Liquid Natural Rubber as a Viscosity Modifier in Nitrile Rubber Processing

Polymer International 42 (1997) 289È300

Liquid Natural Rubber as a ViscosityModifier in Nitrile Rubber Processing

N. Radhakrishnan Nair,a* Sabu Thomasb & N. M. Mathewa

a Rubber Research Institute of India, Kottayam 686 009, Kerala, Indiab Mahatma Gandhi University, Priyadarsini Hills PO, Kottayam 686 650, Kerala, India

(Received 12 March 1996 ; revised version received 24 July 1996 ; accepted 23 August 1996)

Abstract : The use of thermally depolymerised liquid natural rubber as a plastici-ser in nitrile rubber has been investigated with special reference to the rheologi-cal aspects. A comparative evaluation has been made with dibutylphthalate, theester-type plasticiser commonly used with nitrile rubber. The study revealed thatthe use of liquid natural rubber as a plasticiser and viscosity modiÐer improvedthe processability of the compounds and also enhanced many of the vulcanisateproperties.

Key words : liquid natural rubber, plasticiser, nitrile rubber, viscosity modiÐer.

INTRODUCTION

Oligomers are used as process aids in various com-pounded rubbers and plastic formulations. The advan-tages over conventional oil and ester-type plasticisersare their easier addition, improvement in properties foroil-resistant applications, where the plasticiser is not tobe leached by the surrounding oil medium, enhance-ment of physical properties due to their polymericnature, and better processability. The heterogeneousnature of rubber compounds, the strong interactionbetween the various formulation components, the visco-elastic nature and poor understanding of Ñow behav-iour of such complex materials, make the processing ofrubber compounds a complex procedure. Changes inthe recipe are often made, either to meet new speciÐ-cations for Ðnished products or to compensate forvariations in properties or quality of raw materials.Improvement in rubber processing properties hasbecome necessary owing to the need for high per-formance rubber formulations and because of competi-tiveness with the introduction of new challengingmaterials such as thermoplastic elastomers. ModiÐ-cation in bulk viscosity of compounds can result ineasier processing, fewer rejects and reduction in energyconsumption. An improvement of Ñow behaviour canallow higher output, or simpler operation, throughgreater latitude in mould design.

* To whom all correspondence should be addressed.

Annual consumption of natural rubber (NR) is aboutÐve million tons and it remains as a major elastomerwith unique properties for certain applications, despitecontinuous progress with synthetic polymers. The pro-duction potential of NR is larger than the present con-sumption and, because of its natural photosyntheticorigin, it is an interesting renewable source of raw poly-meric materials. The origin of natural rubber is unique

TABLE 1. Properties of LNR

Ash content (%) 0·06

Volatile matter (%) 0·1

Specific gravity 0·92

Colour light brown

Brookfield viscosity (cps at 38¡C) 112 000

M1w

44 210

M1n

5 929

Polydispersity index 7·48

TABLE 2. Properties of NBR

Volatile matter (%) 0·75

Ash (%) 1·0

Soap (%) 0·05

Antioxidant (%) 1·0

Bound ACN content (%) 33

Mooney viscosity ML(1 ½4)=100¡C 50

289Polymer International 0959-8103/97/$09.00 1996 SCI. Printed in Great Britain(

290 N. Radhakrishnan Nair, S. T homas, N. M. Mathew

TABLE 3. Formulation of mixes

Compound C L5 L10 L15 L20 L25 D5 D10 D15 D20 D25

NBR 100 100 100 100 100 100 100 100 100 100 100

ZnO 5 5 5 5 5 5 5 5 5 5 5

Stearic acid 3 3 3 3 3 3 3 3 3 3 3

Liquid natural — 5 10 15 20 25 — — — — —

rubber

Dibutyl — — — — — — 5 10 15 20 25

phthalate

Accelerator 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5

CBS

Sulphur 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5 1·5

among the widely used polymers, because it occurs asthe polymer itself, while others are manufactured fromtheir respective monomers, mostly of petroleum origin.In order to obtain oligomers of natural rubber, depoly-merisation methods must be resorted to. Syntheticrubber oligomers are obtained by controlling the pro-cessing conditions during the polymerisation process.

