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Technology of Rubber

This page gives some technical explanations pertaining to, and getting an understanding of what Rubber is all about

RUBBER, WHAT IS IT?

Well, it's like nothing else you've encountered before
Rubber is essentially an elastic material which is also a ....... sealant, abrasion resistant, energy absorbing and impact resistant, and corrosion resistant ....... quite a mouthful!

And it is the fact that rubber is a highly amorphous (highly random at molecular level) substance that gives it these properties.

What makes rubber even more interesting is that it is referred to as a visco elastic material...... another mouthful. This essentially means that it is largely in it's uncured form a (highly) viscous liquid, and in it's cured form a highly elastic solid.
This has to be taken with a pinch of salt.....
-Rubber isn't actually a liquid in it's uncured form, hence the reference as HIGHLY VISCOUS liquid
-Rubber isn't actually highly elastic in it's cured form like for instance a metal as qualified by Young (credited with the modulus of an elastic material being named after him), if anything is it actually highly extensible.

But let's not split hairs on the subject, because rubber is very unique in so far as it's (let's call it) "mechanical" properties, and not at all badly described as a visco elastic material....... it certainly serves to impress the non-rubber people..... especially once you try and explain that this is a state somewhere in between a solid and a liquid..... even the chemists now become confused!

THERMOSET vs. THERMOPLASTIC
Being a highly viscous and elastic material, must rubber invariably be processed on rather robust machinery. which brings us to another feature of rubber, nl. that it tends to (and needs to) soften at elevated temperatures to in fact be at all processible, but needs to be dimensionally stable in order to be suitable for most applications. Getting this dimensional stability can be achieved in a number of different ways, which we will discuss in the following sections....

Rubbers are generally Thermoset, meaning that they are such that a chemical reaction takes place which changes their state from a viscous liquid, to an elastic solid.
Certain types of more modern rubbers (like most plastics) are Thermoplastic. They rely instead simply on change in temperature to convert them from a soft processible material at higher temperatures to a more dimensionally stable material at lower (around room) temperatures.
Needless to say is the Thermoset process irreversible and can such rubbers not be re-processed, whereas the Thermoplastic process is reversible and can such rubbers largely be re-processed.

Another important differentiating feature is that Thermoset rubbers can be used at a reasonbly high temperatures (80 to 140'C depending on polymer), whereas Thermoplastic Rubbers (because of the very fact that they are thermoplastic) cannot work at much higher than room temperature...... but more on these rubbers later.

COMPATIBILITY

Compatibility mainly refers to the extent to which the basic polymer is resistant to certain substances. Although compounding can change these properties to a lessor or greater extent, does it complicate things so much, that we will not attempt even to discuss this here.
Compatibility can be qualified on two "fronts".... the extent to which a rubber can withstand inorganic substances and is aging resistant, and the extent to which a rubber is resistant to organic substances. This is differentiation is made, because two very different mechanisms are involved.....

Inorganic and ageing resistance
This is largely determined by the extent to which the polymer is unsaturated. You will recall that all Vinyl/ Addition polymers are produced from monomers that contain a double bond. Perhaps more by bad luck than design, do many of these monomers contain two double bonds, of which one remains after polymerisation. This double bond is very reactive, and will tend to react with other substances (mainly oxidising agents like oxygen, acids, etc.), causing a break in the C-C backbone, which is called de-polymerisation. this results in shorter chains that greatly reduce the properties of the rubber, to the point that it causes the material to break apart.
So, generally the lower the unsaturation (double bond content), the more resistant will the rubber be to inorganic substances, ageing, and temperature.
This is why rubbers like NR, SBR, BR, IR are not very resistant to acids and can only withstand temperatures of around 80'C, while EPDM, IIR, CR and CSM are more resistant to acids and can withstand temperatures around 120 to 150'C. CR can withstand approx. 100'C because although it contains a large amount of unsaturation (double bonds), are these screened / protected by the large Chlorine atom.

Organic resistance
This is determined by the polarity of the rubber as compared with the polarity of the substance. Polarity refers to the extent that a substance is "lopsided" as a result of containing large atoms or molecules.
The simple rule of "like likes like" might help you to remember that substances with similar polarities will dissolve each other and hence not be considered to be resistant to each other. Substances with different polarities will therefore be resistant to each other..... obviously could such a substance not be used as a solvent for the particular rubber
For this purpose is a list of polarities for a range of rubbers and organic substances available, to help determine resistance vs. solubility.
It is of interest to note that inorganic substances have a rather high polarity, and hence are rubbers (which have comparitively low polarities) highly resistant to inorganic substances from a polarity point of view.

CONVENTIONAL RUBBERS

Conventional rubber include Natural Rubbers, and all the Synthetic Rubbers produced via the Vinyl/Addition polymerisation process. This process simply involves a Carbon based monomer containing a double bond, reacting and linking up to form the required long chainlike polymer. The rubbers include the ......

