However, for applied use, several simpler classifications have been devised, grouping several elements and compounds. For instance Ashby uses the following diagram in his classification:
Classification of the Material World
In order to come to grips with the almost unlimited variety of matter,
it is common, for practical purposes, to divide the material world into
a manageable number of classes. One of the great achievements of science
has been the discovery of the periodic
table of elements, which is the "keyboard" for all material science,
physics and chemistry.
However, for applied use, several simpler classifications have been
devised, grouping several elements and compounds. For instance Ashby
uses the following diagram in his classification:
If the periodic table of elements is looked at as the 'Bible', then Ashby's and many other classifications can be looked at as an organization analogous to the Books of the Bible.
It is worthwhile to look at the role composites play in this organization. The five material groups arranged around the central composite core of the pentagon all have limits specific to their 'nature'. By skillfully combining two or more of the five perimeter materials, it is possible to create 'new' materials with very favourable qualities.
Ashby places these materials groups in an illuminating historical perspective
in the following diagram:
This graph shows in a glance a number of interesting trends: It shows that at the first recorded labour strike in history, when in Old Testament |Egypt the Jews brought on the wrath of the Pharao by not producing their quota of straw and clay for the fabrication of straw reinforced mud bricks, this material was quite popular. It also shows that material use by a population is always variable, depending on demand, availability, technology, and the balance between cost and benefit. For instance, the supremacy of metals over all other groups in the late 1950's can be explained by the booming post-WWII economy which created a high demand, fueled by plentiful supplies and ready technology 'left over' from the war. In addition, there were no environmental laws in place to curtail production of metals, nor the use of metal products, like heavy and polluting automobiles. Materials use around the end of the 20th century , has changed dramatically by an evolving technology in a developing world. Driven by the push into aero-space, technology and industry now search for high strength/weight ratios, as well as resistance to very high temperatures. In a very short time this search has produced dramatic developments in alloyed metals, plastics, ceramics and composites. The energy crisis of the 1970's combined with the newly prominent environmental movement made oil suddenly expensive and pollution suddenly a real variable. As a result, high-strenght, low weight materials are increasingly replacing bulky mild steel and cast iron components.
For the purposes of this project Ashby's model of six sections is simplified to four:
Polymers/ Elastomers, Metals, Ceramics/Glasses and Composites.
The performance of the materials in these groups can be roughly evaluated
as follows:
| Relative cost | Density | Elasticity | Strength | corrosion -
resistance |
thermal conductivity | Electrical conductivity | thermal expansion/contraction | |
| Polymers/
Elastomers |
low | low | very high/
extremely high |
high/very high | high/very high | very low | very low | very high |
| Metals | high | high | high | high/very high | very high (gold)
very low (iron) |
high/very high | high / very high | low /moderate |
| "old" Ceramics
"new" Ceramics |
low
high |
very high
varies |
low
varies |
high (in compression only!)
very high |
very high
very high |
very low
but high in some 'new' ceramics |
very low
varies |
low
low |
| Composites | varies | moderate/
low |
depends, can be high | very high
in compression+tension |
very high | depends | depends | depends |
A basic overview of the four basic materials groups
Some of the information in this section has been taken
from the excellent MIT site which is no longer available.
Polymers
 |
 |
| 1929 bakelite phone- a classic example
of man-made polymer use |
arrangement of cut lumber in a tree trunk- a classic
example of natural polymer use |
natural polymers, such as wood, wool, silk, leather . It must be realized that wood has partly composite qualities the way it combines lignin and keratin. An important new development in polymers is the 'reconstituting' of natural polymers, in particular wood products. Plywood, chip board, particle board, hard board, and even paper are ways of changing the nature of wood, by slicing, shredding or grinding wood into small elements, and reconstituting it by gluing the material in layers or compacting it in random order. This process 'homogenizes' the wood in that it distributes any natural unevenness, turning anisotropic wood ( wood has very different properties along the grain from across the grain) into a largely isotropic material ( some plywoods have 'strong' and 'weak' directions as a result of the orientation of the face veneers; particle boards are by nature homogeneous)
artificial polymers
An excellent short overview of the history, current use and future of
plastics is given by the Association of Plastics Manufacturers in Europe
(APME) The following
diagram is taken from that source, indicating the process of
plastics manufacture.
