Rheometry evaluation of PP/SEBS (20% St.) and PP/SEBS (30% St.)
Torque rheometry is an important analysis to monitor the processability of PP upon addition of a crosslinked material (SBRr), especially with high contents such as 30% of the weight. Additionally, the SEBS effect in PP/SBRr can be elucidated through torque plots, in order to understand possible interactions. Figure 4 (a) shows the torque plots of PP and SEBS with 20% and 30% styrene.
As reported [27–29], torque is proportional to the polymer viscosity, i.e., higher torque corresponds to higher viscosity. In Fig. 4 (a), it was verified that SEBS (30% St.) presented higher torque, consequently, higher viscosity among the neat materials. PP displayed intermediary torque, while SEBS (20% St.) has the lowest one, suggesting greater fluidity for the experimental conditions applied.
In Fig. 4(b), it was verified that PP presented a terminal torque around 23.7 N.m, while the PP/SBRr slightly higher, i.e., 25.8 N.m, suggesting increased viscosity. This behavior can be attributed to SBRr, considering that it is a vulcanized residue that does not flow and does not melt, acting as elastomeric filler and increasing PP viscosity. However, considering that 30% of SBRr weight was added into PP matrix, the increase in torque was not significant. This reflected positively during the injection molding process, since there were no problems with mold filling or formation of defective specimens, as seen in Fig. 1
It is important to point out that SEBS (20% and 30% St.) was added to PP/SBRr at 3 min, as the compound was already melted. PP/SBRr/SEBS compounds (20% St.) with 5%, 7.5% and 10% presented torques of 24.1 N.m, 24.7 N.m and 25.1 N.m after 10 min of processing, respectively. There was a slight reduction in the viscosity of the PP/SBRr/SEBS systems (20% St.), related to the non-compatibilized (PP/SBRr), contributing to improve the processability. On the contrary, PP/SBRr/SEBS (30% St.) with 5%, 7.5% and 10% of SEBS exhibited torques of 28 N.m, 28.3 N.m and 29.1 N.m, respectively, in 10 min. These compounds became more viscous, especially with 10% SEBS (30% St.). As seen in Fig. 4 (a), SEBS (30% St.) displayed the highest torque and, consequently, the lowest fluidity, promoting more visible increases in torque for PP/SBRr/SEBS (30% St.), related to PP, PP/SBRr and PP/SBRr/SEBS (20% St.).
SEBS is a non-reactive copolymer with groups that interact with PP and SBRr. The interactions in SEBS-compatible PP/SBRr compounds involve the miscible styrene groups of SEBS and SBRr and, at the same time, the ethylene-butylene (EB) blocks with PP matrix, which takes place during melt mixing. Figure 5 shows MFI as stabilized torque (8–10 min) torque function of PP and investigated compounds. MFI is inversely proportional to the viscosity, in other words, the higher the flowability, the material will be less viscous. PP presented the lowest stabilized torque and, consequently, the highest fluidity. PP/SBRr and PP/SBRr/SEBS (20% and 30% St.) showed lower fluidity related to PP, since SBRr is vulcanized and does not flow, imposing greater difficulty for PP matrix to flow, confirming the reported trend [18].
For PP/SBRr/SEBS (20% St.), in the range of 5–10% SEBS (20% St.), higher fluidity was observed, related to PP/SBRr. This indicates that addition of SEBS (20% St.) contributed to increase MFI and reduce the viscosity, corroborating the torque rheometry. Regarding the PP/SBRr/SEBS (30% St.), there was a trend to reduce the flow rate upon SEBS concentration increasing, indicating increase in viscosity, even surpassing the non-compatibilized compound (PP/SBRr).
Figure 6 shows the impact strength results of PP, PP/SBRr and PP/SBRr/SEBS compounds (20% and 30% St.). PP displayed impact in the order of 39.5 J/m, while in PP/SBRr it reduced to 34.3 J/m. Although SBRr presents elastomeric character, addition of 30% SBRr was not able to toughen PP. Such reduction can be attributed to the structural difference between SBRr and PP (see Table 1), i.e., there is no strong interaction, thus providing an immiscible and incompatible compound. Similar trend already verified by Costa et al. [17], in PP compounds with rubber waste from the tire industry.
The compatibilization of PP/SBRr with SEBS (20% and 30% St.) resulted to be effective, since there was a very significant increase in the impact strength of PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.), related to PP and PP/SBRr. As SEBS content increased, regardless the styrene content, a continuous increase in impact strength was observed. Therefore, it is suggested that SEBS promoted interfacial interactions between PP and SBRr, strengthening the interface, as verified later on in SEM images. These interactions and then the stronger interface improved the energy transferring mechanisms from PP to SBRr, generating greater crack dissipation and distribution within PP matrix, which led to higher impact.
