Effect of an ionomer on the thermal stability, fire and mechanical behaviour of phosphorous flame retardant ABS formulations

Acrylonitrile butadiene styrene (ABS) is a widely used thermoplastic copolymer due to its desirable properties such as good mechanical performance (toughness, impact resistance, among others), ease of processability, chemical resistance, and low cost. The most important problem of ABS is its inherent flammable characteristic which restricts the application areas of ABS. Therefore, research about flame retardancy of ABS becoming more important according to its developing using areas. Phosphorus flame retardants (PFRs) have been utilized for decades and they are also incorporated with ABS as copolymers. These flame retardant ABS copolymers are eco-friendly alternatives to other halogenated ones. One of the main drawbacks of incorporating PFRs into ABS is the deterioration of its mechanical performance. Therefore, the use of compatibilizers is a possible strategy to minimize the negative effect of PFRs on ABS. In the present work, ABS and flame retardant ABS formulations were prepared and the effect of adding a commercial ionomer on the ABS and ABS phosphorus flame retardant (PFR) formulations (ABS-PFR) was analyzed. The main objective of this study was to analyze to effect of a commercial ionomer on the thermal stability, mechanical and fire behaviour of ABS and ABS-PFR formulations. All formulations were prepared by means of a melt blending process. A 10 and 20 wt.% of an ammonium polyphosphate (APP) and an aluminium diethylphosphinate (AlPi) were added, in a 1:1 proportion, into ABS. Also, a 5 PHR of an ionomer of ethylene methacrylic acid copolymer (EMAA) was incorporated into the ABS and ABS phosphorus flame retardant (PFR) formulations. Specimens of ABS and ABS formulations were prepared using a compression-molding technique. It must be said that two different conditions were applied to the melt blending formulations before compress-molding them: a) a pre-dried (PD) conditioning of 95 ºC during 24h and b) non pre-dried (NPD). These last formulations were directly molded after the melt-blending process. In order to analyze the ionomer effect on the compatibility of PFR and ABS scanning electron microscopy (SEM) was used. Poor interaction between PFR additives and matrix was observed for formulations without ionomer samples. On the other hand, for the ionomer included formulations, the interaction between APP and polymer matrix was improved. This indicated that ionomer could partially compatibilize PFR additives and ABS matrix. Furthermore, thermal stability of ABS and ABS-PFR formulations with and without ionomer, and with different conditioning, were analyzed using a thermogravimetric analysis (TGA). According to TGA analysis, the starting temperature of the decomposition of ABS decreased with the presence of the PFR additives. An increase of residue was also observed according to an increase of wt.% PFR at 900 °C. This observation could be attributed to synergistic interactions between the polymer matrix and the PFR additives during the thermal decomposition process. In addition, according to TGA results, inclusion of ionomer in PFR formulations increased the amount of residue and the degradation starting temperature for both PD and NPD conditionings. The PFR content and ionomer effects on the mechanical and fire behaviour of ABS-PFR formulations (with and without pre-drying) were analyzed by means of three-point bending flexural / charpy impact tests and cone calorimeter, respectively. It was observed that the presence of PFRs reduced the flexural strength, but slightly increased the flexural modulus. Analysis also showed that NPD formulations, with ionomer presented higher strength and flexural modulus. On the other hand, PD conditioning decreased the flexural properties of ABS and flame retardant ABS formulations. Impact strength decreased with the increase of wt.% PFR. Analysis also showed that ionomer addition decreased the impact strength of all samples. All flame retardant formulations showed an increase in the amount of char with increasing PFR. The presence of ionomer was observed to delay the ignition times of all ABS formulations, while also increasing the fire growth rate index.