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TOPIC 13 NATURAL MATERIALS- RUBBER

Synopsis

There are many things around you that are made from rubber. In this topic, you will be learningabout the composition of rubber, how it is processed, its properties and its uses in our dailylives.

Learning Outcomes

1. State the composition of rubber2. Explain the processing of rubber

3. State the properties of rubber4. State the uses of rubber in our daily life

Overview

Figure 13.1 Overview of contentContent

Take a rubber band and stretch it. What do you observe? Release the rubber band. What doyou observe? The rubber band stretches when pulled and when the force is released it returnsto its original length or shape. The rubber band is made from rubber, which is a type of polymercalled elastomer. As an elastomer, the rubber band is different from other solid materials in thatit can be stretched to more than four times its original length and when it is released it will

readily return to its original length. This property is called elasticity.

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13.1 Composition of rubber

Natural rubber is a addition polymer of isoprene, C5H8. The IUPAC name for isoprene is 2-methylbut-1,3-diene. Rubber, for example, is very elastic and is made from isoprene monomers.

In naturally occurring rubber, the chain extensions are on the same side of the double bond.This form is called poly-cis-isoprene, with a molecular weight of 100,000 to 1,000,000. Becauserotation about the carbon-carbon double bond does nor readily occur, the orientation of thegroups bound to the carbons is rigid. This makes rubber products easily oxidized by air, heatand mechanical movements like bending. The oxidized rubber cracks and peels off easily.

Polyisoprene is also created synthetically, producing what is sometimes referred to as "syntheticnatural rubber".Synthetic rubber is produced from the polymerisation of 2-chlorobuta-1,3-diene.

n CH2=CH-C=CH2 ~(CH2-CH=C-CH2) ~

Cl Cl

2-chlorobuta-1,3-diene neoprene

The product is known as neoprene. It is resistant to chemicals and used to make hose for petroland containers for corrosive chemicals.

Another synthetic rubber is styrene-butadiene rubber (SBR), a copolymer betweenphenylethene (30%) and 1,3-butadiene (70%).

CH2=CH-CH=CH2 + CH2=CH CH2-CH=CH-CH2-CH2-CH

1,3-butadiene phenylethene styrene-butadiene rubber

13.2 Processing of rubber

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http://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Isoprenehttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Isoprene

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Rubber is a natural polymer, formed from a liquid resin or latex derived from the inner bark ofthe Hevea brasiliensis tree. Latex or natural rubber is made from the sap of the wild rubber tree.The bark of the tree is 'tapped', this means cutting small V shape slits into the bark, so that thesticky, white sap, known as latex, drips out and is caught in collecting cups tied to the tree truck.Figure 13.2 shows the tapping of rubber trees.

Rubber trees Latex being collected from Knives used for tappinga tappedrubber tree rubber

Figure 13.2 Tapping of rubber trees(Source: http://images.google.com)

Latex consists of 35% rubber particles and 65% water. Every rubber particle contains rubbermolecules which are wrapped by a layer of negatively charged protein membrane. It issometimes necessary to add a few drops of ammonia solution to the cup, or to the transporttank, to prevent pre-coagulation of the latex before it reaches the factory. It can also be left inthe cup to coagulate naturally (bacteria action which produces acid) into cup lump for collectionbefore the next tapping, although this will produce a lower grade of product.

The collected latex is then brought to a collection centre and poured into a big tank. The latex isthen filtered to remove impurities and then passed through centrifugal machines to remove mostof the water. Rubber is obtained when methanoic or ethanoic acid is added to latex to causecoagulation of rubber molecules. The positively charged hydrogen ions from acid will neutralisethe negative charges of rubber molecules.

The coagulated latex is then rolled into sheets using rollers. The sheets are cut into suitablesize and sent to the rubber factory for processing. The rubber sheets are washed with machinesusing soap and water to remove external dirt. Then, they are hanged on bamboo stems fordrying for about 3-5 days depending on the thickness of the rubber sheets. After that, they aredried in the smoke house, maintained at a temperature of 70 0C, using wood from older rubber

trees or coconut husks. This process is called smoking.

