Isothermal dynamic time test

Arrelano et al. (1989) evaluated the gel points of various DGEBA-based epoxy-resin systems via the crossover method. They also defined the vitrification point as the maximum in G" in an isothermal dynamic time test. In all these gel-point measurements the frequency must be chosen such that the relaxation of the network is enabled during data sampling. This is represented as... 

The effect of fillers on the gel point of thermosets has not been studied extensively. Ng and Manas-Zloczower (1993) used isothermal dynamic time tests to measure the crossover point for a silica-filled epoxy resin. They noted a decrease in gel time with increasing filler loading. Metzner (1985) also noted that the storage modulus and loss modulus increased by different amounts with filler loading. Therefore, the gel-point tests for highly filled systems must involve knowledge of the effect of filler characteristics at various levels. 

Winter (Winter and Chambon, 1986, Winter, 1987) also described an isothermal dynamic relaxation test to measure the gel point. He noted that the gel point coincides with a power-law relationship between the relaxation modulus (G) and relaxation time (t) G = St ", where S and n are constants. An isothermal step strain test measures the relaxation modulus as a function of time after an instantaneously applied strain. The gel time can be measured as the point at which the profile of the relaxation modulus can be expressed by this power law. This model is equivalent to tan <5 being independent of frequency. 

Thin-fllm samples of specific Isothermal cure histories were prepared for dynamic tensile testing on the Rheovibron by compression molding 0.6 to 0.7 mm thick samples at a pressure of 14 MPa. The samples were air quenched after various times at constant cure temperature. The thin-fllms were then mounted In the Rheovibron and the temperature Increased from room temperature to approximately 31 0 C at 2 C/m1n In a dry nitrogen atmosphere. 

Thin-Films. Dynamic tensile testing was carried out on thin-films of Kesin 5208 molded with specific isothermal cure histories. The temperature dependence of the loss tangent at several frequencies of one of these samples cured at 124 C for 5 hours is shown in Figure 13. The state of the cure of the sample corresponds to that of a resin studied isothermally by DSA at a time between the two observed DSA tan 5 peaks. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

Differential Thermal Analysis (DTA). The differential thermal analysis test serves to examine transitions and reactions which occur on the order between seconds and minutes, and involve a measurable energy differential of less than 0.04 J/g. Usually, the measuring is done dynamically (i.e., with linear temperature variations in time). However, in some cases isothermal measurements are also done. DTA is mainly used to determine the transition temperatures. The principle is shown schematically in Fig. 2.20. Here, the sample, S, and an inert substance, /, are placed in an oven that has the ability to raise its temperature linearly. 

Fig. 30. Schematic representation of the change of the dynamic Young modulus E with time of isothermal cure (right part) and with test temperature (left part). A(Te) temperature interval of glass...

Isotherms are normally developed to evaluate the capacity of the carbon for the adsorption of different contaminants. Data are obtained in batch tests, which determine the equilibrium relationship between the compound adsorbed on the carbon and that remaining in solution. The isotherms are used as screening tools to determine which carbon is suitable for a given application. Batch equilibrium tests are often complemented by dynamic column studies to determine system size requirements, contact time, and carbon usage rates [19]. Other parameters that are used to characterize activated carbons for water treatment include phenol number, an index of the ability to remove taste and odor, and molas.ses number, which correlates with the ability to adsorb higher molecular weight substances. However, these parameters still do not reflect performance in service, and they can only be considered as guidelines. 

