Recent developments in flame retardant polymeric coatings

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Contents lists available at ScienceDirect

Progress in Organic Coatings

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ecent developments in flame retardant polymeric coatings

huyu Liang, N. Matthias Neisius, Sabyasachi Gaan ∗,1

hemistry Group, Advanced Fibers, EMPA Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, 9014 St. Gallen, Switzerland

r t i c l e i n f o

rticle history:eceived 28 January 2013eceived in revised form 17 July 2013ccepted 17 July 2013vailable online xxx

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This paper reviews the recent developments (last decade) in flame retardant polymeric coatings thatmostly work by formation of conventional char (condensed phase) and or radical species in gas phase.Advancements in the method of application of such coatings on various substrates, problems of existingflame retardant coatings and new technological developments in terms of flame retardant chemistryare briefly discussed. This review focuses on various approaches in development of flame retardantcoatings on various substrates i.e. incorporation of reactive and non-reactive organic compounds andorganic/inorganic compounds (hybrid systems) based on metal, Si, P, N and halogens in suitable

ondensed phaseadical specieson-intumescentlame retardant coatingayer-by layerlasma

polymeric matrices and evaluation of their flame retardant characteristics using various analyticaltechniques.

© 2013 Elsevier B.V. All rights reserved.

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ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Flame retardant application areas and application technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Application areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2. Application technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.2.1. Ultraviolet curing technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.2. Plasma technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.3. Physical and chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.4. Sol–gel process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.5. Layer-by-layer assembly approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Traditional flame retardant coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004. Development in flame retardant chemistry for application in coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4.1. Halogen based formulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.2. Inorganic additive incorporated systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.3. Phosphorus based coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.4. Nitrogen based coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.5. Phosphorous–nitrogen based coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.6. Silicon-based coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.7. Multi element FR systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.8. Nanocomposite based coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.9. Other coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5. Further perspectives on non-intumescent coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Please cite this article in press as: S. Liang, et al., Recent developmenhttp://dx.doi.org/10.1016/j.porgcoat.2013.07.014

5.1. Ease of application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.2. Cost-saving strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.3. Sustainable flame retardant systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +71 2747 611; fax: +71 2747 862.E-mail address: [email protected] (S. Gaan).

1 Group Leader (Additives and Chemistry).

300-9440/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.porgcoat.2013.07.014

ts in flame retardant polymeric coatings, Prog. Org. Coat. (2013),

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dx.doi.org/10.1016/j.porgcoat.2013.07.014


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5.4. Compatible coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.5. Synergistic coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.6. Multifunctional coating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005.7. Nano-technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

6. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

With the increasing trend of more stringent fire safety regu-ations, demands for reduction of the fire hazard posed by highlyombustible materials such as wood, plastics, textiles, etc., haveained importance in recent years. A suitable flame retardant (FR)reatment might be able to retard the ignition of these materialsnd/or decrease flame spread, thereby obviating fire hazards andoss of life and destruction of property [1].

Since a longtime a great deal of effort has been invested inroviding different materials with fireproof properties, e.g. wood,lastics, textiles, etc. Three types of approaches have been quiteell accepted and commonly used in various domains, such as elec-

rical and electronics, building construction and transportation [2].he first common approach involves mechanical incorporation ofame retardant additives into the bulk polymeric matrix, which isostly low cost and fast blending technique. However, the load-

ng of FR needed to be effective is usually too high, which can leado a significant influence on the strength and elastic modulus ofhe materials [3,4]. The second way to reduce the flammabilityf the matrix is to bind units chemically to it by using FR seg-ents that contain functional groups. Through this approach the

R element becomes an integral part of the polymer chain and usu-lly results in higher efficiency and longer durability of FR effect5]. Such incorporation could change the morphology and physicalroperties of the bulk polymer, such as melting point, density andlass-transition temperature and presents relatively higher diffi-ulties in industrial manufacturing for certain materials, e.g. fibers,extile and flexible foams, etc. [6,7]. The third approach which

ostly involves surface modification is widely exploited in variousommercial applications. The use of fireproof coatings has becomene of the most convenient, economical and most efficient wayo protect the substrates against fire. Some of its advantages areiven as follows: FR coatings allow the concentration of fireproofroperties at the surface of the substrate along with preservinghe bulk properties of the material (e.g. mechanical properties),nd can generally be combined with an attractive aesthetic feature8–10].

In most of the cases a fireproof coating represents the onlyarrier between the fuel and a possible fire source, thus it mustithstand effectively throughout the fire, delaying ignition of the

ubstrate, reducing the heat-and-mass transfer between the gasedium and the condensed phase, and hindering propagation of

he flame. Based on the flame retarding mechanism, “flame-safe”oatings are classified as either intumescent or non-intumescentypes. Intumescent coating can be described as a mixture that hasapability to swell and form a three dimensional char layer on topf the substrate when exposed to fire. Traditional intumescent sys-ems consist of a carbon source that acts as a char former (e.g.entaerythritol), an acid source that acts as a dehydrating cata-

yst (e.g. polyphosphate) and a blowing agent that helps form theorous barrier (e.g. melamine, guanidine) [11,12]. This carbona-


eous cellular/porous-like residue acts as a barrier to heat, air andyrolysis products, and finally shields the underlying substraterom fire spread [13]. Bourbigot’s group in Lille [14–19] has exten-ively investigated such kind of systems, while a state of the art

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review on this topic has been recently reported by Weil [20]. Incontrast to that, there is a lack of comprehensive reviews of non-intumescent flame coating systems published in the last 10 years.Unlike intumescent system such coating exhibits a different modeof action on exposure to heat, where it may release active speciesacting in gas phase for flame inhibition, catalyze decomposition ofthe surface of the material to form non-voluminous glassy/char lay-ers, or act as insulative mirror for protection against radiation fromheat source.

As stated earlier, conventional non-intumescent coatings con-tain flame retardant additives and hinder flame spread, however,are in absence of providing a significant voluminous protection tothe substrate like intumescent one. Therefore the efficiency of thesolely use FR compound in non-intumescent system is insufficientfor certain applications and currently experiencing wide revolu-tions in terms of FR chemistry, which will be shown in a laterpart.

In this review we summarized the various technologicalindustrial or academic development carried out in the field of non-intumescent coatings over the past 10 years. A brief summary ofvarious application areas and application techniques, a detaileddescription of development of non-intumescent flame retardantchemistry and their formulations have been presented in thisreview.

2. Flame retardant application areas and applicationtechnologies

2.1. Application areas

Flame retardant coatings have many applications that can befound in different sectors. Popular flame retardant market areasinclude building construction, electrical applications, electronicsand transportation [2,21–24], while textiles represent anotherlarge area where flame retardants are used in coatings. In the build-ing and transportation industry, a popular way to impart flameretardancy is by mixing FR additives in the paint/lacquer whichare used to coat surfaces. Many formulations have been describedthat utilize components such as chlorinated paraffin’s, antimonytrioxide, and titanium dioxide for FR latex and alkyd based paints[24,25], which in case of fire hazard help reduce the flame spreadin the gas phase and catalyze decomposition of the surface of thematerial to form non-voluminous char in the condensed phase. Inelectrical applications, flame retardant coatings are often used toprovide cables and wires with needful properties, as can be seenin a patent by Galletti et al. [26], and can be found in the litera-ture, such as briefly discussed by Coaker and Hirschler [27]. Dueto the high flammability of textiles, fabrics are often treated withreactive FR or back coated with a polymer matrix to provide flameretardancy. Such matrix may be composed of polyacrylates, sili-cones, epoxides, polyurethanes or PVC. Especially PVC based FRcoatings are commonly utilized for architectural textiles, as they are


able to provide useful weathering properties, as well as adequateflame retardancy over a long time under adverse environmentalconditions [28].


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.2. Application technologies

Traditional FR coatings can be mostly considered as polymeratrices based on acrylic, epoxy, urethane, or silicone, typically a

ew tens to hundreds of microns thick. They are normally appliedo the substrates by means of spray-, brush-, or roller techniques.nother method is dipping the material surface directly into theoating solutions, followed by drying process to remove the solvent.hese methods are generally multi-step processes that require aarge amount of energy and chemicals and usually bring in the con-erns of toxicity and environmental problems. A common problemor some FR coatings is the delamination under repeated thermalycling which might be caused by limited adhesion [29]. Recently

number of advanced techniques like plasma deposition, physicalnd chemical vapor deposition, layer by layer and sol–gel etc. arettracting increasing attention for coating application in FR domain.

.2.1. Ultraviolet curing technologyUltraviolet (UV)-curing technique is being increasingly used

n industrial flame FR coating applications. In recent years,dvantageous characteristics such as rapid curing, low energyonsumption, reduced environmental pollution, high chemical sta-ility, low volatile organic compounds (VOC) emission and a broadange of applicable changes in formulation and curing conditionsave made various UV-curable compositions highly attractive forR applications [30,31]. By adding the appropriate FR elements intoV-curable matrices, i.e. boron, phosphorus, silicone, the result-

ng coating can provide the underlying material with good flameetardant properties [32–34].

.2.2. Plasma technologyVarious researchers have recently used plasma technology as

n environmentally friendly alternative for application of flameetardant coatings on substrates. Via this technique a thin networkoating can be deposited or grafted on any surface that comes inontact with corresponding plasma source [35–37]. It was reportedhat low pressure plasma technique had been used in FR finishingomain where it conferred highly durable flame retardant proper-ies to textiles [38,39]. However, as a result of its added cost andomplexity due to the vacuum operation, atmospheric pressurelasma has been further employed as a promising alternative to theormer process for industrial applications [40–44]. Schartel et al.ave conducted an investigation based on surface-controlled fireetardancy of polymer using plasma polymerization, where theyllustrated the chance and limits of this concept [45].

.2.3. Physical and chemical vapor depositionNowadays physical vapor deposition (PVD) and chemical vapor

eposition (CVD) have been widely used to modify textile mate-ials and electrical devices due to their inherent merits [46–49],.g. environmental friendliness, various functions and flexibilityo coat a variety of substrates. Sputter coating is one of the mostommonly applied PVD techniques, with which new concepts ofoute to confer flame retardancy to different substrates are alwaysccompanied, e.g. textile fabrics, nano-fibers or plastic substratesor specific FR applications [50,51]. Thermal resistance coatingsing CVD technique are also reported in literature to create thinoatings on various substrates [39,52,53]. It has the advantage tooat complex surfaces uniformly with excellent coverage and thus


ffer a great potential for FR coating applications [39,54].Plasmanhanced chemical vapor decomposition (PEVCD) is one of theommonly used process for the deposition of films to confer flameetardancy [38].

Fig. 1. Schematic representation of LbL assembly. The process is repeated until thedesired number of bilayers is reached.

