ENE2503 Materials Properties and Recycling
ENE2503 Materials Properties and Recycling
Exam Study Guide
Table of Contents
PART 1: POLYMERS ........................................................................................ 2
LECTURE 1: INTRODUCTION ..................................................................................... 2
LECTURE 2: ATOMIC & MOLECULAR BONDING RELATED TO POLYMER STRUCTURE .............. 4
LECTURE 3: POLYMER STRUCTURES ......................................................................... 10
LECTURE 4: PROCESSING OF POLYMERS ................................................................... 16
LECTURE 5: POLYMER PROPERTIES .......................................................................... 22
LECTURE 6: BLENDS AND COPOLYMERS .................................................................... 26
LECTURE 7: COMPOSITES ...................................................................................... 32
LECTURE 8: POLYMER RECYCLING, COMBUSTION & LANDFILL ....................................... 38
LECTURE 9: THE WASTE STREAM AND THE SEPARATION OF PLASTICS.............................. 42
LECTURE 10: CASE STUDIES I ................................................................................. 48
LECTURE 11: CASE STUDIES II ................................................................................ 51
LECTURE 12: GLASS RECYCLING ............................................................................. 54
PART 2: METALS .......................................................................................... 57
LECTURE 1: INTRODUCTION TO METALS AND THE ENVIRONMENT .................................. 57
LECTURE 2: CASE STUDIES ON MATERIALS SELECTION FOR BEVERAGE CONTAINERS ........... 61
LECTURE 3: THE TENSILE TEST ............................................................................... 63
LECTURE 4.A: STRENGTHENING- MICROSTRUCTURE PROPERTY RELATIONSHIPS ................ 68
LECTURE 4.B: RELATING STRUCTURE TO PROPERTIES- PHASE TRANSFORMATIONS ............ 71
LECTURE 5: PHASE TRANSFORMATIONS CONTINUED ................................................... 74
LECTURE 6: FRACTURE AND FAILURE BEHAVIOUR ....................................................... 77
LECTURE 7: ATOMS TO GRAINS .............................................................................. 81
LECTURE 8: METAL PHASES ................................................................................... 85
LECTURE 9: METALS LIFE CYCLE ............................................................................. 88
LECTURE 10: METALS LIFE CYCLE CONTINUED… ........................................................ 90
LECTURE 11: LIFE CYCLE ANALYSIS.......................................................................... 92
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ENE2503 Materials Properties and Recycling
PART 1: POLYMERS
Lecture 1: Introduction
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Manufacturing and waste creates greenhouse gases (eg. Co2 and methane)- effect
on global warming
Recycling would greatly reduce greenhouse gas emissions and increase jobs – more
jobs than in other waste management methods (incineration, landfilling)
Recycling started in WW2, separation of reusable materials
REDUCE, REUSE, RECYCLE
Plastics represent approx. 7% by weight of solid waste steam in Australia
More waste coming from new plastic materials and applications
Waste is getting more complex (composite materials: fibre reinforced polymers,
foams, multi-layering, etc.)
Plastics are used in a wide range of products
Plastic goods have a finite lifetime and hence turn into waste
In Australia, approximately 70% of plastics go into long-life applications and hence
do not enter the waste-stream until some time in the future
Waste generation in Australia has increased
Victorian population has grown in recent years –> more waste generation
Recycling and composting
Thermoplastic
• can be re-melted back into liquid after it hardens, melt-solidify processing is
repeatable
Thermosets
• remain in a permanent solid state, undergo chemical reaction when heated and
transform from a liquid into a solid (cross-links form), this change is permanent and
irreversible
PVC- Polyvinyl Chloride
• rarely used in packaging
• long-term waste
• usage has increased greatly
Monomer
• a molecule that can be bonded to other identical molecules to form a polymer
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ENE2503 Materials Properties and Recycling
PET- water and soda bottles
HDPE- cloudy milk and water jugs and opaque food bottles, big rubbish bins
PVC- cling wrap, soft beverage bottles, toys, plumbing pipes
LDPE- plastic grocery bags, plastic wrap, flexible lids
PP- ice cream tubs, yoghurt cubs, screw on caps, toys, drinking straws
PS- egg cartons, clear take out containers, plastic cutlery
From Hydrocarbons to Plastics
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ENE2503 Materials Properties and Recycling
Lecture 2: Atomic & Molecular Bonding related to Polymer Structure
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materials are elastic because of forces between atoms & molecules
these interatomic & intermolecular forces are called Bonds
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bonds can be stretched or compressed and can have their angles distorted (or both
at the same time)
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Types of Atomic & Molecular Bonding
Ionic Bonding
• electrons transferred, localized electrons, non-directional (attraction occurs
independent of the position of the atoms)
Metallic Bonding
• shared electrons, non- localized (mobile), ‘sea of electrons’ holding positive ions
together
Primary (between atoms) vs. Secondary (between molecules) bonding
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Primary Bonding (between atoms) – Covalent Bonding
• sharing of electrons to fill unfilled orbitals
• highly directional
Polymerization
• monomer is subject to heat and pressure
• eg. double bonds split to single bonds attach to further monomers
Polymers
• long chain molecules
Polymer Examples:
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Secondary Bonding (between molecules):
1. Van der Waals Bonds (Non- Polar)
- weaker than primary bonds
- due to momentary displacement of electron clouds- temporary dipole results in
attraction
- occurs between all atoms and molecules
- C-C, C-H, H-H, all non-polar
2. Hydrogen Bonding/Polar Bonding (Polar) -> POLAR: N, O, Cl
- stronger than van der Waals, but weaker than primary bonding
- occurs if atoms with different electronegativity (electron attraction) are bonded
- strongest case of polar bonding is HYDROGEN BONDING (when H is involved in the
bond)
- results in dipoles:
This clearly shows secondary Hydrogen Bonding between pairs of polar primary bond
chains:
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Shape of Polymer Molecules:
• Carbon bonding is tetrahedral
• Carbon bonding: By rotation about the C-C bonds, the chains are extremely flexible,
whilst still not changing bond angles
• allows for different arrangements in a covalently bonded C-C chain
• Rotation about C-C chains, but bond angles don’t change
• Primary Covalent bonds WITHIN –C-C-C- chain, Secondary bonds (non-polar van der
Waals and polar Hydrogen bonding) BETWEEN molecules
Carbon Bonding
• many polymers crystallize (commonly from 50%to 90%)
• some polymers remain completely amorphous
• long chains prevent complete crystallization
• semi- crystalline polymers: two phases- crystalline and amorphous
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Amorphous and Crystalline Regions
• semi- crystalline polymer crystallites joined by tie molecules
• amorphous part stretches when stressed
• crystallites & tie molecules combine to hold polymer together
Facts:
• large phenyl side groups inhibit chain motion, making a material rigid
• regular repetition of a chain structure permits extensive crystal formation in the
polymer, making it rigid
• this is especially the case when one n- section already has full repetition in it
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ENE2503 Materials Properties and Recycling
Lecture 3: Polymer Structures
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Primary covalent carbon bonding within –C-C-C- chains
Easy C-C bond rotation
Flexible chains
Secondary bonds (van der Waals and polar/hydrogen) between molecules (chains)
Molecular Weight:
• PD (Polydispersity)-typical polymer formed from monomer has a range of molecular
weights -> Average molecular weight is used
• DP (Degree of polymerization)- the number of repeated units per AVERAGE polymer
chain
Chemical Structure:
• a single unit is called the repeat unit or mer unit
• in a polymer chain there are many mer units – poly-mer
• polymers are made by reactively (heat and pressure) joining together many, many
monomer units
Modulus
• how stretchy the material is àhigher modulus if it is harder to pull
• primary bonds are much harder to pull apart and hence have higher modulus than
secondary bonds
• when stress is applied on POLYMERS, it acts on secondary bonds between chains,
bond angle bending and a small component along the primary covalent bond
• MECHANICAL PROPERTIES DETERMINED MAINLY BY SECONDARY BONDS
• for metals and ceramics, stress acts on primary bonds!
Molten Polymer
• secondary bonds break (lower modulus)
• chains can move past each other
• polymer chain is of random shape
• “freely rotating” coil
• rotation about bonds is easy
• there are still entanglements though (entanglements also there in solid state in
addition to secondary bonds)
• more entanglements mean increased viscosity (make it harder to flow than when
there are no entanglements)
• the higher the temperature, the more entanglements free themselves- lower
viscosity
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Cooling down a molten polymer
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Solid polymer has the same structure as the random melt only there are secondary
bonds between chains and no bond rotation (entanglements also in solid state)
Why are polymer moduli low if they all have covalent bonds?
