Comparative study on flame retardancy and thermal degradation of phosphorus- and silicon-containing epoxy resin composites

Exploration of the Fire-Retardant Potential of Microencapsulated Ammonium Polyphosphate in Epoxy Vitrimer Containing Dynamic Disulfide Bonds

Epoxy vitrimers appear as a promising alternative to common epoxy thermoset composites. Nevertheless, the possibilities of applying these materials are limited due to their high flammability which may cause high fire risks. To date, the flame-retardant epoxy vitrimer systems reported in the literature almost all rely on intrinsic flame retardancy to achieve high fire safety; however, the complex and expensive synthesis process hinders their large-scale application. In this work, disulfide-based epoxy vitrimer (EPV) was fabricated with 4, 4'-dithiodianiline as the curing agent, and microencapsulated ammonium polyphosphate (MFAPP) was employed as a potential additive flame retardant to improve their fire retardancy. As a comparative study, common epoxy (EP) composites were also prepared using 4,4'-diaminodiphenylmethane as the curing agent. The results showed that the introduction of dynamic disulfide bonds led to a reduction in the initial thermal decomposition temperature of EPV by around 70 °C compared to EP. Moreover, the addition of 7.5 wt.% of MFAPP endowed EP with excellent fire performance: the LOI value was as high as 29.9% and the V-0 rating was achieved in the UL-94 test (3.2 mm). However, under the same loading, although EPV/MFAPP7.5% showed obvious anti-dripping performance, it did not reach any rating in the UL-94 test. The flame-retardant mechanisms in the condensed phase were evaluated using SEM-EDS, XPS, and Raman spectroscopy. The results showed that the residue of EPV/MFAPP7.5% presented numerous holes during burning, which failed to form a continuous and dense char layer as a physical barrier resulting in relatively poor flame retardancy compared to EP/MFAPP7.5%.

Research on Properties of Silicone-Modified Epoxy Resin and 3D Printing Materials

A kind of organosilicon intermediate was prepared using isophorone diisocyanate (IPDI), hydroxyl silicone oil (HSO), and hydroxyethyl acrylate (HEA). The organosilicon modification of epoxy resin was realized by introducing a -Si-O- group into the side chain of epoxy resin by chemical grafting. The effects of organosilicon modification of epoxy resin on the mechanical properties systematically discuss its heat resistance and micromorphology. The results indicate that the curing shrinkage of the resin was decreased and the printing accuracy was improved. At the same time, the mechanical properties of the material are enhanced; the IS and elongation at break (EAB) are enhanced by 32.8 and 8.65%, respectively. The brittle fracture is changed to a ductile fracture, and the tensile strength (TS) of the material is decreased. The glass transition temperature (GTT) of the modified epoxy resin increased by 8.46 °C, and T_50% and T_max increased by 1.9 and 6 °C, respectively, indicating that the heat resistance of the modified epoxy resin was improved.

Preparation and properties of nano-TiO2-modified photosensitive materials for 3D printing

The surface of nano-TiO2 was modified by a silane coupling agent KH570, and the photosensitive resin was modified by blending the modified nano-TiO2 with three-dimensional (3D) printing light-curing resin. The modified nano-TiO2 powder was characterized by infrared spectrum, X-ray diffraction, contact angle test, and scanning electron microscope. The effects of different content of modified TiO2 on the viscosity, curing shrinkage, tensile strength, elongation at break, hardness, thermal stability, and cross-section morphology of 3D printing photosensitive resin were studied. The results showed that the mechanical properties of epoxy resin were improved obviously after surface modification with a silane coupling agent. When the mass fraction of TiO2 was 1.5%, the mechanical properties of the molded parts were the best. The tensile strength, impact strength, and elongation at break were increased by 51.1, 43.8, and 10.8%, respectively, and the hardness value was maintained at 81–83 HD. The addition of modified TiO2 can improve the heat resistance of the epoxy resin. When the amount of TiO2 is 1.5%, the T 50%, T max, and carbon residue rate of the epoxy resin are increased by 3.44°C, 6.34°C, and 25.3%, respectively.

A Novel Synergistic Flame Retardant of Hexaphenoxycyclotriphosphazene for Epoxy Resin

Hexaphenoxycyclotriphosphazene (HPCP) is a common flame retardant for epoxy resin (EP). To improve the thermostability and fire safety of HPCP-containing EP, we combined UiO66-NH₂ (a kind of metal-organic frame, MOF) with halloysite nanotubes (HNTs) by hydrothermal reaction to create a novel synergistic flame retardant (H-U) of HPCP for EP. For the EP containing HPCP and H-U, the initial decomposition temperature (T_5%) and the temperature of maximum decomposition rate (T_max) increased by 11 and 17 °C under nitrogen atmosphere compared with those of the EP containing only HPCP. Meanwhile, the EP containing HPCP and H-U exhibited better tensile and flexural properties due to the addition of rigid nanoparticles. Notably, the EP containing HPCP and H-U reached a V-0 rating in UL-94 test and a limited oxygen index (LOI) of 35.2%. However, with the introduction of H-U, the flame retardant performances of EP composites were weakened in the cone calorimeter test, which was probably due to the decreased height of intumescent residual char.