Comparison of viscoelastic and dielectric relaxations as a function of carbon black loading in butyl and chlorobutyl rubber

Journal of Non-Crystalline Solids 131-133 (1991) 877-882 North-Holland

877

Comparison of viscoelastic and dielectric relaxations as a function of carbon black loading in butyl and chlorobutyl rubber R.N. Capps Naval Research Laboratory, Orlando, FL 32806, USA

J. B u r n s Florida Institute of Technology, Melbourne, FL 32901, USA

Dynamic mechanical and dielectric spectra have been measured for a number of the simpler amorphous polymers such as acrylate compounds. Many complex polymer systems such as synthetic rubbers have not been as extensively studied. A transfer function technique and time-temperature superposition were used to measure the frequency-dependent Young's modulus and loss tangent as a function of carbon black loading and type in chemically cross-linked butyl and chlorobutyl rubbers. Dielectric permittivity and loss were measured as functions of frequency and temperature. The dielectric behavior was significantly influenced by the type of cross-linking system and by filler loading. The Williams-Watts 'stretched exponential' function was found to be a reasonable fit to the dielectric spectra of the unfilled systems. The effects of molecular motions and rubber-filler interactions on the two types of relaxations are discussed.

1. Introduction Natural and synthetic rubbers are an i m p o r t a n t group of polymers from both a practical and a scientific standpoint. The influence of cross-link density and types u p o n viscoelastic behavior of elastomers is well characterized [1,2]. The role of particulate fillers in the d y n a m i c mechanical behavior of these materials has been extensively studied [3,4]. Fewer studies have been reported of dielectric behavior [5-9]. Relatively few of these have been d o n e in a systematic fashion to examine crosslinking agents and fillers. Few comparisons with viscoelastic behavior have been made. Commercially available butyl rubbers are rand o m copolymers of isobutylene and isoprene, with 0.6-2.5 mol% isoprene in the chain. Pure butyl rubber is non-polar, and should exhibit no dielectric relaxation except for minor surface oxidation effects. Cross-linked systems should exhibit dielectric behavior characteristic of the cure system and any fillers used. Chlorobutyl is chemically similar

to butyl, but contains approximately 1.1-1.3 wt%chlorine as b o t h tertiary allylic and isomeric secondary allylic chlorides, attached to the polymer chain. These two c o m p o u n d s were used as model systerns to study whether the a relaxation mechanism associated with motions of the p o l y m e r chains in tensile and shear relaxations also controls dielectric behavior. Additionally, effects of cross-link types, as well as c a r b o n black loading and particle size, on dielectric and viscoelastic behavior were examined.

2. Experimental The p o l y m e r systems and loadings of fillers studied are summarized in table 1. C o m p o u n d e d rubber samples were obtained from the Burke R u b b e r C o m p a n y of San Jose, CA. Proper cure conditions were determined with a M o n s a n t o oscillating disk rheometer as per A S T M D2084. Samples used for experimental purposes were

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

R. Capps, J. Burns / Butyl and chlorobutyl rubber

878

using a Polymer Laboratories dynamic mechanical thermal analyzer. A transfer function technique

Table 1

Formulations for IIR and CIIR rubbers studied System Exxon Butyl 268 ChlorobutylHT1066 Zinc Oxide Schenectady SP1055 a)

Component (phr) IIR CIIR 100.0 100.0 5.0 5.0 10.0 4.0

MBTS b)

was also used to measure the frequency-dependent Young's modulus and loss tangent as a function of temperature. Time-temperature superposition was used to construct extended master frequency curves [10]. Silver paint was used to spray coat electrodes onto masked areas of 5.08 cm diameter samples. Micrometer calipers were used to determine exact

CIIR2 100.0 5.0 4.0

2.0

Stearic Acid Diphenyl guanidine

1.0

1.0 0.5 5.0 0-45.0 0-45.0

5.0 0-45.0 0-45.0

1.0

a) Brominated methylol phenol resin, b) 2,2'-Benzothiazyl disulfide, c) Paraffirtic oil.

