Layered double hydroxide derivatives as flame retardant for flexible PVC

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Synthesis of LDH and hydromagnesite

The layered double hydroxide (LDH) [Mg0.667Al0.333(OH)2](CO3)0.167⋅mH2O] was synthesised according to the method described by Labuschagné et al. (24). The procedure was as follows: The Al(OH)3 and light MgO powders were mixed in the required 2:1 stoichiometric ratio. The powder mix was slowly added, while stirring, to one litre of distilled water in a 1.6 L Parr autoclave. The final solids concentration of the slurry was 15 wt.%. A 60 mol.% excess of NaHCO3 was added to the mixture as the source for the intercalate anion. The reaction was conducted under vigorous stirring at a temperature of 180 °C and a pressure of approximately 14 bar. The autoclave was kept at this temperature and pressure for approximately 5 h. Thereafter heating was discontinued and the reaction mixture was allowed to cool overnight while stirring. The solid product was removed from the autoclave, filtered and washed several times with distilled water to remove residual Na2CO3. Finally it was dried in an oven at 80 °C for at least 48 h.
The hydromagnesite was also synthesised according to the method described by Labuschagné et al. (24) as follows. Magnesium oxide stock from Chamotte Holdings was calcined at 800 °C for 10 min. A total of 100 g of the cooled MgO was suspended in 1.5 L distilled water using a Silverson disperser at 6700 rpm. An ice bath was used to keep the temperature of the mixture at approximately 23 °C. Carbon dioxide was bubbled through the mixture to form hydromagnesite. The pH gradually dropped and stabilised at 8.2. It was maintained at this value for another 30 min. The mixture was filtered and the precipitate dried in an oven at 140 °C for approximately 12 h. Finally the dry sample was ground to a fine powder.

Preparation of PVC-composites

DINP plasticiser (130 g) was weighed into a 600 mL beaker. Next small portions of the PVC powder (up to a total of 130 g) were added and mixed using a high-speed Anvil milkshake mixer. The dispersion was de-aerated for about 30 min in a Speedvac vacuum chamber. Then the LDH filler powder (39 g) was incorporated. The dispersion was again de-aerated but this time for about 1 h.
Cast PVC composite sheets were made in a three-step pressing process. The paste mixture was poured into a mould measuring 100 mm × 100 mm × 3.5 ± 0.1 mm. The mould was closed and placed in a convection oven set at a temperature of 130 °C for 10 min. Then it was hot pressed at a pressure of 10 MPa at 150 °C for 5 min. The mould was then removed from the press and a heavy weight placed on the top plate. The moulding was allowed to cool down at ambient conditions before it was removed.

Heat stability assessment

The heat stability of the PVC compounds was evaluated on a Metrohm 895 Professional PVC Thermomat according to ISO 182 Part 3 (25). The method is based on the fact that PVC releases HCl when it decomposes at high temperatures. The evolved hydrochloric acid is flushed with a stream of nitrogen gas and passed through a measuring vessel where it is absorbed in purified water. The progress of the decomposition is tracked by measuring the change in the conductivity of this water. Performance is quantified in terms of either the induction time (i.e. the time that is required to reach the break point in the conductivity curve) or a stability time, i.e. the time until a conductivity difference of 50 μS cm−1 is reached. The PVC compound sample amount tested was 0.50 ± 0.05 g. The samples were cut into small pieces less than 1 mm in size. The stability was determined in triplicate at 200 °C. Nitrogen flow was controlled at 7 L h−1 and 50 mL deionised water was used to trap the HCl.

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Cone calorimeter fire testing

The ISO 5660 standards (26-28) were followed in performing the cone calorimeter tests using a Dual Cone Calorimeter (Fire Testing Technology (UK) Ltd.). Three specimens of each composition were tested. The sheet dimensions were 100 mm × 100 mm × 3.5 ± 0.1 mm. They were placed on aluminium foil and exposed horizontally to an external heat flux of 35 kW m−2. This heat flux was chosen on the basis of the study conducted by Wang et al. (29). They studied 3 mm to 10 mm thick PVC sheets at heat fluxes of 25, 35 and 50 kW m−2. They found that the time to ignition varied linearly with the inverse of the cone calorimeter heat flux. Furthermore, the minimum heat flux for ignition of sheets in this thickness range was found to be about 19 kW m−2. The smoke photometer used a helium-neon (He-Ne) laser that emits red light with a wavelength of 632.8 nm.

Chapter 1: Outline of the thesis
1.1 Introduction
1.2 Scope and focus of the work
1.3 References
Chapter 2: The influence of stearic acid coating on the properties of magnesium hydroxide, hydromagnesite and hydrotalcite powders
2.1 Introduction
2.2 Experimental
2.3 Characterization
2.4 Results
2.5 Discussion
2.6 Conclusion
2.7 Acknowledgements
2.8 References
Chapter 3: The effect of magnesium hydroxide, hydromagnesite and layered double hydroxide on the heat stability and fire performance of plasticised PVC
3.1 Introduction
3.2 Experimental
3.3 Characterisation
3.4 Results
3.5 Discussion
3.6 Conclusion
3.7 References
Chapter 4: Layered double hydroxide derivatives as flame retardant for flexible PVC
4.1 Introduction
4.2 Experimental
4.3 Characterisation
4.4 Results
4.5 Discussion
4.6 Conclusions
4.7 Acknowledgements
4.8 References
Chapter 5: Heat stabilising flexible PVC with layered double hydroxide derivatives
5.1 Introduction
5.2 Experimental
5.3 Characterisation
5.4 Heat stability assessment
5.5 Results
5.6 Discussion
5.7 Conclusion
5.8 Acknowledgements
5.9 References
Chapter 6: Conclusions and Recommendations
6.1 Overall conclusions
6.2 Future Research


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