From Natural Rubber to High-Tech Material

1 0 0 JYAE H
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Looking Back. Only just turned one hundred years old, synthetic rubber raw materials have over the decades been transformed into high-tech materials without
which many applications of modern technology would be unthinkable. In contrast
to natural rubber, which is set narrow limits in its chemical modification, synthetic
rubbers offer a wide range of possibilities for increasing the performance envelope
of elastomers. Today the principle of tackling new challenges in the rubber sector
at the molecular level is firmly established.
From Natural Rubber to
High-Tech Material
MARTIN MEZGER
here are many reasons why the high
performance of synthetic rubbers
is not widely known today: Parts
that are made out of them are not especially eye-catching since they are often
black and carry out their assigned tasks
out of the limelight. Another possible
cause lies in the history of rubber.
Particularly in the early days of these
materials they were intimately linked to
natural rubber which is produced from
the sap of the rubber tree. Hard rubber
manufactured from this sticky substance is regarded as one of the first polymeric materials. Thus natural rubber,
from which hoses, tires and waterproof
clothing, but also combs and housings
were manufactured, achieved a great
economic significance as early as the
19th century. For many laypersons rubber is still a material that “grows on
trees”.
T
In the Early Days Simply a
Substitute
The price of natural rubber swung widely through shortages caused by war and
the dependence on living sources meaning that chemical companies were interested in a substitute. The first patent for
Translated from Kunststoffe 4/2010, pp. 14–18
Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE110363
a method for manufacturing a synthetic
rubber, in spite of all the efforts to produce a usable surrogate, was only granted in 1909 to Elberfelder Farbenfabriken
Friedr. Bayer & Co. The legacy of this
company, held in the interim by Bayer
AG, is today continued by Lanxess, Leverkusen, Germany. At the time a chemist
called Fritz Hofmann had discovered a
method of linking isoprene, one of the
building blocks of rubber and identical to
the one found in nature, to make a sticky
substance. It was for this that the first synthetic rubber patent was granted (Fig. 1).
Hofmann’s work however went much
further: As it became clear that isoprene
could not be manufactured economically he turned to another building block,
methyl isoprene, which was much easier
to come by.
Several tonnes of methyl rubber were
actually produced according to Hofmann’s method and even used in the
manufacture of tires. Although for various reasons methyl rubber was not able
to hold its own in the market place for
long, Hofmann and this new material
had opened the door to further synthetic rubbers. Despite the fact that later generations of chemists conceived of new
and much more effective methods for
combining suitable chemical building
blocks to produce synthetic rubber, the
basis of nearly every synthetic rubber
even today are still monomers that are
chemically different to those found in
natural rubber.
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W 2010 Carl Hanser Verlag, Munich, Germany
Fig. 1. 100 years ago the chemist Fritz Hofmann
laid the foundation stone for the development
of synthetic rubbers with his methyl rubber
(photo: Lanxess)
Breakthrough with Styrene and
Nitrile Rubbers
Amongst the first synthetic rubbers that
could beat natural ones on the basis of
technical advantages was the styrene rubber Buna S. Its butadiene and styrene
monomers were combined with the help
of sodium catalysts and a great deal of
process technology know-how to make a
material that gave vehicle tires a longer
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for perfecting an important component
without which many technical applications would not be possible and that is the
shaft seal ring, also called a Simmer ring
(Fig. 3). Due to the oil sensitivity of “classic” natural rubber shaft seal rings had
even been made of leather before the discovery of NBR. Thus nitrile rubber was
one of the first new elastomers to shake
off the image of a “substitute” for natural
rubber and succeed in the market place
in spite of its higher price.
Ever More Materials Available
Fig. 2. The growing demand for automobile tires
has been driving the development of rubber
since the 1930s (photo: Lanxess)
life than natural rubber. The first products with a Buna S tread were launched
in 1936. The patent for the copolymerization of butadiene and styrene was granted on June 21, 1929, which once again
shows how far ahead of his time Hofmann was. Even today modified versions
of styrene rubber are still a major component of automobile tires (Fig. 2).
A further technical milestone was the
nitrile rubber Buna N made from butadiene and acrylonitrile (patented on April
26, 1930). For the first time elastomers
produced from these materials were resistant to non-polar liquids such as oil.
Natural and styrene rubbers in contrast
swell in oil and are therefore not suitable
for sealing oil transmission lines or tanks.
Nitrile rubber (NBR) came just in time
Over the following years the availability
of these “new”synthetic rubbers was continuously expanded. New building blocks
were utilized that gave the elastomers additional and in some cases very specific
properties. Buna EP, an ethylene propylene diene rubber, showed excellent aging
and temperature resistance. The polarity
of EVA rubbers such as Levapren – made
from ethylene and vinyl acetate – can be
varied by adjusting the proportion of the
polar monomer. In the 1970s post hydrogenation of HNBR rubber removed a pivotal starting point for aging reactions in
nitrile rubbers. The result was an elastomer that had both the oil resistance and
excellent dynamic properties of NBR as
well as increased service life under particularly tough conditions.
