Expandable Graphite in Polyethylene: The Effect of Modification, Particle Size and the Synergistic Effect with Ammonium Polyphosphate on Flame Retardancy, Thermal Stability and Mechanical Properties

Rathberger²,

Schöffel³

et al. 2021

A new expandable graphite (EG) type was studied as a flame retardant additive in Polyamide 6 (PA6). The fire behavior was characterized by a cone calorimeter using external heat fluxes of 35, 50 and 65 kW/m2, limiting the oxygen index (LOI) and UL-94 burning tests. Additionally, electric and thermal conductivity as well as rheological properties were characterized to provide a general property overview. Fire tests were conducted using dry and humid conditioned samples. Cone Calorimeter tests showed a minimum filling degree of 15 wt.% (8.6 vol.%) EG was required to achieve a significant fire inhibiting effect in PA6 independent of the sample condition. UL-94 fire tests show a V0 classification at filling degrees greater than 20 wt.% (humid) and 25 wt.% (dry), although the associated LOI values of 39% and 38% demonstrate good flammability inhibition. Correlation analyses were conducted to identify major influences given by the sample condition for most important key figures measured in cone calorimeter tests. Accordingly, humid-conditioned samples containing between 2.5 (PA6 + 25 wt.% EG) and 4.2 wt.% (PA6) water were found to reduce the total heat evolved (THE) on average by 16% and the total smoke production (TSP) on average by 22%.

Section: Discussionsupporting

confidence: 62%

Expandable Graphite for Flame Retardant PA6 Applications

Rathberger²,

Schöffel³

et al. 2021

“…Expandable graphite provides excellent flame retarding properties, which are particularly effective in the long term to reduce fire spread. The fire protection effect, as reported for PA6 [ 10 ] as well as for other polymeric material systems (e.g., PE [ 15 , 16 , 17 , 18 ], PP [ 19 , 20 ], PS [ 21 ], PVC [ 22 ], ABS [ 23 ], PA6 [ 24 ]), is based solely on the physical formation of a thermally stable, voluminous residue. Since the residue initially starts to form when exposed to heat, an efficient flame retardancy effect initially becomes effective with a certain time delay.…”

Section: Discussionmentioning

confidence: 90%

“…When incorporated into a polymer, the expanded structure forms a voluminous char, thermally isolating lower polymer layers and thus effectively reducing flame spread by lowering the pyrolysis gas flow feeding the flame. Due to its solely physical effect, expandable graphite can be used in various polymeric systems (e.g., PE [ 15 , 16 , 17 , 18 ], PP [ 19 , 20 ], PS [ 21 ], PVC [ 22 ], ABS [ 23 ], PA6 [ 24 ]). Please note that an appropriate temperature stability is mandatory for processing.…”

Section: Introductionmentioning

confidence: 99%

Expandable Graphite, Aluminum Diethylphospinate and Melamine Polyphosphate as Flame Retarding System in Glass Fiber-Reinforced PA6

Schoeffel²,

Rathberger³

et al. 2022

A flame retardant system based on expandable graphite (EG), aluminum diethylphosphinate (AlPI) and melamine polyphosphate (MPP) was investigated in glass fiber- (GF) reinforced polyamide 6 (PA6). Burning characteristics were evaluated via cone calorimeter, limiting oxygen index (LOI) and UL-94 tests. Thermogravimetric analysis (TGA) and coupled Fourier transform infrared spectroscopy (FTIR) was used to investigate the decomposition process as well as flame retardant modes of actions. Specifically, in the cone calorimeter tests, formulations containing EG showed excellent flame retardant properties for non-reinforced and reinforced PA6. The best performance was achieved for 25 wt.% glass fiber-reinforced PA6 containing solely 20 wt.% EG, corresponding to a measured pHRR of 134 kW/m2 and a total smoke production of 1.2 m2. Higher glass fiber contents of 45 wt.% (30 vol.%) revealed a lower char volume, which was attributed to both the limited space available for expansion and the sheer-induced reduction in particle size during processing. All of the reinforced PA6 formulations only achieved V2 classifications, but this was at low filling degrees (10 wt.%) for both net EG or EG/AlPi/MPP combinations. For GF-reinforced PA6 containing EG/AlPi/MPP mixtures, a synergistic effect was found to improve the oxygen index up to 30.6%.

“…Within this paper we discuss an efficient multi-material flame retarding system for polyamide 6 based on expandable graphite (EG) as main ingredient as well as AlPi/MPP mixtures as synergistic component. EGs have been proven to be an excellent flame retardant additive in various polymers as individual component and as multi-material flame retardant system, e.g., PE [ 10 , 11 , 12 , 13 ], PP [ 14 , 15 ], PS [ 16 ], PVC [ 17 ], ABS [ 18 ], PA6 [ 19 ]. When a critical temperature is reached, blowing agents intercalated between the graphene layers react, forming a voluminous and thermally stable char-residue with a typically worm-like structure.…”

Section: Introductionmentioning

confidence: 99%

A Synergistic Flame Retardant System Based on Expandable Graphite, Aluminum (Diethyl-)Polyphospinate and Melamine Polyphosphate for Polyamide 6

Schoeffel²,

Rathberger³

et al. 2021

Expandable graphite (EG), aluminum (diethyl)polyphosphinate (AlPi) and melamine polyphophate (MPP) was used as flame retardant multi-material additive in a polyamide 6 (PA6) matrix. Flame inhabitation performances were conducted by cone calorimeter, LOI and UL-94 tests, synergisms identified analyzed by TGA-FTIR and TGA-GC/MS and effects found were comprehensively discussed. SEM images were used for char residue characterization. For PA6 containing 20 wt.% EG and 5 wt.% AlPi/MPP (3:2), a well working synergism in limiting oxygen indices could be identified exhibiting the highest oxygen index (OI) measured: 46%. The study shows that the synergism due to the partial substitution of EG by AlPi/MPP can be attributed to two effects: (1) When PA6/AlPi/MPP mixtures decompose predominantly CO2 evaporates in early decomposition stages. CO2 evaporations was found to be sensitive to the heating rate applied, whereas specifically high heating rates increased the CO2 yield measured. (2) Solid decomposition products of AlPi/MPP act as “glue” between expanded graphite and thus increase the mechanical residue stability.