**1. Introduction**

By virtue of its excellent performance and environmental protection characteristics, SBS-modified asphalt has become the most commonly used modified asphalt around the world by blending SBS modifier and base asphalt [1–3]. SBS polymer is used as a dispersed phase and physically dispersed into the continuous phase of base asphalt. Due to the large differences between SBS polymer and matrix asphalt in chemical structure and physical parameters such as density, polarity, solubility, and molecular weight, SBS is distributed only in the form of particles in asphalt medium. It has a two-phase structure, which is an unstable thermodynamic incompatible system [4]. Therefore, in order to reduce the segregation and promote the swelling of the polymer in the production process of the modified asphalt, a certain amount of additive such as solubilizer and stabilizer may be added. The solubilizer usually uses petroleum distillate with high aromatic hydrocarbon content and low asphaltene content to improve the ratio of four components in the base asphalt [5–7]. The aromatic hydrocarbon can promote the dissolution of polystyrene blocks in the SBS polymer, making the SBS modifier more compatible with the matrix asphalt. Among all the solubilizers used in the market, rubber oil is widely accepted because of its low cost and good solubilizing performance. In terms of stabilizer, it is often used to improve the storage stability of SBS-modified asphalt and reduce the problem of segregation. The stabilizers currently used for SBS-modified asphalt mainly include the following four categories [8]: sulfur containing stabilizer, such as elemental sulfur and sulfur compounds, polyolefin stabilizer, like polyisobutene, inorganic acid and inorganic

**Citation:** Xu, G.; Gong, X.; Yu, Y.; Chen, X. A Rapid Method for the Determination of SBS Content Based on the Principle of Orthogonal Testing. *Appl. Sci.* **2021**, *11*, 10911. https://doi.org/10.3390/ app112210911

Academic Editors: Amir Tabakovic, Jan Valentin, Liang He and Luís Picado Santos

Received: 15 August 2021 Accepted: 11 November 2021 Published: 18 November 2021

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**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/).

metal oxide stabilizers, such as phosphoric acid and zinc oxide, and inorganic clay stabilizer like montmorillonites and kaolin clay. Among them, sulfur-containing stabilizers, especially sulfur, stand out and are widely used in the industrial production of SBS-modified asphalt because of construction convenience, low requirements for equipment and low cost. It can be used directly after mixing with modified asphalt, and the vulcanization crosslinking reaction is mild and easy to control [5,9,10].

SBS is a block polymer of polystyrene-polybutadiene-polystyrene. After being added to asphalt, it is sheared into fine particles and swelled, gradually forming a network-like crosslinked structure. The modification effect of asphalt will be affected by its content. Studies have shown that the microstructure of SBS modified asphalt is largely determined by the content of SBS [11]. The morphology and interaction between modified asphalt molecules will be different with a different SBS content, thus affecting the rheological properties and mechanical properties of modified asphalt. At present, the main methods to determine the modifier content of SBS-modified asphalt include fluorescence microscopy, gel permeation chromatography, chemical analysis, and infrared spectroscopy [12–15]. Fourier-Transform Infrared (FTIR) spectroscopy stands out among various methods, because of the simple test process, quick and easy operation, short data processing time and little error. Being able to quickly determine the content of SBS modifier with higher accuracy, it has become the main method for rapid determination of SBS content in engineering.

However, existing studies on the rapid determination of SBS content by FTIR had only considered a single factor of SBS content and had not considered the influence of admixtures on the test results [16,17]. Therefore, for the SBS modified asphalt with additives such as sulfur and rubber oil, the results of determining SBS content by FTIR may have a certain bias, and the accuracy is not known. It is necessary to design tests to explore the influence of additives on the accuracy of using FTIR to determine the polymer content in modified asphalt. For the case with multiple influencing factors, the commonly used test methods include comprehensive test and orthogonal test. Compared with comprehensive test, the orthogonal test can reduce test times and improve efficiency, which is essential for the cases with multiple influencing factors and levels.

Therefore, in view of this problem, this paper firstly analyzes the effect of SBS content on the high and low temperature performance of modified asphalt, and then establishes the calibration curve of SBS content and characteristic absorption peak absorbance in modified asphalt according to the infrared spectrum. Finally, the orthogonal test is designed, and the results are analyzed by the multivariate analysis of variance to explore the influence of additives on the determination results of SBS in modified asphalt.
