Note: Descriptions are shown in the official language in which they were submitted.
CA 02312467 2000-06-27
FLEXIBLE LOW BULK PRE.IMPREGNATED TOW
BACKGROUND OF THE INVENTION
3 I. ~ Field of the Invention
The present invention relates to a method of producing towpregs and, more
particularly, to a method of producing towpregs having low bulk and low
bending
rigidity which utilizes powder coating or a similar coating process. The
method of the
present invention involves the steps of converting unsized, high-performance
tows
t0 into a preselected cross-sectional form. such as flat ribbon form, coating
only the
outer fibers of the tow form with resin particles while leavns the interior
fibers of the
tow form uncoated, partially melting the resin particles onto the tow to
crezte a
discontinuous sheath of resin shout the tow, znd winding the resulting
flexible low
bulk to«preg onto take-up bobbins. The resulting towpreg can be processed
further
t s into other useful, typically stiffer and lighter, components.
2. Prior Art
Towpregs comprising fibers combined with a matrix resin are one Corm of
preprcg. Conventional prepreg consists of hundreds or thousands of fibers
embedded
in a continuous mass of matrix. The reinforcing fibers ypically used sre
awilzble
0 commercially in continuous form in bundles know as tows, which vay widely in
number of fibers per tow. :lrtanv matri~c resins are available: however m~o
kinds of
matrix resin sytems dominate the prior art: thermoplastic znd partisllv cured
thcm~osct pol~~mcrs.
_CA 02312467 2000-06-27
2
Thermoplastic polymers have advantages over thermosetting materials is
fracture toughness, impact strength and environmental resistance.
Thermoplastics
also provide prcpregs with indefinite shelf life, give the fabricator better
quality
assurance and avoid the storage and refrigeration problems associated with
3 thermosetting prepreg. The disadvantage of thermoplastic polymers as a
matrix
material is the difficulty of uniformly coating the fibers due to the high
viscosity of
the molten polymer. Thermoplastic prepregs also typically are rigid and less
well-
suited for weaving or braiding. and the resulting fabrics are stiff Similarly.
the
rigidity of thermoplastic coated' prepregs complicates the formation of
complex
to shapes; heat must be focused at the point of contact to achieve
conformability during
lay-up.
Preprcgs containing thermosetting prepolymers, although relatively flexible,
may be tacky, thus requiring a protective release coating, typically a release
paper or
film, which must be rcmoued prior to use. While thermoset prepregs are
acceptable
t 5 for many applications, their tackiness and the requirement of a protective
release
coatin~e have made thermoses tow~pregs unfeasible for v~eaving and braiding.
Two other t,pes of towpreg are metal matrix to«preg, which is simile to
thermoplastic toyreg, and partially cured ceramic toyrcg. which is similar to
thermoset to"preg. Due to the limited markets for metal and ceramic to~~pregs
and
'0 composites, these materials are not generally available and have not been
the focus of
developments in manufacturing technology. :~Tetal and ceramic matrix toyre=s
can
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3
be manufactured in a manner and form analogous to thermoplastic and
thermosetting
towpregs.
Continuous fiber prepregs can be produced by a number of impregnation
methods including hot melt, solution, emulsion, slurry, surface
polymerization, fiber
commingling, film interleaving, electroplating, and dry powder techniques.
In hot melt processing, impregnation can be accomplished by forcing the fiber
and resin through a die at high temperature under conditions that create high
shear
rates. Due to the high temperatures of this process, the thermoplastic
material can
degrade. Other disadvantages of this process include the high stress applied
to the
fibers and difficulties in impregnating the fiber tours with thermoplastics,
leading to
low processing speeds.
In solution coating, the matrix material is dissolved in solvent and the fiber
is
passed through this solution and then dried to evaporate the solvent. Two
disadvantabes of this process are that thermoplastics usually exhibit limited
solubility
i5 at high concentration, and most engineering thermoplastics cannot be
dissoted in a
low boiling solvent at room temperature. Additionally, high solution viscosiy
results
in the same impregnation problems as with hot melt. as wellvas causing the
fibers to
stick together. Another problem is the dif~iculh~ in removing the solvent.
Further.
traces of solvent left in the prepreg lead to undesirable porosity in the
composite
'0 structures.
An emulsion process is one way to apply particulate polymer matrix material
with a very small particle size to tov~preg fiber by svnthesizins: the resin
as an aqueous
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4
emulsion with. a surfactant. The problem with this process is that the removal
of the
surfactant from the final towpreg is difficult.
Slurry coating or wet powder processing is a non-solvent coating technique
designed to resolve the problem of the insolubility of most thermoplastics in
a solvent
s at room temperature. In slurry coating, the powder is suspended in a liquid
medium,
wherein no solvency exists behaeen the resin and the medium, and the fibers
are
drawn through the slurry. The slurried particulate matrix does not
substantially
wetout the fiber, resulting in the need for higher pressures to consolidate
the matrix
and fibers into a prepreg. This prepreg can be tacky, vrhich is not suitable
for weaving
to or braiding. Other disadvantages include the necessity for the removal of
the liquid
medium, volatiles, and dispersants or' surfactants which are used to form the
polymer/Iiquid colloidal state, the likelihood of aggregates in the slurry
caused by
poor mixing, and the possibility that polymer particles will settle during
processing.
