Mechanical Properties of Polypropylene Reinforced with
Short Graphite Fibers
R. A. WEISS
Corporate Research-Science Laboratories
Emon Research and Engineering Company
Linden, New Jersey 07036
The me2hanical properties of injection molded and compression molded graphite fiber-reinforced polypropylenes are
discussed. In general, the tensile properties of polypropylene
are significantly improved by reinforcement with graphite
fibers and to a greater extent than achieved by glass fiber
reinforcement. The failure mechanism of these materials involved a combination of matrix fracture and debonding of the
fiber-polymer interface. The use temperature of these materials is shcwn to increase with increasing fiber content and the
notch sensitivity is reduced.
INTRODUCTION
T"
e increasing demand in recent years for new lightweight, high performance materials has stimulated
considerable research and developme i t of reinforced
polymeric systems. Of greatest interest from an engineering viewpoint are fiber-reinforced polymers using
strong, stiff filaments such as glass, boron, or graphite.
The majority of the literature on fiber-reinforced polymer composites is concerned with cont nuous filaments
in thermoset polymer matrices such as cured epoxy or
polyester resins. Less work has been done with short
filamentary reinforcement of thermoplastic polymers,
though even in this area, the literature liver the past ten
years is substantial. Most of this literature, however, is
concerned with fiberglass-composites and several reviews of these materials have appeared recently (1-3).
Relatively little work has been done wii h short-graphite
fibers in thermoplastic matrices; cf., T u d e 1. The reason
for this is simple: although the cost of gi aphite fibers has
been reduced considerably in the past t,:nyears, it is still
prohibitively high at $10-100/lb (4). It has been projected, however, that graphite fibers ail1 become available for less than $1O/lb in the near fu:ure (5), and this
fact, coupled with t h e outstandin; properties of
graphite, e.g., high modulus and strength, low density,
high electrical conductivity, low thermal coefficient of
expansion, low coefficient of friction, and excellent
chemical resistance (6),make composit 3s based on these
fibers considerably attractive. In addition, short fiberreinforcement of thermoplastics is desirable because of
the processing flexibility, e.g., extrusion and injection
molding, and the economics attained with such compositions.
In this paper, the preparation and properties of
short-graphite fiber reinforced polyp ropulene are reported. In subsequent papers, other topics related to
these materials will be discussed, s u c l ~as the improvement of the fiber-matrix interfacial strength, temperaPOLYMER COMPOSITES, JULY, 1981, Yo/. 2, Clo. 3
ture dependency of the mechanical properties, and the
rheological and viscoelastic behavior of these composites.
EXPERIMENTAL
Materials
The starting isotactic polypropylenes were commercial materials obtained from Exxon Chemical Company
and were designated CD-481 and CD-460. The melt
flow rates of these polymers measured at 230°C and 298
kPa (26) were 10.8 dg/min and 18.8 dg/min, respectively. Molecular weights (M,) were calculated from the
intrinsic viscosities (IV) indecalin at 135°C ( M , = [IV]'.25
x 10') and were 2.3 x 105forCD-481 and 2.0 x lo5for
CD-460.
Table 1. Literature References for Short Graphite
Fiber-Reinforced Thermoplastics
~~
~
~
~~
Matrix
Nylon 11
Nylon 616
Polysulfone
Poly(buty1ene terephthalate)
Poly(pheny1ene sulfide)
Ethylene-tetrafluoroethylene copolymer
Vinylidene fluoride-tetrafluoroethylene
copolymer
Impact polystyrene
Polyacetal
Polyethylene
Polycarbonate
Poly(4-methyl-pentene-1)
ABS
Carboxylated ethylene ionomer
Poly(methy1 methacrylate)
Natural rubber
Nitrile rubber
Poly(pheny1ene oxide)
Polypropylene
~~
~
Reference
20, 21
7,8,14, 15, 16,20.
23, 24, 25
7, 8, 17, 19
7
7, 25
7
7
8
8
8, 9, 18, 20
8, 9, 11, 12, 13, 21
8
8
9
9
10
10
13, 17, 21
18, 22, 24
95
R . A . Weiss
Table 2. Graphite Fibers
Fiber designation
Manufacturer
Precursor
Cross section
Sizing
Starting length (in.)
