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2 Advantages of supercritical CO2 (scCO2) for the devulcanization of sulfur cross-linked rubber

2 Advantages of supercritical CO2 (scCO2) for the devulcanization of sulfur cross-linked rubber

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Recycling of sulfur cross-linked natural rubber using scCO2



439



nonflammable and chemically inert, and its critical point occurs at relatively

mild conditions (the critical temperature and pressure are 31°C and 74 kg/

cm2, respectively). A recycling process using scCO2 can, therefore, be

considered an acceptable one from the engineering viewpoint. The removal

of this swelling solvent is very easy because CO2 is gaseous at ambient

temperature. In addition, using scCO2 as a solvent was reported to be similar

to using typical hydrocarbon solvents such as toluene.23 scCO2 is, therefore,

expected to be used for swelling the rubber vulcanizates and for promoting

several chemical reactions in rubber vulcanizates in order to make the rubber

recyclable.

In our study,24 a devulcanization reaction of a model polymer network,

i.e., sulfur-cured unfilled synthetic polyisoprene rubber (IR), was carried out

using scCO2. It was demonstrated that scCO2 works very well in facilitating

the penetration of the devulcanizing reagent into the IR vulcanizate. Diphenyl

disulfide (DD) was found to be one of the most effective devulcanizing

reagents in scCO2 when compared to other reagents. DD has also been proven

to be an effective reagent for the devulcanization of NR vulcanizate in some

organic solvents.6,8 In this chapter, the usefulness of the devulcanization

process using scCO2 is examined with a focus on sulfur cross-linked NR.



17.3



Devulcanization of sulfur cross-linked NR in

scCO2



Sulfur cross-linked unfilled NR was subjected to a devulcanization reaction

in scCO2 using DD.15 The sample was prepared by mixing NR (RSS#3) with

2 parts per hundred rubber by weight (phr) of stearic acid, 5 phr of zinc

oxide, 3 phr of sulfur and 1 phr of N-cyclohexyl benzothiazyl sulfenamide

(CBS) in a Banbury mixer and by heat-pressing at 141°C for 30 min.

The cross-linking density was 1.93 ¥ 10–4 mol/cm3. The devulcanized

product was fractionated into sol and gel components. The fraction of sol

component containing reusable linear polymer increased with the increase

in scCO2 pressure, especially over the critical pressure. The molar mass of

the resulting sol component was approximately tens of thousands, and the

cross-linking density of the gel component decreased. It is worth noting that

the NR vulcanizates that were cross-linked with a shorter cure time were

substantially devulcanized and could be converted into reusable materials

when DD was used as a devulcanizing reagent, in scCO2, at 180°C under

10 MPa for 60 min (Fig. 17.2). The devulcanized rubber showed a slightly

higher Tg (−51°C) than that of raw NR (−59°C). The obtained sol component

had a main structure of cis-1,4-polyisoprene containing 7% trans-isomer,

which was identified by solid 13C-NMR. It was, however, reported that the

Tg of the trans-polyisoprene (−69.8°C) was a little lower than that of the

cis-isomer (−63.4°C).25 The increase of Tg (8°C) is, therefore, not ascribable



440



Chemistry, Manufacture and Applications of Natural Rubber

180°C, 10 MPa



Polymer main chains



Cross-links



Vulcanizate



CO2



Heating



Devulcanizing

reagent



Devulcanization



Recovery



Reclaimed rubber

: Diphenyl disulfide (DD)

: scCO2



17.2 Devulcanization of sulfur cross-linked NR using DD as a

devulcanizing reagent under 10 MPa at 180°C in scCO2 for 60 min.



to cis-trans isomerization during the devulcanization. It is supposed that the

addition of DD onto the polymer main chains during the devulcanization

resulted in a decrease of the molecular mobility, leading to the higher Tg.

The presumed cross-link cleavage reaction of this devulcanization is

shown in Fig. 17.3 and described here. First, some DD molecules should

be dissolved in scCO2. The solvated molecules will penetrate into the NR

vulcanizate swollen with scCO2. DD in the NR vulcanizate may then attack

the sulfur−sulfur bond of the cross-links. This additional reaction will lead

to the cross-link cleavage in the NR vulcanizate. Our experimental results of

the effect of mono-, di- and poly-sulfidic linkages on the devulcanization in

scCO2 supported this theory since the NR vulcanizates with predominately

mono-sulfidic linkages tended to resist the devulcanization.



