Thermal properties of polypropylene/rice husk ash composites

Polymer International 38 (1995) 33-43

Thermal Properties of Polypropylene/Rice Husk Ash

Composites

M . Y. Ahmad h a d . * J. Mustafah, M . S. Mansor

Plastics Technology Centre, Standards and Industrial Research Institute of Malaysia (SIRIM), PO Box 7035,4091 1, Shah Alam, Malaysia

Z. A. Mohd lshak & A. K. Mohd Omar

School of Industrial Technology, Universiti Sains Malaysia, 11800, Penang, Malaysia

(Received 30 January 1995; revised version received 7 March 1995; accepted 18 March 1995)

Abstract: Incorporation of rice husk ash (RHA) fillers into polypropylene affected some of the thermal properties of polypropylene composites. Addition of the black rice husk ash (BRHA) filler raised the thermal degradation temperature while maintaining the oxidative stability. The thermal degradation temperature of the white rice husk ash (WRHA) composites was found to be independent of filler loading but the oxidative stability deteriorated with increasing filler content. DSC studies indicated that both white and black RHA fillers act as weak nucleation agents and increase the degree of crystallinity of polypropylene by a small margin. Addition of the RHA fillers reduced the linear thermal expansion coefi- cient of the composites. Dynamic mechanical studies showed that the RHA composites with higher filler content have higher storage modulus. Tan 6 curves of the composites indicated that WRHA filler increased the damping property while the BRHA filler had an opposite effect.

Key words : polypropylene composites, rice husk ash, filler, thermal properties.

INTRODUCTION properties of the filled polymers too. Consequently, results and trends normally observed during studies of

It has been widely recognised that the incorporation of unfilled polymers may not necessarily apply to polymer disperse fillers into polymers induces substantial composites. changes in terms of their physicochemical and mechani- There is a wide array of thermal analysis techniques cal properties. These changes are due to several factors, that are commonly employed for the study of polymeric such as variation in the mobility of the macromolecules materials. In this paper, four of the techniques are in the boundary layers, the orientating influence of the applied to analyse the thermal properties of poly- filler surface, the different types of filler-polymer inter- propylene (PP) filled with rice husk ash (RHA) fillers. actions, as well as the effect of fillers on the chemical Thermogravimetric analysis (TGA) is used for thermal composition and structure of the polymers. It is degradation study while differential scanning calorime- believed that the above factors affect the thermal try (DSC) analysis is used for oxidative stability and

crystallinity studies. Thermomechanical analysis (TMA) * To whom correspondence should be addressed. is employed for linear coefficient of thermal expansion

Polymer International 0959-8 103/95/$09.00 0 1995 SCI. Printed in Great Britain 33

34 M . Y . Ahmad Fuad et al.

determination and dynamic mechanical analysis (DMA) characterises the viscoelastic properties of the PP/RHA composites.

Application of heat energy to polymers initially induces degradation, i.e. the break-up of macro- molecular chains and the formation of low-molecular- mass products. The initiation of the degradation consists of the cleavage of the carbon-carbon bond of the macromolecules to form free radicals. In the polypropylene molecular chain, the cleavage occurs at the bond adjacent to the tertiary since the bond is the weakest. The thermal degradation of polypropylene is known to be activated by various inorganic compounds such as metallic oxides, aluminosilicates and zeolites.' Polypropylene was reported to degrade as low as 200"C, the normal processing temperature, in the presence of aramid fibres.2

The thermo-oxidative degradation of polypropylene in the presence of disperse inorganic fillers has been extensively studied using DSC and TGA techniques. Dispersed aluminium and silicon decreased the rate of thermo-oxidative degradation of polypropylene while zinc increased it. Metal oxides exert a negligible action, while aluminosilicates result in a substantial increase in the reaction rate.3

The addition of various types of fillers is known to significantly influence the crystallinity of polymers serving as the matrix materials, thus affecting the mechanical properties of the composites. For instance,the modulus increases rapidly because of cross- linking and filler effects of crystallites. An understanding of the crystallisation behaviour of polypropylene is important in order to explain the mechanical properties of the composites. Kowalewski & Galeski4 and Mit- suishi et al.' studied the influence of calcium carbonate and its surface treatment on the crystallisation of filled polypropylene.

