The direct coordination between polyhedral oligomeric
silsesquioxane
(POSS) and Co forms an assembly of nanoparticles with low specific
surface area and leads to a poor dispersion state in the epoxy resin
matrix, resulting in unsatisfactory flame-retardant efficiency. Metal–organic
frameworks (MOFs), for instance, ZIF-67, provide not only the cobalt
element but also the porous framework that endows the nanocomposite
of MOFs and POSS with high specific surface area and abundant Co sites
in the silica skeleton. Herein, ZIF-67 is hybridized with octacarboxyl
POSS, resulting in the removal of the alkaline ligand to form novel
metal POSS–organic frameworks (MPOFs). The size differences
for organic groups and silica nanocages of POSS vs. micropores of
ZIF-67 gave rise to a reverse click reaction, reforming octavinyl
POSS isolated on the outer surface of the Co complex, which could
be further modified by a phosphorous flame retardant using an addition
reaction. The obtained MPOFs-P with 2 wt % loading in epoxy resin
could improve the limiting oxygen index value of the composites to
27.0% and pass the V-0 rating in the UL-94 test. Meanwhile, the peaks
of the heat release rate and especially the total smoke production
were reduced by 46.6 and 25.2%, respectively. The robust char layer
reduces the emission of toxic gas CO by 39.8%. The above epoxy product
with promising flame retardancy also improved mechanical properties,
thanks to the filler with a unique nanostructure. The ingenious work
offers enlightenment for the hybridization method of MOFs and POSS
to fabricate a multielement flame-retardant system for epoxy resin
with high efficacy.
To explore the influence of the crystal structure difference of melamine trimetaphosphate (MAP) on the application performance of its polymer composites, an intumescent flame retardant with the optimal crystal type was designed and synthesized to improve the mechanical properties and flame retardancy of polyamide 6 (PA6). I-MAP and II-MAP were obtained using different concentrations of MA and sodium trimetaphosphate (STMP) in an acidic aqueous solution. The morphology, chemical composition, and thermal stability were comprehensively characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The dispersion, mechanical properties, and flame retardancy of PA6/I-MAP and PA6/II-MAP were evaluated by SEM, stress and strain, limiting oxygen index test (LOI), vertical burning test (UL-94), cone calorimetry (CONE) test, and char residue analysis. The conclusion is as follows: I-MAP and II-MAP have a greater influence on the physical properties of PA6 but less influence on the chemical properties. Compared with PA6/I-MAP, the tensile strength of PA6/II-MAP is 104.7% higher, the flame rating reaches V-0, and PHRR is reduced by 11.2%.
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