PP/SBRr/SEBS (5% − 20% St.) and PP/SBRr/SEBS (5% − 30% St.) showed gains of 149.6% and 124.3%, respectively, compared to PP; these are significant increases upon addition of only 5% of SEBS, indicating potential applications where medium impact PP is required. At addition of 7.5% SEBS (20% St.) and SEBS (30% St.) impact values of 112.4 J/m and 108 J/m were computed. From a practical point of view, impact results of PP/SEBS (7.5% − 20% St.) and PP/SEBS (7.5% − 30% St.) are comparable to the typical impact strength values of commercial high impact polystyrene references (HIPS) [30, 31] targeting general-purpose applications such as housewares and furniture accessories.
PP/SBRr/SEBS (10% − 20% St.) significantly increased the impact related to PP reaching 164.4 J/m, while in PP/SBRr/SEBS ( 10% − 30% St.) it was 137.7 J/m. SEBS added content is of great importance to maximize the impact, especially as it provides greater synergistic between PP and SBRr. As seen in Fig. 2, SEBS was able to toughen PP, but it was not as expressive as PP/SBRr/SEBS. In addition, SBRr individually did not improve impact in PP (34.3 J/m). Therefore, it is reasonable to suggest that SEBS was able to interact with both PP and SBRr, as discussed in torque rheometry section. As result, PP/SBRr/SEBS (10% − 20% St.) and PP/SBRr/SEBS (10% − 30% St.) can be considered toughened at room temperature, surpassing, for example, polyamide 6 [32, 33], HIPS [31], polypropylene copolymer [34] and PA6/ABS/SMA [35]. Regarding the styrene content in the SEBS, it was observed that, in general, 20% of St. promoted greater impact strength in PP/SBRr/SEBS (20% St.) compounds, most due to the greater amount of the flexible stylene-butylene (EB) central block, related to PP/SBRr/SEBS systems (30% St. ).
Table 3 and Fig. 7 show the mechanical properties for PP, PP/SBRr and PP/SBRr/SEBSr. PP presented elastic modulus of 1287 MPa, quite similar value as verified by Nascimento et al. [36]. Addition of 30% of the weight of SBRr to PP matrix reduced the elastic modulus to 839.7 MPa, due to SBRr elastomeric character behavior. In addition, when SEBS was added to PP/SBRr the elastic modulus reduced more, indicating greater flexibility. As PP content was reduced in PP/SBRr/SEBS (20% and 30% St.), the elastic modulus was lower than in PP/SBRr. It was observed that PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) showed similar trends, that is, there was a decrease in the elastic modulus upon increasing SEBS content. However, PP/SBRr/SEBS (20% St.) exhibited higher losses in elastic modulus related to PP/SBRr/SEBS (30% St.). This behavior can be explained by the greater amount of ethylene-butylene (EB) in the SEBS (20% St.), providing greater flexibility and less resistance to the elastic deformation, corroborating the impact test. Addition of 10% of SEBS (30% St.) in PP/SBRr improved the elastic modulus by 13.8%, related to PP/SBRr/SEBS (10% − 20% St.). This indicates that 10% difference of styrene in SEBS can contribute to adjust the elastic modulus for a specific application.
In Table 3, it is verified that PP presented tensile strength of 36.5 MP, while addition of 30% of SBRr reduced it to 26.2 MPa. This trend is in agreement with the literature [37], where the authors state that by adding a residue with elastomeric characteristics to PP, it contributes to increase flexibility and, consequently, deformation at lower stresses.
PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) exhibited continuous reduction in tensile strength upon SEBS increasing content. This suggests that there is intensification in the flexibility of these compatibilized compounds, related to PP/SBRr. Therefore, it is believed that increasing SEBS content saturates PP/SBRr/SEBS interface, providing greater interfacial mobility and reducing tensile loads to deformation. At the same time, part of the SEBS may have been dispersed in PP matrix, which also favored flexibility, as reported by Mello et al. [38]. PP/SBRr/SEBS (30% St.) showed higher tensile strength than PP/SBRr/SEBS (20% St.), most due to the higher styrene content, which increases rigidity. It was observed that, for the composition range 5–10% of SEBS (30% St.) in PP/SBRr, there were no statistically significant differences for the three compounds produced.
