Ultraviolet absorbers and optical brighteners

Among the numerous additives commonly used in plastic materials, the ultraviolet absorbers are increasing in importance because they are often used in food packaging materials to protect the plastic material as well as the foodstuff packaged from the actinic action of ultraviolet radiation. Actinic effects may cause discolouration of both the plastic material and the foodstuff and, on occasion, also changes in taste and loss of vitamins in the food. 

Methods 17 and 18, respectively, determine Cyasorb UV 531 (2-hydroxy-4-n-octoxybenzophenone) in polyolefins by direct infrared spectroscopy and thin-layer chromatography. Phenolic and organophosphorus type antioxidants do not interfere in these procedures. Method 19 describes an ultraviolet spectroscopic method for the determination of Tinuvin 326 ultraviolet absorber in polypropylene. 

The fluorescence emitted by optical brighteners under ultraviolet light on a thin-layer plate has been utilized as a means of estimating these compounds (method 19). To estimate Tinuvin 326 7(6-butoxy-5-methyl-benzotriazol-2-yl) 3-phenyl-coumarin) in polymer granules, the sample is extracted from the polymer with chloroform. The extract is applied to two Kieselgel G plates and chromatograms are developed with benzene-chloroform (2 3) and benzene, respectively the spots being detected by their fluorescence in ultraviolet radiation. 

Lappin and Zannucci determined 4-(dodecyloxy)2-hydroxybenzophenone (DOBP) and 2,6-di-tert-butyl-p-cresol (BHT) in polypropylene by this technique (Table 1.4). The precision of the BHT determination is good but the quantity found (0.02%) was less than the amount added (0.05%) possibly due to some losses of volatile BHT during polymer compounding. DOBP determinations ranged from 0.20 to 0.35% for a sample originally containing 0.30% of the additive. 

Denning and Marshall devised a method in which toluene extracts of polyethylene are examined for ultraviolet absorbers (and antioxidants) at two column temperatures in order to overcome the problem that whereas some compounds have a relatively low retention time, others of higher molecular weight have very long retention time at 250 C. Relative retention data obtained for ultraviolet absorbers, are given in Table 1.5. 

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