Solutions for high quality printing papers
Coated woodfree (Coated Freesheet) papers grades are medium to heavy weight papers produced from chemical pulp. Typical paper weights range from 90-300 gsm. These papers are generally the highest quality printing papers available. They are very smooth, usually have a blue-white shade and can produce very high resolution prints.
Most commonly these papers are offset printed using sheet fed or heat set presses. Coated Woodfree grades are mainly used for commercial printing applications such as company brochures, annual reports and other higher quality advertising, but high end magazines and books and magazine covers are also a major end use.
In Europe coated woodfree grades are classified by the finished paper weight , gloss level (matt, silk or gloss) and printing type - heat set offset (reels) or sheet fed offset (sheets). The highest qualities are often referred to as “Art” grades. In North America classifications were traditionally based on brightness with #3, #2 and #1 being classed as coated freesheet. In general brightness, grammage and quality increase in moving from #3 to #2 to #1 grades, but #3 is by far the most common grade. In Japan grades are classified on the basis of coat weight rather than finished weight. Coated woodfree grades are typically A1, A2 and A3 grades with A1 being higher quality and coat weight. Some of the very light Bitoko grades are also based on chemical fibre. In the rest of Asia, coated woodfree can mean anything from light weight coated (LWC) grades to higher “Art” qualities, but finished weight remains a key classification.
Much of the coated woodfree produced globally is multi-coated with two or three coating layers applied to each side of the paper. For multilayer coating, coverage and performance is built up in layers. The role of the precoat and mid coat layers is principally to provide physical coverage of the base and optical performance, while the role of the topcoat is to produce the desired surface finish (gloss, silk or matt) and printability. However, single coating is also common in this sector, particularly in North America, where the majority of coated freesheet is single coated. For this reason pigment selection for North American coated freesheet more closely resembles that used in heavier weight single coated mechanical grades than it does other multi-coated woodfree.
Precoats and Midcoats
For precoats and midcoats the pigment choice is usually 100% coarse or engineered carbonate such as Carbital 60, Carbital 75 or Carbopaque 90. Kaolins can be used to further improve coverage and performance on more demanding basestocks and here products such as Capim RG, Capim NP, Astra-Plate and the Contour clays can be used to good effect at 20-30% addition in the coating pigment recipe. Use of kaolin in woodfree precoating is most common in Japan.
In topcoating coating formulations are generally based on high carbonate levels, but the choice of pigments will depend on the surface finish required. For glossy paper the requirement for sheet gloss development is the key driver for pigment selection and because coverage is less critical, pigments tend to be finer than those used in single coating. For commodity coated woodfree applications, Carbital 90 and Carbital 95 are typical topcoating carbonates which would be used together with low levels of fine glossing clays such as Astra-Glaze or Astra-Sheen. However, ultrafine carbonates such as Carbilux can be used successfully in topcoating without the need for glossing clay, but in this case the binders and additives in the coating color also need to be optimised to maintain gloss and print quality. In the highest quality applications where brightness is critical then engineered carbonates are often needed. Imerys recommends its CarboFlex PCC/GCC concept for these applications as a cost effective way of generating gloss together with light scatter and optics. In this case engineered kaolins such as Capim DG or Contour Xtreme become a better choice for the partner kaolin than glossing clays.
Matt / Silk Topcoats
For matt or silk topcoats generating good smoothness and printability with low sheet gloss is the key driver for pigment selection. Often, the lower gloss finish is achieved simply by adding coarser carbonates such as Carbital 60 or Carbital 75 to the topcoat recipe. However, the best approach for achieving low gloss with good smoothness is to use coarse, higher shape kaolins together with fine or steep carbonates. Depending on the gloss level sought, Imerys recommends use of Capim NP, Capim SP and Capim RG for these applications, together with Carbital 90 or Carbopaque 90.
Single Coated Woodfree (Freesheet)
In single coated woodfree (freesheet) applications the pigment selection is based on the need to balance optical requirements with basepaper coverage, paper gloss and print gloss development. Kaolin levels are typically higher than in topcoating applications and kaolin choice is typically fine engineered products such as Capim DG, Astra-Print GS or Contour Xtreme which has introduced a new level of design flexibility in this sector. This fine platy pigment is highly efficient and allows higher levels of carbonate while achieving good printability, gloss and smoothness. Requirements for high paper brightness and opacity in this segment have increasingly driven carbonate selection towards engineered products such as PCCs (Optical 600 and Optical-400), steep GCCs (Carbopaque 95) and CarboFlex. These help generate light scatter in the coating, thus minimising use of expensive opacifiers. They also open up the coating structure to improve blister resistance in heat set grades. Calcined kaolins such as Alphatex and Deltatex can also be used to further improve blister resistance and opacity in these applications.