Lecture 12, 13 - Polymers

8/12/2019 Lecture 12, 13 - Polymers

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MENG 3319 MATERIAL SCIENCE AND MANUFACTURING

Polymers

Dr. Tafesse Borena


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Lecture outline

Polymeric MaterialsClassification of polymersMolecular StructureChemistry of polymersPlastics and additivesCr stallinit of ol mers

Additives to polymersMechanical and physical properties of plasticsThermoplastics,Thermoset plastics and ElastomersPlastic IdentificationPlastic recyclingBiodegradable polymers


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Plastic facts We live in a plastic society.

Everything around us is plastic. Could you go for a day without

plastic? Tooth brus h , c lo th ing , food

con tainers , coo k ing spa tu las ,pans , bo t t led w ater, autom ob i le

par ts , b icyc le par ts , eye glasses ,iPod, ca lcula tor, m ou se , co m pu ter parts , prin ter, s tapler, head ph on es , TV, c loc k, f lash m emo ry

h o u sin g , u s b c o n nec to r,keyboard , shoes , backpack p ar ts ,ce l l ph on e, credi t cards ..


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Polymer Structures

Polymer Greek root poly (many), mer (unit)Substance made up of many of repeating units (mers)Synonym for plastic

PlasticGreek, plastikos, means to mold and shape

a er a sMost polymers are based on carbon and are therefore considered organicchemicals

Solid material

Metals Ceramics Plastics Composites

Thermoplastics Thermosetts Elastomers


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Examples of Polymers

Thermoplastics:Polyethylene, polyvinylchloride, polypropylene, polystyrene,and nylon

Thermosets:Phenolics, epoxies, and certain polyesters

Elastomers: Natural rubber (vulcanized)

Synthetic rubbers, which exceed the tonnage of naturalrubber


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Polymer StructuresMetal: single atoms, metallic bondCeramic: metallic oxides, ionic bond or dipoleinteractions, van der Waals bonds

Polymer: long molecule chain

-[C2H4]n- , poly[ethylene]

Spaghetti


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Polymer StructuresPolymer

Covalent Bonding: link monomers to form a chain

Occurs when two nonmetal atoms are in close proximity

-[C2H4]n- , poly[ethylene]

Strong Bond


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Polymer StructuresSecondary Bonding: hold polymer chains

Van der WaalsDue to the attraction of all molecules have for each other,e.g. gravitational. Forces are weak since masses are small

Secondary Bonding Hydrogen bonds Ionic bonds Weaker than metallic,

covalent


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Molecular shape

The angle between the singly bonded carbon atoms is ~109 o carbon atomsform a zigzag pattern in a polymer molecule.

While maintaining the 109 o angle between

bonds polymer chains can rotate aroundsin le C-C bonds double and tri le bonds

Random kinks and coils lead toentanglement, like in the spaghetti

structure:

are very rigid).


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Most polymers are organic, and formed from hydrocarbonmolecules

Each C atom has four e - that participate in bonds, each H atomhas one bonding e -

Hydrocarbon molecules

Examples of saturated (all bonds are single ones) hydrocarbonmolecules:

Methane, CH 4Propane, C 3H 8Ethane, C 2H 6


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Double and triple bonds can exist between C atoms (sharing

of two or three electron pairs).These bonds are called unsaturated bonds. Unsaturatedmolecules are more reactive


H-C C-H

Ethylene, C 2H 4ce y ene, 2 2

Isomers are molecules that contain the same atoms but in adifferent arrangement. An example is butane and isobutane:

Butane C4H

10 Isobutane


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Other organic groups can be in a polymer. R represent radical:organic groups that remain a unit during reactions(e.g. CH3, C2H5, C6H5)


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Chemistry of polymer molecules

Replace hydrogen inpolyethylene

polytetraflouroethylene (PTFE) Teflon

Replace every fourth hydrogen polyvinyl chloride(PVC)

Replace every fourth hydrogenin polyethylene with CH

3methyl

group polyproplylene


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Addition and Step PolymerizationModel of addition polymerization(chain ) polymerization: (1)initiation, (2 ) rapid addition of monomers, and (3) resulting long chainpolymer molecule with n mers at termination of reaction (e.g.,Polyethylene, paolypropylene, polyvinylchloride, polyisoprene)

