Improving The Flame Retardancy Of Polypropylene Composites By Introducing Muti-dimensional Kaolinite-based Materials

Intumescent flame retardant (IFR) have been widely used to improve flame retardancy of polypropylene (PP) due to its environment, non-toxic,low smoke release and anti-dripping properties compared to halogen flame retardants. However, high loading (>25%) is usually required to obtain effective flame retardant level, which will damage mechanical properties of PP composite. Silicate clay has attracted much attention because it can significantly decrease the heat release (HRR). However, the effect of dimensional and detailed flame retardant mechanism of clay have not been systematically investigated so far. In this work, a series of muti-dimensional materials with different chemical compostion and dimensions are designed and prepared from kaolinite-based; and they are used as synergistic agents with IFR to improve the flame retardancy of PP.Moreover, the comprehensive performance of composite was also studied.Muti-dimensional modified kaolinite was prepared by different methods. Their chemical structures were analyzed by X-ray diffraction(XRD) and Fourier transition infrared spectroscopy (FTIR) and so on.Their synergistic effect on improving the thermal stability and flame retardancy of PP/IFR was investigated by thermogravimetric (TGA),limiting oxygen index (LOI), vertical burning behavior (UL-94) and cone calorimetry (Cone). The morphology and chemical constitution of residue char were also investigated by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and FTIR. Moreove, the synergistic mechanism was also proposed and discussed. The main content of this work includes four parts:1. One-dimensional nanotuber Kaol (N-Kaol) was prepared by muti-step intercalation and calcinationfrom raw Kaol, and then it was introduced into the PP/IFR composite. The flammability and thermal behavior of PP,PP/IFR and PP/IFR composites containing Kaol or N-Kaol were characterized by LOI, UL-94, Cone and TGA. The results showed that decomposition temperature of PP/IFR/Kaol and PP/IFR/N-Kaol were both higher than that of PP/IFR composite, and N-Kaol showed better than Kaol.The LOI value of PP/25wt% IFR was 31.1 with a UL-94 grade of V-2, and it was increased to 34.5 with a UL-94 grade of V-0 by substitution of 1.5 wt% IFR with N-Kaol. Cone data showed that the N-Kaol could effectively reduce the HRR, total heat released (THR) and so on. It was suggested the presence of N-Kaol was beneficial to the formation of a compact and homogeneous char layer of PP/IFR during burning, which could effectively prohibit the transfer of heat, mass and other combustible gases between the substrate and outside environment.2. Exfoliated kaolinite (E-Kaol) or intercalated two-dimensional kaolinite (I-Kaol) was prepared by intercalating dimethyl sulfoxide(DMSO) and potassium acetate (KAc)/ methyl alcohol (MOH) into kaolinite (Kaol) successively, followed by irradiation under ultrasonic or intercalated with target melecules such as ammonium sulfamate (AS)/thiourea (TU). The peak heat release rate (pHRR) of PP/IFR composite was decreased above 80% when the substitution of IFR with 1.5wt.% I-Kaol compared to that for PP control sample. The TGA results demenstrated that the presence of I-Kaol could enahnce the themorstability and improve char residues of PP/IFR composites. The dispersion of I-Kaol in PP/IFR composites was observed by SEM and XRD. It was suggested that exfoliated I-Kaol structure was formed during the extrusion. This structure was beneficial to form more dense and compact char layers which can protect matrix. The effect of E-Kaol on the fire performance of PP composites were similar to that of I-Kaol.3. 4A zeolite was synthesized from Kaol by calcination followed successive by a hydrothermal process, and it was then used as the precursor in order to obtain 3A zeolite and 5A zeolite through ionic exchange respectively. Three kinds of A-type zeolite were then introduced into PP incorporation with IFR respectively. It had been found that the flame retardancy and thermostabily PP/IFR composite containing 4A zeolite were much better compared with that of the sample containing other two kinds of zeolites. In order to further enhance the fire performance of PP/IFR composite. The rare earth element (La) was introduced into 4A zeolite by ion exchange and obtained 4A-La. The LOI value of PP/25 wt.%IFR composite was 31.1, but it can increase to 34.1 by 1.5 wt.% 4A substitution with IFR and further increase to 35.4 by 1.5 wt.% 4A-La substitution with IFR. The 4A-La could effectively decrease HRR and THR and could enahnce the themorstability and improve char residues of PP/IFR composites. FTIR spectra of PP/IFR/4A-La residue at different temperature showed that 4A-La can promote char formation by reacting 4A-La with degradation compounds from PP/IFR during heating. It was also benefitted to formation of more compact and continuous char residues during burning.4. Modified kaolinite containing Lewis acidic sites (Acid-Kaol) was prepared by calcination of Kaol and successive acid treatment. Acid-Kaol was then introduced into PP/IFR. The LOI value of PP composite containing 23.5 % IFR and 1.5 % Acid-Kaol was increased to 34.9, which was 3.8 units higher that that of the PP sample containing 25% IFR. The morphology and chemical composition of char residues after Cone tests were characterized by SEM, XPS and FTIR. It was indicated that Acid-Kaol accelerated crosslinking during combustion and promoted char formation, which could act as an insulating barrier to protect the matrix from further decomposition.According to the flame retardant effect of muti-dimensional kaolinite-based and flame retardant mechanism in PP composites, the difference of flame retardancy effect were compared, and their flame retardant mechanisms were discussed respectively.