PP had an elongation at break of 24.7%, suggesting ductile behavior. Addition of 30% SBRr to PP matrix promoted a deleterious effect on elongation at break, conducting to 17.1%. This reduction in the elongation of PP/SBRr reveals the immiscibility and incompatibility of this compound, confirming the morphology seen in SEM images as further presented.
Upon SEBS (20% and 30% St.) addition to PP/SBRr, trend to synergism in elongation was verified, since 5% was enough to achieve the result of PP. The elongation at break of PP/SBRr/SEBS (10%-20% St.) and PP/SBRr/SEBS (10%-30% St.) was maximized with this content, surpassing the value obtained with PP. As elongation at break is a ductility indicator under tension, the compatibilized compounds displayed improved deformation level, confirming acquired data under impact.
In comparison with the styrene content in SEBS, the PP/SBRr/SEBS (20% and 30% St.) containing 5% and 7.5% showed similar results. Apparently, the difference started to become more significant in the elongation at break from 10% SEBS (20% St.) in PP/SBRr/SEBS, due to the higher value (47.1%). This is taken as an indication that the interaction of SEBS (20% St.) with PP and SBRr increased, as discussed in torque rheometry, directly reflecting the greater deformation capacity of PP/SBRr/SEBS (10% -20% St.).
In Fig. 7(a), it was verified that PP presented stress versus strain plot typical of ductile material, with yielding and deformation until failure. PP/SBRr also showed ductile behavior under tension, but with a lower deformation level related to the compatibilized compounds. The degree of ductility of PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) was improved, indicating that SEBS acted as compatibilizer for PP and SBRr. There was an increase in deformation upon SEBS addition, especially for 10% of this copolymer. PP/SBRr/SEBS (30% St.) presented greater inclination and higher tensile strength related to PP/SBRr/SEBS (20% St.), suggesting that they present greater resistance to elastic deformation, i.e., they are more rigid.
Table 3
Mechanical properties of PP, PP/SBRr and PP/SBRr/SEBS compounds.
Compounds | E (MPa) | TS (MPa) | ɛ (%) |
PP | 1287.0 ± 71 | 36.5 ± 2.0 | 24.7 ± 2.1 |
PP/SBRr | 839.7 ± 21 | 26.2 ± 1.3 | 17.1 ± 1.8 |
PP/SBRr/SEBS − 20St (5%) | 707.3 ± 18 | 22.4 ± 1.1 | 23.2 ± 1.8 |
PP/SBRr/SEBS − 20St (7.5%) | 701.5 ± 11 | 19.8 ± 0.8 | 32.2 ± 1.6 |
PP/SBRr/SEBS − 20St (10%) | 683.7 ± 13 | 18.7 ± 1.0 | 47.1 ± 2.2 |
PP/SBRr/SEBS − 30St (5%) | 826.9 ± 11 | 24.1 ± 0.8 | 25.1 ± 1.2 |
PP/SBRr/SEBS − 30St (7.5%) | 802.0 ± 14 | 23.2 ± 1.0 | 33.6 ± 1.7 |
PP/SBRr/SEBS − 30St (10%) | 778.3 ± 16 | 23.8 ± 1.1 | 38.3 ± 1.5 |
E = elastic modulus under tensile; TS = tensile strength; ɛ = elongation at break. |
Shore D hardness is an important property for the plastic or rubbery products, since a material that is too hard or too soft can provide to the product distinct characteristics from those desired. Therefore, hardness measurement during the design, manufacturing process and quality verification is critical. Figure 8 shows the Shore D hardness of PP, PP/SBRr and PP/SBRr/SEBS (20% and 30% St.).
PP showed greater hardness related to PP/SBRr and PP/SBRr/SEBS (20% and 30% St.), with a value around 68 Shore D, indicating greater resistance to penetration. When modifying PP with 30% of SBRr, the hardness was reduced about 15%, indicating that PP/SBRr has softer surface, requiring less load to be penetrated [39]. Such behavior can be attributed to SBRr has elastomeric character, contributing to reduce the stiffness and, consequently, the hardness related to PP. A similar trend observed by Silva et al. [40] with PP/rubber waste from tires compatibilized with maleic anhydride-grafted polyethylene. When the PP/SBRr was compatibillized with SEBS, shore D hardness of PP/SBRr/SEBS compounds (20% and 30% St.) was reduced.
