Recent Advances for Flame Retardancy of Textiles Based on Phosphorus Chemistry

polymers

Review Recent Advances for Flame Retardancy of Textiles Based on Phosphorus Chemistry

Khalifah A. Salmeia 1,*, Sabyasachi Gaan 1 and Giulio Malucelli 2,*

1 Additives and Chemistry, Advanced Fibers, Empa, Swiss Federal Laboratories for Materials Science and Technology, St. Gallen CH-9014, Switzerland; [email protected] 2 Department of Applied Science and Technology, Local INSTM Unit, Politecnico di Torino, Viale T. Michel 5, 15121 Alessandria, Italy * Correspondence: [email protected] (K.A.S.); [email protected] (G.M.); Tel.: +41-5-8765-7038 (K.A.S.); +39-0131-229369 (G.M.); Fax: +41-5-8765-7499 (K.A.S.); +39-0131-229399 (G.M.)

Academic Editors: Baljinder Kandola, Abderrahim Boudenne and Paul Kiekens Received: 30 June 2016; Accepted: 19 August 2016; Published: 25 August 2016

Abstract: This paper aims at updating the progress on the phosphorus-based flame retardants specifically designed and developed for fibers and fabrics (particularly referring to cotton, polyester and their blends) over the last five years. Indeed, as clearly depicted by Horrocks in a recent review, the world of flame retardants for textiles is still experiencing some changes that are focused on topics like the improvement of its effectiveness and the replacement of toxic chemical products with counterparts that have low environmental impact and, hence, are more sustainable. In this context, phosphorus-based compounds play a key role and may lead, possibly in combination with silicon- or nitrogen-containing structures, to the design of new, efficient flame retardants for fibers and fabrics. Therefore, this review thoroughly describes the advances and the potentialities offered by the phosphorus-based products recently developed at a lab-scale, highlighting the current limitations, open challenges and some perspectives toward their possible exploitation at a larger scale.

Keywords: ﬂame retardants; ﬁbers and fabrics; phosphorus-based products; biomacromolecules; cotton; polyester; cotton-polyester blends; polyamide

1. Introduction Textiles play an important role in everyday life: one of their main drawbacks refers to their structure, as they are mainly made of organic polymers, which conversely, if not inherently flame-retarded (such as polyaramides and polyphosphonate fibers), are flammable and potentially dangerous species. Specifically referring to fibers and fabrics, the annual UK fire statistics have clearly demonstrated that most of fire incidents occur in houses, involving upholstering furniture, bedding and nightwear [1]. In this context, the ease of flammability of textiles has been faced by designing and synthesizing suitable flame retardants (FRs), i.e., additives that are able to suppress or delay the appearance of a flame and/or reducing the flame-spread rate (flame retardants) or delaying ignition or reducing the rate of combustion when needed (fire retardants) [2]. From an overall point of view, combustion in the presence of flames is a gas-phase process exploiting the oxygen taken from the surroundings. As a consequence, before the occurrence of the combustion process, the textile undergoes degradation: some of the so-obtained degradation products turn into combustible volatile species that, in combination with oxygen, fuel the flame. If the heat generated in the combustion is sufficient, it can be easily transferred to the textile substrate,

Polymers 2016, 8, 319; doi:10.3390/polym8090319 www.mdpi.com/journal/polymers Polymers 2016, 8, 319 2 of 36

Polymers 2016, 8, 319 2 of 36 hencehence giving giving rise rise to furtherto further degradation degradation phenomena phenomena and and supporting supporting a self-sustaining a self-sustaining combustion combustion cyclecycle (Figure (Figure1). 1).

Air (oxygen supply)

GAS PHASE

Products Volatile species Flame (CO, CO2, ...)

Heat to the atmosphere

Textile substrate Char CONDENSED PHASE

FigureFigure 1. 1.Scheme Scheme ofof thethe textile combustion combustion cycle. cycle.

On the basis of the chemical composition and their thermal and fire characteristics, if they are On the basis of the chemical composition and their thermal and fire characteristics, if they not inherently flame retarded, fibers and fabrics have to be treated with additives that may contain are nothalogen, inherently nitrogen, flame phosph retarded,orus, sulphur, fibers andboron, fabrics metals, have etc., tohence be treatedbecoming with flame additives retarded. thatThe may containaforementioned halogen, nitrogen, additives phosphorus, can be added sulphur, during spin boron,ning metals,processes etc., performed hence becoming on synthetic flame fibers, retarded. or The aforementioneddeposited on the additivessynthetic or can natural be added fiber/fabric during su spinningrface, hence processes creating performed a protective on layer/coating. synthetic fibers, or depositedBoth finishing on the and synthetic coating or methods natural fiber/fabriccan be exploited: surface, concerning hence creating the former, a protective the fiber/fabric layer/coating. is Bothimpregnated finishing and with coating a solution methods or a stable can suspension be exploited: that contains concerning the FR the additive. former, Conversely, the fiber/fabric the is impregnatedcoating method with ainvolves solution the or application a stable suspensionof a continuous that or contains discontinuous the FR layer/film additive. on Conversely, both the coatingsurfaces method of the textile. involves the application of a continuous or discontinuous layer/film on both From an overall point of view, the self-sustaining combustion cycle of Figure 1 can be broken in surfaces of the textile. the presence of FR additives, thus achieving the extinction of the flame or reducing the burn rate. In From an overall point of view, the self-sustaining combustion cycle of Figure1 can be broken particular, they can: in the presence• Lower of FR the additives, developed thus heat achievingto below that the nece extinctionssary to carry of the on flame the combustion or reducing process. the burn rate. In particular,• theyModify can: the pyrolysis process to lower the quantity of flammable volatiles developed, favoring, at the same time, the creation the char, i.e. a carbonaceous residue that also limits the heat • Lowerand mass the transfer developed between heat the to textile below material that necessary and the flame. to carry on the combustion process. • Modify• theIsolate pyrolysis the flame process from the to loweroxygen/air the quantitysupply. of flammable volatiles developed, favoring, at the• sameRelease time, flame the creationinhibitors the (like char, chlorinated i.e., a carbonaceous and brominated residue compounds) that also when limits the the textile heat and massapproaches transfer the betweenignition temperature. the textile material and the flame. • • Isolate theLower flame the from heat theflow oxygen/air back to the fabric, supply. hence limiting or preventing further pyrolysis. • Favor the formation of a char or an intumescent protective layer when the textile interacts • Release flame inhibitors (like chlorinated and brominated compounds) when the textile with a flame or a heat source. approaches the ignition temperature. A review published in 2011 succeeded in summarizing the state of the art for the different • Lowercommercially the heat available flow back flame to retardants the fabric, for hence textile limiting materials, or which preventing were classified further pyrolysis.according to the • Favorfollowing the periods formation [3]: of a char or an intumescent protective layer when the textile interacts with a flame• or1950–1980: a heat source. The ”golden period” of flame retardant research, involving the appearance of the first patents on FRs based on organophosphorus compounds for cellulosic textiles (i.e. cotton). ADuring review this publishedperiod, inhere inntly 2011 FR succeeded synthetic fibers in summarizing bearing aromat theic structures state of were the artalsofor developed. the different commercially• 1980–late available 1990s: flame Very retardants limited foradvances textile of materials, the research which in FRs were were classified achieved according during this to the followingperiod. periods [3]:

• 1950–1980: The “golden period” of flame retardant research, involving the appearance of the first patents on FRs based on organophosphorus compounds for cellulosic textiles (i.e., cotton). During this period, inherently FR synthetic fibers bearing aromatic structures were also developed. • 1980–late 1990s: Very limited advances of the research in FRs were achieved during this period. Polymers 2016, 8, 319 3 of 36

• 2000 onward: Several efforts were carried out in the design of char-former flame retardant additives, possibly containing phosphorus-based products. Another goal was the investigation into the possibility of replacing bromine derivatives with other less toxic and efficient products. Furthermore, during this period, nanotechnology was demonstrated to show outstanding potential for conferring flame retardant features to fibers and fabrics, through the use of nanoparticles having different aspect ratios. In particular, the exploitation of both top-down (using preformed nanoparticle suspensions) and bottom-up (exploiting the generation of single nanoparticles or nanoparticle assemblies—even hybrid organic-inorganic structures) was successfully considered [4,5].

Specifically referring to the textile field, FRs can also be classified according to their “laundry durability”: indeed, a non-durable FR is washed off immediately when soaked in water, but may resist dry cleaning. Conversely, semi-durable FRs are able to resist water-soaking and possibly a few washes, while durable FRs endure some 50 or 100 washing cycles. However, according to the very stringent directives recently promoted by the EU community and USA, some of the halogenated compounds (such as brominated diphenyl derivatives) have been banned, as they have clearly shown a high toxicity for both animals and humans [6]. The aforementioned disadvantages stimulated the scientific community toward the design and development of phosphorus-based compounds, which seem less toxic and may represent a suitable alternative to their halogen-based counterparts. Though it is not a general case that all phosphorus compounds are non-toxic, the development of new flame retardants based on phosphorus compounds has shown that they have lower toxicity profiles as compared to halogen-based counterparts [7,8]. In general, the development of any new flame retardant should involve a complete assessment of its performance in material as well as its toxicity. In this context, some new products have been designed and nowadays are commercially available. ® In particular, Trevira CS , which is based on the use of a phosphorus-containing comonomer (in the form of propionylmethylphosphinate), has been exploited for conferring flame retardant properties to polyester fibers and fabrics [9,10]. Regarding cotton and cellulosic-rich substrates, the present focus is either on the synthesis of effective non-halogenated additives for coatings and back-coated fabrics or on the utilization of hydroxymethylphosphonium salts (Proban®) or N-methylol phosphonopropionamide derivatives (Pyrovatex®). The chemistry of the Proban® process exploits a tetrakis(hydroxymethyl) phosphonium–urea condensate, which, after padding, is crosslinked by ammonia gas in a dedicated plant and then subjected to peroxide oxidation for stabilizing the resulting polymeric matrix [11]. The washing fastness of this treatment is due to the deposition of the chemical within the fibers by a construction of a polymer network during the heating process: as a consequence, Proban® is not linked to the fibers but is mechanically retained within the fiber interstices. One of its major disadvantages refers to the possible release of formaldehyde during the fabric use [12]. Conversely, the chemistry behind Pyrovatex® is based on a conventional pad-dry-cure process in the presence of a methylolated crosslinking agent, which is responsible for the formation of covalent bonds with the hydroxyl groups of the cellulosic substrate. Nonetheless, about 50% of Pyrovatex® FR treatment has been reported to be lost during the first laundry occurrence, because of the extraction of unreacted products, though it remains stably linked thereafter. Despite the efficiency of such commercially available treatments, both academic and industrial researchers are still seeking worthy alternatives, also taking into account the requirements that have to be fulfilled by the new products. In particular, with respect to the already existing FRs:

• Any new flame retardant should show equivalent or superior ease of application. • Any new flame retardant should not emit formaldehyde during application or service. • Any new flame retardant should provide comparable textile service-life features, specifically referring to durability, comfort, tensile properties, outward appearance and aesthetics. Polymers 2016, 8, 319 4 of 36

• Any new flame retardant should possess an overall comparable cost-effectiveness to the already existing chemicals and preferably be less costly. • Any new flame retardant should possess equivalent or even lower toxicity and environmental impact. • Air permeability of the treated textiles should be maintained, irrespective of the possible high amounts of chemicals needed to provide flame retardant features. • Any new flame retardant should not cause any alteration in the hue of the dye and/or dyeability of the fibers/fabrics.

Nowadays, the approach adopted by the scientific community is being slightly changed: indeed, the durability of any new flame retardant is still needed, but the novel processes and methods developed in last five years seem to be more addressed to the design of low impact and eco-friendly systems. In this context, the present paper aims at providing an overview of the recent advances in the design of phosphorus-based FRs, also in combination with nitrogen- or silicon-containing structures, for different fibers and fabrics: in particular, the evolution from chemical to low environmental impact products will be thoroughly described, highlighting the current achievements and limitations, as well as the open challenges and perspectives.

2. Assessment of the Fire Behavior of Textiles This section summarizes the current methods that allow assessing the reaction of a textile toward the exposure to a flame or a heat flux. All the developed methods take into account that, for textiles, the high fiber surface to mass ratio favors their easy ignition; in fact, these materials burn faster than other bulk polymers. Generally speaking, ignition occurs when a small flame is applied to flammable fabrics for no more than 12 s and the textile continues to burn after the removal of the flame. Therefore, most of the work on flammable fabrics focuses on the evaluation of the facility of ignition, the rate and extent of flame spread, the duration of flame propagation, the heat release and heat of combustion. All these parameters are merged with a quantitative portrayal of burning wreckages, such as melt dripping. It is very difficult to find a single test method able to measure all the aforementioned parameters. Concerning the fabrics that show self-extinction, such as flame-retarded fabrics, tests comprise the evaluation of time of afterflame and afterglow and extent of fire damage (specifically referring to char length, dimension of holes, or damaged sample length). The measurement of textile flammability involves either scientific (i.e., research) test or the standard test methods. The former provide information suitable for assessing the burning behavior and are exploited for the design of new FRs or fire-retardant treatments. Limiting oxygen index (LOI), also called oxygen index (OI), is one of the most popular scientific methods, used in many standards, such as ISO 4589 and ASTM D2863. LOI denotes the minimum concentration (vol %) of O2 in a mixture of O2 and N2 that will just sustain flaming combustion of a material in a candle-like manner. Textile materials burn rapidly when they exhibit LOI values up to 21 vol %, while they burn slowly when LOI is in between 21 vol % and 25 vol %. LOI values beyond 26 vol % indicate some flame retardant features [13]. The obtained LOI values may be affected by several fabric structural parameters, when measured for the same fiber type: this makes LOI values relative and not absolute data. In addition, the textile material is ignited at the top and thus it burns vertically downward (i.e., in candle-like manner) which is opposite to the burning of any material freely suspended. Simple ignition tests (used in many standards, such as BS 5438 and EN ISO 6941) represent another usual approach for assessing the flammability of a textile material: more specifically, a standard gas flame is applied to the face or lower edge of a vertically oriented fabric sample; ignition is examined by visual observations and the time needed to ignite the specimen is recorded. The textile does not pass the test when, after the removal of ignition source, the flame achieves any end of the sample. If the Polymers 2016, 8, 319 5 of 36

