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SYNTHETIC RUBBER

Rubber is essentially a susbtance that consists of long chain like Hydro-Carbon molecules with a very uncrystalline amorphous structure. If anything like this structure could be synthesized, could a similar product be obtained...... which led to the birth of Synthetic Rubber

BULK POLYMERISATION
First attempts to produce a synthetic rubber was prompted by the First World War, when Germany was cut off from Natural Rubber supplies. This first attempt simply involved polymerising Methyl to form what was called Methyl Rubber .... but this was neither a commercial nor technical success, although much was learnt about the concept of producing rubber synthetically
In 1925 was Butadiene polymerised producing a more viable material using Sodium as a catalyst, hence known as BuNa. Although it exihibited rubbery properties, was it's use very limited since it had very low physical strength
In 1929 however when Butadiene and Styrene was co-polymerised (known as BuNaS), did a tougher product result that could be used for it's rubbery properties in limited applications.
The problem with the above (bulk polymerised) product was mainly due to the fact that the material became more viscous as the monomer polymerised causing heat build up and subsequent de-polymerisation and chain branching
EMULSION / FREE RADICAL POLYMERISATION
In 1930 was Emulsion polymerisation developed that essentially allowed the polymerisation to proceed in a water medium. This solved the problem of heat build up, producing a synthetic Latex that could be further processed like a Natural Rubber latex.
The process could be allowed to happen at a temperature as low as 5'C (before risk of the water freezing), to minimize the effects of depolymerisation and chain branching. It however necessitated the use of a peroxide catalyst (also known as a Free Radical)
The first material to be produced using this process was a co-polymer of Styrene and Butadiene, which subsequently became known as Styrene Butadiene Rubber (abbreviated SBR)
This was the first rubbery substance that remotely resembled Natural Rubber, and could be considered as a substitute. This material although much improved from the original development is still in use today, and in fact the most widely used Synthetic Rubber
Subsequent variations were produced that displayed the required rubbery properties but had enhanced compatability properties...
- Chloroprene Rubber (CR) better known as Neoprene originally developed and patented by Du Pont, produced from 2-Chloro Butadiene displayed enhanced temperature, chemical, ageing and non-flammable properties and is still in use today, although expensive and used mainly in non-flammable applications.
- Nitrile Butadiene Rubber (NBR) generally known as Nitrile, produced as a co-polymer of Acrylo Nitrile and Butadiene, displayed excellent oil and solvent resistance, still the only commonly used rubber with this property

Another product manufactured using this method was Poly Ethylene (PE), the first plastic which subsequently became known as Low Density Poly Ethylene (LDPE).

SOLUTION / IONIC POLYMERISATION
In 1933 did Esso develop a process that used an organic solvent as it's medium, which firstly allowed the polymerisation temeparture to be lowered to -90'C, and secondly allowing an ionic catalyst to be used.

Using this method was IsoButylene polymerised producing poly iso butylene (PIB), a very sticky substance with much improved ageing properties which found use in outdoor sealing applications.
By co-polymerising approx. 2% of Isoprene with the Isobutylene, was a material produced that could be vulcanised with sulphur and used in more conventional rubber applications.

In 1953 using a Lithium Alkyl anionic catalyst, was a pure poly Butadiene produced, which found limted application in supplementing Natural Rubber.

Also in 1953 was a major breakthrough made in the production methods of synthetic rubbers when Prof. Ziegler and Dr. Natta based in Italy tested and developed a range of cationic catalysts that allowed for the successful polymerisation of non-polar monomers and reaction temperatures at a more reasonable 50'C without any chain branching or de-polymerisation.
This method firstly resulted in the production of a much improved plastic, subsequently named High Density Poly Ethylene (HDPE), another plastic Poly Propylene (PP), then a co-polymer of the above two plastics resulting in a more random structure and rubbery material named as Ethylene Propylene Rubber (EPR), from which EPDM developed being a very versatile and commonly used rubber today.

Subsequent developments led to.....
-an improved high-cis poly Butadiene
-polyIsoprene Rubber (IR), being the synthetic rubber with almost an identical molecular structure to Natural rubber, but quite expensive and with no improved properties, not very commonly used.
-Solution SBR (S-SBR), fast becoming the replacment for emulsion SBR, largely because of it's slightly better physical properties and lower cost to produce

IN CLOSING
Synthetic Rubber had to comply with one main requirement, nl. it had to have molecular amorphous properties similar to that of Natural Rubber in order to display the same elastic properties.
If it was too crystalline, would it be more like a plastic (a product that developed very much at the same time / along the same lines as synthetic rubber). If it had a too low molecular mass, would it not have the same physical properties, and be more like a jelly.
In the beginning years were such materials produced more by chance, and those with the required properties retained and developed further.