The linear thermal expansion coefficient (LTEC) is one of the characteristic properties of considerable importance for polymeric materials. It directly relates to component compatibility and is strongly influenced by structural and compositional changes as well as the physical or chemical state of the material, such as per- centage crystallinity, degree of cure, etc. Incorporation of an inorganic filler to a polymer is known to reduce the LTEC of the polymeric system.6 Prediction of this behaviour in a polymer composite is important since it influences the dimensions and dimensional stability of products directly. Unequal expansion or contraction of constituents of a composite system may lead to un- acceptable stresses, thus matching of the LTECs is usually desirable. Even, if the stresses generated do not result in failure, they may significantly affect the mechanical properties of the constituents.

The dynamic mechanical properties of polymers and composites, especially damping, are extremely sensitive to all kinds of transitions, relaxation processes and

structural heterogeneities. They are also particularly sensitive to the morphology of multiphase systems such as crystalline polymers, polyblends, and filled or composite materials.' As such, the DMA techniques have been widely employed by a great number of researchers to study the effect of fillers on the viscoelastic behaviour of polymer composites.8-''

EXP ER I M ENTAL

Preparation of the PP/RHA composites has been described in previous publications.' ','

The thermogravimetric analyser used for the thermal degradation study was a Mettler TG50 thermobalance. Approximately 10-2Opg of the RHA composites and the neat polypropylene samples were scanned from 50 to 800"C, at a heating rate of 20"C/min. The purge gas used was oxygen-free nitrogen at a flow rate of 200ml/ min. The onset of degradation analyses were performed on the thermogravimetry curves at the point where the major loss of mass is experienced.

DSC analysis was carried out using a Mettler DSC20. For oxidative stability analysis, the samples were scanned in air running at a flow rate of 200mllmin. The scan was performed from 50 to 3WC, at a heating rate of 10"C/min. The oxidation of the sample is indicated by a sudden exothermic increase in the thermal curve. To study the effect of the RHA fillers on the crystallinity of the polypropylene matrix, PP/RHA composite samples were cut from injection-moulded test specimens into 5-1Opg pieces and placed inside an aluminium cru- cible. Each sample was subjected to the following scanning cycle: initial heating from ambient to 200°C at a heating rate of 10"C/min, and maintained isothermally at 250°C for 1Omin; cooling to 70°C at a cooling rate of 10"C/min and final heating again to 200°C. For each sample, three specimens were scanned to ensure repro- ducibility and to acquire the mean values of the crystallinity parameters. The scan was carried out in nitrogen atmosphere at a flow rate of 200ml/min. Various crystallisation parameters such as degree of crystallinity, temperature at the peak of the crystallisation exotherm (Tp), and onset of crystallisation (T,,,,,) temperature were evaluated. To ensure consis- tency between the various samples evaluated, the integ- ration of the cooling curve was performed over a fixed temperature range of 90-130°C.

For the LTEC study, a Perkin Elmer DMA 7 instru- ment was set to constant force mode (TMA mode), i.e. a static load was applied to the sample as the sample was subjected to a temperature profile. In this mode, the frequency is set to zero and the stress is held constant, i.e. under static load condition. An expansion measuring probe (3mm sphere probe) was used. Samples were cut into 20mm x 5mm x 3mm pieces from the injection- moulded test bar specimens prepared earlier. Scanning

POLYMER INTERNATIONAL VOL. 38, NO. 1, 1995

Polypropylene/rice husk ash composites 35

was performed from 5 to 120°C at a heating rate of 5"C/ min. Prior to scanning, the sample was maintained at 0°C for 10min to achieve thermal equilibrium and constant initial height. The linear thermal expansion coefficient (LTEC) was computed from the gradient of the expansion versus temperature curve determined at 5°C intervals. Only the LTEC values well within the temperature range, i.e. from 20 to 90"C, were considered for each sample and the values were plotted against the temperature scale.

