polymers

Chemistry in Action (Polymers)

Polymers• Polymers are macromolecules formed by repeated joining of many

small molecules.

• Polymerisation is the process of joining together many small molecules

repeatedly to form very large molecules like polymers.

• Monomers are compounds that join together repeatedly to form a

polymer in the process of polymerisation.

Polymers can be natural or synthetic. The natural polymers covered include

proteins, polysaccharides and nucleic acids

The most important naturally occurring polymers are:

Proteins

Polysaccharides (e.g. cellulose, starch)

Nucleic acids (e.g. DNA, RNA)

Rubber

Synthetic polymers are produced commercially on a very large scale.

They have a wide range of properties and uses.

Plastics are all synthetic polymers

Synthetic polymers can be made from monomers by two basic polymerisation

processes:

(a) addition polymerisation which produces addition polymers

(b) condensation polymerisation which produces condensation polymers

Well-known examples of synthetic polymers are:

Polyethene (PE)

Polystyrene (PS)

Polyvinyl chloride (PVC)

Nylon

Urea-methanal


Natural PolymersAmino acids and proteinIntroduction

Amino acid are bifunctional compounds containing both the amino (-NH2) and

carboxylic (-COOH) groups.

NH2 COOH

Classification of amino acids

1. Neutral amino acids:

Number of amino groups = number of carboxylic groups

E.g. Glysine NH2CH2COOH

2. Basic amino acids

Number of amino groups > number of carboxylic groups

E.g. Lysin

3. Acidic amino acids

Number of amino groups < number of carboxylic groups

E.g. Aspartic acid

Stereochemistry of Amino Acids

All amino acids except aminoethanoic acid contain an asymmetric atom and

exhibit optical isomerism.

Example: Alanine


They are optical isomers, but optical inactive, since they are racemic mixture.

Laboratory synthesized amino acids are ONLY optically inactive because of

the formation of race mixture.

Physical properties of Amino Acids

The dipole moments of the amino acids are very large. For example,+NH3CH2COO- CH3CH2COOH CH3(CH2)2CH2NH2

Dipole Moment 14D

Ionic

Compound

1.7D

Acid

1.4D

Base

In fact, in the solid state and in solution, amino cids exist as internal ionic

salts, called Zwitterions.

So,

Amino acids are high melting point solids.

e.g. Glycine melts at 235℃

They are very soluble in water, but they only dissolove slightly in organic

solvents.

They have a very large dipole moment.

Chemical properties of Amino Acids

Amphoteric nature of amino acids

At some intermediate pH value, a dipolar (zwitterions) form is produced.

OH

H+

OH

H+H2N – CH – COO

|R

H3N – CH – COOH|

R

+ H3N – CH – COO

|R

+OH

H+

OH

H+

OH

H+

OH

H+H2N – CH – COO

|R

H3N – CH – COOH|

R

+H3N – CH – COOH

|R

+ H3N – CH – COO

|R

+H3N – CH – COO

|R

+

The hydrogen ion from the carboxyl group is trasnsferred to the basic amino group within the molecule.


The existence of the zwitterionic form can be explained in terms of acid-base

theory:

1. –NH2 is a stronger base than –COO-

2. –COOH is a stronger acid than –NH3+

Further evidence for zwitterions formation is electrophoresis.

Methods to separate a mixture of amino acids.

Paper chromatography will be used to separate amino acids.

There is a thin film of water on the chromatography paper.

The amino acids distribute themselves between the stationary phase (water

on the paper) and the moving phase (the solvent/eluent)

To make the amino acid spots visible to naked eyes, spray chromatography

paper with ninhydrin solution which reacts with amino acids to give purple

coloured compounds .

(also accept using UV radiation/ iodine vapour to detect the amino acid spots.)

Reactions of Amino acids

Two main types reaction of the Amino Acids

1. reaction of the carboxyl group

2. reactions of the amino group


Showing acidic

Properties

Showing basic

Properties

Peptides, Polypeptides and protein

Dipeptide

The (-NH2) group of one amino acid can react with the (-COOH) group of

another to form an amide.

