U.S. patent application number 14/755943 was filed with the patent office on 2015-10-22 for thermoplastic prepreg containing continuous and long fibers.
The applicant listed for this patent is Ticona LLC. Invention is credited to Aaron H. Johnson, Jeremy J. Maliszewski, Timothy L. Tibor.
Application Number | 20150298400 14/755943 |
Document ID | / |
Family ID | 44628058 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150298400 |
Kind Code |
A1 |
Maliszewski; Jeremy J. ; et
al. |
October 22, 2015 |
Thermoplastic Prepreg Containing Continuous and Long Fibers
Abstract
A prepreg that contains a plurality of unidirectionally aligned
continuous fibers embedded within a thermoplastic polymer matrix is
provided. In addition to continuous fibers, the prepreg also
contains a plurality of long fibers that are combined with the
continuous fibers so that they are randomly distributed within the
thermoplastic matrix. As a result, at least a portion of the long
fibers become oriented at an angle (e.g., perpendicular) relative
to the direction of the continuous fibers. Through such
orientation, the long fibers can substantially increase the
mechanical properties of the prepreg in the transverse direction
(e.g., strength) and thus achieve a more isotropic material.
Although unique isotropic prepregs are one aspect of the present
invention, it should be understood that this is not a requirement.
In fact, one notable feature of the present invention is the
ability to tailor the mechanical properties of the prepreg for an
intended application by selectively controlling certain process
parameters, such as the type of long fibers employed, the type of
continuous fibers employed, the concentration of the long fibers,
the concentration of the continuous fibers, the thermoplastic
resin(s) employed, etc.
Inventors: |
Maliszewski; Jeremy J.;
(Winona, MN) ; Johnson; Aaron H.; (Winona, MN)
; Tibor; Timothy L.; (Winona, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ticona LLC |
Florence |
KY |
US |
|
|
Family ID: |
44628058 |
Appl. No.: |
14/755943 |
Filed: |
June 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13698382 |
Feb 12, 2013 |
9096000 |
|
|
PCT/US2011/041454 |
Jun 22, 2011 |
|
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14755943 |
|
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|
61357301 |
Jun 22, 2010 |
|
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Current U.S.
Class: |
264/171.11 |
Current CPC
Class: |
B29C 48/0018 20190201;
B29K 2105/107 20130101; Y10T 428/249945 20150401; B29C 48/06
20190201; B29C 48/08 20190201; B29C 48/916 20190201; B29C 48/154
20190201; B29K 2105/08 20130101; B29C 48/157 20190201; B29K
2105/105 20130101; B29C 48/156 20190201; B29C 48/919 20190201; Y10T
428/249942 20150401; Y10T 428/24124 20150115; B29C 70/08 20130101;
B32B 5/12 20130101; B29C 48/04 20190201; B29C 48/695 20190201; B29C
48/345 20190201; B29C 48/908 20190201; B29B 11/16 20130101; B29C
48/0011 20190201; B29C 48/905 20190201; B29K 2101/12 20130101; B29C
48/21 20190201; B29C 48/07 20190201; B29C 48/705 20190201; B29C
48/71 20190201; B29B 11/10 20130101; B29C 48/397 20190201; Y10T
428/249946 20150401; B29B 15/122 20130101; B29K 2105/0872 20130101;
B29K 2105/0881 20130101; B29C 48/05 20190201; B29C 70/523 20130101;
B29C 70/081 20130101 |
International
Class: |
B29C 70/08 20060101
B29C070/08; B29B 11/16 20060101 B29B011/16; B29B 11/10 20060101
B29B011/10 |
Claims
1-15. (canceled)
16. A method for forming a thermoplastic prepreg, the method
comprising: supplying continuous fibers and long fibers to an
extrusion device; supplying a thermoplastic feedstock to the
extrusion device, wherein the feedstock comprises at least one
thermoplastic polymer; and extruding the continuous fibers, long
fibers, and the thermoplastic polymer within an impregnation die to
form an extrudate in which the continuous fibers are intermixed
with the long fibers and embedded with a matrix of the
thermoplastic polymer.
17. The method of claim 16, wherein the feedstock comprises the
long fibers.
18. The method of claim 16, further comprising pulling the
extrudate through a nip defined between rollers to consolidate the
extrudate into the form of a sheet.
19. The method of claim 16, wherein the continuous fibers
constitute from about 10 to about 80 wt. % of the prepreg and the
long fibers constitute from about 2 to about 35 wt. % of the
prepreg.
20. The method of claim 16, wherein the continuous fibers, the long
fibers, or both, include glass fibers, carbon fibers, or a
combination of glass and carbon fibers.
21. The method of claim 16, wherein the thermoplastic polymer
includes a polyolefin, polyether ketone, polyetherimide,
polyarylene ketone, liquid crystal polymer, polyarylene sulfide,
fluoropolymer, polyacetal, polyurethane, polycarbonate, styrenic
polymer, polyester, polyamide, or a combination thereof.
22. The method of claim 16, wherein a manifold assembly supplies
the thermoplastic feedstock to the extrusion device, the manifold
assembly comprising branched runners through which the
thermoplastic feedstock flows.
23. A method for forming a thermoplastic prepreg, the method
comprising: supplying continuous fibers to an extrusion device;
supplying a thermoplastic feedstock to the extrusion device,
wherein the feedstock comprises at least one thermoplastic polymer;
and extruding the continuous fibers and the feedstock within an
impregnation die to form an extrudate in which the continuous
fibers are embedded with a matrix of the thermoplastic polymer;
thereafter, applying long fibers to the extrudate to form a
composite.
24. The method of claim 23, further comprising pulling the
composite through a nip defined between rollers to consolidate the
composite into the form of a sheet.
25. The method of claim 23, wherein the continuous fibers
constitute from about 10 to about 80 wt. % of the prepreg and the
long fibers constitute from about 2 to about 35 wt. % of the
prepreg.
26. The method of claim 23, wherein the continuous fibers, the long
fibers, or both, include glass fibers, carbon fibers, or a
combination of glass and carbon fibers.
27. The method of claim 23, wherein the thermoplastic polymer
includes a polyolefin, polyether ketone, polyetherimide,
polyarylene ketone, liquid crystal polymer, polyarylene sulfide,
fluoropolymer, polyacetal, polyurethane, polycarbonate, styrenic
polymer, polyester, polyamide, or a combination thereof.
