Visco handbook THEORY AND APPLICATION OF VISCOMETRY WITH GLASS CAPILLARY VISCOMETERS Welcome to SI Analytics! We express our core competence, namely the production of analytical instruments, with our company name SI Analytics. SI also stands for the main products of our company: sensors and instruments. As part of the history of SCHOTT® AG, SI Analytics has nearly 80 years experience in glass technology and in the development of analytical equipment. As always, our products are manufactured in Mainz with a high level of innovation and quality. Our electrodes, titrators and capillary viscometers will continue to be the right tools in any location where expertise in analytical measurement technology is required. In 2011 SI Analytics became part of the listed company Xylem Inc., headquartered in Rye Brook / N.Y., USA. Xylem is a leading international provider of water solutions. We are pleased to introduce you to the Visco Handbook! It replaces the previous brochure "Theory and Practice of Capillary Viscometry". This has been updated, redesigned and restructured. The information content of some areas such as polymer analysis was increased and specially themed areas such as hydrodynamic principles were moved into their own appendices so as not to deter the reader by too much theory. The focus was placed on the practical and general information needed by each user of capillary viscometry. Both laboratory experiences and policies of the relevant standards are considered here. We thus hope to provide you with a faithful companion for everyday laboratory work, one that you can use profitably. We of SI Analytics are pleased to work with you successfully in the future. SI Analytics GmbH Dr. Robert Reining CEO CONTENTS CHAPTER 1 Viscosity - Rheology 1.1 Introduktion .................................................................................... 7 1.2 Non-Newtonian flow behavior ...................................................11 1.3 Principles of viscosity measurement ..........................................13 CHAPTER 2 Fundamentals of capillary viscometry 2.1 Measurement principle ................................................................14 2.2 Designs of glass capillary viscometers ......................................16 CHAPTER 3 Measurement of the flow time 3.1 Manual timing ................................................................................19 3.2 Automatic timing ...........................................................................19 CHAPTER 4 Method for determining viscosity 4.1 Neglect of the kinetic energy correction ..................................23 4.2 Application of the kinetic energy correction ..........................25 4.3 Experimental determination of individual kinetic energy correction (according to DIN 51562-3) ..........26 4.4 Examples of viscosity determination .........................................28 CHAPTER 5 Calibration ............................................................................................................ 29 CHAPTER 6 Handling of capillary viscometers 6.1 General guidelines for the selection of the measuring system ..............................................................................................32 6.2 Cleaning of capillary viscometers ..............................................35 6.3 Preparation of the measurement ...............................................38 6.4 Performing the measurement .....................................................41 CHAPTER 7 Sources of error and special corrections 7.1 Correctable errors and corrections ............................................47 7.2 Uncorrectable errors .....................................................................51 7.3 Trouble shooting table .................................................................53 CHAPTER 8 Special application 8.1 Testing of plastics ...........................................................................57 8.2 Viscosity determination of oils and additives ............................. 68 8.3 Food testing ....................................................................................71 Appendix A Symbols and units used Bibliography Standards Authors edition: Prof. Dr.-Ing. habil. Jürgen Wilke Dr.-Ing. Holger Kryk Dr.