Chapter 7 Chapter 7 Conclusions and Future Scopes
2.4 Investigation on Effect of Temperature on Composite Materials
2.4.1 Thermogravimetric Analysis (TGA)
Thermal stability is also an important property of thermoplastic polymer composite requires to be investigated before various applications in an environment prone to temperature
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fluctuation. thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis, thus, carried out to investigate the thermal behaviors of polypropylene and its cement filled composite. TGA is carried out on a Mettler Toledo TGA/SDTA 851e/LF/1100, Switzerland, instrument in Nitrogen gaseous environment. The composite samples are converted into powder form of an amount of 12-35 mg to provide a suitable sample for the test. While DSC analysis is carried out on NETZSCH instrument, model no. STA 449 F3, Jupiter®, (Germany), instrument in Argon gaseous environment. Both the cases, the heating rate is kept at 10ºC/min. and temperature is maintained at 25ºC to 600ºC. Figure 2.8(a) and 2.8(b) show the TGA and differential thermo-gravimetric (DTG) patterns of polypropylene and the composite respectively. Figure 2.8(a) shows the weight loss of pure polypropylene starts at a temperature of 271ºC and continue till to full degradation is completed. For 5%, 10% and 15% cement reinforced composites, it is observed that the starting and peak point of thermal degradation is observed to be around 312-490ºC, 360-492ºC, and 376-488ºC respectively. The initial shift of decomposition temperature is observed to be 41ºC, 89ºC and 105ºC for 5%, 10% and 15% cement reinforced composites respectively with respect to pure polypropylene, indicate better thermal stability of the composite due to the presence of cement particles.
Temperature (ºC)
100 200 300 400 500 600
-20 0 20 40 60 80 100 120
Weight (%)
0%
5%10%
2710C 15%
3120C 3600C 3760C
4070C
4900C
4920C 4880C
(a)
Chapter 2 Fabrication and Characterizations…
200 300 400 500 600
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4
DTG (%) / min
0%
5%
10%
15%
Temperature (ºC) 387ºC
464ºC
469ºC 462ºC
(b)
Figure 2.8 (a) TGA patterns of polypropylene and composites and (b) DTG patterns of polypropylene and composites
It is further observed that the thermal degradation of 5% cement reinforced composite starts much earlier although the stability and sustainability of the composite much longer as compared to other cement reinforced composites and pure polypropylene. For 15% cement reinforced composite, it is observed that thermal degradation starts lately at 376ºC indicates better sustainability at high temperature. Thus, with the percentage increase of cement materials to the composite, dramatic shift of thermal degradation is observed. Figure 2.8(b) shows the degradation rate of all composite samples. The maximum degradation rate of pure polypropylene is observed to be at 387ºC, while 462ºC, 464ºC and 469ºC for 5%, 10% and 15% cement reinforced composites respectively.
The degradation starting temperature (Tds), maximum degradation temperature (Tdmx), temperature corresponding to weight loss (Twl) and the differential thermo-gravimetric peak temperature (dW/dT) (Tdtg) are obtained from the DTG curve (Figure 2.8(b)) and summarized in the Table 2.2. As given in the Table 2.2, for example, 6% weight loss of 10% cement filled polypropylene composite material takes place when the temperature is 372ºC.
Similarly, the weight loss (%) of the material can be calculated at a particular temperature.
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Table 2.2 Characteristic thermal decomposition temperatures of polypropylene and cement reinforced composites
Filler
content Tds (ºC) Tdmx (ºC) Tdtg (ºC)
Twl (ºC) at weight loss (%)
2% 4% 6% 8% 10%
0% 271 407 387 231 268 282 295 302
5% 312 490 462 262 292 321 343 364
10% 360 492 464 287 359 372 391 403
15% 376 488 469 290 365 387 401 410
2.4.2 Differential Scanning Calorimetry (DSC)
The DSC patterns of polypropylene and the composite are shown in Figure 2.9. It is anticipated that addition of cement particles in polypropylene decreases the enthalpy of fusion, which attribute to the non-ignition property of the composite.
