| As one of the maximal output resin, polypropylene (PP) is widely used in many fields such as housing, automobiles, electronic and electric industry, wire and cables due to its low cost, low density, low toxicity, excellent electrical resistance and ease of processing and moulding and so on. However, the flammability of PP limits its application fields.Intumescent flame retardant (IFR) is usually composed of phosphorus, nitrogen and carbon. To impart flame retardancy of PP, IFR can afford a swelling char layer, which can provides a thermal and physical barrier to the underlying material or bulk polymer during combustion. This dissertation mainly studies the synthesis, application and flame-retardant mechanism of three kinds of phosphorus-containing caged bicyclic flame retardants. The main contents are summarized as follows: 1. As an intermediate, 1-oxo-4-hydroxymethyl-2, 6, 7-trioxa-1-phosphabicyclo [2.2.2] octane (PEPA) was synthesized from pentaerythritol and phosphorus oxychloride. Phenylphosphonic dichloride (PPD), phenylphosphonothioic dichloride (PPTD) and phenyl dichlorophosphate (PDCP) were reacted with PEPA to produce bis (2, 6, 7- trioxa- 1- phosphabicyclo [2.2.2]octane- 1- oxo- 4- hydroxymethyl) phenylphosphonate (BCPPO), bis (2, 6, 7- trioxa- 1- phosphabicyclo [2.2.2] octane- 1- oxo- 4- hydroxymethyl) phenylphosphine sulfide (BCPPS) and bis (2, 6, 7- trioxa- 1- phosphabicyclo [2.2.2] octane- 1- oxo- 4- hydroxymethyl) phenyloxyphosphine (BCPPOO), respectively. Furthermore the routines were optimized, and the structures were characterized by FT-IR and 1H-NMR. The result of TGA showed that all of BCPPO, BCPPS, and BCPPOO had good thermal stability and high residues at 600℃, respectively.2. A large amount of flame retardant PP formulations based on BCPPO, BCPPS, BCPPOO, APP, MA or MP had been studied. Based on the results of LOI and UL-94 tests, three good flame retardant formulations were obtained: APP: MA: BCPPO= 3: 1: 1, APP: MP: BCPPS= 2: 1: 2 and APP: MP: BCPPOO= 4: 2: 9 when keeping the total loading of 30 wt.%, whose LOI values were 30.3, 31.3 and 27.1, respectively. The cone calorimeter tests showed that heat release rate (HRR), mass loss rate (MLR) and rate of smoke release (RSR) were decreased largely compared to pure PP.3. The thermal degradation process and residue after combustion of three flame-retardant PP based on BCPPO, BCPPS and BCPPOO as carbonization resource were studied by TG and SEM, respectively. The influence of different structures on flame retardancy was discussed.4. In order to improve further the flame retardancy of IFR-PP system, some potential synergistic flame retardants such as boric acid, zinc oxide and nickel acetate were added to IFR-PP systems. When small amount of boric acid was added to the BCPPO-based IFR-PP system (APP: MA: BCPPO=3: 1: 1), much better flame retardant effect was obtained than the corresponding system without boric acid. The mechanism analysis indicates that boric acid could be decomposed into boron oxide, which could paste the char and make the char more compact. However, zinc oxide and nickel acetate could react with APP to produce cross-linking materials to decrease the speed of the melt to flame area.5. The thermal behaviors were investigated by means of TG, and the results showed that the flame retardant PP degraded ahead of pure PP under both air and nitrogen atmosphere, but the final residue was much higher than that of neat PP. The Broido method was used to calculate the activation energy of PP/APP/MP/BCPPOO system and pure PP, and the incorporation of IFR could decrease the activation energy.6. The flame retardant mechanism of various systems was studied by means of SEM, FT-IR, XPS and XRD. Results showed that the compact char layer was a crucial factor for flame retardancy. Phosphorus-containing compounds were detected in intumescent char, and they could delay the oxidation of PP. The XRD analysis of intumescent char containing nickel acetate showed that phosphorus-nickel oxide and inorganic salts were detected.7. A capillary rheometer was used to investigate the rheological properties of various IFR-PP systems. Both neat PP and flame-retardant PP systems exhibit the pseudoplastic flow behavior, and the apparent viscosity decreases with the increase of shear rate. At a certain temperature, the IFR-PP systems had much higher apparent viscosities than neat PP, but there were not obvious differences of apparent viscosities among various IFR-PP systems. XRD was used to investigate the crystalline behaviors of various IFR-PP systems. IFR had no effect on the crystalline structure of PP. The investigation of mechanical properties showed that the tensile strength, elongation at break and Izod impact strength of IFR-PP systems decreased, especially elongation at break, but flexural strength increased compared to those of neat PP. However, the mechanical properties of the IFR-PP systems were a little better than those of some commercial flame-retardant PP products.
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