28. The method of claim 23, wherein a manifold assembly supplies
the thermoplastic feedstock to the extrusion device, the manifold
assembly comprising branched runners through which the
thermoplastic feedstock flows.
Description
RELATED APPLICATIONS
[0001] The present application claims priority to Provisional
Application Ser. No. 61/357,301, filed on Jun. 22, 2010, the entire
contents of which are incorporated herein by reference thereto.
BACKGROUND OF THE INVENTION
[0002] Reinforced composite materials (or "prepregs") have
conventionally been formed from fibers (e.g., carbon or glass
fibers) that are impregnated within a resinous matrix. Thermoset
resins (e.g., unsaturated polyester, epoxy, or polyimide) are often
employed as the matrix, particularly in applications requiring a
high level of strength. One problem with prepregs formed from
thermoset resins, however, is that they are generally brittle and
have poor impact resistance. Furthermore, the prepregs are often
difficult to store due to the short shelf life of the resin. In an
attempt to overcome these issues, efforts have recently been made
develop thermoplastic prepregs. One such prepreg is formed from a
thermoplastic resin and unidirectionally aligned continuous fibers.
Such a prepreg has excellent performance in terms of modulus of
elasticity and strength in the fiber axis direction. However,
because such prepregs have anisotropic mechanical properties,
multiple prepreg layers are required during use that are oriented
in different direction(s). This inevitably causes an increase in
the cost and thickness of the resulting part. Other attempts to
solve the problem associated with thermoset prepregs involve the
use of a thermoplastic resin and chopped fibers formed by cutting
unidirectionally aligned strands. While such prepregs exhibit
better isotropic strength properties, the maximum volume fraction
of the reinforcing fiber is generally low, which results in
relatively poor modulus of elasticity and strength. Furthermore, it
is difficult to tailor the mechanical properties of the prepreg
through manipulation of the volume of such chopped fibers.
[0003] As such, a need currently exists for a method of forming
thermoplastic prepregs that allows for selective control of its
mechanical properties depending on the particular application. A
need also exists for thermoplastic prepregs that have isotropic
mechanical properties.
SUMMARY OF THE INVENTION
[0004] In accordance with one embodiment of the present invention,
a thermoplastic prepreg is disclosed that comprises a plurality of
continuous fibers that are substantially oriented in a longitudinal
direction and a plurality of randomly distributed long fibers, at
least a portion of which are oriented at an angle relative to the
longitudinal direction. The continuous fibers constitute from about
10 wt. % to about 80 wt. % of the prepreg and the long fibers
constitute from about 2 wt. % to about 35 wt. % of the prepreg. The
prepreg also comprises a resinous matrix that contains one or more
thermoplastic polymers and within which the continuous fibers and
long fibers are embedded, wherein the thermoplastic polymers
constitute from about 10 wt. % to about 80 wt. % of the prepreg.
The ratio of the maximum tensile stress of the prepreg in the
longitudinal direction to the maximum tensile stress of the prepreg
in the transverse direction is from about 1 to about 40.
[0005] In accordance with another embodiment of the present
invention, a method for forming a thermoplastic prepreg is
disclosed. The method comprises supplying continuous fibers and
long fibers to an extrusion device and supplying a thermoplastic
feedstock to the extrusion device, wherein the feedstock comprises
at least one thermoplastic polymer. The continuous fibers, long
fibers, and the thermoplastic polymer are extruded within an
impregnation die to form an extrudate in which the continuous
fibers are intermixed with the long fibers and embedded with a
matrix of the thermoplastic polymer.
[0006] In accordance with yet another embodiment of the present
invention, a method for forming a thermoplastic prepreg is
disclosed. The method comprises supplying continuous fibers and a
thermoplastic feedstock to the extrusion device, wherein the
feedstock comprises at least one thermoplastic polymer. The
continuous fibers and feedstock are extruded within an impregnation
die to form an extrudate that in which the continuous fibers are
embedded with a matrix of the thermoplastic polymer. Thereafter,
long fibers are applied to the extrudate to form a composite.
[0007] Other features and aspects of the present invention are set
forth in greater detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A full and enabling disclosure of the present invention,
including the best mode thereof to one skilled in the art, is set
forth more particularly in the remainder of the specification,
including reference to the accompanying figures, in which:
[0009] FIG. 1 is a schematic illustration of one embodiment of an
impregnation system for use in the present invention;
[0010] FIG. 2A is a cross-sectional view of the impregnation die
shown in FIG. 1;
[0011] FIG. 2B is an exploded view of one embodiment of a manifold
assembly and gate passage for an impregnation die that may be
employed in the present invention;
[0012] FIG. 2C is a perspective view of one embodiment of a plate
at least partially defining an impregnation zone that may be
employed in the present invention;
[0013] FIG. 3 is a transverse cross-sectional view of one
embodiment of a prepreg made with the system of FIG. 1;
[0014] FIG. 4 is a longitudinal cross-sectional view of the prepreg
of shown in FIG. 3;
[0015] FIG. 5 is a schematic illustration of one embodiment of a
pultrusion system that may be employed in forming a profile from
the prepreg of the present invention;
[0016] FIG. 6 is a transverse cross-sectional view of anode
embodiment of a prepreg made of the present invention;
[0017] FIG. 7 is a longitudinal cross-sectional view of the prepreg
of shown in FIG. 6;
[0018] FIG. 8 is a schematic illustration of yet another embodiment
of an impregnation system for use in forming the prepreg the
present invention;
[0019] FIG. 9 is a transverse cross-sectional view of one
embodiment of a prepreg made with the system of FIG. 8;
[0020] FIG. 10 is a longitudinal cross-sectional view of the
prepreg of shown in FIG. 9; and
[0021] FIG. 11 is a cross-sectional view of one embodiment of a
profile that may be formed in accordance with the present
invention.
[0022] Repeat use of reference characters in the present
specification and drawings is intended to represent the same or
analogous features or elements of the present invention.
DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS
[0023] It is to be understood by one of ordinary skill in the art
that the present discussion is a description of exemplary
embodiments only, and is not intended as limiting the broader
aspects of the present invention.
