about l.S-4
times greater in the fiber direction than in transverse directions.
In a further ~ t of the invention, the plastic
compo~ites incorporating unidirectional discontinuous reinforcing
fibers are formed from pultruded pellets consisting of strands of a
reinforcing material surrounded by a coating of a ~h~ ylastic
material. Thi~ process uses subst~nt;~lly the same thermoplastic
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materials and reinforcing materials as the process described above.
Although this proces begins with reinforcing materials which are
again in the form of continuous fibers or strands, the thermoplastic
material need not be in fiber form, but rather may be provided in the
form of pellet~, granules, flakes, powders, or any other divided form.
In the first stage 200 of this process, the thermoplastic
material and reinforcing fibers are coextruded to form a continuous
string 210 consisting of the continuous fiber 212 of the reinforcing
material surrounded by a coating 214 of the ~h~ plastic material.
This coextrusion step may comprise a conventional pultrusion process
for forming such continuously coated extrudates. In such process, the
~h~ ~la~tic material is heated to form a molten masQ. A continuous
fiber of the reinforcing material i~ pulled through this molten mass
and then through a shaped orifice 220 to form a uniform coating of the
the plastic material entirely around the fiber. In preferred
processes, a plurality of the reinforcing material fibers are first
collected into strand~ prior to the pultrusion process. Depen~i ng on
the particular reinforcing material selected, these strands may
include as few as two such fibers or as many as several thousand of
such fibers.
Once the th~ plastic material has cooled below it5 heat
distortion temperature, the pultruded string is cut in planes trans-
verse to the elongation direction of the reinforcing material strands
into a plurality of pellets 230, each consisting of a relatively short
strand surrounded on its periphery with a layer of thermoplastic and
~Ypo~e~ on its ends. Preferably, the length of these pellets is
between about 0.60 cm and about 6.0 cm, and more preferably between
about 1.3 cm and about 4.0 cm.
A~ will he_ - clear as the description of this process
progres~e~, it is not necessary that the cut be made entirely through
~he pultruded strings to sever Qame into separate and discrete
pellets. Rather, it will be sufficient for this process to cut the
strings to a sufficient depth to completely sever only the reinforcing
material strands. Thus, the pultruded product may consist of segments
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each having a relatively short strand of the reinforcing material
currounded by a coating of the thermoplastic material, each of the
~ -ts being held together by a thin web of the thermoplastic
material.
The pultrusion proce~ may include pultruding the thermo-
plantic material with a plurality of ~eparate fibers or strands of the
reinforcing material, all arranged codirectionally with one another,
to form a profile. The ~hape of these pultruded profiles will be
dete i n~d by the shape of the orifice in the pultrusion die. Regard-
lesG of its shape, the pultruded profile will consist of the plurality
of fibers or ~trands of the reinforcing material extending codirec-
tionally in the pultru~ion direction and ~urrounded by a ~ubstantially
continuou~ phase of the ~he plastic material. After the th~ -
plastic has cooled, the profile can be cut into a multiplicity ofpiece~, each including relatively ~hort strands of the reinforcing
material : '-dded within the th~ - plastic material. Again, the
profile need not be entirely severed during this cutting procedure, 50
long as the cutting procedure completely sever~ all of the reinforcing
3trands in the profile.
The pellet~ or pieces 230 are then juxtApo~ed with one
another, as at 240, ~o that the relatively short strands or fibers
therein extend ~ubstantially codirectionally with one another in the
fiber direction. The th~ ~lastic material in the juxtaposed pieces
is then fused to form a unitary mass including a substantially contin-
uous pha~e of the the~ plastic material surrounding the discontinuous
fibers or strands of the reinforcing material, wherein the fibers or
strands ~tLll extend codirectionally in the fiber direction. This
fuuing step may be performed in substantially the same manner as
described above in connection with the previouR process. That is,
after heating the juxtaposed pieces 240 to soften the thermoplastic
material (but not reinforcing material) these pieces may be subjected
to a compres~ive load 250, applied transversely to the fiber direc-
tion, which will cause the ~h~ plastic material to flow together and
fuse into a unitary mas~ 255. Although the unitary ma3s will include
the discontinuou~ strands or fibers ext~n~ing substantially in the
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same direction within the t~e ~lastic phase, these fibers or strands
may be arranged in substantially end to end disposition at locations
correspon~ing to the ends of the original individual pieces. Such end
to end disposition materially reduces the phyaical p.~e.Lies of the
composite. It is therefore preferable to subject the unitary masq 255to a shearing ~tep 260 which redistribute~ the di~continuous fibers or
strands to side-by-side, overlapping and interweaved disposition while
maint~ining their substantially codirectional ali~ L.
