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PRINCIPLES OF OPERATION
As polymerisation proceeds in a liquid polymer a point will be reached at which on average there will be
one cross link per molecule and at this point gelation will be observed. The gel formed may be soft but
it will have the property of a solid of elastic recovery from deformation. This change from liquid to a solid
is preceded by a considerable rise in viscosity.
An instrument to measure the time of gelation must be able to discriminate between a state of high
viscosity and that of elasticity. A gelation timer operated primarily by change in viscosity would be wrong
in principle and, although it might give useful control tests on a particular product, it could not be
expected to give times in accordance with the fundamental theoretical analysis of gelation.
The design of the Techne Gelation Timer
1
was based on the above considerations. A flat weighted disc
or paddle is connected by a link with end play in it to a crank turned by a synchronous motor that falls
under gravity in the polymer liquid. The disc is pulled up on the upstroke by the synchronous motor. At
the gel point the rigidity of the polymer is sufficient to support the weight of the disc, causing the link
to be compressed and to close an electric circuit. This operates a relay which stops the synchronous
motor and lights a neon lamp. The gelation time can be read on a digital counter connected to the
synchronous motor.
The motion of the disc is resisted both by elastic and by viscous forces but the size, weight and time have
been chosen so that the magnitude of a purely viscous force sufficient to trip the mechanism would be
far greater than is observed in practice before gelation has taken place.
The work of B.A. Hills
2
has shown that gelation times measured by this instrument with suitable choice
of disc diameter, are in excellent agreement with predicted values based on the analysis of P. J. Flory
.
3
Measurements of gelation time with the Techne Gelation Timer are thus a very simple method of making
fundamental control tests on a polymeric products.
The Techne Gelation Timer conforms to the requirements of BS 3532 : 1990 and BS 2782 : Part 8 :
Method 835C : 1980 for gelation timing of unsaturated polyester resin systems. It is equally useful for
epoxies, urea formaldehyde, resorcinal formaldehyde and other cross linking products.
1 N.A. de Bruyne, modern plastics 27, No.9, May, 1950.
2 B.A. Hills, J. Oil & Colour Chemists Assoc. 45, 251-260 (1962).
3 P.J. Flory, J. American Chemical Soc. (1941) 63, 3083.
The Gelation Timer is designed so that tests may be carried out at constant temperature. The sample cup
may stand directly in a laboratory water bath and the gelation timer is relatively narrow so that several
instruments may, if necessary, stand together over quite a small bath.
An alternative way of keeping sample cups at constant temperatures is to use a Techne Dri-Block
®
which
is held at accurately controlled temperatures and which has a hole drilled in it of a diameter to accept the
sample cup.
Techne manufactures a wide range of both water baths and Dri-Block
heaters suitable for gelation tests
®
at constant temperatures. Details of these are available on request.
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