After smoking, the rubber sheets become more transparent, and impurities can be more easilyseen and removed. These smoked sheets are called Ribbed Smoked Sheets or RSS. They aregraded as RSS1, RSS2, RSS3, RRS4 and RSS5 depending on their colour, purity andelasticity. The dried rubber sheets are then pressed into cubes using a hydraulic press. Thisform of rubber is called bulk rubber which is used to make tyres after vulcanisation of therubber. The processed rubber sheets are then sold to factories to be made into a variety ofsubstances.

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http://en.wikipedia.org/wiki/Rubber_tappinghttp://en.wikipedia.org/wiki/Par%C3%A1_rubber_treehttp://images.google.com/imgres?imgurl=http://img.alibaba.com/photo/11254902/Banana_Knives_And_Knives_For_Rubber_Tree.jpg&imgrefurl=http://www.alibaba.com/product/cuchilleria-11254902-10836744/Banana_Knives_And_Knives_For_Rubber_Tree.html&usg=__5JJmI_8em0K-mCcRPbKTzXnvt1c=&h=400&w=500&sz=30&hl=en&start=18&tbnid=_VFkeSDWBka-uM:&tbnh=104&tbnw=130&prev=/images%3Fq%3Drubber%2Btree%26gbv%3D2%26hl%3Den%26sa%3DGhttp://images.google.com/imgres?imgurl=http://www.rbgkew.org.uk/plants/images/rubber.jpg&imgrefurl=http://www.rbgkew.org.uk/plants/rubber.html&usg=__-Z7vEupfQEpHNqSA8wsC_CIvS2A=&h=274&w=274&sz=28&hl=en&start=3&tbnid=fjnSkGzCWiebTM:&tbnh=113&tbnw=113&prev=/images%3Fq%3Drubber%2Btree%26gbv%3D2%26hl%3Den%26sa%3DGhttp://www.eds-lab.jp/images/photo_rubber01.jpghttp://en.wikipedia.org/wiki/Rubber_tappinghttp://en.wikipedia.org/wiki/Par%C3%A1_rubber_tree

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Today, Asia is the main source of natural rubber. The three largest producing countries(Indonesia, Malaysia and Thailand) together account for around 72% of all natural rubberproduction.

13.3 Cross-linking in rubber

Raw natural rubber is a thermoplastic and it becomes soft and sticky on hot days and hardwhen it is cooled to lower temperatures. Therefore its uses are limited. It can be made stiffer byintroducing chemical bonds between the polymer chains. Forming bonds between chains iscalled cross-linking, which is shown in Figure 13.3 The greater the number of cross-links in apolymer, the more rigid the material.

Figure 13.3 Cross-linking in polymers

13.4 Vulcanisation of rubber

The vulcanisation of natural rubber, a process discovered by Charles Goodyear in 1839, is anexample of cross-linking. Goodyear accidentally discovered that adding sulfur to rubber andthen heating the mixture makes the rubber harder and reduces its susceptibility to oxidation orother chemical attack. The sulfur changes rubber into a thermosetting polymer by cross-linkingthe polymer chains through reactions at some of the double bonds. Cross-linking of about 5percent of the double bonds creates a flexible, resilient rubber. When the rubber is stretched,the cross-links help prevent the chains from slipping, so that rubber retains its elasticity.Vulcanised rubber is stronger, harder and elastic and these properties do not change withtemperature. Vulcanised rubber is used to make tyres and seat-belts. Figure 13.4 shows eachdisulphide group connecting two polyisoprene molecules.

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Figure 13.4 Cross-linking in rubber

13.5 Properties of rubber

Rubber exhibits unique physical and chemical properties.Some of the properties of naturalrubber are:

Flammable

Elastic and can be stretched

Reactive to chemical agents such as oil, grease and petrol

Flexible since it is a thermoplastic that softens on heating and hardens on heating,can be melted and reshaped to be reused again and again

Impermeable to water. This makes it an excellent barrier against pathogens such asHIV virus which causes AIDS.

Sensitive to ozone cracking due to the presence of a double bond in each and everyrepeat unit

Thinking (1 hour)

Can you state some of the properties of synthetic rubber? Use agraphic organiser to compare and contrast the properties of synthetic

rubber, natural rubber and vulcanised rubber.

