The Development of A New Thermoplastic Elastomer (TPE)-SBS Modified Asphalt

The use of (recycled) plastics and (waste) vulcanized rubber powder is the main polymer of raw materials, and composite organic additives are selected to fully combine with asphalt components. The physical and chemical reactions between different components are completed in dynamic mixing, establishing a morphology structure similar to thermoplastic elastomers (TPEs), and a thermoplastic highly asphaltized alloy material. TPE-modified asphalt not only significantly improves the high-temperature stability of the base asphalt, but also has the social and economic value of rational utilization of resources and turning waste into treasure. There are very few studies on the preparation of modified high-viscosity asphalt formulations using rubber and plastic as modifiers. In this study, rubber, plastic, and plasticizers were added to the base asphalt, and the TPE modifier formulations were developed through the research of new TPE modifier series and functional formulations, preparation process, and its modified asphalt properties. Meanwhile, the preparation method of the rubber–plastic alloy modifier was determined. The performance of the TPE-modified asphalt was verified through performance verification tests to evaluate the modification effect of the modifier on the base asphalt. The test results showed that the penetration, softening point, ductility, and viscosity indexes of the TPE-modified asphalt developed through the proposed formulation, and it met the specification requirements for high-viscosity modified asphalt. Rubber and plastic modifiers significantly improved the high-temperature stability of the base asphalt. In addition, the rubber–plastic modifier had a significant tackifying effect, with a dynamic viscosity of 60°C and a Brinell rotational viscosity much greater than asphalt and rubber asphalt. The microscopic mechanism of the newly developed TPE-modified asphalt was analyzed by fluorescence microanalysis. The results showed that the rubber–plastic modifier fully swelled in the asphalt and was uniformly dispersed in the asphalt as a floc. The network structure of activated waste rubber powder-modified asphalt was more uniform and dense, resulting in good performance of the modified asphalt, and stable storage of modified asphalt was obtained. Through appropriate formulation, the comprehensive performance of the TPE-modified asphalt obtained met the requirements of pavement application and construction, providing a good theoretical basis for promoting TPE-modified asphalt.

Currently, various waste rubber products and waste plastic products (commonly known as waste rubber and plastic materials) and other materials are increasing in quantity. The amount of waste rubber and tires produced each year is conservatively estimated to be over 15 million tons. With a production of plastic goods that exceeds 21 million tons yearly and is growing at a rate of around 10% annually. China is another important plastics-producing nation in the world. A large amount of waste gradually intensifies “black pollution” and “white pollution”, which inevitably result in more catastrophic social and environmental issues .

Currently, the recycling of waste plastics and rubber is an effective way to protect the environment, and is also the sole method for sustainable development. Among them, using waste rubber powder to produce modified asphalt plays an important role in resource conservation and environmental protection. Through a large number of experimental studies and engineering practices, it has been shown that mixing the rubber powder obtained by grinding the rubber tire into petroleum asphalt can improve the high and low-temperature performance, viscoelastic performance, and anti-aging performance of asphalt. A large number of studies on rubber asphalt mixtures have shown that rubber asphalt mixtures have a longer service life compared with ordinary asphalt mixtures, and it can reduce driving noise, slow reflective cracks, improve the adhesion of binder materials with aggregates, improve pavement skid resistance and wear resistance, as well as improve driving comfort, absorb light, and reduce the stimulation of strong light on eyeglasses, among others benefits . On the other hand, with the development of the plastic industry, plastic-modified asphalt already occupies a place in the road material market. Plastics can greatly improve the high-temperature performance of asphalt, while plastic-modified asphalt plays a significant role in improving the high-temperature performance of the mixture. However, plastics do not help improve the low-temperature performance of asphalt, instead plastics reduce the low-temperature performance of asphalt .

According to relevant studies, TPE modifiers significantly increase the resilience of asphalt at high temperatures. Styrene-butadiene-styrene (SBS), which is very widely used, is one of the so-called “TPE” materials, or thermoplastic elastomers . Waste rubber and plastic can now be used to change asphalt thanks to advancements in polymer chemistry technology, which have led to steady advances in this field. Especially in the field of additive reinforcement, stability, and uniform dispersion, the development of materials that have not been successfully explored in the past, has now provided better application conditions. In this context, it is necessary to expand the material selection, in recent years, SBS-modified asphalt has been the absolute main variety in the construction of high-grade highways in China . Through the comparison of cost performance of different materials under the new technical status quo and the study of applicable conditions, it is necessary to diversify the application types of modified asphalt and refine their respective application directions.

With the development and use of new polymers, the use of rubber asphalt has gradually declined, leading to an increase in its usage for preparation of asphalt, with PE-modified asphalt emerging as a “new player” in the field of modified asphalt technology . The United States has conducted extensive research on linear low-density polyethylene (LLDPE)-modified asphalt. This research has added LLDPE to asphalt to obtain modified asphalt with high tensile strength, cold resistance, and creep resistance. Austria’s polyethylene-modified asphalt has obtained a patent, named Novophalt, and is widely used in asphalt pavements around the world . Japan uses waste polyethylene scraps to prepare modified asphalt, greatly improving the flexibility and durability of asphalt at low temperatures, making the road surface free from obvious cracks, fractures, and pits in winter due to severe cold, and easier to lay . At present, one of the most significant problems with PE-modified asphalt is that PE cannot naturally maintain a persistent and uniform dispersion of particles in the asphalt, meaning that it lacks storage stability.

