Thermal and mechanical characterization of polypropylene/basalt fiber/ethylene propylene diene monomer rubber hybrid composite

Polypropylene (H110MA) with a melt flow index of 11 g 10 min⁻¹ was used as a base material. EPDM (Density = 0.857 g cm⁻³, ENB = 5%, E/P = 70/25%,) at 5, 15, 20, and 30 wt% was used. PP and EPDM were purchased from SB Enterprises, Mysuru, India. Silane-treated short-length Basalt fiber (BF) with 3.65 g cm⁻³ density,6 mm length and 13 μm diameter purchased from Nickunj, Mumbai was used as reinforcement material. Maleic anhydride (MA) compatibilizer at 3 wt% supplied by Molychem, Mumbai was utilized to promote intermolecular interaction between the filler and matrix.

All the components were pre-heated at 80 °C for 6 h before the composite preparation. Polypropylene was mixed with 3 wt% MA and 40 wt% BF via a twin-screw extruder. The obtained pellets were then melt mixed with various wt% of EPDM (5, 15, 20, and 30) in a twin-screw extruder. The extruded pellets were cooled, granulated and dried out in an oven to remove moisture. The dried pellets were subjected to an injection molding machine to prepare specimens as per ASTM standards required for tensile, flexural and impact tests. The compositional details of PP-MA-BF-EPDM composites are represented in table 1.

The tensile properties of the composite having a dimension of 200 × 20 × 3 mm³ were evaluated as per ASTM638 on a tensile testing instrument with 12 mm gauge length, 5 mm min⁻¹ crosshead speed, and using a 50 kg load cell. For impact testing samples having measurements of 65 × 12.7 × 3 mm³ and an angle of 45 degrees, a notch depth of 2.5 mm, were evaluated as per ASTMD256 on a pendulum impact testing machine with a strike energy of 7.5 kJ. The flexural strength of the specimens with dimensions of 127 × 12.7 × 3 mm³ was evaluated as per ASTMD790 on a flexural testing machine with 1 mm min⁻¹ crosshead speed.

The temperatures at which the PP and composite crystallized and melted were determined using DSC analysis. DSC (Perkin-Elmer, model Pyris 1) was utilized in an inert condition. About 6–8 mg of the samples were heated in a pan of alumina at a rate of 10 °C min⁻¹ from 25 °C – 200 °C.

The DMA of the composite was investigated using a DMA Q800 T.A machine. 3-point bending tests were performed on the samples at a rate of 5 °C min⁻¹ from 25 °C – 200 °C. The strain level and frequency were chosen as 0.1% and 1 Hz, respectively.

The fractured texture of the composite was analyzed with SEM (TESCAN VEGA3 LMU). The specimens were gold coated to avoid electrostatic discharge. The images were captured at 30 kV accelerating voltage in a high vacuum medium.

The TEM image of the composites was obtained using a TEM machine (JEM 2100) with a 200 kV. Powder samples of PP-MA-BF-EPDM composites were examined using TEM.

A PAN Analytical XRD was utilized to analyze the crystalline structure of the composite at a current of 30 mA, voltage of 40 kV and wavelength of 0.154 nm. The XRD patterns were scanned at a rate of 0.5° min⁻¹ with 0.001° increments from 10° to 80°.

The stress-strain graph for PP-MA-BF-EPDM composite and PP reinforced with various weight percent of EPDM is indicated in figure 1. The composite's tensile properties were calculated using the stress-strain graph.

Zoom In Zoom Out Reset image size

Figures 2 and 3 show the tensile characteristics of PP-MA-BF-EPDM composites and PP. The tensile strength of the material diminishes as the EPDM filler content rises. Similar observations have been made by other researchers in the case of aramid fibers [28], and wollastonite [15] reinforced with rubber-toughened polypropylene composites. The tensile strength decreased from 33 MPa for 5 wt% EPDM to 15 MPa for 30 wt% EPDM i.e., there was an overall reduction in tensile strength by 54%. This decline in tensile strength is due to the low tensile strength of EPDM and the reduction in the crystallinity (refer to 3.3) of the composite.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

By the addition of 5 wt% EPDM, the tensile modulus increased; however, as EPDM content increased further, the modulus dropped in comparison to 5 wt% EPDM. But the modulus for other wt% was greater than PP. The tensile property decreases as filler loading increases because the matrix around the fillers is unable to efficiently transfer load and balance the stress of the fillers, resulting in filler failure and a reduction in the tensile property [29]. As per Oksuz et al [30], the elastomeric structure, which facilitates the mobility of the polypropylene chain, is responsible for the loss of tensile characteristics. These results agree well with other researchers [30–32]. The tensile modulus increased drastically with an increase in EPDM wt% compared to PP/EPDM reinforced with fibers like jute [33], latania [19] and kenaf [21]. The drop in tensile properties can be correlated with the reduction in the crystallinity of the composite. Table 2 shows the tensile properties obtained by other researchers.

