BACKGROUND OF THE INVENTION The present invention relates generally to multi-copy forms, and more particularly to a multi-copy form which may be used with laser printers and electrostatic (xerographic) copiers to make duplicate copies with only one pass of the form through the printer or copier. The multi-copy form has at least one thermally sensitive imaging sheet sandwiched between two sheets of paper.

FIELD OF THE INVENTION

In the past, pressure-sensitive carbonless paper has been used to make duplicate copies using typewriters, printers, pencils, pens, or the like. Such carbonless paper is described in United States Patent Nos. 4,906,605, issued to Kraft on March 6, 1990; 4,398,954, issued to Stolfo on August 16, 1983 (multi-copy form); and 4,046,404, issued to Treier on September 6, 1977. United States Patent No. 4,434,198, issued to Clark on February 28, 1984, discloses a duplicating stencil made by fusing toner of an electrostatically produced image on a sheet of copy paper to a stencil sheet by sending the copy paper and stencil sheet through an electrostatic copier, and subsequently separating the sheet of copy paper from the stencil sheet, thereby forming the stencil.

SUMMARY OF THE INVENTION ^ere is a need for a multi-copy form that does not use presε ^v *e-sensitive carbonless paper.

__,ε is an object of this invention to provide a multi¬ copy form that may be used with laser printers or

electrostatic (xerographic) copiers to make duplicate copies by passing the form only one time through the printers or copiers.

It is another object of this invention to provide a multi-copy form that may be used with laser printers or electrostatic copiers for the purpose of. making copies by transferring heat and pressure from toner fused to a top sheet of copy paper to one or more underlying thermally sensitive imaging sheets. It is still another object of this invention to provide a multi-copy form that may be used to make di ferent colored copies.

These and other objects and advantages are attained by a multi-copy form which may be used with laser printers or electrostatic (xerographic) copiers to make duplicate copies by passing the form only one time through the printers or copiers. The multi-copy form has at least one thermally sensitive imaging sheet sandwiched between two other sheets, preferably of paper. The imaging sheet may be releasably held to the two sheets of papers by any laminating adhesive which allows the sheets to be peeled apart. As one specific example, "dry tack," which is a form of shellac, may be used. This laminating adhesive may also function as a release layer between the sheets. Toner is fused to the top sheet of paper by heat and pressure from top and bottom rollers, making images thereon, and heat and pressure transferred from the toner to the underlying thermally sensitive imaging sheet forms substantially duplicate images on the imaging sheet. The thermally sensitive imaging sheet has a thermal image layer applied to it which forms an image when heat is transferred to the imaging sheet. The bottom sheet of paper furnishes insulation against heat and pressure from the bottom roller. Different colored copies may be made by varying the dye formers and developers used for the thermal image layer.

In accordance with a broader aspect of the invention, only two sheets need to be used, an upper sheet upon which the toner is fixed by a heated roller, and a lower sheet having the thermal sensitive imaging material on its upper surface. Using this embodiment, the lower sheet may have relatively low thermal conductivity and/or the lower roller may be maintained somewhat cooler than the upper roller so that the thermally sensitive imaging material is not activated except in the areas below the fixed toner patterns. The reduced or lower thermal conductivity may be obtained either intrinsically from the nature of the sheet material used, or by coating the back side with a thermal insulating layer, such as may be formed by a foamed polymer or hollow, particulate-filled coating. The various features of the present invention will be best understood together with further objects and advantages by reference to the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a multi-copy form illustrating the principles of the present invention showing one corner of the form turned upward to show how a thermally sensitive imaging sheet is sandwiched between top and bottom sheets of the form;

FIG. 2 is a perspective view of a copying machine showing the multi-copy form exiting the machine;

FIG. 3 is a cross-sectional view of the multi-copy form of FIG. 1 showing heat and pressure being applied to the form by the rollers of an electrostatic copier or laser printer;

FIG. 4 is a cross-sectional view like FIG. 3 showing another embodiment of the multi-copy form having two thermally sensitive imaging sheets sandwiched between top and bottom sheets of the form; and

FIG. 5 is a fragmentary view of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following specification taken in conjunction with the drawings sets forth the preferred embodiments of the present invention in such a manner that any person skilled in the art can make and use the invention. The embodiments of the invention disclosed herein are the best modes contemplated in a commercial environment, although it should be understood that various modifications can be accomplished within the parameters of the present invention.

