Chapter 4
Use of Layered Double Hydroxides as Polymer FireRetardant Additives: Advantages and Challenges
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Charles Manzi-Nshuti, Liying Zhu, Calistor Nyambo, Linjiang Wang, Charles A. Wilkie*, and Jeanne M . Hossenlopp* Department of Chemistry and Fire Retardant Research Facility, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881
Layered Double Hydroxides (LHDs) have been identified as a promising new additive class for generating polymer nanocomposites with enhanced thermal stability and improved flammability properties. An advantage of these materials over structurally similar smectite clays is the ability to tune physical and chemical properties via simple synthetic strategies that can modify the metal hydroxide layer and/or the identity of the charge-balancing interlayer anion. Recent advances in development of LDHs for polymer fire retardancy applications are reviewed here and a discussion of future challenges is provided.
© 2009 American Chemical Society
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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36 Organically modified smectite clays have been extensively studied as polymer nanocomposite additives and the have been shown to improve thermal stability, flammability, and mechanical properties (1). Layered double hydroxides, LDHs, can be categorized as anionic analogs of clays. In an L D H , the metal hydroxide layer consists of a combination of divalent and trivalent cations and have the general formula [ M M i . (OH) ] (A "x/ )mH 0 where A * represents the intercalated anion (2). Due to the abilility to alter the identity and relative stoichiometry of the metals and/or the interlayer anion, L D H s and related anionic clays have been proposed for a number of applications, including fire retardancy of polymers (3). The most common L D H used in polymer nanocomposite applications is hydrotalcite, Mg6Al (C0 )(OH) -4(H 0), or a modified M g / A l L D H where the carbonate is replaced by an organic species to promote dispersion. In literature reports, these additives were found to improve thermal stability and some flammability properties of a variety of polymers (4). The addition of iron to hydrotalcite was found to provide improvement in fire properties of an ethylenevinyl acetate copolymer (4j). While M g / A l L D H provides some promising improvements in selected polymer fire properties, use of these additives with other fire retardants as also begun to attract attention as a strategy for producing effective formulations (5). While LDHs have shown some promise as fire retardant additives, a number of issues remain in characterizing and optimizing L D H s for this purpose. Recent work in our laboratories has focused on identifying how factors such as the L D H structure (metal ion composition and anion size and structure), loading, and dispersion quality affect the thermal stability and flammability of selected model polymers. Key results are reviewed here. 2 +
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Downloaded by CORNELL UNIV on June 28, 2012 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch004
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Experimental Methods LDHs were synthesized via coprecipitation or by exchange of the desired anion with a nitrate-containing L D H precursor. Syntheses were carried out under nitrogen in order to exclude C 0 and thus minimize carbonate contamination, following literature methods (6). Relative amounts of divalent to trivalent metal ions in the product were controlled by the initial concentrations of metal nitrates. L D H products were characterized via powder x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Polymer (nano)composites can be formed via melt blending, in situ polymerization, or solution blending methods; the work reviewed here focuses on melt blended samples. The L D H was mixed at different mass ratios with commercial polymer samples in a Brabender Plasticorder for approximately 10 minutes at 60 rpm and 185 ° C. Dispersion and nanocomposite formation were assessed using X R D and transmission electron microscopy (TEM). Thermal 2
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
37 stability was assessed via thermogravimetric analysis (TGA). T G A experiments were performed on a SDT 2960 machine at the 15 mg scale under air or a flowing nitrogen atmosphere at a scan rate of 20 °C/min. Flammability was assessed using cone calorimetry. Cone calorimeter measurements were performed on an Atlas CONE-2 according to A S T M Ε 1354 at 35 or 50 kW/m incident flux using a cone shaped heater; the exhaust flow was set at 24L/sec. The specimens for cone calorimetry were prepared by the compression molding of the sample (about 30 g) into 3 χ 100 χ 100 mm square plaques. Typical results from cone calorimetry are reproducible to within about ± 10%; these uncertainties are based on many runs in which thousands of samples have been combusted. 2
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Results and Discussion The advantage of using an L D H is the wide range of tunability - one can vary the stoichiometry, the identity of the divalent and/or trivalent metal ion, and the identity of the anion and all of these have been studied. The initial investigation was to evaluate whether an L D H could work with either polar or non-polar polymers. Accordingly, a magnesium-aluminum undecenoate L D H was melt blended with four different polymers, polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(methyl methacrylate) ( P M M A ) and the morphology and fire performance were evaluated . From T E M , only in the case of P M M A is the L D H well-dispersed in the polymer; for all three of the non-polar polymers, only immiscible systems are obtained. From cone calorimetry, minimal changes (
