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Mechanics, Materials and

Machine Elements (MME M1)

Polymers 1

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ADVANTAGES OF PLASTICS VS STEEL

Low density and high specific strength

Electrical and thermal insulation

Design flexibility

Easy to shape

Coloring possibilities

Reduced total energy cost

Easy processing/forming

Cheaper

Corrosion resistance Chemical resistance

Tailored properties

Transparency (in some cases)

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Relatively low maximum allowable service temperatures

Moderate creep resistance and specific stiffness

Significant influence of temperature on mechanical

properties

Flammability

UV resistance (for some materials)

Higher thermal expansion rate

DISADVANTAGES OF PLASTICS

VS STEEL

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6Source: http://www.sciencedirect.com/science/article/pii/S0141391004001776

http://www.google.dk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=Fyzd4n38plAQYM&tbnid=-9yUp84dVC_0SM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0141391004001776&ei=67BvUe3SMMHfswb38YHQDw&bvm=bv.45368065,d.Yms&psig=AFQjCNHZw_ColUK45PpI0W68hlpMYInlPA&ust=1366360599982137

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DENSITY

8

Why lower

density than

metals?

Lighter

atoms

Largerinteratomic

space

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WHAT IS A POLYMER?

Poly merosmany unit

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C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Poly(vinyl chloride) (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

monomer monomer monomer

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DEGREE OF POLYMERIZATION, DP

DP= average number of repeat units per chain

11

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) DP= 6

DP can be as high as 100.000, depending on plastic type.

POLYMER COMPOSITION

Most polymers backbone contain C, sometimes combined

with O and/or N. Other elements can be attached to the

backbone, e.g. H, N, Cl, and F, or groups of elements.

"Backbone" should probably be called monomer!

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POLYMERS

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POLYMERS

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Naturally occurred polymers:

rubber

cotton

wool leather

silk

wood

proteins

enzymes, starches, cellulose

Synthetic produced

polymers:

plastics

rubbers fiber materials

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BULK OR COMMODITY POLYMERS

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Again, repeat unit = monomer!

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BULK OR COMMODITY POLYMERS

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Repeat unit = monomer!

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BULK OR COMMODITY POLYMERS

Repeat unit = monomer!

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POLYMERS

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POLYMERS

Thermoplastics

Can be re-melted

Low melting temperature

Long chains of monomers

Can be recycled

Little crosslinking

Ductile

Soften w/heating

Ex.: polyethylene, polypropylene,

polycarbonate, polystyrene

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Thermosets

Cannot be re-melted

Strong cross-linked, 3D intermolecular

bonds => one large molecule

Are only partially polymerized beforeprocessing

Significant crosslinking (10 to 50% of

repeat units)

Hard and brittle

Do NOT soften w/heating

Ex.: vulcanized rubber, epoxies,

polyester resin, phenolic resin

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INTERATOMIC BONDS

Covalent bonding

The atoms get a stableouter shell by sharingelectrons. Ex. CH4.

In thermosets between

molecules. In thermoplastics within

molecules.

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Secondary bonding In thermoplastics between the molecule chains.

Types: van der Waals, hydrogen, dipole.

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THERMOPLASTICS

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Amorphous

Can not be 100% crystalline.

High crystallinity:

Increase of density

Higher melting point

Higher E-modulus

Higher tensile strength

Lower ductility

Better resistance against

organic solvents.

Semicrystalline

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SEMICRYSTALLINE POLYMERS

Polyethylene (PE) :

Polyetrafluoroethylene (PTFE) (Teflon)

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SEMICRYSTALLINE POLYMERS

Polyamide (PA)

Polyoxymethylene (POM)

Polypropylene (PP)

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AMORPHOUS POLYMERS

Polystyrene (PS)

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PS art by Tara Donovan

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AMORPHOUS POLYMERS

Polystyrene (PS)

Polyvinylchloride (PVC)

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AMORPHOUS POLYMERS

Acrylonitrile butadiene styrene (ABS)

Poly(methyl methacrylate) (PMMA)

Polycarbonate (PC)

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MOLECULAR STRUCTURES FOR

POLYMERS

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Branched Cross-Linked NetworkLinear

secondarybonding

Linear: HDPE, PVC, PS, PA

higher density; can be easier close packed

Branched: LDPE.

less density

Cross-linked: Rubbers

Network: Polyurethanes, epoxies, polyesters, etc. (thermosets)

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COPOLYMERS

Two or more monomerspolymerized together

randomA and B randomlypositioned along chain

alternatingA and B

alternate in polymer chain blocklarge blocks of A units

alternate with large blocks ofB units

graftchains of B unitsgrafted onto A backbone

A B

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random

block

graft

alternating

TEMPERATURE BEHAVIOR OF

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TEMPERATURE BEHAVIOR OF

AMORPHOUS VS. SEMICRYSTALLINE

PLASTICS

TgC TmC

PE -120 137

PP -15 176

POM -50 180

PA 50 225

PBT 55 225

PMMA 100 205-230

PVC 80 200-220

ABS 110 230-270PC 149 290-320

PS 100 204-251

Amorphous

Semicrystalline

Rubberyor

Viscoelasticstate

Temperature

Semicrystalline

Tg

morphous

T

g

E-Modulus

Semicrystalline

Tm

Glassy state Meltstate

Data not 100% reliable!

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POLYMER ADDITIVES

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Improve mechanical properties, processability, durability, etc.

Fillers

Added to improve tensile strength & abrasion resistance,toughness & decrease cost

ex: carbon black, silica gel, wood flour, glass, limestone,

talc, etc. Plasticizers

Added to reduce the glass transition temperature Tgbelow room temperature

Presence of plasticizer transforms brittle polymer into aductile one

Commonly added to PVC - otherwise it is brittle

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POLYMER ADDITIVES (CONT.)

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Stabilizers

Antioxidants

UV protectants

Lubricants

Added to allow easier processing Polymer slides through dies easier ex: sodium

stearate

Colorants

Dyes and pigments

Flame Retardants

Substances containing chlorine, fluorine, and boron

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PLASTIC

IDENTIFICATION

CODE