The DMA equipment used to study the viscoelastic property of the PP/RHA composites was a Perkin- Elmer DMA 7 system. It was a vertical DMA system that consists of four major components: a central vertical core rod and probe assembly, a temperature con- trolled displacement detector (LVDT), a linear force motor and a furnace. The test specimen was cut down to a rectangular bar of approximate dimensions 20mm x 5mm x 2mm. The specimen was positioned between the two supports of the 15mm bending plat- form and a lOmm knife probe tip was used. Highly purified helium gas was used for purging the sample environment.

RESULTS AND DISCUSSION

Thermal degradation

Figure 1 indicates that the degradation temperatures of the black RHA samples improved (increased) with filler loading while the white RHA samples showed no significant changes. One probable explanation may be offered in terms of the heat absorption capacity of the RHA fillers. Black RHA particles being black in colour

are excellent absorbers of heat energy (black body principle). As their quantity increases, more heat is taken up by the BRHA component of the composite. Thermal degradation of the polypropylene matrix occurs only after a certain amount of heat energy has been absorbed by the material. The heat initiates the degradation process and breaking down of the matrix structure by causing molecular chain ruptures or scis- sion. With increasing filler content, more heat is absorbed by the black RHA particles. A higher temperature is therefore required to supply the threshold energy for commencement of the degradation process. Thus as black RHA filler content increases, there is a gradual favourable shift of the degradation temperature.

According to B r ~ k , ~ introduction of fillers into poly- alkenes should result in an increase of the thermal stability of the polymer. Incorporated fillers cause a reduction in chain mobility in the adsorption and boundary layers. This leads to a decrease in the tension induced to the carbon-carbon chain by the thermal excitation and since the majority of bond breakings are via this mode, less degradation will occur. In other words, thermal stability is improved by the grafting of macromolecules on to the filler surface and the formation of spatial chemical structures in the filled polymer.

Oxidative stability

Differential scanning calorimetry (DSC) analyses revealed that as WRHA filler loading increased, there was a gradual shift in the onset of oxidation temperature towards the lower end of the temperature scale (Fig. 2). The decrease in the onset of oxidation was quite significant, i.e. from 224°C in case of the unfilled polypropylene down to 209°C in the highest loading composi te.

460 I I

450 ?? 3 c

440 E P 0 430

F

C

m U - 0)

420

41 0 0 10 20 30 40

F i I i er Content (Yo) Fig. 1. Onset of degradation temperature versus filler content.


36

225

220

G 8 0

m X 0

c 215 - 0

0 210 ~

0

c

I

a, 0) c

205 -

200 -

. Tvpe of filler ‘-_.

0 10 20 30 40

Filler Content (%)

Fig. 2. Effect of filler content on the onset of oxidation of RHA composites.

M . Y. Ahmad Fuad et al.

I

Polypropylene experiences thermooxidative degradation by an autocatalytic process, i.e. a free radical reaction with random chain breaking. Factors affecting the oxidation rate include the physical state, structure and ~rystal1inity.l~ Incorporation of filler into a polymer matrix probably loosens the structure of the polymer, causing it to ‘crumble’ more easily upon application of heat and thus rapidly exposing the inner structure to air and heat, facilitating the thermooxidation process. In the molten phase and solid state, the oxidation is an heterogeneous reaction with rate depending not only on kinetic parameters but also on transport parameters. Increasing the proportion of filler decreases the volume fraction of polypropylene and this, possibly, leads to an increase in the surface area to volume ratio of the resin exposed to air. As a result there is an increase in the oxygen diffusion to the polymer and diffusion of the volatiles from the polymer, leading to earlier oxidation.

The BRHA composites did not show significant changes in the onset of oxidation temperature with filler loading. This stabilising effect is believed to be because the BRHA filler acts as an excellent absorber of heat. Much of the heat supplied to the composite was absorbed by the BRHA particles themselves. Thus the overall effect on the onset of oxidation of the polypropylene matrix was less and the BRHA composites exhibited greater stability. This effect is in line with the earlier observation in the TGA study that the BRHA composites have higher thermal degradation stability than the WRHA composites.

