Doc Sol

Doc Sol is the superhero in the background. As an expert in pressure sensitive adhesive applications, he answers questions on release liner and their optimal usage. Should you have a question, simply send it to Doc Sol via email. See below for his answers to a few frequently asked questions;

How is it possible for Sappi Sol silicone base papers to consume significantly less silicone?

The key factor is the oil uptake and the oil penetration as well as the microporosity of the clay-coated surface. The pigmentation of the clay coating uses hexagonal kaolin platelets that ensure very even coverage of the raw paper surface, and with the blade coating process forms a positive, smooth surface. Sappi Sol’s high quality clay coating is applied to ensure low oil uptake and oil penetration for an exceptionally high proportion of coating agent as compared to the norm for the paper industry.

This coating agent has a low pigment/binder ratio. Customer experience confirms our claim of up to 25% less silicone consumption whilst simultaneously achieving a very homogeneous silicone surface. This ensures excellent quality of the self-adhesive films removed. The reduced silicone consumption results in significant cost savings, as well as having a positive effect on the eco-balance.

How is the exceptional surface of the Sappi Sol silicone base papers achieved for the siliconisation?

Achieving the exceptional surface of our Sappi Sol silicone base papers begins with the manufacture of the base paper used. A key point here is the use of a Yankee cylinder in the paper machine. In the production of our Sappi Sol family, the smooth side of the raw coated paper, dried on one side in contact with a Yankee cylinder interface, is used as the surface for the application of the water-based coating. Unlike traditionally dried machine finished papers, there is no significant roughening of the paper fibre surface due to the swelling of the fibres on contact with humidity. Instead, the permanent smoothness of the coated paper surface is retained. Another factor affecting the excellent surface is the blade coating process. All Sappi Sol substrates have a clay top layer. Sappi, and the plant in Alfeld in particular, has decades of coating experience that enable us to achieve optimal results. It can be challenging to apply these kaolin pigment based, binder-enriched special coatings onto base paper in an optimum manner, and our expertise and investment in modern paper manufacturing technologies give us an edge in this regard. Our modern paper machine 2 with integrated blade coating units has the latest coating technology and ensures the highest quality levels.

What effect does the coating have on the quality of Sappi Sol silicone base papers?

The quality of the clay coatings applied to Sappi Sol substrates naturally has a significant effect on the excellent ability to siliconise the papers. We take great care to ensure the purity of the coating components in the coating formula; the curing process of the silicones applied by the converter must not be impaired in any way by interactions between the clay coating and the silicone. With all coatings applied to our Sappi Sol substrates, we make absolutely sure that only silicone-compatible components are used. It takes considerable experience and applications knowledge to create the correct composition of these components.

How is the quality of Sappi Sol silicone base papers verified?

The quality control of our papers is assigned the highest possible priority. We undertake high intensity quality testing of our papers and the online accuray measurement scanners integrated into the production lines are especially important. These are used to continuously monitor the paper profile via measurement of basis weight, thickness, humidity, coating weights and ash levels and these factors are checked at each incremental production stage.
Every production batch is also subjected to precise testing in our climate lab. Typical physical paper characteristics such as thickness, basis weight and tear strength are also tested in this process. Detailed measurements are carried out and evaluated immediately across the entire drum width. After every check on a jumbo roll, the corresponding production batch is assigned a certificate. At the same time, application-specific tests on Sappi Sol substrates for oil uptake, oil penetration and even lab siliconisation are carried out, combined with tests on the silicone curing and silicone anchorage.

How easy is PE anchoring (extrusion) on the reverse side of Sappi Sol papers and which applications is this useful for?

Some converting customers extrude polyethylene (PE) onto the reverse side of clay-coated Kraft papers to ensure the flatness of laminates on wide width flat bed cutting plotters as well as the evenness of laminate formats in critical outdoor conditions; e.g., in warm, damp climates. The wear on cutting plotter blades with this type of hybrid solution with a clay coated front has also been shown to be lower than in the case of siliconised PE surface finishes. The anchoring of the polyethylene extruded onto the back of the CCK paper is important, for example, to counteract bubble formation during siliconisation or adhesive coating that can occur due to inadequate PE adhesion onto the rear of the paper. There are advantages provided by Corona treatment prior to PE extrusion and a raw paper back with a large surface for the physical anchoring of the extruded plastic. To achieve this, Sappi has developed special Sappi Sol paper types uncoated on the back that -- thanks to their high splitting strength -- are ideally suited for cutting plotter applications.

