Dynamic and thermo-mechanical properties of polypropylene reinforced with date palm nano filler

1 Introduction

An increasing focus on polymer composites based on natural fibers/fillers has been stimulated by the necessity of low carbon generation, sustainability consideration, combined with high performance that can be utilized for high engineering applications. Natural fibers/fillers reinforced polymer composites offer great potential in automotive, aerospace, prosthesis, sports and ballistic applications where light weight and high resistance are crucial parameters (Al Rashid, Khalid, Imran, Ali, & Koc, 2020; de Queiroz, Banea, & Cavalcanti, 2021; Malalli & Ramji, 2022; Mansor, Nurfaizey, Tamaldin, & Nordin, 2019; Marichelvam et al., 2021; Nurazzi et al., 2021). In fact, all major automotive manufacturers, including Toyota, Ford, Audi, Volkswagen, Volvo, Porsche, and McLaren, now using natural fiber reinforced polymers in non-structural body components for vehicles such as the under roof, instrument and door panels, sun visor, seat backs, oil air filters, boot liners, internal engine covers, and preforms (J. S. Neto, de Queiroz, Aguiar, & Banea, 2021).

Generally, plant fibers are classified based on the parts of the plant from which they are isolated, for example, bast, straw, leaf, seed, fruits and wood fibers. Several types of plant fibers such as flax, jute, sisal, kenaf, oil palm are already being well established and used in a various application. Additionally, the academic community continues to investigate other waste crop residues as fillers to determine their suitability for polymer reinforcement. One example is the date palm plant which is abundantly available in arid regions, and produces bast fibers that have been used for household applications since centuries. Aside from this, its high cellulose content, durability, and good tensile strength, make it unique plant parts (Faiad et al., 2022). Nonetheless recent studied have demonstrated that date palm agro residue can be used in high engineering applications due to their competitive technical characteristics (Benzidane et al., 2022).

Furthermore, the molecular and microscopic characteristics of the polymer matrix can be altered to varying degrees by the addition of these fillers. Similarly, among commodity polyolefins, polypropylene has appealing attributes such as low cost, a wide processing temperature range, thermo-oxidative stability, and resistance to various chemicals. Furthermore, analyzing and validating the characteristics and performance of natural filler-based composites are crucial, especially when such composites are subjected to periodic stresses such as damping. It is well known that dynamic mechanical analysis is widely used for determining such properties as a function of temperature, frequency and time by applying sinusoidal force on composite materials. The modulus of the polymer is divided in to storage modulus (É or G’) and loss modulus (É́ or G”). As temperature increases, the Éof polymers decreases while the É́and damping factor (tan δ) increases to the glass transition (Tg) point (Seth, Aji, & Tokan, 2018). The stiffness of the particular composite of polymer is known to responsible in decreasing É while É́is associated with energy dissipation induced by the viscous portion of the polymer (Haris et al., 2022). However, various factors such as composite’s morphology, heterogeneity, morphological transformation and relaxation causes internal molecular friction of the molecular chains and thereby effects dissipation of energy (J. S. Neto et al., 2021). Similarly, the damping factor (δ), which is calculated by dividing the storage and loss modulus (tan = E“/E'), indicates the interaction of the filler/matrix interactions, and associates with the internal mobility of polymeric molecular chains.

To determine how thermoplastic and thermoset composites perform in terms of thermomechanical performance, numerous amounts of research have been conducted in the past. This study, however, will focus primarily on lignocellulosic biomass based nanofiller and polypropylene matrix composites. Mohd Izwan S et al., (S. Mohd Izwan, S. Sapuan, M. Zuhri, & A. Mohamed, 2021) investigated the dynamic mechanical and thermal stability of polypropylene composites reinforced with chemically treated sugar palm and kenaf fiber (first alkalization, then benzoylation). These fibers were melt blended with polypropylene in varying ratios up to 20 wt% to construct hybrid composites. It was shown that an equal proportion of these fibers, whether treated or untreated, results in the highest É in a PP composite. The PP composite with untreated fibers have Éof 1300 MPa, 958Mpa, and 695 at 20 °C, 40 °C, and 60 °C, respectively, while treated fiber composites have storage moduli of 1360 MPa, 991Mpa, and 711 at the same temperature range. Furthermore, the loss modulus (E“) of the same composites with untreated and treated fiber is 80.7Mpa and 86.2 MPa, respectively, while the damping factor (δ) was reported to be 0.0585 Pa and 0.0531 Pa.

