News
The role of silicones in release coatings
02.12.2008 (x-head)
The role of silicones in release coatings
Although the specialisation of tapes is growing constantly, tape producers – due to logistical reasons – wish to keep the number of individual liners in use low. The technical demand for quality levels have been enhanced by optimised production processes at both converters and end users of siliconised release liners.
Apart from mechanical properties of the used substrate, the silicone coating is obviously very important. A release line producer has to consider various influencing factors, such as the type of substrate, the silicone formulation and all relevant processing parameters and keep them in balance.
For C2, Jörg Soding and Volker Kienitz from B. Laufenberg GmbH discuss the role of silicones in release coatings and the numerous factors and influences that have to be considered.
PLEASE BOX OUT INFORMATION
(x-head)
Four groups of silicones
In release coatings, these four main groups of silicones are generally used:
• Solvent-based silicones
• Emulsion silicones
• UV/EB curable silicones
• Solvent-less silicones
BOX ENDS
Silicone itself contributes strongly to release forces and release performance. More than 70% of the world market of release coating is covered by solvent-less silicones. These consist of five components.
The polymer (vinyl functionalised polymethyl siloxane) and the crosslinker (Si-H functionalised siloxane) form a 3-dimensional matrix during the curing process, in which the temperatures are ranging between 100 and 230°C. The platinum catalysed reaction usually takes place within a few seconds in the drying oven.
To avoid premature reaction or solidification of the mix, an inhibitor slows down the reaction time. It evaporates at temperatures of 100-120°C. These four components usually compose easy release coatings. As an additional component, the producer can add silicone resin (CRA) to adjust the release force. The more CRA is added to the formulation, the stronger the release force against the adhesive will be.
(x-head)
Influencing factors to release forces
The five factors mentioned above have to be adequately adjusted in order to develop a new silicone coating or to modify an existing formulation to a customer adhesive. Because silicone systems from different silicone producers can provide different performance and behaviours towards an adhesive, a test program should be installed.
Different types of silicone resins obviously differ in their chemical constitution and performance. The release is strongly influenced by the reaction between CRA and adhesive, although this depends mostly on the actual adhesive composition that is used.
So, in addition to initial release values, ageing effects can be critical and have to be considered. Release values have to be as constant as possible throughout prolonged storage times, from manufacturing until the final use.
Depending on the adhesive in use, not only CRA, but also the other components have an influence on the release properties. For example, the vinyl content and the formation of the vinyl groups in polymers are not to be underestimated. Their chemical constitution influences the relation between release force and delamination speed.
Especially with acrylic adhesives, the type and amount of crosslinker also influences release values. Remaining Si-H groups can react with components of the adhesive so that the release force may be increased. But as the Si-H groups dissipate over time, this is no suitable adjustment method and not reproducible.
Another way to control the release force in addition to the silicone formulation is the silicone coat weight, usually specified in g/m2. If it is too low, the variability of surface roughness or porosity of the base material through insufficient coverage can result in variable release forces.
A considerable influence to the release also comes from the liner that is used, even though the carrier is protected by the silicone coat from the adhesive. There are some specific properties of release performance that can be detected apart from the general type of substrate that show the importance to select a suitable liner:
• The surface roughness influences the release even at good silicone coverage. Silicone is usually coated by 5- or 6-roller coating head unit. The heads apply a coat over the surface contour with a layer thickness of about 1-3 microns. Because peaks and valleys cannot be dressed completely, an uneven silicone film remains on the liner surface. For hard adhesive tapes laminated on rough silicone surfaces, this results in low release values because the adhesive only comes into contact with the peaks of the liner.
• Considering the chemical composition or properties, granulate blends and additive can affect the release, especially with films. Anti-friction agents and plasticizers can inhibit the catalyst that can result in insufficient cure and subsequent transfer of silicone to the adhesive. Furthermore, chemical components within the film can migrate to the silicon surface, causing disturbances of the release properties in the contact area between silicone and adhesive.
Production parameters selected during the silicone coating process, such as oven temperatures, coating head adjustments, machine speed and re-conditioning, resemble further influences to release properties.
If the oven temperature is set high on production with papers, the paper will absorb less silicone. Drying temperature and time can also influence the release force of CRA containing silicone formulations.
Regarding machine speed, different roller speeds are usually needed in coating head systems with five or six rollers to consistently spread out the silicone film and thus achieve a specific silicone coat weight. For the surface and coating quality, the correct selection of the differential roller speeds and the quality and hardness of the applicator roller rubber is key.
The re-conditioning of the paper is an often unconsidered, yet important effect. The paper loses moisture during the silicone cure and would only contain around 2 or 3%. So in order to provide enough dimensional stability and depending on the paper type, the final moisture has to be brought back to about 4-6%.
PLEASE BOX OUT INFORMATION
(x-head)
From easy to tight release
The B. Laufenberg GmbH, a leading supplier of specialist silicone coated products, is able to offer a range of several thousand product combinations thanks to 12 different release levels – from super easy to very tight – for every silicone system and a wide range of different substrates.
Regardless of the adhesive, the company aims to propose the suitable combination of liner and silicone formulation with the help of self-designed re-conditioning units, elaborate test methods and intensive partnerships with both customers and suppliers.
