Natural Rubber

Natural rubber is a natural polymer.

Natural rubber is obtained from the latex secreted by rubber tree.

Latex is a colloid. It consists of rubber particles dispersed in water.

Natural rubber is poly(isoprene).

Its monomer is 2-metyhlbuta-1,3-diene or isoprene. Each isoprene molecule contains two pairs of double bonds.

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The isoprene molecules undergo addition polymerization to produce a long-chain molecule called poly(isoprene)

Each rubber particles is made up of many long-chain rubber molecules, enclosed by a protein-like membrane which is negatively charged.

Negatively-charge protein membrane

Long chain rubber molecules

Rubber particle

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Latex

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Polymerization of Isopropene

H CH3 H H│ │ │ │C ═ C ― C ═ C │ │H H

+ H CH3 H H│ │ │ │C ═ C ― C ═ C │ │H H

H CH3 H H│ │ │ │C ═ C ― C ═ C │ │H H

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H CH3 H H │ │ │ │― C − C ═ C − C ― │ │ H H

H CH3 H H │ │ │ │― C − C ═ C − C ― │ │ H H

Isoprene

H CH3 H H │ │ │ │― C − C ═ C − C ― │ │ H H

Repeated unit in polymer

polymerization

Isoprene Isoprene

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Coagulation process of latex

Rubber particle has negatively-charged protein membrane.

The repulsion between the negatively-charged particles prevents the rubber particles from coming close to each other.

Latex could not coagulate.

Rubber particle─

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Q: So what causes the latex to coagulate?

When an acid is added, the hydrogen ions, H+ neutralize the negative charges on the protein membrane.

Q: Can you named the acid that usually used to coagulated the latex in rubber industry?

A: Ethanoic acid also known as acetic acid. [notes: all acid solutions can make latex coagulate]

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Q: What do you think will happen next?

The rubber particles now come close together.

This enable them to collide with one another resulting in the breakage of the protein membrane.

The rubber molecules/polymer combines with one another and entangle.

Thus, causing the latex to coagulate.

EASY lah

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Q: The coagulation of latex will also occur if latex is exposed to air. Why?

A: Bacteria from air can enter latex. The growth and spread of bacteria produce lactic acid that causes the process above.

Q: Named the substance can be used to preserve latex in liquid state? Explain.

A: Ammonia, NH3. NH3 solutions contains hydroxide ions, OH- that neutralised the acid/hydrogen ions, H+ produced by the bacteria. The rubber particles remain negatively charged and the coagulation is prevented.

[notes: all alkali solutions also can be used]

Properties of natural rubber- Soft- Elasticity decreases over time.- Easily oxidized by air. - Sensitive to heat. When heated, it becomes sticky.

When cooled, it becomes hard and brittle.

Vulcanization of rubber7

Properties of rubber can be improved through vulcanization.

Vulcanization is a process whereby rubber is reacted with sulphur to improve its properties.

Q: How the vulcanization process is carrying out in industry?

A: 1st method: Latex is heated with sulphur, or

2nd method: Rubber products are exposed to disulphur dichloride, S2Cl2.

Q: Compare and contrast the properties of vulcanised and 8

unvulcanised rubber.

Similarities Vulcanised and unvulcanised rubber is elastic, and heat and electrical insulator

Differences [notes: draw a table]

PropertiesVulcanised

rubberUnvulcanised

rubberElasticity More elastic Less elastic

Hardness Harder Softer

Tensile strength Stronger Weaker

Resistance to heat

Can withstand higher temperature

Cannot withstand temperature

Resistance to oxidation

Less easily oxidized

Easily oxidized

Effect of organic solvent

Does not become soft and sticky easily

Become soft and sticky easily

Q: Why?

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vulcanisation

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C═C C═C

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C═CC═C C═C

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Natural rubber Vulcanised rubber

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Rubber molecules

Cross-linkage of sulphur atoms

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A: Improved properties of vulcanised rubber is due to the presence of cross-linkage of sulphur atoms between the rubber molecules.

Q: How the cross-linkage of sulphur atom improve elasticity and strength of the vulcanized rubber:

A: When vulcanized rubber is stretched and released, the cross linkage of sulphur atoms pull the chains back to their original arrangement.

Q: Why vulcanized rubber more resistant to heat and organic solvent?

A: The presence of sulphur cross-linkage increases the size of rubber molecules.

Q: Why vulcanized rubber more resistant to oxidation?

A: Vulcanized rubber has lesser carbon-carbon double bond.

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Uses of natural rubber

Home WorkReview Questions pg 100 - 102

i. objective (no. 1-8)ii. subjective questions (1 and 2)iii. essay (no. 1c)

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SPM Question (2006)Diagram 2 shows the stretching phases of a vulcanized rubber and an unvulcanized rubber strands.

Stretching phases

Length of vulcanized rubber

Length of unvulcanized rubber

Before

During

After

Diagram 2Plan an experiment to compare one characteristic shown in Diagram 2 for both types of rubber.

Your planning should include the following aspects: (a) Aim of the experiment(b) All the variables(c) Statement of the hypothesis(d) List of substances and apparatus(e) Procedure of the experiment(f ) Tabulation of data [17 marks]

45 mm 45 mm

59 mm 60 mm

45 mm 50 mm

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Answer(i) Problem statement: Does vulcanized rubber is more elastic than unvulcanized rubber?

(a) Aim: To compare the elastic properties between vulcanized rubber and unvulcanized rubber.

(b) List of variable:

Manipulated variable = Type of rubber Responding variable = Elasticity // length of rubber strip during and after stretching phase.

Fixed variable = Original length of rubber strip // Length/size/mass of rubber

(before stretching phase)

(c) Hypothesis:

Vulcanized rubber is more elastic than unvulcanized rubber. // sk3

The more elastic of the rubber, the less length of the rubber during and after stretching phase. // sk2

Rubber that is more elastic shows less length when/after it has been stretched. sk3 (iv) List of materials and apparatus:

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Vulcanized rubber, unvulcanized rubber, Meter ruler, 50 g weight, retort stand and clamp, clip, thread.

(vi) Procedure :1. Clip is used to hung the strip of vulcanized rubber.2. The original length of the vulcanized rubber is

measured.3. A weight of 50 g is hung on the strip of vulcanized rubber. 4. The increased in length of the vulcanized rubber

strip is measured. 5. The weight is removed. 6. The final length of the vulcanized rubber strip is measured. 7. Step 1 to 6/ experiment is repeated using

unvulcanized rubber.

Retort stand with clamp

Clip

Rubber strip

Thread

Weight 50 g

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(vii) Tabulation:

Type of rubber strip

Original length /mm

Final Length when weight is hung /mm

Increase in length /mm

Final length when weight is removed /mm

Vulcanized rubber 45 59 14 45

unvulcanized rubber 45 60 15 50

HOT SPOT SPM 2011

Try this one

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Manipulated variable: Action to be taken:

Responding variable: Action to be taken:

How the variable responds?:

Fixed variable: Action to be taken:

Prepared by;Kamal Ariffin B SaaimSMKDBLhttp://kemhawk.webs.com

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