devulcanized rubber natural rubber effects of

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Devulcanized rubber natural rubber diphenyl disulfide devulcanization mechanical properties

Thermo-chemical devulcanization of used truck tires was conducted using various amounts of diphenyl disulfide (DPDS) as a devulcanization aid. The ef-fects of different DPDS loadings on de-vulcanization efficiency and on the me-chanical and thermal properties of de-vulcanized rubber/natural rubber (D-GTR/NR) blends were studied. It was found that the soluble fraction increa-sed while the crosslink density decrea-sed with increases in the DPDS loading, indicating the breakdown of the rubber network and an increase in the devulca-nization efficiency. Upon blending the devulcanized rubber with virgin natural rubber, it was apparent that the DPDS affected the mechanical properties and had a significant influence on the ther-mal properties of the D-GTR/NR blends.

Effekte von Devulkanisations-hilfsmitteln auf die mechani-schen und thermischen Eigen-schaften von devulkanisiertem Gummi / Naturkautschuk-Ver-schnitte Devulkanisierter Gummi Naturkaut-schuk Diphenyldisulfid Devulkanisa-tion mechanische Eigenschaften

Thermo-chemische Devulkanisation von gebrauchten LKW-Reifen wurde mit verschiedenen Anteilen an Diphenyldi-sulfid (DPDS) als Devulkanisationshilfs-mittel durchgeführt. Es wurde der Ef-fekt von verschiedenen DPDS – Anteilen auf die Devulkanisationseffizienz und auf die mechanischen und thermischen Eigenschaften von devulkanisierten Na-turkautschuk (D-GTR/NR) Verschnitten untersucht. Es wurde festgestellt, dass die lösliche Fraktion zunimmt während die Vernetzungsdichte mit zunehmen-dem DPDS-Anteil abnimmt. Dies zeigt den Abbau des Netzwerks an, wobei die Devulkanisationseffizienz zunimmt. Beim Verschneiden des devulkanisier-ten Kautschuks mit ursprünglichen Na-turkautschuk war es offensichtlich, dasssich DPDS auf die mechanischen Eigen-schaften auswirkt.

Figures and Tables: By a kind approval of the authors.

IntroductionUsed tires are usually discarded, causing both a disposal problem and a waste of rubber. The difficulty in directly recycling waste tires stems from the three-dimen-sional polymer chain networks created by vulcanization. Therefore, over recent years, a variety of rubber recycling pro-cesses have been investigated and devel-oped [1-16]. These include thermo- and mechano-chemical processes [1-3], mi-crobial [4,5], ultrasonic [6,7], and other methods [8], of which comprehensive summaries have been published by Adhikari et al. [9] and Myhre et al. [10].

One well-known alternative is ther-mo-chemical devulcanization. This is per-formed by combining thermal treatment with the use of small amounts of devul-canization aids and this method has been found to give superior results [11-16]. Disulfides are recognized as a class of de-vulcanization aids and have been exploit-ed in rubber recycling as radical stabilizing agents. Several types of disulfide have been used in various types of waste rubber such as vulcanized natural rubber (NR) [11-13], styrene butadiene rubber (SBR), [14] and ethylene propylene diene rubber (EPDM) [15, 16]. Among the various types of disulfides, diphenyl disulfides (DPDS) have been claimed to provide the most effective devulcanization for NR [11-13].

In this study, thermo-chemical devul-canization of NR from used truck tires was performed using various amounts of DPDS as the devulcanization aid. The ef-fects of the DPDS loading on devulcani-zation efficiency were analyzed via both the soluble fraction and the crosslink density in the devulcanizates. Further, the potential of the devulcanizates in blends with virgin NR was also assessed, by inspecting the mechanical and ther-mal properties of the blends.

