Characteristics of polypropylene-based antistatic bio-nano composites reinforced with mono-diacylglycerols and cellulose nanocrystals

Polypropylene (PP) is known as a polymer without antistatic properties that is susceptible to the use of high temperatures. Therefore, to improve the thermal and antistatic properties of PP, it is necessary to modify PP to antistatic bio-nanocomposites with mono-diacylglycerols (M-DAG) as an antistatic agent and cellulose nanocrystals (CNC) as a reinforcement. This research aimed to characterize the electrical resistivity and thermal properties of PP-based antistatic bio-nanocomposites reinforced with M-DAG and CNC at different concentrations of CNC (0%–5%), and 2% of M- DAG, compared to pure PP. The results showed that the addition of 2% CNC (AS-BNC-2) gave the melting temperature of 125.0 °C, which was higher than pure PP of 118.3 °C. The thermal stability of the antistatic bio-nano composites with 3% CNC (AS-BNC-3) was 457.10 °C, which was higher than pure PP of 441.56 °C. The electrical resistivity of the antistatic bio-nano composites from all treatments was still in the range of the antistatic category of 1010–1012 Ω/sq. The melting temperature and thermal stability of bio-nano composites were higher than those of pure PP and they have antistatic properties. This indicates the potential application of these materials in the electronics devices and packaging industries.


Introduction
Polypropylene (PP) is one of the most widely used thermoplastic polymers in various industries.However, PP is known as a polymer that is susceptible to the use of high temperatures and does not have antistatic properties.Besides that, PP is harmful to the environment due to its non-degradable nature [1,2], but PP is recyclable [3].In this work, Antistatic bio-nano composites (AS-BNC) reinforced by mono-diacylglycerols (M-DAG) as an antistatic agent [4] and cellulose nanocrystals (CNC) as a reinforcement [4][5][6][7][8][9], where PP serves as the matrix.These biomaterials have been investigated in systems and materials aspects for exploring and choosing new suitable materials for future applications.
M-DAG is a mix of mono-acyl glycerol (MAG) and di-acyl glycerol (DAG) [10] which has one or two free hydroxyl groups at glycerol which is unesterified with fatty acid.The presence of free hydroxyl groups makes it a biocompatible and biodegradable surfactant or emulsifier, and it is widely used in foods, cosmetics, and pharmaceuticals [4].M-DAG can be produced from palm fatty acid distillate (PFAD), a by-product of crude palm oil refining [11][12][13].
CNC is a cellulose-based nanomaterial that has better mechanical characteristics such as tensile strength (7.5 GPa) [14], tensile modulus (100-140 GPa) [14], and thermal stability (283.5 °C) with a surface area of 569 m 2 g −1 [15] compared to other cellulose-based nanomaterials such as cellulose nanofiber (CNF) which has 0.38 GPa of tensile strength [16] and 23.9 GPa of modulus tensile [16] with a surface area of 430 m 2 g −1 [17].However, CNC has a low stability (>280 °C) which causes it very susceptible to high heat treatment when used as reinforcing material in polymer matrices.Therefore, a stabilizer is needed that can protect CNC from thermal degradation during the process.M-DAG can act as a stabilizer and lubricant so that the processing time is shorter and reduces thermal degradation.
The addition of M-DAG as an antistatic agent [4] and CNC as a reinforcement [4][5][6][7][8][9] to the PP as a thermoplastic matrix [4][5][6][7][8][9] had a positive impact on the characteristics of the resulting PP-based antistatic bionano composites and PP-based bio-nano composites [4][5][6][7][8][9].The combination of the added M-DAG and CNC is expected to produce a synergistic effect to improve the quality of the antistatic bio-nano composites.Adriana et al [18] reported the addition of CNC and glycerol monostearate (GMS) as an antistatic agent on a polystyrene (PS) matrix and showed a positive effect on PS-based antistatic bio-nano composites.However, there are no reports regarding the use of these two materials simultaneously in PP matrix.In addition, the use of M-DAG synthesized from palm fatty acid distillates (PFAD), a byproduct from palm oil refinery, is of particular interest, because it increases the added value of this material.
Presumably, the addition of M-DAG and CNC will improve the electrical resistivity and thermal properties of PP-based antistatic bio-nano composites.Therefore, this study aimed to identify the electrical resistivity and thermal properties of composites at different concentrations of CNC compared to that of pure PP.