[0024] Generally speaking, the present invention is directed to a
prepreg that contains a plurality of unidirectionally aligned
continuous fibers embedded within a thermoplastic polymer matrix.
In addition to continuous fibers, the prepreg also contains a
plurality of long fibers that are combined with the continuous
fibers so that they are randomly distributed within the
thermoplastic matrix. As a result, at least a portion of the long
fibers become oriented at an angle (e.g., perpendicular) relative
to the direction of the continuous fibers. Through such
orientation, the long fibers can substantially increase the
mechanical properties of the prepreg in the transverse direction
(e.g., strength) and thus achieve a more isotropic material.
Although unique isotropic prepregs are one aspect of the present
invention, it should be understood that this is not a requirement.
In fact, one notable feature of the present invention is the
ability to tailor the mechanical properties of the prepreg for an
intended application by selectively controlling certain process
parameters, such as the type of long fibers employed, the type of
continuous fibers employed, the concentration of the long fibers,
the concentration of the continuous fibers, the thermoplastic
resin(s) employed, etc.
[0025] Various embodiments of the present invention will now be
described in more detail.
I. Continuous Fibers
[0026] The term "continuous fibers" refers to fibers, filaments,
yarns, or rovings (e.g., bundles of fibers) having a length that is
generally limited only by the length of the part. For example, such
fibers may have a length greater than about 25 millimeters, in some
embodiments about 50 millimeters or more, and in some embodiments,
about 100 millimeters or more. The continuous fibers may be formed
from any conventional material known in the art, such as metal
fibers; glass fibers (e.g., E-glass, A-glass, C-glass, D-glass,
AR-glass, R-glass, S1-glass, S2-glass), carbon fibers (e.g.,
graphite), boron fibers, ceramic fibers (e.g., alumina or silica),
aramid fibers (e.g., Kevlar.RTM. marketed by E. I. duPont de
Nemours, Wilmington, Del.), synthetic organic fibers (e.g.,
polyamide, polyethylene, paraphenylene, terephthalamide,
polyethylene terephthalate and polyphenylene sulfide), and various
other natural or synthetic inorganic or organic fibrous materials
known for reinforcing thermoplastic compositions. Glass fibers and
carbon fibers are particularly desirable for use in the continuous
fibers. Such fibers often have a nominal diameter of about 4 to
about 35 micrometers, and in some embodiments, from about 9 to
about 35 micrometers. The fibers may be twisted or straight. If
desired, the fibers may be in the form of rovings (e.g., bundle of
fibers) that contain a single fiber type or different types of
fibers. Different fibers may be contained in individual rovings or,
alternatively, each roving may contain a different fiber type. For
example, in one embodiment, certain rovings may contain continuous
carbon fibers, while other rovings may contain glass fibers. The
number of fibers contained in each roving can be constant or vary
from roving to roving. Typically, a roving may contain from about
1,000 fibers to about 50,000 individual fibers, and in some
embodiments, from about 2,000 to about 40,000 fibers.
II. Long Fibers
[0027] The term "long fibers" generally refers to fibers,
filaments, yarns, or rovings that are not continuous and have a
length of from about 0.5 to about 25 millimeters, in some
embodiments, from about 0.8 to about 15 millimeters, and in some
embodiments, from about 1 to about 12 millimeters. The long fibers
may be formed from any of the material, shape, and/or size as
described above with respect to the continuous fibers. Glass fibers
and carbon fibers are particularly desirable for use as the long
fibers.
III. Thermoplastic Matrix
[0028] Any of a variety of thermoplastic polymers may be employed
to form the thermoplastic matrix in which the continuous and long
fibers are embedded. Suitable thermoplastic polymers for use in the
present invention may include, for instance, polyolefins (e.g.,
polypropylene, propylene-ethylene copolymers, etc.), polyesters
(e.g., polybutylene terephalate ("PBT")), polycarbonates,
polyamides (e.g., Nylon.TM.), polyether ketones (e.g.,
polyetherether ketone ("PEEK")), polyetherimides, polyarylene
ketones (e.g., polyphenylene diketone ("PPDK")), liquid crystal
polymers, polyarylene sulfides (e.g., polyphenylene sulfide
("PPS")), fluoropolymers (e.g.,
polytetrafluoroethylene-perfluoromethylvinylether polymer,
perfluoro-alkoxyalkane polymer, petrafluoroethylene polymer,
ethylene-tetrafluoroethylene polymer, etc.), polyacetals,
polyurethanes, polycarbonates, styrenic polymers (e.g.,
acrylonitrile butadiene styrene ("ABS")), and so forth. ABS is a
particularly suitable thermoplastic polymer.
[0029] One or multiple thermoplastic matrices may be employed in
the prepreg. For example, in one embodiment, the long fibers are
initially pre-impregnated with a first thermoplastic matrix, such
as in a manner that will be described below, and thereafter cooled
and chopped into to pellets having a length of about 25 millimeters
or less. These pellets may be subsequently combined with the
continuous fibers as they are impregnated with a second
thermoplastic matrix. Thus, in this embodiment, the long fibers are
actually embedded within two thermoplastic matrices. The first
thermoplastic matrix employed for the long fibers may be the same
or different than the second thermoplastic matrix. Alternatively,
the first thermoplastic plastic matrix used to pre-impregnate the
long fibers may constitute the only thermoplastic matrix present in
the prepreg. In still another embodiment, the long fibers are not
pre-impregnated and are simply combined with the continuous fibers
as they both are impregnated with the thermoplastic matrix.
IV. Prepreq Formation
[0030] As stated above, the prepreg is generally formed in a manner
that is capable of randomly distributing the long fibers. This may
be accomplished in a variety of ways. In one particular embodiment,
for example, long fiber thermoplastic pellets may be supplied to
the hopper of an extrusion device and thereafter melt blended with
continuous fibers. The pressure and force of the extrusion process
cause the desired random orientation of the long fibers within the
resulting prepreg. Referring to FIG. 1, for example, one embodiment
of such an extrusion device is shown. More particularly, the
apparatus includes an extruder 120 containing a screw shaft 124
mounted inside a barrel 122. A heater 130 (e.g., electrical
resistance heater) is mounted outside the barrel 122. During use, a
thermoplastic polymer feedstock 127 is supplied to the extruder 120
through a hopper 126. In this particular embodiment, the feedstock
127 also contains long fibers. The long fibers may, for example,
constitute from about 5 wt. % to about 60 wt. %, in some
embodiments from about 10 wt. % to about 50 wt. %, and in some
embodiments, from about 20 wt. % to about 40 wt. % of the
feedstock. Alternatively, the feedstock 127 may be free of long
fibers, and such fibers may be supplied at another location (not
shown), such as downstream from the hopper 126 and/or other feed
ports.