2.2.4. Sol–gel processSol-gel technique is a novel wet-chemical technique for fab-

rication of environmentally friendly flame retardant coatings ontextiles. This process is traditionally used to develop polymeric net-works of organic co inorganic composites, while some advantagesof organic polymers emerge those of inorganic part to enhancethe overall properties of the hybrid coatings [55]. Furthermore, asa result of the usually appeared synergistic effects derived fromorganic and inorganic moieties under the molecular scale, thecoated system is then able to achieve high levels of flame retardancy[56,57]. A state of art based on sol–gel technique for FR textiles hasbeen published in ref. [58].

2.2.5. Layer-by-layer assembly approachMore recently, a highly tailorable coating technique, layer-by-

layer (LbL) assembly has been used as a simple, highly inexpensiveas well as environmentally friendly FR treatment method fortextiles [59,60]. The FR modification can be conducted throughalternative immersion, or spraying, of the substrate with oppositelycharged polyelectrolyte solutions or suspensions that create pos-itively and negatively charged multilayers on the surface (Fig. 1)[61]. In addition to the assembly force from electrostatic attractions,multilayered coatings can also be built through donor/acceptorinteractions, hydrogen-bonding, and covalent bonds [62]. Recentstudies revealed that this technique is extremely advantageouswhen used for cotton fabric and has attractive potential for fur-ther application on some other substrates, such as polyesters,polyamides and polyurethane foams [63–65].

Besides, there are also reports in literature which describe thecombinatory use of above mentioned advanced application tech-nologies e.g. sol–gel plus UV radiation-curing technique [66,67],sol-gel plus plasma technique [68] and plasma plus UV curing [42]technique to develop novel FR coatings. Furthermore for certainadvanced research of nano-FR coating new technologies such asvacuum-assisted resin transfer molding process and electrospin-ning have also been used [69].

3. Traditional flame retardant coatings

Flame retardant coatings, as mentioned earlier, preserve thebulk properties of the coated material while providing a highconcentration of flame retardant on the surface of a material. Tra-ditional non-intumescent flame retardant coatings typically utilizecompounds that contain halogens, i.e. chlorine or bromine, or phos-phorous, or inorganic metal compound as the main FR componentin the formulation, the presence of which usually help inhibit the


flame spread via radical quenching and/or forming glassy protec-tive layers instead of blown voluminous char during combustion[21,70]. In case of intumescent FR coatings, an optimal selectionof compatible intumescent ingredients in terms of physical and


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hemical properties is critical to obtain high fire protection effi-iency. Non-intumescent FR coating systems, usually contain lessngredients and thus are more compatible with the matrices whichan lead to better mechanical and fire performance properties [71].

Halogenated FR systems are popular, but have come undercrutiny in recent years. Most of the halogen based compoundsreferably work in the gas-phase by acting as free radical scav-ngers and, in most of cases, were shown to be efficient for flameetardation [72–75]. Phosphorous flame retardants are predom-nantly represented by phosphate esters and demonstrate greatotential of being used as efficient FR additive for coatings. Dur-

ng heat exposure P-flame retardants can interact with the coatingatrix to improve its charability for surface protection [76]. Within

his class particularly halogenated phosphate esters have foundommon use in FR coating formulations [77]. Antimony oxide is

widely used synergist for halogenated flame retardant systems76] and basically works via an intermediate formation of anti-

ony trihalide which eventually act as a strong flame-quenchern the gas-phase [78]. In addition, inorganic compounds such asluminum hydroxide (ATH) and zinc borates are also known as FRllers for coating applications, within which the flame spread issually inhibited by endothermic/cooling effects on surface.

. Development in flame retardant chemistry forpplication in coatings

Over the past 10 years there has been increasing trend in sci-ntific community to develop non-intumescent FR coatings withajor focus on halogen free systems. Until now numerous novel

ormulations of organic and inorganic compounds based on P, N, Sind metals have been reported in literatures, while certain amountf studies for halogenated systems are still involved. Those men-ioned FR compounds may also contain combinations of above

entioned elements for synergistic interaction.

.1. Halogen based formulations

Although halogenated flame-retardant systems are popular andidely used in FR coating formulations, their applications are still

eing restricted due to the environmental problems. Over theast 10 years there were only few published research articles onevelopment of new halogen based systems for applications in FRoatings. As discussed in earlier section these FR systems workredominantly in the gas phase, forming halogen radicals that acto eliminate free oxygen and hydroxyl radicals formed in the fire,hereby interrupting the combustion process.

Errifai et al. used plasma technique to graft/deposit a fluorinatedcrylate monomer (AC8) onto the PA 6 surface (Scheme 1) [79]. Theone calorimeter results in Table 1 (1) show that the incorporationf the FR coating onto the PA substrate results in the decreases ofoth peak heat release rate (HRR) and time to ignition (tig) by 50%.

t is further proposed that such significant reduction was due tohe dilution of combustible gases caused by reaction of CFx radicalsith the degraded polymer fragments.


Though it is rare to see development of novel flame-retardantormulations that include halogens, it is worth noting that thendustry still uses halogen based compounds for flame retardation.n a recent patent, halogenated epoxy resins have been used as

Scheme 1. 1,1,2,2, tetrahydroperfluorodecyl acrylate (AC8).

PRESSCoatings xxx (2013) xxx– xxx

flame retardant coatings for optical fibers [80], while Serafein et al.[81] have reported an invention related to FR plastisol compositionsbased on polyvinylchloride (PVC), plasticizer and ion-exchangezeolites that are useful as coatings, backings and adhesives for flexi-ble woven and non-woven fabric, carpets and the like. In this systemzeolites can act as a replacement of Sb2O3 for PVC matrix in termsof flame retardation. However, such replacement seems to be notperfect enough. As can be seen in Table 1 (2), The LOI value slightlydecreases from 28% to 25%, when Sb2O3 is partially replaced byZeolite.

Another PVC formulation containing 2-decabromodiphenyloxide and antimony trioxide as main FR parts, 2-Ethylhexyldiphenyl phosphate as plasticizer has also been patented and pro-posed as FR coating for various substrates [82]. As an example,all the coated polyethylene foams exhibited in UL-94 fire test thetop flammability (V-0) as compared with a V-2 level for uncoatedspecimens and thus turned to be a testimony to the efficient FRcharacteristic of the halogenated coating under invention.

However, as a result of the found drawbacks of antimony tri-oxide compound such as increased smoke production, possibletoxicity of the volatiles, and a relatively high cost, Giúdice et al.have made effort in substituting such synergist with zinc boratesof formulas 2ZnO·3B2O3·3.5H2O and 2ZnO·3B2O3·7.5H2O on theflame retardation of chlorinated alkyd coatings. They found thatthe efficiency of zinc borate incorporated coatings were inferior tothat of antimony trioxide (ATO) assisted system but the formersstill offered advantage of better cost/performance ratios (LOI, UL94–45◦, and FSI test). 2ZnO·3B2O3·7.5H2O exhibited better flameretardant properties than 2ZnO·3B2O3·3.5H2O (Table 1 (4)), possi-bly due to the higher amount of hydrated molecules that is able toabsorb a greater amount of heat released at high temperatures [83].Meanwhile Horrocks et al. investigated possible synergistic effectsof zinc stannate (ZS), zinc hydroxystannate (ZHS), zinc borate (ZB)and ATO in PVC coatings of glass fabrics substrates. As shown inTable 1 (5), only the plastisol reference and ZB containing systemyielded the ratings (M3) below all the others which achieved thehighest possible M1 rating. LOI values are significantly increased byall synergist formulations apart from ZB-PVC system. In this studyZS and ZBS demonstrated great potential as promising alternativesto ATO in PVC coating system [84].

4.2. Inorganic additive incorporated systems

In addition to acting as a synergistic co-additive (discussed inSection 4.1) that helps increase the existing FR efficacy of the coat-ing system, inorganic additives such as aluminum or magnesiumhydroxyl are used as the main FR element within the coating sys-tem [25,83]. However, inorganic fillers are often required at highloadings which may adversely affect the physical properties of thecoatings. Additionally such additives may be incompatible with thebinder system (such as UV curable coating) which leads to bloomingand other similar undesirable effects [31].

There are many disadvantages from the current methods offlame retarding wood for construction applications. These includecomplexity of the treatment, short supply of materials and dete-rioration of the mechanical and aesthetic properties of the wood[85,86]. Plotnikova et al. have studied the effect of natural mineralfilled urea-formaldehyde resins on the FR efficacy of the coatings[87]. They varied the composition of the fillers in the coating andkept the flame retardant concentration constant (12% diammoniumhydrogen phosphate, DAHP). The fillers that were utilized in thecoating included phlogopite (KMg3AlSi3O10 (F, OH)2; phyllosili-


cates), mineral wool, basalt fiber and graphite, and the coatingswere tested using a ceramic tube apparatus. They were able togain the best FR effect when phlogopite + mineral wool/or Basaltwas used as filler. As noted in Table 2 (1), the system containing




S. Liang et al. / Progress in Organic Coatings xxx (2013) xxx– xxx 5

Table 1Results of flammability tests of halogen based systems.

Coating system Flame retardant components in coating(wt%)

LOI (%) Flammability test/Cone calorimeter(CC) test

1 [79] F-based acrylate coating Virgin PA6 – CC: heat flux = 35 kW/s:HRR = 960 kW/m2; THC = 945 kJ;tig = 125 s

AC8 coated PA6 – HRR = 394.9 kW/m2; THC = 714 kJ;tig = 62 s

2 [81] PVC coating on tufted pile carpet Antimony trioxide: 3.5% Zeolite: 0 28 NFPA flame test: pass; stopped burningin 2.94 s

Antimony trioxide: 2.5% Zeolite: 1% 27 pass; 3.00 sAntimony trioxide: 1% Zeolite: 3% 25 pass; 4.58 s

3 [82] PVC coating deposited on foam materials 0% – UL 94 vertical test: V-2 rating2-decabromodiphenyl oxide: 4.92%2-Ethylhexyl diphenyl phosphate:3.88%

– V-0 rating

4 [83] PVC coating on wood Antimony trioxide: 9% 46 UL 94—45◦ test: pass and 127 rating;FSI: 0.39

2ZnO·3B2O3·3.5H2O: 9% 42 pass and 107 rating; FSI: 0.542ZnO·3B2O3·7.5H2O: 9% 41 pass and 117 rating; FSI: 0.51Sb2O3: 3% 2ZnO·3B2O3·3.5H2O: 3%2ZnO·3B2O3·7.5H2O: 3%

56 pass and 142 rating; FSI: 0.08

5 [84] PVC coating on glass fabrics Plastisol coating reference 23.6 NFP-92-503 test: M3 ratingPlastisol-ZHS 27.6 M1 ratingPlastisol-ZS 28.0 M1 ratingPlastisol-ATO 29.5 M1 ratingPlastisol-ZB 23.6 M3 ratingPlastisol-ZHS/ZB 28.6 M1 rating

TR


Plastisol- ZS/ZB

Plastisol- ATO/ZB

able 2esults of flammability tests of inorganic additive incorporated systems.

