Method of enhancing the visibility of diffraction pattern surface embossment

#This invention relates to the decoration of sheeting and, more particularly, to the decoration of materials such as thin transparent plastic films. This invention also relates to the embossment of plastic film and more particularly to improved methods for making embossed patterns visible to the human eye.

#There are wide-spread applications for embossed plastic films. One example is provided by plastic films embossed with holographic patterns. These are used in application where security sensitive items such as holographic labels are used. They are also used for decorative applications such as self adhesive stickers and gift wrap.

#The above-described embossments on plastic films are usually-made clearly visible by depositing a thin layer of metal (usually aluminum) on the embossed surfaces to give a mirror-like luster which intensifies the visibility of the embossments. This thin layer of metal is generally applied to the surface by a process known as vacuum metallization. In that process, the film to be metallized is placed in a large vacuum chamber and, after a high vacuum is achieved, is passed over a series of crucibles or evaporation boats containing molten metal. The metal deposits from the vapor phase onto the moving plastic web. This method of metallization can be conducted at high speeds (up to 2000 ft. per min.) on wide webs, but has several significant disadvantages. One disadvantage is that the metallizing equipment is very expensive. A second disadvantage is that the metal can only be applied as a uniform coating or, by using appropriate masks, in swipes running in the machine (long) direction of the plastic film web. If it is desired to create a discreet metallized pattern, this is usually accomplished by first metallizing the surface overall, then etching away the unwanted metal by use of a corrosive etchant such as caustic soda or an acid. The etching process is expensive because it entails an additional manufacturing step, specialized machinery, and handling and disposal problems with the etchant. Another disadvantage of the vacuum process is that the metal can only be applied at one level of deposition, whereas it is often desirable to have different levels of metal deposited at different points on the plastic film to produce partial transparency or other special effects. Still another disadvantage of the vacuum metallization technique is that it is not suited for use on narrow webs or small rolls of film, and is especially uneconomical to use when the film is in sheet form.

#The character of the invention, however, may best be understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

#FIG. 1 is a cross-sectional view of a laminated decorative sheet embodying the principles of the present invention,

#FIG. 2 is a cross-sectional view of a second laminated sheet embodying the principles of the present invention,

#Referring first to FIG. 1, wherein are best shown the general features of a laminated decorative sheet embodying the principles of the present invention, the sheet, denominated generally by the numeral 10, is shown to include a backing sheet 11, and a transparent cover sheet 12 adhesively bonded to one surface of the backing sheet 11 by adhesive 19.

#The inside surface of the cover sheet 12 is embossed with a diffraction grating or holographic image. This inside surface is printed with a layer of the type one, two or three ink 13, which simulates the optical effect of metallizing that surface, when the cover sheet 12 is viewed from its outer surface. In this simple laminate, the optical effect not only provides a decorative aspect, but also is difficult to duplicate by forgers.

#A more complex structure is presented ha FIG. 2. In this structure, the inside surface of the backing sheet 21 is printed with information 24 which requires protection from forgery or alteration. The transparent, embossed cover sheet 22 is printed with the layer of type one, two or three ink 23, but not uniformly. In zone 25 and 26, the ink is thick and essentially opaque. The printing 24 does not show through the cover sheet 22. In zone 27, the ink is thin and translucent so that the printing 24 is visible but somewhat obscured. In zone 28, there is no ink so the printing is clearly visible through the adhesive 29 and the embossing is invisible. This structure is very difficult and complex to duplicate or forge. It therefore provides an effective anti-forgery and anti-alteration system. Because the ink is applied using a printing process, precise control of the zone or windows in the sheet and the pattern of decoration or obscurity can be achieved.