Combine flake like magnesium hydroxide and cubic like aluminum hydroxide nanostructures in order to improve fire retardant of PVC and PMMA

INTRODUCTION

A fire retardant is a material that reduces flammability of polymer or delays their combustion. This includes chemical agents, but may also include substances that work by physical action, such as cooling the polymer, such as clay. There are several reports for improvement properties of composite materials by using nano-fillers which lead to the development of nanocomposites. Improvement the fire retardant behavior of polymers is still a major challenge for extending their use for most applications and various flame retardant additives like halogenated compounds are restricted with regard to the environmental demands. One of the most commonly used mineral flame retardants is magnesium hydroxide because when the temperature raises magnesium hydroxide decomposes endothermically (about 330◦C with an endothermic of 1.356 kJ/g) and absorbs energy [1-4]. Moreover, it provides water which dilutes combustible gases and provides heat insulation by reflecting heat when it accumulates on the surface. The main problem of using magnesium hydroxide is that for successful flame retardancy tests, 35 to 65 wt% of metal hydroxide is required. Increasing the loading of inorganic metal hydroxides will result in a significant decrease in physical properties [5-9]. Nanostructures could solve this problem by providing high surface/ volume rate. Recently, Baglioni et al. synthesized several metal hydroxide nanoparticles by various chemical reactions [10- 14]. These products were used as consolidants for wall paintings (frescoes), stone, paper, and wood. Many attempts have been put on the shape controlled fabrication of nanostructures [15-17].

Poly(vinyl chloride) (PVC) products are widely used in a large range of applications such as coatings, medical devices, paints, films, and food packaging and their thermal degradation has been the subject of some reviews [18,19]. Poly(methyl methacrylate), or PMMA, is an important thermoplastic for numerous uses and especially for optical applications due to excellent transparency in the visible region. Nevertheless, PMMA has limiting properties, particularly its low toughness and poor thermal stability.

MATERIALS AND METHODS

Sodium hydroxide, magnesium nitrate (Mg(NO₃)₂, aluminum nitrate [Al(NO₃)₃], EDTA (ethylenediaminetatraacetic acid), PVC, PVD, PMMA, graphite, ethanol, cyclohexanol were purchase from Merck company. All of the chemicals were of analytical grade and used as received without further purification. Deionized water (DI) was used throughout the experiment.

Synthesis of aluminum hydroxide

66.4 g of aluminum nitrate was dissolved in water. Then 110 mL of NaOH solution (9 M) was added to the above solution and heated at 115 C for 30 min. Afterward, solution was cooled down to room temperature by adding distilled water. Finally, EDTA was added and stirred for 25 h. Final products were washed and dried.

Synthesis of magnesium hydroxide

0.5 mol of magnesium nitrate was dissolved in water. Then 220 mL of NaOH solution (9 M) was added to the above solution and heated at 115 for 30 min. Afterward, solution was cooled down to room temperature by adding distilled water. Finally EDTA was added and stirred for 25 h. In the final step, the products were washed and dried.

Prepare nanocomposite

First, melted PVC, PMMA, and graphite were added into beaker and stirred under 80 rpm. Then, aluminum hydroxide nanoparticles and cyclohexanol were added to the above solution. The obtained solution was transferred to the watch glass. In addition, composite with different ratio of PVC/Mg(OH)₂/Al(OH)₃was prepared.

RESULT AND DISCUSSIONS

The samples were characterized by X-ray diffraction (XRD) patterns were recorded by an X’pert pro diffractometer using Ni-FILTERED Cu Kα radiation (l= 1.54^o A). the morphology and structure of nanoparticles were characterized using a scanning electron microscopy (SEM) images using an LEO instrument model 1455VP (Prior to taking an image, to obtaining a better contrast and to make the sample surface conducting and prevent charge reposition the sample was coated with a very thin layer of Au) and energy dispersive spectrometry analysis (EDS) was investigated by XL30, Philips microscope. Thermogravimetric–differential thermal analysis (TG–DTA) was carried out using an instrument (Shimadzu TGA-50H) with a flow rate of 20.0 mL min^-1 and a heating rate of 10 C min-1 under nitrogen atmosphere.

The XRD pattern of flake like magnesium and aluminum hydroxide is illustrated in Fig. 1. The XRD pattern of as-prepared magnesium hydroxide shows a good agreement with literature values (JCPDS No. 33-0018) with pure cubic phase while aluminum hydroxide shows pure monoclinic phase. The SEM image of magnesium and aluminum hydroxide nanostructure is shown in Fig. 2 and Fig. 3. As seen from Fig. 2, 3, flake like magnesium hydroxide was formed. In this work, flake like magnesium hydroxide and aluminum hydroxide were used as flame retardants to improve thermal stability of PVC, PMMA. Firstly, the effect of magnesium hydroxide on fire retardant of PVC was investigated. The TGA, DTG and DSC results are shown in Fig. 4 a-c. Base of these results, two possible reactions were introduced during burning composite. Reaction 1 (eq. 1) refer to the reaction of PVC with oxygen which lead to produce CO₂, HCL and H₂O. Base of DTG and DSC curve in Fig. 4, this reaction is exothermic and release 4.7kcal/gr energy. The second reaction (eq. 2) is related to the burning of magnesium hydroxide and produced magnesium oxide. Reaction 2 is an endothermic reaction and need 1.356 kJ/g.