Material and method
The main raw materials for the synthesis of antistatic bio-nano composites were CNC, M-DAG, and PP (see figure 1 and table 1).Then, the supporting materials were maleic anhydride polypropylene (MAPP), antioxidant (AO), and mineral oil (MO).Main and supporting materials were determined based on the initial identification results that have been carried out by Sarfat et al [19] and the results of other relevant studies [5-7, 9].In addition, this is also based on materials commonly used in the plastics industry such as PT Intera Lestari Polimer, where  this research was conducted.MAPP, AO, and MAU were used for synthesis with the purposes listed in the table 2.

Antistatic bio-nano composites production
Synthesis of antistatic bio-nano composites is carried out in three stages, namely the masterbatch process (including the mixing, air knives, and pelletizing process), the moulding process, and the cooling process.This process is commonly used in the commercial industry to produce plastic-base products.Therefore, this process was adapted for plastic composite synthesis in this research.Figure 2 shows the steps of the synthesis of antistatic bio-nano composites.A mixture of PP, M-DAG, CNC, MAPP, AO, and MO was extruded using a twin-screw extruder at a temperature of 160 °C-220 °C and a screw speed of 150 rpm to produce a product in the form of a masterbatch pellet.The main raw materials formulation for the synthesis of antistatic bio-nano composites (M-DAG, CNC, and MAPP) which entered the masterbatch process determined based on practices in industry and trial-error.The formulation of supporting raw materials (MAPP, AO, and MO) was determined based on the standard use of each ingredient contained in the Technical Data Sheet.
The masterbatch pellets were injected using an injection molding machine at a temperature of 160 °C-220 °C to produce antistatic bio-nano composite sheets at a concentration of 0%-5% CNC and 2% M-DAG with dimensions of 5 × 8 cm.The antistatic bio-nano composites sheets cooled to room temperature of 25 °C, with a pressure of 3 MPa for ±24 h at 50% humidity before finally characterizing the antistatic bio-nano composites.Table 3 shows the symbols of each treatment.The formulation of each treatment was based on the ratio of molarity concentration of CNC with maleic anhydride (MA) in MAPP, which was 1:1.It is expected that the availability of 1 mole of MA can bind to 1 mole of CNC in the form of a single ester bond during the extruding process.
In this study, the range of CNC concentration referred to the results of previous studies conducted by Adriana et al [18], while the determination of 2.00% M-DAG concentration referred to Salsabila et al [4].For supporting materials, the concentration was based on the usage dosage listed on the technical data sheet of each supporting material.

Antistatic bio-nano composites characterization
Characteristics of the antistatic bio-nano composites consist of thermal properties analysis using differential scanning calorimetry (DSC), melt flow index (MFI), electrical resistivity, and visual analysis.
Thermal properties as indicated by the DSC curves were obtained from 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 −1 .The crystallization and melting behaviours were recorded in a nitrogen atmosphere, at the range mass used of 21.80 to 29.00 mg.
MFI method was ASTM D-1238.Measurements were carried out at a temperature of 230 °C with a weight of 2.16 kg.The extrude limit time interval setting is one minute for all samples.Five replicates of extruding were collected for each sample and weighed close to 1 mg.MFI is defined in standard units of g/10 min.
The electrical resistivity method was ASTM D-257-99 (Advantest R8340 Ultra High Resistance Meter).The electrical resistivity of antistatic bio-nano composites (before and after exposure to room temperature for 1-7 weeks) was determined using equipment 1 [20].The thickness of the specimen, test voltage, and test time were 1 mm, 500 V, and 1 min, respectively.
where s r is the surface resistivity (Ω/sq), Rs is the surface resistance (ohms), π is the ratio of the circumference of a circle to its diameter equal to 3.14, D is the inner diameter of the protective electrode (cm), d is the diameter of the main electrode (cm).
Finally, visual characterization of antistatic bio-nano composites was carried out to identify colour, odour, and texture using visual observation.