17.4



Devulcanization of carbon black-filled sulfur

cross-linked NR



Most practical rubber products, such as tire rubber, contain carbon black

(CB) as the reinforcing filler. CB has a beneficial effect on the chemical

and physical properties of a rubber compound in that its presence causes

a significant increase in the cure rate by the chemical groups on the

surface.26 CB tends to reduce the swelling of a vulcanizate in a manner

that is proportional to the filler content, but which leaves the cross-linking

density in the rubber compound unchanged.27,28 The diffusivity of gases for



Recycling of sulfur cross-linked natural rubber using scCO2



S



S



Sx–1



S

Sx–1



PhSSPh

in scCO2



PhS



S



S



S



SPh



PhS



Sx–1 Sx–1



SPh



441



17.3 Cross-link cleavage reaction in the devulcanization of sulfur

cross-linked NR.

Table 17.1 Recipe for CB-filled NR vulcanizates in phra 16

Sample code



NR-0



NR-20



NR-40



NR-60



100

0

2

2

0.3

1.5

1.5



100

20

2

2

0.3

1.5

1.5



100

40

2

2

0.3

1.5

1.5



100

60

2

2

0.3

1.5

1.5



Ingredient

NR

Carbon black

Zinc oxide

Stearic acid

Diphenyl guanidine

N-Cyclohexyl-2-benzothiazole sulfonamide

Sulfur

a



The values are in parts per one hundred rubber by weight.

Source: Reprinted from Kojima, M., Tosaka, M., Ikeda, Y. and Kohjiya, S.,

‘Devulcanization of carbon black filled natural rubber by using supercritical carbon

dioxide’, J. Appl. Polym. Sci., 95, 137–143, copyright © 2003, with permission from

John Wiley & Sons.



NR vulcanizate also depends on the CB content; the higher the fraction of

CB the less diffusivity.29 CB may, therefore, affect the devulcanization in

scCO2. Interestingly, however, CB-filled NR vulcanizates were devulcanized

sufficiently in scCO2 at 180°C under 10 MPa for 60 min in the presence of

DD.16 Note that the model samples with various contents of high abrasion

furnace (HAF) CB were prepared according to the formula shown in Table

17.1, which involved mixing in a Banbury mixer and press-heating at 141°C

for 30 min to produce a vulcanizate sheet. Regardless of the CB content in

the NR vulcanizates, sol fractions of 20–40% were obtained. The recycled

CB-filled NR is shown in Fig. 17.4. The swelling ratios of the gel components

were higher than in the original vulcanizates. In dynamic mechanical analysis,

the devulcanized rubbers showed a slightly lower shear storage modulus (G¢)

and a slightly higher tan d than those of the initial compounds as well as a

much lower G¢ and much higher tan d than those of the vulcanizates. These

results indicate that the presence of CB in NR vulcanizates does not disturb

the devulcanization. The devulcanized rubbers with various CB contents had

good processability. Conventional silica was also found not to prevent the

decross-linking reaction of NR vulcanizate in scCO2.30



442



Chemistry, Manufacture and Applications of Natural Rubber



17.4 Recycled CB-filled NR.



17.5



Devulcanization of an NR-based truck tire

vulcanizate



The devulcanization of an NR-based truck tire vulcanizate, which is a

typical CB-filled NR product, was investigated as a practical application for

our devulcanization method in scCO2. In addition to the ingredients in the

model vulcanizates of CB-filled NR, butadiene rubber (BR), aromatic oil

and antioxidant were compounded into the NR-based truck tire vulcanizate.

The formulation is shown as compound ini-TT in Table 17.2. The truck

tire vulcanizate was sufficiently devulcanized under the same conditions

as those of model vulcanizates.16 The obtained devulcanized rubber was

blended with virgin rubber according to the formulations TT-re20, TT-re40,

and TT-re60 shown in Table 17.2. The compounds were vulcanized again

to become recycled rubber.

The tensile properties of recycled rubber made from virgin rubber and

devulcanized truck tire rubber are shown in Fig. 17.5. No deterioration of the

tensile property was observed for TT-re20. The stresses at 100% elongation

(M100) of TT-re40 and TT-re60 were considerably higher than those of

the original vulcanizate (ini-TT). For TT-re40, the M100 increased by 28%

compared to the original vulcanizate. The hardness of all the recycled rubber



Recycling of sulfur cross-linked natural rubber using scCO2



443



Table 17.2 Recipe for an NR-based truck tire and recycled rubber vulcanizates

made of virgin NR and devulcanized rubber materials in phra 16

Sample code



ini-TT



93

NR

7

BR

0

DRb

HAF Carbon black

70

20

Aromatic oil

2

Zinc oxide

2

Stearic acid

2

Antioxidantc

Diphenyl guanidine

0.3

N-Cyclohexyl-2-benzothiazole sulfonamide

1.5

Sulfur

1.5



TT-re20



TT-re40



TT-re60



83.7

6.3

20

63

18

2

2

2

0.3

1.5

1.5



74.4

5.6

40

56

16

2

2

2

0.3

1.5

1.5



65.1

4.9

60

49

14

2

2

2

0.3

1.5

1.5



a



The values are in parts per hundred rubber in weight (phr).