• chains held together by secondary bonds, pull acts on these bonds not on the
covalent bonds
• rotation prevented
• polymer is rigid or glassy (amorphous)
Tg- glass transition temperature
• secondary bonds melt at this temperature
• chains become more flexible
• Entanglements hold system together after secondary bonds melt
• Polymer becomes rubbery at Tg and then molten as T increases to Tm
For amorphous polymer:
• at Tg, the modulus drops suddenly as the secondary bonds break
• the occurrence at Tg is called the glass transition
• Decreasing modulus: glassy > rubbery > viscous flow (melt)
• Glassy: rigid, cold, secondary bonds
• Large side groups decrease chain mobility- higher Tg
• A plasticizer lubricates polymer chains and hence reduces Tg, plasticizer sits
between molecules and breaks secondary
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Plasticizer
• Increasing from left to right
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Crystallites are rigid up to melting temperature, Tm (in comparison to amorphous
polymers that are only rigid up to Tg)
Tm is always greater than Tg, because bonds in crystalline region are stronger since
it is more close packed
Above Tg, crystallites and tie molecules hold polymer together after secondary
bonds have broken
Amorphous region secondary bonds break at Tg but crystalline region has stronger
bonds due to close packing
Highly crystalline polymers are useful up to Tm
Polymers above Tg and Tm can be formed into moulds (eg. injection moulding) –
these are known as THERMOPLASTICS – melt and solidify (reversible)
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Thermoplastics vs. Thermosets
Thermosetting Polymers
• remain in a permanent solid state, undergo chemical reaction when heated and
transform from a liquid into a solid (cross-links form), this change is permanent and
irreversible -> IRREVERSIBLE CHEMICAL REACTION
• have extensive crosslinks
• made by mixing low molecular weight liquids
• good as matrix for composite with fibres
• cannot be remelted
• irreversible process
• eg. epoxy, polyester
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Optical Properties
Scattering
• due to density (refractive index) fluctuations, and reflection and refraction at
interfaces
• doesn’t happen if material is homogeneous
• doesn’t happen if fluctuations are too small (<<400nm)
Reflection
• it first vibrates electrons at the surface and they reradiate at the same frequency
• electrons must be available to accept the energy
Absorption
• by interactions between the light and electrons
• electrons must be available to accept the energy of the radiation
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ENE2503 Materials Properties and Recycling
Metals (opaque)
• reflection
• electrons can acquire any amount of energy
• these are free to vibrate
• each reradiates at the same frequency
• combine to form reflected wave
• nothing penetrates the skin layer, hence metals are opaque
• metals have distinctive colour due to bound valence electron absorbing
wavelengths
Insulators/Non-conductors
• absorption and scattering
• no free electrons
• covalent, ionic or secondary bonded crystals (not metallic bonding)
• many ceramics & materials are transparent
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Amorphous polymers are clear/transparent – light passes through them
Polycrystals (100% crystalline!!!) are opaque (scattering/reflection from grain
boundaries)
Semi-crystalline polymers are milky (scattering from two phase structure)
Filled polymers (minerals, fibres, etc.) are coloured- absorption of certain
wavelengths
è amorphous is transparent, semi-crystalline is milky (unless crystals are very small (PET
bottles)), polycrystalline is opaque
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transparent materials have no boundaries, so light passes right through
eg. amorphous or in a single crystal
semi-crystalline polymers have an amorphous and crystalline stage
crystal grain boundaries scatter light and hence they are white/milky
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free electrons absorb light and reflect it through re-radiation (metals)- opaque
bound electrons don’t absorb (covalent, ionic, secondary bonded materials)
but refractive index fluctuations scatter light (polycrystals, semicrystalline polymers,
mixed materials)
Fact: Polyethylene is a milky coloured solid polymer at normal room temperatures –
because it is semi-crystalline and the combination of crystals and amorphous material make
the polymer appear milky
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ENE2503 Materials Properties and Recycling
Lecture 4: Processing of Polymers
1. Extrusion
• most basic plastics processor
• continuous operation
• powder or granules are fed to the extruder, continuous, rotation screw
conveys plastic pellets which melt, the melt conveyed through die (an
opening in the form of the wanted product)
• profile produced can be rod, cylinder or sheet
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Pipe extrusion conducted with a mandrel (solid object in the middle for melt
to flow around), held in place by a spider screw
The motion of plastic through the screw is helical
Good mixing possible- extruder used as blender- can compound in additives
like plasticizers, UV stabilizers or other polymers to form blends
Spaghetti die and chopper allow for pellets at the other end- ready for
injection moulding, etc.
Multi-layer Extrusion
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2. Continuous Film Blowing/Blow Film Extrusion
• form bubble, blown up and collapsed
• guide rolls are heated
• bottom section is cold- cooling air on outside
• multi-layer continuous film blowing is also possible
3. Injection Moulding (LARGE GREEN BINS)
• molten polymer from extruder is injected into cooled mould (clamp or press)
• small and huge objects can be moulded (eg. little containers, bins, car panels,
aircraft components)
• Process: Heat, Mould to Shape, Cool
• ram moves forward and fills mould
• mould pressure maintained
• article solidifies
• screw rotates and moves back
• die opens and ejects moulding
• ready for next shot
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4. Thin Fibres
• Very high polymer molecular alignment
• polymers can be made into very thin fibres and they have very high modulus
Why do thin fibres have higher modulus than injection moulded objects? highly ORIENTED
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ENE2503 Materials Properties and Recycling
5. Extrusion Blow Moulding
• Blow Moulding is the method of manufacture of most bottles (Extrusion or
Injection)
• makes use of PLASTIC MELT STRENGTH
• entangled polymer melt extruded into mould
• no pre-form, extruded material directly moulded
• how to distinguish between extrusion and injection blow moulding: extrusion
blow moulding has the little imperfection on the bottom where it was
pinched (bottom of bottles)
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ENE2503 Materials Properties and Recycling
6. Injection Blow Moulding
• Pre-form is moulded
• 2 stages: Injection Stage (moulding of pre-form) and Inflation Stage (blowing
to take shape of mould)
7. Rotational Moulding
• Polymer inserted in form of powder
• Rotation around several axes
• Used to make big agricultural containers, canoes
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ENE2503 Materials Properties and Recycling
8. Vacuum Forming (Heat and Air Pressure)
• Flat sheet is a thermoplastic
• Thermoplastic can be made rubbery, thermoset reacts and goes directly from
liquid to solid
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ENE2503 Materials Properties and Recycling
Lecture 5: Polymer Properties
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Tg (smaller than Tm) is the UPPER USE temperature of an amorphous polymer =>Tg
must be greater than the highest use temperature
Crystalline polymers can be of used up to Tm
Tg (glass transition temperature) = temperature at which polymer goes from glassy
to rubbery, temperature at which secondary bonds break and chain rotation further
increases
Increasing temperature (secondary bonds break at Tg, starts melting at Tm):
Amorphous Polymer
• Modulus decreases immensely at Tg
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ENE2503 Materials Properties and Recycling
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Dynamic Mechanical Thermal Analysis (DMTA) is a good way of getting modulus vs.
temperature and thus Tg
Sinusoidal stress input, what output strain like in terms of amplitude (Young’s
Modulus) and phase (loss modulus)
maximum loss Modulus (phase difference) gives Tg
Loss Modulus measures energy dissipated as heat
How does chain structure affect Tg?
1. Big Side groups
- decrease flexibility, lower mobility, weigh down the chain and require more
thermal energy ->higher Tg
2. Polar Groups
- increase secondary bonding between chains, higher Tg
- Note: Cl is very polar, has higher Tg than big side group
3. Chain Length
4. Molecular Weight
5. Chain flexibility/rigidity
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ENE2503 Materials Properties and Recycling
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polymers can be used above their Tg if they have some crystallinity
Polymers can crystallize if the chains are REGULAR IN SHAPE & ABLE TO PACK
CLOSELY, CHAIN FOLD
Crystalline regions: form zig-zag pattern due to valency of carbon, long chains stack
next to each other
Polymers are NEVER 100% crystalline, always only semi-crystalline
Crystalline regions are chain folded, so crystallize quickly
Crystalline regions and amorphous regions connected by tie molecules
Since semi-crystalline polymers have both amorphous and crystalline regions, they
have both Tg and Tm, where Tm is always higher than Tg
Why? – more and stronger secondary bonds to break in crystalline region, because it
is better packed (chain folded) and there are tie molecules
Semi-crystalline polymer
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crystals have higher density and greater modulus than amorphous region(due to
better packing and more secondary bonds)
amount of crystallinity depends on how fast you cool from melt
Faster cooling-> less time to crystallize-> lower modulus
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ENE2503 Materials Properties and Recycling
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metals and ceramics show same behaviour as 100% crystalline polymer because they
have a crystal lattice
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Polyethylene can be polymerized in two ways – HDPE & LDPE
- HDPE- 80% crystalline
- LDPE- 50% crystalline
- Method of synthesis influences the amount of side branching and hence the ease
of crystallization
- HDPE is linear- easy for chains to pack closely, chain fold and crystals to form –
higher crystallinity- higher density and higher modulus- stiff LDPE is highly branchedhard for chains to pack closely and form crystals- lower crystallinity- lower density
and lower modulus- less stiff
Melt flow Index (MFI) measures the easy of flow of a thermoplastic
- defined as mass of polymer (g) flowing through capillary in 10 minutes
- high MFI means low viscosity (easy flow)
- high MFI corresponds to low MW
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Low Molecular Weight- Low viscosity – low melt strength- high Melt Flow Index
High Molecular Weight- High viscosity- high melt strength- low Melt Flow Index
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ENE2503 Materials Properties and Recycling
Lecture 6: Blends and Copolymers
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We can vary a given polymers properties (if it is crystallisable, and not all polymers
are) by the rate of cooling or post-processing annealing between Tg and Tm
Annealing
• re-heat a metal and allow it to cool slowly (to allow for greater crystallinity – higher
modulus and transition temperature)
Other ways to change properties of plastics (modulus and transition temperature):
1. Copolymerization
• (so far everything has been about homopolymers, where one type of monomer is
used and repeated)
• the combination of different monomers at the time the plastic is synthesized
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Block and Graft copolymers have 2 distinct phases, so it has 2 Tgs, while the others
have only 1 Tg
copolymer properties are often an average of homopolymers (eg. softening
temperature, mechanical properties, etc.)
