Torsion tests, gels

In addition, other measurement techniques in the linear viscoelastic range, such as stress relaxation, as well as static tests that determine the modulus are also useful to characterize gels. For food applications, tests that deal with failure, such as the dynamic stress/strain sweep to detect the critical properties at structure failure, the torsional gelometer, and the vane yield stress test that encompasses both small and large strains are very useful. 

ISO 2535 [68] describes a method using a prescribed cure system with measurement of time from point of accelerator addition to onset of gelation at a temperature of 25 C. The resin is contained in a test tube with temperature control using a water bath. A glass rod is rotated at 1-2 rev/min by a torsion wire connected to a geared electric motor. The gel time is taken as the time taken for the wire to twist an amount corresponding to a liquid viscosity of 50 Poise for measuring this point and is shown in the standard. 

A torsion pendulum is a particular useful device for examining the modulus and internal friction of polymers. When the sample geometry is in the form of a cone-and-plate or concentric cylinders, the pendulum can be used for very weak gels, which are hard to characterize in conventional testing machines. Commercial models are available which give a complete record of amplitude versus time. However, even mannal estimation of the frequency of oscillation and the number of cycles to decrease the amplitude by some factor can be quite useful. 

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