Investigations on synergistic flame retardancy and mechanisms of halogen-free flame retardant polyester composites
Student thesis: Doctoral Thesis
Related Research Unit(s)
Represented by poly(ethylene terephthalate) (PET) and poly(1,4-butylene terephthalate) (PBT), partly aromatic polyesters are widely applied as engineering plastics in textile, high volume automotive, electrical, and other fields, owing to their good dimensional stability and thermal resistance. However, the development and application of polyesters are greatly limited due to its flammability and serious dripping when subjected to elevated temperatures or combustion. Based on systematically reviewing previous studies on flame retardant techniques used in halogen-free flame retarded polymeric materials including polyesters, several flame retardant techniques or systems which were suitable for polyesters were firstly introduced into polyesters in this study. The thermal decomposition and flame retardancy mechanisms were then studied. Finally, novel compounds used as flame retardants in polyesters were synthesized, and the flame retardancy mechanisms were investigated. In the present study, the following work was finished. (a). The effect of microencapsulated APP with silica gel shell (MAPP(Si)) and polyurethane shell (MAPP(PU)) in combination with MC on flame retardancy of PBT was firstly studied. The experimental results showed that both MAPP(Si) and MAPP(PU) significantly improved the flame retardancy of PBT in the presence of MC compared with APP. Based on the excellent fire retardancy of the intumescent flame retardant (IFR) system containing MAPP(PU) and MC, organo-modified montmorillonite (OMMT) was then introduced to the PBT/IFR system. The synergistic effect between OMMT and IFR was also found that the addition of OMMT further improved the flame retardancy of PBT/IFR composites. A mass of microcomposite structure particles formed in the heating or combustion process of PBT/IFR/OMMT nanocomposites were the first time to be found in the SEM images. It was a strong evidence to confirm the migration or accumulation of montmorillonite and carbonaceous-silicate materials during the heating or combustion process. (b). A novel halogen-free flame retardant system was established, composed of aluminum hypophosphite (AHP) and melamine derivatives (MD) to develop halogen-free fire retarded PBT and GRPBT composites. For the PBT composites with the incorporation of AHP and melamine derivatives, the heat release capacity (HRC) which is an indicator of a material fire hazard from microscale combustion calorimeter (MCC) testing was significantly reduced. The intensities of a variety of combustible or toxic gases detected by Thermogravimetric analysis/Fourier transform infrared spectrometry (TGA-FTIR) technique were remarkably decreased. Moreover, a loading of 20 wt% flame retardant mixture fulfilled the PBT composites high limiting oxygen index (LOI) and V-0 classification in Underwriters Laboratories 94 (UL-94) testing. The flame retarded GRPBT composite containing AHP, MD and PC can achieve a V-0 classification in UL-94 testing (the thickness of testing bar is 0.8 mm). In MCC and cone calorimeter testing, both of the two different PHRRs (peak of heat release rate) for GRPBT composites are significantly reduced by the incorporation of the novel system. Thermogravimetric analysis and residue characterization revealed that the addition of PC promotes the formation of char residues leading the reduction of mass loss rate, which results in the improvement of fire retardancy. (c). Nanoclay and AHP were used to develop halogen-free fire retardant GRPBT composites with enhanced fire retarded performance. Exfoliated clay nanocomposites of flame retarded/GRPBT were fabricated through melt blending process. TGA-FTIR analysis showed that the incorporation of AHP and nanoclay remarkably reduced the release of esters decomposed from PBT. MCC results indicated that the HRC was reduced by 51%. Moreover, significant improvements were obtained in LOI along with maintained UL-94 ratings. Thermogravimetric analysis and residue characterization revealed the flame retardancy mechanism in which the well dispersed layered silicates in polymer matrix probably play an important role of “microreactor” during the combustion. In addition, the substitution of a certain fraction of AHP by nanoclay could recover the interface between glass fiber and polymer matrix leading to the improvement of tensile strength. (d). Inspired by the flame retardant effects provided by AHP, another metal hypophosphite was taken into account to improve the fire retardancy performance of GRPs. Two kinds of rare earth metal hypophosphite, lanthanum (Ⅲ) hypophosphite and cerium (Ⅲ) hypophosphite, were successfully synthesized and characterized. Thermogravimetric analysis illustrated that both the incorporation of LHP and CHP could improve the thermal stability of GRPBT. TGA-FTIR analysis showed that all the volatilized products were reduced by the addition of LHP or CHP. For the composite containing 20 wt% of LHP or CHP, it could achieve a V-0 classification with a high LOI (about 28%). Additionally, both the PHRR and total heat release (THR) values of GRPBT composites were significantly reduced by the addition of LHP and CHP. The incorporation of the flame retardant mixture containing LHP or CHP with MC can further improve the flame retardancy of GRPBT. (e). Comparative study on the thermal decomposition and combustion behavior of CHP and AHP in GRPET composites was completed. TGA results showed that both the introduction of CHP and AHP reduced the thermal stability of GRPET, and improved the char residues. LOI and UL-94 testing indicated that both the addition of CHP and AHP could enhance the flame retardancy of GRPET. GRPET containing 10 wt% of AHP achieved V-0 classification with a high LOI of 30%, while only V-1 classification was obtained for GRPET with 10 wt% of CHP. Cone calorimeter testing results showed that CHP and AHP can both reduce the PHRR and THR of GRPET, and GRPET/AHP showed lower THR. For the PET/GF composites with a low loading of the flame retardant mixture containing AHP and MC, the HRC value was reduced by 47%, meanwhile, the intensities of a variety of combustible or toxic gases detected by TGA-FTIR technique were remarkably decreased. Thermal decomposition and residue analysis revealed that the flame retardancy of GRPET composites was enhanced by the condensed-phase action of AHP and the gas-phase dilution effect of MC.
- Fire resistant polymers