1 Queensland University of Technology CRICOS No. 000213J Polymers in Extreme Environments Graeme George School of Physical and Chemical Sciences Queensland University of Technology, Brisbane, Australia CRICOS No. 000213J a university for the world real R In the 21st Century Polymers underpin our society • Packaging and hygiene • Computers and Telecommunications • Transportation • Biomedicine and Health But: There are some environmental issues: - Litter - Clean production and disposal CAN WE CONTROL THE USEFUL LIFETIME OF POLYMERS? CRICOS No. 000213J a university for the world real R The synthetic Polymer industry is just 100 years old 1907-1909: Leo Baekelund developed Bakelite The world’s first synthetic plastic OH C O H H + Phenol Formaldehyde Brittle thermosetting polymer reinforced with wood flour ie. a Composite OH ־-H 2 O CRICOS No. 000213J a university for the world real R Thermoplastics vs. Thermosets Short chains cross- linked chemically Consist of (long) entangled chains Brittle if not reinforced Often ductile and energy absorbing Cannot be recycled Can be recycled and reprocessed Heat: Little effect until decomposition Heat: Distortion and melting Thermosets Thermoplastics
Transcript
1
Queensland University of Technology
CRICOS No. 000213J
Polymers in Extreme EnvironmentsGraeme George
School of Physical and Chemical SciencesQueensland University of Technology,
Brisbane, Australia
CRICOS No. 000213Ja university for the worldreal R
In the 21st CenturyPolymers underpin our society
• Packaging and hygiene• Computers and Telecommunications• Transportation• Biomedicine and Health
But:There are some environmental issues:
- Litter- Clean production and disposal
CAN WE CONTROL THE USEFUL LIFETIME OF POLYMERS?
CRICOS No. 000213Ja university for the worldreal R
The synthetic Polymer industry is just 100 years old
1907-1909: Leo Baekelunddeveloped Bakelite
The world’s first synthetic plasticOH
C O
H
H
+
Phenol Formaldehyde
Brittle thermosetting polymer reinforced with wood flour ie. a Composite
OH־ -H2O
CRICOS No. 000213Ja university for the worldreal R
Thermoplastics vs.Thermosets
Short chains cross-linked chemically
Consist of (long) entangled chains
Brittle if not reinforcedOften ductile and energy absorbing
Cannot be recycledCan be recycled and reprocessed
Heat: Little effect until decomposition
Heat: Distortion and melting
ThermosetsThermoplastics
2
CRICOS No. 000213Ja university for the worldreal R
Linear
Cyclic
BranchedShort
Long
•
••
••
••
•
Network
•
•
•
•
Thermoplastic Thermoset
Simplest Molecular Architectures
CRICOS No. 000213Ja university for the worldreal R
Structure-Property Relationships
Factors that control properties:
• The length of polymer chain (MW; crosslinking: Mc)
• The repeat unit and forces between chains(polarity; H bonding)
• The type and degree of crystallinity
CRICOS No. 000213Ja university for the worldreal R
H
H H
HH
H H
H H
H H
H
1.54Å
2.54Å
109.5o
H H
H
HH
H
H
H
H
H
(a) All trans conformation(extended chain)
(b) Cis conformation(chain kink)
-(CH2-CH2)n-Poly(ethylene)
CRICOS No. 000213Ja university for the worldreal R
Stephen Z. D. Cheng Nature 448, 1006-1007( 2007)
Polyethylene single crystalsgrown from dilute solution
Hindered C-C backbone due to pendant methyl groups
0Poly(propylene) PP
Flexible C-C backbone-125Poly(ethylene) PE
Structural featuresTg (oC)Polymer
Effect of structure on Tg
CRICOS No. 000213Ja university for the worldreal R
Brittle fracture of polymers
• Strength much less than calculated from atomic structure• Crack propagation from surface flaws• Toughness depends on crack blunting by fibrillation etc.• Tough polymers tolerate large cracks eg PE: 17 mm
cf. PS: 0.3 mm
Glass: brittle fracture
CRICOS No. 000213Ja university for the worldreal R
Effect of test speed on mechanical properties
Impact: Brittle; elastic response
Normal: Ductile; visco-elasticresponse
Slow: Creep; Viscous response
Finite time is taken for chains to disentangle and absorb energyThere is an effective shift in Tg to higher T with increased rate of test
6
CRICOS No. 000213Ja university for the worldreal R
Rubber elasticity
Deform
Heat
• Light crosslinking (1%) of low Tg macromolecule inhibits creep
Random Coil (high entropy) Oriented chain (low entropy)
Strain induced crystallization
Other systems:Thermoplastic rubbers(EPR; TPU)
Mw ∞
CRICOS No. 000213Ja university for the worldreal R
HARD
HARD
SOFT
SOFT
SOFT
SOFT
HARD
HARD
HARD
HARD
150 nm
> 1000 nm
Thermoplastic poly(urethane) elastomer (TPU)
Hard segments crystallize (Aromatic)
Soft segments extensible (Polyol)
Crystalline blocks act as crosslinks giving an elastomerOn heating the blocks melt for processing
CRICOS No. 000213Ja university for the worldreal R
Transmission Electron Micrograph of high impact polystyrene (HIPS) A tough immiscible blend
CRICOS No. 000213Ja university for the worldreal R
High performance composites• Brittle Fibres: Carbon, Boron, Glass• Brittle Resins: Epoxy, Polyimide.• Each of these alone is strong, but brittle, but the
composite is TOUGH. Why?• The strategy is to control interfacial adhesion to initiate
debonding, absorb energy and deflect cracks to give a fail-safe structure.
