Abstract
Aluminium diethylphosphinate (AlPi-Et) and inorganic aluminium phosphinate with resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H+RXP) were compared with each other as commercially available halogen-free flame retardants in poly(butylene terephthalate) (PBT) as well as in glass-fibre-reinforced PBT (PBT/GF). Pyrolysis behaviour and flame retardancy performance are reported in detail. AlPi-H+RXP released phosphine at very low temperatures, which can become a problem during processing. AlPi-Et provided better limiting oxygen index (LOI) values and UL 94 ratings for bulk and PBT/GF than AlPi-H+RXP. Both flame retardants acted via three different flame-retardancy mechanisms in bulk as well as in PBT/GF, namely, flame inhibition, increased amount of char, and a protection effect of the char. AlPi-Et was more efficient in decreasing the total heat evolved of PBT in the cone calorimeter test. AlPi-H+RXP reduced the peak heat release rate of PBT more efficiently than AlPi-Et. An optimum loading of AlPi-Et in PBT/GF was found, which was below the supplier’s recommendation. This loading provides a maximum increase in LOI and a maximum decrease in total heat evolved.
1 Introduction
Poly(butylene terephthalate) (PBT) is one of the most important engineering thermoplastics. It is widely used in electrical and electronic products as well as in automotive and transportation applications because of its combination of advantageous properties (1). In the past, the good flame-retardancy performance required for most applications was achieved using halogenated flame retardants (2). Nowadays, flame-retardant polymers are expected by the market and environmental regulations to be halogen free (3). Phosphorus-based compounds are the most promising alternative among the halogen-free flame retardants currently in use.
Today, metal phosphinates play a major role in the flame retardancy of PBT. They can be divided into organic phosphinates and inorganic phosphinates. Organic phosphinates are salts of dialkyl, diaryl, or alkyl-aryl phosphinic acid, respectively. Today’s products are based on formulations containing aluminium or zinc organic phosphinates alone (4–6) or in combination with other substances such as melamine cyanurate (7). Inorganic phosphinates, also called hypophosphites, are salts of non-substituted phosphinic acid; that is, mainly aluminium hypophosphite has been proposed with and without synergists as flame retardants for polyesters (8, 9).
The mode of action of organic metal phosphinates in polyesters is known to a certain extent. Aluminium diethylphosphinate (AlPi-Et) and zinc diethylphosphinate (ZnPi-Et) show a strong gas-phase activity in PBT and a slightly increased residue (10). The formation of intumescent residue was reported for AlPi-Et in PBT (11) and for ZnPi-Et in poly(ethylene terephthalate) (12). Synergistic improvements in flame retardancy occurred for the combinations of AlPi-Et with nanometric metal oxides in PBT (13, 14). ZnPi-Et combined with nanoparticles is proposed in polyesthers (15, 16). Most studies on inorganic metal phosphinates deal with aluminium hypophosphite (AlPi-H); lanthanum and cerium hypophosphite, however, have also been investigated (17). The flame retardancy mechanisms of AlPi-H have been discussed in very few works only. The condensed-phase activity of AlPi-H in glass fibre-reinforced PBT (PBT/GF) was proposed, whereas gas-phase activity was not discussed (18). The degree of understanding concerning inorganic metal phosphinates is somewhat lower than that for organic metal phosphinates. Furthermore, works in which organic and inorganic metal phosphinates were compared directly with each other are very rare, although such comparison is very interesting, especially with regard to application.
Thus the aim of this work was to provide a better understanding of AlPi-H’s mode of action in PBT, as well as in PBT/GF, and to show a direct comparison with AlPi-Et. The pyrolysis of the two flame retardants in PBT and the possible interactions during decomposition were assessed in detail using thermogravimetry coupled with FTIR spectroscopy. The flammability and burning behaviour were assessed by LOI, UL 94, and the cone calorimeter test, respectively. The active flame-retardancy mechanisms were identified and quantified. The efficiency of the flame retardants, i.e., their performance per unit of phosphorus, was assessed. AlPi-H+resorcinol-bis(di-2,6-xylyl phosphate) (RXP) released phosphine at relatively low temperatures, which could cause problems during compounding. AlPi-Et provided better LOI values and UL 94 ratings than AlPi-H+RXP. An optimum loading was found for the use of AlPi-Et in PBT/GF, which was even below the supplier’s recommendation. Thus, this study provides some very important results for the efficient application of the two flame retardants.
2 Experimental
2.1 Materials and sample preparation
The poly(butylene terephthalate) used in this study was of commercial injection moulding grade (Ultradur B4520, BASF SE, Germany; TM=223°C). Exolit OP 1240 consisting of aluminium diethylphosphinate (AlPi-Et) was obtained from Clariant Produkte Deutschland GmbH, Germany. Phoslite B85AX consisting of 85 wt.% aluminium phosphinate and 15 wt.% resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H+RXP) was obtained from Italmatch Chemicals, Italy. The materials were melt compounded using a co-rotating twin-screw extruder (Megacompounder ZSK 26 MCC, Werner & Pfleiderer, Germany) with a length-to-diameter ratio of 44 at a maximum temperature of 250°C. The dried polymer and the flame-retardant additives were added through the main feeder.
PBT/GF was produced by adding 30 wt.% glass fibres from PPG (Fibre Glass ChopVantage HP 3786) with a diameter of 10 μm and an initial length of 4.5 mm. Glass fibres were added via a co-rotating side feed near the die.
Test specimens were produced by injection moulding after the compounded material was dried for 4 h at 100°C. An injection mould machine (Allrounder 320 S 500-150, ARBURG, Germany) with a melt temperature of 250°C and a mould temperature of 70°C was prepared to mould the compact and the reinforced specimen. Tables 1 and 2 provide an overview of the composition of the compact and glass fibre-reinforced materials, respectively.