It has been observed from the available data on the thermal and thermo-oxidative degradation of polypropylene that it is even more sensitive to the surface chemistry of the fillers and their concentration in the systems than polyethylene. The presence of tertiary carbon atoms bearing CH, substituents in the micro-

chains of polypropylene leads to a decrease of the strength of the C-C bonds and to abstraction of the hydrogen atom from the tertiary carbon atoms. It is believed that introduction of filler into the polymer results in the appearance of uneven tensions along the chain due to the interaction of certain segments with the solid surface, which promotes thermal cleavage of weaker bonds (at tertiary carbon atoms). For thermo- oxidative degradation, fillers exhibiting surface active sites participate in the catalytic decomposition of the hydroperoxides or in redox reactions and promote decomposition of the weakest bonds of the polymer.

Crystallinity

The degrees of crystallinity of the untreated WRHA and BRHA composites are shown in Fig. 3. Addition of the WRHA filler increased the overall crystallinity though the increase was neither linear nor continuous. There was an initial increase of crystallinity up to 20% filler content at which the crystallinity reached a maximum before it began to decrease and level off. A similar trend was observed with the BRHA composites except that the crystallinity was at its maximum at 10% loading before levelling off. Up to the maximum filler loading of 40 wt% investigated in this study, the crystallinity of the RHA composites was still higher than that of the neat polypropylene. Maiti & Mahapatro” reported that although at lower filler content the crystallinity of calcium carbonate/polypropylene composites was higher than the neat polypropylene, beyond 30 wt% loading the crystallinity decreased steadily and was sub- sequently lower than that of the unfilled polypropylene. On the other hand, Alonso et a1.I6 reported a continual increase in the crystallinity of talc-filled polypropylene with filler content while Burke et ~ 1 . ’ ~ found no change


Polypropylenelrice husk ash composites

58

37

rn -

124

122

120 c 5 118

E 116 I-

114

112L

h

3

2

54 j.

---/ -

-h -

-

-

4 -

' ' ' ' ' I '

c

53 PP w1 wz w3 w4 P1 P2 P3 P4 L1 u L3 L4 B1 BZ 93 84

PPlRHA Composites

Fig. 3. Degree of crystallinity of WRHA (W series) and BRHA (B series) composites versus filler loading. P and L refer to WRHA composites treated with Prod 2020 and Lica 38 coupling agents respectively.

at all for the crystallinity of polypropylene filled with rutile titanium dioxide at any filler level. Petrovic et

reported a steady decrease in the crystallinity with filler content of polypropylene filled with carbon black. It should be remembered though that the BRHA filler is not purely carbon but more than half of the constituents are made up of amorphous silica.

The effect of fillers on the peak of the crystallisation exothermic curves (T,) of the neat and filled poly- proylene is shown in Fig. 4. The trend in the changes of Tp follows those of crystallinity, i.e. Tp increased with filler content with a maximum increase at 20% loading for the WRHA and at 10% for the BRHA composites. T, is a measure of the supercooling phenomenon, i.e. cooling of a polymer melt below its melting point without crystallisation. An increase in T, (i.e. reduction in supercooling) usually indicates the occurrence of the nucleation process where, in this case, the RHA particles act as heterogeneous nuclei that induce the crystallisation process of the matrix polypropylene. The observed change in T, was, however, quite small. This indicates that the magnitude of their nucleating ability is small in comparison with the nucleating ability of good nucleating agents. For instance, Beck & Ledbetterlg reported that a small quantity of an effec-

tive nucleating agent, aluminium dibenzoate, incorporated into polypropylene yielded a shift of T, by 20°C.

Figure 5 shows the temperatures at which the onset of crystallisation began, Tonset. The parameter Tonset was measured at the point where the DSC curve first devi- ated upward from the baseline during the cooling process. Incorporation of the RHA fillers seemed to increase the values of T,,,,,. It might be inferred that addition of the fillers resulted in earlier commencement of the crystallisation process, i.e. supporting the earlier observation that the fillers serve as nucleating sites for the polypropylene to crystallise. Since the margin for the temperature differences in the T, and Tonse, was not big for either RHA composites, the RHA appeared to serve only as a weak nucleating agent. This is quite understandable since the sizes of the RHA particles (6.6pm for WRHA, 19pm for BRHA) were rather big, beyond the customary sub-micron range that is characteristic of a good nucleating agent.

