Preparation of Pyrolytic Carbon Black from Scrap

Tire Rubber Crumb and Evaluation in New

Rubber Compounds

Franco Cataldo

Trelleborg Wheel Systems, Via Casilina 1626/A, 00133 Rome, ItalyE-mail: cdcata@flashnet.it

Received: December 13, 2004; Revised: February 13, 2005; Accepted: February 16, 2005; DOI: 10.1002/mame.200400388

Keywords: carbon black; pyrolysis; reinforcement; rubber compounds

Introduction

Pyrolytic carbon black can be produced in relatively high

yields from scrap tires pyrolysis. The economy of the py-

rolytic process for tire recycling depends on the application

of the resulting products which consists in gases, a liquid

hydrocarbons mixture and residual pyrolytic carbon black

other than the steel and textile fibers which can be recovered

from tires prior pyrolysis.

Different pyrolytic conditions have been proposed as

the most suitable for the production of a series of pro-

ducts from scrap tires. It could be distinguished between

processes operating under low pressure or vacuum,[1–3]

processes under nitrogen blanket[4,5] and processes under

hydrogen pressure.[6–8] None of the proposed processes has

shown a clear economical return to find an immediate

application.

However, in the new economic situation with the high

cost of raw materials and petroleum-based chemicals the

scrap tires pyrolysis may reach a certain economic con-

venience. Furthermore, the environmental impact of the

new growing economies in China, India and South-East

Asia, will imply a deeper utilization of the resources and

an increasing number of scrap tires per year worldwide.

In these perspectives it is reasonable to believe that in the

future the pyrolytic processes will find an industrial

application.

In the present paper we are focusing on the preparation of

pyrolytic carbon black from scrap tire rubber under nitrogen

atmosphere and in the evaluation of the resulting carbon

black in a rubber compound formulation.

Experimental Part

Pyrolytic carbon black (CBp) was obtained in the sameapparatus described in detail previously.[9] The apparatusconsisted in quartz tube equipped with a quartz crucible fittedinto an electric furnace.

Summary: Pyrolytic carbon black (CBp) has been preparedby rubber crumb pyrolysis under nitrogen flow at 700 8C. TheCBp obtained by this process had an average surface areaof 81 m2 � g�1 and was obtained in 43% yield over the start-ing rubber crumb. Although the CBp surface area can beincreased up to 109m2 � g�1 by washing away the Zn- and Si-based ashes with HF treatment, the CBp was tested in astandard NR/SBR-based formulation without any purifica-tion and ash extraction. CBp was tested at increasing loading

levels as partial or full replacement of a standard N339furnace carbon black. CBp depresses the physical propertiesof the rubber compound in a way which is directly pro-portional to the amount added. The reason of this result andthe limited reinforcing effect is discussed in terms of lowsurface area and low structure in comparison to N339 carbonblack as well as in terms of low surface activity, theinterference of the ashes and the poor dispersion. Ideas offurther development works are outlined.

Macromol. Mater. Eng. 2005, 290, 463–467 DOI: 10.1002/mame.200400388 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper 463

As rawmaterial for each pyrolysis experiment usewasmadeof a rubber crumb derived from scrap tires free from textilefragments and steel. The rubber crumb derived tire tread wassupplied byAmbra company (Italy). The fineness specificationcalled for 1% residue on the 420 micron sieve. Other specifiedproperties of the rubber crumb were as follows: Carbon blackcontent 28% typical, ash content 10% maximum, acetoneextract 20% maximum and rubber hydrocarbon content 45%maximum.

Each batch for the preparation of CBp involved the pyrolysisof 25–30 g of rubber crumb. The pyrolysis was conductedunder a nitrogen flow by heating the sample at 50 8C �min�1 toreach as soon as possible 700 8C. That temperature was thenmaintained isothermally for 10 minutes. The cooling stepinvolved the switch-off the electric furnace and waiting acomplete cooling down to 50 8C under nitrogen flow. A total of84 batches weremade to produce about 1 kg of CBp, a quantitysuitable for compound evaluation. The average yield in CBpfrom the starting rubber crumb was 43.1%. Since about 10%was ash, the real amount of CBp produced was about 33%.

The CBp was used as filler in rubber compound formulationwithout any treatment, without removing the ash.

The testing formulationwas as follows: natural rubber (SIR-20), 59.8 phr; S1712, 55.4 phr; carbon black N339, 75 phr;aromatic oil (DAE) 17 phr; stearic acid, 2 phr; zinc oxide, 3 phr;wax, 2.0 phr; TMQ, 1 phr; 6PPD, 1.5 phr; CBS, 1.8 phr; sulfur,1.5 phr.

N339 carbon black was replaced partially or totally by theCBp in the above formulation as shown in Table 1.