[0031] Regardless, the thermoplastic feedstock 127 is conveyed
inside the barrel 122 by the screw shaft 124 and heated by
frictional forces inside the barrel 122 and by the heater 130. Upon
being heated, the feedstock 127 exits the barrel 122 through a
barrel flange 128 and enters a die flange 132 of an impregnation
die 150. A continuous fiber roving 142 or a plurality of continuous
fiber rovings 142 are supplied from a reel or reels 144 to die 150.
The rovings 142 are generally kept apart a certain distance before
impregnation, such as at least about 4 millimeters, and in some
embodiments, at least about 5 millimeters. The feedstock 127 may
further be heated inside the die by heaters 133 mounted in or
around the die 150. The die is generally operated at temperatures
that are sufficient to cause melting and impregnation of the
thermoplastic polymer. Typically, the operation temperatures of the
die is higher than the melt temperature of the thermoplastic
polymer, such as at temperatures from about 200.degree. C. to about
450.degree. C. When processed in this manner, the continuous fiber
rovings 142 become embedded in the polymer matrix, which may be a
resin 214 (FIG. 2A) processed from the feedstock 127, and
intermixed with the long fibers contained therein. The mixture is
then extruded from the impregnation die 150 to create an extrudate
152.
[0032] A pressure sensor 137 (FIG. 2A) senses the pressure near the
impregnation die 150 to allow control to be exerted over the rate
of extrusion by controlling the rotational speed of the screw shaft
124, or the federate of the feeder. That is, the pressure sensor
137 is positioned near the impregnation die 150 so that the
extruder 120 can be operated to deliver a correct amount of resin
214 for interaction with the fiber rovings 142. After leaving the
impregnation die 150, the extrudate 152, or impregnated fiber
rovings 142, may enter an optional pre-shaping, or guiding section
(not shown) before entering a nip formed between two adjacent
rollers 190. Although optional, the rollers 190 can help to
consolidate the extrudate 152 into the form of a ribbon (or tape),
as well as enhance fiber impregnation and squeeze out any excess
voids. In addition to the rollers 190, other shaping devices may
also be employed, such as a die system. The resulting consolidated
ribbon 156 is pulled by tracks 162 and 164 mounted on rollers. The
tracks 162 and 164 also pull the extrudate 152 from the
impregnation die 150 and through the rollers 190. If desired, the
consolidated ribbon 156 may be wound up at a section 171. Generally
speaking, the ribbons are relatively thin and typically have a
thickness of from about 0.05 to about 1 millimeter, in some
embodiments from about 0.1 to about 0.8 millimeters, and in some
embodiments, from about 0.2 to about 0.4 millimeters.
[0033] Within the impregnation die, it is generally desired that
the rovings 142 are traversed through an impregnation zone 250 to
impregnate the rovings with the polymer resin 214. In the
impregnation zone 250, the polymer resin may be forced generally
transversely through the rovings by shear and pressure created in
the impregnation zone 250, which significantly enhances the degree
of impregnation. This is particularly useful when forming a
composite from ribbons of a high fiber content, such as about 35%
weight fraction ("Wf") or more, and in some embodiments, from about
40% Wf or more. Typically, the die 150 will include a plurality of
contact surfaces 252, such as for example at least 2, at least 3,
from 4 to 7, from 2 to 20, from 2 to 30, from 2 to 40, from 2 to
50, or more contact surfaces 252, to create a sufficient degree of
penetration and pressure on the rovings 142. Although their
particular form may vary, the contact surfaces 252 typically
possess a curvilinear surface, such as a curved lobe, rod, etc. The
contact surfaces 252 are also typically made of a metal
material.
[0034] FIG. 2A shows a cross-sectional view of an impregnation die
150. As shown, the impregnation die 150 includes a manifold
assembly 220, a gate passage 270, and an impregnation zone 250. The
manifold assembly 220 is provided for flowing the polymer resin 214
therethrough. For example, the manifold assembly 220 may include a
channel 222 or a plurality of channels 222. The resin 214 provided
to the impregnation die 150 may flow through the channels 222.
[0035] As shown in FIG. 2B, some portions of the channels 222 may
be curvilinear, and in exemplary embodiments, the channels 222 have
a symmetrical orientation along a central axis 224. Further, in
some embodiments, the channels may be a plurality of branched
runners 222, which may include first branched runner group 232,
second group 234, third group 236, and, if desired, more branched
runner groups. Each group may include 2, 3, 4 or more runners 222
branching off from runners 222 in the preceding group, or from an
initial channel 222.
[0036] The branched runners 222 and the symmetrical orientation
thereof generally evenly distribute the resin 214, such that the
flow of resin 214 exiting the manifold assembly 220 and coating the
rovings 142 is substantially uniformly distributed on the rovings
142. This desirably allows for generally uniform impregnation of
the rovings 142.
[0037] Further, the manifold assembly 220 may in some embodiments
define an outlet region 242, which generally encompasses at least a
downstream portion of the channels or runners 222 from which the
resin 214 exits. In some embodiments, at least a portion of the
channels or runners 222 disposed in the outlet region 242 have an
increasing area in a flow direction 244 of the resin 214. The
increasing area allows for diffusion and further distribution of
the resin 214 as the resin 214 flows through the manifold assembly
220, which further allows for substantially uniform distribution of
the resin 214 on the rovings 142.
[0038] As further illustrated in FIGS. 2A and 2B, after flowing
through the manifold assembly 220, the resin 214 may flow through
gate passage 270. Gate passage 270 is positioned between the
manifold assembly 220 and the impregnation zone 250, and is
provided for flowing the resin 214 from the manifold assembly 220
such that the resin 214 coats the rovings 142. Thus, resin 214
exiting the manifold assembly 220, such as through outlet region
242, may enter gate passage 270 and flow therethrough, as
shown.