Results and discussion
3.1.Thermal properties and melt flow index analysis DSC is a popular analytical technique for investigating the thermal properties of nanocomposites.Thermal characteristics determined by DSC were melting temperature (T m ), melting enthalpy (ΔH m ), peak temperature (peak max), peak height, onset temperature (T onset ), offset temperature (T offset ), and thermal degradation (T inf ).Thermal properties results of antistatic bio-nano composites can be seen in figure 3, table 4, figure 4, and table 5.
Based on the results of thermal properties (figure 3 and table 4), the melting temperature of the antistatic bionanocomposites AS-BNC-2 was 177.40 °C higher than other antistatic bio-nano composites but lower than pure PP 181.10 °C with a decrease of 2.96%.This indicates that there is an opportunity to decrease the melting temperature with increasing CNC concentration.According to Al-Haik et al [21], the melting temperature of bio-nano composites with the addition of 2.00%, 4.00%, and 5.00% CNC on the PP matrix showed a greater value when compared to pure PP and the addition of 4.00% CNC had the higher melting temperature, but there was a decrease in melting temperature with the addition of 3.00% CNC.According to Yousefian and Rodrigue [22], the distribution of CNC particles in the polymer matrix greatly influences the thermal properties of the resulting bio-nano composites.
The use of M-DAG and CNC simultaneously on the PP matrix, of course, will form a unique pattern of interaction or reaction which can certainly have an impact on the thermal properties of antistatic bio-nano composites.A similar case was found by Adriana et al [18].This causes the highest melting temperature of antistatic bio-nano composites AS-BNC-2 treatment (addition of 2.00% CNC and 2.00% M-DAG).Based on the results of thermal degradation (figure 4 and table 5), the thermal stability of the antistatic bionano composites treated with AS-BNC-0, AS-BNC-2, AS-BNC-3, and AS-BNC-5 were 456.75 °C, 456.50 °C, 457.10 °C, and 452.50 °C higher than that of pure PP with an increase of 3.33%, 3.27%, 3.40%, and 2.36% respectively.However, when compared with AS-BNC-1, it had lower thermal stability than pure PP with a decrease of 6.32%.Therefore, the addition of 3.00% CNC and 2% M-DAG to the PP matrix can increase the thermal degradation of the resulting antistatic bio-nano composites.According to Al-Haik et al [17], the thermal stability of bio-nano composites with the addition of 1.00%, 2.00%, and 3.00% CNC on the PP matrix showed a greater value when compared to pure PP and the addition of 4.00% and 5.00% CNC, with the highest thermal stability on the addition of 3.00% CNC.CNC particles are thought to increase the thermal resistance of antistatic bio-nano composites 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 [23,24].In addition, the presence of M-DAG can inhibit the thermal degradation of the antistatic bio-nano composites.The reduced thermal resistance in the antistatic bio-nano composites with the treatment of 2.00% M-DAG, and 1.00% CNC may have been due to the non-uniform dispersion of the CNC particles [23].
This also happens to the melting temperature of antistatic bio-nano composites, where the addition of 3.00% CNC and 2.00% M-DAG to the PP matrix can increase the thermal degradation of the resulting antistatic bio-nano composites.The use of M-DAG and CNC simultaneously on the PP matrix, of course, will form a unique pattern of interaction or reaction which can certainly have an impact on the thermal properties of antistatic bio-nano composites.
Based on the results of MFI analysis (table 6), the MFI of AS-BNC-0 antistatic bio-nano composites was lower than pure PP with a reduction of 10.70%.While the antistatic bio-nano composites with the treatment of AS-BNC-1, AS-BNC-2, AS-BNC-3, and AS-BNC-5 had higher MFI values than pure PP with an increase of 21.75, 39.85, 24.62, and 40.39%, respectively.M-DAG has a lower melting temperature value than pure PP, so the addition of M-DAG to the PP matrix can reduce the MFI value of the produced antistatic bio-nano composites.On the other side, CNC has a higher melting temperature than pure PP, so the addition of CNC to the PP matrix can increase the MFI value of the antistatic bio-nano composites.The specific gravity is also influenced by CNC concentration, the higher CNC concentration, the lower the specific gravity of antistatic bionano composites.This showed that the density of antistatic bio-nano composites was getting smaller possibly because the CNC molecules can form a three-dimensional structure among the PP molecules.