All polymers, natural or artificial, consist of large molecules built up by the repetition of small, simple chemical units called monomers. In some cases the repetition is linear, much as a chain is built up from its links. In other cases the chains are branched or interconnected to form three-dimensional networks. The length of the polymer chain is specified by the number of repeat units in the chain. This is called the degree of polymerization (DP). The molecular weight of the polymer is the product of the molecular weight of the repeat unit and the degree of polymerization. Most high polymers useful for plastics, rubbers or fibers have molecular weights between 10,000 and 1,000,000.
In some types of plastics, polymerization produces cross linking between the long chain molecules. Cross linkage produces thermosetting plastics which are hardened permanently by heat. These plastics will remain permanently hard and will not soften upon subsequent heating. Examples of thermosetting plastics are polyesters, amines, and urethanes.Plastics which are not cross linked are known as thermoplastics. Thermoplastics can be softened upon heating and hardened upon cooling; this cycle can be repeated indefinitely. Examples of thermoplastics are poly amides, polyethylene's, and polystyrenes. Another type of polymers are the elastomers. Elastomers such as rubber can be stretched to many times their initial length and still spring back to their original length when released.
The formation of larger molecules from smaller ones is known as polymerization. The processes of polymerization are divided into two groups:
In condensation polymerization, condensation takes place between two poly functional molecules to produce one larger poly functional molecule, with the possible elimination of a small molecule such as water. The reaction continues until almost all of one of the reagents is used up; an equilibrium is established which can be shifted at will at high temperatures by controlling the amounts of the reactants and products.
Two types of condensation polymers are Nylon and Kevlar.
Kevlar is an aromatic nylon prepared from terephthalic acid. Aromatic polyamids (aramids) are high melting, high in thermal stability, and generally have high performance properties. Kevlar by Dupont melts at over 500 degrees Celsius, and is exceptionally high in strength. It finds use in heavy duty conveyor belts, and in composite structure with casting resins such as epoxies. It competes with steel in radial tire reinforcement.
Polyester
Polyester has a high melting point, good mechanical properties to about 175 degrees Celsius, and good resistance to solvent and chemicals. Polyester fibers have good crease resistance, low moisture absorption, and good resistance to abrasion. Polyester film has high tensile strength, high resistance to failure on repeated flexing, fair tear strength and high impact strength. Some typical uses of polyester are in garments, rope, fish nets, and high grade films.
Poly(ethylene terephthalate) (PET) is the best known polyester being used as a film and as a fiber. It can be rapidly prepared from the acid chloride using aqueous interfacial system. PET is blow molded to produce soft drink bottles. It is also used for a variety of things such as distributor caps, fender extensions, home appliances, plumbing components and sports equipment.
The phenylene groups in PET provide stiffness and the two methylene groups in the chain provide a small degree of flexibility.
Addition or Chain Reaction Polymerization
Polytetrafluoroethylene, or Teflon, is a linear polymer like polyethylene, the only difference being that instead of hydrogen atoms there are fluorine atoms. It has a very high crystallinity as manufactured - about 90 percent. Teflon is insoluble in most solvents, uniquely non adhesive, and has low friction properties. Teflon has constant electrical and mechanical properties from 20 to about 250 degrees Celsius, and it has high impact strength. Teflon is typically used as coatings for frying pans, for wire and cable insulation, and insulation for motors and generators.
Polyethylene is a polymer in which branching has been accomplished. As a result of branching, side groups become attached to the main chain. Branching leads to a decrease in crystallinity, lowered density, and impaired stiffness. Branched, low density polyethylene has good toughness and pliability. It has outstanding electrical properties, it is resistant to acids and bases, and has high tear strength. Branched polyethylene is used for films, drapes, table cloths, squeeze bottles, and coatings for foil.
Linear, high density polyethylene has high crystallinity and high melting temperature. Linear polyethylene has a greater hardness and tensile strength than branched polyethylene. It is used in bottles, house wares, toys, pipes, and wire and cable insulation.
