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

A good example of recycling of paper-based release liners through a dedicated repulping process is the one being used by three WEPA’s recycling mills (in France, Germany and the Netherlands). These recycling mills have a wide range of different repulping units varying in type and throughput and are recycling many different paper types from different sources. The choice of which pulping unit to use depends on a number of different factors, but the two key factors are the type of paper being recycled and the volume of that paper stream that is available. WEPA currently receive a range of paper-based silicone release liners from a number of different industrial sources. Many of these are glassine based, silicone coated, release liners from label applicators which have been used to carry self-adhesive labels, but there are also other grades of release liners from other industrial applications.

After delivery of the spent release liner to the WEPA facility, the first stage is to separate the liner into two main ‘streams’ based on the colour of the paper being recycled. Where the release liners are ‘light’ coloured (white, yellow and honey), they can be directly pulped with streams intended for high brightness grades of pulp including a deinking process and disperser. Where the release liners are darker coloured, the fibres are blended with other coloured recycled paper streams to produce low bright recycled pulp.

The paper-based silicone release liners are fed directly into a high consistency pulper which is designed to separate the paper fibres from each other, using recycled water from the end of the process. After separation of the fibres, the non-fibrous material is removed as a waste stream (sometimes referred to as sludge). This stream includes a portion of the silicone release coating along with fillers/binders from the paper. The paper fibre stream is then passed through a further washing step where paper fibres are further cleaned and refined (based on fibre size) before being dried and formed into pulp. For the sludge stream there is further washing and separation before final 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 pulping process for re-use.

The output from the process consists of dried pulp and sludge. The dried pulp is used as a feedstock for paper production. The choice of use depends on the final end markets of WEPA, but typically their pulp is used for a combination of graphics paper grades and tissue paper grades.

The process is already running at a significant scale and planned to grow still further in the next few years. The current limitations for WEPA are concerning access to supplies of waste liner to recycle. The volume of the waste stream they are receiving is important in determining their choice of re-pulping unit as they need sufficient scale to match the scale of the specific re-pulping unit for optimal operational efficiency. WEPA have many recycling facilities in Europe which would be capable of the same recycling process for release liner using high consistency pulpers, but the main challenge is to source a reliable volume of the release liner for recycling.