From devulcanization to revulcanization: Challenges in getting

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From devulcanization to revulcanization: Challenges in getting recycled tire rubber for technical applications Fabiula Danielli Bastos de Sousa, Aline Zanchet, and Carlos Henrique Scuracchio ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

From devulcanization to revulcanization: Challenges in getting recycled tire rubber for technical applications

Fabiula D. B. de Sousa†§*, Aline Zanchet‡ and Carlos H. Scuracchio#

†

Technology Development Center, Universidade Federal de Pelotas, Rua Gomes Carneiro, 1,

96010-610, Pelotas – RS, Brazil §

Center of Engineering, Modeling and Applied Social Science, Universidade Federal do ABC,

Avenida dos Estados, 5001, 09210-580, Santo André – SP, Brazil ‡

Polytechnic School of Civil Engineering, IMED, Rua Senador Pinheiro, 304, 99070-220, Passo

Fundo – RS, Brazil #

Materials Engineering Department, Universidade Federal de São Carlos, Rodovia Washington

Luís, Km 235, 13565-905, São Carlos – SP, Brazil

Corresponding author e-mail address: [email protected] (F. D. B. de Sousa)

ACS Paragon Plus Environment

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ABSTRACT. For a sustainable development, recycling is a very important step, which, beyond saving the use of natural resources can also be a solution for low-income families, providing them earnings. Recycling is also considered a category of green chemistry, and contributes to the reduction of pollution, bringing the improvement to the public health. However, recycling of a material trying to contribute to the environment will not bring positive results if the final properties of the recycled material do not point to an application that is viable. In this sense, the aim of this work is to present a comprehensive study of the revulcanized ground tire rubber (GTR) samples, previously devulcanized by the action of the microwaves under different exposure times. It is extremely important to know in depth the modifications occurred in the recycled material during all the stages of the recycling process to which the material was subjected, in order to select the ideal application for the recycled material. The obtained results pointed out to modifications as result of the revulcanization process, such as in the chemical structure, composition, thermal stability, and morphology.

KEYWORDS. Ground tire rubber, recycling, devulcanization, revulcanization, microwaves, sustainable development.

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Introduction The search for new environmentally friend processes and materials must necessarily address the problem of reducing impacts all over the life cycle of the product. Polymeric materials have several advantages over other classes of materials at this point, such as low density, low processing temperatures, high specific resistance, low prices, among others. However, in the last years, mainly due to the quick increase in the production and incorrect disposal of these materials, polymers are often referred to as environmental villains. Regarding the classes of polymeric materials, the recycling of rubber waste has some peculiarities not found in thermoplastics. The most obvious one is the thermoset nature of the vulcanized rubber, which the main consequence is the inability to be remolded into a new product without some sort of chemical and/or physical treatment.1 A possible way to overcome this fact is to pass the rubber through one of the various devulcanization processes available. These processes, theoretically, can restore the flow capability of the rubber and, consequently, its ability to be remolded and reshaped into a new product. Several techniques to achieve these goals are described in the literature, such as by using chemicals,2,3 ultrasound,4–6 shear and heating in a extruder,7–9 microwaves,10–17 among others. In this work, microwaves devulcanization was the chosen technique for the study. Regardless the technique, a devulcanized rubber must fulfill at least two requirements:4 (i) flowability like a thermoplastic, and (ii) ability to be vulcanized (revulcanized). If, on one hand, the ability to flow is important to produce a new product, the vulcanization is a key step to obtain a useful product. These chemical reactions guarantee the unique mechanical properties of the rubber, due to the formation of a tridimensional network that serves as shape memory and enhances the modulus and strain recovery.18 This is true not only when raw rubber is used, but also for the devulcanized flowable rubber.



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Comparison between the vulcanization of a raw rubber and the revulcanization of a devulcanized one shows that the revulcanization of the devulcanized rubber has specific characteristics that are not found in raw rubber vulcanization: the presence of remaining additives from the first vulcanization;19,20 original cross-linkings preserved from the devulcanization process;21 reduced molar mass of the polymer main chains;5,19,20 chemical groups attached to the chains not found in the raw one,22 among other factors which are able to influence the extent, the chemistry and the kinetics of the revulcanization process.23 In general, revulcanization of devulcanized rubber, in comparison with the vulcanization of a similar raw rubber, presents lower scorch time and faster vulcanization rate.22,23 Although some tendencies in the revulcanization are relatively constant for almost every devulcanization process used, there are some peculiarities that must be investigated for each specific technique. One important characteristic when a high temperature is used to devulcanize the rubber (like in the case of microwaves devulcanization) is the change in carbon black/polymer ratio.12,24 This factor has a great influence on the final properties of the rubber good made of revulcanized rubber.25 In this work, ground waste truck tire rubber, called ground tire rubber (GTR), was previously devulcanized by microwaves at different levels of devulcanization (thanks to changes in the exposure times to the microwaves), the results of this stage are discussed in our earlier work.24 So, in order to deeply understand the modifications attained by the rubber during the revulcanization process, the samples were revulcanized by using an accelerated sulphur system. Characterization in terms of chemical structure, morphology, composition and dynamic-mechanical properties were performed. When compared to the properties of the devulcanized GTR, it was possible to observe several modifications as result of the revulcanization process, such as in the chemical structure, composition, thermal stability, and morphology. Additionally, the results also pointed out to the influence of the exposure time of the GTR to the microwaves during


