Structure and properties of polymers

What are polymers?

Includes plastics (polypropylene, nylon, PVC, polystyrene….), natural polymers (shellac, amber, cellulose…) and biopolymers (proteins etc).

Mechanical properties vary from one polymer to another, and depend on temperature and processing.

Mostly long-chain hydrocarbon molecules with:

Strong covalent bonds between the molecules along the chain, and

EITHER weak secondary bonds between the carbon chains

OR strong covalent bonds cross-linking adjacent carbon chains

If chains can be aligned, the structure can become partially crystalline.

But frequently the chains are entangled (amorphous)

Tensile deformation of a semi-crystalline polymer

• Elastic deformation:– Amorphous regions elongate

i.e. untangling of the carbon chains

• Plastic deformation:– Crystalline regions align

i.e. orientate in direction of stress

– Cold drawingCrystalline regions slide past each other andChains in the amorphous regions are drawn out

• BUT, to move molecules around (plastic deformation) needs time and temperature (see later)

Force vs. extension for polyethylene stretched at different strain-rates:

(ii)

Mechanical properties of polymers tend to be very sensitive to (i) temperature and (ii) rate of deformation:

PMMA

(i)

Visco-elastic behaviour

Loadcycle

Elasticbehaviour

Above the polymer’s “glass transition temperature” Tg, polymers show a visco-elastic behaviour as the polymer chains only slowly respond to the applied stress:

Viscoelastic

Viscous

PE low density, low strength & impact resistant, Tmelt ~ 120oC

Bottles, toys, film-wrap

PVC high strength, stiff unless plasticizer added, Tmelt~210oC

Flooring, pipes, wire insulation, old LPs

PTFE polytetrafluoroethylene -

high density, low stiffness, Tmelt ~327oC, chemically inert, good electrical insulator, low friction

Seals, pipes, bearings, high-T electronics

PP low density, Tmelt~175oC, cheap Bottles, packaging film, luggage

PS low ductility, cheap Wall tiles, toys, indoor lighting

PMMA medium strength, low ductility, transparent, weather resistant

Lenses, outdoor signs

Popular polymers

polyethylene

polyvinyl chloride

teflon

polypropylene

polystyrene

polymethyl methacrylate

http://video.google.co.uk/videoplay?docid=5994300959507748421&ei=SkL0SrTdPNCr-Aa7nt28BA&q=polymer+history+and+nylon&hl=en#

http://www.youtube.com/watch?v=7nCfbZwGWK8&feature=player_embedded

Polyethylene

HIGH DENSITY POLYETHYLENE

Linear carbon chains

Shorter, stronger secondary bond

Higher strength and density

High levels of crystallinity poss.

LOW DENSITY POLYETHYLENE

Branched carbon chains

Longer, weaker secondary bond

Lower strength and density

Low levels of crystallinity

PVC and polypropylene

PVC

# Very strong dipole (secondary bond)

# Strong, stiff, brittle

POLYPROPYLENE

# Regular side groups~>low density

# Weak secondary bond

# Strength due to entangling caused by side groups

Polystyrene & PMMA

POLYSTYRENE POLYMETHYLMETHACRYLATE

Strong dipole; large side group

Brittle; amorphous

(Further reading: William D Callister Chapters 14 & 15)

Structure and properties of ceramics

Types of ceramics

Glasses

Clayproducts Abrasives

Advancedceramics

Glasses

Glassceramics Fireclay Basic Cements

Structuralclay

productsWhite-wares Silica Special

Refractories

Ceramic materials

Inorganic, non-metallic materials: mostly compounds between metallic and non-metallic elements e.g. oxides, nitrides, carbides etc

Glasses• Glasses

– Non crystalline (amorphous) silicates

– contain other oxides (notably CaO, Na2O, K2O and Al2O3) which influence properties

• Glass ceramics– Fine grained polycrystalline

structure– Formed by heat treatment

of glasses– Better strength and thermal

shock resistance

Refractories and AbrasivesREFRACTORIES

Can withstand high temperatures without melting or decomposing

Used for furnace linings

ABRASIVES

Used to wear, cut or grind away material

Require high hardness plus some toughness and “refractoriness”

Mostly based upon alumina (Al2O3) and silica (SiO2)

Basic refractories based on MgO

Silicon carbide (SiC)Tungsten carbide (WC)Corundum (Al2O3)SilicaDiamond

Clays

Structural clay: bricks, tiles, sewer pipes

Whiteware: tableware, sanitary ware

A “plastic”, earthy natural mineral of alumina and silica found in the ground.

Formed from rocks that were slowly dissolved in water. When the solutions get supersaturated, tiny clay crystals start to grow from the solution.

Cement

Microstructure:

• voids (black)

• hydrated cement (grey)

• unhydrated cement (white)

Produced by mixing clay and lime in proportion and heating together (calcination)

Principle constituent are calcium silicates.

Hardening due to the hydration reactions of calcium silicates (chemical reactions with water) NOT from drying.

Constituent of concrete: added to bind aggregate particles (sand, gravel) to form a composite material.

