Absorbers, ultraviolet

UV absorbers are used to protect the plastic material as well as the foodstuff packaging from the actinic action of UV radiation. Actinic effects may cause discoloration of both the plastic material and the foodstuff, and may also occasion changes in taste and loss of vitamins in the food. 

In a typical method, 0.2 g of the PS sample was shaken with chloroform to achieve a solution. The sample was excited by UV radiation of wavelength 370 nm from a mercury vapour lamp and the fluorescence spectrum of the sample recorded over the range 400-440 nm. The reading from the fluorimeter was noted and the Uvitex OB concentration in the PS determined by reference to a prepared calibration graph. 

A clean-up MALDI time of flight mass spectrometry [91] used in conjunction with SFC extraction has been used to determine UV stabiliser and antioxidants 

Juo and co-workers [94] have demonstrated that ammonia desorption chemical ionisation was a better ionisation technique than fast atom bombardment (FAB) for the determination of additives in PE extracts. 

2a Ultraviolet Spectroscopic Procedure. By their nature, many of these types of compounds are amenable to analysis by fluorimetric analysis, thus Uvitex OB has an intense ultraviolet absorption at a wavelength of 378 nm which is high enough to be outside the region where many potentially interfering substances present in the polymer extract would be excited to fluoresce. This is illustrated in the fluorimetric procedure described in Method 32 for the determination of down to 10 ppm Uvitex OB in polystyrene. 

In many cases the use of pigments is not a convenient way to prevent the absorption of damaging radiation by polymers. However, substances that are transparent in the visible region but have a high absorption coefficient in the ultraviolet can be used as filters for that part of the solar spectrum which is responsible for the deterioration of most polymers. Although many organic compounds have a suitable absorption spectrum for that purpose only very few can be used as ultraviolet absorbers a severe restriction is that they should be photochemically stable, otherwise the stabilization effect will not be durable and sensitization may occur. Ultraviolet absorbers that have found commercial application belong to five main families. 

These are the most widely used ultraviolet absorbers. Some of them are exemplified below and their absorption spectra are shown in Fig. 25 

The photochemistry of o-hydroxy-aryl ketones and aldehydes has been extensively studied by several authors. It has been found that salicyl-aldehyde, 2-hydroxybenzophenone and 2,4-dihydroxybenzophenone are different from the unsubstituted compounds and from the p-hydroxy or o-methoxy derivatives in that they are completely inactive as sensitizers for the isomerization of piperylene [130]. Moreover, 2,4-dihydroxybenzophenone gives no luminescence in a rigid hydrocarbon glass at 77°K, whereas 4-hydroxybenzophenone exhibits an intense phosphorescence in 

It has also been suggested that in the ground state the enol form is energetically preferred, whereas the reverse is true for the first excited singlet state. On absorption of a photon, the following cycle would then ensue [132]  

According to Beckett and Porter [131] the less probable intersystem 

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Hindered amines, such as 4-(2,2,6,6-tetramethylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin Aims under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or elecbonically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

Analytical Techniques. Sorbic acid and potassium sorbate are assayed titrimetricaHy (51). The quantitative analysis of sorbic acid in food or beverages, which may require solvent extraction or steam distillation (52,53), employs various techniques. The two classical methods are both spectrophotometric (54—56). In the ultraviolet method, the prepared sample is acidified and the sorbic acid is measured at 250 260 nm. In the colorimetric method, the sorbic acid in the prepared sample is oxidized and then reacts with thiobarbituric acid the complex is measured at - 530 nm. Chromatographic techniques are also used for the analysis of sorbic acid. High pressure Hquid chromatography with ultraviolet detection is used to separate and quantify sorbic acid from other ultraviolet-absorbing species (57—59). Sorbic acid in food extracts is deterrnined by gas chromatography with flame ionization detection (60—62). 

The exterior durabiHty of relatively stable coatings can be enhanced by use of additives. Ultraviolet absorbers reduce the absorption of uv by the resins and hence decrease the rate of photodegradation. Eurther improvements can be gained by also adding free-radical trap antioxidants (qv) such as hindered phenols and especially hindered amine light stabilizers (HALS). A discussion of various types of additives is available (113). 

