Inhibition of light-induced colour reversion of wood-containing papers by means of coating

Abstract: The main purpose of this thesis was to find ways to maintain a low level of light?induceddiscolouration at an increased addition of mechanical and chemimechanical pulps in coated highqualityfine paper and magazine paper grades. Current technology allows the production of highyieldpulps such as thermomechanical and chemimechanical pulps with properties suitable formanufacturing high?quality paper or paperboard with a low basis weight. Coating of woodcontainingpaper will probably be necessary for photo?stability reasons if lignin?containing pulps areto be used as the main fibre furnish in long?life and high?value products.In order to find the most suitable pulp for this purpose, light?induced discolouration of a variety ofpaper samples from unbleached and bleached softwood and hardwood pulps was studied under bothaccelerated and long?term ambient light?induced ageing conditions. Hardwood high?yield pulps,especially aspen pulps, were proven to be more photo?stable compared to softwood pulps. Hardwoodpulps should therefore be the first choice for applications where a high permanence is desirable.Evaluating ageing characteristics using the CIELAB colour system showed that accelerated ageingconditions tend to mainly increase the b' value and decrease the L' value (i.e. yellow the pulp),whereas long?term ambient ageing also increases the a' value, which makes the pulp more reddish.A new method for studying the influence of the UV?screening properties of coating layers on abase paper was developed, and used to investigate the effect of pigment, pigment size distribution,binder and UV?absorbing additives. The coat weight and pigment type were found to be the mostimportant factors for reducing the transmittance of UV?radiation. Coating colours containing kaolinpigments had a lower UV?transmittance than calcium carbonate pigments. Of the calcium carbonates,precipitated calcium carbonates were better than ground calcium carbonates and the difference wasgreater at higher coat weights. The particle size distribution should preferable be narrow. When thebest pigment (bleached kaolin) and the best binder (styrene butadiene latex) were combined withtitanium dioxide, the UV?transmittance could be reduced by about 90% at a coat weight of ~10 g/m2.At a coat weight close to 20 g/m2, the transmittance was close to zero. This shows that it is possible tomore or less fully protect a double coated base paper from harmful UV?radiation, when the coatinglayer has an optimum composition for that purpose. A prerequisite to reach so far is that the coatinglayer has an even coat weight.