ENVIRONMENTAL BEHAVIOUR OF POLYMERS

HNC/D Applied Chemistry

Polymer Chemistry

By Sabrina Lee

DEGRADATION

WHAT IS DEGRADATION?

Degradation is a type of organic chemical reaction in which a compound is converted into a simpler compound. [1]

EXAMPLES OF DEGRADATION?Examples of degradation include environmental degradation e.g. the

hole in the o-zone layer caused by car emissions and pollution (physical) and chemical degradation.

DEGRADATION IN POLYMERS

POLYMER DEGRADATIONDegradation in polymers refers to a change in the properties of

a polymer under the influence of one or more environmental factors

FACTORS Heat Light

Chemicals – Acids, Alkalis and some salts

IS DEGRADATION DESIRED OR UNDESIRED?

Are the changes in the properties of a polymer due to degradation desired or undesired?

UNDESIRABLE CHANGESX CrackingX Chemical disintegration of

products

DESIRABLE CHANGESBiodegradation –

deliberately lowering the molecular weight of a polymer for recycling, often termed “aging”.

TYPES OF DEGRADATION IN POLYMERS

PHOTOINDUCED DEGRADATIONDegraded by photolysis to give a lower molecular weight – often via visible light or UV, X-rays or gamma rays.

THERMAL DEGRADATIONDegraded by thermolysis at high temperatures to give monomers, oils, gases and water.Includes: Pyrolysis, Hydrogenation and Gasification

CHEMICAL DEGRADATIONDegraded by solvolysis and mainly hydrolysis to lower molecular weight

ANTI-DEGRADANTS

WHAT ARE ANTI-DEGRADANTS?Deterioration inhibitor in rubber compounds to deter the aging of

rubber products

HOW DO THEY WORK?Anti-degradants cover the surface of the rubber product to prevent

direct contact between rubber and oxygen, thus preventing the degradation, or “aging” of the product i.e. wax products

EXAMPLES OF WAXES

ANIMAL WAXESBeeswax

Chinese wax

Lanolin (wool wax)

Shellac wax

VEGETABLE WAXESBayberry wax

Carnauba wax

Castor wax

Jojoba oil

Soy wax

MINERAL WAXESCeresin waxes

Montan wax

Ozocerite

Peat waxes

PETROLEUM WAXESParaffin wax

Microcrystalline wax

Petroleum jelly

SYNTHETIC WAXESPolyethylene waxes

Chemically modified waxes

Substituted amide waxes

Polymerised α-olefins

ENVIRONMENTAL DEGRADATION

WHAT IS ENVIRONMENTAL DEGRADATION?

• The deterioration of the environment through depletion of resources, such as air, water and soil.

• The destruction of ecosystems and the extinction of wildlife

• When natural habitats are destroyed or natural resources are depleted, the environment is degraded.

FACTORS OF ENVIRONMENTAL DEGRADATION

NATURAL FACTORS

• Droughts

• Storms – e.g. hurricanes, tornadoes, volcanic eruptions

These factors lead to land degradation caused by erosion.

HUMAN FACTORS

• Deforestation

• Industrialisation

• Urbanisation

These factors lead

to water, air and

land pollution.[3]

ENVIRONMENTAL STRESS CRACKING (ESC)

[2]

WHAT IS ESC?

• Unexpected brittle failure of thermoplastic polymers

• Accounts for 15-30% of all plastic component failures in service

• Caused by exposure to liquid chemicals• Occurs mostly in amorphous, brittle

polymers like polystyrene and polycarbonate

ESC AND POLYMER DEGRADATION

ESC and polymer degradation are not the same thing.

Polymer degradation involves breaking polymer bonds, whereas ESC does not.

ESC breaks the secondary linkages between polymers instead.

EXAMPLES OF ESC

ESC is seen in various polymers from plastic drinking glasses to the automotive industry.

The need to resist ESC in the automotive industry is high as a number of different polymers are subjected to a number of fluids.

Examples:• Petrol• Brake fluid• Windscreen cleaning solution

HOW DOES STRUCTURE AFFECT REACTIVITY?

The structure of a polymer depends upon various factors:• Number of branches (methyl groups)• Type of branching – long or short branched polymers• Number of cross links within a polymer

All of these factors contribute to physical and mechanical properties of a polymer.

HOW DOES STRUCTURE AFFECT REACTIVITY? (Cont.)

BRANCHING

• Branching makes a polymer less dense resulting in low tensile strength and low melting points.

• Branching also reduces the free volume of a polymer, resulting in a higher glass-transition temperature, Tg.

HOW DOES STRUCTURE AFFECT REACTIVITY? (Cont.)

CROSS LINKING

Cross linkage is described as a short side chain of atoms linking two longer chains in a polymeric material [1]. Cross links can either be covalent or ionic bonds.

Polymers with a high enough degree of cross-linking have "memory." When the polymer is stretched, the cross-links prevent the individual chains from sliding past each other. The chains may straighten out, but once the stress is removed they return to their original position and the object returns to its original shape.

HOW DOES STRUCTURE AFFECT REACTIVITY? (Cont.)

FUNCTIONAL GROUPSDifferent functional groups can affect a polymers properties by

• Different groups lending the polymer to ionic or hydrogen bonding, depending upon where the bond is formed, resulting in higher

tensile strength and crystalline melting points.

• Amides or carbonyl groups form hydrogen bonds between adjacent chains while polymers such as Ethene have no permanent dipole,

resulting in VDW forces which are much weaker. Hydrogen bonding is stronger than VDW, resulting in higher MP & tensile strengths.

REFERENCES

[1]

Oxford Dictionary of ChemistrySixth EditionPublished 2008Edited by John Daintith[2]

http://upload.wikimedia.org/wikipedia/en/thumb/4/42/Crazes1.jpg/417px-Crazes1.jpg[3]

http://thenovocastrian.files.wordpress.com/2012/07/urbanisation.jpg