What are thixotropic agents?
Ryen Caenn
DUDLEY) writes:
>
>I have run across a reference to thixotropic agents. I cannot find a
>definition for this term, although the article says that fumed silica
and
>organoclay are both materials of this nature. Unfortunately, I can't
find a
>reference as to what fumed silica and organoclay are either. Any
assistance
>would be appreciated.
>
>Thanks,
>
>Marshall
Thixotropy is the characteristic of a fluid increasing in viscosity as
it sits quiesent (sp?). I don't know about fumed silica, but
organoclays are not really thixotropic. Bentonite clays in water, at a
certain concentration, are thixotopic (they increase in vicosity with
time, but can be thinned by increased shearing), but not organoclays
which are dispersed in oils (more Newtonian than non-Newtonian).
Organoclays can developed gel strengths which is not neccesarily the
same as being thixotropic. One of the fine points of rheology.
BTW - Back in the good old days before everyone had there own computer
and word processor, I wrote a report describing the rheological
properties of a drilling fluid. The secretary typed it as theological
properties. Since rheology is really a religion with its own belief
structure, I decided to leave it as it was.
My spell checker still tries to change rheology to theology!
Ryen Caenn
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Steve Eisenstein
>time, but can be thinned by increased shearing), but not organoclays
>which are dispersed in oils (more Newtonian than non-Newtonian).
Correction - you have never worked with clay greases - formed from the
dispersion of bentonite or hectorite clays in oil. They are
decidedly NON-Newtonian.
Steve Eisenstein
Alan "Uncle Al" Schwartz
network in the absence of shear, leading to very high viscosities at low
loadings (try 1% fumed silica in mineral oil - it's a solid).
When shear (differential flow) is applied, the hydrogen bonded network
breaks down and the liquid flows freely. Remove the shear, and it sets
up again. That is how exterior housepaint works, loading the brush but
not dripping thereafter.
Consider the ususal ketchup joke. Instead of pounding on the bottom of
the inverted bottle, tilt at about 45 degrees and rapidly tap the neck in
the horizontal plane. The stuff will shear thin and flow.
--
Alan "Uncle Al" Schwartz
uncl...@ix.netcom.com ("zero" after "uncleal")
http://vvv.com/adsint/freehand/uncleal/
"Quis custodiet ipsos custodes?" The Net!
Bruce Hamilton
>I have run across a reference to thixotropic agents. I cannot find a
>definition for this term, although the article says that fumed silica and
>organoclay are both materials of this nature. Unfortunately, I can't find a
>reference as to what fumed silica and organoclay are either. Any assistance
>would be appreciated.
The following is from the sci.chem FAQ. Note that the first reference used
to be free upon request from agents for Brookfield viscometers, and is a
good general introduction. I'm sure Jim Oliver would be happy to clarify
- but please remember he also pays for both incoming and outgoing email.
His email address is jim.o...@welcom.gen.nz
Bruce Hamilton
19.1 Rheological properties and terminology
Contributed by Jim Oliver
RHEOLOGY
What is RHEOLOGY ?
RHEOLOGY describes the deformation of a material under the influence of
stresses. Materials in this context can be solids, liquids or gases. In this
FAQ we will be concerned only with the rheological properties of liquids.[1]
Perry discusses the some aspects of the behaviour of gases, and Ullmann
discusses elastic solids.
When liquids are subjected to stress they will deform irreversibly and flow.
The measurement of this flow is the measurement of VISCOSITY. IDEAL liquids
are very few, whereas non-ideal examples abound. Ideal liquids are : water
and pure paraffin oil. Non-ideal examples would be toothpaste or cornflour
mixed with a little water. [2]
What is VISCOSITY ?
VISCOSITY is expressed in Pascal seconds (Pa.s) and to be correct the
conditions used to measure the VISCOSITY must be given. This is due to the
fact that non-ideal liquids have different values of VISCOSITY for different
test conditions of SHEAR RATE, SHEAR STRESS and temperature. [3,4]
A graph describing a liquid subjected to a SHEAR STRESS (y axis) at a
particular SHEAR RATE (x axis) is called a FLOW CURVE. The shape of this
curve reveals the particular type of VISCOSITY for the liquid being studied.
[3]
What is a NEWTONIAN LIQUID ?
NEWTONIAN LIQUIDS are those liquids which show a straight line drawn from the
origin at 45 degrees, when graphed in this way. Examples of NEWTONIAN liquids
are mineral oil, water and molasses. (Issac NEWTON first described the laws
of viscosity) [1] All the other types are NON NEWTONIAN.
What does NON NEWTONIAN mean ?
a. PSEUDOPLASTIC liquids are very common. These display a curve starting at
the origin again and curving up and along but falling under the straight
line of the NEWTONIAN liquid. In other words increasing SHEAR RATE results
in a gradual decreasing SHEAR STRESS, or a thinning of viscosity with
increasing shear. Examples are toothpaste and whipped cream.
b. DILATANT liquids give a curve which curves under then upward and higher
than the straight line NEWTONIAN curve. (Like a square law curve) Such
liquids display increasing viscosity with increasing shear. Examples are
wet sand, and mixtures of starch powder with small amounts of water. A car
may be driven at speed over wet sand, but don't park on it, as the car may
sink out of sight due to the lower shear forces (compared to driving over)
the wet sand.
There are other terms used which include :
THIXOTROPY - this describes special types of PSEUDOPLASTIC liquids. In this
case the liquid shows a YIELD or PLASTIC POINT before starting to thin out.
What this means is the curve runs straight up the y axis for a short way then
curves over following ( but higher and parallel to ) the PSEUDOPLASTIC curve.
This YIELD POINT is time dependant. Some water based paints left overnight
develop a FALSE BODY which only breaks down to become usable after rapid
stirring. Also: the curve describing a THIXOTROPIC liquid will be different
on the way up (increasing shear rate) to the way down (decreasing shear rate).
The area inside these two lines is a measure of it's degree of THIXOTROPY.
This property is extremely important in industrial products, e.g to prevent
settling of dispersed solids on storage. [3]
A RHEOPECTIC liquid is a special case of a DILATANT liquid showing increasing
viscosity with a constant shear rate over time. Again, time dependant but in
this case _increasing_ viscosity.
Why do some liquids become solid ?
A few special liquids (dispersions usually) display extraordinary DILATANT
properties. A stiff paste slurry of maize or cornflour in water can appear to
be quite liquid when swirled around in a cup. However on pouring some out
onto a hard surface and applying extreme shear forces (hitting with a hammer)
can cause a sudden increase in VISCOSITY due to it's DILATANCY. The
VISCOSITY can become so high as to make it appear solid. The "liquid" then
becomes very stiff for an instant and can shatter just like a solid material.
It should be noted that the study of viscosity and flow behaviour is
extremely complex. Some liquids can display more than one of the above
properties dependant on temperature, time and heat history.
[1] Solutions to Sticky Problems. Booklet
More Solutions to Sticky Problems. Booklet
BROOKFIELD Engineering Laboratories Inc, MA, USA.
[2] Introduction to Practical Viscometry. Booklet
HAAKE, Mess-Technik GmbH.
[3] Principles and Applications of Rheology.
A.G.Frederikson
Prentice-Hall Inc. NJ.
[4] Viscosity and Flow Measurement.
Van Wazer, Lyons, Kim, Colwell.
Interscience Publishers. NY