Fiber diameter (pm)'
Cross sectional area (pmz)
Specific gravity3
Tensile modulus (GPa)I
Tensile strength (GPa)'
~~~~~~~~
~~~
~~
Fortafil CG-3
Fortafil CG-5
Great Lakes Carbon
Orlon PAN
dogbone
epoxy
.25
(a)2 13.5
(b)* 5.3
55
1.8
207
2.5
Great Lakes Carbon
Orlon PAN
dogbone
epoxy
.25
(a) 14.7
(b) 5.8
65
1.8
331
(b) 2.8
~
~~
' Nomlnal valuer reported by manufacturer.
* For Fortafll fibers (a) is the major diameter of the dogbone and (b) is the minor dlameter.
Taken from Dlefendorf and Tokarsky, Ref. (27).
Chopped graphite fibers (6.4 mm lengths) were obtained from Great Lakes Carbon Corporation and are
described in Table 2. These fibers are prepared from an
Orlon poly(acry1onitrile) precursor (27) which yields a
fiber with adogbone cross-section, Fig. 1. The fibers had
an epoxy resin sizing.
Masterbatches of graphite fiber and CD-481 polypropylene powder containing 30-40 weight percent fibers
were prepared by dry blending, followed by extrusion
either at 250°C with a Sterling, two-in., single screw
extruder or at 200°C with a Brabender l%-in., single
screw extruder. Compositions ranging from 10 to 40
weight percent fibers were prepared by let-down of the
masterbatch with CD-460 polypropylene pellets. For
comparison, polypropylenes containing 10 to 40 weight
percent glass fibers (Owens Corning Fiberglass 452AA)
were prepared in the same way.
Sample Preparation
Injection molded test specimens were prepared with
a Boy, 22 ton, reciprocating screw injection molding
machine equipped with a Boy open nozzle (29). The
molding conditions are given in Table 3 . Compression
molded flexural specimens were cut from a 1.8 mm pad
molded at 170°C. Prior to testing, all specimens were
conditioned at room temperature and 50 percent humidity for at least 48 h.
Measurements
Fiber concentrations were determined by pyrolyzing
the polymer matrix in air at 500°C and weighing the
recovered fibers. Graphite fibers treated under the
same conditions showed a negligible weight loss. The
fiber length distribution of the recovered fibers was
determined by measuring the lengths of 300-700 fibers
with an optical microscope.
Tensile measurements were made on type-I tensile
bars with an Instron Universal Testing Machine using a
crosshead speed of 5.08 mm/min. A strain-gauge extensometer attached to the tensile specimen was used to
measure strain. The modulus was determined from the
linear portion of the stress-strain curve and the engineering tensile strength was calculated from the
maximum tensile force and the original cross-sectional
area of the sample (30).
Three-point flexural measurements were made with
an Instron at a crosshead speed of 1.27 mm/min. Test
specimens were cut either from the narrow section of a
Type I tensile bar or from the center portion of a 12.7
mm wide rectangular bar. The modulus of elasticity, the
flexural strength and the ultimate strain were determined from the load-deflection curves as described in
ASTM Standard D-790.
The heat deflection temperature under a flexural load
of 455 kPa was determined using an American Instruments Softening Point Tester by the technique described in ASTM Standard D-648. The values of deflection temperature given in this report are the average
Table 3. Injection Molding Conditions Used for Sample
Preparation
Type I
tenrlle
bar (30)
Fig. 1 . Scanning electron micrograph (2000X)of unsized, Fortufil CG-3, graphite fibers.
96
Melt temperature, "C
Mold temperature, "C
Injection pressure (oil), MPa
Back pressure
Screw speed, rpm
Injection time, s
Cooling time
Rectangular
bar
127 x 12.7
x 3.2 mm
210
210
60
60
4.9
minimum
100
15
6.0
minimum
100
12
20
20
POLYMER COMPOSITES, JULY, 1987, Vol. 2, No. 3
Mechanical Propevties of Polypropylene Reinforced with Short Gruphite Fibers
from two determinations using injectica molded rectangular bars.
