Flame retardant effect of zinc borate in polyamide 6 containing aluminum hypophosphite

Highlights

• ZnB improves the thermal stability of AlHP.

• Chemical interaction is observed between ZnB and AlHP.

• LOI value steadily increases up to 7 wt% ZnB addition.

• Fire performance index greatly improves with the use of ZnB.

Abstract

The effect of zinc borate (ZnB) on the thermal and flame retardant properties of polyamide 6 (PA 6) composites containing aluminum hypophosphite (AlHP) was examined. The properties of the composites were investigated using thermogravimetric analysis (TGA), limiting oxygen index (LOI), horizontal burning test (UL-94 HB), vertical burning test (UL-94 V), and mass loss calorimeter (MLC) studies. According to the thermal test results, ZnB improved the thermal stability of the AlHP. Chemical interaction was observed between ZnB and AlHP. According to the flammability test results, the highest LOI value of 26.7 is observed with the addition of 5 wt% ZnB. However, UL-94 V rating turned to burn to clamp owing to the ceasing of dripping. All samples were classified as HB in UL-94 HB test. According to the MLC results, the lower peak heat release rate (pHRR) and total heat evolved (THE) were obtained with the addition of ZnB. Fire performance index (FPI) greatly improved with the use of ZnB. ZnB showed adjuvant effect in the condensed phase via increasing the residue yield with compact structure.

Introduction

Polyamide 6 (PA 6) is an important engineering polymer and finds applications where the fire retardant property is required. Various phosphorus based flame retardants can be used as environmentally friendly flame retardant additive in PA 6 [[1], [2], [3], [4]]. Metal hypophosphite compounds are emerging class of phosphorus containing flame retardants and show flame retardant effect especially in heteroatom containing polymers [[5], [6], [7]]. Aluminum hypophosphite (AlHP) is the most widely studied metal hypophosphite compound in the literature.

The flame retardant effect of AlHP is studied in various thermoplastic polymers including polyethylene [8], polystyrene [9], polyvinyl alcohol [10], thermoplastic polyurethane [11], acrylonitrile butadiene styrene [12,13], poly (butylene terephthalate) [14], poly (lactic acid) [15] and PA 6 [[16], [17], [18], [19], [20], [21]]. It is concluded from these studies that AlHP shows its flame retardant effect both in the condensed phase via the formation of intumescent char structure and in the gas phase via radical trapping effect. Limited number of studies are performed in order to improve flame retardant effect of AlHP [[22], [23], [24], [25], [26], [27], [28]]. In these studies, expandable graphite [22,23], melamine cyanurate and silicon based additive [24], nanoclay [25], pentaerythritol [26], graphene oxide [27], intumescent flame retardant [28] and nanosilica [29] are used as synergistic additive.

Various grades of zinc borate (ZnB), which are considered as ecofriendly, are used either solely or with other flame retardant additives as a synergistic agent [30]. The synergistic interaction between ZnB and commercially available phosphorus based flame retardants including ammonium polyphosphate [[31], [32], [33], [34], [35]], red phosphorus [36,37], melamine polyphosphate [38] and aluminum diethylphosphinate [39,40] is studied in the literature. According to the results of these studies, it is concluded that ZnB shows synergistic interaction with these phosphorus based additives in different manner depending upon the amount of ZnB and the kind of the matrix material and phosphorus based additive. In the current study, it is considered that ZnB may also show adjuvant and/or synergistic interaction with AlHP via different mechanisms. To that end, AlHP and ZnB are used together at constant loading of 15 wt%. The fire retardant performances of the composites were characterized by limiting oxygen index (LOI), horizontal burning test (UL-94 HB), vertical burning test (UL-94 V), and mass loss calorimeter (MLC). Thermal stability of the additives and composites are investigated using thermogravimetric analysis (TGA). The structural changes occurred with the heat treatment in AlHP, ZnB, and AlHP/ZnB mixtures are examined with XRD analyses. Detailed analyses are performed on the char residues remained after MLC test for explaining the possible interaction mechanism between AlHP and ZnB. The char residues are investigated by attenuated total reflectance – Fourier-transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy with wavelength dispersive X-ray spectrometer (SEM-WDX).