Depolymerisation of natural rubber at di†erent stageshas been extensively studied in the past. The Ðrst com-mercial production of depolymerised natural rubber inthe USA was by depolymerising solid rubber.1 A pilotplant for latex stage depolymerisation was set up on theIvory Coast.2 Production of liquid natural rubber bythe combined e†ect of mechanical, chemical andthermal energies has been reported by Claramma et al.3

Nitrile rubber is generally difficult to process owingto the high viscosity of the compounds. The use of lowmolecular weight polymers in rubber compounding hasbeen reported by Royale4 and Hashimoto.5 Thermallydepolymerised natural rubber as a reactive plasticiser innitrile rubber has been studied by Radhakrishnan Nairet al.6 Compounding nitrile rubber with furfuryl alcoholas a plasticiser has been reported by Magrupov et al.7The use of latex-stage depolymerised natural rubber,using a phenyl hydrazine redox system as a bulk vis-cosity modiÐer in rubber compounding, has beenreported by Le Blanc et al.8 The present work on theplasticisation of nitrile rubber using liquid naturalrubber produced by thermal depolymerisation wasundertaken with a view to increasing the techno-economic potential of natural rubber and gaining aninsight into easier processing of nitrile rubber.

EXPERIMENTAL

Materials

Thermally depolymerised liquid natural rubber (LNR)was used for the studies. The starting material for thepreparation was ISNR-5. Depolymerisation was carriedout in a thermal depolymeriser. A liquid natural rubber

sample in the medium viscosity range was used for plas-ticising nitrile rubber. The physical properties of LNRare given in Table 1. Nitrile rubber of medium acrylo-nitrile content (grade N-339, manufactured by Syn-thetics and Chemicals, Bereilly, India) was used for thepreparation of the compounds. SpeciÐcation parametersof the nitrile rubber sample are given in Table 2.Dibutyl phthalate (DBP) used for compounding waslaboratory grade N-dibutyl phthalate supplied by SyscoChem. Industries, Bombay. Other chemicals used wereof industrial purity.

Preparation of compounds

A nitrile rubber formulation with N-cyclohexylbenz-thiazylsulphenamide (CBS)Èsulphur cure system wasused. An unÐlled recipe was selected in order to observethe changes without the inÑuence by Ðller. Mixing wasdone on a laboratory-scale two-roll mill with a frictionratio of 1 : 1É25. A compound containing no plasticiserwas taken as standard. Plasticiser content in the range 0to 25 phr loading was investigated. Recipes of the di†er-ent formulations are given in Table 3.

Testing

For the determination of cure time, an oscillating discrheometer was used. The viscosity of the compoundswas determined using a Mooney viscometer (SPRI Ltd,Negretti Automation, UK). A capillary rheometerattached to a Zwick UTM model 1474 was used for therheological studies. The dimensions of the capillaryrheometer are given in Table 4. The sample for testingwas placed in the barrel, which was maintained at the

TABLE 4. Physical dimensions of capillary rheo-

meter

Diameter of capillary (mm) 1·27

Length of capillary (mm) 50·08

Diameter of plunger (mm) 15·99

Diameter of cylinder (mm) 16·00

POLYMER INTERNATIONAL VOL. 42, NO. 3, 1997

L iquid natural rubber as a viscosity modiÐer 291

TABLE 5. Properties of compounds

Compound Scorch time (min) Cure time (min) Mooney viscosity

C 5·25 21·5 38

L5 5·25 24·0 32

L10 5·50 22·5 28

L15 6·75 23·0 20

L20 7·00 22·5 15

L25 7·00 25·0 10

D5 5·25 19·5 33

D10 5·50 23·0 25

D15 5·75 22·0 21

D20 6·00 22·0 18

D25 6·00 19·5 15

test temperature. The sample was forced down the cap-illary using the plunger attached to the Ðxed cross-head.After a warming-up period of 3 min, the melt wasextruded through the capillary at preselected speeds ofthe cross-head. The melt height in the barrel beforeextrusion was kept the same in all experiments and themachine was operated to give ten di†erent plungerspeeds. Each plunger speed was maintained until therecorded force stabilised, before changing to the nextspeed. Forces corresponding to speciÐc plunger speedswere recorded. The force and the cross-head speedswere converted into corresponding apparent shearstress and the shear rate at the wall by using(qw) (cw)the following equations involving the geometry of thecapillary and plunger :

