Rubber Processing Through Rheology - American Chemical Society

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11 Rubber Processing Through Rheology

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JEAN L. LEBLANC Monsanto Europe S.A., 270-272, Avenue de Tervueren, B-1150Bruxelles, Belgium

This paper gives some insights into the growing under­ standing of the particular flow phenomena associated with rubber processing. Attention is focused on two particular operations, i.e. internal mixing and extrusion. Using simple capillary rheometry experimen­ tal results, the importance of elongational flow and energy absorption processes is demonstrated in the mixing of natural rubber. The rôle played by the converging zone at the entrance of short extrusion dies is underlined and estimated using experimental data. In addition, some particular effects typical of heterogeneous rubbery materials are presented and their significance in rubber processing discussed. The discussion i n rheological terms of the problems associated with rubber processing i s r e l a t i v e l y recent and results from theoretical and experimental progress i n understanding flow properties of pure polymers, and p a r t i c u l a r l y thermoplastics. However the rheology of elastomers i s further complicated by the necessary presence of f i l l e r s , p l a s t i c i z e r s and other ingredients. These lead to peculiar flow properties associated with heterogeneous matter and therefore not yet well understood. The aims of this presentation are to give some insights into the growing understanding of the p a r t i c u l a r flow phenomena involved i n rubber processing. Rather than give a general survey of the numerous flow situations i n rubber processing, attention w i l l be focused on two p a r t i c u l a r operations, i . e . internal mixing and extrusion. Using experimental r e s u l t s , the main rheological effects associated with rubber processing w i l l be demonstrated and their significance underl i n e d . In addition, some p a r t i c u l a r effects t y p i c a l of heterogeneous rubbery materials, such as thermoplastic elastomers, w i l l be presented and their significance i n rubber processing discussed. Most of the results discussed i n this paper have been obtained through a suitable use of the Monsanto P r o c e s s a b i l i t y Tester, and therefore this paper w i l l also consist i n an i m p l i c i t demonstration of the usefullness of this v e r s a t i l e instrument. 0097-6156/ 84/ 0260-0183S07.00/ 0 §> 1984 A m e r i c a n C h e m i c a l Society

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

184

POLYMERS

FOR FIBERS A N D

ELASTOMERS

Rheology and mixing rubber

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Preparing rubber compounds by blending of raw elastomers and mixing f i l l e r s , o i l s and other ingredients into rubbers i s a fundamental operation i n the rubber industry which, s u r p r i s i n g l y , has been the object of l i t t l e work u n t i l recently. The aims of mixing are to incorporate and uniformly disperse the various ingredients within the elastomer matrix, i n such a way that easy processing, e f f i c i e n t curing and adequate end-use properties w i l l be obtained. This operation i s generally achieved i n internal mixers which are thoroughly described elsewhere (1, 2). Physics of mixing. The mixing of rubber compounds i s a very complex operation about which i t i s d i f f i c u l t to have an o v e r a l l and clear understanding (3-5). However, i t i s useful to consider that four basic physical operations take place during the mixing cycle (1). For the sake of discussion, those four operations can be best described with respect to the mixing power curve, as shown i n Figure 1: 1. The incorporation of ingredients, either s o l i d or l i q u i d , i n the elastomer takes place at the beginning of the mixing cycle. More s p e c i f i c a l l y , this operation concerns the 'wetting of s o l i d particles which, according to Tokita and P l i s k i n (6) and Nakajima (8), can occur following two mechanisms: (i) large deformation of the elastomer increases the contact area with f i l l e r p a r t i c l e s and seals them inside, ( i i ) the elastomer breaks down into small pieces, mixes with the f i l l e r agglomerates and once again sealing occurs. If the former mechanism can e a s i l y be observed i n an open m i l l , the l a t t e r i s not necessarily apparent because these phenomena occur on a micro-scale. The incorporation involves a decrease i n the s p e c i f i c volume of the mix (6). 2. The dispersion results i n progressive spreading of f i l l e r p a r t i c l e s . This operation involves a reduction i n size of f i l l e r agglomerates, possibly to their ultimate p a r t i c l e size and, generally takes place between the two power peaks. An attempt i n t h e o r e t i c a l l y predicting the dispersive mixing process from fundamental considerations has recently been published (7). 3. The p l a s t i c i z a t i o n modifies the rheological properties of the mix mainly a v i s c o s i t y decrease by mechano-chemical degradation of the polymer and modification of i t s v i s c o e l a s t i c properties. 4. The d i s t r i b u t i o n or simple mixing involves the moving of p a r t i c l e s throughout the compound, without changing their physical shape, i n order to increase the randomness of the s p a t i a l d i s t r i b u t i o n and therefore the entropy of the mix. This elementary physical process takes place during the whole mixing cycle. To describe p r a c t i c a l mixing with a set of four basic physical operations i s c l e a r l y an o v e r s i m p l i f i c a t i o n since, to a c e r t a i n extent, a l l actions occur simultaneously. This description helps however i n introducing the rheology of mixing. 1