flame reaches extinction, the char length, dimension of holes, afterglow, and type of any wreckage (molten drops, etc.) are thoroughly evaluated. Flame spread (UL-94, which contains EN 60695 11-10, ASTM D 635-03 and D 3801-00) is a bench-scalePolymers 2016 test, 8 which, 319 measures the rate of flame spread usually calculated as the ratio of the5 of distance 36 to the time taken of the advancing flame front to reach defined distances marked on the fabric specimen. Flame spread (UL-94, which contains EN 60695 11-10, ASTM D 635-03 and D 3801-00) is a The upward fire spread is far faster than downward and horizontal flame spread and, hence, adopted bench-scale test which measures the rate of flame spread usually calculated as the ratio of the as a betterdistance means to the of time assessing taken of the the fire advancing hazard offlame a fabric. front to reach defined distances marked on the Althoughfabric specimen. not specifically The upwarddesigned fire spread for is far fabrics, faster conethan downward calorimetry and tests horizontal (according flame to spread ISO 5660) haveand, become hence, a standardadopted as bench a better scale means model of assessing of early the flaming fire hazard [14,15 of]. a In fabric. particular, the cone calorimeter mimics theAlthough penetrative not specifically burning seen designed as fire for burning fabrics, intocone a calorimetry specimen. Ittests evaluates (according the to heat ISO release 5660) rate and thehave effective become heata standard of combustion bench scale from model a burning of early material flaming exposed [14,15]. toIn aparticular, controlled the radiant cone heat sourcecalorimeter (ISO 5660 mimics part 1). the Usually, penetrative such burning parameters seen asas Timefire burning To Ignition into (TTI),a specimen. Total HeatIt evaluates Release the (THR), Heatheat Release release Rate rate and and corresponding the effective peakheat (pkHRR),of combustion Effective from Heat a burning of Combustion material (EHC),exposed Massto a Loss controlled radiant heat source (ISO 5660 part 1). Usually, such parameters as Time To Ignition (TTI), (ML) and Mass Loss rate (MLR) can be evaluated. The cone calorimeter can also be utilized to evaluate Total Heat Release (THR), Heat Release Rate and corresponding peak (pkHRR), Effective Heat of smokeCombustion generation (EHC), (ISO 5660 Mass part Loss 2): in(ML) this and case, Mass the measuredLoss rate parameters(MLR) can includebe evaluated. the determination The cone of CO andcalorimeter CO2 concentrations, can also be utilized as well to as evaluate the assessment smoke generation of smoke density(ISO 5660 (Specific part 2): Extinction in this case, Area–SEA, the Totalmeasured Smoke Production–TSP, parameters include etc.). the determination of CO and CO2 concentrations, as well as the Theassessment micro-combustion of smoke density calorimeter (Specific Extinction (MCC) hasArea recently – SEA, Total been Smoke standardized Production (ASTM – TSP, etc.). D7309-13) and exploitedThe micro-combustion for evaluating the calorime flammabilityter (MCC) of has polymers recently [ 16been,17 ].standardized In this process, (ASTM a small D7309-13) specimen (aboutand 2–10 exploited mg) undergoes for evaluating pyrolysis the flammability through of a fastpoly heatingmers [16,17]. up in In inertthis process, atmosphere a small (with specimen a heating (about 2–10◦ mg) undergoes pyrolysis through a fast heating up in inert atmosphere (with a heating rate below 1 C/s). The obtained pyrolysed products are then mixed with O2/N2 mixture to expedite combustion.rate below The 1 °C/s). oxygen The concentration obtained pyrolysed and flowproducts rates are of then the combustionmixed with O gases2/N2 mixture are evaluated, to expedite and the combustion. The oxygen concentration and flow rates of the combustion gases are evaluated, and amount of generated heat is calculated on the basis of oxygen consumption calorimetry. the amount of generated heat is calculated on the basis of oxygen consumption calorimetry. 3. Phosphorus Chemistry in FRs: An Overview 3. Phosphorus Chemistry in FRs: An Overview Phosphorus-basedPhosphorus-based flame flame retardants retardants are quiteare quite versatile versatile in their in flametheir retardantflame retardant action. Phosphorusaction. compoundsPhosphorus often compounds exhibit both often condensed exhibit both and conden gas phasesed activityand gas [phase18]. A activity simplified [18]. scheme A simplified of various flamescheme retardant of various actions flame of phosphorus retardant actions is presented of phosphorus in Figure is presen2. ted in Figure 2.

Figure 2. Mode of action of phosphorus-based flame retardants. Figure 2. Mode of action of phosphorus-based ﬂame retardants.

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An additive is considered to be active in condensed phase if it alters the thermal decomposition characteristics of the polymer by a chemical reaction. Hydrolysis, dehydration, chain scission or de-polymerization are some of the main chemical reactions occurring in condensed phase activity. This activity is usually characterized by a reduction in the decomposition temperature of the polymer and increased formation of char residue at elevated temperatures [19]. In some cases, the de-polymerization of thermoplastic polymer chains in the presence of a heat source reduces the viscosity of the system and enables it to retreat from the fire without producing any residue. The efficiency of phosphorus compounds to change the decomposition and combustion characteristics of polymers makes their fire suppressant use imperative. Depending on the substrate and their chemistry, there could be chemical interactions in the condensed phase at elevated temperatures, which lead to changes in the decomposition pathway of the polymer and possible formation of carbonaceous char residues on the surface of decomposing polymer, hence preventing its further oxidation. In other instances, the phosphorus compounds and some of their decomposed products preferably volatilize from the polymer substrate when heated. These phosphorus species further decompose to release reactive phosphorus species, which then interact with the combustion intermediates in the gas phase as inhibitors. In most cases, such interactions lead to recombination of the H and OH radicals and prevent their oxidation. The condensed or gas phase activity of phosphorus compounds significantly depends on their structure, as well as on the polymer substrate. For example, in case of natural polymers like cellulose and wool, the phosphorus compounds primarily exhibit condensed phase activity where dehydration of the polymer, leading to the formation of a thermally stable char, is the predominant mechanism. Referring to synthetic polymers containing oxygen and nitrogen atoms in their structure, catalytic hydrolysis of the ester or amide groups by phosphorus acids promotes an enhanced melt dripping and fast shrinkage from flame. As far as olefin-based polymers are considered, the phosphorus compounds mainly act in the gas phase by recombining the key fuel species such as H and OH radicals and preventing their oxidation. Some minor physical effects due to volatilization of phosphorus compounds and dilution of the fuel can also occur.

4. Chemical Phosphorus-Based FRs for Textiles The following paragraphs will describe the recent advances concerning the use of phosphorus-based chemical products, suitable for conferring flame retardant properties to different fibers and fabrics. In general, Table1 summarizes the recent findings of P-based flame retardants and their performance on textile fabrics. Polymers 2016, 8, 319 7 of 36

Table 1. Main results achieved by treating fabrics with phosphorus-based ﬂame retardants.

Durability Type of P-based FR Textile material Highlights Ref. (washing fastness) New oligomers were synthesized and their burning behavior was compared to Antiblaze 19®. The new oligomers showed higher thermal stability and more char residue comparing to Dioxaphosphorinane Antiblaze 19®. PET ND * [20] derivatives The thermal stability and flame retardant properties were studied only on the oligomeric derivatives. Treated PETs with these oligomers were not investigated. Cotton fabric was treated with UV-curable flame retardants and cured under UV-lamp in presence of photoinitiator. LOI values increased with increasing the FR content. Cotton MCC showed a decrease in HRC and HRR and THC with increasing the FR content. Yes [21–23] As increasing the monomer concentration and UV exposure time, the coating yield increased. Relatively small monomers were not suitable for UV-curing as they might evaporate during curing process. UV-curable flame Allyl-functionalized polyphosphazene additive was investigated, avoiding the disadvantage of retardants Cotton and Cotton/ small molecules. Yes [24] polyester blend The treated fabrics showed good flame retardancy with char formation. The additive acts in the condensed phase mode of action UV-curable epoxy based oligomer formulation. Vinyl phosphonic acid (VPA) was incorporated in the formula. Polyester/ The thermal stability, char formation and LOI values of the treated fabrics increased with ND [25] Polyamide blend increasing the concentration of the VPA acid The peel strength values increased gradually up to 50.8 N/cm with increasing VPA. The triazine-based flame retardants are derivatives of cyanuric chloride. These additives are considered as active flame retardants, forming ether bond by replacement the chlorine atoms with the hydroxyl groups pf cellulose. Triazine-based flame The fabrics were treated with flame retardants by impregnation. Cotton Yes [26–33] retardants Thermal analysis showed a char formation which proves the condensed phase mode of action. LOI values increased with increasing the add-on of flame retardants. With increasing the flame retardant concentration, the treated fabrics did not show any afterglow, which can be considered as self-extinction. Polymers 2016, 8, 319 8 of 36

Table 1. Cont.

Durability Type of P-based FR Textile material Highlights Ref. (washing fastness) The fabrics were treated with flame retardants using the sol-gel technique. Increasing the sol-gel precursors on treated fabrics, showed a decrease of the HRC, PHRR and Cotton TGA onset and an increase of the char residue. Yes [34–41] Increasing the solid dry add-on increased relatively the after flame with no afterglow. Hybrid Increasing the solid dry add-on increased the LOI value and decrease of the THR and HRC. organic-inorganic flame retardants The PA6 samples were treated with different concentrations of equimolar mixtures of the flame retardant and TEOS. PA6 Combination of P- and Si-based additives improved the thermos-oxidative stability and ND [38] combustion behavior by increasing the char residue and reducing the HRC and PHRR of treated samples, respectively. The cotton fabrics were treated by dipping/soaking in a solution of the polymeric flame retardants. Binders or crosslinkers were used when needed to bind the polymer permanently onto fabrics. LOI values of the treated fabrics showed an increase with increasing the add-on. Cotton Partially studied [42–49] Polymeric flame The vertical burning test of the treated fabrics showed a reduction of the afterglow time and retardant additives char length. Cone calorimetry showed a decrease of HRR THR and CO2/CO ratio with increasing the add-on. The nylon fabrics were dip-treated in a solution containing FR and crosslinker. Nylon fabrics Yes [49] MCC analyses showed a decrease of HRR, THR and HRC. LOI values of the treated fabrics increased with increasing the phosphorus content. The treated fabrics with add-on beyond 5 wt % were found self-extinction. Phosphoramidate Cotton Partially studied [50–57] The thermal stability and the flame retardancy performance of phosphoramidates are derivatives influenced by the nature of chemical substituents and type of cotton fabrics. Nylon and polyester The vertical flame test showed better flame retardancy of treated nylon fibers. ND [56] * Not determined. Polymers 2016, 8, 319 9 of 36 Polymers 2016, 8, x FOR PEER 9 of 36 Polymers 2016, 8, x FOR PEER 9 of 36 4.1. Dioxaphosphorinane Derivatives for PET Fibers 4.1 Dioxaphosphorinane Derivatives for PET Fibers These new flame retardants were designed to exhibit similar performances in their These new flame retardants were designed to exhibit similar performances in their activity to activity to Antiblaze 19®, i.e., a trimethylolpropane methylphosphonate oligomer obtained Antiblaze 19®, i.e. a trimethylolpropane methylphosphonate oligomer obtained from the reaction of from the reaction of trimethylolpropane phosphite with dimethyl methylphosphonate [54], trimethylolpropane phosphite with dimethyl methylphosphonate [54], employed for employed for poly(ethyleneterephtalate) (PET) fibers. In particular, Negrell-Guirao and poly(ethyleneterephtalate) (PET) fibers. In particular, Negrell-Guirao and co-workers [20] co-workers [20] synthesized 2,5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (1), 2-butyl- synthesized 2,5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (1), 5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (2) and 2-benzyl-5-ethyl-5(allyloxymethyl)- 2-butyl-5-ethyl-5-(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (2) and 2-oxo-1,3,2-dioxaphosphorinane (3) (Figure3). 2-benzyl-5-ethyl-5(allyloxymethyl)-2-oxo-1,3,2-dioxaphosphorinane (3) (Figure 3).

Figure 3. Representative synt synthesishesis of monomers (1), (2) and (3).

Due to the two pseudo-asymmetric centers fo forr dioxaphosphorinane monomers, monomers, compounds compounds ( (11),), (2) andand ((33)) existexist asas diasterioisomers.diasterioisomers. However, thethe radicalradical polymerizationpolymerization ofof dioxaphosphorinanedioxaphosphorinane monomers shows the influence influence of the presence of a chain transfer agent (CTA) on the efficiencyefficiency of the radical radical polymerization polymerization reaction. reaction. Moreover, Moreover, dime dimethylthyl phosphite phosphite can can play play a role a role by enhancing by enhancing the thefire fireretardant retardant efficacy efficacy of dioxap of dioxaphosphorinanehosphorinane derivative derivatives.s. As the As CTA the CTA concentration concentration increases, increases, the themonomer monomer conversion conversion increases increases as well as well and and the thedegree degree of polymerization of polymerization decreases decreases (Figure (Figure 4). 4).

Figure 4. Radical polymerization of dioxaphosphorinane monomers, using dimethyl phosphite as Figure 4.4. RadicalRadical polymerization polymerization of of dioxaphosphorinane dioxaphosphorinane monomers, monomers, using us dimethyling dimethyl phosphite phosphite as CTA. as CTA. The products of the polymerization reaction were isolated as mono- and di-adduct, rather than The products of the polymerization reaction were isolated as mono- and di-adduct, rather than high MW polymers. Furthermore, the flame retardancy of these dioxaphosphorinane derivatives was high MW polymers. Furthermore, the flame retardancy of these dioxaphosphorinane derivatives was nothigh assessed: MW polymers. therefore, Furthermore, it was not the possible flameto retardancy make a real of comparisonthese dioxaphosphori with Antiblazenane derivatives 19®. was not assessed: therefore, it was not possible to make a real comparison with Antiblaze 19®. 4.2. UV-Curable Flame Retardants Coatings 4.2 UV-Curable Flame Retardants Coatings Xing and co-workers designed and applied UV-curable flame retardants coatings able to protect Xing and co-workers designed and applied UV-curable flame retardants coatings able to cottonXing fabrics and fromco-workers heat penetration designed and flameapplied spread UV-curable [21]. In flame particular, retardants as shown coatings in Figure able 5to, protect cotton fabrics from heat penetration and flame spread [21]. In particular, as shown in Figure triprotect (acryloyloxyethyl) cotton fabrics from phosphate heat penetration (4) and triglycidyl and flame isocyanurate spread [21]. acrylate In particular, (5) were as synthesizedshown in Figure and 5, tri (acryloyloxyethyl) phosphate (4) and triglycidyl isocyanurate acrylate (5) were synthesized and employed5, tri (acryloyloxyethyl) for impregnating phosphate cotton (4 fabrics,) and triglycidyl using 1:1 isocyanurate weight ratio acrylate of the two (5) monomerswere synthesized in acetone and employed for impregnating cotton fabrics, using 1:1 weight ratio of the two monomers in acetone solutionemployed with for 0.05impregnating g/mL, 0.10 cotton g/mL fabrics, and 0.20 using g/mL 1:1 of reactiveweight ratio compounds. of the two The monomers monomers in were acetone then solution with 0.05 g/mL, 0.10 g/mL and 0.20 g/mL of reactive compounds. The monomers were then curedsolution using with 4 0.05 wt % g/mL, of photoinitiator 0.10 g/mL and under 0.20 a g/mL UV-lamp. of reactive compounds. The monomers were then cured using 4 wt % of photoinitiator under a UV-lamp.

Polymers 2016, 8, 319 10 of 36 Polymers 2016, 8, x FOR PEER 10 of 36

Polymers 2016, 8, x FOR PEER 10 of 36

FigureFigure 5. 5.Schematic Schematic synthesissynthesis of UV-curable monomers monomers (4 (4) )and and (5 ().5 ). Figure 5. Schematic synthesis of UV-curable monomers (4) and (5). TheThe formation formation of theof the UV-cured UV-cured flame flame retardant retardant coatings coatings on theon the cotton cotton surface surface was was confirmed confirmed using using Attenuated Total Reflectance (ATR) and Scanning Electron Microscopy (SEM) analyses. The AttenuatedThe formation Total Reflectance of the UV-cured (ATR) and flame Scanning retardant Electron coatings Microscopy on the cotton (SEM) surface analyses. was confirmed The flame flame retardancy of the treated fabrics was also assessed: in particular, LOI values were found to retardancyusing Attenuated of the treated Total fabricsReflectance was (ATR) also assessed: and Scan inning particular, Electron LOIMicroscopy values were(SEM) found analyses. to increase The increase with increases in the flame retardants’ content. A similar trend was also noticed from withflame increases retardancy in the of flamethe treated retardants’ fabrics content.was also Aas similarsessed: in trend particular, was also LOI noticed values fromwere Micro-scalefound to Micro-scale Combustion Calorimetry measurements. Furthermore, cotton fabrics treated with flame Combustionincrease with Calorimetry increases in measurements. the flame retardants’ Furthermore, content. cotton A similar fabrics trend treated was withalso flamenoticed retardant from retardant monomers (4) and (5) showed good durability to laundry. The MCC results after monomersMicro-scale (4) Combustion and (5) showed Calorimetry good durability measurements. to laundry. Furthermore, The MCC cotton results fabrics after treated laundering with flame showed retardantlaundering monomers showed a (4slight) and increase (5) showed in Heat good Release durability Capacity to laundry.and Tota Thel Hydrocarbons MCC results values, after a slight increase in Heat Release Capacity and Total Hydrocarbons values, which, however, were launderingwhich, however, showed were a slightstill lower increase than in those Heat of Releaseuntreated Capacity cotton. andIn genera Totall, Hydrocarbons the UV-curable values, flame still lower than those of untreated cotton. In general, the UV-curable flame retardant coating based which,retardant however, coating were based still on lower monomers than those (4) ofand un treated(5) showed cotton. condensed In genera l,phase the UV-curable mode action flame as on monomers (4) and (5) showed condensed phase mode action as confirmed by the increased char retardantconfirmed coatingby the increased based on char monomers formation ( 4of) theand treated (5) showed cotton. condensed phase mode action as formation of the treated cotton. confirmedVery recently,by the increased Edwards char et al. formation [22] synthesized of the treated novel UV-curablecotton. flame retardants obtained from Very recently, Edwards et al. [22] synthesized novel UV-curable flame retardants obtained from the reactionVery recently, of hexachlorotriphosphazene, Edwards et al. [22] synthesized in the novelpresence UV-curable of triethylamine, flame retardants with different obtained molar from thetheequivalents reaction reaction of ofof hexachlorotriphosphazene, 1-(acry-loyloxy)-3-(methacryhexachlorotriphosphazene, inloyloxy)-2-propanol, the presence of of triethylamine, triethylamine, affording monomers with with different different (6), ( 7molar) and molar equivalentsequivalents(8), (Figure of 6).of 1-(acry-loyloxy)-3-(methacryloyloxy)-2-propanol, 1-(acry-loyloxy)-3-(methacryloyloxy)-2-propanol, affordingaffording monomersmonomers ((66),), (7(7)) andand ( 8 ), (Figure(8), (Figure6). 6).

