Polypropylene-based Bio-nano Composites Reinforced with Cellulose Nanocrystals and Added with Mono-diacylglycerols as Antistatic Agent

This research aims to identify the performance and mechanical characteristics of polypropylene-based antistatic bio-nano composites reinforced with 2% mono-diacylglycerols (M-DAG) and 0, 0.5, 2.5% cellulose nanocrystals (CNC) and compared to pure PP. Based on the results of SEM on cross-section, there was an agglomeration of M-DAG and CNC on the PP matrix. XRD diffractogram pattern of antistatic bio-nano composites showed peaks representing the diffraction structure of glycerol monostearate and cellulose I. The FTIR spectrum of each formulation was very similar to the characteristic peaks of PP and showed three distinct peaks compared to pure PP.  The melting temperature of antistatic composites without CNC (176.54^oC) was higher than pure PP (175.44^oC). Thermal stability of antistatic bio-nano composites with 0, 0.5, and 2.5% CNC were 472.07, 470.25, and 475.15 ^oC, respectively, higher than pure PP (468.27^oC). The best mechanical properties were 2.5% CNC with 11.071 MPa flexural modulus, 30.376 MPa tensile strength, 23.796% tensile elongation, 1.659 J/m² impact strength, which were higher than pure PP, and it generated antistatic activity of 10¹⁰ - 10¹² /sq resistivity.

Antistatic bio-nano composites; Biopolymers synthesis; Cellulose nanocrystals; Mono-diacylglycerols; Polypropylene

M-DAG is an ester of glycerol and free fatty acid (FFA) which has unreacted or free hydroxyl groups. MAG, or monoglyceride, has a single fatty acyl chain, while DAG, or diacylglyceride, has two fatty acyl chains (Sarfat et al., 2022). This free hydroxyl group makes M-DAG a non-ionic surfactant that is degradable and bio-compatible, so it is widely used in the food, cosmetic, and pharmaceutical industries. This free hydroxyl group allows M-DAG to be used as a stabilizer and an antistatic agent in bio composites or plastics (Salsabila et al., 2021). M-DAG used in this research was produced from palm fatty acid distillates (PFAD) from the refining process of crude palm oil (CPO) (Setyaningsih, Suwarna, and Muna, 2020; Setyaningsih et al., 2020).

CNC is a cellulose-based nanomaterial that has better mechanical characteristics such as tensile strength (7.5 GPa) (Tang et al., 2017), tensile modulus (100 - 140 GPa) (Tang et al., 2017), high surface area (569 m²/g) (Brinkmann et al., 2016), with a diameter average of 8 nm (Rochardjo et al., 2021) compared to other cellulose-based nanomaterials such as cellulose nanofiber (CNF) which has a tensile strength of 0.3833 GPa (Kafy et al., 2017), a tensile modulus of 23.9 GPa (Kafy et al., 2017) and surface area of 430 m²/g (Moser, Henriksson, and Lindström, 2016). However, there are disadvantages of CNC, namely low stability starting from 283.55 ⁰C, which causes CNC to be very susceptible to high heat treatment when used as a reinforcing material in polymer matrices. M-DAG can be used as a lubricant and stabilizer to protect CNC from thermal degradation during processing.

Therefore, a combination of M-DAG and CNC as additive materials for the synthesis of PP-based antistatic bio-nano composites has a prospect as next-generation material that is more flexible in use and exhibits superior performance and mechanical characteristics compared to pure PP. There has never been researched that combines M-DAG and CNC simultaneously as reinforcement in the PP matrix. Therefore, this study aims to evaluate the performance and mechanical characteristics of PP-based antistatic bio-nano composites reinforced with varying concentrations of M-DAG and CNC, and compare them to those of pure PP.

2.1. Raw Materials    

        The raw materials for synthesizing antistatic bio-nano composites were PP, M-DAG, and CNC. PP (PT Chandra Asri Petrochemical Tbk) has a melt flow index of 10 g/10 minute and a density of 0.903 g/cm³. M-DAG (SBRC-LPPM-IPB) has a crystallinity index of 92.85%, a diameter of 0.11–1.78 nm, and a thermal degradation rate of 200.50 ^oC. CNC (CelluForce Co.) has a density of 1.5 g/cm³, a crystallinity index of 98.95%, and diameter of 3.39–12.72 nm, a length of 44 – 108 nm, and thermal degradation of 296.96 ^oC. The supporting materials consist of maleic anhydride polypropylene (MAPP)(BYK Chemie GmbH), antioxidant (AO) (BASF Schweiz AG, Switzerland), and mineral oil (MO) (Arkema France).

