Preparation And Properties Of Star-branched Nylon 6 Material And Additive With Hexafunctional Cyclotriphosphazene Core

Star-branched nylon 6 has attracted considerable research interest due to its simple design, novel structure, unique property and promising potential.Cyclotriphosphazenes have excellent thermal stability, because nitrogen and phosphorus atoms arrange alternately to form a rigid six-member ring. In this study,we first chose hexafunctional cyclotriphosphazene as a core to synthesize a series of star-branched nylon 6 with different molecular weights, and then systematically investigated how star-branching and molecular weight influence its properties by comparing with linear nylon 6. In order to broaden its application, two kinds of modification methods were used. Moreover, a hindered amine multifunctional stabilizer with hexafunctional cyclotriphosphazene core was synthesized and then used in nylon 6, and their ultraviolet aging and thermo-oxidative aging properties were studied. Five parts of this work are stated as below:1. A hexafunctional agent HCPCP was successfully synthesized via two-step reaction with high yield and low cost. First, HAPCP was synthesized by a nucleophilic substitution reaction between hexachlorocyclotriphosphazene and4-hydroxy benzaldehyde using anhydrous K2CO3 as the deacid reagent, and then HAPCP was oxidized by KMnO4 to form HCPCP. The TG result indicates that HCPCP has excellent thermal stability with the initial thermal decomposition temperature above 300 oC. Therefore, HCPCP can be used as a core during high-temperature melting polycondensation process of nylon 6.2. A novel star-branched nylon 6 with hexafunctional cyclotriphosphazene core was synthesized using hexa(4-carboxylphenoxy)cyclotriphosphazene(HCPCP) with six carboxyl groups as multifunctional agent in the hydrolytic ring-opening polymerization of ε-caprolactam, and then used for the investigation of mechanical properties, crystallization and rheology behaviors. Star-branching structure and molecular weight have great effects on its properties. Compared with linear nylon 6,star-branched nylon 6 has lower relative viscosity and higher melt flow rate while its mechanical properties can be almost retained by the use of star-branching and an appropriate molecular weight. Research on crystallization behavior indicates that the degree of crystallinity(Xc) of star-branched nylon 6 decreases slightly, but its crystal structure still belongs to α form. The peak crystallization temperature(Tc) andcrystallization rate(1/t1/2) of star-branched nylon 6 are significantly higher than those of linear nylon 6 because of heterogeneous nucleation induced by HCPCP core, which is beneficial for the use of rapid molding process. As the molecular weight increases,the Tc and 1/t1/2 of star-branched nylon 6 first increase and then decrease. Capillary rheometer measurement exhibits that the shear viscosity of star-branched nylon 6decreases with decreasing molecular weight, and the shear viscosity of star-branched nylon 6 with an appropriate molecular weight shows a low value and little or no sensitivity to shear rate and temperature. Such rheology behavior allows for processing at low temperature and low pressure and reduces system cost.3. A star-branched nylon 6 with hexafunctional cyclotriphosphazene core(SPA6)was synthesized at large scale in 50 L autoclave, and then SPA6 was mixed with glass fiber(GF) by twin-screw extruder to prepared SPA6/GF composites with 10, 20, 30,40 and 50 wt% GF content. The results indicate that the melt flow rates of SPA6/GF composites which decrease with increasing GF content are still higher than those of LPA6/GF composites. The tensile strength, bending strength, bending modulus and impact strength of SPA6/GF composites increase with increasing GF content. When GF content is less than 30 wt%, the mechanical properties of SPA6/GF composites are almost the same with LPA6/GF composites. When GF content is more than 30wt%, the mechanical properties of SPA6/GF composites are higher than those of LPA6/GF composites, particularly for bending strength and bending modulus. For SPA6/GF composites with high GF content, GFs are uniformly distributed in SPA6 matrixs, and their surfaces have no GF revealed phenomena and show excellent glossiness, which solve the problems of GF revealed phenomena and poor glossiness for SPA6/GF composites with high GF content.4. Nylon 6-grafted sulfonated graphene(SGgraft) was prepared using sulfonated graphene(SG) as a precursor by in situ hydrolytic ring-opening polymerization ofε-caprolactam. SPA6/SGgraft(SSG) composites were prepared by melt blending in twin-screw extruder. The SEM images indicate that SSG composites exhibit obvious lamellar structure and SG sheets are uniformly distributed in SPA6 matrixs. The tensile strength and bending strength of SSG composites increase with increasing SG content due to the reinforcement effect of SG. However, impact strength and elongation at break of SSG composites decrease with increasing SG content. The initial thermal decomposition and the amount of residual char of SSG composites increase with increasing SG content. With the increase of SG content, the Tc of SSGcomposites increases due to the heterogeneous nucleation effect of SG, and Tm of SSG composites first increases and then decreases. The relative viscosity of SSG composites decreases with increasing SG content, and the melt flow rate increases with increasing SG content. The thermal conductivity(λ) of SSG composites increases with increasing SG content. The λ of SSG composite with 0.8 wt% SG content is 0.364 W·m-1·K-1 with the increment of 71.7 %. The excellent thermal conductive properties of SSG composites should be attributed to the large specific surface areas of SG sheets and the homogeneous dispersion of SG sheets in SSG composites.5. A hindered amine multifunctional stabilizer with cyclotriphosphazene core(HPD) was synthesized and then used in nylon 6 to stabilized PA6 by twin-screw extruder. The results indicate that ΔΕ changes and mechanical properties changes of stabilized PA6 after ultraviolet aging and thermo-oxidative aging are slower than those of pure PA6. The addition of HPD depresses the aging rate of PA6. By analyzing the structures of the samples before and after two kinds of aging, the results indicate the intensity of absorption band at 1730 cm-1 both weaken obviously by comparison with pure PA6. This result further confirms the addition of HPD depresses the formation of chromophore containing carbonyl group in PA6 chains.