Mixing rheology - l u b r i c a t i o n theory. When considering only the nip region, i t i s possible to develop a rheological approach based on the l u b r i c a t i o n theory (11). Analyses by Bernhardt (3), Bolen and

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Rubber Processing Through Rheology

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LEBLANC

Mechanisms

of

filler

incorporation

(i)

(

In f a c t , as seen i n the previous section (see Figure 6), the o v e r a l l extrusion pressure AP consists of several components, i . e . AP - AP + AP,. + P . (15) ent die exit where A P , AP^^g and it Pressure drop, the pressure gradient within the die and the exit pressure respectively. Since P £ i s generally very small, i t can be neglected i n f i r s t approximation and i t follows that: AP . = AP - AP (16) die ent ' And since the Bagley correction b a s i c a l l y takes into account the entrance pressure drop effects (and the n e g l i g i b l e exit e f f e c t s ) , i t can be written that: p

e n t

e x

a

e

r

e

t l i e

e

n

t

r

a

n

c

e

x

t

A

AP

4 -

+

l

V

4i

AP - AP _

4i

Equation (17)allows an estimation of the entrance pressure drop, A P , to be calculated, knowing the extrusion pressure and the Bagley correction, as follows: ent

(—) . o ent D

AP

AP

ent

1 •

D

(18)

«o ent

o

At the entrance of a die, there i s a converging f i e l d which produces a strong extensional flow. Cogswell (24) has suggested a procedure for c a l c u l a t i n g an 'apparent extensional v i s c o s i t y , ^ E C * from measurements of entrance flow. Since neither the l o c a l normal stresses nor the l o c a l extension rate can be calculated d i r e c t l y from measurable quantities, i t i s necessary to make a number of assumptions about the flow. Cogswell assumes that the entrance pressure drop, A can be represented as a sum of two terms, one related to shear and the second to extension. He further assumes that the f l u i d follows streamlines that r e s u l t i n the minimum pressure drop, and for a f l u i d having a power law v i s c o s i t y function, he derived the following equation for an 'apparent extensional 1

p

e

v i s c o s i t y ' , TI Q E

z

9(n+l) (AP_J ent "EC

32 n

(19)

1

where n i s the flow index, and n « .the apparent v i s c o s i t y at shear rate Y . a^ #

Y

a

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

LEBLANC

11.

