Significant Factors Affecting the Thermo-Chemical De-vulcanization Efficiency of Tire Rubber

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1118-1123

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

Significant Factors Affecting the Thermo-Chemical

De-vulcanization Efficiency of Tire Rubber

Anuwat Worlee , Sitisaiyidah Saiwari , Wilma Dierkes , Siti Salina Sarkawi

Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani, 94000, Thailand.

Department of Mechanics of Solids, Surfaces and Systems, Elastomer Technology and Engineering, University of Twente,

500AE Enschede, the Netherlands

Rubber Research Institute of Malaysia, Malaysian Rubber Board, P.O. Box 10150, 50908 Kuala Lumpur, Malaysia

Received: 13/05/2020

Accepted: 09/07/20xx

Published: 20/09/2020

Abstract

In this study, the influence of the molecular structure of the rubber, the carbon black loading and de-vulcanization time and temperature

on the thermo-chemical de-vulcanization efficiency of whole tire rubber was investigated by correlating sol fraction and crosslink density

(Horikx-Verbruggen method). Differences in molecular structure influence the de-vulcanization mechanisms of rubbers as well as the

efficiency. Increasing carbon black loadings result in higher crosslink densities due to a deactivation of the de-vulcanization aid. Variation

of de-vulcanization temperature and time results in different degrees of heat accumulation in the rubber during de-vulcanization and thus

leads to different de-vulcanization efficiencies.

Keywords: Tire rubber; De-vulcanization; Recycling; Carbon black

Introduction¹

comparable de-vulcanized and virgin rubbers due to the

uncontrolled polymer scission which occurs during the reclaiming

process. De-vulcanization targets at the sulfuric crosslinks in the

vulcanized rubber, to selectively cleave C-S and S-S bonds. These

strength of these bonds differs: -C-S-C (285 kJ/mol), -C-S-S-C-

When mentioning the environmental pollution problems that

almost every country is facing today, it is inevitable to discuss the

issue of non-biodegradable waste like vulcanized elastomers and

plastic. The molecular structure of these materials results in

outstanding water resistance and in-conduciveness to growth of

microbes. In addition, elastomers have a very strong network. As

a result, these materials when becoming waste are difficult to

decompose: some types may take more than 100 years to

biodegrade completely in the environment.

Used tire rubber is one of polymer materials that is difficult to

decompose. It is a durable material made of complex components

consisting of various types of rubbers, reinforcing fillers and

fabric. Therefore, disposal in the environment or usage of

inappropriate methods of removal may cause environmental

pollution in the future. Recycling and re-utilization of used rubber

pose a great challenge. For end-of-life tires, incineration is

currently the main outlet, impeding the re-use of this valuable raw

material in new rubber products. Two recycling processes of end-

of-life tires are well known: reclaiming and de-vulcanization.

These two methods are often referred to as similar processes, but

they are fundamentally different concerning the chemical reaction

to break sulfur crosslinks, the ratio of crosslink scission to

network breakdown, and the molecular structure of the polymeric

material (Fig. 1). Reclaiming is usually accompanied by

considerable scission of the polymeric chains resulting in a lower

molecular mass fraction and poorer mechanical properties than

(268 kJ/mol) or –C-S -C- (251 kJ/mol). This can be one way to

selectively break sulfur bonds in a crosslinked elastomer (1). A

considerable share of material recycling can only be achieved if

tire material can be used in real recycling loops: tires back into

tires. This requires high-quality recycled rubber products, which

can only be produced by a tailored de-vulcanization process.

