Page 160 - CW E-Magazine (27-5-2025)

Free to Die

Why synthetic rubber has such a high molecular

weight?

This monthly column is all about Permeation Chromatograph (GPC) as
openly sharing expertise and knowhow also lab facilities required for prepar-
with our readers, regardless of industry, ing emulsion polymers. One student
product or service. The aim is to plant who had fi nished her Ph.D. work but not
the seeds of inspiration, strengthen the fi nished her viva-voce was available for
knowledge base, and hopefully, lead to four to six months. Since I wanted work
new opportunities and innovations. badly, I made a proposal that my friend
could not refuse. I only asked for 5 litres
While the column is being rolled out of several monomers, a monthly stipend
with the ideas of Prof. V.C. Malshe, an for the student, and some funds for other
academician, researcher, innovator, consumables. In those days, a research
consultant and businessman, it is open guide could not include consultancy fees
to one all. Simply write to editorial@ in the research proposal (now it can be
chemicalweekly.com. done); so, my services were available
In 1995, I was new to the job as Pro- for free!
fessor in UDCT. I had only a couple of It was agreed that several polymers
Ph.D. students, just four to fi ve lectures and copolymers of acrylic monomers I knew something about rubber
every week and few consultancies. In would be prepared by emulsion poly- compounding and processing. I knew
short, I had lots of free time, and able merisation using AMS dimer as chain that the fi rst step of rubber compound-
to accept whatever work came my way. transfer agent at various concentrations ing was to masticate rubber with a free
radical generator to reduce the mole
and without it; the polymers would
At such a time, one of my friends cular weight, so that mixing of the various
working in a petrochemical industry then be isolated, purifi ed by repeated components to the formulation becomes
precipitation; and the molecular weight
asked me about the use of alpha-methyl- and molecular weight distribution easy. Rubber compounding is relatively
styrene (AMS) dimer for molecular determined by GPC. simple – rubber, carbon black, silica,
weight control of vinylic polymers. I and other additives such as masticating
told him what I knew, which was not a The project was fi nished on time, a agents, vulcanising agent, antioxidants,
whole lot. Then I came to know that his report was prepared, and several copies antiozonants, sulphur, lubricants, zinc
employer was getting about 150-tonnes were handed over to the sponsor. They oxide, stearic acid, rosin esters, etc. are
of AMS as a by-product, which they used it to promote use of AMS dimer mixed in 30:30:30:10 (the last being
were selling at a price cheaper than of to regulate molecular weight of emulsion all the additives put together), though
styrene monomer. AMS does not homo- polymers. there are some minor variations to this
polymerise and acts as an inhibitor for As per the contract with them, we composition.
vinylic polymerisation by formation of could not publish that work. I did not The fi rst step of processing all rub-
stable free radicals. AMS dimer, which even keep a copy for future use. bers is mastication with free radical gene-
is a mixture of two dissimilar structures, rators. The purpose is to bring down the
had been reported for molecular weight Controlling the molecular weight of molecular weight. Most rubbers as pro-
control of vinylic polymers, but exact synthetic rubber duced have very high molecular weight.
details were not known. On his request, For next several years they did not Natural rubber has a molecular wight in
I gave him a research proposal to study raise the subject. But sometime in 2002, the range of 5-6 million Dalton. Synthetic
the eff ect of AMS dimer on the mole- the company proposed to increase their varieties are about a million; give or take
cular weight of acrylic and styrenic AMS dimer capacity. So, a delegation a few thousand.
emulsion polymers. met me to fi nd out avenues for increas-
ing the market for AMS dimer. I gave This fi rst step is energy intensive.
The then Paints, Pigments and Var- them several ideas. One possibility was Typically, in a high shear mixer, 2,700-
nishes (PPV) Section of the Plastics and for controlling the molecular weight of kWh of energy is required to masticate
PPV Department at UDCT had a Gel synthetic rubber. one tonne of rubber to make it process-

160 Chemical Weekly May 27, 2025

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