Similarly, polypropylene-based biocomposite was made by mixing 5 wt% to 20 wt% Carpinus betulus L. (CB) powder. It was found that increasing the weight fraction of the filler enhances the stiffness of the composite, as evidenced by the composite with the maximum storage and loss modulus values having 20 wt% CB (Atagur et al., 2020). Another study investigated the dynamic mechanical characteristics of polypropylene biocomposites composed of alkali and silane treated bagasse microfibers (Hidalgo-Salazar, Luna-Vera, & Correa-Aguirre, 2018). The storage modulus (E’) and loss modulus (E“) decreased to 55 % and 52 %, respectively, as the temperature rose from 25 °C to 75 °C. Furthermore, the increase in the value of α relaxation and the stability of E' with temperature was linked in this work to bagasse fiber, that improves the stability of the storage modules of the PP matrix with the temperature.

In contrast, thermomechanical analysis (TMA) is another methodology for examining how a composite material's dimension vary when it subjected to both temperature and a fixed load. Besides this, it can also be used to study volumetric changes, molecular structure, surface roughness, curing of polymers, composites and cross-linking polymerization under dynamic as well as static loads. Nonetheless, the coefficient of thermal expansion (CTE) of a composite made of polypropylene and treated sugar palm-kenaf fiber with various ratio was also examined by Mohd Izwan S et al., (S Mohd Izwan et al., 2021). In order to determine the CTE from the linear slope of the strain-temperature curve, the specimen of these composites was heated from −50 °C to 100 °C with 0.05 N loading of the probe. It was stated that, porosity of the samples began to shrink during the testing when an external load was applied in the axial direction with temperature, and the samples displayed deformities in various phases. Consequently, it was reported that thermal expansion coefficient (CTE) of the PP composite reinforced with an equal amount of chemically treated fibers at 45 °C was 7.32 as compared to 3.21 for untreated fibers. Furthermore, the thermal expansion coefficient rose from 24.93 to 30.11 at 105 °C for untreated and treated fibers, respectively. At lower temperature, minor changes in CTE were linked to transition state and moisture evaporation, which prohibited excessive expansion of the composite, however at higher temperature, significant increases were reported owing to melting of composite.

In another study, the coefficient of thermal expansion (CTE) of a polypropylene/rice husk composite was determined by adding up to 50 % of the filler. The CTE of pure PP was reported to be 123 × 10⁻⁶/°C in the temperature range of −10 to 50 °C, increasing to 163 × 10⁻⁶/°C in the temperature range of 50–100 °C. These values were reported to decrease by 30–62 % percent after addition of the 50 wt% of the filler. A similar observation was made with the PP-based composite material, where the thermal expansion decreased as amount of reinforcement increased (Atagur et al., 2020; Doan, Brodowsky, & Mäder, 2016; Reixach et al., 2015).

The significance of this study in this context is, in the development of biocomposites/green composites reinforced with the least amount of date palm nanofillers in polypropylene. This filler was made using a simple mechanical approach, avoiding the use of hazardous chemicals for the treatment process and consequently eliminating the multi-step purification step (Alothman, Shaikh, Alshammari, & Jawaid, 2022). To the best of our knowledge, no relevant research on the thermo-mechanical characteristics of date palm nanofiller/polypropylene composites has been reported. The information gained from these analyses is crucial to understand how the reinforcing of such fillers influences the properties of the polypropylene matrix, which can be helpful in outdoor applications in arid climates with high temperature variation.

2 Experimental

2.1

2.1 Materials

The homopolymer polypropylene (PP 3030, melt flow index (MFI) of 3 g/10 min and a density of 0.9 g/cm³) obtained from the National Industrialization Company (TASNEE, Saudi Arabia) was utilized to formulate biocomposites. Maleic anhydride grafted polypropylene compatibilizer (PRIEX-25097, The Netherland) was purchased and used from Addcomp, The Netherland. Similarly, the nano size filler was also produced using planetary ball milling with 99 cycles, each lasting 15 min at 300 RPM (Pulverisette 7 Premium, Fritsch Co. Germany). Fig. 1 display the scanning and transmission electron microscopy (SEM and TEM) images of these nanofillers. These fillers typically had a diameter of 30–50 nm and a length of 1–10 mm.

