Eur. Polym. J. Vol. 19, No. 10/11, pp. 919-921, 1983 Printed in Great Britain. All rights reserved
0014-3057/83 $3.00 +0.00 Copyright y'~)1983 Pergamon Press Ltd
REGENERATED CELLULOSE IN ELASTOMER COMPOUNDS E. B. MANO and R. C. R. NUNES Instituto de Macromol~culas, Universidade Federal do Rio de Janeiro, CP 68525, Rio de Janeiro, Brazil
Abstract--Natural rubber vulcanizates showed an unexpected reinforcement effect with regenerated cellulose as filler, with maximum tensile properties at 15 phr cellulose. These rubber compounds were compared with analogous mixtures containing SRF black, and a similar system in which the elastomer was SBR. The swelling behaviour of these compounds was studied. The density of crosslinks was determined in heptane and benzene using the equation of Flory and Rehner. The crosslink density is higher in vulcanizates containing regenerated cellulose than in vulcanizates containing carbon black. It is also higher when the swelling is done in heptane than when it is done in benzene; SBR shows a higher crosslink density than NR when heptane and benzene were used. The crosslink density increases when the amount of filler increases for the experimental conditions.
INTRODUCTION Carbon black is the usual reinforcing ingredient for rubber vulcanizates, especially for most of the synthetic rubbers. On the other hand, crystallizing rubbers, such as natural rubber, due to the very regular p o l y - c i s - i s o p r e n e structure in pure gum vulcanizates, show crystallization by stretching, thus providing self-reinforcement with corresponding enhancement of the tensile strength without the need of a reinforcing filler. Up to one third of the elastomer chains can be involved in the crystallite domains. These represent areas of extra strong cohesion between the elastomer chains, which accommodate the stresses and thus distribute them very uniformly. These crystallites perform the same function as a reinforcing filler, as evaluated by tensile strength and elongation. Besides carbon black, any particulate filler having sufficiently large surface area will be expected to reinforce a non-crystallizable elastomer, such as SBR or N B R , provided that the particles are wetted by the rubber . Fibrous materials have been used for incorporation in the rubber compounds to provide reinforcement. Short fibres of cellulose have been studied extensively [2 9]. G o o d results were found with treated cellulose fibres (Santoweb), which are easier to incorporate into the elastomeric mass than the untreated fibres. It was shown that the addition of small amounts (about 2'~) of treated short cellulose fibres to natural rubber and SBR compounds markedly improved the resistance to crack growth, although other physical properties were practically unaffected . Alignment of reinforcing fibres by milling creates a significant anisotropy in the composite properties. Fibre-to-matrix adhesion is a problem. No evidence of consistently good fibre-matrix adhesion is observed except for precoated cellulose fibres. The interaction between fibre and elastomer was improved only when a coating or sizing, compatible with both the fibre and its matrix, was used . Complete, homogeneous dispersion of the fibres
into the polymer matrix is important fully to utilize the performance of the composite. Poor dispersion causes surface defects besides decrease in tensile properties. Polymer mixing has been discussed in the literature without a completely satisfactory solution [121. We have been studying the performance of fibrous filler in natural as well as synthetic rubber vulcanizates [13-15]. In this paper, we present some data on the effect of regenerated cellulose in natural rubber and SBR, as compared to S R F black equivalent compounds. EXPERIMENTAL
To overcome the difficulty of obtaining a homogeneous dispersion by the incorporation of short fibres of cellulose in the elastomer compound, we carried out the mixing process in liquid phase: the fibrous material was reduced to a viscous, alkaline aqueous solution of sodium cellulose xanthate, and the elastomer used as an acid-sensitive aqueous emulsion--that is, natural rubber latex or SBR 1502 latex. The apparently homogeneous, viscous, milky mixture was slowly coagulated by addition of a 1: 1 molar mixture of sulphuric acid and zinc sulphate with stirring at room temperature. The precipitated particles, with the aspect of yellowish crumbs, were rather uniform; the dimensions were dependent on many variables, such as the dilution, the rate of coagulation, the speed and shape of the stirrer and the temperature. The crumbs do not aggregate easily and can be utilized as a particulate rubber master batch for the usual compounding purposes in a roll-mill. Solid rubber or crumbs were compounded according to the formulation presented in Table 1. Vulcanized samples were prepared and submitted to tests. Swelling tests were performed according to the literature  in benzene and heptane for 7 days at room temperature.
An unexpected, reinforcing action of the regenerated cellulose when compared to S R F carbon black is shown in Fig. 1, for natural rubber compounds containing 0-60 phr filler. It is interesting that the
E.B. MANO and R. C. R. NUNES
Table 1. Formulation of the natural rubber and SBR compounds Components
Elastomer Zinc oxide Stearic acid Antioxidant black or Filler 5SRF [regenerated cellulose
100 5 1 1
17.5 A o
Benzothiazyl disulphide Sulphur
maximum effect is obtained with only 15 phr regenerated cellulose as filler. No such effect is presented by SBR compounds as seen in Fig. 2, where SRF exhibits the expected behaviour and the presence of regenerated cellulose does not affect substantially the mechanical properties of SBR pure gum vulcanizates. The relationship between reinforcement and swelling in vulcanized rubber compounds has been reported [16-20]. Although the effect of polymer network structure on swelling is clearly understood in the case of unfilled systems, the incorporation of fillers cause equilibrium swelling changes which have been the subject of considerable speculation. It was noted that fillers reduce the equilibrium swelling volume and that reinforcing fillers, particularly carbon black, show this property to a more pronounced degree than inert fillers. This phenomenon suggests a relationship between reinforcement and swelling since the reduction in swelling could be attributed to an increase in the amount of crosslinking or effective network chain density. It must be pointed out that the principal mechanism which has been proposed to explain the role of
_o I0.0 ~-
e 7.5 I5.0
Fig. 2. Stress vs strain curves for SBR vulcanizates with SRF black and regenerated cellulose as fillers.
