Polymers - process & post-processing treatments induced failures

1. Polymers - process & post-processing treatments induced failures Dr Xi Peng Polymer Scientist

2. Agenda • Polymer and Plastics • Product Concept • Plastics Processing and Issues • Post-processing Treatment • Testing and Problem Solving

3. Polymer and Plastics • Polymer • A polymer is a long chain of small molecules repeatedly joined together • Plastics • A plastic is a mixture of a polymer material and additives (anti-oxidants, fillers, plasticisers, pigments, UV stabilisers, etc..) Polyethylene (PE)

4. Product Concept • Material Selection • Design • In-service Conditions • Processing

5. Material Selection There are approximately 100 generic types of plastic e.g. polypropylene, Nylon, phenolic resin. Within these types are subgroups e.g. Nylon: Nylon 6, Nylon 6-6, Nylon 12 For each subgroup there will be many grades available: e.g. easy flow, food grade, UV stabilised, fire retardant, etc.

6. Influences of Design • Item size • Assembly methods (e.g. screw fixing, welding, adhesives, snap fitting, etc.) • Transparency • Surface finish • In contact with other materials • Expected life • End of life – recycling, biodegradable

7. In-service Conditions • Temperature requirements – minimum/maximum temperature; continuous or occasional exposure • Chemicals from surroundings • Stress – residual stress from moulding; stress from assembly method; applied stress in the application • Outdoor application – UV resistance • Fire retardant • Electrical properties

8. Plastics Processing • Moulding - Injection, Blow, Transfer, Rotational • Extrusion - Continuous long items of uniform section, e.g. film, sheet, pipe, profile, coating • Forming - Vacuum Forming, Pressure Forming • Other - Spreading, Dipping, Sintering, Casting

9. Plastics Processing • Extrusion – Film • Extrusion – Pipe • Extrusion – Profile • Other Extrusion • Injection Moulding • Blow Moulding • Thermoforming • Other Processes SmithersRapra

10. Injection Moulding • Probably the most common plastic processing technique • Rapid cycle time with good dimensional control & part to part consistency • Materials must be well mixed to ensure uniform melting • Items can be identified by the marks of gate & ejector pins

11. Injection Moulding Common Issues: • Short shot – incomplete mould filling • Flashing – leakage of molten plastic around split line or core pins • Weld lines – poor weld line strength • Sinking & Voiding • Poor dimensional tolerances • Warpage • Excessive residual stresses • Brittle mouldings

12. Extrusion • Continuous long items of uniform section • Rods & tubes • Gas & water pipes • Window frames • Cable sheathing • Films & Sheets • Compounding of filled or reinforced grades of plastic materials

13. Extrusion Common Issues: • Contamination • Polymer chain scission & cross-linking • Surface instability – sharkskin • Cooling rate control • Co-extrusion problems • Uneven mixing in compounding

14. Post-processing Treatment One-off Process • Annealing – heating plastic part at an temperature close to its Tgthat makes the residual stresses relieved and increases its ductility • Machining – controlled material-removal process in which a component is cut, drilled or polished into a desired final shape and size • Coating – a process of applying covering to the surface of an object for decorative and/or functional purposes

15. Post-processing Treatment contd. Multiple Repeat Process • Cleaning – a process using devices and cleaning agents to clean items • Sterilisation – any process that eliminates microbiological organisms present on the surface of device

16. Cleaning • To remove any unwanted substances – the contaminations from the part surface • May involve heat, pressure & chemicals • Cleanliness validation – surface analysis

17. Issues Caused by Cleaning Process • Degradation – can be caused by heat (thermal oxidation), chemical attack and/or hydrolysis; leading to reduction of molecular weight and deterioration of the physical/chemical properties • Material migration – relates to low molecular weight species present in plastics, e.g. residual monomers, additives, low molecular weight oligomers, reaction/breakdown products, etc.; migration increased due to heat and/or polymer swelling

18. Issues Caused by Cleaning Process • Environmental stress cracking (ESC) – exposed to chemical environment in combination with tensile stress • Delamination – co-extruded or over-moulded items; thermal expansion of polymer ~ 10 times metal insert • Discolouration – yellowing due to generation of coloured species during degradation

19. Sterilisation Main methods: • Heat (e.g. dry, steam, boiling) • Pressure (e.g. autoclave) • Chemicals (e.g. ethylene oxide) • Ionising radiation (e.g. Gamma rays, electron beam) Effects on polymers vary depending on the polymer structure

20. Issues Caused by Sterilisation Process • Degradation leading to polymer chain scission and/or cross-linking • Change in colour – e.g. yellowing • Generation of odour due to volatiles formed by reactions • Leave toxic residuals or by-products on the polymer surface • Influence on extractables and leachables • Disintegration of multi-layer and over-moulded part

21. Analysis Techniques for Process-Induced Failures Polymer Degradation • Gel permeation chromatography (GPC): measurement of molecular weight (MW) and molecular weight distribution • Rheological testing: e.g. melt flow index (MFI), rotational viscometry • Mechanical testing: e.g. tensile strength and modulus, elongation

22. Analysis Techniques for Process-Induced Failures Contamination • Infrared spectroscopy (FT-IR): identification of polymers & contaminants • Microscopy combined with elemental analysis technique (e.g. scanning electron microscopy/energy dispersive X-ray analysis, SEM/EDX): detection & identification of any inclusions and unspecified materials • Chromatography (e.g. gas chromatography with mass spectrometry, GC-MS): separation & identification of low molecular weight contaminants

23. Analysis Techniques for Process-Induced Failures Porosity & Voids – may cause structural weakness & stress concentration • Optical and/or electron microscopy (e.g. SEM) • Pressure leak testing

24. Analysis Techniques for Process-Induced Failures Excessive Residual Stresses – may cause environmental stress cracking (ESC) • Dip testing: by applying a known ESC agent (solvent) to the part any high stress areas will tend to crack; the time-to-crack is indicative of the level of residual stress • Thermal analysis (e.g. differential scanning calorimetry, DSC): indication of processing history that determines the level of residual stress • Impact test: high residual stress low impact strength

25. Analysis Techniques for Process-Induced Failures Uneven Mixing of Components – may cause structural weakness & stress concentration • Optical and/or electron microscopy (e.g. SEM) • Thermal analysis (e.g. thermogravimetric analysis, TGA): providing information about additives/fillers

26. Analysis Techniques for Process-Induced Failures Low or Excessive Crystallinity high crystallinityembrittlement low crystallinity loss of properties • X-ray diffraction (XRD): determination of crystallinity and crystallite size • Thermal analysis (e.g. differential scanning calorimetry, DSC): providing information about relative crystallinity

27. Analysis Techniques for Process-Induced Failures Material Migration Can alter the extractables and leachables profile – generally include processing aids, polymerisation residues, additives & reaction products • Profiling of chemical species by GC-MS • Identifies relatively volatile, thermally stable compounds • Molecular weight range 50 to~500 Daltons

28. Material Migration contd. • Profiling of chemical species by LC(liquid chromatography)-MS • Identifies volatile to oligomeric sized compounds • Can be used for thermally unstable compounds, polar compounds and aromatic hydrocarbons • Molecular weight range 200 to >1000 Daltons • Quantitative determination of trace elements by inductively coupled plasma (ICP) • Quantitative determination of non-volatile residues (NVR) • Infrared (FT-IR) analysis of NVR – overall chemical nature of residues

29. Summary

30. Summary contd.

31. Thank you