Molecular simulation of the interaction mechanism between wollastonite and silane and the properties of modified powder filled nylon 6

China is the world's largest exporter and importer of wollastonite. Studying the surface modification process and formula of wollastonite and its application in plastics is of great significance to reduce the dependence of enterprises on imported products, saving foreign exchange for the country, and improving the technological content of related enterprises' products. In recent years, there have been many studies on the high value-added application of modified wollastonite filled polymer, but the molecular simulation of the bonding mechanism of silane and wollastonite surface has not been reported. Based on this, the surface modification of wollastonite with 3-aminopropyltriethoxysilane (SCA1113) was carried out in this study. The effects of modification time, modification temperature and silane dosage on the oil absorption value of wollastonite were studied. Nylon 6 composite was prepared by filling the modified powder into nylon 6. The mechanical properties of the composite were investigated. The microscopic mechanism of the surface interaction between silane and wollastonite was analyzed by means of FTIR and molecular simulation.

The effects of modification temperature, modification time and silane amount on the modification effect of SCA1113 modified wollastonite were investigated using the oil absorption value as the index. As 30% of wollastonite is added to nylon 6, and the inorganic filler used in polymer base material, the oil absorption value of filler will directly affect the processing properties and filling amount of the material. When the filler and plasticizer are used at the same time, if the oil absorption value of the filler is high, the plasticizer will adsorb the plasticizer, reduce the plasticizer's plasticizer effect on the resin, or the amount of plasticizer needs to be increased. The oil absorption value can be used as an evaluation method to compare the formulations of modifiers. According to the national standard, the mechanical properties of nylon 6, wollastonite/nylon 6 and modified wollastonite/nylon 6 composites were tested respectively. The CASTEP module of Material Studio software was used to simulate the modification process, and the functional group changes of samples before and after modification were compared with Fourier transform infrared change spectrometer to reveal the microscopic mechanism of silane surface modification of wollastonite.

①When the dosage of SCA1113 was 0.8 % of the mass of wollastonite, the modification time was 20min, and the modification temperature was 80℃, the oil absorption value of the modified powder reached the lowest 0.275 ml/g, and the modification effect was the best. ②The impact strength of nylon 6 is 6.8 KJ/m, the tensile strength is 60.56 Mpa, the bending strength is 72.54 MPa, and the bending modulus is 1955.41 MPa. The impact strength of wollastonite/nylon 6 is 4.4 KJ /m, the tensile strength is 60.82 Mpa, the bending strength is 109.96 MPa, and the bending modulus is 3280.27 MPa. The impact strength of modified wollastonite powder/nylon 6 is 7.38 KJ/m, the tensile strength is 75.6 Mpa, the bending strength is 108.05 MPa and the bending modulus is 3439.4 MPa. ③The Fourier transform infrared spectrum of the modified wollastonite powder is as follows: the characteristic peak of the OH of 1421.78 cm shifts to 1423.51 cm. The -OH characteristic peak of 3459.73 cm was shifted to 3421.92 cm. There are more characteristic peaks of -CH- and -CH- at 2922.50 cm and 2853.47 cm. ④The simulation of wollastonite crystal shows that the maximum change of atomic coordinates of the model is less than 0.005 ; The surface structure of wollastonite is simulated. It is found that the surface energy of crystal face (100), (010) and (001) is -23.30 eV/, -21.60 eV/ and -21.34 eV/, respectively. The upper atoms of crystal face (100) have longitudinal shrinkage and transverse stretching. It shows that it has the potential to adsorb external molecules. It was found that the bond length of Si-[OH] terminal of SCA1113 is close to that of the silico-oxygen chain in wollastonite crystals during the adsorption process, and the binding energy of SCA1113 and wollastonite is -9.43 eV, which is much higher than the energy contribution of a single hydrogen bond.Conclusion: (1) The optimum conditions for the modification of wollastonite by silane SCA1113 are as follows: the modification temperature is 80 ℃, the modification time is 20 min, and the amount of silane is 0.8 % of the mass of wollastonite. (2) Compared with the original wollastonite filled nylon 6, the modified wollastonite filled nylon 6 can form a flexible bonding interface to reduce the damage of impact resistance, and can significantly improve the mechanical properties of nylon 6. (3) The binding of silane SCA1113 to wollastonite is a chemical adsorption. When modified by silane SCA1113, the reactivity of wollastonite does not come from inside the wollastonite crystal, the crystal surface (100) is the most reactive, and the exposed calcium atoms in the wollastonite surface structure bind to the hydroxy group of SCA1113 to form Si-O-Ca bonds.