Design, Preparation And Property Studies Of Novel Transparent Flame-retardant Polymethyl Methacrylate With Enhanced Thermal Property

As a typical transparent amorphous polymer, polymethyl methacrylate (PMMA) has been widely used in building construction, plastic optical fibers and optical lenses because of its excellent characteristics including high strength, good flexibility, weather resistance, and dimension stability. However, the poor thermal stability and flammability limits its further application. Consequently, improving the thermal properties and flame retardancy of PMMA has been attracting increasing attention. To avoid environmental pollution, there is a trend toward using halogen-free flame retardants (FRs) in PMMA resins. Phosphorus-, nitrogen-, or silicon-containing compounds as promising "green" flame retardants are mostly used to replace halogenated compounds in polymers.In this dissertation, different reactive FRs containing phosphorus, nitrogen and/or silicon (APEEA, SNP and TMSAP) were synthesized by the way of molecular design and well characterized. The non-halogenated FRs were added into PMMA matrix by chemical incorporation including copolymerization or/and silane cross-linking method to improve the flame retardancy, thermal property and mechanical property. By comparison, TMSAP exhibits the best flame-retardant efficiency. Considering the advantage of nanocomposite technique, two layered nanoaddtives comprising layed aluminophosphate (LAP) and graphene was firstly organic-modified and then dispersed into PMMA. Compared to heat-conductive graphene, the addition of small amounts of non-conductive LAP nanolayers better improve the flame retardancy and thermal stability. To achieve higher flame-retardant efficiency, chemical incorporation method and nanocomposite technique are combined. The research work of this dissertation is composed of the following parts:1. Three phosphorus-, nitrogen-and/or silicon-containing reactive flame retardants, noted as APEEA, SNP and TMSAP, were synthesized and well characterized using FTIR,’H NMR,31P NMR and29Si NMR. The APEEA and SNP were synthesized by the esterification of phenyl dichlorophosphate (PDCP) with hydroxyethyl acrylate (HEA), and followed by the reaction with diethylamine (APEEA) or3-aminopropyltriethoxysilane (APTES). The TMSAP was synthesized by a Kabachnik-Fields reaction. The oligomers of these three compounds were prepared by homopolymerization and/or silane cross-linking. The thermal stability and flammability of the oligomers were evaluated by TGA and MCC. The TGA results showed that the char residues of Olig-APEEA, Olig-SNP and Olig-TMSAP were18.6%,34.6%and63.5%, respectively. The HRC of the three oligomers are304J/K-g,93J/K-g and75J/K-g respectively. Both the TGA and MCC results showed the three monomers had high char formation and low flammability.2. The phosphorus-and nitrogen-containing flame retardant, APEEA, was copolymerized with MMA monomers to prepare a novel linear PMMA-based copolymer. The morphology and structure of the copolymers were characterized using FTIR. The copolymers exhibit relatively high transparency and significant improvements to the thermal stability and fire retardancy when compared to PMMA. However, the mechanical properties and glass transition temperature (Tg) are reduced remarkably. From the mechanism analysis, the char formation of copolymers caused by APEEA during degradation plays a key role in the flame retardancy enhancement. The increased molecular flexibility and copolymer free volume is responsible for the deterioration of mechanical properties and Tg.3. Two novel reactive flame retardants (SNP and TMSAP) containing phosphorus, nitrogen, and silicon were incorporated into PMMA matrix through copolymerization and the sol-gel method to produce PMMA based organic-inorganic hybrids. The29Si MAS NMR results for the hybrid materials suggested the formation of cross-linked networks in the hybrids. A morphological study showed that the inorganic particles were well distributed in the PMMA matrix. The hybrids retained a high transparency and exhibited a significant improvement in glass transition temperature, thermal stability, hardness, and flame retardancy upon the incorporation of flame retardants into the PMMA matrix. The network structure, homogeneous distribution, and char formation during degradation were proposed as three key reasons for the improved properties.4. PMMA/DDA-LAP intercalated nanocomposites are prepared by in situ bulk polymerization of MMA. The intercalated structure is characterized by XRD and TME. With the intercalation of DDA-LAP in PMMA matrix, the Tg obtained from DSC are increased. From UV-vis, TGA and MCC results, the nanocomposites obtained keep relatively high transparency in optical property and have a significant improvement in mechanical properties, thermal stability and flame retardancy. The mechanism for the properties enhancement is investigated. The strong interfacial interaction between the aluminophosphate layers and the PMMA chains, the homogeneously distribution and the graphitized char formation during heating are three key roles for the properties improvement.Graphene is a kind of very promising filler for polymer composites, but its irreversible aggregation when introduced into polymers is a challenge for property enhancements and limits its industrial application. To address this, we report one-step covalent functionalization and simultaneous reduction of graphite oxide (GO) with hydroxyethyl acrylate (HEA), resulting in a functionalized graphene with double bonds. The functionalized graphene obtained, noted as FGN, is successfully incorporated into polymethyl meth-acrylate (PMMA) matrix by latex technology and melt blending. Latex technology is used for the pretreatment of FGN through emulsion copolymerization between methyl methacrylate (MMA) monomers and FGN double bonds. After pretreatment of FGN, covalent attachment of PMMA particles to the edges of FGN sheets can effectively prevent their agglomeration and markedly improve their dispersion in the polymer matrix. Since these PMMA particles act as good compatibilizers in the interface between FGN and PMMA matrix during the melt mixing process, the PMMA/FGN composites obtained exhibit exfoliated morphology and very good dispersion, as evinced by the results from X-ray diffraction (XRD) and transmission electron microscopy (TEM). When even a small amount of FGN (<1.0wt%) is incorporated, the thermal properties and mechanical properties of PMMA/FGN composites are enhanced significantly. The Tg increases from103.8to110.5℃, while the tensile strength increases by31.0%(1.0wt%FGN addition). Moreover, the storage modulus of PMMA/FGN composites increases by27%(1.0wt%FGN addition) at room temperature. These enhancements are attributed to the strong chemical interaction between the FGN sheets and PMMA and the good distribution of FGN sheets in the PMMA matrix. However, due to the high heat conductivity of FGN sheets, the flammability of composites is a little decreased, which suggest that the graphene sheets do not exhibits good flame-retardant effect on PMMA resin.5. To further improve the flame-retardant efficiency,15%DDA-LAP/TMSAP mixture with different DDA-LAP contents was incorporated into PMMA matrix by sol-gel method to prepare a novel PMMA based composite (poly (MMA-co-MSMA)/TMSAP/DDA-LAP). XRD and TEM results showed that the LAP nanolayers were intercalative or/and exfoliated in the composites, exhibiting high dispersion degree. The29Si MAS NMR results for the composites suggested the formation of cross-linked networks. From hardness test, DSC, TGA, LOI, UV-vis and MCC results, the composites obtained possess enhanced hardness, glass transition temperature, thermal stability and flame retardancy, and keep high transparency. The degradation process of composites was studied by RTIR. Raman and SEM were used to study the components and structure of char residue. Possible mechanisms for the performance enhancements of composites are proposed. Moreover, it may be observed that much more PHRR reduction and LOI increment were observed in the composites with the addition of15%DDA-LAP/TMSAP than those with the addition of15%TMSAP, suggesting that a synergistic effect occurs between DDA-LAP and TMSAP in enhancing flame retardancy of PMMA. The physical barrier effect of LAP nanolayers combining with the graphitized char catalyzed by DDA-LAP and TMSAP during combustion plays key roles in the significantly enhanced flame retardancy.