International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012 1
ISSN 2229-5518
Development of Stearic Acid Coated Fly Ash Reinforced Recycled Polypropylene Matrix Composites and their Thermal Analysis
Shubhalakshmi Sengupta, Pulakesh Maity, Dipa Ray, Anirudhha Mukhopadhyay
Fly ash (FA) and polypropylene (PP) are industrial wastes which affect the environment of our planet. A proper management and commercial utilization of fly ashes will provide alternative value added products and also reduction of environmental and economic impacts from the cost of its disposal. Fly ashes are mainly by-products of the thermal power plants which are produced from the combustion of carbon and fossil fuels and are comprised of primarily of SiO2 along with lower contents of Al2O3, Fe2O3, Na2O, MgO, K2O etc. [1].Attempts are going on in establishing fly ash as attractive mineral filler in various polymer matrix composites [1-3]. Earlier, a lot of studies were conducted on the mechanical properties of fly ash filled polymer matrix composites [4]. Sridhar et al. reported from the thermogravimetric analysis of fly ash / waste tire powder/ isotactic PP composites that there was a progressive increase in the activation energy of the composites with increase in fly ash loadings [3]. Pardo et al. reported that improved stiffness and strength of materials were observed when silane coupling agents of three types were used in mixing fly ash with isotactic polypropylene matrix along with improved thermal stability. This improvement was greater in the composites containing
vinyl and amino silane treated fly ashes, which showed higher onset temperature and was strongly dependent on
filler-matrix adhesion [1]. Das et al reported utilization of recycled polypropylene by forming composites with fly ash in 1:1 weight ratio and using two types of coupling agents (vinyl trimethoxy silane coupling agent, VTMO and maleated polypropylene, Epolene G3003). Thermal stability was found to be much higher in the VTMO treated composites compared to the untreated ones [5]. In our earlier study, the mechanical properties of furfuryl palmitate coated fly ash reinforced recycled polypropylene matrix composites were reported [6]. In that earlier study it was found that the highest enhancement in mechanical properties was observed in 2 wt% FP (FP2) coated fly ash filled composites. In this study, stearic acid (SA) was used to coat the fly ashes and staeric acid coated fly ash was then used as reinforcement in RPP matrix composites. Mechanical and thermal properties of the composites were studied in order to analyze the efficiency of this low cost easily available organic acid as a coupling agent in FA/RPP composites. The composites were tested for their dynamic mechanical behaviour (DMA) and thermogravimetric analysis (TGA).
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Shubhalakshmi Sengupta, Pulakesh Maity, Dipa Ray, Department of Polymer Science and Technology, University of Calcutta,92, A.P.C Road, Kolkata-700009.
Aniruddha Mukhopadhyay, Department of Environmental Science,
University of Calcutta, 35, Ballygunge Circular Road, Kolkata-
700019.
E-mail: sengupta.shubha@gmail.com, roy.dipa@gmail.com.
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International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012 2
ISSN 2229-5518
Fly ash used was collected from Kolaghat Thermal Power station, India. This ASTM class ‘F’ fly ash (as per ASTM-C 618) was found to have different proportions of oxides. The ash particles had a particle size distribution: 42 wt% of the particles had a particle size between 104 and 152
µm, 25 wt% were between 76 and 104 µm and the rests
were below the range. Recycled polypropylene (RPP) was obtained from post-consumer plastic products and was used as the matrix material. Stearic acid and the solvents, acetone and toluene, were obtained from Loba chemie.
The fly ash particles (100 gms in each case) were surface coated by immersing them in the solutions of stearic acid (SA) (1, 2, 3, 5 gm of SA in 100 ml of acetone and toluene in the ratio of 3:1) separately under constant stirring for 15 min. It was kept for drying for 2 days at room temperature. Then it was vacuum dried to completely remove the solvents. The surface coated fly ash particles were designated as FASAX, where X denoted the wt% of stearic acid with respect to the weight of fly ash coated.
The surface coated fly ash (FASAX) samples and the recycled polypropylene (RPP) were taken in a 1:1 weight ratio. Uncoated fly ash (designated as FASA0) and RPP were also taken. RPP and FASAX were mixed in an internal mixer (Brabender 30/50 E, Germany) for 20 minutes at a temperature of 170oC. The resultant product was compression moulded at 170oC for 10 minutes to form composite laminates of 1.5 mm thickness. The composites were designated as RFASA0, RFASA1, RFASA2, RFASA3 and RFASA5.
Dynamic Mechanical Analysis of the composite samples was carried out in a DM Q 800 in nitrogen atmosphere in a fixed frequency flexural mode of 1.0 Hz. The samples were evaluated in the temperature range of -
50oC to 150oC with a heating rate of 10oC per minute. The thermal degradation behaviour was studied in a TGA HI Res TGA 2950 Thermogravimetric analyzer. The samples
were scanned from 25oC to 600oC at a heating rate of
10oC/min.
The Dynamic mechanical analysis of the composite samples was carried out to investigate the properties of the composites under dynamic loading condition with increase in temperature. The glass transition temperature (Tg) was measured from the loss modulus data. The values are given in TABLE 1. From this a delay in transition from glassy to rubbery state in all the SA coated FA/RPP composites is visible indicating a restricted chain mobility, which can be attributed to effective interfacial bonding with highest being in the case of RFASA1.
