Polymers: Structure, General Properties, and Applications.
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Transcript of Polymers: Structure, General Properties, and Applications.
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Polymers:
Structure, General Properties, and Applications
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Polymer
• Plastics meaning can be molded and shaped
Most products have a component made of some type of polymer.
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generally characterized by:
• Low density.
• Low strength and stiffness.
• Low electrical and thermal conductivity.
• Good resistance to chemicals.
• High coefficient of thermal expansion.
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(cont’d)
• Many colors and transparencies
• Low in cost.
• Can be machined, cast, formed, and joined
• Minimal surface-operations required -
advantage over metals
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Structure of Plastics
• Plastics are essentially a series of linked or cross-linked molecules
• Poly-mer, meaning many-units (mers).
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Monomer:
• basic building block of polymers
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Polymerization:
• Chemical reaction where monomers are linked to form larger molecules.
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Bonding
• Determines overall strength of a
polymer
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Classification of Polymers
• Thermoplastics
• Thermosets
• Elastomers
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Thermoplastics• Can be reshaped after heated to
glass temperature, returning to original hardness and strength after cooled.
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• Repeated heating/cooling leads to degradation (thermal aging).
• Can undergo large uniform deformations in tension before fracture, which enables the forming of complex shapes.
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• Like metals, when deformed, molecules align in the direction of the elongation, becoming stronger along the stretched direction, yet weaker in the transverse direction (anisotropic behavior).
•Deformation leads to strength in the direction of elongation
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Thermosets
• Curing reaction is irreversible,
shape is permanently set, thus
the term thermosetting plastic.
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Behavior of Thermosets
• Generally, thermosetting plastics have better mechanical, thermal, and chemical properties, electrical resistance, and dimensional stability than thermoplastics.
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• If temperature is increased sufficiently, thermosetting polymers burn, degrade, and blacken.
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Elastomers (Rubber)• Amorphous polymers (low glass-
transition temperature).
• Ability to undergo large elastic deformation without rupture, recovering original form and dimension after loads have been removed, even after large deformations.
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• Elastomers are Cross-linked at elevated temperatures (vulcanization), cannot be reshaped.
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Types of Elastomers
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Natural rubber• Latex base, sap obtained from tropical
tree bark, good resistance to abrasion and fatigue, low resistance to oil, heat, ozone, sunlight.
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Synthetic rubber• Further developed natural rubber• improved resistance to heat, gasoline,
and other chemicals.
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Additives
• Used to impart specific properties to polymers
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Fillers
• Reduce cost, improve properties
• fine sawdust, silica powder, clay, cellulose, glass, asbestos
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Plasticizers
• impart flexibility and softness by lowering the glass transition temperature.
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Antioxidants
• Used to prevent or slow degradation from ultraviolet radiation and oxygen.
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Colorants
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Flame Retardants
• Chlorine, bromine, phosphorus
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Lubricants
• Reduce friction during processing
• Linseed oil, waxes, soaps
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Traditional Thermoplastic Processing Techniques
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Extrusion• Screw extruders
• Feed section• Melt section – friction and heating
elements• Metering or pumping section – pressure
buildup at the die• Water and air cooling often used after the
extrusion
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Extrusion Processes
• Plastic tubes and pipes• Use of a spider die
• Coextrusion• Simultaneous extrusion of 2 or more
polymers• Plastic coating electrical wire
• Electrical cables and paper clips• Polymer sheets and films
• Flat extrusion die (coathanger die)
• Sheet = thicker than .5 mm
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Extrusion Processes Cont.
• Thin film• Film = thinner than .5 mm• Blown film process
• Plastic films• Such as teflon• Produced by shaving the circumference of a solid plastic billet• Much like veneers from logs• Called “skiving”
• Pellets• Used as the raw material for many thermoplastics processing
techniques• Small diameter is extruded and chopped into pellets
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Injection Molding• Use of hydraulic plunger or rotating /
reciprocating screw (extruder)
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250 Ton Machine
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Molds• complex, several cavities• “multicomponent injection molding – various
colors and shapes (rear light covers – autos)• Insert molding – metal hardware – (auto elect
components• Overmolding – process of making hinges and
ball & socket joints in one operation• Use of different materials that won’t bond• Use of cooling inserts between the material to
avoid bonding called “ice cold molding”
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Blow Molding
• Modified extrusion and injection molding• Extrusion blow molding
• Tube (preform) is extruded into mold cavity• clamped in mold cavity with a larger
diameter• blown outward with hot air blast to fill mold
cavity• Drums up to 5oo gal.
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Extrusion Blow Molding
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Injection Blow molding• Parison is injection molded (may be
stored for later use)• Mold opens – parison loaded and mold closed• Hot air blown into parison and expands to
parison to mold walls• Beverage bottles, shampoo, etc.
