Experimental Investigations of Polypropylene and

Poly(vinyl chloride) Composites Filled with Plerospheres

Mingzhu Wang, Zhigang Shen, Chujiang Cai, Shulin Ma, Yushan Xing
Beijing Key Laboratory for Powder Technology Research and Development, Beijing University of Aeronautics and
Astronautics, Beijing 100083, People’s Republic of China

Received 20 March 2003; accepted 28 August 2003

ABSTRACT: Plerospheres, defined here as superfine the content was increased from 0 to 30 wt % and further
spherical particles (0.5–5 ␮m) separated from fly ash (rather increased with the addition of a coupling agent. Differential
than as other solid spherical particles, as some have used the scanning calorimetry indicated that the thermal properties
term), are separated from coal fly ash but are dramatically of the plerosphere/PP composite improved. The surface
different from it. Plerospheres can be used as polymer fillers characteristics and morphology of the impact fracture sur-
to improve the properties of composites. With plerospheres face were examined in detail with scanning electron micros-
used as fillers for polypropylene (PP) and unplasticized copy. The rheological performance of plerosphere/UPVC
poly(vinyl chloride) (UPVC), the effects of the filler content, pipe composites obviously improved; the plasticizing time
the particle sizes of the plerospheres, and the coupling agent was shortened, and the maximum torque was reduced.
on the composite properties were studied. The particle sizes © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 126 –131, 2004
of the plerospheres were 2 and 5 ␮m. The results suggested
that the notched impact properties both at a normal temper-
ature and a low temperature and the tensile and flexural Key words: poly(propylene) (PP); poly(vinyl chloride)
properties of plerosphere/PP increased significantly when (PVC); composites; mechanical properties

INTRODUCTION the conditions of turbulence, surface tension, and

emergency cooling. The main components of plero-
Coal fly ash, an industrial solid-waste byproduct, is
spheres (80 –90%) are silicon dioxide (SiO2) and alu-
produced in large quantities during the combustion of
mina (Al2O3). The mineralogical compositions found
coal by thermal power plants. Thousands of millions
of tons of fly ash are generated in the world each year. by X-ray diffraction analysis are mullite, sillmanite,
A huge source of pollution, fly ash can also be a huge and quartz. The surface Mohs’ hardness of plero-
resource. The storage and disposal of coal fly ash, spheres is 6 – 8. Plerospheres also possess many other
designed to avoid environmental pollution, have be- excellent characteristics, such as sphericity, high
come a worldwide problem, and the reuse of fly ash is strength, high resistivity, and low thermal conductiv-
becoming increasingly important. During the last few ity. By air classification, a large quantity of plero-
decades, the wide use of fly ash has been studied by spheres can be obtained with a diameter ranging from
many researchers, and fly ash has been used in many 0.5 to 5 ␮m. Plerospheres are dramatically different
areas, such as cement and concrete applications, brick, from coal fly ash. One of the most important uses of
ceramic tile, light-weight aggregates, highway pave- plerospheres is as fillers for plastic products. Nowa-
ment, road bases, subgrades, and backfills.1–3 Most of days, mineral fillers are widely used in plastic prod-
these are low-added-value uses. Recently, many re- ucts to improve the performance and to reduce the
searchers have used fly ash as a filler for plastics, costs. More than 17% of plastic products contain min-
paints, insulating materials, and metal matrix compos- eral fillers. Sometimes, the addition of mineral fillers
ites and have made beneficial use of fly ash.1,4 – 8 may lead to a reduction of the impact strength.9,10
There are many superfine spherical particles in coal Plerospheres represent a potential replacement for
fly ash; these are called plerospheres. Plerospheres are these mineral fillers and may improve both the
formed during the burning of coal at 1500 –1800°C. strength and toughness of plastic products. They may
When molten incombustible minerals in coal pass have more advantages than traditional fillers.
through a flue, many plerospheres are formed under Polymers filled with fly ash have been investigated
by a numbers of researchers,2,7,8,11 but so far, not much
work has been reported on the use of plerospheres as
Correspondence to: Z. Shen (szg@buaapowder.net.cn).
fillers for polymers. Chand7 introduced fly ash into a
Journal of Applied Polymer Science, Vol. 92, 126 –131 (2004) thermosetting polyester and found that the impact
© 2004 Wiley Periodicals, Inc. strength and ultimate tensile strength decreased with
PP AND PVC COMPOSITES FILLED WITH PLEROSPHERES 127

