This is an Accepted Manuscript of an article published by Taylor & Francis in  Polymer - Plastics Technology and Engineering  on 20 October 2016, available online: https://doi.org/10.1080/03602559.2016.1233262


Synthesis and application of hexaallylamino-cyclo-triphosphazene as flame retardant in latex coatings

Jana Machotova 1, Lucie Zarybnicka 1, Adela Ruckerova 1,  Radka Bacovska 2, Jozef Rychly 3, Ales Imramovsky 4

1 Institute of Chemistry and Technology of Macromolecular Materials, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
2 Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic
3 Polymer Institute, Slovak Academy of Sciences, Dubravska Cesta 9, 845 41 Bratislava 45, Slovak Republic
4 Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic

Correspondence to: Jana Machotová; e-mail address: jana.machotova@upce.cz, 
tel.: +420 46 603 7194, fax: +420 46 603 7068.

Abstract The paper deals with a novel flame retardant and its application in waterborne coatings. The flame retardant was synthesized from hexachloro-cyclo-triphosphazene by nucleophilic substitution with allyl amine. Functionalized emulsion copolymers bearing in the structure hexaallylamino-cyclo-triphosphazene were prepared by the semi-continuous emulsion polymerization. The latexes were used as the main component of transparent coating systems cured by melamine-formaldehyde resin. The incorporation hexaallylamino-cyclo-triphosphazene did not affect transparency, flexibility and toughness of resulting coatings and increased their flame stability in terms of the amount of smoke release and maximum average rate of heat emission indicating a slower flame spread during the material combustion.

Keywords: emulsion polymerization, core-shell latex flame retardant, phosphazene, cone calorimeter

Introduction
During the last decades, increasing numbers of coating suppliers are moving to waterborne technology in order to meet volatile organic compounds (VOC) targets set by the government regulation laws. Thus, traditional solvent-based coatings are being substituted by environmentally friendly waterborne coatings offering a wide application range of products, starting from paints and plasters for the construction industry, through wood paints and varnishes to paints for metal protection and decorative systems [endnoteRef:1],[endnoteRef:2]. However, performances such as quick drying, adhesion strength, water resistance, and mechanical properties of the latter are generally inferior to those of the former and many attempts to improve them are being continued [endnoteRef:3],[endnoteRef:4],[endnoteRef:5],[endnoteRef:6].  [1: .  Itoh Y., Akasaka R.: Hydrolysable emulsifier-containing poly(meth)acrylate lattices for paper coating. International Journal of  Polymeric Materials and Polymeric Biomaterials, 63,137-142 (2014).]  [2: . Peruzzo P.J., Anbinder P.S., Pardini O.R., Vega J., Costa C.A., Galembeck F., Amalvy J.I.: waterborne polyurethane/acrylate: comparison of hybryd and blend systems. Progress in Organic Coatings, 72, 429-437(2011).]  [3: . Steward P.A., Hearn J., Wilkinson M.C.: An overview of polymer latex film formation and properties. Advances in Colloid and Interface Science, 86, 195-267 (2000).]  [4: .  Bal A., Güçlü G., İyim T.B., Özgümüş S.: Effects of nanoparticles on film properties of waterborne acrylic emulsions. Polymer-Plastics Technology and Engineering, 50:990-995 (2011).]  [5: . Xu H., Yang D., Guo Q., Wang Y., Wu W., Qiu F.: Waterborne polyurethane-acrylate containing different polyether polyols: preparation and properties. Polymer-Plastics Technology and Engineering, 51, 50-57 (2012).]  [6: . Liu J., Ma J., Bao Y., Zhu Z.: Synthesis and application of polyacrylate/nano-SiO2 composite leather finishing agent with polymerizable surfactant. Polymer-Plastics Technology and Engineering, 51, 1460-1467 (2012).] 