The surface area of the N339 carbon black measured byiodine absorption (ASTM D1510) was 90 g � kg�1 (which cor-responds to about 90 m2 � g�1). On the other hand the pyrolyticcarbon black showed a iodine absorption of 81 g � kg�1 cor-responding to a surface area of about 81 m2 � g�1.

The compoundsweremixed in a 2.2 L lab internalmixer andall specimens were prepared and tested according to thestandard ASTM or ISO procedures.

Results and Discussion

General Aspects of Pyrolytic Carbon Black (CBp)Prepared with our Process

The analyses on CBp from scrap tire pyrolysis have

evidenced the fact that CBp is characterized by an higher

content of elemental S and N in its composition in

comparison to furnace or other industrial carbon black

types.[1,10] In particular, the surface analysis with several

techniques including XPS (X-ray photoelectron spectro-

scopy) has revealed an abundant content of oxygenated

groups on the surface of CBp even if it was prepared under

inert atmosphere or under hydrogen atmosphere.[1,10] Pre-

vious studies on CBp have clarified that higher surface area

Table 1. Test results on CBp mixed in a NR/SBR compound as replacement at different levels of N339 carbon black.

Sample A B C D E

N339 carbon black/phr 74.8 0 37.4 56.1 65.45CBp from tire pyrolysis/phr 0 74.8 37.4 18.7 9.35Rheometer (151 8C, 30 min)ML 2.68 1.9 2.19 2.44 2.64MH 12.41 8.13 9.93 10.7 11.55MH-ML 9.73 6.23 7.74 8.26 8.91TS1 5.52 5.41 5.22 5.30 5.41TS2 6.32 6.3 6.1 6.11 6.2T10 5.48 4.90 4.91 5.09 5.32T30 6.79 6.18 6.32 6.41 6.58T50 7.86 7.21 7.45 7.5 7.64T60 8.59 7.82 8.16 8.24 8.37T90 14.31 11.82 12.87 13.14 13.39

Mooney viscosity at 100 8C 50.4 51.7 49.8 49.7 50.8Mooney scorch at 127 8C 26.08 27.3 25.54 25.51 25.39Cure (151 8C, 30 min, dumbbell)Density 1.13 1.142 1.138 1.133 1.133Hardness IRHD 62 50 56 59 61Tensile/MPa 13.15 3.51 7.86 10.28 11.52Elongation 463 411 476 481 472M25%/MPa 0.91 0.52 0.65 0.82 0.85M50%/MPa 1.30 0.75 0.98 1.19 1.22M100%/MPa 2.22 1.19 1.71 2.01 2.09M200%/MPa 4.72 2.04 3.23 3.85 4.2M300%/MPa 7.76 2.79 4.88 6.01 6.76

Tear strength unaged/kN 65.21 17.77 35.5 51.45 64.81Tear strength aged (3 d, 100 8C)/kN 45.92 15.3 23.42 25.89 42.53RPA2000 tan d at 60 8C 0.191 0.111 0.132 0.153 0.180Dispergrader/(% undisp filler) 7.1 25.4 22.7 14.9 10.1

464 F. Cataldo

Macromol. Mater. Eng. 2005, 290, 463–467 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

can be achieved by conducting pyrolysis at relatively high

temperatures for instance 600–700 8C, preferably by work-ing under reduced pressure, even if it has been demonstrated

that the decomposition of the rubber component occurs

completely already at 420 8C.[1] Higher pyrolytic tempera-

tures, above 420 8C, are needed to avoid the char formation

from the carbonization of the rubber components and to

remove the rubber hydrocarbons from the carbon black

surface and pores. However, the utilization of too high

temperatures in conjunction with relatively long residence

times produce a CBp with undesirable low reinforcing

properties.[1] One approach to increase the surface area of

carbon black and hence its reinforcing properties involved

the low temperature pyrolysis of tire rubber for instance at

500 8C in vacuum followed by a post-treatment at higher

temperatures 670–860 8C.[3] In such way it was possible

to increase the surface area from 53 to 80 m2 � g�1.[3]

Another approach involved the tire pyrolysis at 500 8Cunder nitrogen blanket followed by the char activation

with steam at 850 8C to produce a mesoporous activated

carbon having a surface area of 737 m2 � g�1versus the

starting surface area (previous the steam activation) of 30–

90 m2 � g�1.[5] ACBp with a so high surface area is no more

suitable for rubber compounding but may find application

in other fields.

In our study, we have selected conditions closer to those

of proposed by Kaminsky and Hemmerich, i.e. pyrolysis at

700 8C under a nitrogen flow.[4] Under similar conditions

to those employed here, the mentioned scientists[4] were

able to obtain a CBp with relatively high surface area

(85m2 � g�1) and very low extractablematter (0.2%), which

is an indication of the very low amount of char formed on

the black surface.

