Agent for protecting the surface of lithographic printing plate comprising a plasticizer containing oil phase and a surfactant and a hydrophilic high molecular weight compound containing aqueous phase

DETAILED DESCRIPTION OF THE INVENTION The plasticizer which is the most characteristic feature of the plate protective agent of this invention constitutes the oil phase of an emulsion and is required to solidify at a temperature lower than about 15.
degree.
C.
Such plasticizer can be selected from what are generally called low-molecular-weight plasticizers.
The plasticizers used in this invention boils at atmospheric pressure at a temperature which is preferably at least about 200.
degree.
C.
, more preferably at least 300.
degree.
C.
Examples of the preferred plasticizer include a phthalic acid diester such as dibutyl phthalate, diheptyl phthalate, di-n-octyl phthalate, di(2-ethylhexyl)phthalate, dinonyl phthalate, didecyl phthalate, dilauryl phthalate or butylbenzyl phthalate; an aliphatic ester of a dibasic acid such as dioctyl adipate, butylglycol adipate, dioctyl azelate, dibutyl sebacate, di(2-ethylhexyl)sebacate, or dioctyl sebacate; an epoxidized triglyceride such as epoxidized soybean oil; a phosphate ester such as tricresyl phosphate, trioctyl phosphate or trischloroethyl phosphate; and a benzoic ester such as benzyl benzoate.
Particularly preferred are dioctyl adipate, dibutyl sebacate and dioctyl azelate which give off the least odor and can be handled with maximum safety.
According to this invention, these plasticizers can be used independently or as a mixture and in either case, they are used in an amount within the range of from about 0.
1 to about 10 wt%, preferably from 0.
5 to 5 wt%, based on the total weight of the plate protective agent.
As the amount of the plasticizer to be used decreases to less than 0.
1 wt%, the resulting plate protective agent will lose its stability as an emulsion and it also decreases the lipophilicity of the image area of a lithographic printing plate.
On the other hand, as the amount of the plasticizer increases to greater than 10 wt%, more plasticizer will stick to the image area and, as a result, the image area becomes so greasy to the touch that the lithographic printing plate coated with such protecting agent is difficult to handle