DR denotes the devulcanized rubber material.

c

Santflex 6PPD.

Source: Reprinted from Kojima, M., Tosaka, M., Ikeda, Y. and Kohjiya, S.,

‘Devulcanization of carbon black filled natural rubber by using supercritical carbon

dioxide’, J. Appl. Polym. Sci., 95, 137–143, copyright © 2003, with permission from

John Wiley & Sons.

b



samples was also higher than that of the original vulcanizate. Residual sulfur

in the devulcanized tire rubber is thought to have worked as a crosslink

agent in the recycled compounds.15,26 The tensile strength at break (Tb) of

the recycled rubber decreased slightly with the increase in devulcanized tire

rubber content. This decrease of Tb may have been caused mainly by the

low molecular weight component in the devulcanized rubber. The relatively

poor Tb of the recycled rubber is strongly supposed to be due to the polymer

structural changes that occurred during the devulcanization process. The

decrease in Tb, however, was only approximately 10% up to 40 phr of the

devulcanized tire rubber content. De et al. successfully devulcanized NR

vulcanizate by mechanical milling in the presence of a reclaiming reagent

and prepared recycled rubber with it.9 In their report, at a devulcanized NR

content of 40 phr, the tensile strength decreased to around 80%. In our case,

approximately 90% of the original tensile strength was retained, as shown in

Fig. 17.5. The fact that our devulcanization process did not involve intensive

mechanical shearing meant that the scission of polymer main chains may

have been reduced.



17.6



The role of scCO2 in the devulcanization of

sulfur cross-linked rubber



In order to establish what factors are important for the useful decross-linking

reaction of rubber vulcanizates and to reveal the role of scCO2 in this reaction,



444



Chemistry, Manufacture and Applications of Natural Rubber

35

ini-TT

TT-re20

TT-re40

TT-re60



30



Stress (MPa)



25

20

15

10

5

0



0



100



200

300

Strain (%)

(a)



400



500



600



35



500



25

20



400



15

Tb



10



Eb



5

0



Eb (%)



Tb, M300 (MPa)



30



300



M300

0



200

20

40

60

80

Content of recycled rubbers (phr)

(b)



17.5 Tensile properties of the recycled rubber vulcanizates made

from virgin rubber and devulcanized truck tire rubber: (a) stress–

strain curves and (b) effect of the amount of recycled rubber on Tb,

Eb and M300. (Reprinted from Kojima, M., Tosaka, M., Ikeda, Y. and

Kohjiya, S., ‘Devulcanization of carbon black filled natural rubber by

using supercritical carbon dioxide’, J. Appl. Polym. Sci., 95, 137–143,

copyright © 2003, with permission from John Wiley & Sons.)



the impregnation of a decross-linking reagent into a peroxide cross-linked

synthetic NR, i.e., isoprene rubber (IR) in scCO2, was studied as a model for

sulfur cross-linked NR.31 The sample was prepared by mixing IR and 1 phr

of dicumyl peroxide on a two-roll mill and by heat-pressing the mixture at

170°C for 10 min. The cross-linking density of the sample was 8.4 ¥ 10–5

mol/cm3. An effect of the CO2 pressure on the mass uptake of DD into the

IR matrix at 40°C is shown in Fig. 17.6. The mass uptake of DD was almost

zero at 0.1 MPa, i.e. under ambient pressure, and gradually increased with

the increase of CO2 pressure up to ca. 6 MPa. At the near critical pressure

of CO2 (7.38 MPa), the mass uptake of DD abruptly increased, and after



Recycling of sulfur cross-linked natural rubber using scCO2



445



Mass uptake of DD into IR



1.0

0.8

0.6

0.4

0.2

0.0



0



2



4

6

8

Pressure (Mpa)



10



12



17.6 Effect of pressure on the mass uptake of DD into the IR matrix

at 40°C for 10 h soaking in CO2. Mass uptake = mt/M0, where mt and

M0 stand for the mass of low molar mass molecule impregnated into

the peroxide cross-linked IR at soaking time (t), and the mass of low

molar mass loaded in a reaction vessel, respectively. (Reprinted from

Kojima, M., Kohjiya, S. and Ikeda, Y., ‘Role of supercritical carbon

dioxide for selective impregnation of decrosslinking reagent into

isoprene rubber vulcanizate’, Polymer 46 (7), 2016–2019, copyright ©

2005, with permission from Elsevier.)



