Synthesis of phosphorus-based flame retardant systems and their use in an epoxy resin

Flame retardancy and toughening properties of epoxy composites containing ammonium polyphosphate microcapsules and expanded graphite

Ammonium polyphosphate microcapsules (BM (polybenzoxazine modified) APP) were prepared through the in situ ring-opening polymerization of allyl group containing benzoxazine monomers on the surfaces of ammonium polyphosphate (APP), and they were significantly hydrophobic than the APP. A flame retardant system of epoxy (EP) resin was prepared with BMAPP and expanded graphite (EG). Flame retardancy, the thermal degradation behavior, a mechanical property of EP and EP/BMAPP/EG composites was investigated through limited oxygen index, vertical burning test, cone calorimetry (CONE), and the thermogravimetric analysis (TGA). The flame retardancy tests indicated that the EG could improve the thermal performance, promote the charring, and enhance the char quality of EP/BMAPP. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze the morphology and composition of the char residue formed during CONE testing, and to understand the mechanism of char formation. The results of TG-FTIR confirmed the possible mechanism of flame retardancy of EP/BMAPP/EG in the gas phase during combustion. The EG content effects on Young’s modulus, the tensile strength, and the fracture toughness ( KIC) of the EP/BMAPP composites were also investigated. The KIC of the composites containing 1% of EG and 10% of BMAPP increased by approximately 76% and 153%, respectively, compared to the neat matrix and EP/BMAPP-10%. The SEM images of the fractured surface indicated that the enhanced toughness of EP/BMAPP/EG composites mainly attributed to the debonding of the BMAPP and the subsequent plastic void growth of the matrix, as well as the crack deflection effect of the BMAPP/EG.

Intrinsic Flame-Retardant and Thermally Stable Epoxy Endowed by a Highly Efficient, Multifunctional Curing Agent

It is difficult to realize flame retardancy of epoxy without suffering much detriment in thermal stability. To solve the problem, a super-efficient phosphorus-nitrogen-containing reactive-type flame retardant, 10-(hydroxy(4-hydroxyphenyl)methyl)-5,10-dihydrophenophosphazinine-10-oxide (HB-DPPA) is synthesized and characterized. When it is used as a co-curing agent of 4,4'-methylenedianiline (DDM) for curing diglycidyl ether of bisphenol A (DGEBA), the cured epoxy achieves UL-94 V-0 rating with the limiting oxygen index of 29.3%. In this case, the phosphorus content in the system is exceptionally low (0.18 wt %). To the best of our knowledge, it currently has the highest efficiency among similar epoxy systems. Such excellent flame retardancy originates from the exclusive chemical structure of the phenophosphazine moiety, in which the phosphorus element is stabilized by the two adjacent aromatic rings. The action in the condensed phase is enhanced and followed by pressurization of the pyrolytic gases that induces the blowing-out effect during combustion. The cone calorimeter result reveals the formation of a unique intumescent char structure with five discernible layers. Owing to the super-efficient flame retardancy and the rigid molecular structure of HB-DPPA, the flame-retardant epoxy acquires high thermal stability and its initial decomposition temperature only decreases by 4.6 °C as compared with the unmodified one.

Flame retardant and toughening mechanisms of core–shell microspheres

Epoxy (EP) composites containing polystyrene-ammonium polyphosphate core–shell microspheres (CSP_PS-APP) were developed for flame retardant and toughening effects.

Phosphorylation of bio-based compounds: the state of the art

The aim of this review is to present both fundamental and applied research on the phosphorylation of renewable resources, through reactions on naturally occurring functions, and their use in biobased polymer chemistry and applications.

Preparation and properties of epoxy resin composites containing hexaphenoxycyclotriphosphazene

Hexaphenoxycyclotriphosphazene (HPCTP) was synthesized by a nucleophilic substitution reaction between hexachlorocyclotriphosphazene and phenol. The chemical structure of HPCTP was then characterized by Fourier transform infrared spectroscopy as well as 1H and 31P nuclear magnetic resonance spectroscopy. The synthesized HPCTP was incorporated into the diglycidyl ether of bisphenol A to prepare epoxy resin (EP) composites using 4,4′-diamino diphenylmethane as curing agent. The thermal and flame-retardant properties of cured samples were investigated by differential scanning calorimetry, thermogravimetric analysis, UL-94 vertical burning test, and cone calorimetry. Evaluation of thermal properties demonstrated that the resulting composites achieved high thermal stability with high char yields. The UL-94 V-0 rating was easily achieved with the addition of HPCTP, and the heat release intensity simultaneously decreased. Results of scanning electron microscopy and energy-dispersive x-ray spectroscopy confirmed that HPCTP can enhance the thermal stability and flame retardancy of epoxy resin.