Composition of the investigated bulk materials.
| Material | P-content (wt.%) | PBT (wt.%) | AlPi-Et (wt.%) | AlPi-H+RXP (wt.%) |
|---|---|---|---|---|
| PBT | 0 | 100 | – | – |
| PBT/AlPi-Et6.3 | 1.5 | 93.7 | 6.3 | |
| PBT/AlPi-Et20 | 4.8 | 80 | 20 | – |
| PBT/AlPi-H+RXP4.2 | 1.5 | 95.8 | – | 4.2 |
| PBT/AlPi-H+RXP20 | 7.1 | 80 | – | 20 |
Composition of the investigated glass fibre-reinforced materials.
| Material | P-content (wt.%)a | PBT/30% glass fibre (wt.%) | AlPi-Et (wt.%) | AlPi-H+RXP (wt.%) |
|---|---|---|---|---|
| PBT/GF | 0 | 100 | 0 | – |
| PBT/GF/AlPi-Et4.4 | 1.0 | 95.6 | 4.4 | – |
| PBT/GF/AlPi-Et6.3 | 1.5 | 93.7 | 6.3 | – |
| PBT/GF/AlPi-Et14 | 3.3 | 86 | 14 | – |
| PBT/GF/AlPi-Et20 | 4.8 | 80 | 20 | – |
| PBT/GF/AlPi-H+RXP4.2 | 1.5 | 95.8 | – | 4.2 |
| PBT/GF/AlPi-H+RXP20 | 7.1 | 80 | – | 20 |
aWith respect to the whole material.
2.2 Morphology
The morphology of the flame retardants in PBT was characterised by transmission electron microscopy. The samples were cut by a microtome UltraCut E instrument from Leica and, afterwards, investigated on a Zeiss 902 transmission electron microscope with an acceleration voltage of 80 kV. The fracture surfaces of the glass fibre-reinforced samples were analysed by a scanning electron microscope (JSM-IC 848, Jeol) with an acceleration voltage of 15 kV.
2.3 Thermal analysis
Thermal analysis experiments were performed on a thermo-microbalance instrument with an automatic sample charger (TG 209 F1 Iris, Netzsch, Selb, Germany). Cryo-milled samples of 10 mg were heated in alumina pans from 30°C to 900°C at a heating rate of 10°C/min and a constant nitrogen flow rate of 30 ml/min. Blank measurements were used to correct apparatus-specific deviations including buoyant forces.
Fourier transform infrared (FTIR) spectrometry (TENSOR 27, Bruker Optics, Ettlingen, Germany) coupled with thermogravimetric analysis was used to investigate volatile decomposition products. The coupled measurements were run with the same parameters as the thermal analysis, except that a sample weight of 5 mg was used. The entire purge-gas flow was transferred to the spectrometer’s gas cell. The transfer line and the gas cell were maintained at 270°C during the measurements. Such a high temperature was necessary to successfully detect diethylphosphinic acid and vaporised AlPi-Et. These compounds would not be able to reach the gas cell if a temperature of only 230°C was used. FTIR spectra were recorded with an optical resolution of 4 cm-1 and in the spectral range of 600–4000 cm-1. The spectra were evaluated with regard to characteristic absorption bands. Reference spectra from databases were used to identify the decomposition products.
A Vertex 70 FTIR spectrometer (Bruker Optics) equipped with an FTIR600 Linkam stage (Linkam Scientific Instruments Ltd., Chilworth, UK) was used to analyse changes in the condensed phase. Sample powder was melted on a KBr disc to prepare thin films. The disc was then fixed by a clamp in the Linkam stage. A nitrogen flow rate of 150 ml/min was purged through the Linkam stage during heating from 35°C to 600°C at a heating rate of 10°C/min. The spectra were recorded in the spectral range of 400–4000 cm-1 using the transmission mode with a resolution of 4 cm-1.
Attenuated total reflection (ATR) IR spectra were recorded on a Tensor 27 FTIR spectrometer (Bruker Optics) in the spectral range of 400–4000 cm-1 with a resolution of 4 cm-1.
2.4 Fire behaviour
The flammability (reaction to a small flame) of the materials was determined by the limiting oxygen index (LOI) according to ISO 4589 (specimen dimensions 150×10×4 mm) and by UL 94 according to IEC 60695-11-10 (specimen dimensions 150×12.9×3.2 mm). Cone calorimeter testing (Fire Testing Technology, East Grinstead, UK) according to ISO 5660 was used to assess the fire behaviour under forced-flaming conditions. Plate-shaped specimens (100×100×3 mm) were placed in aluminium trays and exposed to the irradiation (50 kW/m2) of the cone heater. Each material was tested only twice if the reproducibility was satisfactory (below 10% deviation for all characteristics).
3 Results and discussion
3.1 Morphology of the materials
Both systems, PBT/AlPi-Et and PBT/AlPi-H+RXP, showed a composite structure with particle sizes between a few micrometers and a nanometre scale (Figure 1A and B). The composites differed in terms of the shape and size of the particles. The AlPi-Et particles showed a rod-like shape with a median length of 240 nm. In contrast, the AlPi-H and RXP particles, respectively, had a significantly lower median length of 110 nm and a spherical shape. Nevertheless, the particle dispersion in the two composites was quite similar.
Morphology of the investigated materials: Transmission electron microscopy images of (A) PBT/AlPi-Et6.3 and (B) PBT/AlPi-H+RXP4.2. Scanning electron microscopy images of the fracture surface of (C) PBT/GF/AlPi-Et20 and (D) PBT/GF/AlPi-H+RXP20.
In Figure 1C and D, the morphology of two glass fibre-reinforced composites is presented as scanning electron microscopy images of the fracture surface. The AlPi-Et composites show a strong fibre-matrix adhesion, which can be seen in wetted fibres. In contrast, the glass fibres in the composites with AlPi-H+RXP show relatively poor adhesion to the matrix. Therefore an influence on the mechanical behaviour is expected, perhaps in combination with an influence on melt flow and dripping.
3.2 Pyrolysis: mass loss and residue
PBT decomposed via one decomposition step (Figure 2A and B) at a temperature of maximum mass loss rate (Tmax) of around 400°C, leaving a 7.5 wt.% residue (Table 3). The addition of AlPi-Et slightly decreased the starting temperature of decomposition, whereas Tmax remained unchanged. The materials containing AlPi-Et showed a small second decomposition step (3.7 and 7.5 wt.%, respectively) subsequently to the main decomposition step (Tmax ∼478°C). Adding 20 wt.% AlPi-Et to PBT increased the residue to 12.6 wt.%. The addition of AlPi-H+RXP decreased the starting temperature of decomposition more strongly than that of AlPi-Et. The temperature of the maximum mass loss rate was not changed for PBT/AlPi-H+RXP4.2, and a small “early” decomposition step prior to the main step was indicated. PBT/AlPi-H+RXP20 clearly showed this small decomposition step prior to the main step. The main decomposition step of PBT/AlPi-H+RXP20, i.e., the temperature of the maximum mass loss rate, shifted to temperatures lower than for PBT and the residue increased to 18.8 wt.%. Thus PBT/AlPi-H+RXP20 produced more residue than PBT/AlPi-Et20. Nevertheless, the amount of residue corresponds to the calculated residue, assuming a superposition of the contributions of the individual components. Thus there is no evidence for any interaction between PBT and AlPi-H+RXP that increases charring. The starting temperature of decomposition was 8°C lower than expected from superposition of the individual components of PBT/AlPi-H+RXP. The temperatures of the maximum mass loss rates of both decomposition steps were 12°C lower than expected from a superposition. Thus AlPi-H+RXP slightly destabilised PBT.