TABLE 1.Tg values of the composite samples
Composite | Tg (oC) |
RFASA0 | - 1.6 |
RFASA1 | +6.3 |
RFASA2 | +5.61 |
RFASA3 | +5.16 |
RFASA5 | +4.21 |
From TGA analysis the temperatures for onset of degradation was obtained. The temperature at which the rate of degradation was highest was also derived. These values are indicative of the thermal stability of the composites. The values are given in TABLE 2.
TABLE 2.Data from TGA analysis
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International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012 3
ISSN 2229-5518
RFASA5 190 411, 440
From the data, it could be understood that
RFASA1, RFASA2 and RFASA3 were thermally more stable than RFASA0. This could be attributed to their mechanical properties which were studied in earlier. In our earlier study both RFASA1 and RFASA2 had shown enhancement in mechanical properties due to better interfacial bonding between the filler and matrix. In case of RFSA5 which showed the lower onset of thermal degradation, higher concentration of coupling agent might have facilitated thermal degradation and consequently lowered its thermal stability. In RFASA0 the rate of degradation reached its peak at 443oC but in all the SA coated FA/RPP composites two degradation peaks were observed. This could be due to the thermal degradation of the less compact RPP chains at a lower temperature followed by the degradation of closely packed RPP chains at a higher temperature. In an earlier study by Manna et al, the DTGA curve showed the degradation peak over a range of temperature for silane treated clay reinforced epoxidized natural rubber (ENR) matrix composites and it was reported to be due to breaking of bonds between filler and the coupling agent and then breaking of the matrix bonds [7]. In our case strong physical interaction between the filler and the coupling agent and the matrix was discussed earlier. Thus, breaking of such interactions and then the RPP chains might have resulted in such multistep higher rate of degradation. However, RFASA1 showed delayed onset of this degradation peak among all the composites rendering it to be the most stable among all the composites.
0
-2
RFASA1
60 40
% Residue
35
50
30
40
25
30
20
Activation energy
20
15
10 10
RFASA0 RFASA1 RFASA2 RFASA3 RFASA5
Fig.2.The activation energy and % residue left in the composite samples
The activation energy of the composites were also studied which revealed information about the thermal degradation behavior of the composites. The present calculation is based on the equation derived by Broido [8],
ln[ln(1/y)] = E/R [1/T+K],
where, y = Wt/Wo, Wt = weight remaining at time t, Wo = initial weight, T = temperature in Kelvin. From the residual weight of the samples, ln[ln(1/y)] was plotted against 1/T (Fig.1.). From the slope of the linear plot, which was –E/R (R=universal gas constant), the activation energy E was
-4
-6
-8
-10
-12
-14
RFASA0
RFASA5
RFASA2
RFASA3
0.0010 0.0015 0.0020 0.0025 0.0030 0.0035
1/T (K-1)
calculated.
The variation of the activation energy of the composites and the % residual weight derived from TGA analysis are shown in Fig.2. The activation energy was found to be 20 KJ/mol, 56 KJ/mol, 44 KJ/mol, 57 KJ/mol and
43 KJ/mol in case of RFASA0, RFASA1, RFASA2, RFASA3 and RFASA5 respectively. Thus the SA coated FA/RPP composites required higher amount of activation energy for their degradation. Thus, they are thermally more stable.
Fig.1. Plot of ln[ln(1/y)] vs [1/T] for the composite samples
Increase in activation energy was marked with a decrease
in percent residue of the thermally degraded composite. In SA treated FA/ RPP composites the percent residue was found to be much less. Thus, they thermally degrade well
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International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012 4
ISSN 2229-5518
but require more energy for their degradation as a result of strong interfacial bonding between the filler and the matrix.
Stearic acid was used as a coupling agent between recycled polypropylene and fly ash particles. The concentration of the coupling agent was varied from 1 wt% to 5 wt% with respect to the fly ash weight. The glass transition shifted to a higher temperature in all the treated composites, the highest being in RFASA1 at + 6.3oC from that of -1.6oC in RFASA0. This reflects restricted chain mobility in the treated composites due to enhanced interfacial interaction. The thermal stability also increased with maximum delay in the onset of degradation in RFASA1 at 269o C due to better interfacial bonding between the filler and matrix. The coupling agent also increased the activation energy of the composites. Thus, a renewable, low cost chemical like stearic acid can be used as an effective coupling agent for fly ash in place of expensive synthetic coupling agents which are more commonly used in order to make materials from recycled plastics (here RPP) with tailor made properties specially in case of better thermal stability.
Dipa Ray is thankful to Department of Science and Technology (DST), Government of India, for granting her the project. Shubhalakshmi Sengupta is
thankful to DST for her fellowship.
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[2] D. Ray, S. Banerjee, A.K. Mohanty, and M. Misra, “Thermal and electrical behaviour of vinylester resin matrix composites filled with fly ash particles’’,Polym Compos, Vol. 29, pp. 58 -62, 2008.
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[7] D. Ray, S .Banerjee, A. K. Mohanty, M. Misra, “Thermal and electrical behaviour of vinylester resin matrix composites filled with fly ash particles”, Polym. Compos,Vol. 29, pp.58-62,2008.
[8] A.K. Manna, D.K. Tripathy, P.P. De, S.K. De, M.K. Chatterjee,D.G. Peiffer, “Bonding between epoxidized natural rubber and clay in presence of silane couplingagent”, J. App. Polm. Sci.Vol. 72, pp.1895-1903, 1999.
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