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

The peroxide decomposer will drastically reduce the number of radicals, which can then be more effectively mopped up by the chain-breaking materials. A widely used combination is 4-methyl-2,6,di-t-butylphenol and dilauryl thiodipropionate. It is possible to envisage most powerful combinations where a chain-breaking antioxidant, a regenerating agent, a peroxide decomposer, a metal deactivator and an ultraviolet absorber are all employed together. 

Reactions (d) and (e) occur much less frequently than reactions (a)-(c) but do so at a sufficient rate to cause changes in most polymers. There are three methods available for combating such effects light screens, ultraviolet absorbers and quenching agents. 

Ultraviolet absorbers are a form of light screen which absorb primarily in the ultraviolet range. It is a requirement for most ultraviolet absorbers that absorption in the visible range should be negligible if this were not so the resultant instant colour formation could be worse than that experienced after prolonged exposure of the polymer. 

The ultraviolet absorber could dispose of absorbed energy by radiation, by conversion into heat or by chemical changes leading to stable products. The most important commercial absorbers, such as the o-hydroxybenzophenones, o-hydroxyphenylbenzotriazoles and salicylates, appear to function by conversion of electronic energy into heat. The properties of the main types of ultraviolet absorbers are summarised in Table 7.7. 

To improve the resistance to ultraviolet light carbon black is often useful as a light screen. Its use in fibres and films is clearly very restricted and in these instances ultraviolet absorbers and/or quenching agents are used. Recent developments include the greater use of hindered amine and nickel compounds. 

Low levels of ultraviolet absorbers (typically 0.2-0.8 pphr [parts per hundred resin]) can also be useful in preventing initiation of degradation mechanisms. Modified benzophenones and benzotriazoles are in widest commercial use. 

Further improvement in light stability may be achieved by addition of small quantities of ultraviolet absorbers. Typical examples include phenyl salicylate, 2,4-dihydroxybenzophenone, resorcinol monobenzoate, methyl salicylate and stilbene. 

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. 

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

The alternative approach is to add a photodegradant which is an ultraviolet light absorber. However, instead of dissipating the absorbed energy as heat (as with conventional ultraviolet absorbers) the aim is to generate highly reactive chemical intermediates which degrade the polymer. One such material is iron dithiocarbamate. 

Paper chromatography (benzene-chloroform 1 1—formamide system) of representative chromatogram fractions indicates the presence of a small quantity of a more polar ultraviolet absorbing component that gives a negative blue tetrazolium test and a very polar component (no ultraviolet negative tetrazolium test). These materials have not been characterized. 

Infrared spectra and the degree of specific rotation show typical features of the malic acid polyester (Table 3). Ultraviolet absorbance spectra of )3-poly(L-malate) from both P. polycephalum and Aureobasidium sp, A-91 are similar and are reminiscent of malate itself [4,5]. For a solution of 1.0 mg/ml polymer, absorbance increases from 0.40 units at 230 nm to 10 units at 190 nm wavelength. After saponification and pH-neutralization, the absorbance increased from 8.7 units at 230 nm to 100 units at 190 nm. 

Paints are complex formulations of polymeric binders with additives including anti-corrosion pigments, colors, plasticizers, ultraviolet absorbers, flame-retardant chemicals, etc. Almost all binders are organic materials such as resins based on epoxy, polyurethanes, alkyds, esters, chlorinated rubber and acrylics. The common inorganic binder is the silicate used in inorganic zinc silicate primer for steel. Specific formulations are available for application to aluminum and for galvanized steel substrates. 

XCPS-1 Controlled pore Glass, CPG-10 (324 and 1038 A) Electronucleonics 0.05 M Na2S04 Ultraviolet absorbance (A, = 206 nm)... 

See Free-radical scavengers, Peroxide decomposers, Quenchers, Ultraviolet absorbers, Ultraviolet screening agents. 

Nagels, L J., Creten, W.L., Vanpeperstraete, P.M. (1983). Determination limits and distribution function of ultraviolet absorbing substances in liquid chromatographic analysis of plant extracts. Anal. Chem. 55, 216. 

He, Y., Yeung, E.S., Chan, K.C., Issaq, HJ. (2002). Two-dimensional mapping of cancer cell extracts by liquid chromatography-capillary electrophoresis with ultraviolet absorbance detection. J. Chromatogr. A 979, 81-89. 

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