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1. A method of manufacturing a substrate with an embedded, UV-visible pattern, the method comprising the following steps: a) providing an uncoated substrate comprising at least one optical brightener and a filler, wherein the filler comprises from 0 to 60 wt.-% of a salifiable alkaline or alkaline earth compound, based on the total weight of the substrate, b) providing a liquid treatment composition comprising at least one acid, and c) applying the liquid treatment composition onto at least one region of the substrate in form of a preselected pattern to form an embedded, UV-visible pattern.

2. The method of claim 1 , wherein the filler comprises the salifiable alkaline or alkaline earth compound in an amount of at least 1 wt. %, based on the total weight of the substrate.

3. The method of claim 1 , wherein the optical brightener is present in an amount of at least 0.001 wt. %, based on the total weight of the substrate.

4. The method of claim 1 , wherein the optical brightener is selected from the group consisting of stilbene derivates, pyrazolin derivates, cumarin derivates, benzoxazol derivates, naphthalimide derivates, pyrene derivates, derivatives of diaminostilbenedisulfonic acid, derivatives of diaminostilbenetetrasulfonic acid, derivatives of diaminostilbenehexasulfonic acid, 4,4′-diamino-2,2′-stilbenedisulfonic acid, 4 4′-bis(benzoxazolyl)-cis-stilbene, 2 5-bis(benzoxazol-2-yl)thiophene, 5-[(4-anilino-6-methoxy-1,3,5-triazin-2-yl)amino]-2-[(E)-2-[4-[(4-anilino-6-methoxy-1,3,5-triazin-2-yl)amino]-2-sulfonatophenyl]ethenyl]benzenesulfonate (leucophor PC), and mixtures thereof.

5. The method of claim 1 , wherein the substrate is selected from the group consisting of paper, cardboard, containerboard, or plastic.

6. The method of claim 1 , wherein the salifiable alkaline or alkaline earth compound is an alkaline or alkaline earth oxide, an alkaline or alkaline earth hydroxide, an alkaline or alkaline earth alkoxide, an alkaline or alkaline earth methylcarbonate, an alkaline or alkaline earth hydroxycarbonate, an alkaline or alkaline earth bicarbonate, an alkaline or alkaline earth carbonate, lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, calcium magnesium carbonate, calcium carbonate, is a ground calcium carbonate, a precipitated calcium carbonate, a surface-treated calcium carbonate, or mixtures thereof.

7. The method of claim 1 , wherein the salifiable alkaline or alkaline earth compound is in form of particles having a weight median particle size d 50 from 15 nm to 200 μm.

8. The method of claim 1 , wherein the at least one acid is selected from the group consisting of hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, citric acid, oxalic acid, acetic acid, formic acid, sulphamic acid, tartaric acid, phytic acid, boric acid, succinic acid, suberic acid, benzoic acid, adipic acid, pimelic acid, azelaic acid, sebaic acid, isocitric acid, aconitic acid, propane-1,2,3-tricarboxylic acid, trimesic acid, glycolic acid, lactic acid, mandelic acid, acidic organosulfur compounds, acidic organophosphorus compounds, HSO 4 − , H 2 PO 4 − , HPO 4 2− , being at least partially neutralized by a corresponding cation selected from Li, Na + , K + , Mg 2+ or Ca 2+ , and mixtures thereof.

9. The method of claim 1 , wherein the liquid treatment composition further comprises a fluorescent dye, a phosphorescent dye, an ultraviolet absorbing dye, a near infrared absorbing dye, a thermochromic dye, a halochromic dye, metal ions, transition metal ions, lanthanides, actinides, magnetic particles, quantum dots, or a mixture thereof.

10. The method of claim 1 , wherein the liquid treatment composition comprises the acid in an amount from 0.1 to 100 wt.-%, based on the total weight of the liquid treatment composition.

11. The method of claim 1 , wherein the preselected pattern is a continuous layer, a pattern, a pattern of repetitive elements, a repetitive combination(s) of elements, a one-dimensional bar code, a two-dimensional bar code, a three-dimensional bar code, a QR-code, a dot matrix code, a security mark, a number, a letter, an alphanumeric symbol, a logo, an image, a shape, a signature, a design, or a combination thereof.

12. The method of claim 1 , wherein the liquid treatment composition is applied by spray coating, inkjet printing, offset printing, flexographic printing, screen printing, plotting, contact stamping, rotogravure printing, spin coating, reverse (counter-rotating) gravure coating, slot coating, curtain coating, slide bed coating, film press, metered film press, blade coating, brush coating, stamping and/or a pencil.

13. The method of claim 1 , wherein the method further comprises a step d) of applying a protective layer above the embedded, UV-visible pattern.

14. A substrate comprising an embedded, UV-visible pattern, obtainable by a method according to claim 1 .

15. A product comprising a substrate according to claim 14 , wherein the product is a branded product, a security document, a non-secure document, a decorative product, a perfume, a drug, a tobacco product, an alcoholic drug, a bottle, a garment, a packaging, a container, a sporting good, a toy, a game, a mobile phone, a compact disc (CD), a digital video disc (DVD), a blue ray disc, a machine, a tool, a car part, a sticker, a label, a tag, a poster, a passport, a driving licence, a bank card, a credit card, a bond, a ticket, a postage or tax stamp, a banknote, a certificate, a brand authentication tag, a business card, a greeting card, a voucher, a tax banderol, or a wall paper.

16. The substrate of claim 14 , wherein the substrate is suitable for use in security applications, in overt security elements, in covert security elements, in brand protection, in microlettering, in micro imaging, in decorative applications, in artistic applications, in visual applications, in packaging applications, or in track and trace applications.

17. The method of claim 1 , wherein the salifiable alkaline or alkaline earth compound is a precipitated calcium carbonate.

18. The method of claim 1 , wherein the salifiable alkaline or alkaline earth compound is in form of particles having a weight median particle size d 50 from 100 nm to 10 μm.

19. The method of claim 1 , wherein the at least one acid is selected from the group consisting of sulphuric acid, phosphoric acid, boric acid, suberic acid, sulphamic acid, tartaric acid, and mixtures thereof.

20. The method of claim 1 , wherein the at least one acid is phosphoric acid and/or sulphuric acid.