What effect does humidity have on Sappi Sol silicone base papers?

Sappi Sol substrates are coated with a moisture barrier on the back and offer a high level of dimensional stability. Experience has shown that with appropriate humidity control in the lamination process for finished composite films/paper laminates, there is no formation of undesirable bumps in the paper. The evenness of the laminate formats meets market requirements even in critical markets. The reverse side coating applied to these papers results in retardation of moisture penetration. This even permits paper re-humidification with vapour or water film. Its blocking effect against migrating contaminants prevents printing errors on the cover film and also minimises problems due to uncured silicone drips in the top film printing. With the surface coated on both sides, this also prevents the occurrence of mechanical embossing effects (orange peel) on soft printing films.

Can chemical interactions occur in the siliconisation? Of course, but we are working to minimise this.

Due to the stringent selection process for the types of pigments and binding agents used as well as co-binder types and appropriate heating of the coatings in the coat drying process, undesirable chemical interactions are avoided between the clay coated paper surface and the silicones used. Over the long term we aim to provide our customers and converters with the fastest possible silicone curing and very consistent release forces required when removing the self-adhesive film from the carrier paper.

What needs to be considered with intentional back scoring for improved removal of the carrier paper?

In the case of Sappi Sol substrates, we can create this cracked back feature with the filler content and the use of special pulp materials in the base paper. According to our lab tests with special groove tests, the paper volume is the key factor. Pre-conditioning of the paper before the actual grooving process is also important in practice to ensure good cracking noise of the grooves.

Which applications are PE coated papers used for, and do CCK papers such as Sappi Sol represent an alternative to this?

PE coated papers have an advantage in terms of dimensional stability. This means they prevent problems with evenness and the formation of humidity bulges in composite laminates, especially in critical climate zones. An important disadvantage of PE coated papers is that these silicone carrier materials are not suitable for applications with high heat exposure, such as for the laser printing of self-adhesive films. Due to the high proportion of plastic, they are not actually sustainable or environmentally friendly products, either.

Traditionally, PE coated carrier papers are preferred in certain areas where they are not always necessary. In other words, less critical applications can be replaced with dimensionally stable clay coated Kraft papers. Due to their very good dimensional stability and the low hygro expansion, Sappi Sol carrier papers can frequently be used as an excellent alternative. This can result in a more sustainable and more environmentally friendly product as well as reducing costs. For this reason, the application should be thoroughly analysed to determine whether PE coated carrier papers are actually necessary.

Are Sappi Sol release liners available with FSC and/or PEFC certification?

Yes absolutely. Sappi Sol release liners are available with FSC® and/or PEFC™ certification. Sappi takes its responsibility in using sustainable materials and renewable raw materials very seriously. Our customers aim for and demand a high proportion of sustainable and renewable raw materials. Therefore, Sappi sources its pulp from certified forestry as a matter of principle, never losing sight of its long-term strategy of reducing fossil-based solutions or replacing them with renewable raw materials. The principles of sustainability are also reflected in the low silicone consumption of Sappi Sol release liners. Converters using Sappi Sol silicone base papers require significantly less silicone. Despite the low silicone consumption, a very homogeneous silicone surface is achieved. The reduced silicone consumption results in significant cost savings and has a positive effect on the eco-balance.

Which applications require high flatness? How is this achieved?

Sappi Sol release liners are most commonly used in complex applications in the graphic arts industry to produce self-adhesive PVC foils in reel or sheet formats. This includes digital, offset and screen printing applications, as well as the converting of large-format advertising banners on flat bed plotters. Good flatness is particularly important for reel format applications, since it would otherwise not be possible to maintain the high printing speeds of offset and flexo printing (and now digital printing as well). The benefits of the Algro Sol release liner reverse side coating really come into play here, preventing the laminates from buckling and holding them in place on the vacuum-based converting table. In addition, the reverse side coating ensures that moisture is released from the paper slowly and evenly, preventing possible foil contraction during digital printing, where the foil is exposed to higher temperatures.