EXCEPT FOR GRAFT COPOLYMERS and (block copolymers?). Get domains of each, bit
like a BLEND of both polymers
BLEND= physical mixing, COPOLYMERIZATION= reaction, molecular- primary bonds
Flory-Fox equation for Tg of RANDOM (statistical) Copolymers -Average:
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ENE2503 Materials Properties and Recycling
2. Plasticisation
• Plasticiser= small molecules which sit between chains and decrease secondary
bonds
• Cause lower modulus (more flexible) and lower Tg
• Eg. rigid white PVC pipe compared with plasticized garden hose
• Plasticisers are chemicals (not polymers) that have a high molecular weight, high
boiling and vaporization points
3. Crosslink the polymer
• Crosslink the polymer
• Crosslinking means joining up different polymer chains by PRIMARY BONDS, not just
secondary bonds
• Crosslinked polymers CANNOT crystallize (100% amorphous), thus they have Tg only
• Modulus increases, so does glass transition (Tg) because there are more bonds to
break and limited chain rotation
• “holds rubbery tyres together”
• crosslinking of a thermoplastic polymer produces increased rigidity
• not all cross-linked or network plastics involve crosslinking of linear chains
• some form crosslinks straight from the monomer stage (thermosets), ie. Epoxies
(“super-glue”)
• although in theory crosslinked polymer is “one big molecule”, imperfections such as
chain ends and loops mean this isn’t true
Elastomer
• a lightly crosslinked rubber
• 100% amorphous, so amorphous characteristics overpower the light cross links
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ENE2503 Materials Properties and Recycling
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Tg is less than room temperature, sample is rubbery
Eg. car tyres, rubber bands
Low modulus
High extensions (600% elongation)
Recovers shape when released
Effect of cross-linking:
4. Polymer Blends (physical mixture of plastics)
• plastics seldom used on their own
• if a homopolymer cannot be found to do the job, it is expensive to design and
synthesize a new plastic; instead existing plastics are blended together (cheap and
easy)
• may be able to blend a cheaper polymer with a more expensive one and maintain
properties
• a polymer that is hard to process (high viscosity leading to degradation) may be
blended with lower viscosity polymer
• each polymer may have good properties in its own right – combine to get best of
both worlds
• eg. Polymer 1 (high modulus) blended with Polymer 2 (chemical resistance) to form a
stiff polymer with good chemical resistance
• Polymer blends may be ONE PHASE (like scotch and coke) or TWO PHASE (like oil and
water)- depends on Enthalpy and Entropy
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G- Gibbs Free Energy of mixing (How does energy change when things are mixed?)
Enthalpy (H)- sum of internal energy, interactions between different polymer chains (what
secondary bonds are possible?) –change in H<0 favors mixing
Entropy (S)- what is the change in disorder on mixing? (always increases over a process,
more possible microstates of how something can exist means higher entropy)
2nd law of Thermodynamics
• spontaneous processes always result in greater disorder- things always go to their
greatest disordered state
• for a blend, the entropy of mixing is always greater than before (more possible
microstates)
• for any blend, the entropy of mixing is always greater than 0, hence free energy of
mixing is less than 0
• if change in G <0, =0 or slightly >0 materials are miscible (one phase)
• most polymer blends are immiscible (two phase) Why?
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some polymer blends are miscible but only 1 or 2 are commercial. Most blends are
immiscible
Miscible blends: occur when different chains are intimately mixed on a molecular
level
Immiscible phase- has gross phase separation on micron scale
How to tell if they are miscible or not?
1. By microscopy (optical or microscopic) – can you see one or two distinct phases?
2. By looking at how many Tg’s temperature there are (from modulus vs temp curves
or loss modulus vs temp curves)
- 1 Tg => miscible blend (molecular mixture)
- 2 Tg’s => immiscible blend (two phase)
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Immiscible blends often show additive (average) behaviors in terms of modulus BUT
have poor interfacial adhesion of phases, leading to poor failure properties (eg. low
tensile strength and low impact strength)
Rules for miscibility and adhesion:
• “likes dissolve likes” - miscibility
• “likes attract likes” – adhesion (greater failure properties)
• eg. 2 polar polymers are miscible or at least adhere well, 2 non-polar polymers are
miscible or at least adhere well, a polar and non-polar polymer are immiscible and
usually do not adhere well
Compatibiliser
• a low molecular weight polymer (usually a copolymer) that sits between phases of
an immiscible blend and improves interfacial adhesion
• “glues” the two phases together
• improves impact strength and failure strain
Summary of ways to change polymer properties:
• change crystallinity by rate of cooling (also annealing and then cooling again)
• copolymerization
• plasticization
• crosslinking
• physical blending
Case Study: Bisphenol A
• a colorless solid
• soluble in organic solvents, but poorly soluble in water
• used to make plastics and epoxy resins
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BPA based plastic is clear and tough- found in water bottles, sports equipment, CDs
and DVDs
Epoxy resin containing BPA- lining of water pipes, coatings on food and beverage
containers, thermal paper
Use in some products has been banned due to hormone-like properties
Plastic water bottles
• made with chemicals known as plasticizers
• with the purpose to make them strong and flexible
• contain bisphenol- A or phthalates – both are known hormone disrupting chemicals
• to find out what the bottle is made of, check the number on the bottom
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ENE2503 Materials Properties and Recycling
Lecture 7: Composites
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Eg. tires, tennis racket frames, fiberglass, airplane components
STIFF, STRONG and FRACTURE TOUGH (ductile), BUT EXPENSIVE
can be Fiber- reinforced or Particle-reinforced
Composite Material: any mixture of materials (concrete, reinforced concrete,
fiberglass, metal alloys, etc.)
Matrix is usually plastic (polymer matrix)
Materials reinforced with fibers
Fibers can be plastic, glass, liquid crystal polymer, or ceramic whiskers
Most composites involve fibers
Fibers may be CONTINUOUS or CHOPPED
Fibers are very stiff and thus increase the modulus of ductile (fracture tough) resins
Continuous fibers offer better reinforcement than the chopped fibers
Better alignment of fibers (chopped or continuous) means higher modulus-> but only
in the direction of the fibers
One way to get rid of anisotropy (different value in different directions)
Isotropic- having the same value when measured in all directions
And to have better properties in all directions is to use laminates: mats are put
together with a range of orientations
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Matrix
• has to wet and impregnate fibers
• thermoset polymer (epoxy, polyester, etc.)
• Tg matrix >> Tg use, so the matrix is glassy and rigid at use
• Modulus of matrix, E~2-5 GPa
• Matrix is the material that seeps between the fibers to hold them all together
Different structures:
• unidirectional composite, showing fibres and matrix
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random direction (matrix not shown)
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no matrix, waved bundles, fibres in two directions (horizontal, vertical)
6ft Tensile Properties of Composites
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load acts on matrix (low modulus) when fibres are not oriented in direction of pull
load acts on fibres (high modulus) when oriented in the direction of pull
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Fracture Properties of Composites
• synergy (combined to produce greater result) in composites is best in fracture of
fibre reinforced composites such as fiberglass
• these have a good strength to stiffness ratio (it is often hard to have both, for other
materials it is usually one or the other)
• resin (plastic) is brittle, glass fibre is brittle but together it is fracture tough- ductile
• fracture toughness (DUCTILITY) is due to (1) fibre pull out (friction, energy absorbed)
and (2) crack size increases, grows around fibres and absorbs more energy
• high stiffness
• high strength
• however also high cost
Ways to manufacture Composites:
1. Pultrusion
• to make a reinforced plastic article by drawing resin-coated glass fibres through a
heated die
• combination of fibres with thermosetting resin (monomer)
• only for continuous fibres
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2. Hand Lay Up
• laying down of fibreglass cloth and impregnating it by pouring monomer onto
it and rolling for compaction
3. Spray-up Process
• can also process fibres and mats by hand
• fibres are chopped inside the spray machine
• spray-up technique by firing glass reinforcement through a nozzle with
monomer
• fibre through one hole, monomer through another – sprayed simultaneously
into mould
4. Helical Winding- Cylinders
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5. Autoclave
• a strong heated container used for chemical reactions and other processes
using high pressures and temperatures
• material with a matrix in which the polymer is still soft because it hasn’t
reacted yet is entered into the autoclave
• put into autoclave to react and become hard. To form into mould the high
pressure is applied.