In the shearing step 260, the unitary mass 255 is heated
above the heat distortion temperature of the thermoplastic (but, where
applicable, below the heat distortion temperature of the reinforcing
material) and engaged between the confronting surfaces of a pair of
opposed I - 8 while the surfaces are moved relative to one another
in the fiber direction. One such shearing process may include feeding
the unitary mass 255 through a nip 262 defined between a pair of
opposed rollers 264 and 266 which are rotated at unequal surface
velocities to form a sheet 268. As the maas passes through the nip
262, the surface in contact with the roller 266 having the greater
surface velocity will be pulled relative to the surface in contact
with the roller 264 having the lower surface velocity. The relative
displ~r- --t of theae surfaceq with respect to one another will result
in a redistribution of the discontinuous fibers or strands within the
sheet 268, but will not affect the substantially codirectional align-
ment of these strands or fibers with one another.
The fusing step and shearing step may be performed in a
~ingle operation. In one such operation, the individual pellets or
pieces 230 may be fed through a roll mill (not ~hown) having a nip
defined by a pair of heated rollers rotating at unequal surface
velocities. As they pa~s through the heated nip, the thermoplaatic in
each of the pieces will be heated to a flowable condition and will
fuse with the thermoplastic in the adjacent pieces. At the same time,
the different surface velocities of the rollers will apply a shear
force to the mass to substantially redistribute the discontinuou~
fibera therein with respect to one another.
W O 94/09972 P ~ /US93/10313
i 4~ 16 -
The sheared, unitary sheet~ 268 may serve as a preform for
molding composite articles to a final shape. Thu~, the ffheet 268 may
be cut into a plurality of panels 270 which can be stacked on top of
one another so that the discontinuous fiber~ in all of the panels
extend in ~ubstantially the same direction, until a predet~ ed
thic~ne~ i~ reached. The stack can then be heated to place the
~- plastic material in a flowable condition (but not the reLnforc-
ing material fiberq) and compacted by applying a load tran~versely to
the fiber direction, such as in a compres~ion mold 280, to form an
article 290 in the desired shape. This article 290 will consist of
discontinuous reinforcing fibers exten~ing codirectionally with one
another and surrounded by a ~ubstantially continuous phase of the_ -
plastic .
The proces~ employing the coextrusion step forms unidirec-
tional discontinuous composites which also have physical properties
which generally are greater with re~pect to loads applied in the fiber
direction than with re~pect to loads applied in directions tran~verse
to the fiber direction. The magnitude of these properties will again
depend upon the particular th~ ylastic and reinforcing materials
from which these composites are fabricated.
In both of the processes de~cribed above, i.e., the process
employing the carding step and the process employing the coextrusion
step, the physical properties of the composite in the fiber direction
may be further enhanced through the use of continuous reinforcing
fiber~ which have a non-uniform cro~-section in the length direction.
Examples of such fibers include fibers having a diameter which
~ ~tes along the length of the fiber; twisted or spiraled fibers;
fibers having a zig-zag or accordion-shaped profile; and fibers having
radially protruding lobes at spaced distances along their length.
The following examples illustrate certain features of the
invention as described above.