Electrical resistivity analysis
The electrical resistivity analysis of antistatic bio-nano composites was carried out at regular intervals of 0, 7, 14, 21, and 30 days (table 7 and figure 5).Based on the results of the electrical resistivity of antistatic bio-nano composites, the values were in the range of the antistatic category of 10 10 -10 12 Ω/sq.In addition, the surface resistivity value of antistatic bio-nano composites was the static dissipative value range of 10 6 -10 12 Ω/sq which has the potential used for electrostatic discharge prevention or the presence of a sudden electric current caused by an electric short circuit, a dielectric fault, or the contact between two electrically charged objects [25].In general, the polymer matrix is highly insulated.Therefore, the presence of electrically conductive nanomaterials with a large aspect ratio dispersed in small quantities can drastically increase the electrical conductivity to a level that can support use for electrostatic discharge protection [26].Adriana et al [18] reported that the electrical resistivity of PS nanocomposite decreased sharply with the addition of glycerol mono stearate (GMS) up to 3.00%, and was almost stable until the addition of 5% GMS.Meanwhile, at the same concentration of GMS (3.00%), the electrical resistivity of PS nanocomposite decreased with the addition of a not-so-much variation of the CNC concentration.

Proposed mechanism of interaction between M-DAG and CNC on PP matrix
The interaction between M-DAG and CNC on the PP matrix begins with the interaction of CNC on the PP matrix-assisted by MAPP.The presence of MA makes CNC able to bind to the PP matrix through ester bonds (see figure 6).The presence of the -OH group on the CNC provides an opportunity to graft polymer chains via ester linkages in MAPP.The -OH group is very susceptible to reacting with anhydrides at high temperatures.Consequently, in the modification of several cellulose derivative products, this process has been widely used [27].
Another possibility occurs in the interaction between M-DAG and CNC on the PP matrix during the antistatic bio-nano composites synthesis process, that there is 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, while the polar groups (glyceryl) are oriented away from the PP matrix towards the antistatic bio-nano composites surface, and it is possible to physically interact with CNC.Another possibility is that the polar group (glyceryl) reacts with the O group of the maleic anhydride during the synthesis process.
In the second stage, M-DAG will be distributed evenly on the PP matrix reinforced with CNC during the extrusion and moulding process.The hydrophilic groups in M-DAG will move toward the PP surface reinforced with CNC and form a conductive layer that develops antistatic property (see figure 7).M-DAG which is added to  the PP reinforced with CNC during the extrusion process would work 24-48 h after the extrusion process to migrate to the surface of the PP reinforced with CNC, forming a hygroscopic film that attracts water, so that has a conductive function, as it discharges static electricity and reduces the level of plastic charge [28].
Based on the initial indication, M-DAG acts as a barrier to thermal degradation during the production process.This was demonstrated during the antistatic bio-nano composites synthesis process, where at the same CNC concentration (15.00%), without the addition of M-DAG to the raw material mixture, there was no formation of masterbatch pellet because the extruded product was crushed and charred to a dark black color.Meanwhile, the addition of 5.00% M-DAG showed the formation of extruded pellets, although the color was dark brown (see figure 8).
The thermal degradation inhibition by M-DAG is probably because of the presence of a free hydroxyl group in M-DAG, and makes it function as a heat stabilizer [4] and lubricant to shorten the synthesis time of antistatic bio-nano composites.Of course, the presence of free hydroxyl groups on M-DAG will protect the surface materials of CNC and PP from exposure to excessive heat.However, using too high a temperature can reduce the availability of free hydroxyl groups on M-DAG.This can have an impact on reducing the antistatic properties of antistatic bio-nano composites.

