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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
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 ½ minute
extra mixing time and a higher discharge temperature (+50c).
From the results, it is seen that on Masticating without any peptiser, even with
½ 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.
DETAILS GIVEN IN ATTACHMENT I
Various Dosage Combination Of Zinc Oxide And Stearic Acid On Plasticity And Pri
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.
THE COMPOSITIONS STUDIED ARE:
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).
TEST RESULT GIVEN IN ATTACHMENT I
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
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
1. Mixing Characteristics
2. Processing Characteristics
3. Phsical Properties
THE FINDINGS ARE:-
a. The Mixing characteristics are comparable.
b. The processing characteristics with special reference to tack and handling
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.
FLEX CRACKING TEST:
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:-
1. MASTICATION PROCESS 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
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: