Experimental investigation on the spectral, mechanical, and thermal behaviors of thermoplastic starch and de-laminated talc-filled sustainable bio-nanocomposite of polypropylene

3.1 Spectral characterization

The plastic identification test was done by Fourier transform infrared (FTIR) spectroscopy as per test standard ASTM E 1252-98 (Ra 2021), samples were scanned in the wavelength range 400–4,000 cm⁻¹, and the spectrum was evaluated to obtain stretching/bending vibrations for identification of the molecular structure of the composite materials.

Bio-nanocomposite specimens were also subjected to an X-ray diffraction (XRD) pattern, and the intensity of X-rays diffracted by the specimen in 2θ range 5° to 80° was recorded to assess its atomic orientation/structure, crystallinity, and phase purity. XRD spectrum gave insight into the crystallinity and molecular orientation of bio-nanocomposite material.

UV-Visio range UV–visible [UV–Vis] spectrophotometry of the samples was also done to obtain their UV–Vis range optical absorbance, and specimens were scanned in the wavelength 200–800 nm for absorbance spectrum.

3.2 Thermal characterization

Differential scanning calorimetry (DSC) test has been done as per standard ISO 11357-1:2023. The thermal behavior of the bio-nanocomposite was obtained as a thermogram which provided insight into its melting and crystallization behavior. Thermogravitational analysis (TGA) was also done to examine the pattern of thermal degradation; results indicated the temperature range within which bio-nanocomposite shall remain stable. Test has been done as per ASTM 1131:2000 test standards to ascertain its decomposition behavior, thus indicating its reliability for optimum performance.

3.3 Mechanical characterization

Mechanical characterization of bio-nanocomposite was done by measurement of its tensile strength, elongation, flexural properties, and melt flow index (MFI). Tensile strength and elongation tests were done following ISO 527-1:2019 standards, whereas measurements of flexural properties and MFI were done following ISO 178:2019 and ISO 1133-1:2022 (procedure B), respectively.

4.1 FTIR spectroscopy

Figure 1(a) depicts the FTIR spectra of PP, PPST-15, 20, and 25, this spectrum helps identify the presence of PP and thermoplastic starch in the bio-composite. Absorbance peaks near 972.261 and 1168.149 cm⁻¹ may be attributed to the stretching vibration because of C–C bond, which is the backbone of the PP matrix. C–H bond in the CH₃ (methyl group) and CH₂ (methylene group) can be attributable to the C–H stretching vibration linked to an absorbance peak between 2865.123 and 2993.56 cm⁻¹.

Figure 1

(a) FTIR spectrum of PP, PPST-15, 20, and 25. (b) FTIR spectrum of PPSTTL 2001, 2003, and 2005.

Absorbance peaks around 1455.88 and 1376.234 cm⁻¹ can also be attributed to the bending vibration due to the C–H bond of the PP matrix [35,36,37]. Absorbance peak around 3422.648 to 3790.027 cm⁻¹ indicates O–H stretching vibration; this can be attributable to the hydroxyl group associated with thermoplastic starch molecule [35,36,37], and another absorbance peak around 2837.139–2948.357 cm⁻¹ indicates stretching vibration due to its C–H bond. The carbonyl group and glycosidic bond are starch components, for which, C═O stretching vibration at 1707.736 to 1747.918 cm⁻¹ indicates carbonyl group and C–O stretching vibration at 1011.726–1254.253 cm⁻¹ indicates glycosidic bond. Upon successful verification of the presence of PP matrix and starch as filler, let us consider Figure 1(b) which depicts the FTIR spectra of bio-nanocomposite PPSTTL 2001, 2003, and 2005; this contains PP as a base matrix, 20% thermoplastic starch as bio-filler, and de-laminated talc as nano-filler in concentration 1, 3, and 5%.

From the FTIR spectra shown in Figure 1(b), it looks like that all the absorbance peaks for PP and thermoplastic starch, which were indicated in Figure 1(a), are present in Figure 1(b) as well; those absorbance peaks are indicative of the presence of PP and starch in the bio-nanocomposite. In addition to all the absorbance peaks marked in Figure 1(a), spectrum at Figure 1(b) contains a sharp and strong absorbance peak at 1016.031 cm⁻¹ corresponding to the stretching vibration of the Si–O bond and a peak at 3676.656 cm⁻¹ corresponding to the stretching vibration because of O–H without hydrogen bond; both these peaks are indicative of the successful presence of talc as nano-filler [38,39,40] in all the bio-nanocomposite.