Coating system Flame retardant components incoating (wt %)

LOI (%) Flam

1 [87]Urea-FomaldehydeResin on fabric

Graphite 16% – Tortemburdam

Graphite 16% and DAHP 12% – Fluselfext

Graphite 10%, phlogophite 12%and DAHP 13%

– Fluselfext

Graphite 16%, phlogophite 16%and DAHP 12%

- Fluselfext

Basalt fiber 3%, graphite 16%,phlogophite 16% and DAHP 12%

– Fluselfext

Mineral wool 6%, phlogopite44% and DAHP 12%

– Fluselfext

2 [88] Metal alkoxidebased coating

Cotton reference 19 ISObur

Si-sol coated sample 22 Totres

Ti-sol coated sample 22 Totres

Zr-sol coated sample 21 Totres

Al-sol coated sample 22 Totres

3 [90] UV-cured epoxyacrylate coating

LDH: 0% 20 TGA(60

LDH: 1% 21.5 T-10

LDH: 2% 23.5 T-10

LDH: 4% 24.5 T-10

LDH: 8% 25 T-10


28.0 M1 rating30.3 M1 rating

mability test/Cone calorimeter (CC) test

ch flame test: Flue gasperature = 253 ◦C; self-maintainedning time = 20 s; extent ofage = 68%

e gas temperature = 290 ◦C;-maintained burning time = 69 s;ent of damage = 90%e gas temperature = 265 ◦C;-maintained burning time = 48 s;ent of damage = 82%e gas temperature = 260 ◦C;-maintained burning time = 39 s;ent of damage = 77%e gas temperature = 190 ◦C;-maintained burning time = 0 s;ent of damage = 30%e gas temperature = 219 ◦C;-maintained burning time = 15 s;ent of damage = 44%

151025 vertical flame test: Totalning rate = 20 mm/s; residue = 10%

CC: Heat flux = 35 kW/s: tig = 18 s;PHRR = 88 kW/m2; THC = 2.8 kJ

al burning rate = 11 mm/s;idue = 30%

tig = 28 s; PHRR = 70 kW/m2;THC = 2.0 kJ







: T-10% = 368 ◦C; residue0 ◦C) = 13.3%

CC: Heat flux = 35 kW/s;PHRR = 332 kW/m2; HRC = 342 J/gK;

% = 363 ◦C; residue = 16.1% PHRR = 295 kW/m2; HRC = 301 J/g K% = 356 ◦C; residue (600 ◦C) = 16.9% PHRR = 269 kW/m2; HRC = 271 J/g K% = 350 ◦C; residue (600 ◦C) = 16.9% PHRR = 250 kW/m2; HRC = 252 J/g K% = 353 ◦C; residue (600 ◦C) = 18.1% PHRR = 228 kW/m2; HRC = 232 J/g K


IN PRESSG Model

P

6 ganic Coatings xxx (2013) xxx– xxx

3etIdp

tcatpwppcmwtb1tr

rticptm

mwsLsrcishwod

ltwiacmisfvani(ttt‘c

boards which utilized a halogen-free flame retardant resin (deriva-tive of DOPO; 9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide) (Scheme 4) [98]. The resin consisted of 5% to 70%phosphorous-containing epoxy resin and exhibited excellent heat



% Basalt, 16% graphite, 16% phlogophite and 12% DAHP did notxhibit self-maintained burning (0 s) and showed lower flue gasemperature (190 ◦C) and smaller extent of damage by length (30%).t can also be seen that the flame retardancy of the coating systemepends more on the nature of the fillers than on their constitutingercentages in the formula.

Alongi et al. investigated the effect of several metal alkoxides onhe thermal and FR properties of cotton [88]. The inorganic metalompounds were principally based on, titanium, zirconium, andluminum, and were incorporated into the cotton using the sol–gelechnique described in the earlier section. It was found that, in theresence of metal compounds the thermal properties of coatingsere improved except the case of the aluminum compound. Theeak of HRR of coated fabrics was reduced for most treated sam-les. As compared to pure cotton, all the sol–gel coated fabrics afteruring led to increase in water content by at least 24% (as deter-ined by Karl–Fisher titration) and showed higher LOI values asell as longer TTI values by around 10–50% in Cone Calorimeter

ests. In the vertical flame tests their burning rates all decreasedy >50%, while the char residues after test increased by around00–200% (Table 2 (2)). These metal alkoxide coating systems arehus considered to be quite helpful in rendering fabric good flameetardancy.

A patent by Nosker et al. describes an invention of flame-etardant coating formulation that is composed of a cross-linkedhermosetting polymer (polyurethane based) and a metal hydrox-de (magnesium hydroxide) [89]. This patent describes theomposition of coating precursor, along with a method that coatslastic composite plank with a flame-retardant layer. It’s claimedhat the metal hydroxide concentration of around 30% in the coating

atrix provides self-extinguishing character to the substrate.As an important part in the development of FR systems for poly-

ers, new anionic clays, namely layered double hydroxide (LDHs)as applied in the epoxy acrylate (EA) thin film, coated on sub-

trate and cured by UV curing technique [90]. In this work, theDHs of formula M2+

1-x M3+x (OH)2 An-

x/n. mH2O were synthe-ized, where M2+ and M3+ were di- and trivalent metal cations,espectively, and An− was an interlayer anion compensating for theharge on the layers. Increasing the content of LDHs in the coatingsmproved their thermal stability and flame properties. In TGA mea-urement the T-10% got lower with increasing DLH content, but hadigher residue at 600 ◦C (from 13.3% to 18.1% at 8% loading), asell as higher LOI values (from 20% to 25%). In the measurements

f micros-scale combustion calorimeter PHRR and HRC were bothramatically reduced by around 31% and 32%, respectively.

A patent published recently by Galletti et al. deals with cables forow-voltage electrical energy distribution or telecommunicationshat exhibit self-extinguishing and low-smoke properties [26]. Itas shown that application of magnesium hydroxide in the coat-

ng matrix which had a higher decomposition temperature (340 ◦C)llowed for better processing of cables. The cable consists of aonductor and a flame retardant coating that comprises of a poly-er matrix and the natural magnesium hydroxide (Brucite). To

ncrease compatibility with the polymer matrix, natural magne-ium hydroxide was surface-treated with different coupling agents,or example organic silanes or titanates i.e. vinyltriethoxy-silane,inyltriacetylsilane, tetraisopropyltitanate, tetra-n-butyltitanate,nd alike. It was found that a better compatibility of treated mag-esium hydroxide with the matrix did not result in a significant

mprovement in flame retardancy of the coated cable specimens40% loading of MgOH), but in a much better mechanical and elas-ic properties. As a result magnesium hydroxides can be added into


he cable matrix in higher amount while having not deterioratedhe suitable mechanical properties and it is thus considered to beone way’ for the enhancement of the extinguishing feature of theable.

Scheme 2. The phosphate acrylate monomer (TGMAP).

4.3. Phosphorus based coating systems

Environmental concerns related to the use of halogenated flameretardants have given impetus to the development of novel phos-phorous FRs [2]. Phosphorus based FRs are versatile as they canexhibit both condensed phase and/or gas phase flame retardantaction [91]. As compared to other flame-retardants, phosphorusFRs are also found to generate less toxic gases and smoke duringcombustion [92].

Chen et al. have synthesized a multifunctional flame retar-dant monomer which was introduced into the polymer chainfixed on the substrate through UV curing [93]. The phosphateacrylate monomer (TGMAP in Scheme 2), was synthesized bythe reaction between phosphoric acid and glycidyl methacrylate,an easier route than the complicated synthesis route previouslyreported for UV curable monomer: tri-(acryloyloxyethyl) phos-phate [94,95]. The resulting coating exhibited a high level of flameretardance through thermal analysis with TG and TG-IR. It wasfound that the mode of flame retardation was mostly attributedto the increased char formation catalyzed by poly (phosphoricacid) formed in the condensed phase during thermal degrada-tion (char residue >36.3% in TGA). C akmakc i et al. studied theeffect of the presence of allyldiphenyl phosphine oxide (ADPPO)(Scheme 3) on the flame retardancy of epoxy acrylate coatings[96]. A thiol-ene system was utilized to enhance the polymeriza-tion of ADPPO, as it has been previously reported that the ADPPOmostly participated in dimerization instead of the desired poly-merization with the epoxy acrylate resin [97]. Trimethylolpropanetris-(3-mercaptopropionate) (TMPMP) was for cross-linking andalso facilitating the polymerization of allyl moiety in this polymer-ization process. As can be seen in Table 3 (1), the LOI values ofthe films increased with increasing content of the ADPPO, from thevalue of 19.5 for the ADPPO free system to that of 27.5 when 4.4%P content is added.

Fung et al. have invented a dielectric coating for printed circuit


Scheme 3. Allyldiphenyl phosphine oxide (ADPPO).





Table 3Results of flammability test of phosphorus based coating systems.

Coating system Flame retardant components in coating (wt %) LOI (%) Flammability test/Cone calorimeter (CC) test

1 [93] Polymeric free coating/film ADPPO: 0% P content 19.5ADPPO: 2.2% P content 25ADPPO: 4.4% P content 27.5

2 [98] Epoxy resin film P- content: 0% – UL 94: V2 rating: Thermal conductivity

rosa

oabsc[mi

4

asasb[imLwdpei2r3odMg

P- content: 70%

esistance and good flame retardancy (UL 94 V0 rating). However,nly a rating of V2 is reached for P free coating (Table 3 (2)). Suchystems are environmentally friendly as the material is halogen freend generates no toxic or corrosive gases upon burning.

Although phosphorus-based compounds have showed seriesf advantages to be the promising alternatives to halogen-freedditives, some still received from public the critical argumentsased on their potential environmental and health concerns,uch as acidic erosion, eutrophication from hydroxylation of theompounds, or even some potential carcinogenic activities, etc.99–101]. As a result more studies need to be further conducted to

inimize all those problems when using phosphorus compoundsn coatings.

.4. Nitrogen based coating systems

Nitrogen based flame retardants are becoming more populars they are considered as an environment friendly and non-toxicubstitutions for existing halogen formulations. Besides, materi-ls based on nitrogen containing flame retardants are furthermoreuitable for recycling [102]. Most of the commonly used nitrogenased compounds for FR application are derivatives of melamine103] and they exhibit both gas and condensed phase flamenhibition [5]. The use of melamine phosphoric acid salts in the intu-

escent coating has been reviewed by Weil [104] while recentlyiang et al. developed a novel melamine containing monomerhich is applied for non-intumescent systems [105]. They haveeveloped methacrylated phenolic melamine (MAPM) and incor-orated it in UV curable systems utilizing a commercially availablepoxy acrylate, EB600, as shown in Scheme 5. MAPM could greatlymprove the flame retardancy of EB600; the LOI values ranged from1.5% for pure EB600, to 26.5% when 50 wt% MAPM were incorpo-ated (Table 4 (1)). It was found that MAPM decomposed at about00 ◦C to form melamine, which further condensed and simultane-


usly released nitrogen volatiles; the condensation product furtherecomposed with no residue after 600 ◦C. It was concluded thatAPM works only in the vapor phase by releasing non-flammable

asses like ammonia with no increase in char formation.

Scheme 4. Phosphorus containing epoxy resin.