Syntheses and characterization of two novel 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-based flame retardants for epoxy resin

Two novel phosphrous–nitrogen-containing flame retardants (3) and (4) were successfully synthesized from two steps: condensation reaction between 4-aminopenol and 1,4-phthaladehyde (or 4,4′-biphenyldicarboxaldehyde) and addition reaction between the above condensation compounds and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. The structures of (3) and (4) were characterized by Fourier transform infrared, nuclear magnetic resonance, and mass spectroscopy methods. The results show that (3) and (4) can serve as co-curing agents of diaminodiphenylmethane (DDM) for the curing reaction of epoxy resin. The epoxy thermosets-containing DDM and (3) or (4) exhibit moderate changes in glass transition temperature, thermal stability, and show improvement in flame retardancy: UL94 V-0 grade can be obtained when phosphorus content of the epoxy thermoset is only 1.0%; the limiting oxygen index of epoxy resin system containing (4) reaches 40.9 when phosphorus content is up to 1.5%.

Morphology, thermal properties, and fire behavior of epoxy resin nanocomposites containing octaammonium polyhedral oligomeric silsesquioxane-modified montmorillonite

A series of epoxy resin (EP) nanocomposites containing octaammonium polyhedral oligomeric silsesquioxane-modified montmorillonite (OAPOSS-MMT) was prepared via in situ polymerization using 4,4′-diaminodiphenyl sulfone as the curing agent. X-Ray diffraction and transmission electron microscopy were used to characterize the morphology of the resulting nanocomposites. Characterization results indicated the formation of intercalated and exfoliated structures. Thermogravimetric analysis (TGA) was performed to study the thermal properties of epoxy nanocomposites. Analysis results revealed that OAPOSS-MMT improved the thermal stability of EP matrix. TGA coupled with Fourier transform infrared spectrometry was used to detect pyrolytic gases. The gas species produced by epoxy nanocomposites were the same as those of control EP. A cone calorimeter was used to evaluate the flame retardancy of epoxy nanocomposites containing different concentrations of OAPOSS-MMT. Results showed that OAPOSS-MMT improved the flame retardancy of EP to some extent. The enhanced thermal oxidative stability was also confirmed by the data obtained through x-ray photoelectron spectroscopy.

Novel spirocyclic phosphazene-based epoxy resin for halogen-free fire resistance: synthesis, curing behaviors, and flammability characteristics

A novel halogen-free fire resistant epoxy resin with pendent spiro-cyclotriphosphazene groups was designed and synthesized via a three-step synthetic pathway. The chemical structures and compositions of spiro-cyclotriphosphazene precursors and final product were confirmed by (1)H, (13)C, and (31)P NMR spectroscopy, mass spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy. The thermal curing behaviors of the synthesized epoxy resin with 4,4'-diamino-diphenylmethane, 4,4'-diamino-diphenyl sulfone, and novolac as hardeners were investigated by differential scanning calorimetry (DSC), and the curing kinetics were also studied under a nonisothermal condition. The evaluation of the thermal properties demonstrated that these thermosets achieved a good thermal resistance due to their high glass transition temperatures more than 150 °C, and also gained high thermal stabilities with high char yields. The flammability characteristics of the spirocyclic phosphazene-based epoxy thermosets cured with these three hardeners were investigated on the basis of the results obtained from the limiting oxygen index (LOI) and UL-94 vertical burning experiments as well as the analysis of the residual chars collected from the vertical burning tests. The high LOI values and UL-94 V-0 classification of these epoxy thermosets indicated that the incorporation of phosphazene rings into the backbone chain imparts nonflammability to the epoxy resin owing to the unique combination of phosphorus and nitrogen following by a synergistic effect on flame retardancy. The epoxy resin obtained in this study is a green functional polymer and will become a potential candidate for fire- and heat-resistant applications in electronic and microelectronic fields with more safety and excellent performance.

New Trends in Reaction and Resistance to Fire of Fire-retardant Epoxies

This paper focuses on current trends in the flame retardancy of epoxy-based thermosets. This review examines the incorporation of additives in these polymers, including synergism effects. Reactive flame-retardants—which are incorporated in the polymer backbone—are reported and the use of fire-retardant epoxy coatings for materials protection is also considered.