2.2. Methodology

Figure 1 The Stages of the synthesis of antistatic bio-nano composites

2.2. Antistatic Bio-nano Composites Characterization

    Morphological analysis used scanning electron microscopy (SEM). The SEM used was JEOL Standard on JSM-6510LA (electrical image shift  WD = 10 mm), resolution mode 10 kV and 20 kV). Degree of crystallinity analysis used X-ray diffraction (XRD), Labx - XDR 600 equipment from Shimadzu with radiation in the range between 0^o to 80^o, rate scanning of 2^o/min and the total exposure time of 40 min.  To identify crystal peaks, degree of crystallinity, and amorphous composition, we used Mach! Version 3.11.3.192 and the Crystallography Open Database (COD) inorganic reference database. The crystal size (nm) perpendicular to the lattice plane (002) of cellulose was determined using the Scherrer equation (Saleh, 2021), as calculated in Equation 1. 

where is the crystal size in the perpendicular direction of the hkl lattice plane, K is the Scherrer constant (0.94),is the X-ray wavelength (1.54 A^o), is the full width at having the greatest according to the hkl peak in radians, and is the angle Bragg.

Infrared spectrum analyzed using Fourier transform IR (FT-IR) Thermo Fisher Scientific Nicolet iS5 spectrophotometer with cleaning pump and wavelengths 300 cm^-1 to 4000 cm^-1, 128 accumulated scans, resolution 4 cm^-1, in ATR and transmittance module. Thermal properties analysis used differential scanning calorimetry (DSC) TA Instruments, New Castle, UK model Q200. Dynamic DSC scans were conducted in the temperature range from 23 to 400 °C at a heating rate of 10 °C/min. The crystallization and melting behaviors were recorded in a nitrogen atmosphere, at the range mass used of 21.8 to 29.0 mg.

where W is the corrected energy absorbed by breaking the specimen (J), is the thickness of the specimen (mm), and  is the width of the specimen (mm).

where  is flexure strength (N/mm2/MPa), is flexure modulus (N/mm2/MPa),  is breaking load (N), s is support span (mm), w is beam width (mm),  is the thickness of the beam (mm), and  is the deflection at the center of the span (mm).

where  is the surface resistivity (/sq), R_s is the surface resistance (ohms),  is the ratio of the circumference of a circle to its diameter equal to 3.14, R₁ is the outer radius of the center electrode (cm), R₂ is the inner radius of the outer ring electrode (cm).

3.1. Morphology Analysis

Figure 2 Morphological analysis of antistatic bio-nano composites using SEM

Based on the results of the SEM analysis on the cross-section, M-DAG agglomeration was found on the PP matrix because it shows a morphological form of M-DAG, while the CNC is not visible. This indicates a chemical reaction or physical interaction between M-DAG and the PP matrix. Physical interaction occurs when the polar groups (palmitate) are oriented to the PP matrix. In contrast, the polar groups (glyceryl) are oriented away from the PP matrix towards the antistatic bio-nano composite surface, and it is possible to interact physically with CNC. Another possibility is that the polar group (glyceryl) reacts with the O group of the maleic anhydride during the synthesis process. In addition, there was no fibrillation on the surface of the antistatic bio-nano composites cross-section. This implies that no significant plastic deformation occurred in the antistatic bio-nano composites layer during fracture, although the CNC concentration is increased. According to Shojaeiarani, Bajwa, and Chanda (2021),  the rheological properties of CNC-filled polymer melts depend on factors such as the degree of CNC particle dispersion, their orientation state, and aspect ratio. Consequently, the rheological properties of the antistatic bio-nano composites in our study were also influenced by these factors. In particular, the homogeneous dispersion of CNC particles led to more effective load transfer to the reinforcements.