Experimental

MaterialsGround tire rubber (GTR) from waste truck tires, with an average particle size

of 0.40 mm, was obtained from Sang-Thai Rubber (Bangkok, Thailand). The GTR contained a 46% rubber content which is mainly NR, 32% carbon black, 8% acetone extract and 14% ash. The processing oil used in this study was treated distillate aromatic extract (TDAE) from Hansen & Rosenthal (Hamburg, Germany). The di-phenyl disulphide (DPDS), used as the devulcanization aid, was manufactured by Sigma-Aldrich, (Dorset, England). The virgin NR used was block Standard Thai Rubber (STR 20), manufactured by Tavorn Rubber Industry (Songkhla, Thailand). The zinc oxide and stearic acid used as activators were purchased from Global Chemical (Samutprakarn, Thailand) and Imperial Chemical (Pathumthani, Thai-land) respectively. The N-tert-bu-tyl-2-benzothiazolesulfenamide (TBBS), used as an accelerator, was manufac-tured by Flexsys (West Virginia, USA) and the sulfur used as a vulcanizing agent was supplied by Siam Chemical (Samut Prakarn, Thailand).

Effects of Devulcanization Aid on mechanical and thermal Proper-ties of devulcanized Rubber/Vir-gin Natural Rubber Blends

AuthorsS. Saiwari, K. Waesateh, A. Worlee, N. Hayeemasae, Pattani, C. Nakason, Surat Thani, Thailand

Corresponding Author:S. SaiwariDepartment of Rubber Technology and Polymer ScienceFaculty of Science and TechnologyPrince of Songkla UniversityPattani CampusPattani 94000, ThailandPh: +6673-312-213Fax: +6673-331-099E-Mail: [email protected]

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Preparation of the devulcanized rubberThe devulcanized rubber was prepared by mixing the GTR with 5 wt% TDAE and 30 mmol of DPDS per 100g of rubber. The TDAE was added to facilitate the diffu-sion of the DPDS (i.e. as a carrier) into the rubber. The mixing was first performed at room temperature, following which it was conditioned at 60°C for at least 30 min, in a hot air oven. The ther-mo-chemical devulcanization was then carried out in an internal mixer (Bra-bender® GmbH & Co.KG, Duisburg, Ger-many) at a devulcanization temperature of 220°C for 6 min with a fill factor of 0.60 and a rotor speed of 60 rpm. DPDS loadings at concentrations of 15, 30, 45 and 60 mmol/100g rubber were then added. After thermo-chemical devulcani-zation, the mixed material was immedi-ately removed from the internal mixer and quenched in water in order to reduce degradation and to interrupt the oxida-tion cycle, and it was then dried in an oven at 50°C for 48 hrs.

Preparation of devulcanized ground tire rubber (D-GTR)/NR blendsThe D-GTRs prepared with various amounts of DPDS were blended with virgin NR in an internal mixer (Brabender Plasticorder, model 50EHT 3Z), using a fill factor of 0.75 and a constant rotor speed of 50 rpm at 50°C as rubber formulations shown in Table 1. First the D-GTR and NR with a fixed blending ratio of D-GTR/NR = 20/80 were simultaneously masticated for 2 min in the mixing chamber. Then, the other ingredients (i.e., 2 phr of stearic acid, 5 phr of ZnO, 1.5 phr of TBBS and 2.5 phr of sulfur were sequentially incorpo-rated for 1 min each. The blend was then removed from the mixing chamber and compressed in a laboratory-size two-roll mill into 15 × 33 cm2 sheets with a final thickness of approximately 2 mm.

Testing and characterizationRubber soluble fraction. – The soluble (Sol) and insoluble (Gel) fractions of the devulcanized rubber samples were de-termined by extraction using a Soxhlet apparatus. Each devulcanized rubber sample was initially extracted with ace-tone for 48 h, in order to remove low molecular weight polar substances, such as the remnants of the accelerators and curatives. Then, the acetone-extracted material was re-extracted with tetrahy-drofuran (THF) for 72 h in order to re-move the non-polar components includ-ing oil, non-crosslinked polymer resi-

dues, and soluble polymers that had been released from the molecular net-work because of chain scission during devulcanization. Then, the extracted product was dried in a vacuum oven at 40°C to remove the residual solvent, un-til a constant weight was reached. The Sol fraction was then calculated as fol-low:

Sol fraction ( )% = W0 – W1

W0 × 100 (1)

where W0 and W1 are weights of the sample before and after extraction, re-spectively.