(A) Mass loss and (B) mass loss rate of the bulk materials and (C) gas-phase FTIR spectra of PBT/AlPi-H+RXP20 taken at curve a (30 min, 329°C) and at curve b (36.5 min, 394°C).
Thermal decomposition characteristics of the investigated bulk materials.
| Material | T(2%) (°C) | Tmax early step (°C) | Mass loss early step (wt.%) | Tmax main step (°C) | Mass loss main step (wt.%) | Tmax second step (°C) | Mass loss second step (wt.%) | Residue at 800°C (wt.%) |
|---|---|---|---|---|---|---|---|---|
| PBT | 361 | – | – | 399 | 92.4 | – | – | 7.5 |
| PBT/AlPi-Et6.3 | 357 | – | – | 401 | 88.7 | 473 | 3.7 | 7.0 |
| PBT/AlPi-Et20 | 353 | – | – | 402 | 79.6 | 483 | 7.5 | 12.6 |
| PBT/AlPi-H+RXP4.2 | 353 | a | a | 401 | 91.8 | – | – | 7.7 |
| PBT/AlPi-H+RXP20 | 323 | 322 | 4.5 | 392 | 76.3 | – | – | 18.8 |
Error±2°C and ±1.0 wt.%. aA small decomposition step prior to the main step was indicated, but it was not significant and thus was not considered.
3.3 Pyrolysis: volatile decomposition products
PBT released butadiene, tetrahydrofuran (THF), benzoic acid, butyl benzoate, CO, and CO2 as volatile decomposition products during the maximum of decomposition. These results are in agreement with the literature results (19). The main decomposition step of PBT/AlPi-Et was characterised by the release of the main decomposition products of PBT. Additionally, the release of diethyl phosphinic acid (3649, 854, and 770 cm-1) was detected. A comparison with the ATR-IR spectra of pure AlPi-Et revealed that a part of AlPi-Et itself (1457, 1410, 1136, 1062, and 770 cm-1) vaporised. The second decomposition step was characterised by the release of butyl benzoate, benzoic acid, and diethyl phosphinic acid, as well as by the vaporisation of AlPi-Et. These results are in agreement with the work of Braun et al. (10).
PBT/AlPi-H+RXP released phosphine (2410, 2320, 2249, 1121, 1065, and 991 cm-1) at about 322°C during a small decomposition step prior to the main step (Figure 2C, curve a). Phosphinate or phosphinic acid, respectively, disproportionates to phosphine or phosphonic acid at elevated temperatures. The phosphonic acid disproportionates further to phosphine and phosphoric acid (20, 21). The release of butadiene (3107, 3084, 2996, 1832, 1809, 1605, 1584, 1013, and 909 cm-1), CO (2183 and 2100 cm-1), CO2 (2350, 2308, and 669 cm-1), benzoic acid (3583, 1761, 1182, and 1083 cm-1), butyl benzoate (2968, 1742, 1265, and 1101 cm-1), and THF (2980, 2872, and 1083 cm-1) was detected during the main decomposition step of PBT/AlPi-H+RXP (Figure 2C, curve b). Thus the main decomposition step was characterised by the decomposition of the PBT fraction in PBT/AlPi-H+RXP.
The release of volatile phosphorus compounds by AlPi-Et and AlPi-H+RXP indicates the potential of both flame retardants for flame inhibition in the gas phase.
3.4 Pyrolysis: changes in the condensed phase
The typical absorption bands of PBT’s condensed phase (22), i.e., 2963 cm-1 (aliphatic CH2), 1713 cm-1 (C=O), 1267 cm-1 (C-O ester), 1104 cm-1 (O-CH2), and 728 cm-1 (CH aromatic ring), decreased as decomposition proceeded. The appearance and increase of a band at 1557 cm-1 was attributed to the formation of polyaromatic char structures (23). The formation of terephthalic acid, which was found to condense on the Linkam cells’ window, was indicated by a band at 1690 cm-1. PBT/AlPi-Et formed polyaromatic char structures, as well as terephthalic acid structures. Additionally, phosphorus-related bands at 1371 and 1277 cm-1 (P=Osym, asym) remained after decomposition.
Two bands of AlPi-H (1165 and 1071 cm-1, PO2-) overlapped with the O–CH2-vibration (1103 cm-1) of PBT in PBT/AlPi-H+RXP. The condensed-phase analysis of PBT/AlPi-H+RXP (Figure 3A and B) showed a decrease in the absorption bands associated with P-H bonds (stretching vibration: 2408 and 2385 cm-1; deformation vibration: 827 cm-1). This corresponds to the decomposition of the phosphinate. The bands associated with PBT, e.g., aliphaic C-H bonds (2963 cm-1) and ester bonds (C=O 1714 cm-1, C-O 1269 cm-1), decreased when PBT decomposed. The formation of terephthalic acid was indicated by a band at 1690 cm-1. A broad absorption band at 1094 cm-1, which was attributed to phosphorus, did not decrease as much as the bands of PBT did. This broad absorption band dominated the condensed-phase FTIR spectrum of PBT/AlPi-H+RXP at 600°C (Figure 3C, curve a). The small bands at 1692, 1575, 1508, 1423, 1271, 1020, 885, and 731 cm-1 originated from terephthalic acid, which condensed on the Linkam cell’s window. The broad absorptions centred at 1094 and 400 cm-1 were attributed to aluminium phosphate. The ATR-IR spectrum of the residue, which was obtained from a thermogravimetry experiment with pure AlPi-H+RXP (Figure 3C, curve b), was very similar to the FTIR spectrum of the low cristobalite form of aluminium phosphate reported in the literature (24). This proves the formation of aluminium phosphate from AlPi-H+RXP.
