Recent Advances in Production of Flame Retardant Polyamide 6 Filament Yarns Najsodobnejše raziskave na področju proizvodnje ognjevarnih

Polyamide 6 is one of the key engineering polymers with excellent mechanical properties and resistance which enable its global production and wide use in the industrial and domestic plastic manufacturing. Polyamide 6 also represents an important raw material for the production of technical fi lament yarns. However, an important drawback associated with the fl ammability of PA6 fi bres has not been resolved yet. This paper reviews the most common halogen-free fl ame retardant additives for polyamide 6, their mode of action as well as diff erent strategies for the incorporation of fl ame retardant additives in the production process of fl ame ratardant polyamide 6 fi bres. The most recent research and patents on this topic are critically dis-


Introduction
Polyamide 6 (PA6) is one of the key engineering polymers with excellent mechanical properties and resistance which enable its global production and wide use in the industrial and domestic plastic manufacturing. Excellent processing properties, low moulding shrinkage and simple and low cost processing on one hand as well as its toughness, high tensile strength, abrasion and creep resistance on the other hand enabled PA6 to become one of the most important raw materials in the production of textile fi bres for technical applications, such as technical clothing, carpets and carpet paddings, fabrics for furniture upholstery, transport seats, fl oor coverings and air bags [1]. Whereas PA6 plastics represent the largest segment of the global polyamide market, the PA6 textile fi bre application segment is the fastest growing [2]. Furthermore, PA6 has an exceptional feature, i.e., its chemical recyclability back to monomer ε-caprolactam, which classifi es PA6 as a "sustainable polymer" Izvleček Poliamid 6 je eden ključnih inženirskih polimerov z odličnimi mehanskimi lastnostmi in visoko odpornostjo, ki omogoča njegovo široko uporabo pri proizvodnji industrijskih in gospodinjskih plastičnih mas. Poliamid 6 predstavlja tudi pomembno surovino za proizvodnjo fi lamentnih prej za tehnične namene. Vendar pa pomembna pomanjkljivost, povezana z vnetljivostjo poliamidnih 6 vlaken, še ni bila odpravljena. Članek vključuje pregled najpogosteje uporabljenih ne-halogenih ognjevarnih aditivov za poliamid 6, njihovega načina delovanja in različnih pristopov za vgradnjo ognjevarnih aditivov v proizvodnem procesu ognjevarnih vlaken iz poliamida 6. Predstavljena je kritična razprava o najsodobnejših raziskavah in patentih na področju. Ključne besede: tekstilije, plastične mase, ognjevarnost, ognjevarna sredstva, proizvodne strategije and, consequently, dramatically enhances its reusability and added value [3]. Despite the widespread use of PA6 in various economic areas, a problem associated with the fl ammability of PA6 fi bres has not been solved yet. Specifi cally, a very hazardous drawback of PA6 is its inherent fl ammability, which can lead to rapid burning with an intensive fl ammable melt-dripping and a release of toxic smoke, which may present a great risk and danger for lives and material goods. Despite the rather successful production of fl ame retardant (FR) PA6 moulding plastic materials, the development of FR PA6 textile fi bres remains a challenging scientifi c problem, and commercially available FR PA6 textile fi bres still do not exist [4]. Much eff ort has been put into the production of FR PA6 fi bres during the last decades [4][5][6][7][8]. For eff ective FR action, it is crucial that the FR additive matches the processing and pyrolysis specifi cs of PA6. Accordingly, the FR additive needs to be thermally stable and non-volatile at the processing temperatures of PA6 composites and fi laments. Th e thermal stability and mode of action of the FR additive must match those of PA6 during pyrolysis, during which the mostly volatile cyclic monomer ε-caprolactam, alkyl cyanides and ammonia are produced, leaving no charred residue in the condensed phase [9]. According to the literature, FR loadings higher than 15 wt% are required to achieve an eff ective FR action in the PA6 bulk polymers [4], but these high loadings are unacceptable for the textile fi bres because of their impaired spinnability and tensile properties. Th ese represent the most important limitations in the production of FR PA6 fi bres in comparison with PA6 bulk polymers. Furthermore, compared to bulk plastic, the lightweight PA6 textile fi bres and fabrics have an open structure with a much higher surface, which intensifi es the burning rate.

