U N I V E R S I T Y
F O C U S
Breakdown
service
Pioneering work at the University of Akron promises
advances for recycling devulcanized rubber – but the tire
industry is yet to be convinced
by Keith Read
I
n the USA alone, some 300
million tires are scrapped every
year and a similar number sit in
stockpiles. The vast majority of
these worn-out tires are burned
to produce energy – plus CO2
and ash – while many more end up in
landfill sites. But, stripped of the steel,
textiles, and other components within
modern tires, something like 20 lb of
rubber is available for devulcanization
and more-lucrative recycling. And
a team at the University of Akron,
Ohio, under the leadership of Dr
Avraam Isayev, a distinguished
professor at its department of polymer
engineering, has developed the
ultrasonic devulcanizing technology
that produces rubber of such
quality that it can be added to virgin
rubber to make new products.
“Some people call burning a way
of recycling,” says Isayev. “Looking at
raw material costs – with rubber now
something like US$2 per lb – against
the amount of energy you get from
burning rubber, you get about double
the amount of energy that you get
from burning 1 lb of coal. However,
it’s not really the way to do it because
coal costs something like US$100
per [metric] ton, while rubber costs
US$4,000 per [metric] ton!”
By burning scrap rubber
and worn-out tires, rather than
devulcanizing in order to recoup the
main components for further use,
the world is also being drained of
supplies of rubber. And while coal
is a finite resource and rubber is
42
Right: Akron’s
experimental
ultrasonic twinscrew extruder
Above: Ultrasonic
single-screw barrel
extruder in the lab
not, the price differentials indicate
a far from cost-effective benefit,
even if some would claim ecological
advantages of burning instead of
dumping and producing energy at
the same time. Isayev underlines the
point: “If you burn rubber it goes
into CO2 and H2O and just goes
away. Burning rubber is not a good
idea. Dimitri Mendeleev, the Russian
chemistry professor who created
the Periodic Table in 1869, said that
the burning of gas is the same as
burning money. I say the burning
of tires is the burning of money…”
Devulcanization
There is, therefore, a powerful case for
devulcanization. Indeed, the process is
by no means a concept of 21st-century
need to reduce waste and conserve
resources. Charles Goodyear,
after whom the tire company is
named, started to consider ways of
devulcanization soon after receiving
www.tiretechnologyinternational.com October 2012
U N I V E R S I T Y
October 2012 www.tiretechnologyinternational.com
F O C U S
43
U N I V E R S I T Y
F O C U S
THE CHALLENGE
The major problem encountered with the recycling of thermosets is that, unlike
thermoplastics, they cannot be reprocessed simply by applying heat. This is due to
the presence of their crosslinked structure and three-dimensional network. To recycle
thermosets, the crosslinks have to be broken.
Originally it was thought that rubber is vulcanized by ultrasound rather than devulcanized
by it. However, early experiments showed that a small sample of vulcanized rubber in a
batch process was devulcanized at 50kHz ultrasound waves after treatment for 20 minutes,
breaking down carbon-sulfur bonds, but not carbon-carbon bonds. The properties of
the subsequently revulcanized rubber proved to be very similar to those of the original
vulcanizates.
It was from this initial work that Isayev and his team developed the technology that
currently offers a significantly beneficial alternative to incineration and dumping, as well
as resulting in a more-acceptable and less-polluting method of devulcanization for those
prepared to make the capital investment.
Future development of the technology and machinery will concentrate on increasing the
throughput of vulcanized rubber to the point where it becomes attractive to the tire industry
as a viable recycling system for worn and scrapped tires. n
his US patent for vulcanizing rubber
in 1844. Since then, several different
methods and processes have been
developed with varying degrees
of success and acceptability.
One method of recycling used
with tires is pyrolysis, whereby
whole or shredded tires are heated
in a reactor vessel containing an
oxygen-free atmosphere. The rubber
is softened, after which the polymers
continuously break down into
smaller molecules, which eventually
vaporize. The vapor can be burnt to
produce power or condensed into
an oily liquid used mainly for fuel.
Another is to use chemicals
– highly toxic chemicals – to
devulcanize the rubber. “The
disadvantage of using chemicals
is that you get a nasty smell in the
rubber that is retrieved,” explains
Isayev. “There is also thermodegradation of certain types of
Below: Ultrasonic
industrial singlescrew extruder
Opposite page:
Dr Avraam Isayev,
University of Akron
rubber and a process to make
rubber particles, often referred to
as ‘chipping-up’ the rubber. The
latter can be used in the laying
of cushion surfaces in children’s
playgrounds and other areas
requiring a ‘soft’ surface.”