Model of step polymerization showing the two types of reactionsoccurring: (left) n-mer attaching a single monomer to form a(n+1)-mer ; and (right) n1-mer combining with n 2-mer to form a(n 1+n 2 )-mer (e.g., Nylon, polycarbonate, phenol formaldehyde)


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Polymer Structures

Linear PolymersSequential chainsThermoplastics: acrylics, nylons,

Branched PolymersSide branch chains

Thermoplastics: polyethylene , (Chain packing efficiency is reduced compared to linear polymers- lower density)

Linear Branched


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Polymer Structures

Cross-linked PolymersAdjacent chains linked by covalent bonds(often achieved byadding atoms or molecules that form covalent links betweenchains). Many rubbers have this structureThermosetts: epoxies, silicones,

Network Polymers 3D networks of 3 or more active covalent bonds(

trifunctional mers) Examples: epoxies, phenol-formaldehyde

Cross-linked Network


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Copolymers (composed of different mers)

Copolymers: at least two different types of mers , can differ in theway the mers are arranged:

Random copolymer

Alternating copolymer

Block copolymer

Graft copolymer

Synthetic rubbers are copolymers


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Polymer StructuresThermoplastic (TP) vs.

Thermoset (TS)

TP (Solid materials at roomtemperature but viscous liquidswhen heated to temperatures of only a few hundred degrees)

easily and economically shapedinto productsThey can be subjected to heatingand cooling cycles repeatedlywithout significant degradation

TS - (Cannot tolerate repeated heating cycles )

When initially heated, they softenand flow for moldingElevated temperatures alsoproduce a chemical reaction thathardens the material into aninfusible solidIf reheated, thermosets degradeand char rather than soften


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Crystallinity in PolymersThe crystalline state may existin polymeric materials.However, since it involvesmolecules instead of justatoms or ions, as with metalsor ceramics, the atomicarrangement will be more

comp ex or po ymers .There are ordered atomicarrangements involving molecular chains .Example shown is apolyethylene unit cell(orthorhombic).


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Polymer Crystallinity

Polymers are rarely 100% crystallineDifficult for all regions of all chains tobecome aligned

Degree of crystallinityexpressed as % crystallinity .

crystallineregion

-- Some physical propertiesdepend on % crystallinity.

-- Heat treating causescrystalline regions to growand % crystallinity toincrease. amorphous

region


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Crystal Structures

Fe 3C iron carbide orthorhombic crystal structure


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Crystallinity and Properties

As crystallinity is increased in a polymer Density increasesStiffness, strength, and toughness increasesHeat resistance increasesIf the polymer is transparent in the amorphous state, itbecomes opaque when partially crystallized

Polyethylene type Low density High density

Degree of crystallinity 55% 92%

Specific gravity 0.92 0.96

Modulus of elasticity 140 MPa(20,000 lb/in 2)

700 MPa(100,000 lb/in 2)

Melting temperature 115 C(239 F)

135 C(275 F)


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Factors for CrystallizationSlower cooling promotes crystal formation and growth

Mechanical deformation , as in the stretching of aheated thermoplastic, tends to align the structure andincrease crystallization

Plasticizers (chemicals added to a polymer to soften it)


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r n i n g ,

I n c .

T h o m s o n

L e a r n

i n g

i s a

t r a d e m a r

k u s e d

h e r e

i n

n d e r

l i c e n s e .

Crystal Structures

The effect of temperature on the structure and

behavior of thermoplastics.

2 0 0 3 B r o o k s /

C o l e , a

d i v

i s i o n o

f T h o m s o n

L e

.

u

.


