Elsevier

Journal of Cleaner Production

Volume 237, 10 November 2019, 117739
Journal of Cleaner Production

Reuse of reclaimed tire rubber for gas-separation membranes prepared by hot-pressing

https://doi.org/10.1016/j.jclepro.2019.117739Get rights and content

Highlights

  • Hot pressing was developed as a new synthetic technology for membrane fabrication.

  • Reclaimed tire rubbers was recirculated into the gas membrane by hot-pressing.

  • Reclaimed tires derived membranes showed ideal CO2/N2 separation performance.

Abstract

Waste tires are mainly composed of natural rubber, synthetic rubber, carbon black, steel wires, and fibers. Rubber, which is a valuable resource, is one of the raw materials used for membrane preparation. In this study, a convenient and environmentally friendly synthetic technology, hot-pressing method was developed for reusing reclaimed tire rubber to prepare gas-separation membranes. The effects of the preparation conditions, including hot-pressing temperature, pressure, and time on the membrane structure and separation performance, were investigated. Thermogravimetric analysis and field-emission scanning electron microscopy were carried out. The Thermogravimetric analysis, Fourier-transform infrared spectroscopy, and cross-linking degree results showed that the reclaimed tire rubbers used in this study consisted mainly of styrene-butadiene rubber and polybutadiene rubber. Reclaimed tire rubbers with low carbon contents and low degrees of cross-linking are beneficial for fabricating membranes by hot-pressing. The hot-pressing technique proposed in this study yielded dense gas-separation membranes from the reclaimed tire rubbers, proved to be an efficient approach for simplified recycling of waste tires and can in fact inspire the development of new recycling routes. The hot-pressed membranes derived from reclaimed tire rubber showed competitive results with a CO2/N2 selectivity of 12.8 and CO2 permeability of 161 Barrer.

Introduction

In industrially developed and prosperous countries, transportation is relatively convenient. Tires are an important part of the means of transport used in everyday life such as cars, trucks, and two-wheelers. The increasing transportation demands, though beneficial for the tire industry, are also leading to the accumulation of waste tires (Fang et al., 2001).

These waste tires do not decompose easily as they are complex objects consisting of many components such as natural or synthetic crosslinked rubbers, carbon black, steel, and other additives (Luo et al., 2018), which lead to a long degradation time in nature. The accumulation of waste tires leads to a serious waste management issue and a persistent environmental pollution. To build a sustainable economy, several waste management strategies have been proposed to deal with the increasing amount of waste tires including tire-derived fuel, waste tire reuse/retreat, and reclamation of waste tire rubber (henceforth referred to as reclaimed tire rubber, RTR).

Waste tires are mainly composed of nature rubber (NR) and synthetic rubber, and can thus be used as a candidate material for membrane fabrication. Polymer chain mobility is recovered by transforming three-dimensional structures into two-dimensional structures through cryo-mechanical or mechanical-chemical processes (Zhuang et al., 2016). The author previously developed and fabricated RTR into a gas-separation membrane through the solution-casting method. The membrane exhibited excellent CO2/N2 separation performance (Zhuang et al., 2018). Owing to its efficiency, low operating cost, and easy modularization, membrane-based gas separation is a popular technology applying in industrial process. Over the past few years, membranes have been widely used in high-degree precision separation processes, which cannot be carried out by using traditional procedures. The global demand for membranes is predicted to increase by 8.6% per year and the market output can increase up to USD 2.64 × 1010 by 2019 (Hanft, 2010).

The commonly used membrane materials include polymers, metals, zeolites, and carbon molecular sieves. Among these, polymer membranes are used for a large number of practical applications owing to their easy preparation and low cost. Polymeric materials are mainly classified as plastics or rubbers. Rubber-derived membranes usually exhibit high permeance to gases with high critical temperatures (e.g., CO2 and CH4) owing to their high affinity for polar gases and low crystallinity (Yampolskii, 2012). Accordingly, rubber-derived membranes such as NR, butadiene rubber (BR), and polydimethylsiloxane (PDMS) are popular precursor materials that exhibit high CO2-selective diffusivity and solubility, which stems from their high molecular chain mobility and excellent affinity for CO2 (Zhuang et al., 2018). Rubber is an amorphous elastomeric material with a structure similar to that of entangled yarn. As rubber chains rotate around single bonds, they can move easily. When rubber is subjected to heating or stress, it becomes very sensitive to the strain rate. For example, the glass transition and softening temperatures of cis-polyisoprene are 70 °C and 35 °C, respectively.

Various methods such as leaching, casting, and phase inversion have been used to develop polymeric gas separation membranes. The phase-inversion method is the most commonly used method for preparing membranes but it requires additional organic solvents, which cause a secondary environmental pollution. Another simple polymer processing method, which is more environmentally friendly is the hot-pressing process, as it only uses heat to soften the polymer chains without using organic solvents. After extrusion (between two hot plates) and cooling, the pressed polymer is solidified to obtain a thin film. This process has been used in membrane electrode assemblies (Therdthianwong et al., 2007), fuel cells, and automotive-components (Lee et al., 2009).

In this study, a waste-to-product method was developed using the hot-pressing technique to prepare membranes with gas permeance and separation characteristics using RTR as the polymer precursor. Four kinds of RTRs were purchased from commercial sources. The effects of the RTR composition and hot-pressing parameters including the hot-pressing temperature, pressure, and time on the gas-separation performance of the RTR-derived membranes were investigated to optimize the parameters. This study thus proposes a potential technology to convert waste resources into high-value materials.

Section snippets

Sample collection and characterization

Four kinds of blocky RTRs: RA, RB, RC, and RD were obtained from the local markets (see Fig. 1(a)) and were used as the precursors to prepare gas-separation membranes. These rubber materials were reclaimed from waste tires using the high-temperature desulfurization method. The analysis of the RTRs was carried out by thermogravimetric analysis (TGA, PerkinElmer, STA6000). Prior to the analysis, the samples were dried at 60 °C for 24 h. The dried samples (approximately 6–8 mg) were then placed in

Compositional analysis of the RTRs

Fig. 1(b) shows the TGA results of the RTRs. As can be observed, RA exhibited two stages of thermal decomposition, which is characteristics of a typical RTR (Lee et al., 2009). The first decomposition window was observed between 225 °C and 450 °C with a large weight loss of 60%. The residual weight remained constant until 700 °C. This indicates that RA consisted of 60% polymer and 40% carbon black. All the other RB, RC, and RD exhibited only one decomposition window from 300 °C to 700 °C. There

Conclusions

In this study, the author developed a facile technique to prepare dense gas-separation membranes from RTR, which might open up a new recycling pipeline. The RTRs used in this study showed (1) low carbon black contents, (2) low cross-linking densities, and (3) high degrees of desulfurization. Because of this, they were suitable as the precursors for membrane preparation by hot-pressing. The dense membranes prepared without using a middle hollow mold exhibited high permselectivity. These rubber

Acknowledgement

The authors gratefully acknowledge the financial support from the Environmental Protection Administration Taiwan (EPAT) (No. EPA-106-XA02).

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