Background of the Invention Security documents such as bank notes, stocks, bonds, checks, warrants and identification cards need to be guarded by antifalsification measures and they are often made from a security paper having a security element in the form of fibers, strips or threads embedded therein.

For example, GB 2,213,098 discloses a partially metallized film strip, used as a security thread, which has metal-free portions in the form of a pattern, design, indicia, etc. to provide a continuous metal path along the length of the strip.

Further, Japanese Laid-open Patent Publication No.

216795/1988 teaches a security strip having a metallized pattern on the surface of a transparent film.

However, prior art method of preparing partially metallized films, useful as security threads, comprise complicated and hazardous etching techniques.

Summarv of the Invention It is, therefore, an object of the present invention to provide an improved process for preparing a security thread suitable for use in a security paper.

It is another object of the present invention to provide a security thread having enhanced security features.

It is still another object of the present invention to provide a security paper containing the inventive security thread.

In accordance with an aspect of the present invention, there is provided a process for the preparation of a security thread having a partially metallized layer, which comprises: forming a water-soluble ink layer on a selected portion of the surface of a transparent film; depositing a layer of a metal selected from the group consisting of aluminum, tin, silver and an alloy of cobalt and nickel on the entire surface of the transparent film including the water-soluble ink layer; and washing the metal deposited film with water to'remove the portion of the metal layer deposited on the water-soluble ink layer.

Brief Description of Drawing The above and other objects and features of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawing, wherein:

Figs. 1 to 4 show the steps of forming a partially- metallized film having a transparent region in the form of a set of characters and a dyed region in the form of another set of characters in accordance with one embodiment of the present invention; and Figs. 5 to 8 illustrate other embodiments of security threads prepared in accordance with the inventive process.

Detailed Description of the Invention In practicing the present invention, a water-based ink, which is soluble in water, is printed on a selected portion of the surface of a transparent film substrate, in the form of a character, pattern or figure to form a partial water- soluble ink layer. The substrate may be made from any material known in the art such as a polyester (e. g., polyethylene terephthalate (PET)), polyvinyl chloride, polypropylene and the like. The water-based ink may be any one conventionally used in the art.

Subsequently, à layer of a metal is deposited on the entire surface of the film including the printed ink layer.

The deposition may be carried out by vacuum deposition methods which are well known by a skilled person in the art. Representative examples of the metal suitable for forming the metal layer are aluminum, tin, a metal alloy (e. g., Co and Ni), silver and the like. The thickness of the metal layer ranges from 1 A to 2 Hm.

Thereafter, the metal-deposited film is washed with

water, preferably with brushing, to remove the portion of the metal layer deposited on the water-soluble ink layer, to obtain a film having a partially metallized layer.

In the present invention, a security thread having a enhanced security feature may be obtained by employing a fluorescent pigment or dyestuff to provide a partially metallized film having both a demetallized portion and a dyed portion. In this case, a partial coating of the fluorescent pigment or dyestuff is applied to a selected portion of the film surface prior to the step of printing the water-soluble ink. Alternatively, if desired, the water-soluble ink layer may be so formed as to mask the dyed portion. The fluorescent pigment or dyestuff may be any one conventionally used in the art and the coating thereof may be applied in the form of a character, pattern or figure, or a combination thereof, preferably by a gravure printing method.

The security thread of the present invention has a thickness and width suitable for use in security documents, e. g., a thickness ranging from about 15 to 35 manda width ranging from 0.5 to 5 mm.

The security thread of the present invention may be preferably embedded between two sheets of paper formed separately on a paper making machine and subsequently brought together to form a security paper having an embedded security thread.

One Embodiment of the Invention Figs. 1 to 4 show the steps of forming a partially- metallized film having a transparent region in the form of a set of characters and a dyed region in the form of another set of characters in accordance with one embodiment of the present invention.

Referring to Fig. 1, a fluorescent dyestuff or pigment composition such as Lumilux CD 305, CD 316 or Papilion is applied in the form of a set of characters (2) on the surface of a transparent polyester substrate (10) at a regular interval by a gravure printing method.

Subsequently, as shown in Fig. 2, a water-based ink is printed on the film to form a mask (30) of the printed characters portion (2) and, simultaneously, a design (31) in the form of another set of characters.

Aluminum is deposited on the resulting film by a vacuum deposition method to form an aluminum layer (4) at a thickness of 1 A to 2 pm over the entire surface of the film, as shown in Fig. 3.

Finally, the portions of the aluminum layer deposited on the mask (30) and the design (31) are removed by washing with water, while brushing, to obtain an aluminum-deposited film (1) comprising transparent characters (5) (corresponds to the design (31)) and printed characters (2) positioned in a transparent region (10).

Figs. 5 to 8 illustrate other embodiments of security threads prepared by the inventive process. Specifically,

Fig. 5 shows a security thread (1) having alternating metal- deposited portion (4) comprising transparent characters (5), and transparent substrate region (10) comprising colored characters (2) at a regular interval.

Further, referring to Fig. 6, there is provided a metallized security thread (1) having various patterns of colored regions (2), optionally positioned in transparent regions. Fig. 7 shows a security thread (1) produced by forming metallized bands (4) in a regular pattern on a transparent base film (10), and Fig. 8 shows a security thread (1) obtained by a chain of imprinting alternating colored (2) and metallized links (4) on a base film (10).

While the invention has been described with respect to the specific embodiments, it should be recognized that various modifications and changes may be made by those skilled in the art to the invention which also fall within the scope of the invention as defined by the appended claims.