Durability improvement of poroelastic road surface with treated rubber: Molecular dynamics simulation and experimental observations

https://doi.org/10.1016/j.jclepro.2022.133334Get rights and content

Highlights

  • The introduced oxygen-containing groups on rubber surface improve durability of poroelastic road surface (PERS).

  • MD simulation shows the treated rubber has greater bonding ability with polyurethane.

  • The formation of the carbonyl group on rubber surface is achieved through soak in NaOH solution.

  • Laboratory tests show the treated rubber cause greater tensile strength ratio and less abrasion loss.

Abstract

Poroelastic road surface (PERS) is usually composed of rubber particles, aggregates, and polyurethane. However, the poor bonding strength between rubber granules and polyurethane affects PERS’ durability. This study aimed to improve the durability of PERS with treated rubber using molecular simulation and experimental tests. The cohesive energy density (CED), interaction energy (IE) and shear bonding capacities between two kinds of rubber granules and one-component polyurethane were simulated using molecular dynamics (MD). The hydrophilicity test and Fourier transform infrared (FTIR) spectroscopy test were utilized to demonstrate the formation of oxygen-containing groups on rubber surfaces. The indirect tension (IDT) test and Cantabro test were employed to evaluate the durability of PERS mixtures with treated rubber. The MD simulation results showed that the oxygen-containing groups, including the hydroxyl group (-OH) and the carbonyl group (Cdouble bondO), could improve surface polarity of natural rubber (NR) and styrene-butadiene rubber (SBR) and thereby enhance rubber-polyurethane bonding performance. In particular, hydroxyl groups improved the bonding energy of NR-polyurethane by 59% while the carbonyl groups enhanced the bonding performance of SBR-polyurethane by 20%. The hydrophilicity of the treated rubber granules was effectively improved since new carbonyl groups were introduced on rubber surface. The treatment of rubber with NaOH solution improved the durability of PERS specimens by 8.4% in terms of tensile strength ratio (TSR) and 64.7% in terms of Cantabro abrasion loss. These findings prove the feasibility of designing durable PERS with good functional performance.

Introduction

A large number of waste tires are produced worldwide every year, most of which are landfilled or stacked directly in the open air, causing a large load on our environment (Jena et al., 2020; Wang et al., 2020). Recycling waste tires as a form of rubber granules or rubber powders has significant environmental benefits (Shahjalal et al., 2021; Roychand et al., 2020; Pacheco-Torres et al., 2018).

Poroelastic road surface (PERS) is one type of porous pavement in which rubber particles replace part of aggregates and polyurethane resin replaces asphalt binder. Many practices have proved that PERS outperforms traditional porous asphalt (PA) pavements in terms of noise reduction, anti-icing performance, and raveling resistance (Chen et al., 2018a, 2018b; Wang et al., 2016). However, due to low surface polarities of rubber particles and porous structure, severe bonding failures around rubber granules within the PERS mixture were observed (Wang et al., 2017a). Therefore, the bonding performance between the rubber and polyurethane needs to be improved for the durability of PERS.

Surface modification of rubber granules can increase surface polarity and improve the adhesion between the rubber and other materials (Li et al., 2019). Several modification methods, such as chemical reagent, plasma, and ultraviolet treatments, are often utilized. Many active groups can be formed on the treated rubber surface, which increases the hydrophilicity, adhesion, and compatibility (Cheng et al., 2017; He et al., 2016; Nuzaimah et al., 2020; Ossola and Wojcik, 2014). The ultraviolet light is used to irradiate rubber particles, which increases the water retention ability of rubber particles by 25% and flexural strength of treated rubber cement concrete by 20% (Ossola and Wojcik, 2014). The low-temperature plasma (LTP) treatment is applied to rubber particles to introduce oxygen-containing groups, which causes the water contact angle of rubber to decrease from 122° to 34° (Cheng et al., 2017). The rubber is treated sequentially with NaOH solution, KMnO4 solution, and H2SO4 solution, which reduces the water contact angle of rubber from 95° to 71° (He et al., 2016). Rubber crumbs are treated with NaOH solutions with different concentrations and it is observed that the rubber crumbs treated with 7% and 10% NaOH solutions provide better wettability and hydrophilicity than those treated with 1% and 4% NaOH solutions (Nuzaimah et al., 2020).