(prepreg): 9.7 W/(m K)– Thermal conductivity (prepreg):

<8.1 W/(m K)

Wu et al. have synthesized a novel maleimide modifiedepoxy monomer by reacting N-(4-hydroxylphenyl) maleimide(HPM) with diglycidylether of bisphenol-A [106]. The resultingN-containing monomer which possessed both oxirane ring andreactive maleimide groups was thermally cured to form a highlycross-linked resin. Owing to good thermal stability and improvedflame retardation such resin could find application in encap-sulation/coating materials for green electronic components. Assummarized in Table 4 (2), the initial decomposition temperatureincreased from 369 ◦C to 386 ◦C and LOI increases from 23% to 29.5%when 33% of maleimide was used in the system.

Due to the limited efficiency of nitrogen based FR additives usedin the formula, they are usually combined with the phosphorusbased FR to gain a synergistic effect. However, as a result of exten-sive development in coating technique, the sole use of N basedsystem for efficient surface-protection is still under consideration.Gallo et al. has reported a new route to confer flame retar-dancy through innovative surface coating [107], where polyamicacid (PAA) nanofiber mats and (polyimides) PI nanofiber matsare deposited on polyamide (PA) substrate through electrospin-ning. The results from a series of tests (Table 4 (3)) showed ageneral reduction in the propensity for fire growth and flamespread rate. When PAA (or PI) fiber is electrospun onto the sub-strate, time to inginition (tig) is postponed from 38 to 72 (or 85) s,and fire growth rate (FIGRA) decreased from 9.3 to 7.7 (or 7.6)kW/(m2 s). It is that both electrospun nanofiber mats act not onlyas sacrificial layers but also a protective surface that delays igni-tion.

In the studies of Bourbigot’s group they explored the use ofnitrogen based non-woven (NW) as heat barrier to protect a metal-lic substrate, where Carbtex fibers consisting in a thermoplasticcore inside an oxidized outer shell (polyacrylonitrile or PAN fibers)were selected to make such NW surface. By means of a macro-scopic model study it was revealed that low heat conductivity ofthe fibers and the NW structure with large porosity was crucial toget superior heat barrier performance. The intuitionistic feature isthat below heat radiator this Carbtex layer provides an effectiveheat protection for the aluminum plate, where the heating rate ofthe assembly aluminum + Carbtex is strongly decreased comparedwith the aluminum plates along (0.35 ◦C/s vs. 0.7 ◦C/s) [108].

4.5. Phosphorous–nitrogen based coating systems

Phosphorous and nitrogen containing compounds have beenshown to exhibit synergistic effects in many flame retardant sys-tems [109–112]. This means that the flame retardant efficacy ofthe phosphorous and nitrogen compounds together is more sig-nificant than the theoretical effects of the individual compoundsadded separately. Such synergism is sought after, and P-N systemsare promising substitutes for halogenated systems.


In recent years there has been a considerable interest in thedevelopment of compounds based on phosphazene chemistry.These compounds are believed to exhibit P-N synergism, good ther-mal and chemical stability. A novel cyclic phosphazene derivative,





Scheme 5. Methacrylated phenolic melamine (MAPM) containing resin.

Table 4Results of flammability tests of nitrogen based coating systems.


LOI (%) Flammability test/Conecalorimeter (CC) test

1 [105] Epoxy resin (suggested in coating application) P based MAPM: 0% 21.5 –P based MAPM: 25% 24.5 –P based MAPM: 50% 26.5 –P based MAPM: 100% 29 –

2 [106] Epoxy resin film Maleimide content: 0% 23.0 –Maleimide content: 16.7% 26.0 –Maleimide content: 33% 29.5 –

3 [107] Electronspun ES) fiber coating on PA66 No coating – CC: heat flux = 50 kW/m2:tig = 38 s; fire growth rate

2

precurecurs

aa[c(

TR

PAA (ESPI (ES p

ziridinyl phosphazene (NPAZ) was synthesized by reacting hex-chlorocyclotriphosphazene (NPCl2)3 with aziridine (Scheme 6)


113]. This bi-functional compound was reported to serve as aross-linker and reactive FR for the aqueous based polyurethanePU) curing system. In the literature it was found that the thermal

able 5.1esults of flammability tests of phosphorus-nitrogen based coating systems.

Coating system Flame retardant components in coating (wt

1 [113] P-N free standing coating NPAZ (phr): 0%

NPAZ (phr): 1.5%

NPAZ (phr): 3.5%

2 [114] P-N free standing coating NPHE (phr): 0%

NPHE (phr): 10%

NPHE (phr): 20%NPHE (phr): 40%

3 [115] Epoxy resin coating CPEP: 0%

CPEP: 25%

CPEP: 33%

CPEP: 50%

4 [116] UV-cured acrylate coating TAEP100

HPPA20 TAPE80

HPPA50 TAPE50

HPPA100

5 [117] UV-cured acrylate coating TAEP/TGICA = 1/1 by weight: 0 g/L in preparsolutions for coatingTAEP/TGICA = 1/1: 50 g/L

TAEP/TGICA = 1/1: 100 g/L

TAEP/TGICA = 1/1: 200 g/L

(FIGRA) = 9.3 kW/(m s)rsor solution at 20%) – tig = 72 s; FIGRA = 7.7 kW/(m2 s)or solution at 20%) – tig = 85 s; FIGRA = 7.6 kW/(m2 s)

stability, flame resistance and physical properties of the resultingPU film were significantly improved even with a small percentage


of NPAZ. In presence of 1.5 phr NPAZ, the char yield of the resultingsystem after cone calorimeter test increased from 16.4% to 36.6%,but its total smoke production got much higher (307.5 vs. 77.1 for

%) LOI (%) Flammability test/Cone calorimeter (CC) test

– CC: heat flux = 20 kW/m2: tig = 100 s; THR: 12.2 MJ/m2;TSP: 77.1; Char = 16.4%

- tig = 237 s; THR: 7.4 MJ/m2; TSP: 338.0; char = 36.6%- tig = 187 s; THR: 10.0 MJ/m2; TSP: 307.5; char = 55.7%

22 CC: heat flux = 30 kW/m2: tig = 72 s; THR = 36.6 MJ/m2;TSP = 346.5; char = 5%

24 tig = 53 s; THR = 30.3 MJ/m2; TSP = 698.7; char = 11.1%25 tig = 45 s; THR = 27.5 MJ/m2; TSP = 717.9; char = 12.5%27 tig = 37 s; THR = 19.5 MJ/m2; TSP = 714.3; char = 19.4%

– UL 94 flame test: Failed– V-2– V-1– V-0

4740.53534

ed 21 Microscale combustion calorimeter (MCC):PHRR = 183 W/g; THC = 8.8 kJ/g

22.5 PHRR = 165.8 W/g; THC = 5.1 kJ/g23 PHRR = 138.2 W/g; THC = 4.1 kJ/g24.5 PHRR = 119.2 W/g; THC = 3.6 kJ/g


ARTICLE ING Model


S. Liang et al. / Progress in Organic

rip

aaoh(bpidc0cii

Scheme 6. Synthesis of aziridinyl phosphazene (NPAZ).

eference shown in Table 5.1 (1). They suggested that the flamenhibition characteristics of the film system were attributed to theresence of both the phosphorus and nitrogen components.

In addition, based on the similar phosphazene chemistry,nother UV curable polyurethane coating system was developednd studied by Huang et al. [114]. This system included a PU acrylateligomer and a UV-reactive flame retardant monomer 2,2,4,4,6,6-exakis-(2-oxyethylmethacrylato)-cyclotriphosphazene (NPHE)Scheme 7) for its preparation. The film produced from such com-ination provides better mechanical, physical and flame retardantroperties and demonstrated the flame-inhibition character based

n the improved LOI values. TGA analysis showed that the initialecomposition temperature of the films was lowered and thehar content increased with increase in the NPHE content (from


to 14.7% at 600 ◦C in presence of 40 phr NPHE dosage). Theone calorimeter results in Table 5.1 (2) showed that the time tognition of the coatings decreased from 72 s to 59 , 45 and 37 s withncreasing NPHE content and the total heat released was reduced

Scheme 7. Preparation of UV-reactive 2,2,4,4,6,6-Hexakis (2

Scheme 8. Synthesis of novel phospha

PRESSCoatings xxx (2013) xxx– xxx 9

by 46.7% when 40 phr NPHE is added). They suggested that the firesuppression actions occurred in both gas phase and condensedphase, which blocked air supply to the surface and produced thecarbonaceous residue for effective fire protection.

Liu et al. synthesized a novel phosphazene-modified monomer(CPEP in Scheme 8) to develop a novel P-N based resin [115].Compared to the corresponding commercially available E51-basedthermosets, the cured resins with a mixture of CPEP and E51have better thermal stabilities, higher char yields after decompo-sition at 800 ◦C (increase of 127% when 25% CPEP was introduced).Meanwhile it showed great improvement in flame retardancy(Table 5.1 (3)) while maintaining relatively good mechanical prop-erties, hydrophobicity and electric resistance. They suggested thatthe epoxy resin prepared in this study could be used as a flameretardant coating material in electric and electronic fields.

A series of hyperbranched P-N containing FR coatings have beendeveloped based on UV curable technology. Huang et al. formulateda number of UV curable flame retardant coatings based on hyper-branched polyphosphate acrylate (HPPA) which was produced byreacting tri(acryloyloxyethyl) phosphate (TAEP) with piperazine(Scheme 9) [116]. Blends of TAEP and the HPPA were made atdifferent ratios, exposed to UV light to form films and charac-terized for their photopolymerization kinetics. The formulations


containing both HPPA and TAEP polymerized much faster than thatcontaining individual component. It is worth noting that there wasno phosphorous–nitrogen synergism observed in this system, aswith higher loads of HPPA the LOI value decreased from 47% to

-oxyethylmethacrylato) cyclotriphosphazene (NPHE).

zene-modified monomer (CPEP).





ynthe

3piiob

rcoCiTTpaa

eusteitPcS

Scheme 9. S

4%. It was proposed that HPPA mainly acted in the condensedhase, while TAEP acted mainly in the gas phase, but the gas phase

nhibition activity remained dominant in the blends. Although nomprovement on flame retardancy was found with adding HPPA,ther properties such as polymerizing rate and toughness becameetter via mixing TAEP with HPPA.

By blending TAEP with another monomer triglycidyl isocyanu-ate acrylate (TGICA), Xing et al. formulated another FR coating forotton [117]. In this research the positive effect of the FR coatingn flame retardancy of the cotton fabric was proven by Micro-scaleombustion Calorimeter (MCC) and LOI measurements. With the

ncrease of FR content, LOI value rose from 21% to 24.5%, whileHC decreased from 8.8 to 3.6 kJ/g (Table 5.1 (5)). The results fromGA showed that the coatings lowered the decomposition tem-erature of treated fabric and enhanced the char formation. Theuthors proposed a possible synergistic effect between phosphorusnd nitrogen for the flame retardancy of cellulose.