Izod impact strengths of notched and unnotched samples were determined at room temperature with a
Wiedemann Baldwin Impact Tester. The test specimens
were cut from injection molded recttingular bars, and
the test was carried out as described in ASTM Standard
D-256.
IU
RESULTS AND DISCUSSION
(MPa;
Stress-Strain Behavior
Typical tensile stress-strain c u r i e s for injection
molded graphite fiber-reinforced polypropylenes are
shown in Fig. 2 for various fiber concentrations. The
tensile and flexural properties of composites containing
Fortafil CG-3 and CG-5 fibers are plotted versus fiber
volume fraction in Figs. 3 and 4 . 130th stiffness and
strength are improved by the addition of the graphite
fibers to polypropylene; the higher modulus for the
composites containing CG-5 fibers c2.n be explained by
e n s i l e Strensth
0
0
0.05
0.15
0.10
0.20
0.25
+
Fig. 4 . Ultimute stress of gruphitejiber-reinforced polypropylCG-Sfibers(m).
ene vsfiher volume fruction; C G - 3 j b e r s ( O ,O),
a
a
50-
t
-
0.265
b = 0.188
10-
,
= 0.119
a -
b =
Q.057
m
10
0
0
I
2
3
4
5
7
6
I
9
10
12
11
I STRAIN
F i g . 2 . Typirul &tress-straincurves forgruphitefiber-reinforced
polypropylenes ut vurious fiber colume f;-uctions, 4.
16.0
I
0.05
I
0.10
I
0.15
0.20
0.25
0.30
*
Fig. 3. Modulus of graphite fiber-reinfor-ced polypropylene
fiber ooluine .fraction; CG-3.fiber-s( 0 ,Oi, CG-5fibers (mi,
POLYMER COMPOSITES, JULY, 1981, Vol. ;', No. 3
GS
the higher modulus of these fibers. No difference was
observed, however, between the tensile strengths of
composites using the different Fortafil fibers, even
though the reported strength for the CG-5 fibers is
roughly 10 percent higher than for the CG-3 fibers, cf.,
Table 2. This is most likely d u e to the fact that the
strength of the fibers is not efficiently utilized in these
composites, and this will be elaborated upon in the
following section on the ultimate failure behavior of
these materials.
The flexural moduli were slightly lower than the tensile moduli, cf., Fig. 3, which is not an uncommon
observation for filamentary composites (31). Nevertheless, for a wide variety of samples, the tensile moduli and
flexural moduli of the fiber-reinforced plastics prepared
on our equipment corresponded to within 20 percent.
This was not the case for t h e flexural and tensile
strengths, cf., Fig. 4 . These differences, however, can
be explained by statistical strength theory (32), and for a
number of graphite fiber-reinforced polypropylenes
evaluated in our laboratory, the ratio of the flexural
strength and the tensile strength was 1.72 2 0.12.
The properties of compression molded samples are
given in Figs. 5 and 6 . Because of the fiber orientation
which results from the flow in injection molding, as
opposed to no preferred orientation in compression
molding, it is not surprising that the tensile modulus and
strength of compression molded samples are less than
for injection molded samples. The line for the compression molded data in Fig. 5 corresponds to a quasiisotropic laminate (see Ref. (33) ) which confirm the
random orientation of the fibers in these samples. Compression molded samples evaluated by a tensile test
invariably broke near the jaws, which most likely explains the decrease in tensile strength with fiber concentration in these samples. On the other hand, the flexural
strength exhibits the expected increase with increasing
fiber concentration.
97
R . A. Weiss
I
I
similar to those achieved with glass fiber-reinforced
polypropyl enes
It should be noted, however, that no attempt was
made here to optimize t h e strength of either the
graphite fiber-polypropylene or t h e glass fiberpolypropylene interface and, whereas, no technology
currently exists for improving the strength of graphite
fiber-polypropylene composites, certain materials, i. e.,
coupling agents, are commercially available which when
properly used can greatly improve the strength of glass
fiber-reinforced polypropylene. In a future paper we
will discuss how t h e strength of graphite fiberreinforced polypropylene can be improved in a similar
way.