 (2)

Thermal decomposition of the PVC nanocomposite shift towards higher temperature in the presence of flake like Mg(OH)₂ nanostructures. Magnesium hydroxide has barrier effect to slow down the product volatilization and thermal transport during decomposition of the polymer. So the initial degradation temperature (Tonset) increases. In addition, we studied the effect of cooperation between magnesium hydroxide and aluminum hydroxide on thermal stability of PVC. Fig. 5 a-c show the TGA, DTG, and DSC of Al(OH)₃/Mg(OH)₂/PVC Nano-composite, respectively. As seen from the results, cooperated nanostructures show better results. Exfoliated aluminum hydroxide has a barrier effect to slow down the product volatilization and thermal transport during decomposition of the polymers. Adsorption of polymer chains onto the surface of aluminum hydroxide nanoparticles leads to restriction of the segmental mobility and suppress chain-transfer reactions [20, 21]. The TGA graph indicate that adding of aluminum hydroxide to the PVC leads to a little reduction of the Tonset but enhances the residue at the temperatures above 400 C. In addition, we studied the effect of different ratio of PVC/MgOH)₂/Al(OH)₃ on fire retardant of PVC. As seen from results, best result was obtain for 1:1:1 of PVC:Mg(OH)₂/Al(OH)₃.

The DSC results of PMMA-Mg(OH)₂ nanocomposite is shown in Fig. 6a. As seen, by increasing temperature, two main reactions are occurred. First, reaction of PMMA with oxygen (burning of PMMA) which produces CO₂ (g) and H₂O (g) and releases were about 3120 Cal/gr energy (eq. 3). Second reaction is related to the combine magnesium hydroxide with oxygen and produce magnesium oxide and water (eq. 4).

 (3)

 (4)

According to the above equations and DSC results, peak in 280- 300 is an exothermic and refer to burning PMMA and release CO₂ and H₂O.

The TGA results of PMMA-Mg(OH)₂ nanocomposite is illustrated in Fig. 6b. As seen from figure, the thermal stability of PMMA is improved by the addition of flake like magnesium hydroxide. In the system containing pre-dispersed flake like magnesium hydroxide, mass loss starts very subtly near 100. The Comparing the decomposition of these alumina-filled composites with neat PMMA, the thermal stabilities of main chain decompositions in composites are shifted toward higher temperatures.

Beside this, we studied the effect of cooperation between magnesium hydroxide and aluminum hydroxide on thermal stability of PMMA. TGA, DTG, and DSC of Al(OH)₃/Mg(OH)₂/PVC Nano-composite illustrated in Fig. 7a-c. Cooperate magnesium hydroxide and aluminum hydroxide show similar result on improve stability of PMMA.

In the case of graphene-PVC- magnesium hydroxide nanocomposite, three possible reactions are proposed (Fig. 8a). First two reactions are related to the combine oxygen with PVC and magnesium hydroxide (eq1 and 2). Third reaction is related to the burning of graphene in the nanocomposite (eq. 5).

 (5)

As previously mentioned, the exothermic peaks in 320-400 are related to the burning PVC which released H₂O (g), HCL (g) and CO₂. The main difference between DSC of PVC- Mg(OH)₂ nanocomposite and graphite- PVC- Mg(OH)₂ nanocomposite is the intensity of peak at 410 . In the case of PVC- Mg(OH)₂nanocomposite, intensity is higher than graphite- PVC-Mg(OH)₂ nanocomposite. This happens because energy released by combine oxygen and graphite is absorbed by magnesium hydroxide.

TGA result of graphite-PVC nanocomposite and graphite-PVC-Mg(OH)₂ nanocomposite are shown in Fig. 8b. In below 400 C, both curve for graphite-PVC nanocomposite and graphite-PVC-Mg(OH)₂ nanocomposite are same. In above 400 C, weight losing percentage for graphite-PVC-Mg(OH)₂ nanocomposite is lower than graphite-PVC nanocomposite.

CONCLUSION

Herein, flake like magnesium hydroxide and cubic like aluminum hydroxide were synthesized via a simple surfactant-free precipitation reaction at room temperature. The effect of prepared nanostructures and their cooperation on fire retardant of PVC, PMMA, and graphite- PVC were investigated. The results show that cooperated nano-filler show significant effect on improve thermal stability and flame retardancy of the PVC and graphite- PVC because of endothermic decomposition that release water simultaneously. Nanoparticles were characterized using XRD and SEM techniques.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.