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Thermal Behavior of Polymers

Specific volume(density) -1 as afunction of temperature


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Additives and their FunctionProperties of a polymer can often be beneficially changedby combining it with additives

Additives either alter the molecular structure orAdd a second phase, in effect transforming the polymer into acomposite material

Fillers strengthen polymer or reduce cost Colorants pigments or dyesLubricants reduce friction and improve flowFlame retardents reduce flammability of polymerCross-linking agents for thermosets and elastomersUltraviolet light absorbers reduce degradation fromsunlightAntioxidants reduce oxidation damage


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Mechanical Properties of ThermoplasticsThermoplastic polymers

Heating and cooling can be repeated many times withoutdegrading the polymerReason: TP polymers consist of linear and/or branchedmacromolecules that do not cross-link

Low modulus of elasticity (stiffness)E is much lower than metals and ceramics

Low tensile strengthTS is about 10% of metal

Much lower hardness than metals or ceramicsGreater ductility on average

Tremendous range of values, from 1% elongation for polystyrene to 500% or more for polypropylene


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Strength vs. Temperature

Deformationresistancestren th of

polymers as afunction of temperature


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Physical Properties of Thermoplastics

Lower densities than metals or ceramicsTypical specific gravity for polymers are 1.2 (compared toceramics (~ 2.5) and metals (~ 7)

Much higher coefficient of thermal expansionRoughly five times the value for metals and 10 times thevalue for ceramics

Much lower melting temperaturesInsulating electrical propertiesHigher specific heats than metals and

ceramics


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General Properties of Thermosets

Rigid - modulus of elasticity is two to three times greaterthan thermoplastics

Brittle, virtually no ductility

Less soluble in common solvents than thermoplastics apa e o g er serv ce temperatures t an

thermoplastics

Cannot be remelted - instead they degrade or burn


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Cross-Linking in TS Polymers

Three categories:1. Temperature-activated systems2. Catalyst-activated systems3. Mixing-activated systems

Curing is accomplished at the fabrication plants that supply the starting materials to the fabricator


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Temperature-Activated Systems Catalyst-Activated Systems

Curing caused by heat suppliedduring part shaping operation(e.g., molding)

Starting material is a linearpolymer in granular form

supplied by the chemical plant

Cross-linking occurs whensmall amounts of a catalyst areadded to the polymer, which isin liquid form

Without the catalyst, the

polymer remains stable and

en eate , mater a

softens for molding, butcontinued heating causescross-linking

Most common TS systemsThe term thermoset" appliesbest to these polymers

Once combined with thecatalyst it cures and changesinto solid form


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Catalyst-Activated Systems Cross-linking occurs when small amounts of a catalyst areadded to the polymer, which is in liquid form Without the catalyst, the polymer remains stable and liquid Once combined with the catalyst it cures and changes into

solid form

Mixing-Activated SystemsMixing of two chemicals results in a reaction that formsa cross-linked solid polymer

Elevated temperatures are sometimes used to accelerate thereactions

Most epoxies are examples of these systems


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Elastomers Polymers capable of large elastic deformation whensubjected to relatively low stresses

Some can be extended 500% or more and still return totheir original shapeTwo categories:1. Natural rubber - derived from biological plants2. Synthetic polymers - produced by polymerization processes

e t ose use or an po ymers

Elastomers consist of long-chain molecules that arecross-linked (like thermosetting polymers)They owe their impressive elastic properties to twofeatures:1. Molecules are tightly kinked when unstretched 2. Degree of cross-linking is substantially less than

thermosets


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Elastomer MoleculesModel of long elastomer molecules, with low degree of

cross-linking: (left) unstretched, and (right) under tensile stress

en stretc e , t e mo ecu es are orce to unco anstraightenNatural resistance to uncoiling provides the initial elasticmodulus of the aggregate material

Under further strain, the covalent bonds of the cross-linked molecules begin to play an increasing role in the modulus,and stiffness increasesWith greater cross-linking, the elastomer becomes stiffer andits modulus of elasticity is more linear


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Vulcanization

The term for curing (cross-linking) in the context of natural rubber (and certain synthetic rubbers)Typical cross-linking in rubber is one to ten links perhundred carbon atoms in the linear polymer chain,

depending on degree of stiffness desired Considerable less than cross-linking in thermosets


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Natural Rubber (NR)

NR = polyisoprene, a high molecular-weight polymer of isoprene (C 5H8)