The effects of application of Prosil2020 (silane-based) and Lica 38 (titanate-based) coupling agents to the WRHA composites may be observed in the DSC ther- mograms shown in Fig. 6, and in the various crystallinity parameters versus filler content curves illustrated in Figs 3-5. The DSC thermal curve shows


132 1

/ /.

124 PP w1 w2 w3 w4 P1 P2 P3 P4 L1 L2 L3 L4 81 82 83 84

PP/ RHA Composites

Fig. 5. Onset of crystallisation (T,,,,) temperatures of WRHA and BRHA composites with respect to filler loading.

clearly, for the 40 wt% filler content, the Prosil coupling agent resulted in a shift of the exothermic peak towards the higher end of the temperature scale while Lica 38 had the opposite effect. The overall shift in the exothermic peaks may also be observed in the Tp values of Fig. 4. For the WRHA composites treated with the Prosil 2020 coupling agent, there was a relatively big increase in T, and the increase seemed to be linearly proportional to the filler content, while those treated with Lica 38 had significantly lower values of T,. Similarly, the values of Tonset for the former composites were shifted to the higher side of the temperature scale.

In terms of degree of crystallinity, Prosil 2020 enhanced the crystallinity of the polypropylene further while Lica 38 did not cause significant changes. The increase in crystallinity of the composites treated with the Prosil 2020 coupling agent was reflected in our previous studyz0 by a significant tensile strength improvement of the composites after treatment with the coupling agent. No significant improvement was observed in the tensile strength of the composites treated with titanate coupling agent” as depicted by

the relatively unchanged degree of crystallinity of these composites compared with the crystallinity of the untreated composites.

Linear coefficient of thermal expansion

The effect of filler loading on the linear coefficient of thermal expansion (LTEC) of the white and black RHA composites is shown in Fig. 7. It was observed that incorporation of both types of filler reduced the expansion coefficient of the composites. As the filler loading was increased, the greater was the reduction in the LTEC. This was particularly clear for the BRHA composites. For the WRHA filler, in the low temperature range, the LTEC reduction of the highest loading composite (40 wt%) was similar to that on the 30% composite, perhaps indicating the maximum reduction possible with increasing filler content. Petrovic et ~ 1 . ’ ~ working with polypropylene/carbon black composite made a similar observation, i.e. LTEC reduction was inversely proportional to filler loading but the reduction even- tually levelled off at the highest loading.

0

i\ PP/ I

.I i

I I I 200 180. 160 1 4 0 120. 100. 80 ‘I:

~

Fig. 6. Shifting of the crystallisation peaks due to treatment with coupling agents for RHA composites at 40 wt% filler loading. P2 and L denote WRHA composites treated with Prosil 2020 and Lica 38 coupling agents respectively.

POLYMER INTERNATIONAL VOL. 38, NO. 1. 1995

Polypropylenelrice husk ash composites

I

39

20 , i = . o o

A A

,\ A 0 0

0 iJ 0

m m 0

. 0

. 0

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I . I \ \I

A 0 .-

s 5 v i I . r r r r v m PP 3 81 A 82 , 83 7 84

1 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Tr m perature ('C)

Fig. 7. Linear thermal expansion coefficient of (a) WRHA and (b) BRHA composites as a function of temperature.

The expansion coefficients of the composites at two temperatures approximating the ambient conditions (20 and 30°C) are plotted against the filler concentration in Fig. 8. The effect of filler loading decreasing the LTEC can be seen even more clearly. With the exception of the 40 wt% WRHA composite, the RHA composites exhibited LTEC reduction that was quite linear with the increase in RHA concentration. Okada & Kama122 showed that the LTEC of a fibre-filled system was affected not only by fibre loading but also by fibre orientation. Investigating the LTEC of glass fibre re- inforced polypropylene, they asserted that the LTEC was also influenced by the crystallinity and orientation of both the amorphous and crystalline phases of the matrix.