I
I
.Tensile
Modulus
Flexural Modulus
Fiber Length Distribution
I
1
0.05
1
0.15
0.10
1
0.20
5
0
d
Fig. 5 . Effect of processing on the modulus of gruphitefiberreinforced polypropylene.
//
l n j w t l o n Molded
15
-
01
0
1
0.05
0.10
0.15
0.20
0.25
*
Fig. 6. Effect of processing on the strength of graphite fiberreinforced polypropylene.
The tensile properties of glass fiber-reinforced polypropylene prepared in the same manner as the graphite
fiber composites are also shown in Figs. 3 and 4 . The
tensile modulus for the graphite fiber-filled materials is
roughly 50-70 percent greater than for an equivalent
volume loading of glass fibers, which can be attributed to
the higher modulus of the graphite fibers (200-300GPa)
vs glass fibers (70-80 GPa). Quantitative comparisons
between the properties of the graphite fiber- and glass
fiber-reinforced polypropylenes are difficult because
the average fiber aspect ratio of the glass fibers was
roughly twice that of the graphite fibers, and the fiber
orientation distributions are not known. One might
presume, however, that given equivalent processing
conditions and mold geometry, the fiber orientation
distributions in the two composites are similar. Thus it
appears that all things being equal, the strengths achievable with graphite fiber-reinforced polypropylenes are
98
Although fiber-reinforced thermoplastics are attractive because of their ease of processing, i.e., extrusion
and injection molding, the large shear deformations
which occur during these pr0cesse.s given rise to significant fiber length attrition. This, in turn, has a detrimental effect on such composite properties as modulus
and strength.
The fiber length reduction which occurred during the
preparation of the materials discussed in this paper is
demonstrated by the fiber length distribution shown in
Fig. 7 for the extrusion compounded masterbatch (=
0.220). The starting fiber lengths were 6.4 mm while
after extrusion compounding the fiber lengths were less
than 1 mm with a number average length of 0.21 mm.
The fiber lengthdistribution in the injection molded and
compression molded samples corresponded very closely
to that shown in Fig. 7 . An important implication of
these results is that the fiber lengths in the composites
described here were extremely small, and this is most
likely a consequence of not having optimized the
equipment and processing conditions used to prepare
these samples.
Ultimate Failure Behavior
Failure in tension of a fiber-reinforced plastic can
occur by one or more of four possible mechanisms: (I)
fiber fracture, (2)interface shear fracture or debonding,
(3) matrix shear fracture, and (4) matrix tensile fracture.
A detailed analysis of the f d u r e mechanism in short
fiber-reinforced plastics is complex because of the need
1
x 10’ rn
Fig. 7. Fiber length distribution irr gruplzite.fiberlpolypropyletie
mu.sterlx~tch(+ = 0.220). N , = the fruction of fibers hacing
lengths greuter than 1.
POLYM€R COMPOSITES, JULY, 1981, Vol. 2 , No. 3
M e c h a n i c a l P r o p e r i i e s of Polypropylene R e i n f o r c e d w i t h S h o r t G r u p h i t e F i b e r s
to account for such variables as the dis bribution of fiber
lengths and fiber orientations, stress concentrations due
to the interaction of fiber ends, residual matrix stresses
due to processing, and the strength of the fiber-matrix
interface. The final result, however, is dear-the mechanism requiring the lowest applied stress to initiate and
propagate a crack will dominate.
In order to achieve the maximum reinforcement benefit of strong fibers, the composite failure mechanism
should be fiber fracture. This can be accomplished only
if the stress in the fiber reaches the ultimate fracture
stress of the fiber, which in the case of graphite fibers, is
of the order of 2-3 GPa. Because of the differences in the
moduli of the fiber and the polymer, application of a
uniform axial load results in different ::trains in the two
components. This gives rise to a shl3ar stress at the
fiber-polymer interface which provides the mechanism
by whichlongitudinal fiber stresses build up. In order to
achieve theoretical composite strengths, the stress must
be effectively transferred to the fibers so that the stress
in the matrix does not approach the yield failure stress of
the polymer. In addition, the shear strength of the
fiber-polymer interface must not be exceeded.