At room temperature, most ceramics fracture before any plastic deformation …… --> catastrophic BRITTLE fracture

Deformation of Ceramics

PLASTIC deformation can occur (eg at high temps), but it is difficult as most ceramics are either:

(i) ionic crystals e.g. MgO, NaCl, ZnS etc - dislocations cannot easily move, or

(ii) covalent crystals e.g. silcates, aluminates (rock etc), WC etc with VERY strong bonds between atoms, or

(iii) non-crystalline (amorphous) e.g. fused/vitreous silica glass, - dislocations don’t exist. Plastic deformation is by viscous flow of atoms (like a very viscous liquid)

Strength determined by porosity levels.

For metals, the compressive strength is basically the same as the tensile yield strength. However, CERAMICS are much stronger in compression than in tension, as they are often full of micro-cracks (causing failure in tension). Hence tend not to use the tensile test (gripping ceramic test specimens without breaking them is also difficult)! Instead often use a standard 3-point bend test to measure elastic (Young’s) modulus, and rupture strength (stress at failure):

F

SiC 345 100-1100Al203 360 300-700Glass 70 100(steel 200 200-600)

Rupture stress(MPa)

Young’s modulus (GPa)

Concrete: compressive strength of concrete typically 15-40 MPa (c.f. a tensile strength of only 1-4 MPa), but only achieved after ~4 weeks of curing.

Tension/Compression Testing of Ceramics

World consumption of hydrocarbons (left column), metals, polymers, building materials and C-fibre composites:

Fig 20.1 in Ashby

Structure and properties of composite materials

from Ashby

Materials……….

Tyres are also composites: rayon cloth, steel bands and nylon belts all set in a matrix (binder) of rubber

Bundle of fiberglass

+ plastic

CFRP tail of an RC helicopter

Fabric made of woven carbon filaments

+ plastic

http://science.discovery.com/videos/how-its-made-bicycle-frames.html

What is a composite material?

Two or more individual materials (metal, polymer or ceramic) combined.

Frequently two phases (Matrix & Dispersed reinforcement)

Examples:FRP (fibreglass – glass fibre reinforced polymer)CFRP (carbon fibre)i.e. Ceramic fibres bonded together (reinforced) by a polymer resin

Principle of combined action:composite can exhibit combined properties of constituents, and allows property trade offs.

Example:Polymer (ductile, weak and flexible) + Ceramics (strong, stiff but brittle) Strong, stiff and tough composite.

(Ceramic fibres/particles strengthen & stiffen polymer matrix. The Matrix protects brittle fibres from damage.

Mechanical propertiesFor continuous-aligned long fibre-reinforced composites stressed in the longitudinal direction:

Elastic deformation…..

Summary: for continuous aligned fibre reinforced composites in the longitudinal direction:

Stage II – plastic deformation of matrix; elastic deformation of fibres

Stage I – elastic deformation of fibres and matrix• Modulus of composite:

Ec = EfVf + EmVm

where V is the volume fraction

• Strength of composite:

c = fVf + mVm

Example

• What is the modulus of a FRP containing 40% glass fibres modulus 69GPa in a resin modulus 3.5GPa?

Ec = EfVf + EmVm

Ec = 69 * 0.4 + 3.5 * 0.6

Ec = 27.6 + 2.1

Ec = 29.7 GPa

Ans: 5.6 GPa

Ec

Concrete• Aggregate particles (usually

60-80%) act as (cheap) filler to reduce the amount of cement (expensive)

• Fine particles of sand fill spaces between gravel

• Sufficient cement required to coat aggregate particles and bond them together.

• Water:

• Weak and brittle in tension, so increase strength by (steel) reinforcement

• Good bond between steel and concrete, and similar thermal expansion

Prestressed concrete:• Steel reinforcement stretched

before concrete is poured.• Release tension once concrete

has set Concrete placed into compression.

• Concrete now able to withstand tensile forces

Too little ~> incomplete bondingToo much ~> excessive porosity.Both ~> reduced strength

• a heterogeneous, hygroscopic, cellular and anisotropic material. • composed of fibres of cellulose (40% – 50%) and hemicellulose (15% – 25%) impregnated with lignin (15% – 30%).

WOOD

A diffuse-porous hardwood (Black walnut), showing vessels ("pores"), rays (white lines) and annual rings

Black locust end grain, showing the ring-porous structure.

Standard static-bending, compression, tension and shear testing used…. generally conducted at 12% moisture content and at 20°C.

Strength valueSitka

spruceHoop pine

Density [kg.m-3] 432.5 520.6

Static bending stress at elastic limit 42 MPa 56 MPa

Static bending rupture stress 72 MPa 90 MPa

Static bending modulus of elasticity 10 GPa 13 GPa

Compression parallel to grain:maximum crushing strength

38 MPa 48.7 MPa

Compression strength perpendicular to grain

5.6 MPa -

Tension strength parallel to grain[= modulus of rupture ]

72 MPa 90 MPa

Tension strength perpendicular to grain 0.9 MPa -

Mechanical testing of wood

e.g. soft-woods for structural parts of aircraft:

Parallel to grain

Density

Strength

Stiffness

Fracture Toughness