TGA - Project1

6.2. Thermo Gravimetric analysis (TGA)

The thermal stability of virgin PBAT, PBAT bio-nanocomposite hybrids and MA-g-PBAT bio-nanocomposites are assessed employing TGA. The initial degradation temperature and temperature at 50% degradation and ash content is represented in figure 6 and table 6.

Figure 6: Thermo Gravimetric analysis (TGA) for PBAT and its Bio-nanocomposite

(a). PBAT, (b). PBAT-Na⁺MMT, (c). PBAT-C20A, (d). PBAT-C30B, (e). PBAT-B109, (f). MA-g-PBAT-C30B, (g). MA-g-PBAT-B109

It is evident that the thermal degradation of virgin PBAT starts at 310.58°C and 100% degradation were noticed around 412°C, whereas incorporation of organically modified nanoclays substantially increases the thermal stability of the biopolymer. PBAT/C30B nanocomposite hybrid exhibits the initial degradation temperature around 322.58°C and final degradation temperature around 469.58°C which is comparatively higher than that of virgin matrix. A similar increase in the initial and final degradation temperature of PBAT/B109 nanocomposite hybrid to 326.23°C and 469.24°C was observed. The phenomenon of increase in thermal stability of the biopolymer matrix with the addition of layered silicates is primarily due to the fact that the nanoclays act as heat barrier, thereby increasing the thermal stability of the system as well as assisting in char formation during thermal decomposition.

The grafted bio-nanocomposite hybrids exhibited a further increase in the degradation temperature. MA-g-PBAT/B109 showed maximum initial and final degradation temperature of 339.59°C and 505.82°C. The bio-nanocomoposite hybrid samples prepared using B109 nanoclay exhibited optimum thermal performance owing to its higher surface area and smaller platelets.

In case of the nanocomposite hybrid a char residue was obtained which indicated improved flammability characteristic in the system.

Thermal Characterization - Project1

6. Thermal Characterization

6.1. Differential Scanning Calorimetry (DSC)

Table 5 depicts the melting characteristics of PBAT and various nanocomposite hybrids. DSC thermogram reveals that the co-polyester presents a single transition or Tg, Tc and Tm due to repartition of different sequences. At room temperature PBAT exhibits a rubbery pleatue between Tg and Tm. The enthalpy for 100% crystalline PBAT has been calculated taking into consideration the contribution of ester group, methylene and paraphenylene groups as –2.5, 4.0 and 5.0 kJ/mol respectively. The calculated value has been determined to 114 J/g. The degree of crystallinity has been estimated using the following equation

Xc (%) = ( ∆ H_{f /} ∆H_{100% ) * ( 100 /} (1-Ww) )

Where Xc = % of crystallinity

∆ H_f = Experimental melting heat of fusion

∆H_100% = Heat of fusion of 100% crystalline PBAT

Ww = weight fraction of nanoclay

It is observed that ∆Cp gap at glass transition temperature is very small. The different thermodynamic value is in consistent with the data obtained by other authors.

Test results reported in table 5 and figure 5a indicates that incorporation of organically modified nanoclays result in significant increase in Tg of PBAT matrix from –35.97 to –39.69°C in case of PBAT/C30B and -27.28°C in PBAT/B109 nanocomposite hybrids respectively.

Figure 5a: Glass transition temperature (Tg) for PBAT and its Bio-nanocomposite

(a). PBAT, (b). PBAT-Na⁺MMT, (c). PBAT-C20A, (d). PBAT-C30B, (e). PBAT-B109, (f). MA-g-PBAT-C30B, (g). MA-g-PBAT-B109

Wherein intermolecular interactions between –OH group of C30B and carbonyl groups (>C=O) of PBAT ester functions have been reported. The presence of hydrogen bonds probably result in immobilization of polymer chains, subsequently enhancing the Tg values. Further MA-g-PBAT/C30B and MA-g-PBAT/B109 nanocomposite hybrids exhibit additionally higher Tg values of –-26.31°C and –25.28°C revealing improved interface through the formation of chemical/physical linkage.

Similarly the melting temperature of PBAT matrix (figure 5b) also showed a substantial increase from 109.2°C to 125°C in PBAT/C30B, 126.72°C in PBAT/B109, 138.25°C in MA-g-PBAT/C30B and 139.02°C in MA-g-PBAT/B109 nanocomposite hybrids respectively.

Figure 5b: Melting temperature (Tm) for PBAT and its Bio-nanocomposite

(a). PBAT, (b). PBAT-Na⁺MMT, (c). PBAT-C20A, (d). PBAT-C30B, (e). PBAT-B109, (f). MA-g-PBAT-C30B, (g). MA-g-PBAT-B109

The variation of crystallization temperature (Tc) of virgin matrix and nanocomposite hybrids is presented in table 5 and figure 5c.

Figure 5c: Crystallization temperature (Tc) for PBAT and its Bio-nanocomposite

(a). PBAT, (b). PBAT-Na⁺MMT, (c). PBAT-C20A, (d). PBAT-C30B, (e). PBAT-B109, (f). MA-g-PBAT-C30B, (g). MA-g-PBAT-B109

The virgin matrix exhibits a crystallization peak around 66.14°C which increased substantially with the incorporation of nanoclays as well as functionalization of PBAT with MA. PBAT/C30B bio-nanocomposite hybrid exhibits optimum crystallization peak around 96.45°C. This is primarily due to heterogeneous nucleation effect in presence of nanoclay which increase the nucleation sites in the polymer matrix. However, grafting of virgin matrix does not show any appreciable increase in the crystallization temperature of PBAT in the bio-nanocomposites as compared with the ungrafted bio-nanocomposites.

The melting heat of fusion of virgin matrix was noticed around 13.9 J/gm, which exhibited the marginal decrease with the incorporation of nanoclays. The degree of crystallinity of PBAT also did not show any appreciable change with addition of nanofiller as well as functionalization of polymer matrix.