Within this study, vulcanized rubber was de-vulcanized and

its efficiency was investigated concerning the tendency for

crosslink versus main-chain scission. Thermo-chemical de-

vulcanization using the optimum conditions proposed by Saiwari

and co-workers (2) was applied to rubbers used in passenger ca

tires: styrene-butadiene rubber (SBR), brominated butyl rubber

(BIIR), natural rubber (NR) and butadiene rubber (BR). The goal

of this project was to elucidate the influence of the molecular

structure of the elastomer on the de-vulcanization efficiency, and

to understand the influence of carbon black in a thermo-chemical

de-vulcanization process of the tire rubbers. The effect of carbon

black loading (i.e., 30, 60, 90 phr) and de-vulcanization time and

temperature on the de-vulcanization efficiency were also studied.

The mechanisms behind the rubber network breakdown of the

carbon black filled single rubbers will be investigated and

discussed. The understanding of the factors affecting the de-

Corresponding author: Sitisaiyidah Saiwari, Department of Rubber Technology and Polymer Science, Faculty of Science and Technology,

Prince of Songkla University, Pattani, 94000, Thailand. Email: s.saiwari@gmail.com.

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2020, Volume 8, Issue 3, Pages: 1118-1123

vulcanization mechanism of vulcanized rubber are of major

importance for the development of an efficient tire recycling.

(SBR1502) produced by BST Elastomers (Thailand), Bromo-

butyl rubber produced from KIJ PAIBOON (Thailand) and

Natural Rubber (RSS#3) obtained from Yang Thai PakTai

Company, Thailand. Treated distillate aromatic extract or TDAE

oil (Vivatec 500) was supplied by Hansen&Rosenthal KG

(Hamburg, Germany). Diphenyl disulfide, DPDS was used as the

de-vulcanization aid was obtained from Sigma-Aldrich Company

Ltd., England. The ingredients for preparing rubber compounds

were ZnO (Global Chemical, Thailand), stearic acid (Imperial

Chemical, Thailand), TBBS (Flexsys, Belgium), and sulfur (Siam

Chemical, Thailand). Carbon black (N330) with a particle size of

4-32 nm and a CTAB surface area of 75 - 85 m /g was used.

They were supplied by Polychem Chemicals, Thailand.

Figure 1: Simplified scheme of the two reactions occurring during

rubber recycling processes: reclaming and de-vulcanization (2)

Background

The de-vulcanization of end-of-life of tire is a major challenge

and was studied extensively over the past several decades. Many

researchers attempted to develop a new de-vulcanization method

such as mechanical (3, 4) thermo-mechanical (5) mechano-

chemical (6), and supercritical CO

processes (7). Using microbes

(

8, 9, 10) and ultrasound (11, 12, 13, 14, 15, 16) in de-

vulcanization is also extensively studied. Among the de-

vulcanization processes, several methods were successfully

developed in the laboratory; however, after re-vulcanization, the

material exhibits poor mechanical properties compared to the

starting vulcanizate due to aging during service life, and the fact

that whole tire material is a blend of different compounds,

polymers and fillers. A detailed study of some possibly

complicating factors such as type of rubber: NR, SBR, BR, IIR,

and presence or absence of fillers is needed. Thermo-chemical de-

vulcanization by using diphenyl disulfide (DPDS) or other

disulfides used as de-vulcanization aid is considered as an

alternative way of recycling of used tires, which is widely studied

nowadays. The main reasons to use this process and these de-

vulcanization aids is that it is a rather simple process, applicable

in the industrial sector, and low cost as well. The de-vulcanization

mechanism of this method is shown in Fig. 2. There are two

possible major reactions: The first one is thermal scission of the

rubber network due to the elevated temperature of de-

vulcanization. When thermal vibrations overcome bonding

energies of S-S bonds or C-S bonds, the rubber network

breakdown will occur mainly at the sulfur crosslinks being the

weakest bonds in the network. At excessive temperatures, the

rubber network is subjected to random scission: breakage of the

rubber network will occur both ways, at the main chains of the

polymer (C-C bonds) and at the sulfur crosslinks. For this reason,

the free radicals generated during de-vulcanization include both,

carbon radicals from main chain scission and sulfur radicals from

sulfur bridge breakage. The second reaction is trapping of free

radicals generated in the first reaction by sulfide radicals of

DPDS, to prevent the recombination of broken rubber chains (Fig.

