Plastics Testing: Plastics Testing: Techniques & Instruments Techniques & Instruments

for the Processorfor the Processor

Dr Bob JonesDr Bob JonesDepartment of Mechanical EngineeringDepartment of Mechanical Engineering

University of Texas – Pan AmericanUniversity of Texas – Pan American

OverviewOverview

Material Properties & MicrostructureMaterial Properties & Microstructure

*BREAK**BREAK* Test TypesTest Types Coupon tests versus product testsCoupon tests versus product tests Sample/Specimen preparationSample/Specimen preparation Instrument selection for QC labsInstrument selection for QC labs

Key Mechanical PropertiesKey Mechanical Properties

STIFFNESS (modulus) is resistance to STIFFNESS (modulus) is resistance to deformationdeformation

STRENGTH (yield or flow stress) measures STRENGTH (yield or flow stress) measures resistance to permanent deformationresistance to permanent deformation

ULTIMATE STRENGTH measures resistance ULTIMATE STRENGTH measures resistance to failure.to failure.

TOUGHNESS is a measure of energy TOUGHNESS is a measure of energy absorbed in failure or resistance to growth absorbed in failure or resistance to growth of a crack.of a crack.

StressStress-Intensity of Force-Intensity of Force

-Stress = force /area -Stress = force /area

Types of StressTypes of Stress

Normal : load perpendicular Normal : load perpendicular to cross-sectionto 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

Results of Tensile or Flex TestResults of Tensile or Flex Test

Viscoelastic Creep Viscoelastic Creep

Microstructure ElementsMicrostructure Elements

Molecule Length

Morphology

Molecule Arrangement

Intermolecular BondsFillers & Modifiers

Microstructure TestsMicrostructure Tests

Measurement/characterization of the Measurement/characterization of the condition and arrangement of condition and arrangement of molecules and additives within a molecules and additives within a plastic systemplastic system

Process parameters most often affect Process parameters most often affect properties by altering microstructureproperties by altering microstructure

Molecular WeightMolecular Weight

Average Length of moleculesAverage Length of molecules Determines processing Determines processing

characteristics and some mechanical characteristics and some mechanical propertiesproperties

Usually measured by resin Usually measured by resin manufacturermanufacturer

Effect of Molecular Weight on Effect of Molecular Weight on Mechanical PropertiesMechanical Properties

Molecular MorphologyMolecular Morphology

Thermoset Thermoplastic

Network

Cross-link density

Network strain

Glass transition temperature

Amorphous Crystalline

Degree of crystallization

Type of crystals

Glass transition

Orientation

Semicrystalline Thermoplastic Microstructure

CrystalsCrystals

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 morphologiesTwo crystalline morphologies

SpheruliteSpherulite

(no shear)(no shear)

Row Nucleated Row Nucleated

(shear)(shear)

Shish-kebabShish-kebab

Effects of CrystallizationEffects of Crystallization

IncreasesIncreases Modulus (stiffness)Modulus (stiffness) StrengthStrength DensityDensity Chemical Chemical

ResistanceResistance Thermal stabilityThermal stability HardnessHardness

DecreasesDecreases ToughnessToughness PermeabilityPermeability

Thermoset MorphologyThermoset Morphology

Cross-Link BondBackbone bond

Measurement of Network StructureMeasurement of Network Structure

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

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.

Dynamic Mechanical Analysis:Dynamic Mechanical Analysis:Modulus & TemperatureModulus & Temperature

Molecular OrientationMolecular Orientation

Failure process Failure process depends on depends on molecular molecular orientationorientation

Generally more Generally more orientation = orientation = greater toughnessgreater toughness

Processing Affects MorphologyProcessing Affects Morphology

Crystallization takes time so cooling Crystallization takes time so cooling rate is criticalrate is critical

Shear during mold filling or drawing Shear during mold filling or drawing can create row nucleated structure can create row nucleated structure or orient moleculesor orient molecules

Curing rate and temperature Curing rate and temperature determine network structure in determine network structure in thermosetsthermosets

Issues in Filler/Modifier MorphologyIssues in Filler/Modifier Morphology

Loading level – absolute amount of Loading level – absolute amount of filler or modifierfiller or modifier

Dispersion or distribution – clumping Dispersion or distribution – clumping or orientation of material can or orientation of material can dramatically affect performancedramatically affect performance

