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Zinc Oxide And Stearic Acid Peptising System
For Natural Rubber

(By C. Nageswara Rao)


In the process of Mastication of Natural Rubber, Peptisers are used to increase the efficiency of Mastication (i.e. to increase the rate of molecular breakdown) so as to reduce the mixing time of Mastication.
This paper explains that the Zinc Oxide and Stearic Acid, which are higher to used as activators for activating the organic accelerators in the process of Vulcanisation, can also be used as peptizing agents and the efficiency of peptisation is comparable to the conventional established peptiser. It also shows that the use of fraction of total Zinc Oxide and Stearic Acid in the compound for peptizing function does not in any way affect the activating function in curing. This means that zinc Oxide and Stearic Acid system assumes dual role, namely peptizing function during Mastication and Activating function during Valcanisation.


The idea of using Zinc Oxide and stearic Acid as peptizing system has its origin in a work done to improve SBR Compound mixing characteristics. Sagging of SBR compound on the mills is one of the processing problem. In one of the trials to resolve the said problem, it is observed that when the SBR is prewarmed in the Banbury along with Zinc Oxide and stearic Acid, sagging of compound on the Mills minimized though not fully overcome. The handling of the compound on the Mills namely cutting and rolling, observed to be more easier and the compound sheets were more smoother, compared to one mixed in the conventional way. This observation led to undertake the study, the effect of Zinc Oxide and stearic Acid during Mastication of Natural Rubber.
One Batch each of Natural Rubber, with the following compositions was Masticated and Plasticity checked on Mooney Plastomer. 1.2 PHR of Zinc Oxide and 1.2 PHR of stearic Acid was orbitaraly chosen for the trial.

1. 0.1 PHR Renacit 7
2. 2.0 PHR Aktiplast
3. 1.2 PHR Zinc Oxide and 1.2 PHR Stearic Acid

along with these trials one batch was without any peptiser, but with minute extra mixing time and a higher discharge temperature (+50c).
From the results, it is seen that on Masticating without any peptiser, even with min extra time and 50c higher dump temperature, the resultant batch of Rubber has a significantly higher mooney viscosity than that of standard masticated batch. The use of 2.0 PHR Aktiplast as peptiser still does not match the standard. However with 1.2 PHR each pf Zinc Oxide and Stearic Acid, the plasticity was comparable.


Effect Of Various Dosage Combination Of Zinc Oxide And Stearic Acid On Plasticity And Pri Index

Having observed the peptizing action of Zinc Oxide and Stearic Acid, tests were undertaken with varying dosage of Zinc Oxide and Stearic Acid to study the effect on Plasticity and PRI on resultant rubber.

Plasticity checks were done both on Mooney Plastometer and on Wallace Plastometer. The PRI test was done to check whether the system has any effect on the Heat aging characteristics of the rubber.

i. 0.1 PHR Renacit 7
ii. 1.2 PHR Zinc Oxide and 1.2 PHR Stearic Acid
iii. 1.2 PHR Zinc Oxide and 0.9 PHR Stearic Acid
iv. 0.9 PHR Zinc Oxide and 0.9 PHR Stearic Acid

(many other combinations were tried and the result were not recorded here as they are of no interest).


The Test Results Shows That :-

a. With the 1.2 PHR of Zinc Oxide and 1.2 PHR of Stearic Acid the plasticity is comparable with one obtained having 0.1 PHR of Renacit 7 which is the standard practise.
b. The values of plasticity with 1.2 PHR Zinc Oxide and 0.9 PHR Stearic Acid are some what lower indicating that efficiency of the peptisation is greater.
c. With the systems having 1.2 PHR Zinc Oxide the PRI values are comparable- in fact better than the standard.

Trials In Regular Compounds :-

Masticated Rubber having 1.2 PHR Zinc Oxide and 0.9 PHR of Stearic Acid was used in the compound having 100 N.R. 15 PHR reclaim, 35 PHR SRF, 0.8 PHR of MOR and 2.5 PHR Sulphur and checked for (suitable adjustments were made to the levels of Zinc Oxide and Stearic Acid while mixing stock so that resultant levels are not altered (i.e.) the experimental batches did not contain extra Zinc Oxide and Stearic Acid).
1. Mixing Characteristics
2. Processing Characteristics
3. Phsical Properties


a.   The Mixing characteristics are comparable.
b. The processing characteristics with special reference to tack and handling are comparable.
c.   Physical properties are comparable.

The effect on storage was checked for a period for 6 days by checking the properties every day and do no adverse effect was noticed.


Since plasticity, scorch and vulcanizing characteristics, the physical properties, PRI Index are comparable, test was conducted on the Dematia Machine for studying the Flex Resistance. The results here too was comparable.


1. 1.2 PHR Zinc Oxide and 0.9 PHR Stearic Acid has conclusively established to be a satisfactory peptiser for N.R. on the basis of:

a. Plasticity Tests
b. PRI Test
c. Compound Scorch and Vulcanisation Characteristics
d. Physical Properties of Compound
e. Processing behaviour of Mills, at Calender and at Extruder.
f. Tack and handling during building operations.
g. Anti Flex characteristics of Compound.

2. The use of fraction of the Total Zinc Oxide and Stearic Acid in the compound for peptising function during Mastication does not seem to affect the activating function during curing (the levels of Zinc Oxide and Stearic Acid being suitably adjusted during stock mixing to keep the total levels in the compound unaltered (i.e. compound does not contain extra Zinc Oxide and Stearic Acid).
3. Since no extra materials are required, substantial savings can be achieved.


Money Viscosity Details:-


a. Rubber Using :-  RMA 4 50% RMA 5 50%

b. Plant Details :-
1. 40 Rev. Banbury with 11 Ram Cylinder
2. Ram Pressure-100 psi + 5 psi
3. Batch Weight-165 Kg+ Peptiser
4. Discharge Temp-135 0c
5. Mixing Time-4 min.

C.  Test Results 


0.1      PHR

Renacit 7




extra time

2.0 PHR


1.2 PHR  Zin. Oxide

1.2 PHR  St. acid

ML 4 @ 120

ML 8 @ 120









MONEY Viscosity, Repid Plasticity Number
And Plastrict Retention Index:

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