Radiation Physics and Chemistry 68 (2003) 527–529

Temperature dependence of the lifetime spectrum ofrubber–carbon black composites

Jingyi Wang, C.A. Quarles*

Department of Physics and Astronomy, Texas Christian University, TCU Box 298840, Fort Worth, TX 76129, USA

Abstract

We report preliminary results of the temperature dependence of the lifetime spectra of natural rubber (NR) and NR

loaded with 50 phr of carbon black (CB) from room temperature to below the glass transition temperature (Tg).

Additional polymers to be studied include Sn-SSBR, Duradene 706, 709 and 711 and butyl rubber: both unloaded and

loaded with CB N115 or N762. Different types of CB have very different structure and are expected to have different

effects on the behavior of the lifetime near Tg:r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Rubber; Polymer; Carbon black; Glass transition; Positron lifetime

1. Introduction

While positron annihilation spectroscopy techniques

have been extensively applied to the study of polymers,

there has been much less study of rubber compounds

filled with carbon black (CB) (Abdel-Hady et al., 1993;

Debowska et al., 1999; Kostrzewa et al., 1995; Patnaik

et al., 1998; Quarles et al., 2001; Semaan et al., 2001,

2002; West et al., 1979). When CB is added to a rubber

the main effect is to strengthen the rubber. The purpose

of this research is to investigate the temperature

dependence of the positron annihilation lifetime from

room temperature to below the polymer glass transition

temperature (Tg) for both the polymer and the CB

loaded polymer. We are especially interested in how the

free volume changes when CB is added to rubber.

2. Experimental details

The lifetime apparatus is a typical fast–fast coin-

cidence system (Urban-Klaehn et al., 1999). A Na-22

radioactive source, 10–50mCi deposited on thin kapton

or nickel foil, is sandwiched between two identical pieces

of the sample and placed in the sample chamber between

two scintillation counters, which are Photonis XP2020/

URQ photomultipler tubes coupled to BaF2 scintilla-

tors. The timing resolution of the system is about 350 ps.

The sample chamber is a small vacuum chamber

attached to a dipstick in a 30 liter Dewar and is

evacuated with a LN2 sorption pump. The temperature

is measured with a thermocouple and controlled by a PC

computer that records the temperature and turns a small

heater on and off. The temperature is maintained to

about 5�C, which is adequate for the measurements.

3. Samples

CB is an industrial product of considerable economic-

al and technological importance. For the tire industry,

which uses as much as 80% of the worldwide produced

CB, the reinforcing properties of CB are essential.

Several parameters of CB such as surface area and

structure can predict some properties of the CB–rubber

composites. However, other essential properties such as

wear resistance and traction have not yet been directly

correlated to CB and polymer chemical and physical

ARTICLE IN PRESS

*Corresponding author. Tel.: +1-817-921-7375; fax: +1-

817-257-7742.

E-mail address: c.quarles@tcu.edu (C.A. Quarles).

0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0969-806X(03)00223-8

characteristics. A detailed discussion of CB science and

technology is given by Donnet et al. (1993) and

Gerspacher et al. (1996).

Today, CB is produced mainly in the furnace process.

In this process, the feedstock oil is burned in a non-

stoichiometric reaction to form a very finely divided

material composed of aggregates that are the CB mono-

units. These aggregates are typically submicron size

objects of complex shapes. A typical CB aggregates size

could range from 50 to 300 nm. The surface of these

aggregates is tiled with graphite crystallites and areas of

amorphous carbon. Specific surface area for ASTM

grade furnace CB can vary from 27 to 146 m2/g for CB

types N762 and N115. As for structure, CB particles can

be tightly clustered together, like a bunch of grapes (like

N762) or the same number of nodes can be arranged in a

more open fashion, giving greater bulkiness (like N115).