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devulcanization process in all the properties mentioned above, beyond the dynamic-mechanical ones.

Experimental Materials Ground tire rubber (GTR), a waste from truck tire previously separated from nonelastomeric components (as received), rubber accelerator N-tert-butyl-2-benzothiazole sulfenamide (TBBS) and sulphur were kindly supplied by Prometeon Pneus Ltd. The exact recipe of the GTR is not known. The particle size distribution of GTR before devulcanization ranged between 297 and 37 μm.

Devulcanization of the GTR and preparation of the compounds The GTR was devulcanized by the action of the microwaves, being that the apparatus consisted of a conventional microwave oven (commonly used at homes). The equipment suffered some adaptation to perform the process (a motorized stirring system containing a speed control), being used the maximum power of the oven (820W). The time at which the material was exposed to microwaves ranged from 3, 4, 5, and 5.5 minutes. The devulcanized GTR was mixed with the vulcanization additives by using a laboratory two roll mill PRENMAR for approximately 6 minutes at room temperature, in which were added 1 phr of accelerator TBBS and 1 phr of sulphur. The compounds were vulcanized at 180 °C and pressure of 45 Kgf/cm2 in a hydraulic press Tecnal TE-098-E2 for 5 minutes.



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The nomenclature adopted is: for devulcanized samples, type GTRX; and for revulcanized samples, type GTRX+ad, where 'X' denotes the exposure time of the sample to the microwaves and '+ad' is related to the presence of the vulcanization additives.

Characterization The extraction of the sol phase was performed according to ASTM 2765-11, by using toluene as solvent. About 5 g of sample was placed in a cage (pouch) made of stainless steel blanket, being completely immersed in boiling toluene for 24 h. After the extraction, the samples were dried for 24 h at 40 °C to remove the solvent. After the extraction of the sol phase, the compounds were pressed in a hydraulic press at 180 °C by using a circular mold (Ø25x1 mm) for 5 minutes. 100 mL of toluene was used to immerse the pressed samples for 72 h at 25 °C. The masses of the dry samples (before swelling) and of the swollen samples were measured. The swelling degree, Q, was calculated by using the gravimetric method26 (Equation (1)): (1) where: ms is the mass of the swollen sample, and m0 is the mass of the sample before swelling. ATR-FTIR analysis was performed in order to obtain information about chemical modifications of the GTR as result of the revulcanization process. A Perkin-Elmer Spectrum FT-IR with ATR accessory was used to record the spectra, from 4000 to 500 cm-1, at room temperature and at a resolution of 4 cm-1 over 20 scans. A thermogravimetric analyzer STA 449 F3 Jupiter was used to perform the thermogravimetric analysis (TGA), to make available information about the thermo-oxidative degradation behavior of the revulcanized samples. Around 10 mg of each sample was heated from


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25 °C to 550 °C under a nitrogen atmosphere to control the weight loss of rubbers and oil. Following, the gas flow was changed by an oxygen atmosphere, being the samples heated to 800 °C to monitor the degradation of carbon black. Both heating ramps were of 10 °C/min. Dynamic-mechanical properties of the revulcanized samples as function of the temperature were analyzed by a dynamic mechanical analyzer (DMA) of type DMA Q-800 TA Instruments. The analyses were performed under the following conditions: single cantilever mode, frequency of 1 Hz, temperature range from -100 to 100 °C and a heating rate of 1 °C/min. The dimensions of the samples were approximately 17.5x12.5x2 mm. The surface morphology of the samples was analyzed by Scanning Electron Microscopy (SEM) and by Atomic Force Microscopy (AFM). SEM was carried out on a JEOL microscopy model JSM-6010LV. AFM tapping mode microscopy was carried out on a Park systems microscopy model NX10. For both techniques, the devulcanized and revulcanized samples were pressed in a hydraulic press as mentioned before, resulting in sheets. For AFM, only the GTR5.5+ad sample was analyzed due to technical problems. The pressed samples for SEM were coated with gold by using a sputter coater.