(A) Condensed-phase FTIR spectra of PBT/AlPi-H+RXP20 taken at curve a (0 min, 30°C), curve b (30 min, 329°C), curve c (33 min, 357°C), and curve d (34 min, 366°C); (B) condensed-phase FTIR spectra of PBT/AlPi-H+RXP20 taken at curve e (35 min, 379°C), curve f (40 min, 428°C), curve g (43 min, 456°C), curve h (45 min, 478°C), and curve i (50 min, 528°C); and (C) condensed-phase FTIR spectrum of PBT/AlPi-H+RXP20 taken at curve a (60 min, i.e., 2.5 min after the Linkam cell reached 600°C) and at curve b: ATR-IR spectrum of the residue of pure AlPi-H+RXP obtained from a thermogravimetry experiment.
3.5 Pyrolysis: decomposition pathway
The decomposition mechanism of PBT is well known (19). Chain scission starts via a six-membered transition state, leading to the formation of butadiene and terephthalic acid, which further decompose to benzoic acid and CO2. Alternatively, PBT’s decomposition starts with the hydrolysis of the ester bond leading to the formation of terephthalic acid and 1,4-butanediol. The 1,4-butanediol eliminates either one or two molecules of water to form THF or butadiene, respectively. PBT/AlPi-Et released the same decomposition products as PBT. Thus the addition of AlPi-Et may have changed the decomposition mechanism of PBT to only a limited extent. Some of the AlPi-Et vaporised and some decomposed, releasing diethyl phosphinic acid. The formation of mixed aluminium phosphinate carboxylate as intermediate products in the condensed phase was proposed for PBT/AlPi-Et (10, 25). With the release of ethene, benzoic acid, CO, and CO2, the mixed salts decompose to aluminium phosphates during the second small decomposition step.
The decomposition of PBT/AlPi-H+RXP (Scheme 1) started with the decomposition of aluminium phosphinate to phosphine and aluminium phosphate. AlPi-H decomposed prior to the main step. It slightly destabilised PBT due to phosphorus acids that formed upon the decomposition of AlPi-H, catalysing the hydrolytic scission of PBT’s ester bond. The change in the decomposition of PBT by the addition of AlPi-H+RXP was limited to this slight destabilisation, as the same decomposition products as for pure PBT were detected during the main decomposition step of PBT/AlPi-H+RXP.
Decomposition pathway of PBT with aluminium phosphinate (AlPi-H). Products in frames were proved in the gas phase.
3.6 Fire behaviour of the bulk materials: flammability
PBT is a flammable polymer with an LOI of 23.3% and an HB classification in the UL 94 test (Table 4). The LOI of PBT was increased strongly by adding AlPi-Et, whereas the addition of AlPi-H+RXP yielded only a slight increase in LOI. The UL 94 classification of PBT was improved to V-2 with 6.3 wt.% AlPi-Et and to V-0 with 20 wt.% AlPi-Et, respectively. Adding 4.2 wt.% AlPi-H+RXP to PBT improved the UL 94 classification to V-2. Increasing the amount of AlPi-H+RXP to 20 wt.% yielded no further improvement in the UL 94 rating because flaming dripping still occurred. The formation of a flow limit was reported for PBT/AlPi-Et20 but not for PBT/AlPi-H+RXP20 (26), which explains the different UL 94 ratings of these two materials. The time to ignition was decreased by both flame retardants. The decrease was stronger with 20 wt.% AlPi-H+RXP than with 20 wt.% AlPi-Et. This is consistent with the earlier decomposition of PBT/AlPi-H+RXP in the thermal analysis.
Flammability (reaction to a small flame) and ignitability of the bulk materials determined by LOI (error=±1.0%), UL 94, and time to ignition (tign) in the cone calorimeter with an irradiation of 50 kW/m2.
| Material | P content (wt.%) | LOI (%) | UL 94 | tign (s) |
|---|---|---|---|---|
| PBT | 0 | 23.3 | HB | 54±1 |
| PBT/AlPi-Et6.3 | 1.5 | 27.5 | V-2 | 43±8 |
| PBT/AlPi-Et20 | 4.8 | 52.0 | V-0 | 44±9 |
| PBT/AlPi-H+RXP4.2 | 1.5 | 23.2 | V-2 | 48±1 |
| PBT/AlPi-H+RXP20 | 7.1 | 26.8 | V-2 | 32±2 |
The error of tign was based on the maximum deviation of measured values from the average value. Some specimens deformed strongly upon exposure to the cone heater, thus causing a high deviation in tign.
3.7 Fire behaviour of the bulk materials: forced-flaming combustion
The heat release rate curves (Figure 4A and B) show a similar decrease in peak heat release rate (pHRR) and total heat evolved (THE; i.e., total heat released taken at flameout) of PBT/AlPi-Et6.3 and PBT/AlPi-H+RXP4.2. These two materials have the same phosphorus content of 1.5 wt.%. PBT/AlPi-Et20 showed a strongly decreased pHRR and an HRR curve shape that is typical for non-charring materials (27). This is because charring and intumescence occurred by the time of pHRR (122 s) and thus could hardly influence the HRR. Nevertheless, a protective effect of the intumescent char (Figure 4C) was expected. PBT/AlPi-H+RXP20 showed a strong decrease in pHRR and an HRR curve without a pronounced peak at the end. This curve shape corresponds to a charring material (27). The HRR rose until an efficient compact char layer was formed (Figure 4D and E). Then the HRR remained constant. One side of the char layer curled upward during the cone calorimeter test (Figure 4D). Thus the efficiency of the char’s barrier effect was reduced. The HRR rose slightly and formed a broad peak at 100 s.
(A) Heat release rate and (B) total heat release of the bulk materials during the cone calorimeter test with an irradiation of 50 kW/m2. Fire residues from (C) PBT/AlPi-Et20 and (D) PBT/AlPi-H+RXP20 obtained at 50 kW/m2. (E) Detailed photo shows the upper and the lower sides of the PBT/AlPi-H+RXP20 residue.
Dependence of (A) pHRR, (B) THE, (C) THE/TML, (D) residue on the phosphorus content of the bulk materials. Fire risk in terms of (E) fire load (THE) vs. pHRR/tign and (F) THE vs. FIGRA of the bulk materials.