To achieve intimate mixing in emulsioa or slung coating, the particulate size
i5 of the slurry or emulsion should be smaller than the fiber diameter. For
many of the
thermopi~stics that cannot be made by emulsion or dispersion polymerization,
it is
eWremely di~cult to produce such fine powder. Thus. a coarse blend bet~veen
fibers
and particles is obtained. The qualih~ of the blend decreases as the particle
size
increases. leading to poor matrix distribution in the consolidated towpre_.
and a poor
'0 composite structure:
In surface polymerization. coating results from the polymerization of the
polymer matrix on the fiber substrate. Disadvantas_:es associated with this
process
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include the need to remove undesired materials, such as solvent, inhibitor, or
by-
products of the polymerization reactions. Surface polymerization only can be
conducted by special techniques, such as eiectropolymerization, which cannot
be used
to produce many of the polymeric matrices of commercial interest.
5 In fiber commingling, the polymeric matrix is introduced in fibrous form.
Polymeric and reinforcing fibers are mingled as dry blends; however, mettins
of the
fiber by a process such as melting the polymer fiber is deterred until the
towpreg is
consolidated. Effective impregnation depends on the degree of randomness of
the
intermingling of the resin and fiber throughout the system. Since no wettins
of the
o reinforcing fibers by the matrix material occurs, higher pressures are
needed to
consolidate the to«preg under equivalent processing times and temperatures; as
compared to completely wetted toyregs.
Film casting is one method for producing prepreg which resolves some of the
problems associated ~~7th hot melt impregnation of thermoplastics. It consists
of
~ 5 stacking a film layer of matrix material cast from either hot melt or
solution over the
towpreg fibers. The fibers sandv~ched between I<vo films are heated and
calendered
to force the resin into the fibers.
Powder coating of fibers has significant advantages in that no solvent is
required and no high stress is introduced in the process. The powdered resin
must be
'0 solid at ambient and elevated storace temperatures. and be capable of
melting t~
permit flow and to penetrate the fiber tow when heated. Dy powder coating has
many advantaces because the elimination of a wet base. solvent or water.
facilitates
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G
reclamation of the coating material. This is an important economic advantage
which
promises a high utilization of powder plus elimination of expensive solvents.
Another method of powder coating fibers with matrix is disclosed in GanQa,
U.S. Patent No. 4,614,678 and ~, U.S. Patent No. 4,713,139, the later of which
was a divisional application of the former. This method entails impregnating
fibers
with dry thermoplastic polymer powder and then covering the coated fibers with
a
continuous thermoplastic sheath which leads to higher bulk. However, as tha
matrix
powder is not melted onto the fibers, only fibers in contact with the sheath
may have
adhered to the matrix. This results in a three component product: fiberst
matrix
to powder, and a separate continuous sheath. Consolidating this toyreg also
requires
higher pressures under comparable prodessing conditions than completely wetout
towpreg. Furthermore, as in slurry coating, a fine pol?~mcr powder, generally
less
than 20 microns, is recommended for this process. Producing such fine powder
from
thermoplastics can be very expensive.
t5 Intermediate composite products, such as towpreg. must contain sufficient
matrix. n~picalIy over 15% by volume. to permit consolidation of the
components into
a substantially void free to«preg structure nithout requiring the
incorporation of more
matrix material. Linear toyregs can be com~erted into two and three
dimensional
product forms by weaving, braiding, filament winding. and other known
processes.
3o Alternatively, these toyregs can be used to create a discontinuous fiber
reinforced
feedstock for molding by chopping, cutting, or like know processes.
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7
Towpreg can be converted to a preform of a predetermined shape and fiber
orientation. Preforms can be produced by any one of the conventional textile
prefocming methods, such as weaving, braiding and knitting, or by processes
such as
filament winding and tape laying. Preforms ultimately or concurrently can be
consolidated into composite parts by applying heat and pressure.
Textile preforming operations require that the towpreg be flexible. In a
powder coated tow, the fibers are not necessarily connected by the resin
material at all
places. This provides freedom for the fibers to move relative to each other.
Due to
this structure, tow~preg produced by a powder coating process has the
potential to be
t0 flexible. A powder coating process, such as the one disclosed in Muzzy, et
al., U.S.
Patent No. x,094,883, tropically involves'tHe following four steps:
1. Spreading the high performance filaments into sheet form, almost to
individual filament level;
3. Incorporating the matrix powder particles betveen the filaments;
t s 3. Partially melting the powder particles to the filaments: and
4. Condensing the sheet of filaments back into strand form.
During this pmcess. the powder particles. generally of size 20 to ?00 microns.
are
placed in-between the filaments. Generally of a diameter as small as 5
microns. As a
result. after powder coating. it is not possible for the filaments to pack as
tightly as
.o they were packed before powder coatinG. Ficure 1 illustrates this
phenomenon
schematically. Hence. the resultinG towpreG. while flexible. tends to be
bulky. The
high bulk of the towprees poses various problems in the processing of these
towpreLS.
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8
High bulk towpreg or preforms require molds with deeper cavities than molds
used for lower bulk towpreg or preforms. Therefore, to process the high bulk
tov~~preg
or preforms, the existing molds may need to be redesigned. Another problem
arises
during consolidation. To consolidate the high bulk towpreg or preforms, the
mold
halves have to move larger distances for full consolidation. This can cause
larger
fiber disorientation leading to weaker parts. Another problem is that high
bulk
towpreg and prefocms are difficult to fit into molds used for making smaller
sized
parts W th processes such as expansion molding. Another problem is that
textile
processing of high bulk toyreg results in more processing problems than in
te~ctile
t0 processins of low bulk to~ypres.
