Advances in coating were presented at the 27th Adhesives and Finishing Symposium last October. Michael Boyle reports from Munich
Inatec’s Harald Wallner’s opening presentation emphasized the benefits of non contact glue application curtain coating. These include constant glue film thickness across the substrate width and length, cost savings due to lack of wastage, and avoidance of fringing, and of dirt spots or abrasions being transported along the web.
Jim Constanzo, of Rohm & Haas, provided results of an extensive assessment of both the mechanical and formulation aspects of curtain coating. He claimed that four key factors impact the stability of the curtain: surface tension, foam formation, grit formation and lip design. In addition, as the speed and flow rate of the web is increased, the air/web boundary layer affects the coating, producing a ‘heel’ contact (adhesive flows slightly in reverse direction), a 90 degree contact and an ‘extensional’ contact (liquid is pulled beyond the drop point before contacting the web).
He suggested the process could achieve good results at 1,000m/min and higher.
Ralph Goehlert, from Nordson, presented a paper on innovative spray technology for pressure sensitive hotmelts. In controlled fibreization technology the adhesive is drawn into a single fibre using high velocity air jets. Rotated by the directed air jets, the monofilament forms a helical pattern on the substrate. Summit technology uses four adhesive strands for a full die width pattern. He claimed both these methods continue to represent production proven processes with the highest bond strengths. Applications range from paper and paperboard products to packaging and customized solutions.
Innovative machine technology for the coating of UV curable pressure sensitive adhesives was discussed by Nils Hansen, of Max Kroenert. UV lamps supplied by Drytec can be retrofitted to existing type PAK 400 and 600 multipurpose hotmelt coating machines, so that UV curable pressure sensitive adhesives can also be processed as hotmelt. To meet coating speed requirements up to 1,000m/min, the Kroenert ‘technicum’ has recently been completed with a second test facility with slot die technology especially for hotmelts and UV acrylate adhesives.
Armin Aebi, of Polytype, continued with a paper on in-line siliconizing and coating of aqueous pressure sensitive adhesives. He presented various design requirements that are essential for high speed converting machines. Features such as a non stop unwinder, automatic roll handling, computer controlled drive controls, and splicing and roll diameter measurement, are some of the important features integrated into Polytype five or six-roller coating lines proven for high process productivity and high product quality. Curtain coating PSA emulsions onto siliconized paper, slot coating for hotmelt applications, and laminating stations for manufacturing high quality labelstock were three practical examples of Polytype machine applications.
Gerd Blecken, from ExxonMobil Chemical Films Europe, discussed printing performance of OPP films for the self adhesive labelling market. He revealed the results of five different printing processes (UV screen, UV letterpress, UV offset, UV flexo, and water based flexo) on transparent and white opaque OPP. The claimed that the facestock film design determines the print result with respect to ink adhesion and scratch resistance. The defined surface properties of coatings and polymer surfaces should be adjusted to the requirements and are the basis for good adhesion. In-line corona treatment on polymer surfaces can improve ink adhesion as it increases the surface energy
Degussa-Goldschmidt’s Hardi Döhler looked at improving the speed and efficiency of UV curing systems using silicone acrylates. A new photoinitiator with improved silicone solubility is capable of enhancing the degree of cure and the release performance of silicone acrylates. With the addition of phosphites, it is now possible to run UV units at higher oxygen levels and still achieve high quality. This enables a reduction in nitrogen consumption, saving costs. The usefulness of phosphites is particularly important with UV units running at critical inerting levels, and where rough substrates such as non woven materials and machine grade papers (such as business forms) are used.
Kaj Koskinen, of UPM-Kymmene, reported on recent developments in release materials. According to his figures, the European release liner market is growing at six per cent a year, and 55 per cent of the market is for labelstock. There is extremely tough competition and over-capacity amongst paper stock suppliers.
The pressure to reduce prices has been accompanied by label paper and release liner becoming lighter. To date, label paper weight has reduced from 67 to 58g/m2, whereas release liner is at 50g/m2. Less wood is used, less waste is generated, transport costs per square metre are being reduced, less storage space is required, roller lengths have increased, and less water and energy are used in paper production. This results in substantial savings in materials costs.
Dr Karl-Heinz Schumacher described BASF’s new raw material for pressure sensitive adhesives for tapes, labels and films – acResinUV. These UV acrylic hotmelts are solvent-free and water-free. They are said to combine the economic advantages of hotmelt coating with the high performance of acrylic chemistry, including an excellent resistance to heat and ageing, and optical transparency.