polymer & plastic tech

POLYMER TECHNOLOGY

POLYMER TECHNOLOGYPOLYMER literally means many(POLY) units (MER). Polymers are composed of a large number of repeating units(small molecule) called MONOMER.

The structure of polymers2

Polymers Structures 3Bonding monomers are linked together by covalent bonds, forming a polymer chain (primary bonds). The polymer chains are held together by secondary bonds. The strength of polymers comes in part from the length of polymer chains. The longer the chain, the stronger the polymer. More energy is needed to overcome the secondary bonds. Linear polymers

A sequential structure resulting in thermoplastics like nylon, acrylic, polyethylene. A linear polymer may contain some branched and cross-linked chains resulting in change in properties. Branched polymersSide branch chains are attached to the main chain which interferes with the relative movement of the molecular chains. This results in an increase in strength, deformation resistance and stress cracking resistance. Lower density than linear chain polymers.

Polymers Structures 4Cross-linked polymers

Three dimensional structure, adjacent chains are linked by covalent bonds. Polymers with cross-linked chains are called thermosetting plastics (thermosets), epoxy and Silicones.Cross-linking is responsible for providing hardness, strength, brittleness and better dimensional stability.

A three dimensional network of three or more covalent bonds. Thermoplastic polymers that have been already formed could be cross-linked to obtain higher strength. Polymers are exposed to high-energy radiation.Network polymers

CLASSIFICATION OF POLYMERBasis of classificationPolymer typesOriginNatural, Semisynthetic, SyntheticThermal responseThermoplastic, ThermosettingMode of formationAddition, CondensationLine structure Linear, Branched, Cross-linkedApplication & physical propertiesRubbers, Plastics & FibresTacticityIsotactic, Syndiotactic, atacticCrystallinityNon-crystalline(amorphous), Semi-crystalline, crstalline

Important definitionsCopolymers : Polymers whose molecules are made of repeating units of two different types. Also possible are terpolymers , which consist of mers of three different types. (ex. ABS or acrylonitrile-butadiene-styrene)Elastomers (rubbers) :exhibit extreme elastic extensibility when subjected to low mechanical stressPlastics :Polymers compounded with various additives.Stereoregularity :Concerned with the spatial arrangement of the atoms and groups of atoms in the repeating units of the polymer moleculeCrystallinity: Amorphous and crystalline structures are possible with polymers, although the tendency to crystallize is much less than for metals and nonglass ceramics. Degree of crystallinity (the proportion of crystallized material in the mass)As crystallinity is increased in a polymer, so doesDensityStiffness, strength, and toughnessHeat resistance

CrystallinityCrystallization in linear polymers involves the folding back and forth of the long chains upon themselves to achieve a very regular arrangement of the mers

A number of factors determine the capacity and/or tendency of a polymer to form crystalline regions within the material.As a general rule, only linear polymers can form crystalsStereoregularity of the molecule is criticalCopolymers, due to their molecular irregularity, rarely form crystalsSlower cooling promotes crystal formation and growth

Crystallinity

Thermoplastic PolymerWhen heated under decomposition temperature it becomes soft & easily deformed to desired shape. These polymers can be heated & cooled any number of times. Hence it has a resale or scrape value. Some commercial thermoplastics are Polyethylene, Polyvinylchloride (PVC), Poly styrene, Polytetrafluoroethylene (PTEE).

Thermosetting polymerThermosetting polymers become soft during their first heating & become hard during cooling. They do not soften during subsequent heatings & rather become harder due to completion of cross linking because any left over functional groups in polymer. Hence they do not have a resale or scrap value. Some commercial examples are Phenolics, Aminos, Polyesters, Epoxies.

Polymerization The process of linking together of monomers. Polymerization mechanisms mainly two types Addition Polymerization & Condensation polymerizationADDITION POLYMERIZATIONCONDENSATION POLYMERIZATIONRequires unsaturated molecules.Requires two unlike molecule.It does not yield by-productIt yields a by product.Reaction is very fast & take few seconds.Reaction normally takes hours & days to complete.It is kinetic chain reaction.It involves intermolecular reaction.Polythylene,Polyvinylchloride (PVC),Polypropylene(PP),Teflon, Polystyrene, Acrylics are produced by this.Bakelite(Phenol Formaldehyde), Amine Formaldehyde, Polysters, Epoxy Resins (epoxies) are produced by this.