Crosslink density. – Crosslink densities of filled vulcanizates are typically estimat-ed by the Flory-Rehner equation [17] with the Kraus correction [18]. In this work, the extracted rubber samples were first swollen in toluene for 72 h at room temperature, before the solvent was re-moved by filtration. The weight of the swollen samples was then measured af-ter removing excess solvent from the surfaces. The crosslink density was then estimated from the Flory-Rehner equa-tion, as follows:

νe =Vr + xVr

2 + ln(1 – Vr)

Vs �0.5Vr – Vr

13�

(2)

(3)

where νe is the crosslink density per unit volume; Vr is the polymer volume frac-tion in the swollen sample; Vs is the sol-vent molar volume; mr is the mass of the rubber network; ms is the weight of sol-vent in the sample at equilibrium swell-ing; ρr is the density of the rubber; ρs is the density of the solvent; and x is the Flory-Huggins polymer-solvent interac-tion parameter.

The Kraus correction was also applied to correct the crosslink densities of the filled rubber vulcanizates, as follows:

(4)

(5)

where νapparent is the uncorrected esti-mate of the chemical crosslink density

Fig. 1: Solution fraction as a function of amount of devulcanization aid (DPDS).

1

1 Formulation and blending sequence of D-GTR/NR blends and control sample.

Ingredients Content (phr) Mixing time (min)Control D-GTR/NR Blends

Natural rubber (STR 5L) 100 80 2Stearic acid 1.5 1.5De-vulcanized rubber - 20 1Zinc oxide (ZnO) 5 5 1TBBS 1.5 1.5 1Sulfur 2.5 2.5 1



(from Eq. 2); νactual is the actual chemical crosslink density; K is a constant for a given filler; Φ is the volume fraction of filler in the specimen; and Wb and Wa are the weights of the specimen before and after extraction, respectively.

During the swelling test the soluble fraction is extracted and this needs to be taken into account to correctly obtain the real crosslink density (νreal) after devulca-nization. Its effect is through the total vo-lume of rubber, and the correction to the real crosslink density is as follows:

νreal = Number of crosslinks

Volume of total rubber = νactual × (1-Sol fraction)

νreal = Number of crosslinks

Volume of total rubber = νactual × (1-Sol fraction)

(6)

where νreal is the final corrected crosslink density of the devulcanizate.Cure characteristics. – All the blends were tested for their cure characteristics at 170°C using an oscillating disk rheome-ter (ODR 2000) (Alpha Technologies, Ohio, USA). The scorch time (Ts1), 90% curing time (Tc90), minimum torque (ML) and maximum torque (MH) of the all blends were then established. The blends were then vulcanized according to the cure time (Tc90) plus 5 min in a laboratory compression moulding press at 170°C and 100 bar, to form 2 mm thick sheets.

Mechanical properties. – The tensile properties were determined using dumb-bell shaped specimens according to ASTM D 412, using a Hounsfield Tenso-meter model H 10KS (Hounsfield Test Equipment Co., Ltd., Surrey, England). The tests were performed at a constant cross-head speed of 500 mm/min, using a 500N load cell. The hardness of each sample was measured with a Toyo seiki hardness tester, Shore A type, according to ASTM 2240.

Thermal properties. – Thermal analy-sis of the rubber blends was conducted with a simultaneous thermal analyzer (STA 6000, Perkin Elmer Inc., Norwalk, Connecticut, USA). Each sample of appro-ximate weight 10 mg was placed in a ceramic pan and then heated at a hea-ting rate of 10°C/min across temperatu-res from 30 to 800°C, under nitrogen at-mosphere.

Results and discussion

Devulcanization efficiency of the D-GTRThe soluble fraction and crosslink densi-ty of the D-GTR using various amounts of

DPDS, are shown in Figures 1 and 2, re-spectively. It can be seen that the soluble fraction increased as the crosslink densi-ty decreased with increasing DPDS load-ings. The increase of the soluble fraction of the devulcanizates can probably be attributed to the breakdown of the rub-ber network during devulcanization. Moreover, the significant decrease in crosslink density clearly indicates that the devulcanization aid (i.e., DPDS) as-sisted in disintegrating the rubber net-work, in a dose-dependent manner. These experimental results are in agree-ment with several reports [11, 12, 14], that is, the utilization of de-vulcanization aids increases de-vulcanization efficien-cy. Basically, NR network can be effec-tively cleaved at temperatures above 170°C [11]. Typically, a devulcanization aid is used in order to scavenge the radi-

cals that are formed during devulcaniza-tion, and DPDS significantly enhances the devulcanization process with the temperature seemingly the main factor governing the level of devulcanization. The mechanism of radical scavenging by DPDS in the devulcanization of sulfur cured rubber vulcanizates involves the opening of crosslinks or the scissoring of the rubber main chains by heat and shearing forces. The fragments from this process then react with the disulfide based radicals, which prevents recombi-nation [11]. Thus, increasing the DPDS loading prevents the broken rubber chains from recombining and the soluble fraction is also significantly increased with increasing amounts of DPDS.