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13.6 Uses of rubber

The uses of rubber including natural rubber, synthetic rubber and vulcanized rubber are varied.Natural rubber is used to make objects such as:

Erasers

Water hose Rubber band Soles of shoes Surgical and medical examination gloves and condoms Spring materials for engines to cut down on vibrations

Vulcanised rubber is used to make objects such as:

Car mats

Car tyres

Radiator hoses

Window wiper blades

Seals and suspension mountings

Synthetic rubber such as neoprene is used to make objects such as:

Gaskets

Car belts

Conveyor belts

Flexible pipes or hoses for the petrol industry because it is not attacked by petrol

Insulating materials for electrical connections, especially where the environment haschemicals

Figure 13.5 Uses of rubber

((Source: http://images.google.com)

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http://images.google.com/imgres?imgurl=http://www.lakewoodconferences.com/direct/dbimage/50045567/Full_Dipped_Rubber_Gloves.jpg&imgrefurl=http://www.lakewoodconferences.com/catalog/69/241/147334/full_dipped_rubber_gloves.html&usg=__t9OGp3YqyzLEjWsIs_gxpJQzzOw=&h=360&w=360&sz=21&hl=en&start=53&tbnid=NS11MLzlrjFTyM:&tbnh=121&tbnw=121&prev=/images%3Fq%3Drubber%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN%26start%3D40http://images.google.com/imgres?imgurl=http://www.par-group.co.uk/UserImages/Rubber_Polyurethane/Rubber%2520Bungs.jpg&imgrefurl=http://www.par-group.co.uk/rpe.aspx%3Fpage%3D150&usg=__N5H2LSE7P-T_9hmSdwTG_jQZR4A=&h=572&w=752&sz=71&hl=en&start=44&tbnid=vYMFF0WiX9Sj4M:&tbnh=107&tbnw=141&prev=/images%3Fq%3Drubber%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN%26start%3D40http://images.google.com/imgres?imgurl=http://www.germes-online.com/direct/dbimage/50315828/Metallic_Rubber_Balls.jpg&imgrefurl=http://www.germes-online.com/catalog/62/1169/118940/sell_metallic_rubber_balls.html&usg=__KiMf0tGvDw3maHYvZoo0taMhgQw=&h=360&w=360&sz=38&hl=en&start=29&tbnid=z3fMzn92eQAEzM:&tbnh=121&tbnw=121&prev=/images%3Fq%3Drubber%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN%26start%3D20http://images.google.com/imgres?imgurl=http://www.germes-online.com/direct/dbimage/50248985/Ladies__Rubber_Boots.jpg&imgrefurl=http://www.germes-online.com/catalog/69/74/260/329385/sell_farmer_s_rubber_boots.html&usg=__fyYO8uBVj4FNzE0W0ddaQa-XRMo=&h=360&w=360&sz=18&hl=en&start=5&tbnid=Uar-2jXbtYkuIM:&tbnh=121&tbnw=121&prev=/images%3Fq%3Drubber%26gbv%3D2%26hl%3Den%26sa%3DG

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Collecting Information (2 hours)

Do you know that in the late 1950s, Malaysia produced almost 40% of theworlds natural rubber. It was said that rubber built up Malaysia (Source:Countdown to 50th Merdeka!, The SUN newspaper, Monday August 132007, page 12). Gather information about the development of the rubberindustry in Malaysia.

References

Brown, T.L.; Lemay,H.E.; Bursten, B.E. (2000) Chemistry-The Central Science. Eighth Edition,New Jersey:Prentice Hall.

McMurry,J.; Fay,R.C. (2001) Chemistry. Third Edition, New Jersey: Prentice Hall.

http://en.wikipedia.org/wiki/Rubber(rubber industry)

http://www.irrdb.com/IRRDB/Rubber_Industry/Other_Suppliers/ProcessingChemicals/Vulcanization.htm(vulcanisation of rubber)

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http://en.wikipedia.org/wiki/Rubberhttp://www.irrdb.com/IRRDB/Rubber_Industry/Other_Suppliers/ProcessingChemicals/Vulcanization.htmhttp://www.irrdb.com/IRRDB/Rubber_Industry/Other_Suppliers/ProcessingChemicals/Vulcanization.htmhttp://en.wikipedia.org/wiki/Rubberhttp://www.irrdb.com/IRRDB/Rubber_Industry/Other_Suppliers/ProcessingChemicals/Vulcanization.htmhttp://www.irrdb.com/IRRDB/Rubber_Industry/Other_Suppliers/ProcessingChemicals/Vulcanization.htm