Plastics Testing: Techniques

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Plastics Testing Techniques Instruments for
the Processor

• Dr Bob Jones
• Department of Mechanical Engineering
• University of Texas Pan American

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Overview

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

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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.

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Stress

• -Intensity of Force
• -Stress force /area

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Types of Stress

• Normal load perpendicular to cross-section

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Types of Stress

• Shear load parallel to cross-section

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Strain Intensity of Deformation e dl / lo
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Relating Stress and Strain
S E e Stress Youngs modulus x
strain
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Results of Tensile or Flex Test
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Viscoelastic Creep
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Microstructure Elements
Molecule Length
Molecule Arrangement
Morphology
Intermolecular Bonds
Fillers Modifiers
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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

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Molecular Weight

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

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Effect of Molecular Weight on Mechanical
Properties
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Molecular Morphology
Thermoset
Thermoplastic
Network
Amorphous
Crystalline
Cross-link density Network strain Glass
transition temperature
Degree of crystallization Type of crystals
Glass transition Orientation
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Semicrystalline Thermoplastic Microstructure
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• Crystals

Callister, Materials Science Engineering An
introduction, Wiley
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Chains assume folded chain conformation
These collect into lamellar crystallites
Callister, Materials Science and Engineering An
Introduction, Wiley
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Calister, Materials Science and Engineering an
Introduction, Wiley
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• Two crystalline morphologies
• Spherulite
• (no shear)
• Row Nucleated
• (shear)
• Shish-kebab

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Effects of Crystallization

• Increases
• Modulus (stiffness)
• Strength
• Density
• Chemical Resistance
• Thermal stability
• Hardness

• Decreases
• Toughness
• Permeability

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Thermoset Morphology
Cross-Link Bond
Backbone bond
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Measurement of Network Structure

• Best Indicator of network intensity (crosslink
  density, crosslink functionality) is Glass
  transition temperature

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Glass Transition Temp.

1. Polymer goes from Glassy/Rigid behavior to
   rubbery/flexible
2. Breakdown of Van Der Waals Forces
3. Onset of large scale molecular motions
4. Important in amorphous polymers not as
   significant in crystalline materials.

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Dynamic Mechanical AnalysisModulus Temperature
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Molecular Orientation

• Failure process depends on molecular orientation
• Generally more orientation greater toughness

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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

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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

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Test Classifications

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

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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

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Coupon vs. Article Testing

• Coupon Testing
• Result is usually a continuous variable
• Doesnt necessarily include all processing
  effects
• Will generally be directly related to material or
  process factor
• More useful in process control and product
  engineering

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Test Type (Common at UTPA) Coupon
Article

• Hardness
• DSC
• TGA

• Tensile
• Flexural
• Compression
• Impact
• DMA
• TMA
• DEA

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Differential Scanning Calorimetry

• Small sample (10 mg)
• Heat flow into sample measured
• Heat capacity measured
• Tg, melting, cure crystallization, decomp,
  volatiles

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Thermogravimetric Analysis

• Weight change with temperature

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Dynamic Mechanical Analysis

• Many load configurations
• Oscillating load
• Changing temperature
• Continuous measurement

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DMA Results

• Tg
• Temperature dependence of modulus
• Sensitive to small changes in composition or
  morphology

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Thermo-Mechanical Analysis

• Expansion of sample measured
• Temperature varied
• Continuous measurement

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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

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Minimum Equipment for Plastics QC Lab

• Linear measurement (calipers)
• Analytical balance
• Oven
• Instruments required by customers (colorimeter,
  reflectometer, etc)

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Recommended Equipment for QC Lab

• Differential Scanning Calorimeter (DSC) virtual
  requirement for thermosetting processors
• Durometer or other hardness tester

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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)