3.2. Degree of Crystallinity and Particle Size Analysis

Crystallinity analysis was carried out on AS-BNC using XRD analysis to observe changes in the crystal structure as a function of chemical treatment and to measure the degree of crystallinity (CrI) using the deconvolution method with a ratio between the area corresponding to the crystal peak and the total area (Park et al., 2010). Figure 3A shows the XRD diffractogram of AS-BNC. XRD diffractogram patterns were recorded using Cu K irradiation,   = 1.5418 A.

The results of the XRD diffractogram analysis of AS-BNC showed with seven peaks with peak heights ranging from 87.09 – 1000 I rel of AS-BNC-5, 84.61 – 1000 I rel of AS-BNC-0.5, and 90.91 – 1000 I rel of AS-BNC-2.5 (Table 2). The XRD diffractogram pattern showed peaks representing the diffraction structure of glycerol monostearate and cellulose I. The diffraction characteristic of glycerol monostearate can be observed in the peak range between 5^o to 30^o (Yusuf et al., 2013). The diffraction characteristics of cellulose I could be observed around peaks at 15^o (001), 22.5^o (002), and 34^o (040) (Park et al., 2010), Couret et al. (2017) said the peaks at 15^o (1-10), 17^o (110), 21^o(102/012), 23^o (200), and 34^o (004) represent the diffraction structure of cellulose I.

The XRD diffractogram of PP showed the pattern, which has seven peaks with peak heights ranging from 85.41 – 1000 I rel. According to Guerra, Wan, and McNally (2019), the XRD patterns showed the most intense peaks for PP at  = 16.5^o  (100), 19.2^o  (300), 20^o  (040), 22^o  (130), and for the GNPs at  = 32^o (002).

The identification results of the particle size, degree of crystallinity, and percent of amorphous components in AS-BNC showed the average particle sizes between 15.84 to 16.01 nm, 89.81 to 91.10 % degree of crystallinity, and 8.90 to 10.19 % amorphous components (Table 2). The particle size of AS-BNC was increased with the increase in CNC concentration. The degree of crystallinity of AS-BNC-2.5 is higher than that of pure PP.

3.3. Infrared Spectrum Analysis

The FTIR spectrum of each treatment (Figure 3B) was very similar to the characteristic peaks of PP and depicted distinct tri peaks for the antistatic bio-nano composites compared to the pure PP. The first distinct absorption peak ranging from 1200 cm^-1 to 1000 cm^-1 were primarily assigned to C–O–C bond, C–C bending, and ring structures (with typical sharpening at 1071.50 cm^-1 with %T of 85.14% and 1166.98 cm^-1 with %T of 69.93% (AS-BNC-0), 1080.18 cm^-1 with %T of 83.03% and 1166.02 cm^-1 with %T of 64.77% (AS-BNC-0.5), and 1085.01 cm^-1 with %T of 74.86% and 1166.02 cm^-1 with %T of 51.02% (AS-BNC-2.5), which correspond to typical cellulose and glycerol compound (Al-Haik et al. 2020). In the band around 1080 cm^-1, the %T decreased as the concentration of CNC added to the PP matrix increased, while the %T in Pure PP was lower than each of the AS-BNC, which was 70.79%. This indicated the presence of an increasing C–O–C bond due to ring deformation of maleic acid compounds or ring widening and C–O stretching. The same phenomenon occurred in the band around 1166 cm^-1, which showed a decrease of %T with increasing concentration of CNC addition to the PP matrix, but lower than Pure PP (68.11%), except for AS-BNC-0. This indicated the presence of an increasing C–C bending.

The second distinct band is related to the wavelength near 2900 cm^-1. This broadband was assigned to stretching vibration of C–H hydroxyl groups asymmetric stretching of cellulose and glycerol. In this band, % transmittance (T) decreased as the concentration of CNC increased, while %T in pure PP was higher than AS-BNC. The presence of these bands confirmed the interaction of CNC and the distribution of M-DAG in the PP matrix. Due to this interfacial adhesion, the overall mechanical properties were enhanced for the AS-BNC. Hobuss et al. (2020) determined the asymmetric and symmetric C–H stretching mode of the fatty acid chain methylene group at 2922 cm^-1 and 2853 cm^-1. The third distinct peak was 3315 cm^-1, which is related to O–H [ (O–H)] stretching, a characteristic of hydroxyl groups. This band showed that %T decreased as the concentration of CNC increased, while %T of pure PP was higher than AS-BNC. Hobuss et al. (2020) set the O–H stretch at 3360 cm^-1. 