Figure 2: Simplified reaction scheme proposed for radical scavenging by

DPDS in unfilled rubber (17)

.2 Preparation of vulcanized rubber

Vulcanized tire rubbers were prepared by incorporation of

vulcanization ingredients such sulfur, activators and accelerators,

as well as anti-oxidants into NR, SBR, BR and BIIR in an internal

mixer with a mixing chamber volume of 50 cm . The mixer was

operated at a rotor speed of 60 rpm, a fill factor of 0.75 and an

๐

initial temperature of 50 C.

Table 1: Basic formulations of the rubber compounds

Ingredients

Amount (phr.)

SBR

100

BIIR

Zinc oxide

Stearic acid

TBBS

1.5

100

1.5

100

0.5

2.5

Sulfur

The compounding formulations are simplified as shown in

๐

Table 1. Cure time (tc,90) at a temperature of 170 C was measured

in an oscillating disc rheometer (ODR 2000). The cure time was

used to vulcanize the rubber compounds in a compression

molding machine at 170°C and 100 bar, into 2 mm thick sheets.

). The trapping efficiency of free radicals in this reaction

depends on the amount of sulfide radicals of DPDS.

Experimental

.3 Preparation of de-vulcanized rubber

The vulcanized rubber sheets were subsequently ground in a

.1 Materials

The rubber materials used in this study were butadiene rubber

Fritsch Universal Cutting Mill Pulverisette 19 (Fritsch, Germany)

with a 2 mm screen at room temperature. Thermo-chemical de-

(

BR) and emulsion-polymerized styrene butadiene rubber

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2020, Volume 8, Issue 3, Pages: 1118-1123

vulcanization was performed batch wise in an internal mixer. A

fill factor of 0.7, a constant rotor speed of 50 rpm and a chamber

temperature of 220C was used as de-vulcanization conditions.

The de-vulcanization of ground rubbers were carried out using the

optimized process conditions as elaborated for the gum tire

rubbers (Saiwari et al., 2013) as given in Table 2. DPDS (30

mmol/100 g rubber) was blended with 5 phr of TDAE oil relative

to the polymer content of the rubber, and then mixed with the

Results and Discussion

.1 Influence of rubber molecular structure

The influence of the rubber molecular structure on the thermo-

chemical de-vulcanization efficiency of unfilled vulcanized

rubber can be seen in Fig. 3. The position between the crosslink

and main chain lines of the data points of each sample is different.

This indicates that the de-vulcanization mechanisms of the

rubbers differ. In case of NR, it is known that the molecular chains

have a low thermal resistance and are destroyed easily at higher

๐

ground rubber. It was heated in an oven at 60 C for 30 minutes

before feeding it into the mixing chamber. The de-vulcanization

was carried out at 220C, and the de-vulcanization time was 6

min. After de-vulcanization, the material was taken out of the

internal mixer and directly quenched in liquid nitrogen and

subsequently stored in a refrigerator to avoid an oxidation.

๐

temperatures (220 C). Therefore, almost 100% sol fraction and a

crosslink density reduction to almost zero are observed in unfilled

NR de-vulcanizates. This indicates that the NR network is

completely broken during de-vulcanization. For SBR, the data

point is located on the crosslink scission line, with a remaining

crosslink density decrease of about 65% relative to the untreated

SBR. This is attributed to the de-vulcanization conditions used in

this investigation, as elaborated by for unfilled SBR by Saiwari

(19). The de-vulcanization efficiency for BIIR is rather close to

the value measured for SBR. This might be expected as both

elastomers, SBR and BIIR, are co-polymeric synthetic rubbers

(Fig. 4) with a lower concentration of double bond units in the

carbon backbone compared to BR and NR. As the double bond is

heat-sensitive, these polymers are expected have similar heat

resistance properties. In the case of BIIR, the bromine atom can

be thermally split off and it may interfere with the radical trapping

of DPDS, causing a lower de-vulcanization efficiency.

