Surface chemical behaviour

University dissertation from Institutionen för pappers- och massateknologi

Author: Marie Wickman; Kth.; [1998]

Keywords: ;

Abstract: The objective of the investigations included in this thesisis to show how the properties of alkyd resins, used as bindersin offset printing inks, affect the ink in terms of pigmentdispersion, emulsification of fountain solution, and inksetting.The interactions between alkyd resins and aqueous solutionshave been investigated by interfacial tension measurementsusing the pendant drop technique and by measurement of dropletsize using light diffraction. The interfacial tension dependsboth on the alkyd resin used and the surface active additivesin the fountain solution. Alkyd resins are surface active,acting as emulsifiers to stabilise the emulsion. Similar to oilsoluble surfactants, small amounts of alkyd resin are needed todecrease the interfacial tension substantially. The interfacialtension and droplet size show a strong dependence of hydroxylicand acidic value of the alkyd resins. Polar alkyd resins givelower interfacial tension and smaller droplets.Adsorption of alkyd resin onto pigment surfaces weredetermined by the solution depletion technique. The adsorptionof alkyd resins onto pigment surfaces must be discussed interms of adsorption of polydisperse polymers and the adsorbedamount depends on the total area available for adsorption. Theadsorbed amount increases with alkyd resin polydispersity andhydroxylic value but decreases with increasing oil length. Anempirical equation to determine adsorption of alkyd resins ontocalcium carbonate is presented. This equation may also beuseful to predict differences in adsorbed amount of alkyd resinon other surfaces.When adsorption of alkyd resins onto different pigmentsurfaces are compared, the porosity of the pigments must beconsidered. Larger molecules are unable to penetrate into thesmall pores. Alkyd resins with low molecular mass do notdiscriminate between different surface areas whereas with highmolecular mass alkyd resins the adsorbed amount decreases withincreasing surface area. It is also shown that the adsorbedamount decreases when coating pigments have been pre-treatedwith dispersants.It is believed that alkyd resinshaving strong interactionswith coating pigments would give printing inks with lessset-off and rub-off problems. A method for the directmeasurement of ink setting on coated papers is presented. Theresults show that alkyd resins with strong interactions give abetter phase separation during setting and remain on the papersurface, while alkyd resins with weaker interactions accompanythe oil into the paper.KEYWORDS:Adsorption, Alkyd resins, Binders, Calciumcarbonate, Carbon black, Droplet size, Emulsion, FTIR, Fountainsolution, Interfacial tension, Ink formulation, Ink properties,Ink setting, Offset, Printing ink, Surface tension

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