• Multilayer blow molding• Coextruding tubes (parisons) for multilayer
structures• Packaging for food and beverages
Odor, taste, aroma protection, scuff resistance, capabilty for printing
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Injection Blow Molding
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3 Station Injection Blow Molding
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Rotational Molding
• Large hollow parts
• Premeasured powdered polymer placed inside warm mold
• Mold is heated and rotated in oven
• Catalyst is added for thermosets
• Inserts may be molded integrally
• Trash cans, large hollow toys, carrying cases, footballs
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Thermoforming• Forming thermoplastic extruded sheets or
films over a mold• Use of heat and pressure • Sheet is clamped• Heated to sag point• Forced against mold by vacuum or air
pressure• Packaging trays for cookies & candy• Advertising signs, refrigerator liners, shower
stalls• No parts with holes
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Casting• Thermoplastics and thermosets cast into rigid or
flexible molds• Typcical parts are gears, bearings, wheels,
lenses, etc.• Potting and encapsulation
• Casting the polymer around an electrical component to embed it in plastic (coils or transformers)
• Potting: housing becomes part of final product• Encapsulation: component is coated with a layer of
polymer, surrounding it completely• Both of these processes - the plastic serves as a
dielectric (nonconductor)
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Foam Molding• Raw material is expandable polystyrene beads• Polystyrene beads obtained by polymerization
of styrene monomers are placed in a mold• Exposed to heat (steam)• Beads expand as much as 50 times original
size to fill mold cavity• Thermoplastics & thermosets can be used but
thermoplastics are in liquid form so the process is much like RIM
• Products: styrofoam cups, food containers, insulating blocks, packaging materials
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Foam Molding Cont.
• Structural foam molding• Thermoplastics are mixed with a blowing
agent (inert - nitrogen)• Injection molded into cold molds• Rapid cooling against the cold mold
produces a rigid skin and a cellular core• Products: furniture components, computer
and business machine housings, construction trim and moldings
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Composite Materials
•Combination of two or more
chemically distinct and insoluble
phases whose properties and
structural performance are superior to
those of the constituents acting
independently.
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Examples
• addition of straw (reinforcement) to
clay (matrix) for making mud huts
and bricks for structural usage.
• Concrete reinforced with re-bar or
wire mesh
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Consist of a matrix and reinforcement combined
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Categories
• Polymer matrix composites
• Thermoplastics
• Thermosets
• Metal matrix composites (MMC)
• Ceramic matrix composites
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Types of Polymer Matrix materials
• Polyester
• Epoxy
• Phenolics
• Carbon
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Functions of the matrix material
• Support the fibers in place
• Protects the fibers
• Transfer stresses
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Reinforcements
• Give composites their directional qualities
• Very anisotropic
• Fibers very strong and stiff in tension• Molecules oriented in long direction
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Types of Reinforcements
• Glass
• Carbon
• Aramid (Kevlar)
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Reinforcement Forms
• Fibers (dia usually less than .0004” )
• Particles
• Whiskers (tiny needlelike single crystals grown)
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Also classify fibers as:
• Continuous
• Discontinuous
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Fiber forms
• Chopped
• Yarns and Roving – twisted and untwisted strands of fibers
• Woven fabric
• Mats
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Other Composites
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Metal Matrix Composites
• high resistance to elevated
temperatures.
• high ductility and toughness
• Aluminum, aluminum lithium,
magnesium, titanium
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Ceramic Matrix Composites
• resist high temperatures and
corrosive environments.
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Honeycomb Structures
• core of honeycomb, or other
corrugated shape, bonded (by
adhesives to thin outer skins)
• High strength to weight ratio
• Core carries compressive loads,
faces carry the bending loads
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Applications of composites
• Aircraft / Aerospace
• Sporting goods
• Construction
• Transportation
• Marine
• Biomedical – Artificial limbs
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Composites Processing Techniques
• Thermosetting Polymers
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Composites Raw Material Forms
• Prepregs• Fibers pre-coated in resin and rolled
into sheet or tape.
• Sheet Molding Compounds (SMC)• Chopped fibers deposited on layer of
resin, then pressed on rollers.
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Reinforced Plastics (Cont.)
• Bulk molding compounds (BMC)• Similar to SMC but in billets. Formed
like dough
• Thick molding compounds (TMC)• Combines characteristics of BMC
(cheep) with SMC (strength) and injection molding.
Electrical components.
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TMC
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Molding techniques
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Contact Molding
• Parts with high surface area-to-thickness ratios (swimming pools, tubs, shower units),
• usually hand layup or spray- up
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Vacuum-Bag Molding• Vacuum used to form shape of
plastic (prepregs) to mold
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Automated Tape lamination• Multiaxis CNC Machines
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Compression molding
• Also called matched die molding
• open mold process
• material placed between molds & pressure applied
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Resin Transfer Molding
• Resin mixed with a catalyst is forced through a piston pump into a mold cavity with fiber reinforcements.
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Reaction injection molding (Thermosets)
• Two reactive resin components are mixed and then injected into a closed mold
• Typical parts• Auto bumpers, fenders, steering
wheels, instrument panels, water skis, etc.
• Parts up to approx 110 lbs
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Filament Winding
• Resin and fibers combined and
wrapped around mandrel.
pipes, tanks.
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Pultrusion
• Resin and fibers pulled through a
thermosetting polymer bath and a heated
die.• Polyester and glass reinforcements are the
most commonly used materials.
• Production of long shapes with various
continuous profiles
rods, tubing, ladders, golf clubs, handrails.
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Pulforming
• After resin and fabric (fibers) are
pulled through a polymer bath,
product is clamped by two dies and
then cured.
• Non constant cross sections.
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Metal Matrix Composites
• Liquid Phase Processing• Casting of liquid matrix and solid
reinforcement.
• Solid phase processes• Fibers and powder metals
tungsten carbide in cobalt matrix for tools.
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Ceramic Matrix Composites
• Slurry Infiltration• Fiber preform is hot pressed and
impregnated with a matrix