Printing ink or overprint varnish with renewable binder component

Abstract

The present invention is related to a printing ink, in particular flexographic printing ink, or an overprint varnish, comprising a nitrated lignin as at least one binder component, to a process for preparing the same as well as to the use of a nitrated lignin as a binder component in a printing ink, in particular a flexographic printing ink, or an overprint varnish.

Claims

1. Printing ink, comprising a nitrated lignin as at least one binder component.

2. Printing ink according to claim 1, wherein the nitrated lignin is obtained by nitration of alkaline lignin with nitric acid at a temperature not exceeding 28 C. in the presence of a solvent which is immiscible with water and inert to nitric acid.

3. Printing ink according to claim 2, wherein the alkaline lignin is obtained by subjecting lignin to a chemo-thermomechanical treatment involving mechanical shear at a maximum temperature of about 100-220 C. and a pressure of about 0,5 to 10 atmospheres in the presence of an additive which lowers the softening point of the lignin.

4. Printing ink according to claim 1, wherein the ink comprises at least one additional binder selected from the group consisting of polyurethane resins, ketonic resins or polyamide resins.

5. Printing ink according to claim 1, wherein the ink additionally comprises a colorant.

6. Printing ink according to claim 1, wherein the ink additionally comprises a solvent and optionally additives.

7. Overprint varnish, comprising a nitrated lignin as at least one binder component.

8. Overprint varnish according to claim 7, wherein the nitrated lignin is obtained by nitration of alkaline lignin with nitric acid at a temperature not exceeding 28 C. in the presence of a solvent which is immiscible with water and inert to nitric acid.

9. Overprint varnish according to claim 8, wherein the alkaline lignin is obtained by subjecting lignin to a chemo-thermomechanical treatment involving mechanical shear at a maximum temperature of about 100-220 C. and a pressure of about 0,5 to 10 atmospheres in the presence of an additive which lowers the softening point of the lignin.

10. Overprint varnish according to claim 7, wherein the ink comprises at least one additional binder selected from the group consisting of polyurethane resins, ketonic resins or polyamide resins.

11. Overprint varnish according to claim 7, wherein the ink additionally comprises a solvent and optionally additives.

12. Process for preparing a printing ink or an overprint varnish, comprising the steps of: preparing a nitrated lignin by nitrating alkaline lignin and dissolving said nitrated lignin in acetone to obtain a 50% solution in acetone mixing the solution of nitrated lignin in acetone with one or more components selected from the group consisting of a colorant, at least one solvent, and optional additives.

13. Process according to claim 12, wherein the nitrated lignin is prepared by suspending alkaline lignin in a water-immiscible organic liquid which is inert to nitric acid, and reacting the lignin with nitric acid at a temperature not exceeding 28 C. for 2 to 8 h.

14. Process according to claim 12, wherein after completion of the reaction the nitrated lignin is washed with water and with CC14.

15. Method of use of a nitrated lignin for preparing a printing ink according to claim 1, comprising a step of applying said printing ink or overprint varnish onto a substrate.

16. Method of use of a nitrated lignin for preparing an over-print varnish according to claim 7, comprising a step of applying said printing ink or overprint varnish onto a substrate.

Description

EXAMPLE 1

Synthesis of Nitrated Lignin

(1) Nitrated lignin was prepared according to the method described in the example of GB-866,968. 50 gram of lignin was dispersed in 250 ml of carbon tetrachloride (CCl.sub.4). The dispersion was cooled with water, while a mixture of 40 ml of concentrated nitric acid and 125 ml of CCl.sub.4 was carefully added in small portions under constant stirring into a double layered reactor vessel. In order to avoid any oxidative degradation, the temperature was not allowed to rise beyond 28 C. After 6 hours the products were filtered and washed with water followed by CCl.sub.4 to remove acid and impurities and dried under vacuum. It should be noted that in the example of GB-866,968, the reaction was only conducted for 2 hours. In order to ensure complete nitration, the reaction time was prolonged. Also, in the example of GB-866,968, the reaction product was only washed with CCl.sub.4. This was not found sufficient for obtaining a pure product. Thus, an additional step of washing the product with water was included. During washing with water, it was seen that some portions of the product were soluble in water.

(2) The nitrated lignin was found to be soluble in acetone and partially soluble in alcohol. However, after keeping the solid nitrated lignin for several days, the acetone solubility had decreased. So in order to keep the solubility of the nitrated lignin constant, a 50% acetone solution of the nitrated lignin was prepared. This solution was used throughout the ink following examples.

EXAMPLE 2

Over-print Varnish with Nitrated Lignin

(3) An over-print varnish was prepared as follows: 54,6 wt.-% of nitrated lignin according to example 1 (as a 50% solution acetone) was mixed with 24,4 wt.-% ethanol, 6 wt.-% ethyl acetate and 15 wt.-% n-propyl acetate. 77 wt.-% of the resulting nitro lignin solution were mixed with 7,7 wt.-% of a polyurethane resin (Surkopak 5244), 4,6 wt.-% ethanol, 7,8 wt.-% ethoxy propanol, 1,4 wt.-% of wax (Luwax AF-31 from BASF, a micronized polyethylene wax), and 1 wt.-% of an adhesion promoter (Vertec PI-2 from Johnson Mathey; this is a titanate coupling agent (Ethoxy isopropoxy titanium bis(2, 4-pentanedionate)).

COMPARATIVE EXAMPLE 1

Over-print Varnish with Nitrocellulose

(4) For comparison, an over-print varnish was prepared as described in example 2 with the exception that instead of 77,5 wt.-% of the nitro lignin solution, 77,5 wt.-% of a solution consisting of 39 wt.-% nitrocellulose (SS , IPA damped) 40 wt.-% ethanol, 6 wt.-% ethyl acetate and 15 wt.-% n-propyl acetate was used.

EXAMPLE 3

Heat Resistance Test

(5) Aluminium foil was used as a substrate. On said substrate, a conventional ink formulation (Access-Foil from Siegwerk) was applied.

(6) Onto the conventional ink layer, alternatively the over-print varnish from example 2 and the over-print varnish from comparative example 1 were printed. The resulting product was heat treated. The product comprising the over-print varnish from comparative example 1 was found to be stable up to 200 C. However, product comprising the over-print varnish from example 2 was found to be stable up to 250 C. Thus, the over-print varnish on the basis of a nitrated lignin was much more heat resistant than a conventional over-print varnish on the basis of nitrocellulose as main binder component.