Fire behavior and mechanisms of flame-retardant unsaturated polyester resins by incorporation of novel phosphorus-containing comonomers
Student thesis: Doctoral Thesis
Related Research Unit(s)
Modern polymer materials have been extensively applied in society. However, they are easily ignited and frequently suffer from fire risk issues. As a consequence, these drawbacks limit the application of the polymer materials to a great extent and must be overcome to reach their full potential. Generally, enhancing the flame retardancy of a polymer material can be achieved by an additive or reactive approach. The former, blending flame-retardant additives into the polymer, has demonstrated desirable efficacy in fire resistance. However, it usually encounters with unfavorable reactions between matrices and additives, which give rise to the deterioration of the polymer's properties (e.g., mechanical properties) to some extent. In contrast, the latter, incorporation of flame-retardant moieties into the polymer chains, demonstrates desirable characteristics of tailoring the properties of the material and optimizing its overall performance. From a sustainably developmental perspective, developing halogen-free FRs is a promising trend in both academic and industrial fields. Among numerous candidates, phosphorus-containing compounds have been the subject of intense research in recent years, owing to the environmental friendliness and efficacy in flame retardancy. Particularly, reactive phosphorus-containing FRs exhibit little or no compromise of the matrices' intrinsic properties in addition to a low required composition to achieve prominent nonflammability. Inspired by previous findings, in this dissertation, we synthesized different reactive flame-retardant monomers and then introduced them into unsaturated polyester to prepare flame-retardant unsaturated polyester resins (FR-UPRs). The research work of this dissertation is presented as follows: 1. Two reactive phosphorus- and sulfur-containing monomer, [di(allyloxybisphenol sulfone) phenoxy phosphate (DASPP)] and [bis(acryloxyethyl diphenyl phosphate) sulfone, BADPS], were successfully synthesized and well characterized. Corresponding FR-UPRs with various amounts of monomer were prepared by radical bulk polymerization. The thermal properties and flammability of the FR-UPR samples were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurements, and cone calorimetry. The results showed that the introduction of the phosphorus- and sulfur-containing monomer into unsaturated polyester resin (UPR) can substantially improve its fire resistance and high-temperature stability. Interestingly, a linear increase in the glass transition temperature (Tg ) with increasing incorporated monomer content was observed by DSC. Scanning electron microscopy (SEM) and Raman spectroscopy studies revealed that these monomers can effectively improve the microstructure of UPR char residue and increase its graphitization degree, which can enhance the UPR's thermo-oxidative stability and char yield in high-temperature regions. Furthermore, real-time Fourier transform infrared (RTIR) spectroscopy was employed to study the thermo-oxidative degradation reactions of different UPR samples, providing insight into the degradation mechanism. In addition, results from tensile testing demonstrated the improved mechanical properties for the samples incorporated with these monomers. 2. A reactive cyclic phosphorus-containing monomer [ethyl acrylate cyclic glycol phosphate, EACGP] was synthesized in a facile way, and various amounts of EACGP were combined with unsaturated polyester by radical bulk polymerization. The resulting FR-UPR samples were investigated by TGA, LOI, and microscale combustion calorimetry (MCC) tests. Due to the high phosphorus content of EACGP, incorporation of this monomer led to a marked decrease in the peak heat release rate (pHRR), the total heat release (THR), an increase in the LOI, and the combustion char formation. Furthermore, RTIR spectroscopy was employed to investigate the thermo-oxidative degradation behavior of UPRs, and the charring effect of EACGP as well as the UPR char morphology was studied, illustrating the flame retardancy mechanism in UPR. 3. A reactive phosphorus-containing monomer [1-oxo-2,6,7-trioxa-1- phosphabicyclo-[2.2.2]octane-methyl diallyl phosphate, PDAP] was synthesized, and various amounts of PDAP were combined with unsaturated polyester by radical bulk polymerization. The thermal properties and flammability of the resulting UPR samples were studied. By means of TGA experiments it showed that incorporation of PDAP significantly altered the thermal decomposition pathway of the UPR. This phosphate-based flame retardant reduced the decomposition temperature of the resin and reacted with the polymer chains during the main decomposition step, contributing to a lowered maximum mass loss rate (MMLR) and enhanced char formation. The flammability of the UPRs was investigated with MCC and LOI tests. For the samples containing PDAP, marked nonflammability is attributed to the reduced HRC and THR as well as an enhanced char formation in the condensed phase. Furthermore, thermogravimetry-Fourier transform infrared (TG-FTIR) and RTIR spectroscopy were employed to gain insight into the degradation mechanism of UPRs. These results explicitly described the different chemical decomposition reactions in UPR after incorporation of PDAP. 4. Two phosphorus-containing compounds with equal element content, [methyl cyclic glycol phosphonate, (MCGP) and poly 1,2-dihydroxyethane phosphonate,(PDMP)] were synthesized and then added into unsaturated polyester for curing. The resulting FR-UPR samples were investigated by TGA and MCC tests. Interestingly, while both compounds are identical in element content and chemical bond, their flame retardancy mechanisms are different. Results showed that the flame retardancy mechanism for MCGP was active in gas phase, and PDMP exerted condensed phase flame retardant action, thus they demonstrated different performance in flammability tests. Furthermore, TG-FTIR spectroscopy was used to analyze their decomposition behaviors, elaborating the degradation mechanism of the samples.
- Copolymers, Thermal properties, Phosphorus compounds, Fire resistant polymers