Interpretation of tracer data: significance of the number of terms in specific activity functions

It has already been shown that the number of pools in an open system in the steady state cannot be determined from the number of exponential terms in the specific activity function of a pool, even if the data were free from experimental error. However, some information is conveyed by the number of exponential terms. The information is different depending upon whether the data are obtained from the pool into which the tracer is introduced or from another pool. In the latter case, the number of exponential terms is shown to indicate the maximum number of intermediate pools involved in the shortest path of transfer of material from the injected pool to the pool in question. With regard to the former case, this paper is restricted to functions with two exponential terms and shows which systems of n pools (n >/= 2) are consistent with such data. Consequently, biexponential experimental curves can be interpreted in terms of models consisting of an unrestricted number of pools in which each pool is defined in terms of fast mixing. The generalization to cases of functions with more than two exponential terms can be carried out in a similar manner.

Growth and intracellular development of a new respiratory virus

The multiplication of a new, ether-sensitive, ribonucleic acid virus, 229E, isolated from the human respiratory tract, has been studied in cultures of WI-38 human diploid cells. In thin sections of these cells examined with the electron microscope, particles appeared in vesicles in the cytoplasm of cells at a time corresponding to the initial increase in infectious virus. Antigen was also detected in the cytoplasm of cells by the immunofluorescent technique. Extracellular particles of similar morphology were prominent soon after. These events preceded a detectable cytopathic effect. Later, an electron-dense particle appeared within vacuoles in the cytoplasm but was never found extracellularly. Its role in virus development is not known. Complement-fixing antigen developed along with the increase in infectious virus.

Reinforced Thermoplastics

If a thermoplastic is placed under load, as in a normal tensile tester, the characteristic stress strain curve is of the form given in Figure 1. Whether the material is described as tough or brittle depends on the relation between the fracture stress and the yield stress. In many cases the apparently irreversible deformation which takes place on yield is described as flow, but this is seldom, if ever, the case. The extension which takes place consists in the orientation of the tangled molecular network in a manner similar to that which occurs during extension of rubber, but with characteristic hard thermoplastics the deformation is opposed by viscous forces so that most of the energy applied is irrecoverably lost as heat. Deformations of this type are largely reversed on heating—as shown in the case of high-density polyethylene in Figure 2. In order to modify a brittle polymer to make it tougher we need to alter the relation between deformation and fracture so that, at least under short time tests, deformation takes place before fracture so that enough energy can be absorbed to cope with shock conditions. For this purpose the yield stress must be lower than the fracture stress. Naturally the ideal way of achieving this would be to raise the fracture stress, as any reduction in yield stress will normally lead to a softer and less rigid product. The extent and limitations of this problem will now be examined in relation to the styrene plastics.

Reaction of Ozone with Natural Hevea and Acrylonitrile-Butadiene Rubbers

The cracking of rubber vulcanizates when exposed to the atmosphere in a state of stress is a well-known phenomenon which has been studied extensively. A comprehensive review of the literature on this subject has been published by Newton. Following the lines of earlier work Newton showed conclusively that the cracking is produced by traces of ozone present in the atmosphere. The simultaneous presence of both stress and ozone is required for cracking to occur and the cracks that form are oriented with their lengths perpendicular to the direction of the stress. It has been reported, however, that similar cracking is produced when stretched rubber is exposed to free radicals produced by the thermal breakdown of organic peroxides. Unstretched vulcanizates do not crack when exposed to ozone, nor do they crack when stretched after exposure to ozone. Apart from the frosting effect described by Tuley, which is stated to require the presence of moisture and which may require the presence of small stresses, there seems to be no record in the literature of a visible effect being produced by the action of ozone on unstretched vulcanizates. The effect of ozone on unstretched vulcanizates has been investigated more completely and the results are reported in the present paper. Little is known of the mechanism of the reactions that lead to cracking of vulcanizates and this paper describes experiments designed to throw some light on this. The products of the reaction between ozone and solid rubber have been investigated by means of infrared spectroscopy. Allison and Stanley have previously described the infrared spectra of products formed from solutions of Hevea and various synthetic rubbers and Salomon and van der Schee have recently reported changes which occur in the infrared spectrum of a film of Hevea rubber when exposed to ozonized air.

Analysis of Vulcanized Rubbers. Influence of Accelerators on the Determination of Free Sulfur by the Copper Spiral Method

The copper spiral method is being inserted in a revised edition of the “British Standard Methods of Testing Vulcanized Rubber”, and this raises the question of what sulfur compounds react as “free” sulfur. In this method, “free” sulfur is considered to be that part of the sulfur which is present in the acetone extract and which reacts with a copper spiral, placed in the acetone during the extraction, with the production of copper sulfide, the amount of which can be estimated after its removal from the acetone. Since most accelerators contain sulfur, it is obvious that the presence of accelerators or accelerator fragments is a potential source of error. Fourteen accelerators of various types were therefore examined, and it was found that some react with copper, producing compounds which evolve hydrogen sulfide on treatment with hydrochloric acid.

Identification of Accelerators and Antioxidants in Vulcanizates by Infrared Spectroscopy

Reliable methods for the chemical analysis of vulcanizates have been available for many years, and standard methods have been published in a British Standard which has recently been revised, but these methods do not cover the identification of accelerators and antioxidants and no comprehensive scheme for this type of analysis has been published. The identification of accelerators and antioxidants in vulcanizates presents a difficult problem because of the small concentrations employed, the close chemical similarity of many of them, and the wide variety of substances used. Many chemical tests have been developed for their identification, but difficulties arise when they are applied to vulcanizates. On the whole these chemical tests are unsatisfactory, and it seems unlikely that a comprehensive chemical scheme can be devised for the identification of accelerators and antioxidants in vulcanizates. There are, however, several physico-chemical methods which can be used for such analyses. The work of Proske has shown that accelerators in vulcanizates can be detected and estimated by the polarograph. The method is applicable to thiazoles, thiuram disulfides, and dithiocarbamates, but cannot be used with such accelerators as guanidines. Chromatography has recently been applied to the problem with considerable success. It has been shown that several accelerators give a characteristic chromatogram when the acetone extract of their vulcanizate is chromatographed on alumina, either alone or in the presence of cobalt oleate. In addition, antioxidants can be separated by chromatography from the rest of the acetone extract and can then be identified by color reactions. Although this method has been found in the R.A.B.R.M. laboratories to be very useful, there are serious difficulties in the way of developing a comprehensive scheme of analysis on this basis. For example, distinction between the various dithiocarbamate accelerators is difficult, and the method sometimes gives ambiguous results when applied to complex mixtures. Ultraviolet spectroscopy has been applied to the detection of accelerators and their decomposition products in vulcanizates, to the estimation of accelerators in masterbatches, and to the quantitative estimation of antioxidants. These applications suggest that ultraviolet spectroscopy should be a useful method for the identification of accelerators and antioxidants in vulcanizates.