Preparation, Properties And Flame-retardant Mechanism Of Phosphorus-containing Copolyesters/Inorganic Nanocomposites

Poly (ethylene terephthalate) (PET) fiber has been a kind of synthetic fiber with the biggest yield and widest uses of all the synthetic fibers due to its outstanding characteristics of high strength, resistance to shrinkage and stretch, and chemical resistance, but their uses are limited by their combustibility. From 90's, China became a country with the biggest yield of PET in the world. Meanwhile, the flammability of PET may cause serious calamity for their wide application.Therefore, the study on the flame retardation of PET has been a significant issue. Generally, phosphorus-containing flame retardants are very effective for polyester. They have much more advantages than halogen-containing flame retardants and inorganic flame retardants. Most of the phosphorus-containing flame retardants are additives, which have some disadvantages such as the decrease of mechanical properties owing to the poor compatibility between flame retardant additives and PET. Some existing reactive phosphorus-containing flame retardants can overcome the above drawbacks, but the polyesters flame-retarded by them still behave seriously melt-dripping during burning. On the other hand, people lack a good understanding of the flame-retardant mechanism because there have not been a systematic relevant study on polyesters flame-retarded by phosphorus-containing flame retardants in the literature.The purpose of this thesis is to prepare phosphorus-containing polyesters with both flame retardancy and anti-dripping. Molecular design, in situ polymerization, and nanotechnology have been employed to address the issues mentioned above. The syntheses, characterizations and properties of phosphorus-containing flame retardants, inorganic nanofillers, phosphorus-containing copolyesters and their nanocomposites have been studied, and the relevant flame-retardant mechanisms have been investigated systematically. The main work is summarized as follows:1. Two kinds of reactive flame-retardants, 9,10-dihydro-10[2,3-di (hydroxycarbonyl)propyl] 10-phosphaphenanthrene-10-oxide (DDP) and 2-(6-oxide-6H-dibenz〈c, e〉〈1, 2〉oxaphosphorin-6-yl)-1,4-dihydroxy-phenylene (DOPOHQ), were synthesized and used to synthesize two kinds of phosphorus-containing copolyesters (PET-co-DDP and PET-co-DOPOHQ) by polycondensation with the monomers of PET, terephthalic acid (TPA) and ethyleneglycol (EG), respectively. By in situ polymerization method, the phosphorus-containing copolyesters/MMT nanocomposites (PET-co-DDP/MMT and PET-co-DOPOHQ/MMT) have been synthesized successfully for the first time. The nanostructure has been identified by WAXD, SEM and TEM. At the same time, another foaming and charring agent, poly (2, 2-dimethylpropylene spirocyclic pentaerythritol bisphosphonate) (DPSPB), has been used as flame retardant for polyesters in this study. The results show it has excellent anti-dripping properties as used in the flame retarded PET nanocomposite.2. A novel phosphorus-containing copolyester nanocomposite, PET-co-DDP/ZrP, has been studied for the first time. Its structures and morphology were characterized by FT-IR, SEM, TEM and WAXD.3. A new approach,"double in situ synthesis"for preparing polyester nanocomposites, has been developed in this study, in which nanoparticles and nanocomposites can be formed simultaneously in one process. The structure and morphology of the composites have been investigated by FTIR, SEM and TEM. Furthermore, multi-wall nanotube (MWNT) grafted with polyester has been investigated for the first time. 4. The burning behaviors of neat PET, phosphorus-containing copolyester and phosphorus-containing copolyester nanocomposites have been studied by LOI, UL-94 and Cone calorimeter. Based on those results, the flame retardant performances of the copolyesters and their nanocomposites have been improved obviously comparing with that of neat PET: their LOI values can reach 35.1, and UL-94 test V-0 rating. The HRR and THR decrease by 35-47%, 27-41%, respectively. Meanwhile the anti-dripping behaviors have also been improved.5. The thermal degradation behaviors and crystalline behaviors of pure PET, phosphorus-containing copolyester and phosphorus-containing copolyester nanocomposites were investigated via TG and DSC, respectively. From the TGA's results, the initial decomposition temperature of phosphorus-containing copolyester and phosphorus-containing copolyester nanocomposites are lower than that of pure PET in nitrogen. Based on the thermal dynamic method, the phosphorus-containing copolyester and phosphorus-containing copolyester nanocomposites show lower activation energies than PET at lower conversion. After that, the activation energy of both phosphorus-containing copolyester and phosphorus-containing copolyester nanocomposites present higher than that of PET. The reason for this is duo to the weak bond of P-C in flame retardant, which will be broken firstly when heated or fire. And then, protective layers that prohibit the heat and oxygen transition can be formed on the surface of polyesters. The order of their crystallizability is as follow: phosphorus-containing copolyester nanocomposites > PET > phosphorus-containing copolyester. From the research on the activation energy of nonisothermal crystallization, the introduction of flame retardant into polyester matrix weakens the crystallization because of the irregular molecular chain of the flame retardant, while the case has been changed when adding the nanoparticle.6. The relationship between thermal properties and combustion behaviors of the studied samples has been investigated. The results show that there are some correlations between both of them.7. The decomposition and flame retardation mechanisms were studied on the base of the results mentioned above and the analysis of Py-GC-MS, TG-FTIR, SEM, WAXD and XPS. A novel radical decomposition mechanism in the condensed phase has been proposed for the first time in this study. For phosphorus-containing copolyester, the main flame retarded mechanism is protective char layer in the condensed phase. For phosphorus-containing copolyester nanocomposites, the main mechanism is protective char layer and radical decomposition mechanism in the condensed phase. A continuous and compact protective layer is formed on the surface of phosphorus-containing copolyester when burning. The effective protective layer can prohibit the transition of heat and oxygen. When MMT is added to the phosphorus-containing copolyester, more fused ring compounds can be found in the test of Py-GC-MS, while they are not found in the gas phase from TG-FTIR test. These evidences proved the proposed radical decomposition mechanism in the condensed phase.