Current powder coating methods generally are incompatible mith the
utilization of sizing agents. Since most fibers used as reinforcement are
~~ery fragile,
sizing agents which are compatible with the resin matrix often are used to
protect the
fibers. Howe~~er, sizing tends to glue the fibers together, which makes fiber
spreading
difficult. Sinet prior art powder coating processes require that the fibers be
spread
prior to powder coating. the fibers cannot be sized. This limits the t<~pes of
fibers
which can be used W th current powder coating processes to fibers which are
less
fragile or less easily damaged.
For all of these reasons, it is very important to have towpregs with the least
't) amount of bulk possible. Ho~~rever. as stated abo~~e. ~~~ith the existing
methods of
powder coating. spreading techniques are used to make the powder particles
penetrate
into the fiber bundle. thus makinu it very difficult to reduce the bulk of the
towpregs.
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Therefore, a need exists in the industry for a method of producing relatively
low bulk
and flexible towpreg which can be accomplished by utilizing powder coating
techniques
and the advantages thereof.
SUMMARY OF THE INVENTION
High performance fiber tows may be converted into a predetermined
cross-sectional form by one of a plurality of different methods, powder
coating the
cross-sectional form with preselected resin particles, partially melting the
particles onto
the exterior fibers of the fiber bundle such that the interior fibers remain
uncoated and a
discontinuous sheath of resin is created on the exterior of the fiber bundle.
The resulting
towpreg can be wound onto take-up bobbins. The towpreg then can be further
processed.
The resulting flexible towpreg then can be converted into preforms for
composite applications through one of many conventional textile processes, or
by tape
laying or filament winding. Although the towpregs in the flat ribbon shape
obtained
when the flat ribbon form is used are quite suitable for many applications,
towpregs with
elliptical or more circular sections are preferable in many textile processes
such as
braiding and knitting. A post treatment process, described in detail below,
can be used
to convert the flat ribbons into more of an elliptical section.
2 0 The method can involve coating unspread fiber tows in various cross-
sectional configurations with a matrix resin. The tows are arranged in a
predetermined
cross-sectional shape, such as for example flat ribbon form, tapes, ellipses,
rectangles
and circles. The reshaping of the cross-section of the tow may be obtained by
different
means such as, for example, roller flattening or passing the tow through an
aperture.
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An appropriate sizing agent can be used to hold the fibers together in the
required shape.
During the coating process, since the fibers are not spread, the resin
particles generally
do not enter into the fiber bundle, but rather coat only the surface of the
tows. This
results in substantially dry internal fibers in the core of the towpreg and a
discontinuous
sheath of substantially wet outer external fibers.
The external fibers are mostly wet out on the surface of the towpreg;
however, the resin sheath does not completely coat the entire surface of the
fiber bundle,
resulting in decreased bending rigidity. The resin layer on the surface of the
towpreg is
formed by a partial melting of the resin particles, resulting in the fusing of
the particles
10 onto the tows. Partially melting the particles results in a resin layer
which does not
completely cover the surface of the towpreg. This type of discontinuous sheath-
dry core
structure results in a towpreg with relatively low bulk and comparable bending
rigidity
in comparison to towpregs produced with the existing powder coating method.
It is desirable to provide a method for producing flexible towpregs with
the least possible bulk using high performance fibers such as carbon, aramids,
glass
fibers, etc., and resin materials including both thermoset and thermoplastic
resins.
It is also desirable to provide a method for producing low-bulk, flexible
towpregs having at least 15 °Io resin by volume, thereby permitting
consolidation of the
components into a substantially void-free composite structures.
2 0 It is also desirable to provide a method for producing low-bulk, flexible
towpregs wherein the resin particles can be uniformly distributed across and
along the
exterior fibers of the tow, so that with application of pressure and
temperature, the resin
will be able to melt and flow between the fibers to result in a uniform degree
of wetout
of the fibers when forming a composite article.
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It is also desirable to provide a method fc~r producing low-bulk, flexible
towpregs which method allows the maximum number of fibers to be covered with
the
resin material wii:hout increasing tlm bulk of rigidity ot~ the towpreg.
It is also desirable to provide a method i~~nr producing low-bulk, flexible
towpregs which proposes partial, as opposed to complete, melting of the resin
powder
particles to achieve adhesion of the resin to the fibers.
It is also desirable to provide a tov~preg cornprisir~g a bundle of fibers,
the
interior fibers remaining free. of matrix fiarmi~ag resin lnartic°les
and the exterior fibers
being partially coated with a discontinuous sheath of matrix forming resin.
It is also desirable to prcavide a method tcfr producing low-bulk, flexible
towpregs which is suitable for processing various types of commercially
available
fibers and resin materials in an economically viable manner.
It is also desirable to provide a method for convc;rting flat ribbon towpreg
produced in accordance with the present invention into flexible unidirectional
tape.
Accordingly, in one aspect of the invention, there is provided a method for
producing flexible towpregs from a plurality of fibers, said fibers organized
in a
predetermined cross-sectional arrangement of inner and cuter fibers, said
method
comprising the steps of: Via) coating the outer fihers of the tow with resin
particles; (b)
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old
heating the coated tow to a temperature grad for a tinge tcD at last partially
melt the resin
particles, wherein at least partially melted resin particles are adhered to
the outer fibers
of the tow thereby forming the tc>wpreg; and I.c) c;ooling tlxe towpregs such
that the
outer fibers which are adjacent to each other are at least partially
ernbeddc~d in a
discontinuous sheath of said resin while the inner fibers rE;main
substantially free of
said resin.
In another aspect of the invention there is provided a flexible towpreg
comprising a bundle of fibers coated with a discontinuou s sheath of a matri:~
resin,
wherein said matrix resin adheres to thf° fibers on the exterior of the
bw~dle, while
fibers in the interior of the bundle remain substantially free o~'the matrix
resin.