Results and discussion Chemical modifications Figure 1b presents the resulted FTIR spectra of the revulcanized samples previously devulcanized under different exposure times to the microwaves. For sake of comparison and in order to improve the discussion, FTIR spectra of the devulcanized samples are also presented (Figure 1a).24



FTIR spectroscopy is a significant method to analyze the chemical modifications in organic compounds, being a valuable way to evaluate the chemical modifications on revulcanized GTR previously devulcanized by microwaves. Formerly, our research group presented an in-depth study about the modifications on the structure of GTR due to the devulcanization by microwaves, being that especially the chemical modifications were analyzed by ATR-FTIR.24 As the main conclusion, some bonds were broken and others were formed at the same time by the action of the microwaves, and these chemical modifications influenced the posterior revulcanization of these materials. First of all, it is important to address here that all the revulcanized samples present a very intense spectra, despite some noise observed on it due to the presence of high concentrations of carbon black,12,24 since it is able to highly absorbs the infrared radiation and, consequently, influences on the rubber FTIR spectrum.27 1080

GTR3+ad

(2)

Transmittance (a. u.)

GTR4

GTR5

(7)

(8)

(6)

GTR4+ad

GTR5+ad

(1)

GTR5.5+ad

GTR5.5

(a) 3000

(5) (4) (3)

GTR3

Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b) 2500

2000

1500

1000

500

3000

2500

-1

2000

1500

1000

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 1. FTIR spectra of the: (a) devulcanized, and (b) revulcanized GTR samples.

According to the spectra (Figure 1b), it can be clearly observed the main modifications occurred in the revulcanized samples from 2500 to 600 cm-1. In order to help with the analysis, Table 1 presents the main modified FTIR characteristic peaks.


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Table 1. Main bands observed in the analyzed samples.

Reference number

Wavenumber (cm-1)

Characteristic band

Reference

(1)

2350

–CO2

28

(2) (3)

1670–1540 1436

C=C C–H

29–32

(4)

1365

–CH3

34

(5) (6) (7) (8)

1300–1000 960 1000–800 800–600

S–O–C/C–O–C/S=O/S–O C=C HC=CH

11,21,27,35,36

C–S

27

33

37 35,37–39

The peak present between approximately 2350–2330 cm-1 (small and discreet, not very clear due to the noise), more evident in the samples GTR5+ad and GTR5.5+ad, is attributed to the carbon dioxide formed during thermal degradation of the main chains.28 The peaks present in the region 1670–1540 cm-1 are characteristic of C=C bonds of styrene butadiene rubber (SBR),29,30 as well as to  C=C stretching of natural rubber (NR),39 both present in GTR; to stretching vibration of a methyl-assisted conjugated double bond;31,32 and also related to carbon black aromatic ring stretching.30 According to Li et al., this peak is result of the reversion.40 At 1436 cm-1, the peak is assigned to methylene C–H bending,33 and to –CH2 deformation of NR.41 At 1365 cm-1, it can be assigned to –CH3 bands from NR.34 According to Shi et al., the peaks at 1436 and 1365 cm-1 are assigned to CH2 deformation, typical of NR.39 The peaks present in the region of 1300–1000 cm-1 can be assigned to C–O–C groups of the SBR.21,35 According to Formela et al., the presence of C–O peak is a result of partial oxidation of the main chains during reclamation of the GTR and posterior revulcanization.34 On the other hand, the peaks present in the region of 1160–1030 cm-1, according to some authors, can be assigned to S=O bonds, being also a sign of oxidation of the cross-linkings present in the compounds.36 According to Hirayama and Saron, bands between 1070 and 1370 cm-1 are assigned to chemical