In recent years, researchers have begun to study rubber–plastic composite-modified asphalt. By adding rubber and plastic composite modifier (TPE) to the base asphalt to study the modified asphalt, it was found that the high-temperature stability of modified asphalt is very prominent. Through economic analysis, to illustrate the low cost and economic benefits of rubber–plastic alloy, Fathy et al. conducted further research on rubber–plastic alloy (TPE) and compared and analyzed TPE-modified asphalt with SBS-modified asphalt. It was found that some formulations of TPE-modified asphalt had better high-temperature performance than SBS-modified asphalt, but the addition of TPE would reduce the low-temperature performance of asphalt. Magioli et al. successfully prepared thermoplastic elastomer blends composed of 30% polypropylene and 70% ground rubber tires through dynamic vulcanization process, whether pure or combined with original styrene butadiene copolymer, including different proportions of dicarboxylic peroxide and isophthalimide imide (BMI). This blend exhibited excellent ultimate tensile strength and elongation at break, comparable to other thermoplastic vulcanization systems, and also exhibited good reprocessing properties, which were useful for several potential applications . Nizamuddin et al. studied how the use of innovative compatibilizers can help stabilize recycled rubber–plastic asphalt mixtures during high-temperature storage. At the same time, the storage stability, rheological properties, thermal properties, and physical properties of rubber and recycled low-density polyethylene composite-modified asphalt were studied in the presence and absence of compatibilizers .

While studying waste rubber–plastic alloy-modified asphalt, many scholars successfully borrowed the concept of “alloy” and proposed the proposition of preparing rubber–plastic alloy modifiers and rubber–plastic alloy-modified asphalt. Wang et al. studied the technical performance of PE/waste rubber powder-modified asphalt and demonstrated that it is reasonable to analyze and evaluate the technical performance of composite-modified asphalt through the Strategic Highway Research Program (SHRP) binder specification . Mulage prepared modified asphalt using rubber powder and waste plastics (polypropylene and polyethylene), and studied the reaction mechanism of composite modified asphalt. Maw et al. compared and analyzed TPE-modified asphalt with SBS-modified asphalt, and found that some formulations of TPE-modified asphalt had better high-temperature performance than SBS-modified asphalt, but the addition of TPE would reduce the low-temperature performance of asphalt . Reddy et al. further studied and added rubber powder, PE, rock asphalt, and SBR to the base asphalt for composite modification to obtain a high-viscosity modified asphalt. Through road performance verification, it was found that the performance of the OGFC mixture was excellent [18]. Xu et al. prepared three types of rubber–plastic modifiers and modified asphalt, and conducted road performance tests for three types of modified asphalt. Xu et al. mainly conducted research on the road performance of rubber and plastic alloy-modified asphalt, and its conclusions indicate that as a new modified material, rubber–plastic alloy-modified asphalt has no less high-temperature performance than SBS-modified asphalt, but also good low-temperature performance. Based on the component analysis of waste rubber powder and waste plastics, Ki et al. prepared waste rubber–plastic alloy modifiers with different rubber–plastic ratios, analyzed the modification mechanism of waste rubber–plastic alloy modifiers on base asphalt, and tested the technical performance and storage stability of waste rubber–plastic alloy-modified asphalt. The results showed that when the rubber–plastic ratio was 7:3, the waste rubber powder and waste plastic had good compatibility; the blending process of waste rubber and waste plastics and the modification process of base asphalt by waste rubber–plastic alloy modifiers are mainly physical effects; The waste rubber–plastic alloy modifier has good compatibility with the base asphalt, significantly improving the road performance of the base asphalt . Chen et al. studied the effect of raw material quality ratio on the performance of thermoplastic elastomer-modified asphalt. The experimental results indicated that the mass ratio of water-based adhesive powder and SBS had a more important impact on the permeability, softening point and ductility, viscosity, high-temperature storage performance, elastic recovery rate, and dynamic rheological properties of TPE-modified asphalt .

In recent years, some scholars have conducted systematic research on waste rubber powder-modified asphalt. However, there is a gap between the performance of modified asphalt modified with rubber powder alone and that of SBS-modified asphalt. For instance, as the subject of current research, the high-temperature softening point index of SBS-modified asphalt can reach about 80 °C, while rubber powder-modified asphalt usually only reaches about 60 °C; the low-temperature ductility index is also lower than that of SBS-modified asphalt . Judging from the share of rubber powder-modified asphalt and SBS-modified asphalt used in the world, although rubber powder asphalt has many applications, it cannot fundamentally shake the mainstream position of modified asphalt prepared using thermoplastic elastomer SBS. The rubber–plastic alloy TPE material studied in this paper is an extension of the modification of pure rubber powder. From the perspective of technical principles, rubber can impart elasticity to asphalt, while resin can impart rigidity to asphalt, enabling asphalt to have the characteristics of both rubber and resin, thereby enabling rubber–plastic alloy TPE materials to achieve the performance of SBS block copolymer TPE materials.

In this paper, the serialization and functionalization of novel rubber–plastic alloy (TPE) modifiers were studied, as well as the preparation process and properties; the mix ratio of new TPE modifier asphalt mixture was designed; rubber–plastic alloy modifier formulations were developed, the formulation design plans and processing technology plans were then proposed; subsequently, the preparation method of the rubber–plastic alloy modifier was determined. By analyzing the high- and low-temperature performance, temperature sensitivity, aging performance, and rheological properties of the new rubber–plastic alloy-modified asphalt, the theoretical basis for proposing technical standards related to road performance is improved.