Part B Flashcards
What is polymer degradation?
Chemical change resulting in reduced chain length and mechanical properties like tensile strength.
What mechanical effect does degradation typically have on polymers?
It causes embrittlement and loss of tensile strength.
What happens during polymer degradation that leads to increased brittleness?
Reduction in molecular weight and chain scission.
How does incorrect processing of PLA affect its mechanical properties?
Processing PLA at 230°C for 5 minutes without drying leads to dramatic strength loss.
Why must PLA be dried before moulding?
To avoid hydrolysis from water content, which degrades the polymer.
What are Tg and Tm of PLA?
Tg ≈ 55°C, Tm ≈ 165°C.
What are examples of polymer ageing processes?
Secondary crystallisation and volume relaxation.
What distinguishes ageing from degradation in polymers?
Ageing is physical and reversible; degradation involves chemical change.
How can ageing effects be reversed?
By heating above Tg (volume relaxation) or Tm (secondary crystallisation).
What effect do solvents have on polymers?
They cause plasticisation, lowering stiffness and Tg.
What is a classic example of plasticisation?
PVC plasticised by phthalates.
How does solvent immersion affect amorphous PEEK?
It depresses Tg, enabling crystallisation and improved mechanical properties.
What are common triggers for polymer degradation?
Heat, oxygen, water, radiation, ozone, acids, alkalis, chlorine, microorganisms.
What is solvolysis?
Polymer degradation via reaction with solvents like water (hydrolysis).
What is weathering?
Exposure to multiple degradation triggers like UV, moisture, and oxygen.
What chemical test estimates thermal stability?
Heating polymer in vacuum and measuring temperature at 50% mass loss.
How does bond dissociation energy affect thermal stability?
Higher bond dissociation energy correlates with higher thermal stability.
What hazardous gases are released during PTFE degradation?
Hydrogen fluoride and other toxic byproducts.
At what temperature does PTFE begin to degrade dangerously?
As low as 300°C, even though pronounced degradation occurs near 500°C.
Which functional groups are susceptible to hydrolysis?
Esters and amides.
How does hydrolysis occur in PLA and nylons?
Water acts as a nucleophile attacking ester/amide groups, causing chain scission.
When does degradation typically occur in polymer processing?
During melt compounding or injection moulding at high temperatures and shear.
How can processing degradation be mitigated?
Using stabilisers and selecting high Mw grades.
Why is degradation in service harder to predict?
Due to environmental variability (UV, humidity, temperature).
What can go wrong with PVC in electrical insulation?
Overheating releases HCl and benzene, increasing fire hazard.
Why is PVC degradation dangerous in fires?
Its byproducts produce hazardous, toxic smoke.
What are common ways to detect polymer degradation?
Optical changes, molecular weight reduction (GPC), mechanical testing, thermal analysis (DSC/TMA), viscosity changes, TGA, and IR spectroscopy.
How can optical methods indicate degradation?
Discoloration, surface texture changes, crack formation (seen via optical/SEM microscopy).
Why might discoloration not indicate polymer degradation?
It may be due to additive degradation rather than the base polymer.
What polymer shows visible degradation and fragility upon aging?
Poly(hydroxybutyrate) (PHB).
What does SEC/GPC measure in polymers?
Molecular weight distribution (Mn and Mw) via elution through porous columns.
How does degradation affect SEC traces?
Peak broadening and shift to longer elution times; Mn and Mw decrease.
What might make SEC measurements misleading?
Insolubility due to cross-linking, so only soluble fragments are measured.
How does degradation affect tensile strength?
Reduces chain length and entanglement, lowering tensile strength.
What is the relationship between TS and Mw?
TS = TS∞ − A/Mw, where TS∞is strength at infinite chain length.
How can degradation promote crystallinity in semi-crystalline polymers?
Shorter chains from degradation crystallize more easily (secondary crystallization).
What does DSC measure in polymers?
Glass transition (Tg), melting (Tm), crystallisation, and enthalpy changes.
How does Mw affect Tg?
Lower Mw leads to lower Tg due to more chain ends.
What is the effect of cross-linking on Tg?
Increases Tg by reducing chain mobility.
What is chemi-crystallisation?
Crystallisation driven by degradation-induced chain shortening.
How does degradation affect melting behavior?
Tm may increase (due to secondary crystallisation) or decrease (due to chain erosion or impurity effects).
What is Gibbs-Thomson effect in polymers?
Tm is proportional to lamella thickness; thinner crystals melt at lower temperatures.
How does degradation affect viscosity?
