The unwanted build-up of static electricity on an insulating support has been a continuing problem. It is well known that a thin conductive coating will prevent static build-up but while it is relatively easy to formulate a conductive composition that can be coated on a support, it has been quite difficult to combine these conductive properties with other desirable physical properties.
The stringent physical requirements for the surfaces of photographic elements make the formulation of a suitable antistatic composition for these elements particularly troublesome. Typically, the antistatic composition is coated directly on the support and on the other side of the support is coated the radiation sensitive layers. The radiation sensitive layers frequently comprise a hydrophilic binder, such as gelatin, to facilitate processing. The antistatic layer on the so called "base side" of the support must be compatible with the hydrophilic binder on the so called "emulsion side" so that when the antistatic layer contacts the hydrophilic layer, such as when the film is rolled on itself, no physical defects are produced. With the majority of antistatic compositions, an impasse is reached at this point. It is known that the ionic polymeric compounds that are frequently used antistatic agents, require the presence of moisture to provide conductivity. To allow the moisture to contact the antistatic agent it is commonly thought that any binder that is used must be hydrophilic.
It has been proposed to coat photographic elements with polymers of vinylbenzyl quaternary ammonium compounds and in U.S. Pat. No. 3,399,995 to Winchell there is described such an element. These polymers, which are crosslinked by the inclusion of a small amount (such as 5.0 to 0.01 percent by weight) of a crosslinking divinylbenzene unit in the polymer, are coated directly onto the support without the aid of a binder. While these coatings of vinylbenzyl quaternary ammonium polymers are useful in increasing the conductivity of the support, coating the antistatic polymer without the aid of a binder creates several other physical problems. For example, coatings made in the manner described in the Winchell patent tend to have poor resistance to aqueous processing compositions and cause scumming in photographic films and produce brittle layers having poor adhesion. These coatings also cause severe ferrotyping when contacted with the emulsion side of another element. In a similar patent, U.S. Pat. No. 3,001,918, L. H. Silvernail et al describe another binderless conductive coating of a vinylbenzyl quaternary ammonium compound. These coatings suffer from the same defects.
In U.S. Pat. No. 3,674,711 to Growald et al, there is described a self-supporting clear transparent sheet comprising a vinylbenzyl quaternary ammonium polymer and a binder. Again the antistatic polymer is crosslinked with only a small amount (less than 5% by weight) of divinylbenzene. While useful in forming relatively thick (10 mils) self-supporting sheets, the sheets of Growald et al are not useful for photographic elements in that said elements require thin antistatic coating layers.
In U.S. Pat. No. 3,786,002 to Paesschen et al, there is described an antistatic coating for a polyester film comprising an uncrosslinked vinylbenzyl quaternary ammonium compound along with a chlorine-containing polymer consisting of at least 70 percent by weight of vinylchloride or vinylidene chloride. The use of linear hydrophilic antistats, even if used with a hydrophobic binder, frequently results in impaired physical properties due to the migration of the antistat to the surface of the coating.
While many compositions have provided layers which significantly reduce the static susceptability of photographic elements, the antistatic component is generally coated with a hydrophilic binder. It has been found however, that in a photographic element, if a hydrophilic binder is used for an antistatic layer which comes in contact with the hydrophilic radiation sensitive layer, numerous physical problems frequently result. The two hydrophilic binder materials may stick together, cause ferrotyping or other undesirable defects.
Thus there is a continuing need for antistatic compositions which can be coated on elements to provide the necessary antistatic characteristics without deleteriously effecting the physical properties. It is desirable to have an antistatic composition that can be coated on the base side of a photographic support so that when the element is coiled or rolled on itself the antistatic layer does not stick to or ferrotype the emulsion layer. It would also be desirable to have an antistatic composition that retains substantial conductivity even at low relative humidity.