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Me 131 lecture3

Me 131 lecture3






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    Me 131 lecture3 Me 131 lecture3 Presentation Transcript

    • Nonmetallic Materials Plastics, Elastomers, Ceramics and Composites
    • • Built by joining smaller molecules to produce larger molecules. • Natural or synthetic resins or their compounds that can be molded, extruded, cast or used as thin films or coating. • Offers low density, low tooling costs, good resistance to corrosion and chemicals, cost reduction and design versatility. Plastics
    • Use of Plastics • Artificial organs • Shatter proof glass • Bullet proof vests • Provide thermal insulation • Encapsulate medicines • Form the base material in products such as: – Shower curtains – Contact lenses – Clothing
    • Molecular Structures of Plastics • Paraffin-type hydrocarbons – Carbon and Hydrogen combine/ link together indefinitely to form very large molecules. – Carbon and Hydrogen held together by double or triple bond. Ethane Methane Ethylene Acetylene
    • Isomers • Compounds with the same molecular formula but different structural formulas. • Due to the difference in their crystal structures, isomers do not have the same properties.
    • Polymerization • Also called linking of molecules. • Number of basic units (monomers) link together to form a large molecule (polymer) in which there is a repeated unit (mer). • The degree of polymerization (average number of mers) ranges from 75 to 750 for most commercial plastics. • Increase chain length = increase in toughness, creep resistance, melting temperature, melt viscosity and difficulty in processing.
    • • Copolymers – Two different types of mers are combined into the same addition chain, thus improving the physical and mechanical properties of new types of plastics. • Terpolymers – Combination of three different monomers.
    • Condensation Polymerization • Occurs when reactive molecules combine with one another to produce a polymer and small by-product molecules like water.
    • Thermoplastics • Contains molecules of different lengths therefore they do not have a definite melting point. Different temperatures Temperatures above melting temperature Can be poured and cast or formed by injection molding. Temperature where it is fully solid Material can retain its amorphous structure but with companion properties that are rubbery Much lower temperatures Bonds become stronger and polymer is stiffer and leathery. Below glass transition temperature Linear polymer retains its amorphous structure but becomes hard, brittle and glasslike
    • Crystallized Thermoplastics • Thermoplastics can partially crystallize when cooled below the melting temperature. • Chains closely align over appreciable distances with an increase in density. • Polymer becomes harder, less ductile and more resistant to solvents and heat.
    • Noncrystallized Thermoplastics • Individual molecules are bonded within by strong covalent bonds but the bonding forces between molecules are much weaker. • To increase strength, focus on restricting intermolecular slippage.
    • 4 Most Common Thermoplastics 1. Polyethylene (PE) 2. Polypropylene (PP) 3. Polystyrene (PS) 4. Polyvinyl chloride (PVC)
    • Thermosets • Three dimensional framework in which all atoms are connected by strong covalent bonds. • Produced by condensation polymerization where elevated temperature promotes irreversible reaction. • ONCE SET, subsequent heating WILL NOT produce the softening observed with the thermoplastics.
    • • Stronger and more rigid than thermoplastics. • Can resist higher temperatures and have greater dimensional stability. • Lower ductility and poorer impact properties.