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- I2-
The present invention will become apparent from the following discussion when
read in conjunction with the attached figures, in which like reference
numerals correspond
to like components throughout the several views.
BRIEF DESCRIPTION OF THE FIGURES
Figs. lA-1C illustrate the high bulk of a powder coated tow produced by
conventional powder coating techniques.
Fig. 2 illustrates a first embodiment of the powder coating system and method
of
the present invention.
Figs. 3A-3D illustrate the different types of cross-sectional geometries for
the tows
which are suitable for use with the present invention.
Figs. 4A-4D contain scanning electron micrographs of the surface and cross-
section of a towpreg produced in accordance with the present invention,
illustrating the
discontinuous sheath nature of the outer coating of resin material.
Fig. 5 illustrates a second embodiment of the powder coating system and method
of the present invention.
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13
Fig. 6 illustrates a system for accomplishing serving operations in accordance
with the present invention.
Fig. ? illustrates a system for heating a towpreg wrapped with filament yarn
to
cause the yarn to shrink and wrap around the towpreg tightly.
. .
Fig. 8 illustrates a schematic diagram of a device used to measure the bulk of
towpreg produced is accordance with the present invention.
Fig. 9 illustrates a schematic diagham of the ultraflexible unidirectional
tape
produced is accordance with the post treatment process of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Reducing the bulk of the towpreg without sacrificing the flexibility and other
properties of the towpreg is an important advantage of the present invention.
This is
accomplished by suitably modifying the fiber architecture of the tow before
coating
the fibers with resin material.
As shown in Figs. lA-1C, conventional powder coating involves spreading the
tows to a width typically in the range of 100 to 130 mm for a 12,000 filament
carbon
fiber tow. Other fiber tows are available ranging from 1000 to 300,000 fibers,
all of
which are suitable for the present process: Any of the typical fibers may be
used for
the tow. Preferably, the fibers have a modules of elasticity of at least
approximately
34 GPa and a tensile strength of at least approximately 0.70 GPa. Fibers made
from
carbon, organic compounds, glasses, metals, and ceramics having the above
physical
characteristics are most preferred.
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14
The present inveation can be accomplished using both thermoxt and
thermoplastic resin particles. The preferred thermoxts include epoxies,
polyesters,
' adrylica,' golyimides, and phenolics. The preferred thermoplastics include
nylons,
polypropylenes, polyesters, polycarbonates, acrylics, golyimides,
polyetherimides,
and polyaryl ethers.
In the conventional pcncess, after the powder particles $2 are placed between
the fibers 80 (Fig.1B), typically by using an electrostatic fluidized bed,
these particles
82 are fuxd to the fibers 80 by passing the coated fibers through a hot oven.
The
fibers are condensed (Fig. 1G~ after the resin particles are cooled However,
since the
powder particles are fused to the fibers while the fibers are in a spread
configuration,
the fibers do not pack as closely together after condensation. ~ This
generally leads to
the towpregs having relatively high bulk.
In accordance with the present invention, the spreading process used in
conventional powder coating is eliminated. A uniform distribution of resin
particles
on the fiber tow exterior is obtained by suitably modifying the tow
architecture before
coating tha .tows: Fig..2 is n schematic diagram illustrating the principles
of the
powder coating process of the present invention. The tow 20 from the supply
package
22 is unwound and fed to the powder coating chamber 24, where the
electrostatically
charged resin powder particles (not shown) are deposited onto the fibers of
the tow.
Once the tow 20 emerges out of the powder coating chamber 24, the tow 20 is
pasxd
through an oven or healer 25 where the powder particles are partially melted
onto the
fibers. The
CA 02312467 2000-06-27
hot towpreg 27 is then cooled by passing the tow 20 around a cooling roller
contained
within cooling system 30 before being wound onto a take-up package 32.
The resin picked up by the tow can be controlled by changing the level of
electmstatie charging, the powder feed rate and the tow speed. Higher charge
on the
powder particles, higher powder feed rate and slower tow speed. increase the
powder
pickup. The temperature of the oven is xlected depending on the melting
temperature
of the matrix powder being used. The temperature is selected in such a way
that the
powder particles are partially melted in order to adhere to the tow. Complete
melting
of the resin results in wicking o~the melted resin along the tows and a
stiffer towpreg.
Insu~cient heating of the powder results in poor adhesion of the powder to the
tows
and in powder falling off in subsequent processing. Typically the powder
coating line
is operated in the range of appmxirnately 10 to 60 meters per minute. The
range for
the radiant panel temperature is 650°C to 950°C depending on the
polymer and line
speed
The qualities of the towpreg, such as beading rigidity, and the properties of
laminates produced from these towpregs depend upon the cross-sectional shape
of the
tovv, ~ the resin content and the powder coating conditibns. Different
possible
geometries of ~tha cross sections of the tow are provided in Figs. 3A-3D. Fig.
3A
shows a flat ribbon geometry, Fig. 3H shows an elliptical geometry, Fig. 3C
shows a
circular geometry and Fig. 3D shows a square geometry. These different shapes
can
be obtained by passing the tows through a bath containing sizing agent and
then
through a die with the required cmss-sectional shape, or by other means
without using
.Yo? P'7'\f.~/R~rY
CA 02312467 2000-06-27
15/1
siting agents. The geaeral range in the amount of size used is 0.5°~G
to S.0% by
weight
CA 02312467 2000-06-27
IG
The types of the sizing agents influence the interface of the composite system
to a large extent. A strong interface between the fiber and the resin results
in an
eff~etive.load transfer between the fiber and resin. The interface plays an
important
role in the shear and fatigue performance of the composite. Hence, having the
right
size or sizing agent on the tow to promote adhesion is very important.
