Abstract:

Objective Poly(ethylene terephthalate) (PET), as the major polyester material, has excellent performance but is flammable, and hence it is important to improve the flame retardant properties of PET to satisfy the requirement for various applications. Introducing phosphorus-based flame retardants into PET molecular chains by copolymerization is one of the effective flame retardant modification methods at present. The flame retardant copolyester produced by melt polymerization has a intrinsic low viscosity of 0.6-0.7 dL/g, which cannot meet the flame retardant specifications of copolyester in fields such as engineering plastics, bottles, and industrial filament. This research represents an effort to increase the intrinsic viscosity of copolyester by polycondensation so as to expand the applications.

Method The side group phosphorus-containing flame retardant 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl] 10-phosphorus-phenanthrene-10-oxide (DDP) was copolymerized into PET molecular chain to obtain copolyester (PETD). Nuclear magnetic resonance hydrogen spectroscopy(¹H NMR) and Fourier transform infrared spectrometer characterization methods were adopted to measure the success of copolyester synthesis. The intrinsic viscosity of copolyesters was determined by using viscosity test. The thermal stability, crystallinity, carbon formation capacity, flame retardant properties, actual phosphorus content and carboxyl end-group content of copolyesters were characterized by differential scanning calorimetry, thermogravimetric analysis, limiting oxygen index (LOI) test, inductively coupled plasma-optical emission spectrometer test and carboxyl end-group concentrations test. In order to investigate the intrinsic viscosity changes of flame retardant copolyester containing phosphorus side group after solid-state polycondensation reaction under different reaction conditions, different amount of side group phosphate-containing flame retardant were used for optimisation. Reaction rate constants and activation energy were calculated by analyzing the viscosity increasing effect and reaction kinetics.

Results The flame retardant copolyester containing phosphorus side group was synthesized by copolymerization method. The reaction process of solid-state polycondensation (SSP) of flame retardant copolyester containing phosphorus side groups and the reaction kinetics were studied to establish understanding of mechanism governing the polycondensation technology of flame retardant copolyester. The results showed that the intrinsic viscosity of the prepared PET and copolyester reached 0.6-0.7 dL/g (Tab. 1), meeting the requirements of conventional polyesters. The increase of the DDP content of the flame retardant caused the carboxyl end-group concentrations to increase, the crystallization capacity to be worsened, and the carbon forming capacity and flame retardant property to increase. PETD₅ deminstrated good thermal stability and mechanical properties, and showed 13.6% of carbonization capacity (Tab. 2 and Fig. 6), 15% of crystallinity, and 31.8% of LOI (Tab. 1), proving successful preparation of flame retardant copolyester containing phosphorus side group in preparation for subsequent polycondensation reactions. The research showed that all polyesters achieved the intrinsic viscosity of 1.0 dL/g or higher within 10 h of solid-state polycondensation reaction at 200 and 210 ℃ (Fig. 8), and that the intrinsic viscosity of all polyesters increased with the increase of temperature and reaction time. Correspondingly, the concentration of carboxyl end-group concentrations decreased gradually with the increase of reaction temperature and reaction time. Compared with conventional PET, the reaction rate constant of flame retardant copolyester PETD increased with increasing temperature and decreasing flame retardant amount, and the activation energy increased when increasing flame retardant amount.

Conclusion It is found that the solid-state polycondensation reaction conditions are mild even with prolonged reaction time, and the copolyester demonstrates satisfactory thermal stability. It is easy to adjust the reaction conditions according to different demands in production, and therefore the study of solid-state polycondensation and condensation and adhesion reaction of flame retardant copolyester containing phosphorus side groups is of certain value for its industrial development. There are still many concerns calling for further study on the adhesion reaction of flame retardant copolyesters, and many aspects can be further explored, such as the influence of different reaction factors (vacuum degree, reaction atmosphere, particle size, crystallinity) on the adhesion of flame retardant copolyester. The changes of flame retardant properties, thermal stability, crystallization capacity and mechanical properties of flame retardant copolyester after polycondensation need to be studied in depth. The flame retardant and mechanical properties of the tackified flame retardant copolyesters needs to be evaluated.

Key words: phosphorus-based flame retardant, copolymerization, flame retardant copolyester, solid-state polycondensation, reaction kinetics