RUBBER EXTRUDED TECNOEXTR - Rubber technology, Synthetic rubbers

Rubber technology

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 completely 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 works 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 of them rotates faster than the other generating friction between them.
The mixing action is a shear action and it 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 in opposite ways and while rotating create a 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 antidegradants 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, suddivisi in Shore A e 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

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

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

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

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

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

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

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

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

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

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.

Temperatura

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 at break rise and then decrease, rebound resilience rise until a maximum is reached.

It is necessary to separate effects due to 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.

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.
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.

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