A composition for peelable opaque film used in the preparation of printing plates, and for the varnish used to form such a film

Technical field

The present invention relates to the composition of a peelable opaque film in masks for the preparation and/or correction of printing plates, produced in sheet or strip format and comprising a support of transparent plastic material, generally polyester, to which the film is applied on one face over an interfacing layer of adhesive material that can be detached subsequently together with the film.

Background art

The product in question is known conventionally as a red mask, or as high-opacity masking film, which amply reflects the intended purpose. Such masks find widespread application in printing, offset, silkscreen, etc. , serving in particular for the preparation and/or correction of plates, and for phototypesetting arid developing thereafter.

In practice, the mask is cut initially to desired contours or patterns using a variety of means and techniques, manual or otherwise, in such a way as to pierce through the red film but without touching the transparent support. The selectively cut parts are then peeled off, leaving a sheet that exhibits figures or images defined by contrasting red and transparent areas. This same sheet is then laid over a presensitized plate, and the plate exposed to UV radiation as part of the developing process. The red/transparent, or rather opaque/transparent

contrast, is transferred to the plate; thus, when developed, the plate will reproduce the selfsame figures or patterns as those created previously on the sheet by peeling off the cut parts of the film, whereupon the plate is ready for use in printing. The parts of the film cut out and peeled off will lift the corresponding parts of the adhesive layer also, and these, suitably shaped and proportioned, are used further to cover or mask off those areas of plates requiring modification or correction.

The use of the colour red stems from its excellent opacity to light for the purposes of developing the plate, hence the conventional name of "red mask"; nonetheless it is possible to purchase products with film in other colours -viz, orange, blue or green, affording similar properties of opacity. Given the typical application of the product, which can involve cutting and subsequently repositioning ultrafine fillets of the film, certain properties are essential:

-ease of cutting;

-a low elmendorf tear value;

-a moderate degree of elasticity;

-a suitably measured peeling force; -satisfactory repositioning and adhesion following detachment.

Currently, film is produced using nitrocellulose modified in various ways as a base polymer, with a variety of phthalates as the plasticizer, whilst the adhesive material is a natural rubber blended with small quantities of a hydrocarbon resin. On the practical level, the film is formed by spreading a suitably formulated varnish over the support of transparent plastic material, already coated with the layer of adhesive, which is then

dried; the solvent evaporates, leaving a skin that can be peeled from the support together with the interfacing adhesive. The one serious drawback experienced with products manufactured for general sale stems precisely from the use of nitrocelluse as the base polymer; this results in the formation of a highly inflammable film, hence of serious hazards in both handling and storage for manufacturers and users alike. Accordingly, the object of the invention is to overcome the drawback mentioned above by providing a new composition for peelable opaque film such as will give a non-flammable and self-extinguishing product. More exactly, the object of the present invention is to replace the nitrocellulose used currently in the composition of such products with another base polymer possessing a high resistance to flame and self-extinguishing properties, in combination with a similarly self-extinguishing plasticizer and a compatible adhesive, whilst obtaining a film with the same essential qualities and the same technical and functional properties as hitherto.

Disclosure of the invention

The stated object is realized in a composition, as characterized by the appended claims, for peelable opaque film as used in masks for the preparation of printing plates, which comprises as a base polymer, a strongly flame resistant thermoplastic cellulose resin, classifiable self-extinguishing, and as a plasticizer, an alkyl or aryl phosphate; the layer of adhesive material consists advantageously in a derivative of butyl acrylate, suitably modified and self-reticulating.

A preferred embodiment of the invention will now be described in detail, by way of example. According to the present invention, the composition of the film must comprise a base polymer other than nitrocellulose, i.e. affording a strong resistance to flame, classifiable as self-extinguishing and with chemical and physical properties such as will ensure the technical qualities expected of a mask. Experiment has shown thermoplastic cellulose resins to be suitable for the purpose, and in particular, compositions with a base polymer consisting in a cellulose acetate. Preferred compositions obtained as the result of experiment comprise a base polymer consisting in any one of the following cellulose esters:

-cellulose acetate butyrate

-cellulose acetate propionate

-cellulose vinyl acetate.

Whilst the usual phthalates are suitable for use as plasticizers, these must be discarded due to their low self-extinguishing capability. It has emerged from experiment that alkyl or aryl phosphates are useful and effective, e.g. tricresyl phosphate, alkylaryl phosphate, triaryl phosphate, phosphate of chlorine, etc.. With notable self-extinguishing properties, these phosphates proved to be ideal as plasticizers for thermoplastic cellulose resins in formulating a film with the technical attributes aforementioned. Natural rubber cannot be utilized in the layer of adhesive material, as currently is the case, given its incompatibility with the base polymers and the plasticizers mentioned above. A derivative of butyl acrylate, modified and self-reticulating, as used in the manufacture of oridinary adhesive tapes, was

found to be suitable for the purpose and perfectly compatible with the components in question. To reiterate, a varnish is applied to one face of the transparent plastic support, coated previously with a layer of adhesive material, and then dried. The composition of the varnish must be carefully formulated to the end of obtaining a film with the desired characteristics. The following formulation, obtained by experiment, is expressed in percentages by weight:

In seeking to create a film with superior technical and functional attributes, ideal results were also obtained using a base of cellulose acetate butyrate and adopting a varnish formulated to the following percentages by weight:

-cellulose acetate butyrate 22.2 %

-alkylaryl phosphate 11.7 % and as additives:

-opacifier 1.0 % -antioxidant 0.2 % -leveller/surface adhesion regulator 0.2 %

-antistatic agent 0.1 %

-colouring agent 1.5 % -solvent 63.1 %

Aside from the fact of their inclusion in selected percentage proportions in a composition according to the invention, the additives mentioned above are well known individually to a person skilled in the art as regards their effects and functions.

In accordance with the invention, the selection of the absolute and relative proportions of additives listed in the above formulations will reflect the type and respective proportions of the base polymer and plasticizer adopted, to the end of ensuring the requisite correction of the resulting chemical and physical characteristics of the varnish (density, opacity, viscosity, adhesion, stability etc.), and of the peelable opaque film ultimately obtained. Clearly enough, given the method of production, the composition of a varnish as recited in the above formulations is not quantitatively equivalent to the composition of the resulting film.