Polystyrene is clear, easily colored, and has good resistance to acids and bases. The repeat unit of polystyrene is -CH2-CH-C6H5-. C6H5, a phenyl group, is a benzene ring with one hydrogen atom removed. Polystyrene is available in many forms. General-purpose polystryrene is polystyrene which either has no additives or additives other than rubbers or copolymers. It is a brittle, transparent material with a smooth surface finish that can be printed on. It is used for injection-moulding containers for cosmetics, boxes and ball-point pen barrels. Toughened or high impact polystyrene is a blend of polystyrene with rubber particles. This blending improves the impact resistance but results in a decrease in tensile modulus, tensile strength and transparency. This blend of polystyrene is used to produce cups for vending machines and casings for cameras, projectors, radios, television sets and vacuum cleaners. Polystyrene are attacked by many solvents such as dry cleaning agents, greases, oxidizing acids and some oils. Detergents can lead to stress cracking.
Metals are chemical elements which form solids that are opaque, lustrous,
good conductors of electricity and heat, and when polished, good reflectors
of light. Most metals are strong, ductile, and malleable, and, in general,
of high density. Metals are the primary structural materials of technology,
and are often divided into two groups:
ferrous metals ( cast iron, plain carbon steels, alloy steels).
Quebec Bridge built
of steel
an Indian necklace made with
gold
Metallic crystals consist of positive ions immersed in a "gas" of negative electrons. The attraction between the positive ions and the negative electrons holds the structure together and balances the repulsive forces between the ions and between the electrons. The electrons move freely through the lattice and provide good electrical and thermal conductivity. This is why most metals feel cool. Alloying elements together has led to a large variety of metallic materials that can be designed for a specific property, at least partially realizing the dreams of alchemists..
Ceramics can be identified as any inorganic, nonmetallic solid, at least once processed at high temperatures.
"Everyone fears time, but time fears the pyramids" Arab saying
For the purpose of this classification we can divide ceramics in 'natural' and 'man-made' ceramics. Quarried stone, gravel, field stone are natural ceramics that have been used as materials and tools since the beginning of civilization. Almost as old are pottery, brick, tiles, table china produced by the skillful blending and firing of natural ingredients such as clay, sand and glazes. Ceramics also include materials such as glass, graphite, and cement (concrete). The durability of natural ceramics such as stone are well known, and have been part of the language in all cultures. ( "On Peter I have built my Church" etc...) In this classification all the 'classic' ceramics have been grouped under 'old ceramics'
A very important group of ceramics, the 'new' ceramics are "high tech" applications of oxides, carbides and nitrides. Many of these are of great industrial interest. This is a special and fast growing category of non-metallic inorganic material. These materials are the building blocks of transistors, solid electronics and computers. "Silicon Valley" is named not after a natural resource found there, but after the industry that has sprung up around the new semiconductor technology. The physical and mechanical properties of ceramics stem from their atomic binding and crystal structure. The binding can be ionic or covalent, or intermediate between the two. The absence of free electrons is responsible for making most ceramics poor conductors of electricity and heat. The crystal structures of ceramics are many and varied. The structures are often of low symmetry, which gives some of these materials interesting electromechanical properties. For example, piezoelectricity, the formation of a static charge upon elastic deformation, is used to make sensors and transducers.
Some examples of 'modern' ceramics are:
Silicate Ceramics, Nitride Ceramics, Ferroelectric Ceramics, Superconducting Ceramics.
COMPOSITES
COMPOSITE MATERIALS such as the reinforced concrete bridges shown above are a class of materials that combine two or more separate components into a form suitable for structural applications. While each component retains its identity, the new composite material displays macroscopic properties superior to its parent constituents, particularly in terms of mechanical properties and economic value.
Composites can combine materials of very different groups: straw and mud , steel and concrete, glass fiber and epoxy, the list is ever lengthening.
The combining of high tensile strands and a rigid matrix can be made even more profitable by pre-tensioning or post-tensioning the strands. This is done routinely in high performance concrete construction, and has many advantages. It reduces or even eliminates deflection cracks that are inevitable in non pre-stressed concrete. These cracks often lead to corrosion of the reinforcing bars.
Continue with Chapter 2