An Investigation On Novel Halogen Free Flame Retardant Polyamide Fibers/Fabrics

As the earliest industrializing production of synthetic fiber in the world, polyamide, also called nylon, has been widely used in military and civilian clothing due to its outstanding wear-resisting, high mechanical property, easy dyeing and mothproof. Polyamide serves as the second largest synthetic fibers, and its flame retardant property is regard as a key factor on many occasions. Although it is not combustible fibers, the flammability of polyamide still can’t satisfy the usage requirements. The melting temperature and ignition temperature of polyamide vary widely, leading to a shrink and molten drop during burning and thus causing more damages, which is very dangerous in practical application.In this research, novel flame retardant nylon fibers/fabrics were prepared by both melt blending and afterfinish methods, treated with polyamide 6 (PA6) and polyamide 66 (PA66). The influence of flame retardant and treatment process on spinnability, flammability, thermal stability, mechanical property and washing durability was fully studied, as well as the synergistic effects between flame retardants. Therefore, this thesis includes two parts:Part A, preparation of flame retardant PA6 fibers by melt blending method.For the first section, halloysite nanotube (HNT) was selected as the flame retardant, accompanied with melamine polyphosphate (MPP) to exert their synergistic effects. The results showed that uniform and continuous flame retardant PA6 fibers could be obtained by loading with the combination of MPP and HNT. Further research indicated that the limiting oxygen index (LOI) value could reach 24.0% and UL-94 rating could achieve V2 level at the presence of 12% flame retardants. Cone calorimeter data demonstrated that peak heat release rate was significantly reduced from 554 kW·m-2 of neat PA6 to 367.6 kW·m-2 of the sample containing flame retardants, reducing 33.6%. The MPP/HNT flame retardant system could also improve the thermal stability at high-temperature stage and promote char formation. The mechanical property was enhanced due to the presence of HNT. There existed a synergism between MPP and HNT via interactions between HNT and MPP to form more thermally stable compounds. The thermal gravity analysis (TGA) coupled with Fourier transform infrared spectroscopy (FTIR) results and scanning electron microscopy (SEM) morphology of residue char demonstrate the solid-phase flame retardant mechanism. It was known that HNT could migrate to the surface during combustion and participate in the formation of a protective barrier, which could isolate the heat and oxygen in combustion zone. The decomposition product of MPP was absorbed on the surface of HNT during combustion, which gave rise to a surrounding charred structure.In the second section, multi-walled carbon nanotubes (MWNT) were chosen as flame retardant, and surface modified to improve the compatibility between MWNT and polymer matrix. Two kinds of flame retardants hexa-chlorocyclotriphosphazene (HCTP) and guanidine sulfamate (GAS) were selected as the modifier. X-ray photoelectron spectroscopy (XPS), TGA and FTIR were used to verify the grafting process, and two kinds of modified carbon nanotubes can be obtained:MWNT-HCTP and MWNT-GAS. Modified MWNT could also be used to prepare uniform and continuous PA6 fibers via melt compounding with PA6. Both dispersibility and compatibility of MWNT were improved becaused of the introduction of polar group, reflected in the change of fiber diameter and nano particle size. PA6 fibers were observed by optical microscope and showed that most aggregations in the matrix were disappeared after modification. The calculated average size of nano particles reduced from 346.9 μm2 of unmodified to 20.7 μm2 (MWNT-GAS) and 8.9 μm2 (MWNT-HCTP) respectively. By the presence of 3% modified MWNT, the heat release rate (HRR) of PA6/MWNT-GAS and PA6/MWNT-HCTP composites reduce 24.7% and 35.2% respectively. Modified MWNT could not only reduce the HRR but also increase LOI, which increased from 22.0% to 24.1%(MWNT-GAS) and 25.8% (MWNT-HCTP). It was demonstrated that MWNT-HCTP mainly took effect in solid phase. During the decomposition process, carbon nanotubes could aggregate on the surface of melt polymer and then a network cover layer would be formed. The grafted HCTP worked as patches to fill the meshes of MWNT network and formed more compact protective layer. Besides, the Cl atoms in HCTP could form Cl· during combustion, which would catch free radicals to terminate the chain reaction during combustion. MWNT-GAS took effects in both solid phase and gas phase. In solid phase, the decomposition products of grafting GAS could wrap in the surface of carbon nanotubes, and then promote the formation of more compact protective layer. In gas phase, the decomposition of GAS could release ammonia, and then accelerate the degradation of PA6. The noncombustible volatiles from GAS and PA6, such as NH3, H2O and SO2, were contributed to the fire resistance of polymer matrix in gas phase.Part B, preparation of flame retardant PA66 fabrics by afterfinish treatment.For the afterfinish treatment of PA66 fabrics, the combination of guanidine sulfamate and thiourea (TUR) was firstly used to flame retarding PA66 by their synergistic effects. The combination of two flame retardant could overcome the limitations of single flame retardant (such as blushing or poor hand feeling) and improve the flame retardant property significantly. After treated with 20% flame retardant solution (TUR:GAS=4:2, mass ratio), the LOI of PA66 fabric increased to 30.4% and after flame time decreased to 0 s with a damage length of 6.8 cm. The heat release rate of treated fabric decreased from 436.4 kW·m-2 to 321.9 kW·m-2. There existed a synergistic effect between TUR and GAS, and both of them took effects in gas phase principally. The decomposition gas of TUR and GAS containing sulfur and nitrogen could dilute the combustible gas and oxygen in combustion zone, thus promoting the extinguishment of flame.The simple padding process could give PA66 fabrics ideal flame retardancy, however, the treated fabrics usually showed no durability. In order to improve the washing durability of PA66 fabrics, hydroxymethylated by a 36%formaldehyde aqueous solution and grafted with thiourea was used to improve the traditional thiourea-formaldehyde prepolymer technology and obtained durable flame retardant PA66 fabrics with good hand feeling. The flame retardant property of grafted PA66 fabric was better than the simple padding sample. The LOI of grafted fabric was 36.1% and remaining at 33.4% after 30 times washing. The damage length of grafted fabric decreased to 6.9 cm and peak heat release rate decreased from 436.4 kW·m-2to 324.1 kW·m-2. After treated by this method, PA66 fabrics showed good washing durability. The thermal decomposition kinetics of treated and untreated PA66 fabrics was investigated using the non-isothermal approach according to Flynn-Wall-Ozawa and Kissinger methods. At the main stage of decomposition, the activation energies of treated PA66 fabrics increased sharply compared with that of untreated PA66 fabrics. The flame retardant used in this part was thiourea; therefore, the flame retardant mechanism was gas phase mechanism.