3.1. Samples and X-Ray Diffraction (XRD)
Figure 1a-b shows the images of the polymeric blend specimens, TPU/PVC (100/0, 33/67, 50/50, 67/33, 0/100 wt.%), and TPU/rPVC (50/50, 67/33, 0/100 wt.%), respectively. No pores were found on their surfaces. Moreover, the injection molded plates presented an excellent visual appearance, free of bubbles and lumps and well leveled.
<Insert Fig. 1>
Figure 1 - Visual appearance of the specimens (a) TPU/PVC polymer blends: (1) 0/100 wt.%, (2) 33/67 wt.%, (3) 50/50 wt.%, (4) 67/33 wt.%, (5) 100/0 wt.%; and (b) TPU/rPVC polymer blends: (1) 0/100 wt.%, (2) 50/50 wt.%, and 67/33 wt.%; (c) diffractograms of the polymer samples (pure TPU, TPU/PVC (50/50 wt.%) blend, and TPU/PVC (67/33 wt.%) blend.
In Fig. 1b, the specimens had a purplish color due to the PVC reused in recycling, and no transparency was observed. However, the coloration of the rPVC and its blends was homogeneous, with no thermal decomposition. Regarding texture, both the recycled PVC and its blends were softer to the touch, and no porosity was noticed. The TPU/rPVC (33/67) blend had technical problems and could not be produced with the injection molding machine used in this work (image not shown).
Moreover, the addition of PVC to the TPU did not alter the diffraction patterns (Fig. 1c) of the TPU/PVC blends produced. These results indicated that the blends remained amorphous polymers, independent of the PVC added. In addition, some small diffraction peaks were observed at 42.6 and 49.4° for the pure TPU and its blends, which might be related to the additives used in the formulation of the TPU. According to the manufacturer, up to 2 wt.% of these additives (anti-hydrolysis agents, UV stabilizers, etc.) are added to improve the polymer-specific properties or processing characteristics [39].
3.2. Density
Density measurements are essential to screen the qualified samples prior to further mechanical testing. As the TPU concentration increased, the density of the TPU/PVC polymer blends tended to decrease. The densities of the TPU/PVC polymer blends were 1.209, 1.192, 1.196, 1.199, and 1.183 g cm− 3 as the TPU content increased from 0 to 33%, 50%, 67% and 100 wt.%, respectively. The densities calculated for the mixtures from the densities of the PVC (1.209 g cm− 3) and TPU (1.183 g cm− 3) also predicted these results well. The good agreement between the measured and theoretical density values indicated that the amount of voids in the blends was negligible, indicating that the melt-blended conditions used were suitable for this study.
The density values increased as PVC was added to the blend since TPU is a less dense polymer when compared to PVC [20, 26, 40]. Also, the lower density of the TPU is due to its heterogeneous polymeric chain, with rigid and non-rigid parts that are not easily accommodated in a compact structure, unlike PVC, which, with its linear chains, is more efficiently compacted when going through the extrusion process.
The compatibility of plasticized PVC and TPU differs in relation to the hard and soft segments, as demonstrated by Kim et al. [30]. Compared to the TPU with a greater hard segment, the TPU with a smaller hard segment is more compatible with plasticized PVC.
The values found for the average apparent density of the TPU/rPVC polymer blends were 1.243 g cm− 3 (67/33 wt.%), 1.229 g cm− 3 (50/50 wt.%), and 1.267 g cm− 3 (0/100 wt.%). From then on, the density increased less intensely with the rPVC content, but curiously, in a line almost parallel to the pure polymer blends. These findings support recent research reports that the apparent density of TPU/rPVC polymer blends increased significantly [23, 25, 38].
Recycled PVC is formed from previously shaped PVC, being processed again for recyclable applications, a process that modifies the size of the linear polymeric chains, reducing their size and rearranging the chains to produce a denser PVC. By submitting this PVC to a new injection process with pure TPU, it is more difficult to blend with high TPU contents. When the recycled PVC content is high enough in the blend, the blending behavior predominates, but is similar to pure PVC.
3.3. Thermal Stability
TGA and DTG measurements were used to investigate the thermal stability of the TPU/PVC and TPU/rPVC blends. Figure 2a-b depicts the TGA profiles with four stages of thermal decomposition. The small degradation stage occurred in the temperature range from 100 to 145°C, which can be attributed to the evaporation of water adsorbed on the polymer surface. The next degradation stage was in the temperature range from 145 to 367°C, representing the main stage with a 79.0% mass loss (Fig. 2a). According to Han et al. [41] and Abbas et al. [42], dehydrochlorination and the formation of aromatic compounds from PVC can occur. However, the three peaks that appeared during this stage (at 189, 259, and 285°C) suggest intermediate degradation stages related to interconnected chain scission mechanisms, due to the gradual reaction with oxygen. The third stage started at 365°C with the appearance of three more peaks at 419, 430, and 451°C, and ended at 480°C, with a 13.0% mass loss (Fig. 2a) related to the polyenes suffering scission in the air. Nagy et al. [19] discussed the oxidation mechanisms of polyenes during the thermal degradation of PVC, reporting that they occur in distinct steps. The last degradation stage was noted in the range from 480 to 530°C with an 18.0% mass loss, which can be related to the backbone of the polymer blend.