A useful tool to further assess the de-vulcanization mechanism was developed by Horikx [20] who noted that the rubber

Fig. 2: Crosslink densities as a function of the amount of devulcanization aid (DPDS).

2

Fig. 3: Sol generated during devulcanization versus the relative decrease in crosslink den-sity of D-GTR with various DPDS loadings.

3



soluble fraction in the rubber devulcani-zates and the crosslink density of the rubber Gel fractions are correlated. There is, therefore, a theoretical relationship between the Sol generated by the degra-dation of polymer networks and the rela-tive decrease in the crosslink density with degradation either by main-chain scission or by crosslink breakage.

This form of polymer degradation can also be applied to rubber recycling, whe-re a mix of main-chain scission and crosslink breakage also takes place du-ring the devulcanization process. The re-lationship between the main-chain scis-sion and the relative decrease in cross-link density is given by:

1 − νf

νi= 1 −

⎣⎢⎢⎡�1 − sf

12� �

2

�1 − si

12� �

2

⎦⎥⎥⎤

(7)

where si is the Sol of the rubber network before degradation or recycling, sf is the Sol of the recycled rubber, νi is the crosslink density of the network prior to treatment and νf is the crosslink density of the recycled rubber.

Furthermore, the pure crosslink scissi-on can be estimated via the correspon-ding relationship:

1 − νf

νi= 1 −

⎣⎢⎢⎡γf �1 − sf

12� �

2

γi �1 − si

12� �

2

⎦⎥⎥⎤ (8)

where the parameters γf and γi are the average numbers of crosslinks per chain in the insoluble network after and before recycling, respectively. The values for γf and γi are determined as previously de-scribed by Verbruggen [16].

Figure 3 shows a graphical representa-tion of the calculation based on equa-tions 5 and 6. The two curves in this fig-ure correspond to the extreme pure modes where only main chains are bro-ken (solid curve) or only crosslinks are broken (dashed curve). It is evident that in pure crosslink scission, almost no solu-ble fragments are generated until most of the crosslinks are broken. Only then can the long chains be removed from the network. In main-chain scission, the Sol is immediately produced at the begin-ning of the earlier stage, because ran-dom scission of the polymers in the net-work results in small loose chains, which can easily be removed. From Figure 3, it is clear that all the experimental data points are located on the right hand side of the graph, which indicates cleavage of

the devulcanized rubber network. It can also be seen that the devulcanization of the D-GTR with various DPDS loadings as a function of the relative decrease in crosslink density resulted in more than 60% Sol and the decrease in crosslink density reached 90%. Furthermore, in-creases in the DPDS loadings tend to move the experimental observations up-wards in this plot, indicating an increased Sol. As previously stated, an increase in the rubber Sol indicates more break-down of the rubber network during de-vulcanization. Therefore, more extensive breakdown of the rubber network is also indicated by the shifting of the experi-mental data points to the right in the Horikx plot (i.e., Figure 3). As a result, a decrease in crosslink density was found with increasing DPDS loadings. It should be noted that in a typical thermo-chemi-cal devulcanization, the devulcanization agent (i.e., the DPDS in this case) is added in order to scavenge radicals that are formed during the devulcanization pro-cess. The current results confirm that a more extensive breakdown of the rubber network was clearly evidenced when in-creasing the DPDS content. However, in-creasing the loading of DPDS also causes shifting of the data points towards the theoretical curve representing main-chain scission. Since NR molecular chains are inclined to break at high tempera-tures above 170°C [11] non-selective breakdown of the rubber network should be evident at elevated temperatures.