Figure 6. Representative schematic synthesis of UV-curable monomers (6), (7) and (8). FigureFigure 6. 6.Representative Representativeschematic schematic synthesissynthesis of of UV-curable UV-curable monomers monomers (6 (),6 ),(7 ()7 and) and (8). (8 ).

Polymers 2016,, 8,, 319x FOR PEER 11 of 36

The fabric samples were impregnated with UV-curable monomers in acetone solutions, The fabric samples were impregnated with UV-curable monomers in acetone solutions, containing containing different concentrations of monomers and photoinitiator. All three synthesized different concentrations of monomers and photoinitiator. All three synthesized monomers showed monomers showed an increase in coating yield with increases in the monomer concentration and UV an increase in coating yield with increases in the monomer concentration and UV exposure time; exposure time; conversely, the photoinitiator concentration did not affect the coating yield. As far as conversely, the photoinitiator concentration did not affect the coating yield. As far as flammability tests flammability tests are considered, all the treated cotton samples showed enhanced flame retardancy are considered, all the treated cotton samples showed enhanced flame retardancy with the formation with the formation of a coherent char. In particular, the higher phosphorus content in monomers (6) of a coherent char. In particular, the higher phosphorus content in monomers (6) and (7) allowed and (7) allowed achieving better fire performances as compared with monomer (8). achieving better fire performances as compared with monomer (8). Pursuing this research, the same group [23] synthesized two new UV-curable monomers, Pursuing this research, the same group [23] synthesized two new UV-curable monomers, namely namely di(allylamino)ethyl phosphate (9) and di(allylamino)dimethyl phosphoramide (10), and di(allylamino)ethyl phosphate (9) and di(allylamino)dimethyl phosphoramide (10), and their flame their flame retardant efficacy was compared to that of hexa(allylamino)cyclotriphosphazene (11) retardant efficacy was compared to that of hexa(allylamino)cyclotriphosphazene (11) (Figure7). (Figure 7).

Figure 7. RepresentativeRepresentative schematic synthe synthesissis of UV-curable monomers (9),), ((10)) andand ((11).).

The obtained monomers were dissolved in acetoneacetone together with different amounts of photoinitiators andand thenthen applied applied onto onto cotton cotton fabrics. fabrics. It It was was found found that that monomers monomers (9) ( and9) and (10 )(10 were) were not suitablenot suitable for UV-curing for UV-curing polymerization polymerization reactions: reactions: this finding this was finding attributed was to attributed the fast evaporation to the fast of theevaporation monomers of oncethe monomers they were exposedonce they to UVwere radiation. exposed Thus,to UV cotton radiation. fabrics Thus, were cotton impregnated fabrics thesewere monomersimpregnated and these were monomers tested without and undergoing were tested the UV-curingwithout undergoing process. Although the UV-curing the treated process. fabrics didAlthough not burn the whentreated subjected fabrics did to flame not burn spread when tests, su hencebjected revealing to flame goodspread flame tests, retardant hence revealing features, theygood couldflame notretardant be subjected features, to thethey UV-curing could not process. be subjected to the UV-curing process. In order toto overcomeovercome thisthis problem,problem, Mayer-GallMayer-Gall and co-workers [[24]24] synthesized allyl functionalized polyphosphazenes (12)) startingstarting fromfrom hexachlorotriphosphazenehexachlorotriphosphazene (Figure(Figure8 8).).

Figure 8. Synthesis of allyl-functionalized polyphosphazene (12). Figure 8. Synthesis of allyl-functionalized polyphosphazene (12). The obtained product was then used for cotton and different cotton/polyester blended fabrics, The obtained product was then usedused forfor cottoncotton andand differentdifferent cotton/polyestercotton/polyester blended fabrics, using an impregnation method (1 mL/g of 25 wt % of a solution of monomer (12) in acetylacetone) using an impregnation method (1 mL/g of of 25 25 wt wt % % of of a solution of monomer ( 12) in acetylacetone) followed by UV-grafting in inert atmosphere (argon). The treated fabrics exhibited good flame followed by UV-graftingUV-grafting inin inertinert atmosphereatmosphere (argon).(argon). The The treated fabrics exhibited good flameflame retardancy with char formation, hence, confirming a condensed phase mode of action of the retardancy withwith char char formation, formation, hence, hence, confirming confirming a condensed a condensed phase phase mode ofmode action of of action the designed of the designed flame retardant system. The UV-cured FR coatings showed also a good washing fastness. flamedesigned retardant flame system.retardant The system. UV-cured The UV-cured FR coatings FR showed coatings also showed a good also washing a good fastness. washing fastness.

Polymers 20162016,, 88,, 319x FOR PEER 12 of 36 Polymers 2016, 8, x FOR PEER 12 of 36 Very recently, Yildiz et al. synthesized a UV-curable epoxy based oligomer adhesive Very recently, Yildiz et al. synthesized a UV-curable epoxy based oligomer adhesive formulation formulationVery recently, (13), by Yildizreacting et acrylical. synthesized acid with abisphenol-A-diglycidylether UV-curable epoxy based (DGEBA)oligomer [25].adhesive The (13), by reacting acrylic acid with bisphenol-A-diglycidylether (DGEBA) [25]. The reaction is shown in formulationreaction is shown (13), inby Figure reacting 9. acrylic acid with bisphenol-A-diglycidylether (DGEBA) [25]. The Figurereaction9. is shown in Figure 9. OH O O O O OH OO O O O O O OOn O OH n OH

PPh3 100PPh3 °C-120 °C 100 °C-120 °C

OH OH OH O OH O O OH OOOH O O O O OOn O O O n O O 13 13 Figure 9. Schematic synthesis of oligomer (13). Figure 9. Schematic synthesissynthesis of oligomer (13). The system was employed to adhere cord fabrics (i.e. a polyester/polyamide blend) onto the rubberThe surfaces. systemsystem waswasVinyl employedemployed phosphonic toto adhereadhere acid cordcordwas fabricsfabricsthen incorporated (i.e.,(i.e. aa polyester/polyamidepolyester/polyamide into the epoxyacrylate-based blend) onto the rubberadhesive surfaces.surfaces. at different Vinyl Vinyl weight phosphonic phosphonic ratios acidto enhanceacid was thenwas the incorporatedthen flame incorporated retardancy into the ofintoepoxyacrylate-based the treatedthe epoxyacrylate-based fabrics; 3 adhesive wt % of atadhesivephotoinitiator different at weight different in methyl ratios weight toethyl enhance ratios ketone to the enhancewas flame also retardancy theadded. flame The of retardancy the fabrics treated were of fabrics; the dip-coated treated 3 wt % fabrics; and of photoinitiator the 3 solutionwt % of inphotoinitiatorwas methyl then ethylhomogeneously in ketonemethyl wasethyl spread also ketone added. using was also Thea squeez a fabricsdded.ing The were roller. fabrics dip-coated The were coating dip-coated and layer the solution wasand thethen was solution cured then homogeneouslywasexposing then bothhomogeneously sides spread of usingthe spreadtreated a squeezing usingfabrics roller. ato squeez a TheUV coatinginglamp. roller. Thermogravimetric layer The was coating then cured layer (TG) exposing was measurements then both cured sides ofshowedexposing the treated that both thermal fabricssides ofstability to the a UV treated and lamp. char fabrics Thermogravimetric formation to a UV of thelamp. treated (TG) Thermogravimetric measurementsfabrics increases showed (TG) with measurements increasing that thermal the stabilityconcentrationshowed andthat charthermal of formationvinyl stability phosphonic of and the treatedchar acid. formation fabricsFurtherm increases ofore, the treatedLOI with values increasingfabrics improved increases the concentration with with increasingincreasing of vinyl the phosphonicconcentration acid. ofof vinylvinyl Furthermore, phosphonic phosphonic LOI acid acid. values in theFurtherm improvedadhesiveore, formulation. withLOI values increasing improved the concentration with increasing of vinyl the phosphonicconcentration acid of vinyl in the phosphonic adhesive formulation. acid in the adhesive formulation. 4.3 Triazine-Based Flame Retardants 4.3.4.3 Triazine-Based Triazine-Based Flame Flame Retardants Retardants Cyanuric chloride is the main starting material for obtaining triazine-based flame retardants: indeed,Cyanuric this product chloride isshows the main a high starting chemical material reactivity for obtaining and triazine-basedis suitable forflame flame synthesizing retardants: indeed,phosphorus-nitrogen this this product flame shows retardants a high with chemical differ ent reactivity phosphorus and isisto suitablenitrogensuitable forratios,for synthesizingsynthesizing either via phosphorus-nitrogennucleophilic substitution flame flame or Michaelis-Arbuzov retardants with differentdiffer reaction.ent phosphorus to nitrogen ratios, either via nucleophilicChang substitutionand co-workers or Michaelis-ArbuzovMichaelis-Arbuzov synthesized diethyl reaction.reaction. 4,6-dichloro-1,3,5-triazin-2-ylphosphonate (14), tetraethylChang 6-chloro-1,3,5-triazin-2,4-diyldiphosphonate and co-workersco-workers synthesizedsynthesized diethyldiethyl 4,6-dichloro-1,3,5-triazin-2-ylphosphonate4,6-dichloro-1,3,5-triazin-2-ylphosphonate (15) via Michaelis-Arbuzov reaction (14(as), tetraethylshown in 6-chloro-1,3,5-triazin-2,4-diyldiphosphonateFigure6-chloro-1,3,5-triazin-2,4-diyldiphosphonate 10), and used these monomers for conferring (15 ()15 via) Michaelis-Arbuzovvia flame Michaelis-Arbuzov retardant properties reaction reaction (as to showncotton (as infabricsshown Figure [26].in 10 Figure), and 10), used and these used monomers these monomers for conferring for conferring flame retardant flame propertiesretardant toproperties cotton fabrics to cotton [26]. fabrics [26].

Figure 10. Representative synthesis of monomers (14) and (15) via Michaelis-Arbuzov reaction. Figure 10. 14 15 Figure 10. Representative synt synthesishesis of monomers (14) and (15) via Michaelis-Arbuzov reaction.reaction.

Polymers 2016, 8, x FOR PEER 13 of 36 Polymers 2016,, 8,, 319x FOR PEER 13 of 36 The chlorine atoms of the two synthesized oligomers were exploited to react with hydroxyl The chlorine atoms of the two synthesized oligomers were exploited to react with hydroxyl group of the cellulose, through the formation of ether bonds, hence permanently linking the FR groupThe of chlorinethe cellulose, atoms ofthrough the two the synthesized formation oligomers of ether werebonds, exploited hence permanently to react with hydroxyllinking the group FR additives to the cellulosic substrate [27]. Thermogravimetric analyses showed the formation of a of the cellulose, through the formation of ether bonds, hence permanently linking the FR additives to charadditives at 600 to °C, the which cellulosic proved substrate the condensed [27]. Thermo phase modegravimetric of action analyses of the synthesizedshowed the FRs. formation LOI values of a thechar cellulosic at 600 °C, substrate which proved [27]. Thermogravimetricthe condensed phase analyses mode of showed action of the the formation synthesized of aFRs. char LOI at 600values◦C, were found to increase up to 48% (for treated fabrics) with increases in the flame retardant whichwere found proved to the increase condensed up phaseto 48% mode (for of treated action offabrics) the synthesized with increases FRs. LOI in valuesthe flame were retardant found to concentration. From an overall point of view, the synthesized oligomers acted as good flame increaseconcentration. up to 48%From (for an treated overall fabrics) point withof view, increases the insynthesized the flame retardantoligomers concentration. acted as good From flame an retardants for cotton fabrics when at least 10 wt % add-on was achieved. As triazine-based flame overallretardants point for of cotton view, thefabrics synthesized when at oligomers least 10 wt acted % add-on as good was flame achieved. retardants As for triazine-based cotton fabrics flame when retardants are believed to be of interest for textile industries, the durability of the flame retarded atretardants least 10 wtare % believed add-on wasto be achieved. of interest As triazine-basedfor textile industries, flame retardants the durabilit are believedy of the toflame be of retarded interest cotton fabrics has not been reported yet. forcotton textile fabrics industries, has not thebeen durability reported of yet. the flame retarded cotton fabrics has not been reported yet. Slopek and co-workers exploited a nucleophilic reaction for synthesizing dimethyl Slopek and co-workers exploited a nucleophilic reaction for synthesizing dimethyl (4,6-dichloro-1,3,5-triazin-2-yloxy)methylSlopek and co-workers exploited phosphonatea nucleophilic ( 16reaction), (6-chloro-1,3,5-tr for synthesizingiazine-2,4-diyl)bis dimethyl (4,6-dichloro-1,3,5-triazin-2-yloxy)methyl phosphonate phosphonate (16 ), (6-chloro-1,3,5-triazine-2,4-diyl)bis(oxy)bis(16), (6-chloro-1,3,5-triazine-2,4-diyl)bis (oxy)bis(methylene) diphosphonate (17), diethyl 2-(4,6-dichloro-1,3,5-triazin-2-ylamino) (methylene)(oxy)bis(methylene) diphosphonate diphosphonate (17), diethyl 2-(4,6-dichloro-1,3,5-triazin-2-ylamino)(17), diethyl 2-(4,6-dichloro-1,3,5-triazin-2-ylamino) ethylphosphonate (18) ethylphosphonate (18) and tetraethyl 2,2’-(6-chloro-1,3,5-triazine-2,4-diyl)bis(azanediyl)bis(ethane- andethylphosphonate tetraethyl 2,2’-(6-chloro-1,3,5-triazine-2,4-diyl)bis(azanediyl)bis(ethane- (18) and tetraethyl 2,2’-(6-chloro-1,3,5-triazine-2,4-diyl)bis(azanediyl)bis(ethane- 2,1-diyl) diphosphonate 2,1-diyl) diphosphonate (19). The reactions are shown in Figure 11. (2,1-diyl)19). The diphosphonate reactions are shown (19). The in Figure reactions 11. are shown in Figure 11. O O Cl O Cl Cl P O Cl OP O Cl O N N HO OP Cl 2 HO P O N N O O N N HO O N N 2 HO O N N O O O O Cl N O P 0°C N N 0°C-RT OP O N O PO O Cl N Cl O O Cl N O OP pH0°C = 6 pH0°C-RT = 7-8 PO O N O OP Cl N Cl O O 16 O pH = 6 pH = 7-8 O 17 O 16 17

O Cl H3N Cl H3N O Cl H N O O O O 3 P O Cl H3N P O P Cl Cl O O OP O P O P O Cl Cl O O 2 O O NN N N O H O 2 PO NN N N HN N NH O O O N N Cl P P 0°C-RT Cl N Cl 0°C-RT HN N N O O O HN N Cl N N OP H 0°C-RTpH = 6 Cl N Cl pH0°C-RT = 9-10 O pH = 6 pH = 9-10 N N O Cl 18 Cl19 18 19 Figure 11. Synthesis of flame retardants (16)–(19) via nucleophilic reaction. Figure 11. Synthesis of flame ﬂame retardants (16)–(19) via nucleophilic reaction. The obtained FR products were applied onto cotton fabrics [28,29]. From an overall point of view,The the obtainedobtainedtreated cotton FRFR products products fabrics were showedwere applied applied a fire onto ontobeha cotton viorcotton fabricssimilar fabrics [ 28to ,29[28,29].that]. Fromprovided From an overall anby overallmonomers point point of view, (14 of) view, the treated cotton fabrics showed a fire behavior similar to that provided by monomers (14) andthe treated(15). cotton fabrics showed a ﬁre behavior similar to that provided by monomers (14) and (15). and (15). Recently, dioxo(3-triethylphosphite-5-chlorin-triazine)dioxo(3-triethylphosphite-5-chlorin-triazine) neopentyl neopentyl glycol glycol (20 )( was20) was synthesized synthesized from fromreaction Recently,reaction of monomer ofdioxo(3-triethylphosphite-5-chlorin-triazine) monomer (14) with (14) almost with almost half equivalent half equivalent of neopentyl neopentyl of neopentyl glycol glycol (Figure glycol (20) 12was(Figure); thesynthesized obtained 12); the from reaction of monomer (14) with almost half equivalent of neopentyl glycol (Figure 12); the obtainedproduct was product applied was onto applied cotton onto fabrics cotton [30 fabrics]. [30]. obtained product was applied onto cotton fabrics [30].