Lower Mw reduces viscosity; cross-linking increases viscosity.
What instrument is used to measure polymer viscosity?
Rheometer.
What does TGA detect in polymer degradation?
Mass loss from volatile degradation products.
What are common TGA measures?
Onset temp, temp for X% mass loss, residual mass, isothermal weight loss.
Why use an inert purge gas in TGA?
To prevent oxidative reactions during decomposition measurement.
How does PVC degrade in TGA?
Two-step mass loss: HCl evolution and benzene formation.
How is TGA used in compositional analysis of elastomers?
Reveals plasticisers, natural/synthetic rubber, carbon black, and inorganics via staged heating.
What does FTIR measure in degradation?
Appearance/loss of IR absorption peaks for chemical functional groups.
How does HDPE FTIR spectrum change on degradation?
New carbonyl and hydroxyl bands appear due to oxidation.
What is time-resolved FTIR?
Recording spectra over time or temperature to track degradation.
What causes IR peak shifts in polymers?
Secondary interactions that alter bond vibration frequencies.
Why should multiple methods be used to assess degradation?
Each method has limitations; combining results gives a clearer picture.
What is the key molecular requirement for thermoplastic moulding?
Polymer chains must be mobilised by heating above Tg or Tm to allow flow.
What determines ease of mould filling?
Melt viscosity, which is controlled by molecular weight.
Why can UHMWPE not be injection moulded?
Its very high melt viscosity requires compaction and melt-consolidation.
What degradation occurs during polymer processing?
Thermo-oxidative degradation from heat and oxygen.
What additional degradation occurs if polymer is hygroscopic?
Hydrolysis from absorbed water during processing.
What processing condition promotes crystallisation?
Controlled cooling to allow sufficient time for crystallisation kinetics.
Why is PP more prone to degradation than PE?
Presence of tertiary hydrogen in PP facilitates free radical formation.
What stabiliser prevents PP degradation during processing?
Free radical scavengers like BHT.
How does PVC degrade during processing?
Dehydrochlorination followed by cross-linking and gel formation.
How can HCl formation in PVC be controlled?
Metal stearates remove HCl; organotin compounds limit formation.
What makes PMMA potentially recyclable?
It depolymerises via ‘unzipping’ to yield 90% monomer.
What is PAN’s degradation mechanism?
Extensive cyclisation, enabling carbon fibre production.
What degradation effect occurs in PEEK above 360°C?
Cross-linking that inhibits recrystallisation.
Why must PET and PA be dried before processing?
They are hygroscopic and undergo hydrolysis if moisture is present.
What equilibrium affects PET degradation?
Step-growth equilibrium is shifted away from polymer by water.
What causes PHB chain scission during heating?
β-elimination via six-membered ring transition states.
What is a notable degradation product of PHB?
Crotonic acid.
Why is UHMWPE sterilised with gamma irradiation?
It can’t withstand autoclaving and irradiation improves wear resistance.
How does gamma irradiation affect UHMWPE?
Cross-linking improves wear resistance, but oxidation can cause brittleness.
Why is PVC contamination a problem in PET recycling?
HCl release from PVC degrades PET during high-temperature processing.
What recycling strategy is effective for PA?
Hydrolytic depolymerisation in super/sub-critical water to yield monomer.
How is PHB degraded during composting?
Microorganisms metabolise it to CO₂ and H₂O.
What slows PHB degradation initially?
Colonisation delay by microorganisms.
What are the two stages of PCL biodegradation?
Stage I: Hydrolytic scission; Stage II: Oligomer loss via erosion or phagocytosis.
What polymer features slow PCL biodegradation?
High crystallinity and hydrophobicity.
What accelerates PCL biodegradation?
Enzymatic action by lipases like pseudomonas cepacia lipase.
What polymer groups are attacked by ozone?
C=C double bonds (alkenes).
Why is ozone degradation a safety concern in rubber fuel lines?
Cracking and rupture under load can cause leaks.
How is rubber protected from ozone?
Antioxidants and waxes form a barrier coating.
What triggers surface photodegradation in polymers?
UV radiation, especially in oriented surface layers.
How can UV degradation be limited?
Using UV absorbers (e.g. benzophenone), pigments (carbon black), or stabilisers (HALS).
How can weathering be accelerated in lab tests?
By using high UV intensity, heat, and moisture in enclosed chambers.
What polymers form protective char during burning?
PET, PC, PVC, PI.
How does Mg(OH)₂ improve fire resistance?
It decomposes to absorb heat and release water that hinders ignition.