Close

Fig. 1

SEM and TEM of date palm nanofillers (DPNF).

Export to PPT

3 Results and discussion

3.1

3.1 Coefficient of thermal expansion

The dimensional stability of PP/NFD biocomposites and neat PP can be evaluated through the estimation of the coefficient of thermal expansion (CTE). Fig. 2 showed the dimension change vs temperature of the biocomposites and neat PP. It was showed that the dimension changes of the biocomposites and neat PP increase as temperature increase. It is well known fact that internal stress in polymer composites is formed during processing owing to stretching and quenching. The porosity of the samples collapsed during analysis due to applied force and temperatures, and samples exhibit various deformities. Such deformities were caused by positive strain from elastic creep. The creep strain regained near or at the polymer's glass transition temperature (Tg), between 40 and 105 °C, followed by shrinkage (S. Mohd Izwan, S. M. Sapuan, M. Y. M. Zuhri, & A. R. Mohamed, 2021). The large number of entangled polymeric chain and cross linking contributes to the high modulus of elasticity and reversible deformation (M.R.M. Asyraf, M. Rafidah, A. Azrina, M.R. Razman, 2021).

Close

Fig. 2

Coefficient of thermal expansion (CTE) of nano date filler-based PP composite.

Export to PPT

The coefficient of thermal expansion at temperature 40 °C and 80 °C biocomposites and neat PP was tabulated in Table 2. Among the biocomposites addition of 5 wt% of nan-fillers shows the slightly highest CTE compared to other biocomposites sample. In general, is observed that addition of NFD have negligible effect on the CTE of the biocomposites throughout the temperature range. This showed that the cellulose filler maintained the composite's dimensional stability by reducing significant thermal expansion. It also denotes an effective filler-reinforcing effect, minimizing shape change and warping effect, and therefore these composites can be employed for dimensional stability while subjected to thermomechanical stress. A decrease in CTE of PP composite of bio-flour such as rice husk and wood has already been observed in earlier studies owing to the low thermal expansion of these additives (H.S. Kim, S.W. Choi, B.H. Lee, S. Kim, H.J. Kim, C. W. Cho and D. Cho, 2007). However, Reixach et al (Reixach et al., 2015), on the other hand, observed a contrasting behavior for other polypropylene wood-based composites. They reported that as the amount of wood fiber in the PP composites increased, the thermal expansion decreased. This behavior was attributed to the lower coefficient of expansion of the reinforcement compared to that of the matrix. Similarly, Mohd Izwan et al. (S. Mohd Izwan, S. M. Sapuan, M. Y. M. Zuhri, & A. R. Mohamed, 2021) found that the CTE of sugar palm/kenaf fiber reinforced PP at 45 °C was much lower than that of neat PP. The authors observed that when the temperature raised to 105 °C, a huge expansion occurred for these composites. They attributed this increase in CTE to the melting state of the bio composites. Similar conclusion has reported by Poletto et al. (Poletto, 2018).

Table 2 The coefficient of thermal expansion at temperature 40 °C and 80 °C biocomposites and neat PP.

Sample ID.	CTE at 40 °C	CTE at 80 °C
Neat PP	80.4	124
PP/NFD-1	85.9	120
PP/NFD-2	89.4	128
PP/NFD-3	88.4	119
PP/NFD-4	83.5	121
PP/NFD-5	95.4	138