Figure 12. Schematic synthesi synthesiss of oligomer ( 20). Figure 12. Schematic synthesis of oligomer (20).

Cotton fabrics were impregnated in a solution containing (20) and Na2PO4·H2O as a catalyst at Cotton fabrics were impregnated in a solution containing (20) and Na2PO4·H2O as a catalyst Cotton fabrics were impregnated in a solution containing (20) and Na2PO4·H2O as a catalyst at 40 °C.◦ After drying, the samples were then cured at 160 °C.◦ LOI values increased with increases in at40 40°C. C.After After drying, drying, the the samples samples were were then then cured cured at at160 160 °C. C.LOI LOI values values increased increased with with increases increases in the concentration of the oligomer in the treated cotton fabrics. Furthermore, the effect of the catalyst inthe the concentration concentration of the of oligomer the oligomer in the in treated the treated cotton cotton fabrics. fabrics. Furthermore, Furthermore, the effect the of effect the catalyst of the concentration on the treated fabrics was investigated: it was found that with increases in the catalyst catalystconcentration concentration on the treated on the fabrics treated was fabrics investigated: was investigated: it was found it was that found with that increases with increases in the catalyst in the concentration, the LOI values of the treated fabrics increased. This finding was ascribed to the catalystconcentration, concentration, the LOI thevalues LOI valuesof the oftreated the treated fabrics fabrics increased. increased. This finding This ﬁnding was wasascribed ascribed to the to

Polymers 2016,, 8,, 319x FOR PEER 14 of 36 Polymers 2016, 8, x FOR PEER 14 of 36 presence of the catalyst that promoted the grafting reaction between the oligomer and the fabric presence of the catalyst that promoted the grafting reaction between the oligomer and the fabric thesubstrate. presence Vertical of the flammability catalyst that promotedtests showed the a grafting gradual reaction decrease between of char the length oligomer and increase and the of fabric the substrate. Vertical flammability tests showed a gradual decrease of char length and increase of the substrate.char residue Vertical with flammabilityincreases in teststhe FR showed concentrat a gradualion. Furthermore, decrease of char with length increases and increase in the offlame the char residue with increases in the FR concentration. Furthermore, with increases in the flame charretardant residue concentration with increases to in at the least FR concentration.20 wt %, the Furthermore, treated samples with increasesdid not inshow the flameany afterglow retardant retardant concentration to at least 20 wt %, the treated samples did not show any afterglow concentrationphenomena. Finally, to at least the 20flame wt %, reta therded treated fabrics samples showed did a notsignificant show any washing afterglow fastness phenomena. that makes Finally, the phenomena. Finally, the flame retarded fabrics showed a significant washing fastness that makes the thesynthesized flame retarded oligomer fabrics a durable showed flame a significant retardant washing for cotton fastness fabrics. that makes the synthesized oligomer synthesized oligomer a durable flame retardant for cotton fabrics. a durableZhang flame et al. retardant prepared for a sulfur-Nitrogen cotton fabrics. flame retardant containing triazine ((21), Figure 13) by Zhang et al. prepared a sulfur-Nitrogen flame retardant containing triazine ((21), Figure 13) by reactingZhang cyanuric et al. preparedchloride awith sulfur-Nitrogen sodium 2-aminoethane-sulfanilate, flame retardant containing and triazinethen with ((21 diethanolamine), Figure 13) by reacting cyanuric chloride with sodium 2-aminoethane-sulfanilate, and then with diethanolamine reacting[31]. cyanuric chloride with sodium 2-aminoethane-sulfanilate, and then with diethanolamine [31]. [31].

Figure 13. Synthesis of S-N flame retardant containing triazine (21). Figure 13. SynthesisSynthesis of S-N flame flame reta retardantrdant containing triazine (21). The obtained flame retardant was dissolved in water at different concentrations and applied The obtainedobtained flameflame retardant retardant was was dissolved dissolved in in water water at differentat different concentrations concentrations and appliedand applied onto onto cotton fabrics. The thermal stability of the treated fabrics was investigated using TG analyses in ontocotton cotton fabrics. fabrics. The thermalThe thermal stability stability of the of treatedthe treated fabrics fabrics was was investigated investigated using using TG TG analyses analyses in airin air atmosphere. The treated cotton fabrics showed an increase of the char residue◦ at 600 °C, which airatmosphere. atmosphere. The The treated treated cotton cotton fabrics fabrics showed showed an increase an increase of the of char the residue char residue at 600 atC, 600 which °C, provedwhich proved the condensed phase mode action of the obtained flame retardant. However, no standard provedthe condensed the condensed phase mode phase action mode of action the obtained of the ob flametained retardant. flame retardant. However, However, no standard no standard burning burning tests were reported and therefore it was not possible to assess the flame retardant burningtests were tests reported were and reported therefore and it wastherefore not possible it was to not assess possible the flame to retardantassess the performance flame retardant of the performance of the treated cotton fabrics. performancetreated cotton of fabrics. the treated cotton fabrics. A similar approach was adopted by Zhao and co-workers, who synthesized the flame retardant A similar approach was adopted by Zhao and co-workers, who synthesized the flame flame retardant 22 (shown in Figure 14), suitable for cotton fabrics [32]. 22 (shown in Figure 1414),), suitable for cotton fabricsfabrics [[32].32].

Figure 14. Synthesis of S-N flame retardant containing triazine (22). Figure 14. SynthesisSynthesis of S-N flame flame reta retardantrdant containing triazine (22). The flame retardant performances of the treated fabrics were evaluated using LOI and vertical The flame retardant performances of the treated fabrics were evaluated using LOI and vertical flammabilityThe flame tests. retardant Cotton performances fabrics couldof reach the treatedself-extinction fabrics werewhen evaluated treated with using 10 LOIwt % and add-on vertical of flammability tests. Cotton fabrics could reach self-extinction when treated with 10 wt % add-on of flammabilitythe FR. tests. Cotton fabrics could reach self-extinction when treated with 10 wt % add-on of the FR. the FR.Pursuing this research, (22) was reacted with POCl3, affording 6-chloro-4-(diethylamino Pursuing this research, (22) was reacted with POCl3, affording 6-chloro-4-(diethylamino phosphoratePursuing phosphoryl this research, chloride)-2-(sodium (22) was reacted with POCl4-aminobenzensulfonate)-1,3,5-triazine3, affording 6-chloro-4-(diethylamino ( phosphorate23) (Figure phosphorate phosphoryl chloride)-2-(sodium 4-aminobenzensulfonate)-1,3,5-triazine (23) (Figure phosphoryl15) [33]. chloride)-2-(sodium 4-aminobenzensulfonate)-1,3,5-triazine (23) (Figure 15)[33]. 15) [33].

Figure 15. Representative synthesis of monomer (23). Figure 15. Representative synthesis of monomer (23).

Polymers 2016, 8, 319 15 of 36 Polymers 2016, 8, x FOR PEER 15 of 36 Polymers 2016, 8, x FOR PEER 15 of 36 4.4.4.4 Hybrid Hybrid Organic-Inorganic Organic-Inorganic Flame Flame Retardants Retardants 4.4 Hybrid Organic-Inorganic Flame Retardants Hybrid organic-inorganic flame flame retardants are materialsmaterials that usually favor the formation of a Hybrid organic-inorganic flame retardants are materials that usually favor the formation of a carbonaceous layer upon exposure to a heat source [[34].34]. Among the different strategies that can be carbonaceous layer upon exposure to a heat source [34]. Among the different strategies that can be successfully exploited,exploited, thethe sol-gelsol-gel technique technique is is one one of of many many techniques techniques developed developed for for incorporation incorporation of successfully exploited, the sol-gel technique is one of many techniques developed for incorporation theseof these hybrid hybrid organic-inorganic organic-inorganic flame flame retardants retardants onto onto textiles textiles [35 ].[35]. of these hybrid organic-inorganic flame retardants onto textiles [35]. In this context,context, several phosphorus-based si siliconlicon containing flame flame retardants have been In this context, several phosphorus-based silicon containing flame retardants have been developed. Hu Hu and and co-workers co-workers [36] [36 ]exploited exploited th thee reaction reaction of of isophorone diisocyanate with developed. Hu and co-workers [36] exploited the reaction of isophorone diisocyanate with (3-aminopropyl)triethoxysilane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxideand 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), (3-aminopropyl)triethoxysilane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide obtaining(DOPO), obtaining oligomer oligomer 24 of Figure 24 of16 .Figure 16. (DOPO), obtaining oligomer 24 of Figure 16.

Figure 16. Representative synthesis of hybrid organic-inorganic flame retardant precursor (24) and Figure 16.16. Representative synthesis of hybrid organic-inorganic flameflame retardant precursor (24) andand its hydrolysis. its hydrolysis.hydrolysis. After the hydrolysis of the product in acidic media, a sol-gel solution was obtained and used for After the hydrolysis of the product in acidic media,media, a sol-gel solution was obtained and used for treating cotton fabrics by dipping. The treated samples were then dried in the oven at 80 ◦°C and then treating cottoncotton fabricsfabrics byby dipping.dipping. TheThe treatedtreated samplessamples werewere thenthen drieddried inin thethe ovenoven atat 8080 °CC and then 130 ◦°C. TG analyses showed a decreased onset temperature and an increased char with increases in 130 °C.C. TG analyses showed a decreased onset temperature and an increased char with increases in the concentration of the sol-gel precursor on cotton fabrics: these findings were attributed to the acid the concentration of the sol-gel precursor onon cottoncotton fabrics:fabrics: these findingsfindings were attributed to the acid environment promoted by the degradation of the organophosphorus part and to the thermal environment promotedpromoted by by the the degradation degradation of theof organophosphorusthe organophosphorus part andpart to and the thermalto the stabilitythermal stability of Si–O bonds that exerted a protection on the underlying fabric. A similar trend was found ofstability Si–O bonds of Si–O that bonds exerted that aexerted protection a protection on the underlying on the underlying fabric. A fabric. similar A trend similar was trend found was in found MCC in MCC tests: PHRR and HRC values showed a decrease with increases in the concentration of the tests:in MCC PHRR tests: and PHRR HRC and values HRC showed values ashowed decrease a withdecrease increases with inincreases the concentration in the concentration of the precursor. of the precursor. All these results proved that the sol-gel precursor acts in the condensed phase by Allprecursor. these results All these proved results that theproved sol-gel that precursor the sol-gel acts inprecursor the condensed acts in phase the bycondensed increasing phase the char by increasing the char formation of the treated cotton samples, which reduces the heat release. formationincreasing ofthe the char treated formation cotton of samples, the treated which cotton reduces samples, the heat which release. reduces the heat release. Quite recently, Vasiljević and co-workers [37] exploited an addition reaction of DOPO to Quite recently, VasiljevićVasiljević andand co-workersco-workers [[37]37] exploitedexploited anan additionaddition reaction of DOPODOPO toto vinyltrimethoxysilane in the presence of Azobisisobutyronitrile (AIBN) as initiator, giving rise to the vinyltrimethoxysilane in in the the presence presence of of Azobisisobutyronitrile Azobisisobutyronitrile (AIBN) (AIBN) as asinitiator, initiator, giving giving rise rise to the to formation of the flame retardant (25) shown in Figure 17. theformation formation of the of theflame flame retardant retardant (25) ( 25shown) shown in Figure in Figure 17. 17 .

Figure 17. Synthesis of hybrid organic-inorganic flame retardant (25). Figure 17. Synthesis of hybrid organi organic-inorganicc-inorganic ﬂameflame retardant ((2525).). Different concentrations of the obtained flame retardant were prepared in ethanol and were Different concentrations of the obtained ﬂameflame retardant were prepared in ethanol and were hydrolyzed using HCl. The cotton fabrics were then treated by the pad-dry-cure method at 20 °C, hydrolyzed using HCl. The cotton fabrics were then treated by the pad-dry-cure method at 20 °C,◦C, subsequently dried at 120 °C and then cured at 150 °C. TG analyses showed a decrease of the subsequently dried at 120 °C and then cured at 150 °C. TG analyses showed a decrease of the thermal stability and an increase of the char formation with increases in the amount of dry solid on thermal stability and an increase of the char formation with increases in the amount of dry solid on

Polymers 2016, 8, 319 16 of 36

Polymers 2016, 8, x FOR PEER 16 of 36 subsequently dried at 120 ◦C and then cured at 150 ◦C. TG analyses showed a decrease of the thermal thestability treated and cotton an increase fabrics. of In the addition, char formation the flame with retardant increases performance in the amount of treated of dry solidcotton on fabrics the treated was investigatedcotton fabrics. using In addition, vertical the flame flame spread retardant tests: performance by increasing of treated the solid cotton dry fabrics add-on, was an investigated increased flamingusing vertical combustion flame spread after tests:the removal by increasing of the the fl solidame drywas add-on, found; an conversely, increased flaming LOI values combustion were improved.after the removal of the flame was found; conversely, LOI values were improved. Pursuing thisthis research,research, the the same same group group exploited exploited the the combination combination of theof the sol-gel sol-gel precursor precursor (25) ( and25) andtetraethoxysilane tetraethoxysilane (TEOS), (TEOS), which which was was utilized utilized as a as sol-gel a sol-gel finishing finishing for polyamide for polyamide 6 (PA6) 6 (PA6) fabrics fabrics [38]. [38].The obtainedThe obtained hybrid hybrid system system decreased decreased the Total the Heat Total Release Heat valuesRelease of values the treated of the fabrics, treated increasing, fabrics, increasing,at the same at time, the same the char time, yield thewith char respectyield with to the respect use of to precursor the use of (25 precursor) alone, hence (25) alone, proving hence the provingsynergistic the behavior synergistic of thebehavior combined of the hybrid combined system. hybrid system. Yang etet al.al. [ 39[39]] reacted reacted (3-aminopropyl)trimethoxysilane (3-aminopropyl)trimethoxysilane with with diphenylphosphinic diphenylphosphinic acid inacid order in orderto prepare to prepare the flame the flame retardant retardant precursor precursor (26) shown (26) shown in Figure in Figure 18. 18.

FigureFigure 18. 18. SynthesisSynthesis of of hybrid hybrid organi organic-inorganicc-inorganic flame ﬂame retardant (26).

