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• application notes & technical articles
The influence of thickeners on the application
method of automotive coatings and paper
coatings – rheological investigations
surface coatings industries
Spraying automotive coatings
and the application of paper
coatings are industrial
processes, in which
elongational flows play an
important role.
As a result, the application behaviour of these
fluids often cannot be sufficiently characterised
with traditional shear experiments. Products with
similar shear viscosities can have very different
elongation properties.
With the Thermo Scientific HAAKE CaBER 1
extensional rheometer, a liquid filament is created
that is stretched under the influence of surface
tension. Using the decrease in the filament
diameter as a function of time and the life of the
filament, it is possible to characterise the
elongational behaviour of low viscosity to pasty
liquids in a simple way. The rheological properties
of typical water-based automotive coatings and
paper coatings are determined by the thickeners
used and their interactions with the other recipe
components. In the CaBER 1 experiment, different
types of thickener display a characteristic
decrease in the filament diameter as a function
of time. Different break-up times are desirable,
depending on the application.
When automotive coatings are sprayed, short
break-up times are advantageous in order to
obtain the finest possible drop distribution.
When paper coatings are applied with rollers,
spraying and „misting“ should be prevented as
much as possible, which means that formulations
with long break-up times are advantageous.
A full copy of the application note is available on
request by quoting V-206.
• application notes & technical articles
Slurry rheology and pipeline transport
properties, an overview
surface coatings industries
Slurry transport in the mining
industry is a commonly used
technique for moving partially
and fully processed materials,
and waste.
Often this type of treatment is more economical
and rapid than other transportation methods, such
as those involving conveyor systems or haulage.
Additionally, the materials on a mine-site are often
slurried for processing, and pipeline transport is
necessary.
However frequently utilised, efficient slurry
transport still poses serious engineering problems
through the properties of the slurry. These
include, but are not limited to, particle settling,
attrition, pipe/fitting/impeller wear, degradation of
flocculated or friable solids, and the pumpability of
the slurry. In addition, the effects of these
challenges can be exacerbated, or reduced by
biennial 07-08
process or environmental variables, such as
temperature, pH or solids loading.
In-transit particle settling leads to stratified flow,
which in turn can cause uneven and excessive
wear in the transport system, and in extreme
cases, pipeline blockage due to solids build-up
around fittings or inclined sections. Particle
attrition has several drawbacks. Firstly, the
product before the pump and pipe is not identical
to that afterwards if the particles are friable, this
may be a QC matter both for disposal and
in-process. Secondly, changes in particle size
distribution may affect the ability of the particles to
remain suspended in the carrier fluid. Problems
with the pumpability of slurries potentially include
the fact that they often behave as a solid, e.g.
when at rest, making it expensive to initiate
pumping. Many fluids, including slurries have flow
properties which depend on the ‘rate of deformation’
(analogous to their flow-rate).
Rheological techniques can help the engineer or
technician to design for and manipulate
processes. A range of instruments is available
which measure the effects of solids loading,
carrier fluid, particle size distribution and viscosity
or pH modifiers on the propensity of particulate
matter to flocculate, be suspended, settle etc.
through a knowledge of the characteristic flow
properties of the slurry. These rheometers and
viscometers allow the engineer to directly measure
and assess the causes and effects listed above.
Physical quantities such as yield stress (the
minimum force required for a slurry to flow),
plastic viscosity (how ‘runny’ a slurry will be at a
given laminar flow-rate), hysteresis (the changes
in plastic viscosity and/or yield stress which are
due, usually, to particle attrition or settling) can be
better predicted with the appropriate equipment.
Sensitive, high precision versatile equipment is
available for repeatable and reliable measurement
of all flow properties in the laboratory, and durable,
precise, robust units for in the field use are also
commercially available. These units have been
shown to be of value for both QC and product
development.
A full copy of the application note is available on
request by quoting Rheo013
22 www.rheologysolutions.com