May 2008
By M.E.Marley
The transfer of curtain coating from speciality to mainstream papermaking has become the focus of intensive R&D work around the world, following the expiry of the basic patents held by the photographic sector, where the technology has been in use for some 50 years.
In Europe, Metso and Voith are developing curtain coating technologies for the graphics sector and in particular for use on LWC lines.
At West Michigan University, a major center for coating research in North America, the focus is on barrier coating, especially for the food packaging sector, where the production of a thin, pinhole-free barrier layer could transform the economics of the pizza box and burger wrapper, for example.
The attraction of curtain coating lies in the promise of huge economic, quality and process benefits which derive from a non-contact process which can:
1. Provide high, uniform coverage at low coat weights, even on rough substrates.
2. Apply up to 10 layers of thin, uniform coating in a single pass.
Simultaneous, multi-layer curtain coating has been developed to a high degree of sophistication in the photographic sector, where, conventionally, up to 10, thin, multifunctional layers are applied to the substrate to achieve specific chemical reactions and properties. In the 1980s the technology was transferred to speciality papermaking and it is now an industry standard in the production of carbonless, thermal and high quality inkjet papers, at operating speeds of 1,600 m/min.
It is this multi-layer capability that will keep curtain coating at the forefront of R&D work for the next decade, a period which will see the emergence of new generations of highly engineered paper and board products.
New coating structures are at the center of these developments. They will incorporate the nano-chips and functional chemicals which will impart smartness, interactivity and enhanced barrier properties.
A pre-metered coating process
Curtain coating is a pre-metered process whereby the exact amount of coating to be applied to the web is fed to a coating head, that is to say, a die, of slot or slide design. The slot die can apply three coating layers simultaneously; the slide die can be expanded at will to apply more than 10 layers in one pass.
The different coating liquids for the multilayer structure exit from individual metering slots and fall vertically onto the substrate, one on top of the other. In this way, the coating layer is formed before it comes into contact with the paper web. As the uniform, liquid curtain falls onto the moving web it merges with it, producing a smooth contour coating and an effect which has been likened to a laminated sheet. This virtually guarantees 100% coverage at any coat weight.
Several benefits stem from this process. First, since the curtain coater provides a true contour coating, there are no pinholes, that is to say the bald spots or insufficiently covered base sheet areas which impair barrier properties and may cause the failure of the barrier function in food packaging grades.
Second, there is no mechanical contact between coater and substrate, as is the case with blade, rod and film press coaters. The absence of rigid equipment at the curtain impingement zone – where curtain meets substrate - means that the coating process is unimpeded and this results in a scratch and streak-free sheet. In addition, the film splitting which promotes pinholes is avoided.
Pinholes are an unavoidable consequence of blade coating and other surface coating processes which have to fill all the valleys on the paper surface before an effective barrier layer can be created. This creates the problem of uncovered peaks or pinholes.
In addition, like the hybrid rod coater, which provides a surface-contour coating, the blade coater operates under hydrostatic pressure. As a result, the coating penetrates into the base sheet leaving exposed areas, or pinholes. As for the film press, although it applies a contour coating, the film splitting tendency increases the incidence of pinholes.
The pinhole problem is at its most acute with light coatings and it has lead to a two-stage process whereby the required coat weight is applied in two layers – the coincidence of two pinholes in the separate layers is remote. This is a less cost effective process than the complete, light weight coverage promised by the curtain coater.
Third, the non-contact curtain coating process puts no mechanical stresses onto the sheet. As a result there are fewer web breaks, runnability is enhanced and broke and downtime are minimized.
Fourth, the curtain coater increases flexibility in that weaker base sheets can be run; and, a wide range of coat weights and viscosities can be applied with a single coating head. In fact, the thickness of the coating layer can be controlled by adjusting the flow rate from the coater and the speed of the moving web.
Fifth, the pre-metering of the precise amount of coating required means that there is no excess fluid, as is the case with the roll, blade, rod and air knife processes. This eliminates numerous operational problems such as the recycling of excess fluid, replenishing evaporating solvents, foam and crust formation.
Finally, the ability of the multi-layer curtain coater to apply pre-coat, coat and top-coat in a single pass will bring significant cost and quality benefits. It is this possibility that is driving the research efforts of Metso and Voith in the LWCfield.
Curtain coating for graphic grades
At the Metso Technology Center in Järvenpä, pilot trials on a production-scale pilot machine show that the high speed curtain coating process can produce superior, LWC coating coverage at low coat weights, that is to say a coverage which is 40% better than that achieved by a metered size press.
At constant low coat weights of 5 to 6 g/m², a high coverage of 85% was achieved, even when the coating was applied at a low coating solids of 54% to avoid air bubbles. And, the trials were carried out at web speeds as high as 1,500 m/min.
To achieve these results, it was necessary to remove the air currents which travel along the moving web, disrupting the curtain and causing film splitting. In addition, high speed operation requires a coating colour with low air void volume and viscosity control. The latter is the field of the Emulsion Polymers division of Dow Chemical, a partner with Metso in the curtain coating project.
The Metso development team found that the removal of air bubbles from the coating color was the most challenging issue. Air bubbles can break up the liquid curtain at the exit of the slot die and they can cause defects in the coating layer. While the macrovoids created by large bubbles can be managed easily, the removal of microvoids, which are measured in tenths of millimeters, is more difficult.
Development work on curtain coating for LWC grades is also underway at the Voith Paper Technology Center in Heidenheim. Trials on the pilot coater, which can handle 8 m paper webs at speeds of 2,500 m/min, show that the curtain principle is advantageous for LWC paper, white top liner and woodfree grades.
This project also includes the development of bespoke coating colours, in which Voith's partners are BASF and Omya. The focus is on low surface tension and the formation, stability and elasticity of the curtain.
Elasticity is an important property if sufficient coverage is to be achieved at high speeds and low coat weight. In the impingement zone, the coating colour is stretched as it follows the substrate at speeds of 1,000 m/min and above - a curtain which does not have the necessary stretchability produces micro skip defects. The rheological properties of the coating color will therefore play a major role and the team has developed a rheology additive which enhances the runnability of the color at high speeds and low coat weights.
Voith has a successful record in technology transfer, having developed the first curtain coater for carbonless grades in a joint project with Voith IHI of Tokyo.
The first commercial installation of the Direct Fountain (DF) curtain coater was in a Japanese mill which produced CBcoating for carbonless paper. Since then the application range has been extended to inkjet paper, to carbonless CF and to thermal papers and today Voith's DF machines are running at speeds of 1,600 m/min.
In December 2001, the first curtain coater for thermal paper started up at the Kehl Mill of August Koehler, on the new 1500 m/min machine. It is on the DF coater on this machine that Voith carried out curtain coating trials on 58 g/m² woodfree base at speeds of 1,200 m/min.
The resulting single-coated single curtain coated paper was compared with double coated papers from blade/blade and filmpress/blade coaters, and the papers were analyzed for their coverage, smoothness and printability on a sheet-fed offset press.
The trials showed that the curtain coater can bring a significant improvement in coverage and that gloss, bulk and roughness were within typical industry standards.
Pulp & Paper International is FREE to qualified subscribers.
Click here to find out more.