Thanks to such technical advantages
synthetic rubbers were able to move into
application areas which are closed to natural rubber amongst other things due to
its aging behavior. Synthetic rubbers are
Fig. 4. Rubber has not
had to be black for
quite some time: UV
resistant synthetic
rubbers such as
polyvinyl acetate
(EVA) are even suitable for the manufacture of long lasting
light transmitting
rubber articles such
as bellows for articulated buses
(photo: Contitech)
Fig. 3. The oil resistance of rubber sealing lips
is of fundamental importance for shaft seal
rings. Only with the development of nitrile
rubber (NBR) was it possible to perfect this
important component without which many
technical applications could not be imagined
(photo: Freudenberg)
thetic rubbers would not be anything like
what they are today. Just what a development Fritz Hofmann’s discovery started
can be seen from the current market figures: Synthetic rubbers met around 56 %
of the 22 million tonne worldwide demand for rubber in 2008.
Development Potential
Nowhere Near Exhausted
The development of completely new elastomers using previously unused monomers is currently almost at a standstill.
However, the types currently available offer plenty of opportunity for further optimization. Modern synthetic rubber
manufacturers can point to dozens of special rubber variants that have in some cases been tailored to individual customer
requirements. Nitrile rubber for instance
is no longer a simple copolymer of acrylonitrile and butadiene: In today’s letter
sorting lines conveyor belts made from
carboxylated nitrile rubber guarantee
trouble-free mail transport (Fig. 6). Cured >
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W 2010 Carl Hanser Verlag, Munich, Germany
used for flame retardant cable sheathing,
weathering resistant seals and even UV
stable translucent rubber articles (Fig. 4).
Air springs made from dynamically loadable variants such as Baypren (polychloroprene rubber, CR) can survive millions of working cycles without fatigue
and are not sensitive to oxygen or ozone
(Fig. 5).
Hand in hand with the new rubbers an
increasingly sophisticated application
technology developed: Without detailed
know-how, for instance in maximizing
the fabric adhesion of rubber, and without special chemicals that control the vulcanization process and prevent the breakup of molecular cross-linking sites, syn-
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rubber made out of this elastomer shows
higher resistance to abrasion and dynamic loading as well as a paper-friendly hydrophilic surface.
Achieving this kind of property profile
requires in some cases extensive modification of the elastomer molecule for
which decades of experience with this
class of material are required. An example of this is the higher melt flow of
HNBR elastomers: Lower viscosities
mean that products from the Lanxess
Therban AT range can now flow more
easily into filigree molds than even a few
years ago. This means that they can for
example be made into particularly delicate seals (Fig. 7). The rapid filling of large
volume molds is also eased, which increases the failure safety of solid rubber
components made from this oil and temperature resistant raw material. Energy requirements and cycle times during manufacture are also reduced. It is worth noting that the production process for
Therban AT is based on research results
for which Yves Chauvin, Richard Schrock
and Robert Grubbs received the Nobel
Fig. 6. When transporting paper the exceptionally abrasion resistant functional layer made
from Krynac X 740 carboxylated NBR rubber on
these machine belts ensures long service life
and very good carriage properties (photo: Lanxess)
Fig. 5. The performance of rubber articles is often underestimated – this air
spring made out of
Baypren (CR) has
been designed for
several millions of
load changes (photo:
Lanxess)
solutions. Thus they are suitable for the
production of rubber components that
come into contact with relatively aggressive biofuel. In their fully hydrogenated
form, just like the best “classic” HNBR
variants, they show excellent aging resistance.
Even Old Workhorses Are Good
for a Surprise
HNBR elastomers are comparatively new
rubber materials. However, development
work on the “established” synthetic rubbers has not stood still either. An example of this are functionalized SSBR rubbers, that is styrene rubbers manufactured in solution processes and equipped
with special function groups in order to
improve adhesion to the widely used filler
silica. Up until now only end group functionalized grades were available in the
market. Today it is possible to distribute
anchor groups along the entire length of
the SSBR molecule. This significantly increases their density in the material. Initial laboratory investigations of pilot scale
samples have confirmed that the interaction with silica fillers has been increased
further. Through this the dampening
properties of the rubber are further improved. If the expectations for these materials are also confirmed in the customer
trials currently in progress then it should
be possible to use this rubber for making
tires with improved wet grip without the
rolling resistance suffering. Experts also
believe the abrasion properties of these
products to be very good. It has until now
been difficult to optimize all three of the
properties at the same time.
New approaches have also been found
for the venerable member of the tire rubber family tree,SBR styrene rubber (ESBR)
manufactured in emulsion processes.
This modernized synthetic rubber continues to be considered an all rounder
product for many applications.With their
new developments marketed under the
Nanoprene and Micromorph names,
however, Lanxess and its subsidiary Rhein
Chemie have entered the age of nanotechnology. Both of them have recently started to offer pre-cured ESBR rubbers with
particle sizes of a few dozen nanometers
(Fig. 8). Such microgels are differentiated
according to their chemical composition,
glass transition temperatures and surface
functionality. They open up entirely new
possibilities for the systematic development of rubber mixtures and polymer
blends. For instance tires in which
Nanoprene particles have been added to
the tread formulation show significantly
better grip on dry roads and improved
abrasion resistance without negatively affecting rolling resistance and wet grip.