sample thicknesses and electrode diameters. A General Radio 1690A dielectric holder, General Radio 1689M R L C Digibridge with an IEEE 488 interface, and Zenith 248 computer were used to measure the capacitance and dissipation of the dielectric holder with and without the sample at each frequency of measurement over the range of 102-105 Hz. The dielectric permittivity and loss as

cured to 90% optimum cure. Density was determined by Archimedes' method. Hardness was measured by a constant-rate, constant-load Shore A durometer, Scans of the Young's modulus and loss tangent at 10 Hz as a function of temperature were made

functions of frequency were then calculated. Temperature control was provided by a Thermotron Model S1 with a stability of ___0.1°C. A platinum R T D in the body of the dielectric holder was used to m e a s u r e the t e m p e r a t u r e . P o l y m e t h y l methacralate was used to verify the satisfactory operation of the system.

S u n p a r l 2 0 c)

N347 Black N990 Black Nll0 Black N550 Black

1010

I

I I IIIIII

15.0 15.0 15.0 15.0

I

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I

I I Illlll

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I

45

~;;

phr

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10°

D n v

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15 phr 107

106101

I

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102

I I|tim

I

I IIIIIII

103 REDUCED

I

104 FREQUENCY

I Illilll

1

10 s

I Illil

10 e

(Hz)

Fig. 1. Plot of storage Young's modulus as a function of reduced frequency at different loadings of N347 carbon black at a reference

temperature of 273.15 K.

R. Capps, J. Burns / Butyl and chlorobutyl rubber

3. Results

reduced frequency on tan & The tan ~ curves are asymmetric and skewed toward higher frequencies, and exhibit decreases with increasing carbon black loading. Use of the large particle size, less reinforcing N990 black at loadings of 45 phr, had a much smaller effect upon the modulus and loss tangent, consistent with predictions of the modified Guth-Gold equation [11] relating carbon black particle size and structure to viscoelastic behavior. The modulus and loss tangent were very similar for systems IIR and CIIR in table 1, with the exception that the peak in the loss tangent curve occurred at higher frequencies for the first chlorobutyl system (CIIR) at a fixed reference temperature. All three unfilled compounds had virtually identical Shore A hardness, indicating similar degrees of cross-linking. The second chlorobutyl rubber system (CIIR2) with no MBTS or diphenyl guanidine displayed a peak in the tan 8 curve that occurred at higher frequencies, and a more rapid slope in the modulus-frequency curve. The three systems had similar glass transition temperatures as measured by DSC (202.7 K for systems IIR and CIIR in table 1, 203.7 K for CIIR2 at a scan rate of 20 o C/rain).

3.1. Viscoelastic behavior

Only one transition was observed when the loss modulus at 10 Hz was measured as a function of temperature from 193 to 313 K. Similarly, DSC scans showed only one transition. A secondary/3 transition from rotation of methyl groups pendant to the polymer backbone might be expected. The only experimental manifestation that this might be merging with the primary a glass-to-rubber relaxation was an almost imperceptible shoulder on the low temperature side of the loss tangent curve as a function of temperature at 10 Hz. It was found that time-temperature superposition could be satisfactorily used to construct master frequency curves using the transfer function technique. Either an Arrhenius or WLF shift function could be used, with only very slight differences in the resultant curves at lower frequencies. Figure 1 shows the storage Young's modulus of butyl rubber as a function of reduced frequency for several loadings of N347 black at a reference temperature of 273.15 K. As expected, increased loading of black increased the modulus. Figure 2 shows the effect of

1.2

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IIIIIII

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1.0~ 0.8

I

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IIIIIII

I

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I

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IIII

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Ill

r ~

/ °c

IIIIIII

879

.....~ - -

0.6

t.

-

15 phr

o

45

phr

0.4 7

0.2

I 01

I

IIIllll

I 10 2

I

IIIIIII

I 10 3

I

IIIIIII

I 10 4

I

REDUCED FREQUENC(Hz) Y

IIIIIII

I 10 5

10 e

Fig. 2. Plot o f l o s s tangent as a function of reduced frequency at differentloadings 0 f N 3 4 7 carbon black at a reference temperature of 273.15K.

880

R. Capps, J. Burns

2.95

I

I

I

I

/

B u t y l a n d chlorobutyl rubber

I IIII

I

I

I

[

I IIII

I

I

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(a) ~'~ ~'~.