[0039] Upon exiting the manifold assembly 220 and the gate passage
270 of the die 150 as shown in FIG. 2A, the resin 214 contacts the
rovings 142 being traversed through the die 150. As discussed
above, the resin 214 may substantially uniformly coat the rovings
142, due to distribution of the resin 214 in the manifold assembly
220 and the gate passage 270. Further, in some embodiments, the
resin 214 may impinge on an upper surface of each of the rovings
142, or on a lower surface of each of the rovings 142, or on both
an upper and lower surface of each of the rovings 142. Initial
impingement on the rovings 142 provides for further impregnation of
the rovings 142 with the resin 214.
[0040] As shown in FIG. 2A, the coated rovings 142 are traversed in
run direction 282 through impregnation zone 250, which is
configured to impregnate the rovings 142 with the resin 214. For
example, as shown in FIGS. 2A and 2C, the rovings 142 are traversed
over contact surfaces 252 in the impregnation zone. Impingement of
the rovings 142 on the contact surface 252 creates shear and
pressure sufficient to impregnate the rovings 142 with the resin
214 coating the rovings 142.
[0041] In some embodiments, as shown in FIG. 2A, the impregnation
zone 250 is defined between two spaced apart opposing plates 256
and 258. First plate 256 defines a first inner surface 257, while
second plate 258 defines a second inner surface 259. The contact
surfaces 252 may be defined on or extend from both the first and
second inner surfaces 257 and 259, or only one of the first and
second inner surfaces 257 and 259. FIG. 2C illustrates the second
plate 258 and the various contact surfaces thereon that form at
least a portion of the impregnation zone 250 according to these
embodiments. In exemplary embodiments, as shown in FIG. 2A, the
contact surfaces 252 may be defined alternately on the first and
second surfaces 257 and 259 such that the rovings alternately
impinge on contact surfaces 252 on the first and second surfaces
257 and 259. Thus, the rovings 142 may pass contact surfaces 252 in
a waveform, tortuous or sinusoidual-type pathway, which enhances
shear.
[0042] The angle 254 at which the rovings 142 traverse the contact
surfaces 252 may be generally high enough to enhance shear, but not
so high to cause excessive forces that will break the fibers. Thus,
for example, the angle 254 may be in the range between
approximately 1.degree. and approximately 30.degree., and in some
embodiments, between approximately 5.degree. and approximately
25.degree..
[0043] In alternative embodiments, the impregnation zone 250 may
include a plurality of pins (not shown), each pin having a contact
surface 252. The pins may be static, freely rotational, or
rotationally driven. In further alternative embodiments, the
contact surfaces 252 and impregnation zone 250 may comprise any
suitable shapes and/or structures for impregnating the rovings 142
with the resin 214 as desired or required.
[0044] To further facilitate impregnation of the rovings 142, they
may also be kept under tension while present within the
impregnation die. The tension may, for example, range from about 5
to about 300 Newtons, in some embodiments from about 50 to about
250 Newtons, and in some embodiments, from about 100 to about 200
Newtons per roving 142 or tow of fibers.
[0045] As shown in FIG. 2A, in some embodiments, a land zone 280
may be positioned downstream of the impregnation zone 250 in run
direction 282 of the rovings 142. The rovings 142 may traverse
through the land zone 280 before exiting the die 150. As further
shown in FIG. 2A, in some embodiments, a faceplate 290 may adjoin
the impregnation zone 250. Faceplate 290 is generally configured to
meter excess resin 214 from the rovings 142. Thus, apertures in the
faceplate 290, through which the rovings 142 traverse, may be sized
such that when the rovings 142 are traversed therethrough, the size
of the apertures causes excess resin 214 to be removed from the
rovings 142.
[0046] The impregnation die shown and described above is but one of
various possible configurations that may be employed in the present
invention. In alternative embodiments, for example, the fibers may
be introduced into a crosshead die that is positioned at an angle
relative to the direction of flow of the polymer melt. As the
fibers move through the crosshead die and reach the point where the
polymer exits from an extruder barrel, the polymer is forced into
contact with the fibers. It should also be understood that any
other extruder design may also be employed, such as a twin screw
extruder. Still further, other components may also be optionally
employed to assist in the impregnation of the fibers. For example,
a "gas jet" assembly may be employed in certain embodiments to help
uniformly spread a bundle or tow of individual fibers, which may
each contain up to as many as 24,000 fibers, across the entire
width of the merged tow. This helps achieve uniform distribution of
strength properties in the ribbon. Such an assembly may include a
supply of compressed air or another gas that impinges in a
generally perpendicular fashion on the moving fiber tows that pass
across the exit ports. The spread fiber bundles may then be
introduced into a die for impregnation, such as described
above.
[0047] Regardless of the technique employed, at least a portion of
the long fibers in the prepreg are oriented at an angle relative to
the longitudinal direction in which the continuous fibers are
oriented (the machine direction "A" of the system of FIG. 1). For
example, about 10% or more, in some embodiments about 20% or more,
and in some embodiments, about 30% or more of the fibers may be
oriented at an angle relative to the longitudinal direction of the
continuous fibers. This angle may, for instance, be about
10.degree. to about 120.degree., in some embodiments from about
20.degree. to about 110.degree. C., and in one embodiment, about
90.degree.. Referring to FIGS. 3-4, for example, one embodiment of
a prepreg 200 is shown that contains a plurality of long fibers 220
oriented at various angles relative to a longitudinal direction "L"
in which continuous fibers 240 are aligned. Among other things, the
angle of orientation helps control the transverse strength of the
prepreg.
[0048] In addition, the relative percentage of long fibers and
continuous fibers in the prepreg also help to control the strength
properties. To achieve a good balance between tensile strength and
transverse strength, the ratio of the weight of continuous fibers
to the weight of long fibers is typically controlled within the
range of from about 0.2 to about 10, in some embodiments from about
0.4 to about 5, and in some embodiments, from about 0.5 to about 5.
For instance, continuous fibers may constitute from about 10 wt. %
to about 80 wt. %, in some embodiments from about 20 wt. % to about
70 wt. %, and in some embodiments, from about 40 wt. % to about 60
wt. % of the prepreg. Likewise, long fibers may constitute from
about 2 wt. % to about 35 wt. %, in some embodiments from about 5
wt. % to about 30 wt. %, and in some embodiments, from about 10 wt.