Rubber Processing Through Rheology

203

Using experimental r e s u l t s obtained with the three Santoprene grades studied, the entrance pressure drop and the apparent extensional v i s c o s i t y were calculated according to equations (18) and and (19). Results are given i n graphical terms i n Figures 11 and 12 respectively. Figure 11 shows that, as expected, the entrance pressure drop increases with increasing shear rate. In addition, the AP vs y curves of the three samples are s i g n i f i c a n t l y d i f f e r e n t , obviously related to the rubber p a r t i c l e content. The lower the hardness the higher the entrance pressure loss. As f a r as entrance effects are i n d i c a t i v e of the magnitude of melt e l a s t i c i t y , i t i s e a s i l y understood that the higher the rubber p a r t i c l e content, the higher the melt e l a s t i c i t y . This r e s u l t associated with the fact that the extrudate swell of Santoprene decreases with increasing rubber content indicates the l i m i t s of the analogy between OTV's and highly f i l l e d polymer. Indeed, the analogy can only be correct i f ' e l a s t i c f i l l e r s ' are considered. Furthermore, since the curves i n Figure 11 seem to converge at very high shear rate, i t i s clear that the rheological behaviour of OTV's i s controlled by the existence of a connective structure of rubber p a r t i c l e s , progressively destroyed when shear increases, as suggested by Goettler, Richwine and Wille (32). As far as Cogswell's approach i s correct i n estimating elongational v i s c o s i t y from entrance pressure drop, Figure 12 presents very interesting r e s u l t s . As can be seen, the 'extensional v i s c o s i t y ' increases with decreasing hardness of the material. This can be explained using the connective structure model pointed out above. These p a r t i c l e associations however are more l i k e l y to form while the melt i s not under shear and therefore, the 'apparent elongational v i s c o s i t y ' at die entrance i s decreasing when shear rate increases. As the connective structure i s destroyed, the entrance flow resistance i s weaker. In addition, i t seems that the r w v s Y curves exhibit a maximum, depending upon the rubber content. The higher the rubber content, the lower the shear rate for this maximum. As can be seen i n Figure 12, the 40D grade exhibits an increasing elongational v i s c o s i t y below 30-40 s ~ l and a decreasing n £ at higher shear rate. Lower hardness grades probably exhibit the same behaviour but with the maximum a r i s i n g at lower shear rate. Whilst not yet f u l l y understood these r e s u l t s c l e a r l y demonstrate that the processing rheology of Santoprene thermoplastic rubbers i s dominated by singular e f f e c t s associated with the connective structure of rubber p a r t i c l e s .

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a

E

Conclusions The rheological approach to rubber processing allows a better understanding of the flow behaviour of elastomers to be achieved. Obviously the picture i s f a r from being complete and due to their complexity, rubber compounds are exhibiting phenomena not yet completely understood. The s p e c i f i c examples presented i n t h i s paper, either i n mixing rubber compounds, or i n shaping through short dies or when considering the melt rheology of thermoplastic elastomers, demonstrate c l e a r l y that one has to pay as much attention to elongational flow as to shear flow i n rubber processing s i t u a t i o n s .

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

204

POLYMERS F O R FIBERS A N DE L A S T O M E R S

500 Elongational, viscosity T = 204°C

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100 kPa.s

\ .

SANTOPRENE® GRADE ^

10

20173

201-87

3 1D

1

I

• 2

10 Apparent shear

1

io rate

3

>^

•

S"

203-40 . i

• i i i i 1

4

10

Figure 11 :Entrance pressure drop versus shear rate f o r Santoprene thermoplastic elastomers at 204°C

Apparent

shear

rate

Figure 12 :Apparent elongational v i s c o s i t y versus shear rate f o r Santoprene thermoplastic elastomers at 204°C

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

11.

LEBLANC

Rubber Processing Through Rheology

205

Correct and imaginative use of suitable rheological equipment, such as the Monsanto P r o c e s s a b i l i t y Tester, allows pertinent information to be obtained at lower cost than the c l a s s i c a l t r i a l and-error approach. Furthermore, better cost e f f i c i e n c y can be achieved when interpreting c a p i l l a r y rheometer results i n terms of processing and selecting the best operating conditions with respect to material flow properties.

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Acknowledgements The author wishes to thank Dr. M. Bristow of Malaysian Rubber Producers Research Association - Brickendonbury, U.K. who kindly supplied various grades of natural rubber. He i s grateful to Dr. L.A. Goettler and Mr. J . Sezna from Monsanto Polymer Products Company, Akron, Ohio, f o r the experimental data on Santoprene thermoplastic rubbers used i n this paper.