The surface modifications of rubber granules are usually investigated using macroscopic experiments. However, the fundamental understanding of rubber surface treatment from the molecular level is seldom explored (Garcia and Green, 2012). The interaction between different materials at the atomic scale can be observed through molecular dynamics (MD) simulations, such as carbon nanotube-reinforced styrene-butadiene rubber (Chawla and Sharma, 2018, 2019), rubber modified asphalt (Guo et al., 2020a, 2020b), asphalt-aggregate adhesion (Wang et al., 2017b; Sun and Wang, 2020a); rejuvenator diffusion (Xu et al., 2018; Sun and Wang, 2020b), and asphalt aging and self-healing (Xu and Wang, 2017; Sun and Wang, 2020c). It is expected that the effectiveness of surface modification of rubber particles can be discovered using MD simulations.

This study aims to investigate the durability improvement of PERS with treated rubber using molecular simulation and experimental observations. The surface activity of the rubber and its adhesion to polyurethane after introducing active groups were simulated by molecular dynamics. The variations of cohesive energy density, and interaction energy before and after rubber modification were calculated. Three-layer shear simulation and velocity distribution analysis were performed to quantify the modification effect of oxygen-containing groups on the rubber-polyurethane bonding capacity. On the other hand, the hydrophilicity test, Fourier transform infrared (FTIR) spectroscopy test, indirect tension (IDT) test, and Cantabro test were utilized to demonstrate the formation of oxygen-containing groups on rubber surfaces and to evaluate the durability of PERS mixtures with treated rubber.

Section snippets

Molecular dynamics simulation

To investigate the effects of the treated rubbers on durability in the molecular scale, the interaction between rubber and polyurethane and shear bonding behaviors of the three-layer system were simulated by MD using Materials Studios. During these simulations, the COMPASS force field was used since it is suitable for common organic and inorganic molecules and has been widely used for asphalt materials, mineral aggregates, and polymers in the literature.

Rubber treatment

For applications in asphalt modification, rubber granules are often produced from waste rubber tires by ambient grinding or cryogenic breakdown (Fazli and Rodrigue, 2020). The constituents of as-received rubber granules often include NR and SBR. The size of rubber granules used for laboratory experiments was in the range of 1–6 mm and the apparent density of rubber granules was 1.1 g/cm3.

The as-received rubber granules were soaked in NaOH solution with a concentration of 1.0 mol/l for different

Effects of rubber surface modifications from MD simulation

Fig. 9 showed the variation of the rubber-water and rubber-PUPU interaction energy before and after rubber modification. As can be seen, the hydroxyl group (-OH) contributed the most to the activity of the natural rubber (NR) molecule, which significantly improved the adhesion between NR and water, and the carbonyl group (Cdouble bondO) could slightly increase the interface energy. Similarly, both the hydroxyl group and carbonyl group could significantly improve the surface activity of the SBR molecule.

Conclusions

The durability improvement of PERS with treated rubber was simulated by molecular dynamics and demonstrated by experimental tests. The following conclusions could be drawn.

  • 1)

    The introduction of oxygen-containing groups significantly enhanced rubber-water interaction energy as indicated in MD simulation, improving the hydrophilic property of rubber.

  • 2)

    The formation of oxygen-containing groups effectively improved the bonding ability between rubber granules and polyurethane. Particularly, hydroxyl

CRediT authorship contribution statement

Gongyun Liao: Investigation, Writing – original draft, review & editing. Xin Fang: Conceptualization, Data curation, Investigation. Hao Wang: Methodology, Investigation, Writing – review & editing. Jin Tang: Methodology. Patrick Szary: Writing – review & editing. Jun Chen: Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors kindly appreciate the financial support from National Key R&D Project of China (2021YFB2600602, 2021YFB2600600) and the Science and Technology Plan of the Department of Housing and Urban-Rural Development of Anhui Province, the People's Republic of China (2020-YF05).

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