Innovative, Platy Nano-Additives as Efficient Flame Retardants for Polymer Nanocomposites
The objective of this work was to explore inorganic layered materials (nano-additives) as innovative flame retardants for polymer nanocomposites. In particular, polystyrene (PS) nanocomposites were prepared with a variety of nano-additives including layered double hydroxides (LDHs), hectorite (hect), graphite oxide (GO), or a combination of these. The maximum flame retardant (FR) efficiency can be achieved when these nano-additives are perfectly dispersed in the polymer matrix yielding a maximum specific interface area. Good dispersion is most efficiently achieved by appropriate surface modification lowering the surface tension. Due to the unique 1:1 layered structure of kaolinite that represents two different kinds of external basal surfaces, kaolinite was used as a model to investigate selective surface modification of µ-hydroxy (gibbsite-like) surfaces. The surface modification was achieved by siderophilic ligands such as catechol derivatives. The catechol functional group (1,2-dihydroxyphenyl) forms stable chelate-complexes with the aluminum cation of the octahedral surface (OS). Modification was confirmed by bathochromic shifts of the reflection maxima of the adsorbed catechols. Furthermore, forced sedimentation measurements proved a better stability of the modified kaolinite in an organic solvent (e.g. THF). Later on, this kind of surface modification was applied to modify the structurally related materials with similar surface functionalities such as LDHs. CO3²¯-LDHs with high aspect ratio (α: 20-40) were either directly synthesized via the urea hydrolysis method at low concentration or by a two-step method including post-synthesis milling in a stirred media mill. Selective surface modification was applied using a siderophilic ligand (3,4-dihydroxybenzophenone). Consequently, suspensions of surface modified LDHs showed a better stability in THF as compared to the pristine LDH. Furthermore, surface modification restricted to the external surface diminishes the mass of the easy flammable modificator, expressly, reducing the fire load of the nano-additives. Moreover, the intercalation of LDHs was investigated following the synthesis of anion-exchangeable NO3¯-LDHs using a modified urea hydrolysis method. As a result, anions having FR properties such as phosphorus-containing organic anions were intercalated. Additionally, LDHs were used in combination with other FRs such as GO to investigate possible synergism. In order to prevent heterocoagulation and to achieve stable dispersion in an organic solvent, GO was also surface modified using 1-dodecylamine leading to a reversal of the surface charge (“Umladung”). Furthermore, the ion exchange properties of cationic/anionic clays were harnessed to design multi-component FR systems. Combinations of LDHs and hect having ions with FR properties adsorbed in the interlayer or on the external surfaces were investigated. For instance, LDH intercalated with phenyl phosphate while hectorite modified with melaminium were used to introduce an additional intumescent FR system. In order to maximize the dispersion of nano-additives in PS, all mentioned PS nanocomposites were compounded via solution blending in THF utilizing a three-roll mill (TRM). Thereby, shear forces were sufficient to break aggregates, which were monitored by measuring the particle size distributions (PSDs). The cone calorimeter results highlighted the importance of the aspect ratio and the interlayer anion of LDHs on flame retardancy. This was confirmed by large reductions of peak heat release rates (PHRR). Moreover, synergisms of combined systems of FRs were achieved indicated by enhanced reductions in PHRR (≈ 50 %) at relatively low loadings (≈ 5 wt%) for both combinations LDH/hect and LDH/GO.
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