Literature (1) H. PALMGREN - Rubb. Chem. Technol. 48, 462 (1975) (2) J.M. FUNT - "Mixing of Rubbers" - RAPRA, Shrewsbury, England (1977). (3) E.C. BERNHARDT - "Processing of thermoplastic materials" van Nostrand Reinhold, New York, pp 424-446 (1959). (4) J.M. McKELVEY - "Polymer Processing" - chap.12, Wiley, New York (1962). (5) P.S. JOHNSON - Elastomerics, 115 (1), 9, January 1983. (6) N. TOKITA and T. PLISKIN - Rubb. Chem. Technol.,46, 1166 (1973). (7) I. MANAS-ZLOCZOWER, A. NIR and Z. TADMOR - Rubb. Chem. Technol., 55 (5), 1250 (1982). (8) N. NAKAJIMA - Rubb. Chem. Technol.,53, 1088 (1980). (9) W.M. WIEDMANN and H-M. SCHMID - Rubb.Chem.Techno1. 55, 363(1982) (10) P.K. FREAKLEY and W.Y. VAN IDRIS - Rubb.Chem.Technol.,52, 134 (1979) (11) H. LAMB - "Hydrodynamics" - chap. XI, pp 583, Dover Publ., New York (1945). (12) W.R. BOLEN and R.E. COLWELL - SPE Antec, 14, 1004 (1958). (13) J.L. WHITE and N. TOKITA - J . Appl. Polym. Sc., 12 1589 (1968). (14) J.L. WHITE - Polym. Eng. Sc., H) (11), 818 (1979). (15) K. ODA, J.L. WHITE and E.S. CLARK - Polym. Eng. Sc., 18, 25 (1978). (16) G.R. COTTEN - Plastics and Rubber : Processing, 4 (3),89 (1979). (17) J . MEISSNER - Trans. Soc. Rheol., 26, 405 (1972). (18) N. NAKAJIMA - Polym. Eng. Sc., 19, 215 (1979). (19) G.M. BRISTOW - NR Technology, 10 (3), 53 (1979). (20) J.L. LEBLANC - Plastics and Rubber Processing and Applications, Vol.1, No 2, 187 (1981) (21) G.M. BRISTOW and A.G. SEARS - NR Technology, 11 (3), 45 (1980); ibid, 11 (4), 77 (1980); ibid, 12 (2),36 (1981) (22) S. MONTES and J.L. WHITE - Rubb. Chem. Technol, 55, 1354 (1982) (23) E.B. BAGLEY - J. Appl. Phys., 28, 624 (1957) (24) F.N. COGSWELL - Trans. Soc. Rheol., 16, 383 (1972) (25) J.M. McKELVEY - "Polymer Processing" - Chap. 2, sec. 2-7, p 51, Wiley, New-York (1962)

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

206

POLYMERS FOR FIBERS A N D E L A S T O M E R S

C.D. HAN and M. CHARLES - Trans. Soc. Rheol., 15, 371 (1971) J.L. WHITE and A. KONDO - J . Non-Newtonian Fluid Mech.,3,41(1978) J.L. LEBLANC - Rubb. Chem. Technol., 54, 905 (1981) E.B. BAGLEY, S.M. STOREY and D.C. WEST - J. Appl. Polym. Sc., 7, 1661 (1963) (30) J.L. WHITE; A.P. PLOCHOCKI and h. TANAKA - Polym. Eng. Rev., Vol. 1, N° 3, 218 (1981) (31) A.Y. CORAN and R. PATEL - Rubb. Chem. Technol.,53, 141 (1980) (32) L.A. GOETTLER, J.R. RICHWINE and F.J. WILLE - Rubb. Chem. Technol., 55, 1448 (1982)

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(26) (27) (28) (29)

RECEIVED February 6, 1984

Arthur et al.; Polymers for Fibers and Elastomers ACS Symposium Series; American Chemical Society: Washington, DC, 1984.