During the preparation of a composite, the filler and matrix are heated to a certain processing temperature. During the cooling process, both phases shrink, but the shrinkage of the matrix phase is restrained by the par-

ticles, hence setting up compressive stresses across the interface. When the composite is heated again, the matrix tends to expand more than the filler particles, and if the interface is capable of transmitting the stresses that are set up, the expansion of the matrix will be reduced. This assumes that the adhesion at the interface is strong enough to withstand these thermal stresses. Since thermal expansion involves transmission of stresses across the interface, it may be considered as some kind of indicator to the degree of adhesion between the filler and matrix phases.

Dynamic mechanical properties

The dynamic mechanical data of the unfilled polypropylene and the WRHA composites at various filler loadings are presented in Fig. 9. The corresponding storage modulus, E' and tan 6 values of the BRHA composites are shown in Fig. 10. It may be observed


40 M. Y . Ahmad Fuad et a].

12

E

3 x 6 - 4 -

4 0 0 05 0 1 0 15 0 2 0 25

Volume fraction of filler

0 0 1 0.2 0.3 Volume fraction of filler

Fig. 8. Linear thermal expansion of coefficient of (a) WRHA and (b) BRHA composites versus filler loading at 20°C and 30°C.

that incorporation of the RHA fillers increased the storage moduli of the composite systems within the temperature range studied. This is evident from the increasingly higher modulus values with increasing filler loading for both types of composites.

As the temperature of the composite sample is raised, the storage modulus decreases gradually. At very low sub-ambient temperatures, the magnitude of the decrease is less and this may be attributed to the restricted molecular motion. As the temperature approaches the glass transition temperature of the sample, the drop becomes more apparent. At the glass transition temperature region, there is a big drop in the storage modulus values, indicating phase transition from the rigid glassy state where molecular motions are rather restricted to a more flexible, rubbery state where the molecular chains have greater freedom to move.

Addition of RHA fillers increased the stiffness of the composites, as shown in the flexural modulus study,20 and a parallel observation was made in the present study. As the measuring system for the modulus determination is the same, i.e. by three-point loading system, it is interesting to compare the modulus results acquired

from the DMA analysis (in dynamic mode) and those obtained in the flexural modulus (static mode). The respective flexural modulus results from the dynamic test (at 30°C) and static test (at ambient temperature of 27°C) are shown in Table 1. The moduli of the RHA composites obtained from both dynamic and static modes of measurements (at equivalent temperature) are quite comparable at most filler loadings. The DMA mode tends to give slightly higher modulus values for the filled systems and a lower value for the unfilled polypropylene. Unfortunately no similar comparative study could be found cited in the literature. Probably this is due to the fact that most studies on the dynamic mechanical properties of composites did not use the three-point bending measurement mode. The torsion pendulum, vibrating reed and rotating beam types are the more common and established measuring modes employed in the majority of DMA studies.'

The change in the storage modulus was accompanied by a corresponding change in the values of tan 6 as the latter is a function of the former. However, prominent and sharp tan 6 peaks could not be observed in the case of polypropylene or its composites here as would be


Polypropylenelrice husk ash composites 41

Temperature ('C)

1 .4 l ' l

I I I I I I I -100 .o -75.0 5 0 . 0 -25.0 0.0 25.0 50.0 75.0

Temperature I'C)

Fig. 9. DMA scans of WRHA composites as a function of temperature. (a) Effect of filler loading on storage modulus; (b) effect of filler loading on tan 6.