COMPARISON

General layout of polymer production

13Plastics (Polymers)

The word plastics is from the Greek word Plastikos, meaning able to be shaped and molded

Why Design with Plastics?14

CostRelatively low cost compared to metals and compositesDensity

Light weight, high weight to strength ratio, particularly when reinforced

Why Design with Plastics?Corrosion resistanceLow electrical and thermal conductivity, insulator Easily formed into complex shapes, can be formed, casted and joined.Wide choice of appearance, colors and transparencies15

Disadvantages of using PlasticsLow strengthLow useful temperature range (up to 600 oF)Less dimensional stability over period of time (creep effect)Aging effect, hardens and become brittle over timeSensitive to environment, moisture and chemicalsPoor machinability16

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Mechanical Properties of Various Plastics18

Steel: 350 to 1900 MPaBrass: 200 to 850 MPaAluminum: 100 to 550 MPa

Materials PlasticsThermoplasticThermoplastic propertiesThermoplastic usesExample outcomesAcrylics or Perspex (Polymethyl Methacrylate)Stiff, hard, shiny, brittle in small sections, durable, scratches easily, available in different colours, good electrical insulatorUsed for signs, key rings, lighting, storage containersHigh impact polystyrene (HIPS)Light but strong plastic, available in sheets in a variety of thicknesses and colours and softens at about 95 degreesUsed for vacuum forming and making outer casings and packaging for products AcetateHard, shiny and translucent/transparentUsed in badge-making, packaging and for overhead projector transparenciesExpanded polystyrene (styrofoam)White, lightweight and crumblyUsed for protective or insulating packaging

Material systems which can be conveniently to flow or deform into a desire shape or form, usually under heat and/or pressure, are commonly known as plastic.There are two main classes of plastics: Thermosetting plastics that can only be heated and be moulded once. If re-heated they cannot soften. Thermoplastics plastics that are moulded by heating and can be remoulded if heated again. Thermoplastics soften when heated and can be shaped when hot. The plastic will harden as it cools down.Materials - Plastics

ThermoplasticThermoplastic propertiesThermoplastic usesExample outcomesPolypropylene (PP)Light, hard, flexible but can scratch easily. Durable to wear and has good resistance to chemicalsUsed for different kinds of packaging, chairs, gadgets, textiles and automotive componentsLow density polyethylene (LDPE)Soft, flexible, good resistance to chemicals, good electrical insulator, toughUsed for packaging film, carrier bags, bottles, toysHigh density polyethylene (HDPE)Hard, stiff and strong. Able to be sterilisedUsed for plastic bottles, tubing and household equipmentCorrugated plasticLightweight, rigid and weatherproofUsed for sign boards and folders

Low-tack masking filmFlexible and transparentUsed to position sticky-backed vinyl letters or images onto a chosen surface. Used for creating signs, stencils and vehicle signagePolyvinyl chloride (PVC)Stiff, hard wearing, brittle but can be treated to make it softer and more rubberyUsed for blister packs, window frames, records and clothing

Materials - Plastics

Plastics History

CLASSIFICATION OF PLASTICS

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Thermal & Mechanical Properties

Stress vs. Strain curvePlastics

Applications of Thermoplastics24Design requirement: strengthApplications: Valves, gears, cams, pistons, fan blades, Plastics: nylon, acetal (delrin), polycarbonate, phenolicDesign requirement: wear resistanceApplications: bearings, gears, bushings, wheels, .Plastics: nylon, acetal (delrin), polyurethane, phenolic, polymide

Applications of Thermoplastics25Design requirement: functional and decorativeApplications: knobs, handles, cases, moldings, pipe fittings, Plastics: ABS, acrylic, polyethylene, phenolic, polypropylene, polystyreneDesign requirement: hollow shapes and housingsApplications: pumps, helmets, power tools, cases, Plastics: ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonate

Design requirement: functional and transparentApplications: lens, goggles, signs, food processing equipment, Plastics: acrylic, polycarbonate, polystyrene, polysulfone

Popular Plastics26Polyethylene (LDPE (low density) and HDPE (high density)Properties: good chemical and electrical properties, strength depends on compositionApplications: bottles, garbage cans, housewares, bumpers, toys, luggageABSProperties: dimensionally stable, good strength, impact and toughness properties, good resistance to abrasion and chemicalsApplications: automotive components, helmets, tool handles, appliances, boat hulls, luggage, decorative panels

Acetal (Delrin)Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicalsApplications: mechanical components; gears, bearings, valves, rollers, bushings, housings