The obtained precursor was dissolved in aqueous/ethanolaqueous/ethanol solutions solutions at at di differentfferent concentrations and appliedapplied asas a a hybrid hybrid organic-inorganic organic-inorganic flame flame retardant retardant forcotton for cotton fabrics. fabrics. After treatment,After treatment, the fabrics the fabricswere dried were at dried 140 ◦ C.at The140 thermal°C. The stability thermal and stabilit the flammabilityy and the flammability behavior of behavior the treated of fabricsthe treated were fabricsassessed; were furthermore, assessed; furthermore, the washing the fastness washing was fastness evaluated. was TG evaluated. measurements TG measurements showed an increaseshowed anof charincrease formation of char of formation the treated of cottonthe treated fabrics cotton at 500 fabrics◦C. Vertical at 500 flame °C. Vertical spread testsflame revealed spread tests char revealedformation char quickly formation after ignition quickly andafter the ignition specimens and the achieved specimens self-extinction achieved afterself-extinction the removal after of the removalflame. In of addition, the flame. the In treated addition, fabrics the didtreated not fabric lose theirs did flame not lose retardancy their flame after retardancy several laundry after several cycles, laundrythus proving cycles, the thus durability proving of the the durability proposed of flame the proposed retardant flame treatment. retardant treatment. Very recently, Liu et al. exploited a similar synthetic approach for obtaining hybrid organic-inorganic 3-aminopropyl 3-aminopropyl triethoxysilane triethoxysilane derivatives derivatives on on cotton cotton fabrics fabrics [40]. [40 ].To To this this aim, aim, a nucleophilica nucleophilic substitution substitution was was carr carriedied out out (Figure (Figure 19). 19). A A 10 10 wt wt % % so solutionlution of of the the hybrid hybrid precursor precursor ( (2727)) was prepared and then the cotton fabrics were immersedimmersed for different times before a subsequent thermal treatment. In a similar way, way, Wang and co-workersco-workers synthesized the precursor ( (2727)) shown shown in in Figure Figure 19,19, which was applied onto cotton fabrics at different concentrations [41]. [41]. TG data of the fabrics treated with precursor ( 27) showed an an increase increase of of the the thermal thermal stability stability with with increases increases of of final final dry dry add-on. add-on. Conversely, the cotton fabrics treated with precursor ( 28) exhibited a lower thermal stability as compared to the other hybrid system. However, However, bo bothth flame flame retardants retardants showed showed an an increase of the char residue residue with with increases increases in in the the add-on add-on on on the the treated treated fabrics. fabrics. A similar A similar trend trend was was found found when when the treatedthe treated fabrics fabrics were were subjected subjected to vertical to vertical flammabilit flammabilityy tests. tests.Therefore, Therefore, from an from overall an overall point of point view, of theview, designed the designed hybrid hybrid coating coating systems systems were were able able to provide to provide improved improved fire fire retardancy retardancy to to cotton, cotton, exploiting thethe formationformation of of a thermallya thermally stable stable char ch thatar preventsthat prevents the underlying the underlying substrate substrate from ignition from ignitionand limits and the limits release the of release flammable of flammable gases during gases the during thermal the degradation thermal degradation process. Finally, process. the washingFinally, thefastness washing of the fastness flame of retardant the flame was retardant assessed: was in as particular,sessed: in theparticular, specimens the treatedspecimens with treated the sol-gel with theprecursor sol-gel ( 28precursor) did not exhibit(28) did any not change exhibit of theany phosphoruschange of the content phosphorus after the firstcontent washing after cycle.the first washing cycle.

PolymersPolymers 20162016,, 88,, 319x FOR PEER 1717 ofof 3636 Polymers 2016, 8, x FOR PEER 17 of 36

Figure 19. Representative schematic synthesis of precursors (27) and (28). Figure 19. Representative schematic synthesis of precursors (27) andand ((28).). 4.5 Flame Retardants as Polymer-Additives 4.5.4.5 Flame Flame Retardants Retardants as as Polymer-Additives Polymer-Additives Another possible strategy, differing from those described in the previous paragraphs, exploits the designAnother of possiblepossible phosphorus-based strategy,strategy, differing differing polymeric from from thoseflame those described retardants,described in in therather the previous previous than paragraphs,low paragraphs, molecular exploits exploits weight the designthecounterparts. design of phosphorus-basedof Indeed,phosphorus-based the former polymeric polymericare being flame flamewidely retardants, retardants, investigated rather rather becausethan than lowoflow their molecularmolecular durability weight and counterparts.ecofriendly character: Indeed, Indeed, thisthe the hasformer former pushed are are thebeing being scientific widely widely community investigated to employbecause because different of their polymeric durability flame and ecofriendlyretardants as character: additives this for has either pushed textiles the scientific scientific or engineering community plastics. to employ In addition, different polymeric polymeric flameflame retardantsretardants as asbearing additives additives hydroxyl for for either either or textiles carboxyl textiles or engineering groupsor engineering were plastics. found plastics. In addition,to be In very addition, polymeric effective polymeric flame for retardantscellulosic flame bearingretardantsmaterials. hydroxyl bearing or hydroxyl carboxyl groupsor carboxyl were foundgroups to were be very found effective to be for very cellulosic effective materials. for cellulosic materials.LiuLiu andand co-workersco-workers [[42]42] synthesizedsynthesized poly(1,2-dicarboxylpoly(1,2-dicarboxyl ethyleneethylene spirocyclicspirocyclic pentaerythritolpentaerythritol bisphosphonate)bisphosphonate)Liu and co-workers ((29)) byby reactingreacting [42] synthesized spirocyclicspirocyclic poly(1,2-dicarboxyl pentaerythritolpentaerythritol bisphosphoratebisphosphorate ethylene spirocyclic disphosphoryldisphosphoryl pentaerythritol chloridechloride withbisphosphonate)with tartarictartaric acidacid (Figure( 29(Figure) by reacting20 20).). Cotton Cotton spirocyclic fabrics fabrics were pentaerythritolwere washed washed with with bisphosphorate NaOH NaOH solution solution disphosphoryl and and then then soaked soaked chloride in anin aqueouswithan aqueous tartaric solution solution acid containing(Figure cont aining20). 30 Cotton wt30 wt % offabrics% (of28 )(28 andwere) and a washed catalyst. a catalyst. with After After NaOH drying, drying, solution the the samples samplesand then were were soaked cured cured in at 160anat 160aqueous◦C °C and and washedsolution washed with cont with sodiumaining sodium 30 carbonate wt carbonate % of ( solution28 solution) and anda catalyst. and then then dried After dried at drying, 70at ◦70C. °C. the samples were cured at 160 °C and washed with sodium carbonate solution and then dried at 70 °C.

FigureFigure 20.20.Synthetic Synthetic approach approach of poly(1,2-dicarboxyl of poly(1,2-dicarbo ethylenexyl spirocyclic ethylene pentaerythritol spirocyclic bisphosphonate).pentaerythritol Figurebisphosphonate). 20. Synthetic approach of poly(1,2-dicarboxyl ethylene spirocyclic pentaerythritol Thebisphosphonate). ﬂame retardant performance was investigated by LOI, vertical burning tests and TG analyses. The flame retardant performance was investigated by LOI, vertical burning tests and TG LOI values showed an increase up to 33.8% by increasing the add-on up to 21.2 wt %. Furthermore, analyses.The flameLOI values retardant showed performance an increase was up investigat to 33.8% edby byincreasing LOI, vertical the add-on burning up teststo 21.2 and wt TG%. analyses. LOI values showed an increase up to 33.8% by increasing the add-on up to 21.2 wt %.

Polymers 2016, 8, 319 18 of 36 Polymers 2016, 8, x FOR PEER 18 of 36 Polymers 2016, 8, x FOR PEER 18 of 36 Furthermore, vertical burning tests performed on the treated samples showed a reduction of the verticalFurthermore, burning vertical tests performed burning tests on the performed treated samples on the showedtreated asamples reduction showed of the afterglowa reduction time of andthe afterglow time and of the char length, without after-flame phenomena, as compared to the untreated afterglowof the char time length, and without of the char after-flame length, without phenomena, after-flame as compared phenomena, to the as untreated compared fabric. to the As untreated assessed fabric. As assessed by TG analyses, an anticipation of the onset thermal decomposition, as well as an fabric.by TG analyses,As assessed an by anticipation TG analyses, of the an onsetanticipation thermal of decomposition, the onset thermal as well decomposition, as an increase as ofwell the as char an increase of the char formation by increasing the add-on was found: once again, this latter finding increaseformation of by the increasing char formation the add-on by increasing was found: the once add-on again, was this found: latter once finding again, proves this the latter condensed finding proves the condensed phase mode of action of the flame retardant. phaseproves mode the condensed of action ofphase the flame mode retardant. of action of the flame retardant. Dong et al. reacted phenyl dichlorophosphate with ethylenediamine, in order to obtain the Dong et al. reacted reacted phenyl phenyl dich dichlorophosphatelorophosphate with with ethylenediamine, in in order order to obtain the poly(phoshorodiamidate) (30), shown in Figure 21. The product was applied onto cotton fabrics [43]. poly(phoshorodiamidate) (30), shown in Figure 2121.. The product wa wass applied onto cotton fabrics [43]. [43].

Figure 21. Representative synthesis of poly(phoshorodiamidate) (30). Figure 21. Representative synthesis of poly(phoshorodiamidate) (30). More specifically, cotton fabrics were dipped in solutions of (30) at different concentrations, in More specifically, cotton fabrics were dipped in solutions of (30) at different concentrations, in the presenceMore speciﬁcally, of tetraethyl cotton orthosilicate fabrics were employed dipped inas solutionsa crosslinker of (30 to) permanently at different concentrations, link the polymer in the to the presence of tetraethyl orthosilicate employed as a crosslinker to permanently link the polymer to presencethe fabric of substrate. tetraethyl The orthosilicate samples employedwere then as dr aied crosslinker and cured to permanently at 160 °C. All link samples the polymer were to then the the fabric substrate. The samples were then dried and cured ◦at 160 °C. All samples were then fabricwashed substrate. before testing. The samples The flame were thenretardant dried perf andormance cured at of 160 theC. treated All samples cotton were was theninvestigated washed washed before testing. The flame retardant performance of the treated cotton was investigated beforethrough testing. vertical The burning ﬂameretardant tests: as compared performance with of un thetreated treated fabrics, cotton wasshorter investigated after-glow through time and vertical char through vertical burning tests: as compared with untreated fabrics, shorter after-glow time and char burninglengths were tests: found as compared by increasing with the untreated add-on fabrics, of the treated shorter samples. after-glow time and char lengths were lengths were found by increasing the add-on of the treated samples. foundAbou-Okeil by increasing theet add-onal. of[44] the treatedexploited samples. the bulk polymerization process of Abou-Okeil et al. [44] exploited the bulk polymerization process of methacryloyloxyethylorthophosphortetraethyldiaAbou-Okeil et al. [44] exploitedmidate the monomer bulk polymerization in the presence processof benzoyl of methacryloyloxyethylorthophosphortetraethyldiamidate monomer in the presence of benzoyl methacryloyloxyethylorthophosphortetraethyl-diamidateperoxide as radical initiator, for synthesizing polymer (31) monomerexploiting ina three-step the presence reaction ofbenzoyl (Figure peroxide as radical initiator, for synthesizing polymer (31) exploiting a three-step reaction (Figure peroxide22). as radical initiator, for synthesizing polymer (31) exploiting a three-step reaction (Figure 22). 22).

Figure 22.22.Representative Representative synthesis synthesis of methacryloyloxyethylorthophosphorotetraethyl-diamidateof methacryloyloxyethylorthophosphorotetraethyl-diamidate polymer. Figure 22. Representative synthesis of methacryloyloxyethylorthophosphorotetraethyl-diamidate polymer. polymer. Cotton fabrics were dipped into aqueous solutions of polymer (31) at different concentrations, Cotton fabrics were dipped into aqueous solutions of polymer (31) at different concentrations, in theCotton presence fabrics of a were binder dipped and a crosslinker.into aqueous The solutions treated of fabrics polymer were (31 then) at squeezed,different concentrations, dried and then in the presence◦ of a binder and a crosslinker. The treated fabrics were then squeezed, dried and then incured the atpresence 150 C. of After a binder the treatments, and a crosslinker. the fabrics The increased treated theirfabrics LOI were values; then TG squeezed, measurements dried and revealed then cured at 150 °C. After the treatments, the fabrics increased their LOI values; TG measurements curedthe formation at 150 °C. of a After stable the char. treatments, Despite these the results,fabrics theincreased observed their changes LOI werevalues; not TG very measurements signiﬁcant as revealed the formation of a stable char. Despite these results, the observed changes were not very revealedcompared the to untreatedformation cotton.of a stable char. Despite these results, the observed changes were not very significant as compared to untreated cotton. significantRecently, as compared Dong et al. to [45 untreated] synthesized cotton. a guanidyl- and phosphorus-containing polysiloxane ﬂame Recently, Dong et al. [45] synthesized a guanidyl- and phosphorus-containing polysiloxane retardantRecently, (32): forDong this et purpose, al. [45] accordingsynthesized to Figurea guanidyl- 23, 3-aminopropylethoxysilane and phosphorus-containing was polysiloxane hydrolyzed flame retardant (32): for this purpose, according to Figure 23, 3-aminopropylethoxysilane was flameand then retardant reacted with(32): dicyandiamidefor this purpose, and according P2O5 (Figure to Figure23). 23, 3-aminopropylethoxysilane was hydrolyzed and then reacted with dicyandiamide and P2O5 (Figure 23). hydrolyzed and then reacted with dicyandiamide and P2O5 (Figure 23).

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Figure 23. Schematic synthesis of guanidyl- and phosphorus-containing polysiloxane (32).

After washing, cotton fabrics were soaked in a bath containing the FR polymer, After washing, cotton fabrics were soaked in a bath containing the FR polymer, 2-phosphonobutane-1,2,4-tricarboxylic acid and urea at room temperature, then squeezed in 2-phosphonobutane-1,2,4-tricarboxylic acid and urea at room temperature, then squeezed in laboratory laboratory scale padder, rinsed with water, dried at 80 °C and cured at 160 °C. The flame retardant scale padder, rinsed with water, dried at 80 ◦C and cured at 160 ◦C. The flame retardant performance of performance of the treated samples was tested by cone calorimetry and thermogravimetric analyses. the treated samples was tested by cone calorimetry and thermogravimetric analyses. Cone calorimetry Cone calorimetry measurements showed that by increasing the add-on of the FR polymer, HRR and measurements showed that by increasing the add-on of the FR polymer, HRR and THR decreased THR decreased accordingly, hence, indicating that the flame retardant hinders the formation of accordingly, hence, indicating that the flame retardant hinders the formation of combustible volatile combustible volatile species. In addition, the CO2/CO ratio decreased dramatically with increases in species. In addition, the CO2/CO ratio decreased dramatically with increases in the FR add-on. the FR add-on. TG measurements performed on the treated cotton fabrics showed an increase of the TG measurements performed on the treated cotton fabrics showed an increase of the decomposition decomposition temperature with, at the same time, an increase of char formation. It is worthy to temperature with, at the same time, an increase of char formation. It is worthy to mention that, mention that, in addition to the enhanced flame retardant performances, the treated fabrics showed in addition to the enhanced flame retardant performances, the treated fabrics showed an effective an effective antimicrobial behavior against E. coli and S. aureus [46]. antimicrobial behavior against E. coli and S. aureus [46]. Very recently, the same group synthesized a novel nitrogen and phosphorus-containing Very recently, the same group synthesized a novel nitrogen and phosphorus-containing polysiloxane ((33), Figure 24), starting from poly(4-iodo-n-butoxy)methylsiloxane and polysiloxane ((33), Figure 24), starting from poly(4-iodo-n-butoxy)methylsiloxane and N-methylo-3-(dimethoxy dibenzyloxyphosphoryl) acrylic amide [47]. N-methylo-3-(dimethoxy dibenzyloxyphosphoryl) acrylic amide [47]. By adopting a similar approach, the polymeric additive ((34), Figure 24) was also synthesized By adopting a similar approach, the polymeric additive ((34), Figure 24) was also synthesized using poly (4-iodo-n-butoxy) methylsiloxane and guanidine sulfamate [48]. using poly (4-iodo-n-butoxy) methylsiloxane and guanidine sulfamate [48]. Both polymeric flame retardant additives were applied onto cotton fabrics, using an Both polymeric flame retardant additives were applied onto cotton fabrics, using an impregnation impregnation bath also containing urea and zirconium oxide. The samples were then dried and bath also containing urea and zirconium oxide. The samples were then dried and cured at 160 and cured at 160 and 150 °C (for treated fabrics with (33) and (34), respectively). Cone calorimetry and 150 ◦C (for treated fabrics with (33) and (34), respectively). Cone calorimetry and TG analyses were TG analyses were exploited for assessing the performances of the samples treated with polymer (33): exploited for assessing the performances of the samples treated with polymer (33): more specifically, more specifically, the treated fabrics showed an enhancement of the flame retardancy by decreasing the treated fabrics showed an enhancement of the flame retardancy by decreasing HRR, THR and HRR, THR and CO2/CO ratio. TG data showed a decrease of the onset decomposition temperature CO2/CO ratio. TG data showed a decrease of the onset decomposition temperature and an increase of and an increase of the char yield for all treated samples. Furthermore, LOI values of the treated the char yield for all treated samples. Furthermore, LOI values of the treated samples did not show samples did not show a significant drop after several washing cycles, hence, proving the durability a significant drop after several washing cycles, hence, proving the durability of flame retardant (33). of flame retardant (33). A similar FR performance was also assessed for the cotton fabrics treated A similar FR performance was also assessed for the cotton fabrics treated with polymer (34): unlike with polymer (34): unlike (32), the treated fabrics also showed water repellency, which was (32), the treated fabrics also showed water repellency, which was attributed to the reaction of the flame attributed to the reaction of the flame retardant with cotton fibers, leading to methyl group retardant with cotton fibers, leading to methyl group orientation on the fiber surface. orientation on the fiber surface.