High structure CBs give increased viscosity to uncured

rubber stocks and increase smoothness of extruded

stock. Increasing structure increases hardness, abrasion

resistance and electrical conductivity, but decreases flex

resistance in vulcanized compounds.

The Sid Richardson Carbon Co., Fort Worth, TX,

provided the samples. The rubber, vulcanizing additives

and CB are mixed in an internal mixer (Haake Rheocord

90) according to the ASTM D3191 standard recipe. The

samples were then cured in a mold at 160�C, which

produced cylindrical samples with parallel surfaces of

diameter 38 mm and thickness 15 mm. Two identical

samples were used for each experiment.

We plan to investigate seven polymers: natural rubber

(NR), polybutadene (BR), butyl rubber, solution SBRs

(Sn-SSBR, Duradene 706, Duradene 709, Duradene

711); and two CB types: N115 and N762. Only the

preliminary results for NR are presented here.

4. Results and discussion

We analyzed the lifetime spectra into three compo-

nents using the LT program (Kansy, 1996). The shortest

lifetime (t1) is fixed at the para-positronium lifetime of

125 ps to stabilize the non-linear fit. The longest lifetime

(t3) is identified as due to the ortho-positronium (o-Ps)

pick-off process. The intermediate lifetime (t2) is due to

direct annihilation and trapping in the sample.

In Fig. 1, the o-Ps lifetime (t3) is plotted versus

temperature. The o-Ps lifetimes of NR and NR with

either 50 phr of N762 or N115 CB are shown. The o-Ps

lifetime does not depend on the sample at room

temperature. This is consistent with what had been

reported for different rubbers and CB types at room

temperature, where t3 was found to be independent of

both the wt% and the type of CB and just depends on

the polymer (Quarles et al., 2001; Semaan et al., 2001,

2002). The lifetime decreases with temperature until Tg

is reached. Near or below Tg; however, the values of t3

for the samples differ. Below Tg; the radius of the free

volume appears to be lower in the samples with CB. We

do not yet have an explanation for the onset of

dependence of the free volume radius on CB type below

Tg: The Tg point marked in the figure is from DSC

measurements. From DSC the values of Tg are �55.7�C

for NR, �56.5�C for NR with 50 phr N762 and

�57.6�C for NR with 50 phr N115. The value of Tg

measured by o-Ps lifetime appears somewhat higher

than that from DSC. Also, the difference in Tg between

the CB loaded and the NR sample is larger than seen

with DSC.

In Fig. 2, the intensity I3 of the o-Ps component is

plotted versus temperature. I3 is a measure of the

amount of free volume in the sample. At room

temperature I3 is lower with CB loading since CB

ARTICLE IN PRESS

Temperature (°C)

-80 -40 0 20

1.0

1.5

2.0

2.5

3.0

NR with 50 phr N762NR

Tg from DSC

-60 -20

τ3 (

nsec

)

NR with 50 phr N115

Fig. 1. Plot of the o-Ps lifetime versus temperature for samples

of NR and NR loaded with 50 phr of either N762 or N115 CB.

The glass transition temperature Tg from DSC measurements is

indicated.

Temperature (°C)

-80 -60 -40 -20 0 20

o-P

S I

nten

sity

(%

)

0

2

4

6

8

10

12

14

NR NR with 50 phr N762 CBNR with with 50 phr N115

Fig. 2. Plot of the intensity of the o-Ps lifetime component

versus temperature for samples of NR and NR loaded with

50 phr of either N762 or N115 CB.

J. Wang, C.A. Quarles / Radiation Physics and Chemistry 68 (2003) 527–529528

inhibits positronium formation. As the temperature is

lowered, I3 for the NR with CB samples decreases

linearly with temperature. The difference in I3 between

the NR and NR with CB samples persists until about

�40�C. At that point the free volume of the NR sample

decreases quickly to about the same value as the NR

with CB samples. Below Tg; the trend of I3 for the NR

sample appears to be somewhat lower than the NR with

CB sample although the values agree within the errors.

