1. Introduction

Figure 1. Vacuum pyrolysis process of Pyrovac Inc.

Figure 1. Vacuum pyrolysis process of Pyrovac Inc.

2. Experimental

2.2. Sample Preparation

Figure 2 shows the steps carried out in this work during the preparation of the HDPE/rCB composite materials. The average size of rCB particles at the exit of the pyrolysis oven was measured to be at around 380 microns in diameter with a large variability due to the nature of the process and diversity of waste tires. The origin of these large agglomerates is mainly the carbonaceous deposits during the pyrolysis decomposition process. To reduce their size, rCB powder was grinded using a Spex/Mixer Mill 8000M with zirconium oxide beads (step A on Figure 2).

Figure 2. Processing steps for the preparation of composites test specimens.

Figure 2. Processing steps for the preparation of composites test specimens.

After milling rCB, the composite material was prepared by mixing the melted polymer with carbon black particles using a twin-screw extruder (step B on Figure 2). A Leistriz 18HP twin-screw extruder (TSE) with 8 temperature zones was used with optimized processing conditions for HDPE (see Figure 3). HDPE pellets and 6 wt.% of milled rCB were compounded at a temperature varying from 180 to 200°C. Since the virgin HDPE pellets already contain a thermal stabilizer, no additional antioxidant was added to the mix. The extruded filament was pelletized at the exit of the TSW to an average size of 3 mm. To have the same thermal history, the unreinforced HDPE was also prepared using the same processing conditions in the TSE as the composite samples. The manufactured samples are named HDPE for the virgin polymer and R6 for the composite containing 6 wt.% of rCB respectively.

Figure 3. Mixing and extrusion of HDPE/rCB composites with a twin-screw extruder Leistriz 18HP.

Figure 3. Mixing and extrusion of HDPE/rCB composites with a twin-screw extruder Leistriz 18HP.

Polymer and composite pellets manufactured by TSE were molded into plates of 2 mm thickness using a compression molding technique (step C on Figure 2) in a heated hydraulic press (Figure 4). A Carver press with plates heated at 170°C and a closing mass of 2 500 kg was used to press mold rectangular flat plates. The molded plates were then cut in 9 cm per 5 cm test specimens using a hydraulic press and a die cut (step C on Figure 2). These samples were used for accelerated aging experiments of this study.

Figure 4. Plate molding: a) Carver hot press, b) mold and molding principle, c) frame containing composite pellets and molded plates.

Figure 4. Plate molding: a) Carver hot press, b) mold and molding principle, c) frame containing composite pellets and molded plates.

2.3. Sample Aging

Accelerated aging was carried out in a weathering chamber ARTACC from SEVAR. The samples were placed on the outer part of the moving wheel as illustrated in Figure 5. This chamber was built according to ISO 4892-1 standard. The test specimens were held in place with clips and their internal face was exposed to a 400 W medium pressure mercury vapor lamp with an irradiation of 120 W/cm² UVa and 20 W/cm² UVb (EN 60 188 standard requirements). The temperature inside the chamber was 60 ± 0.1°C and water was added in the chamber to simulate natural conditions. The samples undergone six cycles per 24 hours alternating dry and wet cycles, which correspond to 80% of dry irradiation, 5 % of wet irradiation and 15% of wet not irradiated phase. For the non irradiated phase, samples were hidden from the UV lamp by a mask (Figure 5a). The exposed face of the sample to UV light was named Fe and the non-exposed face Fne (Figure 5b).

Figure 5. Artacc Bandol Wheel diagram: a) overview of samples placement, b) sample placement indicated exposed and non-exposed faces.

Figure 5. Artacc Bandol Wheel diagram: a) overview of samples placement, b) sample placement indicated exposed and non-exposed faces.

