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CONTROLLED ORIENTATION OF DISCONTINUOUS FIBERS IN COMPOSITES 


BY 


T. L, TOLBERT 




This document is subject to special export controls and each transmitted «o foreicn 


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of Materiel Sciences, Office of Nava! Research. 


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HPC 70-121 


CONTROLLED ORIENTATION OF DISCONTINUOUS FIBERS IN COMPOSITES 


T. L. TOLBERT 


DECEMBER 1970 


MONSANTO/WASHINGTON UNIVERSITY ASSOCIATION 
HIGH PERFORMANCE COMPOSITES PROGRAM 
SPONSORED BY QNR AND ARPA 
CONTRACT NO, N0001A-67-C-0218, ARPA ORDER 876 
ROLF BUCHDAHL, PROGRAM MANAGER 


MONSANI0 RESEARCH CORPORA II ON 
800 NOR'SH LINDBERGH BOULEVARD 
SI. LOUIS, MISSOURI G5166 









FOREWORD 


The research reported herein was conducted by the 
staff of the Monsanto/Washington University Association 
under the sponsorship of the Advanced Research Projects 
Agency, Department of Defense, through a contract with 
the Office of Naval Research, N00014-67-~C~0218 (formerly 
N0Q014-66-C-0045), ARPA Order No. 876, ONR contract 
authority NR 356-48/4-13-^6, entitled "Development of 
High Performance Composites.." 

The prime contractor is Monsanto Research Corporation. 
The Program Manager is Dr, Rolf Buchdahl. (Phone: Area 
Code 314-694-4721). 

The contract is funded for $7,000,000 and expires 
30 April 1972. 









CONTROLLED ORIENTATION OK DISCONTINUOUS FIBERS IN COMPOSITES 

T. L. Tolbert 

ABSTRACT 

Yarns of uni.directional.ly oriented, discontinuous high- 
modulus fibers and whiskers have been successfully produced 
in the laboratory by a modified vortex spinning process. Both 
core yarns in which the higher modulus fibers are overwrapped 
with organic or small diameter glass fibers and all-whisker- 
yarns have been spun. These yarns can be readily incorporated 
into plastics as reinforcing agents by filament winding and 
related procedures. The strength of resulting composites is 
markedly superior to those of similar materials fabricated by 
other technique! due to well controlled fiber orientation and 
very uniform fiber overlap. 




fONTUOU.KD OK i KMTATION OK H I SC'ONT I NtlOUS K I UK US IN COMi’OS I TKII 

T. !,. TOIjHEKT 

Monsaiito/W.ishi r.gt on tin Ivors i fy Associal i.on 
St. l.oui Missouri U.S.A. 

The t x'nH'mloas potent i n 1 ol high modulus t i bri s and 
whiskers as r»' i.iit.orci nq agents lor plant ics ,ind metals i.s now 
well recognized. In a little inon' than a decade, use of such, 
materials has advanced from laboratory studies aimed primarily 
at extending glass fiber reinforced plastics technology, or 
imitating naturally occurring fiber reinforced systems, to com¬ 
mercial manufacture and acceptance of resulting composites as 
practical engineering materials. Composites containing high 
modulus fibers such as boron and graphite are currently employed 
as primary and secondary structure in such diverse applications 
as components for military and commercial aircraft, sports 
equipment and advanced prosthetic devices. 

Key to the almost explosive growth of the composites field 
has been development of effective methods for producing and 
utilizing high modulus fibers. Small scale manufacture of con¬ 
tinuous fibers such as boron and graphite fiber was achieved 
quite early and production of high quality fiber made routine. 
More recently, procedures for much larger scale operation have 
been dove loped which promise to reduce cost and make use of 
composites reinloieod with t hose ? i be n; tenet.si tv pr.iet i ca I. ioi 










) 


many orw iiu'ivr iruj appi. ! •' ** i ana . Methods tor Labrica tiny boron 
and qrnph.it>' t.' i in' composites, one;' primarily hand operations , 
have been greatly improved and to some extent automated and 
reliable procedures worked out tor incorporating and composites 
into component structure. Utilization of boron and graphite 
composites thus now appears more limited by shortcomings in matrix 
performance and in design methodology than by deficiencies of 
the fibers. 

