Dissolution of adhesive resins present in plastic waste to recover polyolefin by sink-float separation processes

Highlights

• A new approach to dissolution of adhesive resins with renewable solvent.

• Polymeric waste from paper recycling process was separated through the sink-float.

• Characterizations of separated and standard material have a satisfactory similarity.

• The process proposed show economic viability.

Abstract

This study investigated the dissolution of adhesive resins present in polyolefin films that cause plastic materials to adhere to each other. The process of dissolution was made by the use of ethyl acetate and followed by separation through the sink-float process. The objective was to separate and characterize polyolefin films from plastic solid waste derived from recycled post-consumer paper. Through these procedures, 6% polyethylene of high-density (HDPE), 14% polyethylene of low-density (LDPE) and 39% polypropylene (PP) were separated and recovered from plastic waste. Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analyzes (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were conducted to determine the chemical, thermal and mechanical properties of the recovered polymers and to establish a comparison with standard commercial polymers. It demonstrated that recovered material kept their chemical, thermal, and mechanical properties. This process indicates possible economic viability considering the demand, the market value of the PP, and the required investment to be implemented in the recycling process that could be amortized in a short period of time. Moreover, the organic solvent used in the dissolution process can be easily recovered by distillation.

Introduction

Plastic materials are indispensable and play an important role in the modern lifestyle due to its light weight, versatility and low production cost (Al-Salem, 2019; Geyer et al., 2017). Its vast use makes recycling an essential measure for sustainable development (Al-Salem et al., 2017). The waste generated in the world has increased, the world's cities generated 1.3 billion tonnes of solid waste in 2012 and it is projected to rise to 2.2 billion tonnes by 2025 (World Bank, 2017). Recently, the major cellulose and paper industries have enabled the use of recycled paper in their industrial processes (Azevedo et al., 2018). The product based on post-consumer packages processed in those industries has significant contamination of plastics from the most varied types. The separation of paper from plastic is relatively simple and it is a common practice in industries. However, the amount of plastic waste generated from this process is significant and difficult to be reused due to the presence of adhesive resins. The paper sector used approximately 215 Mt of discarded paper for recycling (Van Ewijk et al., 2018; Van Ewijk and Stegemann, 2016), if assumed that paper waste for recycling is contaminated by approximately 1%wt of plastics (Mumbach, 2017) more than 2 Mt was generated, creating high demand for industrial landfills, or leading to inappropriate discard, which can negatively affect the environment (Aljerf, 2016). Therefore, to avoid such issues, sustainable and efficient plastic waste treatment is essential.

Several works in the field's literature report the recycling of post-consumer plastics by thermal, chemical or thermochemical mechanisms (Singh et al., 2017), which usually involve toxic compounds and high costs (Gil-Jasso et al., 2019). In contrast, routes using dissolution by natural solvents combined with mechanical methods are poorly reported. These routes own great environmental relevance, as they are the most environmentally friendly and profitable alternatives (Zhao et al., 2018).

Examples of technologies applied in the separation of polyolefin are the sink-float (Pongstabodee et al., 2008), froth flotation (Wang et al., 2015), and magnetic density, with this last one occurring in the inverse magnetic density separator (IMDS), which uses the combination of a gradient magnetic field and a magnetic liquid (Serranti et al., 2015), more information about techniques for polymer separation are listed in Table 1.

The sink-float method is simple to apply in separation by density in aqueous media (Pongstabodee et al., 2008), and it has shown to be effective for the separation of some polymers, such as polyolefin (HDPE, LDPE and PP) which usually presents low density difference (Hu et al., 2013). The effectiveness of this method depends on the adjustment of the aqueous medium density to the desired fluctuation level and the absence of glue materials, such as adhesive resins, that would disrupt the density separation process. The adhesive resins present in the residue prevent an efficient separation of the plastic materials by density method once it causes plastic materials to adhere to each other. Thus, a natural solvent from a renewable origin (such as ethyl acetate, boiling point 77,1 °C) can be a good complement to this process, as it is able to solubilize resins based on polystyrene (PS) (Cella et al., 2018), and remove paints and other dirt. A great number of studies in the field work with known percentages of each material in the waste mixture (Pongstabodee et al., 2008), however, in this study, the material percentages considered were not controlled, as well as, the amount of each material present in the residue, once it could vary from day to day, according to what was available to conduct this study's experiments.

The objective of this work is to present a study with a new approach to the recovery of plastic materials, derived from the process of recycling paper scrap. The process of recovery combines the dissolving of adhesive resins in ethyl acetate with separation by the sink-float process in an aqueous medium, used to separate the polyolefin of interest from other materials present in the plastic scrap. The chemical, thermal, and mechanical properties of the separated materials were characterized by techniques of Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). The characteristics of the separated polypropylene (PP) presented high similarity with those of the standard PP, which suggests low contamination by other materials. The obtained results were promising and indicated the viability of the process.