Figure 3 (- - - -) Tensile strength and (—) tensile modulus

of PP composites with the addition of modified plero-
Figure 1 Notched impact strength of PP composites with spheres.
the addition of modified plerospheres at (- - - -) 20 and (—)
⫺20°C. air classification and should behave differently than
fly ash. Therefore, plerospheres may produce better
results. There are currently no reports on plerosphere-
the addition of fly ash. Srivastava and Shembekar11 filled polymer composites.
also observed a reduction in the tensile strength when Many researchers have also studied filled PP com-
fly ash was added to an epoxy resin, but this was posites, and numerous fillers, such as talc, calcium
accompanied by an increase in the modulus and an carbonate, silica, and various other minerals, are used
increase in the fracture toughness with higher addi- commercially. These fillers are added to increase the
tions. Wong and Truss8 studied fly-ash-filled polypro- modulus of PP but are generally believed to reduce
pylene (PP) and found that the modulus of the PP the toughness of composites.8 In this study, plero-
composite increased, the yield strength did not spheres were used as fillers for two polymers, PP and
change, and the impact strength decreased with the unplasticized poly(vinyl chloride) (UPVC). The me-
addition of the filler. In their studies, the filler was chanical and thermal properties of the PP composites
normal fly ash that was not well separated. Therefore, and the rheological behavior of UPVC were experi-
the particle sizes were not even, and there were many mentally studied. The effects of the filler content, the
coarse particles in the fly ash material, even many particle size of the plerospheres, and the coupling
irregularly shaped impurities. All of these could have agent on the mechanical and thermal properties were
led to the reduction of the mechanical properties. also studied.
However, plerospheres are separated from fly ash by
EXPERIMENTAL
The main materials used in this investigation were
plerosphere, PP, and UPVC powders. The plerosphere

Figure 2 Effects of the coupling agent on the notched im-

pact strength of plerosphere/PP composites. The filler con- Figure 4 (- - - -) Flexural strength and (—) flexural modulus
tent was 20 wt %. of PP composites with the addition of modified plerospheres.
128 WANG ET AL.

