Case study: the recycling of paper-based silicone release liners
LINER RECYCLING INITIATIVE
RECYCLING PAPER-BASED SILICONE RELEASE LINERS:
HOW THIS IS DONE IN PRACTICE
INTRODUCTION – WHAT IS A PAPER BASED SILICONE RELEASE LINER?
Paper-based silicone release liner is the name given to the part of a self-adhesive label laminate which needs to be peeled away from the label itself before the label is then applied to the final article or surface. It’s role is to protect the self-adhesive part of the label before the label is applied to the final surface, at which point the release liner is no longer needed and ready to be recycled. Typical paper-based silicone release liners are based on a lightweight glassine paper which is coated with a thin layer (roughly 1 micrometer thick, or 0.001 of a millimeter) of a silicone elastomer.
RECYCLING OF PAPER-BASED SILICONE RELEASE LINERS
Paper-based silicone release liners, like many other grades of paper, can be (and indeed are already) recycled.
In some cases, this is occurring where the paper-based release liners are being collected together with other paper grades in large scale paper recycling processes. In some of these paper recycling processes there can be a challenge to break up the silicone coated paper, but provided that sufficiently energetic pulping conditions are utilized, the silicone coated release liner should be relatively straightforward to re-pulp. In this respect it is normally recommended to prefer batch re-pulping equipment over continuous re-pulping equipment in order to overcome the naturally hydrophobic nature of the silicone coating. The concentration level of silicone release liner which can be included in such mixed paper recycling processes varies significantly and depends on the process employed and the intended end application for the recycled paper fibres.
In other cases, the paper-based silicone release liners are handled using dedicated recycling processes where separated streams of 100% paper release liner are taken and recycled using optimized separation processes that have been specifically developed for the release liner. The benefit of this approach is that due to the high quality of the paper fibres used in the production of the base papers for silicone release liner (such as glassine), the high quality fibre that is recovered as pulp can be re-used for a wide range of paper grades including be recycled back into the production of glassine which can then be used as a base paper for release liner once more.
CASE STUDY – RECYCLING OF PAPER BASED SILICONE RELEASE LINERS IN A ‘DEDICATED’ RECYCLING PROCESS
Whilst there are cases today where paper-based silicone release liners are recycled at low concentrations with other paper grades in general paper recycling processes, the most successful recycling approach has been to recycle them in more specialized/dedicated recycling processes.
A current example of this process is the one being used by UPM PLATTLING mill in Germany. The Plattling paper mill receives a range of paper-based silicone release liners from a number of different industrial sources. One major source is the UPM-Raflatac “RafCycle” scheme where release liners are returned from end-users of Raflatac’s self-adhesive materials and there are other sources from industrial users of self-adhesive laminates and labels. These are mostly glassine based papers coated with silicone which have been used to carry self-adhesive labels.
After the labels are removed from the liner during the packaging operation, the rolls/sheets of glassine are then bulked together before shipment to Plattling’s paper mill. There may be some small differences in the grades of silicone coated glassine that are collected, both in terms of their basis weight or colour, but this is not usually something that would prevent their subsequent recycling. The colours typically used for glassines in label applications are generally ‘light’ colours (white, yellow, honey and light blue), which means they can be recycled together in the process. For the darker colours of glassine (such as ‘havana’/’Topaz’), which are typically used for other applications there can be significant problems to recycle with light coloured grades as these are based on ‘colour dyed fibres’ rather than a subsequent ink/coating which might have been removed in a de-inking process. These darker grades would need to be recycled in a separate process to avoid contaminating the other grades.
Contamination of the paper release liner by other materials such as filmic materials, self-adhesive labels and other non-paper contaminants can also pose problems in the recycling process but can still be ‘handled’ at low levels in the current process.
The paper-based silicone release liners are fed directly into a high efficiency batch pulper which is designed to separate the paper fibres from each other, using recycled water from the end of the process. At this stage the materials that have been separated from the paper fibres (including a portion of the silicone release coating and fillers/binders), are held in suspension with the fibres. The mixture is then passed to a flotation process where flotation additives are added to the mix and where, through agitation and inclusion of air, the non-paper materials can be separated from the paper fibres and treated separately. For the paper fibre stream, this is followed by further washing steps where paper fibres are further cleaned before being dried and formed into pulp. For the flotation stream, there is further washing and separation before removal of water to leave a final, dried, ‘sludge’ which is collected as secondary output of the process. The vast majority of the reclaimed water is recycled back to the initial batch pulper for re-use in the next batch of pulping.
The output from the process consists of dried pulp and sludge. The dried pulp is used as a feedstock for paper production. This can be for printing paper grades in Plattling or for new glassine production in other UPM paper mill locations. Whilst there may be a low level of silicone remaining with the recycled pulp, it is not high enough to affect the re-use of the pulp in paper production. The sludge is used as a process aid in another industrial process.
The process is already running at a significant scale and planned to grow still further in the next few years. If you have or know about further examples and best practices offered by label companies around Europe, do not hesitate to reach out to CELAB-Europe.