The film formation of conventional latex coatings primarily relies on coalescence of thermoplastic polymeric particles to provide mechanical properties and chemical resistance. Coalescence is a result of physical entanglement of polymeric molecules, and consequently it is not an adequate substitute for the chemical crosslinking. Hence, the crosslinking technology has frequently been incorporated into latex coatings to improve their performance [endnoteRef:7],[endnoteRef:8]. A widely used thermoset coating technology is based on acrylic latexes containing carboxyl and hydroxyl functional groups. These materials are commonly reacted with melamine formaldehyde (MF) crosslinkers during the curing cycle of the thermoset coating [endnoteRef:9],[endnoteRef:10],[endnoteRef:11],[endnoteRef:12],[endnoteRef:13]. The primary crosslinking reaction takes place between a preferably primary hydroxyl group on the emulsion polymer and an alkoxymethyl on the MF crosslinker. The alkyl is usually a methyl, butyl or isobutyl group. Usually, a high baking temperature (150-200 °C) is required to crosslink the baking finishes currently used in industry. These types of thermoset coatings possess many excellent performance properties including light stability and water resistance, which is important for products that will be subjected to exterior exposure. [7: . Zhang J.D., Yang M.J., Zhu Y.R., Yang H.: Synthesis and characterization of crosslinkable latex with interpenetrating network structure based on polystyrene and polyacrylate. Polym Int, 55, 951-960 (2006).]  [8: . Holub P.: (2004) One-component systems yield good properties. Eur Coat J, 10:21-28]  [9: . Ferrell P.E., Gummeson J.J., Hill L.W., Truesdell-Snider L.J.: (1995) the reactions of amines with melamine formaldehyde crosslinkers in thermoset coatings. J Coat Technol, 67:63-69]  [10: . Harakawa H., Kasari A., Tominaga A., Yabuta M.: (1998) The rheological properties of an aqueous acrylic dispersion suitable for automotive waterborne basecoats. Prog Org Coat, 34:84-90]  [11: . Huang Y., Jones F.N.: (1996) Synthesis of crosslinkable acrylic latexes by emulsion polymerization in the presence of etherified melamine-formaldehyde (MF) resins. Prog Org Coat, 28:133-141]  [12: . Richter G., Doessel K.F., Dutt W.: (2010) Process for the production of a dark-color multi-layer coating. PCT Int Appl WO 2010030970 A2 20100318]  [13: . Dutt W., King J.G.: (2010) Method of producing a polished metal effect finish on a vehicle. US Pat Appl Publ US 20100272986 A1 20101028] 

For special applications, increased flame stability of coating materials may be appreciated. To enhance special safety properties in terms of reduced flammability, the incorporation of derivatives of halogeno-cyclo-phosphazenes into the polymer backbone may offer solutions. Hexahalogeno-cyclo-triphosphazenes contain alternating phosphorus and nitrogen atoms in the cycle with two substituents attached to the phosphorus atoms. These compounds exhibit unusual thermal properties, such as flame retardancy and self-extinguish ability [endnoteRef:14],[endnoteRef:15]. During the endothermic thermal decomposition of phosphazene-based polymers, phosphate, metaphosphate and polyphosphate compounds are generated and a nonvolatile protective barrier layer on the polymer surface is formed, cutting off the supply of oxygen [endnoteRef:16],[endnoteRef:17]. The most significant halogeno-cyclo-phosphazenes include hexachloro-cyclo-triphosphazene (HCCTP) which was first synthesized by J. von Liebig in 1834. The typical reaction of HCCTP is a nucleophilic substitution. The nucleophile, being able to pass its free electron towards the attacking electrophilic phosphorus atom, belongs usually to the group of amines, alcoholates, alcohols, or thiols. Several derivatives of HCCTP have already been synthesized and used as flame retardants and antioxidants in polymers [endnoteRef:18],[endnoteRef:19],[endnoteRef:20],[endnoteRef:21],[endnoteRef:22],[endnoteRef:23],[endnoteRef:24],[endnoteRef:25],[endnoteRef:26]. Nevertheless, the covalent bonding of HCCTP derivatives into emulsion polymers and their utilization as flame retardants in latex coatings has not been reported so far to the best of our knowledge.  [14: . Zarybnicka L., Bacovska R., Vecera M., Snuparek J., Alberti M., Rychly J., Kalenda P.: (2016) Synthesis of curing agent for epoxy resin based on halogenophosphazene. J Appl Polym Sci, 133:42917-42926]  [15: . Allen C.W.: (1991) Regio- and stereochemical control in substitution reactions of cyclophosphazenes. Chem Rev, 91:119- 135]  [16: . Levchik G.F., Grigoriev Y.V., Balabanovich A.I., Levchik S.V., Klatt M.: (2000) Phosphorus–nitrogen containing fire retardants for poly(butylene terephthalate). Polym Int, 49:1095-1100]  [17: . Gu J.W., Zhang G.Ch., Dong S.L., Zhang Q.Y., Kong J.: (2007) Study on preparation and fire-retardant mechanism analysis of intumescent flame-retardant coatings. Surf Coat Technol, 201:7835-7841]  [18: . Gu X.J., Wei H., Huang X.B., Tang X.Z.: (2010) Synthesis and characterization of a novel curing agent for epoxy resin based on phosphazene derivatives. J Macromol Sci A: Pure Appl Chem, 47:828-832]  [19: . Sun J., Wang X.D., Wu D.Z.: (2012) Novel Spirocyclic Phosphazene-Based Epoxy Resin for Halogen-Free Fire Resistance: Synthesis, Curing Behaviors, and Flammability Characteristics. ACS Appl. Mater. Interfaces, 4:4047-4061]  [20: . You G., Cheng Z., Hao P., He H.: (2014) Research progress on synthesis and application of cyclotriphosphazene-based flame retardants. Chin J Appl Chem 31:993-1009 ]  [21: . He Y., Su S.: (2014) Flame retardant resin composition without halogen and its application. PCT Int Appl WO 2014089934 A1 20140619]  [22: . Ru J, Liu S, Wu H (2010) Flame retardant adhesive with low halogen content, and flexible copper clad laminate therewith. Faming Zhuanli Shenqing CN 101892027 A  20101124.]  [23: . Lee C.Y., Chiu Y.S.: (2001) Process for preparing a mixture of amino-​containing phosphazenes from chlorophosphazenes, n-​propanol and ammonia by a water-​free process. US Pat Appl Publ US 6265599 B1 20010724]  [24: . Yoon H.S. (1999) Curing of an DGEBA by diaminotetrachlorocyclotriphosphazene and its thermal properties.  J Korean Fiber Soc, 36:572-579]  [25: . Yoon H.S., Choi K.S., Takahashi K.: (1996) Curing of an epoxy resin by diaminotetrabromophenoxycyclotriphosphaz​ene and its thermal properties. Han'guk Somyu Konghakhoechi, 33:610-617]  [26: . El Gouri M., El Bachiri A., Hegazi S.E., Ziraoui R., Rafik M., El Harfi A.: (2011) A phosphazene compound multipurpose application-Composite material precursor and reactive flame retardant for epoxy resin materials J. Mater. Environ. Sci. 2:319-334] 

