Flame Retardancy and Thermal Stability of Materials 2018; 1: 1–13 Research Article Hao Wu*, Rogelio Ortiz, Renan De Azevedo Correa, Mourad Krifa, Joseph H. Koo Self-Extinguishing and Non-Drip Flame Retardant Polyamide 6 Nanocomposite: Mechanical, Thermal, and Combustion Behavior https://doi.org/10.1515/flret-2018-0001 Received June 7, 2017; accepted October 9, 2017 1 Introduction Abstract: Incorporation of flame-retardant (FR) additives A major drawback of flame retarding thermoplastic and nanoclay fillers into thermoplastic polymers polymers by incorporating particulate additives consists effectively suppresses materials flammability and melt of mechanical property losses [1]. In particular, nylon dripping behavior. However, it largely affects other 6 composite systems treated with non-halogenated properties, such as toughness and ductility. In order to intumescent fillers achieved self-extinguishing non- drip recover the lost toughness and ductility of flame retardant behavior but suffered from significant losses in ductility and polyamide 6, various loadings of maleic anhydride elongation at break [2, 3]. In previous studies, our research modified SEBS elastomer were added and processed by addressed this issue through elastomer toughening of twin screw extrusion. TEM images showed exfoliated the composite systems. Thus, flame retardant polyamide nanoclay platelets and reveals that the clay platelets well 6 was toughened using a maleated triblock copolymer dispersed in the polymer matrix. By balancing the ratio of containing styrene-hydrogenated butadiene-styrene flame retardants, nanoclay and elastomers, formulation (SEBS-g-MA). This approach achieved successful recovery with elongation at break as high as 76% was achieved. of ductility as evidenced by a significant improvement Combining conventional intumescent FR and nanoclay, in Izod impact strength and elongation at break, while UL-94 V-0 rating and the LOI value as high as 32.2 were preserving the flame-retardant performance [4]. In the achieved. In conclusion, effective self-extinguishing and current research, we further explore the combined effect non-drip polyamide 6 nanocomposite formulations with of rubber toughening and incorporation of nanoclay significant improvement in toughness and ductility were additives to compound a polyamide 6 nanocomposite achieved. system with self-extinguishing non-drip performance and recovered ductile property. Keywords: flame retardant, rubber toughening, polyamide The efficiency of nanoparticles in modifying material 6, nanocomposite properties has led to intensifying research interests on polymeric nanocomposites in recent decades. In the area of flame retardant applications, one of the most effective and widely used nano-fillers is nanoclay. Among different types of clay, the montmorillonite (MMT) nanoclay with surface treatment is commonly used to achieve good *Corresponding author: Hao Wu, Materials Science and interfacial properties and high levels of dispersion in the Engineering, Texas Materials Institute, The University of Texas at Austin, 204 E. Dean Keeton Street; Stop C2201 Austin, TX 78712, polymer matrix. For instance, cation exchange is often USA, E-mail: [email protected] carried out on the nanoclay before processing with the Joseph H. Koo, Materials Science and Engineering, Texas Materials polymer matrix [5, 6]. Numerous studies have demonstrated Institute, The University of Texas at Austin, 204 E. Dean Keeton that the incorporation of nanoclay even at very low Street; Stop C2201 Austin, TX 78712, USA loadings could greatly decrease the peak heat release Rogelio Ortiz, Renan De Azevedo Correa, Joseph H. Koo, Department of Mechanical Engineering, The University of Texas at Austin, 204 E. rate as characterized by cone calorimetry or microscale Dean Keeton Street, Stop C2200, Austin TX 78712, USA combustion calorimetry, by creating a carbonaceous- Mourad Krifa, Kent State University, 800 E. Summit St. Kent, OH silicate char layer [2, 7-9]. This clay-rich char layer could be 44240 caused by either pyrolysis of the polymer leaving the clay Open Access. © 2017 Hao Wu et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 3.0 License. 