The TGA plots of the TPU/PVC polymer blends showed a behavior that appeared to be dictated by the PVC, mainly at 250 and 390°C (Fig. 2a). Above this temperature, the stages became similar, be it the pure polymer or the blend. This thermal behavior reinforced the fact that PVC and TPU are poorly miscible, and hence each component of the blends showed independent thermal responses. Adding specific binders or additives to these blends can partially overcome this difficulty. Figure 2b shows a shift of the maximum decomposition temperature (Td = 257°C) as the TPU concentration decreased. The value of Td shifted with increasing PVC or rPVC content due to extra interactions with the TPU, destabilizing the polymer blend. This confirmed the fact that PVC or rPVC makes the TPU less flammable [43]. Moreover, in the temperature range of 490–550 ℃, the TPU/rPVC polymer blends underwent a significant weight loss, estimated at 12% (Fig. 2b). This shift in the decomposition temperature range of the blend can be attributed to the stabilization of the TPU by the migration of stabilization components from the rPVC.
<Insert Fig. 2>
Figure 2 - TGA plot and derivative curves of the blends - (a) TPU/PVC polymer blends: 100/0, 33/67, 50/50, 67/33, 0/100 wt.%, and (b) TPU/rPVC polymer blends: 50/50, 67/33, and 0/100 wt.%.
3.4. Shore A Hardness
The hardness of pure TPU is related to the ratio of the rigid and flexible segments in its polymeric chains. According to Kim et al. (1999) [30], a 2.1 ratio (rigid/soft segments) corresponds to a TPU hardness of 70, in agreement with the TPU evaluated here (Fig. 3a), which presented a Shore A hardness of 76 and a rigid/flexible segments ratio close to 2.8, according to the manufacturer.
<Insert Fig. 3>
Figure 3 - (a) Shore A hardness of the TPU/PVC and TPU/rPVC polymer blends. (b) Abrasion test results of the TPU/PVC and TPU/rPVC polymer blends.
Regarding the TPU/PVC polymer blends, the Shore A hardness decreased slightly with the addition of PVC (TPU/PVC (33/67 wt.%) – Fig. 3a), and up to the TPU/PVC blend composition of (67/33 wt. %), the Shore A hardness remained practically unaltered (Fig. 3a). However, from this point up to the pure PVC, the Shore A hardness drastically reduced (Fig. 3a), characterizing a non-linear behavior for this parameter.
The recycled PVC Shore A hardness was higher than that of virgin PVC (75 > 62), in agreement with the data informed by the suppliers. Thus, the rPVC impacted the Shore A hardness of the TPU/rPVC polymer blends, which remained practically constant at any composition, ranging from 74–76 (Fig. 3a). After reprocessing the PVC (rPVC), the Shore A hardness enhanced the thermal degradation of the process, which can decrease the polymer molar mass by reducing the polymeric chain size [44]. Also, to improve its flexibility, the plasticizer added to the PVC formulation could "detach" from the main PVC polymeric chain, altering the molecular structure and reducing the flexibility of the PVC.
The facts described above produced a significant gain in the mechanical properties of the blends made with recycled material. The rPVC polymer blends showed a Shore A hardness equivalent to that of pure TPU, which positively impacts the footwear industry, allowing for the possibility of using polymeric upcycling strategies [45]. This enables the replacement of virgin polymers (pure PVC) by recycled ones (rPVC) to produce polymeric soles with high-quality properties (Shore A hardness).
3.5. Abrasion Resistance
In the abrasion test, the volume loss of the specimen after testing is inversely proportional to its abrasion resistance [46]. Analyzing the results for TPU, virgin PVC, rPVC, and their blends, an increase in abrasion resistance can be seen as the amount of TPU in the blend increases (Fig. 3b).
Blends prepared from virgin polymers have slightly higher abrasion resistance (≈ 10%) than blends prepared with recycled PVC, except for the rPVC sample itself. The rPVC seems to be more susceptible to abrasion when compared to virgin PVC due to the loss of resistance of the thermoplastic regarding thermal degradation during reprocessing [47]. Pure rPVC is denser and harder than virgin PVC, significantly improving abrasion resistance.