FIG. 1 shows a preferred embodiment of the multi-copy form 10 of the present invention. The multi-copy form 10 has a thermally sensitive imaging sheet 12 sandwiched or disposed between top and bottom sheets 14 and 16, respectively, of paper. Sheets 12, 14, and 16 may be releasably held together by any laminating adhesive 18 (see FIG. 3) , or in any other desired manner, and may be easily peeled apart.

As shown in FIG. 2, the multi-copy form 10 may be used to make more than one copy by feeding the form 10 through a laser printer or copying machine 20 only one time, or in a single pass through the machine 20. As such, the multi- copy form 12 saves time when making copies, and may be used to make different colored copies during a single pass through machine 20, as discussed below.

Any material suitable for copying or printing in an electrostatic (xerographic) copier or laser printer may be used for top copy sheet 14. For example, plastic film, clear or pigmented, or paper may be used for sheet 14. Preferably, smooth paper is used for both top and bottom sheets 14 and 16. The thickness of sheets 14 and 16 preferably has a range of from about 0.5 mil to about seven mils. Preferably, the type of paper used for sheet 14 is:

Bleached Kraft basis weight (25x38x500) = 40 #

Sheffield = 110 - 120

Tear MD/CD = 45/45 gm/in ² MIT fold MD/CD = 90/90

Brightness = 87

Mullen = 28 psi

Tensile #/in ² MD/CD = 32/18

The basis weight (500 sheets, each being 25" x 38") of sheet 14 may vary in a range of from about 20# (pounds) to about 60#. The type of paper used for bottom sheet 16 is the same as used for top sheet 14, except the basis weight may have a range of from about 40# to about 90#. Bottom side 22 of top copy sheet 14 is coated with the laminating adhesive 18, which may be an acrylic-based, blended adhesive such as Robond, sold by Rohm and Haas, Philadelphia, Pennsylvania. "Dry tack," which is a form of shellac, may be used for adhesive 18. "Dry tack" is available from Air Products Co., of Pennsylvania, and is sold under their trade name "Airflex-400." Other adhesives may also be used, if desired, such as a polyvinyl acetate adhesive with a plasticizer, release agents, pigments, fillers, and/or a retarder, allowing sheet 14 to be laminated wet to the thermally sensitive imaging sheet 12. Adhesive 18 is preferably a stable adhesive that will remain stable when subjected to temperatures in the range of up to about 300 to about 400 degrees Fahrenheit to facilitate printing or copying through laser or other high temperature printing or copying equipment. The top side 24 of bottom sheet 16 is also coated with the adhesive 18 allowing sheet 16 to be laminated wet to imaging sheet 12.

The coating weight of the pressure sensitive adhesive 18 is determined so as to provide a sturdy multi-copy form 10 that is sufficiently flexible and will not buckle when passing through a printing or copying machine 20, but will

allow the sheets 12, 14, and 16 to be peeled apart after copies are made. The thickness of the adhesive 18 is preferably less than about 0.5 mil, but may be varied as desired. Preferably, the type of paper used for the thermally sensitive imaging sheet 12 is the same type paper used for top sheet 14, except the basis weight has a range of from about 10# to about 90#. The thickness of sheet 12 preferably is in the range of from about one mil to about four mils. A sealing coating is applied to top side 26 of imaging sheet 12 in order to provide a smoother paper surface. Alternatively, white pigment may be added to the sealing coating, if desired, to increase the paper's brightness. The sealing coating preferably comprises: Parts bv Weight

Water 34.18

Styrene butadiene latex 5.42 Plastic pigments, such as 52.22 styrene micro spheres (about 47% solids) dispersed in the water

Paraffin wax dispersion 8.18

(about 50% solids)

The parts by weight ranges for the above components preferably are: styrene butadiene latex (from about 4.34 to about 5.53) ; polystyrene micro spheres (from about 44.90 to about 59.50); paraffin wax dispersion (from about 7.20 to about 8.20) .