lic of China) with a Zwick pendulum impact tester

(Germany) operated from ⫺20 to 20°C. The impact
specimens were 55 mm long, 6.5 mm wide, and 4.3
mm thick and had a small notch. After fracturing, the
surfaces were studied with scanning electron micros-
copy (SEM), and the fracture surfaces of the specimens
were coated with carbon. Tensile and flexural tests of
PP composites with and without plerospheres were
conducted at 23°C according to GB/T 1040-1992 and
GB 1042-79 (national standards of the People’s Repub-
lic of China), respectively, with an Instron universal
testing machine (Shimadzu Co., Japan) operated at a
10 mm/min crosshead speed. Tensile specimens were
made into dumbbell samples, and flexural specimens
Figure 5 Effects of the coupling agent on the tensile mod-
were made into rectangular strips 55 mm long, 10 mm
ulus and flexural modulus of plerosphere/PP composites.
The filler content was 20 wt %. wide, and 4.2 mm thick. Differential scanning calorim-
etry (DSC) measurements of PP composites with and
without plerospheres were carried out with DSC-SP
particle sizes were 2 and 5 ␮m, and the plerospheres equipment from Rheometric Scientific, Ltd. (USA), at a
mainly consisted of 56% SiO2 and 31% Al2O3. The heating rate of 10°C/min. Rheological tests of the
coupling agent was silane. UPVC composites with and without plerospheres
The plerospheres in this study were separated from were conducted on a Brabender plasticorder (Ger-
coal fly ash by air classification, which was carried out many). The test temperature was 165°C, and the test
by impeller centrifugal airflow classification. Before rate was 5 rpm.
filling the polymers, the plerospheres were modified
with the coupling agent by the aerosol method as
RESULTS AND DISCUSSION
follows. First, the plerospheres were dispersed, and
the coupling agent was atomized by the airflow. Then, Mechanical properties of plerosphere/PP
the atomized coupling agent and the dispersed parti- composites
cles combined to form an aerosol. The temperature of Impact properties
the system was controlled and maintained for a few
minutes, and the modification of the filler was com- Figure 1 shows the notched impact strength of PP
pleted. The modification improved the compatibility composites with various additions of modified plero-
between the plerospheres and the matrix. The selec- spheres (0 –30 wt %). The content of the coupling agent
tion of the coupling agent was very important for the (silane) was 1%. The particle sizes of the fillers were 2
composites. First, the plerospheres were mainly com- and 5 ␮m, and the test temperatures were ⫺20 and
posed of SiO2 and Al2O3. Under the action of air, 20°C.
many SiOOH bonds were generated on the surfaces As Figure 1 shows, the notched impact strength of
of the plerospheres. The SiOOH bonds showed a PP composites increased with the addition of plero-
strong trend of forming new chemical bonds
(SiOOOSi) with a silane. Therefore, a silane was one
of the best choices for the coupling agent. Second, the
coupling agent needed to combine with the inorganic
powder by its polar functional group; meanwhile, it
had to combine with the organic matrix by its nonpo-
lar functional group. Because every polymer matrix
had a different polarity, a silane as a coupling agent
had to meet the polarity requirement. For example, if
the matrix was PP, silane A151 (triethoxy vinylsilane)
was the proper coupling agent because the polarity of
PP was very weak. If the matrix was poly(vinyl chlo-
ride) (PVC), silane KH-550 (␥-aminopropyl triethox-
ysilane) was more proper because PVC was a kind of
semipolar material.
Impact tests of PP composites with and without
plerospheres were conducted according to GB/T Figure 6 DSC endothermic thermograms of plero-
1043-1993 (a national standard of the People’s Repub- sphere/PP composites.
PP AND PVC COMPOSITES FILLED WITH PLEROSPHERES 129

Figure 7 Impact fractographs of PP composites filled with 30 wt % plerosphere and 1% coupling agent: (a) 1000⫻ and (b)
5000⫻.

spheres. At both ⫺20 and 20°C, when the content of that of the base polymer when the filler content was
plerospheres was only 10 wt %, the notched impact maximum in the experiment. Figure 4 shows that the
strength improved greatly. At 20°C, as the filler con- flexural strength and flexural modulus also improved
tent increased to 20 wt %, the notched impact strength with increasing plerosphere contents.
further improved and reached a maximum value of Like the impact properties, the tensile and flexural
28% in the experiment. When the content of plero- properties of the plerosphere/PP material with a par-
spheres increased to 30 wt %, the notched impact ticle size of 2 ␮m were better than those of the material
strength decreased to the value at which it was at 10 with a particle size of 5 ␮m. That is, the finer the filler
wt %. The changes in these properties meant that the was, the better the tensile and flexural properties of
mechanical properties of the composites did not in- the composites were.
crease linearly with increasing filler contents and that Figure 5 shows the tensile and flexural moduli of PP
there was an optimum content for the best impact composites with modified and unmodified plero-
properties. However, when the particle size was 2 ␮m, spheres. Figure 5 demonstrates that the tensile modu-
the notched impact properties of the plerosphere/PP lus increased with the addition of coupling agent I, but
composites were better than those with a particle size it decreased with the addition of coupling agent II.
of 5 ␮m. This meant that the finer the plerospheres Figure 5 also shows that the flexural modulus in-
were, the better the impact properties of the compos- creased with the addition of coupling agent I and
ites were. coupling agent II, although coupling agent II showed
Figure 2 shows the notched impact strength of PP a better effect.
composites with the filler modified and not modified
by the coupling agent. At 20°C, the notched impact Thermal properties
strength increased when the plerospheres were well
modified with a coupling agent. The increase in the Figure 6 shows typical DSC endothermic thermo-
notched impact strength depended on the coupling grams of pure PP, PP composites with 20 wt % un-
agent, and the coupling agent I (A151) showed the
better effect, whereas at ⫺20°C, the notched impact
strength changed little with the coupling agent.