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groups containing sulphur−oxygen bonds such as sulphoxides, sulphones, and sulphonamides.27 Besides, according to Aoudia et al., peaks present between 1300–1000 cm-1 are related to S–O–C bonds.11 The peak at 1068 cm-1 can be also assigned to the symmetric C–S–C group stretching vibrations in the two C–S bonds,42 as the ones at 1024 and 1000 cm-1.27 The axial vibration attributed to monosulphidic and polysulphidic bonds can be observed in the region of 800–600 cm-1.27 The peaks at 1000–800 cm-1 are related to C=C bonds from butadiene present in the SBR and in the NR,35,37–39 and the one at 960 cm-1 is attributed to the trans –HC=CH– groups vibrations of butadiene present in the SBR.37 Also, according to Shi et al., the peaks around 832 cm-1 are assigned to CH bending, and from 2969–2849 cm-1 for C–H saturated stretching, all them typical of NR.39 To quantitatively analyze the oxidation in the revulcanized samples, the peaks correlated to C–O–C, S–O–C and S=O/S–O bonds (1300, 1160 and 1080 cm-1, respectively) were normalized by dividing each respective height by that of the transmittance peak at 2870 cm-1, which matches to the C–H bonds of the main chains of the NR present in the GTR (Figure 2b). In the case of devulcanized samples, the peak correlated to S=O/S–O bonds (1080 cm-1) was normalized by dividing its respective height by that of the absorbance peak at 2870 cm-1 (Figure 2a). According to the results, all the revulcanized samples presented different oxidation levels, being that the GTR4+ad sample seems to be the most oxidized one. However, when the intensity ratios of the S=O/S‒O bonds of the devulcanized samples are compared, the ones of the revulcanized samples are smaller. In our earlier work, the discussion of the FTIR spectra of the devulcanized GTR samples depicted that the formation of the S–O new bonds would, probably, result from the rearrangement of the sulphur free radicals from the devulcanization of the GTR in the presence of oxygen, since during the process, bonds are broken and formed at the same time.24 Based on this, it seems that this rearrangement still occurred during the revulcanization process.


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Being that the peaks S‒O‒C can not be clearly observed in the spectra of the devulcanized samples, and the intensity ratios of the S=O/S‒O bonds in the revulcanized samples are smaller than in the devulcanized ones, this probably depicts this modification. 6

C-O-C S-O-C S=O/S-O

(b)

(a) 1.5

5

Intensity ratio

4

Intensity ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

1.0

0.5

1

0.0

0

GTR 3

GTR 5

GTR 4

GTR 5.5

GTR 3+ad

Sample

GTR 4+ad

GTR 5+ad

GTR 5.5+ad

Sample

Figure 2. (a) Intensity rations of the S=O/S‒O bonds of the devulcanized samples, and (b) intensity ratios of some chemical bonds related to oxidative processes during the revulcanization.

Figure 3 presents the swelling degrees of the devulcanized and the revulcanized GTR samples. According to the results, devulcanized samples (GTRX) presented higher swelling degrees than the revulcanized ones (GTRX+ad). Since the higher the swelling degree, the lesser the crosslinking density, the results proved that the devulcanized samples are able to revulcanize. As discussed before, links present in the structure of the revulcanized samples as S–O–C/C–O–C may also influence on the cross-linking density of these samples. Additionally, the results showed that the devulcanization degree seems to influence the cross-linking of the revulcanized samples. In the sample with a high devulcanization degree, their chains present a higher freedom during the revulcanization stage, which influences on the crosslinking density and swelling degree.



90

GTRX GTRX+ad

Swelling degree (%)

75

60

45

30

15

0 3

4

5.5 6

5

Exposure time of the GTR to the microwaves

Figure 3. Swelling degrees of the devulcanized and the revulcanized GTR samples, under different exposure times to the microwaves.

Thermo-oxidative degradation by TGA Figure 4 shows the TGA curves of the analyzed samples. For sake of comparison and deepening of the discussion of the obtained results, the TGA curve of the GTR devulcanized under different exposure times to the microwaves is also presented.24

GTR3+ad GTR3

100

GTR4+ad GTR4

100

80

Mass (%)

80

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

I

II

III

IV

60

II

I

III

IV

40

40

20

20

(b)

(a) 0

0 0

100

200

300

400

500

600

700

800

0

100

o

200

300

400

500

600

700

800

o

Temperature ( C)

Temperature ( C)


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GTR5+ad GTR5

100

GTR5.5+ad GTR5.5

100

80

Mass (%)

80

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

II

I

III

IV

40

60

III

II

I

IV

40

20

20

(c)

(d)

0

0 0

100

200

300

400

500

600

700

800

0

100

o

200

300

400

500

600

700

800

o

Temperature ( C)

Temperature ( C)

Figure 4. TGA curves of the devulcanized24 and revulcanized samples, exposed for different exposure times to the microwaves (minutes): (a) 3, (b) 4, (c) 5 and (d) 5.5. The graphics were separated by time of exposure of the sample to the microwaves for sake of clarity.