Curing characteristics of the D-GTR/NR blendsFigure 4 shows the cure curves of the D-GTR/NR blends with various DPDS loadings. The cure characteristics are de-picted in Table 2. It can be seen that the maximum torque decreased with in-creasing DPDS loadings. This might cor-respond to an increasing trend in the Sol with higher DPDS loadings (Figure 1). Thus, with a lower amount of DPDS at 15 mmol/(100 g rubber), a low Sol was ob-served, indicating a high fraction of the remaining rubber network. This caused higher stiffness and hence curing torque in the revulcanized rubber blends. On the other hand, the blends obtained from the revulcanizates with higher DPDS loadings showed higher Sol with less re-maining rubber networks. Furthermore, the soluble fragments might behave as a lubricant during the oscillation of the rheometer disk at higher temperatures because of the lower maximum torque.

Fig. 4: Cure curves of D-GTR/NR blends with various DPDS loading.

4

2 Curing characteristics of D-GTR/NR blendsCompound type

ML (dN.m)

MH (dN.m)

MH - ML, ΔT (dN.m)

Scorch time, ts1 (min)

Cure time, tc90 (min.sec)

Pure NR 7.50 25.00 17.50 1.42 2.68A15 8.90 27.90 19.00 1.17 3.63A30 8.50 25.40 16.90 1.55 4.55

A45 8.60 25.60 17.00 1.32 4.05A60 7.90 23.30 15.40 1.54 4.47



In Figure 4, it can also be seen that the scorch time and cure time of the D-GTR were not significantly affected by differ-ent loadings of DPDS. This is because the scorch time and the cure time generally depend on the curing system used par-ticularly the reactivity of the cure accel-erator. Also, the radicals newly formed during the devulcanization of the GTR were more or less completely converted to stable covalent bonds that could not significant influence the cure reactions.

The effect of DPDS loading on the complex viscosity of the D-GTR/NR blends is shown in Figure 5. The presence of DPDS resulted in a reduction in the crosslink density of the D-GTR, making more free molecular chain and curing sites in the D-GTR. Upon revulcanization of the blends, higher crosslinking was clearly observed leading to an increase in the complex viscosity of the blends. However, when more DPDS was added, there were two competitive reactions taking place, crosslinking and chain scis-sion. Further, higher complex viscosity was observed in the D-GTR/NR blends than in pure NR. This finding is consist-ent with the torque difference observed, suggesting that higher crosslinking oc-curred because of the inclusion of DPDS.

Mechanical properties of the D-GTR/NR blendsStress-strain curves, moduli, tensile strengths and elongations at break for the D-GTR/NR blends with various DPDS loadings are shown in Figures 6, 7 and 8, respectively. In Figure 6, it can be seen that the moduli at 100% and 300% strains decreased with increasing DPDS loading. This can be attributed to the formation of short free chains during devulcanization, corresponding to an in-creasing amount of DPDS. In Figure 7, it can also be seen that the tensile strength decreased with increased loading of DPDS from 15 to 30 mmol/(100 g rub-ber), but there were no significant changes in the tensile strength when the DPDS was increased from 30 to 60 mmol/(100 g rubber). The higher moduli and strength properties of the blend with 15 mmol/(100 g rubber) DPDS might be because this blend had the lowest soluble fraction and the highest crosslink density. It should be noted that the lowest Sol in the D-GTR indicates the least main chain scission, which is the optimum condition since it results in the strongest devulcanized rubber, which benefits from long high-molecu-

lar-mass rubber main chains. Further-more, the highest crosslink density indi-cates the greatest content of remaining rubber networks. Therefore, the blend with 15 mmol/(100 g rubber) DPDS had the highest modulus and tensile strength, attributable to the high molec-ular mass of rubber main chains and the high content of remaining rubber net-works.

As can be seen from Figure 7, the elon-gation at break increased with increasing DPDS loadings, which corresponds to the increase in Sol and the crosslink density of the D-GTR illustrated in Figures 1 and 2 respectively. That is, when the soluble

fraction of the D-GTR is high, the chain mobility is also high, and this gives high elongation at break. Therefore, the blend containing 60 mmol/(100g rubber) of DPDS had the highest elongation at break.

