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ABSTRACT:The structures of the intumescent charred layers formed from polyolefin (PO) blends with expandable graphite (EG) and/or the other free-halogen flame retardant (HFFR) and their flame-retardant mechanism were studied by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermal conductivity (TC) measurements. The FTIR, XPS, and LRS data showed that the carbonaceous structures of intumescent charred layers consist of EG and various numbers of condensed benzene rings and/or phosphocarbonaceous complexes attached by the P-O-C and P-N bonds or quaternary nitrogen or dehydrated zinc borate (ZB). These results and the morphologic structures observed by SEM have demonstrated that the compact structures of charred layers slow down heat and mass transfer between the gas and condensed phase and prevent the underlying polymeric substrate from further attack by heat flux in a flame. The DTA data provide the positive evidence for the flame-retardant mechanism of the PO/EG/HFFR systems, which works by increasing the oxidation temperature and decreasing thermal oxidation heat. At the same time, the TC data reveal the flame-retardant essence of the charred layers as good heat-insulated materials whose TC value is only about 1 ⁄10 of the corresponding blend.

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Radical intermediates formed by UV irradiation of low-density polyethylene (LDPE) in the presence of benzophenone (BP) as a photoinitiator have been trapped by using 2,3,5,6-tetramethylnitrosobenzene (ND) and 2,4,6-tri-tert-butylnitrosobenzene (BNB) as spin-trapping reagents. The resulting spin-adduct radicals have been characterized in situ by ESR spectroscopy. Two kinds of spin-adducts are detected and identified: a tertiary carbon radical and a secondary carbon radical which are formed by addition of the spin traps (ND or BNB) to two polymer radicals. These intermediates are produced by hydrogen abstraction of the BP excited triplet state from branch points and methylene groups of the LDPE chain, respectively. This is the evidence that the photo-cross-linking of LDPE mainly takes place at both tertiary carbons (branch points) and secondary carbons along the chains and that H-shaped links dominate. Secondary bonded hydrogens are less reactive but occur in a much larger number than tertiary bonded hydrogens in LDPE.