In practice, most short fiber-reinforced plastics break
by some mechanism other than fiber fracture. If the
interfacial bond between the polymer and the fiber is
weak, the interface fails and the fibers pull out of the
matrix. On the other hand, if interfacial strength is
sufficiently high, failure may occur w thin the polymer
matrix. For example, the interaction of fiber ends can
produce relatively high local stresses in the adjacent
matrix (33)which can result in a matriJ. initiated fracture
mode. In either event, interface or n atrix failure, premature failure of the composite occurs at relatively low
strains.
An examination of the tensile fracture surfaces of the
graphite fiber-reinforced polypropylttnes by scanning
electron microscopy, Figs. 8-10, revells that these materials do not fail by fiber fracture, but rather by either
matrix failure or interfacial debonding. The more highly
filled materials fail at relatively low strains and exhibit
Fig. 8. Scanning electron micrograph (SOOX) of tensile fracture
surface of graphitefiber-reinforced polypropylene (4 = 0.220)
showing fiberfmatrix debonding.
POLYMER COMPOSITES, JULY, 1981, Vol. 2, No. 3
Fig. 9. Scanning electron micrograph ( 4 S O X ) of tensile fracture
surfuce of graphitefiber-reinforced polypropylene ( 4 = 0.045)
showing local matrix yielding.
Fig. 10. Scunning electron micrograph (950X)of tensile fructure
surfuce of graphite fiber-reinforced polypropylene showing
mutrix failure around the fiber ends.
brittle failure. Extensive fiber pull-out and little matrix
drawing are evident in the fracture surfaces of these
samples, cf., F i g . 8, which indicate that failure of the
fiber-polymer interface preceded fracture of the composite specimen.
At lower fiber concentrations in the graphite fiberpolypropylene composites fail at higher strains and in a
ductile manner. In some materials considerable fibrillation of the matrix is observed, F i g . 9, which suggests that
failure was initiated by local yielding of the polymer. In
other materials exhibiting ductile failure, “craters” are
observed surrounding the exposed fibers, F i g . 10, and
the holes from which the fibers have pulled out are
distorted so that they no longer conform to the dimensions of the fiber as in the case where brittle fracture
occurred, cf., F i g . 8. The absence of polymer on the
surface of the fibers in these micrographs may be taken
as further evidence that the fracture occurred at the
fiber-polymer interface, though in the case of ductile
fracture it appears that failure may have been first initi99
R . A. Weiss
ated in the matrix. For example, inFig. 9 where considerable fibrillation of polymer is observed, this occurs
predominantly near fiber ends where stress concentrations are expected to be significant. Thus one scenario
of the failure of these materials might be that concentrated stresses near the fiber ends result in localized
yielding of the polymer, i.e., fibrillation, which results
in the initiation of a crack which then propagates along
the fiber-polymer interface. These results are consistent
with those of Bader et aE. (23) for short graphite fiberreinforced nylon 616 in which the authors present acoustic emission and microscopic evidence for matrix cracking preceding the composite failure.
Heat Deflection Temperature
The heat deflection or distortion temperature (HDT)
of plastics is commonly used to estimate the upper temperature limit at which a polymer can be used as rigid
material.
The HDT's at 455 kPa for t h e graphite fiherreinforced polypropylenes are plotted against fiber concentration inFig. 1 1 . As might be expected, the H D T of
the polymer is increased by the addition of graphite
fibers. For example, a material containing 22 volume
percent graphite fibers exhibits a H D T some 35°C
higher than that of the unreinforced polymer. Similar
improvements in the HDT upon the addition of reinforcing fibers have been reported in the literature, for
example, see Ref. 7. In general, the increase in the H D T
with increasing fiber concentration can be attributed to
an increase in modulus, or to be more exact, changes in
the position of the modulus-temperature curve resulting
from the addition of fibers.