While he accepts that such
uses have their value, Isayev points
out that devulcanization using
ultrasonics allows a percentage of
the reclaimed rubber to be added
to virgin rubber without sacrificing
any of the essential properties
required by producers of rubber
goods. And there is no unpleasant
smell associated with ultrasonically
devulcanized rubber. Laboratoryand small-scale industrial ultrasonic
devulcanization processes, based
on single-screw extruders, have
been assembled by Isayev’s team.
The equipment is particularly
suitable for recycling offcuts and
minor scrappage of material at
companies producing rubber goods.
But tire manufacturers are
reluctant to add it to virgin rubber.
“Its use in tires presents a difficult
issue. There are a lot of liabilities
there,” explains Isayev. “In certain
percentages, yes they possibly would
do so. But I feel there is a lot of
work still to be done in this area in
order to make it a reliable process
for vehicle tires. However, tires for
slow-moving vehicles – trailers,
bikes, and other things – could well
incorporate some rubber reclaimed
by ultrasonic devulcanization.
“Unfortunately, despite the
advantages of devulcanization,
the majority of the tire industry
goes for burning waste rubber and
scrapped tires,” he adds. “Clearly it
is more expensive to devulcanize
because you have to buy the
equipment. However, the majority
of the expense is in capital cost and
that can be recouped over time.”
Ultrasonic power
A long-standing, significant barrier
to ultrasonic devulcanization was
the fact that while laboratory-scale
operations worked well, industrialscale installations were impractical
through insufficient ultrasonic
power. It would have taken many
44
www.tiretechnologyinternational.com October 2012
U N I V E R S I T Y
laboratory-sized ultrasonic machines
to come anywhere near a viable
commercial operation. “However,
at our current stage of development
of the technology, it is possible to
overcome that barrier,” he points out.
“It’s not such a big issue now because
the main issue was in developing
high-power ultrasonic equipment.
That area is now more or less behind
us. You can get high-power ultrasonic
equipment from manufacturers.”
Isayev concedes that current
ultrasonic technology still requires
the use of multiple devices – but not
nearly so many as would have been
required in the past to facilitate an
industrial-scale operation. However,
widespread take-up of ultrasonic
devulcanization by tire makers still
looks some time away. “We have built
an industrial extruder in which we
have put four [ultrasonic] devices.
It processes between 200 and 300 lb
per day. It’s not designed for the tire
industry but for another sector of the
rubber industry to do recycling inhouse. What I’m hearing from the tire
industry is that they want machines
that will process 500 lb, 1,000 lb, or
even 5,000 lb an hour!” says Isayev.
The company for whom the
four-device ultrasonic extruder has
been built is happy with the machine.
Not only does it devulcanize offcuts
and scrapped rubber, producing
valuable component raw materials
for re-use, it also eliminates the
charges it had to pay to have waste
rubber removed from the factory
for burning. “They have seen a
serious cost benefit,” adds Isayev.
“Thermomechanical
devulcanized rubber suffers more
thermal degradation, which is not
beneficial for the final property of
F O C U S
rubber,” he explains. “It also
requires additives to assist the
process. But with ultrasonic
devulcanization, what comes
out is rubber that is not
contaminated with anything.
The beauty of the process is
that what comes out is the
same as what went in.”
Some degradation takes
place during ultrasonic
devulcanization, but not to
the same extent as in other
processes. This means the
product commands a higher
value than rubber devulcanized
by other processes, and as
the price of virgin rubber
increases, it should be possible
to amortize the cost of adopting
ultrasonic devulcanization.
The future
Isayev, who is now a US citizen,
started his research into the use
of ultrasonics for dispersion of
pigments in what was then the
Soviet Union at the end of the
1960s. He took out a patent, but
it was never published because
the authorities of the day
classified it as a ‘secret patent’. He
moved to the USA in 1979 and
resumed his work in polymer
processing. In the early 1990s he
started his pioneering work on
using ultrasonics to devulcanize
rubber. Others around the world
were also exploring ultrasonic
devulcanizing processes and
possibilities. However, Isayev
believes he has managed to
gain an advantage. “I know a
little bit more than they do,”
he says with a chuckle.
In order to maintain the
technological lead, he and his
team at the University of Akron
are currently experimenting
with ultrasonics in a twin-screw
devulcanizing extruder. “It’s
quite an exciting development,”
he enthuses. “There is still
much to do in order that
the tire industry can adopt
ultrasonic devulcanization.
But our work has shown that it
is an effective method for the
recycling of elastomers.” tire
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