Figures 4 and 5 illustrate the effect of filler loading on the flexural characteristics of a PP-MA-BF-EPDM composite. The figure 4 shows that the flexural strength increased for 5 wt% EPDM, but as the filler wt% increased the flexural strength decreased. The flexural strength decreased from 47 MPa for 5 wt% EPDM to 17 MPa for 30 wt% EPDM i.e., there was a reduction in flexural strength by 64%. The flexural modulus for all wt% of EPDM was greater than PP. The initial increase in flexural properties for 5 wt% EPDM might be due to the bonding of polymer chains with reinforcement, resulting in good adhesion with less filler content. The decrease in flexural properties of the composite with a further enhancement in filler wt% is related to the low modulus of EPDM and decreased stiffness and rigidity of the composite. Composites with 5 wt% EPDM shows maximum flexural strength and modulus compared with all other combinations. Researchers have observed similar trends when blending EPDM with polypropylene (figure 5) [34].

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Figure 6 depicts the impact strength for PP-MA-BF-EPDM composites and PP. Figure 6 shows that, in comparison to pure PP, the impact strength enhanced as the weight percentage of EPDM increased. The impact strength improved from 5 kJ m⁻² for 5 wt% EPDM to 11 kJ m⁻² for 30 wt% EPDM i.e., there was an enhancement of impact strength by 120%. The EPDM particles function as stress concentrators, imparting ductility and promoting matrix shear yielding, which increases the composite's impact strength [8]. The rise in the toughness of the composite may be due to the excellent bonding of PP, BF, and EPDM by the inclusion of a compatibilizer [22]. Toughening of PP may also be due to variation in the crystallite size of PP by the addition of elastomer [6]. One of the possible causes for the rise in impact strength could be fiber-like structure formation (see SEM image figure 9). This fiber-like structure can absorb the fractured energy and prevent the propagation of cracks further. The polypropylene matrix is made more ductile and more favourable to shear yielding by the discrete rubber particles that are present in the brittle PP-MA-BF-EPDM composite. This is accomplished by creating numerous stress concentrate fields from EPDM rubber particles that interact well, increasing impact strength. It's also possible that basalt fibers with a higher modulus concentrate stress at the fracture point, avoiding crazing or shear yielding. The energy dissipation throughout the fracture's progression is confined to a very limited area [23].

Zoom In Zoom Out Reset image size

Sang et al [15] reinforced PP with 15 wt% EPDM and obtained an impact strength of 2 kJ m⁻² while Anuar et al [16] reinforced PP with 30 wt% EPDM and obtained an impact strength of 5.5 kJ m⁻². In contrast to the aforementioned literature, the impact strength of the PP-MA-BF-EPDM composite has improved. In comparison with the PP/EPDM [13] composite having a tensile strength of 15 MPa and an impact strength of 6 kJ m⁻², the PP-MA-BF-EPDM composite had a tensile strength of 21 MPa and an impact strength of 8 kJ m⁻² for 20 wt% EPDM. Hence the inclusion of basalt in the PP-MA-BF-EPDM composite helped in balancing the strength and stiffness. By comparing the result with PP/BF composites, the impact strength was found to increase for PP-MA-BF-EPDM composite with an increase in EPDM wt%.

The XRD peaks for PP-MA-BF-EPDM composites with variation in EPDM wt% is shown in figure 7. The XRD diffraction peaks observed at 2θ 14.5°, 17.3°, 19.0°, 21.7°, 25.8° and 29.1° corresponds to (110), (040), (130), (111), (060) and (220) planes of PP-MA-BF-EPDM composites [35]. The XRD peaks show that the planes correspond to α crystalline structure of PP (JCPDS 00–050–2397). Figure 7 shows that the peak intensity reduced with an increase in EPDM content. This decrease in the intensity of the peak is related to the reduction in crystallinity of the composite as the weight percentage of EPDM increases [36].

Zoom In Zoom Out Reset image size

The fractured surface morphology of the PP-MA-BF-EPDM composite is shown in figure 8. Figure 8 shows that there is no phase separation of the matrix by the addition of EPDM. With the increase in EPDM wt%, wrinkles in the composites increased, indicating that the composites underwent ductile fracture. The composites had a rougher surface by increasing the EPDM content, indicating that the composite is ductile. The composites containing 30 wt% EPDM are more ductile compared to 5 wt% EPDM composite. The impact property of the composites is enhanced with the inclusion of EPDM because the deformed rubber domains form crazes in the surrounding matrix [37]. Another reason may be that EPDM serves as a toughening agent [22]. The improvement in impact strength PP-MA-BF-EPDM composite may be attributed to the development of fibrils. The fibrils formed, correspond to elongated EPDM, which absorbs the impact energy and hinders crack propagation [38].

