Thermographic Image Formation Utilizing A Copy Sheet Of Discrete Thermoplastic Particles And A Powder Developer

Abstract

A copying process wherein a copy sheet comprising a support and layer thereon of discrete thermoplastic particles is exposed to infrared radiation while in heat conductive contact with an original bearing an infrared absorbing image, thereby causing the coalescence of the particles in the areas corresponding to the original image. A finely divided thermoplastic powder is applied to the exposed copy sheet and selectively adheres to the coalesced areas, thereby forming a powder image. The powder image is then transferred to a receptor sheet.

Description

IMAGE FORMATION

The present invention relates to image formation, and refers more particularly to materials and methods for reproducing graphic images by the use of heat.

In the prior art there are techniques for reproducing graphic information utilizing various procedures, but these prior processes lacked the utility, convenience, and ease of operation of the present invention. For example, there are thermographic papers which produce images directly upon being heated in image-wise fashion by means of infrared-absorbing graphics in heat conductive relation therewith. The disadvantage of such thermographic papers is that they are not fixed after exposure and any subsequent heat discolors the paper. Furthermore, there are xerographic processes for copying graphic information which do not have this fixing problem, but involve rather expensive and complex reproduction machines, not a simple source of actinic radiation such as is present in a thermal copying machine.

Therefore one object of the present invention is to provide materials and methods for reproduction of graphic images by the use of heat, which overcome the disadvantages of the prior art.

Another object is to provide methods and materials for reproducing stable copies of graphic originals by means of actinic radiation.

Another object is to provide methods and materials for making multiple copies.

Another object is to provide methods and materials for making reversal copies.

Another object is to provide methods and materials for making a printing plate.

Other objects will become apparent during the course of the following specification.

Normally when polymer layers are coated from emulsions or dispersions, they are designed so that the disperse phase coalesces on drying to form a uniform continuous layer. It is obvious that an emulsion or dispersion which forms a discontinuous layer is useless for binding purposes as in paints, paper coatings, and the like. It has now been found, however, that the use of a discontinuous layer enables the preparation of a reproduction material of unusual merit and convenience.

The drawing depicts a copying process according to the present invention wherein a copy sheet 10 comprising a support 12 and a layer 14 thereon of discrete thermoplastic particles is exposed to infrared radiation while in heat conductive contact with an original 8 bearing an infrared radiation absorbing image 9, thereby causing the coalescence of the particles in areas 16 corresponding to the original image. Upon separation of original 8 and copy sheet 10 bearing the latent image of coalesced thermoplastic powder is applied to surface of copy sheet 10 and selectively adheres to coalesced areas 16, thereby forming a powder image 18. Subsequent transfer of the powder image to a receptor sheet 20 and fusion of the powder by the application of heat results in a copy 26 of the original.

The following examples are given for purposes of illustration and are not intended to limit the scope of the present invention.

EXAMPLE 1

A very dilute suspension of an acrylic resin latex such as Rhoplex B-85 was coated upon a support such as a sheet of infrared transparent paper and dried at a temperature of less than 100.degree. C. to produce a layer of discrete particles on the support. The coated sheet was placed in heat conductive relation with an original and the sandwich exposed to heat rays. Upon separation of the latex-coated paper from the original, a very slightly visible image of the heated areas was noticeable on the paper. Present, however, was a very intense latent image corresponding to the original image, which was developed by a finely ground thermoplastic powder containing a dark colored pigment or dye sprinkled over the surface of the coating. The powder adhered only to the image areas.

It appeared that the brief heating rendered the heated areas permanently sticky. Sixteen hours after heat exposure, it was still possible to dust the sticky image and develop an excellent reproduction of the original. The dark colored thermoplastic powder may be fused if necessary to fix the image to the paper. Where the thermoplastic powder contains an infrared absorber such as carbon black, the fusing can take place by exposure to infrared radiation. A suitable developing powder comprised about 4.5 percent carbon black dispersed in a thermoplastic styrenated terpene resin. Where the support is visually transparent or translucent, the developed image may be used as a diapositive, and where the powder is oleophilic, the record sheet may be used as a printing plate.