100 200 300 400 500 600
0 10 20 30 40
DSC (mW/mg)
5%0%
10%
15%
Temperature (ºC) Tc = 117ºC
Tm = 195ºC
T c =
139ºC
Tc = 139ºC Tm = 196ºC Tm = 196ºC
Tb = 402ºC
Tb = 465ºC Tb = 465ºC
Tb = 465ºC
Figure 2.9 DSC patterns of polypropylene and cement reinforced polypropylene composites
The melting points of composites demonstrate the possible degree of crystallization of the composites (Kanagaraj et al., 2007). The crystallization onset temperature (Tc) rises about 22ºC (from 117ºC for the pure polypropylene to 139ºC for composite). The evaporation temperature (Tb) of the samples are 402ºC, 465ºC, 465ºC and 465ºC for pure polypropylene,
Chapter 2 Fabrication and Characterizations…
5%, 10% and 15% cement reinforced composites respectively. It is also observed that the melting points of the composite samples are significantly affected by the presence of cement particles. Even though, the melting points of the composites are higher compared to pure polypropylene, although the normalized enthalpy of fusion decreases for each composite. The degree of crystallinity of the composites are calculated from the DSC analysis taking into account the heat of fusion 209 J/g for 100% crystalline polypropylene (Garcia et al., 2004;
Jose et al., 2008) and the results are summarized in the Table 2.3.
Table 2.3 Results obtained from continuous DSC analysis for fabricated composite materials
Filler content
Melting temperature (Tm)
[ºC]
Crystallinity temperature(Tc)
[ºC]
Evaporation temperature(Tb)
[º C]
Enthalpy of fusion (ΔHm)
[J/g]
Crystallinity (Xc) [%]
0% 178 117 402 96.92 46
5% 195 139 465 71.28 36
10% 196 141 465 78.60 42
15% 196 141 465 78.56 44
The degree of crystallinity can be expressed as eqn. (2.1) (Jose et al., 2008):
m c
c
X H 100%
H
(2.1) Where, Xc is the degree of crystallinity, ΔHm is the enthalpy of fusion of the sample and ΔHc is the normalized enthalpy of fusion of 100% crystalline polymer (Jose et al., 2008). For composite samples, the degree of crystallinity is given as (Fortunati et al., 2010):
m
c
f c
X H 100%
100 W H
100
(2.2)
where, Wf is the weight percentage of filler material. It is observed that the enthalpy of fusion decreases as percentage of cement increases and the degree of crystallinity changes significantly.
2.4.3 Evaluation of Activation Energy (Ea) of Composites
The activation energy of thermal decomposition of the composites is obtained from the TGA analysis using Horowitz–Metzger eqn. (Jose et al., 2008; Sridhar et al., 2009).
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n 1
2
a a
1 1 R 2RT E
ln ln 1 0.434
E E RT
T
(2.3) where, T is temperature (K), Eα is activation energy in J/mol, R is universal gas constant 8.3136 J /mol-K, n is the order of reaction and α is the fractional weight loss at a particular temperature. The fractional weight loss (α) can be expressed in eqn. (2.4):
i t
i f
W W
W W
(2.4) where, Wi and Wf are the initial and final weight of the sample respectively. While Wt is the weight at a given temperature. For n = 0, eqn. (2.3) is reduced to:
a
2
a a
ln 1 R 2RT E
ln ln 1 0.434
E E RT
T
(2.5)
The n-value can be estimated using the following Expression (Saikrasuna and Saengsuwan, 2009):
a max
E RT
max a
max
n 1 E e e
R d dt
(2.6) where, αmax is the degradation quantity at maximum degradation rate. The exponential factor Ф can be calculated using the following expression (Saikrasuna and Saengsuwan, 2009):
max
n 1a
ln n 1 R
E
(2.7)
where, ψ is the pre-exponential factor. Therefore, a plot of
2
ln ln 1 T
against 1000
T is constructed, as shown in Figure 2.10. Activation energy Ea is calculated from the slope using the expression as eqn. (2.8):
Ea 2.303 R m (2.8) where, m is the slope, obtained from the approximated straight line as shown in the Figure 2.10.
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1 1.5 2 2.5 3 3.5
12 14 16 18 20 22
1000/T (1/K)
k
0%
5%
10%
15%
Figure 2.10 k [where k =
2
ln ln 1 T
] against 1000
T
(1/K) curve
Activation energy is the least amount of energy required to initiate a chemical reaction for a material. Figure 2.11 demonstrates the gradual increment of activation energy of the composite due to percentage increase of filler material which attributes the uniform dispersion of the filler material throughout the polypropylene matrix. The activation energy (Ea) of polypropylene is calculated and it is found to be approximately 63 kJ/mol. The activation energy of 5% cement reinforced composite is considerably high as 72.5 kJ/mol.
0 10 20 30 40 50 60 70 80 90 100
0 5 10 15
Activation energy [Ea] (kJ/mol)
Cement content (wt%)
Figure 2.11 Variation of activation energy with respect to filler loading weight fraction
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While, for 10% and 15% cement reinforced composites, the activation energy is obtained as 76 kJ/mol and 82 kJ/mol respectively. The fact is attributed that the addition of cement particles significantly enhanced the thermal stability of the composite.