Effects of a Phosphorus Flame Retardant System on the Mechanical and Fire Behavior of Microcellular ABS

Abstract

The present work deals with the study of phosphorus flame retardant microcellular acrylonitrile⁻butadiene⁻styrene (ABS) parts and the effects of weight reduction on the fire and mechanical performance. Phosphorus-based flame retardant additives (PFR), aluminum diethylphosphinate and ammonium polyphosphate, were used as a more environmentally friendly alternative to halogenated flame retardants. A 25 wt % of such PFR system was added to the polymer using a co-rotating twin-screw extruder. Subsequently, microcellular parts with 10, 15, and 20% of nominal weight reduction were prepared using a MuCell^® injection-molding process. The results indicate that the presence of PFR particles increased the storage modulus and decreased the impact energy determined by means of dynamic-mechanical-thermal analysis and falling weight impact tests respectively. Nevertheless, the reduction of impact energy was found to be lower in ABS/PFR samples than in neat ABS with increasing weight reduction. This effect was attributed to the lower cell sizes and higher cell densities of the microcellular core of ABS/PFR parts. All ABS/PFR foams showed a self-extinguishing behavior under UL-94 burning vertical tests, independently of the weight reduction. Gradual decreases of the second peak of heat release rate and time of combustion with similar intumescent effect were observed with increasing weight reduction under cone calorimeter tests.

Keywords: MuCell® injection-molding foaming; flame-retardant ABS microcellular foams; phosphorus flame retardants.

Figure 1

Scheme representing the square-shaped injection-molded part and the sample taken for dynamic-mechanical-thermal analysis (in blue). Blue arrow indicates the surface of the sample taken for the analysis of the cellular morphology of foams by scanning electron microscopy. VD: Vertical direction; WD: Width direction.

Figure 2

Low magnification SEM images showing the characteristic structure of injection-molded foams (ABS/PFR-15): (a) Skin and transition zone; (b) Core zone.

Figure 3

SEM images showing the characteristic core cellular morphology of ABS and ABS/PFR foams.

Figure 4

SEM images showing PFR particles throughout the cell walls of ABS/PFR foams.

Figure 5

Evolution of the storage modulus with temperature for unfoamed and foamed (a) ABS and (b) ABS/PFR.

Figure 6

Variation of the storage modulus of ABS and ABS/PFR with the nominal weight reduction at (a) −90 °C and (b) 30 °C.

Figure 7

Evolution of the tan δ with temperature for the unfoamed and foamed (a) ABS and (b) ABS/PFR.

Figure 8

Variation of the glass transition of the (a) rubbery (T_g1) and (b) rigid (T_g2) phases of ABS and ABS/PFR with the nominal weight reduction.

Figure 9

Variation of the intensity of tan δ of the (a) rubbery (T_g1) and (b) rigid (T_g2) phases of ABS and ABS/PFR with the nominal weight reduction.

Figure 10

Characteristic force versus time falling weight curve of unfoamed and foamed (a) ABS and (b) ABS/PFR.

Figure 11

Characteristic images of unfoamed and foamed ABS and ABS/PFR parts after impact testing.

Figure 12

Characteristic HRR versus time curves of unfoamed and foamed ABS and ABS/PFR.

Figure 13

Characteristic HRR curves of unfoamed and foamed ABS/PFR parts showing the three main combustion stages.

Figure 14

Residues of unfoamed and foamed ABS/PFR parts after the cone calorimeter tests.

Figure 15

SEM micrograph showing the characteristic foamed morphology of a ABS/PFR char residue after cone calorimeter test.