Layered Double Hydroxides a Potential Synergistic Flame Retardant for Polyolefins

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1 Layered Double Hydroxides a Potential Synergistic Flame Retardant for Polyolefins Udo Wagenknecht 1,*, Francis R. Costa 1, Gert Heinrich 1, Stefan Reinemann 2 and Ute Schwarz 2 1 Leibniz-Institut fuer Polymerforschung Dresden e.v., Hohe Str. 6, Dresden, Germany 2 Thueringisches Institut fuer Textil- und Kunststoff-Forschung, Breitscheid Str. 97, D Rudolfstadt-Schwarza, Germany * Corresponding author: Phone : , Fax: , wagenknt@ipfdd.de The use of metal hydroxides, like Mg(OH) 2 (MH) and aluminum trihydrate (ATH) as flame retardants in polyolefin composites is very common. However, in almost everywhere a high dose, often about 60 wt%, of these metal hydroxides are required for obtaining satisfactory flame-retardant behavior. This on the other hand deteriorates the mechanical properties and processibility of the final composites [1, 2]. These metal hydroxides exhibit flame retardant effect mainly through endothermic decomposition and barrier formation by the metal oxide residue. Because of their very mechanism of flame inhibition, they are far less efficient as flame retardant compared to halogenated flame retardants, which directly terminate the flame propagating reactive radical species in the flame zone. As a result, these metal hydroxide are required in large quantity for effective flam retardation. In addition, their poor compatibility with non polar polymer matrices results inhomogeneous dispersion, which further reduces their efficiency as flame retardant. Therefore, extensive researches have been made and still continuing to improve the interfacial adhesion between metal hydroxide filler and non polar polymer matrix [3, 4]. Magnesium-Aluminum layered double hydroxide (Mg AlLDH) with its potential as nanofiller could be an interesting development in this regard. The problem of inhomogeneous particle dispersion could be addressed to some extent through reduced particle size and layer exfoliation in LDH based nanocomposites. However, LDH alone may not be sufficient for improving flammability, especially in testing like LOI and UL94 are not satisfactory. This is due to the indirect mechanism of flame retardation (similar to MH) and lower processible limit of filler loading in LDH based polyethylene nanocomposites. The latter is very important criteria because loading LDH beyond 10 wt% makes the final material extremely brittle and almost impossible to process using extrusion technique. The alternative way of

2 utilizing the flame retardant potential of LDH could be its use in combination with MH with an aim of reducing the overall filler loading in a PE/MH type conventional flame retardant composites. In the present work, MH was partially replaced by an increasing amount of LDH and the flammability performance of the composites were investigated. The maximum amount of LDH used for such investigation was 10 wt%. 1 The LOI results for composites based on 40, 50 and 60 wt% MH are shown in Figure 1. As the LDH content in all these composites (by which an equivalent amount of MH is replaced) increases starting from 0 to 10 wt%, the LOI value also increases progressively in comparison to the corresponding MH based composite. Figure 1 also shows that the composition PE/10LDH/30MH has similar LOI value as that of the composition PE/50MH and the similar is true also in case of PE/10LDH/40MH and PE/60MH. This means that overall filler loading in a PE/MH composite could be significantly reduced in presence of LDH to obtain desired LOI values. The explanation of such synergism could be obtained if the burning process is observed carefully. In case of composites containing only MH, the burn residue/char is held less firmly with the sample stock. As a result, with increasing volume of the char it fall quite easily under its own weight exposing fresh surface for burning. On the other hand presence of LDH enhances the melt viscosity and also the strength of the char, which shows more efficient barrier effect. This means the mechanical strength of char improves as magnesium

3 hydroxide is replaced partially with LDH clay. Also, the improved dispersion of LDH particles causes better distribution of cooling effects during burning throughout the matrix. Figure 2

