Preparation and physical properties of flame retardant acrylic resin containing nano-sized aluminum hydroxide
Introduction
PMMA, an acrylic resin, is the most popular material of this type in many fields, and has good material properties; however, since PMMA is a thermodegradable polymer, a large monomer is formed during combustion, and hence PMMA is flammable. The use of halogenated compounds, phosphorus compounds and inorganic hydroxides as flame-resistant additives in PMMA is well established [1], [2], [3], [4], [5], [6]. Halogen fire retardants give flame resistance by suppressing the radicals which arise during combustion. However, the possibility of generation of hazardous gases and compounds such as dioxin has been indicated with general flame retardants [7]. For this reason, the introduction of inorganic flame retardants to organic polymer materials has been widely investigated [8], [9], [10], [11], [12], [13]. The addition of inorganic hydroxides increases flame resistance due to a decrease in the proportion of combustible polymer present. In addition, due to the endothermic nature of inorganic hydroxide dehydration, ignition and temperature increase to decomposition temperature of the polymer are suppressed. Because the flame retardant effect depends on dehydration of the inorganic hydroxide, decomposition of the polymer around the surface of the inorganic compound is proportional to the specific surface area. In fact, the fire retardant effect becomes greater as the size of the inorganic hydroxide decreases. Aluminum hydroxide and magnesium hydroxide are generally used for this purpose. The dehydration temperature of aluminum hydroxide is between 523 K and 593 K, and that of magnesium hydroxide between 613 K and 763 K. Aluminum hydroxide appears to be an effective fire retardant additive for PMMA, because the decomposition temperature of aluminum hydroxide corresponds approximately with that of pure PMMA which is about 523 K.
The aim of our study was to prepare transparent and fire-resistant hybrid materials. To this end, hybrid materials were prepared using nano-sized aluminum hydroxide. To obtain high performance nano-sized hybrid materials, homogeneous dispersion of aluminum hydroxide in the PMMA matrix is important; the fire resistance and physical properties of the hybrid materials will be improved by bonding between the PMMA matrix and aluminum hydroxide at the interface. Aluminum hydroxide modified at the surface with methacrylate containing phosphoric acid moieties was prepared and dispersed in MMA. In this way, polymerizable groups were introduced to the aluminum hydroxide surface. Organic–inorganic nano-hybrid materials were obtained by bulk polymerization with benzoyl peroxide as an initiator in the presence of surface-modified aluminum hydroxide. The fire resistance, transparency, thermal and mechanical properties of the resulting hybrid materials were evaluated by horizontal flammability testing, UV–vis measurement, thermogravimetric analysis, dynamic mechanical analysis, and three-point bending test.
Section snippets
Materials
Aluminum hydroxide sol (particles ca. 47 nm in diameter, specific surface 400 m2/g, 3.4 wt%, pH 3.0) was supplied by Sumitomo Chemical Co., Ltd. Phosmer PP was provided by Uni-Chemical Co., Ltd. All other solvents and reagents were purchased from Nacalai Tesque Inc. and used without further purification.
Surface modification of aluminum hydroxide with phosmer PP
Based on previous reports that phosphorus compounds can adsorb to aluminum [14], [15], one such compound was used as surface modification agent for aluminum hydroxide. The reaction between aluminum
Preparation and characterization of organo-AH
Phosmer PP is adsorbed onto the Al–OH groups of aluminum hydroxide by ionic bonding. IR measurements were carried out to confirm that phosmer PP was adsorbed on the aluminum hydroxide. Fig. 1 shows IR spectra of aluminum hydroxide, phosmer PP and organo-AH. The spectrum of organo-AH shows characteristic absorption peaks at 1070 cm−1 (ν, Al–O), 1170 cm−1 (ν, COC) and 1720 cm−1 (νas, CO). From the IR results, it was confirmed that phosmer PP was adsorbed onto the Al–OH groups of aluminum hydroxide
Conclusion
PMMA/organo-AH hybrid materials were prepared by bulk polymerization. The prepared hybrid materials showed high transparency, similar to that of pure PMMA, due to the uniform dispersion of aluminum hydroxide in the PMMA matrix. Moreover, the addition of 3 wt% inorganic material in organo-AH resulted in a considerable reduction in horizontal burning rate. In comparison with the burn rate of pure PMMA, that of the prepared hybrid materials was almost halved. This is due to the joint use of
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