Plastics for multi-layer composite packaging, usually composed of multiple materials such as plastic, paper and metal, can cause recycling barriers for plastics. In this case, safe handling is required to avoid environmental contamination, and pretreatment steps before recycling may be critical.
This article will take Tetra Pak as an example to introduce 6 ways of recycling multi-layer composite packaging.
Tetra Pak is a multi-layer packaging mainly used in the food, pharmaceutical, chemical and daily necessities industries. Tetra Pak bags are usually composed of three materials: cardboard, aluminium and low density polyethylene. Its composition is as follows: 71% cardboard, 24% plastic, 5% aluminum foil.
Tetra Pak Packaging Laminate
Cardboard, plastic, and aluminum foil make up the six layers of the Tetra Pak. Each layer has a specific purpose. But this is not absolute. For example, some products with a short shelf life do not require the protection of an aluminum layer. On the other hand, when the above-mentioned shelf life needs to be extended, the LDPE layer can be replaced by a PP layer, which provides an opportunity for further heat treatment of the product. HDPE, PET and PA are also possible options to replace the LDPE layer. Finally, polyurethane and EMAA are often used as adhesives between layers, while Tetra Pak may also contain various chemical additives such as plasticizers, stabilizers, lubricants, fillers, blowing agents, colorants, flame retardants agent and antistatic agent.
Tetra Pak packaging is mainly composed of paper, and the removal and recycling of the cardboard layer is very important. Therefore, there are two main processing routes: anhydrous pulping and hydropulping recycling. The initial procedure treated the carton as a whole, while the latter used hydropulping technology to first separate the fibers from the Al-LDPE laminate.
1. Anhydrous pulping recycling
The main purpose of following this route is energy recovery or cycle reduction. Energy recovery is achieved through pyrolysis, gasification or incineration in combination with municipal solid waste. However, this approach has many disadvantages. Paper - the main component of Tetra Pak boxes, has low thermal combustion (16MJ/Kg), high moisture content and significantly high ash value. This makes the whole process inefficient, so it's usually not widely used.
2. Hydraulic pulping recycling
Before making this selection, it is necessary to briefly review the hydropulping process. When the material to be recycled first arrives at the recycling unit, the hydropulper uses rotating blades to separate the paper, using high-pressure water to create a fibrous pulp. Further processing is followed by centrifugal cleaners that remove heavy materials such as sand, adhesives, staples, etc. The end result of this process is a cellulose fiber pulp that can be used as a substitute for wood pulp for the production of kraft paper and wood pulp boards. The rest of the process is the outer LDPE layer and the Al-LDPE laminate. However, residual cellulose fibers can account for up to 5% of the finished product.
3. Pyrolysis
The appeal of this approach is its simplicity and cost-effectiveness. The pyrolysis process is divided into two steps: (a) degradation of paper (200-400 °C) and (b) devolatilization of LDPE (420-515 °C). It should be noted that temperature plays an important role in the composition of the final product. For example, the higher the temperature, the less coke is formed, and the opposite is the case for wax.
The next solid products are aluminum, coke from paper degradation, wax and tar from LDPE degradation. A large number of gaseous products are formed, mainly composed of CO2, CO, H2, CH4, C2-6 hydrocarbons and volatile substances. Finally, there is an aqueous phase, consisting of water and phenol.
These extracted pyrolysis products have many uses. The gas produced can be used to sustain the pyrolysis process itself, or it can be used elsewhere entirely. Coke and tar can be developed as solid fuel and petroleum fuel, respectively, and coke can also be used as the main resource for the production of carbon-based materials. Waxes and water phases can easily be used as raw materials for the chemical industry.
Researchers in Mexico and Spain have taken a novel approach, using coke and aluminum from the pyrolysis process, allowing them to act as sorbents for mercury in an aqueous solution. Through trial and error and thermodynamic analysis, they concluded that the semi-char obtained by pyrolysis at 600 °C for 3 h showed the most promising mercury adsorption capacity at 21.0 mg/g. The field of coke adsorbents is still expanding, and Tetra Pike pyrolysis coke is expected to become the main adsorbent for the industry in the future.
4. Thermal selective dissolution-precipitation process
The basic principle of this method is to soak Al-PE laminates in a carefully selected solvent under specific temperature conditions, with the aim of dissolving LDPE in the solvent. The next step is to remove additives and impurities by filtering. The anti-solvent is added last and the dissolved polymer then precipitates. To maximize the recovery of LDPE and pure aluminum, the SDP process was repeated three times.
The produced LDPE is of comparable quality to the original product, and the recycled aluminium is also of high purity. Together with the hydropulping process, this is a very promising option for Tetra Pak packaging recycling. However, this process is not without its drawbacks: firstly, because of the cost-effective energy consumption required to separate the solvent-antisolvent mixture, and secondly, because of its high environmental impact, the economic viability of the technology depends on the solvent-antisolvent mixture can be separated cheaply.
5. pH stratification
The technique, developed by researchers in China, separates LDPE and aluminum through a separation reagent, primarily an aqueous organic acid solution, or even an acid mixture. The procedure works by breaking the mechanical bonds so that the laminations come together, which allows the product to be recycled.
The yield of this process depends to a large extent on the reaction conditions. During this process, some of the aluminum will be dissolved by the acid and the acid will be consumed, so losses are inevitable. However, this depends on many factors such as the acid used, temperature, etc. Product purity is also related to these factors. After trial and error, methane acid was found to be the best separation reagent for Tetra Pak. Finally, there appears to be a high correlation between the rate of separation, the temperature at which the reaction occurs, and the concentration of reagents. More specifically, the reaction time decreased with increasing reagent concentration and temperature.
6. Heat recovery and material recovery
Due to the high calorific value (40 MJ/Kg) of the Al-LDPE layer, it can be used as a sufficient fuel source. This is a precedent, especially in Europe. While laminate can be used directly after the hydropulping process, it is often used with other fuel sources. This recycling method can be considered environmentally friendly because Tetra Pak's LDPE burns cleanly and does not produce fumes containing elements such as sulfur, nitrogen or halogens.
Furthermore, under high temperature conditions, through the reaction of aluminum and moisture, Al2O3 produced during pyrolysis is largely utilized by the cement industry as an ideal component for cement production. Finally, there is an option to use laminates directly for roof tile production, injection and rotational molding, PE-Al agglomeration and pulverization.
