| Natural rubber,as an important industrial raw material,has played an important role in various fields.In order to improve the mechanical properties of the natural rubber matrix,some fiber reinforcement materials are added to prepare high-performance natural rubber composites.Ultra-high molecular weight polyethylene(UHMWPE)fibers are widely used as the reinforcement of resin matrix because of their excellent properties such as high specific strength,low density,high impact resistance,high chemical stability,and low friction,but there are few research reports on the use of UHMWPE fiber in natural rubber matrix.This is mainly attributed to the melting point of UHMWPE fibers being lower than the vulcanization processing temperature of natural rubber,which leads to their melting and damages their own performance.In addition,the surface of UHMWPE fibers is smooth and chemically inert,which limits the formation of good interfacial bonding between them and the natural rubber matrix,which is not conducive to the enhancement of performance.Based on the above issues,this paper proposes a two-step strategy for the preparation of UHMWPE fiber/natural rubber composite materials.Firstly,a new type of material is innovatively introduced into the vulcanization system of natural rubber.By adjusting the vulcanization formula,the natural rubber can undergo efficient vulcanization reaction at lower temperatures to ensure the structural integrity of UHMWPE fibers during processing.Secondly,surface modification of UHMWPE fibers is carried out to improve the interfacial adhesion between them and the natural rubber matrix,obtaining a high-performance natural rubber based composite material.The specific research content and results are as follows:(1)Three kinds of Zn-ZIFs materials(ZIF-8,ZIF-11 and ZIF-14)based on the zinc ions coordinating with different imidazole ligands were prepared.The surface energy of ZnZIFs was calculated through the contact angle results to study the dispersion and flocculation of Zn-ZIFs materials with natural rubber matrix.The results showed that ZnZIFs had good dispersion in natural rubber matrix and were less prone to flocculation after dispersion.(2)By adjusting the vulcanization system of natural rubber and introducing Zn ZIFs to explore the low-temperature vulcanization formula of natural rubber,a low-temperature and efficient vulcanization system based on dibenzothiazole disulfide(accelerator DM)/tetramethylthiuram disulfide(accelerator TMTD)/ZIF-8 was obtained.The effects of the types and additions of Zn-ZIFs on the vulcanization behavior of the natural rubber matrix was explored,and the results showed that the addition of ZIF-8 or ZIF-14 could achieve rapid vulcanization of natural rubber at 130 ℃,while ZIF-11 did not show significant accelerated performance.The imidazole molecules were introduced in natural rubber matrix instead of ZIF-8,ZIF-11 and ZIF-14,respectively,and the accelerating result of imidazole molecule in vulcanization is better than the corresponding Zn-ZIFs based on it.But the resulted natural rubber composites with Zn-ZIFs shows better mechanical properties than that with imidazole molecule.It is concluded that Zn atom and its coordination porous network in ZIFs play synergistic effects in accelerating vulcanization and enhancing mechanical properties of natural rubber matrix besides its role of reinforcement filler.This is the reason that we chose to add Zn-ZIFs to natural rubber(rather than imidazole molecules).(3)The effects of the types and amounts of Zn-ZIFs on the mechanical properties of natural rubber were studied and the reasons for the differences in mechanical properties were explored.Among the three types of Zn-ZIFs,the natural rubber composite material containing ZIF-8 has the best mechanical properties,which can be attributed to the topological structure of ZIF-8.Due to the polysulfide species encapsulated inside in the cage of ZIF-8 probably make ZIF-8 as a lock to connect the chains of natural rubber,which is conducive to the improvement of mechanical properties.In addition,the influence of ZIF-8 on the thermo-oxidative aging resistance and dynamic mechanical properties of natural rubber was also explored.The addition of ZIF-8 distinctly improved the thermo-oxidative aging resistance and dynamic mechanical properties of natural rubber composites.Finally, the comprehensive performance of ZIF-8 in Zn ZIFs is the best,and the optimal addition amount is 0.6 phr.(4)Then the above Zn-ZIFs/DM/TMTD vulcanization system was used to prepare the UHMWPE fibers/ natural rubber composites.Based on the chemically inert and extremely smooth surface of UHMWPE fibers,the UHMWPE fibers were modified with piranha solution,plasma and dopamine solution.The modified fiber surface introduces various polar groups and increases the surface roughness,which can promote the interfacial adhesion between the fiber and the natural rubber matrix.The differences in the properties of the natural rubber,the UHMWPE fiber/ natural rubber,and the modified UHMWPE fiber/natural rubber composites were compared.Compared with pure natural rubber matrix,the mechanical properties of modified fiber/natural rubber composites were significantly improved(vulcanization temperature was 130 ℃).In addition,the tensile strength of composites vulcanized at 130 ℃ was compared with those vulcanized at 150 ℃.By comparison,the UHMWPE fibers did not perform the expected reinforcement towards natural rubber matrix under a vulcanization temperature of 150°C due to the possible impairment of the mechanical strength of UHMWPE fiber by undergoing a vulcanization process above its melting point.This study provides a novel strategy and attempt to realize the fast vulcanization of natural rubber at a low temperature with a low dosage of Zn-ZIFs rather other only adjusting the traditional accelerator formula.It opens a new path for ZIFs materials to step into the rubber fields and innovate a technical approach for the development of low-melting fibers/rubber composites with high-performance. |
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