WO 94/09972 PCI/US93/10313
~1~6~.35
EXAMPLE 1
Extruded polypropylene fibers having a diameter of about
28 microns are collected into strands of 72 fibers each. The strands
are then heated above the heat distortion temperature in an oven
heated to a t~ -~ature of about 130C and texturized by subjecting
same to bla~ts of air. After cooling to about room temperature, the
~trands are cut into discrete lengths. A cardable fiberglass produced
by Owen~-Corning Fiberglass, Inc., 10 microns in diameter, is collect-
ed in strands of about 2,000 fibers each and cut to predete ine~
lengths. Cardable fiberglass is fiberglass which has been coated with
a suitable surface agent or sizing which enables the strands to be
unbundled into individual fiber~ during a carding process.
In different runs of thi~ example, the predete in~d length
of the fiberglass strands are about 0.5, 1.0, 1.5, 2.0 and 2.5 inches,
and the di~crete length of the poly~ylene strands are about 2.0
inches. By weight percentage, 30% of the cut fiberglass stands and
70% of the textured and cut poly~ ylene strands are loaded into a
precarder in which a majority of the poly~.opylene and fiberglass
strands are at least partially separated into individual fibers and
combined in a three-dimensional f~5hion to yield a homogeneous mixture
in which the fibers are randomly oriented.
The hl -4E-ecug mixture i8 then fed into a carding machine in
which the fibers are pulled apart and aligned substantially codirec-
tio~lly with one another in the -rh;n~ direction to yield a con-
tinuous veil or web one meter wide and several fiber diameters thick
at the output of the carder. The bulk density of the web ranges
beL~aan about 0.2% and about 0.7% of the true density of the mixture,
depen~ing upon the strand lengths for the different runs. This web is
then fed through the orifice of a die which collects the web to form a
continuous sliver in which the fibers extend codirectionally parallel
to the elongation direction of the sliver. The bulk density of these
slivers is between about 0.004 gm/cm3 and about 0.01 gm/cm3, depen~ing
upon the strand lengths and the degree of compaction imparted to the
sliver during h~n~ling ~ubsequent to the carding process.
WO 94/09972 ~ PCr/US93/10313
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- 18 -
The continuous ~liver, having a diameter of about 4 cm, is
then cut into lengths of about 30 cm. A plurality of these lengths
are juxtaposed to form a layer 20 cm wide and 30 cm long, with the
sliver lengths all exten~; ng codirectionally with one another. Thirty
of such layers are stacked on top of one another to form a parallel-
piped shape in which all of the sliver lengths extend in the same
direction. The thus formed stack is placed between two sheets of
Teflon-coated fabric and heated for about 5 minutes in an oven at a
temperature of about 240C to soften the polypropylene fibers to a
flowable ~tate. The heated stack iff placed between the opposed
members of a compression mold (which members are at a temperature of
about 70C) and compression molded at an applied pressure of about 200
ATM for about 30 seconds, during which time the the plastic fibers
are fused to form a substantially continuous ~he ~plastic phase
aurrounding the discontinuous fiberglas~ fibers. This compacting
process yields a composite 30 cm long X 20 cm wide X 3.5 mm thick.
Table 1 qhows the physical propertieq of the compo~ites
formed in the different runs of Example 1, in both the fiber direction
and in the direction tran~verse to the fiber direction. It can be
~een that regardleas of the length of the discontinuous gla~s fibers
therein, each of the composites exhibited significantly superior
ntrengths and toughn~ss in the fiber direction than in the transverse
direction.
Table 1
Glass Fiber Flex Modulus Flex Strength Tensile Strength Toughnes~
Length (IN) MPa MPa MPa KJ/M2
FD TD FD TD FD TD FD TD
0.5 8155 5539 193125 93 62 58 37
1.0 8197 5315 184118 116 71 60 36
1.5 6324 1968 15451 86 16 50 14
2.0 6698 1747 18358 92 19 47 12
2.5 8551 5503 191116 84 50 59 35
FD = Fiber Direction
TD = Transverse Direction
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-- 19 --
Although the invention herein has been described with refer-
ence to particular : 'o~ Ls, it is to be understood that these
~ -L~ are merely illustrative of the principle~ and application~
of the present invention. It is therefore to be understood that
numerous modifications may be made to the illustrative - 'c'i -~ts and
that other arranyf Ls may be devised without departing from the
spirit and scope of the present invention a~ defined by the appended
claims.
