Wood-containing Paper Grades

For cold offset printing, Bendtsen roughnesses of the order of 150-200 ml min are required. This roughness level can easily be achieved on a two-roll online soft calender. The linear loads here are between 40 and 100 N mm . The calendering temperature corresponds approximately to the web temperature. 

For heat offset printing, lower roughness is required, i.e. Bendtsen values of approximately 100 ml min . To meet this requirement, on line soft calenders with a total of two nips are used, so that each side of the web contacts a heated roll once. With a suitable layout - linear load, roll surface temperature - even rotogravure-capable SC-B papers (see below) can be produced 

SC papers can be subdivided into two main grades, SC-B with a high percentage of recovered paper (up to 100%) and SC-A with a high percentage ofwoodpulp (TMP or groundwood up to 80%). 

LWC papers with a typical basis weight of 50-70 g m , which are bIade oated on off-line coating machines, are still calendered off-line today. The extreme demands made on the smoothness of these papers provided with coating of only 6-8 g m per side - the PPS-IOS roughness values are around 0.7-0.8 pm - have until now prevented on line calendering at high speed. For the off-line calendering 

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Paper grades that contain pulp produced by mechanical or chemimechanical processes are referred to as wood-containing papers. Mechanical pulps are mainly used in short-Hfe printing papers, hygienic papers and board. 

This is also predominantly applied in wood-free uncoated paper, wood-containing uncoated paper (in combination -with kaolin), wood-free coating base paper, directory grades and white top linerboard [8]. In order to control porosity and burning rate, PCC is also widely used as a filler in the manufacture of cigarette paper. 

Table 4.2 Quality parameters of European ONP/OMG mix and DIP for different wood-containing graphical paper grades.

Charcoal is generally satisfactorily activated by heating gently to red heat in a crucible or quartz beaker in a muffle furnace, finally allowing to cool under an inert atmosphere in a desiccator. Good commercial activated charcoal is made from wood, e.g. Norit (from Birch wood), Darco and Nuchar. If the cost is important then the cheaper animal charcoal (bone charcoal) can be used. However, this charcoal contains calcium phosphate and other calcium salts and cannot be used with acidic materials. In this case the charcoal is boiled with dilute hydrochloric acid (1 1 by volume) for 2-3h, diluted with distilled water and filtered through a fine grade paper on a Buchner flask, washed with distilled water until the filtrate is almost neutral, and dried first in air then in a vacuum, and activated as above. To improve the porosity, charcoal columns are usually prepared in admixture with diatomaceous earth. 

Industrial Papers. The industrial papers comprise one of the largest uses of paper products worldwide. Industrial papers are not subject to the refining, bleaching, and other finishing treatments required for finer papers. Instead, some industrial grade papers may actually contain small wood fibers among the cellulose. This class of papers is normally used for packaging material and utility applications that do not call for a more refined paper. 

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