Optical and Colour Stability of Graphic Paperboards and Prints

<p>In the study we investigated the influence of surface treatment on permanence and stability of various graphic paperboard types - board for books, brochures, business cards, calendars, protection package - under the conditions of accelerated ageing with increased temperature and relative humidity in the absence of light according to ISO/5630/3, and as specified for material on paper/board of permanent value by EN/ISO 9706 (8). Paperboards were coated with various coating colours, and subsequently lab printed in offset technique. The effect of artificial ageing on surface properties as well as on image optical and colour stability was monitored.</p><p>The changes observed in surface characteristics such as smoothness, roughness, air permeability, absorptivity before and after the accelerated ageing process revealed a strong influence of coating components (pigments, binders) and surface treatment type. Increase in roughness, alteration of board absorption behaviour and change in optical and colour parameters - decrease in brightness, increase in yellowness, colour shift with samples containing higher amount of optical brightening agents - were the most pronounced results.</p><p>Measured differences in printing colour (dE) and optical density values indicated that colour stability of offset image approached the limit values, which are set for an image of still acceptable quality. Black (K) exhibited the highest stability among CMYK colours. Image colour stability significantly depends on surface and optical board characteristics, especially on gloss and optical brightener agent, as well as on board fibre composition and coating pigments.