A thermal image layer is coated over the sealing coating applied to the imaging sheet 12. The composition of the thermal layer is preferably as follows:

Parts bv Weight

Water 3485

Color developer dispersion 4470

Stearamide dispersion 194 (about 37% solids) Zinc stearate dispersion 476 (about 37% solids)

Polyvinyl alcohol 10% 670 fully hydrolyzed

Dye Dispersion Suspension 705

The parts by weight ranges for the above components preferably are: color developer dispersion (from about 4470 to about 5017) ; stearamide dispersion (from about 184 to about 204) ; zinc stearate dispersion (from about 476 to about 524); polyvinyl alcohol 10% (from about 670 to about 536) ; dipropyl . . . . (from about 705 to about 811) .

Dye dispersion suspension preparation for the thermal image layer is preferably as follows:

Parts bv Weight Dialkyl sodium sulfo succinate 20

Polyvinyl alcohol 200

Water 1800

Dipropyl (amine-6-methyl)

-7- amino fluoran 2000

This preparation is milled in a sand mill (glass bead media) until a particle size of about 3 microns is obtained.

The parts by weight ranges for the above components preferably are: dialkyl sodium sulfo succinate (from about 17 to about 23); polyvinyl alcohol 10% (from about 2000 to about 2200) ; dipropyl . . . (from about 1580 to about 2500) .

The color developer dispersion for the thermal image layer is prepared by charging the following into a sand mill containing glass media and grinding until a particle size of about 3 microns is obtained:

Dialkyl sodium sulfo succinate Commercial fluorescent brightener 1, 1, 3 tri (methyl-4-hydroxy-5- cycloxehyl phenyl) butane Calcium carbonate (<150 mesh) Tetrabromo bisphenol A Methyl bias (4-hydroxy phenyl) acetic acid methyl ester Polyvinyl alcohol 10% hydrolyzed Water

The parts by weight ranges for the above components preferably are: dialkyl sodium sulfo succinate (from about 0.025 to about 0.035); commercial fluorescent brightener. . . (from about 0.40 to about 0.54); 1, 1, 3 tri . . . (from about 1.55 to about 1.61); calcium carbonate (from about 18.9 to about 20.8) ; tetrabromo bisphenol A (from about 1.57 to about 1.63); methyl bias . . . . (from about 7.74 to about 8.06) ; polyvinyl alcohol 10% (from about 1.35 to about 1.65) .

Wax and stearamide dispersions are made by dissolving in water a suitable dispersing agent such as Tamol sold by Rohm and Haas. The solution is agitated and the temperature raised to about 150-190 degrees Fahrenheit. Then the melted wax or stearamide is added to the stirring water solution. An oil in water emulsion forms and is stable when the temperature drops.

The coating operation for the thermally sensitive imaging sheet 12 is as follows: applying the sealing coating, applying the thermal image layer, and finally subjecting sheet 12 to an anti-curl treatment. Anti-curl may be achieved by using water or an additional insulating layer on the back of sheet 12. Following the coating operation, the imaging sheet 12 is calendared to further smooth out the surface so as to ensure complete contact with

another sheet during printing or copying. Calendaring is adjusted to yield a Bekk smoothness for sheet 12 preferably in a range of from about 500 to about 1,200.

Alternatively, top and bottom sides 26 and 28 of the imaging sheet 12 may be coated with a mild silicon release agent or coating made out of a mixture of aliphatic and aromatic solvents so as not to affect the thermal image layer. The silicon release agent comprises:

Parts bv Weight Dow Corning release 20 25 sold by Dow Corning

Toluene 12

Xylene 8

VM & P Naptha 40 Heptane 40

The parts by weight ranges for the above components preferably are: Dow Corning release 20 (from about 25.0 to about 26.3) ; toluene (from about 10.2 to about 13.8) ; xylene (from about 6.8 to about 9.2); VM & P (from about 34.0 to about 46.0); heptane (from about 34.0 to about 46.0).

