The most important reinforcing fillers used in rubber industry are carbon black and silica. There are issues such as air pollution and higher energy consumption when reinforcing fillers are incorporated in dry rubber. The change in mechanical and dynamic properties accompanying filler incorporation has been known for about the past one hundred years. The demanding requirements for modern tyres like low rolling resistance, good traction and excellent abrasion resistance depend very much on the dynamic properties of rubber vulcanisates.
Compared to carbon black, silica is less compatible with non- polar elastomers such as natural rubber due to its polar nature. During the conventional mixing process, silica remains highly aggregated due to filler –filler interaction resulting in a dispersion which is not favourable for reinforcement. Therefore several attempts like use of silane coupling agent or special surface treatment etc had been attempted to enhance the dispersion of silica in the rubber.
In the case of carbon black, production of latex-carbon black master batch by the addition of carbon black as a slurry has been suggested as one of the methods to avoid some of the problems arising during filler incorporation. From the earlier works, it is clear that latex master batch process can deliver a premix which can be given a shorter mixing cycle and still result in an extremely well-dispersed compound.
It is reported that coagulation time of latex is reduced by the presence of suitable surfactants and this effect can be made use in latex stage incorporation of fillers. It is expected that if time of coagulation of latex filler slurry after addition of a coagulant is reduced then the fillers will be more uniformly distributed in rubber matrix as compared to conventional coagulation methods where the coagulation time is high.
There has been no systematic study on the production of filler batches from fresh natural rubber latex through a quick coagulation process. In this paper, an attempt is made to prepare latex carbon black/silica dual filler master batch from fresh NR latex by a modified coagulation process.
Quick coagulation method
Fresh natural rubber latex used in the study was obtained from Rubber Research Institute of India (RRII), Kottayam. High abrasion furnace black (N330) was obtained from Phillips Carbon Black Limited, Kochi, India. Precipitated silica used was ULTRSIL VN3. Other ingredients used were rubber grade chemicals.
Dispersions of carbon black and silica were prepared separately by the conventional ball milling process in the presence of a suitable surfactant. The dispersion was added slowly into fresh natural rubber latex under stirring and coagulated by addition of acid to produce the filler master batch. In this new method, the filler-latex mixture is coagulated chemically almost immediately after addition of acids.
The coagulum was washed well to remove the acid and dried in an air oven maintained at 70 ºC. The fillers were incorporated in latex so as to have levels of 40-60 parts per hundred parts of dry rubber (phr). The dried rubber was mixed , and vulcanised by conventional methods. Control mixes with loadings of 40, 50 and 60 phr fillers were also prepared by conventional mill mixing process.
The particle size of filler dispersion and latex was determined using a particle size analyzer model Nanosizer, from Malvern U.K., based on dynamic light scattering technique. The cure behaviour was determined at 150 ºC using a moving die rheometer model MDR2000 of Alfa Technology USA.
The Mechanical properties were determined from relevant ASTM standards. The dynamic properties were determined using Dynamic Mechanical Analyser model 01 dB DMA 50N of Metravib, France . The test was conducted at a frequency of 10 Hz and dynamic strain of 0 .12%. Filler dispersion was studied on vulcanized films using Dispersion Analyser from Tech Pro USA.
Where T is the weight of the test specimen, D its deswollen weight, F the weight fraction of insoluble component and Ao is the weight of the absorbed solvent, corrected for the swelling increment, pr and ps are the densities of rubber and solvent respectively.
The experiment yielded the following results:
a) Coagulation characteristics
Both silica and carbon black dispersions contain filler particles that have comparatively lower size (average 100 nm). The particle size of fresh NR latex-carbon black slurry mixture varies from about 90-6000 nm.
This shows that after addition of filler dispersions to latex, the size of filler particles increased to a small extent due to a certain degree of aggregation. However it is expected that there is uniform mixing of filler in the latex stage.
Filler dispersion-fresh latex mixture coagulated immediately on addition of acid leaving a clear serum. On addition of surfactants to latex, they cause displacement of protein and get strongly adsorbed on rubber particles. In this way, the protein stabilised latex gets transformed into a surfactant stabilized system.
On addition of acids to surfactant containing latex, the adsorbed anions react with acid to form undissociated surfactant, and deprive the latex particles of stabilisers. As a consequence, latex coagulates immediately. Due to quick coagulation, it is expected that the uniformly mixed carbon black/silica remains unaggregated during coagulation and further processing.
b) Vulcanisation characteristics
In the master batch mixes containing 40 phr of silica/carbon black dual fillers a comparatively higher torque is recorded when the carbon black content is higher. For pure silica system, a lower minimum torque and lower scorch time are recorded for the master batch as compared to mill mixed one. In the same filler loading, the master batch mix recorded a higher rheometric torque and lower cure time as compared to dry rubber mixed one showing higher levels of vulcanisation.
A similar observation of a higher rheometric torque is seen for both 50 and 60 phr filler containing master batch mixes. During coagulation a portion of the surfactant gets converted into the corresponding fatty acid. This helps in better vulcanisation characteristics of rubber as fatty acids are activators of vulcanisation. Due to higher level of vulcanisation the volume fraction of rubber in the swollen gel for master batch is higher than the corresponding conventional mix.
e) Mechanical properties
The vulcanisates prepared from the master batches by the new method showed higher tensile strength, higher modulus, higher hardness and higher tear strength along with lower heat build-up and abrasion loss .The improvement in mechanical properties is attributed to better filler dispersion and higher level of vulcanisation. Comparatively better dispersion and lower aggregation are shown by master batch mix as compared to mill mixed one.
Dynamic mechanical analysis
For master batch mix there is a lowering of tan delta peak height compared to the control mix showing that there is better polymer filler interaction for the master batch than the control mix. The storage modulus is also higher for the master batch mixes compared to the control. This is in line with the observations on the tensile properties.
It is also observed that the temperature showing the maximum tan delta is lower for master batch than control showing higher chain flexibility possibly due to lower filler-filler interactions. Higher chain flexibility is known to correlate with higher abrasion resistance and lower heat build-up characteristics.
Ideal filler for tyre tread compounds are those which possess high polymer–filler and low filler–filler interactions. The former ensures higher abrasion resistance and the latter is necessary for lower rolling resistance or lower tan delta. It is observed that a lower tan delta at 600C is obtained for the master batch compared to the control, showing a lower rolling resistance for the master batch.
The experiments show that a uniformly mixed carbon black/ silica dispersion and fresh latex in the presence of suitable surfactant coagulate quickly on addition of acids. The filler master batch prepared by this new method shows enhanced cure characteristic, filler dispersion and superior dynamic and mechanical properties as compared to conventionally prepared mixes. This is a simpler and cheaper method of master batch preparation as compared to the earlier methods.
These are extracts from a paper based on the original research work done jointly by Dr. Arup K Chandra, Head (RM/R&D), Apollo Tyres Ltd, Dr. James Jacob, Dr. Rosmma Alex and K K Sasidharan, Director, Dy. Director (Rubber Technology) and Research Scholar respectively of Rubber Research Institute of India (RRII) and Prof. Thomas Kurian, Department of Polymer Science & Rubber Technology, Cochin University of Science & Technology. This paper was adjudged the best technical paper in the all India competition held
as part of the 21st National Rubber Conference organised by Indian Rubber Manufacturers Research Association (IRMRA) in Mumbai.