The following examples illustrate the effect of the cross-sectional geometry
and the effect of having the right sizing agent on the fibers while forming
the desired
cross-sectional shape of the to«~s.
EXAMPLE I
t0 Circular cross section with no sizing agent. Unsized 12K G30-X00 (Toho
lndusiries, Japan) carbon fibers were unwound from the supply package. Each
carbon
fiber has a diameter of approximately 8 microns. Application of 90g tension to
z tow
of these fibers brought the tow to a circular section having a diameter of
about 2.2
mm. This tow was then passed through a powder coating chamber containing a
I3 charged powder cloud of the matrix resin panicles. The line speed was 18.3
meter/minute (60 ft/minute), hence the residence time for the tows inside the
powder
coating chambeF was 3.6 seconds. The toga was passed through an oven once it
emerged from the powder coating chamber. The temperature of the heatin_
clement
v~~as kept aE approximately 900°C. The residence time inside the oven
was ?.0
30 seconds. The powder used in this case was nylon 6 ~~ith a mean particle
size of 100
microns with a range of'_0 to X00 microns. The powder content in the towpreg
~~~as
32.0% by weic;ht. Assuming hexagonal close packing of the fibers and complete
wet
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17
out of the surface layers of fiber, the result is a towpreg wherein 1 out of
every 16
fibers is completely wet out, these being the exterior or surface fibers.
The bending rigidity of the resulting towpreg was 305 x 10's Nm=. Uncoated
12K G30-500 carbon tow has a bending rigidity of 14 x 10's Nm=. Bending
rigidity
s was measured using a pure bending mode in a Kawabata Evaluation System, a
highly
sensitive method ustd for measuring bending rigidity of textile strands. The
method
is described in Ramasamy, A., Y. Wang, and J. Muzzy, Characterization ojPowder
Cnatcd To»press and ?D Pr~jorms. 38 Int'1 SAMPE Symposium 188? - 91 (1993).
Laminates were made with the towpregs under the follow mg
t0 processing conditions: consolidation pressure 700 kPa; temperature
?GO°C: and time
20 minutes. The laminates were tested'id a three point bending configuration.
The
ultimate strength in the longitudinal direction was 1.4 GPa and that in the
transverse
direction was 0.060 GPa. There was a significant amount of dry fibers in the
cross-
section of the laminate. Increasing time, temperature and/or pressure would
eliminate
t3 these dry fibers and increase the strength of the laminates.
EXAMPLE 11
Ribbon cross section W ch epoxy sizing. Epoxy-sized 12K T700 carbon fiber
tows (Toravca. Japan) having flat ribbon shape in cross section were powder
coated
with melon 6 powder. The uncoated tow had a width of approximately 6.3; mm and
a
.0 thickness of about 0.13 mm. The amount of size present in the tow was 1.0%
of the
weight of the fibers. The towpregs were produced using conditions similar to
those
used in Example 1. The powder content in the tow in this case was .iG.?%. The
CA 02312467 2000-06-27
18
increased powder pickup, as compared to Example I, was due to the increased
surface
area of the tow. For this flat ribbon cross-section, assuming hexagonal close
packing
of the fibers and complete wet out of the surface layers of fibers, should
result in a
towpreg wherein 1 out of every 6 fibers is completely wet out by the resin,
these also
being the exterior or surface fibers.
Figs. 4A-4D show scanning elcctroa micrographs of both the surface (Figs, 4A
and 4B) and the cross-section (Figs. 4C and 4D) of the towpreg of this
example. Figs,
4A-4D show the presence of dry fibers in the core of the towpreg and at least
one
layer of fibers on the surface being substantially but not completely wet out
by ~ the
resin. Figs. 4A-4D also show the discontinuous nature of the resin sheath
about the
exterior of the tow. The discontinuous sheath results is a towpreg having a
lower
bending rigidity.
During consolidation, the dry fibers in the core are wet out completely by the
resin due to the ~coasolidation pressure and temperature. The gaps between the
fibers
in the sheath assist in resin flow from the surface of the towpreg to the
core, resulting
in wet~out of the dry fibers under consolidation pressure. The bending
rigidity of the
tow was 112 x 10's Nms. Laminates were made with these towpregs using the same
consolidation conditions as in Example I. The ultimate strength in the
longitudinal
direction was 1.6 GPa and that in the transverse direction was 0.06 GPa. Dry
fibers
were not observed in the cross-section of the laminates.
EXAMPLE III
Ribbon cross section with nylon sizing. Unsized 12K G30-500 (Toho
Industries, Japan) carbon fiber tows were sized with a nylon terpolymcr sizing
agent
CA 02312467 2000-06-27
19
using the system shown in Fig. 5. The tows 35 were dipped by a dip roller
assembly
36 into a size bath 38, passed through a set of flattening rollers 40, passed
over drying
-- ~ .
rollers 43 contained in oven 42, and they to a take up winder where they were
taken
up on take up bobbins (not shown). The resulting tow from this setup was a
flat
.. ,
ribbon, approximately 5.80 mm wide and about 0.152 mm thick. The tow had a siu
content of 1.5% of the total weight of the tow. These tows were powder coated
with
nylon 6 using conditions similar to those used in Example I. The powder
content in
the tow in this case was 34.5%. The bending rigidity of the resulting tow was
135 x
10'~ Nm=. Laminates were made with these towprega under the same consolidation
conditions as in Example I. The ultimate strength in the longirirdinal
diration was
2.0 GPa and that in the transverse d'uection was 0.16 GPa. Hence, the presence
of
nylon size when producing a nylon matrix composite results in higher
mechanical
properties. Dry 5bers were not observed in the cross-section of the laminates.
EXAMPLE IV
Ribbon cross section with on-line sizing. Unsized 12K G30-500 (Toho
Industries, Japan) carbon fiber tows were sized with a nylon terpolymcr sizing
agent
using an on line sizing with the powder coating process. In this case the
sizing unit
shown in Fig.. 5 was placed on line with the powder coating line, except that
the
drying rollers 43 were removed and the tows were dried by blowing hot air on
the
flattening rollers 40. The line speed in this case was 1.2 meter/minute. The
infrared
heating clement temperature was 520°C. The powder pickup in this
example was
40.1% because of the slower line speed. The bending rigidity of the resulting
towpreg
CA 02312467 2000-06-27
was 298 x10' NmI. Laminates were made with these towpregs under the same
consolidation conditions as in Example I. The ultimate strength in the
longitudinal
directioe' was 1.6 GPa and that in the transverse direction was 0.13 GPa. This
example demonstrates the feasibility of online sizing and restructuring of the
tow
s cross-section along with powder coating.
The above examples illustrate different methods to make flexible toyrcgs
with low bulk without spreading the tows.. Similar types of structures also
can be
obtained using resin material in forms other than ponder. One method is to
cover the
surface of the tows W th very fine short fibers made out of resin by processes
such as
0 melt blowing. In the melt blowing process, the polymer is melted and
extruded
tlvough spinnerets while compressed hot~air is forced into the spinnerets.
This causes
the resin to be deposited on the tow surface in the form of a net,~~ork of
short, fine
fibers. For example, see U.S. Patent No. 4,380,570 to Sch~~~arz,.
The present invention eliminates the need for sprcadins the tows, thereby
~ 5 allowing multiple tows to be run simultaneously through the powder coating
tine
close together to achieve high production rates. The present invention further
permits
pow~det coating of filament t<~pes which are difficult to spread. The
following
examples demonstrate these advantaees.
EXAMPLE V
Coating of multiple ends of aramid fibers with melon 1 l powder. Twelve ends
of Kevlarn 49 aramid fibers of 2840 Denier were powder coated simultaneously
with
nylon 11 powder. (n this example, powder coating was carried ouc in z
horizon«1
CA 02312467 2000-06-27
21
conFguration. The length of the oven used for fusing the powder to the tows
was 2.44
meters. All the tows were run at 13.7 meters/minute. The temperature in the
oven
was not 'measured but controlled only as the fraction of power input to the
heating
elements. The tows were passed around fixed steel rollers before they entered
the
s powder booth. The action of the tows around the steel rollers flattened the
tows to a
width of about 4 to ~ mm. The powder pickup was about 32%. The tows ~~ere then
woven into a fabric (2 by 2 hvill with 3.8 by 3.8 picks per centimeter) and
the fabrics
were consolidated into laminates. These laminates were tested in a 3-point
bending
configuration. The ultimate breaking strength was 0.3 GPa. The laminates were
to substantially void free (less than 2% voids) when the cross-sections were
examined
under the microscope.
EXAMPLE VI
Coating of multiple ends of E-Glass with polyester powder. Forty-cibht ends
of E-Glass of 1600 yield were powder coated simultaneously Wth polyester
t 5 (polyethylene terephthalate) powder. The powder coating system used is
similar to
the one used in the experiment discussed in Example V. In this case. 8 of the
to«~s
were combined into one tow before going into the powder chamber. This combined
tow was passed around a fixed steel bar. The action of the tow around the
steel bar
flattened the combined tow to a width of about ?0 mm. The processing
parameters
'0 were kept at the same level as in the e~cperiment of Example V. The amount
of
powder pickup in this case mas about :6%. These to«pregs were consolidated
into
unidirectional laminates and tested in a three point bending conr'_uration.
The
CA 02312467 2000-06-27
12
ultimate breaking strength in the axial direction was 1.5 GPa and 0.07 GPa in
the
transverse direction. The laminates were substantially void free (less than 2%
voids)
when the cross-stctions were examined under the microscope.
These examples clearly demonstrate that, with the present invention, powder
coating of tows in flat ribbon form results in better towpreg properties.
However, for
textile preforming processes such as braiding, a circular or elliptical cross-
sectional
shape is often preferred to that of flat ribbon. shape. In order to obtain a
cross section
closer to the circular or elliptical cross-sectional shapes. sending and heat
shrinking
operations are carried out in~line W th the powder coating.
Fig. 6 illustrates a schematic diagram of a system 45 used for the sen-ing
operation. In this operation, the flat ribDdn towpreb 48 is passed throush s
3.3 mm
(0.125") diameter hole in a Teflon' plug 50 and through the central hole in a
hollow
spindle 52, as shown in Fig. 6. The hollow spindle also is fitted with a
similar Teflon"'
plug 54 at the top. As the towpreg 48 is passing through the holes in the
Teflon' plugs
13 ~0 and 54, it is folded as a result of the diameter of the hole (3.2 mm)
being smaller
than the width of the towprcg (6.4 mm). The toyreg 48 then is wrapped with
fine
filament yarn (preferably but not necessarily made of the same material as the
matrix
material: other thermoplastic materials can he used) in its folded form by the
rotation
of the hollow spindle 52. The wrap density (number of NTaps per unit length of
the
'0 towprcg) can be changed by changing the speed of the hollow spindle ~3 or
by
chancing the line speed of the tow. A typical range of wrap density is -10 to
.00
CA 02312467 2000-06-27
wraps pet meter. Altemaeively, a tow bundle having a circular crow-section may
be
wrapgcd with the filament yarn.
