Infl uence of Flame Retardant Additive on Thermal Behaviour and Stability of Fibre-Forming Polyamide 6

This work presents a study of the infl uence of diethyl aluminum phosphinate (EOP) and sodium alumino silicate (ZP) as the novel green fl ame retardant spinning additives on the thermal properties of polyamide 6 (PA6) fi bres. The PA6/additive composite fi laments were prepared at 4 wt% concentration of additives and their mixture by melt spinning. The results show that the additives were physically incorporated into the PA6 fi lament, resulting in an insignifi cant change of the melting temperature. The presence of EOP decreased Tonset and increased Tmax2 compared to pure PA6, which indicates that the degradation process started at lower temperature, whereas the thermo-oxidative stability in the second decomposition step increased. Contrary to EOP, ZP did not cause any noticeable changes in the decomposition temperatures comparing to pure PA6, but signifi cantly increased the fi nal char amount. Both phenomena were also observed when the additives were used in combination. Whereas EOP did not signifi cantly aff ect the mechanical fi lament properties, the incorporation of ZP resulted in the reinforcement of fi bres.


Introduction
Polyamide 6 (PA6) is a widely used polymer which is synthesised by ring-opening polymerisation of caprolactam.PA6 fi bres are tough, possessing high tensile strength, elasticity and lustre.Th e application of PA6 can be found in various sectors, e.g. transportation (automotive industry), home textiles, clothes and construction.However, due to its organic nature, PA6 is fl ammable, which represents its important disadvantage.At this time, no acceptable fl ame-retardant solutions exist for PA6 fi bres mainly due to the processing issues, i.e. melt spinning of the fi bres containing fl ame retardant additives [1][2][3].Flame retardancy of fi bres or fabrics can be achieved by using several approaches: coatings, back coatings and/or fi nishing treatments (reactive or unreactive treatments) are applied onto fabrics (nonwovens, knitted or woven fabrics); materials able to dissipate a signifi cant amount of heat (e.g.metal foil) are layered onto the fabric; in the case of synthetic fi bres, the graft ing of fl ame retardant (FR) molecules on the polymeric chain or the direct incorporation of FR additives during the processing step of the fi bre can be considered [4][5][6][7][8][9][10][11].Th e aim of our study was to produce a PA6 fi lament with increased thermal stability by using the commercially available fl ame-retardant additives based on phosphorus and silica.Th e PA6/additive composite fi laments were produced in a melt spinning process.Th e infl uence of the amount of additives in the composite fi lament on the thermal and mechanical properties of composite fi laments was investigated.

Masterbatch preparation
Th e pellets of PA6/EOP with 80/20 (w/w) composition and PA6/ZP with 90/10 (w/w) composition were used in combination with pure PA6 pellets to obtain diff erent composite formulations.To have a minimal eff ect on the mechanical fi bre properties, the total amount of additives was fi xed to 4 wt%.Th e masterbatch was dried for 5 h at 90 °C prior to use.

Processing of fi bres
Th e pure PA6 and PA6/additive composite fi laments were produced in a melt spinning process using a laboratory melt spinning device (Extrusion System Ltd; Figure 1).Th e spinning temperature was 250 °C in all zones of extruder, spinning pump and spin pack.Th e spinneret had 10 holes with 0.35 mm in diameter.Th e godet velocity was 70 m/min and the winding speed was 350 m/min.Th e sample codes are presented in Table 1.

Scanning electron microscopy (SEM)
SEM images were obtained for all samples using a JEOL JSM 6060 LV scanning electron microscope, operating with a primary electron beam accelerated at 10 kV and with the working distance of 17 mm.Th e samples were coated with a thin layer of gold before the observation to increase the clarity of images.

Optical microscopy
Optical images of samples were made with an optical microscope and Laboratory tool Axio Vision REL Leica Microscopy soft ware was used to made photos.

Fourier-transform infrared spectroscopy (FTIR)
FTIR spectra were obtained on a Spectrum GX I spectrophotometer (Perkin Elmer, Great Britain) equipped with an attenuated total refl ection (ATR) cell with a diamond crystal (n = 2.0).Th e spectra were recorded over the range from 4,000-600 cm -1 at the resolution of 4 cm -1 .

Th ermogravimetric (TG) and diff erential scanning calorimetric (DSC) analyses
Th e TG and DSC analyses were performed using an STA 449c Jupiter instrument (NETZSCH) at the temperatures ranging from ambient temperature to 800 °C at the heating and cooling rates of 10 °C/min in an open alumina pan (sample mass = 1 mg) with the samples under an air atmosphere.Th e DSC analyses were also performed from 0-300 °C in a Mettler Toledo instrument to measure the glass transition temperature and infl uence on the crystallinity of fi bres.Th e heating and cooling rates were 10 °C/min in an open alumina pan (sample mass = 15 mg) with the samples under a nitrogen atmosphere (10 mL/min).Th ree measurements were recorded for each sample and the mean value of the measured quantities was calculated.Mechanical and dynamic mechanical analysis (DMA) Tensile strength and elongation at break were measured on a STATIMAT dynamometer (Textechno).Th e DMA tests were performed on TA equipment DMA Q800 (USA), with a controlled gas-cooling accessory (GCA).Th e samples were heated from 0-220 °C at the constant rate of 2 °C/min.During the heating, the test samples were deformed (oscillated) at constant amplitude (strain) of 10 μm at a frequency of 10 Hz and the dynamic mechanical properties were measured.

