| Styrene-ethylene/butylene-styrene (SEBS) block copolymers are new styrenic-based thermoplastic elastomers with good comprehensive properties, and the investigations involved SEBS modified by rubber extender oils and thermoplastic resins are drawing more and more attentions. However, the range of applications of SEBS and its blends is restricted by their flammability. Flame-retardant polymer materials producing halogen-free, non-toxic or corrosive gases and smoke during burning have became a trend for environmental requirements. Magnesium hydroxide (MH) has been one of the most promising flame retardants for halogen-free flame retarded polymer materials due to the good smoke suppressibility, loading and low cost. But up to now, MH has been rarely used for the flame retarded SEBS-based compounds.In this study, the blends consisted of SEBS modified by paraffin oil and polypropylene (O-SEBS/PP) and MH were adopted as matrix and main flame retardant, respectively, and the mixtures of polymers and flame retardants were processed in a double-roll plasticator. The effects of MH content on the combustion, mechanical and processing properties of the composites were studied. Besides, the influences of surface modification of MH, incorporations of some different inorganic flame retardants, various ratios of compositions in matrix and the use of compatibilizer on the performances of the O-SEBS/PP/MH were also investigated, and the halogen-free flame retarded O-SEBS/PP composites with good mechanical properties were finally obtained. Moreover, by the means of limiting oxygen index (LOI), UL-94 vertical test, cone calorimeter test (CCT) and thermogravimetric analysis (TGA), the combustion properties and thermal stability of flame-retarded composites were evaluated, and the combustion behavior of composites was discussed. The relationships between microstructure and macroscopic properties of composites were characterized by scanning electron microscopy (SEM).At the beginning, micro-sized MH was used as a single flame retardant. The effects of the amount of MH and several MH treated by different surface modifiers on the combustion properties, thermal stability, mechanical and processing performances of O-SEBS/PP composites were studied. The results showed that 60 wt% MH loading was required to make the composites achieve UL-94 V-0 rating, whereas the mechanical and processing properties were deteriorated seriously. When MH were treated by different surface modifiers, the tensile strength of O-SEBS/PP composites filled with 60 wt% MH was improved to some degree, but the LOI values decreased, and the melt viscosity increased slightly. SEM observations revealed that the modified MH particles could be dispersed more evenly in the matrix, and the interface of two-phase became fuzzy, indicating that the silane coupling agent could lead to an improvement in the interface adhesion between the fillers and O-SEBS/PP blends.In order to reduce the total content of flame retardants while maintain the flame retardancy, the influences of the incorporations of MH with zinc borate (ZB) or expandable graphite (EG) on the combustion properties, thermal stability, mechanical and processing properties of flame-retarded O-SEBS/PP composites were studied, respectively. It is noted that there was no clear synergistic effect between ZB and MH on increasing flame retardancy of the composites. In contrast, EG could significantly improve the flame-retardant efficiency, but it also led to the decrease of mechanical properties. Based on it, the addition of EG was maintained at 5 wt%, and the effects of decrease in MH content on the properties were further investigated. It is found that the composites could also reach UL-94 V-0 rating and exhibit good mechanical properties when the total amount of flame retardants was 40 wt%. Furthermore, compared to the O-SEBS/PP composites filled with 60 wt% MH, the tensile strength, elongation at break and tear strength of O-SEBS/PP/MH/EG composites increased by 41.2%, 20.4%, and 31.2%, respectively. At the same time, the processing performance of composites was improved significantly due to decrease in total amount of the flame retardants.The influences of various ratios of polymer components and PP-g-MAH used to substitute equivalent some PP in the matrix on the mechanical, processing, combustion performances and thermal stability of O-SEBS/PP/MH/EG composites were studied. The results suggested that the lower oil content in the matrix, the better the mechanical strength and thermal stability of composites, but resulted in poor processing performance and high hardness. When the ratio of SEBS to Oil in the matrix was retained at 100/65, the addition of an appropriate amount of PP was able to increase the mechanical strength, processing performance and initial decomposition temperature of composites. The composites had better comprehensive performances when the proportion of SEBS/Oil/PP was 100/65/50. After PP in the matrix was substituted by 14~21 phr PP-g-MAH with the same content, the mechanical properties of composites were enhanced while the good flame retardancy was also maintained. The tensile strength, tear strength, elongation at break and Shore-A hardness of flame retarded composites were 13.4 MPa, 47.9 kN·m-1, 231%, and 85, respectively.
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