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Friday, April 1, 2011

Rubber Chemistry: Part 1

Rubber! You probably first encountered it when sucking on a bottle as a baby, or later in the classroom to rub out errors in your pencil work. Indirectly we encounter rubber many times a day - perhaps most noteworthy is in getting to your destinations - tyres - they are made from rubber with additives.

Try think of 10 other uses of rubber. Do it now before your read on.......


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Perhaps you saw some rubber in the video clip of my last post - on my speargun - see it even finds use in fishing! My wetsuit kept me warm. What is it made from?

I will now cover some chemistry of natural rubber.
Man has learned to make his own rubber from crude oil, but first it was taken from plants. That is the focus of this post.

Indians in South America used rubber sourced from the sap of trees to make shoes and coat the paddles of their canoes to waterproof them - the smoke of their fires cured the rubber. Explorers and early settlers in South America saw the Indians using rubber and some were intrigued sent samples home to Europe. The industrious western mind turned such rubber into big industry over two centuries.

The milky sticky sap that is found in many plants contains some form of rubber. Just the other day I was gardening (pulling out weed creepers that entangle my garden plants). I got my hands full of sticky, yukky, white "milk". Next the soil got stuck in the sap that had lodged on my hands and my hands were a mess! Soap refused to remove it - only hot water and hard rubbing with a hot rough cloth helped remove the sticky adhesion.

What type chemistry underlies this sticky sap?

Isoprene (2-methyl-1,3-butadiene) is a common, naturally occurring organic compound with the formula:
CH2=C(CH3)CH=CH2

Plants produce and release large amounts of isoprene into the atmosphere. Rainforests make a huge contribution. The haze one sees on a hazy day in regions where there is extensive plant life - may be partly due to isoprene. A variety of pretty complex reactions form such hazes. I will save that discussion of atmospheric chemistry for another day. Back to isoprene...

At room temperature it is a colorless liquid but it boils easily at 34 degrees Centigrade. So it is very volatile - this explains how plants can release it with relative ease.
Isoprene is the monomer of natural rubber. Monomers are molecular building blocks from which bigger building blocks are made. Just as bricks make up walls, and walls make up buildings, so isoprene units can be linked together (polymerized) into rubber.

In biological systems such as plants, the isoprene structure is found repeated in many polymers. It occurs often as dimethylallyl diphosphate (DMADP) [click on this link for more data] and its isomer isopentenyl diphosphate (IDP). An isomer is a molecule with the same formula but a different shape or structure.


Early scientists actually extracted isoprene from rubber by decomposing it and correctly assumed that rubber was made up from repeated units of isoprene. Natural rubber, or polyisoprene exists as long chains - thousands of isoprene units bound together by the carbon carbon double bonds. Some double bonds remain in the structure of polyisoprene - these can be used to change the properties of rubber.

Charles Goodyear used sulphur to harden rubber in a process called vulcanization - where sulphur is added to rubber and it is heated to harden the rubber. Vulcanization is derived from the name of the roman god of fire "Vulcan". Volcano has the same source. Sometimes Man learns from his blunders and Charles Goodyear was such a person, he had lead oxide mixed with sulphur and rubber. Accidentally some fell onto a stove and hardened and the rest is history the future of rubber had been secured.

The long, relatively strait rubber chains (polyisoprene) are secured in position by crosslinking sulphur bonds. These bonds prevent the rubber from becoming sticky and softening with heat. A clear diagram is illustrating this in the Wikipedia definition of vulkanization
Mr Goodyear; yes, the same name as Goodyear tyres, he developed the vulcanization process but sadly he did not make a fortune out of it and he died a poor man in 1860. Patent infringments were problematic in his endeavours. At least his name lives on in the Goodyear Tyre company.

Now that rubber had been made hard and resistant to heat it found many uses especially in the tyre industry. Here in Port Elizabeth we have a big tyre industry where rubber is vulcanized on a daily basis in the tyre factories. 

Next time you take a drive in a car or bicycle think about the polyisoprene and sulphur that hardened the rubber that supports and carries you.


 

1 comment:

  1. Interesting post. Love your whole blog. Thanks for sharing.

    -Mr Acetate

    ReplyDelete