Visual analysis of antistatic bio-nano composites
Visual analysis of antistatic bio-nano composites was carried out to identify colour, odour, and texture using visual observation (see figure 9).Based on colour analysis, the antistatic bio-nano composites have a darker colour (brown to dark) along with the high concentration of CNC used.This is due to degradation by sulfates contained in CNC [29].Therefore, pre-treatment or modification of the CNC surface and temperature control during the synthesis process need to be considered, so the thermal degradation can be minimized.According to Shojaeiarani et al [30], surface modification of CNC sometimes can substantially affect the thermal characteristics of native CNC.Generally, functional groups are attached to the CNC surface via surface modification.These functional groups can change the thermal stability and crystallization behaviour of native CNC due to chemical reactions.Based on odour analysis, the antistatic bio-nano composites have a slightly woody and fatty smell compared to odourless pure PP.It probably comes from CNC and M-DAG.Meanwhile, based on the texture analysis, the antistatic bio-nano composites have a coarser texture along with the high concentration of CNC used.The slightly rough texture of antistatic bio-nano composites is possibly caused by friction between CNC and the moulding surface because CNC has a very hard texture [14] and high surface area [15].Therefore, it is necessary to pay attention to the odour and texture of the bio-nanocomposite because it affects the acceptability of products made with these materials.
Based on the characteristics of the antistatic bio-nano composites produced, indicate the reliability of antistatic bio-nano composites for application in the industry such as electronics industries.In particular, this material can be used as electronic device packaging with the category of electronic device packaging level 5 (outer casing or enclosures), which protects a single electronic device, because of their superior electrical, thermal, and mechanical properties [31].However, to ensure its reliability as an electronic device packaging, it is necessary to carry out further verification.

Conclusion
Antistatic bio-nano composites reinforced by CNC and M-DAG (AS-BNC-2) showed a melting temperature of 125.00 °C higher than pure PP of 118.30 °C with an increase of 6.42%.The melting temperatures of AS-BNC-0, AS-BNC-1, and AS-BNC-3 were also higher than that of pure PP with an increase of 2.20%, 1.44%, and 0.59%, respectively.However, the addition of a higher CNC concentration (5.00%) can reduce the melting point.The thermal stability of the antistatic bio-nano composites AS-BNC-0, AS-BNC-2, AS-BNC-3, and AS-BNC-5 were higher than that of pure PP with an increase of 3.33%, 3.27%, 3.40%, and 2.36%, respectively.The electrical resistivity value of the antistatic bio-nano composites from the different concentrations of CNC were still in the range of the antistatic category of 10 10 -10 12 Ω/sq.The addition of M-DAG can reduce thermal degradation due to its nature as a lubricant in the moulding process.
The findings in this study show that the melting temperature, thermal stability, and electrical resistivity of antistatic bio-nano composites are better than those of pure PP.This indicates potential applications in the electronics and packaging industries.In addition, the use of natural materials, M-DAG and CNC, is expected to increase the biodegradability of antistatic bio-nano composites.Optimization of M-DAG and CNC concentration should be done further, completed with morphological analysis using scanning electron microscopy (SEM) to understand the morphology and surface topography of antistatic bio-nano composites.

Figure 1 .
Figure 1.Physical of the potential raw materials for the synthesis of antistatic bio-nano composites.

Figure 2 .
Figure 2. The steps of the synthesis of antistatic bio-nano composites.

Figure 7 .
Figure 7. Interaction between M-DAG and CNC on PP matrix.

Figure 8 .
Figure 8. Visuals of PP-based antistatic bio-nano composites pellet with the addition of 15% CNC and 5% M-DAG.

Table 1 .
Characteristics, function, and suppliers of the potential raw materials for the synthesis of antistatic bio-nano composites.

Table 3 .
Symbols of each treatment.

Table 4 .
DSC analysis of antistatic bio-nano composites.

Table 5 .
Thermal degradation of antistatic bio-nano composites.

Table 6 .
MFI and specific gravity analysis of antistatic bio-nano composites.