Green thermoplastic vulcanized based on recycled polyethylene and waste tire powder

ThermoPlastic Vulcanized (TPV) is a class of polymeric materials capable of combining the high elasticity of elastomers with the recycling of thermoplastics. The production of TPV with recycled material contributes to the reduction of polymeric waste on the planet, reducing its environmental impact. In this study, recycled TPV samples were produced by combining recycled polyethylene and waste tire powder. The TPV samples were obtained in an internal mixer, changing the processing conditions, during the vulcanization and stabilization stages of the final torque. The results showed that by reducing the processing speed from 60 to 40 rpm, TPV samples were obtained with higher tensile strength and low swelling in oil. ANOVA statistical analysis confirmed that significant changes occurred due to processing speed variations. The DUNCAN mean parity statistical model was used for comparisons between pairs of TPV samples. Frequency sweeping rheological analysis confirmed the effect of adding tire powder on the samples’ elastic modulus. There were no changes in the viscous and elastic modules of the samples. The absence of significant changes in the final morphology of the TPV samples was attributed to the tire powder size. The increased properties of TPV samples are attributed to new crosslinking of the elastomeric phase during dynamic vulcanization.


Introduction
Vulcanized elastomers are materials with specific mechanical properties, such as high elasticity and low modulus.
However, the vulcanized material cannot be reprocessed, since there are sulfidic bonds, which makes them insoluble and infusible. To minimize this problem, researchers in the areas of thermoplastics and elastomers have joined efforts in the development of dynamically vulcanized thermoplastic elastomers (TPV). These materials have synergistic properties of elastomers and thermoplastics. They are elastic materials capable of reprocessing (Soares, 2008;Mondal, 2008, Ranjbar, 2012Carvalho, 2016;).
The first industrially produced TPV were based on polypropylene (PP) and ethylene-propylene-diene rubber (EPDM), due to the good compatibility between the two polymers. This material presented excellent dispersion of the vulcanized phase in the thermoplastic matrix, without compatibilizers (Coran, 1980;Naskar, 2004). Other compositions of TPVs, however, need compatibilizing agents in the formulations due to differences in polymer polarity like PP / nitrile rubber (NBR) (Naderi, 1999Soares, 2006Soares 2008), Polyamide (PA) / NBR (Chatterjee et. al., 2016) and PP / acrylic rubber (ACM) (Soares, 2008b;Celestino et. al., 2009. For the development of TPV with good mechanical properties, it is necessary to observe the compatibility between the polymers and the process of production (Lyer, 2021).
Recently, a TPV was produced based on the swelling of the vulcanized elastomeric phase, using tire powder and styrene monomer (Lima, 2016). The polystyrene polymerization was carried out in suspension, but the thermoplastic phase had low molecular weight, drastically reducing the mechanical properties of the TPV. The swelling methodology of the elastomeric phase is not economically advantageous, in addition to the usage of organic solvents. The use of these solvents should be avoided in synthetic routes. The development of new products and reuse of materials are recommended by the principles of green chemistry. Another methodology used in the production of TPV was by electron radiation, absence of solvents. As a result, the elastomeric phase crosslinking occurs by radiation, combining the breaking of the polymeric chain with the formation of lattices (Shen, 2013). This technique does not require vulcanization additives, such as zinc oxide, synthetic plasticizers, accelerators, and sulfur. However, the TPV has low mechanical performance and tendency of aging. Among the processes to produce TPV samples, the physical mixture is considered the most versatile, due to the ease of processing and to the countless possibilities of polymeric combinations.
The physical mixtures of the TPV samples are performed on internal mixers and extruders. The most used internal mixers are Banbury and Kneader models, which are capable of producing multiphase polymeric mixtures with excellent dispersion (Silva, 2007). The stress-strain profile in the Banbury mixing chamber is high for the TPV production. TPV samples consume high energy during processing, due to the disruption of the elastomer chains and to misorientation of the vulcanized particles in the thermoplastic matrix. The dispersion process involves the separation of the vulcanized particles from the solid, where there is a mechanical work on the cohesive forces that maintain the particles agglomerated (Ning, 2018). The multiphase polymeric systems of TPV samples consist of two phases: dispersed phase and matrix phase (Moreira, 2002;Zhao, 2015). The formation of the dispersed phase occurs in two stages, the first one is the dispersion of the thermoplastic phase in the elastomeric matrix. The division of a drop of immiscible polymeric fluid involves the generation of stresses in the interphase space and requires high mechanical work. This division will occur in the presence of the elastomeric matrix, where the cohesive forces of the viscous polymeric fluid (interfacial tension) will agglomerate the macromolecules of the thermoplastic (Minale, 2010). Then, the second dispersion stage occurs: there is the dynamic vulcanization process of the elastomeric phase, the matrix being the thermoplastic and the dispersed phase being the vulcanized elastomer. The inversion phase is decisive to obtaining TPV with increased mechanical properties (Magioli, 2010;Tian, 2012;Li, 2017).
Our research group has studied the PP / NBR TPV samples. This polymeric pair, due to the difference in polarity, requires the use of compatibilizing agents: the addition of PP grafted with maleic anhydride (PPgMA) reduces the size of the vulcanized rubber domains in the PP phase, improving the final properties (Soares, 2008;Soares, 2007;Carvalho, 2018;).
Another TPV developed in the group involves PP, residues of vulcanized elastomers (tire powder) and the styrene-butadienepolymer cup (SBR) (Cossa, 2009;Leite, 2011). The results showed the successful replacement of the virgin elastomer by tire powder. The production of TPV with industrial application using polymeric material totally recycled is new. This research is in line with the principles of green chemistry, which advocates the reuse of materials. In this work, the recycled TPV was developed from the physical mixture of recycled polyethylene, from irregularly discarded plastic bags, and tire powder. This pair does not require compatibilizing agents, the studies were focused on the processing speed of the TPV, to improve its mechanical performance.

