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Viscosity Training

Viscosity – What is it?

  • The term “Viscosity” is a parameter that describes a fluids resistance to flow
  • This resistance is a measure of the friction and the forces of interaction between the layers of a fluid
  • This friction is apparent when one layer of fluid is made to move against another
  • The greater the friction, the greater the amount of force required to cause the movement – known as “Shear”
  • Shearing occurs whenever a fluid is moved or distributed i.e. Poured, Spread, Sprayed etc.
  • Highly viscous fluids require a larger force to move them than do less viscous fluids
  • When a shearing force is applied to a fluid it is deformed causing the layers to move according to a velocity gradient relative to the force imposed
  • Viscosity therefore is the relationship between shear stress and shear rate

Viscosity - Some History

  • In 1687 Sir Isaac Newton defined the fundamental unit of Viscosity - the poise.
  • A material requiring a shear stress of one dyne per square centimeter to produce a shear rate of one reciprocal second has a viscosity of one poise, or 100 centipoise.
  • Pascal-Seconds – an SI unit of viscosity is also used in preference to the metric designation.
  • One Pascal-second is equal to ten poise; one milli-Pascal-second is equal to one centipoise.

History Continued

Newton knew that the viscosity of a fluid changed with temperature however he assumed that it was independent of shear rate.

Hence the term “Newtonian” characterised most materials at that time …….. However this was only partly correct!

Flow or not to Flow... That is the question

The viscosity of Newtonian fluids are constant at a given temperature regardless of the shear rate applied

The viscosity of Non-Newtonian fluids exhibit a change in viscosity when a change is made to the shear rate applied.

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Shear Thinning (Psuedoplastic)

  • Where the viscosity of a fluid decreases   with   an increase in shear rate
  • A common behaviour in coatings including paints, emulsions and dispersions of many different types

Shear Thickening (Psuedoplastic)

  • Where the viscosity of a fluid increases with an increase in shear rate.
  • Although not common in coatings, food products, including corn starch in water, exhibit this behaviour.
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Plastic

  • Where a fluid appears solid when static and only flows when a certain amount of force is applied.
  • A good example of this is Tomato Ketchup. It will only flow when a force is applied greater than its resistance to flow(Yield Value).

Thixotropic

  • Where the viscosity of a fluid decreases under a constant shear rate over a period of time.
  • This behaviour is commonly observed in greases, heavy printing inks, and paints.
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Rheopectic

  • Where the viscosity of a fluid increases under a constant shear rate over a period of time
  • Although there are not many examples, there is ongoing research into military uses of this behaviour. Body armour and combat vehicle armour are key areas where efforts are being made to use rheopectic materials.

Ok – so that’s the theory

How does it apply to Coatings and why measure it?

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Modern coatings contain many different raw materials each having differing properties.

Each coating has a use dependant on its industry and application.

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Coatings cost money to produce therefore optimisation of the raw materials and manufacturing processes used is essential.

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Coatings cost money to produce therefore optimisation of the raw materials and manufacturing processes used is essential.

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Inter-batch inconsistencies in viscosity causes problems during application, including sagging, poor hiding power and drying time.

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Ok – I now get it ...

What instruments are used for different types of coatings?

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Rotational Viscometer based on the Stormer type Krebs viscometer

One of the earliest controlled stress rotational methods

Complies to ASTM D562

Measurement of Newtonian coatings

Used to determine the viscosity of paints, ideal for measuring the in-can feel of paint

Stormer Krebs Viscometer Mk1- 1942 with paddle type rotor and stroboscopic timer

Select weights (to the nearest 5 g) that it produces a 200-r/min pattern.

(Use table to convert from grams to Krebs)

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Rhopoint Krebs Viscometer Features -

  • Fully automatic operation
  • Paddle lowers into sample to correct height for each test
  • Test commences when sample has reached required temperature
  • Paddle automatically raises to drain off when test has completed
  • Offset T shaped paddle rotating at 200rpm provides constant shear rate to the sample under test
  • Direct measurement of torque required to keep paddle rotating at constant speed
  • High stability motor speed control ensures accuracy and repeatability during each test
  • Quick release magnetic paddle holder allows rapid changeover of paddle…
  • High stability motor speed control ensures accuracy and repeatability during each test
  • Quick release magnetic paddle holder allows rapid changeover of paddle…
  • Saving time during cleaning
  • Integrated temperature probe accurately measures and records sample temperature for each test as detailed in ASTM D562

 

 

High contrast colour screen simultaneously displays measurement results in KU (Krebs Units), cP (centipoise), and g (grams), including sample temperature if selected

 

 

 

Real time graphing allows monitoring of viscosity during measurement

 

 

Real time graphing allows monitoring of viscosity during measurement

 

 

Measurement data including time, date and test temperature can be printed to an external label printer (supplied as an optional accessory).

 

 

Data can also be transferred directly into Excel using the optional data transfer cable

 

 

Touch sensitive buttons ensure trouble free operation

Eliminating issues caused by mechanical switch failure

Safe to use with most solvents

Manufacturer Rhopoint Brookfield Viscotech Sheen
Model Krebs KU-2 VK2000 480
Operation Automatic Manual Automatic Manual
Measurement Method Digital-Brushless
Swiss Motor
Analogue-Spring +
DC Motor
Digital-DC Motor
Torque
Digital-DC Motor
Torque
Graphing Yes No No No
Speed 200rpm 200rpm 200rpm 200rpm
Display 4.5 inch colour 4 Digit LED 4 Line LCD 1 Line LCD
Operation 6 x Cap Sense Button/Knob 5 x Dome switches 4 Button
Interface USB RS232 RS232 RS232
Printer Yes-Labels Yes Yes Yes
Time Stamped Yes No Yes No
Accuracy ±1% ±1% ±1% ±2%
User Recal Yes with key No No Yes with key
Temperature Measurement Yes No No No

Specific product information retrieved from manufacturer’s published information on 19th April 2016’