3. Results and Discussion

3.1. Carbon Black Analyses

The average diameter of rCB particles from pyrolysis is too large to be directly processed into a thermoplastic resin. Large agglomerates >500 microns are observed and most probably linked to residual carbonaceous deposits from the process which promote the agglomeration of the particles. Carbonaceous deposits are the result of thermal cracking in the pyrolysis oven, they are hard as contain a high amount of carbon, and shear forces generated during the extrusion process are not sufficient to deagglomerate the particles. A secondary upstream process is needed to reduce the size of the agglomerates before they can be incorporated into the polymer resin. Size reduction was carried out using a ball mixer with zirconium oxide balls in dry conditions. Figure 6 illustrates the volumetric ratio of the powder distribution of the rCB before and after milling and the cumulative percentage. A shift in the average particle size toward lower sizes is observed at more than 1 order of magnitude. The mean size of the agglomerates is reduced by 94% from 380 to 20 microns. The milled rCB also shows more than 20% of particles whose size is less than 10 microns which is expected to improve mechanical performance of the composite.

Figure 6. Volumetric ratio of powder distribution (line) and cumulative percentage (dash) of recycled carbon black (rCB) before (black) and after (red) ball milling.

Figure 6. Volumetric ratio of powder distribution (line) and cumulative percentage (dash) of recycled carbon black (rCB) before (black) and after (red) ball milling.

Energy Dispersive X-ray spectroscopy (EDS-X) analyses were performed on rCB particles (Figure 7) to verify the composition of the particles and whether there may have been contaminated by the zirconium oxide beads. The rCB shows that it contains mainly elemental carbon, oxygen, and the remaining solid residues from the pyrolysis process that enters the original waste tires composition. The solid additives based on the total remaining residues (removing oxygen and carbon), contain an atomic percentage of 27 ± 3% for silicium, 35 ± 1 % for sulfur, 31 ± 3 % for zinc and 6 ± 1 % for calcium. The EDS-X analysis reveals no significant contamination by the zirconium oxide ZrO₂(Y₃O₃) ceramic beads during the milling process. Thus, grinding should not make any changes other than size reduction of the agglomerates.

Figure 7. Typical spectrum ESEM EDS-X analysis on rCB particulates.

Figure 7. Typical spectrum ESEM EDS-X analysis on rCB particulates.

An additional verification was carried out to for the morphology of the carbon black particles. This has been investigated using HR-TEM as illustrated in Figure 8. The milled rCB particles (Figure 8b) were compared to a typical commercial carbon black grade used for automotive tire tread, the ASTM N330 (Figure 8a). As illustrated, the two types of particles are very similar, both showing stacked graphitic layers in concentric circles, a structure typical of carbon black [30-33].

Figure 8. HR-TEM image: a) N330 commercial carbon black used for automotive tire treads, b) recycled carbon black.

Figure 8. HR-TEM image: a) N330 commercial carbon black used for automotive tire treads, b) recycled carbon black.

3.2. Evolution of Sample Topography with Ageing

The virgin polymer resin HDPE and the composite R6 samples were taken periodically out of the aging chamber and their surface appearance was observed by optical microscopy. Figure 9 and Figure 10 show the optical micrographs recorded respectively on HDPE and R6 exposed and non exposed surfaces at different aging times, up to 114 days. The non-exposed (Fne) and exposed (Fe) faces are presented in the left and right columns respectively for comparison.

The virgin HDPE (Figure 9), shows that the exposed face starts to crack at 22 days whereas the non-exposed face shows cracks at 36 days of aging. The cracks on Fe at 36 days of aging appear much larger than on Fne. At 64 days, the network of cracks is largely visible and even worse after 114 days of UV exposure. Similar observations were made for polypropylene-polyethylene copolymer exposed to UV accelerated aging conditions in a study by Pertin [34]. Crack propagation is indeed the dominant mechanism of polymer degradation under UV radiation [35,36].

The HDPE/rCB samples R6 show the non exposed face Fne remains unchanged over time with no signs of cracks, even at 114 days of aging as illustrated in Figure 10. For the R6 exposed face Fe, cracks start to appear after 64 days of UV exposure and are largely visible after 114 days. Furthermore, cracks do not propagate in the R6 composite the same way they do in virgin HDPE: in HDPE, cracking is significantly important resulting in macro cracks propagating into micro cracks, which is not the case for R6 where only macro cracks are observed on the surface. This highlights the role of carbon black in filtering UV and consequently reducing polymer degradation and crack propagation.

