Thermoresponsive paper coatings based on cellulose derivatives

Abstract

The present invention relates to a heat-sensitive recording material comprising a carrier substrate, which is black or coloured on at least one side, and a thermoresponsive layer on the at least one black or coloured side of the carrier substrate, wherein the thermoresponsive layer comprises nanoparticles of at least one cellulose ester, and to a method for producing this material, and to a heat-sensitive recording material that can be obtained by this method.

Claims

1. Heat-sensitive recording material comprising a carrier substrate which is black or coloured on at least one side, and a thermoresponsive layer on the at least one black or coloured side of the carrier substrate, wherein the thermoresponsive layer comprises nanoparticles of at least one cellulose ester comprising cellulose acetate butyrate, and wherein the nanoparticles have a number-average particle size of 50 to 400 nm, measured by means of dynamic light scattering (DLS).

2. Heat-sensitive recording material according to claim 1, characterised in that the thermoresponsive layer has a transparency, measured according to DIN 53147:1993-01, of less than 35%.

3. Heat-sensitive recording material according to claim 1, characterised in that the at least one cellulose ester further comprises cellulose acetate, cellulose acetate propionate, and/or cellulose butyrate.

4. Heat-sensitive recording material according to claim 1, characterised in that the at least one cellulose ester has a Tg of 45? C. to 150? C. and/or a Tm of 100? C. to 185? C. determined according to DIN 53765:1994-03.

5. Heat-sensitive recording material according to claim 1, characterised in that the at least one cellulose ester is contained in the thermoresponsive layer in an amount of 35 to 70% by weight in relation to the total weight of the thermoresponsive layer.

6. Heat-sensitive recording material according to claim 1, characterised in that the thermoresponsive layer additionally comprises polyvinyl alcohol.

7. Heat-sensitive recording material according to claim 6, characterised in that the thermoresponsive layer comprises 5 to 50% by weight of the polyvinyl alcohol in relation to the total weight of the thermoresponsive layer.

8. Heat-sensitive recording material according to claim 1, characterised in that the thermoresponsive layer additionally comprises at least one of a kaolin, or an alkali and/or alkaline earth salt.

9. Heat-sensitive recording material according to claim 1, characterised in that the thermoresponsive layer additionally comprises a poly(vinylamine-vinylformamide) copolymer or a cationic poly-acrylamide polyelectrolyte.

10. Heat-sensitive recording material according to claim 9, characterised in that the poly(vinylamine-vinylformamide) copolymer or the cationic poly-acrylamide is in an amount of 5 to 35% by weight in relation to the total weight of the thermoresponsive layer.

11. Heat-sensitive recording material according to claim 1, characterised in that the carrier substrate comprises paper, synthetic paper and/or a plastics film.

12. Heat-sensitive recording material according to claim 1, characterised in that the thermoresponsive layer comprises 0.05 to 10% by weight of at least one of a kaolin, or an alkali and/or alkaline earth salt in relation to the total weight of the thermoresponsive layer.

13. Heat-sensitive recording material according to claim 12, characterised in that the alkali and/or alkaline earth salt is selected from NaCl, CaCO.sub.3 and/or CaCl.sub.2.

14. Method for producing a heat-sensitive recording material comprising applying an aqueous suspension having a solids content of 15 to 65% by weight and containing nanoparticles of at least one cellulose ester comprising cellulose acetate butyrate to a black or coloured side of a support substrate, the aqueous suspension being applied and dried according to coating methods which produce a contour coating or a levelling coating, and wherein the nanoparticles have a number-average particle size of 50 to 400 nm, measured by means of dynamic light scattering (DLS).

15. Heat-sensitive recording material obtained by the method according to claim 14.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows images recorded by light microscopy of a printed heat-sensitive recording material according to the invention.

(2) Top: laser power 80%

(3) Bottom: laser power 70%

(4) Left: Without thermal treatment

(5) Right: With thermal treatment (30 min at 70? C.)

(6) FIG. 2 shows an illustration of the opacities. These are the grey values of a horizontal line. The laser power was 70%. The grey value is a value between 0 and 255, where 255 reflects a completely black pixel and 0 reflects a completely white pixel.

(7) The invention will be explained in detail below using unrestricted examples.

EXAMPLES

Formulation 1

(8) An aqueous coating suspension was prepared by mixing 100 parts of nanoparticies of cellulose acetate butyrate having an average particle diameter of about 170 nm (?40 nm), which were precipitated as described above in the presence of 0.1% polyvinyl alcohol, with THF as solvent and a water/isopropanol mixture in a ratio of 1.2 to 2.8 as non-solvent, with 40 parts polyvinyl alcohol, 5 parts Styronal D 517 as binder, 15 parts Sterocoll as viscosity regulator and 3 parts 1M NaOH.

(9) For the coating formulation, a ratio of 11.75 weight % solid/liquid was chosen. This value was chosen because, after production, the particles were present as a ?15% by weight suspension. The solids contents of the additives and coatings were determined with a dry balance. The polyvinyl alcohol used was polyvinyl acetate saponified to 84% (Mn 100,000 g/mol). In a typical test formulation, the sample vessels were filled with 100 mg nanoparticles of cellulose acetate butyrate, the respective additives were added and the solids content (SC) was adjusted to 11.75% by weight with distilled water. The formulation was then homogenised using a vortex shaker and ultrasonic bath. The coating was applied with the help of an automatic film applicator from BYK Additives & Instruments to a Hostaphan film type RNK 50.0 2600, pre-coated for line application. 100 mm min.sup.-1 was selected as the feed speed, and 90 ?m as the squeegee gap.

(10) Heat-sensitive recording materials were produced, with the application rate of the thermoresponsive layer being 2.5, 4 and 6 g/m.sup.2.

(11) After the paper coatings were prepared and dried at room temperature, the coated substrates were cut in half with scissors. One half of a substrate was placed for 30 min in a drying oven at 70? C. to simulate simple drying conditions. Then, both samples were printed with a 30-watt CO.sub.2 laser (parameters in Tab. 1).

(12) Here, 10 different amounts of energy (0.43-4.3 mJ/mm.sup.2) were deposited, and with each amount of energy 12 lines were written (printed) into the coating.

(13) These two samples were examined in more detail using a light microscope.

(14) A light microscope in transmitted light mode was used to analyse the prints.

(15) The evaluation was carried out with the Open Source image analysis program Im-ageJ. The brightness was adjusted in such a way that the brightest areas did not overload the sensor. Based on the grey values, relative opacities between the melted and untreated areas could be calculated.

(16) TABLE-US-00001 TABLE 1 CO.sub.2 laser printing parameters Parameters Laser settings Line spacing [mm] 0.35 Height [mm] 4.2 Width [mm] 10.33 Power [%] 10-100 Deposited energy [mJ/mm.sup.2] 0.43-4.3 Frequency [Hz] 100 Speed [mm/s] 20,000 Print time [ms]. 803

(17) Under the light microscope, the paper coatings of formulation 1 showed promising results, as can be seen in FIG. 1. As already noted in the coating formulation, the patterns are macroscopically very homogeneous. With a laser power of 70%, sharp fine profiles could already be seen. Increasing the laser power to 80% reduces the distance between the individual lines.

(18) FIG. 2 serves to illustrate the opacities. These are the grey values of a horizontal line. The relative opacity of PVA coatings reached peak values of up to 95%. The thermal treatment showed no negative influence.