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 from “ ” “PANTA RHEI” Heracleitus, 5th Century B.C. “All Things Flow”
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
p vj tij = 0 - = 0 xi xj xj . . T T T ( ) + Cp vj = 2 eijeij xj xj xj . tij tij tij = tji + K(tij) = 2 eij t t )e-(t-t’) dt’ ( tij = K1(CI,CI-1) CI + K-1(CI,CI-1)CI-1) - Mathematics is complicated
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
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
Paper Thin Viscosity [ Pa s ] • 10 20 GLASS 600 YEARS • 10 15 • 10 10 3 mm • 105 polymer melts Glass Windows of the Cathedral of Chartres in France have “Flowed” since they were Produced 600 years ago. • 10 0 ENGINE OIL WATER • 10 - 5 AIR IF YOU WAIT LONG ENOUGH Glass behaves like Fluids !!!
Tim’s Definition of Rheology • The flow characteristic of a polymer at various shear and temperature conditions.
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
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
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
n = 1 n = .3 Extrusion Process window n = .5 MFI region Materials with different Viscosity shear thinning (Power-law Index Role) PET LDPE PP
Screw Operating Range Not Shear Sensitive but Temperature Sensitive Polyester
Screw Operating Range Not Temperature Sensitive, but Shear Sensitive
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
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.
VISCOSITY 0 sec-1 10 sec-1 SHEAR RATE . app = 2.4 MFI What does MFI mean? Not Much!
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.
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.
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
Resin Drying Vacuum Dryer Desiccant Dryer