In our case the surface area of the carbon black obtained

from tire rubber crumb pyrolysis at 700 8C reached the

interesting surface area of 81 m2 � g�1 (measured by

iodine absorption), a value not so far from that found by

the other scientists.[4] The yield of CBpþAsh produced

with our process was 43.1%. Since we know that the ash of

our tire rubber crumb was about 10%, the carbon black

yield was about 33% a value which suggests that a limited

amount of char was formed and present on the surface of

our CBp.

A limitation of our work was that the CBp obtained was

used without any further treatment and purification. In

particular, in our test in a rubber compound formulation we

have not removed the ashes which are constituted mainly

but not exclusively by ZnO, SiO2 and ZnS.

In any case the treatment of the CBp with conc. HCl

followed by a treatment with HF causes the complete

removal of the ashes and an increase of the surface area

from 81 m2 � g�1 to 109 m2 � g�1. The ash-free CBp was

prepared just in sufficient amounts to measure the

surface area and was not tested in a rubber compound

formulation.

Mechanical Properties of the Rubber CompoundsFilled with CBp

In Table 1 are reported all the test data made on the CBp

mixed in the rubber compound. In terms of scorch safety

the use of CBp does not give significant differences in

comparison to N339 carbon black. This fact is also con-

firmed by looking at the TS1 and TS2 parameters of the

rheometer curve. However, in Table 1 it is possible to

observe higher speed cure kinetics for the samples prepared

exclusively with or having high levels of CBp. This

behaviour may be attributed to the low level of purity of

CBp in comparison to a normal furnace black. Especially

the presence of an excess of elemental nitrogen and sulfur

content may affect negatively the cure kinetics.

From the rheometer curve, the level of maximum torque

(MH, Table 1) already anticipates that CBp has detrimental

effects on the mechanical properties of the rubber

compound in comparison to N339. This fact is illustrated

more clearly in Figure 1 and 2, where the drop in tensile

strength and in hardness has been measured as function of

the amount of CBp present in the compound and can be

observed also in Table 1. The higher the level of CBp in the

compound more pronounced is the drop in the physical

properties. It is also interesting to observe that the drop is

linear with the amount of CBp used and involves both the

ultimate physical properties (tensile strength) as well as the

moduli at very low extension like the hardness (see Table 1).

A similar trend was also found in the case of tear strength

(Figure 3) both for samples not aged and aged. Therefore,

it appears very clear the fact that CBp does not have a

comparable reinforcement effect as its furnace counterpart.

Another very clear indication about the poor reinforcing

effect of CBp derives from the hysteresis measurement

made by the RPA2000 rheometer (see Figure 4). The tan dparameter which is the ratio of the viscous modulus and the

elastic modulus decreases by increasing the CBp level. It is

well known that highly reinforcing, high surface area

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80

PYROLYTIC CARBON BLACK (PHR)

TE

NS

ILE

ST

RE

NG

TH

(MP

a)

TENSILE

TENSILE AGED

Figure 1. Effects on the tensile properties of a standard testingformulation as function of the amount of pyrolytic carbon black(CBp) added as replacement of N339 carbon black.

Preparation of Pyrolytic Carbon Black from Scrap Tire Rubber Crumb and Evaluation in New Rubber Compounds 465

Macromol. Mater. Eng. 2005, 290, 463–467 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

carbon blacks give high hysteresis (high tan d levels) in a

rubber compound while this is not the case for low surface

area carbon blacks. The limiting result is offered by graphite

or graphitized carbon blacks which have very low re-

inforcing properties in a rubber compound and offer also a

very low level of tan d parameter and hence a low level of

hysteresis.

The disappointing results achieved with CBp have

several reasons. First of all the measurement of the surface

area has shown that for CBp it is significantly lower than

N339 carbon black (about 81 m2 � g�1 versus 90 m2 � g�1 of

N339). The surface area is only one parameter which affects

the reinforcing properties of a carbon black in a rubber mix.

Another important parameter for instance the ‘‘structure’’

of carbon black, a parameter which is measured by the DBP

or oil absorption; although not directly measured in our

CBp it is expected to bevery lowbecause of the ash and char

present in the CBpwhich of course reduces the void volume

available. An indirect confirmation of this fact derives

from the dispergrader measurement (Figure 5) where the

percentage of the undispersed filler has been estimated by

optical measurements using the software and the statistics

treatment of our Dispergrader. In Figure 5 it appears very

clearly that the amount of undispersed filler increases by

increasing the loading of CBp in a trend which follows the

drop in physical properties. It is now clear that the dis-

appointing physical properties are certainly due also to the

difficult dispersion of CBp; poor carbon black dispersion

are always due to the low structure of the filler and hence

low DBP absorption number.

