Effect of expanded vermiculite on aging properties of bitumen

Abstract

Expanded vermiculite (EVMT) clays modified bitumen was prepared by using EVMT, cetyltrimethyl ammonium bromide (CTAB)–EVMT and octadecyl dimethyl benzyl ammonium chloride (ODBA)–EVMT as modifiers. The morphology characteristic of the modified bitumen was investigated by X-ray diffraction (XRD). The binders were aged by pressure aging vessel (PAV), situ thermal aging and ultraviolet (UV) radiation. The phase-separated structure is observed in EVMT modified bitumen, while CTAB–EVMT and ODBA–EVMT modified bitumen form the intercalated and exfoliated nanostructure, respectively. The mass change rate, viscosity aging index and softening point increment of the binders can be effectively decreased after PAV simulation and situ thermal aging. The improved aging resistance of the binders after thermo-oxidative aging (PAV simulation and situ thermal aging) is more obvious than that after UV aging. Compared with EVMT, organic expanded vermiculites (OEVMTs) have more pronounced improvements in aging resistance of bitumen. In addition, the morphology and aging resistance of OEVMTs modified bitumen depend on the nature of the organic modifiers in OEVMTs.

Introduction

Bitumen is a petroleum material composed of mixture of various organic chemical compounds [1]. It has been widely used as the binder of aggregate in road pavement due to its good viscoelastic properties [2]. However, the increasing demands of traffic on road building materials in recent years have resulted in a search for binders with improved performance relative to normal penetration grade bitumen. Consequently, more and more modified bitumen is used in road pavement [3]. For more than 15 years, polymer/layered silicate nanocomposites have attracted great interest from many researchers. The main layered silicates include montmorillonite (MMT), rectorite, vermiculite (VMT) and kaolinite clay. It has been found that the addition of clay with tiny amounts (usually less then 5 wt%) and its ultimate dispersion as 1 nm-thick nanolayers in a polymer matrix allow many properties, such as stiffness, fire resistance, fluid and gas barrier properties,… to be increased [4]. However, the interlayers of layered silicate are hydrophilic, and the interlayer spacing is small. This makes the polymer chains insertion into its interlayers difficult [5]. To increase the ease of insertion, layered silicate is commonly exchanged with organic cations, particularly alkylammonium ions, making the layered silicate become lipophilic, and the interlayer spacing is enlarged. This results in easy insertion [6], [7]. It has been found that the nature of the organic modifiers has an obvious effect on the morphology and properties of the nanocomposites [4], [8].

Recently, layered silicates have been used to modify bitumen. More attention of the researchers has been paid to MMT modified bitumen. It has been found that physical properties, rheological behaviors and aging resistance of bitumen and polymer modified bitumen could be obviously improved due to barrier properties of MMT [9], [10], [11], [12]. Like MMT, VMT is a mica-type silicate, belongs to the general family of 2:1 layered silicates. Each layer consists of octahedrally coordinated cations (typically Mg, Al and Fe) sandwiched by tetrahedrally coordinated cations (typically Si and Al). The isomorphous substitution of Si⁴⁺ by Al³⁺ leads to a net negative surface charge that is compensated by an interlayer of exchangeable hydrated cations (Ca²⁺, Mg²⁺, Cu²⁺, Na⁺) [13]. When pristine VMT flakes are strongly heated at high temperature (about 900 °C) during a short period of time, the water situated between layers is quickly converted into steams, exerting a disruptive effect upon the structure. As a consequence, a highly porous material named expanded vermiculite (EVMT) is formed and it is an efficient thermal insulator [14]. Recently, EVMT has been widely used for the modification of polymers. It was found that the mechanical properties and the thermal properties of the polymers were enhanced greatly with the introduction of EVMT [15]. Furthermore, the heat resistance and barrier properties of EVMT layers are more obvious than that of MMT due to its much higher aspect ratio [16]. However, there has no report about the preparation of the modified bitumen with VMT as well as the aging properties of the VMT modified bitumen till now.

This study aims at determining the effect of EVMT organic modification and organic modifiers on the morphology and aging properties of the bitumen.