Both flame retardants decreased pHRR and THE as increasing amounts of flame retardant were added (Table 5). The decrease in pHRR was stronger with AlPi-H+RXP. In the comparison of materials with the same phosphorus content (1.5 wt.% P), the decrease in THE was stronger with AlPi-Et than with AlPi-H+RXP. With the materials with 20 wt.% of flame retardant, the reduction in THE was stronger with AlPi-H+RXP than with AlPi-Et. The amount of residue was increased by both flame retardants as increasing amounts were added. The increase in residue was stronger with AlPi-H+RXP. Hence, the condensed-phase activity of AlPi-H+RXP was stronger. Both flame retardants released compounds containing phosphorus into the gas phase, where they acted as flame inhibitors, decreasing the effective heat of combustion (THE/TML). The decrease in THE/TML was similarly strong for AlPi-Et and AlPi-H+RXP. Hence, the two flame retardants showed similarly strong gas-phase activity. Flame inhibition led to incomplete combustion. The increase in total smoke release (TSR) and CO yield was due to an increased amount of incomplete combustion products and thus supports flame inhibition as one of the main flame-retardancy mechanisms active in PBT/AlPi-Et and PBT/AlPi-H+RXP.
Cone calorimeter results of the bulk materials obtained using an irradiation of 50 kW/m2 including peak heat release rate (pHRR), total heat evolved (THE), total mass loss (TML), and total smoke release (TSR).
| Material | P content (wt.%) | pHRR (kW/m2) | THE (MJ/m2) | Residue (wt.%) | THE/TML (MJ/m2 g) | TSR (m2/m2) | CO yield (kg/kg) |
|---|---|---|---|---|---|---|---|
| PBT | 0 | 1812±100 | 76±1 | 4±1 | 2.1±0.1 | 1525±100 | 0.06±0.01 |
| PBT/AlPi-Et6.3 | 1.5 | 1412±100 | 60±1 | 5±1 | 1.7±0.1 | 2488±100 | 0.10±0.02 |
| PBT/AlPi-Et20 | 4.8 | 657±70 | 45±1 | 11±1 | 1.3±0.1 | 3486±100 | 0.16±0.02 |
| PBT/AlPi-H+RXP4.2 | 1.5 | 1188±100 | 66±1 | 7±1 | 1.9±0.1 | 2360±100 | 0.09±0.01 |
| PBT/AlPi-H+RXP20 | 7.1 | 400±40 | 41±1 | 20±1 | 1.3±0.1 | 3333±100 | 0.16±0.01 |
The errors were based on the maximum deviation of measured values from the average value.
3.8 Fire behaviour of the bulk materials: quantitative assessment of flame-retardancy mechanisms
Gas-phase activity and charring were quantified by the decrease in the effective heat of combustion and the increase in the amount of residue, respectively. The residue was increased from 4 wt.% (PBT) to 11 wt.% (PBT/AlPi-Et20). Hence, the amount of fuel released by PBT/AlPi-Et20 was reduced to 93% compared to PBT. The THE/TML of PBT/AlPi-Et20 was reduced to 62% due to flame inhibition. Combining reduced fuel release and flame inhibition yielded a reduction to 58% (0.93×0.62=0.58), which corresponds well with the reduction in THE (59%). Thus the reduced THE of PBT/AlPi-Et20 in comparison to PBT is explained well by flame inhibition and reduced fuel release. The pHRR was decreased to 36%. In the first approximation, a reduction in pHRR to 58% was due to gas-phase activity and increased charring as discussed for THE. The protective effect of the intumescent char caused a further reduction in pHRR from 58% to 36%, i.e., a relative reduction of 38%.
Analogous considerations were made concerning PBT/AlPi-H+RXP20. The increase in residue to 20% corresponded to a reduction in fuel release to 83%. THE/TML was decreased to 62% due to flame inhibition. The combination of flame inhibition and reduced fuel (0.62×0.83=0.51) offers a good explanation for the reduction in THE (54%). The pHRR of PBT/AlPi-H+RXP was reduced to 22%. In the first approximation, a reduction in pHRR to 51% was explained by flame inhibition and reduced fuel. The additional decrease in pHRR from 51% to 22% corresponded to a relative reduction of 57% and was due to the protective effect of the compact char layer.
Three different flame retardancy mechanisms are active in PBT/AlPi-Et20 and PBT/AlPi-H+RXP20. Condensed-phase activity (charring) is strong in PBT/AlPi-H+RXP20 (20 wt.% residue), whereas it plays a minor role in PBT/AlPi-Et20. Gas-phase activity (flame inhibition) was equally strong (38%) in both materials. The protective effect of the char was strong (38%) for the intumescent residue of PBT/AlPi-Et20 and even stronger (57%) for the compact char layer of PBT/AlPi-H+RXP20. The three different flame-retardancy mechanisms influenced the different fire properties. The protective effect of the char decreased pHRR, whereas flame inhibition and charring decreased pHRR and THE.
3.9 Fire behaviour of the bulk materials: assessment of efficiency
The dependence of burning parameters on the phosphorus content of the materials was analysed to compare the efficiency of AlPi-Et and AlPi-H+RXP (Figure 5A–D). PBT/AlPi-Et showed a decrease in pHRR that was linearly dependent on phosphorus content (Figure 5A). The pHRR of PBT/AlPi-H+RXP decreased non-linearly with rising phosphorus content and a levelling-off was indicated for high phosphorus concentrations. For concentrations of up to 5 wt.% phosphorus, AlPi-H+RXP was slightly more efficient at decreasing pHRR than was AlPi-Et. The reduction in THE showed a linear dependence on phosphorus content for the materials containing AlPi-Et, whereas the dependence was non-linear for the materials with AlPi-H+RXP. AlPi-Et was more efficient than AlPi-H+RXP in reducing THE for phosphorus concentrations up to about 6 wt.%. THE/TML decreased (i.e., gas-phase activity increased) non-linearly with rising phosphorus content for PBT/AlPi-Et. A linear dependence of the reduction in THE/TML on phosphorus content was found for the materials containing AlPi-H+RXP. AlPi-Et showed a higher efficiency in decreasing THE/TML than AlPi-H+RXP, i.e., the gas-phase activity of AlPi-Et per unit of phosphorus was stronger than that of AlPi-H+RXP. The increase in the amount of residue was linearly dependent on phosphorus content for both flame retardants. AlPi-H+RXP was more efficient in promoting charring than AlPi-Et, i.e., the condensed-phase activity of AlPi-H+RXP per unit of phosphorus was stronger than that of AlPi-Et.