In order to quantify the crack density, image analysis was carried out using Python software. Figure 11 illustrates the evolution of the surface percentage of cracks during aging for HDPE and R6 samples. Each surface ratio is the average of 3 representative images and the error bars is the associated standard deviation. Hence, the calculation considers the differences in surface homogeneity. The results confirm the microscopy observations. For HDPE, the surface ratio of cracks evolution with aging time is similar for both Fe and Fne surfaces, the cracks density being slightly higher on the exposed face. From 30 days of aging, the crack ratio increases almost linearity with time, reaching 20 % at 114 days of UV exposition for Fe surface. Such a percentage reflects a strong degradation of the surface of the sample. For the Fne surface, the same trend is observed with a slightly lower percentage of cracking which reaches a value of 13 % at 114 days of aging. For R6-Fe, the surface ratio of cracking is significantly lower, not exceeding 5 % at 114 days of aging.

The standard deviation of the measurements reflects the inhomogeneity of the surfaces of the aged samples. For HDPE-Fe, this standard deviation is low at the beginning because the surface only starts to weakly crack, evenly on the plate. Thereafter, it becomes important because the cracking develops mainly in the center of the plate. At 114 days of aging, the surface is completely covered with cracks, which results in a low standard deviation. This is not the case for the Fne side, which is not yet totally degraded. In the case of R6-Fe, the appearance of cracks remains inhomogeneous and concentrated in the center of the sample, which results in a significant standard deviation.

The apparition of cracks in probably due to a gradient of elasticity in the sample thickness which is shown by a variation in the Young’s modulus value as reported by Pertin et al. [37]. During artificial aging, samples undergo thermal expansion and mechanical stresses are build-up on the UV exposed layer and probably released by the occurrence of cracks [37-39].

Figure 9. Optical microscopy images of HDPE samples at different aging times.

Figure 9. Optical microscopy images of HDPE samples at different aging times.

Figure 10. Optical microscopy images of R6 samples at different aging times.

Figure 10. Optical microscopy images of R6 samples at different aging times.

Figure 11. Evolution of the average surface percentage of cracks during photooxidation for HDPE and R6.

Figure 11. Evolution of the average surface percentage of cracks during photooxidation for HDPE and R6.

3.3. Degradation Kinetic and Formation of Oxidative Products

A typical FTIR spectrum of an 85 days aged HDPE is shown in Figure 12a. Three main regions are connected to the oxidation process: the C=C bands (700cm^-1 to 1300 cm^-1) the C=O bands (1500cm^-1 to 1850 cm^-1) and the O-H bands (3200cm^-1 to 3600 cm^-1). Table 1 summarizes the FTIR absorption spectrum of the HDPE and its main oxidation products usually reported in literature [24,29,40-43]. In this work we have focused on the C=O bands region only, which is the band were the oxidation first appears in HDPE [24,29] and remains predominant in this work. An enlarged view of the carbonyl oxidized region between 1500-1850 cm^-1 wavelength is illustrated in Figure 12b. The band deconvolution shows three mains peaks: carboxylic acid/ketone around 1715 cm^-1, γ-lactone at 1780 cm^-1 and vinyl at 1640 cm^-1. These peaks are characteristic of HDPE polymer oxidation products and the cumulative band (red curve) fit well the experimental data (black curve). The carbonyl region is the most apparent for both the virgin HDPE and composite R6 and more specifically the band C=O at 1715 cm^-1. The other absorption bands are less well defined, especially for R6, where they are sometimes even confused in the baseline. The baseline indeed presents variations in absorbance of 0.0076 unity of absorbance (u.a.) in the initial FTIR spectra of both HDPE and R6 inside de carbonyl region (1500-1850 cm^-1). Absorbance bands below this value are therefore considered as signal noise. Oxidation was quantitatively analysed using the carbonyl index (CI) using the SAUB method [44] and defined as:

$$CI={\displaystyle \frac{{Area under band at 1715 cm}^{-1}}{{Area under band at 2912 cm}^{-1}}}$$

(1)

It is the ratio between the area under the absorbance of C=O at 1715 cm^-1 and the area of the methylene stretching peak at 2912 cm^-1. This latter was chosen as the reference band because it is not altered during aging [40].