The first weight loss (from 25 to approximately 300 °C) submits to the decomposition of the processing oil and other organic polymeric additives25 (region I in the Figure 4), the second weigh loss submits to the NR decomposition (from around 300 to 400 °C)24 (region II in the Figure 4), the third one submits to the synthetic rubber decomposition (from 400 to 550 °C)24 (region III in the Figure 4) and the last one (from 550 to 800 °C) submits to the carbon black degradation 24 (region IV in the Figure 4), which takes place under oxidizing atmosphere. In the Figure 4, it can be observed that the devulcanized samples presented a higher thermal stability than the revulcanized ones, especially in the region III, with the exception of the sample exposed to the microwaves for 5.5 minutes (Figure 4d). The higher thermal stability of the devulcanized samples can be, probably, due to the high level of oxidation observed in the revulcanized samples by FTIR results. TGA and DTG curves of the sample GTR3+ad are shown in the Figure 5. In the Figure, the regions (a) to (e) are related to the decomposition of the different material present in the samples, as follows: (a) processing oil and other organic polymeric additives; (b) NR phase; (c) SBR or SBR1



(styrene butadiene rubber type 1) phase, (d) SBR2 (styrene butadiene rubber type 2), polybutadiene rubber (BR) phase or carbon black; and (e) carbon black. A deeper explanation about the meaning of each SBR type will be given in the sequence. 2 100

0

-6

60

-8 40

-10 -12

20

o

-4

Derivative mass (%/ C)

-2 80

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-14 -16

0

(b)

(a) 0

100

200

300

(c) (d) 400

(e) 500

600

-18 700

800

o

Temperature ( C)

Figure 5. TGA and DTG curves of the sample GTR3+ad.

TGA curves of the revulcanized samples are shown in the Figure 6a. In the Figure 6b, DTG curves of the samples are presented, between 300‒450 °C. Table 2 presents the weight loss percentages and the temperature ranges from TGA curves of the devulcanized samples (left column) and the revulcanized samples (right column).


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0

o

80

GTR3+ad GTR4+ad GTR5+ad GTR5.5+ad

-1

Derivative mass (%/ C)


100

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

-2

-3

-4

-5

(b)

(a) -6

0 0

100

200

300

400

500

600

700

800

300

320

340

360

380

400

420

440

o

o

Temperature ( C)

Temperature ( C)

Figure 6. (a) TGA and (b) DTG curves of the revulcanized samples.

Concerning the devulcanization, it was depicted in our earlier work24 that the NR phase was more degraded by the action of the microwaves, given that carbon black was preferably placed on the NR phase.12 Comparing the weight losses of the devulcanized and the revulcanized samples (300‒400 ºC), it can be observed that the value is lesser in the revulcanized samples, showing that the NR phase was even more degraded during the revulcanization process, but in a lighter way. Concerning the SBR phase, the weight loss of the SBR in the revulcanized samples is approximately the same than in the devulcanized ones, however, the ∆T was completely changed. In the devulcanized samples, around 25% of weight reduction was observed between 400‒550 ºC, while in the revulcanized samples, approximately 25% of weight loss was observed in the range of 400‒450 ºC, showing a strong decrease in the thermal stability of this phase in the revulcanized samples.



Page 16 of 37

Table 2. Weight loss percentages (m) and the temperature ranges (ΔT) from TGA curves of the devulcanized samples (left column)24 and the revulcanized samples (right column).

Sample

GTR3

GTR4

GTR5

GTR5.5

Δm (%)

ΔT (°C)

6.4 29.3 25.7 30.0

25‒300 300‒400 400‒550 550‒800

Residue: 8.6 4.3 25‒300 27.8 300‒400 26.3 400‒550 30.4 550‒800 Residue: 11.2 6.6 25‒300 22.9 300‒400 25.5 400‒550 36.4 550‒800 Residue: 8.6 5.6 25‒290 26.7 290‒390 27.4 390‒550 31.3 550‒800

Sample

GTR3+ad

GTR4+ad

GTR5+ad

GTR5.5+ad

Residue: 9.0

Δm (%)

ΔT (°C)

7.7 27.9 28.1 3.3

25‒300 300‒400 400‒450 450‒465

30.0 468‒800 Residue: 3.0 7.3 25‒300 24.2 300‒400 29.5 400‒450 4.0 450‒465 29.0 465‒800 Residue: 5.0 9.0 25‒300 23.0 300‒400 24.0 400‒450 3.0 450‒465 36.0 465‒800 Residue: 5.0 8.0 25‒300 22.0 300‒400 25.0 400‒450 6.0 450‒465 34.0 465‒800 Residue: 5.0

A new peak, compared to the devulcanized samples,12 can be observed in the DTG curve of the revulcanized samples (Figure 5), between 450‒465 ºC. Even not knowing the exact formulation of the samples, it is known that a huge amount of materials (such as antioxidant, antiozonant, fillers such as talc, silica, calcium carbonate, among many others) can be added to the elastomeric compounds, especially tires. It is also known that the main rubbers present in tires are NR, SBR and/or BR. However, previously, it was proved that the elastomeric phases present in the GTR (discussed in the present work) were only NR and SBR.24 As this peak was not observed in the


