Isothermal steady time test

Isothermal steady time test Used for suspensions and curing... 

Isothermal steady time tests are used to determine the gel point of a thermoset system as the point at which the shear viscosity tends towards infinity. In these tests the viscosity is measured as a function of time at a constant shear rate. This method has the following major disadvantages. Firstly, the infinite viscosity can never be measured due to equipment limitations and thus the gel time must be obtained by extrapolation. Secondly, shear flow may destroy or delay network formation. Finally, gelation may be confused with vitrification or phase separation since both these processes lead to an infinite viscosity (St John et al., 1993). However, some work by Matejka (1991) and Halley et al. (1994) has shown that extrapolation to zero values of reciprocal viscosity or normal stress (i.e. extrapolation to infinite viscosity and normal stress) can be used with some success. 

Kwak et al. [44] used an isothermal steady shear test to monitor the copolymerization of sUoxane-containing oligomers and diglycidylether of bisphe-nol A (DGEBA) epoxy. The gel time of the blends was obtained from values of the storage modulus (G ) and loss modulus (G"). Based on the concept of Eq. (4.3),... 

Compared to rubber, thermoplastic-modified systems show a much more complicated phase separation and structure evolution, and consequently very few studies were initially undertaken related to phase separation and rheological behavior. Examples of these studies included Pethrick et cd. [47,48], who applied a curome-ter to monitor the pot life and gel time of thermoplastic-modified epoxy systems. In 1999, Bormet and Pascault et al. [49] were the first to conduct a study of the relationship between rheological behavior and thermoplastic concentration in pol-yetherimide-modified epoxy-amine systems. By using an isothermal steady shear test with parallel plates at low deformation (1%), these authors showed that the rheological behavior at phase separation was greatly dependent on the initial concentration of thermoplastics. 

The effect of time averaging on yields in transient tests can be minimized by shortening the duration of the test. Also, a fixed bed test is superior to an FFB activity test in that backmixing is minimized. Furthermore, an isothermal fixed bed test would be easier to interpret than the adiabatic MAT test. This work shows that from the point of view of a catalyst characterization test, a small steady state riser will give the most direct information for catalyst performance in a commercial riser. 

The isothermal version of this test can also be used to detect induction time effects. In this test the sample is held for long periods under conditions of minimal heat loss. This can be achieved by placing the tube in a Dewar flask, which is then placed in an oven held at a steady temperature. Sample temperature and pressure inside the tube are recorded continuously. A rise in either temperature or pressure indicates onset of decomposition. Usually it is found that ... 

The results were compared with the oxygen diffusion, solubility, and permeability coefficients obtained for PET films under the same conditions. The water sorption isotherm for polylactide films was also determined. Diffusion coefficients were determined with the half-sorption time method. Also, a consistency test for continuous-flow permeability experimental data was run to obtain the diffusion coefficient with the lowest experimental error and to confirm that oxygen underwent Eickian diffusion in the polylactide films. The permeability coefficients were obtained from steady-state permeability experiments. The results indicated that the polylactide films absorbed very low amounts of water, and no significant variation of the absorbed water with the... 

---