New advances in paper coating technology
A new patented nozzle for curtain coaters allows precise sectional volumetric correction of flows to ensure even coating across the entire web surface. Another advancement makes it possible to apply barrier layers in the inboard mode without defects at the edges.
NEW NOZZLE FOR MULTI-LAYER COATING
Curtain coating transfers a thin film of falling liquid (the “curtain”) from the applicator die to a moving web of paper or board. When applying multiple layers simultaneously, the thickness and composition of each layer must be constant over the entire web surface. Achieving this can be particularly difficult with wide webs and/or a wide range of coating weights.
Certain packaging materials require environmentally friendly and sustainable barrier layers to prevent the migration of oxygen and/or oils. ANDRITZ has developed a new type of nozzle for these multi-layer barrier applications which has advanced crossdirection (CD) profiling. For the first time, it is possible to make volumetric corrections to the CD profile of inner layers and produce a completely uniform coating without defects.
THE LABYRINTH BREAKTHROUGH
In collaboration with research institutes, ANDRITZ tested different variations of diffuser block and nozzle geometries. In 2013, there was a breakthrough. A new type of diffuser block was produced with a labyrinth geometry.
Numerous tests were conducted on a prototype unit to confirm the CFD simulations. Figure 1 shows the prototype with eight diffusers in the block and a labyrinth with three expansion chambers. Extensive tests confirmed that the labyrinth renders the flow absolutely evenly.
A range of very thin to very thick coating layers can now be applied with high profile accuracy to eliminate defects or streaks. The deviations in CD profile are typically in the range of ± 1% for coating weights between 2 gsm to 14 gsm.
EDGE GUIDE SYSTEM FOR INBOARD BARRIER COATING
The challenges of inboard coating ( i.e., coating within the width of the web) center around variations in the coating at the edge of the web, and the potential damage to the uncoated portion of the paper/board substrate.
ANDRITZ developed a new edge guide system that uses advanced water jet technology. This makes it possible for the first time to apply barrier layers in inboard mode without coating defects at the edges. It can be used with a wide range of coatings – from very thin to very thick barrier layers – and it can operate continuously without downtime for cleaning.
Unlike conventional water jet systems where the curtain is cut by the kinetic energy of the jet, this new system stretches the curtain before gently cutting it. In this way, the coating color on the inside is spread over a larger area. Since there is only a small amount of color at the edges, no significant beading occurs.
Numerous pilot tests with different barrier colors and viscosities confirm the performance of this new edge guide system. The system is robust in handling these changes and no adjustments were required in the setup of the water jet for the different coatings.
Figure 4 shows the web immediately after coating. While edge beading is apparent with the web treated using a conventional mechanical device (left photo), the web from the new edge guide system has no edge defects (right photo).