184.108.40.206.2 Woodfree Coated Paper Grades
With these papers a differentiation must be made between single-coated, double-coated and triple-coated papers. In addition, it is also of significance whether the papers are to be used in rolls or as cut-size sheets.
Single-coated, woodfree papers are generally calendered on-line. The machines used for this are mostly 2 V 2-roll soft calenders. At speeds up to 1300 m min–1, Gardner gloss values of 55–65 % can be achieved. To insure good flatness – this applies above all to cut-size sheets – the papers must be subjected to only moderate linear loads.
Up to the mid-1990s, double or triple blade-coated woodfree high-gloss papers with coating per side of 20–30 g m–2 were calendered on supercalenders. Even today, they are still calendered off-line. However, 8–10-roll multi-nip calenders, whose resilient rolls are covered with synthetic material, have ousted the classic supercalenders. In contrast to conventional supercalenders, these new types can be operated at higher temperatures, i. e. temperatures of 100–130 °C. Conse¬quently, the desired calendering parameters – PPS-10S roughness 0.7–0.9 mm and Gardner gloss 75–85 % – can now be achieved at much higher speeds, around 700–1000 m min–1. A further increase in speed would theoretically be possible, but presupposes an increase in the linear load. With rising linear load, however, the flatness, which is particularly important for cut size paper, would be negatively affected. Linear loads of 200–300 N mm–1 have proven to be optimal.
Usually, these calenders are equipped with two cooling rolls.It goes without saying that in this premium segment there is, in addition to high-gloss paper, always a certain portion of matte or satin papers. Two different calendering processes exist for these. Either an on-line soft calender with two synthetic-covered rolls is built into the coater – by this soft/soft calendering the matte character of the paper is hardly changed, but the abrasion resistance is significantly improved – or the a. m. multi-roll off-line calender is used by cal¬endering the web in its uppermost and lowermost nips while the intermediate nips are “run through” – in this case both the gloss and smoothness of the web are increased.