the critical point it gradually increased again. The supercritical state of CO2

was found to significantly affect the selective impregnation of DD into the

cross-linked IR. The solubility of DD in CO2 seems to be a little under

the ambient pressure, but it may become larger with the increase of CO2

pressure, because the solubility of CO2 is generally reported to increase with

the increase in pressure.32,33 It is therefore speculated that, once the DD was

dissolved in the CO2, the transference of DD into the IR matrix must have

happened quickly, which might explain the increased mass uptake of DD

in the IR network.

The effect of soaking time on the mass uptake of DD into the IR matrix at

40°C under 10 MPa is shown in Fig. 17.7. The mass uptake of DD increased

with the increased soaking time and the equilibrium of the mass uptake

was reached after ca. 22 h, where it remained constant at ca. 0.91. DD was

apparently efficiently transferred and impregnated into the IR matrix, and

most of the DD was present in the cross-linked IR rather than in the scCO2

at equilibrium. The diffusion coefficient of DD in the cross-linked IR under

scCO2 was estimated to be 3.2 ¥ 10–11 m2/s.

The distribution coefficient of DD for the IR matrix in scCO2 was

calculated to be 1,150 and 0.28 for scCO 2 and toluene, respectively.

Surprisingly, the former was about 4,000 times larger than the latter,

although the degree of swelling of the IR vulcanizate in scCO2 was very

low (1.15) and that in toluene was 7.10. As clearly observed in Fig. 17.8,

the images of the cross-linked IR before and after the swelling in scCO2



446



Chemistry, Manufacture and Applications of Natural Rubber

Mass uptake of DD into

peroxide-cured IR



1

0.8

0.6

0.4

scCO2



0.2

0



Toluene

0



20



40

Time (h)



60



80



17.7 Mass uptake of DD into the peroxide cross-linked IR under 10

MPa at 40°C in scCO2 and at 40°C in toluene. (Reprinted from Kojima,

M., Kohjiya, S. and Ikeda, Y., ‘Role of supercritical carbon dioxide

for selective impregnation of decrosslinking reagent into isoprene

rubber vulcanizate’, Polymer 46 (7), 2016–2019, copyright © 2005,

with permission from Elsevier.)



(a)



(b)



17.8 Images of cylindrical cross-linked IR (a) before swelling in scCO2

at equilibrium at 40°C under 10 MPa. The same bar was set inside

for comparison. (Reprinted from Kojima, M., Kohjiya, S. and Ikeda,

Y., ‘Role of supercritical carbon dioxide for selective impregnation of

decrosslinking reagent into isoprene rubber vulcanizate’, Polymer 46

(7), 2016–2019, copyright © 2005, with permission from Elsevier.)



were not much different. The solubility of scCO2 was lower for DD and it

is considered that DD is transported from the CO2 phase to the IR matrix.

It is therefore concluded that the major function of scCO2 is not to dissolve

DD, but to accelerate DD permeation into the rubbery matrix. It should be

pointed out that this phenomenon was only detected when using DD. As



Recycling of sulfur cross-linked natural rubber using scCO2



447



Mass uptake of molecule into IR



illustrated in Fig. 17.9, other low molar mass molecules such as tetradecane,

docosane, xylene and phenyl ether show fewer mass uptakes at 40°C under

10 MPa in scCO2 than DD. The modest solubility of DD in scCO2 and

the high affinity of DD for IR are considered to result in the high mass

uptake of DD in the IR network. These results mean that scCO2 is the most

efficient solvent for the impregnation of the decross-linking reagent ‘DD’

into the poly (isoprene) segment for the chemical recycling of IR and NR

products.

An infrared spectroscopy of the cross section of a spherical sample showed

the dispersion of DD into the IR matrix to be homogeneous. The absorbance

ratios between the peaks of out-of-plane deformation vibration for aromatic

CH of DD at 740 cm–1 and out-of-plane deformation vibration for olefinic

CH of poly (isoprene) at 837 cm–1 were used for the quantitative analysis

of the concentration of DD in the IR matrix. As shown in Fig. 17.10, the

ratios remained almost constant at all points from the surface of the spherical

cross-linked IR for both samples, which were soaked in scCO2 for 24 h and

96 h. The effect of the soaking time on the amount of DD impregnated into

the IR matrix was clearly observed for the spherical-shaped samples. It was

possible, however, for DD to be transferred into the IR matrix homogeneously

during the impregnation process of this study without dependence on the

soaking time. This can be attributed to the unique properties of scCO2. DD

may easily be diffused into the rubber matrix with scCO2 owing to the high

diffusivity and zero surface tension of scCO2.



