Key role of magnesium hydroxide surface treatment in the flame retardancy of glass fiber reinforced polyamide 6
Introduction
Polyamides are widely used in many industrial fields including building, home textile, automotive and electrical industries. However, due to their organic nature, these polymers are highly flammable which can be an issue for some of these applications. Among the different fire retardant additives that can be used in polyamide 6, magnesium dihydroxide (MDH) has attracted attention of industrial manufacturers due to its chemical nature of being nontoxic and noncorrosive [1]. These additives mainly act in a physical way, undergoing endothermic dehydration. The temperature of the material is then decreased due to the release of water and the flame is diluted by water vapors, leading to a delayed ignition. A ceramization effect, due to the formation of magnesium oxide (MgO), is also obtained, which leads to the production of a ceramic protective structure characterized by a high heat capacity [[2], [3], [4]].
However, high loadings (>50 wt.-%) of MDH are generally needed to obtain fire performance and more precisely a V0 rating at UL-94 test, resulting generally in reduced mechanical properties. Moreover, it was shown [5] that MDH leads to a potential degradation of PA6 during processing, which could reduce the viscosity and increase dripping during the UL-94 test. To decrease the loading level of MDH, the use of synergists was investigated. Song [6] showed that the combination of MDH and red phosphorus (RP) increases greatly the flame retardancy of PA6. The additional use of organophilic montmorillonite [6] leads to a further decrease of the peak of heat release rate and to an improvement of the mechanical properties. However, the reddish-brown color of RP itself restricts the extensive applications. Miyata [7] showed that magnesium hydroxide activated with small amounts of catalytic metals, for instance Mg0.96 Zn0.02 Ni0.02 (OH)2, was effective in giving a V0 rating at loadings as low as 33 wt.-%. More recently [8], the combination of 5 wt.-% bisphenol A bis(diphenyl phosphate) (BDP) and 50 wt.-% MDH allowed obtaining a UL-94 V0 rating as well as a decrease of the peak rate of smoke release (pRSR) and of the total smoke release (TSR) in PA6. On the other hand, a flame retardant system composed of a silane treated MDH and methyl-locked novolac could completely eliminate the melt dripping during the combustion of PA6, and also greatly shorten the flame time [9].
The flame retardancy of glass fiber reinforced polyamide 6 (PA6 GF) is even more challenging due to the 'candlewick effect' caused by glass fibers [10]. V0 rating at UL-94 test can be achieved using a combination of MDH in platelet form with 4% of ethylene propylene copolymer. Baierweck [11] patented a formulation composed of 15% MDH with 10% melamine cyanurate (MCA) which allows reaching a glow wire flaming index of 960 °C. But recent studies have proved that repeated exposure to MCA could lead to high risks on human health [12]. Moreover, this additive is very persistent in the environment [13] and thus the environmentally-friendly benefit associated to the use of MDH is lost.
In this context, the purpose of this paper is to investigate the influence of the MDH surface treatment on the fire retardant properties of PA6 GF/MDH blends. In the literature, a few studies are dealing with the modification of MDH to increase its thermal stability to enable application in aliphatic nylons. Kirschbaum [14] reported that, depending on the surface treatment, only a V1 or V2 rating could be achieved at 50% loading in polyamide 6. Ziegan and Hondgesberg [15] demonstrated that the use of a silane-treated MDH was leading to a decrease of the UL-94 rating. However, the use of surface-treated MDH for the flame retardancy of PA6 GF has never been reported. In this work, two MDH grades of similar particle size were used: H5 and H5A, corresponding respectively to an untreated MDH and a vinylsilane coated MDH.
First, the fire performance of the PA6 GF/MDH formulations in terms of UL-94 rating, glow wire and mass loss calorimetry (MLC) was determined. Then, the whole mechanism of action of the fire retarded (FR) materials was investigated to explain the differences in the fire properties observed with the two types of MDH. More precisely, the thermal stability was studied; the gas and condensed phases were carefully analyzed and the properties of the protective layer formed during the degradation were evaluated.
Section snippets
Materials
Polyamide 6 reinforced with 30% glass fibers (Technyl C216) and polyamide 6 without glass fibers (Technyl S27) were kindly provided by Solvay. Two grades of magnesium dihydroxide (MDH), provided by Albermarle under the trade name Magnifin H5 and H5A, were used in this study. They both have similar particle size (Table 1) but H5 is an uncoated MDH whereas H5A is coated with a vinylsilane treatment. Technyl C216, Technyl S27, H5 and H5A were dried at 70 °C for 24 h prior to extrusion.
Preparation of samples
Formulations
Dispersion of MDH in PA6 GF
Silicon-based organic compounds such as silanes are known to be widely used as coupling agents, changing the properties of solid particle surfaces on which they are applied [20]. According to Shokoohi [21], silane can act as a kind of bridge between the additive and the polymer, improving the properties of the composite system. Practically, they are widely used as glass fiber coatings because of their good influence on the mechanical properties as well as electrical and water repellent
Conclusion
The present study investigated the influence of a MDH surface treatment on the fire performance of PA6 GF/MDH formulations using two grades of MDH, one being surface treated with vinylsilane (H5A grade). It was shown that the surface treatment positively acts as a dispersing agent, limiting agglomeration of particles in PA6 and increasing the compatibility between the fillers and the polymer. Moreover, the fire properties were increased with the surface treated MDH compared to the uncoated
Acknowledgements
The work was financially supported by Schneider Electric.
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