The resultion molecule is a dimmer containing two amino acid units which is

describes as a dipeptide.

In the process, the two amino acid molecules are joined by the condensation

reaction. A water molecule is eliminated.

H O+NH3 C-C-OCH3

R

H-Cl+NH3 CCOOH R

HNH2 CCOOH R

Dil. NaOH(aq)

SOCl2 or PCl5

Fusing with soda lime

CH3OH/H+

Dil HCl

CH3COCl

HNH2 CCOO-Na+

R

HNH2-C-H R

O H HCH3-C-N-C-COOH R

H ONH2 C-C-Cl R


Amino acids are linked together by peptide linkage.

Polypeptide

Amino acids undergo condensation polymerization to form long-chain

polyamide molecules.

dipeptide

Polypeptide/protein

Further reaction of each end


(1) If n< about 50, the product is a polypeptide.

(2) If n> about 50, the product is a protein.

Structure of proteins

Protein structure is describe at 4 levels: 1 , 2 , 3 &∘ ∘ ∘ quaternary.

Being polyamides, both proteins and nylon can be hydrolysed and are thus

broken down to their constituent amino acids.

For example,

Polypeptide

Dipeptide


Amino acids

The peptide linkages in a protein molecule can be broken by hydrolysis using

mineral acids or some enzymes.

On complete hydrolysis, the protein is broken down into amino acids. By

analyzing the resulting amino acids, the composition of the protein molecule

may be deduced.

Carbohydrates

Monosaccharide, disaccharide and polysaccharide

Sugar, starch and cellulose are carbohydrates. Carbohydrates are important

in the diet as a source of energy. They are compounds containing carbon,

hydrogen and oxygen with the general formula CxHyOz.

Carbohydrates may be divided into three groups,

Monosaccharides

Disaccharides

Polysaccharides

The simplest carbohydrates are the sugars. (glucose, fructose and ribose)

Monosaccharides

The monosaccharides consist of a single polyhydroxyaldehyde or

polyhydroxyketone.

Monosaccharides are a group of sweet, soluble crystalline molecules with

relatively low molecular masses. They cannot be hydrolyzed into simpler

compounds. The monosaccharides commonly found in food have the general

formula C6H12O6. Two most important examples are glucose and fructose.

They are found in many fruits and in honey. Glucose is also found in the blood


of animals (including humans)

Each monosaccharides molecule contains one carbonyl group. All the other

carbon atoms are bonded to hydroxyl groups. There are aldose and ketose,

for which the carbonyl group is and is NOT terminal respectively.

Open chain and ring structures of glucose and fructose

Glucose can exist in acyclic and cyclic forms:

aldehyde

ketone

0.02%

36% 64%


Glucose contains an aldehyde group in its acyclic form. Glucose is an

aldohexose

Most of the reactions of glucose in aqueous solutions are due to presence of

the free aldehyde group of the acyclic form.These reactions include its

reducing action

Fructose can exist as acyclic form, as well as cyclic forms of 6-membered

rings and 5-membered rings

Fructose contains a keto group in its acyclic form

fructose is an ketohexose

Most of the reactions of fructose in aqueous solutions are due to:

presence of the free keto group of the acyclic form

Disaccharides


Disaccharides are sweet, soluble and crystalline.

They have the general formula: C12H22O11

Disaccharides can be formed from the condensation reaction of two

monosaccharide molecules a water molecule is eliminated

Common disaccharides include Sucrose (Source: sugar cane), Maltose

(Source: malt) and Lactose (Source: milk)

Glycosidic Linkage in Carbohydrates

Common disaccharides are formed from the condensation reaction between

two monosaccharide molecules and a water molecule is eliminated

The bond formed between two monosaccharides is called a glycosidic linkage

A sucrose molecule is formed by the condensation reaction of a glucose

molecule and a fructose molecule

A maltose molecule is formed by the condensation reaction of two glucose

molecules

Polysaccharides

Polysaccharides are polymers of monosaccharides

General formula: (C6H10O5)n where n is a large number (up to thousands)

Examples of polysaccharides: starch and cellulose

Starch is commonly found in rice, bread and potatoes

Cellulose is found in fruits, vegetables, cotton and wood


The condensation process can be repeated to build up giant molecules of

polysaccharides

e.g. Starch

Cellulose

Adjacent chains of cellulose molecules are linked up by hydrogen bonds.


