practical rheology and its role in polymer processing
DESCRIPTION
Practical Rheology and Its Role in Polymer Processing. By Tim Womer. Outline. Definitions Basic Equipment Viscosity vs. Shear Rate Curves What they tell us How we can use them Melt Index: “What is it?” Moisture?. RHEOLOGY. The Science of Deformation and Flow of Matter. - PowerPoint PPT PresentationTRANSCRIPT
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By
Tim Womer
Practical Rheology and Its Role in Polymer Processing
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Outline
• Definitions
• Basic Equipment
• Viscosity vs. Shear Rate Curves
• What they tell us
• How we can use them
• Melt Index: “What is it?”
• Moisture?
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RHEOLOGY
The Science of Deformation and Flow of Matter
from “ ”
“PANTA RHEI”
Heracleitus, 5th Century B.C.
“All Things Flow”
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Rheology is:
• Science of material flow behavior• Uses very complicated and difficult to
understand mathematical background (tensors, co-rotational or co-deformation curved space, etc.)
• Can be presented in very complicated way which is hard to understand for non-specialists
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Mathematics is complicated
xi
ij
xj
- p
= 0 vj
xj
= 0
ij + K(ij) = 2 eij
. ij
tij = ji
ij = K1(CI,CI-1) CI + K-1(CI,CI
-1)CI-1) ( )e-(t-t’) dt’
t
-
T
xj
( T
xj
) + Cp
T
xj
vj = 2 eij eij . .
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Explanation of Rheology
• Mathematics is overemphasizedComplicated mathematical background leads sometimes to “scientific exhibitionism” of the presenter
• Such an approach is useful for scientific people and conferences but usually useless for practical people
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Explanation of Rheology
• Understanding of the physics is emphasized– the mathematics is avoided as much as
possible– the explanation is based on physical
understanding what is happening
• Such an approach cannot be used for development in rheology but may be useful for understanding material behavior
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AIR
10 0
105
10 10
10 15
10 20
10 - 5
Viscosity [ Pa s ]
WATER
ENGINEOIL
polymer melts
GLASS
Glass Windows of the Cathedral of
Chartres in France have “Flowed”since they were Produced 600
years ago.
3 mm
600YEARS
Paper Thin
Glass behaves like Fluids !!!
IF YOU WAIT LONG ENOUGH
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Tim’s Definition of Rheology
• The flow characteristic of a polymer at various shear and temperature conditions.
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Crystallinity (semi-crystalline)
Definition: A state of molecular structure in some resins attributed to the existence of solid crystal with a definite geometric form. Such structures are characterized by uniformity and compactness.
Semi-Crystalline Resins:
• LDPE,HDPE, PP, FEP, Nylon, PET, PU
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Amorphous (non-Crystalline)
Definition: Devoid of crystallinity or stratification. Most plastics are amorphous at processing temperatures, many retaining this state under all normal conditions
Amorphous Resin:
• PS, PVC, ABS, PC, PMMA
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Resin Pyramid
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Viscosity
• A measure of the internal friction resulting when one layer of fluid is caused to move in relationship to another layer.
• Example:The flow characteristic of 50 wt. motor oil at 50°F in the winter versus its flow characteristic when it is drained out of the oil pan after the motor has been warmed up to 200°F
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Capillary Rheometer
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Materials with different Viscosity shear thinning
n = .3n = .5
n = 1
MFI regionExtrusionProcess window
(Power-law Index Role)
PET
LDPE
PP
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What are the curves telling me?
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Not Shear Sensitive but Temperature Sensitive
Polyester
Screw Operating Range
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Not Temperature Sensitive, but Shear Sensitive
Screw Operating Range
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Viscosity of 4 different Resins
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Melt Fracture
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Melt Density using a Capillary Rheometer
• Definition: The density of a polymer when it is in a molten state. The melt density is totally dependent to the temperature at which is it measure or processed.
• ei, HDPE typically has a solid density of about .96 gm/cc but in the molten/process range it has a density of .75 gm/cc
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Melt Index(MI, MFR, MFI)
The terms “melt index” (MI), “melt flow rate” (MFR) and “melt flow index” (MFI) refer to the same test. MFR was introduced to replace MFI. The term MFI is used to refer to the flow rate of PE obtained under Condition 190/2.16 (formally know as Condition E). The use of such terms is not encouraged for other materials. It is suggested, by ASTM D 1238 that the term melt flow rate (MFR) be used for other plastic materials.
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Melt Indexer
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What does MFI mean? Not Much!
VIS
CO
SIT
Y
SHEAR RATE
app = 2.4 MFI.
0 sec-1 10 sec-1
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Hygroscopic
• Definition: Having the tendency to absorb moisture from the air. Some resins are hygroscopic, thus requiring drying prior to processing.
• “Wet” resin will decrease the physical properties of the resin, typically embrittlement will occur.
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Moisture Content
• Definition: The amount of moisture in a material determined under prescribed conditions and expressed as a percentage of the weight of the moist specimen, that is, the original weight comprising the dry substance plus any moisture present.
• Hygroscopic Resins– HIPS, ABS, Nylon, PET, PETG
• In some processes, HDPE is dried.
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Moisture Analyzer
• Moisture Analyzer is used to measure the moisture content of a hygroscopic material
• Moisture content of resins can range from .005% to .18% measured in parts per million depending on the resin being processed
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Resin Drying
Desiccant Dryer Vacuum Dryer
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Are There Any Questions?