The reinforcement is not due only to surface area and

‘‘structure’’ parameter but is also due to the surface activity

of a carbon filler.[11] The surface activity is in relation with

the presence of certain surface defects in carbon black like

the fullerene-like structures which are of paramount impor-

tance in the black rubber interaction.[11] The absence of

45

47

49

51

53

55

57

59

61

63

0 10 20 30 40 50 60 70 80

PYROLYSIS CARBON BLACK (PHR)

HA

RD

NE

SS

(IR

HD

)

Figure 2. Effects on the hardness IRHD of a standard testingformulation as function of the amount of pyrolytic carbon black(CBp) added as replacement of N339 carbon black.

Figure 3. Effects on the tear strength unaged and aged of astandard testing formulation as function of the amount of pyrolyticcarbon black (CBp) added as replacement of N339 carbon black.

0,1

0,11

0,12

0,13

0,14

0,15

0,16

0,17

0,18

0,19

0,2

0 10 20 30 40 50 60 70 80

PYROLYSIS CB ADDED (PHR)

TA

N-D

AT

60°C

Figure 4. Effects on the hysteresis of a standard testing formu-lation as function of the amount of pyrolytic carbon black (CBp)added as replacement of N339 carbon black.

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80

PYR

% O

F U

ND

ISP

ER

SE

D F

ILL

ER

OLYSIS BLACK (phr)

Figure 5. Effects on the filler dispersion of a standard testingformulation as function of the amount of pyrolytic carbon black(CBp) added as replacement of N339 carbon black.

466 F. Cataldo

Macromol. Mater. Eng. 2005, 290, 463–467 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

these defects which include also the fullerene-like struc-

tures due to their thermal annealing implies a significant

reduction in the reinforcement effect.[11,12]

Another reason which may justify the insufficient per-

formances of CBp are for sure linked to the relatively high

ash content; this means that the real amount of carbon black

added to the rubber mix is practically a fraction of that used

in the reference compound filled with N339.

Conclusion

It has been shown that pyrolytic carbon black (CBp) can

be easily produced by pyrolizing rubber crumb under a

nitrogen blanket under optimal conditions selected from

literature studies.

The resulting CBp showed a surface area of 81 m2 � g�1

and was used in rubber compound without any further

purification. From the study of the physical properties of the

resulting rubber compound it has been found that CBp

cannot replace current furnace carbon black in a formula-

tion at equal part but may be used as replacement of less

than 9 phr of N339 carbon black, otherwise the drop in the

mechanical properties become unacceptable.

This work represents just a starting point of our research.

If carbon black is purified from the ashes, it can reach a

higher surface area which may be beneficial on its reinforc-

ing effect; similarly also its ‘‘structure’’ as measured by the

DBP absorption number can be improved by ash removal

and this will lead to a better dispersion and, at the end, to

a better reinforcing effect. It may also be that a milling

procedure of the resulting blackmay have further beneficial

effects to its regeneration.[12]

Acknowledgements: I wish to thankmy co-workerMr.DanieleCaon for the patient work spent in the pyrolysis experiments.Furthermore I wish to thank all the Trelleborg Wheel Systemslaboratory staff for the enthusiasm dedicated to this research work.

[1] C. Roy, A. Rastegar, S. Kaliaguine, H. Darmstadt, V. Tochev,Plast. Rubber Compos. Proc. Appl. 1995, 23, 21.

[2] C. Roy, A. Chaala, H. Darmstadt, J. Anal. Appl. Pyrol. 1999,51, 201.

[3] D. Pantea, H. Darmstadt, S. Kaliaguine, C. Roy, J. Anal.Appl. Pyrol. 2003, 67, 55.

[4] W.Kaminsky, C. Hemmerich, J. Anal. Appl. Pyrol. 2001, 58/59, 803.

[5] P. Ariyadejwanich, W. Tanthapanichakoon, K. Nakagawa,S. R. Mukai, H. Tamon, Carbon 2003, 41, 157.

[6] A. M. Mastral, R. Murillo, M. S. Callen, T. Garcia, FuelProcess. Technol. 1999, 60, 231.

[7] A. M. Mastral, R. Murillo, M. S. Callen, T. Garcia, FuelProcess. Technol. 2001, 69, 127.

[8] Y. Shi, L. Shao, W. F. Olson, E. M. Eyring, Fuel Process.Technol. 1999, 58, 135.

[9] F. Cataldo, submitted.[10] W. H. Lee, J. Y. Kim, Y. K. Ko, P. J. Rencroft, J. W. Zondlo,

Appl. Surf. Sci. 1999, 141, 107.[11] F. Cataldo, Carbon 2002, 40, 157.[12] F. Cataldo, G. Abbati, A. Santini, F. Padella, Fullerenes

Nanotubes Carbon Nanostruct. 2003, 11, 395.

Preparation of Pyrolytic Carbon Black from Scrap Tire Rubber Crumb and Evaluation in New Rubber Compounds 467

Macromol. Mater. Eng. 2005, 290, 463–467 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim