In recent years, with the acceleration of urbanization and the rapid development of the construction industry, the generation of waste brick powder has increased dramatically. However, their disposal has been a challenge because they occupy a large amount of storage space and release hazardous substances that pollute the environment[1, 2]. Therefore, finding a sustainable and environmentally friendly method to reuse waste brick powder is one of the most effective methods compared to burying and incineration, which has become one of the hotspots of contemporary research. However, the interfacial compatibility of waste brick powder with the substrate is full of uncertainties due to its complex composition, irregular particle shape and rough surface[3–5]. [6, 7] Some research groups have used silane coupling agents to modify substances with a similar composition to brick powder to study the mechanical properties of composites, and the results showed that the mechanical properties of the modified composites were significantly improved over the untreated composites. Before this waste brick powder was often used in concrete, mortar, wall materials, road materials, and recycled cement[8], but its use as a rubber filler has rarely been reported.
Over the years, carbon black (CB) and white carbon black (WCB), among others, have been added in large quantities to rubber as fillers to enhance its mechanical properties[9]. Other fillers such as carbon nanotubes[10], fibers[11], montmorillonite[12], calcium carbonate[13], starch[14], etc., have also been widely used in rubbers to enhance their mechanical properties or to reduce the production cost. In addition to this, more and more researchers are exploring the possibility of recyclable and reused wastes as substitutes for rubber fillers. In a study added fly ash as a core to silicone rubber and found that silicone rubber filled with composite fillers exhibited excellent thermal stability, flexibility, environmental resistance, and hydrophobicity, which not only responded to green chemistry to achieve efficient fly ash recycling, but also designed a new strategy to prepare microwave absorbing materials with strong potential for civil applications[15]. Lignin, as the second most abundant natural resource, is generated as a by-product in the paper and pulp industry and is produced in large quantities, its disposal is often by incineration or landfill, as it is formed from cheap materials, is abundant in renewable resources every year, is lightweight, biologically efficient, and ecologically adapted to a wide range of applications and is added to rubbers to replace carbon black[16]-[18]. Waste brick powder, as a cheap and renewable resource, has attracted much attention for its potential utilization in rubber products. Due to its rich content of silica and metal oxides, it has the conditions to become a rubber filler. The interfacial bonding with rubber will be further enhanced by the secondary functionalization modification of the waste brick powder. Tang D et al[19]added silane coupling agent KH-550 modified waste brick powder into styrene-butadiene rubber, and the results showed that the rubber filled with the modified brick powder exhibited better mechanical properties than the unmodified rubber. Shafik E S et al[20]made red brick waste powder as butyl acrylonitrile rubber The tensile strength and hardness of the composite were improved with the addition of brick powder and showed good electrical insulation properties
Dopamine (DA) can spontaneously oxidatively polymerize under alkaline conditions to form polydopamine (PDA) films, and PDA coatings are often used as a platform for secondary reactions due to their simple composition, mild reaction conditions, ability to adhere to almost any type of material and the presence of abundant catechol and amine groups[21–27]. Yang D et al[28] generated high thermal conductivity natural rubber (NR) dielectric elastomer composites by forming silver nanoparticles on the surface of polydopamine via glucose reduction after coating Al2O3. However, despite the excellent adhesion properties of polydopamine, its high price limits its wide application. Fortunately, it was confirmed that dopamine forms catechol groups and crosslinked networks through oxidative polymerization, which is the main reason for its excellent adhesion properties[29]. Polycatechol/polyamine (PCPA) has a similar catechol structure and amino-terminal structure to PDA and has excellent adhesion capacity. Moreover, PCPA has a shorter reaction time than modification with PDA and the price of PCPA is only 8% of the price of PDA[30, 31]. Wang L et al[32]formed a poly catechol/polyamine (PCPA) layer on the surface of aramid fibers by UV irradiation of catechol/polyamine (CPA), which was then further grafted with ethylene glycol diglycidyl ether (EGDE) to bind the aramid fibers to the catechol/polyamine layer. Grafting with ethylene glycol diglycidyl ether (EGDE) resulted in a maximum increase of 85.6% in the adhesion of aramid fibers to the rubber matrix, which was even better than that of dopamine (67.5% increase). In the literature[33], Al2O3 nanoparticles will be modified using poly (catechol/polyamine) (PCPA) and subsequently, Al2O3/XNBR composites will be fabricated, and the results show that when the content of Al2O3-PCPA nanoparticles is 30%, the thermal conductivity of Al2O3-PCPA/XNBR composites is as high as 3.19 times higher than that of pure XNBR. The poly(catechol/polyamine) (PCPA) layer is a versatile platform with abundant hydroxyl and amine groups, which can be further functionalized by reacting with functional monomers. It has been shown that the epoxy groups can react with the amine groups on the PCPA layer can react with can react with the vulcanization of rubber[34–36]. The functionalized material ethylene vinyl acetate-glycidyl methacrylate terpolymer (EVMG) has carbon-carbon double bonds and epoxy groups[37]. Among them, the epoxy group can react with the amino group, and the carbon-carbon hydrogen bond can react with the sulfur in the rubber, which not only improves the compatibility with the rubber, but also helps to form a three-dimensional crosslinked network with the rubber matrix[38, 39]. In addition, EVMG has a volume repulsion effect and exhibits a synergistic toughening effect, which allows for the formation of a more homogeneous crosslinked network[40]. However, there are fewer studies using EVMG as rubber filler modifiers.
In this study, a two-step modification method was designed to improve the compatibility problem between WBP and rubber matrix, thus enhancing the performance and application value of the composites. Firstly, catechol/polyamine (CPA) successfully modified the surface of WBP into a poly (catechol/polyamine) coating (PCPA), which was used as a platform for macromolecular grafting to improve the grafting rate of EVMG. Second, after the first step of modification, EVMG was grafted onto the PCPA coating to form an adhesive film. The epoxy groups of EVMG could undergo a ring-opening reaction with the amine groups on the PCPA coating, which further increased the interfacial bonding between WBP and the rubber matrix. In this process, EVMG also has a spatial site resistance effect, which is beneficial to improve the dispersion of the filler. Finally, NR/WBP@EVMG composites were prepared by mechanical blending method. The effects of EVMG content on the interfacial interaction between NR and WBP as well as the static and dynamic mechanical properties of the composites were systematically investigated.