.4 Characterization of the de-vulcanizates

.4.1 Rubber soluble fraction

The soluble (Sol) and insoluble (Gel) fractions of the

vulcanized and de-vulcanized materials will be determined by

extraction in a Soxhlet apparatus by initial extraction for 48 hours

in acetone in order to remove low molecular polar substances like

remains of accelerators and curatives, followed by an extraction

for 72 hours in tetrahydrofuran (THF) to remove the polar

components: oil, non-crosslinked polymer residues, and soluble

polymer released from the network by the de-vulcanization

process. The extraction will be followed by drying the samples in

a vacuum oven at 40°C and determining the weight loss until

constant weight is achieved. The sol fraction is defined as the sum

of the soluble fractions in acetone and THF.

.4.2 Crosslink density

The extracted rubber samples are swollen in toluene for 72

hours at room temperature. The weight of the swollen

vulcanizates will be measured after removal of surface liquid with

absorption paper. The crosslink density will be calculated

according to the Flory-Rehner (18) equation and corrected using

the Kraus-correction (19).

Table 2: De-vulcanization condition

Factors

Conditions

De-vulcanization aid

De-vulcanization oil

De-vulcanization

temperature

De-vulcanization

atmosphere

DPDS 30 mmol/100 g rubber

TDAE 5 phr

220C

Figure 3: Sol fraction generated during de-vulcanization versus the

relative decrease in crosslink density of unfilled rubber de-vulcanizates

(De-xx: devulcanized polymer)

With nitrogen gas purging

In liquid nitrogen

Dumping condition

.5 Analysis of the de-vulcanization efficiency

The de-vulcanization efficiency was analyzed using the

method developed by Horikx (20) and Verbruggen (21): the

rubber sol fraction of the de-vulcanizate and the decrease in

crosslink density of the rubber gel fraction were correlated. The

positioning of the data points for a certain de-vulcanizate is

indicative of the ratio of crosslink scission to polymer scission:

the lower line as shown in Fig. 3 represents crosslink scission,

while the upper line represents random scission.

Figure 4: Basic molecular structure of each rubber

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2020, Volume 8, Issue 3, Pages: 1118-1123

The data point of BR is located left of the points of SBR and

BIIR, and close to the random scission line. It indicates a higher

crosslink density compared with SBR and BIIR, thus decreased

de-vulcanization efficiency. Since the molecular structure of this

rubber contains a double bond in every unit of the repeating

monomer, this results in lower heat resistance than SBR and BIIR.

Therefore the rubber network is subjected to random scission: the

breakage of the rubber network will occur at both, the main chains

of the polymer and the sulfur crosslinks. Recombination of chain

fragments will also occur, resulting in an increase in crosslink

density.

the de-vulcanization time and temperature are major factors

affecting the de-vulcanization efficiency. At a constant de-

vulcanization temperature of 220°C with time variation from 3

min to 15 min (see Fig. 6), the data points are shifted to the left in

the graph, which indicates lower de-vulcanization efficiency.

Longer times contribute to a higher degree of recombination of

broken rubber chains resulting in higher crosslink density.

However, this recombination is rather uncontrolled, therefore the

network structure of the devulcanized rubber is different from the

starting network. At an increasing de-vulcanization temperature

with short de-vulcanization time (3 min.), a contradictory trend is

observed as shown in Fig. 7: The data points significantly shift to

the right, indicating improved de-vulcanization efficiency. It was

expected that an increased temperature but short de-vulcanization

time causes a higher degree of network scission and, as a

consequence, a larger sol fraction. This is attributed to

uncontrolled generation of broken rubber chains at this excessive

temperature.

