Release coatings

Release coatings are release coatings made from silicone.

These silicones always consist of at least one polymer , which is polymerized to a polydimethylsiloxane skeleton by means of a crosslinker and in the presence of a catalyst either thermally at temperatures above 100 ° C or under UV light (slightly different reaction chemistry). Both solvent-free, water-based ( emulsions for, among other things, baking paper) and solvent-free silicones, which represent the largest market share, are used as release coatings. The “controlled release effect” is described as an outstanding property of adapting to the different adhesive strengths. Defined (controlled) release properties can be set here by the suitable choice of certain resins in a silicone polymer. When an adhesive is easily separated (delaminated) from a release paper (or film), one speaks of an easy release coating. The higher the proportion of resins in the silicone polymer, the more force has to be applied to loosen the adhesive from the siliconized surface. For most adhesives (which should be constant in formulation and application), this process is reproducible and therefore also "controllable".

Reaction mechanism for thermal polyaddition-crosslinking systems

...- Si-CH= CH2 + H- Si- ... CAT → ...-Si-CH2-CH2- Si- ...

Typical silicone application weights are 1 - 2 g / m². This corresponds to a layer thickness of only one to two micrometers (= 0.001 - 0.002 mm). These thin layers are achieved through special application units in which the silicone is thinned through a kind of “stretching” and “rolling”, in order then to be transferred to the paper or film in the desired layer thickness . This requires rolling mills that have five to six rollers for thinning the film. Once applied, the coated substrate runs through a dryer at over 100 ° C (usually over 150 ° C) and only lingers there for a few seconds to chemically react (film hardens). The dwell time in the dryer (oven) depends on the production speed and dryer length. With ever increasing production speeds and thus ever shorter dwell times, new requirements are constantly being placed on modern silicone coatings. Furthermore, the high temperatures in the dryer place some demands on the substrates. Papers dry out and become brittle. To counteract this, the papers must be re-moistened after the silicone has cured. This takes place in the running production process in chambers saturated with steam. The paper absorbs the moisture through condensation of the steam on the paper surface and thus recovers its initial moisture. In the case of films , the melting point of the carrier (substrate) determines the temperature to be set in the dryer. Polyolefin substrates like polyethylene are the most sensitive with less than 100 ° C and polyester or even polyimide with over 200 ° C the least sensitive. These temperature properties then also determine the speed with which the silicone can be cured.

Worldwide, several billion m² of paper and film are currently coated every year - and the trend is rising. This also makes it understandable why tens of thousands of tons of silicone are required for use on release papers and films with an application weight of just 1 g / m².