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DTG curves of the devulcanized samples, and as the best of our knowledge, it was not observed before in the literature, some assumptions can be made: (a) Apparently, the total mass reduction related to the SBR phase in the devulcanized samples has become two distinct mass losses, which can be verified in the DTG curves (Figure 5). It seems that the first mass loss (400‒450 ºC) is related to a more degraded SBR phase (called SBR1), and the second one (450‒465 ºC) is, probably, the SBR phase itself, i.e., a SBR phase with a lesser degradation degree (called SBR2). So, probably, it occurred the formation of two distinct groups of SBR with different average chain sizes. (b) Another possibility is, since the carbon black content on the samples increased due to the higher degradation level, the second mass loss can be a small amount of SBR phase containing a high concentration of carbon black (called SBR2). It is also important to mention the influence of the carbon black in the analysis. Garcia et al. proposed a schema showing the barrier effect and the increase of the mean free path of volatiles, as well as the adsorption of gas molecules in the carbon black pores.12 It can be clearly observed by the increase of noise (Figure 6b) in the curves as the exposure time of the sample to the microwaves increased during the devulcanization process, since the higher the degradation observed, the higher the amount of carbon black in the sample. (c) As mentioned above, the revulcanization process seems to act as an extra degradation step for the elastomeric phases, especially for the NR phase. Concerning the devulcanization process, it is known that, for example, the rearrangement of the sulphur free radicals from the devulcanization of the GTR in the presence of oxygen can originate new S‒O bonds,24 modifying the structure during the process. The modifications still happened during the revulcanization process, as depicted by the FTIR results previously discussed. So, since the chemical structure of SBR and BR are close, it is possible that a BR phase could have been formed as a recombination way of the NR and SBR broken chains. In addition, Lin et al. showed in their work that the degradation temperature of BR is



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around 450 ºC.43 Since BR shows low hysteresis combined with good wear characteristics,44 this possibility would bring together a high advantage. (d) The gas flow was changed by an oxygen atmosphere at 550 °C and the samples were heated to 800 °C in order to observe the carbon black degradation. However, according to the results (Figure 4), it was observed that the carbon black loss mass occurred in lesser temperatures (around 465 °C). Based on this, another possibility is the modification of the chemical structure of the filler itself, being that a small phase, possibly more oxidized and with a lesser thermal stability, could be formed along the revulcanization process, which presents a lesser degradation temperature. Some authors45 have studied the oxidative degradation of carbon blacks and found the main conclusions: (i) increase in the density of carbons on oxidation; (ii) formation of oxygenated functional groups; (iii) removal of amorphous carbon; and (vi) change of closed pores into open ones, being that the increase of the density would be caused by the change of the closed pores into open pores by the crevasses formation. All the mentioned factors may have influenced the obtained results. The deep understanding of this new phase will be the goal of a forthcoming work. When the DTG curves of the samples are analyzed (Figure 6b), it can be observed a kind of a 'peak' around 400 ºC, related to the intersection point between the DTG curves of the NR and SBR phases (kind of two valleys). This 'peak' is very clear for the sample GTR3+ad, and it tends to become less clear with the increase of the exposure time of the sample to the microwaves during the devulcanization process. This is, possibly, due to the enlargement of the DTG peaks of both phases, due to the increase of the degradation level of the chains, and consequent reduction of the chain size. Thus, with the increase of the exposure time of the samples to the microwaves and subsequent degradation of the chains during the revulcanization process, the division between the phases becomes more subtle, and apparently the final revulcanized material becomes a more homogeneous sample, similar to a 'pure polymer', losing the characteristic of polymeric blend, to be verified ahead


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in the AFM results. This behavior can be clearly observed in the literature in blends containing different concentrations of NR and SBR phases,46 showing that, in this case, the occurrence is due to the reduction of the phases as result of degradation processes. As discussed before, it can be clearly observed the increase of noise (Figure 6b) in the curves as the exposure time of the sample to the microwaves increased during the devulcanization process, since the higher the degradation observed, the higher the amount of carbon black in the sample. Additionally, the change of closed pores into open ones, as well as the change of closed pores into open ones by the formation of crevasses45 could be influenced too.

Morphological properties Garcia et al. deeply analyzed both physical and chemical changes of the GTR after different microwave exposure times.12 Given that GTR is a blend composed of NR, SBR and carbon black, these phases were clearly distinguished from the AFM tapping mode images and, consequently, its morphology could be observed (Figure 7).

Figure 7. AFM tapping mode phase images of the samples. Reprinted from12 with permission from Express Polymer Letters. Copyright 2015, Express Polymer Letters.



