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 Synthetic rubbers are obtained from low
molecular weight fluids named
monomers to get, after chemical reactions,
high molecular weight substances
named polymers, one can imagine as
chains of monomers linked between
them by chemical bonds.
The elastic properties of a synthetic rubber are reached compounding the raw
polymer with additives, heating the compound
and vulcanizing it so.
During vulcanization the molecular
chains crosslink themselves one to each
other avoiding a reciprocal sliding.
Vulcanized rubber, on the contrary of
uncured rubber, is able to deform itself
quite elastically and to return to its former
shape and dimensions once the load
is removed.
Usually the polymers can be subdived
into four families, according their solid
state properties.
The plastomers, known even as termoplastics,
are made of entangled macromolecules
held together by intermolecular
bonds.
Due to these weak bonds, the macromolecules
can slide one onto each other and
consequently the deformations are not
reversible.
Varying the temperature and inside a certain
temperature range for each material,
some chemical/physical changes can
occur, so that scraps and defected parts
cannot be recycled.
As it’s been anticipated, the elastomers
and so the synthetic rubbers are materials which have a recovery quite com-pletely elastic and which are rather
impossible to melt or cast.
These unique properties are due to the
fact that the macromolecules are entangled
and bonded between them by strong
chemical bonds (covalent).
These bridges between the molecules
contrast the reciprocal sliding of the
molecules during loading of the part and
make impossible casting in solvents or
melting because of heat cession to the
material.
The thermoplastic elastomers have properties
similar to the ones shown by the
before mentioned elastomers, from
room temperature to about 70°C.
Their elastic properties are due to weak
bonds (Hydrogen bond) between the
molecules, which spoil their effect over a
certain temperature and form again
decreasing the temperature instead.Thermoplastic elastomers can be recycled because of absence of crosslinking.
The thermoset polymers are stiff materials, made using special reagents.
Giving up heat to the material, a modification of the chemical structure similar to vulcanization takes place, but however the number and the kind of bonds which create are so that the stiffness increases so much that the material doesn’t show a behaviour similar to that of the elastomers. Like elastomers they are impossible to melt, so they cannot be recycled.
A masterbatch is an uncured polymer which has to be used compounded with other ingredients according a recipe for the manufacturing of rubber products.
The first step to manufacturing a rubber compound consists in the softening of
the masterbatch in a mill, to get an easier consequent addition of the other ingredients
at the same time.
Additional ingredients can be subdivided,
according their specific function, as
fillers, plastifiers, antidegradants, vulcanizing
agents and special ingredients.
Black fillers consist merely of carbon
black, white fillers include, for example,
Calcium Carbonates or silicates.
Fillers are used both for technological
than for economical reasons, some of
them to increasing the density of the
compound and to decreasing its own
cost, some others to get the compound
stiffer.
As stiffening it is intended the increasing
of mechanical properties like, for example,
tensile strenght or abrasion resistance.
Plastifiers can be liquid or solid and they can be incorporated in a compound for several different reason: to increasing the density, to getting the process easier, to modifying some of the properties of the vulcanized product. Petrol based oils are the plastifiers most commonly used for both cases.
Other substances commonly used are fats, vegetal oils, waxes, soaps and resins.
Antidegradants are organic substances added in small amounts to slow down deterioration, increasing the expectation of life of the part. They protect the compound from undesidered effects of oxygen, ozone, heat, sunlight, humidity and high frequency radiations.
The antioxidants are some of the most widely used substances and they protect rubbers from oxidation and heat.
Antiozonants, instead, slow down ozone effect on the surface of the part when it
friworks
in a tension state in air.
Vulcanizing agents, futhermore, are
responsible for the compound crosslinking.
Sulphur is the principal vulcanizing
agent for such those materials which
contain a sufficient amount of double
bonds in their structure.
To get a balanced and correct vulcanization
it is anyhow necessary to using even
some other substances known as accelerators
and activators. The combination
between vulcanizing agent, accelerator
and activator is said vulcanizing system.
 Saturated elastomers cannot be
crosslinked by traditional sulphur based
systems, because of the absence of available
double bonds in the chains.
They are crosslinked using organic peroxides,
sometimes assisted by co-agents
or donators to increasing the peroxides
efficiency.
