Recycling of post-consumer multilayer Tetra Pak® packaging with the Selective Dissolution-Precipitation process

Highlights

• Hydropulping and selective dissolution/precipitation separate Tetrapak® constituents.

• Effective separation of paper, AL and LDPE is achieved.

• LDPE from inner layers exhibits properties comparable to virgin LDPE grades.

Abstract

Tetra Pak cartons, i.e. multilayer packaging consisting of paper, low density polyethylene (LDPE) and aluminium, are widely used as aseptic packages in the food and beverage industry. Paper can be easily recovered from this type of post-consumer beverage cartons (PCBCs) by the pulp industry. The remaining LDPE/aluminium by-product, however, cannot be easily separated to its constituents so currently it is mostly downcycled or used for energy recovery. The aim of this work is to develop an effective process that separates and recovers LDPE and aluminium of high purity and good quality from Tetra Pak cartons. To this purpose, following paper recovery by hydropulping, LDPE and aluminium are separated using a selective dissolution-precipitation (SDP) process. The recovered LDPE is characterized by FTIR spectroscopy, thermogravimetric analysis (TGA), differential scanning (DSC) and melt flow rate (MFR) measurements. The findings reveal that the SDP separation process is an effective method to recover LDPE and aluminium from PCBCs.

Introduction

The composite packaging of liquid food, commonly known as Tetra Pak®, was an idea of Erik Wallenberg, patented by Ruben Rausing and introduced into the market in 1952 (Rausing, 1969; Tetra Pak International, 2020a). Although this original packaging was initially made of waxed paper, today multilayer Tetra Pak® packaging is a result of laminated stiff paper (75 wt%), LDPE (20 wt%) and aluminium foil (5 wt%) (Rausing, 1969; Tetra Pak International, 2020b). A typical structure consists of 6 layers which are shown in Fig. 1 while the role of each layer is presented in Table 1.

According to Tetra Pak Company – part of Tetra Laval Group – in 2019 over 160 countries around the world were supplied with more than 190 billion Tetra Pak® packages while post-consumer Tetra Pak® cartons were recycled by Tetra Pak itself with a rate of 26% (Tetra Pak International, 2020d).

The technologies generally applied to PCBCs are divided into those that reprocess them as a mixture and those that include a prior step for removing the cellulosic fibers.

In the first case, PCBCs are subjected to thermal processes (incineration, pyrolysis, gasification) for energy recovery along with other municipal solid waste (MSW). Such a treatment, however, is not efficient due to the low heating value and other characteristics of paper (moisture content, high ash value) which constitutes the main material of beverage cartons. Alternatively, PCBCs are downcycled and used for the production of laminated boards or various products of different shapes by hot pressing shreds of PCBCs (Zawadiak et al., 2017).

In the second case, removal of paper is carried out through hydropulping: in the presence of water, cellulosic fibers are separated from plastic and aluminium layers by centrifugal forces (Selke, 2016). Residuals of paper recovery, i.e. outer LDPE layer and Al-PE laminate, usually contain up to 5% paper fibers. They are mostly used in thermal processes for energy recovery due to their high heating value. A typical application is their use in cement industry, since the Al₂O₃ that is formed is useful for cement production (Zawadiak et al., 2017). Gasification applications also exist, such as Alcoa Aluminio, Klabin and TSL Ambiental plant in Brazil which uses plasma gasification to transform plastic into paraffin and recover aluminium in its pure form (BusinessWire, 2005). Also, pyrolysis is applied in Stora Enso's facilities in Spain exposing the Al-PE residue from the hydropulping process to 400 °C in an oxygen free chamber. The plastic evaporates and is then used to generate electricity and steam while Al remains and can be used to make new aluminium products (Project CLEAN, European Commission).