Figure 12. IR spectra of weathered HDPE sample at 85 days: a) global experimental curve of aged (black line) and non aged (black dash) samples and b) zoom on the carbonyl region of the aged sample and peak deconvolution: experimental curve (black line), cumulative fit (thick black line), vinyl (gray dash dot), carboxylic acid / ketone (black dash), γ-lactone (gray line).

Figure 12. IR spectra of weathered HDPE sample at 85 days: a) global experimental curve of aged (black line) and non aged (black dash) samples and b) zoom on the carbonyl region of the aged sample and peak deconvolution: experimental curve (black line), cumulative fit (thick black line), vinyl (gray dash dot), carboxylic acid / ketone (black dash), γ-lactone (gray line).

Figure 13 illustrates the evolution of CI of virgin HDPE (Figure 13a) and composite R6 (Figure 13b) of the exposed face (plain symbol) and non exposed face (open symbol) for a maximum aging time of 114 days. The CI was calculated using equation (1). The data shown are the results of the average of 3 measurements for each of the faces and the error bars represent the associated standard deviation. The evolution of the CI of Fe and Fne of HDPE is very similar (Figure 13a). Initially, the CI values of Fe and Fne are overlapped below 20 days, then differ slightly. The CI of the Fe faces starts to be higher than that of the Fne faces, after 30 days of aging although differences are very close to error bars. The induction period is less than 10 days, and the CI reaches a saturation plateau value of 30 after 60 days of exposure on both faces. Since ATR analyses is a surface measurement, the saturation observed indicates that the surface layer analysed are fully oxidized.

Figure 13. Evolution of the carbonyl index with aging time for a) HDPE b) R6. Open symbols are for the non-exposed face and plain symbols are for the exposed face.

Figure 13. Evolution of the carbonyl index with aging time for a) HDPE b) R6. Open symbols are for the non-exposed face and plain symbols are for the exposed face.

CI measured on the Fe faces of R6 samples show similar behavior than that of the HDPE Fe faces, as illustrated in Figure 13b. The induction period is less than 10 days, and a Cl maximum value of 30 occurs at 60 days. Up to 100 days of ageing, the Cl values are identical to those measured on the Fe face of HDPE, considering the errors bars. A slight decrease is observed for aging times greater than 100 days as a Cl value of 24 is measured at 114 days of aging. The reason of this small Cl reduction value is under investigation. The quasi identical results obtained on Fe face of HDPE and R6 samples indicated no significant influence of the rCB particles on the oxidation process of the UV-exposed faces. On the other hand, a quite different behavior is observed for the Fne face of R6 samples. The induction period is longer, close to 20 days (instead of 10 days), and the CI values are significantly lower than those on the Fe face. The saturation value is below 10. This result clearly indicates that the recycled carbon black plays a key role in the antioxidant process for the bulk.

To investigate the oxidation beyond the surface, analyzes were carried out through the thickness of HDPE and the R6 composite samples. For this purpose, the samples were gradually polished and measured for each thickness in FTIR spectroscopy. The CI was calculated using equation (1). Figure 14 presents the evolution of the carbonyl concentration through the thickness of HDPE and R6 at 114 days of aging. The surface directly exposed to UV light, Fe, is referred to 0 mm depth and the non exposed surface Fne to 2 mm depth. The oxidation profiles measured on both samples are U-type curves as already observed for thick polyethylene [22] and polyolefins [34,45]. The experimental data of the oxidation profiles have been fitted using an exponential curve of the Beer-Lambert law as already reported in other works [37,46]. The following formula was used:

$$CI\left(x\right)=A\exp \left(-x/B\right)$$

(2)

With x is the sample thickness, A is the Cl value at the surface and B is the characteristic depth of the oxidation profile. The corresponding fitting curves are shown in Figure 14, with the blue curve for HDPE and the red one for R6. The values of A and B calculated for the Fe and Fne faces of HDPE and R6 samples are reported in Table 2. The HDPE curves are quasi symmetrical for Fe and Fne and the characteristic depths of oxidation are similar (162 ± 33 µm for Fe compared to 146 ± 19 µm for Fne).