The mixing of a compound is made using
a rotating mill, whose cylinders are
shaped differently depending on the
application.
The opened mill is composed of two steel
cylinders, polished and water cooled,
rotating in opposite directions. One
them rotates faster than the other generating
friction between them.
The mixing action is a shear action and takes place inside the gap between the cylinders. The closed mill is instead composed by two special shaped rotors, water cooled them too, which rotate opposite ways and while rotating create kind of variable volume zones.
In both cases the ingredients are loaded between the cylinders, so the compound is masticated and consequently released when an uniform dispersion of the ingredients is reached.
After the mixing operation the compound is shaped in such a way to get the easier feeding possible of the machines used to manifacture the desired parts.
For this operation a calander or an extruder may be used.
At this point the compound is ready for being transformed into a finished product using the appropriate technology, press moulding or extrusion, and for being vulcanized, or cured, to get the needed physical, chemical and mechanical properties.
As previously anticipated, all the elastomers
are made of a combination of
ingredients.
The masterbatch gives to the compound
the principal characteristics, for example
oil or ozone resistance, low temperature
flexibility and so on, but even the other
ingredients like plastifiers, fillers or anti-gy
degradants contribute to the behaviour of
a compound and consequently it is clear
that one can develop an infinite number
of compounds whose characteristics are
different and so it becomes furthermore
clear that it is possible to manufacture
compounds for specific reasons and use.
Rubber bases are identified by codes
according ISO 1629-87.
• M Group, with a saturated
polymethilene chain.
• N Group, with nitrogen, without oxygen
and phosphorus.
• O Group, with oxygen.
• Q Group, with silicon and oxygen.
• R Group, with unsaturated carbon.
• T Group, with sulphur.
• U Group, with carbon, oxygen
and nitrogen.
• Z Group, with phosphorus
and nitrogen.
Each group includes different rubbers
one can identify inserting other letters
before the group symbols.
Elastomers shall be furthermore classified
into groups depending upon their
behaviour or their chemical characteristics,
for example according oil
resistence, or their service performances.
Elastomers can be classified
according service performance in
three different groups.
• General purpose elastomers, for example
NR or SBR, which deteriorate in
aggressive media like hot air, mineral
oils, fuels, oxidants, ozone. The main
advantage of these materials is their
cheap price and their fair performances at
room temperature.
• High performance elastomers, for
example CR, NBR or EPDM, able to show
good performances even in an aggressive
media, to the prejudice of a slight price
increase if compared to NR od SBR.
• Special elastomers, like FFKM, FPM,
FMQ or VMQ, fulfil specific needs of the
designer according to the application
required. The material cost is however
still increased.
Hardness: hardness is defined as the
resistance showed by the tested surface
to the penetration of a calibrated tail
under a certain load. As a consequence,
this magnitude is measured as a linear
dimension.
A certain number of different hardness
scales do exist:
• IRHD (International Rubber
Hardness Degrees).
• Shore hardness degrees,
divided in Shore A and Shore D.
The IRHD test is based on the measurement
of the penetration depth in the tested
piece of a little steel sphere due a constant
load applied.
Shore durometers, instead, apply the
load by means of a spring.
 On the type A durometer the tail is dull
conical and the scale arrives to 90 Sh A.
On the type D durometer the tail isn’t dull
and it can be used over 90 Sh A.
The Shore A scale is similar to the IRHD
scale, but a perfect correlation doesn’t
exist.
In most of the application hardness vary
between 40 and 90 ShA.
Mechanical properties include tensile
strength, elongation at break and the tensile
modulus and they are determined
with a dinamometer by means of
monoaxial tensile tests of standard test
pieces according standard test methods.
Tensile strength: it is the strength,
expressed in Mpa or N/mm2, required for
a standard specimen to break under tension
and under constant elongation.
This characteristic may usually vary from
5 to about 50 n/mm2, depending upon the
compound and the additives used.
Elongation: it is the axial deformation of
the specimen under the applied load and
it is expressed in percentage compared to
the initial dimension.
Elongation at break is determined in correspondance
of the tensile strength.
It usually varies between 100% and even
more than 1.000%, depending upon the
tested compound.
 Tensile modulus: it is defined as the force
to get a certain elongation.
It is measured in N/mm2 and it is typically referred to a 100% or 300% elongation.