In addition these shear resistant nanoparticles confer typical elastomeric properties in other polymeric matrices with-
Prize for chemistry in 2005. It took only
a few years to progress from the first ideas
to the application-ready high performance rubber. The potential for this technology that could in the future even deliver “fluid” silicone-like HNBR rubbers
has nowhere near been exhausted.
The same is true for other new HNBR
variants that feature raised acrylonitrile
contents of up to 50.5 % (upper specification limit for Lanxess). Until recently
such grades were seen as difficult to manufacture, but within broad limits they are
resistant to biodiesel and indeed ethanol
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W 2010 Carl Hanser Verlag, Munich, Germany
Fig. 7. The particularly high flow HNBR
rubber Therban AT
3400 VP is suitable
for liquid injection
molding (LIM), the
manufacture of inplace gaskets (IPGs)
and the production of
particularly filigree
seals (photo: Lanxess)
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100 YEARS OF KUNSTSTOFFE
of these devices significantly above 150°C
and thus improve their efficiency.
The Classics Are Still in Demand
Fig. 8. The polymer additive Nanoprene is composed of organic particles at the nano-scale.
With this microgel the properties of elastomers
and thermoplastic materials can be improved in
a targeted manner (photo: Lanxess)
out the formation of unwanted phases
feared in blend formulation since these
pre-cured particles cannot “melt”into the
matrix. Thus for example the toughness
of polyamides can be increased in a very
elegant manner. In this case as well application areas with many faceted potential
are opened up: For example suitable functionalized ESBR nano-particles can much
more effectively transport charge carriers
through the proton conducting membrane in fuel cells than water which was
previously typically used. With these it is
possible to raise the working temperature
Naturally, “classic” synthetic rubbers are
still being used to solve important problems. In energy generation ethylene vinyl
acetate copolymers such as Levapren
could ease the manufacture of photovoltaic modules: Solar cells can be embedded in these UV resistant elastomers
in an elegant manner (Fig. 9), without substantially reducing the performance of the
cells in service as is the case for instance
with the well known yellowing of many
adhesives. Adhesive coatings in similar
elastomers allow their adhesion to be optimized for the widest possible range of
surfaces.
Future orientated manufacturing can
also make its contribution to more environmental friendliness. A good example
of this is the use of “safe” process oils.
Some oils that are co-blended for technical reasons have been found to be previously underestimated sources of polyaromatic hydrocarbons (PACs) and so were
banned in European tires in 2010. Lanxess
was one of the first companies to react to
this challenge: Since 2006 rubbers with
significantly less contaminated oils have
been available. After a transition period
the company will have completely dispensed with the use of oils highly contaminated with PACs.
Natural Rubber –
Still a Challenge
In spite of everything some areas remain
where natural rubber continues to outperform synthetic ones: After all in 2008
9.8 million tonnes of this renewable rub-
Fig. 10. Safe, long
lasting and energy
efficient – without
the use of synthetic
rubbers the performance of modern
tires would not be
possible (photo: Lanxess)
ber raw material were still sold. Natural
rubber has for example an as yet unbeatable combination of elasticity and tensile
strength which is due to the carefully
maintained cis-bonding pattern of the
isopren units of this molecule. This still
justifies the use of natural rubber if the
external conditions allow: Nature continues to work on the polymerization more
accurately than any artificial catalysts. For
example natural rubber still performs a
valuable service in truck tires.
Thus a considerable challenge for the
future lies in the continuing refinement
of catalysts for the manufacture of synthetic rubbers. In this respect the chemists
have already gained ground: Good neodymium polybutadiene rubbers (Nd-BR)
Fig. 9. Levapren is a UV resistant rubber and is
used in adhesives and for embedding solar
cells during the manufacture of solar modules
(photo: Lanxess)
already have a cis-content of up to 99 %.
At the same time it has already been possible to substantially reduce the concentration of vinyl bonds in their molecules.
These are particularly disruptive for the
crystallization of the rubber and therefore have a decisive influence on the tensile strength of the material. Thanks to the
very latest catalysts Nd-BR can also be
manufactured with a very narrow molecular weight distribution which helps to
reduce the rolling resistance of tires since
short molecular chains act as plasticizers
absorbing energy because they can only
ineffectively transmit it. With the ever increasing significance of rolling resistance
synthetic rubbers like this one should be
able to gain ever more market share in this
headline application for natural rubber
(Fig. 10). In tire retreading, which due to
predictable increasing production and
disposal costs is going to gain ever more
significance, butadiene based rubbers
have already caught up. THE AUTHOR
DR. MARTIN MEZGER is a rubber expert in the
Technical Rubber Products Business Unit at Lanxess
Deutschland GmbH, Leverkusen, Germany;
martin.mezger@lanxess.com.
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