STANDARD RUBBERS

Natural Rubber (NR)
-obtained from the "rubber" tree, the sap processed to yield a rubbery substance.
-cheap and most commonly used rubber
-excellent abrasion resistant properties, but other properties fairly standard in comparison to the Synthetics.

Styrene Butadiene Rubber (SBR)
-the first rubber produced synthetically, a co polymer of Styrene and Butadiene.
-cheap, and presently the synthetic rubber used most, mainly as a NR substitute, particularly in motor vehicle tyres

Butadiene Rubber (BR)
-a pure Butadiene polymer -comparatively cheap, seldom used on it's own, mostly used together with SBR to enhance dynamic properties.

Isoprene Rubber (IR)
-the synthetic rubber that has (virtually) the identical molecular structure of NR.
-fairly expensive to produce, has no better properties than the likes of NR and SBR, as a result not used in great quantities ... making it even more expensive.

IMPROVED RUBBERS

Ethylene Propylene Diene Modified Rubber (EPDM)
-The first of the medium speciality rubbers, a co-polymer of Ethylene and Propylene giving it amorphous rubbery properties.
-Contains only enough unsaturation (double bonds) to cure with Sulphur, thus giving it much improved temperature, ageing and chemical resistance.
-Comparatively cheap, making it very popular with medium quality articles

Butyl Rubber (IIR)
-A slightly more temperature, ageing, and chemical resistant polymer
-Comes in two variations, as either..... +pure Butyl which is very slow curing, +halogenated Butyl, either Chlorinated (CIIR) or Brominated (BIIR) with have very similar properties, but cure at more reasonable rates.

SPECIALITY RUBBERS

Chloroprene Rubber (CR)
-Essentially a Butadiene with a Chlorine atom on every monomer molecule.
-the presence of the Chlorine gives it good ageing, chemical and temperature properties.
-a speciality polymer because of it's non-flammability / self extinguishing properties, but comparitively expensive and only used where it's properties uniquely required.
-More generally known by it's DuPont tradename of Neoprene

Nitrile Butadiene Rubber (NBR)
-A co polymer of Nitrile and Butadiene
-moderately priced, with the speciality feature that it is superbly oil resistant
-but has comparatively poor ageing, chemical and temperature properties similar to the Standard rubbers.

Chloro Sulphonated PolyEthylene (CSM)
-a superb age, temperature, chemical, oil and flame resistant rubber
-quite expensive, not easy to process and only used where all of the above properties are uniquely required
-more generally known by it's DuPont tradename of Hypalon

SUPER SPECIALITY RUBBERS


SILICONE RUBBERS
-In many ways similar to the conventional synthetic rubbers, but having a Silicon backbone, and being produced instead via a condensation process.
-has superb temperature resistant properties, as well as chemical and ageing .... what about oil, flammability?
-extremely expensive, and only used where it's unique properties are required.

POLY URETAHNE RUBBERS (PUR's)
Based on Urethane technology (which produces many hard rigid materials and foam), but slightly changed to give rubbery properties Poly Urethan relies mainly on changing it from a liquid to a solid by the mixing and reaction of two components.
PUR's come in two variations....
-Castable PUR's where two liquid components are mixed, initiating the crosslinking process giving a rubbery material
-Mixable PUR's where the material is already in a rubbery solid form, and the curing agent is mixed in and the crosslinking process initiated by heat

THERMO PLASTIC RUBBERS (TPR's)
-TPR's are not crosslinked, and rely for dimensional stability on low levels of crystalinity that develops at room temperatures. As the temeparture is increased does this crystalinity break down, resulting in a softer material that can be processed.
-Because TPR's often need to be processed at around 200'C, have they become more the domain of the plastics industry, because they can with relative minor problems be process on plastics processing machinery. So, although displaying very rubbery properties, and to all intents and purposes a rubber (albeit only to be used at room tempartures), do TPR's not fit in well with the traditional / conventional rubber products manufacturer.

RUBBER TERMINOLOGY


POLYMER: The base material that is used for a rubber. Polymer actually refers to the fact that this material is a very high molecular mass, long chain like molecule.
COMPOUNDING: The process of physically mixing various ingredients together with the base polymer. Because the polymer is often a very highly viscous and tough material, does it require extremely high forces and hence a very strong and robust mixing machine. Mixing was traditionally done on a "two roll mill", but is to a large extent being replaced by "internal mixers"
COMPOUND: What the material is referred to after it has been compouded
TWO ROLL MILL: A fairly robust machine, consisting of two rolls mounted horisontally in a frame with a small gap in between of approx. 5-10mm, referred to as the nip. The rolls rotate towards each other, forcing the rubber rubber and ingredients through the nip, and resulting in the desired mixing action
INTERNAL MIXER: A machine which has two rotating rolls mounted inside a body (referredto as rotors), sealed with a door at the bottom and a vertically moving ram at the top. The main difference is that mixing happens under pressure ) approx. 5-10 Bar resulting in shorter mixing times