qw \ F/4Ap(lc/dc) (1)

cw \ [(3n ] 1)/4n]32Q/n dc3 (2)

where F is the force applied at a particular shear rate,is the cross-sectional area of the plunger, is theAp lc

length of the capillary, is the diameter of the capil-dclary, Q is the volumetric Ñow rate, n is the Ñow behav-iour index deÐned by d(log is thecw)/d(log cwa), cwaapparent shear rate at the wall, and is the actualcwshear rate at the wall. The value of n was determined byregression analysis of the values of and obtainedqw cwafrom the experimental data. The shear viscosity g wascalculated as :

g \ qw/cw (3)

The melt viscosity of the compounds was also calcu-lated at shear rate 333 s~1 and 120¡C. The principalnormal stress di†erence, was calculated fromq11 [ q12 ,the formula :

q11 [ q12\ 2 ] qw[2(de/dc)6[ 2]1@2 (4)

where is the shear stress at the wall, is the diam-qw deeter of the extrudate and is the diameter of the capil-dclary. Recoverable elastic shear stress, SR, was calculated

from the formula :

SR \ (q11[ q12)/2qw (5)

Elastic shear modulus, G, was calculated using theequation :

G\ qw/SR (6)

The samples extruded through the capillary were col-lected at selected shear rates and their diameters weredetermined using an optical microscope. Die swellvalues were calculated from the formula :

die swell (%)\ [(de [ dc)/de]] 100 (7)

The free vibration method on a Yerzely oscillographhas been used for measuring the important engineeringproperties of the vulcanisates. Static modulus (I) hasbeen determined from the slope of the loading curve.Resilience (Y R) has been expressed as a percentage,determined as the average computed from the secondand third half cycles :

Y R (%)\ [(CD/BC)] (DE/CD)]] 50 (8)

where BC is the vertical distance of the Ðrst half cycle ofthe damped sinusoidal wave, CD is the vertical distanceof the downstroke of the second cycle and DE is thevertical distance of the upstroke of the second cycle.

Natural frequency was determined by counting a con-venient number of complete cycles, measuring the hori-zontal distance JK traversed by this number of cycles,X, along the centre axis. The frequency was calculatedfor a drum of 15 inch circumference and 4 rpm rotation,using the equation :

f (Hz)\ X/JK (9)

The loss factor was calculated according to the methodsuggested by Scott9 from the formula :

A3/A2\ e~1@2n tan d (10)

where and are the amplitudes of the third andA3 A2second vibrations and tan d is the loss factor.



Fig. 1. E†ect of plasticiser on cure.

Scanning electron microsopy studies were carried outon a general purpose scanning electron microscope(Hitachi model S-200). The equipment was one of theintelligent series with features such as high resolution,

superauto stigma and superauto focus. The fracturedface of the extrudate sample was coated with gold undervacuum and photographed at appropriate magniÐ-cations.

RESULTS AND DISCUSSION

Cure characteristics

Table 5 shows the processing parameters of the nitrilerubber compounds. Scorch time of the compounds indi-cated that the increased addition of dibutyl phthalate tonitrile rubber compounds had little e†ect. Slightlyhigher cure times were recorded on the rheometer forthe LNR- containing compounds. Figure 1 shows therheographs of compounds plasticised with 10 and20 phr of DBP and LNR in comparison with theunplasticised stock. The torque showed a steadydecrease with increased loading of the plasticisers. Com-paring DBP and LNR, the latter showed morereduction of torque, especially at higher loadings.

Mooney viscosities of the compounds are also givenin Table 5. From the data it can be inferred that addi-

Fig. 2. Shear stress versus shear rate at 90¡C.