VM & P Naptha is a refined petroleum solvent such that: 5% distills at 130" F 90% distills at 145° F 5% distills at 155° F.

This solvent is aliphatic in nature, and is sold by Exxon,

Shell and Ashland. The thickness of the silicon release coating is preferably about a few ten thousands of an inch. However, any desirable thickness may be used.

FIG. 3 shows the multi-copy form 10 having one thermally imaging sheet 12 sandwiched between, or releasably held by adhesive 18 to, top copy sheet 14 and bottom sheet

16. Note that the drawing is not to scale, but illustrates the principles of the present invention.

When the multi-copy form 10 is fed through a laser printer or xerographic copying machine, rollers 30 are used to apply heat and pressure to toner material 32 causing the material 32 to fuse to top side 34 of copy sheet 14, thereby forming images, indicia, or a copy due to the fused toner 32. The toner material 32 may be a dry powder which is a heat-fusible material such as a mixture of carbon, iron oxide, and a synthetic resin, or the like. Such toner material 32 may be made out of a pigment such as carbon black in a polymer such as a styrenebutyl methacrylate copolymer. However, any suitable toner material may be used that fuses to paper upon the application of heat and pressure.

Since the toner material 32 is black and absorbs, transmits, and radiates heat from the roller better than the white top side 34 of copy sheet 14, heat transfers downward through sheet 14, as illustrated by arrows 36 shown in FIG. 3, due to the heat differential between the imaged and non-imaged areas. Also, the presence of molten toner material 32 improves heat transfer in the image areas. The presence of the toner material 32 also compresses the sheets

12, 14 and 16 or other compressible material in the image areas, thereby further improving heat transfer. When the heat reaches the top side 26 of imaging sheet 12, the thermal image layer at side 26 reacts chemically as the color developer melts, and the dye forms an image 38 at side

26 which is a substantially duplicate image of the image formed by the toner material 32.

It is preferred that the color developer have a narrow melt point range so that the optical density of the thermal paper changes from no image to fully developed over a temperature range of less than 30° C. One way which this may be accomplished is by having a narrow molecular weight distribution for the color developer.

If a black dye is used, then a black image will be formed. However, different colored dyes may be used. As a result, a second copy is formed on sheet 12 after the fused toner material 32 forms the initial copy on top copy sheet 14. After the copies are made, sheets 12, 14, and 16 may be peeled apart.

As mentioned above, different dye formers may be used in the dye dispersion to make different colored copies. Some examples of dye formers are:

Color of Copy Dye Formers

Blue second copy 3 - ( 4-diethylamino-2- ethoxyphenyl) -3 (l-ethyl-2- methylindol-3-yl) -4- azaphthalide Red second copy 3-diethylamino-7, 8- benzafluoran Green second copy 3-diethylamino-7-chloro- anilinofluoran

These dye formers are dispersed in the same manner as the black dyes and are used in the same manner.

As discussed above, rollers 30 apply heat and pressure to the multi-copy form 10. If the bottom roller 30 is hot, then bottom sheet 16 provides insulation against heat from the bottom roller 30. Therefore, the type of paper chosen for bottom sheet 16 may be different and may offer better insulation properties than the paper used for top sheet 14. In addition, insulating material is optionally located between sheets 12 and 16 and may be applied to the bottom sheet 16. The insulating material may be a micro sphere "plastic pigment" latex sold by Rohm and Haas under the trade name ROPAQUE. When dry, the insulating material contains packed hollow spheres which allow for insulation.

Alternatively, the multi-copy form 10 may be used without bottom sheet 16 if the bottom roller 30 is maintained somewhat cooler than the upper roller 30. In this case, the thermally sensitive imaging sheet 12 is connected by pressure-sensitive adhesive 18 only to the top sheet 14. Also, the bottom sheet, (of a two- or multiple-sheet assembly) may have relatively low thermal conductivity, if desired. The heat transfer from the lower roller may be further reduced by utilizing a rough surface on the lower surface of the bottom sheet.

