SOLIDS, LIQUIDS, GASES AND PLASTIC (POLYMERIC)
Since the earliest days of the evolution of mankind, the main distinguishing feature between human beings and other mammals has been the ability to use and develop materials to satisfy our human requirements. Woven cloths took the place of animal skins and manufactured goods became increasingly more sophisticated. Nowadays we use many types of materials, fashioned in many different ways to satisfy our requirements for housing, heating, furniture, clothes, transportation, entertainment, medical care, defence and all the other trappings of a modern, civilized society.
Engineers can play an important part in this conservation of material resources by a understanding of the materials used. This understanding is necessary in order to enable them to select the most appropriate materials and to use them with the greatest efficiency in minimum quantities whilst causing minimum pollution in their extraction, refinement and manufacture.
SOLIDS, LIQUIDS AND GASES
Most substances can exist as solids, liquids or gases depending upon their temperature. A notable exception is iodine which sublimes directly from the solid state into the gaseous state when heated without becoming a liquid. Most substances behave like water. Below its freezing point water is a solid (ice). Above its freezing point and below, its boiling point is in the liquid state. If its temperature is increased still further it boils and becomes vapour (steam) before turning into a gas with further heating.
The fact that a substance can exist in the solid state, the liquid state and the gaseous state is due to the fact that the atoms and molecules of substances are in a permanent state of vibration, providing the temperature is above absolute zero (-273 oC), at which temperature, all atomic and molecular movement stops.
When the temperature is low for a given substance to be in its solid state, the vibration is of small amplitude and the atoms and molecules only move to a small extent about a fixed point. When the temperature of a solid is raised to above its melting point, the atoms and molecules of the substance vibrate more violently. They no longer move about a fixed position but are free to move about within the constraints of the container holding the liquid. Finally, if the temperature is raised still further until it is above the temperature of vaporization for substances, the atoms and molecules move so freely that
they can disperse until they completely fill the vessel containing them and if they escape they continue to disperse throughout the atmosphere indefinitely.
This is due to a change in state being accompanied by the taking in or the giving out of latent heat – that is, the heat energy associated with a change of state without an accompanying change of temperature. Heat energy which causes a change of temperature without a change of state is referred to as sensible heat.
POLYMERIC (PLASTIC) MATERIALS
There is an ever-increasing number of synthetic, polymeric materials available under the popular name of plastics. This is a misnomer since polymeric materials rarely show plastic properties in their finished condition. In fact many show elastic properties. The name ‘plastics’ comes from the fact that during the moulding process by which they are shaped, they are reduced to a plastic condition by heating them to just above the temperature of boiling water.
The properties of plastic materials vary widely depending upon composition. A snooker ball is made from hard plastic such as melamine-formaldehyde and has obviously different properties from the soft plastic insulation of a flexible electric cable. For this latter application a plastic material of very different composition is used such as polymerized vinyl chloride (PVC). There are three main groups of polymeric or ‘plastic’ materials which are :-
i. Thermosetting Plastic (Thermosets)
Thermosetting Plastic (Thermosets)
This group of polymeric materials undergoes chemical change during the moulding process and can never again be softened by reheating. These materials are generally hard, rigid and rather brittle. A typical example is melamine formaldehyde used for making such articles as snooker and billiards balls, table-wear and domestic electrical fittings. The strength of thermosetting plastics can be greatly increased by reinforcing them with fibrous materials.
These become soft and can be remoulded each time they are reheated. They are not so rigid as thermosetting plastics but tend to be tougher. For example, rigid polymerized vinyl chloride (PVC) is used for rain water guttering and down-piping on buildings. Thermoplastics can also be soft. For example, the non-rigid PVC is used for the insulation of flexible cables.
The elastomer, or rubbers are cross-linked polymeric materials in which they are not in sufficient to make them as rigid as the thermosetting plastics, but one just sufficient to make them return to their original dimensions when the deforming load is removed. Elastomers are usually addition polymerized as thermoplastics and then vulcanised with sulphur at approximately every five-hundredth carbon atom. Increasing vulcanization is increase the stiffness and reducing the elongation properties of the materials.
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