The blends prepared with pure PVC had no linear trend, which might be related to an incompatibility between the PVC and TPU thermoplastics [27]. This is evident for blends with a higher proportion of virgin PVC. However, when the percentage of PVC is above 50 wt.%, the abrasion resistance starts to vary at a lower rate (Fig. 3b).
Another study demonstrated superior abrasion resistance for TPU/rPVC polymer blends (70/30 and 50/50 wt.%) as compared to pure and virgin PVC [48]. These blends reduced the need for solvent cleaning before use as an adhesive, reducing the amount of volatile organic compounds (VOCs) in the workplace. In addition, the costs of applying this blend were much lower than applying pure TPU. Ames [48] suggested that the blend TPU/rPVC (70/30 wt.%) showed the best processing conditions, as also observed in the present work for the TPU/rPVC blend (67/33 wt.%).
Abrasion resistance is an essential property in the footwear industry, more specifically for shoe soles, since it is directly related to the main mechanical stress the sole suffers, extending its life cycle. According to the results presented here, blends containing rPVC (30–50 wt%) could be a good option for a line of sustainable casual shoes, being directly linked to upcycling and sustainable development.
3.6. Tensile Strength
The visual appearance of the specimens (Fig. 4a and d) after the tensile test was registered to assess the type of fracture and the stress caused by stretching. Table S1, from the supplementary material, summarizes the tensile test data for the polymers and blends evaluated. Also, Figures S1-S3 present the force x deformation curves for all the specimens and their replicates evaluated.
<Insert Fig. 4>
Figure 4 - Specimens after applying the tensile test: (a) TPU/PVC polymer blends (0/100, 33/67, 50/50, 67/33, 100/0 wt.%); (b) TPU/rPVC polymer blends (0/100, 50/50, 67/33 wt.%). Force (N) versus deformation (mm) curves of: (b) TPU/PVC polymer blends (0/100, 33/67, 50/50, 67/33, 100/0 wt.%); (e) TPU/rPVC polymer blends (0/100, 50/50, 67/33 wt.%). Curves represent average behavior for each group. The magnified area is shown in blue and green in (b) and (e).
For the virgin PVC and rPVC blends, elongation at rupture increased with an increase in the proportion of TPU in the blend, consistent with the flexible nature of TPU. Virgin TPU did not fracture during the test, demonstrating greater resistance than the blends and pure PVC, evidence of its highly elastic behavior [49].
The addition of virgin PVC shifts the force-deformation curves upwards (Fig. 4b and c), with the material becoming more resistant to deformation due to the increase in Young's modulus, but not linearly as expected. In general, the blends have greater or equal tensile strength, deforming less at a specific applied force than pure PVC or TPU. This means that the interaction between the flexible/rigid segments of linear PVC or TPU chains, provides materials with structures that are more resistant to deformation than the individual polymers. The linear portion of the curves referring to virgin PVC enlarged (Fig. 4c), with elastic deformation occurring up to 25 mm (applied force between 23–27 N). With the exception of pure TPU, the elastic limit occurred with 10 mm deformation (≈ 12 N) with a slight variation for the TPU/PVC (67/33 wt.%) blend. This blend (Fig. 4b) behaved similarly to TPU, but the presence of PVC made the material more resistant than all the other polymers and blends. The addition of 33 wt.% of PVC to the TPU provided desirable characteristics for better resistance of a shoe sole.
The addition of recycled PVC led to blends that were more deformation-resistant than pure TPU (Fig. 4e), showing strength that was intermediate between TPU and recycled PVC, especially regarding the plastic deformation region. More significant tensile strengths were gradually obtained as the proportion of rPVC in the blend increased, with the linear chains of recycled PVC (smaller and more compact) gradually interreacting and intertwining with the flexible/rigid TPU segments, providing stronger materials as the proportion of recycled PVC increased. The elastic region behaved slightly differently from the plastic region. The elastic region (the linear portion of the curve) occurred up to ≈ 20 mm (applied force ≈ 30 N), except for pure TPU, and in this region the TPU/rPVC (50/50 wt.%) blend was slightly more resistant to deformation (black line) than pure PVC or the TPU/rPVC (67/33 wt.%) blend. In this case, the addition of 50 wt.% rPVC to the TPU might be interesting for some applications.