Fig. 3. Viscosity versus shear rate at 90¡C.

tion of the plasticisers decreased the Mooney viscosity :the greater the quantity of the plasticiser, the greaterwas the reduction. Comparing the plasticisers, the inÑu-ence was more pronounced for LNR at higher loading.

Capillary rheometric studies

Shear stress versus shear rate. Shear stress values at dif-ferent shear rates for the compounds containing LNRand DBP plasticisers and the control compoundwithout any plasticiser were measured at 90¡C. Theresults are given in Fig. 2. The shear stress values at allshear rates were highest for the control and lowest forthe compounds containing LNR, indicating more e†ec-tive plasticiser action by LNR then by DBP. Compari-son of the shear stresses at di†erent shear ratesindicated a shear rate thinning trend with increasingshear rate. This indicates pseudoplastic behaviour. Therandom ground state structure is progressively brokendown with increase in shear rate and a shear orientedstructure is formed, which o†ers reduced resistance tothe force Ðeld. Pseudoplasticity represents the behav-iour of a Ñuid which, when subjected to a shear ratefrom zero upwards, shows a convex curvature with

respect to the shear rate axis. This can be representedby the power law equation

qP cn (11)

where n is the Ñow behaviour index. Another possibilityis that the curvature of the shear stress/shear rate curveis due to some limiting mechanical degradationoccurring in the capillary at higher shear rates.

The e†ect of shear rate on viscosity of the compoundscontaining 15 phr of LNR, DBP and the control com-pound are shown in Fig. 3. The viscosities at low shearrates were highest for the unplasticised compound, fol-lowed by DBP and LNR. As the shear rate increased, adrop in viscosity was noticed in all cases. The pseudo-plastic nature of the compounds can be attributed tothe structured state of entanglement at zero shear ratebeing progressively destroyed upon increasing shear.The LNR-containing compound showed the minimumviscosity.

E†ect on Ñow index. Figure 4 shows the change in Ñowindex with increasing plasticiser content. In general, theÑow index was higher for LNR compounds at all load-ings. In fact the values increase linearly with LNR



Fig. 4. InÑuence of plasticiser on Ñow index.

Fig. 5. Viscosity and Ñow index versus temperature.



Fig. 6. Log viscosity versus 1/T .

content. In the case of DBP the increase in the Ñowindex was nominal, indicating little impact of the plasti-ciser on the pseudoplastic Ñow behaviour of the com-pounds. LNR-containing compounds revealed thatplasticiser loading has a more profound e†ect on theÑow behaviour. For materials with pseudoplasticbehaviour, the value of n in eqn (11) should be belowunity. When high shear rates are operating on apolymer melt, the chain segments of the polymer mol-ecules are aligned along the stream lines and thusincrease the Ñow. The presence of LNR also inÑuencesthese orientations and contributes towards the Ñowbehaviour.

InÑuence of temperature. Figure 5 shows the Ñow indexof the compounds containing 15 parts of LNR andDBP as a function of temperature. With increase intemperature, the Ñow index increased in both cases. Aconsistent reduction in viscosity values was alsoobserved with temperature. At all temperatures, theDBP compounds showed higher viscosities. A small lev-elling trend was observed at elevated temperatures,which was more prominent for LNR compounds andwas attributable to scorching. Increase in temperatureincreases the Brownian motion in polymeric liquids andsoft solid materials and the free volume also increases.Increase in molecular mobility enhances the Ñow behav-

iour. Viscosity temperature relations are governed bythe Arrhenius equation :

g \ A e~E@RT (12)

where E is the activation energy of Ñow and T is theabsolute temperature. In order to obtain the activationenergy, log viscosity was plotted against 1/T at a shearrate of 333 s~1 (Fig. 6). The slopes of the lines werecalculated and the activation energy values of LNR andDBP samples were found to be 1128 and 846 kcal/mol~1 respectively. The dependence of Ñow propertieson temperature is of interest. Temperature is an e†ectivemeans of controlling melt viscosity in processing oper-ations. The optimisation of factors such as time, costand money is necessary, and a control on thermalenergy is required to save the polymer from degradationdue to excessive heat and to provide long life to theproducts.

