| Clay is an important mineral resource in our country. It is a mineral of hydrated magnesium aluminum silicate. Nanoclays have many advantages such as extensive sources, abundance, low cost, and so on. Its chemical composition includes hydrated silicates, alumina (Al2O3), alkali metal oxides (K2O, Na20), alkaline earth metal oxides (CaO and MgO), coloring oxides (Fe2O3, TiO2). It is often contains quartz, feldspar, mica, sulfate, sulfide, carbonate and other impurities.In this paper, one-dimensional structure of attapulgite (AT) and two-dimensional structure of montmorillonite (MMT) were used as reinforcing agent filled to natural rubber (NR). Three type fillers including calcination attapulgite, attapulgite modified by silica (AS) and montmorillonite modified by silica (MS) were prepared. The structure and morphology of the filler particles were characterized by X-ray fluorescence spectrometer (XRF), infrared spectrometer (FTIR), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and surface area instrument. The reinforcing effects of different fillers with different content to natural rubber were studied. The vulcanization performance of mixing rubber was studied. The mechanical properties, swelling properties, cross-linking density, heat resistance and wear resistance of vulcanized rubber were measured. The dispersion properties of nanoclays in natural rubber composites were analyzed. The reinforcing mechanisms of different fillers on natural rubber were discussed.(1) In the purified attapulgite, the fiber structure was very obvious when many impurities were removed. Compared with attapulgite before and after calcination, it was found that the surface colors changed from dark gray to dark brown and the content of SiO2and Fe2O3increased by4-5%and1-2%, respectively. The structure of attapulgite after calcination changed obviously. When calcining temperature was450℃, the crystal water and part of configurations water were lost. The characteristic peaks of hydroxyl at3614cm-1,3581cm-1and3552cm-1disappeared. Although rod structure of attapulgite was still found, some folding happened between layer and layer which caused some particles agglomeration, decrease of specific surface area and increase of pore diameter. When calcining temperature of attapulgite was increased to850℃, all of the characteristic peaks of hydroxyl disappeared and silicon dioxide was produced. The fiber hole collapsed completely and specific surface area fell sharply, but the material had good dispersibility. Compared with pristine natural rubber, the vulcanizing properties and mechanical performances of natural rubber nanocomposites filled with purified and calcined attapulgite were significantly improved. Curing rate index was increased from50min-1to90-130min-1. With the increasing of filler content,300%tensile modulus, tensile strength, hardness, wear resistance and swelling index of natural rubber nanocomposites increased firstly and then descended or tendency gently. When the content of PAT, PAT-450and PAT-850was lphr, the tensile strength, cross-linking density, wear resistance, solvent resistance and heat resistance of natural rubber nanocomposite were higher than those of pristine natural rubber. The tensile strength of PAT-450/NR nanocomposite reached the best value which was about29.69MPa, which was one time better than that of pristine natural rubber. The akron abrasion volume and cross-linking density were0.0431cm3and2.25×10-4mol/cm3. Compared to pristine natural rubber, T5%and Tmax were increased by94℃and17℃, respectively. It was also found that the attapulgite had good dispersion performance in natural rubber which increased the interaction between filler and rubber chain. Thus, the attapulgite calculated at450℃was a good reinforcing agent on natural rubber.(2) FTIR analysis showed that the structure of AS still contained typical four hydroxyl peaks. The XRD studies found that the amorphous diffraction peaks of SiO2were appeared at AS powder. With the increase of silica concentration, the strength of peaks was reduced gradually. TEM indicated that silica nanoparticles had a good dispersion on the surface of attapulgite powder. The smooth surface of attapulgite became rough. The specific surface area analysis found that the BET and Langmuir surface area of AS was higher than that of SiO2and PAT.With increasing amount of AS powder, vulcanization rate of mixing rubber rises in the first stage, and then decreases. When the filler content was lphr, the curing rate of composites was the fastest. Compared with blank sample, the performance of AS/NR nanocomposites improved significant. The tensile strength,300%tensile modulus, wear resistance, cross-linking density of the vulcanized nanocomposites were increased firstly and then went down with increase of filling content of AS. When filled with1phr AS, the properties of composites reached to optimum. The tensile strength of AS41/NR nanocomposite was up to26.10MPa, increased by70%compared to the pristine natural rubber sample. The solvent resistance of natural rubber composites filled with AS powder improved. The tensile fractures of nanocomposites specimens were measured by SEM. The roughness of the tensile fractures were increased significantly, especially AS41/NR and AS51/NR which showed larger holes. It proved that there was strong interaction between AS and natural rubber molecular chain. The heat resistance of natural rubber composites filled by AS particles improved. T5%and Tmax were increased40-70℃ and20℃, respectively.(3) The infrared spectra of MS and MMT showed little change.XRD analysis shown that the lamellar spacing of MS increased from1.87nm to2.09nm with added the content of SiO2. It was proved that intercalated MMT were made. TEM analysis showed that MS surface loaded with large amounts of SiO2, and the SiO2particles had a good dispersion on the surface of montmorillonite. SEM indicated that the agglomeration phenomenon of montmorillonite modified by silicon dioxide was significantly improved. The value of BET and Langmuir surface area of MS was between silica and MMT.MS particles enhanced the vulcanization rate of natural rubber composites. Curing rate reached to maximum when MS content was1phr. Compared with original natural rubber sample, the tensile strength,300%tensile modulus, reinforcement coefficient, wear resistance and cross-linking density of MS/NR nanocomposites first increased and then decreased with increase amount of MS. The mechanical properties of nanocomposites reached an optimum when the filled amount of MS was9phr. The tensile strength of NR/MS51nanocomposite was reached the best value. It was about28.62MPa. There were many pores on the tensile fracture surface of MS/NR composites. The pore size was about1-5microns.The pure natural rubber was a continuous parallel grooves structure. The distance between lines was about0.5mm. But the wear surface morphology of MS/NR nanocomposites was discontinuous short grooves structure and had SiO2concentrated point. Compared with natural rubber, Ts%of MS/NR composites increased about80-90℃, but Tmax was unchanged.(4) The reinforcement model of natural rubber filled with different structure nanoclays was established. In the course of tension, the molecular chains of rubber were restrained by different structure fillers. When dual structure filler particles were used to reinforce natural rubber, interfacial affinity and compatibility between filler and rubber chain were enhanced. |
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