Significant advances have also been made in employment of 
whisker fibers as reinforcing agents, although overall progress 
has not been as great as with boron and graphite. The. concept 
of using monocrystalline fibers as reinforcing agents is extremely 
attractive, of course, and so has been the subject of some ex¬ 
cellent research, especially in the U.K. Good progress has been 
made in solving problems related to whisker fiber quality, in 
scaling-up manufacture and even in bringing the price of such 
materials to practical levels. For example, Norton Aesearcn 
Corporation, Cambridge, Mass., North American Licensee of the 
Phillips AG Silicon Carbide Whisker Process, is said to have 
under development a process for producing extremely uniform 
good quality whiskers on a pilot scale at projected commercial 
prices of less than $50.00 per pound. Uni or tunate 1y, however, 
progress toward utidi -’.ing whiskers as reinforcing agents has 




teg** ifrteim i ‘Win n i ■ 



been much slower; pract ical, methods are still needed for con¬ 
verting whiskers and resin into high performance composites. 

As single crystals, whiskers exhibit equivalent moduli 
and far higher strengths than other fibers of similar composi¬ 
tion. It was the strength of these fibers, often more than a 
million psi, that initially generated so much entnusiasm for 
the idea of whisker reinforced systems. Exploiting this is 
another matter, however; whisker strength has proved to be 
extremely difficult to translate into composite strength. dnlike 
boron filament and graphite fibers, whiskers currently are pro¬ 
duced only in discontinuous form and are usually extremely small 
in size, often only a few microns in diameter and a few millimeters 
long. Both factors greatly complicate the problem of controlling 
directional orientation, placement, overlap and distrioution of 
the fibers in composites to the degree necessary for good reinforce¬ 
ment efficiency, and hence good composite properties. Control 
of fiber orientation has proved particularly difficult. As would 
be expected, these problems are not limited to whiskers, but 
also have been encountered in attempts to employ a variety of 
other small diameters discontinuous fibers as reinforcing agents. 

A number of approaches to overcoming these fabrication 
problems have been reported, the two most effective being in-situ 
growth of reinforcing whiskers in a metallic matrix and (low 





T 


- 4 - 

orientation of short, high modulus fibers in a viscous, coagulabte¬ 
res in. The latter approach, pioneered with whisker ad a asbestos 
reinforced systems by workers at the Explosives Research and 
Development Establishment, Waltham Abbey, Exxex, England, has 
proved particularly versatile. Strands of oriented fibers suitable 

for subsequent processing can be prepared in this way by "wet 
„ 1-4 

spinning techniques or, at the other extreme, finished com¬ 
posites in which the fibers are flow oriented can be prepared 
directly by modified injection molding procedures.^ Unfortunately, 
these techniques are not suitable for processing high modulus 
fibers greater in length than about 10 mm, and so development 
of an alternate method for handling these materials has beer, 
necessary. The technique found most successful in studies 
conducted by the Monsanto/Washington University Association is 
based on a textile process, "vortex" or "open end" spinning; 
this is described below. 

Spinning of Whisker Fibe r Yarns 

Impetus for development of an alternate method for pro¬ 
cessing discontinuous high modulus fibers, such as whiskers, 
into composites came in cur program from a study on the 
reinforcement efficiency of such fibers. Long staple (10-10 nun) 
3-silicon carbide whiskers had been made avai lable 1 or the work, 
but no satisfactory method was available lot converting them 
into the required uniditeet i i >n t! ly reinforced pi.isl ic specimens. 








While flow orientation techniques were quite satisfactory for 

preoaring directionally reinforced composites from short, fibers, 

scuh an approach could not be employed with the longer material 

due to excessive breakage of fibers caused by the high shear 

forces generated in these systems. Following the lead of 

6 1 

several earlier workers ' in the U.S.A. , attention was given 
instead to study of several textile processes which might be 
modified to produce resin-free "packages" or yarns of axially 
oriented fibers suited to filament winding and related composite 
fabrication procedures. The most promising of these proved to 
be a form of "vortex" or "open end" spinning. 

Initially, a simple laboratory vortex spinner, similar in 
principle to that developed by Strang for textile yarns, was 
used to prepare yarns containing whisker fibers entrapped among 
helically twisted textile (binder) fibers. In this, fibers dis¬ 
persed in a viscous medium were formed into a strand by the action 
of hydrodynamic forces on the suspension as it passed through a 
rapidly rotating tube in the spinner head. The yarn was collected 
as it emerged from the tube assembly, washed free of the sus¬ 
pending liquid (usually corn syrup for convenience) in a gentle 
stream of solvent, and taken up on a rotating drum. Yarns were 
reasonably uniform and, even though the binder 1 Leers were un~ 


6 


crimped, of sufficient strength tor composite preparation. Un¬ 
fortunately, however, axial orientation of the entrapped whisker 
fibers was quite imperfect due to a tendency of the whiskers to 
follow the helical path of the oinder material. In addition, 
fiber breakage was excessive. 