Methodology
The raw materials used to study the processing conditions of recycled vulcanized thermoplastic elastomers were: recycled high density polyethylene supplied by Peterlu®, fluidity index (IF) of 15 g/10 minutes, waste tire powder supplied by Artgoma do Brasil, AG 40 (particle size between 100 and 700 microns), dicumyl peroxide with 40% calcium carbonate, supplied by Retilox and liquid from the cashew nut shell (CNSL) provided by Irmãos Fontenele S.A. Ceará. CNSL was used as a vegetable plasticizer between waste tire powder and recycled HDPE. Table 1 show the formulation used to obtain the TPV samples. Processing was carried out in an internal Haake mixer, equipped with Banbury-type rotors, with a temperature of 185 °C and 85% of mixer chamber occupation. The processing speed was changed during processing, during dynamic vulcanization and during final torque stabilization of the recycled TPV samples (Carvalho, 2018). Research, Society andDevelopment, v. 11, n. 4, e50011427421, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i4.27421 4 The speed profile during processing is shown in Table 2. The mixtures were prepared in triplicates, where mixtures with final mass variation greater than 5% were rejected, keeping strict mass control of the suitable formulation.

Results and Discussion
The MFI is a direct measure of the viscosity of melted polymers, widely used in the industry for quality control of the raw material, classification according to the processing ability and application of the final product. Comparing the samples obtained with the same speed in all processing steps, the samples TPV 1 (60 rpm) and TPV 2 (40 rpm) presented a reduction in MFI. Reducing the processing speed results is a more effective dynamic vulcanization, leading to high crosslinking. This behavior can also be observed in TPV 3 (reduction from 60 to 40 rpm during vulcanization). However, by reducing the rotation speed from 60 to 20 rpm (TPV 5), the MFI value increases, matching the MFI value of TPV 1 (60 rpm). The increase of polymer viscosity or the MFI reduction is the result of polymer presenting, high degree of chemical interactions, high molar mass or strong physical bonds (Yuliestyan, 2016). It is expected that TPV samples with low MFI values present high mechanical strength. The effect of increasing processing speed was also assessed. In TPV 6, the processing speed was the highest one, 80 rpm, resulting in the lowest MFI value. The analysis of the fluidity index is inaccurate to fully understand the behavior of the TPV samples, where the frequency sweep test was performed on an oscillatory disc rheometer for further information regarding TPV's elastic and viscous modulus.