These cellulose chains intertwine into fibrils of considerable strength.

DNA as Nucleic acidNucleic acids are the molecules that preserve hereditary information,

transcribe and translate it in a way that allows the synthesis of all the various

proteins of a cell

Nucleic acid molecules are long polymers of small monomeric units called

nucleotides.

The monomers of nucleic acids, called nucleotides, are formed from the

following units:

1. A phosphate unit

2. A five carbon sugar

3. A nitrogen – containing organic base.

Two kinds of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid

(RNA)

The sugar component of RNA is ribose, whereas that in DNA is deoxyribose.

The following nitrogen bases are found in DNA and RNA:

RiboseDeoxyribose


DNA is the nucleic acid that most genes are made of

DNAs have four different kinds of nucleotides as the building blocks

All the four kinds of nucleotides have deoxyribose as their sugar

component

They differ in their nitrogen-containing bases

Adenine (A) and guanine (G)

have double-ring structures

known as purines

Cytosine (C) and thymine (T)

have single-ring structures

known as pyrimidines

Formation of the nucleotide of a DNA molecule

The nucleotides within

a DNA molecule are joined together

through condensation reactions between

the sugar of a nucleotide and the

phosphate group of the next nucleotide in

the sequence long chain (i.e. a polymer)

(in DNA)

(in RNA)


of alternating sugar and phosphate groups is formed

Two hydrogen bonds are formed between A in one chain and T in the other

Three hydrogen bonds are formed between G in one chain and C in the other

Hydrogen bonding between complementary base pairs. The hydrogen bonds

are responsible for formation of the double stranded helical structure of DNA.

DNA replication

The original DNA double helix partially

unwinds, and new nucleotides line up on

each strand in a complementary manner.

Hydrogen bonds help align the new

nucleotides with the original DNA chain.

When the new nucleotides are joined by

condensation reactions, two identical double


helix DNA molecules result.


Synthetic PolymersAddition PolymersFormation and Uses of Addition Polymer

Addition polymerization is a chemical process in which monomer molecules

are joined together to form a polymer without elimination of small molecules.

The resulting polymer will therefore have the same percentage composition

as the reactant monomers.

1. Polyethene, or Polyethylene [PE]

Polyethene has many useful properties:

– it is easily moulded;

– it is an excellent electrical insulator;

– it does not corrode;

– it is tough;

– it is not affected by the weather;

– it is durable.

Ethene is the monomer that is used to synthesize polyethene

Depending on the manufacturing conditions, two kinds of polyethene can be

made

low density polyethene (LDPE)

• Molecular mass between 50 000 and 3 000 000

• Light, flexible

• Low melting point

• Used to make soft items (e.g. wash bottles, plastic bags and food

wraps)

high density polyethene (HDPE)


• Molecular mass up to 3 000 000

• Tougher

• Higher melting point

• Used to make more rigid items

(e.g. milk bottles and water buckets)

Polyethene is a thermoplastic

It softens at a high temperature

Uses of polyethene

Insulate telephone line

Its unique electrical properties were essential during the development of

radar.

Plastic bags

It used in supermarket for packing various food product.

milk bottles and water buckets

Hard and rigid, not poisonous

Mechanism for the addition polymerization: Free Radical Addition

Polymerization of Ethene

The reaction mechanism consists of three stages:

chain initiation

chain propagation

chain termination

Chain initiation

diacyl peroxide molecule as a initiator


Chain propagation

Chain termination steps

Addition polymers formed from these substituted ethenes (H2C=CHX) have a

range of properties predictable based on the structure of –X (non polar

substituent). –X group like –CH3 or –C6H5 are soluble in organic solvents like

acetone or propanone.

