Reentry research

Knaak, J. B. Schlocker, P. Ackerman, C. R. Seiber, J. N. Reentry Research Establishment of safe pesticide levels on foliage. Bull. Environm. Contam. Toxicol., 1980, 24, 796. 

Reentry research establishment of safe pesticide levels... 

There has been a sharp debate for many years on the best description of the real macroconformation. Much experimental research has been carried out on pure polymers using different techniques (225) [small angle and intermediate angle neutron scattering (226), electron microscopy, IR, etc.]. Yoon and Flory (40, 228-231) and Gawrisch et al. (232) held the view that the probability of adjacent reentry in polymeric lamella is rather low (<50%) and does not justify the validity of such a model. The trajectory of the chain extends across numerous lamellae and its macroconformation is not far from that of the random coil. In the view of Keller and co-workers (224, 233-236) the adjacent reentry, although not complete (3 1 with respect to other possibilities) largely prevails. 

The F.W.G.P.M. Task Force addressed itself mainly to the problem of organophosphates, and its terms of reference were to identify areas in which information on occupational exposure to workers was unavailable, to make recommendations for the development of research protocols to determine safe reentry levels for the protection of agricultural and forest workers, and to suggest interim reentry standards based on existing knowledge. The report was controversial but drew attention to the lack of a substantial data base and to the urgent need for surveillance of pesticide-related morbidity and mortality and for research to identify factors influencing safe worker reentry levels. 

It is now understood and accepted that the dermal route is the route of exposure in field situations. There is ample research data to back this up. Also, this fact is reflected in the California regulations on reentry. This point will be discussed later. 

The special needs of the space program motivated the search for composite materials for other reasons also. For example, during tests of the first Atlas ICBM (intercontinental ballistic missile), engineers were concerned that the rocket s metallic components would not survive the missile s reentry into the atmosphere they feared it would melt down because of the intense heat to which it was exposed. By the late 1950s, therefore, aerospace researchers had begun to look for satisfactory substitutes for metal alloys for such applications. With that research, the modern held of composite design was horn. One of the first composites tested consisted of pieces of glass embedded in melamine, purported to be the first composite material developed for aerospace applications. 

From all these data analyses, we can definitely say that the D and H chain stems are distributed statistically randomly in the crystalline lamellae of the D/H cocrystallized blend. This conclusion is quite important in relation with the chain-folding problem, a controversial research theme that had been discussed for a long time (30). The random distribution of the D and H chain stems naturally supports the idea that the D and H chains reenter randomly into and out of the crystalline lamellae as shown in Fig. 5.7. The regular adjacent reentry model is impossible to apply at all as for as the melt-crystallized sample is concerned. 

Analytical Aspects - Barrie Webster, University of Manitoba, Canada Hamdy Baiba, Uniroyal Chemical Allan Cessna, Agriculture Canada Research Station, Regina, Saskatchewan. Reentry Standards - Marie Siewierski, Rutgers University Gunter (Jack) Zweig, Environmental Protection Agency. 

This paper summarizes the development and application of both a philosophic and quantitative framework for unifying research approaches and findings in residue decay, exposure assessment, and cholinesterase response (Popcndorf Lefflngwell. Res. Rev. 82 125, 1982). Examples are provided for using this model to Interpret the potential cholinesterase response from a known foliar residue and to establish reentry intervals to prevent excessive cholinesterase Inhibition. The potential and limitations of extrapolating this approach to other settings is also discussed, as are the needs for future research to support a comprehensive approach to pesticide use, residues, and exposure controls. 

It is clear that many assumptions were needed to calculate this reentry value. If these assumptions are reasonable, the calculated T=17 days contrasts sharply with the current EPA required reentry Interval for Systox of 2 days. These assumptions provide some clues to the kinds of further research necessary to implement this model more broadly. These topics for further research will be reviewed shortly. But first by way of sunmarizlng the current uses of the unified field model, I will review recent research related to each mechanism. 

By way of summary, for dilute solution-grown crystals a modified regular reentry model fits best, with the same molecule forming a new stem either after immediate reentry or after skipping over one or two nearest-neighbor sites. For melt-formed crystals the concept of folded chains is considerably modified. Since active research in this area is now in progress, perhaps a more definitive set of conclusions will be forthcoming. 

The NASA polynomials are usually fitted in the temperature range 300 to 5000 K. The reason for choosing this range is practical. Combustion calculations require thermodynamic and thermochemical properties between room temperature and 3000 or (for special fuels or detonations) 4000 K. In the course of automatic calculations, as well as in some exotic conditions such as spaceship reentry, knowledge of properties to 6000 K is required. Thus, the polynomials discussed here follow the bulk of existing tables (such as JANAF and TSIV as discussed later) by being fit in the range 300-5000 K. Extrapolation to 6000 K is easily done with little error. Extrapolation below 300 K, seldom needed in combustion research, is less accurate. In some cases the polynomials were fit up to 3000 K only. 

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