POLYMERS - PROCESS & POST-PROCESSING TREATMENTS INDUCED FAILURES

Dr Xi PengPolymer Scientist

Agenda

− Polymer and Plastics

− Product Concept

− Plastics Processing and Issues

− Post-processing Treatment

− Testing and Problem Solving

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)

Product Concept

− Material Selection

− Design

− In-service Conditions

− Processing

Material Selection

There are approximately 100 generic types of plastice.g. polypropylene, Nylon, phenolic resin.

Within these types are subgroupse.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.

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

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

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

Plastics Processing

− Extrusion – Film

− Extrusion – Pipe

− Extrusion – Profile

− Other Extrusion

− Injection Moulding

− Blow Moulding

− Thermoforming

− Other Processes

Click icon to add picture

Smithers Rapra

Film 35%

Pipe 10%

Prof ile 2%Other Ext.

9%T'forming 4%

Blow Mld. 11%

Inj.Mld 26%Other 3%

W.Europe Thermoplastics Demand by Process (Total 36mt)

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

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

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

Extrusion

Common Issues:

− Contamination

− Polymer chain scission & cross-linking

− Surface instability – sharkskin

− Cooling rate control

− Co-extrusion problems

− Uneven mixing in compounding

Post-processing Treatment

One-off Process

− Annealing – heating plastic part at an temperature close to its Tg that 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

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

Cleaning

− To remove any unwanted substances – the contaminations from the part surface

− May involve heat, pressure & chemicals

− Cleanliness validation – surface analysis

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

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

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

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

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

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

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

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

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

Analysis Techniques for Process-Induced Failures

Low or Excessive Crystallinity

high crystallinity embrittlement

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

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

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

Summary

Polymer & polymer product developmentPolymer processing Injection moulding

ExtrusionProcessing issues Processing imperfection

Material degradationExcessive residual stresses ContaminationUneven mixing of components Under/over crystallisation

Polymer post-processing treatment

ClearingSterilisation

Summary contd.

Issues from post-processing treatment

DegradationMaterial migration ESCContaminationDiscolourationDisintegration

Analysis techniques Spectroscopy (e.g. FT-IR)Chromatography (e.g. GC-MS)Microscopy (e.g. SEM)Thermal analysis (e.g. DSC, TGA)Elemental analysis (e.g. EDX)Physical testing (e.g. tensile, impact)

Thank you