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Figure 24. Representative synthetic approach for flame retardants (33) and (34). Figure 24. 33 34 Figure 24.Representative Representativesynthetic synthetic approach for flame ﬂame retardants retardants (33 ( ) )and and (34 ( ). ). Very recently, Chen and co-workers synthesized a polymeric sulfur-containing flame retardant Very recently, Chen and co-workers synthesized a polymeric sulfur-containing ﬂame retardant ((35),Very Figure recently, 25) Chenfor andnylon co-workers fabrics, synthesize using dthiourea, a polymeric phosphoric sulfur-containing acid, flamewater retardantsoluble ((35), Figure 25) for nylon fabrics, using thiourea, phosphoric acid, water soluble isocyanate-terminated ((isocyanate-terminated35), Figure 25) for crosslinker nylon (WIT)fabrics, and using epichlorohydrin thiourea, [49].phosphoric acid, water soluble crosslinkerisocyanate-terminated (WIT) and epichlorohydrin crosslinker (WIT) [49 and]. epichlorohydrin [49]. O O H H N H P H N Cl O OO H O H N H Cl OH S PO N OP O S n Cl O OH S O H POH H N NH S n O Cl OH O 2 2 OH OH H N NH HO OP OH OH 2 2 OH HO OHP OH Cl OH Cl

O S O O O WIT H OP S O WIT O O O WIT O N N N N N N H Nylon 66 H OP n WIT Op H H H H H H Nylon 66 O N N N N N N O p H O H H n H H H H O O H H H H O N N N N O H Cl O H WIT H H H O N N N N p H Cl O O O O WIT p O O 35 O 35

H N H O N S ONa O O OS ONa O NH O O NH O O O O O O H O OS WIT = O N N O H O ONa NaO H H N N O = S N O O H S WIT NaO O N H O ONa SO O H O N N O H O O O

Figure 25. Synthetic route of polymeric additive (35). Figure 25. Synthetic route of polymeric additive (35). Figure 25. Synthetic route of polymeric additive (35).

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In particular, nylon fabrics fabrics were were dip-treated dip-treated in in a a finishing finishing solution solution containing containing the the FR FR polymer, polymer, a acrosslinker crosslinker and and potassium potassium thiocyanate. thiocyanate. After After dryi drying,ng, the the samples samples were were cured cured at at 150 150 °C◦C and then soaked in water/soap water/soap solution, solution, rinsed rinsed with with water water and and dried dried again. again. As Asassessed assessed by TG by TGanalyses, analyses, the theonset onset decomposition decomposition temperature temperature of ofthe the treate treatedd fabrics fabrics was was anticipated, anticipated, notwithstanding a substantial increase of the char formation with respectrespect to thethe untreateduntreated counterparts.counterparts. MCC analyses showed an important decrease of HRR, THR and HRC, together with a significant significant char formation. The durabilitydurability ofof thethe flame flame retardant retardant treatment treatment was was also also assessed: assessed: 10 washing10 washing cycles cycles did did not havenot have any detrimentalany detrimental effect effect on the on flamethe flame retardants’ retardants’ performances. performances.

4.6.4.6 Phosphoramidate Phosphoramidate Flame Flame Retardants Retardants Phosphoramidate derivatives are promising flame ﬂame retardant additives, du duee to the synergistic effect provided by phosphorusphosphorus andand nitrogen.nitrogen. Gaan andand co-workers co-workers [50 ][50] synthesized synthesized different different phosphoramidate phosphoramidate derivatives derivatives as ﬂame as retardant flame additivesretardant foradditives cotton fabrics,for cotton following fabrics, the following Atherton-Todd the Atherton-Todd reaction in the presencereaction ofin dialkylphosphites the presence of asdialkylphosphites starting materials as (Figurestarting 26 materials). (Figure 26).

R O H N OH O 2 P R=methyl;36 N OH O H R = ethyl; 37 R

O 1/2 HN NH R O O R R=methyl;38 CCl OPNNP O OPH 4 R = ethyl; 39 R O O O R R R

R NH R O 2 O H R=methyl;40 1/2 H2N O N P R P N O R = ethyl; 41 H O O R

Figure 26. Scheme of the synthesis of phosphoramidate derivatives (36)–(41).

The productsproducts were were applied applied onto cottononto fabricscotton fromfabrics acetone from solutions acetone with solutions different with concentrations different ofconcentrations phosphoramidates of phosphoramidates (36)–(41). LOI ( values36)–(41 were). LOI found values to were increase found by to increasing increase by the increasing phosphorus the contentphosphorus of the content treated of cotton. the treated cotton. Nguyen and co-workers co-workers further further investigat investigateded the the flame flame retardant retardant performance performance of of(36 (36) and) and (37 ()37 at) atdifferent different add-ons add-ons on oncotton cotton fabrics fabrics [51]: [51 in]: particular, in particular, the fabrics the fabrics treated treated with with an add-on an add-on beyond beyond 5 wt 5% wt showed % showed no afterflame no afterflame or afterglow or afterglow time, time, providing providing the thefabrics fabrics with with self-extinction. self-extinction. The The effect effect of ofchemical chemical structure structure on on the the performance performance of of these these flame flame retardants retardants was was stud studiedied using using MCC. MCC. It was found that THR valuesvalues decreaseddecreased withwith increasesincreases inin thethe add-onadd-on ofof productproduct ((3636).). Conversely, THR values werewere found found to to increase increase by increasingby increasing the add-on the add-on of (37). of Product (37). Product (39) promoted (39) promoted char formation char andformation decreased and decreased the THRof the the THR treated of the samples treated withsamp increasesles with increases in the add-on in the onadd-on the treated on the treated fabrics. Onfabrics. the otherOn the hand, other vertical hand, flammability vertical flammability tests performed tests performed on cotton fabricson cotton treated fabrics with treated (39) showed with (39 an) increaseshowed an after increase flame andafter no flame afterglow and no phenomena afterglow phenomena [52]. [52]. As a comparison to (39), the same group [[53]53] synthesized new flameflame retardants usingusing dimethyl phosphorochloridothionate andand diethyl diethyl phosphorochloridate phosphorochloridate as as starting starting material material (Figure (Figure 27 ).27). From From an overallan overall point point of view, of view, it was it foundwas found that the that flame the flame retardant retardant performance performance of the fabricsof the treatedfabrics withtreated39 waswith much 39 was better much than better that ofthan the that substrates of the substrates treated with treated same add-onwith same of (42 add-on). Furthermore, of (42). Furthermore, when cotton fabricswhen cotton were treatedfabrics withwere (treated39) and with (43), ( they39) and showed (43), they almost showed the same almost FR performances.the same FR performances.

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Figure 27. Synthesis of (42) and (43). FigureFigure 27.27. Synthesis of (4242)) andand ((4343).). The phosphoramidate (43) was also investigated as flame retardant additives for cotton twill TheThe phosphoramidatephosphoramidate ( 43(43)) was was also also investigated investigated as as ﬂame flame retardant retardant additives additives for cottonfor cotton twill twill and and cotton print cloth [55]. The flame retardant performance of the treated fabrics was found to cottonand cotton print clothprint [cloth55]. The [55]. ﬂame The retardantflame retardant performance performance of the treated of the fabrics treated was fabrics found was to depend found onto depend on the type of treated fabrics: in particular, the add-on being equal, the treated cotton twill thedepend type on of treatedthe type fabrics: of treated in particular, fabrics: in the particular, add-on being the add-on equal, the being treated equal, cotton the treated twill showed cotton better twill showed better FR performances as compared with the treated print cloth. FRshowed performances better FR as performances compared with as compared the treated with print the cloth. treated print cloth. Recently [56], ditrimethylolpropane di-N-hydroxyethyl phosphoramide (44) was synthesized RecentlyRecently [[56],56], ditrimethylolpropaneditrimethylolpropane di-N-hydroxyethyldi-N-hydroxyethyl phosphoramide (44) waswas synthesizedsynthesized using ditrimethylolpropane and phosphoryl chloride as starting materials (Figure 28). usingusing ditrimethylolpropaneditrimethylolpropane andand phosphorylphosphoryl chloridechloride asas startingstarting materialsmaterials (Figure(Figure 28 28).).

Figure 28. Representative schematic synthesis of (44). Figure 28.28. Representative schematicschematic synthesissynthesis ofof ((4444).). The obtained FR additive (44) was applied to cotton, nylon and polyester fabrics, using a The obtained FR additive (44) was applied to cotton, nylon and polyester fabrics, using a dippingThe method. obtained The FR additivefabrics were (44) wasthen applieddried and to cotton,cured at nylon 160 °C. and The polyester flame fabrics,retardant using performance a dipping dipping method. The fabrics were then dried and cured◦ at 160 °C. The flame retardant performance ofmethod. the treated The fabrics was were tested then using dried vertical and cured flammability at 160 C. tests: The flame it was retardant found that performance the treated nylon of the of the treated fabrics was tested using vertical flammability tests: it was found that the treated nylon fabricstreated showed fabrics was better tested flame using retardant vertical performances flammability as tests: compared it was foundwith the that other the treatedtreated nylonfabrics. fabrics fabrics showed better flame retardant performances as compared with the other treated fabrics. showedShariatinia better flame and co-workers retardant performances [57] synthesized as compared different with phosphoramidates the other treated and fabrics. phosphoramides Shariatinia and co-workers [57] synthesized different phosphoramidates and phosphoramides startingShariatinia from their and corresponding co-workers [57 phosphoramidic] synthesized different dichloride phosphoramidates in an ultrasonic and bath, phosphoramides obtaining the starting from their corresponding phosphoramidic dichloride in an ultrasonic bath, obtaining the flamestarting retardant from their additives corresponding (45)–(49 phosphoramidic) shown in Figure dichloride 29. All in these an ultrasonic flame retardant bath, obtaining additives the flamewere flame retardant additives (45)–(49) shown in Figure 29. All these flame retardant additives were appliedretardant to additives cotton fabrics. (45)–( 49The) shown cotton in fabrics Figure were 29. Alltreated these with flame an retardant aqueous additivessolution of were each applied flame applied to cotton fabrics. The cotton fabrics were treated with an aqueous solution of each flame retardantto cotton fabrics.additive The at different cotton fabrics concentrations were treated and within the an presence aqueous of solution a thermal of eachinitiator. flame The retardant treated retardant additive at different concentrations and in the presence of a thermal initiator. The treated samplesadditive were at different then dried concentrations and cured andat 121 in °C the and presence their flame of a thermal retardant initiator. performance The treated was assessed samples samples were then dried and cured◦ at 121 °C and their flame retardant performance was assessed throughwere then vertical dried burning and cured tests: at 121the flameC and retardant their flame additives retardant (46) performance and (48) showed was a assessed lower degree through of through vertical burning tests: the flame retardant additives (46) and (48) showed a lower degree of graftingvertical burningand an tests:increased the flame char retardant length, as additives compared (46) andwith ( 48the) showed samples a lowertreated degree with ofthe grafting same grafting and an increased char length, as compared with the samples treated with the same concentrationand an increased of (45 char), (47 length,) and ( as49) compared additives. withIn general, the samples the char treated length with was thefound same to decrease concentration with concentration of (45), (47) and (49) additives. In general, the char length was found to decrease with increasesof (45), (47 in) andthe (degree49) additives. of grafting, In general, which thein turn char depended length was on found the additive to decrease concentration with increases in the in increases in the degree of grafting, which in turn depended on the additive concentration in the correspondingthe degree of grafting, treatment which solutions. in turn depended on the additive concentration in the corresponding treatmentcorresponding solutions. treatment solutions.

Polymers 2016, 8, 319 23 of 36 Polymers 2016, 8, x FOR PEER 23 of 36

Figure 29. Representative synthesis of flame retardant additives (45)–(49). Figure 29. Representative synthesis of ﬂame retardant additives (45)–(49).

4.7.4.7 Miscellaneous Miscellaneous and and Potential Potential Phosphorus-Based Phosphorus-Based Flame Flame Retardants Retardants for for Textile Textile Applications In additionaddition toto the the list list of of chemical chemical structures structures and and chemical chemical classiﬁcations classifications that are that mentioned are mentioned earlier, itearlier, is worth it listingis worth some listing miscellaneous some miscellaneous and promising and ﬂame promising retardants flame based retardants on their chemical based structureon their andchemical behavior structure (Table and2). behavior (Table 2).