The bands at 1377.23 cm^-1 and 1457.28 cm^-1 in all treatments were characteristics of PP (Fang et al., 2012). The absorption bands at 1738.90 cm^-1 (Pure PP), 1742.76 cm^-1 (AS-BNC-0), 1738.90 cm^-1 (AS-BNC-0.5), and 1742.76 cm^-1 (AS-BNC-2.5) were observed, which were assigned to the absorption of carbonyl groups (C=O) of maleic anhydride (MA) (Rahman, Hassan, and Heidarian, 2018; Zhou et al., 2013). Finally, the spectrum on the peak 1166.02 cm^-1 indicated the C-C bending, which was the backbone of PP (Fang et al., 2012).

The infrared (IR) spectrum of the antistatic bio-nano composites revealed several characteristic peaks. Specifically, peaks between 3300 cm^-1 and 3250 cm^-1 were assigned to O–H stretching modes, while those between 3000 cm^-1 and 2750 cm^-1 corresponded to the stretching modes of CH, CH₂, and CH₃ groups. The peak at 1750 cm^-1 was indicative of carbonyl stretching (C = O). The peaks between 1500 cm^-1 to 1250 cm^-1 were characteristic of the deformation of the CH₂ and CH₃ groups. The peaks between 1250 cm^-1 to 1150 cm^-1 were referred as the C–O and C–C bonds. The peak at 1100 cm^-1 was also characteristic of the strain of the C–O bond and ester group (–C–O–C–). The "wag" vibration and asymmetric angular deformation of CH and CH₂ groups were found at 750 cm^-1 (Hobuss et al., 2020).

3.4. Thermal Properties and Melt Flow Index Analysis

Based on the identification results of the thermal properties (Figure 3C and Table 3), it was found that the melting temperature of AS-BNC-0 was 176.54^oC higher than that of pure PP 175.44 ^oC. The melting temperature of AS-BNC-0.5 and AS-BNC-2.5 were 171.70^oC and 174.38^oC, respectively, slightly lower than that of pure PP. However, when compared between AS-BNC-0.5 and AS-BNC-2.5, AS-BNC-2.5 had a higher melting temperature than AS-BNC-0.5. This indicated an opportunity for increasing melting temperature with an increase in CNC concentration. According to Al-Haik et al. (2020) and Hejna et al. (2017), the melting temperature of bio-nano composites with the addition of 2%, 4%, and 5% CNC on the PP matrix showed a higher value when compared to pure PP and the addition of 4% CNC had the higher melting temperature than 3% CNC. According to Yousefian and Rodrigue (2016), the distribution of CNC particles in the polymer matrix greatly influenced the thermal properties of the resulting bio-nano composites.

Thermal stability of AS-BNC-0, AS-BNC-0.5, and AS-BNC-2.5 was 472.07 ^oC, 470.25^oC, 475.15^oC respectively, higher than pure PP (468.27 ^oC). Therefore, the addition of 2.5% CNC and 2% M-DAG to the PP matrix can increase the thermal degradation of the resulting antistatic bio-nano composites. The higher the degree of crystallinity of the antistatic bio-nano composites, the higher the thermal stability, except for the antistatic bio-nano composites treated with AS-BNC-0. According to Al-Haik et al. (2020), the thermal stability of bio-nano composites with the addition of 1%, 2%, and 3 % CNC on the PP matrix showed a higher value when compared to pure PP and the addition of 4% and 5 % CNC. It showed that the addition of 3 % CNC has the highest thermal stability. CNC particles were thought to increase the thermal resistance of AS-BNC by inhibiting the diffusion of volatile decomposition products or by forming a charred CNC surface that dissipates heat by absorbing it in the inorganic phase (Thomas et al., 2018; Ng et al., 2017). In addition, the presence of M-DAG can inhibit the thermal degradation of AS-BNC. The reduced thermal resistance in AS-BNC with 2% M-DAG and 0.5% CNC may have been due to the non-uniform dispersion of the CNC particles (Ng et al., 2017).