Nature and Constitution of Shellac III—Some Observations on Development of Opacity in Clear Varnish Films^1,2

Abstract

INDUSTRIAL AND ENGINEERING CHEMISTRYYol. 23, No. 12 experiments will be necessary to determine under what conditions the use of ultra-violet glass in winter in New York or Chicago will be of value in protecting against rickets.Summary 1-This study shows that when albino rats are exposed to Chicago sunshine at the street level during the month of February and behind commercial ultra-violet glass or ordinary glass, they are not protected against rickets; whereas similar rats under similar conditions during a March-April period are fully protected against rickets when behind the ultra-violet glass, but not protected when behind ordinary glass.2-Spectrophotographs comparing the transmitting efficiency of the ultra-violet glasses used in the previously mentioned rat experiments, with a variety of other samples of commercial ultra-violet glass show that in the region 3000 to 3100 Á. the transmission is between about 30 per cent for the poorest to about 52 per cent for the best of the glasses which were used; and that, therefore, there are among these glasses some which are average and representative of commercial ultra-violet glass.3-These results indicate that there is a critical difference between the rickets-preventing efficiency of February sunshine and March-April sunshine behind commercial ultraviolet glass in Chicago at street level.t Literature Cited(1) Blunt and Cowan, "Ultra-Violet Light and Vitamin D in Nutrition,"

Permeability to Moisture of Organic Surface Coatings

“…In general, the moisture adsorption of a coating is related to its permeability, but there are many exceptions to this. For example, a short oil-limed rosin varnish will have a high moisture adsorption but a fairly low permeability, but the most striking contrast may be found in shellac which has a very high moisture adsorption and surprisingly low permeability (8,9,20,21). These variations in permeability and moisture adsorption, together with the demand for high water resistance in surface coatings, have made necessary more precise methods for measuring the water sensitivity of a coating under all possible conditions.…”

Permeability of Varnish Films

“…Studies of the sorption of moisture by films {5, 8, 17) and of the reasons why varnish films become opaque (7) indicate that the inherent structure of these films (25) and the changes which may occur during adsorption of vapors probably have a profound effect on permeability. This has been recognized to some extent by recent investigators (20), but the complete picture cannot be obtained from a limited study of a single permeating substance (15).…”

Nature and Constitution of Shellac: XI. Rates of Adsorption of Moisture by Shellac Films1

“…Shellac films should therefore adsorb moisture without transmission in much the same manner as any other adsorbent, unless attendant swelling effects should alter the structure of the capillary spaces. A study, therefore, of moisture adsorption at low humidities where whitening does not take place might explain why shellac films are one of the best known protective coatings as long as they remain intact, a fact which is frequently overlooked in practical application (4).…”