Thermogravimetric analysis of D-GTR/NR blendsThermogravimetric analysis measures the amount and rate of the mass transi-tion of a sample as a function of tem-perature or time in a certain atmos-phere and is used primarily to deter-mine the thermal stability and composi-tional properties of materials. The

Fig. 5: Complex viscosity of D-GTR/NR blends with va-rious DPDS loa-ding.

5

Fig. 6: Stress-strain curves of D-GTR/NR blends with various DPDS loadings.

6



thermal decomposition behaviour and derivative weight thermograms of the D-GTR/NR blends are shown in Figure 9. The decomposition temperatures at on-set and the maximum weight loss and char residue are also listed in Table 3. A single region of degradation was ob-served with an initial minor weight loss at around 180–200°C followed by the loss of volatile matter such as water, stearic acid and oil at around 300°C. The major region of degradation of the blends was from 330°C to 450°C (see Figure 9A). This region of the decompo-sition was due to the degradation of the NR segments that corresponds to the major peaks observed in the DTG curve (Figure 9B). The degradation of the NR segment is sensitive to the presence of oxidized structures as well as the deple-tion of sulfidic crosslinks in the NR. It is clear that the degradation tempera-tures) of the D-GTR/NR blends which were around 390°C were higher than that of the pure NR of about 375°C. However, there were no significant dif-ferences in the decomposition tempera-tures of the D-GTR/NR blends with vari-ous DPDS loadings.

The higher degradation temperatures of the D-GTR/NR blends were attributed to successful devulcanization of the D-GTR which increased the curing sites on the rubber main chains. As a result, more crosslinking took place after revulcaniza-tion and it is interesting to highlight that the major peak of degradation was rela-ted to the depletion of sulfidic crosslinks in the rubber [11]. The higher crosslin-king found resulted in the devulcanized rubber requiring higher temperatures to

Fig. 7: Modulus of D-GTR/NR blends with various DPDS loadings.

7

Fig. 8: Tensile strength and elongation at break of D-GTR/NR blends with various DPDS loadings.

8

Fig. 9: Thermogravimetric analysis of D-GTR/NR blends with various DPDS loadings: (a) weight loss curves, and (b) their derivatives.

9



3 Onset of degradation temperature (Tonset), maximum degradation temperature (Td fs)and char residue of D-GTR/NR blends.Sample Tonset (°C) Td fs (°C) Char residue (%)NR Pure 345.77 375.20 3.52A15 344.65 389.52 14.79A30 355.69 390.38 13.16A45 355.77 390.91 13.96A60 351.51 390.17 13.44

cleave crosslinks, even though the ther-mal stability reduced with further incre-ases in the DPDS loading. This is a very good indication that successful devulca-nization was achieved and the lower de-gradation temperature found is respon-sible for the cleavage of crosslinking gai-ned at the curing site of the devulcanized rubber, which would not have occurred with less efficient devulcanization.High-er thermal degradation accompanied by low devulcanizing efficiency may be due to the original crosslinking prior to the devulcanization process.

ConclusionsThermo-chemical devulcanization of NR from used truck tires, using various amounts of DPDS as the devulcanization aid, was investigated experimentally. The effects of the DPDS loading on devulcan-ization efficiency were analyzed via both the Sol and the crosslink density in the devulcanizates. It was found that the sol increased while the crosslink density de-creased with increases of the DPDS load-ing. These results indicated that the breakdown of rubber network increased with an increasing DPDS content; that is, non-selective breakdown of the rubber network may be predominant. In addi-tion, blends of the devulcanizates with virgin NR were prepared and various properties investigated including their mechanical and thermal properties. It was found that the different DPDS load-ings solely affected the mechanical prop-erties but did not affect the thermal properties of the D-GTR/NR blends. Fur-thermore, the moduli at 100% and 300% strain and the tensile strength decreased, while the elongation at break increased with increasing DPDS loadings. It is inter-esting to highlight that even though the tensile strength and thermal stability re-duced after increasing the amount of DPDS, it was still higher than that of pure NR. Therefore, it can be concluded that a low amount of DPDS (15 - 30 mmol) is optimal for D-GTR/NR blends and this system is also recommended based on the higher strength and lower manufac-

turing costs of the D-GTR/NR blends produced.

AcknowledgementsThe authors would like to thank the High-er Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission. Contract No. SAT580649S for financial support.

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