Adhesives make life simple – a maxim that applies in many technical applications just as it does in everyday life. In this case, however, “simple” refers only to the handling: self-adhesive products such as labels are actually sophisticated, multilayer systems. In addition to the adhesive and the label itself, a laminated label also includes a backing – also known as a release liner – made of paper, plastic, or other materials. If the label is to work properly, however, an invisible yet crucial fourth layer also comes into play: thanks to this silicone layer, adhesive products can be peeled off of backings quickly and without leaving residue. Applications for these silicones other than labels include adhesive tape, and release papers and films used in the home and in industry.

The global market for self-adhesive products is growing continuously at a rate of four to five per cent each year. Volume currently amounts to roughly 40 billion sq m of material a year, which equates to several billion US dollars in sales worldwide – 30 per cent of which is now generated in Asia. Growth in demand for modern labeling systems has been especially great in emerging markets, where the market for consumer goods is growing at a remarkable pace. Packaging applications play a particularly important role here. According to the German Packaging Institute, 180 billion packages are produced each year in Germany alone. Most of these require one or more labels bearing important logistics, commercial, and consumer information.

The release coating is the deciding factor when it comes to how self-adhesive products work and how they are applied. While traditional label systems incorporate thermally cured silicones, a critical disadvantage of these materials is their highly energy-intensive curing process – crosslinking does not proceed until temperatures reach 100-degC or more. Plus, thermally curing release coatings are not suitable for the growing thermal paper market, as the elevated temperatures of the curing process render the thermally sensitive paper unusable.

UV-C COATINGS SAVE ENERGY

Release liners made of radiation-curing (RC) silicones therefore represent a superior alternative. It is UV-Curing (UV-C) radiation rather than heat that causes the silicone pre-polymers in these products to crosslink and thus to cure, a process that takes place in a fraction of a second using the medium-pressure mercury-vapour lamps commonly used in industry. RC silicones allow manufacturers to make flexible, versatile backings not only from paper, but also from temperature-sensitive plastics. Radiation curing also requires less energy than its thermal counterpart.

Evonik, an industrial group from Germany and one of the world leaders in specialty chemicals, leads the global market for RC silicones. The Tego product line of RC silicones has been put to the test for many years now to determine whether it perfectly meets customers’ sophisticated technical demands, as well as their commercial and environmental expectations. RC silicones fall into two different groups based on their chemistry. The first are the silicone acrylates: Because substances in this group must be in an oxygen-free environment to undergo free-radical curing, Evonik has developed a special nitrogen purge that keeps residual oxygen content below 50 ppm. The second group is made up of epoxy silicones, which undergo cationic polymerisation and, as a result, do not require the reaction chamber to be flushed with nitrogen. Over the past several years, Evonik specialists have developed around a dozen different radiation-curing silicones, each with different release values tailored to customers’ specific requirements.

GLASSINE PAPER

It remains an undisputed fact that traditional self-adhesive products have been perfected and have stood the test of time. They do, however, have one serious disadvantage: the backings used in traditional systems are the source of roughly 40 per cent of these systems’ material costs and weight. In standard products, the backing is made of glassine paper, and after the labeling process, it becomes garbage. Roughly 400,000 metric tonnes (mt) of glassine paper are disposed of each year in the EU alone, a figure that rises to around 1.2 mt worldwide.

Supermarkets, which use thermal labels quite frequently for labeling perishables, have already gone linerless

To put it another way, each year more than one million mt of high-quality cellulose paper is simply thrown away. Attempts at reducing costs and waste have consisted of making the release paper thinner, or switching over to other types of paper or recyclable plastics. At best, however, these approaches can lessen the waste problem, but not eliminate it.

Systems that use no backing at all – known as linerless technologies – are far more sustainable. Evonik has been working with machine manufacturers and printing firms to press ahead with bringing these alternatives to market. With linerless products, labels are no longer fixed to a paper or plastic backing (release liner), but are instead wound up much like a roll of tape. The role that RC silicones play in linerless labels is likewise to ensure that, prior to processing or dispensing, the layers peel off of each other quickly, cleanly, and without leaving a residue. The key difference here, however, is that the release coating is applied directly onto the face of the previously printed label, or soon to be printed thermal image label.

OMITTING LINERS

The advantages are obvious: for customers, doing away with this layer eliminates the costs of the paper or plastic liner, reduces the costs of materials and disposal, and prevents considerable amounts of waste. Storage and logistics expenses decrease as well. And last but not least, linerless applications represent a more compact way of dispensing labels, saving space, and making production more efficient.

Evonik’s linerless method has already become established for thermal labels, which are commonly used in the food, transport, and logistics industries. Production of linerless thermal labels is currently the industry’s fastest growing market segment and has become the standard for new thermal printers. Customers  appreciate how easy to use, lightweight, and compact linerless labels are.

Linerless systems are also part of a clear trend that has been emerging on the market over the past several years: More and more printing firms are using machines that siliconize material immediately after printing. This in-house siliconization process liberates printing firms from label laminate manufacturers and adds value to printers’ processes.

Linerless application is also on the rise in the packaging sector, with Evonik supplying raw materials in close cooperation with other players in the value-added chain. Foremost among these players are machine manufacturers, who develop equipment such as label applicators, and silicone application and curing systems.

One of Evonik’s most important partners is the British firm Ravenwood Packaging Ltd, which develops a variety of machine systems for linerless label production and use. Ravenwood uses only Evonik RC silicones in its machines, as this is where the advantages of these silicones versus thermal curing release coatings come into play.

Evonik partner ETI is taking a different approach. For several years, this Canadian machine manufacturer has been producing machines for in-house siliconization and has recently begun making label dispensers as well.

The Catchpoint company has patented a concept of its own. Here the printer’s production machine includes a special roller that creates microperforation lines on the label roll, and these lines serve as tearing points, making it much easier to detach labels from an automated dispenser. Existing labeling lines can be reversibly converted to linerless materials with Catchpoint perforations.

USABLE ON OLD EQUIPMENT

New labeling machines are not the only kind of equipment suitable for linerless labels – Evonik launched a pilot project in 2013 to determine whether Tego RC silicones could also be used on existing equipment. An application system at the Krefeld site was retooled to affix new linerless labels to soap bottles. Pilot operation lasted for two weeks and fully automated application worked perfectly during this time. The main aspect to take away from the project was that even existing labeling machines can be retooled quickly and economically, allowing manufacturers to switch between normal labels and linerless labels quickly.

Evonik is working with industrial partners to continue moving linerless technology forward. The importance of linerless systems will continue to grow insofar as they improve equipment efficiency and fully automate machinery, and as expectations of sustainability rise. And last but not least, self-adhesive products – regardless of how simple they appear at first glance – clearly show how important it has become to harmonise raw materials and production processes. Only by working together closely can the players involved secure markets for the future and ensure an innovation process that will lead to solutions meeting all of the customer’s needs: considerable leeway for design, highly efficient application, maximum label quality, and preservation of our finite resources.

Authors: Dr Winfried Hamann, Georg Michels, Liz Patterson, Jürgen Pomorin and Dr Stefan Stadtmüller