TABLE 1. Comparison of modulus values acquired by dynamic and static modes

Filler WRHA composites BRHA composites content (wt%) Storage modulus Flexural modulus Storage modulus Flexural modulus

(GPa) (GPa) (GPa) (GPa)

0 1.58 1.23 1.58 1.23 10 1.51 1.64 2.49 1.90 20 2-28 1.94 2.80 2.31 30 2.45 2.29 2.97 2.64 40 2.89 2.54 3.27 3.1 2



-100.0 -75.0 -50.0 -25.0 0.0 25.0 50.0 75.0

Temperature ('C)

1.2 1 I

1.1 - 1.0 -

0.9-

0.8 - 0.7 -

0 . 1 -

- -. -25.0 0.0 25.0 50.0 I I 75.0 I I I

-100.0 -75.0 -50.0

Temperature ('C)

Fig. 10. DMA scans of BRHA composites as a function of temperature. (a) Effect of filler loading on storage modulus; (b) effect of filler loading on tan 6.

observed clearly with other materials such as poly- styrene, polyamide and r ~ b b e r s . ~ ~ * ~ ~ The values of tan 6 (which indicates the damping phenomenon) displayed broad peaks : unfilled polypropylene has a maximum tan 6 at about 30°C while the RHA composites have slightly lower values of tan 6, scattered within the range 15-25°C. Since the tan 6 peaks were not precisely defined and were rather scattered, quantitative assess- ment and detailed analysis of the peaks was not attempted to relate the peaks to filler loading. Although the DMA technique is very sensitive to phase transition such as the glass transition (compared with DSC), it suffers from substantial temperature imprecision. Unlike the DSC technique which has a typical temperature pre-

cision and accuracy of O.l"C, the precision of the DMA temperature measurement is of the order of 5°C since the temperature probe is not in direct contact with the sample. Nevertheless, Petrovic et al." reported tan 6 values of 16-18°C for polypropylene filled with various loadings of carbon black filler.

The maximum value (peak) of tan 6 is commonly taken to be the glass transition temperature, <, of the material. Nielsen & Lande17 disagreed and pointed out that the maximum value of tan 6 is generally about 15- 20°C higher than the T, determined by dilatometry and DSC techniques. They preferred the maximum value of loss modulus, E", to be considered as the < value. According to Cheng,25 the Tg measured by DMA is


Polypropylenelrice husk ash composites 43

usually approximately 20-50°C higher than that observed by DSC because in the former technique, frequency is used to detect molecular motion, while the DSC measurement is carried out in a static situation. Only at very low frequency (about 0.001 Hz) will the T, value determined by DMA be close enough to that measured by DSC analysis.

Careful scrutiny of the tan 6 curves of the WRHA and BRHA composites, however, yields an interesting observation. Generally the tan 6 curves of all the WRHA composites were greater in magnitude than the tan 6 curve of the unfilled polypropylene. On the other hand, the opposite trend might be noted regarding the tan 6 curves of the BRHA composites. Almost all the tan 6 curves of the BRHA composites were lower in magnitude than the neat polypropylene.

As the value of tan 6 is a good indicator of the damping property of the material, it has been shown here that incorporation of the WRHA filler increases the damping behaviour of polypropylene but addition of the BRHA filler has the opposite effect, i.e. reduces it. In the case of the WRHA composites, the magnitude of the damping increased with filler loading, particularly at temperatures above the glass transition temperature. The reverse may be seen for the BRHA composites-the higher loading composites tend to have a greater reduction in the damping property. Ishak & Berry26 while studying the dynamic mechanical properties of nylon 6.6 filled with short carbon fibre observed similar damping behaviour as the BRHA composites. It is worth noting that almost half of the chemical composition of the BRHA filler is made up of carbonaceous matter, hence the possible cause of such semblance. No plausible explanation may be given yet as the cause of the antagonistic damping effects by the white and black RHA fillers.

CONCLUSIONS

The RHA fillers used in this study did affect, to a varying degree, the thermal properties of the composites. The BRHA composites seemed to have better thermal degradation and thermo-oxidative stabilities than the WRHA composites. The RHA fillers also acted as weak nucleation agents and increased the degree of crystallinity of polypropylene. Reduction of the linear thermal expansion coefficient of the composites upon addition of the RHA fillers indicates some improvement

in the dimensional stability of the composites against thermal effect. Dynamic mechanical studies show that stiffness of the RHA composites may be improved by increasing filler loading and that the fillers cause significant modification to the damping property of the matrix material. Overall it may be inferred that incorporation of the RHA fillers does not result in any detri- mental effect to the thermal behaviour of the polypropylene matrix.

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