YELLOWING AND BLEACHING OF PAINT FILMS
Henry W. Levison
THE MOST IMPORTANT CONCLUSION from this investigation is that the net degree to which films are capable of discoloring in the dark does not seem to be a function of the age of the film or the extent and frequency of photochemically induced bleaching the films may have undergone in the intervening time. Moreover, in spite of as many as four cycles of discoloration in the dark and subsequent bleaching, the discolored films proved to be capable of being bleached, at any stage, practically back to their initial degree of whiteness.
The alkyd mediums exhibited the least yellowing throughout, followed closely by the paints made with safflower or poppyseed oil. The linseed oil paint yellowed the most, but recovered extremely well. The extent of discoloration in the case of the painting mediums apparently was related to the content of linseed oil. On the other hand, resins, other than the alkyds, do not appear to affect the degree of yellowing if the initial color due to the added resin is taken into account.
If cobalt drier is used in the minimum effective amount, its effect on yellowing was not noticeable. In all instances, the mixtures containing cobalt drier bleached as though they were without drier. With the zinc-zirconium drier in the safflower oil paint, the initial yellowness was less than the yellowness of the paint without drier and continued that way throughout four cycles.