Fire load and flame spread are two of the main fire risks. Fire load was directly measured as THE, but there is no way to measure flame spread directly in the cone calorimeter test. That is why indices like pHRR/tign and FIGRA [fire growth rate=maximum ratio HRR(t)/t] were proposed to assess flame spread or fire growth, respectively (27). It was additionally proposed to assess both the main fire risks at the same time by plotting THE over pHRR/tign (28). Then, ideal flame retardancy corresponds to a shift towards the origin of this plot.
The decrease in PBT’s two main fire risks by AlPi-Et and AlPi-H+RXP, respectively, is assessed in Figure 5E and F. If flame spread was assessed by pHRR/tign, PBT/AlPi-H+RXP4.2 showed a decrease in fire load and flame spread. PBT/AlPi-Et6.3 exhibited a stronger reduction in fire load than did PBT/AlPi-H+RXP4.2, but no reduction in flame spread was observed. Increasing the additives’ content to 20 wt.% led to a strong decrease in fire load and flame spread for both flame retardants. The reduction of both fire risks was slightly stronger for PBT/AlPi-H+RXP20 than for PBT/AlPi-Et20 due to the stronger condensed-phase activity (charring and protective effect of the char) of PBT/AlPi-H+RXP20. With the THE vs. FIGRA plot, AlPi-Et and AlPi-H+RXP were assessed as ideal flame retardants because they decreased both fire risks with rising amounts of flame retardant added. At a phosphorus content of 1.5 wt.%, AlPi-Et decreased fire load more strongly than AlPi-H+RXP, whereas AlPi-H+RXP reduced fire growth rate slightly more than AlPi-Et did. At 20 wt.% loading of the flame retardants, fire load and fire growth rate were reduced further. AlPi-Et decreased the fire growth rate more than AlPi-H+RXP did. AlPi-H+RXP performed better at reducing fire load than AlPi-Et because of the stronger condensed-phase activity (charring and protective effect of the char) of PBT/AlPi-H+RXP20.
3.10 Fire behaviour of the composites: flammability
The addition of glass fibres increased the flammability of PBT, as demonstrated by the lowered LOI (19.8% instead of 23.3% for pure PBT). Glass fibres suppress melt flow and dripping (29). This effect was also reported for carbon fibres and multi-walled carbon nanotubes (30, 31). Hence, heat is no longer taken away from the pyrolysis zone by melt flow. In addition, a “wick effect” of the glass fibres and improved heat transmission are considered as reasons for the increased flammability of PBT/GF.
Adding AlPi-Et to PBT/GF increased LOI, but only until a phosphorus concentration of 3.3 wt.% was reached (Table 6). The use of phosphorus concentrations higher than 3.3 wt.% led to a decrease in LOI. The dependence of LOI on phosphorus content showed an optimum at about 3 wt.% phosphorus (Figure 6). A levelling-off or even a decrease in flame-retardancy performance for high phosphorus concentrations has been reported before and ascribed to the inherent flammability of phosphorus (32). The addition of AlPi-H+RXP to PBT/GF increased LOI, but this increase was less strong than with AlPi-Et. The dependence of LOI on phosphorus content was linear for AlPi-H+RXP. No levelling-off or decrease for high concentrations was indicated. The UL 94 rating of PBT/GF was improved to V-0 by adding 14 wt.% AlPi-Et, 20 wt.% AlPi-Et, or 20 wt.% AlPi-H+RXP, respectively. The addition of AlPi-Et to PBT/GF did not affect the tign in the cone calorimeter test, whereas tign decreased with rising amounts of AlPi-H+RXP added.
Dependence of LOI on the phosphorus content of the composite materials.
Flammability (reaction to a small flame) and ignitability of the composite materials determined by LOI (error=±1.0%), UL 94, and time to ignition (tign) in the cone calorimeter with an irradiation of 50 kW/m2.
| Material | P content (wt.%) | LOI (%) | UL 94 | tign (s) |
|---|---|---|---|---|
| PBT/GF | 0 | 19.8 | HB | 40±4 |
| PBT/GF/AlPi-Et4.4 | 1.0 | 33.9 | HB | 43±6 |
| PBT/GF/AlPi-Et6.3 | 1.5 | 39.0 | HB | 43±1 |
| PBT/GF/AlPi-Et14 | 3.3 | 44.3 | V-0 | 40±5 |
| PBT/GF/AlPi-Et20 | 4.8 | 36.7 | V-0 | 44±1 |
| PBT/GF/AlPi-H+RXP4.2 | 1.5 | 21.7 | HB | 33±1 |
| PBT/GF/AlPi-H+RXP20 | 7.1 | 31.9 | V-0 | 22±3 |
The error of tign was based on the maximum deviation of measured values from the average value. Some specimens deformed upon exposure to the cone heater, thus causing a high deviation in tign.
3.11 Fire behaviour of the composites: forced-flaming combustion
The HRR of PBT/GF showed an initial increase in HRR, forming a pHRR quite at the beginning of burning (Figure 7). Then, as the level of the burning melt decreased below the upper layer of glass fibres, this upper layer with some char on it (as it appeared black) formed a protective layer at the surface. When more and more material was consumed, the layer thickened and HRR decreased. The shape of HRR changed upon addition of AlPi-Et. The pHRR at the beginning decreased. The formed glass fibre/char layer was more compact, with rising amounts of AlPi-Et added, because there was more char “gluing” the glass fibres together (Figure 7). PBT/GF/AlPi-Et materials showed a second small pHRR at the end due to thermal feedback from the insulation on the sample holder. As no cracking of the char was observed, this was ruled out as a reason for the second pHRR (27). The addition of 4.2 wt.% AlPi-H+RXP decreased the HRR compared to PBT/GF (Figure 7). The HRR curve of PBT/GF/AlPi-H+RXP4.2 was characterised by a peak of HRR at the beginning. As soon as an efficient glass fibre/char layer was formed, HRR decreased and then remained constant until the end, when it exhibited a small second peak. The HRR of PBT/GF/AlPi-H+RXP20 showed only a small peak at the beginning. Then, the HRR remained constant due to the protective effect of the very compact, thick char layer (Figure 7). Similar heat release characteristics were reported for glass fibre-reinforced polyamide flame retarded with red phosphorus (33).
Heat release rate of the composite materials during the cone calorimeter test with an irradiation of 50 kW/m2, and fire residues from PBT/GF (bottom left), PBT/GF/AlPi-Et20 (bottom centre), and PBT/AlPi-H+RXP20 (bottom right) obtained at 50 kW/m2.