Popular Plastics27PolycarbonatesProperties: very versatile and has dimensional stability, good mechanical and electrical properties, high resistance to impact and chemicalsApplications: optical lenses, food processing equipments, electrical components and insulators, medical equipments, windshields, signs, machine componentsNylonsProperties: good mechanical and abrasion resistance property, self-lubricating, resistant to most chemicals but it absorbs water, increase in dimension is undesirableApplications: mechanical components; gears, bearings, rollers, bushings, fasteners, guides, zippers, surgical equipments,

Applications of Thermosetting Plastics28EpoxiesProperties: good dimensional stability, excellent mechanical and electrical properties, good resistance to heat and chemicalsApplications: electrical components requiring strength, tools and dies, fiber reinforced epoxies are used in structural components, tanks, pressure vessels, rocket motor casingPhenolicsProperties: good dimensional stability, rigid, high resistance to heat, water, electricity, and chemicals Applications: laminated panels, handles, knobs, electrical components; connectors, insulators

Applications of Thermosetting Plastics29Polyesters (thermosetting, reinforced with glass fibers)Properties: good mechanical, electrical, and chemical properties, good resistance to heat and chemicalsApplications: boats, luggage, swimming pools, automotive bodies, chairsSiliconesProperties: excellent electrical properties over a wide rang of temperature and humidity, good heat and chemical properties Applications: electrical components requiring strength at high temp., waterproof materials, heat seals

31ABSAcetal (Delrin)AcrylicCellulosicsFluoroplasticsNylonPhenylene OxidePolycarbonatePolyesterPolyethylenePolyimidePolyenylene sulfidePolypropylenePolystyrenePolysulfonePolyurethanePolyvinyl chloride

PhenolicPolyesterPolyurethane

Thermoplastics

ThermosetsStructural and mechanical Appl.Gears, cams, pistons, rollers, fan blades, rotors, pump impellers, washing machine agitatorsXXXXHandles, knobs, steering wheel, tool handles, pipe fittings, camera cases, eyeglass frames

XXXXXLight duty mechanical & decorativeXXX



Thermoplastics

ThermosetsParts for wear applicationsGears, bearings, bushings, tracks, wheels, ware stripsXXXXXXXXXLenses, safety glasses, signs, refrigerator shelves, windshields

XXXXXOptical and transparent parts



Thermoplastics

ThermosetsSmall housing & hollow shapesPhone and flashlight cases, helmets, housings for power tools, pumps, small appliancesXXXXXXXXXXBoat hulls, large appliance housings, tanks, tubs, ducts, refrigerator linersLarge housing & hollow shapes

XXXXXXXXX



Thermoplastics

ThermosetsSmall housing & hollow shapesXXXXXXXXXXLarge housing & hollow shapes

XXXXXXXXXParts for wear applicationsXXXXXXXXXXXXXXOptical and transparent parts

Light duty mech & decoStructural & Mechanical

XXXXXXXXXXXX

Plastic

Commercial Mass Plastics production.

Most modern plastics are derived from natural materials such as oil, coal and natural gas with crude oil remaining the most important raw material for their production. The starting point for the production process is the distillation, in petrochemical refineries, of the raw material into fractions (different parts).The heavy fractions give us lubrication oils and the heavy oils used for heating fuels. The lighter fractions give us gas, petrol, paraffin and naphtha. The chemical building blocks for making plastics come mainly from naphtha.

Commercial Mass Plastics productionThe start of making plastics is to subject naptha to a cracking process in which complex organic chemical compounds are separated into smaller molecules, dependent on their molecular weight. These smaller molecules include ethylene, propylene, butene and other hydrocarbons. The compounds produced through the cracking process are then further refined to produce thebase plastic materials.

Schematic diagram of Cracking naptha.

Modifying polymers/ Compounding AdditivesPolymers are initially produced as powders, granules or laces (like straws). However they are seldom used in their pure form but are changed by adding or compounding with a host of additives to give them their special properties & achieve perfect rheological behaviour before moulding, forming, coating etc. ADVANTAGESReduction of material costImprovements in properties, performance & service life of end products.Achieving ease of processing & reduction of processing cost.Minimizing degradation during processing & use.

Brief description on additivesFILLERFillers are solid additives, used as loading agent with the objective of lowering material cost. Additionally improves physical properties such as tensile strength, modulus, abrasion resistance & tear strength(reinforcing filler). Different types of filler are in use viz, particulate, rubbery, resinous or fibrous.PLASTICIZERThese are added to plastic material to improve flow and processability, flexibility, ductility, toughness & to reduce the brittleness, hardness, stiffness of product. Usually, plasticizer is high boiling nonvolatile liquid having low vapour pressure & low molecular weight. It occupies the position between the large molecular chain & reduce secondary intermolecular bonding.