• Pre-pegs used to give it an initial shape before putting into the autoclave
6. Injection Moulding of short fibre reinforced plastics
• polymer granules impregnated with short fibres can be injection moulded
• used for automotive applications (door modules, seating units, engine bay
parts)
• higher stiffness than pure polymer
• high turn-round time
• hard to inject (high viscosity)
• harsh on equipment
Summary of ways to make composites:
1. Pultrusion
2. Hand lay-up
3. Spray-up
4. Helical Winding
5. Autoclave
6. Injection Moulding
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ENE2503 Materials Properties and Recycling
Lecture 8: Polymer Recycling, Combustion & Landfill
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Plastics recycling is a multi-step process
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different types of plastic are distinguishable by a number coding system
Plastic waste is a PROBLEM
• non- biodegradable rubbish soup in water bodies
• marine mammals are killed by ingesting or getting tangled in plastic
• SOLUTIONS: producing new plastic from recycled materials- uses only 2/3 of the
energy required to manufacture it from raw materials
Australian Plastics scene
• Plastic represents approximately 7% by weight of the solid waste stream in Australia
• increasing level of imports
• major component is packaging (mainly LDPE and HDPE)
• today many households recycle domestic plastic waste
Plastic as Waste
• BAD: Low density-> takes up lots of volume, Awkward shapes -> hard to handle
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GOOD: Easy to compact (but this takes effort and is expensive), Light to transport
(this is cheap and lowers fuel costs)
Government Action through policies
• Certain amount of all plastics packaging must be recycled
• goals set for large proportion of cars to be recycled
• use of recycled rubber in highway asphalt
• in Australia recycling and reuse in households has increased immensely in recent
years
• common items: paper, cardboard or newspapers, plastic bottles, glass, plastic bags
• recycling is facilitated by municipal kerbside recycling services
Methods of Plastic Waste Management
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Landfill
major move against landfill-> Concern about environment, less space, few jobs
the process is not cheap: cartage, collection, tipping fees
low value of materials recovered!!
In Australia, 48% of all waste going to landfill – increase in recent years – Australia
uses landfills more than most countries
Move against landfills has diverted waste away, but the increasing amount of waste
produced means there is still a net increase in waste fed to landfills
Alternatives for Polymers:
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Reduce, Reuse and Replace with better material
use less through changes in design and material (material properties)
reuse on a personal level
downsizing of size – requires less material
better mechanical properties (yield, strength, processing conditions) allows for size
reduction
3. Combustion of Polymers
GOOD:
• most plastic waste readily burns
• burning roughly recovers most of the energy used in making them -> Convert waste
back to energy
• the recovered energy can be used for producing electricity
• burning can reduce landfill waste by 50% by weight and 95% by volume
• since many plastics are made from petroleum, burning them produces much the
same energy
BAD:
• bad image due to recollection of old-fashioned incinerators with smoke
• even though filters are implemented, there are still some health concerns
• some plastics produce toxic gases arising from toxic chemicals in the material
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Steps:
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the answer to toxicity can be chemical removal from output of dangerous gases, or
more careful sortation of what goes in initially
there can be dangerous ash residue
polymers themselves do not form ash, ash results from metals etc., which do not
burn. They may occur due to additives in plastic (stabilizers, pigments, heavy metals,
etc.)
today there are certain requirements for incineration facilities:
must have burning as complete as possible
this can be done by: holding material at correct temperature, for sufficient time,
turbulence in combustion region so temperature gets well distributed
there are different types of incinerators – batch or continuous
must be able to dry incoming feed
Energy conversion goes into heating water which converts to steam in boiler tubes
Particles can be removed by secondary combustion chambers, scrubbers or
precipitators
Scrubbers: pass the gas through droplets or neutralizing materials through which the
gases pass
Precipitators: electrostatic- induce charge on particles and sweep them onto plates
truck delivers waste to storage pit
loaded via hooper to charging chute
drying
volatiles are combusted
combustion of solids
steam generated to turn turbine for electricity or to heat boiler for domestic heating
Ash and gas discharged
Summary of undesirable aspects of incineration
• high cost of operation
• high cost to build facility
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high transport cost as materials from a wide area are needed
unburnable material must go to landfill – leaching into water table
carbon dioxide air emissions and other GHG
removes incentive to conserve or recycle!!!
4. Recycle- chemically depolymerizing, melt processing
5. Make plastics degradable (biodegradable or UV degradable)
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ENE2503 Materials Properties and Recycling
Lecture 9: The Waste Stream and the separation of Plastics
Primary Recycling
• reusing plant scrap, never sold and used (eg. trimmings)
Secondary Recycling
• post-consumer waste, physical treatment and cleaning needed
Tertiary Recycling
• depolymerization by treatment with heat and chemicals
Gross (large scale) Sortation of Plastics from other material
1. Hand sorting (physical properties, code imprinted)
2. Electromagnetic for steel cans
3. Eddy current induction system for aluminium cans
4. Air blowing for plastic vs. glass separation
5. Mechanical gravity system for plastic vs. glass separation glass crusher
Often one after the other or a combination of some is used in a separation facility:
Eddy current:
• a secondary magnetic field is induced around the non-ferrous particle (eg.
aluminium). This field reacts with the magnetic field of the rotor, resulting in a
combined driving and repelling force -> this ejects the conducting particles from the
stream of mixed materials
• sorting of different plastics is critical to all recycling programs
• the sorting stage is the MOST EXPENSIVE
• need to separate resins by generic type and color
• sorting may occur at: point of discard or generation (eg. at home), collection at pickup point (bin), centralized location, at a specific commercial location
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Once plastics have been separated from other materials: 2 options for separation of
plastics (eg. containers):
• Macroseparation- separate containers
• Microseparation- separate shredded containers -> chips/flakes
Macroseparation Techniques (manual or machine)
• Clarity – translucent vs. clear, by shining light emitting diodes along conveyor
surface
• Fluorescence- separate PVC from other bottles, by producing low voltage x-rays
which make the C-Cl bond fluoresce
• Color, through visible light system, color cameras
• Shape, a scanner inputs and digitizes the shape of well-known containers
• Infrared spectroscopy, by shining of a laser particular chemical groups (especially
near infra red) are able to be identified
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easiest by hand on conveyor belt- but this is expensive and boring
most new solid object separation is based on some chemical sensor shining on
bottles (or other methods discussed above) passing by on conveyor
need good, even, spatial distribution of objects on a moving conveyor
automatic arm or air jet sweeps off article if found
usually a number of aspects used at once to sort
problems can occur if materials are dirty and hard to recognize
Microseparation
• key to recycling
• chip/cut materials up into flakes or pellets
• assume that materials differ with regards to at least one physio-chemical property->
Density, Wettability, Magnetisability, Electrical Properties, Chemical Properties,
Optical Properties
• Properties – Magnetic, Electrical, Chemical, Optical
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1. Wash/Float Method in gravitational field
• Density separation through wash/float method resulting from gravity- floats if
gravity is lower than that of water and sinks if its higher
• Difficulties: additives in plastic vary densities; adhesion of labels, inclusion of fillers
or dirt also vary densities
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Sink/Float Method in centrifugal field
increase force field above that of gravity, i.e. centrifugal force
uses a device called hydrocyclone
either the device rotates or a fluid velocity is fed
heavy particles move outwards, lighter ones to the centre
same as gravitation sink-float method, expect forces are higher and there is radial
rather than vertical motion
• lighter materials go UP, heavy materials go DOWN
• limited amount of material, recirculate liquid
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Froth Flotation
stirred tank with air fed in from bottom
works on differential affinity of materials for air bubbles
materials with lower wettability cling to bubbles and rise
related to polarity/hydrophobic (repel water)/hydrophilic (mixes with water)
most polymers are fairly hydrophobic, thus not much differentiation – most will rise
with the bubbles
problem if plastics have similar density
also related to particle size and shape
add frothing agents to promote bubbles, small bubbles cling to plastic
materials with similar wettability are not easily separated
while most plastics are hydrophobic, surface treatment can be used to make some
hydrophilic
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in playing around with this technique, must manipulate wetting agents, frother, and
pH condition
rising agent can be heavier and lighter
sinking material must be heavier
4. Electrostatic sorting
• plastics rubbed or impacted in fluidized beds to cause electrostatic charge
generation
• depends on different electron affinity in plastics
• two requirements for this method to work: separate, non-aggregated particles &
different conductivity or triboelectric (charge on rubbing) charging behavior
Roll separators:
• roll separator used for plastics to separate conductivities
• material fed past an emission electrode
• polarizes particles on the roll by emitted ions
• high voltage electrode induces charges
• as the roll moves, conducting particles release charge and are attracted to the
electrode
• non-conductive particles continue around and are brushed downwards
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Free Fall Separators
• due to different triboelectric (charge on rubbing) charge capability
• some polymers charge differently and hence have different polarity
• electrodes cause separation
• some polymers charge positively and others negatively
• charge depends on the chemistry of the polymer
• additives affect characteristics
• opposites attract in the electrode – positive charge to negative plate, negative plate
to positive charge
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5. Chemical Dissolution
• either selectively dissolve different polymers
• OR dissolve all polymers, and selectively precipitate