particulate fillers like carbon black depends on the assumption of linkages between the filler particles and the rubber macromolecules, the nature of which is still a matter of discussion. Some authors suggest the linkages have a chemical nature; others consider them as physico-chemical in character. One of the methods for the determination of crosslinking was proposed by Flory  through the swelling of the vulcanized elastomeric compound, after submitting it for a long period (7-10 days) to the action of solvents. The solvents are chosen on the basis of the solubility of the rubber in the medium, which depends on the free energy level of the rubber in the solid state and in the solution. The number of effective network chains per unit volume rubber (~) is given in terms of swelling volume by the well known Flory-Rehner equation : - I n (1 - Vr) -- Vr-- #V~
where: vr = the volume fraction of rubber in the swollen network; V0 = the molar volume of the solvent; # = the parameter characteristic of interaction between the rubber and the swelling agent.
Fig. 1. Stress vs strain curves for natural rubber vulcanizates with SRF black and regenerated cellulose as fillers.
The application of the Flory-Rehner equation to the crosslinked systems under investigation, using as solvents benzene and heptane, is shown in Table 2. For the systems under investigation, the crosslink density is higher in vulcanizates containing regenerated cellulose than in those containing carbon black. It is also higher when the swelling is run in
Regenerated cellulose in elastomer compounds Table 2. Crosslinking density in N R and SBR vulcanizates with SRF black or regenerated cellulose as fillers* U × 104 (mol/cm 3) Composition
N R pure gum NR + 5 SRF NR + 15 SRF N R + 20 SRF N R + 25 SRF N R + 40 SRF NR + 60 SRF N R + 5 cell. N R + 10 cell. N R ÷ 15 cell. N R + 20 cell. N R + 25 cell. NR + 40 cell. N R + 60 cell.
1.368 1.514 1.591 1.728 1.674 2.554 2.459 1.435 1.539 3.205 3.706 4.215 -4.828
1.318 1.928 2.066 1.686 2.294 2.843 2.942 1.960 2.062 3.313 3.583 4.461 4400 4.885
SBR SBR SBR SBR SBR SBR SBR
0.794 1.967 4.235 4.786 3.291 5.194 5.829
0.875 1.272 3.688 3.898 3.478 5.637 7.250
pure + 20 + 40 + 60 + 20 + 40 + 60
gum SRF SRF SRF cell. cell. cell.
*28"; 7 days; darkness.
h e p t a n e t h a n w h e n it is d o n e in b e n z e n e ; S B R s h o w s a higher crosslink density than NR when heptane and b e n z e n e were used. T h e crosslink d e n s i t y increases w h e n t h e a m o u n t o f filler increases. W o r k o n this subject is in p r o g r e s s in this l a b o r a t o r y using o t h e r e l a s t o m e r s . Dedication---This paper represents the participation of some members of the Brazilian polymer community to
honour Professor Oto Wichterle in the year of his 70th birthday, for his excellent work and great dedication to the development of the macromolecular science in the world.
1. B. B. Boonstra, Polymer 20, 691 (1979). 2. (3. C. Derringer, J. Elastoplast. 3, 230 (1971). 3. K. Boustany and P. Hamed, Rubber World 171, 39 (1974). 4. A. Y. Coran, K. Boustany and P. Hamed, Rubber Chem. Techn. 47, 396 (1974). 5. K. Boustany and P. Hamed, Rubber Ind. 27, 14 (1975). 6. K. Boustany and R. L. Arnold, J. Elastomers Plast. 8, 160 (1976). 7. G. Anthoine, R. L. Arnold, K. Boustany and J. M. Campbell, Rer. gen. Caoutch. Plast. 564, 77 (1976). 8. F. Manceau, Rev. gen. Caoutch. Plast. 592, 95 (1979). 9. P. Lion, Rev. gen. Caoutch. Plast. 570, 61 (1977). 10. J. R. Beatty and P. Hamed, Elastomeries 110, 27 (1978). 11. J. E. O'Connor, Rubber Chem. Techn. 50, 945 (1977). 12. K. S. Shen and R. K. Rains, Rubber Chem. Techn. 52, 764 (1979). 13. E. B. Mano, R. C. R. Nunes and L. C. O. Cunha Lima, Braz. Pedido P1 7502.614 (Dec. 21, 1976), Chem. Abs. 87, 69534r. 14. R. C. R. Nunes, Mieroestrutura Polissacaridica e Aq~o Re[brcadora em Elast6meros. M.S. Thesis, Universidade Federal do Rio de Janeiro, Brazil (1974). 15. E. B. Mano and R. C. R. Nunes, Rev. Bras. Tech. 8, 39 (1977). 16. G. Kraus, Rubber World 135, 67 (1956). 17. G. Kraus, Rubber World 135, 254 (1956). 18. M. Porter, Rubber Chem. Techn. 40, 866 (1967). 19. B. B. Boonstra and E. M. Dannenberg, Rubber Chem. Techn. 32, 825 (1959). 20. R. Mukhopadhyay and S. K. De, Rubber Chem. Techn. 52, 263 (1979). 21. P. J. Flory, Principles o f Polymer Chemistry. Cornell University Press, New York (1953).