Tensile and flexural properties

Figures 3 and 4 show the tensile and flexural proper-
ties of PP composites with various additions of mod-
ified plerospheres (0 –30 wt %). The content of the
coupling agent (silane) was 1%.
Figure 3 shows that the tensile properties of the PP
composites improved with the addition of the filler.
The tensile modulus increased quickly as the plero-
sphere content increased and was 80% higher than Figure 8 Impact fractograph of pure PP materials (1000⫻).
130 WANG ET AL.

showed that the endothermic peak was independent

of the coupling agent, whereas the heat of fusion
increased with the addition of the coupling agent.
Figure 6 also shows that the endothermic peaks of
pure PP and plerosphere/PP composites ranged from
164 to 168°C, and this indicated that PP, both in the
pure state and in the composites, exhibited only the
crystal form ␣ because the melting temperature of ␣
crystals is 160 –176°C.12

SEM observations
Figure 9 Impact fractograph of PP composites filled with
30 wt % plerosphere without the coupling agent (1000⫻). The impact fracture surfaces of plerosphere/PP
composites at room temperature were studied with
SEM after the fracture surfaces were coated with
carbon.
modified plerospheres, and PP composites with 20 wt Figure 7(a,b) shows the impact fractographs of PP
% modified plerospheres. The test temperature ranged filled with 30 wt % modified plerospheres at different
from 60 to 200°C, and the heating rate was 10°C/min. magnifications. The filler particles were dispersed well
In Figure 6, the DSC thermogram of pure PP shows in the matrix, and the spherical particles of the plero-
that the endothermic peak was 164.3°C and that the spheres were well wetted with PP material. There was
heat of fusion was 96.3 J/g. The endothermic peak of no clear separation or void at the interface of the
the PP composites increased by 3– 4°C and the heat of composite. Figures 8 and 9 show SEM micrographs of
fusion decreased by 13–20 J/g with the addition of the fracture surfaces of impact samples of pure PP and
plerospheres. The endothermic peak of PP composites PP with 30 wt % unmodified plerospheres, respec-
with the 20 wt % unmodified filler was 167.7°C, and tively. In contrast, when the coupling agent was
the heat of fusion was 75.7 J/g. This indicated that the present, the adhesion between the filler and matrix
thermal properties of the PP material were improved was much stronger. Figure 7 also shows that there
and that the degree of crystallinity decreased with the were some pores at the interface, but there were many
plerospheres. However, the endothermic peak of the more pores in the pure PP material (Fig. 8). These
PP composite with the 20 wt % modified filler was pores were produced during preparation because of
167.4°C, and the heat of fusion was 83.2 J/g; this the high shrinkage of PP.

Figure 10 Torque versus time for PVC pipes filled with plerospheres.
PP AND PVC COMPOSITES FILLED WITH PLEROSPHERES 131

TABLE I
Rheological Performance of the Plerosphere/PVC Composites Described in Figure 10
Maximum torque Relevant time Balanced torque Relevant time
Specimen (N m) (min) (N m) (min)