    • COMMON TYPES OR FAMILIES OF PLASTICS Thermoplastics and Thermosetting Plastics
    • ABS Low weight, good strength and very tough; resists heat, weather and chemicals; dimensionally stable but flammable Acrylics Common name: LUCITE and PLEXIGLAS; high-impact, flexural, tensile and dielectric strengths Cellulose acetate Good insulating qualities, easily molded, high moisture absorption in most grades Cellulose acetate butyrate Higher impact strength and moisture resistance; will withstand rougher usage Ethyl cellulose High electric resistance and impact strength; retains toughness at low temperatures Thermoplastics
    • Fluorocarbons Inert to most chemicals, high temperature resistance, very low coefficients of friction (Teflon) Nylon (polyamides) Low coefficient of friction, good strength, abrasion resistance and toughness, excellent dimensional stability, good heat resistance Polycarbonates High strength and outstanding toughness, good dimensional stability, transparent or easily colored Polyethylenes Inexpensive, tough, good chemical resistance to acids , bases and salts; high electrical resistance, low strength, easy to shape and join; reasonably clear in thin-film form  LDPE  HDPE  UHMW
    • PMMA(polymethyl methacrylate) Hard, brittle at room temperature, transparent or easily colored Polypropylene Inexpensive, stronger, stiffer and better heat resistance than polyethylene; transparent Polystyrenes High dimensional stability and stiffness with low water absorption; best all around dielectric; clear, hard and brittle at room temperature; burns readily; softens at about 95% Polyvinyl chloride (PVC) General-purpose thermoplastic; good for outside applications; easily molded/ extruded; used with fillers, plasticizers and pigments Vinyls Tear resistant; good aging properties; good dimensional stability and water resistance in rigid forms
    • Thermosets Epoxies Good strength, toughness, elasticity, chemical resistance, moisture resistance and dimensional stability; easily compounded to cure at room temperature; used as adhesives, bonding agents, coating and in fiber laminates Melamines Excellent resistance to heat, water and many chemicals; used extensively in treating paper and cloth to impart water- repellant properties Phenolics Oldest of the plastics; hard, strong, low cost and easily molded but rather brittle; resistant to heat and moisture
    • Polyesters Strong and good resistance to environmental influences Silicones Heat and weather resistant; low moisture absorption; chemically inert; excellent sealants Urea-formaldehyde Properties similar with phenolics but available in lighter colors.
    • • Light weight • Corrosion resistant • Electrical resistance • Low thermal conductivity • Variety of optical properties • Formability or ease of fabrication • Surface finish • Comparatively low cost • Low energy content
    • Additive Agents in Plastics 1. Impart or improve properties 2. Reduce cost 3. Improve moldability/ ease in fabrication 4. Impart color
    • Additives • Fillers and reinforcements • Plasticizers • Lubricants • Coloring agents • Stabilizers • Antioxidants • Flame retardants
    • Fillers • Improve strength, stiffness or toughness • Reduce shrinkage • Reduce weight • Serves as an extender • Provides cost-saving bulk
    • Most Common Fillers Wood Flour General purpose filler; low cost with fair strength; good moldability Cloth Fibers Improved impact strength; fair moldability Macerated Cloth High impact strength; limited moldability Glass fibers High strength; dimensional stability; translucent Mica Excellent electrical properties and low moisture absorption Calcium carbonate, silica, talc and clay Serve primarily as extenders
    • Coloring Agents • Dyes or insoluble pigments which impart color to the plastics.  Dyes – for transparent plastics  Pigments – for opaque • Optical brighteners • Carbon black
    • Plasticizers • Reduce viscosity • Improve flow of the plastic during molding • Increase flexibility of thermoplastic products • Used for molding purposes
    • Lubricant • Waxes and soap • Improve moldability of plastics • Facilitate removal of parts from the mold • Used to keep thin polymer sheets from sticking to each other when stacked or rolled
    • Other Additives • Stabilizers and Antioxidants – Added to retard the degrading effect of heat, light and oxidants. • Flame Retardants – Added when nonflammability is important • Antistatic Agents – Allow migration of electrical chargeand may be incorporated into plastics used for applications such as electronics packaging.
    • • Antimicrobial additives – Provide long-term protection from both fungus and bacteria • Fibers – Increase strength and stiffness – Can modify electrical and magnetic properties
    • Oriented Plastics • Plastics that undergoes orientation process. • Orientation Process – Process that aligns molecules parallel to the applied load. – Give long –chain thermoplastics high strength in a given direction – Sretching, rolling or extrusion – Increase the tensile strength by more than 50 % but a 25% increase is more typical.
    • Engineering Plastics • Group of plastics that has been developed with: – improved thermal properties (up to 350C or 650F) – enhanced impact and stress resistance – High rigidity – Superior electrical characteristics excellent in processing properties and little dimensional change with varying temperatures and humidity
    • Engineering Plastics • Polyamides • Polyacctals • Polyacrylates • Polycarbonates • Modified polyphenylene oxides • Polybutylene terephtalates • Polyketones • Polysulfones • Polyetherimides • Liquid crystal polymers
    • Plastic Adhesive • For bonding of dissimilar materials (metals to nonmetals) • Involves consideration of the manufacturing conditions, the substrates to be bonded, the end-use environment and cost 1. Epoxies 2. Urethane 3. Cyanoacrylates 4. Acrylics 5. Annaerobics 6. Hot melts 7. Silicones
    • Plastic for Tooling • For applications where pressures, temperatures and wear requirements are not extreme. • Costs can be reduced and smaller quantities of products can be economically justified. • Can be produced in a much shorter time, enabling quicker production.