2. Polypropene (PP)


With the use of Ziegler-Natta catalyst, propene can be polymerized to

polypropene

Polypropene can exist in different configurations depending upon the

orientation of the methyl groups in the polymer.

The properties of polypropene can be modified by adjusting the manufacturing

conditions

In isotactic polypropene, all methyl groups are on the same side of the carbon

chain.

Using Ziegler Natta Catalyst, the methyl groups all arrganged on one side of

the carbon chain.

In atactic polypropene the methyl groups are randomly arranged

Isotactic Polypropene Atactic polypropene

More symmetrical Less symmetrical

Molecules pack together closely. Poor packing

High melting point Low melting point

Greater strength sticky


It is more rigid than HDPE and used for moulded furniture

High mechanical strength and strong resistance to abrasion. It is used for

making crates, kitchenware and food containers

Spun into fibres for making ropes and carpets especially useful for

making athletic wear. They do not absorb water from sweating as cotton

does

3. Polystyrene (PS)

Styrene is made from the reaction of benzene with ethane, followed by

dehydrogenation.

The styrene produced is polymerized by a free radical mechanism into

polystyrene at 85 – 100°C using dibenzoyl peroxide as the initiator

Mechanism:


Polystyrene is transparent, brittle and chemically inert. It used to make toys,

specimen, containers and cassette cases

By heating polystyrene with a foaming agent,expanded polystyrene can be

made.

Expanded polystyrene is extremely light, white solid foam. It mainly used to

make light-weight ceiling tiles in buildings, and food boxes and shock

absorbers for packaging

4. Polyvinyl Chloride (PVC)

PVC is produced by addition polymerization of the choroethene monomers in

the presence of a peroxide catalyst (e.g. hydrogen peroxide at about 60°C)


Presence of the polar C – Cl bond is considerable

dipole-dipole interactions exist between the polymer chains makes PVC a

fairly strong material

The large size of Cl atom means the chains cannot easily be moved over one

another. This result in the polymer being rigid and brittle.

PVC is hard and brittle and used to make pipes and bottles

When plasticizers are added, the effectiveness of the dipole- dipole

interactions is reduced. PVC becomes more flexible

• Used to make shower curtains, raincoats and artificial leather

• Used as the insulating coating of electrical wires

(a) (b)

Products made of PVC without plasticizers


Despite its extensive uses, one problem with the use of PVC is its disposal.

PVC is non-biodegradable, normally not disposed of by land-filling.

Incineration of PVC produces HCl (g)

This HCl (g) produced is usually absorbed by the wet scrubber filled with an

alkali such as Na2CO3 (aq).

Also, monomer of PVC is a carcinogen.

5. Polytetrafluoroethene (PTFE)

PTFE is produced through addition polymerization of the tetrafluoroethene

monomers under high pressure and in the presence of a catalyst

Products made of PVC with plasticizers


C—F bonds are exceptionally strong & resistant to heat and chemicals

PTFE has a relatively high melting point and is chemically inert

Its non-stick properties make it an ideal material for the coating of frying pans

ÞSince fluorine is highly electronegative atom an evenly distributed layer of

negative charge is developed on the surface of PTFE

Þ Layers of negative F atoms repel almost all other materials

Þ Thus preventing them from adhering to PTFE

Þ PTFE has very slippery surface

To make Teflon stick to the surface of a newly made cooking pan,

Teflon is heated to a very high temperature

pressed firmly onto the surface of the item.

This film pressing increases the area of contact between Teflon and the

surface of the pan

Increasing the strength of van der Waals’ force

Another technique :

Teflon with oxygen-containing group is polar

This helps strengthening the attraction between Teflon and the surface.

Other polarized group: methyl methacrylate can also be incorporated into the

Teflon polymer chains to achieve the above purpose.