% to about 25 wt. % of the prepreg. Thermoplastic polymer(s) may
constitute from about 10 wt. % to about 80 wt. %, in some
embodiments from about 20 wt. % to about 70 wt. %, and in some
embodiments, from about 40 wt. % to about 60 wt. % of the
prepreg.
[0049] The prepreg also has a very low void fraction, which helps
enhance the mechanical properties of the prepreg. For instance, the
void fraction may be about 3% or less, in some embodiments about 2%
or less, and in some embodiments, about 1% or less. The void
fraction may be measured using techniques well known to those
skilled in the art. For example, the void fraction may be measured
using a "resin burn off" test in which samples are placed in an
oven (e.g., at 600.degree. C. for 3 hours) to burn out the resin.
The mass of the remaining fibers may then be measured to calculate
the weight and volume fractions. Such "burn off" testing may be
performed in accordance with ASTM D 2584-08 to determine the
weights of the fibers and the thermoplastic matrix, which may then
be used to calculate the "void fraction" based on the following
equations:
V.sub.f=100*(.rho..sub.t-.rho..sub.c)/.rho..sub.t
where,
[0050] V.sub.f is the void fraction as a percentage;
[0051] .rho..sub.c is the density of the composite as measured
using known techniques, such as with a liquid or gas pycnometer
(e.g., helium pycnometer);
[0052] .rho..sub.t is the theoretical density of the composite as
is determined by the following equation:
.rho..sub.t=1/[W.sub.f/.rho..sub.f+W.sub.m/.rho..sub.m]
[0053] .rho..sub.m is the density of the thermoplastic matrix
(e.g., at the appropriate crystallinity);
[0054] .rho..sub.f is the density of the fibers;
[0055] W.sub.f is the weight fraction of the fibers; and
[0056] W.sub.m is the weight fraction of the thermoplastic
matrix.
[0057] Alternatively, the void fraction may be determined by
chemically dissolving the resin in accordance with ASTM D 3171-09.
The "burn off" and "dissolution" methods are particularly suitable
for glass fibers, which are generally resistant to melting and
chemical dissolution. In other cases, however, the void fraction
may be indirectly calculated based on the densities of the
thermoplastic polymer, fibers, and ribbon in accordance with ASTM D
2734-09 (Method A), where the densities may be determined ASTM
D792-08 Method A. Of course, the void fraction can also be
estimated using conventional microscopy equipment.
[0058] Through control over the various parameters mentioned above,
the mechanical strength properties may be tailored to the desired
application. In certain embodiments, for example, it is desirable
that the prepreg exhibits relatively isotropic strength properties.
More particularly, for such isotropic prepregs, the ratio of the
maximum tensile stress (or "ultimate strength") in the longitudinal
direction to the maximum tensile stress in the transverse direction
is typically from about 1 to about 40, in some embodiments from
about 2 to about 30, and in some embodiments, from about 4 to about
20. In certain embodiments, the prepreg of the present invention
may exhibit a maximum tensile stress in the longitudinal direction
of from about 250 to about 3000 Megapascals (MPa), in some
embodiments from about 400 to about 2500 MPa, and in some
embodiments, from about 600 to about 2000 MPa, and in the
transverse direction of from about 0.5 to about 50 MPa, in some
embodiments from about 1 to about 40 MPa, and in some embodiments,
from about 2 to about 20 MPa.
[0059] In the embodiments described above and shown in FIGS. 3-4,
the long fibers are incorporated into the extrusion device during
impregnation and therefore become intermingled with the continuous
fibers and generally distributed throughout the entire prepreg. It
should be understood, however, that such a configuration is not
required. In certain embodiments, for example, the long fibers may
be combined with the continuous fibers in such a manner that they
form separate layers. The layers may be "non-discrete" in the sense
that at least a portion of the long fibers extend into the
continuous fiber layer. One embodiment of a system for forming such
a non-discrete layered prepreg may include an additional device
(e.g., extruder) that is employed to apply the long fibers
downstream from the die for impregnating the continuous fibers.
Long fibers applied in this manner are still randomly distributed
within the prepreg, but form a separate layer. This is illustrated
in more detail in FIGS. 6-7. As shown, a layer 260 is formed that
contains the long fibers 262 and a layer 270 is formed that
contains continuous fibers 272. Due in part to the manner in which
they are applied, a portion 230 of the long fibers 262 may also
extend into the continuous fibers layer 270.
[0060] Of course, the layers may also be "discrete" in the sense
that substantially all of the long fibers are contained within the
layer. One embodiment of a system for forming such a discrete
layered prepreg is shown in FIG. 8. In this particular embodiment,
a pre-consolidated continuous fiber prepreg 320 is unwound from a
reel 330 and heated to or above the softening point of the
thermoplastic matrix within an oven 340 (e.g., infrared oven).
Thereafter, the softened prepreg 320 is supplied to an extrusion
device 350 where long fibers (not shown) may be applied. The
resulting layered prepreg 360 may then be supplied between two
consolidation rolls 290 as described above. Once again, the long
fibers applied in this manner are still randomly distributed within
the prepreg, but form a separate "discrete" layer. This is
illustrated in more detail in FIGS. 9-10. As shown, a layer 420 is
formed that contains long fibers 422 and a layer 430 is formed that
contains the continuous fibers 424.
V. Articles Formed from the Prepreq
[0061] The prepreg of the present invention may generally be
employed in a variety of different applications and parts. For
example, the prepreg may be formed into a profile, injection molded
part, compression molded, part, etc. A "profile" is hollow or solid
pultruded part that may possess a wide variety of cross-sectional
shapes, such as square, rectangular, circular, elliptical,
triangular, I-shaped, C-shaped, U-shaped, J-shaped, L-shaped,
slotted, etc. In hollow profiles, at least a portion of the
interior of the profile is a voided space. The voided space may
optionally extend the entire the length of the profile. The
profiles may also be "lineal" to the extent that they possess a
cross-sectional shape that is substantially the same along the
entire length of the profile, or they may have a varying
cross-sectional shape, such as curved, twisted, etc. Regardless,
such profiles may be employed as a structural member for window
lineals, decking planks, railings, balusters, roofing tiles,
siding, trim boards, pipe, fencing, posts, light posts, highway
signage, roadside marker posts, etc.
