In India’s expanding paper industry there is boo increasing pressure to improve paper quality and cost efficiency. Paper properties such as brightness, opacity, print quality, as well as machine runnability, need to improve in order to match imported competitive papers.
Recent years have seen step changes in papermaking practices in order to meet these demands, and this has generated a move toward calcium carbonate based fillers.
This is especially true for uncoated woodfree (UWF) papers. With coated woodfree (CWF) there is a similar trend, as carbonates take a larger share of both coating and filler. Similar developments in pigment usage have been seen in almost every significant paper-producing region.
Why Switch?
What motivates the move to carbonate based fillers as opposed to more traditional minerals such as talc?
The availability and sustainability of carbonate supply is one key reason. Equally critical is the potential to improve important paper properties, such as brightness, opacity (light scattering capacity of the pigment), and print quality combined with the ability to run at higher machine speeds due enhanced dewatering.
According to Kapil Dev, Technical Sales Manager, Imerys India, “With the advancement of processing technology it is now possible to produce high quality wet ground carbonates that are easier to retain and of lower abrasive content than their predecessors.”
These products have a much lower impact on the life of paper machine equipment and clothing than dry ground products, or even early technology wet ground GCC. Adds Danny Ingle, Technical Marketing Manager, Imerys Asia, “The new products are retained in the sheet more readily, especially with onsite applications, where there is no longer a requirement for dispersants in the filler slurry. There is also generally a lower content of insoluble residues and significantly less coarse abrasive material free in the water system, when carbonates are produced using more tightly controlled grinding conditions.”
PCC has also found it’s niche in UWF where the ability to produce crystal morphologies that maximize light scattering and increase the sheet bulk have proven extremely valuable.
Talc’s Traditional Role
Talc has primarily always been favored over GCC due to the low abrasive character. However it has been losing ground in many areas of the region due to availability concerns.
Why is there less paper quality talc available?
The demand for talc in other market segments, like plastics, as well as over mining and flooding of old mines, has lead to a fall in quality (greater content of abrasive contamination). Some areas of northern Asia have almost depleted their supply ability for paper quality grades.
Talc is, in the respect of removing sticky extracts from mechanical and recycled furnish, a superb additive. It also has a recognized role in paper coating, competing with kaolin as a means of controlling ink holdout, in an application where demands on brightness have also seen the requirement for engineered GCC coating pigments increasing annually.
Rising Share of Carbonates In Asia
Throughout Asia,GCCand PCC together already represent 70% of the filler used in UWF and 100% of the filler on the region’s top 15UWF production machines.
With a predicted pigment market share growth of 12-15% continuing through 2013. Talc has lost ground and will fall a further 5% over the next five years, as Indian and Korean papermakers continue to convert from acid to neutral or alkaline papermaking conditions.
According to Dr. Andrew Findlay, Global Research and Development manager Ciba Chemicals, “Employing quality carbonates, leading Indian papermakers are achieving a global standard with domestically produced grades, comparable to top imported UWF’s and CWF’s.”
Finding a Balance
Quality improvements, economic factors and sustainability all create a drive towards using higher mineral filler levels in paper. The main challenge of increasing filler loading is to find an acceptable compromise of paper properties and paper machine runnability. Numerous papermakers are investigating increased filler loadings at this time.
Dewatering Gains&EfficiencyWith Chemistry
Efficient dewatering, good runnability and low chemical consumption are essential factors in paper machine performance. Filler type and content plays an important role here. Chemical consumption is mostly affected by the specific surface area, increasing surface area usually increases chemical consumption. But, in a highly competitive market there is a limit to how much compromise to quality can be afforded. It is therefore essential to develop a strong understanding of the attributes of different fillers and the properties they can contribute to the sheet to achieve the desired balance of paper properties.
Imerys’ Dev notes that blending of pigments of different size distribution; aspect ratios and morphologies can be used as an additional tool to achieve quality goals and can potentially provide a unique combination of bulk, optics, stiffness and strength properties.
Coating Options forWoodfree Grades
Coated woodfree papers grades are typically 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 higher solution 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.
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. Says Chris Nutbeem, Global Applications and R&D Team Manager, Imerys, “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 not uncommon in this sector. In this case pigment selection is based on the need to balance optical requirements with basepaper coverage, paper gloss and print gloss development.”
Precoats & Midcoats
For Precoats and Midcoats the pigment choice is governed by the need to provide cost effective base paper coverage enabling application of fine topcoats. Brightness is also important. For this reason pigment choice is usually 100% coarse standard carbonate, 60 and 75<2 μm.
Dr. Rob Hill, Regional Applications Specialist for Coated Paper & Business Development Manager, Imerys India, suggests that with more demanding applications, principally at the lighter weights, where higher opacity is needed, engineered carbonates such as narrow p.s.d. GCCs or PCCs have been preferred. In this case, although quality is improved, he points out that the cost of the precoat also increases, not only because the pigments are more expensive, but because these narrow p.s.d pigments also require lower running solids (more drying) and higher binder levels. In recent times we have see a marked shift away from light weight CWF production (<90 gsm). This is presumably because of the increased formulation costs and loss in machine efficiency associated with these grades. As a result, the use of engineered GCC and PCC in CWF globally is generally declining.
However, basesheet characteristics can also govern precoat pigment selection. Where rough or porous basesheets are used, there are greater demands on the precoat for coverage. Here, platey products such as kaolins can be used to good effect at 20- 30% addition in the coating pigment recipe. By adding the kaolin to a coarse GCC, a bulkier precoat structure is achieved which not only improves coverage, but also improves opacity. The increase cost of using kaolin in the precoat can be set against the potential for cost reduction through increased flexibility in fibre selection and topcoat design.
In topcoating coating formulations are generally based on high carbonate levels for cost and brightness demands, but the final choice of pigments will depend on the surface finish required.
Overall it is clear that the choice of pigment has a big influence upon the quality of the coating. However, it must be remembered that the effect of the pigment is dependent upon a number of other factors including the other components of the coating color, the way the color is applied, and the substrate to which it is applied. Knowledge of the way in which pigments behave takes us only part of the way to understanding how a particular coating system works. It is necessary to understand how the pigment interacts with binders, cobinders and other additives and how it behaves with different types of applicators and substrates in order to understand the coating process completely.