Whatever the technique, there is still the chance of Teflon peeling off from

cooking ware after extended use, because

ÞTeflon decompose at high temperature

ÞIts coefficient of expsion is different from that of the material of the cooking

surface

6. Polymethyl Methacrylate ( Perspex ) (PMMA)

PMMA is formed by the free radical addition polymerization of methyl

methacrylate in the presence of an organic peroxide at about 60°C


PMMA is a dense, transparent and tough solid makes it a good material for

making safety goggles, advertising sign boards and vehicle light protectors

However, it has poor scrtching resistance and can be dissolved by a number

of organic solvents.

Condensation PolymersFormation and Uses of Condensation Polymer

Condensation polymerization is a chemical process in which monomer

molecules are joined together to form a polymer with elimination of small

molecules such as water, ammonia and hydrogen chloride

Each monomer molecule must have at least two functional groups

1. Polyamide

1. Nylon

Nylon 6,6

Ethanoic acid

Ethanol Ethyl ethanoate(An ester)


When a solution of hexane-1,6-dioyl dichloride in hexane

is poured gently onto a solution of 1,6-diaminohexane in

water, a white film of nylon is formed at the interface

between the two layers. The film can be pulled up as a

string and wound onto a stirring rod.

Used for making carpets, thread, cords and various kinds

of clothing from stockings to jackets

Advantages:

drips dry easily

not easily attacked by insects

resists creasing

faster


There are interchain hydrogen bond so Nylon6, 10 is expected to have a

lower tensile strength than nylon6,6. There is decrease in the number of

hydrogen bond per unit length, as a result of the longer carbon chain in nylon

6,10

3. Kevlar

Aramid is a synthetic poly amide

Aliphatic hydrocarbon unit within the polmer chain has been replaced by an

aromatic unit in Aramid

• Kevlar is an aromatic polyamide

• The structure of Kevlar is similar to nylon-6,6

• The two monomers of Kevlar are benzene-1,4-dicarboxylic acid and

1,4-diaminobenzene

Both monomers are bifunctional

In Kevlar, the starting material was modified to create straighter chains in the


polymer. A polyamide was produced with the heat resistance of asbestos.

Strength was much greater than steel.

In Kevlar the aliphatic hydrocarbon chain parts of the poly amide are replaced

by benzene rings. These parts of the polymer chain make the chains inflexible

due to delocalized bonding.

Some of this delocalization extends beyond the benzene rings and onto part

of the amide link resulting in long, rigid molecules that do not easily flex or

twist.

This extended delocalization also leads to enhanced intermolecular hydrogen

bonging between the adjacent Kevlay polymer chains.

This hydrogen bonding network causes the chains to interlock each other,

forming a sheet structure.

All the C = O and – N – H groups in the polymer chains are on opposite sides.

This makes the chains highly symmetrical. The regular structure of the

polymer chains allows them to interlock with each other.


Applications:

(a) Kevlar is an unusual polymer with fire resistant properties and also

great strength. It is found in the crash helmets of Formula I racing

drivers as well as in the suits of racing motorcyclists.

(b) The hull of this offshore racing craft is also reinforced with Kevlar.

(c) Kevlar is used in making bullet proof vests. A more recent innovation is

to use carbonanotubes to make fibres for these bullet proof vests.

These new bullet proof vests can be made 30 % lighter, but 1.5 times

more bullet resistant than conventional Kevlar vests.


2 . Dacron

Formed by repeated condensation reactions of benzene-1,4-dicarboxylic acid

(also called terephthalic acid) and ethane-1,2-diol (also called ethylene glycol)

in the presence of a catalyst and at a low pressure and moderate temperature

(about 250°C)

The two monomers of Dacron are:

The polymerization begins with the formation of an ester

A water molecule is eliminated

Due to polarization of the carbonyl groups C=O, Dacron chains are cross-

(a)

(b)

(c)


linked by strong dipole-dipole attractions

Properties of Dacron:

High tensile strength

High resistance to stretching

Low absorption of moisture

Garments made of Dacron:

are tough

can resist wrinkling

can be washed and dried easily and quickly

Excellent for making trousers and skirts, sheets and boat sails

Can be used alone or blended with cotton to make it absorb sweat better

3 . Urea-methanal

Produced by the condensation polymerization of urea and methanal under

heat and pressure

When an urea molecule joins up with a methanal molecule, water molecule is

eliminated

In the presence of excess methanal, further condensation reactions between

the polymer chains and methanal occur.Cross-linkages between the polymer

chains are formed. A rigid structure of urea-methanal is produced


• Urea-methanal is a thermosetting plastic and cannot be softened or

melted again by heating once they have been set hard

• Excellent electrical insulator

• Resistant to chemical attack

Effect of Structure on Properties of Polymers

• Polymers are long-chain giant molecules

• The final form and the properties of the polymers depend on how these

long polymer chains are packed together

• If the polymer chains do not have a specific arrangement but are

loosely packed together the polymer is said to be amorphous

• Amorphous polymers are generally transparent, flexible and less dense

• When the polymer chains are regularly packed together, the polymer is

said to be crystalline

• Polymers with a high degree of crystallinity are translucent or opaque,

harder and denser

• The attractive forces holding polymer chains together also affect the

properties of polymers


• Polymer chains containing carbon and hydrogen atoms only are held

together by weak van der Waals’ force

• slow melting points

• low mechanical strength

• If polymer chains are held together by stronger van der Waals’ forces

or hydrogen bonds, the mechanical strength of the polymers would be

stronger

• If cross-linkages are present between polymer chains, the polymers

would be mechanically stronger, more elastic or more rigid ,depending

on the extent of cross- linkages in the polymer

Low Density Polyethene and High Density Polyethene

High Density Polyethene

• When Ziegler-Natta catalysts are used, the

polymer chains produced are long

molecules with very little branching. The

polymer chains can pack closely together

into a largely crystalline structure

Thus, the polymer has a higher density

• Compared with LDPE, HDPE

is harder and stiffer

has a higher melting point

has greater tensile strength

has strong resistance to chemical attack

has low permeability to gases

blow-molded objects: bottles for milk, soft drinks, shampoos, bleaches and so

on


Low Density Polyethene

• When ethene is polymerized at 200°C and 1000 atm using peroxide as

the catalyst, low density polyethene (LDPE) is made

• Under these reaction conditions, highly branched polymer chains are

formed

• The branches prevent the polymer chains from getting close to each

other. The polymer chains do not pack together well and creates a

significant proportion of amorphous regions in the structure

• Thus, the polyethene made has a low density

Low density polyethene is a

Waxy

Semirigid

Translucent material

Low melting point

Nylon and Kevlar

• Nylon is a group of polyamides

• It contains a relatively large number of crystalline regions arranged in a


random manner

• When nylon is spun into fibres and is drawn

• the crystalline regions are aligned

• leads to an increase in the tensile strength

In the stretched or drawn nylon, the polymer chains line up and are parallel to

each other. The amide groups on adjacent chains form strong hydrogen

bonds with each other

These hydrogen bonds hold the adjacent chains together making nylon thread

strong

The structure of Kevlar is basically the same as nylon-6,6

When molten Kevlar is spun into fibres, the polymer has a crystalline

arrangement and the polymer chains oriented parallel to each other

Non-aligned crystalline region

Aligned crystalline region


• Kevlar is much stronger than nylon

• The difference in their strength is due to

the orientation of the amide groups along the polymer chains

• In nylon, between the amide groups are the carbon chains the ¾ C = O

and ¾ N ¾ H groups can be on opposite sides or on the same side

• When the ¾ C = O and ¾ N ¾ H groups are on the same side, the

polymer chain would not be straight and the number of hydrogen bonds

formed between adjacent chains would be less

• Kevlar has a regular structure

the polymer chains interlock with each other

Kevlar fibres are very strong

used for making reinforced rubbers and bullet-proof vests

Vulcanization of Polymers

Natural rubber is a polymer of the monomer

2-methylbuta-1,3-diene (isoprene)

• Poly(2-methylbuta-1,3-diene) or polyisoprene can exist in two isomeric

forms

• Natural rubber is the cis-form

2-Methylbuta-1,3-diene


Natural rubber is not a useful polymer because it is too soft and too chemically

reactive. The long chain molecules can be coiled twisted and interwined with

one another

Vulcanization of natural rubber is the chemical process that confers cross-

linkage among the polymer chains of rubber, turning natural rubber into a

flexible elastic material.