Structures of FR additives and their mode of action
Although halogenated FR additives are very eff ective and have been some of the most important FR additives for PA6 over the decades, they have been prohibited as substances of Very High Concern (REACH SVCH), and due to their persistence, bio-accumulation and high toxicity (PBT), there is a gap in the market for FR additives. Th e most intensively investigated halogen-free FR additives for PA6 include phosphorus (P)-and nitrogen (N)-based FRs, inorganic hydroxides and diff erent nanoparticles [4][5][6][7][8][10][11][12]. Th e most common FRs for PA6 are summarised in Table 1, and some of them are presented in Figure 1. FR mechanisms and modes of action depend on the chemical structures of the FR additives as well as the structure and thermal decomposition pathway of the polymers. Accordingly, FR additives act chemically and/or physically in the gas phase and/ or in the condensed phase ( Figure 2) [4][5][6][7][8][10][11][12].
It is known that the same FR additive can provide diff erent fl ame retardancy for diff erent polymers. In general, P-based FR additives are active in both the condensed phase and the gas phase depending on the oxidation state of the P atom [67]. According to the condensed-phase mode of action, phosphorous compounds, i.e., phosphates and phosphonates, promote char formation by infl uencing the decomposition pathway of the polymers. If this reaction is accompanied by the release of water, the combustive vapours are also diluted. According to the gasphase mechanism, phosphorous compounds, i.e., phosphinates and phosphine oxides, decompose to radical scavengers, which terminate the oxidative radical chain reactions in the combustion cycle. In the case of PA6, the gas phase contributes more to fl ame retardancy than char formation because the polymer chain scission, which occurs during thermal decomposition, leads to the generation of ε-caprolactam and other volatiles from the shorter chain fragments, and very little char residue is formed. Th erefore, char-promoting phosphorous FRs alone are not enough to be eff ective in PA6. In contrast, the gas phase active P-based FR additives are of great importance for the protection of PA6. Specifi cally, phosphorous radicals in the fl ame, i.e., HPO 2 ⋅, PO⋅, PO 2 ⋅ and HPO⋅, can scavenge H⋅ and OH⋅ radicals that propagate fuel combustion. Th is leads to a reduction in concentrations of H⋅ and OH⋅ radicals and the quenching of the fl ame. Among N-based FR additives, MeCy has special importance since it is considered as one of the most effective FR additives for aliphatic polyamides [68]. MeCy exhibits a strong condensed-phase mode of action; however, it is also active in the gaseous phase [18]. At higher temperatures, MeCy undergoes endothermic decomposition to melamine and cyanuric acid. Melamine partially sublimes at approximately 350 °C, which is accompanied by a signifi cant absorption of energy. In the condensed phase, melamine undergoes endothermic self-condensation with the release of ammonia, which volatilises and dilutes the fuel gases that support combustion. MeCy also decreases the thermal stability of PA6 and catalyses the chain scission of PA6 macromolecules to oligomeric segments, which are less fl ammable than caprolactam. Th e chain scission also decreases the melt viscosity and accelerates melt fl ow and dripping. Because the melt drips remove heat from the polymer matrix, the phenomenon of fl ame self-extinguishment occurs. MeCy also promotes the formation of a cross-linked structure in the self-condensation reaction with the generated oligomers. Th is promotes the formation of a closed char layer. Nitrogen ammonium sulfamate (AS) [14][15][16][17] melamine cyanurate (MeCy) [18][19][20][21][22][23][24][25][26][27] Phosphorous, nitrogen ammonium polyphosphate (APP) [28] melamine polyphosphate (MPP) [29][30][31][32] 6- 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxide-based diepoxide (DEP) [34,35] cyclotriphosphazene [36][37][38] Phosphorous, aluminium aluminium dialkylphosphinate (AlPi) [ MPP, as a representative of the P/N-based FR additives, was developed as a char former in the intumescent FR action. Th e latter is characterised by swelling and expanding at temperatures higher than critical temperature, leading