Apart from the P-N based flame retardant systems as discussedarlier, there have also been considerable research efforts on thetilization of phosphorous based flame retardants in PU binderystems. It is worth noting that polyurethane contains nitrogen,hus in this case there may also exist a phosphorous–nitrogen syn-rgistic effect during fire exposure [118]. Much of the researchn this area is based on reactive phosphorous compounds where


he phosphorous is chemically incorporated into the PU matrix.ark et al. prepared a two-component polyurethane coating systemonsisting of pyrophosphoric lactone-modified polyester (TAPT incheme 10) and isophorone diisocyanate isocyanurate. The reactive

sis of HPPA.

flame-retardant component had no negative impact on the phys-ical properties of the film [119]. The flame retardant properties ofthe coating were improved with increased phosphorous content, asinvestigated through TGA, Cone Calorimeter test, LOI, and the 45◦

Meckel burner method. It has been found that with higher TAPTcontent, the LOI values increased, the peak heat release reducedand the char length decreased Table 5.2 (1).

In another study conducted by Park et al., a similar flameretardant polyurethane coating was synthesized based on atriphosphorous structural unit instead of diphosphorous as men-tioned above [120]. In this research they have synthesized amodified polyester (ATBTP) by polymerizing tetramethylene-bis-(orthophosphate) (TBOP), 1,4-butanediol, trimethylolpropane,adipic acid with phenylphosphonic acid (PPA). Table 5.2 (2) showedthat with an increasing amount of PPA (increase in phosphorouscontent), both the LOI value (from 25% to 31%) and the char forma-tion increased. However addition of PPA to the matrix resulted ina decrease in some of the physical properties (pencil hardness andgloss retention) of the films. All coatings except the one with 30wt% PPA could meet the technical specification requirements.

Using bio-based materials like castor oil, a series of phospho-rous based flame retardant polyurethanes were prepared by Patelet al. via two different approaches [121]. In the first approach,castor oil was reacted with tris(m-hydroxy phenyl) phosphate


(THPP) to obtain a phosphorous-based polyol or an transesterifi-cation product (EERP in Scheme 11), which was then mixed withvarious diisocyanates to form polyurethane films (EERPPUs). Thesecond method involved the mechanical blending of the castor


Please cite this article in press as: S. Liang, et al., Recent developments in flame retardant polymeric coatings, Prog. Org. Coat. (2013),http://dx.doi.org/10.1016/j.porgcoat.2013.07.014




Scheme 10. Structure of pyrophosphoric lactone-modified polyester (TAPT).

Table 5.2Results of flammability tests of phosphorus-nitrogen based coating systems.

Coating system Flame retardant componentsin coating (wt %)

LOI (%) Flammability test/Cone calorimeter (CC) test

1 [119] Polyurethanecoating

TAPT: 0% 17 45◦ Meckel Burner test: Burnedcompletely

CC: heat flux = 50 kW/m2: PHRR = 1090 kJ/m2

TAPT: 10% 24 Pass; Char length = 18.5 cm PHRR = 854 kJ/m2

TAPT: 30% 30 Pass; Char length = 3.9 cm 3.9 cm PHRR = 711 kJ/m2

2 [120] Polyurethanecoating

PPA in ATBTP: 0% 25 45◦ Meckel Burner Test: Charlength <3.9 cm

PPA in ATBTP: 30% 31 Char length = 4.8

3 [121] Biobasedpolyurethane coating

EERPPU-1 type 29 UL 94 flame test: V0 rating

EERPPU-2 type 27 V1 ratingEERPPU-3 type 26 V1 ratingEERPPU-4 type 30 V0 rating

[121] Mechanical blendbased- PU coating

COPU50THRPUU50 37 UL 94 flame test: V0 rating

COPU60THRPUU40 35 V0 ratingCOPU70THRPUU30 33 V0 rating

4 [122]Polyester-urethanecoating

PP-1 TDI 33 UL94 flame test: V0 rating

IPDI 30 V0 ratingHMDI 29 V1 rating

PP-2 TDI 34 V0 ratingIPDI 32 V0 ratingHMDI 28 V1 rating

PP-3 TDI 32 V0 ratingIPDI 31 V0 ratingHMDI 29 V1 rating

Scheme 11. Preparation of castor oil based polyol.





f casto

optIt(ra322rrd

Scheme 12. Preparation o

il-based polyurethane (COPU) (Scheme 12) with the THPP basedolyurethanes in different proportions. The flame retardancy ofhe films was tested using LOI test and UL 94 flammability test.t can be noted from Table 5.2 (3) that the EERPPUs containingoluene diisocyanate (TDI) and methylene diphenyl diisocyanateMDI): EERPPU-1 and EERPPU-4 respectively, achieved better flameetardancy than those involving isophorone diisocyanate (IPDI)nd hexamethylene diisocyanate (HMDI): EERPPU-2 and EERPPU-

respectively. EERPPU-1 and EERPPU-4 achieved LOI values of9% and 30%, whereas EERPPU-2 and EERPPU-3 achieved 27% and


6%. In the UL 94 testing, EERPPU-1 and EERPPU-4 achieved a V-0ating, while EERPPU-2 and EERPPU-3 were rated V-1. These fireesults were supported by the fact that the char yields observeduring the thermal degradation analysis using TGA were higher for

Scheme 13. Synthetic ap

r oil based polyurethane.

EERPPU-1 and EERPPU-4 (10.09% and 11.41% respectively). In caseof mechanical blend based PU coatings, all the resulting systemsshowed V-0 ratings and the LOI values increased with increasedTHPPPU content. Besides, it was also observed that all EERPPUsprepared via approach one had better scratch resistance than theblends developed by the second method but the blends exhibiteda higher impact resistance.

In a similar study conducted by Patel et al., polyester-urethaneswere also prepared from phosphorous-containing polyester poly-ols [122]. Various phosphorous-containing polyester polyols were


reacted with multiple diisocyanates in order to obtain superior FRcoatings. It was found that, like in the study mentioned above, thecoating using TDI as reactants performed better as compared toIPDI and HMDI based coatings. The LOI value of TDI based coat-

proach of TEMPS.





Table 6Results of flammability tests of silicon based coating systems.

Coating system Flame retardant components in coating (wt %) LOI (%) Flammability test/Cone calorimeter (CC) test

1 [128] Si based curedcoating

EP828 (0% Si) 22 Char yield in TGA at 800 ◦C (air) = 0%

TEMPS40 (2.30% Si) 26 Char yield = 7%TEMPS100 (5.76% Si) 32 Char yield = 16%

2 [129] Si based curedcoating

EB600 (0% Si) 21 Char yield in TGA at 800 ◦C (air) = 0%

EB/TAEPS40 (2.49% Si) 26 Char yield = 9%EB/TAEPS85 (5.30% Si) 32 Char yield = 11%EB/DAEMPS40 (3.2%Si) 26 Char yield = 13%EB/DAEMPS85 (6.81%Si) 30 Char yield = 16%

3 [130] APP assistedSiO2coating

Substrate reference – 45◦ angle flammability test (ASTM D1230-99):Time of burning from bottom to top = 9 s

SiO2 precursor: 20% – Burning time = 25.25 sSiO2 precursor: 40% – Burning time = 30.80 s

4 [38] Organosilicon OnPA 6

Flow rate of O2 added in plasma source: 0 sccm 23 CC: heat reflux = 35 kW/m2:PHRR = 1850 kW/m2; THE = 30 kJ

10 sccm 20–25 PHHR = 1105 kW/m2; THE = 39 kJ50 sccm PHRR = 1300 kW/m2; THE = 23 kJ

[38] Organosilicon OnPA 6- nanocomposites

Flow rate of O2 added in plasma source: 0 sccm 23 PHRR = 1100 kW/m2; THE = 35 kJ

ipfpcsrptt

4

tsrooas

10 sccm

50 sccm

ngs ranged from 32 to 34% depending on the type of the appliedolyol. The LOI for HMDI based coatings ranged from 28 to 29% andor IPDI based ones from 30 to 32%. Both TDI and IPDI based sam-les achieved V-0 ratings in the UL94 burning test whereas HMDIoatings were ranked V-1 (Table 5.2 (4)). On exposure to fire, theseystems were believed to function via decomposition of phospho-ous compounds to release active species which could act in gashase to capture oxygen and hydrogen radicals on the surface ofhe film and also produce phosphoric acid derivatives that catalyzeshe char formation thus improving the barrier effect.

.6. Silicon-based coating systems

Silicon-containing FRs are considered as very promising alterna-ives to halogenated FR compounds as they do not release corrosivemoke during combustion [123,124] and are considered to be envi-onmentally friendly for use in coatings [125]. The preparation


f these silicon-based coatings mainly involves the incorporationf silicones, silicates, organosilanes or silsesquioxanes as fillersnd copolymer or as the main polymeric matrix into the wholeystem.

Scheme 14. Synthetic route

43–45 PHRR = 1051 kW/m2; THE = 34 kJPHRR = 819 kW/m2; THE = 31 kJ

Developments of silicone containing epoxies which can be curedon their own or mixed with other epoxy components have grownrapidly. Hydroxyl-terminated polydimethylsiloxane was used as achemical modifier to effectively introduce silicon into an epoxycoating, of which the thermal stability and anticorrosive proper-ties were significantly improved [126,127]. Cheng et al. [128] havesynthesized a novel silicon-containing tri-functional cycloaliphaticepoxy resin based on tri-(3,4-epoxycyclohexylmethyloxy) phenylsilane (TEMPS) by a simple method (Scheme 13) and thenapplied it as a reactive-type flame retardant in cationic UV-curable coating systems. It’s observed from Table 6 (1) thatthe addition of TEMPS to the commercial epoxide resin DGEBA(EP828) improved the char content (from 0% to 16% after TGAmeasurement) as well as the flame retardancy of the result-ing coating (LOI from 22% to 32%). Based on a similar principle,this group developed another two novel silicone-based multi-functional acrylate monomers, tri-(acryloyloxyethyloxy) phenyl-


silane (TAEPS) and di(acryloyloxyethyloxy) methyl phenylsilane(DAEMPS) (Scheme 14) [129]. Films formed from blending of thecopolymers T85 with commercial oligomer epoxy acrylate (EB 600)also resulted in higher char yield by 16% at 800 ◦C and better flame

of TAEPS and DAEMPS.





rai

chcpaitnioc

iompptccrnrfifi

4

ttPfltdf[apvaasc

Scheme 15. Hexachlorodiphosph(V)azane (Types I–III).

etardancy (Table 6 (2)). Interestingly, in both of their studies theddition of trifunctional silicon-containing blocks led to an increasen the elongation-at-break of the final coatings.

Flame retardant coatings have also been formulated using sili-on dioxide (SiO2) “network armor”, which was obtained throughydrolysis and condensation of the precursor tetraethyl orthosili-ate (TEOS) and then cross-linked on the surface of fibers (sol–gelrocess) [130]. In this article Totolin et al. have described the use oftmospheric pressure plasma (APP) technique to apply this coat-ng on cellulosic materials (Fig. 2) [130]. Table 6 (3) showed thathe modified substrate exhibited improved flame retardancy. SiO2etworks attached to the substrates were still present even after

ntense ultrasound washes and it has been suggested that such kindf coatings could find great applications in upholstered furniture,lothing, and military domains.

