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=Ee
Stress = Young’s modulus x strain
Results of Tensile or Flex Test
Viscoelastic Creep
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
Effect of Molecular Weight on
Mechanical Properties
Molecular Morphology
Thermoset
Thermoplastic
Network
Amorphous
Cross-link density
Network strain
Glass transition
temperature
Crystalline
Glass
transition
Degree of
crystallization
Orientation
Type of
crystals
Semicrystalline Thermoplastic Microstructure
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
Effects of Crystallization






Increases
Modulus (stiffness)
Strength
Density
Chemical
Resistance
Thermal stability
Hardness


Decreases
Toughness
Permeability
Thermoset Morphology
Backbone bond
Cross-Link Bond
Measurement of Network Structure

Best Indicator of network intensity
(crosslink density, crosslink
functionality) is Glass transition
temperature
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:
Modulus & Temperature
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
Test Type (Common at UTPA)
Coupon
Article







Tensile
Flexural
Compression
Impact
DMA
TMA
DEA



Hardness
DSC
TGA
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)