2 H. Wu, et al. platelets on the surface or bursting bubbles of the polymer of loss of toughness and ductility, rubber toughening melt pushing the clay towards the surface [10, 11]. is used as an approach to tailor mechanical behavior. However, materials incorporating nano-fillers alone Details about structural, morphological, mechanical have been found to fall short in flammability tests such as and flammability characterization of the nanocomposite UL-94 [12, 13]. On the other hand, combinations of nano- material are discussed in this paper. additives and conventional flame retardants, such as intumescent fillers have shown potential synergistic effects and high performance in reducing total heat release, which 2 Methods equates to the highest UL-94 rating [6, 9, 14-16]. Another challenge for flame retardant incorporated polymer 2.1 Materials composites is the significant reduction in mechanical properties, such as ductility and elongation at break. Medium viscosity nylon 6 (density 1.13 g/cm3), was obtained The majority of the past researches in flame retardant from Honeywell, Inc. and dried at 80°C for 24 hours prior polymers only focused on reduction in flammability. to processing. Flame retardant additive Exolit® OP1312 However, a balance of properties, such as mechanical, consisting of a mixture of aluminium diethyl phosphinate flammability, and cost are often required in commercial (AlPi), melamine polyphosphate (MPP), zinc and boron systems. Only a few publications have addressed this oxide (ZnB) [18] was provided by Clariant International issue. Shi et al. modified MMT nanoclay with a cationic Ltd. The SEBS elastomer Kraton FG1901 G was provided by vinyl acetate copolymer to make a masterbatch and Kraton Polymers, Inc. Montmorillonite (MMT) organoclay then mix with Ethylene vinyl acetate (EVA). They found Cloisite 30B was provided by Southern Clay Products. desirable flammability and mechanical properties in these Cloisite 30B is produced by cation exchange and treated nanocomposites [17]. Thermoplastic elastomers can be with 90 mequiv./100 g clay of Ethoquad T12 with methyl used as modifiers for engineering thermoplastics including bis-2- hydroxyethyl tallow quaternary ammonium chloride nylons to tailor the mechanical properties. In our previous [19]. Flame retardant, elastomer, and nanoclay were used work, a maleic anhydride modified SEBS elastomer was as received. incorporated into FR nylon 6 the system by twin-screw melt extrusion. The addition of the elastomer succeeded in significantly enhancing the Izod impact strength and 2.2 Compounding partially recovering the elongation at break without compromising the flame-retardant performance [4]. The formulations compounded are shown in Table 1. The objective of this research is to prepare a Previous studies on rubber toughened flame retardant nano-filled nylon 6 (PA6) with balanced properties in polyamide 6 have shown that 15wt% or lower elastomer flammability and mechanical properties. To optimize its loading would generate consolidated char layer and flame retardant properties a combination of nanoclay retain the UL-94 V-0 rating [4]. Considering the potential and non-halogenated conventional intumescent flame synergistic effect between OP1312 and nanoclay, the retardant additives were used. To overcome the problem FR loading was lowered to 15wt% in all formulations Table 1. Formulation table Sample # Designation Flame Retardant Kraton Elastomer Nanoclay Nylon 6 (wt%) (wt%) (wt%) (wt%) 1 Neat - - - 100 2 20FR 20 0 0 80 3 15FR_5KR_2.5NC 15 5 2.5 77.5 4 15FR_5KR_5NC 15 5 5 75 5 15FR_10KR_2.5NC 15 10 2.5 72.5 6 15FR_10KR_5NC 15 10 5 70 7 15FR_15KR_2.5NC 15 15 2.5 67.5 8 15FR_15KR_5NC 15 15 5 65 Self-Extinguishing and Non-Drip Flame Retardant Polyamide 6 Nanocomposite 3 containing nanoclay. Two nanoclay loadings were chosen 3.3 Thermal Properties (2.5wt% and 5wt%) in order to provide enough post burning protective char layer. Thermogravimetric analyses (TGA) of all samples were All formulations were processed using a Thermo- performed using a TGA-50 from Shimadzu Scientific Scientific Process 11 parallel co-rotating twin screw Instruments. The samples were heated in air environment extruder (L/D=40). The co-rotating twin screw extruder from room temperature to 1000°C at a heating rate of provides high shear mixing which ensures proper 10°C/min. The air flow rate was 20ml/min. dispersion of the nano-fillers [20]. Materials were melted DSC was carried out using a Q20 from TA Instruments. and mixed at 240oC, with screw speed at 150 rpm. To For the DSC tests, injection molded samples were cut into ensure a homogenous dispersion, formulations with 7-8 mg square pieces and were directly put into the crucible varying concentrations were pre-mixed using a Thinky® for testing. To erase the thermal history, all samples were planetary centrifugal mixer prior to melt compounding. first heated from room temperature to 260°C at 10°C/min; Extruded composites were water cooled, pelletized, and after holding at 240°C for 2 min, the samples were cooled dried before injection-molding into ASTM D638 standard down to room temperature at -10°C/min. A second heating tensile bars and UL-94 specimens. A Mini-Jector injection cycle then started at the same rate to the maximum of molding