Jama et al. conducted investigations on the thermal stabil-ty of organosilicon thin films obtained from the polymerizationf 1,1,3,3- tetramethyldisiloxane (H(CH3)2Si-O-Si(CH3)2H) (TMDS)onomer doped with oxygen using the cold remote nitrogen

lasma (CRNP) process [38]. It was found that atmospheric oxygenarticipates in the degradation mechanism, leading to the forma-ion of more thermally stable products. From LOI test and conealorimetry results it was revealed that such deposits were effi-ient coating for polyamide 6-nanocomposites in terms of flameetardancy. As noted in Table 6 (4), LOI values for coated PA 6anocomposites were drastically increased by almost 100%. Suchesult was suggested to be the result of good adhesion quality of thelm and/or of possible synergistic reactions between the depositedlm and the clay nanocomposites.

.7. Multi element FR systems

The need to have ‘multi-element’ flame retardant formulationso achieve acceptable levels of flame retardancy has been illus-rated in the work of El-Wahab et al. [131,132]. In their work onU coatings they have developed a series of novel multi-elementame retardant compounds based on P, N, S and Cl. In one ofheir study they have synthesized hexachlorodiphosh(V)azaneerivatives which include chlorine, nitrogen and phosphorousor application in emulsion and solvent based alkyde paints132]. Hexachlorodiphosh(V)azane derivatives (I–III) of formulas shown in Scheme 15, were synthesized and mixed in theaints by grinding in a pebble mill (particle size -38 �m). The LOIalues of the alkyd and emulsion paints with no flame retardants


re 15% and 20%, respectively, but increased drastically withddition of these flame retardants. The type II compound washown to be the best flame retardant in both kinds of paint. Aoncentration of only 0.3% of type II flame retardant could improve

Scheme 16. Chemical structure of cyclodiphosh(V)azane of types (I–III).

the LOI of coatings by around 27% for the alkyd paint, and byaround 60% for the emulsion paint (Table 7 (1)). In a subsequentstudy, three flame retardant additives, cyclodiphosph(V)azaneof sulfaguanidine,1,3-di-[N/-2-pyrimidinylsulfanilamide]-2,2,2.4,4,4-hexachlorocyclodiphosph(V) azane and1,3-di-[N/-2-pyrimidinylsulfanilamide]-2,4-di[aminoacetic acid]-2,4-dichlorocyclodiphosph(V)azane (Scheme 16) were synthesizedby the same group for use as flame retardants in polyurethanecoatings [133]. It is evident that the incorporation of these multi-element additives into the coating system resulted in excellentflame retardant properties, as shown in Table 7 (2).

In the recent past there was an increased interest of researchersto develop novel halogen free flame retardants based on synergis-tic combinations of silicon and other elements. In a recent workit has been showed that siloxane derivatives exhibited a goodsynergistic effect with phosphorus compounds when applied inepoxy resins [134]. In this study a silicon-based acrylate (SHEA)was synthesized via the reaction between 2-hydroxylethyl acrylateand dimethyldichlorosilane and then blended with phosphorus-containing tri-(acryloyloxyethyl) phosphate (TAEP) in variousratios to obtain series of UV-curable fireproof coatings (Scheme 17).Analysis of such coatings using Micro-scale Combustion Calorime-ter indicated that with addition of TAEP to SHEA it was possible todecrease the heat release rate, heat release capacity and total heatof combustion (Table 7 (3)). Among all the TAEP/SHEA combina-tions, the system with TAEP: SHEA ratio of 1:1 showed the highestinitial decomposition temperature (304 ◦C) and left the most charresidue at 800 ◦C (17.3%). Real-time TGA analysis/infrared spec-trometry (FT-IR analysis) indicated that TAEP could promote thedecomposition of SHEA to form a comparatively stable char layer. In


another study conducted by Cireli et al. [135], a novel halogen-freeP-doped SiO2 thin film was deposited on the fabrics using sol–geltechnique to improve their flame retardancy. The treated cotton





ose m

fwtwmcto

TR

Fig. 2. Coating process for cellul

abrics acquired non-flammable properties and durability againstashing. In the Flame Spread test, the time needed to burn through

he fabric is increased from 5 s to more than 20.19 s when dopedith P-SiO2, and reduced to 8 s after 10 times’ wash, which is stillore than 5 s. (Table 7 (4)) Moreover the subsequent treatment of


oating with PU as a second step provided not only increase in theensile strength of the fabrics, but also maintained adequate levelf flame retardancy (Nonflammable in the Flame Spread test).

able 7esults of flammability tests of multi-element FR coating systems.

Coating system Flame retardant compo(wt %)

1 [132] P,N,S,Cl containing alkyd paint 0%

Compound I: 0.3%

Compound II: 0.3%

Compound III: 0.3%

[132] P, N,S,Cl containing emulsion paint 0%

Compound I: 0.3%

Compound II: 0.3%

Compound II: 0.3%

2 [133] Polyurethane Coating Blank coating

Compound I: 1.3%

Compound II: 1.3%

Compound III: 1.3%

3 [134] P-Si UV cured coating TAPE0SHEA6

TAPE2SHEA4

TAPE4SHEA2

TAPE6SHEA0

4 [135] P-doped SiO2 thin film Untreated fabric

SiO2 film

P-doped SiO2 film with

P- SiO2 film ethyldichloP-doped SiO2 film withacid and then with PU

5 [66] B-Si UV cured coating Boron content: 0

Boron content: 0.2

Boron content: 0.4

Boron content: 1.1

6 [136] Si, B, Ti, F containing UV cured coating 3 wt% Si/Ti ratio (wt%):Si/Ti ratio (wt%): 1/0

Si/Ti ratio (wt%): 1/1Si/Ti ratio (wt%): 1/2

Si/Ti ratio (wt%): 1/2 + 1

7 [138] P, N, Si- containing coating TAEP

SHUA10TAEP90:

SHUA20TAEP80:

SHUA40TAEP60:

aterials based on APP technique.

Mülazim et al. have reported organic- inorganic hybrid flameretardant coating systems based on boron and silicon [66]. Suchcoatings were prepared from a mixture of an acrylated bisphenol-A based epoxy resin, methacryloxymethyl triethoxysilane andboric acid in the presence of a cross-linker and a photoinitia-


tor (Scheme 18). It was reported that the thermal stability andflame retardancy of the hybrid coating materials improved withthe increasing boron content in coating formulation. LOI values

nents in coating LOI (%) Flammability test/Cone calorimeter(CC) test

15 –22 –27 –18 –20 –39 –62 –22 –

18 –34 –37 –29 –

– Microscale combustion calorimetry(MCC): PHRR = 280 W/g; THR = 13.4 kJ;HRC = 282 k/gK

– PHRR = 77 W/g; THR = 6.1 kJ;HRC = 78 k/gK



– Flame spread tests (ISO 6941): Time offlame spread (The time of burningthrough) = 5 s

– Time of flame spread = 15.64 sphosphoric acid – Non-flammablero phosphate – Time of flame spread = 20.19 s

phosphoric – Non-flammable

21.1 Char yield in TGA at 700 ◦C (air) = 6%26 Char yield = 11.3%27 Char yield = 15.2%31.6 Char yield = 23.4%

0/0 20 –22.5 –23.3 –24 –

F 24.1 –

43.0 Char yield in TGA at 850 ◦C (air) = 2.1%38.5 Char yield = 4.4%35.5 Char yield = 6.9%32 Char yield = 11.9%





curab

iTtr

Uwtsnprei

cttispcartrn

Scheme 17. Formulation of UV

ncreased from 21.1% to 31.6% while char yield at 700 ◦C (air inGA) increased from 6% to 23.4%, as observed in Table 7 (5). Addi-ionally these coatings brought in significant enhancement of theadiation-shielding properties of the substrate.

In another study conducted by Altıntas et al. [136], a series ofV curable silica–titania hybrid coatings were prepared togetherith the addition of various ratios of fluoro acrylate oligomers in

he coating formulations and then applied onto the polycarbonateubstrate. It was found that upon adding such inorganic compo-ents to the coating materials, the thermal, mechanical, and otherhysical properties, such as hardness, gloss, contact angle and flameesistance (12.5% increase in Table 7 (6)) were all improved. How-ver, incorporation of titanium and fluorine did not significantlymprove the flame retardancy of the coatings.

Kumar et al. showed within their research efforts that theyould improve the flame and moisture resistance as well as thehermo-mechanical behaviors of epoxy resin coatings [137]. Inheir study the principal modification of epoxy resin involvedncorporating siloxane, phosphorus and maleimido-containingkeleton into the coating system. Diglycidylether terminatedoly-(dimethylsiloxane) (DGTPDMS) and phosphorus-nitrogenontaining bismaleimide (PBMI) were used as the main chemicaldditives in the coating formulation (Scheme 19). The incorpo-


ation of DGTPDMS and PBMI into the epoxy resin enhanced thehermal stability and increased degradation temperature of epoxyesin which may be due to the formation of an inter cross-linkedetwork between siliconized epoxy and bismaleimides. The char

Scheme 18. Preparation of the boron-con

le P-Si flame retardant coating.

yields as measured in the TGA of modified epoxy matrices werehigher than that of the unmodified epoxy resin (16.87% compared to3.92% at 800 ◦C). DGTPDMS and PBMI incorporation into the epoxyresin further decreased the moisture absorption, due to hydropho-bic nature of the siloxane moiety and the bismaleimide moiety.

Cheng et al. have recently designed new multifunctionaloligomers containing both organic and inorganic components todevelop novel silicon based hybrid flame retardant coatings [138].A silsesquioxane-based hybrid urethane acrylate (SHUA) moietywas synthesized by modifying silsesquioxane hybrid polyol (SBOH)with isophorone diisocyanate (IPDI) and 2-hydroxyethyl acrylate(HEA) (Scheme 20), and was then blended with a phosphorus-containing monomer tri-(acryloyloxyethyl) phosphate (TAEP) indifferent ratios to form series of UV curable resins. A series offlammability results have been summarized in Table 7 (7): The charresidue of the polymer measured at 850 ◦C increased (from 2.1% to11.9%), whereas the LOI decreased (from 43.0% to 32%) with increas-ing SHUA content (from 0 wt% to 40 wt%). It was postulated thatthe main flame retardant activity of the phosphorus additive was bygas phase inhibition of radicals. The decrease in LOI with increasedSHUA content was attributed to the decreased phosphorus con-tent and the increased silicone content in the matrix, both of whichresulted in a weakening of gas phase activity of the active phos-


phorus species. As proposed in literature, the release of these activephosphorus volatiles can be retarded by more char-formation. Themechanical and physical properties of the resin were also improveddue to the synergistic effect of organic and inorganic components.

taining silicon based hybrid coating.





re of D

4

oomcn

Scheme 19. Structu

.8. Nanocomposite based coating systems

Over the last two decades, research and development in vari-us polymer nanocomposites have shown great promises in termsf improving the flame retardant properties of several polymer


atrices. Typically nano-sized inorganic fillers such as layered sili-ates [139], silsesquioxane derivatives [140], TiO2, SiO2 and carbonanotubes have been extensively used [141,142]. However, studies

Scheme 20. Synthetic

GTPDMS and PBMI.

have also come up with the potential issues of toxicity, workplacesafety, and environmental impact deprived from commercial use ofsome nanoparticles, such as the possible emerging of new classes ofnon-biodegradable pollutants, the worry of such materials infiltrat-ing humans and series of unknown effect after long-term contact


with nanoparticles, etc. [143–148]. It is also to be noted that thenanoparticles are commercially not very successful in FR appli-cations, since they did not help pass some important regulatory

route of SHUA.