Figure 5a shows the elasticity modules of the blends. Comparing PVC and recycled PVC, the latter showed greater tensile strength and a larger elasticity modulus. For each type of PVC blend, the highest tensile strengths were TPU/PVC (67/33 wt.%) and TPU/rPVC (50/50 wt.%). Polyurethanes (TPU) generally show remarkable mechanical properties, with tensile strengths ranging from 20–60 MPa and elongation from 300 to 650% [50]. On the other hand, PVC has a tensile strength between 10–25 MPa and elongation between 150–400% [51]. The rupture strength curve (Fig. 5b) for the TPU/PVC polymer blend (blue line) exhibited a rupture strength that has an inverse relationship with PVC content. This inverse correlation arises from the increased flow of TPU polymeric chains, necessitating a higher force for rupture. Notably, the TPU/rPVC blend with a TPU/rPVC composition of (67/33 wt.%) showed the highest rupture strength evaluated, surpassing that of pure TPU. The rupture strength of rPVC (14.73 MPa) was higher than that of virgin PVC (9.11 MPa), and the rPVC polymer blends with compositions similar to those of virgin PVC blends also showed higher tensile strengths. This result reinforces the idea that the denser and more entangled structure of rPVC provides greater resistance and, consequently, greater tensile strength. The zero values obtained for the tensile stress measurements with strains of 10, 100, and 300% mean that rupture of the specimen occurred until reaching a specific strain.
<Insert Fig. 5>
Figure 5 – The results of the mechanical testing of TPU, PVC, rPVC, and their blends: a) Young’s module; b) rupture strength; c) elongation at rupture.
The values for elongation at rupture of the polymeric blends (Fig. 5c) increased with the TPU content of the blend, in agreement with previous results, which showed that the blends underwent greater elongation since the entanglement of PVC with TPU provided a greater flow of polymeric chains. The results for virgin PVC, TPU, and their blends (blue line) showed a gradual increase in elongation at rupture up to 33 wt.% TPU, followed by an almost linear behavior with further increases in this polymer content. For recycled PVC, TPU, and their blends (red line), the elongation at rupture presented a practically linear behavior with increased TPU content in the polymer blend, probably because the elongation of the blend formed was directly controlled by the interaction of the TPU with the structures formed, mainly by the TPU/rPVC proportions, making it easy to predict the elongation at break by a simple linear regression.
Figure 6 presents the results obtained in the tensile tests for 10%, 100%, and 300% of the applied load. The tensile strength is evaluated from the load applied per unit area at the time of failure. The elongation represents the percentage increase in the length of the specimen under tension. The stresses at 10%, 100%, and 300% are related to the load necessary to produce the deformations related to these stresses. The results obtained with the load of 300% occurred because the specimen fractured before reaching the end of the test, but some specimens withstand great stress before they fracture. The strain values applied at 10%, 100%, and 300% followed technical standards that the market typically reports in the technical bulletin of the polymer sector.
<Insert Fig. 6>
Figure 6 - Results of the stress registered at 10%, 100%, and 300% deformation for (a) TPU, PVC, and the TPU/PVC blends, and (b) TPU, PVC, and the rPVC blends.
Figure 6a presents the stresses required for a 10% deformation of the polymers and their blends. All the stresses were below 1.9 MPa for this condition. Blends based on recycled PVC required greater tension than those required for virgin PVC blends. The TPU/rPVC (50/50 wt.%) blend was the most resistant in the tensile test under this condition. The virgin PVC blends showed increasing resistance with the TPU content.
Figure 6b shows the stresses required for 100% deformation of the polymers and their blends. All stresses were between 4.9 and 12.2 MPa for this deformation, and the blends based on recycled PVC were more deformation-resistant. Under this tension, recycled PVC showed the greatest resistance to deformation of the polymers evaluated in this study.
The TPU/PVC (67/33 wt.%), TPU/PVC (50/50 wt.%), and recycled PVC blends fractured before reaching 300% elongation (Fig. 6). All stresses were below 21 MPa for this deformation except for the fractured polymers, and the blends based on recycled PVC proved to be more resistant to deformation, especially the TPU/rPVC (50/50 wt.%) blend, which withstood more than 20 MPa of tension without breaking.
Polymer blends based on recycled PVC are generally more resistant and less elastic, so it is up to the footwear producer to adapt resistance, elasticity, and comfort to achieve an excellent cost-benefit ratio, maintaining the quality demanded by the consumer.
This study presents more favorable results than those already presented in the literature [52–54]. A spider chart (Fig. 7) is presented to summarize the main findings of this study. The origin of the recycled PVC used here (shoe soles from the footwear industry), was probably less heterogeneous in terms of manufacture, having undergone fewer structural alterations than the recycled PVC used by previous authors, which originated from landfills and civil construction, and was processed in cooperatives by mechanical recycling. This leads to the conclusion that the origin and quality of the recycled PVC used are crucial for obtaining the TPU/PVC polymer blends suitable for the different applications intended by the market. The blends proposed in this work, using a recycled PVC which was more resistant to tensile stress and less elastic, must be better studied, and a greater number of tests carried out to verify the real possibility of using such a product to obtain footwear, whose processing is inserted in a sustainable production cycle.
<Insert Fig. 7>
Figure 1 - Spider chart illustrating the mechanical properties of the polymers and blends investigated in this work.