Derived from latex, a milky substance produced by variousplants, most important of which is the rubber tree that grows

in tropical climates weight), plus various other ingredientsRubber is extracted from latex by various methods thatremove the water


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Vulcanized Natural Rubber

Properties: noted among elastomers for high tensilestrength, tear strength, resilience (capacity to recovershape), and resistance to wear and fatigueWeaknesses: degrades when subjected to heat,sunlight, oxygen, ozone, and oil

Some of these limitations can be reduced by additives

Market share of NR 22% of total rubber volume(natural plus synthetic)


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Natural Rubber Products

Largest single market for NR is automotive tiresOther products: shoe soles, bushings, seals, and shock absorbing componentsIn tires, carbon black is an important additive

It reinforces the rubber, serving to increase tensile strength and resistance to tear and abrasion

Other additives: clay, kaolin, silica, talc, and calciumcarbonate, as well as chemicals that accelerate and

promote vulcanization


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Synthetic RubbersDevelopment of synthetic rubbers was motivated largelyby world wars when NR was difficult to obtainTonnage of synthetic rubbers is now more than threetimes that of NRMost important synthetic rubber is styrene-butadienerubber SBR a co ol mer of butadiene C H and

styrene (C 8H8)As with most other polymers, the main raw material forsynthetic rubbers is petroleum


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Thermoplastic Elastomers (TPE)

Thermoplastic that behaves like an elastomerElastomeric properties not from chemical cross-links, butfrom physical connections between soft and hard phasesin the material

Cannot match conventional elastomers in elevated temperature strength and creep resistance

Products: footwear; rubber bands; extruded tubing, wirecoating; molded automotive parts, but no tires


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Plastics Identification Code (PIC)In 1988 the Society of the Plastics Industry developed a numericcode to provide a uniform convention for different types of plastic containers.The symbol consists of a triangle formed by three bent arrowsenclosing a number to identify the plastic:These numbers can be found on the underside of containers.

1. PET; PETE (polyethylene terephthalate): plastic water and soda bottles.

2. g ens ty po yet y ene : aun ry s etergent3. V (Vinyl) or PVC: Pipes, shower curtains4. LDPE (low density polyethylene): grocery bags, sandwich

bags

5. PP (polypropylene): Tupperware, syrup bottles, yogurtcups,6. PS (polystyrene): Coffee cups, disposable cutlery 7. Miscellaneous: any combination of 1-6 plastics


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Plastics Identification Code (PIC)

All of the plastics covered by the PIC are thermoplasticpolymersThe PIC facilitates sortation of items made of differentplastics for reprocessing

Sorting is a very labor intensive activity Recycling of thermosets and elastomers is more

difficult because of cross-linking (cannot be remelted)Recycling involves retreading of tires, grinding the TS

polymers to be used as fillers, etc.


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Plastics Recycling

Approximately 200 million tons of plastic products aremade annually throughout the worldU.S. production is about 25 million tonsOnly about 6% of the U.S. tonnage is recycled as plastic waste

The rest either remains in products or ends up in garbage landfills

Recycling means recovering the discarded plastic itemsand reprocessing them into new products


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Problems in Plastics Recycling

1. Economics of metal recycling are more favorablebecause the items are usually larger

2. Economics of glass recycling are more favorable becauseglass products are all based on silicon dioxide

3. y contrast, p ast c pro ucts are genera y sma anmade of a great variety of chemical compositions thatdo not mix well


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Biodegradable PolymersDefined as plastics that decompose due to the

actions micro-organisms in natureConventional plastics are not biodegradableTwo forms of biodegradable plastics:

1. Partially biodegradable consist of a conventionalpolymer plus biodegradable filler

2. Completely biodegradable (a.k.a. bioplastics) consist of po ymer p us er, o o w c are o egra a e

Of greatest interest are the completely biodegradableplasticsBoth polymer and filler are derived from natural and renewable sources

Polymer various agricultural products can be used as the raw material for biodegradable plastics

Starch from corn, wheat, rice, potatoesFiller cellulose from flax or hemp


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Documents

Lecture 12, 13 - Polymers