On the other hand, Al and PE can be also recovered from the Al-PE laminates by breaking the mechanical bonds that keep the layers together. Zhang et al. (2009) achieved Al-PE delamination using aqueous solutions of organic acids. Al partially dissolved in the solution, so both Al and acid were consumed to some extent. Reaction time, temperature, solid size and the type of acid affected separation time and Al loss, which for the optimum conditions was 4.7%. Delamination using mixed organic solvents has been also investigated by Zhang et al. (2014). In this case delamination occurred due to the swelling of PE. Some material loss was observed, lower that the one obtained with separation with organic acids. Concerning the efficiency of the separation, SEM images of the PE and Al surfaces indicated molecular structure consisting of C, O and Al (Zhang et al., 2015). Al-PE delamination has also found industrial application in China. (Zawadiak et al., 2017).

Finally, solvent-based PE extraction methods have been studied for the separation of Al-PE laminates coming from PCBCs, where PE is dissolved in an organic solvent and separated from the undissolved Al (Kaiser et al., 2018; Lindner, 2011; APK AG). This type of processes, referred to as Selective Dissolution-Precipitation (SDP), is grounded on controlling and adjusting polymer's solubility by changing solvent and/or dissolution conditions and can be effective either for separating mixtures of plastics or for removing impurities/additives from post used plastics. The basic steps of SDP for recovering a certain polymer with high purity involve dissolution of the polymer of interest in properly selected solvent (S) and conditions, removal of the non-dissolved material through filtration and addition of a non-solvent or anti-solvent (AS) so that the polymer precipitates again (Zhao et al., 2018). Starting from four decades ago, SDP has been extensively studied and successful experimental work has been carried out for recycling a variety of high quality polymers from post-used plastics, such as PVC (Achilias et al., 2009b; Kampouris et al., 1986; Papaspyrides and Diakoulaki, 1986), PS (Achilias et al., 2009a, 2009b; Kampouris et al., 1987;1988; Schlummer et al., 2017), LDPE (Achilias et al., 2009b; Achilias et al., 2007; Papaspyrides et al., 1994), HDPE (Achilias et al., 2007; Poulakis and Papaspyrides, 1995), PP (Achilias et al., 2009b; Achilias et al., 2007; Poulakis and Papaspyrides, 1997), PET (Achilias et al., 2009b; Poulakis and Papaspyrides, 2001), PC and ABS (Achilias et al., 2009a). Also, successful separation of mixtures comprising the most common plastics found in municipal solid waste has been achieved, such as separation of LDPE/HDPE/PP (Hadi et al., 2012) and LDPE/PS/HDPE/PETE mixtures (Kannan et al., 2017) in laboratory scale but also separation of LDPE/PP mixture (Pappa et al., 2001) in pilot scale.

Interest on SDP has gradually faded, mainly because of the low cost of polymers, but lately it has boosted again, since the need to obtain high purity recyclates from plastic waste is recognized (Sherwood, 2019; Ügdüler et al., 2020). Introduction of a variety of chemical additives into polymers to improve their performance depending on the application, as well as development of composite materials with specific characteristics have resulted to plastic waste of complex composition, which cannot be effectively sorted by mechanical methods and still end up in landfill or are recycled into poor quality plastics (Wagner and Schlummer, 2020). SDP offers the ability to remove additives (Schlummer and Mäurer, 2006) and reinforcements from polymers (Cousins et al., 2019; Knappich et al., 2019; Tapper et al., 2019) and recycle polymers with quality that meets that of their virgin counterparts, broadening the closed-loop recycling options.

This work aims to the effective separation of PCBCs to their constituents. To this purpose hydropulping is applied for paper removal followed by SDP to separate the Al-PE laminate. Hydropulping is a simple, well-known and widely applied technique, also for paper removal from PCBCs, so in this study it is used to provide the Al-PE input material for SDP and is examined only in terms of the recoveries it yields. On the other hand, with regards to SDP, this paper aims to prove its technological feasibility for the separation of Al-PE to recyclates of high quality with high recovery ratios. In this context, the purity of the separated Al and LDPE are examined while for LDPE several properties related to its quality and reprocessability are also investigated.