BAD:
• toxicity means expensive and complicated measures to ensure health
• solvent handling required (costly, loss of solvent), explosion-proof rooms
• high energy cost to evaporate solvent
GOOD:
• very pure separation possible, contaminants and additives filtered out
6. Thermal Separation
• different polymers have different softening points
• contacted heating rolls or belts, different adhesion
• temperature increases from top to bottom
• those materials that don’t adhere are thrown off and those that do go around and
are scraped off
• lowest melting polymer sticks to the first belt and is scraped off while higher melting
polymers stick to the belt at lower levels
Summary of Microseparation Techniques
• Wash/Float Method in gravitational field
• Sink/Float Method in centrifugal field
• Froth Flotation
• Electrostatic Sorting
• Chemical Dissolution
• Thermal Separation
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Lecture 10: Case Studies I
1. Polyethylene
• made from ethylene gas (by-product of oil or natural gas)
• Low density polyethylene (highly branched- 50% crystalline)- made under high
pressure
• High density polyethylene (linear-80% crystalline)- made under low pressure
• Reactor can affect: molecular weight, molecular weight distribution, amount of
crystallinity
• These characteristics affect processing (viscosity), mechanical properties,
permeability, opacity, melting point
• World’s most recycled polymer
• Easy to recycle
• Very cheap polymer- difficult to compete with virgin material
• Main feedstock for recycling: rigid containers, film
• Main endpoint of recyclate is packaging
• Often recyclate blended with virgin materials for better properties
Properties
• good thermal resistance (both have high melting temperatures)
• poor solubility (good chemical resistance)
• low weathering rate
• biodegradable (slowly), UV- degradable (slowly)
HDPE (80% crystalline)
• majority pre-consumer industrial and post-consumer domestic
• dairy containers (milk cartons)
• used for: pipes, garbage bins, milk bottles, crates, sheets/panels, freezer and
shopping bags
• Issues: coloration, only minor amounts used in application
• Before breaking point, elastic modulus INCREASES with number of processing
operations – recycling makes it stronger
• Elongation DECREASES with number of processing operations- recycling makes it less
ductile
LDPE (50% crystalline)
• Shrink and stretch wrap from packaging
• Used for: builders film, cling wrap, toys, tubes, saline drips
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Issues: contamination, multi-layer films
Elongation at break decreases with expose time
Strength and elongation at break DECREASE when recycled
most common: degradation through chain scission from multiple processing!!!
used milk bottles are collected and recycled- cannot be used as more milk bottles
due to contamination and degradation-> need larger items to use up polymers (bins
are a good idea, but do not pass the drop test)
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Polypropylene
fourth biggest selling plastic
higher melting point due to CH3 unit
higher modulus than polyethylene
similar processing issues to polyethylene EXCEPT the methyl group (CH3) encourages
much greater breakdown during processing (anti-oxidants)
majority pre-consumer and post- consumer use is industrial
used for: batteris, bumper bars, pallets, crates, flower pots, buckets
limited kerbside collection due to low volumes of this material
Issues: variety of grades, contamination in consumer waste, polymer degradation,
low cost of virgin PP
Can come as neat material (carpets, ice cream containers) but often blended
(bumper bars, blended with rubber)
To improve toughness blended with rubber, but may be copolymerized with
ethylene
Recycling issues: separate acids from batteries and paint from bumper bars
Molecular weight DECREASES when recycled (lower viscosity)
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3. PET
• made by a process that is reversible under certain conditions
• big possibilities to return to original monomer
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can also be remelted in usual way
linear, thermoplastic, semi-crystalline polymer
amorphous region is glassy, discourages gas permeability, as does crystalline region
crystals are very fine and hence don’t scatter light (that is why it is see-through)
very low viscosity after melting, because melting point is much higher than Tg
Uses of virgin PET: fibres in carpet and film for videos and cameras, bottles
Bottles made from PET: pre-form is injection moulded, then bottle is blow moulded
Benefits of PET for bottles: lighter than glass (low transport cost), excellent barrier
properties, good mechanical properties, high shatter resistance, good chemical
resistance, easy to recycle, potential for reusable bottles
PET can be recycled in 2 ways
1) Cleaned and preprocessed by melt blending (IN EXTRUDER)
- product used for fibres in carpet, pillow stuffing, roof insulation, non-food bottles
2) Depolymerization (tertiary recycling)
- three main ways, depending on addition and chemicals added
- conversion of PET into raw materials
- readily recleaned
- can be used to make new bottles (hygienic to reuse again for drinks)
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Lecture 11: Case Studies II
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Polystyrene
on its own its brittle with high Tg
mainly toughened with rubber particles -> High Impact Polystyrene (HIPS)
HIPS is created by adding poly(butadiene) to the monomer
Applications of HIPS: injection mouldings, thin food containers, fridge boxes,
cassette boxes
HIPS is thermally very stable
Morphology doesn’t change much with reprocessing
Recycling doesn’t change the properties of the material much
Applications for recycled HIPS are non-food, flower pots, binder for particle board
Foamed Polystyrene
• used in fast food containers, egg trays, protective wrapping
• known as EPS (expanded Polystyrene)
• foams are granulated, washed and compacted under heat and pressure (to collapse
cells)
• densified PS reused for egg cartons, desk trays and waste-paper baskets
• can be used in non-densified form for insulation
• also used in garden soils (odorless, chemically neutral)
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Polyvinyl chloride (PVC)
rigid, polar molecule
high Tg due to polar side group (Cl)
biggest seller after PE
PROBLEMS: hard to process near, degradation (oxidative)
Incineration: produces HCl (acid rain), dioxins, heavy metal stabilisers
Recycling: contaminants, chars at PET processing temperatures, bubbles, degrades
and gives off gas
Often has additives: stabilizers, plasticisers (reduce Tg), fillers, impact modifiers, UV
absorbers
Rigid PVC- pipes, bottles, gutters, roof sheeting
Plasticized PVC- cables, hose, shoe soles
Primary use in the building and construction industry
Recycled PVC is often mixed with ~75% virgin PVC to make: PVC bottles, flooring,
window mounts, pipes
6. Commingled Plastic Waste
• avoids problems of separation (technology required and cost)
• useful for multi-component materials (blends, rubber-toughened materials, coextrusion)
• most polymers are incompatible, thus commingled products are designed with large
cross-sections and low strength demands
• problem of processing temperatures because different plastics have different
melting temperatures (PVC degrades)
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commingled waste is comprised mainly of HDPE
used to make- bumper bars, park benches, flower pots, fence posts, plaster, bins,
sound barriers, poles
PET and HDPE are very incompatible:
properties are improved through the use of compatibilisers (usually copolymers)
properties are better when there is more of one of the two components, not
half/half
7. Recycling of Rubber
• crosslinked plastic, hardened with rubber by processing it with sulphur -> can’t be
melted or reprocessed
• however it is easily identified and separated
• 50% of worlds rubber used in tyres
• tyres are loaded with steel wire, fabric and carbon black (improves properties,
affinity)
• burned for fuel and energy recovery
• must be shredded and dewired before burning
• better calorific value than coal- more recoverable energy
• less CO2 and sulphur than coal
• can have toxic heavy metals, NO and SO2 fumes
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Uses for recycled tyres: cores for solid tyres, rubberized asphalt, flowerpots and bins
when mixed with plastics, drainage, footpaths, tennis courts, flowable concrete
Tertiary recycling – devulcanisation – to get rid of sulphur crosslinks
Scrap tires may be cut, punched and stamped into various rubber products after
removal of the steel bead – eg. floor maths, belts, shoe soles, seals
Whole tyres can be used as highway crash barriers or boat bumpers in docks
Shredded tyres used in many different types of applications
Currently half of used tyres go to landfills for energy recovery
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Recycling of Thermosets
as with rubber can’t be remelted
epoxy resins, polyester resins (fiberglass)
usually full of fillers and fibres
some thermosets can be broken down chemically (tertiary recycling)
aim is to develop new thermosets which break down more easily
early idea was to grind it into fine particles and remix as high modulus additives in
other materials – however this turned out to act more as a filler than reinforcement
and made processing of the materials difficult
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Biodegradable Plastics
plastics made from plants – cheap and sustainable
most synthetic polymers are NON-biodegradable
naturally- occurring polymers are more biodegradable
push to starch- based polymers to or blends (eg. starch + PE for supermarket bags)
starch is cheap and is eaten by soil micro-organisms in landfills
in modern, well-defined landfill, biodegradation is unlikely (no moisture or air)
problem if they get into commingled stream
10. Photodegradable Plastics
• most polymers slowly degrade in UV (and oxygen)
• must be in main chain of the polymer
• can add chemicals to encourage UV absorption and degradation
• plastics are not out in the open for long enough for them to photodegrade
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Lecture 12: Glass Recycling
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earliest artificial material
most common (soda glass) made of : silica from sand, limestone, sodium ash
sodium ash lowers melting point and prevents crystallization
limestone (calcium carbonate) reduces water solubility
amorphous – Tg about 700℃
if crystalline, melting point about 1000℃
Production of glass:
• 4m wide strip floats on molten tin
• flown on slowly, remove irregularities
• solidifies, drawn slowly through annealing rollers
• cooled down and cut into pieces
• modify surface by putting ions (copper) in bath and putting voltage over glassreplace existing ions- anti-glare glass
• tints used to modify the colour of the glass
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Manufacture of glass Bottles
• press and blow method (one method)
• the parison is shaped by a metal plunger
• pressing operation pushes the glass into mould
• the parison transferred to final mould and air blown into mould
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Glass Recycling
• glass comprises about 25% of all recyclables collected (primarily municipal waste)
• recycling glass saves 74% of the energy it takes to make glass from new materials
FOR ALL MATERIALS, ITS IMPORTANT TO ADD SOME VIRGIN MATERIAL TO THE RECYCLE