Source: Authors
The rheological frequency sweep test was performed on samples of TPV with recycled material, Figure 2. The experimental conditions used were within the regime of linear viscoelasticity, deformation of 1%. The rheological curve behavior, for recycled HDPE, is typical of molten thermoplastic (Magioli, 2010). The recycled HDPE, at low oscillation frequencies, has viscous modulus (G") superior to the elastic modulus (G'), the dissipated energy is greater than the stored modulus. Then, the material has viscous fluid behavior. However, at frequencies above 100 Hz, the inversion of the modulus occurs, the stored energy is higher than the dissipated energy and the material has viscous-solid behavior (Schramm, 2006).
However, with the addition of tire powder in the recycled HDPE, the cross over point of the modulus absented. The TPV samples present values of elastic modulus superior to the viscous modulus, behavior characteristic of elastic polymeric materials, elastic solids (Tian, 2012). There were no significant variations in the values of modulus with the change in processing speed, the determining factor in this analysis was the tire powder content used. The presence of 34% of crosslinked material did not reveal changes in the modulus trend, regardless of processing variations.  produced with constant speed. Note that by reducing the processing speed to 40 rpm (TPV 2), the fracture strength is increased of, approximately, 25%. Therefore, it can be deduced that the simple reduction in processing speed results in TPV samples with higher tensile strength. Studies involving PP / NBR-based TPV samples showed an increase in mechanical properties by reducing the processing speed (Ning, 2018). This behavior was similar to that one observed for TPV3, TPV4 and TPV5, where the processing speed was reduced during dynamic vulcanization. It is important to highlight the increase in tensile strength by reducing the speed during the vulcanization of the mixture. The analysis of the p-values validates the hypotheses corresponding to the significance level of 0.05: comparing the values according to the standard deviation, or the variance, it is not possible to ensure with statistical precision if there is a significant change in the mean value. The analysis of variance indicates that there is a significant difference between the mean values of the factor levels. There are numerous statistical tests that find the differences between the mean values. The Duncan method is widely used procedure for comparisons between all pairs of mean values, regarding the analysis of variance, allowing the maintenance of 95% level of significance (Montgomery & Runger,199). The results in Table 4   The results of swelling in oil are shown in Figure 3. In the TPV 2, TPV 3 and TPV4, the lowest values of swelling are observed; the amount of solvent absorbed in these samples was reduced. This result is important for automotive applications, since smaller the swelling, more resistant the material will be when exposed to oils and solvents. TPV samples with a degree of swelling above 1% were considered inadequate for the automobile industry (Carvalho, 2016). Research, Society andDevelopment, v. 11, n. 4, e50011427421, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i4.27421 9 The ANOVA test was applied to the swelling results, Table 5, where the mean values presented statistical differences. Table 5 shows the results of parity of TPV samples. There is no statistical difference between samples 1x6, 1x5, 2x3, 2x4 and 3x4 indicating that these pairs are similar. The lowest values of swelling were found for samples 2 and 3, where there is no difference between them. The crosslink density of these samples was relevant for the processing speed. In Figure 4 is observed the behavior in the swelling and strength, both properties increased with the processing speed.
Therefore, the reduction in processing speed during the dynamic vulcanization process results in TPV with higher mechanical performance (Carvalho, 2018).

Conclusion
Variations in processing speeds (during vulcanization and torque stabilization) directly affect the final properties of the TPV samples. The reduction in processing speed during vulcanization increases the viscosity of the material, lowering the flow rate. By reducing the processing speed, the tensile strength increases. The low swelling was caused by the increase in crosslinking. The TPV samples present values of elastic modulus superior to the viscous one, due to the presence of tire powder which alters the rheological behavior. Among the values elastic modulus, no variation was observed between the samples studied, as the material's morphology was not modified. The increased properties of TPV samples are attributed to new crosslinking of the elastomeric phase during dynamic vulcanization. News processing will be done to characterize the morphology before, during and after the dynamic vulcanization.