| Most of the high refractive index materials available for optical applications are inorganic-based glasses which are heavy and brittle. Organic polymers offer the advantages of lightness of weight, toughness, and ease of fabrication. However, one disadvantage of classical organic polymeric materials is their relatively low refractive indices. In addition, typical organic polymers are limited in chemical stability, thermostability and flame resistance. These shortcomings have prevented the use of many such polymers for critical optical applications in preference to high refractive index inorganic glasses.The -P=N- construction of cyclotriphosphazene derivatives possesses the relatively high electron density and is optically transparent in the region between 220 nm to 800 nm, which result in the potential of generating high refractive index (nd). Moreover, the nitrogen–phosphorus (P–N) synergistic effect makes the cyclotriphosphazene derivatives obtain high heat resistance and halogen-free flame resistance. Through the molecular structural design, the high reactivity of P-Cl bond in hexachlorocyclotriphosphazene makes it possible to introduce the reactive groups and different side chains on the phosphazenes ring, then the optical resins with the higher refractive index, better heat-resistance and halogen-free flame resistance can be prepaired by homopolymerization of the before-mentioned functional monomers or their copolymerization with the traditional monomers. In addition, the halogen-free flame resistance of the traditional optical resins can be improved by mixed the cyclotriphosphazene derivatives as flame retardant.Two cyclotriphosphazene derivatives with allylic group [C=C] and different carbon chain lengths were synthesized by nucleophilic substitution of hexachlorocyclotriphosphazene, and a series of halogen-free flame retardant optical resins were prepared with the homoplymerization of the double bond and the copolymerization with methyl methacrylate (MMA) or styrene (St). The molecular structures of the compounds were characterized by 1H,13C,31P-NMR and mass spectrometry. The optical properties, the water resistance, the mechanical performance and the the thermal and flame-retardant characteristic of the cured resins are studied. The kinetics and mechanism of thermal decomposition of the phosphazene cyclomatrix homopolymers and copolymers were investigated using integrated thermogravimetry (TG) and FTIR (TG-FTIR) analyses and morphologies of the solid residues after pyrolysis by scanning electron microscope (SEM). The flame retardant mechanism was also discussed.It was found that the phosphazene cyclomatrix homopolymer (Ⅴ) of hexa-(Allyl 4-hydroxybenzoate)cyclotriphosphazene (Ⅲ) exhibited the higher refractive index (nd), good transparency, the higher surface hardness, the lower water absorption, especially the higher thermal stability both in nitrogen and in air atmosphere at elevated temperature, and the higher char yield. The transmittance optical resins were prepared by copolymerization of the cyclotriphosphazene derivative (Ⅲ) with MMA or St. Comparing with the phosphazene cyclomatrix homopolymer (Ⅴ), the phosphazene cyclomatrix homopolymer (Ⅵ) of the cyclotriphosphazene derivative (Ⅳ) with the longer carbon chain also showed the good thermal stability, whereas its refractive index, transparency, char yield and surface hardness were inferior to that of homopolymer (Ⅴ). Further more, the copolymers of the cyclotriphosphazene derivative (Ⅳ) with MMA or St showed the poor performances.The copolymers were prepared from compound (Ⅲ) with MMA or St, which transmittance was higher than 87%. The refractive index, the thermal stability and the surface hardness of the copolymers were higher than that of the corresponding homopolymers (PMMA or PS). With increasing the content of cyclotriphosphazene in the copolymers, the refractive index and the char yield increases. The water resistance of the copolymers of compound (Ⅲ) and MMA was higher than that of PMMA. The results of TGA-FTIR and SEM analyses showed that the phosphazene cyclomatrix homopolymers and the copolymers were intumescent flame retardant mechanism. Two derivatives (additive A and additive B) of hexachlorocylotriphosphazene as halogen-free flame retardants were synthesized via nucleophilic substitution of hexachlorocyclotriphosphazene, and optical resins based on PMMA and the flame retardants were prepared. The effects of the additives on the optical properties, the mechanical performances, the thermal and flame retardant were discussed. As compared with pure PMMA, the as–prepared resins with the two additives exhibited higher refractive index and the longer UV cutoff wavelength. The visible light transmittance of the samples is higher than 90% for all formulas concerning additive A, the same results were obtained when additive B is lower than 30% by weight in formulas. The water resistance and the surface hardness of the resins were improved with increasing of the additives,the surface hardness is increased from HB to 2H. The non-flammability of the two flame retardants was characterized by limited oxygen index (LOI) tests. In the cases using 20 wt% of the additives, the LOI of those using additive A and additive B were 26 and 22, respectively. The LOI of the fomula with 50 wt% additives were 28%. The impact and the tensile strength changed little for the formulas of the additives lower than 30% by weight. The mechanical properties would be improved by preparing lightly crosslinking-PMMA composites through adding the double functional acrylate as crosslink agent. In conclusion, when the dosage of the additives was lower than 30% by weight in fomula, additive A and additive B could be used as potential environment-friendly flame retardant for PMMA to obtain the highly transparent optical resins.
|
PDF Full Text Request