PELLETS TO KEEP GOOD PROPERTIES (RECYCLED PELLET HAVE UNDERGONE DEGRADATION)
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Cullet
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glass as packaging material has decreased immensely
glass products are useful
inert, not affected by acid, good for food and drink
easy to sterilize and clean
allow hygienic re-use after cleaning
approximately 10% of waste stream is glass, mainly bottles
glass is much heavier than steel and plastic
crushed waste glass for recycling
(Pellets/Flakes – for polymers)
cullets also used on the roads and construction
problems with imbalance of colours (not enough clear recycled)
good to sort coloured glass at collection, not always possible
may sort bottles by colour, before crushing
other than colour, easy to separate from other metals and plastics -> NO ADDITIVES
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PART 2: METALS
Lecture 1: Introduction to Metals and the Environment
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materials engineering is the exploitation of the relationship between the structure
and properties of materials with the aim of conferring an engineering and economic
advantage
environmental engineers aim to preserve materials within the process stream and
minimize adverse environmental effects of their production, thereby yielding an
economic and engineering advantage
Ceramics- brittle, hard
Polymers- deformable (ductile), soft
Metals- deformable (ductile), hard
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ceramics are the material used most globally (dominated by concrete), glass is also a
ceramic
global metal production has increased over time
metal usage is dominated by steel
Resource Availability:
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Reserves can be increased by: finding new definite sources, improving technology
that can handle lower ore grades, improve energy efficiency so it is economically
viable to extract lower ore grades
Reserves can be made to last through reduce, reuse, recycle
Australia is one of three largest iron ore producers and is the largest bauxite
producer
Resource availability is limited, wants are unlimited
Exponential growth can be used to describe a constantly increasing production rate
Calculating rate of consumption:
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NOTE: r is a percentage, NOT the percentage converted to a fraction
non- renewable energy sources (coal, gas, oil) account for 86% of the world’s total
energy consumption
Materials- Energy- Carbon triangle:
• Carbon indicates the emissions from the production of materials and usage of
energy
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Embodied energy
• the energy requires throughout the entire process from getting raw materials to the
finished good; the energy consumed by all the processes associated with the final
good’s production and selling
• only a small percentage is actually recoverable
• sources that are more difficult to extract or of lower grades will require more energy
to produce
The material life cycle:
Energy required to manufacture materials
• energy losses can be significant
• depending on the product, manufacture can be a significant proportion of the
energy consumed throughout a product “life cycle”
• energy consumption in products is greatest in different stages
• Stages:
1) Material
2) Manufacture
3) Transport
4) Use
5) Disposal
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in energy intensive applications the impact is on increasing the efficiency (eg. of a car
motor)
recycling metals consumes much less energy than using virgin materials
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Lecture 2: Case Studies on Materials selection for Beverage Containers
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there are 3 things that can be done to cope with future shortages and to minimize
the environmental impact of engineering materials
à REDUCE, REUSE, RECYCLE (replace, redesign)
Life Cycle Analysis
• looks at the stages and how much energy is needed or consumed at each stage
• majority of energy consumed at different stages of the life cycle (materials,
manufacture, transport, use, disposal)
• in non-energy intensive applications, the focus is on reducing the impact of and
amount of material used
Case Study: materials selection for beverage containers
• to minimize the environmental impact of engineering materials
• REDUCE, REUSE, RECYCLE
Questions asked when choosing a material:
1. What is the function of the object?
2. What material properties are required?
3. What is the most environmentally friendly way?
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energy calculations have driven a reduction in weight of beverage containers (less
energy required for less material and hence lower weight) -> weight minimization
Rolling of sheets for can production and then manufacture as a multi-step operation
(including redrawing and ironing)
Recycling cans requires 5-10% of energy needed for new can
Can body needs to be strong but ductile (high toughness), lid needs to be strong but
brittle to break on demand
Designing a minimum weight cable
• performance equations, eg. to minimize mass: write an equation for mass and there
will be another equation (eg. yield stress=F/A), combine equations to eliminate free
variable and find material index by isolating variables that depend on material
properties-> Minimize performance equation but maximize material index
• cost considerations may influence the choosing of a material -> trade-offs
• eliminate the variable component by substitution (usually radius, r)
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metals have a high density and high strength
often must find a compromise between the cheapest and the strongest material
where load-support is less of an issue, castings are usually significantly cheapercastings have lower modulus than formed metals
pouring molten metal into a mold and cooling it
Wrought
• beaten out or shaped by hammering
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trade off between production cost and desired properties
castings produce more scrap but are cheaper to make
wrought products allow us to use less material, but are more expensive
for the production of beer, bottle production is the subsystem that mostly gives rise
to environmental impacts
greatest energy use however occurs in the transport and distribution phase
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ENE2503 Materials Properties and Recycling
Lecture 3: The Tensile Test
Stress- size-dependent measure of load (Force/Area)
• positive for tension, negative for compression
• in a tensile test, the sample is pulled in tension
Strain
• size-dependent measure of displacement
• change in length over initial length
Elastic Behavior
• reversible, shown as a linear relation between stress and strain
• bonds are stretched and then return to initial when unloaded
Elastic Modulus (stiffness)
• stress/strain in the elastic region, the steeper the linear line, the greater the
modulus and the less the material deforms
• to minimize deformation, select a material with a large elastic modulus
• greater modulus- steeper gradient- greater stiffness
• stiffness- force/elongation
• Elastic Modulus- stress/strain
Plastic Behavior
• non-reversible, permanent deformation, occurs when the tensile uniaxial stress
reaches σy- yield strength
• bonds stretch and planes shift, so bonds cannot fall back into place
Toughness
• the energy needed to break a unit volume of material (area under stress strain
curve) up to breaking point
Ductility
• the plastic strain at failure (total elongation- elastic snap back)
• in a tensile test, uniaxial tension is applied
• Stress- Strain Curve:
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elastic limit can be difficult to define exactly – depends on the measurement
accuracy and can be a gradual change rather than an abrupt change in slope
0.2% proof strength- draw a line parallel to elastic region at 0.002 strain
at this point noticeable plastic deformation will have occurred
Necking
• begins at ultimate tensile strength until breaking point is reached
• strain localizes (NOT STRESS) and true stress increases
• necking occurs when an increase in strain produces no increase in load supported by
the beam
Toughness
• how much energy is required to break material, approximated by the area under the
stress- strain curve
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Ductility
• a measure of the ability of a material to undergo plastic strain under stress before it
fractures
• measures of ductility: Strain after fracture, % elongation, percentage reduction in
area
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polymers are more ductile but less strong than metals- METALS HAVE GREATER
TOUGHNESS! (larger area underneath curve)
Nominal/ Engineering Stress
• a measure relative to the original cross sectional area
True stress
• measures the instantaneous behavior (instantaneous cross sectional area)
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similarly nominal strain is related to the initial length while true strain is related to
small strain increments
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Work hardening
• the increase in stress needed to produce further strain in the plastic region. Each
strain increment strengthens or hardens the material so that a larger stress is
needed for further strain
• work hardening occurs between elastic limit and ultimate tensile strength
• a measure of storage of plastic energy
• work hardening is the result of cold-working
• the rate right after yield strength
• dislocations entangle, dislocation density increases- impedes slip of planes
• the increase in stress from the yield strength up to the ultimate tensile strength
indicates that the specimen hardens during deformation
• after UTS is reached, the metal continues to work harden, but at a rate that is too
small to compensate for the reduction in cross-sectional area of the piece
Plastic region
• movement of dislocations is the basis for deformation in metals
• metals have defects in their lattice- dislocations
• metals deform due to the movement of dislocations
• planes slip in plastic deformation, causing this movement
• dislocation is an extra half plane of atoms
• dislocations are brought to the outside, this is the plastic deformation
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Cold Work (Strain Hardening)
• dislocation structure changes during cold working
• dislocations entangle with one another and pile up
• dislocation motion becomes more difficult
à once yield strength is reached, planes start slipping in the plastic region and dislocations
move. Dislocations pile up and become entangled, which leads to greater stress needed for
further strain à WORK HARDENING
•
dislocations entangle with one another during cold work- dislocation motion
becomes more difficult
Work Hardening
• increase in dislocation density and dislocations entangle! This leads to HIGHER
STRENGTH, LOWER DUCTILITY (modulus is however not affected- depends on type
and number of bonds, not dislocations)
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Lecture 4.A: Strengthening- Microstructure Property Relationships
Movement of dislocations is the basis for deformation of metals
•
defects in the regular lattice structure which disrupt and impede the movement of
dislocations, making slip and therefor plastic deformation more difficult
Think of dislocations as the carriers of deformation. When you push on a
1. Cold working
dislocation it can move. Their motion allows atomic planes to move with respect
to each other.
