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Summary:
Change in pH of water based inks and varnishes upon their contact with paper were investigated. Very thin films of the inks and varnishes were applied on the paper surface and their pH was followed within the first second of the contact. A mechanism responsible for acid-base reactions at the ink/paper interface and slightly beyond thereof was proposed. The impact of acid-base interactions on ink spreading on the paper surface was investigated using dynamic contact angle measurements. Properties of surface carboxylic groups of water based polymers were studied using a contact angle titration method and polymer solution/dispersion pH stability profiles. The acid-base reactions appeared to be very fast processes controlled by the rate of water penetration into the bulk of the paper and acidic ion diffusion from the paper into the ink/varnish film. However, the impact of these interactions on ink spreading on the paper surface was found to be secondary to the dynamic surface tension role. It was proposed that pH stability profiles of inks and varnishes could be useful in formulating ink for a given paper.