Therefore, if for example 8 n/mm2 are requested to get an elongation of 100%,
this material shows an 8 N/mm2 modulus under 100% of elongation.
The modulus may be considered an indicator of the degree of crosslinking in a compound and it is determined during the tensile test. It varies from 1 N/mm2 to 13 N/mm2, depending on the chemical composition of the compound.
 Rubber parts may break during their service
life because of propagation of notches,
especially if close to edges.
Tear strength: it is a force per thickness
unit required to tear a specimen in two.
It’s usually expressed in N/mm.
Friction and abrasion: they are correlated
phenomenas because abrasion consists
of in a removal of material particles from
the surface due to friction. In absence of
friction abrasion doesn’t take place.
For a rubber part, the friction coefficient
depends upon several factors, like shape,
composition, temperature, pressure and
surface quality.
Friction generates heat and a long period
of permanence at high temperature may
bring to an accelerated aging of the part.
Friction may be reduced by means of an
appropriate compounding using specific
additives or with a surface treatment of
the part.
Rubber materials are viscoelastic materials
so they behave partially as viscous liquids
and partially as elastic solids.
Protracted deformations bring to a certain
degree of permanent deformation,
which means that the deformation is partially
stored by the structure and partially
it is elastically returned.
The chacteristics known as set, relaxation
and creep are effects due to long
time application of load or deformation.
Set: it is the residual deformation on the
part after the removal of the applied load.
If the rubber part is stretched and, when the load is removed, a deformation is measured, this residual elongation, expressed in percentage and compared to the original dimension, is named Tension Set.
On the contrary, if a compression loading is applied, the residual deformation results as a Compression Set.
A compression set of 100% means a total residual deformation without elastic recovery, instead a 0% value means
absence of deformation and a perfect elastic behaviour. The set is a function of the imposed temperature and deformation and of the duration of the period under those conditions.
Low compression set values are essential
to guarantee the appropriate sealing
action under service temperature,
because it is the elastic recovery of rubber
material which grants the required
force for this purpose.
Relaxation: it is the negative variation of
the mechanical characteristics of a rubber
part when subjected to a constant
deformation for a certain period of time.
Of course, this characteristic plays a fundamental
role on a rubber seal.
Creep: it is the inverse of relaxation, so it
represents an increasing of deformation
when a constant load is applied for a certain
period of time.
Rebound resilience: it is the concept of
resilience applied to rubber materials, so
it measures the capacity stored in the
material to returning the fastest to its
previous shape after a certain deflection,
so, if the compression set indicates a
degree of elastic recovery, the rebound
resilience measures its speed.
As per every single balance of energy, the
energy spent to deforming a rubber part
is dissipated partially in friction as heat
and partially in an other form, in this case
mechanical work, as material resilience.
So, when a deformation results after an
impact, the ratio between returned energy
and applied energy is the above mentioned
rebound resilience.
At room temperature, depending on the
material, it may vary from about 5 to 75%.
 Hysteresis: as mentioned before, when a
rubber part is deformed and than
unloaded, a part of the stored energy
becomes heat.
This energy being spent for each cycle of
imposed deformation is defined hysteresis
and it amounts to 100% minus the
resilience percentage.
This heat storage is due to the temperature
increasing of the part and it is the
result of a rapid cyclic deformation and it
is even due to the fact that rubber materials
have usually a poor heat conductivity.
When a rubber part is deformed cyclically,
the heat which generates cannot be
dissipated, so this bring to an increasing
of the temperature, especially in the
thicker sections.
This may bring to an accelerated ageing
and sometimes to the complete distruction
of the part.
A remedy for such these kind of applications
(i.e, vibration dampers) may consist
in choosing materials with the highest
resilience or in an accurate design of
the critical areas of the part.
Permeability: usually rubber materials
consent the permeation of gas and
vapour, even if this phenomena takes
place differently depending upon the
material being used and it is assumed as
important only while designing and manufacturing
tubes, hoses or diaphragms.
In general silicone compounds have the
highest permeability coefficient, butyl
rubbers the lowest.
Anyhow, depending upon the ingredients
of their own recipe, different compounds
with a common masterbatch may have
different behaviour.
For example, some fillers added in a certain
concentration lower the permeability
and viceversa usually plastifiers raise it.
