The following work was in part performed in cooperation with Dronco AG, Wunsiedel. The focus of the work was laid on the synthesis of fillers for polymer nanocomposites, being appropriate in particular to enhance thermal and flammability properties. Generally, three classes of fillers were used. Two types of silicates and one hydroxide were synthesized and afterwards dispersed in different polymeric matrices. The choice of the polymeric matrix was not the main issue of the presented work.
To synthesize one silicate filler a molecular silane was used to form a silsesquioxane network in situ in a PF precursor. The mechanism of formation and the properties of the synthesized nanocomposite were analyzed and elucidated. By hydrolysis and condensation of the triethyl-aminopropyl-silane added, a silsesquioxane network is formed in-situ, which penetrates the PF homogenously. However, distinct particulate silsesquioxane entities could not be detected. The homogenous dispersion of the silsesquioxane network leads to enhanced mechanical and thermo-oxidative properties of the nanocomposite. Improved flame retardant properties could, however, not be achieved.
The second type of silicate fillers used, were synthetic layered silicates. The synthesis of these layered silicates has been optimized at the AC I department for years. These layered silicates are very interesting starting materials for the development of nanofillers. In detail, a Na-hectorite and a Li-hectorite were used because of their superior properties, in particular huge aspect ratios, compared to their natural counterparts like MMT.
The Na-hectorite used was initially cation exchanged by Mg2+ ions to achieve shear-labile tactoids. Then, these were exfoliated in a stirred media mill to maximize the aspect ratio followed by a subsequent cation exchange by K+ to lower the intracrystalline reactivity. Consequently, the following organic modification by an oligomeric modifier was limited to the outer surface of the tactoids, which allows for the phase transfer in the organic matrix (PS). The synthesized surface-modified nanoplatelets show good dispersibility and a huge aspect ratio leading to improved mechanical, thermal, and flammability properties of the nanocomposite. Due to the specific modification restricted to outer surfaces, the organic fraction of the pristine filler is much lower as compared to completely exchanged MMT fillers, where outer and inner surfaces are exchanged.
The Li-hectorite was used for a comparative study with a natural MMT. The advantage of the Li-hectorite is the huge aspect ratio (typically 1000), which influences the flammability of the PS-Li-hectorite nanocomposite. Here, the crucial impact of the aspect ratio was clearly proven for the first time in detail. Additionally, the influence of the blending methods was evaluated showing that melt mixing is incapable to yield highly homogenous polymer dispersions. Drying the filler prior melt blending leads to agglomerates, which cannot be redispersed completely during the mixing step. The high aspect ratio and the good homogeneity of the solution blended PS nanocomposite leads to intumescent-like behavior during the burning test. This was described for the first time for layered silicates nanocomposites in literature.
The third class of fillers used were LDHs. Due to their platy morphology, the high concentration of structural hydroxyl groups that may be thermally decomposed, and their variable composition, including transition metals that may act as radical scavengers, these hydroxides should be ideally suited as flame retardant fillers. Consequently, iron containing LDHs with high aspect ratio were sought to be synthesized by a novel chelating-agent assisted route. The direct synthesis with established methods proved impossible because of the low solubility of the intermediary precipitated FeOOH. The novel chelating-agent assisted route overcomes this problem to yield LDHs with good aspect ratios, which are promising fillers for flame retardant nanocomposites.
This work is a cumulative dissertation, which describes the results explicitly in the attached publications.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Subjects:||Inorganic chemistry, Nanotechnology|
|Keywords:||Polymer nanocomposites, Phase transfer, Development of nanofillers|
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