Polymers 2016, 8, x FOR PEER 24 of 36 Polymers 20162016,, 88, ,x 319 FOR PEER 24 of 36 24 of 36 Polymers 2016, 8, x FOR PEER 24 of 36

Table 2. Miscellaneous and potential promising flame retardants for textile applications. TableTable 2. Miscellaneous 2. Miscellaneous and andpotential potential promising promising flame flameretardants retardants for textile for textileapplications. applications. Table 2. Miscellaneous and potential promising flame retardants for textile applications. Chemical structure Textile material Highlights Ref. Chemical structure Textile material Highlights Ref. Chemical structure TextileTextile material material HighlightsHighlights Ref. Ref. The FR monomer was grafted onto cotton fabrics using gamma chamber. The FR monomer was grafted onto cotton fabrics using gamma chamber. TGThe analysisFR monomer of the was treated grafted fabrics onto showed cotton fabrics a decrease using of gamma the onset chamber. decomposition temperature cotton TG analysisThe FR of monomer the treated was fabrics grafted showed onto cotton a decrease fabrics of using the onset gamma decomposition chamber. temperature [58] cotton andTG analysisincreaseTG analysis ofof thechar of treated formation. the treated fabrics fabrics showed showed a de acrease decrease of the of theonset onset decomposition decomposition temperature temperature [58] cottoncotton and increase of char formation. [58] [58] Theand burningincreaseand increase testof char showed of formation. char that formation. treated fabrics burnt much slower than untreated fabrics. The burning test showed that treated fabrics burnt much slower than untreated fabrics. The burningThe burning test showed test showed that treated that treated fabrics fabrics burnt burntmuch muchslower slower than untreated than untreated fabrics. fabrics. The FR was applied to viscose fiber fabric through grafting polymerization. The FR was applied to viscose fiber fabric through grafting polymerization. LOIThe FRvalues was increased applied to by viscose increa fibersing thefabric grafting through percentage. grafting polymerization. LOI valuesThe FR increased was applied by increa to viscosesing the fiber grafting fabric percentage. through grafting polymerization. Viscose fiber LOI values increasedwere almost by unchangedincreasing the after grafting many percentage.washing cycles which showed the durability of Viscose fiber LOI valuesLOI valueswere almost increased unchanged by increasing after many the grafting washing percentage. cycles which showed the durability of [59] Viscosefabric fiber theLOI covalent valuesLOI values were bond. almost were almost unchanged unchanged after many after manywashing washing cycles cycles which which showed showed the durability the durability of [59] Viscosefabric fiber fabric the covalent bond. [59] [59] fabric Conethe covalent ofcalorimetry the covalentbond. data bond. of treated fabric showed a significant decrease of PHRR and THR Cone Conecalorimetry calorimetry data dataof treated of treated fabric fabric showed showed a significant a significant decrease decrease of of PHRR andand THRTHR comparedCone calorimetry to untreated data fabric.of treated fabric showed a significant decrease of PHRR and THR compared to untreated fabric. Thecompared FRcompared was to used untreated toas untreated sol-gel fabric. precursor. fabric. The FR was used as sol-gel precursor. ItThe was FR Thehydrolyzed was FR used was as usedwith sol-gel as3-aminopropyltriet sol-gel precursor. precursor. hoxysilane (APTES) in deionized water in presence of It was hydrolyzed with 3-aminopropyltriethoxysilane (APTES) in deionized water in presence of HClIt was followed Ithydrolyzed was hydrolyzed by the with addi 3-aminopropyltriet withtion of 3-aminopropyltriethoxysilane a crosslinker.hoxysilane (APTES) (APTES) in deionized in deionized water in water presence in of HCl followed by the addition of a crosslinker. TheHCl fabricsfollowedpresence were by of treatedthe HCl addi followed bytion impregnation of bya crosslinker. the addition in solutions of a crosslinker. and cured at 150 °C after processing. cotton The fabricsThe fabrics were treated were treated by impregnation by impregnation in solutions in solutions and cured and curedat 150 at°C 150 after◦C processing. after processing. [60] cottoncotton The fabricsburning were test treated showed by thatimpregnation all treated in cotto solutionsn fabrics and curedburned at completely150 °C after withprocessing. significant [60] [60] cotton The burningThe burning test showed test showed that thatall treated all treated cotto cottonn fabrics fabrics burned burned completely completely with with significant significant [60] reductionThe burning of burning test showed time andthat burning all treated rate cottocomparedn fabrics to untreated burned completelyfabrics. with significant reductionreduction of burning of burning time and time burning and burning rate compared rate compared to untreated to untreated fabrics. fabrics. Thereduction FR exhibited of burning condensed time and phase burning mode rate of compared action as toTG untreated analysis fabrics.showed an increase of char The FRThe exhibited FR exhibited condensed condensed phase phase mode mode of action of action as TG as analysis TG analysis showed showed an increase an increase of char of formation.The FR exhibited condensed phase mode of action as TG analysis showed an increase of char formation.char formation. Theformation. cotton fabrics were treated by soaking in finishing baths of FR, each at different The cottonThe cotton fabrics fabrics were were treated treated by by soaking soaking in finishingfinishingbaths baths of of FR, FR, each each at different at different concentrationsThe cotton fabrics but all were at pH treated = 5. by soaking in finishing baths of FR, each at different Polymers 2016, 8, x FOR PEER concentrationsconcentrations but all butat pH all at= 5. pH = 5. 25 of 36 Theconcentrations FRThe can FR react can but with react all atcellulose with pH cellulose= 5. without without using usingother crosslinkers. other crosslinkers. cottoncotton The FR can react with cellulose without using other crosslinkers. [61] [61] cotton LOIThe FRvaluesLOI can values ofreact treated with of treated fabrics cellulose fabrics increased without increased by using increasing by other increasing crosslinkers. the FR the concentration. FR concentration. [61] cotton LOI valuesTable of treated 2. Cont. fabrics increased by increasing the FR concentration. [61] TheLOI treatedvaluesThe treated offabrics treated fabrics were fabrics found were increased founddurable durable by up increasing to up50 washing to 50 the washing FR cycles concentration. cycles as no aschange no change had been had beennoticed The treated fabrics were found durable up to 50 washing cycles as no change had been noticed Chemical structure Textile material onThe the treated noticedLOI values fabrics on the or were LOIthe physicalfound values durable or performance the physical upHighlights to 50 of performance washing the treated cycles fabrics. of the as no treated change fabrics. had been noticed Ref. on the LOI values or the physical performance of the treated fabrics. on the LOI values or the physical performance of the treated fabrics. The cotton fabrics were treated by impregnation in an aqueous solution of FR, binder, The cotton fabrics were treated by impregnation in an aqueous solution of FR, binder, crosslinkercrosslinker and pyrovatex. and pyrovatex. cottoncotton [44] [44] LOI valuesLOI values of the oftreated the treated fabrics fabrics with withdifferent different binders binders and andcrosslinkers crosslinkers were were below below 21 21which which meansmeans they can they ignite can igniteeasily easilyand burn and rapidly. burn rapidly.

The fabrics were treated by immersion in an aqueous solution of the FR in the presence of a buffer and shrinking agent. LOI measurements showed an increase in LOI values with increases in the FR concentration. cotton The treated fabrics showed durability up to 30 laundry cycles which was considered as [62] semi-durable flame-resistant.

The vertical burning test showed no afterflame or afterglow and a decrease in the char length with an increase in the FR concentration. The fabrics were treated by immersion in an aqueous solution of the FR in the presence of a shrinking agent The treated fabrics showed durability up to 30 laundry cycles which was considered as cotton [63] semi-durable flame-resistant The vertical burning test showed no afterflame or afterglow and a decrease in the char length with an increase in the FR concentration.

Meltable flame retardant which facilitates the compounding process. Rheological measurements showed that this FR behaves as a plasticizer for PA6. PA6 fibers TG analysis showed higher char formation of formulated PA6 with the FR compared to neat [64] PA6, while PCFC showed a predominant action in the gas-phase mode. UL-94 test showed a V0 rating with 15 wt % of FR in PA6.

PET and PBT The UL-94 test for the formulated PET and PBT fibers with FR showing a V0 rating. [65] fibers

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Table 2. Cont. Table 2. Cont. Polymers 2016, Chemical8, 319 structure Textile material Highlights Ref. 25 of 36 Chemical structure Textile material Highlights Ref. The cotton fabrics were treated by impregnation in an aqueous solution of FR, binder, The cotton fabrics were treated by impregnation in an aqueous solution of FR, binder, crosslinkerThe cotton andfabrics pyrovatex. were treated by impregnation in an aqueous solution of FR, binder, cotton crosslinker and Tablepyrovatex. 2. Cont. [44] cotton LOIcrosslinker values andof the pyrovatex. treated fabrics with different binders and crosslinkers were below 21 which [44] cotton LOI values of the treated fabrics with different binders and crosslinkers were below 21 which [44] meansLOI values they ofcan the ignite treated easily fabrics and burnwith rapidly.different binders and crosslinkers were below 21 which Chemical structure Textile material means they can ignite easily and burn rapidly.Highlights Ref. means they can ignite easily and burn rapidly.

The fabricsThe fabrics were treated were treated by immersion by immersion in an in aque an aqueousous solution solution of the of theFR FRin the in the presence presence of ofa a The fabrics were treated by immersion in an aqueous solution of the FR in the presence of a buffer bufferand shrinking and shrinking agent. agent. buffer and shrinking agent. LOI measurementsLOI measurements showed showed an incr anease increase in LOI in values LOI values with increa with increasesses in thein FR the concentration. FR concentration. LOI measurements showed an increase in LOI values with increases in the FR concentration. cottoncotton The treatedThe treated fabrics fabrics showed showed durability durability up upto to30 30 laundry laundry cycles whichwhich waswas considered considered as as [62] [62] cotton The treated fabrics showed durability up to 30 laundry cycles which was considered as [62] semi-durablesemi-durable flame-resistant. flame-resistant. semi-durable flame-resistant. The verticalThe vertical burning burning test showed test showed no afterflame no afterflame or afterglow or afterglow andand a decrease a decrease in the in the char char length length The verticalwith an burning increase test in theshowed FR concentration. no afterflame or afterglow and a decrease in the char length with an increase in the FR concentration. with an increase in the FR concentration. The fabrics were treated by immersion in an aqueous solution of the FR in the presence of a The fabricsThe fabrics were treated were treated by immersion by immersion in an in aque an aqueousous solution solution of the of theFR FRin the in the presence presence of ofa a shrinking agent shrinkingshrinking agent agent The treatedThe treated fabrics fabrics showed showed durability durability up upto to30 30 laundry laundry cycles whichwhich waswas considered considered as as cottoncotton The treated fabrics showed durability up to 30 laundry cycles which was considered as [63] [63] cotton semi-durablesemi-durable flame-resistant flame-resistant [63] semi-durable flame-resistant The verticalThe vertical burning burning test showed test showed no afterflame no afterflame or afterglow or afterglow andand a decrease a decrease in the in the char char length length The vertical burning test showed no afterflame or afterglow and a decrease in the char length with anwith increase an increase in the inFR the concentration. FR concentration. with an increase in the FR concentration. Meltable flame retardant which facilitates the compounding process. MeltableMeltable flame retardant flame retardant which whichfacilitates facilitates the compounding the compounding process. process. Rheological measurements showed that this FR behaves as a plasticizer for PA6. RheologicalRheological measurements measurements showed showed that this that FR this behaves FR behaves as a plasticizer as a plasticizer for PA6. for PA6. PA6 fibers TGRheological analysis measurementsshowed higher showed char formation that this ofFR formulated behaves as aPA6 plasticizer with the for FR PA6. compared to neat [64] PA6PA6 fibers fibers TG analysisTG analysis showed showed higher higher char charformation formation of formulated of formulated PA6 PA6 with with the theFR FRcompared compared to toneat neat [64] [64] PA6 fibers PA6,TG analysis while PCFC showed showed higher a predominchar formationant action of formulated in the gas-phase PA6 with mode. the FR compared to neat [64] PA6, whilePA6, whilePCFC PCFCshowed showed a predomin a predominantant action actionin the ingas-phase the gas-phase mode. mode. UL-94PA6, while test showed PCFC showed a V0 rating a predomin with 15ant wt action% of FR in in the PA6. gas-phase mode. UL-94 UL-94test showed test showed a V0 rating a V0 ratingwith 15 with wt % 15 of wt FR % in of PA6. FR in PA6. UL-94 test showed a V0 rating with 15 wt % of FR in PA6.

PET and PBT PET and PBT The UL-94 test for the formulated PET and PBT fibers with FR showing a V0 rating. [65] PETfibers and PBT ﬁbersThe UL-94The UL-94test for test the for formulated the formulated PET and PET PBT and fibers PBT ﬁberswith FR with showing FR showing a V0 rating. a V0 rating. [65] [65] fibers

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5. Phosphorus-Containing Biomacromolecules as Flame Retardants The design of environmentally friendly flame retardant systems for textiles involves another possible strategy, recently studied [66–68], which is based on the use of some biomacromolecules (in particular proteins like caseins, hydrophobins and whey proteins, and DNA—deoxyribonucleic acid). Indeed, the chemical structure of some of these products shows the presence of phosphorus and other elements (like nitrogen and/or sulphur), which can confer flame retardant features to different fibers and fabrics. The use of biomacromolecules as FRs represents a potential innovative strategy that goes far beyond the chemical approach behind the design of standard flame retardants for textiles. Up to 5 years ago, the utilization of these biomacromolecules was undoubtedly delineated to other application fields. Among them, biomacromolecules have been used as edible films, adhesives, food emulsifiers, printing, papermaking, leather finishing systems, as well as for the design of biosensors and environmental monitoring systems. Several advantages can be conferred from the exploitation of proteins and DNA in providing FR features to textiles [67]: in particular, their ease of manipulation, the possibility of exploiting application techniques that are already designed and optimized for fabric finishing (like impregnation/exhaustion, spray, or even the layer-by-layer deposition [69]) and the set-up of low impact/sustainable finishing recipes (thanks to the use of water-based solutions/dispersions). In addition, caseins and whey proteins are somehow by-products or even wastes recovered from the agro-food industry: therefore, their use as possible FRs may represent a new way toward the valorization of agro-food crops, reducing and/or preventing their landfill confinement. Finally, the high price of DNA with respect to standard chemical FRs is being overcome, as large scale extraction and purification processes have been developed [70]. The next paragraphs will describe the main outcomes and accomplishments resulting from the use of these P-based sustainable products as a valuable, potentially eco-friendly alternative to traditional chemical FRs for cotton, polyester and cotton-polyester blends. Table3 summarizes the main results achieved so far. In particular, some fresh examples of textile fireproofing provided by these phosphorus-based biomacromolecules will be thoroughly described, highlighting some structure–fire behavior relationships for the chosen biomacromolecules. Polymers 2016, 8, 319 27 of 36

Table 3. Main results achieved by treating fabrics with biomacromolecules.

Durability Type of P-based FR Textile material Highlights Ref. (washing fastness)

Anticipation of cotton degradation as assessed by TG analyses (T10%onset values: 285 ◦C vs. 329 ◦C for treated and untreated fabrics, respectively). High residue (18%) at 600 ◦C in nitrogen (TG analyses). Significant increase of the total burning time (+40%), and strong reduction of the Casein Cotton No [71] total burning rate (−35%) with respect to untreated cotton in flammability tests. Remarkable increase of the final residue after flammability tests (+34%). Strong reduction of TTI (−52%) and of PHRR (−27%) with respect to untreated cotton (Cone calorimetry tests).

Anticipation of polyester degradation as assessed by TG analyses (T10%onset values: 315 ◦C vs. 400 ◦C for treated and untreated fabrics, respectively). Increase of LOI after the biomacromolecule treatment (from 21% to 26%). High residue (22%) at 600 ◦C in nitrogen (TG analyses). Significant reduction of the total burning rate (−67%) with respect to untreated Casein Polyester polyester in flammability tests. No [72] Remarkable increase of the final residue after flammability tests (+77%). In cone calorimetry tests, strong reduction of TTI (from 112 to 62 s, for untreated and treated polyester) and increase of the residue (from 2, neat polyester, to 11%, treated fabric). Melt dripping phenomena also observed for the treated fabrics.

Anticipation of fabric blend degradation as assessed by TG analyses (T10%onset values: 332 ◦C vs. 304 ◦C for treated and untreated fabrics, respectively). High residue (22%) at 600 ◦C in nitrogen (TG analyses). Remarkable increase (+64%) of the total burning time and decrease of the total Cotton-polyester Casein burning rate (−36%) in ﬂammability tests. No [72] blend (35%–65%) Cone calorimetry tests: strong reduction of TTI (from 30 to 12 s, for untreated and treated fabric blend) and of PHRR (−15%) and increase of the residue (from 3, neat blend, to 5%, treated fabric). Slight increase of LOI after the biomacromolecule treatment (from 19% to 21%). Polymers 2016, 8, 319 28 of 36

Table 3. Cont.

Durability Type of P-based FR Textile material Highlights Ref. (washing fastness)

Anticipation of cotton degradation as assessed by TG analyses (T10%onset values: 244 ◦C vs. 335 ◦C for treated and untreated fabrics, respectively). High residue (34%) at 600 ◦C in nitrogen (TG analyses). DNA Cotton No [73] Self-extinction; flame out reached within 2 s. Cone calorimetry tests: no ignition under a heat flux of 35 kW/m2. Significant increase of LOI (from 18, untreated cotton, to 28%, DNA-treated cotton). 10 wt % is the minimum biomacromolecule add-on necessary to reach the flame out of cotton. DNA Cotton No [74] Cone calorimetry tests performed at 50 kW/m2: remarkable decrease of PHRR (−60%).

