Thermal stability and flame retardant effects of halloysite nanotubes on poly(propylene)
Introduction
Since Blumstein first reported the enhanced thermal stability of poly(methyl methacrylate) (PMMA)/montmorillonite (MMT) clay nanocomposites in 1965 [1], thermal stability and flame retardant properties of nanoparticle reinforced polymer composites have attracted considerable attention in polymer area. In 1976, Fujiwara and Sakamoto [2] showed the potential flame retardant properties of nylon-6/clay nanocomposites. Giannelis [3], Gilman et al. [4], [5], have done much work on the thermal stability and flame retardant of polymer nanocomposites. The most common thermal stability and flame retardant fillers at nanoscale include natural inorganic clay minerals, especially those with layered structures, magnesium hydroxide, aluminium hydroxide and so on. More recently, followed by the discovery of multi-wall and single-wall carbon nanotubes (CNTs), the unusual thermal stability and flame retardant properties of CNTs filled nanocomposites have inspired many scientists’ interests [6], [7], [8]. Reported by Kashiwagi et al. with a little incorporation of CNTs, thermal stability and flammability properties of poly(propylene) increased significantly [6].
However, it is well known that the preparation process of nanoparticles is relatively complex and costly. And for the natural layered clay minerals, the generally required organic treatment is not sufficient to generate well-intercalated and/or exfoliated morphologies in polymer matrix. To achieve satisfactory flame retardant properties of polymer/clay composites, intercalative polymerization or more sophisticated treatment of clay is necessary. As for the CNTs, their relatively higher price and pigmentation ability for polymers, however, will restrict their application as flame retardant fillers in polymers [6]. As a consequence, easily available and effective flame retardant nanofillers are still lacking and highly desired.
Halloysite nanotubes(HNTs) is a kind of aluminosilicate clays with hollow nanotubular structure mined from natural deposits in countries such as China, America, Brazil, France and so on. HNTs are chemically similar to kaolinite. Mainly of hollow microtubules have typical dimensions of nanoscale [9]. Typically, HNTs are used in the manufacture of high quality ceramic white-ware [10]. In recent years, HNTs are used as nanotemplates or nanoscale reaction vessels instead of CNTs or boron nitride nanotubes (BNNTs) [11], [12].
Recently, the authors attempted to utilize HNTs as nanofiller polymers, such as natural rubber, nitrile rubber and polypropylene [13]. It is found that with the incorporation of HNTs in PP, the thermal stability and flame retardant of the PP/HNTs nanocomposites are increased significantly. In this work, we report the effects of HNT loading and surface modification on the thermal stability and flame retardant properties of PP/HNTs nanocomposites.
Section snippets
Materials
The isotactic PP was manufactured by Guangzhou Petro-chemical incorporation, with a melt flow index of 2.84 g/10 min (after ISO-1133: 1997(E)). The HNTs, grade of Ultrafine, were provided by Imerys Tableware Asia Limited, New Zealand. Brightness of 98.9% as measured by a Minolta CR300 using D65 light source. The elemental composition is as follows (wt%): SiO2, 49.5; Al2O3, 35.5; Fe2O3, 0.29; TiO2, 0.09 [14]. γ-methacryloxypropyltrimethoxysilane, with tradename of Z6030, was manufactured by Dow
Morphology of PP/HNTs nanocomposites
The morphology of the HNTs with a lumen structure is shown in Fig. 1. However, apart from the nanotubes, a few particles and agglomerates also could be found. Fig. 2 shows the X-ray diffraction (XRD) spectra of halloysite nanotubes. The basal space reflections indicate a sharp peak at 12.05°, corresponding to a 0 0 1 basal spacing of 0.73 nm. These results confirm the multiwall nanotubular structures at nanoscale of halloysite.
Fig. 3 shows the effects of HNTs loading and surface modification on
Conclusions
PP/HNTs nanocomposites have been prepared by the incorporation of naturally occurring HNTs. The HNTs with tubular structure show promising properties for improving the thermal stability and decreasing flammability of polypropylene. The temperature at 5% weight loss in nitrogen of the nanocomposite filled with 10 phr modified HNTs was 60 °C higher than that of neat PP, and temperature at maximum weight loss rate in air of the nanocomposites filled with 10 phr modified HNTs was 74 °C higher than that
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