    • Foamed Plastics • Extremely versatile with properties ranging from soft and flexible to hard and rigid. • Generally used for cushioning in upholstery and automobile seats and in various applications such as vibration absorbers. • Semirigid foams find use in floatation devices, refigerator insulation, disposable food trays and containers, building insulation panels and sound attenuation • Rigid foams have been used as construction materials for boats airplane components, electronic encapsulation and furniture.
    • Polymer Coatings • Enhance appearance • Provide corrosion protection • Replaces chromes and cadmiums due to environmental concerns relating to the heavy metals.
    • Plastic vs Other Materials • Glass containers – plastic containers • PVC pipes and fittings – copper and brass in plumbing applications 1. Ability to be fabricated with lower tooling costs 2. Ability to be molded at the same rate as product assembly, reducing inventory 3. Possible reduction in assembly operations and easier assembly through snap fits, friction weld or the use of self-tapping fasteners 4. Ability to reuse manufacturing scrap 5. Reduced finishing costs
    • Metals • Cheaper and offer faster fabrication speeds • Greater impact resistance • Stronger and more rigid • Can withstand paint cure temperatures • Resistance to flames, acids and various solvents
    • Elastomers • Elastic polymer • Display an exceptionally large amount of elastic deformation when a force is applied.
    • Elastomeric Polymers Applied force stretches the polymer by uncoiling Remove load  molecules recoil
    • Cross-linking Linking coiled molecules to one another by strong covalent bonds. • Small amount  soft and flexible elastomers • Additional amount  harder, stiffer and more brittle
    • Under constant strain STRESS-RELAXATION
    • Rubber • Outstanding for their: – Flexibility – Electrical insulation – Low internal friction – Resistance to most inorganic acids, salts and alkalis. • However – Poor resistance to petroleum products such as oil, gasoline and naphtha. – Lose their strength at elevated temperatures. (80°C or 175°F)
    • As an engineering material 1839 Charles Goodyear
    • Rubber Soft Gummy Extremely Hard
    • Artificial Elastomers Polyisoprene Synthetic that is closest to duplicating natural rubber Styrene-butadiene Oil-derivative, high volume substitutefor natural rubber. (passenger-car tires) Neoprenes With properties similar to natural rubber but with better resistance to oils, ozone, oxidation and flame. (automotive hoses and belts, footwear, tires, mounting cushions and seals) Silicone rubbers Based on linear chain of silicon and oxygen atoms
    • Selection of an Elastomer • Shock absorption • Noise and vibration control • Corrosion protection • Abrasion protection • Friction modification • Electrical and thermal insulation • Waterproofing
    • Considerations • Mechanical and physical service requirements • Operating environment • Desired lifetime • Ability to manufacture the product • Cost
    • Elastomers for Tooling Application • Can be compounded to range from very soft to very hard. • Hold up well under compressive loading • Impervious to oils, solvents and other similar fluids • Can be made into desired shape quickly
    • Properties – High temperature usage – Hard and brittle – High melting point – Low thermal expansion – Good creep resistance – High compressive strength
    • Nature and Structure of Ceramics • Compounds of metallic and nonmetallic elements • Primary bonds have high strength – High melting temperatures – High rigidity – High compressive strength • Noncrystalline structures are possible – Amorphous condition is observed in glasses • Both crystalline and noncrystalline ceramics are brittle • Clay and whiteware are commonly used ceramics
    • Special Ceramics • Refractory materials – Ceramics that have been designed to provide mechanical or chemical properties at high temperatures – Three categories • Acidic, basic, and neutral • Abrasives – Ceramics have high hardness – Can be used in grinding applications – Diamond and cubic boron nitride are superabrasives • Ceramics may be used for electrical and magnetic applications – Dielectric, piezoelectric, and ferroelectric properties
    • Special Ceramics • Glasses – Soft and moldable when hot; easily shaped – Strong in compression but brittle and weak in tension – Excellent corrosion resistance • Cermets are combinations of metals and ceramics – Crucibles, nozzles, aircraft brakes • Cements – Plaster of paris • Ceramic coatings – Enamels, porcelains, glazes
    • Ceramics for Mechanical Applications • Ceramics have strong ionic or covalent bonds – Most ceramics have small cracks, pores, and impurities – Act as mechanical stress concentration points • Advanced ceramics – Alumina – Silicon carbide – Sialon – Zirconia • Advanced ceramics may be used as cutting tools
    • Advanced Ceramics Figure 8-8 Gas-turbine rotors made of silicon nitride. The lightweight material (one-half the weight of stainless steel) offers strength at elevated temperature as well as excellent resistance to corrosion and thermal shock. (Courtesy of Wesgo Division, GTE, Hayward, CA.) Figure 8-9 A variety of components manufactured from silicon nitride, including an exhaust valve and turbine blade. (Courtesy of Wesgo Division, GTE, Hayward, CA.)