• dislocations entangle during cold work
• makes dislocation movement more difficult
2. Effect of reducing grain size on dislocations
• grain boundaries are barriers to slip
• barrier “strength” increases with angle of misorientation
• smaller grain size: have more slip barriers
• therefore planes are harder to slip
3. Strengthening by solid solution alloying
• a point defect
• this technique works by adding atoms of one element to the crystalline lattice of
another
• melt up the solid to mix with other material and then cool again to make solid
• can be either a smaller atom than the surrounding matrix (creates tension in the
nearby lattice- balances out compressive forces)
• or a larger atom than the surrounding matrix (creates compression in the nearby
lattice- balances out tensile forces)
•
compressive and tensile forces around the dislocation cause it to move
#If we can modify the microstructure to make it more difficult for dislocations to
move, then we will be able to increase yield stress
#Single crystal after plastic deformation by tensile stress in the direction of the
arrow. Slip occurs on distinct parallel planes.
#Close packed plane where the plane has the most of atoms it is easy for the
atoms to move ex:BCC structure
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•
•
•
•
smaller atoms can cause partial cancellation of dislocation compressive strains and
impurity atom tensile strains
larger atoms have a similar effect, reducing the tensile strain of dislocations and
impurity atom compressive strains
both impurities types (as solute) reduce mobility of dislocations and increase
strength
eg. tensile strength and yield strength increase with wt% Ni in Cu
4. Strengthening by adding a secondary hard phase
• can increase strength and/ or stiffness as well as impede motion of dislocations
• eg. for aluminium (primary phase) and silicon (secondary phase)
• dislocations in the aluminium pile-up against the harder silicon- stress builds up until
the particles fracture
• as more particles break, the overall material edges closer towards total failure
•
•
•
•
if the second phase has fine particles, the small particles ‘hold up’ the dislocations
dislocation approaches
dislocation must bow around to pass
dislocation past, but small dislocation left
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•
for very fine (coherent) second phase particles
More force or more stress to move or deform the metal
Summary: Types of obstacles or defects that strengthen metals
1. Cold Working
2. Point defect (solute atoms)
3. Decrease grain size- Line defects (grain boundaries, other dislocations)
when dislocation density is too high or linear
4. A second phase (coarse)
defects: more barrier more strength is needed to
5. A second phase (fine or very fine)
deform the metal
-> they are all defects in the regular lattice structure which disrupt and impede the
movement of dislocations, making slip and therefor plastic deformation more difficult
Strategies for material strengthening:
1. Reduce grain size (impede slip)
2. Form solid solutions- point defect (impede slip)
3. Add secondary phase (composite type strengthening)
4. Small secondary phase (obstacle strengthening)
5. Cold working – dislocations become entangles and disrupt movement (strain
hardening)
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Lecture 4.B: Relating Structure to Properties- Phase Transformations
Equilibrium phase diagrams
• they are like ‘road maps’ telling us what to expect under ideal conditions
• Fe-C phase diagram is crucial for steels
• Steel- primarily composed of iron, Fe (phase diagram is for steel)
• Tells us the composition to expect on slow cooling
•
•
•
•
alpha= ferrite
gamma= austenite
Fe3C= cementite
pearlite = ferrite + cementite (alpha + Fe3C)
•
EUTECTOID is the important one at the scale we are looking at – PEARLITE
production
pearlite is a composite-like material, a lamellae of alternating ferrite and cementite
phases
•
•
•
•
Ferrite=soft
Pearlite= medium (because soft is mixed with hard)
Cementite= hard
•
as the metal undergoes the Eutectoid transformation, austenite turns into pearlite
(alternating layers of alpha and Fe3C phases)
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Hypoeutectoid Steel
• cooled at a lower C composition than at Eutectoid transformation
• C composition is lower than 0.76 wt%
• goes through a phase of austenite+ ferrite in between
• austenite changes to austenite and ferrite, then left over austenite changes to
pearlite
• AUSTENITE (GAMMA) IS THE ONLY TRANSFORMING PHASE
• overall output: pearlite and ferrite
• NOTE: in the end we have pearlite and ferrite, NOT ferrite and cementite (this would
be the wrong way of explaining it)
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Hypereutectoid Steel
• cooled at a higher C composition than at Eutectoid transformation
• C composition is greater than 0.76 wt%
• goes through a phase of austenite+ cementite in between
• austenite changes to austenite and cementite, then the left over austenite changes
to pearlite
• overall output: pearlite and cementite
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Lecture 5: Phase Transformations continued
Mechanical Properties: Influence of C content
• when concentration of C increases, strength increases (harder material) but
percentage elongation decreases- trade-off (more brittle)
Mechanical Properties: Fine Pearlite vs. Coarse Pearlite
• pearlite= ferrite + cementite
• the finer the pearlite, the better the cementite particles fit in between, making
material harder but less ductile
•
while equilibrium phase diagrams tell us the compositions to expect on slow cooling,
they don’t tell us anything about the morphology of the phases, the scale of the
phases or what to expect under severely non-equilibrium conditions
TTT diagram (Time-Temperature-Transformation diagram)
• illustrates the trade-offs between nucleation and growth
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•
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•
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•
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nucleation is rapid at higher undercooling
growth is rapid at higher temperature
to avoid austenite turning into pearlite, quick immediate cooling is needed
COARSE pearlite at top right, FINE pearlite at bottom right
must always start at eutectoid temperature line
the only one that can change is austenite (gamma)
Microscopic structure: austenite-> austenite + pearlite -> pearlite
• this diagram shows an isothermal transformation of internal structure
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Schematic of pearlite formation
• Nucleation of pearlite at austenite grain boundary
• C depletion causes ferrite nucleation adjacent to pearlite
• Spacing depends on temperature (i.e. undercooling and growth rate)
Failure
• stress fracture is linked to geometry & grain structure
• Degradation & ultimate failure comes in many forms:
• Creep, fatigue, fracture, impact, mechanical overload, corrosion, stress corrosion
cracking, thermal shock, wear, yielding
Yielding
• giving in under pressure
Ductile vs. brittle failure
• ductile failure absorbs most energy- greater toughness (given by the area under the
stress- strain curve which is
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Lecture 6: Fracture and Failure Behaviour
Brittle Fracture
• trans granular (through grains)
• inter granular (between grains)- at grain boundaries
•
flaws are stress concentrators in tension
Stress intensity at the top of a crack:
• Kc indicates toughness
• The greater the crack length, the lower the strain needed for fracture
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Criterion/ Design for Crack Growth:
Measuring Toughness
• metals are usually tested under impact conditions
• pass through a ductile to brittle transition- generally an ‘S’ shaped curve
• ductile at high temperature, brittle at low temperature
• DBTT- Ductile Brittle Transition Temperature (the temperature where the metal
turns from brittle to ductile) à 25 degree for many steels
•
steel composition strongly affects the Ductile Brittle Transition Temperature (DBTT) high sulphur, carbon and phosphorous increase DBTT
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Fatigue
• materials often fail below their yield strength under cyclical loads
• fatigue occurs by the slow growth of cracks at stress concentrators usually on the
surface
• fracture surface has beach marks or striations which are related to the gradual
growth of these cracks
Fatigue strength
• the stress at which failure will occur for a specified number of cycles
Fatigue life
• number of cycles to failure for a particular stress
How to improve fatigue strength:
• reduce mean stress level, eliminate sharp surface discontinuities, improve the
surface finish by polishing, impose surface residual compressive stresses by shot
peening, case harden the steels by using a carburizing or nitriding process
Creep
• creep is time dependent deformation which occurs at temperature above about
0.4Tm (Tm is the melting temperature)
• stress is static and usually below the yield stress
• the rate of creep increases with increases in stress and temperature
Creep Behaviour
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Influence of stress and temperature
Grain Size influences Properties
