**1. Introduction**

Asphalt pavement has become the main expressway pavement structure in China due to its characteristics of superior mechanical strength, low noise, driving safety and ease of mechanized construction. However, a variety of pavement stresses such as ruts, cracks, loosening and water damage have emerged due to the rapid increase in heavy traffic and the fluctuations caused by extreme climate. To resolve these issues, researchers have adopted various methods to improve the performance and longevity of asphalt pavement. Polymer modification is the most commonly used method to provide asphalt binder with a wide operating temperature range.

Depending on the polymer modifier types, thermoplastic elastomers and plastomers are the two main categories [1]. Thermoplastic elastomers, specifically styrene–butadiene– styrene (SBS), represent the most successful asphalt modifiers to date. Such modifiers can effectively enhance the resistance of asphalt binder against rutting, fatigue cracking and low-temperature cracking. However, the main drawback of these polymers is their partial miscibility with asphalt binder, which leads to poor storage stability of polymer modified binder. Plastomers, such as polyethylene (PE) and ethylene–vinyl–acetate (EVA), can significantly improve the rutting resistance of binder, but the improvement of low-

**Citation:** Pei, X.; Fan, W. Effects of Amorphous Poly Alpha Olefin (APAO) and Polyphosphoric Acid (PPA) on the Rheological Properties, Compatibility and Stability of Asphalt Binder. *Materials* **2021**, *14*, 2458. https://doi.org/10.3390/ ma14092458

Academic Editor: Francesco Canestrari

Received: 8 April 2021 Accepted: 7 May 2021 Published: 10 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

temperature anti-cracking properties is limited. Its incompatibility with asphalt binder also results in severe separation between polymer and asphalt binder, which is undesirable and restricts its application [2]. Moreover, these polymers must be blended with asphalt binder by a colloid mill or high-speed shearing equipment [3–5]. Equipment investment and high shearing temperatures increase the production cost of modified asphalt binder [6]. In addition, these modified asphalt binders can only be stored for a short period of time and therefore must be used as soon as possible to avoid undesirable phase separation. Although the application of a stabilizer can improve the storage stability of the modified binder, it also leads to an increase in production cost. Therefore, high production cost and poor storage stability are significant constraints currently faced in the manufacture of modified asphalt binder. To reduce the production cost without compromising on the properties of modified asphalt binder, the development of new types of modifiers to replace colloid mills and stabilizers and allow for lower processing temperatures and simplified production processes is of significant research interest in the field.

Amorphous poly alpha olefin (APAO) is a low molecular weight amorphous plastic material, which is highly compatible with asphalt binder [7,8]. It is formed by the copolymerization of α-olefin and has the characteristics of high fluidity, low crystallinity and high randomness. Previous studies have shown that APAO can be completely dissolved in asphalt binder when the APAO content is no more than 6 wt.% [7,9]. It has also been found that APAO modified binder with high stability can be processed without shearing and stabilizer. Kong et al. [10] showed that the processing temperature of APAO modified binder can be controlled at 165 ◦C, which is lower than the processing temperature of 180 ◦C for SBS modified binder. These results demonstrate that APAO is capable of producing modified asphalt binder at a lower processing temperature without shearing and stabilizer. Regarding the impact on asphalt binder performance, numerous studies on the mechanical properties of APAO modified binders have been conducted. Wei et al. [7,9] showed that APAO enhanced the elastic property and lowered the temperature susceptibility of binder, but had adverse effects on the creep stiffness and creep rate. Yan et al. [11] and Liu et al. [12] determined that APAO improved the high-temperature performances, storage stability and aging resistance of waste tire rubber (WTR) modified binder. However, its effect on low-temperature properties remains uncertain and is highly debated. Liu et al. [13] concluded that SBS/APAO modified binder had superior high- and intermediate-temperature properties and storage stability in comparison to SBS modified binder, while its low-temperature performance was equivalent to that of SBS modified binder. You et al. [14] found that APAO enhanced the high- and low-temperature properties as well as storage stability of terminal blend rubberized binder (TB). Yan et al. [15] showed that the incorporation of APAO strengthened the elastic recovery and aging resistance as well as low-temperature cracking resistance of EVA modified binder. Moreover, the results demonstrated that APAO was a good stabilizer to improve the stability of asphalt binder. Evidently, APAO is a modifier that satisfies the requirement due to its superior compatibility with binder, simple modification process and excellent modification effect. In addition, APAO has a comparable price to SBS. In order to further reduce the cost, it is important to choose another modifier that has good compatibility with asphalt binder and combine it with APAO for the modification of neat binder.

Polyphosphoric acid (PPA), a mineral acid, has good compatibility with asphalt binder and has been widely applied in asphalt binder modification [16,17]. The preparation process of PPA modified asphalt binder is relatively simple, which involves uniform mixing through stirring at 150–160 ◦C instead of high-speed shearing [18,19]. PPA does not separate from the asphalt phase when it modifies the asphalt binder in the absence of a stabilizer [20]. Previous research has shown that a small amount of PPA can markedly improve the Superpave performance grade (PG) of asphalt binder [18,21]. The useful temperature interval (UTI) is extended by PPA due to its characteristics of increasing the high-temperature stiffness without decreasing the low-temperature flexibility [22–25]. PPA was also adopted to modify asphalt binder with another polymer to strengthen

its rheological behavior and reduce the cost [26–29]. Moreover, PPA can enhance the compatibility between polymer and asphalt binder during the composite modification process [27,30]. Overall, PPA is a desirable modifier that has similar characteristics to APAO in terms of good compatibility and a simple modification process.

The literature review shows that both APAO and PPA can produce highly stable modified asphalt binder at relatively low processing temperatures without shearing and stabilizer. The recent studies on APAO and PPA modification are summarized in Table 1. Thus, the combination of APAO and PPA can provide some enlightenment for improving the performance of modified asphalt binder and simplifying the process technology. To date, investigations regarding APAO/PPA modified binders have not been published, and the modification effect and mechanism of this unique modification are still unknown. Hence, systematic and comprehensive studies on the interaction between APAO/PPA and asphalt binder are still needed.

**Table 1.** The recent studies on APAO and PPA modified asphalt binder.


This study aims to explore the compound modification effect of APAO and PPA on the base binder. The conventional properties, rheological behavior, storage stability, compatibility and microstructure of composite modified binders were evaluated. Furthermore, the modification effects of APAO/PPA compound modification were compared with those of APAO modification alone and PPA modification alone. The optimal proportions of APAO and PPA were also determined to provide basic parameters for industrial production. Finally, variations in functional groups of asphalt binder were revealed by FTIR spectroscopy measurements.