As the SBRr content was fixed at 30%, the reduction in Shore D hardness of PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) can be associated with the decrease in PP content and the increase of SEBS copolymer, providing greater flexibility, as verified in impact strength and elongation at break. Although it is a slight increase, higher styrene content in SEBS promoted a subtle increase in the Shore D hardness of PP/SBRr/SEBS (30% St.), related to PP/SBRr/SEBS (20% St.). PP/SBRr/SEBS (30% St.) (10%) showed a hardness of 53.1 Shore D, while PP/SBRr/SEBS (20% St.) (10%) indicated 49.1 Shore D. This indicates that the styrene content from 20–30% in the SEBS was able to increase the penetration resistance of PP/SBRr/SEBS (30% St.) compounds, indicating greater surface stiffness.
HDT determines the temperature at which a deformation occurs in the specimen when submitted to a pre-established stress. This is a very important property for the quality control, as it indicates structural stability [41]. Figure 9 demonstrates HDT results for PP, PP/SBRr and PP/SBRr/SEBS (20% and 30% St.) compounds. HDT of PP was 57.8°C, a quite value similar to that already reported [42]. The addition of 30% SBRr to PP matrix subtly decreased HDT to 55.2°C.
Regarding PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) compounds, HDT also subtly reduced upon increasing of SEBS content, related to the non-compatibilized compound (PP/SBRr). This trend can be attributed to the flexibility increase with SEBS addition, as verified in the elongation at break.
PP/SEBS (10% − 20% St.) and PP/SEBS (10% − 30% St.) exhibited HDT of 52.9°C and 54.1°C, respectively. Considering that these compounds have 30% of weight of recycled material, the reduction was not significant compared to traditional elastomeric impact modifiers.
Chiu et al. [43] observed that adding 30% of SEBS to PP matrix resulted in a decline in HDT of 18%, due to greater flexibility. The production of PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) compounds indicated that when adding a vulcanized residue with a gel content in the order of 88.4% (SBRr) it presents the advantage of not severely reducing HDT. Comparatively between SEBS (20% St.) and SEBS (30% St.) copolymers, slight non-significant changes were observed in HDT.
HDT of PP/SBRr/SEBS (20% St.) and PP/SBRr/SEBS (30% St.) are important from a technological point of view, since they are comparable to polyamide 6 (PA6) [44 .45] and poly(lactic acid) (PLA) [46, 47]. Therefore, SBRr reuse as elastomeric filler for PP is of great importance to reduce environmental impacts, especially to produce new materials, which have the potential to be reused in the new products manufacture.
Figure 10 illustrates XRD plots of PP, SBRr, PP/SBRr and PP/SBRr/SEBS compounds (20% and 30% St.). The commercial styrene-butadiene copolymer (SBR) is typically amorphous, but the SBRr used came from the footwear industry and, therefore, is a complex mixture of styrene-butadiene, mineral fillers, processing additives, curing agents and stabilizers [48–50]. SBRr showed peaks at 2θ = 21.1° and 26.4°, indicating the presence of SiO2 (JCPDS − 33-1161) typically used as an active reinforcing filler. The diffraction peaks at 2θ = 29.7° and 36.4° are attributed to MgO (JCPDS − 30–794) and ZnO (JCPDS − 5-664) [51], respectively.
PP diffractogram indicated diffraction peaks at 2θ = 14.1°; 16.2°; 17.8°; 21.1° and 25.7°, which refer to the (110), (040), (130), (111) and (131) planes of α crystalline phase [52.53], respectively. PP/SBRr and PP/SBRr/SEBS compounds (20% and 30% St.) showed intense diffraction peaks at 2θ = 25°, indicating that the mineral fillers of SBRr were added to PP matrix. This finding becomes important since it possibly contributed to keep the elastic modulus performance, as seen in Table 3.
In Fig. 10, PP/SBRr presented the main diffraction peaks of PP and SBRr, confirming an additive and immiscible effect of the system. However, when SEBS (20% and 30% St.) was added to PP/SBRr, the crystalline peaks changed. PP/SBRr/SEBS compounds (20% St.) showed a tendency to destroy the main crystalline peaks of PP matrix, as seen in Fig. 10(a). As the non-compatibilized compound PP/SBRr did not show this behavior, it is suggested that when adding SEBS there was a more efficient accommodation of SBRr within PP matrix, which possibly inhibited the formation of PP crystalline peaks. Apparently, this trend of PP/SBRr/SEBS compounds (20% St.) contributes to increase the degree of flexibility, generating significant gains in impact. Regarding PP/SBRr/SEBS systems (30% St.) with 5% and 10% of SEBS, almost no change in PP crystalline peaks were verified, only at 7.5% of SEBS (30% St.) it was verified an inhibition of the peaks referring in PP matrix.