The first pHRR of PBT/GF was decreased by both flame retardants (Table 7). At a phosphorus content of 1.5 wt.%, the reduction in the first pHRR was similar for AlPi-Et and AlPi-H+RXP in PBT/GF. With a flame retardant content of 20 wt.% in PBT/GF, AlPi-H+RXP reduced the first pHRR more strongly than AlPi-Et did. The second pHRR was decreased only slightly by AlPi-Et, whereas a clear reduction was observed for AlPi-H+RXP. PBT/GF/AlPi-Et showed a decrease in THE with rising AlPi-Et content of up to 14 wt.%. Then THE increased for PBT/GF/AlPi-Et20. AlPi-H+RXP reduced the THE of PBT/GF, with increasing amounts of AlPi-H+RXP added. The residue of PBT/GF (30 wt.%) was due mainly to glass fibres. AlPi-Et increased the residue slightly, with rising amounts of flame retardant added. AlPi-H+RXP increased the residue strongly, with increasing amounts of flame retardant added. Hence, AlPi-H+RXP had stronger condensed-phase activity in PBT/GF than did AlPi-Et. The effective heat of combustion (THE/TML) decreased with rising AlPi-Et content until a content of 14 wt.% was reached. The decrease up to this point corresponded to an increase in gas-phase activity or flame inhibition, respectively. Flame inhibition decreased for AlPi-Et contents higher than 14 wt.%, i.e., for PBT/GF/AlPi-Et20. The addition of 4.2 wt.% AlPi-H+RXP to PBT/GF decreased THE/TML compared to PBT/GF. Thus AlPi-H+RXP showed flame inhibition as well. Increasing the content of AlPi-H+RXP to 20 wt.% decreased flame inhibition. This suggests that 20 wt.% AlPi-H+RXP was already beyond the optimum concentration for PBT/GF.
Cone calorimeter results of the composite materials obtained using an irradiation of 50 kW/m2 including pHRR, THE, TML, and TSR.
| Material | P content (wt.%) | First pHRR (kW/m2) (tpHRR in s) | Second pHRR (kW/m2) (tpHRR in s) | THE (MJ/m2) | Residue (wt.%) | THE/TML (MJ/m2 g) |
|---|---|---|---|---|---|---|
| PBT/GF | 0 | 677±40 (70±2) | 457±40 (133±5) | 59±1 | 30±1 | 1.9±0.1 |
| PBT/GF/AlPi-Et4.4 | 1.0 | 438±40 (72±3) | 442±40 (106±11) | 41±1 | 33±1 | 1.4±0.1 |
| PBT/GF/AlPi-Et6.3 | 1.5 | 367±30 (71±2) | 362±30 (114±12) | 39±1 | 31±1 | 1.3±0.1 |
| PBT/GF/AlPi-Et14 | 3.3 | 351±30 (69±3) | 402±30 (121±4) | 37±1 | 37±1 | 1.3±0.1 |
| PBT/GF/AlPi-Et20 | 4.8 | 310±30 (73±7) | 356±40 (113±10) | 40±1 | 40±1 | 1.5±0.1 |
| PBT/GF/AlPi-H+RXP4.2 | 1.5 | 378±30 (45±1) | 253±20 (141±9) | 44±1 | 34±1 | 1.5±0.1 |
| PBT/GF/AlPi-H+RXP20 | 7.1 | 165±15 (30±3) | 169±15 (95±10) | 35±1 | 55±1 | 1.7±0.1 |
The errors were based on the maximum deviation of measured values from the average value.
3.12 Fire behaviour of the composites: quantitative assessment of flame- retardancy mechanisms
The increased residue of PBT/GF/AlPi-Et20 (40 wt.% instead of 30 wt.%) corresponds to a reduction in fuel release to 86%. THE/TML was reduced to 79% due to flame inhibition. The combination of reduced fuel and flame inhibition led to a reduction to 68%, which explains the reduction in THE (68%) well. The pHRR of PBT/GF/AlPi-Et20 was reduced to 46%. In the first approximation, a reduction to 68% was due to reduced fuel release and flame inhibition. The further reduction to 46% corresponded to a relative reduction of 32% and was due to the protective effect of the char.
Analogous considerations were made for PBT/GF/AlPi-H+RXP20. The increased residue (55 wt.%) corresponded to a reduction in fuel release to 64%. THE/TML was decreased to 89% due to flame inhibition. Combining reduced fuel and flame inhibition yielded a reduction to 57%, which explains the reduction in THE (59%) as well. The pHRR of PBT/GF/AlPi-H+RXP20 was reduced to 24%. In the first approximation, a reduction in pHRR to 57% was due to reduced fuel and flame inhibition. The additional reduction to 24%, i.e., a relative reduction of 58%, was due to the protective effect of the char.
The three different flame-retardancy mechanisms that were active in the compact materials were also active in the glass-fibre composites PBT/GF/AlPi-Et20 and PBT/GF/AlPi-H+RXP20. Condensed-phase activity (charring) was strong in PBT/GF/AlPi-H+RXP20 (36% reduced fuel), whereas it played a minor role in PBT/GF/AlPi-Et20 (14% reduced fuel). Gas-phase activity (flame inhibition) was stronger (21%) in PBT/GF/AlPi-Et20 than in PBT/GF/AlPi-H+RXP20 (11%). The protective effect of the char was strong (32%) for the residue of PBT/GF/AlPi-Et20 and even stronger (58%) for the char of PBT/GF/AlPi-H+RXP20. The three different flame-retardancy mechanisms influenced the different fire properties of the glass-fibre composites, as was also observed for the compact materials. The protective effect of the char decreased pHRR, whereas flame inhibition and charring decreased pHRR and THE.
3.13 Fire behaviour of the composites: assessment of efficiency
The decrease in pHRR was non-linearly dependent on phosphorus content for both flame retardants (Figure 8A). The reduction in pHRR of PBT/GF/AlPi-Et levelled off at a lower phosphorus content than did the decrease in pHRR of PBT/GF/AlPi-H+RXP. AlPi-H+RXP was more efficient than AlPi-Et in decreasing the pHRR of PBT/GF for phosphorus concentrations higher than 2 wt.%, and the maximum possible decrease in pHRR was higher for AlPi-H+RXP than for AlPi-Et. THE decreased non-linearly with rising phosphorus content for both flame retardants (Figure 8B). It showed an optimum at about 3 wt.% phosphorus for PBT/GF/AlPi-Et, whereas levelling-off was observed for high phosphorus concentration in PBT/GF/AlPi-H+RXP. Up to a phosphorus content of 3 wt.%, AlPi-Et was more efficient than AlPi-H+RXP in reducing the THE of PBT/GF. The THE/TML of PBT/GF/AlPi-Et showed a non-linear dependence on phosphorus content with an optimum at 3 wt.% phosphorus (Figure 8C), as was already observed for THE and LOI. The dependence of the THE/TML of PBT/GF/AlPi-H+RXP on phosphorus content was non-linear. An optimum was indicated, but two different phosphorus concentrations were not enough to determine its occurrence with any certainty. The amount of residue increased linearly with rising phosphorus content for both flame retardants. AlPi-H+RXP was more efficient than AlPi-Et in increasing the residue of PBT/GF (Figure 8D).