This behavior is interesting and suggests that the free

volume of the NR sample changes quickly and the onset

of the change is at a temperature higher than Tg:

5. Conclusions

The preliminary results illustrate that PAS is useful to

investigate the temperature-dependent behavior when

CB filler is added to the polymer. The main conclusions

are: (1) The o-Ps lifetime decreases linearly with

temperature above Tg in both the NR and the NR with

CB samples. Below Tg; the radius of free volume does

not change and is smaller in the NR with CB sample. (2)

The intensity of the o-Ps lifetime component or the

amount of free volume is higher, in the NR sample. The

free volume of the NR with CB samples decreases

linearly with temperature. The free volume of the NR

sample remains higher until about 10� or so above Tg:At that point, the free volume quickly decreases,

reaching that of the NR with CB samples.

Further work on lifetime and Doppler broadening

with other polymers and CB types are planned.

Acknowledgements

The authors thank Michel Gerspacher and Leszek

Nikiel of the Sid Richardson Carbon Co., for their

continued interest in and support of this research and

for providing the samples. We also thank the TCU

Research Fund.

References

Abdel-Hady, E.E., Abo-Hashem, A., El-Sayed, A.M.A.,

Moshen, M., Ghani, A.A., 1993. Microstructure analysis

of the effect of SRF carbon black on butyl rubber (IIR) by

positron annihilation lifetime spectroscopy. Polym. Degra-

dation Stability 41, 65–68.

Debowska, M., Rudzinska, J., Pasternak-Girulska, A., Poz-

niak, R., 1999. Carbon black–polyethylene composites

studied by positron annihilation. Acta Phys. Pol. A. 95,

509–514.

Donnet, J.-B., Bansal, R.C., Wang, M.-J. (Eds.), 1993. Carbon

Black Science and Technology. Marcel Dekker, New York.

Gerspacher, M., O’Farrell, C.P., Nikiel, L., Yang, H.H., 1996.

Furnace carbon black characterization: continuing saga.

Rubber Chem. Technol. 69, 569.

Kansy, J., 1996. Microcomputer program for analysis of

positron annihilation lifetime spectra. Nucl. Instrum.

Methods A 374, 235–244.

Kostrzewa, M., Michno, Z., Majcherczyk, T., 1995. Positron

annihilation studies of polyethylene–carbon black compo-

sites. Acta Phys. Pol. A 88, 193–197.

Patnaik, A., Zhu, Z., Yang, G., Sun, Y., 1998. Positron lifetime

as a nanoprobe for free volume distribution in high-density

polyethylene–carbon black conducting composites. Phys.

Status Solidi A 169, 115–125.

Quarles, C.A., Urban-Klaehn, J.M., Gerspacher, M., Nikiel,

L., Semaan, M.E., 2001. Positron annihilation lifetime

spectroscopy of carbon black- and silica-filled rubber.

Mater. Sci. Forum 363–365, 343–345.

Semaan, M.E., Nikiel, L., Quarles, C.A., 2001. Doppler

broadening spectroscopy of carbon black and carbon

black-filled rubbers. Carbon 39, 1379–1385.

Semaan, M.E., Quarles, C.A., Nikiel, L., 2002. Carbon black

and silica as reinforcers of rubber polymers: Doppler

broadening spectroscopy results. Polym. Degradation Sta-

bility 75, 259–266.

Urban-Klaehn, J.M., Quarles, C.A., 1999. Positron annihila-

tion spectroscopy of sandstone and carbonate rocks. J.

Appl. Phys. 86 (1), 355–363.

West, D.H.D., McBrierty, V.J., Delaney, C.F.G, 1979. A

positron annihilation study of carbon black and carbon

black–filled polybutadiene. Appl. Phys. 18, 85–92.

ARTICLE IN PRESSJ. Wang, C.A. Quarles / Radiation Physics and Chemistry 68 (2003) 527–529 529