MECHANICAL PULPING: Changes in extractives and wetting properties of TMP paper during ageing

TMP-based newsprint paper was submitted to accelerated ageing at 60 degrees C for up to 7 days, natural ageing at room temperature for a year, and storage in a freezer for two years. Already after 2 days of accelerated ageing, only 20% of the original lipophilic extractives could be extracted with acetone and analysed by gas chromatography. The contact angle with water increased remarkably during accelerated ageing, already within 2 h, indicating a rapid migration of lipophilic extractives to the surface of the fibres. After this, the contact angle continued to rise slightly, and the contact angle remained high even after extraction of the paper with acetone. During this later phase the extractives are oxidised and further also polymerised, thus preventing their extraction. Similar changes were observed also in paper stored at room temperature (natural ageing), and even in paper stored in a freezer. For pre-extracted paper, the contact angle decreased, verifying that the extractives play the key role in the so-called self-sizing of paper during storage. To manage and control the surface and printing properties of paper, appropriate attention should be paid to the amount and composition of extractives and to the storage conditions, especially for paper made from mechanical pulps.