Sappi’s Paper Machine 2 is used to produce silicone raw papers under the brand name of Algro Sol. According to Gunnar Sieber, Sappi's product group manager, Release Liner, Agro Sol papers feature high dimensional stability and outstanding surface properties “The production of these special papers requires considerable expertise and many years of experience, both of which we are fortunate to have at Sappi,” Sieber says.
Sappi has gained a lot of experience and special applications knowledge with silicone raw papers over the decades and now occupies a leading market position in Europe. “Before the conversion of PM2 in Alfeld, whose completion was celebrated in March 2014, the production of silicone raw papers was concentrated primarily on production line 4. The limited production capacity of this production line did not offer much scope for further growth outside Europe,” explains Sieber. “Thanks to the production capacities gained by the conversion, long-term and sustainable global deliveries can now be assured to meet the needs of a dynamically changing marketplace.”
Silicone raw papers are generally used for self-adhesive applications. These carrier papers are divided primarily into three paper types: Glassine or Super Calendered Kraft paper (SCK), PE coated kraft paper (PCK) and Clay Coated Kraft paper (CCK). Glassine and SCK papers are used primarily for label and sticker applications. Says Sieber, “They are available in weights between 58 and 90 g/m² and are typically used as carriers for self-adhesive products as well as price stickers and bottle labels. The second group includes PCK papers that are used primarily for applications in environmentally sensitive areas with warm and humid environmental conditions.”
Sappi's silicone raw papers belong to the third group of CCK papers. These are primarily used for graphical self-adhesive applications through to the processing of composite materials. Other applications for CCK papers include adhesive tapes and office materials.
According to Sieber, Algro Sol silicone papers are most commonly found in the graphics sector as carriers for self-adhesive PVC films. These include plotter films for lettering on windows and vehicles, printed films for car wraps and other technical advertising applications, or security films to protect against stone strikes. “Algro Sol substrates, often coated on both sides with silicone, are used as process papers for pre-impregnated fibers (prepregs) with composite materials. Carbon, glass fiber and plastic fiber materials are supplied to the shipping and aircraft industries, and these types of composite fiber materials are also used in the production of sports equipment such as tennis racquets,” Sieber says.
The carrier paper must be appropriate for siliconization and downstream processing. “Properties such as an even surface, dimensional stability, low diffusion of the surface and chemical purity are basic requirements for a complete, functioning system,” Sieber explains. “The fine balance among these properties resulted in Algro Sol developing an excellent reputation for decades with end users. To meet the special requirements of industry, the product line has been expanded over the years to include a large number of individual products to address specific applications. They vary in the formula of the pulp types and the coating structures used. These have been carefully developed in line with the needs of customers and their individual applications.”
Sappi’s experience and applications knowledge have resulted in what Sieber points out as these important basic properties of Algro Sol papers:
- Low silicone consumption and simultaneously homogeneous silicone surface
- Rapid silicone hardening
- Good silicone anchoring
- Excellent downstream processing properties
- High dimensional stability with good flatness of the self-adhesive bond
- Low shrinkage of the paper on heat loading
- High tear strength
“In addition, processors confirm and value the consistently high quality of Algro Sol papers that contribute to a high level of process reliability and process efficiency,” Sieber says.
Paper and silicone interaction
“All Algro Sol substrates feature excellent siliconization capabilities,” states Sieber. “The very pure product formulation inhibits plating and guarantees very good silicone hardening. In addition, special additives in the clay coating result in optimum silicone anchoring without silicone wear-off. These features ensure that polar groups of silicone modules find a high number of adhesion points on the clay surface.”
An important attribute for silicone raw paper processors is the low silicone consumption that they achieve with Algro Sol substrates during the siliconization process. “The specified consumption values are between 1 and 1.3 g/m². However, consumption measurements in actual practice have resulted in values of < 1 g/m². This is up to 25% less than other standard carrier papers available on the market. This is achieved with a double layer of clay and the satin appearance of the page being silicone-coated. Despite the low silicone consumption, a very homogeneous silicone surface is achieved. This ensures excellent quality for the removed self-adhesive films.”
“The reduced silicone consumption results in significant cost savings, and has a positive effect on the eco-balance,” Sieber adds. “Another advantage of Algro Sol is its excellent suitability for Corona pretreatment similar to film substrates as well as the two-sided siliconisation capability for the Algro Sol substrates with a functional coating on the back.
Due to the very high dimensional stability and the low humidity expansion of Algro Sol carrier papers, it is also appropriate for applications where PE extruded carrier papers would normally be used. This can reduce costs and result in a more sustainable and more environmentally friendly overall product.
Fit for many applications
Depending on the individual requirements of various self-adhesive applications, Sappi delivers optimum Algro Sol silicone raw papers for the task. For example, in graphics applications, Algro Sol DN and Algro Sol DNN are used with different color shades. They offer the high dimensional stability, good mechanical strength and flat position required in these applications. The rear coating applied to these papers results in retardation of the humidity penetration.
“This even permits paper re-humidification with vapor or water film. Its blocking effect against contaminant migration prevents printing errors on the cover film and also minimizes problems due to unhardened silicone drips in the top film printing,” Sieber explains. “The surface coating on both sides also prevents the occurrence of mechanical embossed effects (orange peel) on soft printing films. The flat weights of these papers range between 120 and 135 g/m2. Algro Sol DN and Algro Sol DNN can be silicone coated on both sides and process papers are also frequently used for prepreg applications.”
Algro Sol PCC with a flat weight of 90 g/m² has a satin finished clay coated side with excellent silicone coating. This creates even, visually perfect, transparent, self-adhesive protective films for masking digital prints in outdoor applications.
In the plotter sector, Algro Sol PC is used with a weight of 120 g/m². This is a silicone raw paper for what is known as hybrid solutions where the PE extrusion is applied onto the back by the processor. “There is very good PE adhesion on the back. On the front is a clay coating for siliconization. This coating must have good siliconisation capability with both solvent-free and solvent-based silicone systems. These must be hardened at low drying temperatures to avoid PE softening. The benefits of this Algro Sol paper include good dimensional stability following PE extrusion, high tear strength and lower wear of the cutting plotter blade due to the silicone-coated, clay-coated front,” says Sieber.
For paper facestock products such as stickers and other applications, Algro Sol 81 NC and Algro Sol SNC are recommended. Sieber adds, “They are suitable for roller and format goods and offer high volume, low dust, even thickness for punching, good rear scoring capability as well as good mechanical strength and rigidity. The weights are between 70 and 98 m/2.
Sieber stresses that Algro Sol silicone raw papers are known in the European market for their consistently high quality, which is valued by Sappi customers. He concludes, “This quality standard is also incorporated into future development of the Algro Sol papers. Whether in the improvement of formulas for the pulp or the optimization of paper machines, a consistently high level of quality must be guaranteed. The primary objective of ongoing development is further optimization of the surface, further minimizing the silicone consumption. Requirements from non-European markets are also being addressed and implemented in a solutions-based manner.”