The Effect of Accelerated Ageing on Graphic Paperboards Degradation
| Main Authors: | , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0034-5806 |
| Online Access: | http://www.viks.sk/chk/res_2_02_118_132.doc |
| Abstract: | CONCLUSIONS. The investigation of the influence of accelerated ageing upon the chemical stability, mechanical resistance, surface and optical stability of the boards can be summarises as follows: ˙ None of the investigated boards reached the quality standard for permanence recommended by EN/ISO 9706. ˙ The best results were obtained for samples 3 and 4 (matt and gloss coated boards) produced from high quality bleached fibre, but these samples contain optical brighteners in the surface coating which are not recommended for permanent and durable paper and board. ˙ Changes in lignin and hemicelluloses content during accelerated ageing indicated quite high degradation especially for coated and uncoated samples of board with CTMP and recycled fibre. ˙ All the boards were produced under neutral or alkaline conditions with CaCO as a filler or coating pigment. In general, as a result of ageing, the pH decreased slightly, but it still remained in the alkaline range. ˙ The degree of polymerization was lower for the coated than for the uncoated boards, both before and after accelerated ageing. The retention of DP is relatively good for coated boards produced from recycled and CTMP fibre, but degradation by oxidation indicated by the Cu number is very high ˙ The decrease in folding endurance during ageing indicates that the boards became somewhat 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. ˙ 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 recycled fibre with a high content of lignin and hemicelluloses. ˙ During ageing, the surface roughness of the coated and uncoated boards increased. Aged coated boards have a more hydrophobic surface, which may be considered as being favourable for practical use. ˙ The changes in optical properties of the investigated boards were relatively high. Brightness decreased and the yellowness increased, both due to chemical changes in the fibre, the coating surface and to 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, which are meant to last, on the other hand, high quality primary pulp fibres must be used in a neutral medium with calcium carbonate as an alkaline reserve and without optical brightness. SUMMARIES. Paper and board, now and most probably also in the future the most important carriers of written and printed cultural heritage, can be of very different permanence and durability, depending on their composition (fibres, fillers, sizing agents, dyestuffs, pigments) and on the technological conditions of production. We studied coated and uncoated boards from various producers and of the same grammage, (ca. 250 g/m2). The samples were submitted to accelerated ageing according to ISO 5630/3. From testing several chemical, mechanical, optical and surface qualities it became obvious that their retention depended mainly upon fibre composition and type of coating pigments. Low chemical and mechanical stability is proportional to the content of thermomechanical pulp (TMP), chemo-thermo-mechanical pulp (CTMP) and recycled fibres. The changes of surface properties are strongly influenced by the type of coating (binders, pigments). After ageing the surface roughness increased while absorbency both increased and decreased. The changes in optical properties of the boards were quite high; brightness decreased while yellowness increased, both due to chemical changes in the fibre composition and the surface coating and due to the degradation of optical brighteners. |
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| ISSN: | ISSN 0034-5806 |