to the formation of a foamed cellular charred layer [4,9,69]. In the intumescent FR formulations, MPP acts as an acid source because the release of phosphoric acid promotes the creation of a carbonaceous shield on the polymer surface, which acts as a heat-barrier and physically prevents contact between the polymer and the fl ame and hinders the fl ow of oxygen and heat to the polymer surface. As an acid source, ammonium phosphate, ammonium polyphosphate or AS can also be used [8]. For eff ective intumescent FR action, the acid source-containing FR additives are usually combined with the carbon source-containing additives, for instance, pentaerythritol, as well as a blowing agent, mostly melamine or guanidine, which releases non-combustible gases. Th e FR action of inorganic hydroxides, such as AlO(OH) and Mg(OH) 2 , involves heat sink and heat-barrier eff ects [8,54,56]. At temperatures between 340-350 °C and above 300-320 °C, boehmite and magnesium hydroxide, respectively, undergo endothermic decomposition (absorbing heat) in which free water is produced. Th is results in the cooling of the polymer and the diluting of the combustive gas mixture by the release of the vapours. Th e water release could also enhance the decomposition of PA6. In addition, a mineral layer formed on the polymer surface acts as a barrier that prevents contact between the heat and the polymer. Silica as well as diff erent clays exhibit the heat-barrier FR eff ect. Th ey contribute to the formation of a reinforced insulating charred layer, which acts as a heat and gas barrier and protects the polymer against thermal oxidation and mass loss during combustion [30,61,70]. Th e presence of the protective layer increases the thermal stability of PA6 and signifi cantly reduces the heat release rate during combustion relative to pristine PA6, suggesting improved fi re behaviour. Additionally, smoke obscuration is signifi cantly lowered in the presence of clays.
Well-dispersed particles of clay can act as nucleating sites for bubble formation in the residue during the decomposition process, leading to an expanded carbonized material composed of a large bubble covering the residue. Th e gases trapped under the bubble insulate the surface of the polymer [30]. Th e FR mechanism of CNTs diff ers from that of clays [63]. It is assumed that CNTs act as inert fi llers that do not signifi cantly infl uence the thermal decomposition behaviour of PA6. Th e presence of CNTs in the PA6 composite increases the time to ignition and decreases the heat release rate but does not signifi cantly lower the total heat release in comparison to pristine PA6. Th e reason for the increase in the time to ignition is attributed to the improved thermal conductivity or the increased melt viscosity of the composite. A decreased heat release rate caused by the CNTs in the composite would indicate a reduction in fl ammability. However, on the contrary, during ignition, the CNTs form a thermally stable interconnected network structure, which fi xes the molten material in the pyrolysis zone and prevents melt-dripping. Th is results in the increase in the composite burning intensity. Since CNTs subsequently decompose in the thermo-oxidative reaction, no charred residue is created during the thermal decomposition of the PA6 composite.

Strategies for the production of FR PA6 fi bres
Th e most common strategy for the production of FR PA6 fi bres includes the use of the melt-compounded PA6/FR composites in the melt-spinning process [71]. However, there are several drawbacks in the fibre production process when using the "melt-compounding approach" related to the agglomeration of FR additives in the PA6 matrix [26,32]. Specifi cally, the high melt reactivity of PA6 and its poor compatibility with FRs due to the strong intermolecular hydrogen bonds between the polymer chains cause the agglomeration of FR additive species. Th is results in the non-uniformly dispersed and low-dispersed micro-sized FR additive particles in the PA6 matrix. Th e fl ame-retardant action of the FR molecules entrapped in the micro-sized agglomerates is inhibited because of the inability of the entrapped FR molecules to actively participate in the fl ame retarding action. Th e fl ame retarding action occurs at the nanoscopic level.