Dependence of the (A) first pHRR, (B) THE, (C) THE/TML, and (D) residue on the phosphorus content of the investigated composite materials. Fire risk in terms of (E) fire load (THE) vs. first pHRR/tign and (F) THE vs. FIGRA of the composite materials.
The first pHRR was used to assess flame spread and fire growth rate, respectively. As the first pHRR was higher than the second pHRR and occurred earlier, the first pHRR determined flame spread (pHRR/tign) and fire growth rate [maximum ratio HRR(t)/t].
In the assessment of flame spread by pHRR/tign, AlPi-Et decreased fire load and flame spread (Figure 8F). PBT/GF with 6.3 wt.% and 14 wt.% AlPi-Et, respectively, showed a similar performance. Increasing the amount of AlPi-Et to 20 wt.% increased fire load slightly but further reduced flame spread. AlPi-H+RXP decreased the fire load and flame spread of PBT/GF as rising amounts of additive were added. At a phosphorus content of 1.5 wt.%, AlPi-Et performed much better than AlPi-H+RXP in reducing fire load and flame spread. At a flame-retardant content of 20 wt.%, AlPi-H+RXP performed slightly better than AlPi-Et. As shown in the THE over FIGRA plot (Figure 8E), the addition of 4.4 wt.% AlPi-Et decreased the fire growth rate and fire load of PBT/GF. A further increase in the AlPi-Et content mainly reduced fire growth rate without having much influence on fire load. At an AlPi-Et content of 20 wt.%, fire load was even slightly increased. AlPi-H+RXP decreased fire growth rate and fire load with rising amounts of additive added. At a phosphorus content of 1.5 wt.%, AlPi-Et performed much better than AlPi-H+RXP in reducing fire load and fire growth rate. At a flame-retardant content of 20 wt.%, the overall performance of the two flame retardants was similar. PBT/GF/AlPi-Et20 showed a lower fire growth rate than PBT/GF/AlPi-H+RXP20, whereas the latter material showed a lower fire load than PBT/GF/AlPi-Et20.
4 Conclusion
The pyrolysis and fire behaviour of organic aluminium diethylphosphinate and inorganic aluminium phosphinate was investigated in PBT and PBT/GF. A detailed comparison of the two phosphinates was reported. The results of the pyrolysis investigation support the decomposition mechanisms proposed for PBT and PBT/AlPi-Et. The AlPi-H in PBT/AlPi-H+RXP decomposed to aluminium phosphate, releasing phosphine at temperatures as low as 320°C. This is quite close to the processing temperatures of PBT. Therefore, the release of toxic phosphine and a corresponding loss of phosphorus are problems that may be encountered during the processing of AlPi-H+RXP. AlPi-Et does not suffer from such problems during processing.
A good performance in the flammability tests LOI and UL 94 is crucial for the application of flame-retarded PBT. At 20 wt.% loading, AlPi-H+RXP achieved only a V-2 rating in bulk PBT, which is not sufficient for most applications. The necessary V-0 rating was achieved with 20 wt.% AlPi-H+RXP in glass fibre-reinforced PBT. In contrast to AlPi-H+RXP, AlPi-Et provided a V-0 rating in bulk PBT at 20 wt.% loading. In glass fibre-reinforced PBT, only 14 wt.% of AlPi-Et was necessary to achieve a V-0 rating. AlPi-H+RXP led to only a moderate increase in the LOI of bulk PBT (up to 26.8%) and reinforced PBT (up to 31.9%). AlPi-Et provided very high LOI values for bulk PBT (up to 52.0%) as well as for glass fibre-reinforced PBT (44.3%). Thus, AlPi-Et performed much better than AlPi-H+RXP in flammability tests.
The cone calorimeter results revealed that both flame retardants acted via three different flame-retardancy mechanisms in bulk as well as glass fibre-reinforced-PBT. The three mechanisms are flame inhibition, increased char, and a protective effect of the char. With the bulk materials, both flame retardants provided equally strong flame inhibition (38%). The increase in charring (17% reduction in fuel release for PBT/AlPi-H+RXP20 instead of 7% reduction in fuel release for PBT/AlPi-Et20) and the protective effect of the char (57% instead of 38%) were stronger for AlPi-H+RXP than for AlPi-Et. With the glass-fibre composites, AlPi-Et showed stronger flame inhibition than AlPi-H+RXP (21% instead of 11%). AlPi-H+RXP provided a stronger increase in charring (36% reduced fuel instead of 14%) and a stronger protective effect of the char (58% instead of 32%) than AlPi-Et.
Based on the cone calorimeter results, the efficiency of the flame retardants was assessed, i.e., the decrease in pHRR, THE, and THE/TML, and the increase in the amount of char per unit of phosphorus. AlPi-Et decreased the THE and THE/TML of the bulk and the composite materials more efficiently than AlPi-H+RXP. Concerning the reduction in pHRR and the increase in charring of the bulk and composite materials, AlPi-H+RXP was more efficient than AlPi-Et. Most interestingly, an optimum loading of AlPi-Et was found for glass fibre-reinforced PBT. The highest LOI value and the strongest decrease in THE and THE/TML was achieved with a loading of 12 wt.% AlPi-Et. The performance was decreased if more or less AlPi-Et was used. Thus it is recommended to use 12 wt.% of AlPi-Et for efficient flame retardancy in glass fibre-reinforced PBT. A synergist can be added to further increase flame-retardancy performance if necessary.
Acknowledgements
The authors thank the German Research Foundation (DFG, SCHA 730/11-1, AL 474/17-1) for its financial support. H. Bahr and P. Klack (BAM) are acknowledged for supporting the fire tests and the evolved gas analysis, respectively.
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