[0062] The manner in which a profile may be formed from a prepreg
can vary as is well known to those skilled in the art. One or
multiple prepreg layers may be employed for forming the profile.
Referring to FIG. 5, for example, one particular embodiment of a
system is shown in which a plurality of prepregs 12 are employed to
form a profile. In this embodiment, the prepregs 12 are provided in
a wound package on a creel 20. The creel 20 may be an unreeling
creel that includes a frame provided with horizontal rotating
spindles 22, each supporting a package. A pay-out creel may also be
employed, particularly if desired to induce a twist into the
fibers. It should also be understood that the prepregs may also be
formed in-line with the formation of the profile. In one
embodiment, for example, the extrudate 152 exiting the impregnation
die 150 from FIG. 1 may be directly supplied to the system used to
form a profile.
[0063] A tension-regulating device 40 may also be employed to help
control the degree of tension. The device 40 may include inlet
plate 30 that lies in a vertical plane parallel to the rotating
spindles 22 of the creel 20. The tension-regulating device 40 may
contain cylindrical bars 41 arranged in a staggered configuration
so that the prepregs 12 pass over and under these bars to define a
wave pattern. The height of the bars can be adjusted to modify the
amplitude of the wave pattern and control tension.
[0064] If desired, the prepregs 12 may be heated in an oven 45
having any of a variety of known configuration, such as an infrared
oven, convection oven, etc. During heating, the fibers are
unidirectionally oriented to optimize the exposure to the heat and
maintain even heat across the entire profile. The temperature to
which the ribbons 12 are heated is generally high enough to soften
the thermoplastic polymer to an extent that the ribbons can bond
together. However, the temperature is not so high as to destroy the
integrity of the material. The temperature may, for example, range
from about 100.degree. C. to about 300.degree. C., in some
embodiments from about 110.degree. C. to about 275.degree. C., and
in some embodiments, from about 120.degree. C. to about 250.degree.
C. In one particular embodiment, for example,
acrylonitrile-butadiene-styrene (ABS) is used as the polymer, and
the ribbons are heated to or above the melting point of ABS, which
is about 105.degree. C. In another embodiment, polybutylene
terephalate (PBT) is used as the polymer, and the ribbons are
heated to or above the melting point of PBT, which is about
224.degree. C.
[0065] Upon being heated, the continuous fiber ribbons 12 may be
provided to a consolidation die to help bond together different
ribbon layers, as well as for alignment and formation of the
initial shape of the profile. Although referred to herein as a
single die, it should be understood that the consolidation die 50
may in fact be formed from multiple individual dies (e.g., face
plate dies). The consolidation die 50 may receive the prepregs 12
so that it is guided through a channel (not shown) of the die 50 in
a direction "A". The channel may be provided in any of a variety of
orientations and arrangements to result in the desired
reinforcement scheme. Within the die 50, the prepregs are generally
maintained at a temperature at or above the melting point of the
thermoplastic matrix used in the ribbon to ensure adequate
consolidation.
[0066] If desired, a pultrusion die 60 may also be employed that
compresses the prepregs into the final shape for the profile. The
configuration of the die 60 depends on the desired shape and
properties for the resulting profile. To form hollow profiles, the
pultrusion die typically contains a mandrel within its interior so
that the fiber material flows between the interior surface of the
die and the external surface of the mandrel to form the desired
shape. Further, although referred to herein as a single die, it
should be understood that the pultrusion die 60 may be formed from
multiple individual dies.
[0067] One or multiple layers may be employed for forming the
profile. In one embodiment, for example, multiple layers are
employed and initially spaced apart from each other in the vertical
direction. As they pass through respective channels of the
consolidation die 50, the widths of the layers are optionally
ribboned to help prevent pressure wedges, and to keep the
continuous fibers aligned and twist-free. Although not specifically
shown, a mandrel may also be provided in the interior of the
consolidation die 50 to help guide the layers into contact with
each other on at least one side of the profile. For example, one
side of a prepreg layer and a side of another prepreg layer may be
angled so that they contact each other and form a side of a hollow
profile. The other side of the profile is, however, typically left
open within the consolidation die 50 so that the long fiber
material can be subsequently applied to the interior of the profile
in the pultrusion die. When in the desired position, the prepreg
layers are pulled into a pultrusion die 60 as described above.
[0068] If desired, the resulting profile may also be applied with a
capping layer to enhance the aesthetic appeal of the profile and/or
protect it from environmental conditions. Referring to FIG. 5, for
example, such a capping layer may be applied via an extruder
oriented at any desired angle to introduce a thermoplastic resin
into a capping die 72. The resin may contain any suitable
thermoplastic polymer known in the art that is generally compatible
with the thermoplastic polymer used to form the profile. Suitable
capping polymers may include, for instance, acrylic polymers,
polyvinyl chloride (PVC), polybutylene terephthalate (PBT), ABS,
polyolefins, polyesters, polyacetals, polyamids, polyurethanes,
etc. Although the capping resin is generally free of fibers, it may
nevertheless contain other additives for improving the final
properties of the profile. Additive materials employed at this
stage may include those that are not suitable for incorporating
into the continuous fiber or long fiber layers. For instance, it
may be desirable to add pigments to the composite structure to
reduce finishing labor of shaped articles, or it may be desirable
to add flame retardant agents to the composite structure to enhance
the flame retarding features of the shaped article. Because many
additive materials are heat sensitive, an excessive amount of heat
may cause them to decompose and produce volatile gases. Therefore,
if a heat sensitive additive material is extruded with an
impregnation resin under high heating conditions, the result may be
a complete degradation of the additive material. Additive materials
may include, for instance, mineral reinforcing agents, lubricants,
flame retardants, blowing agents, foaming agents, ultraviolet light
resistant agents, thermal stabilizers, pigments, and combinations
thereof. Suitable mineral reinforcing agents may include, for
instance, calcium carbonate, silica, mica, clays, talc, calcium
silicate, graphite, calcium silicate, alumina trihydrate, barium
ferrite, and combinations thereof.
