The trend in off-line paper coating is towards ever-rising coater speeds with moderate increases of machine widths. Preparatory work is currently underway for the introduction of production machinery operating in the speed range of 2000 to 2500 m/min.
On the VESTRA pilot coater in Munich, which was commissioned in June 1995, a development program for coater speeds of up to 2500 m/min was launched to prepare machinery, products and processes for the challenges lying ahead.
In the first project stage, the VESTRA Technical Committee studied the most widespread coating systems in order to identify the limiting factors affecting paper quality and runnability in the range of 1000 to 2500 m/min. In a second project stage, the equipment and formulations were modified to overcome these limitations.
Besides LWC offset and rotogravure grades, which are reported in this presentation, LLWC gravure papers were successfully produced at speeds up to 2500 m/min in qualities satisfying modern demands.
At coater expansion stage 1, trials were conducted with a stiff blade. The backing roll diameter was 965 mm and the applicator roll diameter 400 mm. Generally speaking, a guide nozzle has to be installed for high-speed operation of roll applicators. The trials revealed that this guide nozzle required modification to accommodate fast speeds. In addition, the web tension, edge control and drive needed adjustment to enable coating of the second side of LWC gravure under the above conditions. All these modifications were successful and no runnability problems were encountered across the speed range up to 2500 m/min.
The test program for LWC offset suggested that the coating color had to be modified. Using a 60:40 blend of calcium carbonate and clay, a medium-viscosity potato starch and an SB latex as binder system, LWC offset was successfully produced at coater speeds up to 2000 m/min under optimized conditions. The product meets current quality requirements on 54 g/m² papers in terms of paper properties and printability in particular. Interestingly, papers coated at speeds exceeding 2000 m/min satisfy all requirements concerning runnability and paper properties such as bulk, gloss or smoothness. The only deficiency is mottling - a problem that will be investigated in-depth shortly and is expected to be resolved.
The LWC rotogravure trials gave the following results: the optimum coating color includes 70 parts of a clay with a high aspect ratio and 30 parts of talc. When this color was applied, bleeding and dry stalagmiting were tolerable. Different binder systems were also tested, because one of the major problems was the low water retention of the system at elevated speeds due to the high shear rates occurring at the applicator roll and the blade. Ultimately, the measured smoothness, gloss, bulk and printability of the test papers fulfilled the requirements on 54 g/m² rotogravure papers across the speed range up to 2500 m/min.
Runnability and Quality Aspects of High-Speed Coating of LWC Papers at Up to 2,500 M/Min, 1998 Coati