Diffusion rate test

Wire cords are particularly subject to degradation of their adhesion values by moisture. To combat this, halogenated butyl (HIIR) is used in tire innerliners because of its property of low air and water vapor diffusion rates. Moisture is present in most air pumps and many tires are mounted with water left in the tire on mounting. For these reasons tires and tire compounds are tested extensively at simulated aging conditions in the laboratory and on test vehicles before they are sold to the customer. 

Data was collected over a two-year period on the effect of water on DuPont s Zytel 101. In an Arrhenius plot of this data the failure point was the time when the elongation and impact strength started to decrease. This is not a chemical degradation, but rather a permeation or diffusion rate phenomenon. It shows that high temperature water tests can be used to predict normal temperature exposure results. 

Diffusion rates for liquids in an elastomer are easily measured by absorption (immersion) testing, a simple process as indicated in Figure 23.6. An initially weighed immersed sheet sample of elastomer is removed from the liquid periodically, rapidly dabbed with tissue paper, reweighed, and replaced. A plot of mass increase versus root time is drawn (also see Figure 23.6), root time being chosen due to the form of appropriate solutions of Fick s laws. 

The position of the precipitation lines depends on, e.g., the diffusion rate of the different HpHb complexes, of the HBG and of the antibodies. The molecular size (the type) of the HpHb complex will therefore influence the result of the diffusion tests in agar. The technique seems to be simple and promising. So far, the data available on the sensitivity and reliability of the method is not sufficient to warrant recommendation of the procedure. 

Reading the literature on mammalian semiochemistry over the past decade, a chemist is impressed by the enormous volume of biological information that has been gathered in well planned and meticulously executed studies of the modulation of the behavior of mammals by the chemicals released by con-specifics. One cannot, however, escape the impression that the chemical basis of many of these studies is lacking. Some of the problem areas were pointed out in the foregoing sections. To a certain extent there seems to be lack of appreciation of the diffusion rates of compounds with different volatilities and of the extent to which these differences can influence the outcome of behavioral tests. It is difficult to make an estimate of the persistence of semiochemicals that are released into the laboratory atmosphere or that are left on objects or surfaces in arenas in which tests are conducted. From what is known about the evaporation rate of some heavy compounds that are considered to be semiochemicals, it could take several weeks or even months for these compounds to be depleted to levels that cannot be detected by currently available instrumentation levels at which meaningful information could still be available to experimental animals. This then leaves the question unanswered as to when it would be safe to conduct behavioral experiments in a laboratory or arena that had been occupied by conspecifics. 

Selected entries from Methods in Enzymology [vol, page(s)] Analysis of GTP-binding/GTPase cycle of G protein, 237, 411-412 applications, 240, 216-217, 247 246, 301-302 [diffusion rates, 246, 303 distance of closest approach, 246, 303 DNA (Holliday junctions, 246, 325-326 hybridization, 246, 324 structure, 246, 322-324) dye development, 246, 303, 328 reaction kinetics, 246, 18, 302-303, 322] computer programs for testing, 240, 243-247 conformational distribution determination, 240, 247-253 decay evaluation [donor fluorescence decay, 240, 230-234, 249-250, 252 exponential approximation of exact theoretical decay, 240, 222-229 linked systems, 240, 234-237, 249-253 randomly distributed fluorophores, 240, 237-243] diffusion coefficient determination, 240, 248, 250-251 diffusion-enhanced FRET, 246, 326-328 distance measurement [accuracy, 246, 330 effect of dye orientation, 246, 305, 312-313 limitations, 246,... 

Estimate the diffusion rate of water from the bottom of a test tube 10 mm in diameter and 15 cm long into dry atmospheric air at 25°C. 

---