The gas diffusion process through a rubber
part takes place in two different phases:
during the first one, gas dissolves
from one side, usually where pressure is
higher and depending on the gas solubility
inside this kind of rubber and during
the second phase it diffuses in the other
environment.
 As one can easily understand, raising the
temperature even the permeability
increases.
Thermal conductivity: although it varies
depending on the ingredients of the compound,
rubbers may be considered as
poor heat conductors.
This characteristic implies that parts
such as vibration dampers or components
designed to withstand cycles of
deflection or friction must have to be
dimensioned to grant a correct heat
transfer between material and cooling
medium.
The coefficient of thermal expansion of a
generic rubber material is roughly 10
times greater than for a steel.
One can understand the importance of
what has been mentioned here above
when considering the service life of a rubber
part which requires a long time being
spent at very high or very low working
temperatures, due to the huge dimensional
variations of the rubber part if
compared to steel or to another metal.
Physical characteristics of rubber materials
depend upon temperature and even if
standard tests are run at room temperature,
it may be helpful to doing them even
under different temperature levels.
Usually, if the tests take place at high
temperature, tensile strength, modulus
and hardness decrease, elongation
break rise and then decrease, rebound
resilience rise until a maximum
reached.
It is necessary to separate effects due
short and to long term permanence under
high temperature.
Short term effects are mostly phisical
and reversible when the temperature lowers
to the room level.
Long term effects are permanent and
implies structural modifications which
lead to poor mechanical performances.
Because of exposition under low temperature,
some changes may occur in a rubber
material, some immediately, some
others after long time.
The lower the temperature, usually hardness,
modulus and tensile strength raise
and elongation decreases.
The rebound resilience lower until a minimum
is reached, then it raises until a
certain point where the material shows a
brittle and fragile behaviour.
This is the so called Glass transition temperature,
Tg.
Crystallization due to low temperature
isn’t usually dangerous for dynamic
applications, because movement generate
heat, but it may cause a loss of elasticity
in static applications.
When discussing about temperature
ranges one has to take in account that all
the rubber materials show a marked
dependance upon the variable time.
Concerning the lowest temperature limit
in service, it is always strongly recommended
a functional laboratory test.
Concerning the upper temperature limit,
instead, it becomes important the expected
service life and the working conditions.
For example, the upper limit for a standard NBR may be considered 100°C
for a service life in mineral oil. This means
that below 100°C life expectation is probably
longer.
The same compound may show a fair
behaviour for 100 hours at 150°C or after
a few minutes at 300°C.
This means that a recommended working
temperature can be increased for short
time applications or reduced for an environment
with aggressive fluids in contact
with rubber.
Dielectrical characteristics: rubbers are
typically good insulating materials and
show a high electrical resistivity.
Non-polar rubbers (non-oil resistant) are usually better insulating materials than polar rubbers (oil resistant).
However the electrical performances depend more from the additives than from the masterbatch.
Compounds containing carbon black have to be avoided if an insulating material is needed and in this case the best choice is represented by silicone rubbers.
• Deformation: rubber parts don’t have to be stressed while on stock. If impossible, reduce stress to a minimum.
• Contact with metals: do not leave in contact, protect with paper or polyethilene, avoid PVC films.
• Contact with other rubber compounds: to be avoided.
• Contact with liquids or vapours: to be avoided.
• Radiations: avoid exposition under radiations.
• Cleaning: clean and wash with water and mild soap only. Do not use solvents.
Dry at room temperature. Rubber parts, if stored in an appropriate condition, may last in perfect conditions for several years.
Saturated backbone elastomers may last from 10 to 20 years, unsaturated rubbers from 2 to 5 years.
It is possible to get rubbers antistatic or conductive compounding them with large amounts of graphite, special carbon
blacks or metal powders, but anyhow their conductivity cannot be compared with metals’ one.
Guidelines for rubber vulcanizates storage:
rubber parts can change their characteristics if stored in an unappropriate way.
All this may be avoided using a correct packaging and a storage system according ISO 2230.
• Temperature: below 25°C, if possible at 15°C.
• Humidity: avoid its condensation.
• Oxygen and ozone: protect from them avoiding air circulation. Keep away from electrical motors or lamps, because of the increase of ozone concentration.
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