FIG. 4 shows another embodiment of the multi-copy form 10 using two thermally sensitive imaging sheets 12 releasably held to each other and to top and bottom sheets 14 and 16, respectively, by adhesive 18. As explained above, after rollers 30 fuse toner 32 to top side 34 of copy sheet 14 forming an image, heat transfers downward from the toner material 32 (as indicated by arrows 36) and forms a substantially duplicate image 38 at side 26 of the immediately underlying imaging sheet 12 due to the thermal image layer applied to the sheet 12. As illustrated by arrows 44, heat will continue to move downward toward top side 42 of bottom imaging sheet 12 due to the heat differential between the two sheets 12. When the heat reaches the thermal image layer at top side 42 of sheet 12, another substantially duplicate image 46 is formed as explained above. Therefore, three copies are made after a single pass of the multi-copy form 10 through the printing or copying machine. As discussed above, bottom sheet 16 may be eliminated if bottom roller 30 is not heated. FIG. 5 shows an embodiment which is similar to that of FIG. 3, with the addition of the toner penetrable insulating layer 52. The layer 52 may comprise hollow, micro sphere "plastic pigment" sold under the trade name ROPAQUE, as mentioned above. When the toner 32 is applied to selected areas, and the heated roller compresses the toner through the layer 52, the hollow micro spheres collapse in the

imaged areas and allow improved heat transfer. In the non-image areas, the uncompressed and intact micro spheres serve to decrease heat transfer. This serves to increase the image contrast in the lower, thermally-sensitive layers. Other types of thermally- differentiating insulating layers may also be employed.

Note that different colored dye formers may be used for the thermal image layers of the two sheets 12. As a result, different colored copies may be produced by a single ulti- copy form 10 as discussed below.

It is important to point out that the amount of heat transmitted by the sheets 12, 14, and 16 may be varied by using different papers for the sheets which have different thermal conductivity properties. Therefore, copies of different colors may be made by varying the thermal conductivity of the imaging sheets 12 and by applying to the sheets 12 thermal image layers having different colored dye formers and different colored developers. In this way, for example, the original copy formed by top sheet 14 may be followed by a red copy and a blue copy, or vice versa. The melting points of the developers determine the thermal sensitivity of the imaging sheets 12. By varying the melting points of the developers, using different colored dye formers, and varying the thermal conductivity of the imaging sheets 12, the multi-copy form 10. may be used to provide copies of any desirable color.

If more than one imaging sheet 12 is used (e.g., see FIG. 4) , the upper sheets 12 and 14 preferably have high thermal conductivity properties, and are preferably thin and dense. Note that any number of imaging sheets 12 may be used (e.g., more than the two sheets shown in FIG. 4). In addition, the melting points of the color developers used for the thermal image layers of the sheets 12 preferably are:

blue = 118° C; red = 215° C; and green = 216° C.

It is important to note that the melting points of the color developers may be varied as desired. The scope of this invention is not intended to be limited by the ranges used for the melting temperatures of the color developers, and/or for the thermal conductivity of sheets 12, 14 and 16. While some particular, preferred thermally-sensitive layers have been disclosed, others which are known and which have been disclosed in the literature may be used. In general, high-sensitivity, heat-responsive color toners, yielding maximum density of color with the least amount of heat, should be used.

The above description discloses the preferred embodiments of the present invention. However, persons of ordinary skill in the art are capable of numerous modifications once taught these principles. Thus, by way of example and not of limitation, instead of using a multiple-layer form or multiple-sheet form with the lowermost layer usually serving heat insulation functions, this lowermost layer may be eliminated, with the adjacent roller being held at a somewhat cooler temperature and/or using thermal copier paper which has low thermal transmissive properties either by being relatively thick, or by the composition of the paper. It may also be noted that the multiple layers or sheets may be secured together only along a limited area or along one edge by the pressure-sensitive adhesive or in any other desirable manner.

Accordingly, it will be understood by those skilled in the art that changes in form and details may be made to the above-described embodiments without departing from the spirit and scope of the invention.