Brief description on additivesSTABILIZERDifferent stabilizers are used to protect them from degradation from during storage & processing and under performing condition. Common stabilizers are i) antioxidants, which protect the polymer against degradation of oxygen attack. These are two types a) Preventive antioxidants which prevent generation of radicals ex dilauryl thiopropionate. b) Chain Breaking antioxidants which function effectively by interrupting the propagation of the oxidative chain degradation ex. Styrenated phenols. ii) UV absorbers, which protect the polymer against degradation by UV rays.Chlorine containing polymers such as Poly Vinyl Chloride & related copolymers are thermally unstable & require stabilization and protection against thermal degradation. The stabilizer commonly used basic lead salts and selected cadmium & barium compound and organo- tin compound.

Brief description on additivesColourants.Plastics can be coloured by using either dyes or pigments. Dyes give transparent colours and pigments give opaque ones. A very wide range of colours can be obtained from clear to opaque black with a naturally clear material such as acrylic.Impact modifiers.Some plastics are designed to have great impact strength; the ABS used in the manufacture of car bumpers for example. Impact modifiers are polymers that when added to other polymers improve the impact resistance of a product. Impact modifiers, based on acrylic polymers, are used in conjunction with PVC in order to get clear rigid bottles with good impact strength.

Brief description on additivesBlowing agents.Many plastics products are cellular in structure. If you look at them closely they look like a chocolate Aero bar. These expanded or foamed products canbe made from base polymers by the addition of what are known as 'blowing agents'. This is done in the moulding process by adding a filler, which foams throughout the hot polymer filling it with bubbles. A good example of this is expanded polystyrene.Lubricants.These additives are widely used to help plastics flow in moulds. They are usually waxy materials that act by reducing the stickiness of the plastic on the mould surface.

Plastics Processing : General layout

Moulding Processing: Compression Molding

A carefully weighed amount of thermosetting polymer is placed into a preheated lower mould cavity. The mould is then closed by the placing of the upper half and subjected to further heat, and pressure provided by a press, often of several hundred tons capacity.The pressure and heat causes polymerisation and the flow of the plasticised material within the mould.

Moulding Processing : Transfer Molding.In transfer process the charge is melted in a separate chamber or in barrel called plasticizing pot or transfer pot which sometimes constitutesa part of heated mould. The melt is forced into the mould cavity through narrow, heated runner/gate flow channels by ram pressure or reciprocating screw plunger action. The mould is kept at a temperature high enough to cause rapid heat curing of the resin system. The transfer moulding also produces moulded pieces of higher density& mechanical strength.

(more complex shapes)

Moulding Processing : Injection molding

Similar to die casting metal. Pellets are fed into heated cylinder where they are melted. The screw rotates much like extrusion molder (it moves back as material in front of it are melted) then it rams forward pushing the melted material into the die. Most wide spread technique for making 3-D configurations. It uses either reciprocating screw or reciprocating plunger (RAM).It is rated by clamping pressure of die & is high speed moulding for thermoplastics. Defects in Injection Molding : Short Shots, Flashing, Sink marks and voids, Weld lines. Molds in injection molding are usually expensive and comple

Moulding Processing: Blow moulding.Blow moulding is a simple process where compressed air is introduced underneath a warmed sheet of thermoplastic material forcing the material into a mould cavity, or allowing it to expand freely into the shape of a hemisphere. It is a good way of forming large domes, which when made out of clear acrylic sheet are often used in shop displays. There are two basic type of technology i) extrusion blow moulding & ii) injection blow moulding.

Fig: Blow moulding

Moulding process: Extrusion blow moulding.This is a combination of extrusion and blow moulding and is often used where the article to be made has a narrow neck, such as a bottle. The plastic material is first extruded as a tube shape into an open die. The die is then closed to seal the ends of the tube and air is blown in forcing the plastic tube to take up the shape of the die cavity. As the material is extruded first and then blow moulded, the process is known as extrusion blow moulding.

Moulding process: Injection blow moulding & Foam MouldingIn injection blow moulding two moulds are used. A mandrel or blowing stick is placed in the first mould, and the thermoplastics material is then injected into the mould flowing around the mandrel to create a tube. This is then transferred to the second mould where air is introduced to expand it to the shape of the mould.A variation on this method is stretch blow moulding whereby the material is biaxially oriented to produce stronger products. This method is particularly used for the manufacture of PET bottles. Applications: Bottles and containers, Automotive fuel tanks,Venting ducts, Watering cans, Boat fenders etc.