Other suitable latexes or emulsions successfully employed were polyvinyl acetate emulsion (Vinac WR-20), nonionic polyethylene emulsion (Poly-EM), polystyrene emulsion (Emulsion M), and anionic polyethylene emulsion (Poly-EM-11). These emulsions may be mixed. In addition, continuous-phase resins (that is, resins soluble in the continuous phase of the dispersion) may be added as a binder. For example, polyvinyl alcohol (Elvanol 52-22) may be added to Poly-EM-11polyethylene emulsion in order to toughen the coating Here the continuous phase is water.

The major criterion of selection of the nonfilm forming emulsion for this coating was the temperature necessary to remove the continuous phase and the temperature required for the disperse phase to reach a sticky, adhesive state. For practical reasons, the drying temperatures range from room temperature to about 100.degree. C. The heating temperature was greater than the drying temperature. Otherwise, coalescence of the discrete particles occurred prematurely.

Although quite unexpected, the explanation of the observed phenomenon is quite possibly explainable by what is frequently called the Kelvin Equation which shows that the vapor pressure of a curved surface such as that found on a drop, is greater than for a plane surface. Analogously, it can be shown that a finely powdered substance melts at a lower temperature than a mass of material, provided a continuous liquid is formed. These surface tension effects reach large proportions when the particle size approaches micron size, (the size of many of the latexes used). It is to be noted that Folman and Shereshefski in the Journal of Physical Chemistry, Vol. 59, p. 607, 1955 noted that vapor pressure effects in fine capillaries of a few microns radius were 10 to 80 times as great as those calculated by the Kelvin Equation.

The coated sheet comprising a support and a layer of discrete particles may be treated instead with a heated stylus to form the adhesive image. Here it was found that the support need not be infrared transparent. Supports such as polymer films, metals, glass, cloth and the like were suitable.

The developing powder may be selected so that it is relatively oleophilic compared to the support material. Then the developed sheet was usable as a printing plate. Conventional lithographic printing processes were then used to print many copies of the original image.

Where the support is visually transparent as well as actinically transparent, the developed image may be used as a photographic negative or diapositive for photoprinting or projection purposes. Suitable supports are transparent polymer films such as polyethylene terephthalate, cellulose acetate and cellophane.

The unheated particles were easily removed by simple mechanical means such as brushing during the developing steps.

EXAMPLE 2

An important variation of this process was suitable for making multiple copies. A latex-coated, translucent paper was exposed by reflex or contact method to heat rays with an original. The copy transfer paper was separated from the original and the former dusted with powder developer as in Example 1. The developed copy transfer paper was then sandwiched to a receiving sheet. The pair was then run through a blanket charging device which placed a high charge on the receiving sheet to attract the toner from the copy sheet. The developer was then fused to the receiving sheet. The copy transfer sheet still had an active, sticky image. This was redeveloped without a second exposure. The process of transferring to another receiving sheet was then repeated. This technique was repeated many times to make many copies of the original.

A satisfactory coating mixture for this process was a mixture of one part polyvinyl acetate emulsion, such as Vinac WR-20, diluted to one-tenth its original solids content with water and one part of acrylic resin emulsion, such as Rhoplex B-85, also diluted to one-tenth its original solids content with water.

When this coating was applied to paper and exposed as described above, the sticky image was developed with a colored powder. When a blanket positive charge was applied before developing to the surface of the copy paper, much greater contrast was obtained in the image, that is, the lines looked darker against the background.

When the above process was repeated with the copy paper given a negative charge before developing, a reversal image was unexpectedly obtained. In other words, the heated areas accepted the developer less readily than the unheated background areas. This provided a method for making reversal copies of the original.

It is apparent that the above-described examples are capable of many variations and modifications. All such variations and modifications are to be included within the scope of the present invention.

Claims

I claim:

1. A method of recording a heat image which comprises the steps of:

a. coating a support with a layer of discrete thermoplastic particles;

b. drying the coating at a temperature below the coalescing temperature of said particles;

c. impressing a heat image onto the layer;

d. applying a finely divided colored powder to said layer to develop the heated areas corresponding to said heat image;

e. transferring the colored powder image from said layer to a receiving layer; and

f. fusing the transferred colored powder image to said receiving layer by means of heat.

2. The invention according to claim 1 in which the receiving layer is charged with an electrostatic charge of sufficient force to cause the transfer of said colored powder image from the layer of discrete thermoplastic particles.