In the case of the micro-sized aggregates, only the outermost molecules can effi ciently participate in the fl ame retarding action. Th us, in the case of the aggregated FRs, increased weight percent loading of the FR additive is unavoidable for effi cient fl ame retardancy. Furthermore, the micro-sized FR agglomerates impair the spinnability of the PA6 composite fi laments since they cause clogging of the fi lters and spinnerets at higher FR additive loadings. Th e agglomerates also signifi cantly impair the physical and mechanical properties of the fi bres. Consequently, the loading of FR additives that are acceptable for the continuous melt-spinning process provides only a poor FR eff ect.
To solve these problems, three main approaches in the production process of FR PA6 fi bres have been introduced: (i) preparation of the melt-compounded PA6/FR composites with the incorporation of FR nanoparticles as well as FR mixtures with a synergistic action, (ii) in situ polymerisation of ε-caprolactam in the presence of the FR additives, and (iii) synthesis of PA6 copolymers with the incorporation of reactive FR co-monomers.

Melt-compounded PA6/FR composites with the incorporation of nanoparticles and synergistic FR mixtures
Th e incorporation of nanoclays into the PA6 composite fi bres was fi rst reported by Bourbigot et al. [61], who prepared a PA6 nanocomposite with 5 wt% the LOI values of the composites but did not significantly improved the UL 94 test ratings, which remained at the V2 level. Th e increase in the wt% ratio of halloysite nanotubes in the FR mixture delayed the time to ignition, decreased the heat release rate and increased the char residue. Th ese results confi rm the existence of FR synergism between MPP and halloysite nanotubes in PA6. Horrocks et al. [16] produced PA6/FR composite multifi lament yarns by using 2 wt% organically modifi ed montmorillonite nanoclay (NC), Cloisite 25A (Southern Clay Products Inc., USA), together with two types of FR formulations: the vapour phase active micro-sized particles of AlPi, Exolit OP935 (Clariant, Switzerland), at 10 wt%; and the condensed-phase intumescent active mixture of AS (Sigma-Aldrich, UK) and dipentaerythritol (DP) (Fisher Scientifi c, UK) at 2.5 wt% and 1 wt%, respectively. All the yarns had acceptable tensile properties and were knitted into knit fabrics. Th is paper is the fi rst to report the burning and extinguishing behaviour of the knit fabrics using the vertical fl ame spread test. Since not all fabrics burned the entire length, the respective burn lengths, the time to burn that length (or extinguishment time), the rate of fl ame spread and the average number of molten drops were recorded for each fabric. Th e results showed that the presence of NC with AlPi in the PA6 composite fabric sample impaired the fl ame retardant activity of AlPi, which resulted in increases in the total burning time and the burn length compared to fabric samples with only AlPi. Th e PA6/ AlPi/NC fabric sample did not show any tendency to self-extinguish. In contrast, the presence of NC with the AS/DP mixture in the PA6 composite fabric further decreased the self-extinction time from 31 s to approximately 23 s and the burn length compared to that of the AS/DP mixture, which confi rmed the compatibility of the FR additives. However, the authors addressed the water solubility of ammonium sulphamate as a possible limitation regarding the wash durability of these fi bres. Šehić et al. [27]  Th e tensile properties of the studied PA6/FR composite fi lament yarns were not signifi cantly deteriorated, and these yarns could be appropriate for knitting into the knit fabrics. Th ere results confi rmed the Horrocks's observations that silicabased FR additives improved the FR action of condensed-phase active FR additives but not the gas-phase active aluminium dialkylphosphinate.