Figure 14. Evolution of the carbonyl concentration through thickness from the exposed face (Fe) to the non-exposed face (Fne).

Figure 14. Evolution of the carbonyl concentration through thickness from the exposed face (Fe) to the non-exposed face (Fne).

For R6 sample the Fne and Fe profiles are asymmetric, the Cl measured for the non-exposed face Fne is 50% lower than that of the Fe face. The depth of the oxide layer close to the Fne-R6 face is also significantly smaller (B =74 ± 6 µm) compared to the Fe-R6 face (B = 120 ± 20 µm). Note that the A and B profile parameters for Fe-R6 are quite similar to those measured on Fe-HDPE sample, although slightly lower. These results highlight that the carbon black particles issued from tires recycling do not have prooxidative action and that, on the contrary, have a totally opposite behavior and act like an antioxidant.

It is well-known that oxidation process is associated to two combined mechanisms: formation of free radical induced by UV absorption and presence of oxygen [36,42]. The quasi symmetric profile observed for HDPE clearly indicates that the photon flux passes through the sample without being significantly absorbed: the quantity of photon being identical on both faces and so the supply of oxygen; the oxidation process is similar. The decrease of Cl value with depth observed for HDPE samples is clearly related to the non ability of oxygen to diffuse into the bulk of the polymer [22,45]. Figure 15a schematically summarizes the symmetric oxygen diffusion profile (black curve), the photon propagation profile (blue dashed line) and the resulting Cl profile (blue line) for HDPE.

Figure 15. Diagram of the evolution of the oxygen diffusion and the photon propagation and CI through thickness for a) virgin HDPE and b) R6 composite.

Figure 15. Diagram of the evolution of the oxygen diffusion and the photon propagation and CI through thickness for a) virgin HDPE and b) R6 composite.

Since the oxygen supply is identical for both Fe and Fne faces of the sample, the lower carbonyl index measured on the surface of R6-Fne is clearly related to a lower flux of photons reaching the Fne face due to UV absorption through the thickness of the R6 sample (red dashed-line reported in Figure 15b). This result confirms the role of rCB as photon absorber. There is also a significant difference in the characteristic depth of the oxide layer of the Fne and Fe faces (11.7 ± 0.4 µm for Fne-R6 and 24 ± 2 µm for Fe-R6), which suggest that the rCB also affect the diffusion of oxygen into the R6 sample. Also, the surface cracks density (see Figure 11) has certainly an influence on the oxidation depth (B parameter). The presence of cracks on the surface of the sample allows the infiltration of the free oxygen, which leads to a higher oxidation depth. This explains the higher B values of Fe-HDPE, Fne-HDPE and Fe-R6 compared to Fne-R6, where no cracks were observed on the sample surface.

In fact, if the oxygen profile was identical for HDPE and R6 samples (black dashed line in Figure 15b), the characteristic oxidation depths would be expected to be identical since photons pass through the total thickness for both samples. In order to explain the lower characteristic depth observed for R6 samples, we suggest that the oxygen diffusion is also limited by the presence of rCB particles: the carbon black would also act as a radical scavenger, preventing the radical propagation and thus the oxidation process [12,27]. In the first layers close to the surface, the polymer would be weakly protected against photo-oxidation by the addition of the recycled carbon black alone. However, in the deeper layers, the carbon black, which acts as a UV and oxygen filter, becomes more effective as an oxidation protector. The present work clearly demonstrates that the carbon black issued from waste tires pyrolysis is a promising antioxidating agent.

4. Conclusions

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