Transcript Polymers: Structure, General Properties, and Applications (l.u. 3/1/10)

Polymers: Structure, General Properties, and
Applications (l.u. 3/1/10)
Polymer
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Poly-mer, meaning many-units (mers).
 Monomer: basic building block of polymers (similar to
unit cell). (E.g. ethylene = carbon + hydrogen)
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Plastics meaning can be molded and shaped
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Most products have a component made of some type of
polymer.
Plastics
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Plastics are generally characterized by:
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Low density.
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Low strength and stiffness.
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Low electrical and thermal conductivity.
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Good resistance to chemicals.
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High coefficient of thermal expansion.
Plastics (cont’d)
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Many colors and transparencies
Minimal surface-operations needed (advantage over metals)
Can be easily machined, cast, formed, and joined
Low in cost
Complex shapes possible
Polymerization (Bonding):
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Chemical reaction where monomers are linked to form
larger molecules (addition/chain growth – step/network
growth). Consumption of mers - initiator
The greater the degree of polymerization, the greater the
polymer’s viscosity (resistance to flow).
(www.rpgroup.caltech.edu)
Bonding
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Plastics are essentially a series of linked or cross-linked
molecules
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Determine overall strength of a polymer
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Primary bonds (covalent).
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Secondary bonds (different chains or overlapping
portions of same chain).
(www.rpgroup.caltech.edu)
Polymer Bonds

Linear Polymers
 Linear chain-like structure
 Thermoplastics
 Acrylics, nylons, polyvinyl fluoride, polyethylene
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Branched polymers
 Side-branch chains attached to main chain
 Greater strength than linear
 Polyethylene
Polymer Bonds (cont’d)
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Cross-linked polymers
 Adjacent chains liked by covalent bonds.
 Cross-linking improves hardness, strength,
stiffness, brittleness, and dimensional stability.
 Often through ultraviolet light or radiation
 Thermosets, Rubbers and elastomers
 Epoxies, phenolics, silicones, (e.g. polycarbonate
safety glasses)
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Network polymers
 Highly branched (e.g. epoxy, resins)
Monomer (repeating Unit)
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Homopolymer: one monomer type
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Copolymers: two monomer types (Styrene-butadiene)
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Terpolymers: three types (ABS acrylonitrile-butadienestyrene)
Crystalline Structures
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Crystallites which arrange themselves in an orderly
manner
Increases polymer stiffness, hardness, density
Reduces ductility, becomes less rubbery
Becomes more heat and solvent resistant.
Crystalline structures have distinct melting point.
Degree is controlled by cooling and branching
Amorphous Polymers
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Random structure of molecules (opposite of crystalline
structures)
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No specific melting point.
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Amorphous polymers can be transparent (polycarbonate,
acrylic)
Glass Transition
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Changes in mechanical behavior of amorphous
structures with respect to temperature
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Decrease in temperature increases hardness and
brittleness in most - rubbery or leathery at higher
temps
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Temperature in which transition occurs is the GlassTransition Temperature
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Specific Volume tends to decrease with a decrease in
temp
Classification of Polymers
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Thermoplastics
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Thermosets
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Elastomers
Thermoplastics
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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).
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Can undergo large uniform deformations in tension
before fracture, which enables the forming of complex
shapes.
Thermoplastics
LDPE Bag
PVC Window (www.diytrade.com)
HDPE Bag (br.tradeholding.com)
Behavior of Thermoplastics
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).
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Deformation
leads to strength in the direction of
elongation
ABS
Thermoplastic Behavior (cont’d)
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Crazing effect: Wedge-shaped, narrow regions of highly
deformed material develop when subjected to bending
or related stresses.
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Environment, solvents, lubricants, and water enhance
the formation of crazes (voids, spongy material).
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Stress whitening from bending (microvoids)
PVC
Thermoplastic Behavior (Cont.)
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Some polymers absorb water, which acts as a
plasticizing agent (makes polymer more plastic –
soft, flexible). Nylon example
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Generally, low thermal and electrical conductivity,
low specific gravity, and relatively high coefficient of
thermal expansion.
Polyethelene
Thermosets
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Curing through heat, pressure, radiation
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Curing reaction is irreversible, shape is
permanently set, thus the term thermosetting
plastic.
Behavior of Thermosets
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Generally, thermosetting plastics have better
mechanical, thermal, and chemical properties, electrical
resistance, and dimensional stability than thermoplastics.
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Strength and hardness are not affected by temperature
or rate of deformation. If temperature is increased
sufficiently, thermosetting polymers burn, degrade, and
blacken.
XPE - (www.tradenote.net)
(www.orthoassociates.com)
Crosslinked Products
Epoxies
X-PE
Polymides
Elastomers (Rubbers)
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Amorphous polymers (low glass-transition
temperature).
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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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Cross-linked at elevated temperatures (vulcanization),
cannot be reshaped.
Vulcanized Products
Types of Elastomers
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Natural rubber
 Latex base, milklike sap obtained from tropical tree
bark, good resistance to abrasion and fatigue, high
friction properties, low resistance to oil, heat, ozone,
sunlight.
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Synthetic rubbers
 improved resistance to heat, gasoline, and other
chemicals.
Plastic Additives
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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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Flame Retardants
 Chlorine, bromine, red phosphorus
Plastic Additives (Cont.)
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Antioxidants
 Used to prevent or slow degradation from ultraviolet
radiation and oxygen.
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Colorants
 Titanium dioxide, dyes
 Polysynthren, Hostasol, Sandoplast for transparency
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Lubricants
 Reduce friction during processing
 Linseed oil, waxes, soaps
Plastic Problems/Challenges
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Liquid hydrocarbons (e.g. gasoline, solvents, petroleum
oil = soil pollution)
Ultraviolet light degradation
Loss of plasticizers
Slow degrade rate
Recycling may be difficult
Limited life in products
(www.resource.nsw.gov.au)
Toxicity/Health issues in products and processes. Food
containment…still in debate. Processing byproducts can
be harmful.
Plastic Problems/Challenges
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Environmental Concerns – filling landfills
(images.newsquest.co.uk)
(plasticisrubbish.files.wordpress.com)
Image References
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Polymerization: http://www.rpgroup.caltech.edu/courses/bootcamp2005/Polymerization/index.htm
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Polymerization: http://www.rpgroup.caltech.edu/courses/aph162/2006/webpages/Projects/Alex-JinHong-Eileen/mt.html
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PVC Window: http://www.diytrade.com/china/4/products/3170849/pvc_window.html
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HDPE bag:
http://br.tradeholding.com/default.cgi/action/viewproducts/productid/46430/productname/Hdpe_plastic_bags_and_
LDPE_Poncho/
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Plastic on beach: http://plasticisrubbish.files.wordpress.com/2008/04/200503-europe-099-bottles-2.jpg
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Landfill: http://images.newsquest.co.uk/image.php?id=526549&type=full