Ultra violet light absorbers

Radiation such as ultra-violet light radiation may also promote reaction (1). This may be countered by the use of ultra-violet absorbers, light screens (which reflect ultra-violet) or quenching agents (which react with ultra-violet excited molecules to produce inert products). 

Unfortunately, the fluorescent effect is not directly proportional to the concentration of colorant present, since there is considerable quenching if quite low concentrations are exceeded. The light fastness of the fluorescent pigments is also less than that of many other organic pigments now available, but improvement can be achieved using overlayers containing ultra-violet absorbers. This is an area in which further research will clearly be needed. 

Different methods have been used to follow crosslinking reactions. When polymers were irradiated by U. V. light in dilute THF solution above 200 nm (Method A) or 300 nm (Method B), the insolubilization reaction was followed by measuring the solution ultra violet absorbance versus time. Likewise the course of disappearance of cinnamic structure was measured on polymers films placed on quartz at 9cm of the lamp and irradiated above 200 nm (Method C). Lamp used for methods A and C was a PCQ 9 G-1 which emits in the U.V. region at 253.7 (2.5 W), 312.5, 365 nm, the other rays being in visible. A Pyrex filter was placed beetween 450 W Hanovia lamp and solution when only higher wave lenghts were expected. Lamps powers were controled before and after irradiation with a Black-Ray Ultra-Violet Intensity Meter. 

Interest in N, - diaryloxamides [67], as the class of non-toxic ultra-violet absorbers possessing light efficiency has raised lately ... 

From the series of highmolecular compounds the most interesting are polyconjugated azomethine compounds (PAC), stabilizing activity of which has been studied in the works [291, 292]. It is proved in these works that stabilizing effect is achieved at relatively small concentrations of azomethines. In the work [162] the author comes to a conclusion that at small concentrations azomethines work as inhibitors of radical processes, besides, they are ultra-violet absorbers. It should be noted that azomethines display light protective action both in vacuum and in the air. 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

The prerequisite of this method is the ability of the molecule concerned to absorb radiation in the ultra-violet or visible range. Thus acetone in the vapour phase is decomposed by light having a wave-length of =320 nm (3200 A - 375 kJ mol 1) ... 

Spectroscopic techniques look at the way photons of light are absorbed quantum mechanically. X-ray photons excite inner-shell electrons, ultra-violet and visible-light photons excite outer-shell (valence) electrons. Infrared photons are less energetic, and induce bond vibrations. Microwaves are less energetic still, and induce molecular rotation. Spectroscopic selection rules are analysed from within the context of optical transitions, including charge-transfer interactions The absorbed photon may be subsequently emitted through one of several different pathways, such as fluorescence or phosphorescence. Other photon emission processes, such as incandescence, are also discussed. 

The ultra-violet light induces electromagnetic excitation in molecules absorbing the laser energy, and by subsequently applying the principle of absorption spectroscopy, kinetic and spectroscopic information relating to the electronically excited states of various energetic molecules have been derived. The systems studied to-date include s-TNB, s-TNT, triphenyl-amine, and mono- as well as di-nitronaphthalenes (Ref 13, 14, 20, 21 28)... 

The absorption of ultra-violet light by colloidal solutions of sulphur has been examined,6 and is found to vary with the size of the colloid particle, approaching a limit corresponding with the amount absorbed by a molecular solution of sulphur in alcohol. 

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