The study of a particular adsorption process requires the knowledge of equilibrium data and adsorption kinetics [4]. Equilibrium data are obtained firom adsorption isotherms and are used to evaluate the capacity of activated carbons to adsorb a particular molecule. They constitute the first experimental information that is generally used as a tool to discriminate among different activated carbons and thereby choose the most appropriate one for a particular application. Statistically, adsorption from dilute solutions is simple because the solvent can be interpreted as primitive, that is to say as a structureless continuum [3]. Therefore, all equations derived firom monolayer gas adsorption remain vafid. Some of these equations, such as the Langmuir and Dubinin—Astakhov, are widely used to determine the adsorption capacity of activated carbons. Batch equilibrium tests are often complemented by kinetics studies, to determine the external mass transfer resistance and the effective diffusion coefficient, and by dynamic column studies. These column studies are used to determine system size requirements, contact time, and carbon usage rates. These parameters can be obtained from the breakthrough curves. In this chapter, I shall deal mainly with equilibrium data in the adsorption of organic solutes. 

Dynamic properties ASTM 4065-90 ASTM 4473-90 Viscoelastic properties Gel point from crossover tests and time for dynamic viscosity to reach 100Pas For supported and unsupported resins Isothermal and non-isothermal tests... 

Another reason that isothermal heating methods are used in the initial screen is to identify materials that have time dependent thermal stability. These materials have a thermal decomposition that does not follow a simple Arrhenius relationship in which the reaction rate increases exponentially with an increase in temperature. Instead an extended induction period is required before the decomposition becomes detectable. An example of this behavior is shown in Figure 2. The DTA isothermal test recorder traces of methane sulfonic acid, 3,7-dimethyloctyl ester at different test temperatures are shown. The induction time varies from less than 1 hr. at 180 C to 46 hr. at 130 C. As with this compound, it is not unusual that once decomposition is detected it proceeds very rapidly, releasing all of the heat in a short period of time. Dynamic heating methods do not indicate if this type of thermal instability is present if it is, the initial detection temperature from dynamic tests will be grossly misleading as to the thermal stability of the material. 

Figure 6 illustrates the application of DSC to the determination of the oxidative stability of oils." Figure 6(a) shows the thermal analysis curve for the isothermal test in which the time to oxidation (toxid) is measured when the sample is maintained at a constant temperature in an atmosphere of O2. An alternative test is dynamic where it is the temperature of oxidation whieh is measured. Specialised equipment is necessary for the isothermal test, which usually employs a pressure of about 3.5 MPa. An obvious advantage of both tests is that the performance of oils can be... 

Yu et al. (2011) studied rheology and phase separation of polymer blends with weak dynamic asymmetry ((poly(Me methacrylate)/poly(styrene-co-maleic anhydride)). They showed that the failure of methods, such as the time-temperature superposition principle in isothermal experiments or the deviation of the storage modulus from the apparent extrapolation of modulus in the miscible regime in non-isothermal tests, to predict the binodal temperature is not always applicable in systems with weak dynamic asymmetry. Therefore, they proposed a rheological model, which is an integration of the double reptation model and the selfconcentration model to describe the linear viscoelasticity of miscible blends. Then, the deviatirMi of experimental data from the model predictions for miscible... 

The oxidative induction time (OIT) is determined from data recorded during the isothermal test. The elapsed time between time zero and the extrapolated onset of oxidation is recorded as a measure of oxidative stability. A dynamic heating method is also used. In this method, the sample is heated at a rate of 10 °C min from ambient temperature in an air or oxygen atmosphere. The temperature at the onset of oxidation is taken as a measure of oxidative stability. When... 

The selection of the AC for a given application (solutes to be retained, concentration, stream flow, etc.) is made by dynamic tests in pilot adsorption units. However, it is more effectively done, with less time consumption, if the adsorption properties of the candidates are established. Properties such as the surface area and PSD, iodine number, bulk density, particle size, hardness and mechanical strength give an idea of the suitability of the AC. The determination of the adsorption isotherm of a given solute over a wide range of concentrations is the method to obtain the equihbrium capacity of the adsorbent. In this way, the amount of solute it can remove from the solution can be estimated and consequently the suitability of the AC for achieving the required removal of the solute. 

Breakthrough times and uptakes from dynamic testing using 2000mg m pentane, compared with uptakes from adsorption isotherms at p/p0=0.001 and 0.95... 

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