Thermographic imaging element

1. A thermographic substrate assembly comprised of a colorant, a flexible substrate, a thermosensitive layer comprised of a binder, a multiplicity of hollow sphere organic pigments, and a thermal solvent, wherein said thermographic substrate assembly also comprises a barrier layer, wherein said thermosensitive layer is disposed on said colorant, wherein said barrier layer is disposed on said thermosensitive layer, and wherein said barrier layer is comprised of a polymeric binder having a coating weight of from about 0.1 grams per square meter to about 4 grams per square meter.

2. The thermographic substrate assembly as recited in claim 1 , wherein said polymeric binder is a chlorine containing binder.

3. A process for producing visible printing on a thermographic imaging substrate assembly, wherein said thermographic imaging substrate assembly is comprised of a colorant, a flexible substrate, and a thermosensitive layer, wherein said thermosensitive layer is comprised of a binder, a multiplicity of hollow sphere organic pigments, and a thermal solvent, wherein said thermosensitive layer is disposed on said colorant, and wherein said thermal solvent has a Hildebrand solubility parameter from about 16 megapascals 1/2 to about 28 square megapascals 1/2 , comprising the steps of applying heat to a selected portion of said thermosensitive layer and inducing said selected portion of said thermosensitive layer to change from opaque to translucent, whereby a contrasting visual mark is produced.

4. The process as recited in claim 3 , wherein said thermosensitive layer is comprised of from about 10 to about 66 weight percent of said hollow sphere organic pigments.

5. The process as recited in claim 4 , wherein said thermosensitive layer is comprised of from about 25 to about 50 weight percent of said hollow sphere organic pigments.

6. The process as recited in claim 5 wherein from about 0.5 to about 1.5 parts of said thermal solvent are present in said thermosensitive layer for each part of said hollow sphere organic pigment.

7. A thermographic imaging substrate assembly that is comprised of a colorant, a flexible substrate, an opaque thermosensitive layer, and a heat resistant topcoating, wherein:

(a) said thermographic imaging substrate assembly has an unimaged color saturation of less than 0.5 Rd,

(b) said heat resistant top-coating is disposed above said opaque thermosensitive layer;

(c) said opaque thermosensitive layer is comprised of a binder, a multiplicity of hollow sphere organic pigments, and a thermal solvent,

(d) said hollow sphere organic pigments have an average particle size of from about 0.1 micron to about 2.0 microns,

(e) said thermal solvent has a Hildebrand solubility parameter of from about 16 megapascals 1/2 to about 28 square megapascals 1/2 ,

(f) said thermosensitive layer is disposed on said colorant, and

(g) when heat is applied to a selected portion of said thermosensitive layer, said selected portion changes from opaque to translucent, thereby producing a contrasting visual mark.

8. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments are comprised of synthetic organic polymers.

9. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments are comprised of an addition polymer.

10. The thermographic imaging substrate assembly as recited in claim 9 , wherein said addition polymer is comprised of a monoethylenically unsaturated monomer.

11. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments are comprised of polymers comprising acrylic polymers, styrenic polymers, vinyl polymers, and copolymers thereof.

12. The thermographic imaging substrate assembly as recited in claim 7 , wherein said thermal solvent is comprised of an aromatic moiety.

13. The thermographic imaging substrate assembly as recited in claim 7 , wherein said opaque thermosensitive layer is comprised of solid particles with a average particle size of less than 2 microns and a refractive index of from about 1.35 to about 1.65.

14. The thermographic imaging substrate assembly as recited in claim 7 , wherein said binder in said opaque thermosensitive layer is comprised of a polymer with a glass transition temperature less than about 50 degrees Celsius.

15. The thermographic imaging substrate assembly as recited in claim 7 , wherein said binder in said opaque thermosensitive layer is comprised of a polymer with a refractive index in the range from about 1.30 to about 1.7.

16. The thermographic imaging substrate assembly as recited in claim 7 , wherein said heat resistant topcoating is comprised of a binder, a lubricant, and an abrasive particle.

17. The thermographic imaging substrate assembly as recited in claim 7 , wherein said heat resistant topcoat is comprised of acrylic binder, lubricant and an abrasive particle.

18. The thermographic imaging substrate assembly as recited in claim 17 , wherein said lubricant is comprised of a silicone compound.

19. The thermographic imaging substrate assembly as recited in claim 17 , wherein said lubricant is comprised of the salt of a fatty acid.

20. The thermographic imaging substrate assembly as recited in claim 19 , wherein said salt of a fatty acid is a salt of stearic acid.

21. The thermographic imaging substrate assembly as recited in claim 17 , wherein said abrasive particle is selected from the group consisting of inorganic particles and organic particles.

22. The thermographic imaging substrate assembly as recited in claim 17 , wherein said abrasive particle has a Mohs hardness of less than about 7.

23. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments have a wall thickness of from about 5 nanometers to about 1 micron.

24. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments have a wall thickness of from about 50 to about 500 nanometers.

25. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments have an average particle size of from about 0.1 to 10 microns.

26. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere polymer pigments have a glass transition temperature greater than 50 degrees Celsius.

27. The thermographic imaging substrate assembly as recited in claim 7 , wherein said hollow sphere organic pigments have a dry density of less than 0.9 grams per cubic centimeter.

28. The thermographic imaging substrate assembly as recited in claim 7 , wherein said opaque thermosensitive layer is comprised of from about 10 to about 66 weight percent of said hollow sphere organic pigments.

29. The thermographic imaging substrate assembly as recited in claim 7 , wherein said opaque thermosensitive layer is comprised of from about 25 to about 50 weight percent of said hollow sphere organic pigments.

30. The thermographic imaging substrate assembly as recited in claim 7 , wherein from about 0.5 to about 1.5 parts by weight of said thermal solvent are present in said opaque thermosensitive layer for each part of said hollow sphere organic pigment.

31. The thermographic imaging substrate assembly as recited in claim 30 , wherein said thermal solvent is an emulsified liquid.

32. The thermographic imaging substrate assembly as recited in claim 30 , wherein said thermal solvent is a liquid thermal solvent that is dissolved in a binder.

33. The thermographic imaging substrate assembly as recited in claim 7 , wherein said thermal solvent is selected from the group consisting of dibenzyl oxalate, propylene carbonate, benzyl alcohol, triethylene glycol, triethylene glycol, dipropylene glycol, dibutyl phthalate, carnauba wax, 1,2-bis(3-methylphenoxy)ethane, polar waxes, and mixtures thereof.

34. The thermographic imaging substrate assembly as recited in claim 7 , wherein said opaque thermosensitive layer is comprised of from about 5 to about 50 weight percent of said thermal solvent.