.2 Influence of carbon black loading

To understand the effect of carbon black loading on the

thermo-chemical de-vulcanization efficiency, carbon black filled

BR vulcanizates were prepared following the formulation as

shown in Table 1 with varied carbon black loading from 30 phr to

0 phr. Figure 5 shows the Horikx-Verbruggen curves of this

rubber. When adding carbon black, most data points get close to

the random scission line. Moreover, they are shifted left as the

carbon black loading is increased. This indicates a decreased de-

vulcanization efficiency with addition of carbon black to BR. This

is due to the two main causes: Firstly, the accumulation of heat

during the de-vulcanization process increases with increasing

carbon black loading, resulting in higher temperatures and hot

spots compared to the unfilled rubber. As a result, the rubber

network is more subject to random scission: breakage of the

rubber network will occur at both, the main chains of the polymer

and the sulfur crosslinks, resulting in a lower ratio of crosslink to

polymer scission. Secondly, some filler aggregates and

agglomerates might be broken and form radicals during de-

vulcanization. The radicals will react with DPDS radicals and

thus reduces the reactivity of DPDS.

Figure 6: Sol fraction generated during de-vulcanization versus the

relative decrease in crosslink density of filled BR with varying de-

๐

vulcanization time at 220 C

Figure 5: Sol fraction generated during de-vulcanization versus the

relative decrease in crosslink density of carbon black filled BR de-

vulcanizate

Figure 7: Sol fraction generated during de-vulcanization versus the

relative decrease in crosslink density of filled BR de-vulcanizates using

3 min. of de-vulcanization time

.3 Influence of de-vulcanization time and temperature

In this study, BR compound with 60 phr of carbon black de-

vulcanized following the formulation and conditions shown in

Table 1 and Table 2, respectively. Fig. 6 and 7 show the

relationship between the soluble fraction generated after de-

vulcanization and the relative decrease in crosslink density while

varying the de-vulcanization time and temperature. It is clear that

5 Conclusion

There are 2 mechanism occurring during a thermo-chemical

de-vulcanization process: firstly, generation of radicals by

breakage of polymer chains caused by high temperatures, and

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2020, Volume 8, Issue 3, Pages: 1118-1123

secondly, trapping of radicals by the sulfide radical of DPDS to

prevent recombination. Polymers with different molecular

structures have different de-vulcanization efficiencies due to the

variation in heat resistance: a lower heat resistance leads to a

higher degree of random scission. When carbon black is present,

uncontrolled thermal elastomer and filler network scission occurs.

Depending on the type of polymer, recombination of the polymer

fragments and formation of a new polymer network can happen

as well. Increased carbon black loadings decrease the de-

vulcanization efficiency. This is due to, firstly, breakage of main

chains caused by excessive heating with increasing carbon black

loading, and secondly, breaking of aggregates and agglomerates

of carbon black to form a reactive surface, to which DPDS

radicals are attached during de-vulcanization reducing the de-

vulcanization efficiency of DPDS. For the factors of time and

temperature, de-vulcanization of the ground rubber at different

temperatures and times causes differences in heat accumulation

in the rubber during de-vulcanization, which consequently leads

to different de-vulcanization efficiencies.

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Aknowledgment

The authors gratefully acknowledge the financial support for

this work of the Thailand’s Education Hub for Southern Region

of ASEAN Countries for PhD Students (THE-AC) and the PSU

Research Fund (Project number: SAT580880S).

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Ethical issue

Authors are aware of, and comply with, best practice in

publication ethics specifically with regard to authorship

54–465.

(avoidance of guest authorship), dual submission, manipulation

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of figures, competing interests and compliance with policies on

research ethics. Authors adhere to publication requirements that

submitted work is original and has not been published elsewhere

in any language.

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and re-utilization of passenger car tires. Thesis Ph.D. May 2013.

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Competing interests

The authors declare that there is no conflict of interest that

would prejudice the impartiality of this scientific work.

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Authors’ contribution

All authors of this study have a complete contribution for data

collection, data analyses and manuscript writing

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