1.0

0.8

0.6

0.4

0.2

0.0



0



10



20

30

Soaking time (h)



40



17.9 Effect of kind of molecules on the mass uptake into the peroxide

cross-linked IR under 10 MPa at 40°C in scCO2. : DD, D: tetradecane,

: docosane, ¥: xylene, : phenyl ether. (Reprinted from Kojima,

M., Kohjiya, S. and Ikeda, Y., ‘Role of supercritical carbon dioxide

for selective impregnation of decrosslinking reagent into isoprene

rubber vulcanizate’, Polymer 46 (7), 2016–2019, copyright © 2005,

with permission from Elsevier.)



448



Chemistry, Manufacture and Applications of Natural Rubber



Absorbance ratio [A(740

cm–1)/A(837 cm–1)]



1.0

0.8

0.6

0.4

0.2

0.0



0



2000

4000

6000

Distance from the surface of

spherical cross-linked IR (µm)



8000



17.10 Dispersion of DD from the surface of spherical peroxide crosslinked IR. ¥: soaking for 24 h, : soaking for 96 h. (Reprinted from

Kojima, M., Kohjiya, S. and Ikeda, Y., ‘Role of supercritical carbon

dioxide for selective impregnation of decrosslinking reagent into

isoprene rubber vulcanizate’, Polymer 46 (7), 2016–2019, copyright ©

2005, with permission from Elsevier.)



17.7



Conclusion: Key issues in ensuring effective

recycling of sulfur cross-linked NR



High efficiencies for both the decross-linking reaction of used rubber products

and for the purification of recycled rubbers are necessary for an effective

recycling process. A combination of scCO2 and reactants is very important

in order to take advantage of the properties of scCO2 for rubber recycling

systems and to produce a high yield and quality of recycled rubber. When

DD is used as a decross-linking reagent for an NR vulcanizate at under

10 MPa at 40°C in scCO2, a selective and homogeneous impregnation of

diphenyl disulfide (DD) into the poly (isoprene) matrix occurs, which is the

first step toward an effective decross-linking reaction. Neither carbon black

nor silica filler can prevent the decross-linking reaction of NR vulcanizate

in scCO2. This devulcanization method was therefore useful for a used

truck tire vulcanizate with a mileage of 150,000 km. The NR vulcanizate

with predominately mono-sulfidic linkages, however, tends to resist the

devulcanization. Devulcanizing reagents that can break the mono-sulfidic

linkages in the sulfur cross-linked rubber therefore need to be found in order

to further develop the chemical recycling of rubber products for a sustainable

society. Since there are still unknown phenomena in the sulfur cross-linking

reaction of rubber, the fundamental reaction mechanism first needs to be

revealed before progress in the decross-linking reaction of NR can be made.

The effects of non-rubber components in NR on the devulcanization reaction

also need to be investigated.



Recycling of sulfur cross-linked natural rubber using scCO2



17.8



449



Future trends



The rubber industry is confronting the ongoing problem of how to handle

used rubber products. In order to achieve an effective recycling system for

sulfur cross-linked NR, the following points are required so that new science

and technology can be generated from the results in the near future.





selective decross-linking reaction for mono-sulfidic linkage in NR

vulcanizate

∑ recycling of isoprene monomer from the used NR products

∑ recycling of fillers from the used NR products

∑ recycling of zinc atom from the used NR products

∑ elucidation on the mechanism of the sulfur cross-linking reaction

In addition to the recycling of NR molecules, the author would like to

emphasize the importance of achieving the sustainable production of NR

for the environment and for the rubber industry. The two processes, the

continuous production of NR and the effective recycling of NR, are necessary

to maintain activity in the rubber industry all over the world and to preserve

the Earth’s limited resources. A total map for the utilization of used tires as

a model for used rubber is shown in Fig. 17.11.



17.9



Acknowledgements



This study was supported by the Industrial Technology Research Grant

Program in ID: 02B67006c from the New Energy and Industrial Technology

Development Organization (NEDO) of Japan. The author thanks John Wiley

& Sons and Elsevier for copyright permissions.



Landfilling



In use



Waste

New tire

Production

Petroleum

resource



Re



Used tire



tre



ad



Recovery and

separation



ing



Fuel



Raw polymer

Reclaiming



Synthetic

rubbers

Natural

resource

Natural rubber



17.11 Total map for the recycling of tire rubber.



Oil



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