Eecause the form of towpreg produced in accordance with this embodiment of
the present invention has a smooth surface, the serving yarn is not held
firstly to the
towpreg surface. 'Ibis makes it easier for the serving yarns to slide during
further
processing, la order to avoid this problem, the towpreg 59 is passed along
guide
roller, which inay include one or more idle rollers 60 sad one or more drive
rollers
62, and between heated rollers 65, which are heated by as air gun 67, as shown
is Fig,
7. This eauscs the fine serving yarn to shrink and wrap around the tows or
towpceg
tightly. Alternatively, glass fiber can be used as the wrapping Yarn. Although
glass
has s mmim~l shrinkage upon heating, the act of wrapping will hold the cow= or
towpreg together.
Fig. 8 illustrates a xhematic diagram of a devica 70 rood for measuri.ag the
bulk of towpreg~producad is accordance with the present invention The bulk
factor,
Ha is defined as the ratio of the thickness (t~ of the preform or towpreg to
that of the
fully consolidated thickness of the laminate (tJ that can be made From this
pcefocm or
towpreg assuming that there are no voids pcexat in the laminate.
Hulk factor, Hr ' to/t,
An Instron~' model 1125, shown in Fig. 8 and indicated generally by numeral
70, is used for measuring ta. A fixture 72 with a cavity 74 of dimensions 6.4
mm
width (w) and 254.0 mm length (unshown-into the page) and 76.2 cam depth is
fabricated to compress the towpregs 76 within an enclosed space, as shown in
Fig. 8.
CA 02312467 2000-06-27
23/1
Towpregs 76 are cut to len~th,s of 254.4 mm and 50 pieces of towpreg 76 are
stacked
inside the cavity ?4. To measure the buUc factor of the
CA 02312467 2000-06-27
Za
towpregs, a pressure of 50.1 Pa is applied to the towpregs. This pressure
level is
chosen so that the measurements are comparable to other reported measurements.
The
displacement values of the towpregs are measured using a Linear Variable
Differential
Trarufocmer (LVDT, not shown). The thickness of the towpregs is measured by
taking the difference in the LVDT readings with and without the towpreg
specimen in
the fixture.
The bulk factor of nylon 6112K G30~~00 powder coated tow with 35'/o resin
by weight, using the known sprcadin~ method. is 7.5. Similar toyreg produced
in
accordance with the present invention in the flat ribhc~n form W th 13K T~700
carbon
fiber/nylon 6 generally is less than 5.0, and has been produced as low as 3.6.
Ny lon
6/2840 denier Kevlar'~ 49 powder coated tow has a bulk factor of 2.4. These
measurements indicate clearly the relatively tow bulk of the toyregs produced
in
accordance v-ith the.method of the present invention.
In sumrttary, the present invention involves a no~~el method for producing
t 3 flexible towpregs having low bulk- This is achieved by powder coating
without
spreading the tows. The basic processing steps involved are powder coating the
tows
and post treatment of the towpregs for further textile processing. Sizing the
tow prior
to powder coating can be undertaken if necessary. Accocdinelv, the towprees
produced by this novel method have a demonstrated lower bulk, low bending
rieidit~~
?0 and improved processibility.
Prior to powder coating unsprcad fiber tows with the matrix resin. the to~~~s
are
arranged in a predetermined cross-sectional shape, such as a flat ribbon form.
The
CA 02312467 2000-06-27
reshaping of the cross-section of the tow may be obtained by different means
such as
roller flattening or passing through shaped apertures. The sizing agent can
hold the
fibers together in the required shape. During the powder coating process,
since the
fibers are not spread, the powder particles do not enter into the fiber
bundle, but rather
5 coat only the surface of the tows. This results in dry fibers in the core of
the towpreg
and a sheath of fibers which are mostly wet out on the surface of the towpreg
surrounding the core of dry fibers. The resin layer on the surface of the
towpreg is
formed by a partial melting of the powder particles. Partially melting the
particles
results in a resin layer which does not completely cover the surface of the
toyreg.
This type of sheath-core structure results in a towpreg with relatively low
bulk and
comparable flexibility in comparison to ~tow~pregs produced with the existing
powder
coating method. The discontinuous surface layer is beneficial in nvo ways: (1)
it
keeps the tow flexible as it provides freedom for the fibers to move relative
to each
other, and (2) it helps the air in the core of the tovpreg to escape while
consolidating
~ 5 these towpregs into composite parts, leading to substantially void-free
parts.
One aspect of the present invention is the post treatment of flat ribbon
to~~~preg
produced in accordance with the present invention to convert the towpreg into
flexible
unidirectional tape. Two features of the flat ribbon toyreg make it amenable
to
conversion into flexible unidirectional tape. First. the to«preg is in a flat
ribbon
shape with consistent dimensions and with all of the fibers laid parallel to
each other.
Second. only the surface of the towpree has been coated with the resin
material.