3.2

3.2 Dynamic mechanical analysis

3.2.1

3.2.1 Viscoelastic response of in the solid state (Storage modulus, É)

The effect of date palm nanofiller (NFD) on the viscoelastic response of the polypropylene in the solid state was also investigated through dynamic mechanical analysis (DMA). Fig. 3 illustrated the effect of NFD on the storage modulus (É) of the polypropylene composite. It is observed that, incorporation of nanofillers showed slightly improvement in Éof polypropylene. Table 3 showed the Éof the PP/NFD biocomposites and neat polypropylene at −50 °C (glassy region) and 150 °C (rubbery region). It worth to mention that storage modulus, however, decreased as the temperature increased. As temperature rises, the E'of composite decreases, which is caused by an increase in molecular mobility and free volume. Nonetheless, temperature has a significant impact on molecular mobility and free volume as molecules absorb more energy at higher temperature. When this energy becomes comparable to the energy barriers, large-scale conformational rearrangements of molecules can occur. This region is referred to as the rubbery region, and has an E' value that is lower than that of the glassy and transition regions. Moreover, at higher temperature (rubbery region) the storage modulus of the composites are dominated by the intrinsic matrix modulus (Karamipour, Ebadi-Dehaghani, Ashouri, & Mousavian, 2011; Majid, Hassan, Davoud, & Saman, 2011). As a result, there are no significant different in Éfor PP/NFD biocomposites are observed. Similar trends of viscoelastic properties of polymer reinforced with different natural fiber have been reported (Hansen, Borsoi, Dahlem Júnior, & Catto, 2019; Haris et al., 2022; Joseph, Mathew, Joseph, Groeninckx, & Thomas, 2003). However, various other factors such as fiber and matrix type, the presence of fillers, fiber content, and fiber orientation, the chemical treatment of the fibers, amount of loading, and size effect the final properties of composite materials.

Close

Fig. 3

Storage modulus (E’) of nano date filler-based PP composite.

Export to PPT

Table 3 The storage modulus of the NFD/PP biocomposites and neat polypropylene at glassy region and rubbery region.

Sample ID.	Storage modulus at Temperature −50 °C, E’G* (MPa)	Storage modulus at Temperature 150 °C, E’R** (MPa)	Percentage difference in storage modulus compared to PP at Temperature −50 °C, E’G* (MPa)
Neat PP	2631	79	–
PP/NFD-1	2812	85	6.87
PP/NFD-2	2864	76	8.58
PP/NFD-3	2758	74	4.82
PP/NFD-4	2739	80	4.10
PP/NFD-5	2783	79	5.78

G*-Glassy region; R**-Rubbery region;

3.2.2

3.2.2 Analysis of loss modulus, (E”) and damping factor (Tan δ)

Similarly, the loss modulus and tan delta of PP/NFD biocomposites along with neat PP are depicted in Fig. 4 and Fig. 5 respectively. It is observed that biocomposites with 1 wt% filler loading shows a slight improvement in loss modulus compared to neat PP, whereas other samples display no significant difference in loss modulus. Also, the tan delta follows the same pattern. The dynamic mechanical characteristics of several natural fiber-based PP composites have been examined, and it was shown that a decrease in tan delta value results in more elastic nature of composites than pure PP matrix (Tajvidi, Falk, & Hermanson, 2006). The decreased in the delta value is attributed to the reduced viscoelastic lag between the stress and the strain (Han, Han, Cho, & Kim, 2008). The glass transition temperature of the biocomposites and neat PP can be determined from the peak of loss modulus and tan delta. Table 4 shows the glass transition(T_g) temperature of the composites. The result demonstrates that the glass transition of the biocomposites is not appreciably influenced by the addition of NFD. The results of earlier research (Modesti, Lorenzetti, Bon, & Besco, 2006) that reported the addition of nano filler had no significant impact on the glass transition temperature also support this fact. Additionally, a neat polymer and its composite may have a glass transition temperature similar due to a neutral or weak interaction between the fillers and polymer matrix (Bashir, 2021). Typically, the glass transition temperature determined using tan delta is 5 °C-10 °C higher than the loss modulus. The tan δ values were lowered in the biocomposites compared to the PP matrix because of less matrix by volume is available to dissipate the vibrational energy.

Close

Fig. 4

Loss modulus(E”) of nano date filler-based PP composite.

Export to PPT

Close

Fig. 5

Damping factor/Tan (δ) of nano date filler-based PP composite.

Export to PPT

Table 4 The glass transition temperature(T_g) extracted from the peak of loss modulus and tan delta.