Modification of the first apparatus to permit introduction 

( 

of whiskers as a separate, more viscous suspension at tfia center 
of the flowing binder fiber suspension prior to entry to the 
rotating tube assembly proved a more satisfactory approach. Fluid 
forces are not great enough in the central region of the sus¬ 
pension to disturb orientation or cause breakage of the whiskers 
as they are introduced, compacted and wrapped by the binder 
material. Resulting yarns, consisting of a core of axially 
oriented whiskers helically wrapped by a thin layer of textile 
fibers, are uniform and strong enough for easy handling. WhisK«?r 
breakage is minimal. 

The modified spinning apparatus is shown in Fig. 2. It 
consists of a vertically mounted vortex spinning assembly led 
under nitrogen pressure from spearato holding tanks for the 
fiber slurries. The spinning assembly is basically; like that 
in the previous apparatus except tii.it a short portion of the 
spinning barrel i mined i a l e 1 y be low the i ot at leg lube is expanded 


i n i u ni i u i in -nrrdttflt’Mtrfti 







7 


to approximately twice the original diameter to provide a chamber 
for combining fiber slurries. In oar particular system, core 1 

slurry is introduced through a 6 sum tube, just below the rotating 

/ 

tube assembly, into the center of a 16 mm slurry chamber. 

The slurry chamber and core slurry tube are connected directly 
to the holding tanks by tubing entering through the top of the 
tanks and ending in a conically flared tip about 12 inni above 
the bottom. Lids of the tanks are fastened with bolts and are 
sealed with rubber "Q" rings capable of withstanding 100 psi 
pressure. In operation, the speed of the rotating tube and 
the rate of slurry flow are controlled (annular flow must exceed 
core flow) so that the fibers twist together in the short con¬ 
verging section of the chamber, rather than in the spinning 
barrel proper, since in this region the outer fibers approach 
tf*e core from a non-axial postion and are wrapped more uniformly 
around the core. 

The speed at which the rotating tube of the spinner head 
must turn in order to impart enough twist for good yarn strength 
varies with each system, depending primarily on the flow rates 
of the slurries, slurry viscosities and the mechanical properties 
of the fibers. With our apparatus, rotational rates ranging from 
1 200- .1700 rpm produce sufficient twist in most cases for yarn 
take-up velocities of several inches par second. Faster spinning 
is cadi Iy possible but requires more precise control of variables. 






H 


Due to the high cost of whiskers, various glass Libers 
were used as model materials in developing the. spinning ap¬ 
paratus and in all process studies. See Table 1. These 
proved to be excellent substitutes for whiskers; no difficulties 
were encountered in switching the spinning operation from one 
material to the other. Several of the glass fiber yarns also 
proved to be interesting in their own right. It was found, for 
example, that the diameter of Beta glass fibers is small enough, 
and fiber flexibility therefore great enough that these fibers 
can be used both as wrapping and core materials. Thus, yarns 

l 

of short "E" glass fibers wrapped with Beta glass fiber and of 
100% Beta fiber can be prepared. The textile fibers employed 
as binder materials included uncrimped acetate, triacetate, 
high tenacity rayon and Acrilan 57. Ail were found to be 
satisfactory for the purpose. 

In producing core yarns, approximately equal concentrations 
of glass or whisker core fiber and of the lower modulus binder 
fibers were used in the spinning slurries. Maximum concentration 
was determined by the amount of the binder fibers which could 
be used since these are the more flexible and exhibit tne greater 
tendency to entangle with resultant Loss of dispersion uniformity. 
For the lour binder materials shown in Table i, the limiting 






9 


r 


I 



i 

i 


concentration for 18 nun staple dispersed in corn syrup (100-350 
poise) ranged from 0.05 to 0.1 volume percent when the spinning 
tube of the Laboratory equipment was 6 mm in diameter. In order 
to minimize damage to the fibers, the viscosity of the core 
slurry was maintained at approximately 1.00 times that of the an¬ 
nular slurry. 