-Ing. Jutta Hartmann Dieter Wagner major rewrite, update and new edition (© 2015): Dr. Andreas Eich Visco handbook CHAPTER 1 Viscosity characterizes the inter- nal friction of liquids and gases. VISCOSITY - RHEOLOGY If a fluid medium is located bet- ween two plane-parallel plates, 1.1 Introduction it will require some amount of force to displace the upper plate. The subject of this Visco Hand- The liquid starts to flow inside book is viscometry with glass the gap. A layered flow builds up capillary viscometers. (Fig. 1). Viscometry deals with the de- y termination of viscosity and is F a branch of rheology, which is scientifically concerned with the v flow and deformation behavior x of materials. In rheology, sub- stances, especially viscoelastic Fig. 1 Basic shearing model in the ("semi-solid"), are examined, case of laminar, stationary flow which are classified as being be- tween a fluid and a solid, based on their mechanical properties. The fluid particles which are di- rectly adjacent to the plates are With glass capillary viscometers, firmly bonded to the surface by only the viscosity of samples with adhesion forces. In this process ideal flow behavior (Newtonian the fluid layer neighboring the liquids) is measured. This section plate being displaced adopts briefly explains the difference the velocity of the plate. All between ideal and non-ideal neighboring layers stay more flow properties. Additional infor- and more behind with the in- mation about rheology may be creasing distance to the plate found in [1,19]. being moved. The cause for this phenomenon can be found in cohesion forces which counter-act the reciprocal dislo- cation of the individual layers. 7 Visco handbook The shear stress τ refers to The relationship between dy- the quotient of force F and namic viscosity η and density ρ is the boundary surface A of referred to as kinematic viscosity the liquid: ν (pronounced "ny"): F η 2 τ = ν = = m / s (1.5) A (1.1) ρ For reasons of convenience, The gratient of velocity, the shear the unit of mm2/s is used, rate γ is the differential quotient: which then corresponds nu- merically to the former unit. dv γ = (1.2) cSt (centistokes). dy In Newtonian fluids, the viscosity According to Newton's Viscosity remains constant with a change Law, the shear stress τ is propor- of the shear rate. As a conse- tional to the shear rate γ : quence of the differences in size, shape, and interaction between the molecules may change τ = η ⋅γ (1.3) η within very wide limits. The proportionality factor η (pro- nounced "eta") is referred to as Examples: dynamic viscosity. n-pentane 0.230 mPas (20 °C) The unit of viscosity is Pa · s, Water 1.002 mPas (20 °C) wherein low-viscosity samples Propanetriol 1480 mPas (20 °C) are usually specified in mPa · s (Glycerol) and thus the former unit cP (centipoise) numerically corre- sponds to: τ 2 η = Ns / m = Pa⋅ s (1.4) γ 8 γ Temperature dependence of This effect is of great practical im- viscosity portance, for example, in lubrica- tion technology, as will be shown During flow, liquid molecules later. slide against one another under the expenditure of energy (ac- In addition to the temperature, tivation energy). The proportion the pressure also has an effect of molecules that have this ener- on the viscosity: An increase in gy depends on the temperature pressure generally leads to in- and is described by the Boltz- creased viscosity. The effect does mann distribution. not appear so much in everyday life since only pressure increases This leads to the relationship: E of 10 to 100 bar (or even higher) - visk RT lead to a significant increase in η = k ⋅e (1.6) viscosity of liquids. The pressure dependence of the viscosity k Proportionality factor therefore plays no role in visco- metry with glass capillary visco- Evisk Activation energy of the viscous fluid meters, which is discussed in this R Gas constant Visco handbook. T Absolute temperature After that, η strongly decreases for liquids with increasing tem- perature. As a rule of thumb, one can say that the higher the abso- lute values of the viscosity and the lower the temperature, the greater is the decrease. 9 Visco handbook Viscosity of mixtures/solutions A particular behavior can be ob- When liquids are mixed, the vis- served with the dependence of cosity can be approximately cal- concentration of viscosity of elec- culated from the viscosities of the trolyte solutions. individual components accord- ing to a logarithmic mixing rule. If the liquid layers are moving at different velocities, the deforma- Example of two components: tion of the ion cloud will cause the occurrence of additional inter- ionic interacting forces which will lnη = w lnη + w lnη (1.7) mix 1 1 2 2 affect friction between the indi- vidual layers. w1 , w2 : weight fractions of the components 1,2 H. Falkenhagen has derived the limiting law of viscosity from the This rule only applies to mixtures theory of inter-ionic interactions of similar components, and does for highly dilute electrolyte solu- not generally apply for aqueous tions: solutions. For a better description of reality, equations must be η = η + K c (1.8) selected with adjustable para- c 0 meters [2]. ηc Viscosity for the ion The viscosity of the solutions of concentration c Viscosity of the pure solid