Plastics (Polymers)

Plastics (Polymers)The word plastics is from the Greek word Plastikos, meaning able to be shaped and molded.

Aplasticmaterial is any of a wide range ofor semi-syntheticorganicsolids that aremalleable .Plastics are typically organic polymersof highmolecular mass, but they often contain other substances. They are usually synthetic, most commonly derived frompetrochemicals, but many are partially natural.

PolymerisationPolymers are formed when small molecules known as monomers join togetherAll the monomers in this kind of polymerisation must contain C = CEach small monomer becomes a part of the polymer

There are two main groups of plastics:thermosetting plasticsthermoplastic plastics.The word plastic means easily shaped or moulded.

Thermoplastic plastics:

Formed by heatMade of long polymer chains with few cross linksMostly recyclableSoften when heatedCan be reheated and reshapedThermoplastic materials soften on heating so can be moulded into new shapesThermosetting plastics:

Initially set by heatCannot be reshaped once setCommon in powder or resin formsConsist of polymer chains with strong bonds between each chainStrong and durableThermosetting plastics that do not soften when heated, so cannot be remoulded. Elastomer:Anelastomeris apolymerwithviscoelasticity ( having bothviscosityandelasticity) and very weak inter-molecularforces, generally having lowYoung's modulusand high failure straincompared with other materials. The term, which is derived fromelastic polymer, is often used interchangeably with the termrubber, although the latter is preferred when referring tovulcanisates. Each of themonomerswhich link to form the polymer is usually made ofcarbon,hydrogen,oxygenand orsilicon. Elastomers areamorphouspolymers existing above theirglass transition temperature, so that considerable segmental motion is possible. Atambient temperatures, rubbers are thus relatively soft (E~3MPa) and deformable. Their primary uses are forseals,adhesivesand molded flexible parts.

Examples of elastomers:

Unsaturated rubbersthat can be cured by sulfurvulcanization:

Natural polyisoprene: cis-1,4-polyisoprenenatural rubber(NR) and trans-1,4-polyisoprenegutta-percha

Synthetic polyisoprene (IR forIsopreneRubber)Polybutadiene(BR forButadiene Rubber)Chloropenerubber (CR),polychloroprene,Neoprene, Baypren etc.Saturated rubbersthat cannot be cured by sulfur vulcanization:

EPM (ethylene propylene rubber, a copolymer ofethyleneandpropylene) andEPDM rubber(ethylene propylene diene rubber, a terpolymer of ethylene, propylene and adiene-component)

Epichlorohydrinrubber (ECO)

Polyacrylic rubber (ACM, ABR)

Silicone rubber(SI, Q, VMQ)Various other types of elastomers:

Thermoplastic elastomers(TPE)TheproteinsresilinandelastinPolysulfiderubberElastolefin, elastic fiber used in fabric production

Properties of PlasticsFlexible you can squeeze the shampoo out of the bottle

Watertight the shampoo will not leak

Shatterproof the bottle wont break if dropped.

Light easy and cheap to transport

Easily moulded can make plastic into complicated shapes

Heat insulators plastic vending machine cups.

Durable, non biodegradable

Electrical insulators plugs and sockets are made of plastic

The majority of common plastics are thermoplastics. Thermoplastics can be heated and reshaped because of the ways in which the molecules are joined together.This can be repeated many times (as long as no damage is caused by overheating).

HeatCoolHardenSoftenThermoplastic plastics are made of long chains of polymers which dont cross over very often. When heated, the molecules slip easily over one another.

individual monomer moleculeThermosetting plastics also have lots of long chain molecules, but there are links between them. These cross links prevent the molecules from moving over one another.

individual monomer moleculeTHERMOSETTINGThe earliest synthetic polymer was developed in 1906, called Bakelite.Polymers are long-chain molecules and are formed by polymerization process, linking and cross linking a particular building block (monomer, a unit cell).The term polymer means many units repeated many times in a chainlike structure.

Popular PlasticsPolyethylene (LDPE (low density) and HDPE (high density)Properties: good chemical and electrical properties, strength depends on compositionApplications: bottles, garbage cans, housewares, bumpers, toys, luggageAcetal (Delrin)Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicals

Applications: mechanical components; gears, bearings, valves, rollers, bushings, housings

ABSProperties: dimensionally stable, good strength, impact and toughness properties, good resistance to abrasion and chemicalsApplications: automotive components, helmets, tool handles, appliances, boat hulls, luggage, decorative panelsUSES OF PLASTICS A chair made with a polypropylene sea Polyester(PES) Fibers,textiles Polyethylene terephthalate(PET) Carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging. High-density polyethylene(HDPE) Detergent bottles, milk jugs, and molded plastic cases.High impact polystyrene(HIPS) -: Refrigerator liners, food packaging, vending cups.

Polyamides(PA) (Nylons) Fibers, toothbrush bristles, tubing,fishing line, low strength machine parts: under-the-hood car engine parts or gun frames.

Acrylonitrile butadiene styrne (ABS) Electronic equipment cases (e.g., computer monitors, printers, keyboards), drainage pipe.

Polyethylene/Acrylonitrile Butadiene Styrene (PE/ABS) A slippery blend of PE and ABS used in low-duty dry bearings.

Polycarbonate(PC) Compact discs,eyeglassesriot shields, security windows, traffic lights, lenses.

Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) A blend of PC and ABS that creates a stronger plastic. Used in car interior and exterior parts, and mobile phone bodies.Polyurethanes(PU) Cushioning foams, thermal insulation foams, surface coatings, printing rollers (Currently 6th or 7th most commonly used plastic material, for instance the most commonly used plastic in cars).

Low-density polyethylene(LDPE) Outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging.

Polypropylene(PP) Bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers),plastic pressure pipe systems.Molding (process)

Moldingormouldin(seespelling differences) is the process ofmanufacturingby shaping liquid or pliable raw material using a rigid frame called a mold or matrix. This itself may have been made using a pattern or model of the final object.Amoldormouldis a hollowed-out block that is filled with a liquid or pliable material likeplastic,glass,metl, orceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is the counterpart to acast. The very common bi-valve molding process uses two molds, one for each half of the object.Piece-moldinguses a number of different molds, each creating a section of a complicated object. This is generally only used for larger and more valuable objects.

Types of molding include:

Blow moldingPowder metallurgyplussinteringCompression moldingExtrusion moldingInjection moldingLaminatingMatrix moldingRotational molding(or Rotomolding)Spin castingTransfer moldingThermoforming ,Vacuum forming, a simplified version of thermoforming

Rotational MoldingA predetermined amount of plastic, powder or liquid form, is deposited in one half of a mold.The mold is closed.The mold is rotated biaxially inside an oven.The plastics melts and forms a coating over the inside surface of the mold.The mold is removed from the oven and cooled.The part is removed from the mold.

Advantages

Molds are relatively inexpensive.Rotational molding machines are much less expensive than other type of plastic processing equipment.Different parts can be molded at the same time.Very large hollow parts can be made.Parts are stress free.Very little scrap is produced

Rock and roll machineVertical wheel machine

Shuttle machine

Turret machine