Die swell. The die swell values of the compounds at dif-ferent shear rates, with varying loading of the plastici-sers from 0 to 20 phr, are given in Fig. 7. Thetemperature at which the extrudates were collected was120¡C. It is observed that the die swell values decreasewith increasing shear rate. A similar trend is observedwith plasticiser loading. Comparing the plasticisers, theLNR-containing compounds exhibited die swell to a



Fig. 7. Die swell at di†erent shear rates at 120¡C.

lower extent than the corresponding DBP compounds.At higher shear rates the dimensional stability of LNR-containing compounds was remarkably better. Thesurface Ðnish of the extrudates was also good (Fig. 8).

Fig. 8. Photograph of extrudates.

E†ect of shear rate on melt elasticity. Melt elasticity ofthe compounds was calculated from the extrusionparameters at a shear rate of 333 s~1 and a temperatureof 120¡C. The results are depicted Fig. 9. The principalnormal stress di†erence, decreased withq11 [ q12 ,increased loading of the plasticiser. The primary orprincipal normal stress di†erence of viscoelastic liquidsrepresents a combination of elastic and viscous e†ects.A large di†erence in the values of withq11 [ q12increase in plasticiser content indicated the inÑuence ofplasticiser loading on the viscosity of the compounds.Recoverable elastic stress showed a decreasing trendwith increasing plasticiser content. Minor inÑections inthe values of shear modulus, G, were also observed.

Morphology of the extrudates. Morphology studies wereconducted on fractured faces of the extrudate samplesrendered brittle cryogenically. A comparison of thescanning electron micrographs (Fig. 10) revealed goodcompatibility of LNR in the nitrile rubber matrix. Noevidence of phase separation could be observed and thematerials were found to be homogeneous at the 20 km



Fig. 9. Melt elasticity at 333 s~1 and 120¡C. ], recoverable elastic shear stress ; \, shear modulus, G.=, q11 [ q12 ,

scale. Good distribution was attained in the case ofboth LNR and DBP. No di†erence was observedbetween the phase interactions of the two plasticisersystems with nitrile rubber. This suggests that LNR isan e†ective plasticiser for NBR compounds.

V ulcanisate properties. StressÈstrain properties of thevulcanisates containing plasticiser loaded to di†erentlevels were measured and are given in Fig. 11. Modulusvalues of the compounds at 300% elongation showed adecreasing trend with increase in plasticiser content.Tensile strength of the compounds showed a marginalreduction on addition of both the plasticisers at 5 to10 phr levels, but remained constant thereafter. Elon-gation at break showed a decreasing trend initially (5 to10 phr) followed by an increase. LNR at high loadingsshowed higher elongation values, attributable to thepolymeric nature of the plasticiser. In agreement withthe general concept of plasticisation, both LNR andDBP made the rubber compounds softer with increas-ing plasticiser content. However, this decrease was lessfor LNR, indicating its use for making relatively hardervulcanisates.

Dynamic mechanical properties determined by usinga Yerzely oscillograph are given in Fig. 12. Yerzelyresilience values were in general higher for LNR-containing compounds than for DBP-containing com-pounds. This may be attributed to the polymeric

contribution of LNR, which is not present for DBP.Correspondingly higher values of loss factor wererecorded for the DBP compounds than for the LNRcompounds. The fundamental frequencies of thesamples were comparable, within minor experimentalÑuctuations. The static modulus showed a slightlydecreasing trend with increasing plasticiser content.

Evaluation of superposition shift factor. The experimentalÑow curves for log versus at temperatures of 90 tocw qw120¡C were constructed for the nitrile rubber com-pounds. The method of superposition has been appliedby the arbitrary choice of a reference temperature. Thevalues of the superposition shift factors were obtainedby choosing shear rate values at 100 and 1000 s~1 onthe reference temperature Ñow curve and shifting thecorresponding points (constant shear rate) on the Ñowcurves for other temperatures to coincide with thisshear stress. The values of were calculated from theaTequation :

aT \ q(ref)/q(T)(constant) cw (13)

where is the shift factor, is the shear stress ataT q(ref)reference temperature and (constant) is the shearq(T)stress at the particular study temperature.

