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Simulation with Silicone
For pressure-sensitive-adhesive (PSA) labels to function properly, the release coatings, adhesives and backing materials must be optimally matched and must suit the industrial processing conditions. WACKER tests this out in its new DEHESIVE® Coating Center.
Not all labels are the same. While product information should permanently adhere to a shampoo bottle, the price label on a water melon must peel off easily. Different adhesive mixtures make specific applications of self-adhesive materials possible, but also present the coatings industry with the challenge of providing a constant stream of new, modified silicone formulations for release liners. In addition, the substrate itself and subsequent processing play a role in deciding the composition of the release coating. For automated dispensing of labels, for example, it is important that the paper lattice between the individual labels – the matrix – can be pulled off easily without tearing or removing the labels from the backing material as well.
At WACKER’s DEHESIVE® Coating Center in Burghausen, engineers work on a 380-square-meter area to simulate industrial processing conditions for silicone release coating and to devise coating solutions to meet various requirements.
Testing and Optimizing
The center combines a pilot coater, a test lab and a large selection of base papers and films typical of the industry under one roof. “The pilot plant allows us to simulate our customers’ coating conditions and to test DEHESIVE® release formulations on the desired substrate and then optimize it,” explains Dr. Hans Lautenschlager, who is in charge of technical support for silicone release coatings at WACKER. The team led by Lautenschlager is currently testing formulations for seven different substrates supplied by a US partner. There is a choice of 30 silicone polymers and five crosslinking agents available for the coating. Depending on their composition, the individual formulations vary with regard to flow properties, release force and curing rate.
The pilot coater at the new DEHESIVE® Coating Center in Burghausen: WACKER engineers work on a 380-square-meter area to simulate industrial processing conditions for silicone release coating.
At the pilot coater, a 1 to 1.3-µm-thin layer is applied to the backing material and then dried in an air-flotation dryer at 100° to 180 °C – depending on the material properties – for 1.2 to 18 seconds. Here, the WACKER experts simulate the customer’s industrial processing conditions exactly.
The subsequent use of the release liner is also of great importance. In industrial labeling, a machine applies up to five labels per second to packaging – such as a shampoo bottle. The skill of the laminate manufacturer now lies in finding a compromise in the release force between the label and the release liner so that both matrix peeling and labeling run smoothly.
Double-sided adhesive tape poses another challenge for laminate manufacturers. The release liner must feature two different release forces for the two sides, so that the adhesive tape peels off of the underside of the release liner first. This allows the double-sided adhesive tape to cleanly peel off for further processing.
Testing with X-Rays
At the DEHESIVE® Coating Center lab, WACKER engineers determine the release-force profiles of different coatings using customary test methods.
On a lab bench of the DEHESIVE® Coating Center, a color test provides information on the level of cover of a coating right after it has been applied. X-rays measure the thickness of the applied silicone release layer. After it has cured, the release coating still contains reactive groups that can interact with the adhesive during storage. That’s why WACKER engineers also perform long-term tests to ensure that the release coating still meets quality requirements after prolonged storage. To determine whether the curing reaction is complete, the coated substrate is placed in solvent, which dissolves out any uncured silicone. The amount of such uncured silicone is then measured analytically. As a result, the amount of platinum required can be determined precisely. Optimizing the formulation can therefore reduce platinum consumption by as much as one-third.
“The paper quality available on the market is constantly changing. And new adhesives are continuously posing challenges, too.”Dr. Hans Lautenschlager, Head of Technical Support for Silicone Release Coatings at WACKER SILICONES
Technical service engineers measure the coating’s release force electronically with the aid of a peel-force measuring device. Different tests show how the coating behaves at a peel angle of 90 degrees or 180 degrees, for example.
The test results are illustrated graphically. The DEHESIVE® Coating Center’s database already contains 50,000 test results. However, if you think that this means that the Care & Coatings business team will soon have covered all conceivable material constellations, you are mistaken. “The paper quality available on the market is constantly changing. And new adhesives are continuously posing challenges, too,” explains Lautenschlager. He believes that, while the test results to date point the way ahead, they are by no means definitive.
Release coatings thus require further research, so that shampoo, melons, etc. will be optimally labeled in the future, too.
The highly branched star polymers of the DEHESIVE® SFX product family yield silicone layers with comparatively flat release-force profiles. For example, the release force of a DEHESIVE® SFX 250-based layer (red line) rises less at an increasing peel rate than the release force of a standard product such as DEHESIVE® 924 (blue line). Both silicones have a viscosity of around 200 mPa·s. The release force was measured with the aid of a Tesa A 7475 adhesive tape. In the diagram, the peel rates that are typical for slow peeling (0.3 m/min), machine labeling (10 m/min) and rapid matrix peeling (300 m/min) are marked.