[0069] While not shown in detail herein, the capping die 72 may
include various features known in the art to help achieve the
desired application of the capping layer. For instance, the capping
die 72 may include an entrance guide that aligns the incoming
profile. The capping die may also include a heating mechanism
(e.g., heated plate) that pre-heats the profile before application
of the capping layer to help ensure adequate bonding.
[0070] Following optional capping, the shaped part 15 may be
supplied to a cooling system 80 as is known in the art. The cooling
system 80 may, for instance, be a vacuum sizer that includes one or
more blocks (e.g., aluminum blocks) that completely encapsulate the
profile while a vacuum pulls the hot shape out against its walls as
it cools. A cooling medium may be supplied to the sizer, such as
air or water, to solidify the profile in the correct shape.
[0071] Following optional capping, the shaped part is then finally
cooled using a cooling system 80 as is known in the art. The
cooling system 80 may, for instance, be a vacuum sizer that
includes one or more blocks (e.g., aluminum blocks) that completely
encapsulate the profile while a vacuum pulls the hot shape out
against its walls as it cools. A cooling medium may be supplied to
the sizer, such as air or water, to solidify the profile in the
correct shape.
[0072] Vacuum sizers are typically employed when forming the
profile. Even if a vacuum sizer is not employed, however, it is
generally desired to cool the profile after it exits the capping
die (or the consolidation or calibration die if capping is not
applied). Cooling may occur using any technique known in the art,
such a vacuum water tank, cool air stream or air jet, cooling
jacket, an internal cooling channel, cooling fluid circulation
channels, etc. Regardless, the temperature at which the material is
cooled is usually controlled to achieve optimal mechanical
properties, part dimensional tolerances, good processing, and an
aesthetically pleasing composite. For instance, if the temperature
of the cooling station is too high, the material might swell in the
tool and interrupt the process. For semi-crystalline materials, too
low of a temperature can likewise cause the material to cool down
too rapidly and not allow complete crystallization, thereby
jeopardizing the mechanical and chemical resistance properties of
the composite. Multiple cooling die sections with independent
temperature control can be utilized to impart the optimal balance
of processing and performance attributes. In one particular
embodiment, for example, a vacuum water tank is employed that is
kept at a temperature of from about 10.degree. C. to about
50.degree. C., and in some embodiments, from about 15.degree. C. to
about 35.degree. C.
[0073] As will be appreciated, the temperature of the profile as it
advances through any section of the system of the present invention
may be controlled to yield optimal manufacturing and desired final
composite properties. Any or all of the assembly sections may be
temperature controlled utilizing electrical cartridge heaters,
circulated fluid cooling, etc., or any other temperature
controlling device known to those skilled in the art.
[0074] Referring again to FIG. 5, a pulling device 82 is positioned
downstream from the cooling system 80 that pulls the finished
profile 16 through the system for final sizing of the composite.
The pulling device 82 may be any device capable of pulling the
profile through the process system at a desired rate. Typical
pulling devices include, for example, caterpillar pullers and
reciprocating pullers. If desired, one or more calibration dies
(not shown) may also be employed. Such dies contain openings that
are cut to the exact profile shape, graduated from oversized at
first to the final profile shape. As the profile passes
therethrough, any tendency for it to move or sag is counteracted,
and it is pushed back (repeatedly) to its correct shape. Once
sized, the profile may be cut to the desired length at a cutting
station (not shown), such as with a cut-off saw capable of
performing cross-sectional cuts.
[0075] One embodiment of the profile formed from the method
described above is shown in more detail in FIG. 11 as element 516.
As illustrated, the profile 516 has a generally rectangular shape
and is formed from a ribbon 514 within which is distributed a
plurality of long fibers 518. The ribbon 514 may be formed from one
or more prepregs of the present invention. A capping layer 519 also
extends around the perimeter of the ribbon 514 and defines an
external surface of the profile 516. The thickness of the ribbon
may be strategically selected to help achieve a particular tensile
strength and transverse strength (e.g., flexural modulus) for the
profile. For example, the ribbon may have a thickness of from about
1.0 to about 4.0 millimeters, and in some embodiments, from about
2.0 to about 3.0 millimeters. The thickness of the capping layer
519 depends on the intended function of the part, but is typically
from about 0.1 to about 5 millimeters, and in some embodiments,
from about 0.2 to about 3 millimeters.
[0076] As will be appreciated, the particular profile embodiment
described above is merely exemplary of the numerous designs that
are made possible by the present invention. Among the various
possible profile designs, it should be understood that additional
layers of continuous and/or long fiber material may be employed in
addition to those described above.
[0077] In addition to pultrusion, other systems may also be
employed to process the prepregs into the desired article. For
example, an injection molding system may be employed that includes
a mold within which one or more prepregs may be disposed. The time
inside the injector may be controlled and optimized so that
thermoplastic resin is not pre-solidified. When the cycle time is
reached and the barrel is full for discharge, a piston may be used
to inject the material to the mold cavity. Compression molding
systems may also be employed in the present invention. As with
injection molding, the shaping of the prepreg(s) into the desired
article also occurs within a mold. The prepreg(s) may be placed
into the compression mold using any known technique, such as by
being picked up by an automated robot arm. The temperature of the
mold may be maintained at or above the solidification temperature
of the thermoplastic resin for a desired time period to allow for
solidification. The molded product may then be solidified by
bringing it to a temperature below that of the melting temperature.
The resulting product may be de-molded. The cycle time for each
molding process may be adjusted to suit the resins used, to achieve
sufficient bonding, and to enhance overall process
productivity.
[0078] The mold used in the systems described above may be a single
cavity or multi-cavity mold. The number of the cavities may be
determined by the resin used, the cycle time, and the output rate
desired. The shape of the mold may be selected to produce products
for specific applications, such as siding panels, fence picket
parts, end caps, joints, hinges, trim boards for interior and
exterior decoration, synthetic roofing shingles, slates, shakes or
panels, etc.
[0079] These and other modifications and variations of the present
invention may be practiced by those of ordinary skill in the art,
without departing from the spirit and scope of the present
invention. In addition, it should be understood that aspects of the
various embodiments may be interchanged both in whole or in part.
Furthermore, those of ordinary skill in the art will appreciate
that the foregoing description is by way of example only, and is
not intended to limit the invention so further described in such
appended claims.
* * * * *