OSS;

fiatfinrifc

n[o(3fntPCa

spChdc[

oiiatnmi

tcib

Scheme 21. (A) Octamethyl-P

re tests and thus may need additional conventional FRs as co-dditives to improve their actions. Because of extensive benefitshat nanotechnology has ever brought into coating societies, sur-ace treatments by nanocomposites formulations are still gainingncreasing attention in FR domain. Several working mechanisms ofanoparticles have been further proposed in detail [149,150]. It iseported that nanocomposites based coating represents a promis-ng system which protects the oxidation of the char structure onceormed and thus reinforce the flame resistant properties of theoatings [151,152].

Devaux et al. have highlighted the potential of the use ofano-additives in flame retardant application for textile coatings153]. In their work, three nano-additives, octamethyl-polyhedralligomeric silsesquioxanes (POSS-MS), poly vinylsilsesquioxanePOSS-FQ) shown in Scheme 21 and montmorillonite clay (Cloisite0B), were mixed with polyurethane to render polyester or cottonabric flame retardant. It was emphasized that the choice of theano-additives and that of coating process were of great impor-ance in providing substrates with desirable flame retardancy.OSS-FQ have better flame retardant effect than POSS-MS andloisite B, since this FR system showed lower peak value of HRRs well as THC than the all the others, as shown in Table 8.1 (1).

One of the best-known methods for incorporating the nano-cale inorganic particles into an organic matrix is the sol–gelrocess, which has been briefly discussed in the previous sections.ombined with chemical nanotechnologies, remarkable progressas been achieved and this technique has taken its place as a fun-amental approach for the development of novel nano-conceptedoatings, which will be further discussed in the following sections154].

By means of sol-gel technique, Zeytuncu et al. prepared a seriesf UV curable boron containing nano-hybrid coatings and clearlyndicated several benefits of the resulting systems [32]. Their phys-cal and mechanical properties such as hardness, chemical andbrasion resistance and adhesion were also improved. Additionallyheir thermal and flame-retardant properties underwent a sig-ificant augmentation along with increasing boron content. Theorphology studies further showed that the nanometer-scaled

norganic particles dispersed homogenously in the hybrid system.Starting from tetraethoxysilane and �-or �,�-triethoxysilane


erminated poly-(1-caprolactone) (PCL–Si), Messori et al. suc-eeded in formulating the nanocomposites with a high level ofnterpenetration between organic and inorganic phases, followedy a dip-coating of poly(methyl methacrylate) slabs with the

B: Poly (vinylsilsesquioxane).

obtained PCL-Si/silica hybrid [154]. All coated PMMAs exhibiteda fairly strong increase in the flame retardant efficacy (70–120%with respect to uncoated PMMA in terms of delay of flame time inTable 8.1 (2)) without any loss in transparency. Via further investi-gation of XPS the improved flame retardancy was attributed to thepreferential segregation of silica on the surface of coated samples.In addition, it was observed that the hybrid coating improved theUV resistance of PMMA.

Another frequently used method to deposit nanocoatings onsubstrates is Layer-by-Layer (LbL) Assembly. It has become a pop-ular method for fabricating multifunctional thin films that aretypically less than one micrometer thick. Carosio et al. have eval-uated the effectiveness using LbL assembly method for coatingsilica nanoparticles to improve the flame retardant properties ofpolyethylene terephthalate (PET) fabrics [155]. Cone calorimetricmeasurements indicated that the coating system containing fivebilayers (5 BL) of the smallest nanoparticles (CL/SM system) pro-vided the best results. As can be seen in Table 8.1 (3), such systemhave the increase of time to ignition (tig) of by 45% and reductionof peak of HRR in comparison to the reference. Moreover it sig-nificantly reduced the burning time by 95% and eliminated meltdripping of the substrates in vertical burning test.

More recently Laufer et al. have developed thin films of colloidalsilica-polyelectrolyte on cotton fibers using LbL technique to reduceits flammability [156]. The coated fabrics left a significant amountof char and still preserved the fabric weave structure after the firetests and it was interestingly found that the mercerization of fabricprior to LbL assembly can further result in better flame-retardantbehavior of the coated fabric (Table 8.2 (1)). In an earlier work, thesame group has also reported the use of LbL technique for coatingbranched polyethylenimine (BPEI) and sodium montmorillonite(MMT) clay on cotton fabric [62]. As indicated from Table 8.2 (2), thecoatings which consist of 5–20 bilayers of BPEI/MMT significantlyimproved the flame retardancy of the fabric, such as low PHRR, THCand char yield measured in Microscale Combustion Calorimeter.The authors mentioned that this technique was versatile in provid-ing flame retardancy and could be applicable to other materials orsubstrates (e.g. polyurethane foams used for insulation).

Attempts in developing green flame retardant technology haveled to researchers combining plasma and nano-technology to


develop coatings with enhanced fire retardant action. Through coldremote nitrogen plasma technique, organosilicon thin films weredeposited on the nylon substrates by polymerization of 1,1,3,3-tetramethyldisiloxane monomer pre-mixed with oxygen [157]. A


Please cite this article in press as: S. Liang, et al., Recent developments in flame retardant polymeric coatings, Prog. Org. Coat. (2013),http://dx.doi.org/10.1016/j.porgcoat.2013.07.014




Table 8.1Results of flammability test of nanocomposite based coating systems.



1 [153] Polyurethanenanocoating

0% – CC: heat Reflux = 3 kW:PHHR = 210 kW/m2; THC = 50 kJ

POSS FQ 1: 10% – PHHR = 200 kW/m2; THC not measured.POSS FQ 2: 10% – PHHR = 115 kW/m2; THC = 25 kJPOSS MS 2: 10% – PHHR = 190 kW/m2; THC = 38 kJCloisite 30B 2: 10% – PHHR = 125 kW/m2; THC = 58 kJ

2 [154] Si containinghybrid nanocoating

UV-cured reference – Glow wire test (730 ◦C) Time toflame = 6.3 s

Cured �-PCL-Si/SiO2 – Time to flame = 10.7–12.7 sCured �,�-PCL-Si/SiO2 – Time to flame = 10.7–14 s

3 [155] LbL assemblycoating

Reference – Vertical flame test: Burning time = 32 s CC: heat flux = 35 kW: tig = 22 s;PHRR = 54 kJ/m2; THC = 1.47 MJ

5 BL (CL/TM) – Burning time = 6 s; tig = 269 s; PHRR = 52 kJ/m2;THC = 1.40 MJ

10 BL (CL/TM) – Burning time = 9 s; tig = 263 s; PHRR = 49 kJ/m2;THC = 1.10 MJ

20 BL (CL/TM) – Burning time = 22 s tig = 218 s; PHRR = 49 kJ/m2;THC = 1.45 MJ

[155] LbL assemblycoating

5 BL (CL/SM) – Burning time = 10 s tig = 321 s; PHRR = 44 kJ/m2;THC = 1.35 MJ



Table 8.2Results of flammability test of nanocomposite based coating systems.

Coating system Flame retardant components incoating (wt%)



Reference – Micro combustion calorimetry (MCC):Char yield = 4.89 wt%; PHRR = 285 W/g;THC = 12.8 kJ/g

CL/SM 10 BL – Char yield = 9.53 wt%; PHHR = 253 W/g;THC = 11.7 kJ/g

20 BL – Char yield = 9.58 wt%; PHRR = 243 W/g;THC = 11.7 kJ/g

CL/TM 10 BL - Char yield = 6.89 wt%; PHRR = 240 W/g;THC = 12.2 kJ/g


PEI/SM 10 BL – Char yield = 13.07 wt%; PHRR = 227 W/g;THC = 10.5 kJ/g


PEI/TM 10 BL – Char yield = 9.59 wt%; PHRR = 258 W/g;THC = 11.6 kJ/g



Reference – Micro combustion calorimetry (MCC):Char yield = 2.88% wt%; THC = 11.63 kJ/g

BPEI pH7 10/0.2% MMT 5BL – Char yield = 6.75% wt%; THC = 11.17 kJ/gBPEI pH7 10/1% MMT 5BL – Char yield = 8.37% wt%; THC = 10.73 kJ/g

3 [157] Plasma depositedthin film

Reference-PA6 21 CC: heat flux: 35 kW/m2 tig = 66 s;PHHR = 1053 kW/m2; THC = 1346 kJ;Char = 1.0%

Coating on PA 6 substrate 21–22 tig = 67 s; PHHR = 967 kW/m2;THC = 829 kJ; Char = 1.9%

Reference-nano PA 6composites

22 tig = 98 s; PHHR = 699 kW/m2;THC = 949 kJ; Char = 4.0%

Coating on nano PA 6 substrate 42–48 tig = 96 s; PHHR = 623 kW/m2;THC = 900 kJ; Char = 4.2%

4 [50] Siliconenanocoating

PA 6 nanofiber – Char yield in TGA (700 ◦C in air) = 7%; Micro combustion calorimetry (MCC):PHRR = 680.7 W/g

Si coated PA6 – Char yield = 9.48% PHRR = 601.3 W/gSi coated PA6/Fe-OMT – Char yield = 13.42% PHRR = 511.1 W/g





Table 9Results of heat reflectivity/flammability tests of the novel coating systems.

Coating system Sample Effective absorptivities (%) for different heat sources

bb700 bb873 bb1051 bb1200 sooty clear

1 [51] VO2 coating White uncoated PMMA 0.96 0.95 0.95 0.93 0.93 0.95Black uncoated PMMA 0.96 0.96 0.96 0.96 0.96 0.96Clear uncoated PMMA 0.96 0.95 0.93 0.90 0.91 0.95White ITO-uncoated PMMA 0.58 0.63 0.66 0.70 0.69 0.66Black ITO-uncoated PMMA 0.59 0.65 0.69 0.73 0.73 0.68Clear ITO-uncoated PMMA 0.60 0.65 0.67 0.68 0.68 0.68

2 [158] IR-mirror coating Cone calorimeter results (50 kW/m2)tig (s) PHRR (kW/m2) FIGRA (kW/s/m2) MARHE (kW/m2) PHRR/tig (kW/m−2s−1)

PA 66 58 ± 10 1286 ± 30 8.8 ± 0.2 521 ± 6 23.1 ± 4.4± 126

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omparison between virgin PA6 and plasma coated PA6 containinganoclay (2%) indicated higly improved flame retardancy in pres-nce of nanocoaitng. In case of plasma-coated PA6 nanocomposites,n increase in LOI (130%) and decreases both in PHRR (41%) and THR33%) are noticeable in Table 8.2 (3). It was suggested that duringombustion, the combined action of plasma coating and nanocom-osite structure led to the formation of a very effective silicone andarbonaceous protective layer for flame retardation.