7
CRICOS No. 000213Ja university for the worldreal R
Failure of a carbon-epoxy composite after impact (collision with another FA-18) showing crack deflection
and energy absorption by debonded fibres
Composite combines strength and stiffness with a high energyof fracture to give a fail-safe structure that is light in weight
CRICOS No. 000213Ja university for the worldreal R
1.141214100020Nylon
1.452.5130270011Kevlar
2.632 – 37224159E-Glass
1.75≤ 123527608Carbon
ρg/cm3
ε%
EGPa
σMPa
Dia.μm
Fibre
Properties of reinforcing fibres
σ = Tensile strength E = Modulus ε = Elongation at break ρρ = density
CRICOS No. 000213Ja university for the worldreal R
Shed surface integrity is vital to prevent failure by:
• Loss of Insulation Properties at 256kV• Insulator surface becomes
hydrophilic• Flashover may occur when wet
• Loss of Mechanical Properties• Surface Cracking of protective elastomer• Ingress of moisture to core leads to fracture
Flashover
10
37
pollution layer forms in time
Recovery process –migration of LMW silicone
encapsulates pollution
hydrophobic surface retained
Unique feature of silicone insulator sheds
38
Flashover and total breakdown of polymer had occurred
A: slight degradation with smooth surface.
B: moderate degradation with small cracks
C: extensive degradation with cracks
D: extreme degradation
This insulator had been used continuously in a polluted environment, protected from UV and rain. Why had it degraded?
Catastrophic failure of silicone elastomer shed
39
Failed silicone elastomer sheds: Analysis of surface degradation products
• Migratable silicones decrease with degradation: 2.7% to 0.3%
• The most degraded Zones (C and D) are hydrophilic with a silica-like surface
• Degradation is consistent with localized intense corona discharges so surface never recoversSample code
V A B C
SMW
S (%
)
0
1
2
3
Most degradedUndegraded
40
Failure mechanism for silicone insulator• LMW silicones migrate to
surface and are oxidised in plasma
• Hydrophilic surface allows flashover and ablation of polymer
• Absence of washing prevents surface debris from being removed
• Rate of LMW silicone migration insufficient to regain hydrophobicity
• Corona discharges occur in local high humidity environment
11
41
Monitoring of insulators by live-line sampling
• Sampling tool removes surface debris and then a section of polymer
• Analysis gives oxidation index and surface silicone distribution
• Trialled in USA for accelerated exposure assessment
42
Micro-cracking is a feature of oxidative failure eg. Surface of UV exposed Polyolefin
Cracks arise from density and polarity changes together with internal or applied stress
43
The Interconnection of Engineering and Chemistry.Mechanical failure follows from morphological changes
resulting from a change in polymer molecular weight due to UV or heat-initiated oxidation
Morphological
Mechanical
Network Structure
Chemical Composition
Fracture Toughness
Crack FormationSurface Structure
Molecular WeightCrosslink Density
Oxidation Products Oxidation Mechanism
Physical Measurements
Chemical Measurements
44
Relating polymer lifetime to chemistry:Changes in mechanical property of a polyolefin
and the extent of oxidation (oxidation index)
0 20 40 60 80 100 120 140 160
Retained mechanical property
Note that the useful life doesnot extend much beyond theend of the oxidation induction period
OxidationIndex
Oxidation time
■ ■■
■
■
■
■
PE Film Unexposed PE Film UV exposed
12
45
The challenge in lifetime prediction is to study oxidation at the earliest times and establish a
rate of chemical change that underpins the critical property loss
THE REACTIONS TO BE FOLLOWED ARE COMPLEX FREE RADICAL CHAIN REACTIONS INITIATED BY
IMPURITIES AND MEDIATED BY ADDITIVES• Oxidation is initiated and propagates in the amorphous
region by alkyl radicals P· and peroxy radicals POO·• The reactive peroxy radicals attack the polymer to
form oxidation products via a hydroperoxide POOH• Decomposition of POOH results in more radicals and
chain scission, lowering polymer strength46
Stabilization strategies to prevent oxidative degradation
• Intercept free-radical chain reaction at earliest stage: scavenging of either alkyl radicals R· or peroxy radicals ROO·– Additives: Hindered amines (nitroxides react with R·)
or hindered phenols (react with ROO·)• Decompose initiators eg hydroperoxides POOH or
superoxide O2·-
– Additives: Phosphites, thio compounds.• Stabilizers increase the induction period before the rapid
loss of properties leading to crack formation.