• In the process of vulcanization,1–3 % by mass of sulphur is added

to natural rubber and the mixture is heated

• Short chains of sulphur atoms (i.e. cross-linkages) are formed between

the polymer chains

• The sulphur changes rubber into a thermosetting polymer by cross

linking the polymer chains through reaction at some of the double

bonds as shown:

This makes the rubber harder and reduces its susceptibility to oxidation or

other chemical attrack.

• When vulcanized rubber gets hot, the polymer chains cannot slip

across one another since they are still held together by short chains of

sulphur atoms

• That is why vulcanized rubber does not melt when heated and does not

become brittle when cooled

Part of a polymer chain of natural rubber


The extent of the cross-linkages formed between the polymer chains affects

the properties of vulcanized rubber

• If the rubber has few cross-linkages, the rubber is softer, more flexible

and more elastic

• If the rubber has many cross-linkages, it is stiffer, less flexible and less

elastic

• Car tyres are made of vulcanized rubber

• Because of the presence of cross-linkages among the polymer chains,

the rubber does not melt when it gets hot

• That is the reason why car tyres do not melt when drivers drive really

fast

Degradable Plastics

• Natural polymers (e.g. wood and paper) are

biodegradable Micro-organisms in water and in the

soil use them as food

• Synthetic polymers (e.g. plastics) are non-

biodegradable can remain in the environment for a

very long time

• Nowadays, plastic waste constitutes about 7 % of

household waste

• In Hong Kong, plastic waste is buried in landfill sites

it remains unchanged for decades

more and more landfill sites have to be found

Uses of plastics in Hong Kong (an approximation)

(a) Hong Kong is a highly densely populated city

(b) The volume of domestic waste generated daily is very great.

(c) Plastic waste contributes to the main bulk of our domestic waste.

(d) Few sites are left to be used for landfill

(e) The building / operation cost of incineration plants is high and recycling

of plastics also involves very tedious procedures.

Car tyres are made of vulcanized rubber


In order to tackle the pollution problems caused by the disposal of plastic

waste, degradable plastics have been invented

Several types of degradable plastics:

• biopolymers

• photodegradable plastics

• synthetic biodegradable plastics

1. Biopolymers

• Polymers made by living micro-organisms (e.g. paracoccus, bacillus

and spirullum)

• e.g. The biopolymer poly(3-hydroxybutanoic acid) (PHB) is made by

certain bacteria from glucose

• When PHB is disposed, the micro-organisms found in the soil and

natural water sources are able to break it down within 9 months

• However, PHB is 15 times more expensive than polyethene

2. Photodegradable Plastics

• Photodegradable plastics have light-sensitive functional groups (e.g.

carbonyl groups) incorporated into their polymer chains

• These groups will absorb sunlight use the energy to break the chemical

bonds in the polymer to form small fragments

(a) Paracoccus (b) Bacillus

(c) Spirullum


3. Synthetic Biodegradable Plastics

• Made by incorporating starch or cellulose into the polymers during

production

Micro-organisms consume starch or cellulose and the plastics are broken

down into small pieces

• The very small pieces left have a large surface area greatly speeds up

their biodegradation

• Drawbacks of this method:

• the products of biodegradation may cause water pollution

• the rate of biodegradation is still too low for the large quantity of

plastic waste generated

• They are much more expensive than ordinary materials.

• When buried in landfill, they will not be exposed to sunlight light

and may therefore remain unchanged for many years.

• Their long term effects on the environment are unknown of any

residues.

• They may encourage a ‘throwaway is OK’ culture.

• They interfere with the present recycle program.

END

polymers

Documents

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basic amino group

rna rubber synthetic