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Plastics Testing: Techniques & Instruments for the Processor. Dr Bob Jones Department of Mechanical Engineering University of Texas – Pan American. Overview. Material Properties & Microstructure *BREAK* Test Types Coupon tests versus product tests Sample/Specimen preparation
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Plastics Testing: Techniques & Instruments for the Processor Dr Bob Jones Department of Mechanical Engineering University of Texas – Pan American
Overview • Material Properties & Microstructure *BREAK* • Test Types • Coupon tests versus product tests • Sample/Specimen preparation • Instrument selection for QC labs
Key Mechanical Properties • STIFFNESS (modulus) is resistance to deformation • STRENGTH (yield or flow stress) measures resistance to permanent deformation • ULTIMATE STRENGTH measures resistance to failure. • TOUGHNESS is a measure of energy absorbed in failure or resistance to growth of a crack.
Stress -Intensity of Force -Stress = force /area
Types of Stress Normal : load perpendicular to cross-section
Types of Stress • Shear : load parallel to cross-section
Strain: Intensity of Deformation e = dl / lo
Relating Stress and Strain S = E e Stress = Young’s modulus x strain
Microstructure Elements Molecule Length Molecule Arrangement Morphology Fillers & Modifiers Intermolecular Bonds
Microstructure Tests • Measurement/characterization of the condition and arrangement of molecules and additives within a plastic system • Process parameters most often affect properties by altering microstructure
Molecular Weight • Average Length of molecules • Determines processing characteristics and some mechanical properties • Usually measured by resin manufacturer
Molecular Morphology Thermoset Thermoplastic Network Amorphous Crystalline Cross-link density Network strain Glass transition temperature Degree of crystallization Type of crystals Glass transition Orientation
Crystals Callister, Materials Science & Engineering An introduction, Wiley
Chains assume folded chain conformation These collect into lamellar crystallites Callister, Materials Science and Engineering An Introduction, Wiley
Calister, Materials Science and Engineering an Introduction, Wiley
Two crystalline morphologies Spherulite (no shear) Row Nucleated (shear) Shish-kebab
Increases Modulus (stiffness) Strength Density Chemical Resistance Thermal stability Hardness Decreases Toughness Permeability Effects of Crystallization
Thermoset Morphology Cross-Link Bond Backbone bond
Measurement of Network Structure • Best Indicator of network intensity (crosslink density, crosslink functionality) is Glass transition temperature
Glass Transition Temp. • Polymer goes from Glassy/Rigid behavior to rubbery/flexible • Breakdown of Van Der Waals Forces • Onset of large scale molecular motions • Important in amorphous polymers – not as significant in crystalline materials.
Molecular Orientation • Failure process depends on molecular orientation • Generally more orientation = greater toughness
Processing Affects Morphology • Crystallization takes time so cooling rate is critical • Shear during mold filling or drawing can create row nucleated structure or orient molecules • Curing rate and temperature determine network structure in thermosets
Issues in Filler/Modifier Morphology • Loading level – absolute amount of filler or modifier • Dispersion or distribution – clumping or orientation of material can dramatically affect performance • Coupling – quality of bonding between polymer and modifier is critical for optimum performance
Test Classifications • Microstructural • Degree of Cure or Crystallization • Additive levels, distribution • Mechanical • Strength,Toughness, Hardness • Physical • Optical properties • Thermal properties • Electromagnetic properties
Coupon vs. Article Testing • Article Testing • Result often just a pass/fail • Test is often awkward or expensive • Includes effect of processing • May allow for testing of complex service and processing interactions • Results are often difficult to relate back to particular process or material problem ---- not good for process control
Coupon vs. Article Testing • Coupon Testing • Result is usually a continuous variable • Doesn’t necessarily include all processing effects • Will generally be directly related to material or process factor • More useful in process control and product engineering
Tensile Flexural Compression Impact DMA TMA DEA Hardness DSC TGA Test Type (Common at UTPA) Coupon Article
Differential Scanning Calorimetry • Small sample (10 mg) • Heat flow into sample measured • Heat capacity measured • Tg, melting, cure crystallization, decomp, volatiles
Thermogravimetric Analysis • Weight change with temperature
Dynamic Mechanical Analysis • Many load configurations • Oscillating load • Changing temperature • Continuous measurement
DMA Results • Tg • Temperature dependence of modulus • Sensitive to small changes in composition or morphology
Thermo-Mechanical Analysis • Expansion of sample measured • Temperature varied • Continuous measurement
Sample Preparation/Collection • Avoid conditions which deviate from actual product conditions – • Temperature • Excess stress or strain application • Chemical exposure • For warranty returns, if possible, test failed components not just samples from same lot
Minimum Equipment for Plastics QC Lab • Linear measurement (calipers) • Analytical balance • Oven • Instruments required by customers (colorimeter, reflectometer, etc)
Recommended Equipment for QC Lab • Differential Scanning Calorimeter (DSC) – virtual requirement for thermosetting processors • Durometer or other hardness tester
Nice to Have in QC Lab • Small universal testing machine (tensile, flex, compression testing) • Pendulum impact tester (Izod) • Melt Indexer (if regrind blending is done or for acceptance testing of non-certified material)