_

2.65

I

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I lill

10

I

i

!

I

I illl

10 3

I

i

I

I

I il

10 4

10 5

FREQUENCY (Hz)

Fig. 3. Plot of dielectric permittivity as a function of frequency for unfilled butyl and chlorobutyl systems listed in table 1: (a) CIIR at 296.15 K; (b) IIR at 305.15 K; (c) CIIR2 at 307.15 K. Lines represent splines through points calculated from Williams-Watts function with fl = 0.40 in (a), and fl = 0.42 in (b) and (c).

mately 2.68-2.95) that varied only slightly with frequency. This is shown in fig. 3 for the three unfilled systems. Figure 4 shows the loss behavior observed in the three unfilled systems, plotted at

3.2. Dielectric behavior

Both the unfilled butyl and chlorobutyl rubbers displayed low relative permittivities (approxiI

I

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I

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to

. ---~ " 6

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t IIII

10 3

I

10 4

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o o

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10 s

FREQUENCY (Hz)

Fig. 4. Plot of dielectric loss as a function of frequency for unfilled butyl and chlorobutyl systems listed in table 1: (a) CIIR at 296.15 K; (b) IIR at 305.15 K; (c) CIIR2 at 307.15 K. Lines represent splines through points calculated from Williams-Watts function with fl = 0.40 in (a), and fl = 0.42 in (b) and (c).

R. Capps, J. Burns / Butyl and chlorobutyl rubber

slightly different temperatures so that maximum losses are observed in the frequency spectra, Well-defined loss maxima are seen to be present, These are asymmetric and skewed toward higher frequencies when represented as Cole-Cole plots, The loss values also increase in amplitude and shift to higher frequencies as the temperature increases, until they pass through a maximum, Plots of log fmax VS. 1 / T gave linear slopes, with activation energies that were different among the three systems. For the unfilled butyl system (IIR in table 1), this was approximately 12 kcal. The unfilled chlorobutyl system with the same cure (CIIR2) had an activation energy of 10.6 kcal. These were less than the activation energies from the Arrhenius shift functions used to construct the dynamic mechanical master frequency curves. Only the chlorobutyl system using MBTS and diphenyl guanidine in the cure system gave a value that was remarkably close to that in the dynamic mechanical experiments (18.1 kcal in both cases), Increased loading of the smaller particle-sized N347 carbon black caused appreciable dc conductivity. Addition of carbon black also caused increases in both the permittivity and loss. Very strong Maxwell-Wagner-Sillars effects [5,6] were observed at loadings of 30 phr of the highly reinforcing N347 black, with the maximum in the loss peak showing little temperature dependence. Little effect was observed for loadings of the large particle size N990 black, even at loadings of 45 phr. As noted earlier, this filler also showed a much smaller effect on the dynamic mechanical behavior. Measurements of the permittivity and loss of the chlorobutyl system containing loadings of 15 phr of different types of carbon black showed different magnitudes and frequency behavior of the loss. By contrast with the unfilled systems, the maxima in the loss spectra tend to broaden and become less well-defined,