4.2 XRD

Figure 2(a) and (b) depict the XRD spectra of PP, PPST-15, 20, and 25 and PPSTTL 2001, 2003, and 2005, respectively, both spectra form the basis to identify the crystallinity and atomic structure of bio-nanocomposite.

Figure 2

(a) XRD spectrum of Pure PP, PPST-15, 20, and 25. (b) XRD spectrum of PPSTTL 2001, 2003, and 2005.

Diffracted X-rays were analyzed in 2θ range 5–80° for the phases plane, crystal size, and interplanar spacing. Interplanar spacing was calculated by applying the following equation and the same is presented in Table 2.

From the Braggs law, we know that

where n is the order of reflection, λ is the X-ray wavelength, d is the interplanar spacing, and θ is the angle of diffraction.

XRD peaks and corresponding 2θ values of pure PP, PPST-15, 20, and 25 were found to be around 14–16°, 21–23°, and 29–31° which corresponds to the interplanar spacing along (110) plane, (200) plane, and (040) plane, respectively. Larger d-spacing found in all these bio-composites compared to the pure PP suggests less dense packing of starch molecules within the crystalline structure. Furthermore, the smaller interplanar spacing in the bio-nanocomposite PPSTTL 2001, 2003, and 2005 indicates the more orderly distribution of starch and delaminated nano-talc molecules within the crystalline structure of the PP matrix; this can also be verified from the crystal size of the starch-based composite and delaminated nano-talc-filled bio-nanocomposite in Table 2. The relative decrease in XRD peaks of the bio-nanocomposite indicates that some nanoparticles may have been exfoliated. Even though FTIR and XRD spectral measurement methods are volume dependent, i.e., the intensity of a given peak of a substance is proportional to the volume fraction of the substance in the sample, still, comparison of the subtracted graph is not common in XRD analysis, which is in contrast to FTIR analysis. Results indicate that the intensity of diffraction peak at 90 due to nano-talc is highest for PPSTTL 2003 (i.e., with 3% loading) in Figure 2(b). This indicates the highest level of dispersion with a 3% loading of nano-talc in bio-composites with 20% thermoplastic starch. The same was also confirmed by improved thermal and mechanical properties of PPSTTL 2003 bio-nanocomposites.

4.3 UV–Vis spectrophotometry

Pure PP and its bio-composites, i.e., PPST-15, 20, and 25, were subjected to UV-Viso spectrophotometry, and the absorbance spectrum is presented in Figure 3(a) for analysis.

Figure 3

(a) UV-Visio absorbance spectrum of Pure PP, PPST-15, 20, and 25. (b) UV-Visio absorbance spectrum of PPSTTL 2001, 2003, and 2005.

The absorbance spectrum was analyzed in the UV short wave range 200–280 nm, middle wave range 280–315 nm, and long wave range 315–400 nm for the effect of increasing thermoplastic starch content on the UV absorption property of the bio-composites. Thermoplastic starch is known to be hydrophilic and possesses photosensitizer properties [37]; both this property causes the starch molecules to absorb energy possessed by UV lights in all three UV ranges, and this energy ultimately gets transferred to the PP matrix. UV range absorbance spectrum for PPST-15, 20, and 25 is shown in Figure 3(a) indicating its increasing absorbance of UV light in all three UV ranges; this can directly be linked to the increasing concentration of the starch in the PP matrix [37]. Increased absorption of UV energy by all the bio-composite may result in breakage of the chemical bonds of the PP matrix and, thus, faster degradation of the material upon UV exposure. To compensate for the increasing absorption of UV energy by the bio-composite, delaminated nano-talc was included as filler in the bio-composite of PP and starch. PP composite having 20% starch concentration exhibited better mechanical and thermal properties than other bio-composites having 15 and 25% starch filler; thus, nano-talc was infused into it at concentrations of 1% (PPSTTL 2001), 3% (PPSTTL 2003), and 5% (PPSTTL 2005), and the specimen was subjected to UV–Vis spectrophotometry; thermal and mechanical studies of the bio-nanocomposite have also been done in the later part of this study and not included in this section. UV-Visio absorbance spectra of PPST-20, PPSTTL 2001, 2003, and 2005 are presented in Figure 3(b).