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The color scale represents the hardness scale in the AFM images, being a mixture of hard and soft segments, in which the darkest domains are the softer materials (in this case, NR phase), the clearest domains are the hardest materials (in this case, carbon black), and the middle color domains are the SBR phase. The images (Figure 7) showed the transformation of the morphologies as a consequence of the devulcanization process, in which the darkest phase (NR) are consumed and the clearest phase (carbon black) is increased as the exposure time of the GTR to the microwaves increased. NR is more susceptible to thermal degradation than the SBR due to its chemical structure,24 having a good affinity with carbon black, remaining carbon black on NR phase. It is known that carbon black is a conductive filler,47 which absorbs the microwave radiation and converts this energy into heat.27,48 So, by increasing the amount of carbon black in the NR, more microwave energy is absorbed and, consequently, the higher the degradation level of the NR sample.13,17 Figure 8 presents the AFM image of the GTR5.5+ad sample. It is important to be pointed out that the material analyzed in the mentioned work above12 is the same one used in the present work, and it was devulcanized by using the same experimental apparatus. In the present work, as mentioned before, due to experimental problems during the AFM procedure, only the image of the GTR5.5+ad sample could be obtained. The AFM surface image (Figure 8) shows the morphology of an intrinsically homogeneous material, quite different from the AFM images of the devulcanized samples (Figure 7), which exhibit characteristic morphology of polymeric blends. In other words, it seems that the revulcanization has changed the initial morphology of a typical polymeric blend to that of a homogeneous material.


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Figure 8. AFM tapping mode phase image of the GTR5.5+ad sample.

As depicted before, Figure 7 showed the consumption of the NR phase and the increase of the carbon black phase as the exposure time of the GTR to the microwaves increased. The TGA results previously discussed proved that the revulcanized samples suffered further degradation during the revulcanization process, probably reducing the chain size of the phases, which reduced their thermal stability. These reduced chains, with a higher freedom level, possibly mixed better, no matter the phase, resulting in a more homogeneous morphology. In any form, the revulcanized sample presents some clear domains, which demonstrates the high concentration of carbon black homogeneously dispersed and distributed in the other elastomeric phases. As the carbon black is a highly porous material and the NR has a great affinity for it, it may also have occurred that its degraded polymeric chains were adhered to its pores, which possibly contributed to the whitening of the image as a whole. As previously discussed, the behavior could be influenced also by the possible change of the closed pores into open ones, as well as the change of the closed pores into open ones by the formation of crevasses in carbon black as a result of oxidation.45



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SEM images of the pressed surfaces of the devulcanized and revulcanized samples are presented in the Figure 9.

(a)

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(g)

(h)

Figure 9. SEM surface images of the samples: (a) GTR3; (b) GTR3+ad; (c) GTR4; (d) GTR4+ad; (e) GTR5; (f) GTR5+ad; (g) GTR5.5; and (h) GTR5.5+ad.

According to our last work,24 GTR3 and GTR4 samples were not able to flow due to the low devulcanization degree, resulting from the microwave devulcanization process. One of the reasons to devulcanize a rubber is to make it flowable again. This can be observed in the Figures 9a to 9d, especially in the samples GTR3 and GTR3+ad (Figures 9a and 9b). The high amount of voids observed in both samples evidence their disability to flow during the pressing process, resulting in surfaces of low quality and high roughness. In addition, in these same samples (Figures 9a and 9b), it can be clearly observed some big particles (some of them are shown by arrows), these being, probably, vulcanized rubber particles which were not devulcanized during the devulcanization process. The impossibility of the GTR3 to flow was also observed by Garcia et al., being depicted by the authors that this sample was "not able to flow neither at high temperatures (190 °C) nor at high shear rates".49 The authors used the same devulcanization apparatus used in the present work. In the same way, the surface images of the samples GTR4 and GTR4+ad (Figures 9c and 9 d) are not regular as well due to the low devulcanization degree achieved by these samples.24 However, the surface is much more regular compared to the samples GTR3 and GTR3+ad (Figures 9a and 9b). In this case, the higher concentration of soluble fraction (sol) probably influenced the



Page 24 of 37

higher fluidity of these samples, allied to the presence of processing oil and the higher devulcanization degree achieved. In the other samples (Figures 9e to 9h), smother surfaces could be clearly observed due to a joint action of factors, such as the higher devulcanization degree of the samples, which provided the ability to flow during the pressing process, and the higher amount of sol fraction compared to the other samples. On the other hand, the surface of the revulcanized samples, GTR5+ad and GTR5.5+ad (Figures 9f and 9h) seems to be more rugged than the devulcanized ones (Figures 9e and 9g), including the presence of cracks (shown by an arrow) in the sample GTR5.5+ad (Figure 9h). The TGA results depicted a higher degradation level of the revulcanized samples, and a higher amount of carbon black present on these samples, which influenced on the flow during the pressing process. Another influence is the less amount of processing oil present on them, since it is evaporated or decomposed during the exposure of rubber to the microwaves.25 The photos of the pressed samples (revulcanized ones) are presented in the Figure 10. They agree with the SEM images previously analyzed. It is important to be noted the high superficial quality of the samples, especially the samples GTR5+ad and GTR5.5+ad (Figures 10c and 10d), since it is an important point to be considered in the application of the revulcanized rubbers. These samples presented smoother surfaces due to the same factors described earlier. Besides, the presence of a crack in the sample GTR3+ad (Figure 10a) evidences its disability to flow.