4. Discussion Butyl and chlorobutyl rubbers are similar to polyisobutylene in their relaxation behavior [12] in

881

that only one major transition is observed. Both the viscoelastic and dielectric relaxations appear to be primarily a in the sense that they are largely controlled by the gross microBrownian motions of polymer chains. When the full widths at half maximum of the normalized loss tangent and normalized dielectric loss are plotted against temperature, t h e y are seen to be quite similar in both the dielectric and dynamic mechanical measurements, although occurring at different places in the temperature and frequency scales. In the dielectric case, the major source of dipole interaction appears to come from molecules used to cross-link the polymer chains. Thus, the butyl and chlorobutyl rubbers (IIR and CIIR2 in table 1) using only zinc oxide and the brominated methylol phenol resin (SP 1055) in the cure exhibit similar relaxation behavior. The molecules are free to rotate about the cross-link points as thermal activation is supplied. The addition of a small amount of paraffinic plasticizer (Sunpar 120) to the butyl rubber gives greater free volume, and higher dielectric losses. The lines in figs. 3 and 4 represent splines through points calculated from the WilliamsWatts distribution function [13]. The unfilled butyl and chlorobutyl rubbers using a common cure system fit a value of 0.42 for the exponent, r , while the chlorobutyl system using the more complex cure system (CIIR in table 1) fit a fl value of 0.40. From this, one can infer a broader distribution of relaxation times. However, it should be noted that fits to such distribution functions exhibit temperature dependence, and are dependent upon the choices of coo and co when these are extrapolated from Cole-Cole plots [13]. The broader distribution of relaxation times and the smaller value of fl in the Williams-Watts function for the chlorobutyl rubber system using MBTS and diphenyl guanidine in the cure system can be understood by noting that it contains additional types of cross-links that differ in the rotational energy required. It is not clear what role zinc oxide plays in the observed dielectric behavior, as it is presumably converted to a soluble zinc soap during the cure process. This will result in abstraction of chlorine from the polymer chain, converting it to zinc chloride. Experiments are in

882

R. Capps, J. Burns / Butyl and chlorobutyl rubber

progress to compare the behavior observed in the present systems with chlorobutyl rubber cured with only zinc oxide, and with higher proportions of SP 1055 resin and varying plasticizer loadings. The higher cross-link density and shortened repeat units in the polymer chains should shift the maxima in the dielectric loss to higher frequencies and a narrower distribution of relaxation times. The applicability of more quantitative treatments relating measured dielectric behavior to viscoelastic behavior is also being investigated [14,15]. 5. Conclusions

References [1] A.Y. Coran, in: Science and Technology of Rubber, ed. F.R. Eirich (Academic Press, New York, 1978) p. 29 and

references therein. [21 H.L. Stephens, in: Rubber Technology,2nd. Ed., ed. M. Morton (Van Nostrand-Reinhold, New York, 1973) p. 19 and references therein. [3] G. Kraus, in: Science and Technology of Rubber, ed. F.R. Eirich (Academic Press, New York, 1978) p. 339 and

references therein. [4] B.B. Boonstra, in: Rubber Technology, 2nd. Ed., ed. M. Morton (Van Nostrand-Reinhold, New York, 1973). p. 51 and references therein. [5] P. Hedvig, Dielectric Spectroscopy of Polymers (Halsted, New York, 1977) ch. 5 and references therein. [6] N.G. McCrum, B.E. Read and G. Williams, Anelastic and Dielectric Effects in Polymeric Solids (Wiley, New York,

For filled rubber systems, the electrical behavior is controlled by agglomerates of carbon black aggregates, rather than by rotation of dipoles in

1967) ch. 10 and referencestherein. [71 A.I. Lukomskaya and B.A. Dogadkin, KoUoid. Z. 22

the cross-links and movements of backbone chains. The initial results indicate that it may be possible to assign different parts of the loss due to different carbon black structure, in the manner described by Lukomskaya and Dogadkin [7]. Strong

[8] K. Nakajima, M. Naoki and T. Nose, Polym. J. 10 (1978) 307. [9] H. Adachi, K. Adachi and T. Kotaka, Polym. J. 12 (1980) 329. [10] R.N. Capps and L.L. Beumel, in: Sound and Vibration

interracial polarization effects may make the assignments problematical at higher loadings, as these effects will tend to dominate the observed dielectric relaxations. The authors are grateful to the ASW Technologies Block Program of the Office of Naval Technology for support of this work. The assistance of Linda Beumel of TRI-TESSCO in performing the viscoelastic measurements and data reduction is gratefully acknowledged.

(1960) 576.

Damping with Polymers, eds. R.D. Corsaro and L.H. Sperling, ACS Symp. Series 424 (American Chemical

Society, Washington, DC, 1990) p. 63. [11] A.I. Medalia, Rubber Chem. Technol. 46 91973) 877. [12] S.P. Kabin and G.P. Mikhailov, J. Tech. Phys. (USSR) 26 (1956) 493. [13] G. Williams and D. C. Watts, Trans. Faraday Soc. 66

(1970) 80. [141 R.D. Calleja, Polymer 19 (1978) 235. [151 B.E. Read, Polymer 30 (1989) 1439.