The average values of shift factors for the compoundswere calculated10 and plotted as a function of tem-perature in Fig. 13. Using the appropriate average



Fig. 10. Scanning electron micrographs of extrudates : (a)control ; (b) L15 ; (c) D15.

values of the shift factors, the master Ñow curve at 90¡Cwas constructed and is given in Fig. 14. The shear ratetemperature superposition method can be used for pre-dicting the viscosities of the polymer melts at anyrequired temperature by determining the viscosity at areference temperature.

Fig. 11. Physical properties of vulcanisates. Modulus at=,300% elongation (MPa) ; ], tensile strength (MPa) ; \, elon-

gation at break ]100%; hardness ]10 (Shore A).K,

SUMMARY AND CONCLUSIONS

The e†ect of liquid natural rubber (LNR) in a nitrilerubber gum recipe was studied. It was compared withdibutyl phthalate (DBP), which is a popular ester-typeplasticiser used with nitrile rubber. Cure characteristicsof the compounds were comparable. LNR at higherloading showed a lower torque value on a rheographthan DBP. Studies on shear stressÈshear rate relations

Fig. 12. Dynamic properties of vulcanisates. Resilience=,(%), ], tan d ; \, I (kg m2) ; Frequency ]10 (H2) ; x,K,

hysteresis (%).



Fig. 13. Shift factor versus temperature.aT

Fig. 14. Superposition master curve.



using a capillary rheometer showed that both DBP andLNR decreased the compound viscosities with increasedloading. Capillary rheometer measurements indicatedthe pseudoplastic nature of the compounds. The LNRcompounds showed lower stress values than DBP com-pounds at equal shear rates. LNR acts as an e†ectiveplasticiser for nitrile rubber and aids the mouldingprocess. The Ñow behaviour index for DBP showedsteadier values than that for LNR on increase in theirweight percentage. Temperature had a profound e†ecton the viscosity of the compounds. The activationenergy of Ñow of LNR compounds was higher than thatof DBP compounds. Compounds containing LNRresulted in extrudates with less swell and good surfaceÐnish. Morphological studies revealed compatibility ofLNR with nitrile rubber. LNR compounds in generalshowed improvement in elongation and lower hardnessreduction due to plasticiser. Dynamic mechanicalproperties were comparable. A master Ñow curve for the

compounds at 90¡C was also constructed. In general,LNR was found to be an e†ective plasticiser and a goodprocessing aid in nitrile rubber compounding.

REFERENCES

1 Hardman, K. V. & Lang, A. J., Rubber Age, 66 (1950) 419.2 Allet D. A. & Lemoine, A., Proceedings of the International Con-

ference on L iquid Natural Rubber, Abidjan, 1986, p. 33.3 Claramma, N. M., Radhakrishnan Nair, N. & Mathew, N. M.

Indian J. Natural Rubber Res. 4 (1991) 1.4 Royale, E., European Patent Appl. 401193 A1, 1990.5 Hashimoto, G., Japanese patent No. 91100048 A2, 1991.6 Radhakrishnan Nair, N., Claramma, N. M., Baby Kuriakoe &

Mathew, N. M. Indian J. Natural Rubber Res. 2 (1991) 81.7 Magrupov, F. A., Abdurashidov, T. & Gorbunova, M. I. USSR

Patent SU 1558934, 1990.8 Le Blanc, J. L., De Livonniere, H. P. & Boccasio, G. Prog. Rubber

T echnol., 8 (1992) 79.9 Scott, J. R. in Physical T esting of Rubbers. Maclaren, New York,

1965, p. 180.10 Anand, J. S., Int. Plast. Eng. T echnol., 1 (1994) 25.


Liquid Natural Rubber as a Viscosity Modifier in Nitrile Rubber Processing

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