A recent study by Cai et al. reports a well-controlled sputter-ng technique that was used to deposit a thin coating of siliconanoparticles on polyamide 6 (PA-6) nanofibers (substrate 1) andA-6/organic-modified Fe-montmorillonite (Fe-OMT) nanocom-osite fibers (substrate 2) [50]. Compared to uncoated PA fibers, theGA analyses of Si coated nanocomposites fibers demonstrated anncrease of char yield by around 100%. As can be seen in Table 8.24), the PHRR of Si sputtering coated PA6/Fe-OMT nanocompos-te fibers decreased notably from 680.7 W/g to 511.1 W/g. It wasuggested that the in-situ generated silicon dioxide, barrier effectsf silicate and the synergistic effects between the Fe-OMT and Sill contributed to the improved flame retardant properties of theesulting coated systems.

.9. Other coating systems

More recently surface materials using transition metals oxideas also found applications in FR domain. Two types of coatingystems were investigated by Michael et al. The first material wasanadium dioxide (VO2) which would start to reflect the incomingeat radiation once exposed to fire. However, the limited reductionf the absorptivity leads to its limited use from a fire safety per-pective. The second material was indium tin oxide (ITO) coating,hich is partially reflective for fire- induced IR radiation. The effec-

ive absorptivity of PMMA was found to have deceased by around0% when coated by ITO and resulted in more than a doubling ofhe time to ignition in the test of cone calorimeter (Table 9 (1)).n addition to the attention of its potential application in domainf fireproof coating, the researcher also emphasized the concernsbout the availability of indium in the future [51].

In research of Schartel’s group they also proposed a novel andnnovative concept in rendering substrate flame retardancy, whereub-micrometer coating can be used as an infrared mirror (IRC).oatings are composed of three-layer system (Cu and Cr layers asell as SiO2 on the top to protects the metal layers against wear

nd corrosion) to ensure adhesion to the substrate, acting as annfrared (IR) mirror and protecting against oxidation. The coating


irror is deposited by physical vapor deposition (carried out in aluster coating system) on polyamide 66 as well as polycarbon-te (PC) plate in sub-micrometer range. The testing results showedhat not only the ease of ignition but also flame spread and fire

1.8 ± 0.2 140 ± 14 2.1 ± 0.44.4 ± 0.1 177 ± 8 5.2 ± 0.21.5 ± 0.1 74 ± 8 0.9 ± 0.1

growth indices were reduced remarkably [158]. All the indices suchas tig, PHRR, fire growth rate (FIGRA), maximum average rate of heatemission (MARHE) and PHRR/tig have been summarized in Table 9(2). In the case of PA 66, the indices for PA 66-IRC are all reduced to arange from around 1/4 to <1/10 of the values for PA 66; in the case ofPC-IRC, down to the range of around 2/5 to <1/5 of the values for PC.

5. Further perspectives on non-intumescent coatings

After reviewing the literature for non-intumescent flame retar-dant coatings from recent times, there are common trends orinteresting paths which can be seen being taken by academia andindustry alike. Here follows a summary of the trends that have beenfound in the literature, and an explanation to where new researchis heading towards in the future.

5.1. Ease of application

The ease of application of a coating system can be an importantfactor for future development and potential commercialization. Asystem which is easy to apply can reduce setup and cleanup laborand could further increase the production speeds and throughput.As discussed in the previous sections, many of the recent devel-opments on flame retardant coating for textile are focused on aUV or UV-assisted curable system, where the padding/immersionare mostly involved, followed by a rapid process of curing underUV irradiation (e.g. Lamp). However, the use of UV-technology forcoating plastic is limited, mainly because of inferior adhesion tothe substrate compared to solvent borne systems. As a result theoptimization of existing binder/reactive diluent systems/surfacepre-treatment will provide better solutions for series of flameretardant plastics in terms of FR coating [159,160]. In case ofLbL technique that involves the precursor deposition, drying andthermal treatment, more effort can be put into simplifying the pre-cursor preparation and its processing procedure, e.g. eliminatingthe required pre-cleaning process [161,162]. Industrial applicationof vapor deposition technology faces several challenges. In case ofCVD technique, they often require high-deposition temperature(>1000 ◦C) and produce chemical waste that is environmentallyunacceptable, while PVD usually shows the shortcomings relat-ing to its low deposition rates and low efficiency in applying oxidecoatings [163]. A simplified versatile/universal process for a broadrange of applications ranging from traditional FR finishing to com-plex 3D-shapes coating, has become one of the main focus in thedevelopment of flame retardant coating.


5.2. Cost-saving strategies

Looking for cost-saving strategies is also of great importance toenrich its further application in industrial domain. With regards to


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et chemical process such as LbL deposition and sol–gel coatings,he cost involved for the chemicals consumption are on an averageigher than the cost of machinery. In case of gas-based processingystems (e.g. plasma, PVD and CVD), the machinery, infrastructurend operation/maintenance (application of high temperature, highressure, etc.) costs are quite high. As a result, more effort haso be made to minimize the expense involved in coating process,hile its further optimization involves seeking the solution of faster

uring, less solvent and reduced energy consumption. Meanwhile,here has been other research conducted to reduce cost, for exam-le, replacing antimony trioxide with zinc borates, with the flameetardant effectiveness of the coating as the trade-off. It addresseshe importance that the low cost flame retardants are still efficientnough to provide the material being coated with passable flameetardancy. Furthermore, looking for novel synergistic systems, i.e.-N, P-Si synergism, would be a long-term solution for cost saving.

.3. Sustainable flame retardant systems

With increasing global environmental awareness, it is becom-ng more important to develop systems which are sustainable. Toevelop these sustainable systems it would be of great interest totilize renewable sources as raw materials. As can be seen in liter-ture [121,164], there are novel polyurethane systems that use oilss a component that come from vegetables or other plants such asastor beans; these polyurethanes can then be used as FR matrix.he goals of this type of research are to reduce our dependency onetroleum resources, reduce the negative impact on the environ-ent, and add more value to the farming and agricultural industry

164,165].

.4. Compatible coating systems

At a general level, it is clear that there have been concernsver the migration of the FR additives, reduction of the coat-ng/substrates adhesion and low durability of the coating posedy the additive types systems [31,129,166]. The net result of a vari-ty of initiatives has been the increasing requirements of the highlyompatible chemical components within the formulations and thushe growing interest of using reactive FR and substrate-reactiveolymeric coating for different underlying substrates.

.5. Synergistic coating systems

The most effective non-intumescent FR systems to date com-rise of halogen-antimony or halogen–phosphorus combinationsut these formulations pose environmental concerns. Efforts areeing made to develop novel synergistic fire-proof formulations,ased on a completely new FR chemistry (multi-element singleame retardant) or combination of new synergistic components.-N based synergistic FR system is one of the most studied systems.n exposure to heat and fire, the FR efficacy of the phosphorous anditrogen components together is superior to the theoretical effectsf the individual components added separately. These systems usu-lly reduces the loading of the phosphorus component (reducesost) of the FR formulation, while simultaneously preserving orven improving their coating performances, e.g. flame retardancy,echanical, physical and chemical properties. In addition, more tai-

ored functionalities can be introduced to such novel coatings, suchs UV reactive dilutent, crosslinker, etc (e.g. P-N system [114]).

.6. Multifunctional coating systems


From the advanced material point of view, there is a growingendency to design multifunctional systems for high-tech textiles,uilding, electronics and aviation, etc. [167–170] In such modules,


new kinds of thermal, electrical, optical and bio-based materialswith ideal abrasive, impact and flame resistant properties are grad-ually under requisition. As discussed in previous sections, someorganic–inorganic hybrid coating systems offer multifunctionalproperties which exhibit great potentials to help create easily pro-cessable, full-range applicable systems for various substrates.

5.7. Nano-technology

As reviewed in the last section, nano-technology is now play-ing an important role in development of novel FR systems andfinds more and more industrial applications. The advantages thatnanocomposites offer far outweigh the cost concerns. It can enableflexible coating architecture and microstructural design to fulfillversatile requirements, e.g. precise-characteristic control (physical,chemical and mechanical properties), micro-thin film tailoring, andmultifunctional assemblies, etc. [162]. With time the technologywill be further refined and better processes developed. However, ascompared to conventional halogen based FR systems, the efficiencyof nano-particles are still insufficient in providing adequate flameretardancy when used alone in coatings. Future research will focuson new types of nano-fillers, allowing new nanocomposites struc-tures with more improved flame retardant properties. Furthermorecombination of nano-particles with other conventional FR contain-ing systems appears as a promising way to confer the substratewith superior flame retardancy. Nowadays the potential environ-mental and health concerns related to the use of nanoparticleshave evoked extensive attention in scientific community and thusdetailed assessment of their environmental/health impacts is stillneeded before putting them in commercial coating applications.

6. Conclusion

This paper began with introducing a series of processing tech-niques that are currently used to formulate FR coatings, such asUV curing, plasma coating, vapor deposition, sol–gel technique,LbL assembly. It was then followed by a detailed review on abroad range of non-intumescent formulations emerging in the 21stcentury: Due to series of environmental and safety concerns ris-ing from halogenated compounds, halogen based formulations arebeing increasingly replaced by other FR systems. Inorganic addi-tives applied to coatings usually work as a cost-saving FR solution.Phosphorus based compounds are developed as efficient flameretardants in coating and can be used together with nitrogen sys-tems to generate synergistic effect for fire protection. Silicone baseddeposits generate efficient protective ‘coating’ in terms of flameretardancy for various substrates. Most multi-element FR coatings(P, N, S, Si, etc.) are characterized by superior flame retardancy, lowsmoke emission and multifunctional properties but it needs furtherefforts before it can be commercialized. Nanocomposites can beapplied as FR coating via different techniques (sol–gel, LbL assem-bly, plasma and sputtering technique, etc.) which represents newrevelatory solutions in providing efficient fire-proof characteristicsto the substrate. Possible developments for the next generationof non-intumescent FR coating were also highlighted in contextto ease of application, cost-saving strategy, better compatibilityto matrix and design of synergistic, multifunctional and sustain-able nano-based systems. It’s considered that a number of diverseapproaches can be combined to achieve enhanced FR efficacy, butonly time will tell which of these approaches may eventually suc-ceed in significantly decreasing the toxicity, environmental andother regulatory concerns of current flame retardant systems.


Acknowledgements

The authors acknowledge Empa (Swiss Federal Laboratoriesfor Materials Science and Technology) and CTI (commission


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or technology and innovation), Switzerland for the financialupport.

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Documents

Recent developments in flame retardant polymeric coatings