The principals of rheology In pharmaceutical technology

Transcription

1 The principals of rheology In pharmaceutical technology Dr. Aleksandar Széchenyi University of Pécs Gyógyszertechnológiai és Biofarmáciai Intézet Institute of Pharmaceutical Technology and Biopharmacy

2 Lecture scheme - Transport processes - Importance of reology in pharmacy - Newtonian fluids (ideally viscous body) - Non-newtonian fluids time-independent: plastic (ideal,real,pseudo) - time dependent: tixotropy rheopexy - Reology testing techniques in practice of pharmacy dilatant

3 Transport processes There are coupled transportprocesses in the LIVING SYSTEMS, mass-,energy-,momentum- and charge transports take place SIMULTANEOUSLY. 3

4 Flux The rate of flow of extensive property per unit area of surface through which it passes is called the flux. Volumetric flux Mass flux Mass flux = density x volumetric flux Momentum flux Momentum flux = mass flux x velocity Energy flux

5 FLUX Transport processes Heat ELECTRIC CHARGE MOMENTUM MASS (DIFFUSION) 5

6 Importance of rheology in pharmacy 6

7 Rheology of dispersions Definition of rheology reo flow, logos science The rheology describes flow of liquids and the deformation of solid materials. 7

8 Introduction to Rheology Rheology describes the deformation of a body under the influence of stresses. Bodies in this context can be either solids, liquids, or gases. Ideal solids deform elastically. The energy required for the deformation is fully recovered when the stresses are removed.

9 Introduction to Rheology Ideal fluids such as liquids and gases deform irreversibly -- they flow. The energy required for the deformation is dissipated within the fluid in the form of heat and cannot be recovered simply by removing the stresses.

10 Introduction to Rheology The real bodies we encounter are neither ideal solids nor ideal fluids. Real solids can also deform irreversibly under the influence of forces of sufficient magnitude They creep, they flow. Example: Steel -- a typical solid -- can be forced to flow as in the case of sheet steel when it is pressed into a form, for example for automobile body parts.

11 Introduction to Rheology Viscosity The viscosity is a RESISTANCE to flow or movement (internal friction) Thin layers (laminae) of fluid which are able to slide over one another due to external shearing force (F) layers of fluids 11

12 Introduction to Rheology 12

13 Introduction to Rheology 13

14 Rheology of disperse systems 14

15 The difference in response to shear of viscous and elastic liquids From ThermoHaake 15

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17 The Reynolds number is defined as where: ρ is the density of the fluid (SI units: kg/m3) u is the velocity of the fluid with respect to the object (m/s) L is a characteristic linear dimension (m) μ is the dynamic viscosity of the fluid (Pa s or N s/m2 or kg/m s) ν is the kinematic viscosity of the fluid (m2/s). 17

18 Small and high shear rate 18

19 Dynamic viscosity Units of viscosity solids 10 18, fluids , gases Pas Kinematic viscosity m 2 s -1 = 10 4 St=10 6 cst (centistokes) 19

20 Absolute and relative viscosity RELATIVE SPECIFIC REDUCED 20

21 Dependence True solutions : temperature concentration Macromolekular solutions: temperature concentration molacular weight structure of molecule or particle solvent 21

22 Temperature-depending of viscosity A ΔE constant, activation energy of liquid molecule mobility for 1mol [kj mol -1 ], R universal gas constant (8,314 J K mol -1 ), T absolute temperature(k). INCREASING TEMPERATURE makes the viscosity of solid bodies and liquids decrease, of GASES INCREASE. 22

23 Determining of molecular weight of macromolecules (Mark-Houwing correlation) intrinsic viscosity 23

24 Rheological subdivision of materials Newtonian systems viscosity INDEPENDENT from shear stress Non-newtonian systems viscosity DEPENDS on shear stress 24

25 Mechanic rheological models 1. A reversible deformation model of a spring Hooke - ideal elastic body A second model of viscous flow at the piston NEWTON Newtonian body 3. A plastic flow model of the friction element Venant - body 25

26 Newtonian materials (ideally viscous) viscosity can be considered a constant characteristic of the material s quality) flow curve viscosity curve The flow curve is linear and crosses the zero. The viscosity curve is independent from shear stress

27 PLASTICS Non-newtonian materials TIME-INDEPENDENT IDEAL ~ STRUCTURAL VISCSITY (η*) Diluted suspensions REAL ~ (e.g. pastes, ointments and ointment materials, suppositories and supposaitory materials, toothpastes, concentrated emulsions, suspensions, gels. PSZEUDO ~ (e.g. paps, emulsions,suspensions) 27

28 IDEAL plastics 28

29 REAL plastics 29

30 PSEUDO plastics 30

31 Pseudoplastical flowing (structural changes viscosity decreasing STRUCTURAL VISCOSITY, η*) FLOW direction Orientation Alignment VISCOSITY DECREASING Deagregation Deformation Polymer chain alignment 31

32 Non-newtonian materials TIME-INDEPENDENT DILATANT 1 n>1 The occurence of this phenomenon is rare in pharmacy practice (e,g,. pigmentsuspensions) 32

33 Non-newtonian materials TIME-DEPENDENT Rheopexy TIME- INDEPENDENT Thixotropy 33

34 Non-newtonian materials TIME-DEPENDENTID THIXOTROPY e.g.. Gelatin solution Typical hysteresis loop Viscosity depends on the DURATION of the influence, not just the magnitude of shearing stress) TIME-DEPENDENT 34

35 THIXOTROPY Thixotropic materials must meet four CONCURRENT criteria: a shearing stress of increasing intensity decreases η, if the duration of continued stress increases, η decreases, under prolonged and intensive stress η converges toward a final limit, after stress is stopped, the material returns to its original state if left alone under isothermic conditions. 35

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41 Non-newtonian materials TIME-DEPENDENT REOPEXIA This rheological behavior is uncommon in practice. Please note, that rheopectic flow curves need critical consideration, as in most cases the phenomenon is caused by other changes (e.g.: sedimentation, evaporation, subsequent polymerization, high instrument attenuation), not true rheopexy. 41

42 Rheology testing techniques Capillary viscometers Ostwald-Fenske Hagen-Poiseuille 42

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44 Rheology testing techniques Falling sphere viscometers Höppler s viscometer falling ball type Höppler s rheoviscozimeter 44

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47 Rheology testing techniques ROTATIONAL vicometers (Searle,inner cylinder rotates, Couette, outer cylinder rotates) Dynamic viscosity of ANY substances OSCILLATING rheometers for viscoelastic materials 47

48 SUMMARY - Transport processes: momentum flux - Importance of reology in pharmacy - Newtonian fluids (ideally viscous body) - Non-newtonian fluids time-independent: plastic (ideal,real,pseudo) - STRUCTURAL VISCOSITY in practice of pharmacy dilatant time dependent: tixotropy (dependens on the duration of the influence, 4 concurrent criteria,hysteresis rheopexy

49 Thank you for your kind attention 49