Filament analysis
Th e SEM/EDS analysis was used to investigate the eff ect of the applied additives on the morphology of the PA6/additive composite fi laments.As it can be seen in Figure 2 and Table 2, both additives were successfully incorporated into the fi laments.Th e optical microscope photographs that show crosssectional and longitudinal view of fi lament samples (Figure 3) also reveal that the incorporation of additives caused visible dark spots in the fi lament.In contrast to the 4000-1500 cm -1 spectral region, in which the spectra are very similar to each other, the presence of EOP caused the appearance of a slight band at 780 cm -1 , corresponding to the P-O-C stretching, and the presence of ZP gave rise to pronounced changes in the 1100-900 cm-1 spectral region due to the asymmetric stretching vibrations of Si-O-Si.Th ese bands are clearly seen in the spectra of PA6/EOP with 80/20 (w/w) composition and PA6/ZP with 90/10 (w/w) composition.

Th ermal properties
Th e results of the DSC analysis ranging from 0-300 °C are presented in Table 3. Th e heating scans were analysed to determine the melting temperature, T m1 and T m2 , and the cooling scans were analysed to determine the crystallisation temperature, Tc.Th e degree of crystallinity of fi laments was calculated according to Rusu and Rusu [12].Th e results show that both additives did not alter the melting temperature of PA6, which remained 223 ± 1 °C, indicating that the additives did not chemically react with the polymer and that they are only physically incorporated into the polymer.Th e incorporation of EOP shift ed the crystallisation temperature to slightly lower temperature, suggesting that the additive caused crystal defects.Th is phenomenon is accompanied by a lower degree of crystallisation.In contrast to EOP, the incorporation of ZP shift ed the crystallisation temperature to slightly higher temperature, suggesting that the ZP particles can represent the nucleation sites for PA6 and therefore accelerate its crystallisation in the composite material.Th is resulted in the increase of the degree of crystallinity.

Th ermo-oxidative stability
Th e results of the thermogravimetric analysis of studied samples are summarised in   the second decomposition step increased.EOP did not increase the amount of char residue at T max2 and at 800 °C.Th e presence of ZP in the composite did not change the decomposition temperatures, but increased the amounts of char residue in both decomposition steps and at 800 °C.Th is phenomenon is associated with the good thermal stability of silica, which forms a heat barrier in the condensed phase.
A diff erent activity of EOP and ZP was also observed in the case of the PA/2P+2SI sample, where the additives were incorporated in combination.

Mechanical properties
In order to examine the potential changes in the mechanical properties due to the incorporation of additive, tensile analysis was conducted on fi lament samples.In Table 5, the tensile strength and elongation at break are given for pure PA6 and composite fi laments.Th e tenacity of composed fi laments decreased compared to the pure PA6 fi lament.Th e most signifi cant changes were observed in the sample containing 4% of the EOP additive.Similar to tenacity, the elongation of composite fi laments also decreased in comparison to pure PA6 fi laments.Th e highest fall of elongation was observed in the samples containing 4% of the ZP additive.Th e results of the DMA analysis are presented in Figure 6.Th e DMA analysis provides information on the changes at the molecular level, enabling the understanding of the mechanical behaviour of studied samples.Th e storage modulus is oft en associated with the "stiff ness" of a material and to the Young's modulus.Th e loss modulus is oft en related to the modulus.Th e loss modulus is oft en related to the emission of heat due to "internal friction" and is sensitive to diff erent kinds of molecular motions, relaxation processes, transitions, morphology and other structural heterogeneities.A drop in the storage modulus was seen between 50-100 °C (Figure 6a), caused by the increased segmental mobility, which is accompanied by the increase in the loss modulus E'' , with a maximum at approximately 80 °C.Th is behaviour shows that the transition is accompanied by the emission of heat and loss of elastic properties.Th e results reveal that EOP and the mixture of EOP+ZP did not significantly infl uence the shape of the storage modulus curve in the 50-200 °C temperature region.An exception is the sample PA/4SI for which a significantly lower decrease in the storage modulus was detected in the measured temperature range compared to other samples.Whereas this phenomenon was accompanied by the shift of the peak in the loss modulus (Figure 6b) to higher temperature and lower heat release, it can be concluded that the incorporation of 4% ZP into the fi lament resulted in the reinforcement of fi laments.

Conclusion
Th e PA6 fi lament fi bres containing fl ame retardant additives were successfully prepared with the melt spinning process.Th e incorporation of additives did not alter the melting temperature of PA6.Th e presence of EOP slightly decreased the crystallisation temperature, suggesting that the additive caused crystal defects.Th is phenomenon is accompanied by a lower degree of crystallisation.In contrast to EOP, the presence of ZP slightly increased the crystallisation temperature, suggesting that the ZP particles accelerate the crystallisation of PA6 in the composite material.Th is resulted in the increase of the degree of crystallinity.Th e incorporation of EOP in fi bres decreased T onset and increased T max2 compared to pure PA6, indicating that the thermo-oxidative stability of the composite material decreased at the beginning of the degradation but increased in the second decomposition step.EOP did not aff ect the amount of char residue at 800 °C.Th e presence of ZP in the composite did not change the decomposition temperatures, but increased the amounts of char residue in both decomposition steps.Diff erent mechanisms of the EOP and ZP activity were also expressed in the case of the sample PA/2P+2SI, where they were present in combination.

Figure 1 :
Figure 1: Melt spinning device and scheme of spinning process (left ) and fi nal bobbins (right)

Figure 5 :
Figure 5: (a) TG and (b) dTG for fi lament samples analysed in air atmosphere

Table 2 :
Elemental surface composition of fi lament samples.Results were obtained by EDS analysis

Table 3 :
Characteristic crystallisation values of pure PA6 and PA6/additive composite fi laments

Table 4 :
TG data for untreated and treated fi lament samples analysed in air atmosphere a)temperature of thermal degradation onset; b)temperature of the fi rst degradation step peak; c) temperature of the second degradation step peak

Table 5 :
Mechanical properties of fi lament samples