In situ polymerization of ε-caprolactam in the presence of FR additives
Th ere is little research that reports on the in situ polymerisation of ε-caprolactam in the presence of FR additives [20,21,23,24,26,72,73]. However, the in situ polymerisation of ε-caprolactam in the presence of additives is advantageous over the melt-compounding method as it enables the production of PA6 nanocomposites with nanodispersed additives [33][34][35][36][37][38]. According to our knowledge, Alfonso et al. [74] were the fi rst to report results regarding the in situ polymerisation of ε-caprolactam in the presence of FR additives. Th ey found that, contrary to MeCy and APP, which strongly inhibit polymerisation, red P and magnesium oxide do not have an adverse eff ect on kinetics or thermodynamics and enhance fi re performance by the thorough distribution of the additives. According to the preparation method introduced by Wu et al. [21], Li et al. [26] synthesised melamine/adipic acid salt and cyanuric acid/hexane diamine salt and used them in the in situ polymerisation of ε-caprolactam. Th e obtained results for the bulk nanocomposite confi rmed the formation of uniformly dispersed MeCy nanoparticles in the PA6 matrix [21]. Th is nanocomposite showed superior fl ame retardancy and signifi cantly less deterioration of the mechanical properties compared to the composite prepared by the common melt-compounding of PA6 with commercial MeCy. Th e PA6/MeCy composites were pelleted and melt spun into the FR composite fi lament yarns [26]. Unfortunately, the fl ame retardant performance of the knitted fabrics was discussed only qualitatively.

Synthesis of PA6 copolymers with the incorporation of FR reactive comonomers
In recent years, a new approach for the preparation of FR PA6 fi bres has been developed based on the introduction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide DOPO-based reactive co-monomers during the melt polycondensation of ε-caprolactam [75,76]. Th is approach was inspired by the synthesis procedure of a DOPO-containing co-polyester [77][78][79], which was commercialised as the DOPObased FR reactive additive, Ukanol ES, and a co-polyester, Ukanol ES-CoPET (Schill+Seilacher GmbH, Germany). Accordingly, DOPO was fi rst reacted with itaconic acid to prepare a phosphorus-containing diacid, 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10-phosphaphenanthrene-10-oxide (DDP) (Figure 3). In the second step, DDP was reacted with a diamine to form a salt containing an amidogen and a carboxyl group at each end. Th e salt was then mixed with ε-caprolactam, and the polymerisation process was carried out under the appropriate conditions. Th e synthesised FR PA6 includes polyamide chain segments and DOPO-based FR chain segments of diff erent weight ratios ( Figure 4). According to this procedure, Liu et al. [76] prepared the PA6/DDP composites containing 2 wt% to 5 wt% DDP, and their thermal stability and fl ame retardancy were determined by thermogravimetry, the UL 94 test and cone calorimetry.
To study the mechanical properties as well as fl ame retardancy using the vertical burning test, fi lament fi bres and knit fabric samples were prepared. Th e results show that the introduction of DDP decreased the initial temperature of the composite decomposition and simultaneously increased its thermal oxidative stability. Th e presence of 5 wt% DDP decreased the total heat release during combustion and signifi cantly increased the LOI value in comparison with pristine PA6. Th e results of the vertical burning test show that the aft er-fl ame time of the knit fabric samples decreased when the concentration of DDP increased and that 5 wt% DDP was enough to preserve the self-extinguishment of the melt drops and reach the LOI value of 28.4 for the fabric. Th e incorporation of DDP decreased the tenacity at break of the fi bres, but it still met the requirements of textiles.

Conclusion
Since halogenated FR additives have been prohibited due to health and environmental concerns, phosphorus (P) and nitrogen (N) based FRs, inorganic hydroxides as well as nanoparticles have been established for PA6. Among them, AlPi, MeCy, AS and DOPO derivatives alone or in combination with nanoclays have been mainly used. FR additives were incorporated into the polymer matrix by mixing FR additives with PA6 pellets in the process of the preparation of melt-compounded PA6/FR composites prior to the melt-spinning of PA6 composite fi bres or by introducing FR additives during the polymerisation of ε-caprolactam. Whereas the in situ polymerisation was characterised for the MeCy incorporation, the reactive DOPO derivatives were included as comonomers in the synthesis of PA6 copolymers. Th e latter approach is the most promising and will therefore provide important research challenges in the future. It represents a powerful tool for highlighting the fl ame retardant mechanisms of the nanodispersed FRs in the PA6 matrix and how these mechanisms function in the open high surface structure textile materials. Th is knowledge will provide the missing blocks to the current state of the art. Development of the fl ame retardant PA6 nanocomposite also provides very important solution for the lowering FR additive loadings in the bulk polymers, which will increase their sustainability.