Chapter 20
Toluene Diffusion in Natural Rubber 1,3
1,4
2
Lawrence S. Waksman , Nathaniel S. Schneider , and Nak-Ho Sung Downloaded by UNIV OF NEW SOUTH WALES on September 14, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch020
1
Polymer Research Branch, SLCMT-EMP, U.S. Army Materials Technology Laboratory, Watertown, MA 02172 Department of Chemical Engineering, Tufts University, Medford, MA 02152
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Immersion swelling and incremental vapor sorption experiments were carried out with toluene on a lightly crosslinked natural rubber sample with varying amounts of carbon black. Only small differences in equilibrium swelling were found. The sorption isotherms were superimposable for samples at all carbon black levels up to an activity of 0.9 and could be fitted with the Flory-Rehner relation. Sorption and desorption curves, above 25% toluene, showed slight "S" shaped curvature. Diffusion constants, D, obtained by the half-time method or the Joshi-Astarita analysis of coupled diffusion and relaxation, showed a similar maximum in D with concentration. When converted to solvent mobilities, D , the values leveled out rather than extrapolating to the self-diffusion coefficient of toluene, D . Application of the Armstrong-Stannett treatment of heating effects during sorption lead to significant corrections in D and to better agreement with an empirical extrapolation to D*1. 1
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Studies o f the d i f f u s i o n of benzene i n natural rubber represent some of the e a r l i e s t d e t a i l e d examinations of the i n t e r a c t i o n of an organic solvent with a polymer. Hayes and Park c a r r i e d out measurements a t low concentrations by the vapor sorption method (1), and a t higher concentrations by determining the concentration d i s t r i b u t i o n using an interferometric method (2). Complementary measurements by vapor transmission to determine the d i f f u s i o n c o e f f i c i e n t from time-lag data were c a r r i e d out a t low concent r a t i o n s by Barrer and Fergusson (3). The main results of these studies have been summarized i n F u j i t a ' s review (4) of organic vapor d i f f u s i o n i n polymers above the glass t r a n s i t i o n temperature. However, the problems with these measurements were not referenced. In the work o f Hayes and Park, the calculated solvent m o b i l i t i e s extrapolated to a value, a t unit solvent volume f r a c t i o n , which 3
Current address: United Technologies, Hamilton Standard, 1 Hamilton Standard Road, Windsor Locks, C T 06096
4
Address correspondence to this author. 0097-6156/90A)423-0377$06.00/0 © 1990 American Chemical Society
In Barrier Polymers and Structures; Koros, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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BARRIER POLYMERS AND STRUCTURES
Downloaded by UNIV OF NEW SOUTH WALES on September 14, 2015 | http://pubs.acs.org Publication Date: May 9, 1990 | doi: 10.1021/bk-1990-0423.ch020
exceeded the s e l f - d i f f u s i o n c o e f f i c i e n t of pure benzene by two orders of magnitude. This lead the authors to question the thermodynamic correction factors used i n computing the solvent m o b i l i t i e s . In Barrer and Fergusson's study, the values of the d i f f u s i o n c o e f f i c i e n t from steady state were higher than from the time-lag, leading to the conclusion that the behavior might be complicated by relaxation e f f e c t s . Thus, i t appears that even i n t h i s c l a s s i c a l system there are problems which deserve consideration. The goals of the present study were to reexamine vapor sorption i n a l i g h t l y crosslinked rubber, both as an u n f i l l e d sample and i n samples containing two types of carbon black i n varying amounts. Complications i n the vapor sorption-rate curves motivated a more d e t a i l e d study of the d i f f u s i o n problems as the main area of concern. EXPERIMENTAL SAMPLE PREPARATION. Samples of natural rubber were prepared by mixing a l l ingredients i n a Haake-Buchler system 40 internal mixer, using an accelerated sulfur cure and excluding any o i l extender or p l a s t i c i z e r which could leach out i n the immersion experiments. Two types of carbon black were used; N110, a fine p a r t i c l e , high structure black and N774, a large p a r t i c l e , low structure black. A two-stage mixing procedure was used to minimize scorch. The carbon black was incorporated i n the f i r s t stage, followed by l a t e r addition of the curatives, since the addition and mixing of carbon black tends to elevate the batch temperature to unacceptable l e v e l s The formulations and the outline of the procedure are summarized i n Table 1. The compounded rubber was m i l l e d to a thickness of 60 mil and cured a t 121 C f o r 84 minutes i n a hydraulic press using a picture frame shim with a thickness of 20 m i l . The low cure temperature was chosen i n order to maximize the scorch time so that the uncured rubber could flow and f i l l the frame.
Table 1. Natural Rubber Formulations and Processing
Stage 1 Natural Rubber Stearic A c i d Zinc Oxide Agerite Resin D Carbon Black Volume Fraction
PHR
TOD 2 4 1
0-50 0-20
Stage 2 Sulfur Santocure
PHR
0.8
Stage 1 mix: 80°C, 77 RPM m i l l :; T