Concentrations of high modulus fibers in the vortex-spun 
yams prepared in our studies ranged up to about 36 voluraer 
percent, the degree of axial orientation being very high at all 
levels. As mentioned, with, smaller diameter glass fibers it 
also was possible to spin 100% glass staple yarns of reasonably 
good uniformity and strength. Similar, although much weaker, 

100% yarns were spun from whisker fibers as well. The supply 
of whiskers was not sufficient to permit, more than a few attempts 

to prepare such yarns, but results were promising enough that 

i 

there is little doubt that conditions could be found for producing 
good quality all-whisker yarns. 

Summarizing efforts on whisker yarn preparation, a 
process has been.worked out for converting discontinuous, high 
modulus fibers into a yarn consisting of a core of these 
fibers in axially alig> ed form helically wrapped with lower 
modulus binder fibers. The yarn is produced by subjecting a 


) 

5 









S 0 


two component suspension, made up of a viscous, familiarly 
flowing slurry of high modulus fibers urrounded by a more 
rapidly flowing, less viscous suspension of much lower modulus 
fibers, to an axial vortieity gradient. The gradient, generated 
in the suspension by flow through a rotating tube, provides the 
twisting force required. Core yarns containing up to 36 volume 
percent of long staple 8-silicon carbide whiskers and even higher 
percentages of glass fiber have been prepared in this way. Strong, 
relatively uniform textile-like yarns of discontinuous Beta glass 
fiber alone also have been produced in this way. 

Vortex spinning of inorganic fiber core yarns is the only 
potentially practical approach known to the author by which 
long staple small diameter high modulus fibers and whiskers 
can be '"packaged" in polymer-free directionally oriented form. 

While strictly a laboratory technique as practiced here and of 
doubtful interest for textile applications, this or a related 
approach could be the key to much larger scale utilization of 
such fibers as reinforcing agents. Certainly, it is one of the 
first methods* to make filament winding and related fabrication 


* The Canadian "Bobtex" Integrated Composite Spinning process' is 
said to be applicable to some inorganic fibers and so, assuming 
that these are fibers of interest as reinforcing agents, may be 
another way of obtaining yarns for composite fabrication. However, 
since a binder polymer must be presen' in "Bobtex" yarns to sup¬ 
plement the twist-applied forces between libers, use may be re¬ 
stricted in some systems to composites of relatively low fiber 
con to lit . 










11 


procedures worth considering as means for controlling the 
directional orientation, placement and spacing of whisker fibers 
in composites. 


Co mpo sites From Whi ske r Ya rns 

Laboratory preparation of composites from thermosetting 
resins, such as the lower viscosity epoxies, and vortex-spun 
whisker fibers is straight forward using either compression 
molding or simple filament winding procedures. Resin wet-out is 
easy, so long as yarn twist is not excessive, due to ready sorption 
and wicking of the uncured liquid resin by the binder fibers. 
Bonding between the binder material and the matrix resin is good 
enough under most conditions that these fibers actually behave as 
if they were part of the matrix itself. 


The orientation distribution of reinforcing fibers and 
overall quality of composites prepared with vortex-spun yarn 
are fully equivalent to those which can be achieved by very 
carefully controlled flow molding. The orientation distribu 
tion of glass fibers in a typical yarn reinforced molding is 
shown in Table 2. Vortex, yarn composites are distinguished 
from other systems, however, by the markedly improved fiber 
overlap achieved. Overlap of the fibers in these yarns is 
statistically uniform, while in extrusion and flow molding 





12 


processes fiber overlap is more random and depeiftis ent irely 
on shear forces acting on fiber bundles and matrix. The result 
is that, the yarn reinforced composites exhibit significantly 
hiyhe: tensile strengths than flow molded materials, as shown 

in Fj.g , 4for composites of epoxy resin and 3/8 inch glass 
fiber (nominal length). In fact, with the exception of 
specially prepared model specimens, the yarn reinforced 
materials proved to be stronger than experimental composites of 
equivalent composition made by any other technique in our 
laboratory. Composite moduli are much less sensitive to localized 
discontinuities and so are roughly equivalent for both yarn 
reinforced and flow molded systems. The moduli of the experimental 
composites from Fig. 4. thus can be plotted on a common line, as 
shown in the comparison with "rule of mixture" values in Fig. 5. 

Studies of whisker fiber composites prepared from vortex- 
spun yarns are still in progress and data not yet complete 
enough to justify publication. However, all indications to 
date are that improvements in composite strength are similar to 
those obtained in the glass systems. Reinforcement efficiencies 
calculated as the percentage of fiber strength utilized in the 
composite are much lower for the whisker reinforced systems, 
of course, but absolute strength markedly improved. 








