Plastics Testing: Techniques & Instruments for the Processor

1. Plastics Testing: Techniques & Instruments for the Processor Dr Bob Jones Department of Mechanical Engineering University of Texas – Pan American

2. Overview • Material Properties & Microstructure *BREAK* • Test Types • Coupon tests versus product tests • Sample/Specimen preparation • Instrument selection for QC labs

3. 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.

4. Stress -Intensity of Force -Stress = force /area

5. Types of Stress Normal : load perpendicular to cross-section

6. Types of Stress • Shear : load parallel to cross-section

7. Strain: Intensity of Deformation e = dl / lo

8. Relating Stress and Strain S = E e Stress = Young’s modulus x strain

9. Results of Tensile or Flex Test

12. Viscoelastic Creep

13. Microstructure Elements Molecule Length Molecule Arrangement Morphology Fillers & Modifiers Intermolecular Bonds

14. 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

15. Molecular Weight • Average Length of molecules • Determines processing characteristics and some mechanical properties • Usually measured by resin manufacturer

16. Effect of Molecular Weight on Mechanical Properties

17. Molecular Morphology Thermoset Thermoplastic Network Amorphous Crystalline Cross-link density Network strain Glass transition temperature Degree of crystallization Type of crystals Glass transition Orientation

18. Semicrystalline Thermoplastic Microstructure

19. Crystals Callister, Materials Science & Engineering An introduction, Wiley

20. Chains assume folded chain conformation These collect into lamellar crystallites Callister, Materials Science and Engineering An Introduction, Wiley

21. Calister, Materials Science and Engineering an Introduction, Wiley

22. Two crystalline morphologies Spherulite (no shear) Row Nucleated (shear) Shish-kebab

24. Increases Modulus (stiffness) Strength Density Chemical Resistance Thermal stability Hardness Decreases Toughness Permeability Effects of Crystallization

25. Thermoset Morphology Cross-Link Bond Backbone bond

26. Measurement of Network Structure • Best Indicator of network intensity (crosslink density, crosslink functionality) is Glass transition temperature

27. 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.

28. Dynamic Mechanical Analysis:Modulus & Temperature

29. Molecular Orientation • Failure process depends on molecular orientation • Generally more orientation = greater toughness

30. 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

31. 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

32. Test Classifications • Microstructural • Degree of Cure or Crystallization • Additive levels, distribution • Mechanical • Strength,Toughness, Hardness • Physical • Optical properties • Thermal properties • Electromagnetic properties

33. 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

34. 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

35. Tensile Flexural Compression Impact DMA TMA DEA Hardness DSC TGA Test Type (Common at UTPA) Coupon Article

36. Differential Scanning Calorimetry • Small sample (10 mg) • Heat flow into sample measured • Heat capacity measured • Tg, melting, cure crystallization, decomp, volatiles

37. Thermogravimetric Analysis • Weight change with temperature

38. Dynamic Mechanical Analysis • Many load configurations • Oscillating load • Changing temperature • Continuous measurement

39. DMA Results • Tg • Temperature dependence of modulus • Sensitive to small changes in composition or morphology

40. Thermo-Mechanical Analysis • Expansion of sample measured • Temperature varied • Continuous measurement

41. 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

42. Minimum Equipment for Plastics QC Lab • Linear measurement (calipers) • Analytical balance • Oven • Instruments required by customers (colorimeter, reflectometer, etc)

43. Recommended Equipment for QC Lab • Differential Scanning Calorimeter (DSC) – virtual requirement for thermosetting processors • Durometer or other hardness tester

44. 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)