    • Composite Materials • Nonuniform solid consisting of two or more different materials – Mechanically or metallurgically bonded – Each of the constituent materials maintains its identity • Properties depend on: – Properties of individual components – Relative amounts – Size, shape, and distribution – Orientation – Degree of bonding
    • Laminar or Layered Composites • Distinct layers of materials • Layers are bonded together • Typical example is plywood • Bimetallic strips have two metals with different coefficients of thermal expansion • Sandwich materials
    • Particular Composites • Consist of discrete particles of one material in a matrix of another material – Concrete • Dispersion strengthened materials have a small amount of hard, brittle particles in a soft, metal matrix • True particulate composites have large amount of coarse particles • Metal-matrix composites
    • Fiber-Reinforced Composites • Discontinuous thin fibers of one material are embedded in a matrix – Wood and bamboo are naturally occurring fiber composites – Bricks of straw and mud – Automobile tires • Fibers of nylon, rayon, Kevlar, or steel to reinforce the rubber – Glass fibers – Graphite – Ceramic fibers, metal wires, whiskers • Common objective is high strength and lightweight – Orientation of the fibers is important
    • Properties of Fiber-Reinforced Composites • Advanced fiber-reinforced composites – Organic or resin matrix composites • Sports equipment, light-weight armor, low-temperature aerospace applications • have a maximum service temperature of about 315ºC (600ºF) – Metal-matrix composites • Nonflammable, do not absorb water or gases, corrosion resistance • operating temperatures up to 1250ºC (2300ºF), where the conditions require high strength, high stiffness, good electrical and/or thermal conductivity, exceptional wear resistance, and good ductility and toughness
    • Properties of Fiber-Reinforced Composites – Carbon-carbon composites • High temperature applications • operate at temperatures above 2000ºC (3600ºF), along with a strength that is 20 times that of conventional graphite, a density that is 30% lighter – Ceramic-matrix composites • High temperature strength, stiffness, and environmental stability • operate at temperatures as high as 1500ºC (2700ºF)
    • Composites • Hybrid composites – Two or more fibers • Alternate layers of fibers • Mixed strands in the same layer • Combination of mixed strands and alternating layers • Selected placements • Fibers are stitched together
    • Design and Fabrication • Select the component materials • Determine the relative amounts of each component • Determine the size, shape, distribution, and orientation • Select the proper fabrication method – Compression molding – Filament winding – Pultrusion – Cloth lamination • Composites may be tailored to specific applications
    • Practical Applications • Manufacturing with composites is labor intensive • Defects – Delamination voids – Missing layers – Contamination – Fiber breakage – Improperly cured resin • Areas of application – Aerospace – Sporting equipment – Automobiles – Boat hulls – Pipes
    • Areas of Application Figure 8-14 Composite materials are often used in sporting goods to improve performance through light weight, high stiffness, and high strength, and also to provide attractive styling. (Left) A composite material snowboard; (right) composites being used in a fishing rod, water ski, and tennis racquet.
    • Summary • Plastics, ceramics, and composites are nonmetallic materials that are commonly used. • Additives are used in plastics to produce desired properties • Composite’s properties depend on the way in which the materials are combined and their orientation • Ceramics are ideal for high temperature applications