Coupling – quality of bonding Coupling – quality of bonding between polymer and modifier is between polymer and modifier is critical for optimum performance critical for optimum performance

Test ClassificationsTest Classifications

Microstructural Microstructural • Degree of Cure or CrystallizationDegree of Cure or Crystallization• Additive levels, distributionAdditive levels, distribution

MechanicalMechanical• Strength,Toughness, Hardness Strength,Toughness, Hardness

PhysicalPhysical• Optical propertiesOptical properties• Thermal propertiesThermal properties• Electromagnetic properties Electromagnetic properties

Coupon vs. Article TestingCoupon vs. Article Testing

Article TestingArticle Testing• Result often just a pass/failResult often just a pass/fail• Test is often awkward or expensiveTest is often awkward or expensive• Includes effect of processingIncludes effect of processing• May allow for testing of complex service May allow for testing of complex service

and processing interactionsand processing interactions• Results are often difficult to relate back Results are often difficult to relate back

to particular process or material to particular process or material problem ---- not good for process controlproblem ---- not good for process control

Coupon vs. Article TestingCoupon vs. Article Testing

Coupon TestingCoupon Testing• Result is usually a continuous variableResult is usually a continuous variable• Doesn’t necessarily include all Doesn’t necessarily include all

processing effectsprocessing effects• Will generally be directly related to Will generally be directly related to

material or process factormaterial or process factor• More useful in process control and More useful in process control and

product engineeringproduct engineering

Test Type (Common at UTPA)Test Type (Common at UTPA) Coupon Article Coupon Article

Tensile Tensile FlexuralFlexural CompressionCompression Impact Impact DMADMA TMATMA DEADEA

HardnessHardness DSCDSC TGATGA

Differential Scanning CalorimetryDifferential Scanning Calorimetry

Small sample (10 Small sample (10 mg)mg)

Heat flow into Heat flow into sample measuredsample measured

Heat capacity Heat capacity measuredmeasured

Tg, melting, cure Tg, melting, cure crystallization, crystallization, decomp, volatilesdecomp, volatiles

Thermogravimetric AnalysisThermogravimetric Analysis

Weight change Weight change with temperaturewith temperature

Dynamic Mechanical AnalysisDynamic Mechanical Analysis

Many load Many load configurationsconfigurations

Oscillating loadOscillating load Changing Changing

temperaturetemperature Continuous Continuous

measurementmeasurement

DMA ResultsDMA Results

TgTg Temperature Temperature

dependence of dependence of modulusmodulus

Sensitive to small Sensitive to small changes in changes in composition or composition or morphologymorphology

Thermo-Mechanical AnalysisThermo-Mechanical Analysis

Expansion of Expansion of sample measuredsample measured

Temperature Temperature variedvaried

Continuous Continuous measurementmeasurement

Sample Preparation/CollectionSample Preparation/Collection

Avoid conditions which deviate from Avoid conditions which deviate from actual product conditions –actual product conditions –• TemperatureTemperature• Excess stress or strain applicationExcess stress or strain application• Chemical exposureChemical exposure

For warranty returns, if possible, test For warranty returns, if possible, test failed components not just samples failed components not just samples from same lotfrom same lot

Minimum Equipment for Plastics Minimum Equipment for Plastics QC LabQC Lab

Linear measurement (calipers)Linear measurement (calipers) Analytical balanceAnalytical balance OvenOven Instruments required by customers Instruments required by customers

(colorimeter, reflectometer, etc)(colorimeter, reflectometer, etc)

Recommended Equipment for QC Recommended Equipment for QC LabLab

Differential Scanning Calorimeter Differential Scanning Calorimeter (DSC) – virtual requirement for (DSC) – virtual requirement for thermosetting processorsthermosetting processors

Durometer or other hardness testerDurometer or other hardness tester

Nice to Have in QC LabNice to Have in QC Lab

Small universal testing machine Small universal testing machine (tensile, flex, compression testing)(tensile, flex, compression testing)

Pendulum impact tester (Izod)Pendulum impact tester (Izod) Melt Indexer (if regrind blending is Melt Indexer (if regrind blending is

done or for acceptance testing of done or for acceptance testing of non-certified material)non-certified material)