Apparatus for the study of the photochemistry of sheet materials

An apparatus is described which was developed for studies of the photochemistry of organic materials in sheet form.The problems involved in stich studies are discu'ssed, and data are presented to show the extent to which various important requirements have been met in the design of the apparatus.One of the most important problems involved in such studies is the elimination of concurrent temperature effects.Temperatures of thin sheet materials-such as paper, cellulose acetate, Cellophane, rubber, felt cloth, and a paint filmunder various conditions of irradiation with the Pyrex-enclosed electric arc, were measured with thermocouples.Temperatures as high as 280° C were found in white paper exposed to a 2,080-watt aI~C, at a distance of 16 cm.Under less severe conditions, some of which were similar to those of the artificial "weathering" and "light" stability tests, in use at the National Bureau of Standards and elsewhere, temperatures between 80° and 150° C were found in the various sheet materials.Since most of the materials named are known to undergo marked thermal decomposition at these higher t emperatures, the control of t emperature is important.An apparatus for controlling the temperatures of sheet materials during irradiation, through intimate contact of the sheet with a thermostated aluminum backing, was found to be effective.Contact was secured by reducing the pressure between the sheet and backing.Temperature rises were thereby kept smaller; in the extreme case cited, the temperature rise was only 10° C above the temperature of the thermostated backing, which was maintained at 30° C.Temperature rises were further minimized by removal of most of the infrared radiant energy by means of a flowing cupric chloride filter.The removal of the infrared alone was found to be insufficient for adequate t emperature control, owing to the heat effects produced by the visible and near ultraviolet, freely transmitted by the filter.When the filter was used in conjunction with the thermostated backing, the temperature rises due to irradiation were only 2° to 3° C above 30° C. Because of the large amount of infrared energy to be r emoved, as much as 19,000 calories per minute for the arc lamp, and because of the corrosive nature of cupric chloride, special eq uipment was required for this filter.An arrangement for controlling the moisture and oxygen content of the atmosphere around the samples, from normal down to very low values, is also necessary for a comprehensive study.For porous materials, su ch as paper, in which gas absorption on the extensive fiber surfaces of the interior of the sheet may be important, direct circulation of the gases through the sheet is desirable.CONTENTS Page 1. Problems involved in studies of the photochemistry of solid materials __ 568 II.Light so urce and measurement of radiant energy __ _________ _________ 569 III.Absorption of infrared radiant energy _____________________________ _ 570 IV.Control of the temperature of sheet mat erials __________ ________ _____ 571 IV.Control of the temperature of sheet materials-Continued.Page 2. Apparatus for control of temperature of sheets-_ _ _ _ _ _ _ _ _ _ _ _ _ _ 573 3. Effectiveness of the apparatus for controlling the temperature __ 574 4. Sources of error in the temperature measurements ____ ________ 574 V. Irradiation chamber and composition of the atmosphere surrounding the samples during irradiation _____ _______ ______________________ 575 VI.Iteferences ________ _____________________________ ___ ______________ 577 Jour nal of Researc h of the N at ional Bureau of Standards FIC U HE I.-T he photochemical apparatus.