Being equal the final dry add-on on the fabrics, low MW DNA (100–200 bp) is more effective in enhancing the fire behavior of cotton, with respect to high MW DNA (3000–10,000 bp), providing the highest residues after horizontal flame spread tests DNA Cotton No [75] and self-extinction for about the 86% of the tested samples impregnated at pH = 4. 2 Cone calorimetry tests performed at 35 kW/m on fabrics treated with low MW DNA: reduction of THR (−36%) and of PHRR (−40%). LbL treatments with chitosan (20 bi-layers, BL) provide self-extinction to the fabric. Cone calorimetry tests performed at 35 kW/m2 show a significant decrease of TTI DNA Cotton (from 39 s, for untreated cotton, to 23 s, for 20 BL-treated fabric), of PHRR (−41%) No [76] and of THR (−32%); the LbL treatment provide an increase of the residue (from 2% for untreated cotton, to 13%, for 20 BL-treated fabric). Polymers 2016, 8, 319 29 of 36

5.1. Caseins Caseins are phosphorylated proteins that represent the main fraction of milk proteins (around 80%) and, possibly, the most widely investigated food proteins, obtained as co-products during the production of skim milk. Their main constituents are:

• αS1-caseins: They contain eight or nine bound phosphate groups/mol and represent the major protein fraction of bovine milk;

• αS2-caseins: They show a structure similar to αS1-casein, as they contains, according to its four differentially phosphorylated isoforms, around 10–13 phosphate groups/mol;

• β-caseins: They are less phosphorylated with respect to αS1- and αS2- counterparts. They include glutamines bearing amino groups and they display a single major phosphorylation site near the N-terminus. Bovine β-caseins can be sourced in a single, fully phosphorylated form containing ﬁve phosphate groups/mol; • κ-caseins: As regard to any other casein, these constituents have the smallest phosphate component. In particular, the phosphorylated regions, present as single sites, are positioned in the C-terminal region of the biomacromolecule.

Notwithstanding their standard cheese farming uses, these proteins have been mainly employed as a food ingredient for improving different physical properties like foaming and whipping, water binding and thickening, texture and emulsification. Furthermore, they have been exploited as coatings, specifically referring to papermaking, leather finishing, printing and manufacturing of synthetic fibers [77]. As far as fire retardancy goes, cotton, polyester and cotton polyester blend fabrics (polyester content: 65 wt %) were treated with a caseins aqueous suspension (5 wt %) in a climatic chamber (30 ◦C and 30% R.H.): the suspension was spread onto the samples with a spatula and the excess was removed by gently pressing with a rotary drum; then the coated samples were dried to constant weight. The final dry add-on was 20 wt % [71,72]. The presence of the casein coatings promoted a strong anticipation of both cellulose and polyester decomposition: this behavior, observed for all the types of treated fabrics, was attributed to the phosphate groups located on the shell of casein micelles, which, upon heating, release phosphoric acid that favors the degradation of cellulose or polyester toward the formation of a stable char. This latter exerts a protective effect on the underlying textile, limiting the oxygen diffusion and absorbing the heat evolved during the combustion [78]. Therefore, the anticipation of the fabric degradation occurs, but, at the same time, is accompanied by the formation of a thermally stable char. The fire behavior of the treated fabrics was assessed by flammability tests performed in horizontal configuration and by LOI tests. In the presence of the casein coatings, a remarkable reduction of the total burning rate and a strong increase of the final residue were observed, irrespective of the type of fabric. In particular, the casein coatings provided self-extinction to cotton fabrics, even after a second flame application; as far as PET is considered, the protein coatings significantly reduced its burning rate and blocked the flame propagation within 30 mm, giving rise, at the same time, to a remarkable increase of the final residue. Conversely, the biomacromolecule was not able to suppress the melt dripping of the synthetic fabric. Furthermore, the protein coatings were responsible for a notable increase of LOI values of both cotton and polyester (respectively +6% and +5% as compared to the untreated fabrics); on the other hand, they did not considerably modify the LOI value of the blend. Finally, cone calorimetry tests were performed at 35 kW/m2 heat flux: both time to ignition and combustion rate were strongly influenced by the presence of the casein coating, irrespective of the type of fabric substrate. In particular, despite a significant TTI decrease (indeed, the fabrics were found to ignite earlier in the presence of the proteins), the peak of heat release rate showed a substantial Polymers 2016, 8, 319 30 of 36 decrease for coated cotton (−19%) and for the blend (−15%). Finally, the char-forming character of the proteins on PET was confirmed by a significant increase of the final residue.

5.2. Deoxyribonucleic Acid It is very well known that deoxyribonucleic acid (DNA) consists of two long polymer chains of nitrogen-containing bases—namely, adenine (A), guanine (G), cytosine (C) and thymine (T)—with backbones made of five-carbon sugars (i.e., the deoxyribose units), as well as of phosphate groups tied by ester bonds. The chains are rolled-up around the same axis and bonded together: this way, a double helix is formed, which exploits the presence of H-bonds between the bases located side by side and paired in a specific mode (cytosine bases are combined with guanine, while adenine bases are paired with thymine). In the obtained 3D organization, the phosphoric residues and deoxyribose units are oriented toward the outside of the biomacromolecule; conversely, the paired bases are located in the inner portion of the polymer and are stabilized by hydrophobic interactions. The ability of DNA to form double-stranded arrangements has been already utilized for obtaining several DNA-based nanomaterials, exploitable for applications ranging from life science to computing: some examples refer to DNA-linked metal nanoparticles, DNA-directed nanowires and DNA-functionalized carbon nanotubes [79,80]. The use of DNA as a sustainable flame retardant is quite recent and can usually be attributed to the structure of this biomacromolecule, which represents an all-in-one intumescent system [73–75,81–83]. More specifically, the double helix comprises all three constituents of an intumescent material, namely: (i) the phosphate groups that form phosphoric acid; (ii) the deoxyribose units, which act as a carbon source and as blowing agents and (iii) the four nitrogen-containing bases that may give rise to the formation of NH3. Like an intumescent material, when DNA is exposed to a heat source, it develops a multicellular foamed char, which limits the heat and mass transfer between flame and polymer, hence providing flame extinction. The first pioneering work dealing with the flame retardant properties of this biomacromolecule was published in 2013 and dealt with the use of DNA derived from herring sperm for self-extinguishing cotton fabrics. To this aim, a standard impregnation/exhaustion method for reaching the desired final dry add-on (19 wt %) was employed [74]. Among the most important achievements, an improved thermal and thermo-oxidative stability of the treated fabrics in nitrogen and air, in terms of char residue formed at high temperatures, was demonstrated. Furthermore, the combustion was blocked, reaching the flame out within 2 s, after the application of a methane flame in horizontal configuration for 3 s to the fabrics treated with this biomacromolecule. The remarkable flame retardant features of DNA were further confirmed by LOI (28 vs. 18% for DNA-treated fabrics and untreated cotton, respectively) and cone calorimetry tests: these latter showed that none of the DNA-treated specimens ignited upon exposure to 35 kW/m2 heat flux. The high char-forming character of deoxyribonucleic acid was attributed to its chemical structure, where the phosphate groups are able to form phosphoric acid that catalyzes the dehydration of cotton, favoring its auto-crosslinking toward the formation of a stable aromatic char (that is also formed by the deoxyribose units) and inhibiting the production of volatile flammable species. This char behaves as a physical protective barrier on the underlying substrate, limiting the heat, fuel and oxygen transfer between the fabric and the flame. At the same time, the decomposition of pyrimidine and purine bases could promote the formation of azo-compounds able to further induce the char development and the formation of non-combustible gases, like nitrogen, carbon monoxide and carbon dioxide. Pursuing this research, the effect of different DNA add-ons on cotton flammability was investigated: for this purpose, three different add-ons (namely 5, 10 and 19 wt %) were deposited on cotton fabrics, exploiting the same impregnation/exhaustion method [75]. Similarly to other biomacromolecules, the presence of the coating strongly anticipated the cellulose decomposition, Polymers 2016, 8, x FOR PEER 31 of 36

biomacromolecule, which decompose at about 200 °C, releasing phosphoric acid, hence favoring the Polymers 2016, 8, 319 31 of 36 dehydration of the fabrics, and finally giving rise to the formation of a residue, thermally stable up to 500–600 °C [73,83]. Flammability tests performed in horizontal configuration clearly demonstrated whichthat wasthe self-extinction directly related of to the the treated DNA add-on: fabrics instrictly particular, depends asassessed on the biomacromolecule by TG analyses in add-on: nitrogen in and air,particular, the higher the was fabrics the biomacromolecule having the lowest add-on,DNA add-on the lower (i.e. 5 was wt %) the burnt temperature, completely, at which while degradation the other startedspecimens to occur. (having This 10 finding or 19 waswt attributed% add-on) towe there phosphateable to achieve groups the of flame the biomacromolecule, out. SEM-EDS whichmeasurements decompose of atthe about char 200zone◦ C,of releasingthe residue phosphoric left by the self-extinguishing acid, hence favoring coatings the dehydration proved that ofthe the original texture of the fabrics was still preserved and the fibers appeared almost undamaged. fabrics, and finally giving rise to the formation of a residue, thermally stable up to 500–600 ◦C[73,83]. Furthermore, their surface was covered by small spherical structures finely dispersed, mainly Flammability tests performed in horizontal configuration clearly demonstrated that the self-extinction consisting of C, O and P elements: this was a clear indication of the intumescent character of the of the treated fabrics strictly depends on the biomacromolecule add-on: in particular, the fabrics biomacromolecule. having the lowest DNA add-on (i.e., 5 wt %) burnt completely, while the other specimens (having Pursuing this research, very recently, Bosco et al. deeply studied the effect of different 10parameters or 19 wt % add-on)(i.e. molecular were able size, to pH achieve of aqueous the flame solutions, out. SEM-EDS number measurements of impregnations) of the on char the zone fire of thebehavior residue of left deoxyribonucleic by the self-extinguishing acid [75]. coatingsFlame spread proved and that cone the calorimetry original texture tests showed of the fabrics that the was stillcoatings preserved derived and from the fibers low molecular appeared size almost DNA undamaged. significantly Furthermore, improved the their fire surfacebehavior was of the covered fabric by smallsubstrate, spherical which structures they were finely applied dispersed, to. In mainly addition, consisting the fire ofbehavior C, O and was P elements:better enhanced this was when a clear indicationmultiple ofimpregnations the intumescent were character used instead of the of biomacromolecule. a single one, for achieving the same final add-on. In particular,Pursuing the this use research, of multiple very impregnations recently, Bosco at et pH al. deeply4 and 8 studied provided the 86% effect and of different74% of the parameters tested (i.e.,samples molecular with self-extinction, size, pH of aqueous respectively; solutions, furthe numberrmore, these of impregnations) treated fabrics showed on the fire45% behavior and 25% of deoxyribonucleicreduction of THR acid and [pkHRR75]. Flame when spreadwere exposed and cone to 35 calorimetry kW/m2 heat testsflux, with showed respect that to the untreated coatings derivedcotton. from These low results molecular were attributed size DNA to significantly the higher ease improved of diffusion the fire of behavior the low molecular of the fabric size substrate, DNA whichcoating they within were the applied fabric microfibrillar to. In addition, surfaces, the and fire to behavior its higher was thermal better stability enhanced in air. when multiple impregnationsQuite recently, were used DNA instead was also of a exploited single one, as fora component achieving theof samelayer finalby layer add-on. (LbL) In coatings: particular, theindeed, use of its multiple assemblies impregnations were found at to pH improve 4 and 8the provided flame retardant 86% and features 74% of theof the tested treated samples fabrics, with self-extinction,keeping, at the respectively; same time, a furthermore, green character. these treated fabrics showed 45% and 25% reduction of THR and pkHRRIn particular, when were Carosio exposed and toco-workers 35 kW/m [76]2 heat combined flux, with DNA respect (negatively to untreated charged) cotton. with These chitosan results layers (positively charged) on cotton, thus building up an assembly containing up to 20 bi-layers. were attributed to the higher ease of diffusion of the low molecular size DNA coating within the fabric The structure of the obtained LbL assembly is schematized in Figure 30. microfibrillar surfaces, and to its higher thermal stability in air. The formation of the LbL architecture was investigated and confirmed through infrared Quite recently, DNA was also exploited as a component of layer by layer (LbL) coatings: indeed, spectroscopy and SEM analyses. In the presence of chitosan, DNA layers were found to favor the its assemblies were found to improve the flame retardant features of the treated fabrics, keeping, at the char forming character of the former biomacromolecule. In particular, the deposited coatings were sameable time, to provide a green cotton character. fabrics with self-extinction in horizontal flame spread tests and to increase LOIIn values particular, from Carosio 18% (uncoated and co-workers cotton) to [76 24%] combined (cotton DNAtreated (negatively with 20 bilayers). charged) Finally, with chitosan cone layerscalorimetry (positively tests charged)showed a on HRR cotton, decrease thus of building 40% and up a remarkable an assembly increase containing of the upfinal to residue 20 bi-layers. for Thethe structure fabrics treated of the obtainedwith the highest LbL assembly number isof schematizedlayers. in Figure 30.

1 cycle

1 Bi-layer

FABRIC

Adsorption Washing Adsorption from Washing from chitosan DNA solution solution

Repeat

FigureFigure 30. 30.Scheme Scheme of the chitosan-DNA LbL LbL assembly. assembly.

6. Conclusions The formation of the LbL architecture was investigated and confirmed through infrared spectroscopy and SEM analyses. In the presence of chitosan, DNA layers were found to favor the char forming character of the former biomacromolecule. In particular, the deposited coatings were able to provide cotton fabrics with self-extinction in horizontal flame spread tests and to increase LOI values from 18% (uncoated cotton) to 24% (cotton treated with 20 bilayers). Finally, cone calorimetry tests Polymers 2016, 8, 319 32 of 36 showed a HRR decrease of 40% and a remarkable increase of the final residue for the fabrics treated with the highest number of layers.

6. Conclusions This work has clearly depicted the wide potentialities offered by the use of novel phosphorus-based flame retardants as a real (and not only potential) alternative to the toxic flame retardant products. Indeed, the wide possibility of tailoring the structures of the phosphorus-based additives allows exploiting an almost infinite number of FR systems that show high efficiency and suitability for different fibers and fabrics. Furthermore, the concurrent presence of phosphorus and other elements (like nitrogen, silicon and sulphur) may be very important in order to develop additive or synergistic effects during the exposure of the treated textiles to a flame or to a heat source. Though the phosphorus based flame retardants are being developed as an alternative to toxic halogenated counterparts, before they can find real application, they need to be screened thoroughly for toxicity. It is encouraging to see that there are some efforts by researchers where the toxicity of new phosphorus based flame retardants has been evaluated and these latter have shown a low toxicity profile [7,8]. Despite the high flame retardant efficiency observed for most of the chemical and low impact FRs, there are still some unresolved issues that deserve further attention. First of all, the design of any new P-based flame retardant product requires large investment either for assessing its suitability for the desired application (especially from a toxicological point of view) or for scaling-up the production from a lab-scale quantity to at least a pre-industrial scale, trying to maximize yields and reduce expenses. Particularly referring to biomacromolecules, the possibility of adjusting the green technological approach to a larger scale than the lab scale (at least to pre-industrialization) is still under assessment and the final decision will be largely based on the cost-effectiveness of the described biomacromolecules. Definitely, the cost of some of the discussed biomacromolecules, such as DNA, is presently very high: therefore, their use as low impact FRs could be foreseen only on the basis of a significant cost reduction. Second, the industrial world working in the field of flame retardant finishing for textiles is fully conscious that any novel flame retardant product should exhibit, apart from a high efficiency, durability to laundering: At present, some of the proposed P-based FRs (like the proposed phosphorus-containing biomacromolecules) cannot accomplish this goal. Indeed, they show no washing fastness at all, or, in some cases, a limited durability up to few washing cycles. This surely represents a current drawback that limits the use of the treated textiles to those restricted applications for which durability to washing treatments is not required. Thus, further research will also have to consider the design of new strategies able to overcome this challenging issue.

Acknowledgments: The European COST Action “Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals”, FLARETEX (MP1105) is gratefully acknowledged. Author Contributions: Khalifah A. Salmeia, Sabyasachi Gaan and Giulio Malucelli equally contributed to the writing of the review manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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