(a)

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(c)

(d)

Figure 10. Photos of the revulcanized samples: (a) GTR3+ad; (b) GTR4+ad; (c) GTR5+ad; and (d) GTR5.5+ad.

Dynamic-mechanical properties The storage modulus and tan delta curves as a function of the temperature of the samples are presented in the Figure 11. This technique is very valuable in analyzing the molecular stiffness of elastomeric materials before and after the glass transition temperature, Tg. The stiffness of the elastomeric material has contributions both from the primary chemical bonds among the chains (cross-linking density) and from the physical connections between the polymeric chains and a filler, in this work the carbon black, due to its high specific surface.50 The storage modulus results (Figure 11a) showed that the samples GTR5+ad and GTR5.5+ad presented the highest stiffness degrees due, as discussed before, to the lowest swelling degrees (which means higher cross-linkings densities), and to the higher carbon black amount present on them. The change of the carbon black morphology45 possibly influenced on the results as well, since the polymeric chains were probably more adhered to their pores. Concerning the tan delta results (Figure 11b), it was observed the widening of the peaks of tan delta with the increase in the exposure time of the sample to the microwaves during



devulcanization. As previously noted, these samples showed higher levels of degradation, which possibly resulted in the widening of the peaks. 0.7

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Figure 11. Storage modulus and tan delta curves as a function of the temperature of the samples.

Tg values of the samples were obtained from the maximum point of the tan delta curves (Figure 12). Since Tg of the sample is a measure of its cross-linking density, the samples GTR5+ad and GTR5.5+ad presented the higher cross-linking densities, as depicted before by swelling degrees results. This result is due to the higher freedom achieved by the polymeric chains during the revulcanization reaction, since these samples presented the highest devulcanization degrees during devulcanization step.24


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Figure 12. Tg of the analyzed samples.

For a sustainable development, recycling is a very important step, which, beyond saving the use of natural resources can also be a solution for low-income families, providing them earnings. Recycling is also considered a category of green chemistry. Adding up, it is known that rainwater can be accumulated in tires disposed in landfills, making them appropriate habitat for the proliferation of insects as the Aedes aegypti mosquito, which is vector of diseases such as dengue, chikungunya, yellow fever, and zika. As a result, recycling contributes to the reduction of pollution, bringing the improvement to the public health. However, recycling of a material trying to contribute to the environment will not bring positive results if the final properties of the recycled material do not point to an application that is viable.51 Thus, it is particularly necessary and of great relevance to know in-depth the modifications that the recycled material suffered during all the stages of the recycling process to which the material was subjected. Knowing its final properties, it is possible to decide the ideal application for the recycled material. As a main conclusion, this study pointed out the importance of the deeply understanding of the modifications occurred during the revulcanization process of devulcanized samples by the



Page 28 of 37

action of the microwaves. When compared to the properties of the devulcanized GTR, it was possible to observe several modifications as result of the revulcanization process, such as in the chemical structure, composition, thermal stability, and morphology. Additionally, the results pointed out also to the influence of the exposure time of the GTR to the microwaves during devulcanization process in all the properties mentioned above, beyond the dynamic-mechanical ones. They are schematically showed in the Figure 13, in which the AFM images of the devulcanized and revulcanized GTR samples are schematically presented.

Figure 13. Modifications in the revulcanized samples as result of the revulcanization process.


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Acknowledgements The authors would like to thank Prometeon Pneus for the material donation; Capes, FAPESP (process number 2010/15799-6) and CNPq (process number 201891/2011-5) for the financial support; Materials Engineering Department of Escola de Engenharia de Lorena - EEL USP and Prometeon Pneus for the laboratory facilities, and Centro de Microscopia Eletrônica da Zona Sul (CEME-Sul) of the Universidade Federal de Rio Grande (Furg) for the SEM images.

Author Contributions The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. All the authors contributed equally.

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TOC/Abstract graphic

Synopsis Recycling is a process towards the sustainable development, as it saves the use of raw materials from non-renewable sources, being also a category of green chemistry.


From devulcanization to revulcanization: Challenges in getting

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