Vesicular images are formed in a photographic film by refraction of light from small bubbles or vesicles of gas which is formed and trapped in those areas of the film exposed to light and thereafter developed. Generally, the film has a colloid or resinous coating which carries the light-sensitive vesiculating agent and is coated on a photographic film or substrate. The light-sensitive vesiculating agent is sometimes referred to as a sensitizer and the most commonly employed sensitizers are diazo-compounds which, upon exposure to light, generate nitrogen gas. The gas does not form vesicles immediately but does so when the film is thermally developed by heating, thus allowing the microdispersed gaseous material to coalesce and form the bubbles. The resulting vesicles or bubbles in the exposed area make the matrix opaque to the transmission of light in such areas.
One of the essential performances influencing elements of a vesicular film is the resin binder, often described as the vehicle, or matrix in which the sensitizer or vesiculating agent is dispersed and within which the vesicles are formed upon exposure and development. The resin matrix must have certain characteristics of gas permeability, for the formation of the vehicles and chemical and physical stability after formation. In general, a low diffusion constant is needed to allow fermentation of the vesicles and an effective glass transition temperature which is high enough to avoid distortion of the unexposed areas or collapse of the vesicles in the reader due to the heat generated therein during operation. In general, an effective glass transition temperature of about 80.degree. to 85.degree. C. is satisfactory for most reader apparatus in general use.
In addition, the resin binder or matrix must be compatible with the film base or substrate, the other components of the coating composition, e.g., nucleating agents, and the coating equipment to be used in laying a smooth, even coating of uniform thickness and appearance.
Many resin compositions are known for use as the binder or vehicle for coating a film composition containing a vesiculating agent. Vesicular imaging materials, including films, are of extreme importance in information storage and retrival because of the extremely stable character of the image. Vesiculating compositions and agents are well known in the art. Early commercial vesicular compositions containing a vesiculating agent were originally formulated with gelatin as the binder material. Ultimately, gelatin was replaced with improved binder materials which did not exhibit the undesirable tendency of gelatin to absorb moisture from the atmosphere causing loss of image due to collapse of the vesicles. Typical improved materials include a variety of synthetic polymers such as polyvinyl chloride, polyvinylidene chloride, polystyrene, and copolymers obtained by copolymerizing acrylonitrile with a vinyl chloride, styrene, vinylidene chloro-fluoride, and the like.
To be suitable for use as a vesiculating binder or matrix, a material must be sparingly permeable to the vesiculating gas, usually nitrogen. This is necessary in order to properly form the vesicles as the gas is released by the light-sensitive vesiculating agent. To assist in the proper formation of the vesicles, various nucleating agents such as stearic acid have been employed to increase the speed and occasionally decrease the contrast.
Patents relating generally to vesicular elements include U.S. Pat. Nos. 3,779,774; 2,699,392; 2,703,756; and 3,149,971.
It is an object of this invention to provide improved vesicular film elements and vesicular compositions for the making of such elements.
Another object of the invention is to provide an improved binder resin suitable for use as the photographic matrix in vesicular films.