13 


Ac knowl edgmen t 

A number of workers in the Monsanto/Washington University 
Association have contributed to the work which has been 
described. Particular credit is due to Dr. Myrne Riley for 
initial development of the vortex spinning process for high 
modulus fiber yarns and Dr. T. B. Lewis for the data on 
composite properties. This work was performed by the Monsanto/ 
Washington University Association sponsored by the Advanced 
Research Projects Agency under Office of Naval Research Contract 
N00G14-67-C-Q218, formerly N00014-66-C-0045. 









14 


REFERENCES 


1. Parratt, N. J. Composites, 1 [1], 25 (1969); 1 [31 

141 (1970). 

2. Hollingsworth, B. h. , Co mposi tes, 1 [2], (1969). 

3- Schierdinq, R. G. and Deex, 0. D., J. Composite Materials, 

3, 618 (1969) . 

4. Schierding, R. G. and Tolbert, T. L., U. S. Government 
Report, AD 866, 167. 

5. Goettler, L. A., Modern Plasti cs, 47 [4], 140 (1970). 

6. Garde11a, J. W. and Freeston, W. D. Jr., U. S. Government 
Report, AD 825, 310L. 

7. Shaver, R. G., U. S. Government Reports, AD 826. 346 and 
AD 839, 519. 

8. Strang, P. M., U. S. Patent 2, 700, 866; America's Textile 
Reporter , Part I, January 2, 1964 and Part II, January 9, 1964. 

9. Bobkowicz, E. and Bobkowicz, A. J,, Canadian Textile Journal, 
April, 1970. Vol. 87 4, pp 77-84. 








TABLE 1 


FIBERS USED 

IN YARN SPINNING 


Fiber 

Average 

mm 

Diameter 

mils 

E-glass 

0.013 

0.5 

S--glass 

0.0105 

0.413 

Beta glass (Owens-Corning 
Fiberglas Corp.) 

0.00414 

0.163 

Triacetate 

0.0239 

0.94 

Acetate 

0.0211 

0.83 

Acrilan 57A {Monsanto) 

0.0183 

0.720 

Fortisan Rayon (Celanese) 

0.00919 

0.36 




! 




TABLE 2 


ORI ENTATION OF GLASS FIBE RS IN A TYPICA L Y ARN COMPOS ITE 

Molding Technique Hand. Lay-up/Compression 

Reinforcing Fiber 0.163 mil E-glass 

Matrix Shell's Epon 823 Z 


es off Axis) 


% Orientation 


0-5' 
6-10 
11-15 
16-20 
21-25 
26-30 
31-35 
36-40 
41-45 
46-50 
51-55 
56-60 
61 - 6 5 
66-70 
71-75 
76-80 
81-85 


12.5 
10 
12 

11.6 
10 

9 

7.7 

3.3 

4.2 

2.4 

3.9 

2.2 

3.4 

1.5 

2.9 

1 . 7 
(. . 7 
1 . 0 


86-9 0 













SCHEMATIC OF OPEN 






















SCHEMATIC OF OPEN-END COMPOSITE SPINNING APPARATUS 












VARIATION OF COMPOSITE STRENGTH WITH FIBER CONTENT 


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fi HEPOnr Date 


De cembr > r 19 70 


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NO0014-67-C-0218 


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7j». TOTAL MO. OF f’AGES /6. MO OF REF5 

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Ua. ORIGINATOR’S REPORT N UM BF H|S) 

HPC 70-121 


Ob. OTHER REPORT NOISI (Any other number* that inny be assigned 
this report) 


ui outribution statement This document is subject to special export controls 
and each transmittal to foreign governments or foreign nationals may 
ae made only with prior approval of the Director of Material Sciences, 
ffice of Naval Research, _ _ _ 



12 SPONSORING MILITARY ACTIVITY 


Office tff Naval Research 
Washington, D. C. 20360 


V)Yarns of unidirectionally oriented, discontinuous high-modulus 
ibers and whiskers have been successfully produced in the laboratory 
y a modified vortex spinning process. Both core yarns in which 
he higher modulus fibers are overwrapped with organic or small 
iameter glass fibers and all-whisker-yarns have been spun. These 
arns can be readily incorporated into plastics as reinforcing agents 
y filament winding and related procedures. The strength of resulting 
omposites is markedly superior to those of similar materials 
abricated by other techniques due to well controlled fiber orientation 
nd very uniform fiber overlap. . -