Figure 3(b) reveals increased UV absorbance of PPSTTL 2001 when compared to that of PPST-20, whereas the same was found to decrease for the PPSTTL 2003 and PPSTTL 2005; the lowest absorbance in the entire UV range was found to be of PPSTTL 2003 which contained 3% talc filler. Talc is characterized by its property to take part in absorbing some UV radiation and, simultaneously, is also characterized to act as a nucleating agent when used below 3% concentrations and act as a reinforcement filler when used at higher concentrations [26]. From Figure 3(b), it can be inferred that bio-nanocomposite with delaminated nano-talc concentration of 3% acted as a nucleating agent for both starch molecules and polymer matrix, thus reducing the amount of UV radiation penetrated in the PP matrix, hence slowing down the UV induced degradation process [26,41,42]. Large crystals scatter incident lights in a way different from that of smaller crystal size, thus influencing their optical behavior. In this study, small crystal size and smaller interplanar spacing (d-spacing) within the crystal structure of bio-nanocomposite PPSTTL 2001, 2003, and 2005 when compared to that of PPST-15, 20, and 15 indicated more uniformly and orderly distribution of delaminated nano-talc molecules within the PP matrix. High orientation in the crystal chain of the bio-nanocomposite may have led to the birefringence of incident light and, thus, an obvious reduction in its UV absorbance.

4.4 DSC studies

DSC thermogram in Figure 4(a)–(d) represents the thermal behavior of pure PP, PPST-15, 20, and 25, respectively; this curve indicates their melting and crystallization phases during the heating and cooling cycle, respectively. The results extracted from the thermogram are summarized in Table 3.

Figure 4

DSC thermogram of (a) pure PP, (b) PPST-15, (c) PPST-20, and (d) PPST-25.

DSC thermogram and its summary in Table 3 reveal that PPST-20 exhibited the highest melting temperature and enthalpy of 167.6°C and 117.3 J/g, but the lowest crystallization temperature of 129.5°C when compared to pure PP, PPST-15, and PPST-25. This is indicative of enhanced processibility and improved mechanical and thermal stability offered by PPST-20. The higher melting temperature of PPST-20 indicates a higher degree of crystallinity in PPST-20, which is indicative of the presence of starch as an additive in the PP matrix. The area under the melting peak was found to be highest in PPST-20; this is indicative of increased heat of fusion, which can directly be related to the contribution of starch as an additive in increasing the crystallinity of bio-composite. Another DSC thermogram in Figure 5(a)–(d) represents the thermal behavior of PPST-20, PPSTTL 2001, 2003, and 2005 while being in the heating and cooling cycle. Data obtained from the thermogram are summarized in Table 4.

Figure 5

DSC thermogram of (a) PPST-20, (b) PPSTTL 2001, (c) PPSTTL 2003, and (d) PPSTTL 2005.

DSC thermogram and its summary in Table 4 reveal that PPSTTL 2003 exhibited the highest T _m among the other bio-nanocomposites, but the same was almost equal to that of PPST-20; however, a significant reduction in the enthalpy of PPSTTL 2003 and 2005 was found when compared to PPST-20 and PPSTTL 2001. Reduction in enthalpy can be attributable to the action of de-laminated nano-talc as a nucleating agent which increased the crystallization rate upon its infusion as nano-filler in PPST-20. Lowering of enthalpy in PPSTTL 2003 and 2005 indicates an increase in thermal stability, and it can lead to better dimensional stability and better stiffness of products developed by these polymeric bio-nanocomposites. The melting and crystallization behavior of bio-composite PPST-20 and bio-nanocomposite PPSTTL 2003 indicates that thermoplastic starch when acting alone in the PP matrix was serving the purpose of filler; however, infusion of delaminated nano-talc as a nucleating agent in the PP matrix improved the crystallization rate, thus reducing the total energy absorbed by the bio-nanocomposite.