Zoom In Zoom Out Reset image size

Figure 9 shows the TEM image of the E4 composite. The PP-MA-BF-EPDM composites have a two-phase morphology, with the rubber phase distributed within the continuous phase of the PP matrix [39]. The dark areas represent EPDM, which is surrounded by basalt fibers. The basalt fibers are located at the junction between PP and EPDM. The EPDM fillers are dispersed inside the PP matrix and there is no phase-separated morphology present.

Zoom In Zoom Out Reset image size

From the mechanical property evaluation, the impact strength for the E4 composite was maximum. Hence the thermal characterization for pure PP and E4 composites were performed and compared.

Figure 10 shows DSC curves for the PP and E4 composites and corresponding data are tabulated in table 3. The melting enthalpy ∆H_m (J g)⁻¹, enthalpy of crystallization temperature ∆H_c (J g)⁻¹, crystallization temperature (T_c), melting temperature (T_m) and glass transition temperature (T_g) were obtained from DSC graphs. The degree of crystallinity (X_c) is calculated with the equation (1).

$$\begin{eqnarray} &&\% Xc=\displaystyle \frac{{\rm{\Delta }}{\rm{Hm}}}{{\rm{\Delta }}Hf\left(1-\alpha \right)}* 100\end{eqnarray} \tag{ 1 }$$

Where ∆H_c and ∆H_m are the heat of crystallization and heat of melting computed by the integration of the equivalent exothermic and endothermic peaks of the crystallization and melting method. The ∆H_f (J g)⁻¹ is the enthalpy of fusion for 100% crystalline PP, and α is the weight fraction of basalt fiber. The ∆H_f value of PP is recorded as 209 J g⁻¹ as per the previous literature [40].

Zoom In Zoom Out Reset image size

The T_g for neat PP is −25 °C and EPDM is −60 °C respectively. Figure 11 shows that the T_g for the E4 composite improved in comparison to PP. The T_g is affected by the movement of chain segments in the amorphous zone. The T_g of the E4 composite improved by the introduction of EPDM due to the enhancement in mobility of the polymer chain.

Zoom In Zoom Out Reset image size

The Tg of the composite increased due to better bonding between the polymer and the elastomer. The increase in T_g increased the rigidity of the composites because of the restrictions imposed by EPDM in cross-linking. Slight variations in the crystallization temperature (T_c) of the composite were observed because of EPDM acting as a nucleating agent in the composite [41, 42]. The heat of fusion ∆H_m is an important parameter as it is correlated to the degree of crystallinity of the composite. A decrement in ∆H_m value from 93 J g⁻¹ for PP to 27 J g⁻¹ for E4 composite was observed. ∆H_m decreases due to EPDM acting as diluents in the composite [43]. The melting temperature T_m decrease with the inclusion of EPDM when compared to PP. The decrease is attributed to an increase in the amorphous content of the matrix by the introduction of EPDM. Another reason may be the good bonding of the filler with the matrix [44]. The presence of one melting peak shows that only one crystalline structure is present in the composite. It is found that crystallinity is reduced with the inclusion of EPDM in the composite due to restrictions imposed by EPDM for the mobility of the polymer chain in arranging and crystallizing [45].

Figure 11(a) shows the storage modulus for the PP and E4 composite. The inclusion of EPDM in the composite enhanced the storage modulus. The storage modulus of PP improved from 1189 MPa to 2161 MPa with the inclusion of the fillers. The introduction of reinforcing elements to the matrix and enhanced stress transfer at the interface may be responsible for the improvement in storage modulus. The surface energy between the fillers and the matrix decreases as the bond between the fillers and the matrix strengthens, increasing the high-temperature modulus and softening temperature of the composites [42]. The temperature rise led to a decrease in modulus due to the breakdown of the bond between the matrix and the reinforcement [46].

Figure 11(b) illustrates the loss modulus of PP and E4 composite. The inclusion of EPDM improved the loss modulus in comparison to the matrix material. The rise in loss modulus is due to the composites' improved damping capabilities when compared to PP. PP has a lower modulus, indicating that it is mobile in the absence of reinforcement [44].

The Tan δ graph for PP and E4 composite is depicted in figure 11(c). The Tan δ value enhanced with the introduction of EPDM when compared to PP. The ratio of the viscous to elastic response of viscoelastic materials is known as Tan δ. The higher the Tan δ, the better the vibration-dampening capabilities of the material. As the value of Tan δ increases, the material's capacity to absorb energy also increases. The E4 composite is viscoelastic and has a higher capacity for dissipating energy than PP because of the higher Tan δ value [47].