Plastic foams used in packaging and the upholstery industry can be produced by using blowing agents that are mixed with the base polymer. When heated these agents release gas which form bubbles in the plastic. Another method is to inject compressed nitrogen gas into molten plastics during the moulding process. A third method is to freeze a gas within the plastics granules which then expands due in the heat of the moulding process.FOAM MOULDING

Moulding process: Rotational Moulding.Rotational moulding is used to produce hollow thermoplastic products such as drums, storage tanks and litterbins. A carefully calculated amount of plastic is placed in a closed mould that is heated in an oven and rotated slowly around both a vertical and horizontal axes. The plastic material fuses and sticks to the hot mould surface, building up the required thickness. The mould is then gradually cooled by air or water while still rotating. The mould is opened, the finished product removed and the mould reloaded and closed for the next cycle. The time it takes to make one of the product is known as the products cycle time

Rotational moulding.

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.52Rotational molding process consists of six steps

Rotational Molding Machines53

Rock and roll machineVertical wheel machine

Shuttle machine

Turret machine

Rotational Molding54AdvantagesMolds 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

Rotational Molding55LimitationsCan not make parts with tight tolerance.Large flat surfaces are difficult to achieve.Molding cycles are long (10-20 min.)MaterialsPolyethylene (most common), Polycarbonate (high heat resistance and good impact strength), Nylon (good wear and abrasion resistance, good chemical resistance, good toughness and stiffness).

Rotational MoldingPolycarbonate wall thickness is typically between .06 to .375 inches, .125 inch being an ideal thickness.

Polyethylene wall thickness is in the range of .125 to .25 inch, up to 1 inch thick wall is possible.

Nylon wall thickness is in the range of .06 to .75 inch.56Nominal wall thickness

Rotational Molding Examples57

Rotational Molding Examples58

Forming Process: EXTRUSION TECHNIQUEExtrusion is a process that can be compared to squeezing toothpaste out of a tube. Thermoplastic granules are forced through a heated barrel and the fused polymer is then squeezed through a die that is the profile of the extruded component. The extrusion is cooled by water or air as it leaves the die and is finally cut to the required length. The shape of the die can be varied from a simple hole with a centrally supported core to produce tubes such as pipes, to very complex sections for curtain tracks or hollow window frames.

Sheet Extrusion, Profile Extrusion, Pipe extrusion, Co-extrusion, Blown Film Extrusion, Cast Film Extrusion, Foam Extrusion, Pultrusion &Calendering

Different Plastic Extrusion

Defects in Extrusion : Melt Fracture, Sharkskin, Bambooing

Defects in Extrusion

Forming Process: Vacuum forming.This is a very common manufacturing process used, for example, to make a range of plastics packaging. Think of the boxes sandwiches come in, or the inner in a chocolate box, or your acrylic bath. It is really the opposite of blow moulding. Instead of the warmed plastic sheet being forced into a mould by air pressure, in vacuum forming the air is drawn out from under the softened plastic sheet, so it is forced over or into a mould by atmospheric pressure. Vacuum forming is a very common and effective way of producing complex shapes in thermoplastic sheeting.

Forming Process: Calendering.Calendering is used to produce plastic sheeting and products such as floor tiles, coated fabrics and coverings for car interiors. Fused thermoplastic is extruded on to heated rotating rollers that squeeze the material into a continuous sheet or film. The film is cooled by jets of air or water, before being cut to suitable lengths or loaded onto rolls.

Other TechniquesLaminating: Laminates consist of layers or laminae, held together by a suitable binder. The base of the layers is made of such materials as paper, woven fabric from cellulose, glass fabric which act as reinforcing agent. The binders are usually synthetic resins used in solution or emulsion from the impregnation into the base laminae. The resins are phenolic, & amino resins, epoxy resins, polyesters, silicones. Finally it is pressed under heat for permanent bonding. Decorative laminates are used in building & furniture industries.Coating: The coating item or formulation based on a resin or polymer may be used in the form of a solution, emulsion, powder, melt or film. Different types of coating are applicable such as Dip coating, Extrusion Coating, Skim Coating, Friction coating etc.Finishing: Finishing operations consist of numerous process after moulding or forming & they may include cutting, trimming, polishing, mechanical fastening, bonding, sealing, welding, dieing, painting, electroplating, vacum metallizing, printing, engraving etc.

Nylon is produced from butadiene, PVC is produced from acetylene.

polymer & plastic tech

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