Durability of Coated and Uncoated Boards

摘要

The investigation of the influence of accelerated ageing upon the chemical stability, mechanical resistance, surface and optical stability of the boards can be summarised as follows: 1. None of the investigated boards reached the quality standard for permanence recommended by EN/ISO 9706. 2. The best results were obtained for samples 3 and 4 (coated boards) produced from high-quality bleached fibre, but these samples contain optical brighteners in the surface coating which is not recommended for permanent and durable paper/board with lasting value. 3. All the boards were produced under neutral or alkaline conditions with CaCO_3 as a filler or coating pigment. In general, as a result of the ageing, the pH decreased slightly, but it still remained in the alkaline range. 4. The degree of polymerisation (DP) as indicated by the CED viscosity is lower for coated than for uncoated boards - after ageing, the DP is lower. The retention of DP is relatively good for coated boards produced from recycled and CTMP fibres, but the degradation by oxidation indicated by the Cu-number is very high. 5. The decrease in folding endurance on ageing indicates that the board has become more brittle. The tearing resistance was practically the same before and after ageing, and the internal bond strength increased in the case of boards produced from virgin cellulose fibres. 6. The retention of chemical, mechanical and optical properties depends mainly upon the board composition and on the type of coating. Low chemical and mechanical stability are related to a high content of CTMP and recyc ed fibres with a high content of lignin and hemicellulose. 7. During ageing, the surface roughness of the coated and uncoated boards increased. Aged coated boards have a more hydrophobic surface, which is still more favourable because it reduces the possibility of the action of external factors on the paper surface. 8. The changes in optical properties of the investigated boards were relatively large - the brightness decreased and the yellowness increased, due both to ayellowing of the board and to a degradation of the optical brighteners. In conclusion, we can say that the use of recycled fibres of undefined origin and of mechanical pulp fibres is adequate and recommendable for printing papers/boards whose estimated lifetime does not exceed a few years. In the case of printed materials of durable value, on the other hand, high-quality primary pulp fibres must be used in a neutral medium in the absence of optical bleaching agents and with calcium carbonate as alkaline buffer reserve.