CA 02312467 2000-06-27
2G
Converting the new towpregs into unidirectional tape involves the following
process
steps:
1. Placing the required number of spools of towpreg in a creel in a
suitable manner,
2. Condensing the towpreg from the creel by any known means and
preferably by a condenser board; the number of towpregs brought together to
fom~ a
unit length across the width of the tape determines the aerial density (weight
per unit
area) of the tape; this step may involve overlapping of the towpregs;
3. Heating the sheet of individual towpregs formed in accordance with
t0 steps 1 and 2 such that the resin material melts: as the resin material is
present only on
the surface of the towpreg, the resin material can be melted with less energy
than that
which would be required if the resin material totally coated the fibers of the
to~preg;
4. Applying pressure on the surface of the sheet of towpreg having the
molten resin material thereon, such as by passing the sheet of towpregs
between a pair
t 5 of hot nip rollers. v~hich makes the molten resin from neiehborine toyre~s
mix: and
Cooling the sheet, resulting in unidirectional tape havin5 a smooth,
thin layer of resin on both the surfaces and dw tibers in the core: the
resulting
structure pro~~iding flexible unidirectional tape.
EX:1!~~1PLE ~'1I
_'o Flexible unidirectional tape. G30-X00 (Toho Industries. Japan) carbon
fiber
tow with 13.000 filaments was converted into flat ribbon form usinst a nylon
sizing
went and coated with melon 6 resin powder. The amount of resin added to the
carbon
CA 02312467 2000-06-27
27
fiber was 35% ~ 3°/. by weight. The towpregs were condensed into shoot
form of 187
tows per meter. The resulting conFguration was a unidirectional tape having a
density
of 244 g/m=.
Tapes produced by this method form kinks when bent around high curvatures.
This kinking phenomenon is a result of the towpregs being held together by
resin
material thereby allowing less freedom between the towpregs or fibers on a
macro
scale to reposition themselves when the tape is bent. Many composite
applications
require relatively high curvatures and therefore unidirectional tapes which do
not kink
when they are bent are needed. Accordingly, the present invention provides ~
as
~ultraflexible unidirectional tape which does not kink vrrhen bent around
relatively high
curvatures.
To produce the ultratlexible unidirectional tape, the towpregs first are
placed
adjacent to each other using an appropriate creeling method. The resin then is
melted
and fused between the towpregs only in selected places. In this way, fibers or
towpregs between the weld points have the freedom to reposition themselves
when the
towpregs are beat~ Fig. 9 illustrates a schematic diagram of the ultraflexible
,
uaidirectionai tape 90 formed in accordance with this embodiment of the
present
invention. The selective molting of the resin and selective fusing of the
towpregs into
a sheet can be accomplished in either of the two following manners:
1. One or a set of reciprocating or rotating hot rods or shoes may come
down onto a slow moving sheet of towpregs 92 and press the towpregs against a
hot
metal surface thereby causing the resin to melt and form bridges or weld lines
94
.,
between the
CA 02312467 2000-06-27
28
towpregs; the distance between the weld lines 94 can be changed by changing
the
frequency of the reciprocating movement or the rotational speed of the hot
rods or
shoes or by changing the speed of the sheet of towpcegs; or
2. Heated rods with helical grooves can be used to apply pressure to pass
the towpregs 92 against a hot metal surface; in this case the weld lines 94
will be
formed at nn angle to the towpregs.
?ho selective melting and fusing also may be accomplished by other methods
such as
by using hot serrated rollers to form the weld lines or by point bonding of
the shat of
towpregs by using a hot plate with the required pattern.
E~CAMPLE VIII
Ultraflexible unidirectional tapes which avoid kinking when bent around
relatively high curvatures. G30-500 (Toho Industries, Japan) carbon fiber tow
with
12,000 filaments w~ias converted into flat ribbon form using a nylon sizing
agent and a
nylon 6 resin powder coating. The amount of resin material added to the carbon
fiber
was 35 ~ 3%. The resulting towpregs were condensed into sheet form with 157
tows
per meter. The aerial density of the resulting tape was limited by the width
of the
individual towpregs. The resulting configuration was unidirectional tape
having a
density of 195 .g/m=.
The differences in flexibility of the unidirectional tape produced by the
first
embodiment (unidirectional tape of Example VII having a density of 244 g/mi)
and
the ultrallexible unidirectional tape produced in accordance with the second
embodiment (unidirectional tape of Example VIII having a density of 195 glm=)
was
CA 02312467 2000-06-27
z9
measured using a cantilever beam principle (ASTM 1388). The deflection of the
tape
(8,~ for an overhang length (L) is measured by letting the tape (cut to a
predetermined
width) overhang. The bending rigidity (En was calculated by using the
following
equation:
EI = qL4/88"
Where q is the weight of the tape per unit length. The bending rigidity of the
unidirectional tape was calculated to be 20T x 10'~ Nm= and that of the
ultraflexible
unidirectional tape was 264 x 10'a Nm~. Therefore, the bending rigidity
calculations
clearly show that the ultraflexible unidirectional tape is significantly more
flexible
to than the unidirectional tape produced in accordance with the first
embodiment.
EXAMPLE tX
The unidirectional tapes produced in accordance with the first and second
embodiments may both be used to make a tubular product with a 5° bend.
The
outside diameter of the tube is 22.2 mm and the inside diameter of the tube is
1 ~.9
t 5 mm. These unidirectional tapes are oriented such that the fiber direction
is along the
tube on top of a braid. V~fien comparing the unidirectional tape produced in
accordance with the first embodiment to the ultraflexible unidirectional tapes
produced in accordance with the second embodiment, the ultraflexible
unidirectional
tape did not form serious kinks in the finished composite part.
'o Although the present im~ention has been described with reference to
particular
embodiments, it will be apparent to those skilled in the art that
modifications to the
CA 02312467 2000-06-27
method, materials, dimensions and conditions discussed above may be made which
are in keeping with the spirit and scope of the present invention.
.. ,
