Theword plastic originates from ‘plastikos’ which means to mold in Greek.
Thecapability to preserve deformation, after the removal of the load that producedthis deformation, is called Plasticity, as we saw in The Properties of Metals. Before the use of synthetic plastic materials, translucentslices of animal horns were used for lantern windows. The Palaquium Gutta tree possessedlatex called gutta-percha, which was used extensively for cable insulation. Thelac bug’s secretion called shellac was also used. The first synthetic plastic was developed by John Wesley Hyatt.It was a cheap replacement for ivory. Created from plant cellulose, it wasnamed Celluloid and patented in 1869. JohnWesley Hyatt (1837 – 1920)Leo Henricus Arthur Baekeland foundedthe modern plastic industry by developing a material named Bakelite.
The firstcompletely synthetic plastic material. It was created from phenol-formaldehyde.It is quite brittle and generally used with mica, asbestos or wood flour asfiller material. Generally employed in electrical moldings, wire insulation,brake pads etc.
Leo Baekeland (1863 – 1944)However,nowadays the word plastics has become a generic name used to define carbonpolymers (molecules with long chains), whether synthetic or natural, that cantake any shape by being cast, pressed or molded. They can also be employed asfibers when elongated into filaments. Thermoplastic and Thermosetting compositions are two importantgroups of plastics but Elastomers, which are synthetic rubbers, are alsoaccepted as a component of the plastic industry.
Thermoplastic materials become soft and malleable when heated,hence their prefix thermo- (which is Greek for heat), but in contrast they arehard in their normal state. Thermoplastic materials moldable and they can begiven any desired shape when softened. Once cooled, they preserve their new shape.This procedure can be applied again and again as long as their heat limit isn’texceeded.
Aircraftwindshields and side windows are made from two kinds of transparentthermoplastic material, which are generally called cellulose acetate andacrylic. Cellulose acetate plastic was used in outdated aircraft becauseof its light weight and transparency but it has some disadvantages like itsinclination to shrink and become discolored over time, thus it has been almostbeen completely phased out. Particularlywhen aged, it can be recognized by its tint which is slightly yellow and alsothe black smoke and sputtering it causes while burning. Some materials likeacetone will also cause it to react and soften upon contact. Thetrade names used to identify acrylic plastics is Perspex in the UK isPlexiglass in the USA. Compared to cellulose acetate it is more transparent,stiffer and basically without color. If burned, it has a clear flame and has arather pleasant smell. Although application of acetone may leave white stains,it will have no impact on the material’s hardness.
Specifictree extracts, as well as other plants are used to obtain natural resins. Theyare used in lacquers, linoleum, adhesives and food glazing agents thanks tobeing translucent, clear, amber, brown and solid/semi-solid agents. As expectedthey are also used in plastics.
Althoughresins and plastics are words that are used interchangeably, in actuality theyare very different. Plastics are used to point out the material in thecompleted items and resins are usually in flake, pellet, syrup or powder form.They are the raw materials. Althoughresins may be used by themselves to create plastic, generally some additivesare used to support the molding quality of raise the quality of the endproduct. Thesupplement of inert fillers, in order to condense the resin, can be used tofill out holes and cracks in the structure.
Cotton, glass flock, micro balloonsand fumed silica (aerosol) are common filler materials. Othertypical additives used with resins are stabilizers, flame retardants, antioxidants,antimicrobial, thickeners,anti-stats,and impact modifiers. Lubricants and coupling agents are used as processingaids in addition to various other additives. Resins alone lack strength so various synthetic fibers or’cloths’ like paper and linen are used in conjunction. Resin is used toimpregnate these fibers.
Aircraft control cable pulleys are, for a very longtime, constructed from thermosetting resins, reinforced with layers of linencloth. Using high temperatures, these pulleys are cured in a mold and thuspossess high strength and don’t damage to the control cables. As thermosetting resin like phenol-formaldehyde or urea-formaldehydeis used to impregnate layers of paper, it can be molded into flat sheets or anyother shape. This material is used as terminal strips or printed circuit boardssince it becomes an extraordinary electrical insulator when hardened. Polyesterresin can be cast into different shapes or woven into fabric such as nylon whenpressed into fine filaments. It can also be used as a lacquer that is heatresistant.
Fortheir weight, glass fibers and mat for instance possess great strength but don’thave enough rigidity. Thus they are impregnated with polyester resin and moldedinto any desired form in order to be used as structural materials. Polyesteris distinct from other materials which cure by evaporation of asolvent or oil, since polyester cures by chemical action. A styrene monomer isadded to the polyester to make it easier to work on by thinning it because it’susually thick and unmanageable. Inhibitors are used to delay the mixture of polyester andstyrene from curing into a solid mass, which it does eventually when leftuntouched, thus these inhibitors improve shelf life. When the inhibitors aren’t wanted any longer and the curingprocess will be started, a catalyst is needed. Depending on the mass of theresin and temperature, an accelerator will of course reduce the curing time ofthe resin. When the catalyst and accelerator produces heat within the resinthrough a chemical reaction, the actual cure of the polyester resin takesplace.
When a thick layer cures faster than a thin layer, this exothermicreaction can be observed. The chemical reaction’s heat may cause the material to ‘rundown’, especially if placed vertically, since it becomes less viscous. In orderto increase viscosity, a thixotropic agent is combined with the resin. Thisgain in viscosity lets it stay in place. In laminated structures, epoxy resin can also be employed,instead of polyester.
Small percentage of shrinkage, capability to adhere tomany types of materials and strength to weight ratio are importantcharacteristics of epoxy resin. Epoxy resins need a curing or hardening agent without therequirement for heat, which is in contrast to polyester resins that need a catalyst. Epoxy and polyester resin mixing ratios also vary. In order toharden, polyester resin has a ratio of 64:1 resin to catalyst ratio while it is4:1 in the case of epoxy resin. Traditional materials such as wood, metal and natural rubbers are beingsucceeded by plastics.
Its use is ever expanding. The properties plastics have,compared to regular aircraft materials, make them a better choice. Theirpopularity can be explained by the