Figure 11 shows TG plots for PP and investigated compounds, and TG parameters are presented in Table 4. PP displayed thermal stability up to 350°C, suggesting absence of moisture or low molecular weight components. PP degraded in the range from 350 to 480°C, with weight loss of approximately 100%. PP/SBRr and PP/SBRr/SEBS compounds (20% and 30% St.) showed lower thermal stability related to PP, as verified for T0.1. Such reduction may be due to the extender oil volatilization as well as low molecular weight additives present in SBRr, which are typically added to vulcanized rubbers [54].
PP/SBRr showed the lowest thermal stability among investigated materials, suggesting that the weak interaction between the phases accelerated the thermal decomposition. Addition up to 5% of SEBS (20% and 30% St.) to PP/SBRr did not improve the thermal stability, and T0.1 was unchanged. For T0.5, PP/SBRr/SEBS (5%-20% St.) and PP/SBRr/SEBS (5%-30% St.) presented 402.5 and 406.2 oC. Upon SEBS addition of 7.5% and 10%, regardless of the styrene content, the compounds showed a significant increase in thermal stability related to the non-compatibilized PP/SBRr system, being the plots quite close to PP. Apparently, SEBS provided interactions between PP and SBRr, promoting a stabilizing effect and increasing the thermal stability. The effect was more prominent for PP/SBRr/SEBS (10%-30% St.), considering that it overcame the PP for T0.1 and T0.5, suggesting synergistic effect for this composition.
Comparing the type of copolymer SEBS (20% St.) and SEBS (30% St.), there was a tendency for the thermal stability to be higher in PP/SBRr/SEBS (30% St.), possibly due to the greater styrene amount which increased the thermal resistance. The main weight loss of PP/SBRr and PP/SBRr/SEBS (20% and 30% St.) was linked to the decomposition of PP and SBRr chains. In addition, the compounds showed at 550°C residual material related to inorganic fillers of SBRr, as verified in XRD.
Table 4
Thermal stability at 10% (T0.1) and 50% (T0.5) of weight loss for PP, PP/SBRr and PP/SBRr/SEBS compounds.
Compounds | T0.1 (°C) | T0.5 (°C) |
PP | 384.8 | 437.9 |
PP/SBRr | 325.3 | 397.5 |
PP/SBRr/SEBS − 20St (5%) | 326.3 | 402.5 |
PP/SBRr/SEBS − 20St (7.5%) | 363.7 | 428.9 |
PP/SBRr/SEBS − 20St (10%) | 363.5 | 432.8 |
PP/SBRr/SEBS − 30St (5%) | 327.6 | 406.2 |
PP/SBRr/SEBS − 30St (7.5%) | 363.0 | 434.7 |
PP/SBRr/SEBS − 30St (10%) | 390.3 | 451.2 |
Figure 12 shows SEM images of PP, PP/SBRr and PP/SBRr/SEBS (20% and 30% St.), as function of SEBS content. PP presented a ductile fracture surface, with high roughness and plastic deformation. PP/SBR displayed phase separation morphology, confirming the immiscibility of this compound. The fracture surface has an irregular characteristic, with poorly adhered SBRr particles to PP matrix. In addition, delamination presence was observed, suggesting low interaction between PP and SBRr, confirming the low impact strength and lower elongation at break.
In Fig. 12 (c-h), addition of SEBS to PP/SBRr improved the fracture aspect, with SBRr particles adhered to the PP matrix, which indicates higher degree of interfacial adhesion. This indicates that SEBS was able to reduce the interfacial tension between the PP and SBRr components, increasing the interface resistance. As consequence, there was more efficient energy transfer from PP matrix to SBRr and, consequently, energy redistribution in the form of microcracks in PP matrix, thus contributing to improve toughening. PP/SBRr/SEBS (10% − 20% St.) and PP/SBRr/SEBS (10% − 30% St.) presented a more homogeneous fracture surface, with the dispersed phase well adhered and refined. This finding corroborates the observations already discussed and explains the improvement in impact strength.
Figure 13 illustrates a schematic representation of the developed morphology in PP/SBRr and PP/SBRr/SEBS. PP/SBRr presented a coarse morphology with large SBRr particles, due to immiscibility and high interfacial tension between PP and SBRr. Compatibilization of PP/SBRr with SEBS promoted a refinement in the dispersed phase, since part of the compatibilizer migrated to PP/SBRr interface. This promoted a greater degree of interfacial adhesion between PP and SBRr, especially improving impact strength. At the same time, part of SEBS copolymer may have been dispersed in PP matrix, producing a third phase and also contributing to toughening.