• metals having SMALL grains are relatively strong and tough at low temperaturesbarriers to slip
• metals having LARGE grains have good creep resistance at relatively high
temperatures
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Lecture 7: Atoms to Grains
Different levels of observation:
Bonding and Modulus:
• both melting point and modulus correlate with the strength of the inter-atomic
bonds
• the magnitude of the modulus is proportional to the slope of each curve at the
equilibrium interatomic spacing
Metallic Crystal Structures (WHEN METAL IS IN SOLID FORM)
• aim is to stack metal atoms to minimize empty space- close packing means stronger
bonds and hence higher modulus
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•
•
•
FCC vs HCP Stacking Sequence
FCC – ABCABC
HCP- ABAB
FCC- Face Centred Cubic
• atoms touch each other along face diagonals
• close packed arrangement of planes
• ABCABC
• Eg. Aluminium, Copper, Gold
• 4 atoms in unit cell
HCP- Hexagonal close packed
• close packed arrangement of planes
• ABABAB
• Eg. titanium, magnesium, zinc
• 6 atoms in unit cell
BCC- Body Centred Cubic
• atoms touch each other along cube diagonals
• not as close packed
• lower atomic packing factor
• 2 atoms in unit cell
•
because there is a close-packed direction metals, the elastic modulus varies with
direction -> anisotropic
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•
•
we don’t normally notice the direction effect because most metals we use are
polycrystalline
some metals are used as single crystals, and these have directional stiffness
Single vs. Polycrystals
• Single Crystals: properties vary with direction (anisotropic), eg. modulus of elasticity
• Polycrystals: properties may or may not vary with direction. If grains are randomly
oriented= isotropic, if grains are textures= anisotropic
Solidification
Metal solidification
• the process is one of nucleation of a solid phase followed by its growth into the
liquid
• nucleation is favored at a surface
•
•
•
•
•
solidification is a result of casting of molten material
heat flow from material to mould
2 steps: (1) Nuclei form, (2) Nuclei grow to form crystals- grain structure
process starts with a molten (all liquid) material
crystals grow until they meet each other
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•
each grain or crystal is an arrangement of atom planes
Hierarchy of packing:
1. Atoms
2. Planes
3. (grains) crystals
4. Polycrystals
Grain Formation
• crystals grow with different orientations
• the number of possible orientations makes it unlikely that a perfect match will occur
once crystals impinge on one another
Solidification of alloys
• pure metals solidify at a single temperature but alloys solidify over a range of
temperatures
• a phase diagram tells us this range
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Lecture 8: Metal Phases
1. Single Phase solidification microstructures
• alloys that form single solid phases
• a two-phase region exists for all alloy compositions, bounded by so-called “liquidus”
and “solidus”- it can be observed during both solidifaction and melting
Equilibrium solidification:
Cu-Ni (Copper-Nickel) alloys
• completely miscible -> single phase
• it is possible to form liquid or FCC solid solutions of any composition
• main application in the marine environment
• used in coins, useful properties:
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•
•
•
•
•
•
•
•
corrosion resistance
electrical conductivity
anti-microbial
durability
malleability
non-allergenic
ease of stamping (impress pattern)
recyclable
2. Two phase solidification microstructures
• Aluminium-Silicon alloys (Al-Si)
• Lead- Tin alloys (Pb-Sn)
•
•
•
•
•
•
Eutectic Point- low temperature point where a certain composition melts most
easily
2 phases- not completely miscible
The composition is defined by the left and right boundary regions
For example in middle sections- there is liquid with a composition of the left/right
liquidus boundary and solid with a composition of the left/right solidus boundary
In the bottom section there are 2 SOLID PHASES- solid with a composition bounded
by the right line and another solid with the composition bounded with the left
For Hypo-eutectic, aluminium solidifies first, then solid silicon forms
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•
For hyper-eutectic, silicon solidifies first, then solid aluminium forms
Precipitations
• alloys amenable to age hardening
• Age Hardening- spontaneous hardening of metal which occurs if it is quenched and
then stored at ambient temperature or treated with mild heat
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Lecture 9: Metals Life Cycle
New Scrap (PRIMARY RECYCLING)
• produced during product production
• left overs from manufacture process (eg. trimmings)
• known alloy composition
• can be returned directly to melt
• little or no loss in alloy quality
Old scrap (SECONDARY RECYCLING)
• after customer use, what goes into the recycling process
• unknown composition
• must be cleaned and sorted
End of life options:
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•
•
•
for cans, the body and end are made of different alloys
Solution: when recycling, use a mixture of recycled and virgin material and alloy as
required
Develop alloys suited to scrap sources
Contamination
• Fe (particularly in Al alloys) and other elements pick up leads to reduce properties
• Dirt, sand and other impurities need to be removed
• Coatings need to be removed
• Solutions: - make into alloys where Fe is useful - reduce melting times to reduce Fe
pick-up - treat metal to remove the impurity elements - treatment before remelting
to remove as many impurities as possible - make products without/with minimal
coatings - molten metal treatment using fluxes to clean melt
Metal Loss issue
• oxide and intermetallic formation during melting
• dross/sludge (1-2% of production)- metal that has oxidized, intermetallic formation
• low quality metal for recycling
• SOLUTION: minimize formation of dross (big GHG savings), recover metallic
component using filtering
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Lecture 10: Metals Life Cycle continued…
•
•
New scrap- aluminium extrusion billet (pole)
Recycled old scrap- aluminium alloys
New scrap (extrusion billets)
- billets sold to be used in extrusion process
•
•
•
•
top and bottom cut off to meet requirements of product (cut offs are new scrap that
is remelted and used again)
Metallurgical Quality Requirements: Grain size, segregation and chill zone, hardness,
intermetallic particles and homogenization precipitates, inclusions and defects
Regular sampling alloys statistical analysis of the process data to be performed
Slices are made for billet assessment:
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VDC- vertical direct chill casting
Etching
• using strong acid to cut into the unprotected parts of a metal surface
• to measure grain size, microsections are anodized and viewed under polarized light
using optical microscope
• defects on surface found using dye penetrant or
Old scrap (recycling after consumer use)
• used in building and construction
• cast and wrought
• regression models for solidus temperatures
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Lecture 11: Life Cycle Analysis
Life Cycle Analysis (LCA)
• tool for environmental or “project” improvement
• process for assessing the possible environmental impacts of an item arising from ALL
stages of its production, use, and disposal
• aid in the choice of the “best” process and product
• looks at: natural resources used, wastes produced, output effects and issues
• a “cradle to grave” analysis of a manufactures product or service
•
LCA considers resources/waste at each stage:
•
looks at the impacts on the environment (eg. GHG production)
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LCA Framework:
Impact Factors:
Metals Fabrication
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Shape casting
• high pressure die casting
• gravity & low pressure
• sand casting
• investment casting
• ADVANTAGES: complicated shapes, low ductility alloys, most economical
• DISADVANTAGES: porosity (form pores) and other defects, limit to mechanical
properties
Forging- mechanically working or deforming a work piece
Rolling- the most widely used deformation process, consist of passing a piece of metal
between two rolls; a reduction in thickness results from the compressive stress
Extrusion- a bar of metal is forced through a die orifice by a compressive force applied by a
ram
Drawing- pulling a metal piece through a die having a tapered bore by means of a tensile
force that is applied on the exit side
High strength steels
• Formed: usually rolled, forged or drawn
• products in the form of sheet, rod, bar or wire
• casting is also possible but not as common
• wide range in properties, because of the many phases possible
Aluminium alloys
• can be formed or cast, mostly formed (extrusions, sheet, rolled plate, etc.)
• casting proceses- sand cast, gravity, high pressure die casting
• wrought alloys have better properties than cast alloys
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Magnesium Alloys
• mainly high pressure die cast
• very good fluidity and castability
• can also be extruded, but that’s more difficult than for Al alloys
• can also be gravity die cast and sand cast
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