Buoyancy Baths

Trouton found that to achieve uniform deformation with lower viscosity materials, Tjo 10 Pa s, he needed to support the sample in a low viscosity liquid (salt water) of the same density. Connelly, et al. (1979) studied samples drawn out horizontally in an air bath. They could not get uniform extension when the extension rate times the fluid s longest relaxation time was less than 1 

Matching density is less difficult with a horizontal bath. The sample merely must be less dense than the fluid. Typically, dimethyl and phenyl silicone oils and perfluorinated polyethers are used for higher density. Clearly the oil must not diffuse into the sample. A sensitive test for diffusion is actually to watch for changes with time in rheological properties like ijq. Other techniques such as infrared have been used (Munstedt, 1979). 

At low stress levels the force due to interfacial tension, F, between the oil and sample can become appreciable. Equation 

Interfacial tension woiks against the pulling force. Typically, interfacial tension is 5 mN/m, so the correction is small in polymer 

Schematic of extensional rheometer with a translating clamp and vertical buoyancy control bath. The temperature control fluid is circulated through a jacket around the buoyancy fluid. An outer vacuum jacket insulates the apparatus. Redrawn from Munstedt (1979). 

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Highest extensional strain e — 3-4 typical with careful sample preparation and 6-7 possible with rotating clamps Buoyancy bath required forik < 1 (eq. 7.2.13)... 

Schematic drawing of the double rotating clamp apparatus (adapted from Meissner, 1971). (a) Top view. The cylindrical sample P of initial length L is stretched by clamps Z and Zi rotating at f2 and S2 . Drive motor M is shown. Spring B and displacement sensor W measure force, (b) A detail of the rotating clamps. Gear teeth are used to prevent slip, (c) End view showing oil buoyancy bath with top surface O. Fourteen pairs of scissors T cut the sample into small lengths for recovery measurement La-...

Thus for lower rate testing some sort of buoyancy bath is required. Figure 7.2.5 indicates a horizontal buoyancy bath of the type most often used with rotating clamp instruments. Both horizontal (Vinogradov et al., 1970, Franck and Meissner, 1984) and vertical baths (Munstedt, 1975,1979) like the one shown in Figure... 

Thermogravimetric analyses were carried out in 10°-30° temperature increments with 200-mg samples using a conventional (Mauer) TGA system. Automatic recording of weight change was used to follow reaction to equilibrium, but actual weighings were recorded only by manual operation. The sample was bathed continuously in air of controlled humidity (Pmo = 7.9 torr) flowing at 180 cc/min. Precautions were taken to minimize drafts and convective currents, and buoyancy correction curve was made to 950°C. Further details on experimental methods are available (12). 

The apparatus should be assembled as shown in Fig. 2. If desired, the carboy may be mounted in a thermostat bath if so it must be clamped securely to overcome buoyancy. The manometer is an open tube manometer, one side of which is open to the atmosphere the pressure that it measures is therefore the difference of pressure from atmospheric pressure. A suitable liquid for the manometer is dibutyl phthalate, which has a density of 1.046 g cm at room temperature (20°C). To convert manometer readings (millimeters of dibutyl phthalate) to equivalent readings in millimeters of mercury (Torr), multiply by the ratio of this density to the density of mercury, which is 13.55 g cm at 20°C. To find the total pressure in the carboy, the converted manometer readings should be added to atmospheric pressure as given by a barometer. It is unnecessary to correct all readings to 0°C, as all pressures enter the calculations as ratios. 

Contrary to our expectations, die propagating front could not be generated near the wall of the test tube. A buoyancy-driven convection prevented the formation of the front. Experiments were carried out changing the size of the test tube and the temperature. In test tubes having inner diameters of 23,13, or 8 mm, AIBN was dissolved in 15.0, 5.0, or 1.7 mL of MMA. The depth of MMA liquid in each test tube and the concentration of AIBN (2.0 wt%) was kept constant. Hie tubes were then placed in a 50°C oil bath to start the polymerization. In all cases, the front was not generated near the wall of the test tubes. 

A modified COF measurement method for testing finished catheters was reported by Kazmierska [24]. The tribology device is shown in Figure 2.2. A motor winds the strand and pulls the catheter against a weighted surface in a water bath. The friction force is measured by a forcemeter and the normal force is the difference of gravity force of the weight subtracted by the buoyancy force. 

It should be realized that the duration of the experiment is limited to the time taken for the moving end of the sample to traverse the length of the constant-temperature bath so that the entire experiment may be over in as few as four or five seconds. Errors arise if the sample temperature rises or if the sample does not deform uniformly. Also, it is necessary for the end of the sample to go instantly from rest to a finite predetermined velocity at the inception of stretching. In addition, especially for vertical instruments, the density of the supporting liquid has to match that of the polymer to prevent the force of buoyancy from influencing the results. Finally, the method is suitable only for polymers having a zero-shear viscosity in excess of about 1(H Pa sec at the temperature of interest. 

Method 1. A fully dense sample of equivalent volume as the anticipated sample is placed in the sample chamber. The system is then sealed and evacuated. All arrangements, such as the liquid nitrogen cooling bath, should be put in place just as if a sample were present. The adsorbate gas is admitted from very low pressures in increments up to nearly P. This should yield a very linear plot of mass gain or buoyancy, versus pressure. The equation is... 

The measured values of He for different bath depths are also approximated by (3.26), as partly demonstrated in Fig. 3.40. Consequently, the bath depth Hi hardly affects the merging length under the experimental conditions considered. The work done per unit time by the buoyancy force acting on bubbles to liquid around a bubbling jet, Pb, is expressed by [30,31] ... 

The rate of energy input to the bath E = lie), which is actually contributes to the mixing process, is the sum of the rate of energy input due to buoyancy effect of rising bubble ( b = 11 b) and a fraction of the rate of total kinetic energy associated with the gas at the nozzle exit ( k = llek). Thus,... 

There exist many metallurgical reactions generating bubbles in a reactor. When the size of a bubble in the reactor is large enough, the buoyancy force being one of body forces is dominant. The bubble is therefore carried up to the bath surface of the reactor and escapes into the atmosphere. This situation cannot be expected when the size of the reactor is very small because the buoyancy force loses its validity. Many kinds of bubble removal methods have been proposed, as listed below. 

Rhodin s balance was made as symmetrical as possible in order to eliminate buoyancy corrections and to minimize thermal eddy currents. The adsorbent and counter weights were matched to within 10- g and immersed to the same depths in identical thermostatic baths and the... 

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