Development of flame retardant synthetic fibres using novel technologies

Sitpalan, Ahilan (2014) Development of flame retardant synthetic fibres using novel technologies. PhD thesis, University of Bolton.

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Abstract

The dispersion of clays at the nanometer level is known to induce a significant improvement in mechanical properties, flame resistance and barrier properties, compared with pure polymer. Application of ultrasound in polymer processing can be used to improve additive dispersion in polymer melts and solutions but may also initiate chemical reactions and modify the rheological and mechanical properties. This thesis studies the effects of applying ultrasound to the molten fibre-forming polymers, polypropylene (PP) and polyamide 6 (PA6) containing nanoclay and flame retardant additives in order to assess whether improved dispersion generates improved flame retardant properties of derived fabrics. Initially, ultrasound was applied to polypropylene during compounding with up to 5 wt% of ammonium polyphosphate (APP) as flame retardant and 2 wt% of Nanomer 1.3T (Nanocor Inc) as nanoclay. Two ultrasonic probes were used having a power of 50 W at 10, 30, 50 and 100% amplitude and 100W at amplitudes of 20 and 90%. For polyamide 6, the additive selected as flame retardant was aluminium phosphinate (Al-Phos) (Exolit OP 935, Clariant) and the nanoclays, Nanomer 1.3T and Cloisite 25A (Southern Clay Products, USA). Compounded polymer chips of the various formulations were extruded into filaments and tapes by using the Labline extruder and filaments were knitted into fabrics, where possible. Nanoclay dispersion was studied by optical imaging, scanning electron microscopic and electron dispersive scattering or SEM-EDS imaging in terms of silicon or Si-dot mapping for selected tape sample areas using the Datacell software. This calculated the number of Si dots and hence clay particles within a given area and a reduction in Si dot intensity following ultrasound exposure was taken as a measure of increased dispersion in that dispersed particles at the nanolevel were now no longer visible because they are beyond the resolution of the SEM instrument. Results indicated that the 100 W probe with 90% amplitude (ie 90 W power) showed greatest dispersion, so this power was selected for further study. Flammability of the PP tape samples was studied by limiting oxygen index and a modified UL-94 test. Results showed that, in the case of polypropylene, LOI values of samples containing clay and / or flame retardant were not significantly affected by additive or the presence of ultrasound.However, the modified UL-94 test showed that presence of nanoclay and / or flame retardant decreased burning rate which further decreased with ultrasonification. Knitted fabrics were tested for vertical flame spread using the sample ignition test rig described in BS 5438 and results showed rate of burning and burning drips reduced for the ultrasonificated samples. In the case of PA6, the same 100W ultrasonic probe with 90% amplitude was used in the compounding stage. Of particular note was that the LOI of the PA6/25A(2 wt%)/Al-Phos(5 wt%) sample increased more than others and the same sample showed reduced flammability in the vertical flame spread test. However, quality of the filaments was poor due to the extrusion process and some of the samples failed to extrude using the Labline extruder. Subsequent work used the recently acquired Fibre Extrusion Technology (FET) extruder. Based on the above results, PA6 containing 2 wt% of nanoclay and 10 wt% of same flame retardant was used with and without presence of 100 W ultrasonic power at 90% amplitude. Compounded samples were extruded into filaments and tapes and filaments were knitted into fabrics. Properties were characterised as above and the PA6/25A(2%)/AlPhos(10%) sample showed superior performance compared to the others in terms of reduced flammability and this performance was enhanced by ultrasonification during compounding. In order to assess whether chemical degradation had occurred during ultrasonification, the relative viscosity of these samples was measured using a ASTM D 445 Ubbelohde viscometer. The results indicated that chemical degradation is negligible or absent at the probe power used. Differential scanning calorimetry (DSC) was used to determine the degree of crystalinity and values decreased with the addition of clay and flame retardant and further decreased in samples exposed to ultrasound. To extend this PA6 study, ammonium sulphamate (2.5 wt% AS) and dipentaerythritol (1 wt% DP) were selected as flame retardants and various nanoparticles including fumed silica were added. Samples were compounded and extruded into filaments and knitted into fabrics. Of special note is that the PA6/AS(2.5 wt%)/DP(1 wt%)/25A(2 wt%) combination was found to have superior tensile properties as filaments and significantly reduced flame spread in fabric form. In the final part of the thesis, the effect of ultrasound during compounding on core-sheath bicomponent fibres containing combinations of nanoclays and flame retardants was considered. Results showed that the addition of nanoclay or flame retardant to either the core (C) or sheath (S) of the fibres slightly reduced their tensile modulus and elongation-at-break values with respect to a control PA6-C/PA6-S yarn and there was little improvement observed with the ultrasound-exposed samples. The result of vertical flame spread testing showed that the PA6-C / PA6/25A(2 wt%)/Al-Phos (10 wt%)/90W-S sample was superior to all others in terms of their reduced flammability properties defined as minimal burn time, although its values of burn length, flame spread rate and number of drops were not the lowest values.

Item Type: Thesis (PhD)
Additional Information: This is an electronic version of the thesis submitted to the University of Bolton in partial fulfilment of the requirements for the degree of Doctor of Philosophy
Divisions: University of Bolton Theses > Engineering and Sciences
University of Bolton Research Centres > Institute for Materials Research and Innovation
Depositing User: Tracey Gill
Date Deposited: 25 Feb 2019 13:15
Last Modified: 25 Feb 2019 13:15
URI: http://ubir.bolton.ac.uk/id/eprint/2146

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