Izod Impact Strength
The energy required to fracture a material can be
conveniently measured from an Izod impact experiment. For a notched specimen, an apparent crack, the
notch, is already initiated and the impacting force is
iI
% I
P
concentrated at the tip of the notch. In this case, the
energy absorbed in order to fracture the material is
dependent primarily upon the energy needed to propagate the crack (34). On the other hand, for an unnotched
specimen the energy required to initiate a crack is included in the impact strength (34).
Analysis of the impact strength of a fiber-reinforced
material is particularly complex. For example, the impact strength of a plastic may be improved by the incorporation of fibers because of debonding or failure of the
fiber-polymer interface. This process not only dissipates
a considerable amount of energy d u e to the large interfacial areas involved, but it also prevents thelocalization of
stresses which may accelerate crack propagation. Alternatively, the addition of fibers to a plastic may result in a
reduction of impact strength because of either matrix
embrittlement or the localization of stresses at regions
around the fiber ends.
The Izod impact strengths of both notched and unnotched specimens of graphite fiber-reinforced polypropylene are plotted vs fiber concentration in Fig. 12.
Whereas the impact strength of unnotched specimens
decreases as the fiber concentration increases, the impact strength of notched specimens increases. One
might expect the presence of graphite fibers in polypropylene to lower the energy required to initiate a crack
because of stress concentrations due to the fibers and the
expected relatively weak adhesion between the two
components. On the other hand, the weak interface
should result in easy debonding of the fiber and the
polymer interface which should dissipate much of the
intended fracture energy. As a consequence, the impact
strength of unnotched specimens should decrease and
that of notched specimens should increase as fibers are
added, exactly the result attained.
CONCLUSIONS
The results presented in this paper provide an indication of the kinds of mechanical properties one may obtain by reinforcing polypropylene with graphite fibers.
Because the fiber orientation in these materials was not
characterized, the absolute values reported here are
8.0
6.0
4.0
2.0
70
1
0.05
0
I
.
I
1
I
1
5
0
Fig. 1 1 . Heut dejection temperature (Ce 455 kPu) of gruphite
fiber-reinforced polypropylene us fiber volume fruction.
100
0.05
0.10
0.15
0.20
0.2s
F i g . 12. Znipuct strengths ut 25°C of gruphite fiber-reinforced
polypropylene us fiber volume fruction.
POLYMER COMPOSITES, JULY, 1981, Vol. 2, No. 3
Mechanical Properties of Polypropylene Reinforced with Short Graphite Fibers
useful only within the context of this report. Similarly,
because of the extremely small fiber 1e.igthsachieved in
these samples, improved properties IIight be expected
for composites prepared on larger commercial equipment or equipment specifically designed for processing
fiber-filled plastics in which longer fiber lengths may be
achieved.
As was expected, the stiffness and the strength of
polypropylene were improved by reinforcement with
graphite fibers and the properties were superior to those
attainable by glass fiber reinforcement. The fracture
mechanism of graphite fiber-reinf0rcc.d polypropylene
appeared to involve both matrix failure and shear failure
of the fibedpolymer interface; no evidence of fiber fracture was found. These results can be explained by the
presence of stress concentrations in the polymer matrix,
especially near the fiber ends, and by ):hepoor strength
of the fibedpolymer interface. Residual stresses in the
matrix resulting from the molding opcration can probably be relieved somewhat by annealin; these materials,
though this was not done in this investi,;ation. The interfacial strength might be improved ky modifying the
chemistry of the polymer and/or the fiber sizing in order
to promote better interaction betwe,:n these components, and this will be the subject of a future communication.
Incorporation of. graphite fibers ini o polypropylene
embrittled the material (decreased the unnotched Izod
impact strength), but also reduced its notch sensitivity
(increased the notched Izod impact strength). In addition, the use temperature limit of the reinforced materials were significantly higher than for unreinforced
polypropylene.
Future studies of these materials will include the
viscoelastic, rheological, and thermal behavior and the
improvement of the fiber-polymer int d a c e d strength.
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,
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,
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P
I
101