No polymer will last indefinitely
47
Polymers in the body: tissue replacement
48
Polymers in the body: an aggressive environment
Polymers chosen to be:• inert
OR• undergo controlled
degradation with notoxic by-products
AND• non-thrombogenic• additive-free
•Enzymes•Superoxide anion
13
49
A “simple” polymer with a major problem for orthopaedic surgeons (and patients): Short service life of sterilized UHMWPE implants
The Ultra-High Molecular Weight Polyethylene (UHMWPE) bearing of a knee implant was sterilized by gamma radiation in air and stored.
After implantation, the devices failed prematurely by severe pittingand delamination.
Hayes and Associates:Orthopaedic Consultants
www.hayesassoc.com/ orthopaedic_examples.htm
50
Glenoid implant for shoulder reconstruction: shallow UHMWPE cup
R. Crawford
51
UHMWPE Implants show oxidation and fracture during wear: eg. Shoulder Implant after 5 years
Delamination and fracture of polymer
SEM of section of surfaceshowing delamination
HYPOTHESIS: Sterilization by γ-irradiation in air gives rapid initiation of oxidation. When in the body further oxidation (by O2·
-) may occur.
S. ConstantC.Lutton 52
Problems with lifetime prediction of UHMWPE
implants• Materials have very high molecular weight and must be
processed by compression moulding with unknown processing degradation
• FDA Regulations do not allow any stabilizer addition (eg Vitamin E: α-tocopherol; a hindered phenol analogue)
• Sterilization by high energy radiation and plasma leads to free radical production and oxidation
• The low amorphous fraction of UHMWPE results in magnification of degradation, particularly of tie molecules
• A lifetime target of 30 years will only be possible if the polymer is stabilized against oxidative degradation
14
53
Putting polymer degradation to use
• Controlled release of drugs and proteins from implants and patches
• Degradation of temporary scaffolds for wound healing and tissue engineered organ replacements
• Controlled environmental degradation of polymers in agriculture and litter control
54
DEGRADABLE POLYOLEFIN FILMS FOR AGRICULTURAL PRODUCTION
Objective: To develop photo- and thermally degradable ultra-thin polyethylene films suitable for significantly increasing the yields of crops
Achieving the resultsCooperative Research Centre for POLYMERS
55
Strategy: Include additive to initiate oxidation
• Select environmentally benign inorganic additives that have photo-redox activity
Current generation are transition metal salts:
• Iron stearate has been successfully used in Europe but is too aggressive for Australia and polymer degrades on storage
• New additives needed with UV sensitivity and high quantum efficiency but long shelf life.
Growing the economyCooperative Research Centre for POLYMERS 56
Titania: Photo-catalytic Reactions
www.lgworld.co.th/Technology.htm
15
57
Total destruction of polyethylene around titania particles
x 2000
x60000
Scanning ElectronMicroscope Images
58
Field trials
Growing the economyCooperative Research Centre for POLYMERS
59
Dalby – Maize, sorghum
Control plot sorghum
Control plot maize
Growing the economyCooperative Research Centre for POLYMERS
Under film
Control – No film
60
Polymers in the environment: Strategies to control lifetimes of plastics
• Use polymers that have a high natural rate of degradation by oxidation and/or hydrolysis (Starch, aliphatic polyesters)
• Use additives that are triggered at end of useful life (Oxo-degradable polyethylene)
• Use polymers that are made and degraded by biological systems (Biopol)
• Should we ban all polymers from packaging?
16
61
Bioerodible Polyesters [PHB & PHV] from biosynthesis (Patented in 1970’s by ICI)
Polyhydroxy butyratePHB-PHV Copolymers
Biopol®
Up to 30% PHV
Degradation rate α [PHV]
All degraded by soil bacteria
PHB
PHV
Cost prevents implementationCRICOS No. 000213Ja university for the worldreal R
Questions we should ask about any new polymer
• What is the stability in the intended application?
• What will be the impact on the operating environment (whether the built environment, the natural environment or the human body)?
• Biomaterials provide the greatest challenges in extreme environments but we can learn from strategies for industrial materials
CRICOS No. 000213Ja university for the worldreal R
INSTITUTE OF HEALTH AND BIOMEDICAL INNOVATION
QUT
Kelvin Grove Urban Village
CRICOS No. 000213Ja university for the worldreal R
Co-operative Research Centre for PolymersQueensland Node