4 The summary of the cone-calorimeter investigation results for PE/MH and LDPE/LDH/MH composites is shown in Figure 2. The both heat release rate and its peak value (PHRR) decreases as MH in the composites partially replaced by an increasing amount of LDH. The burning time is also extended resulting slower burning process. However, introduction of LDH does not change the time of ignition, may be due to their similar mechanism of flame inhibition. The heat released during combustion is also lowered by the addition of LDH. The slower burning rate in presence of LDH also accounts for the slower mass loss with time as compared to the PE/MH composite. Interesting, though the net amount of metal hydroxide remains more or less constant in all the three composites, the increasing amount LDH causes reduced smoke generation. Perhaps, LDH facilitates carbonaceous char formation resulting less smoke generation. The formation of carbonaceous char is also responsible for increase in CO emission rate during the ending stage of the combustion process. The carbonaceous residue may undergo slow oxidation process liberating CO and other volatile materials after all the metal hydroxides are decomposed. Similar observation of increased CO production after flame-out has also been observed in case of polyamide/carbon nanotube based nanocomposites [5]. The slow thermo-oxidation of carbon nanotube residue has been held responsible for increased CO production in such composites. Contrarily, similar increase in CO 2 emission was not observed. The flammability performance of PE/MH and LDPE/LDH/MH composites were also compared usingul94 vertical burn test results, which are summarized in Table 1. The test samples with specified dimensions (125 mm x 10 mm x 4 mm) were tested for both UL94 vertical and horizontal ratings. In UL94 vertical testing, test specimen is exposed to flame for two times each for 10 seconds. From the results shown in Table 1, it is apparent that partial replacement of MH by LDH improves also the performance of the materials in the UL 94 vertical burn test. The presence of at 40-wt% total filler content (MH+LDH) even after introduction of 10-wt % of LDH do not show positive UL 94 rating. However, the dripping resistance of the material improves significant as more and more LDH are incorporated. The sample containing 40-wt% MH shows continuous melts dripping after first heating. While, the sample containing 30-wt% MH and 10-wt% LDH (10L30MH) does not show any melt dripping first heating, but extinguished after some time. However, this samples burns continuously and show dripping after the second heating step. The MH content up to even 50 wt% is not sufficient for getting

5 positive UL94V rating and the sample burns continuously with melt dripping after first heating. When 20% of MH in PE/50MH is replaced by LDH (i.e. 10L40MH in Table 1), Table 1 positive UL94V rating with V0 classification could be achieved. All the compositions containing 60 wt% total filler are self-extinguishing and show UL94V0 rating. However, when MH is replaced by LDH, burning time after each heating steps in UL94V testing are decreased. The main purpose of substituting magnesium hydroxide by Mg Al LDH in polyethylene based composites was to reduce the overall metal filler content to obtain satisfactory flame retardancy. But, in doing so one should not underestimate the effects on other properties, such as mechanical properties and processibility. In fact a big compromise in these properties in would not be acceptable for useful applications. Therefore, increase in proportion of LDH in a

6 Mg(OH)2 /LDH combination is limited by the processibility and the mechanical properties of the composites. Table 2 shows the summary of the mechanical properties of various LDPE/LDH/MH compositions investigated in the present work. Table 2 The incorporation of LDH up to 10 wt% in presence of maximum 50 wt% Mg(OH) 2 does not deteriorate the modulus and yield strength of the composites rather a small increase in both the properties were observed. However, at 10 wt% LDH content in LDPE/LDH/MH composites containing high amount of Mg(OH) 2 (say above 40 wt%), the elongation properties of final composites are affected significantly indicating the lowering of the impact strength of the materials. To obtain a optimum combination of LDH and Mg(OH) 2 in polyethylene and other polyolefin matrices more rigorous and detail investigation are necessary. May be the use of some compatibilizer would be a potential solution for this limitation.

7 Reference [1] R. N. Ronthon (ed). Particle-filled polymer composites. Longman Scientific, London, UK, 1995 [2] S. Zhu, Y. Zhang, and Y. Zhang. Deformation and fracture of Mg(OH)2-filled polyolefin composites under tensile stress. Journal of Applied Polymer Science, 89: , [3] B. Howarth, C. L. Raymond, and I. Sutherland. Polyethylene compounds containing mineral fillers modified by acid coatings. 2: Factors influencing mechanical properties. Polymer Engineering Science, 41: , [4] S. Kim. Flame retardancy and smoke suppression of magnesium hydroxide filled polyethylene. Journal Polymer Science, Part B: Polymer Physics, 41: , [5] B. Schartel, P. Poetschke, U. Knoll, and M. Abdel-Goad. Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites. Europian Polymer Journal, 41: , 2005.