4.5 TGA studies

During TGA testing, samples were heated from 30 to 750°C in the presence of nitrogen; TGA thermogram of the pure PP, PPST-15, 20, and 25 and PPST-20, PPSTTL 2001, 2003, and 2005 was obtained and is presented in Figure 6(a) and (b), respectively, and data from the thermogram are summarized in Tables 5 and 6.

Figure 6

(a) TGA thermogram of pure PP, PPST-15, 20, and 25. (b) TGA thermogram of PPST-20, PPSTTL 2001, 2003, and 2005.

TGA thermogram and the corresponding summary table reveal that the bio-nanocomposite lost its moisture content between 30 and 200°C. Pure PP lost 10% of its weight around 416°C, whereas starch filling increased this temperature to 445–448°C for bio-composite, which was further elevated between 449 and 453 upon the addition of delaminated nano-talc. PP usually loses its significant weight above 300°C, this was verified by 90% weight loss of pure PP at 500°C, but a significant decrease in % decomposition (78–85% for bio-composite and 68–80% for bio-nanocomposite) at this temperature was observed considering the fusion of thermoplastic starch and nano-talc in it. TGA studies reveal the enhanced ability to sustain thermal stress to a great extent by the addition of nano-talc in the PPST-20.

4.6 Tensile strength and % elongation

Tensile strength and elongation of pure-PP and its bio-composite PPST-15, 20, and 25 and bio-nanocomposite PPSTTL 2001, 2003, and 2005 were evaluated and presented as a bar graph in Figure 7(a) and (b), respectively.

Figure 7

(a) Tensile strength of pure PP, bio-composite, and bio-nanocomposite. (b) % elongation of pure PP, bio-composite, and bio-nanocomposite.

The bar graph reveals the tensile strength of PPSTTL 2003 and PPST-20 to be 28.32 and 29.27 MPa, respectively, which is comparable to that of 30.72 MPa of pure PP. As far as % elongation of bio-nanocomposite is concerned, a significant decrease was observed in all the specimens; however, PPSTTL 2003 and PPST-20 exhibited the lowest decrease in elongation, i.e., 1.03 and 2.24%, respectively, and the same was confirmed by the DSC studies also. A higher value of tensile strength of PPSTTL 2003 and PPST-20, but the reduction in the % elongation can be attributable to the high crystalline nature and faster crystallization behavior of materials as is verified in the thermal and XRD studies.

4.7 Flexural properties

Flexural strength and flexural modulus of the pure PP and its bio-composite PPST-15, 20, and 25 and bio-nanocomposite PPSTTL 2001, 2003, and 2005 are presented as bar graphs in Figure 8(a) and (b), respectively.

Figure 8

(a) Flexural strength of pure PP, bio-composite, and bio-nanocomposite. (b) Flexural modulus of the pure PP, bio-composite, and bio-nanocomposite.

The bar graph reveals an expected decrease in the flexural strength but an increase in the flexural modulus of bio and bio-nanocomposite. The decrease in flexural strength of bio and bio-nanocomposite can be attributable to the poor interface and hydrophilic nature of starch; however, talc content of 3% in PPSTTL 2003 seems to have induced higher flexural strength and flexural modulus, attributable only to its function as nucleating agent. Here, de-laminated talc acts as the nucleating center, which was confirmed by the DSC studies also; this is further verified by a reduction in the enthalpy of the bio-nanocomposites. Thus, bio-nanocomposites developed in this study exhibit better processibility, enhanced mechanical properties, better thermal stability, and very good dimensional stability.

4.8 Melt flow properties

The MFI of the pure PP and its bio-composite PPST-15, 20, and 25 and bio-nanocomposite PPSTTL 2001, 2003, and 2005 are presented as bar graphs in Figure 9.

Figure 9

MFI of pure PP, bio-composite, and bio-nanocomposite.

The bar graph revealed almost linear decreases in the MFI of starch-based bio-composite upon increasing starch content; this is in line with the expected starch behavior due to its poor interface and hydrophilic nature, but, infusion of de-laminated nano-talc in PPST-20 increased its MFI due to its nucleating action. Infusion of talc in the PPST-20 increased the processability of bio-composite, thus making it more suitable for industrial processability. Results indicate that bio-filled nanocomposites when compared with inorganic fillers induce reduced friction in the machine, thus increasing the life cycle of the screw and barrel of the twin-screw extruder.