110 46 0 Could not reach a stable gelation state

111 48 0.5 Could not reach a stable gelation state
112 41 4.5 38 13

Rheological behavior of plerosphere/PVC strength of PP composites increased and further in-

composites creased when the plerospheres were modified with 1 wt
The melt rheology of plerosphere/PVC composites % silane A151. The tensile and flexural properties in-
was tested with a Brabender plasticorder. The particle creased with the addition of plerospheres and further
size of the plerospheres was 2 ␮m. The temperature increased with increasing filler content. When the filler
was 165°C, and the test rate was 5 rpm. The measure- content was 30 wt %, the tensile and flexural properties
ment range of the torque was 10 – 60 N m. Figure 10 were best. Moreover, the mechanical properties were
shows the torque curve over time for the PVC pipes enhanced when the particle size decreased. When PP
material with and without plerospheres, and Table I was filled with plerospheres, the shrinkage of PP mate-
lists the relevant parameters for the rheological per- rial decreased, and the thermal properties of PP compos-
formance of the plerosphere/PVC composites de- ites improved. For PVC pipe material, the rheological
scribed in Figure 10. behavior improved greatly when the material was filled
Figure 10 shows that the rheological behavior of a with modified plerospheres, which could be very impor-
raw PVC pipe material (110) was very bad, and the tant to practical production.
maximum torque was 46 N m in the beginning. Even Generally, the toughness of a filled polymer com-
when the test time was 22 min, the system still did not posite decreases when the strength increases. How-
reach a stable gelation state. For the PVC material with ever, in this study, both the strength and toughness of
the addition of unmodified plerospheres (111), the plerosphere/PP composites increased.
maximum torque (48 N m) appeared when the time This study shows the beneficial use of a coal com-
was 0.5 min. When the test time was 20 min, the
bustion byproduct. Plerospheres, a new multifunc-
system also did not reach a stable gelation state. How-
tional material, may bring great social and economic
ever, for the PVC material with the addition of mod-
benefits and, at the same time, prevent environmental
ified plerospheres (112), the maximum torque was
pollution.
only 41 N m when the time was 4.5 min, and when the
test time was 13 min, the system reached a stable
gelation state. The balanced torque was 38 N m.
These results indicated that the rheological behavior References
of the PVC pipe material was better in the presence of
the plerospheres and coupling agent; the viscosity of 1. Hwang, J. Y.; Huang, X. D.; Hein, A. M. The synthesizing
the composites decreased, and the processability of mullite from beneficiated fly ash. JOM 1994, 36.
2. Devi, M. S.; Murugesan, V.; Rengaraj, K.; Anand, P. J Appl
the plerosphere/PVC pipe composites improved.
Polym Sci 1998, 69, 1385.
Most importantly, the composites reached a stable 3. Watts, A. J. C. Trans Br Ceram Soc 1954, 53, 314.
gelation state quickly, and the maximum torque de- 4. Rohatgi, P. K.; Guo, R. Q.; Iksan, H.; Borchelt, E. J.; Asthana, R.
creased. Furthermore, the rheological performance of Mater Sci Eng A 1998, 244, 22.
the plerosphere/PVC composite without the coupling 5. Rohatgi, P. K. Low-cost, fly-ash-containing aluminium-matrix
agent was even worse, in that the compatibility be- composites. JOM 1994, 55.
tween the plerospheres and PVC matrix was poor 6. Guo, R. Q.; Venugopalan, D.; Rohatgi, P. K. Mater Sci Eng A
1998, 241, 184.
when the coupling agent was absent. This suggested
7. Chand, N. J Mater Sci Lett 1988, 7, 36.
that the coupling agent was very important for the 8. Wong, K. W. Y.; Truss, R. W. Compos Sci Technol 1994, 52,
plerosphere/PVC pipe composites. 361.
9. Xavier, S. F.; Schultz, J. M.; Friedrich, K. J Mater Sci 1990, 25,
CONCLUSIONS 2411.
10. Vollenberg, P. H. T.; Heikens, D. J Mater Sci 1990, 25, 3089.
Plerospheres were used as fillers for PP and PVC mate- 11. Srivastava, V. K.; Shembekar, P. S. J Mater Sci 1990, 25, 3513.
rials. With a filler content of 0 –30 wt %, the impact 12. Shieh, Y. H.; Lee, M. S.; Chen, S. A. Polymer 2001, 42, 4439.