Sample ID.	Tg from Loss modulus (℃)	Tg from Tan delta (℃)
Neat PP	18.28	24.07
PP/NFD-1	19.04	24.63
PP/NFD-2	17.75	24.31
PP/NFD-3	18.00	23.86
PP/NFD-4	18.78	24.91
PP/NFD-5	18.13	23.98

3.2.3

3.2.3 Viscoelastic response of in molten state (Storage modulus, É)

The molten polymeric fluid's elastic response is measured by its storage modulus(E’). The storage modulus denotes the capacity to store energy applied by external forces, and it increased with angular frequency over the entire applied frequency range. Fig. 6 displays the variation between the storage modulus (E′) and the frequency. The storage modulus (E′) of the composites filled with 1 wt% loading (PP/NFD-1) of natural fillers shows slightly higher modulus at lower frequency than the neat PP. The storage modulus at 0.1 rad/s of the neat PP is 732 Pa, while the sample PP/NFD-1 show storage modulus 850 Pa, and at 1 rad/s it is 5524 Pa, and 5700 Pa respectively. This behavior could be due to the more interaction between the fillers and the matrix, which caused increased elasticity in this system compared with remaining composite samples. It has been well established fact that the poor compatibility of filler and the matrix could decrease of both storage modulus and loss modulus. The rest of the samples, on the other hand, had a lower storage modulus than the neat PP across the whole frequency range. These results indicate that the storage modulus changes as a result of the formation of the polymer-filler network. The difference in storage modulus between the composites reduced as the frequency rose. These might occur as a consequence of the particles' inherent stiffness or aggregation. It should also be noted that the inclusion of the filler may cause variations in the relaxation time spectrum, resulting in changes in the composites' viscoelastic characteristics. According to Marcovich et al. (Marcovich, Reboredo, Kenny, & Aranguren, 2004), a composites sample comprising 50 % wood flour reinforced PP matrix exhibited solid-like behavior at low frequency. Furthermore, it was reported that the presence of the wood filler had no effect on the relaxing mechanism of the PP. However, when the wood filler loading increases the corresponding relaxation times also increased.

Close

Fig. 6

Comparison of composite’s storage modulus with frequency.

Export to PPT

3.2.4

3.2.4 Analysis of loss modulus, (E”)

As stated earlier, the loss modulus related to the energy dissipation in the system. The loss modulus variation along with the frequency is shown in Fig. 7. The loss modulus increased with increase in frequency. At 0.1 rad/s, the loss modulus of neat PP is 1703 (Pa), 1453 Pa for PP/NFD-1, 677 Pa for PP/NFD −2, 262.6 Pa for PP/NFD −3, 495.2 Pa for PP/NFD −4 and 390.3 Pa for PP/NFD −5 respectively. The discrepancies in loss modulus between these composites can be related to differences in particle size distribution. For all samples, the loss modulus appears to rise linearly with frequency. The loss modulus differences amongst the composites remained almost constant as the frequency increased. As previously stated, a larger particle concentration resulted in more particle–particle interactions. Even at high shear rates, this impact remained consistent as the shear rate increased. It was therefore concluded that both the storage module (E′) and loss modulus (E”) shows the less frequency dependency characteristic. It is clear that E’>E’’ and it has been usually interpreted as the condition at which the natural filler particles are connected throughout the PP composite sample (Ren et al., 2014).

Close

Fig. 7

Comparison of composite’s loss modulus with frequency.

Export to PPT

4 Conclusion

This study investigates utilizations of the date palm waste to make nanofillers as a reinforcement additive for the formulation of biocomposites of polypropylene, which are then analyzed thermomechanical performance. The addition of these fillers imparted good thermal stability of these composites compared to the virgin polypropylene The thermomechanical analysis indicated that these composites exhibits low coefficient of thermal expansion (CTE), and can be used for dimensional stability when subjected to thermomechanical stress. This could be due to lower coefficient of expansion of these nanofillers. Moreover, dynamic mechanical analysis in solid state reveled that these biocomposites shows improvement of storage (E’) modulus at 50 °C which is much higher than the room temperature. Furthermore, rotational rheology indicated that these samples displays high storage modulus (E’) and loss module (E”) with the frequency due to the stiffer nature of the fillers. Thus, our findings demonstrated that date palm nanofillers are suitable for the formulation of thermomechanically stable biocomposites from inexpensive polyolefins, and can be used in the fabrication of many outdoor applications products in dry climates. These fillers can, however, be treated with various sizing agents to further enhance their functionalities.

CRediT authorship contribution statement

Hamid Shaikh: Conceptualization, Methodology, Investigation, Writing – review & editing. Othman Y. Alothman: Funding acquisition, Resources. Basheer A. Alshammari: Formal analysis, Validation. Mohammad Jawaid: Investigation, Data curation, Resources, Writing – review & editing.