Calendering Methods for Various Paper and Board Grades : Specialty Papers

Calendering Methods for Various Paper and Board Grades

Specialty Papers
6.9.4.3.1 Silicone Base Paper
Silicone base papers are used as carrier papers for the later application of a one-sided or two-sided layer of silicone, onto which, in the following operations, fur¬ther layers, e. g. adhesive in combination with label paper, are applied. Silicone base papers presuppose special quality features. These include, above all, high compaction, uniform caliper and moisture, high transparency, strength, good flat¬ness and a low surface roughness which ensures that the later application rates of the cost-intensive silicone are minimized. The basis weight of the silicone base paper is between 60 and 160 g m–2. As in the past, these papers are calendered off¬line on special supercalenders with at least 15 rolls. The resilient rolls consist of a cotton/linen mixture. The surface temperatures are around a maximum of 150 °C and the linear loads around 450 N mm-1. The operating speed is 600 m min–1. It is only in instances where not such high demands are made that modern off-line multi-nip calenders with synthetic-coated resilient rolls are taken into consideration.

6.9.4.3.2 Laminated Base Paper
Laminated base papers are highly absorbent papers for the production of layered materials saturated with synthetic resins. The basis weight of these papers with an extremely high percentage of fillers – up to 40 % – and correspondingly low strength, lies between 40 and 80 g m–2. Laminated base papers are very dry. Their moisture content is only 1–2 %. Characteristic of the production process of lami¬nated base papers is that the trim widths change frequently and to a great extent. The base papers must be opaque and require one-sided smoothing to Bekk smoothness values of 400–600 sec. For this purpose, 5-roll on-line multi-nip calen¬ders with a linear load of 300 N mm–1 are used. The roll surface temperatures of the heated rolls are around 150 °C. The resilient rolls are covered with synthetic materials. A steam blow box is also used.

6.9.4.4 Board
6.9.4.4.1 Uncoated Board
Uncoated boards include a large variety of sub-grades like test liner, kraft liner, white top liner, liquid container board etc. Some of these grades need no calender¬ing at all. Where calendering is required to achieve better surface properties, Yan¬kee cylinders and soft calenders have been used. As with coated grades, this tradi¬tional technology is about to be superseded by extended nip calendering – see Section 6.9.3.5.

6.9.4.4.2 Coated Board
Coated board grades range from one- to five-ply boards and can consist of virgin fibers and/or recycled fibers. The most important properties are high bulk, stiff¬ness and smoothness. The board is usually one-side coated. In some cases, how¬ever, it can also be two-sided.
Traditionally, calendering has been performed with Yankee cylinders or soft cal¬enders. In view of the bulk saving, this classical technology is increasingly being replaced by extended nip calendering – see Section 6.9.3.5.