Solutions for light to medium weight papers
Coated mechanical (Coated Groundwood) grades are light to medium weight papers produced from mechanical or recycled fibre. Typical paper weights range from 35-115 gsm. These grades are mainly used for magazines and higher quality advertising such as inserts, coupon printing and other low lifetime, high resolution print products.
In Europe coated mechanical grades are classified by the finished paper weight (ultra lightweight, light weight and medium weight) and printing type - heat set web offset or rotogravure. In North America classifications were traditionally based on brightness with #5 and #4 being classed as coated groundwood. In general both brightness and grammage increase in moving from #5 to #4 qualities. In Asia most coated mechanical is produced in Japan and based on recycled fibre. Here grades are classified on the basis of coatweight rather than finished weight. Coated mechanical grades are typically B2 and B3 grades with B3 being lighter. Some of the very light Bitoko grades are also based on recycled fibre.
The majority of grades produced in this sector are single coated with double coating restricted to the heavier European and Japanese qualities. It is the prevalence of single coating in this segment as well as choice of printing technology which dictates pigment selection.
Single Coated Offset
In single coated offset applications the pigment selection is based on the need to balance optical requirements with basepaper coverage, paper gloss and print gloss development. In today's multi-pigment environment it is medium fine carbonates (Carbital 90, Carbital 95 and Carbopaque products) that are used to deliver the required paper brightness, while the kaolin gives the more demanding functionality needed in these coatings. A general rule is that as grammage decreases, the kaolin level in the coating formulation should be increased in order to maximise fibre coverage and printability. Typical kaolins used for these applications are sourced from Brazil or North America. Medium fine particle sizes 80-95 < 2 µm, high aspect ratios and engineered size distributions are all key attributes for kaolins in these applications. From Brazil Imerys recommends Capim SP, Capim DG, and Capim GP for light weight single coated offset and from North America, Astra-Plate and the Contour range of products. At heavier weights, Capim DG remains an ideal kaolin choice, but where higher gloss is required then Astra-Print GS and Contour Xtreme are good solutions.
Single Coated Rotogravure
In single coated rotogravure applications high shape is a key requirement for the pigment selection in order to give the smooth surface needed for good rotogravure printability. Carbonate use should be minimised if high print quality is to be maintained. Kaolins such as Capim NP, Capim GP and Capim SP give a good balance between rotogravure printability, gloss and optics and can be particularly effective solutions for machines where both offset and rotogravure grades are produced. However, at lighter weights, even higher shape products such as Capim RG and E160 are often needed to ensure good coverage and smoothness.
Double Coated Medium Weight Grades
In double coated medium weight grades the precoat is usually 100% coarse or engineered carbonate such as Carbital 60, Carbital 75 or Carbopaque 90. The emphasis here is on optical and physical coverage of the basepaper. Kaolins can be used to further improve coverage on more demanding basetocks and here products such as Capim RG, Capim NP and Astra-Plate can be used to good effect at 20-30% addition in the coating pigment recipe.
In the MWC topcoats gloss is a key driver for pigment selection and because coverage is less critical pigments tend to be finer than those used in single coating. Carbital 90 and Carbital 95 are typical topcoating carbonates, but in higher 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. For kaolins, Imerys would recommend Capim DG, Astra-Print GS and Contour Xtreme, but fine #1 kaolins such as Astra-Cote and Astra-Glaze are also effective here.