In the present work, we concentrated on the synthesis of a new flame retardant using nucleophilic substitution of HCCTP with allylamine. The resulting hexaallylamino-cyclo-triphosphazene (HACTP) was incorporated into acrylic polymers during the emulsion polymerization and the latexes were further used for the development of low VOC thermosetting transparent coating systems. The final coating properties were evaluated with emphasis on the flame stability determined using dual cone calorimeter measurements. 

Experimental
Materials
Allyl amine and hexachloro-cyclo-triphosphazene (HCCTP) were used for the synthesis of the hexaallylamino-cyclo-triphosphazene (HACTP) flame retardant and were purchased from Sigma-Aldrich, Czech Republic. Latexes investigated in this research work were synthesized of methyl methacrylate (MMA), butyl acrylate (BA), methacrylic acid (MAA), allyl methacrylate (AMA) and 2-hydroxyethyl methacrylate (HEMA). All the monomers were purchased from Roehm (Germany). Disponil FES 993 IS (BASF, Czech Republic) was used as a surfactant, 4,4´-azobis(4-cyanovaleric acid) (VAZO 68,  Biosynth, Switzerland) was utilized as an initiator of the polymerization reaction and sodium hydrogen carbonate (NaHCO3, Penta, Czech Republic) was used as a buffer. CYMEL 303 LF was utilized as the MF crosslinker and was kindly obtained from Allnex, Czech Republic. Tetrahydrofuran (THF, Penta, Czech Republic) was stored under anhydrous conditions using activated molecular sieves. All the chemicals were utilized as received without any further purification.

Synthesis and characterization of hexaallylamino-cyclo-triphosphazene
The synthesis of HACTP was performed under anhydrous conditions in an inert argon atmosphere using Schlenk containers. The synthesis reaction of the phosphazene derivative is shown in Figure 1. A Schlenk flask was filled in with 0.0014 mol of HCCTP and 20 mL of THF under an inert atmosphere. 0.0073 mol of allylamine was slowly added dropwise into the stirred reaction mixture at the boiling point of THF. The reaction was terminated after 72 h followed by filtration of resulting salts. Three times washing with THF was followed by vacuum distillation to remove THF from the product. The preparation of HACTP was monitored during the synthesis by means of thin layer chromatography and the identity of the prepared derivative was confirmed also by measuring the melting point. The final product was characterized by 31P(H) NMR, elemental analysis, mass spectrometry (MS) and Fourier transform infrared (FT-IR) spectroscopy. 
31P(H) NMR spectra (δ, ppm; J, Hz) were recorded using a Bruker Advance DRX 300 instrument (Bruker Corp., Germany) at the frequency of 31P: 202.46 MHz and 85 % H3PO4 (as the external standard). The samples were sealed in Simax tubes (diameter 4 mm), inserted in NMR cuvettes (diameter 5 mm) filled with D2O (external lock). The spectra were measured in the coaxial NMR cuvette system. The elemental analysis was performed on a FLASH Organic 2000 Elemental Analyzer (Thermo Scientific, USA), which can detect carbon, hydrogen, nitrogen and sulphur. Determination of chlorine was carried out by combustion in oxygen atmosphere followed by titration with sulphuric acid according to Schöniger [endnoteRef:27]. The method was tested on o-chlorobenzoic acid standard containing chlorine at the concentration 21.915 %.  [27: . Francis H.J.: (1971) Dr. Wolfgang Schöniger, Ph.D. 4 August 1920-24 February 1971. Talanta 18:i, doi:10.1016/0039-9140(71)80164-9] 

FT-IR spectroscopy of the samples was performed on a Nicolet iS50 (Thermo Scientific, USA) with integrated diamond ATR FT-Raman module. The FT-IR spectrometer with an integrated module and ATR fully reflective optics has a broadband DLaTGS detector to make measurements in the area 5000–100 cm-1. The spectral resolution of the device is 0.09 cm-1. MS was performed on the instrument MSD Model 5975B (Agilent Technologies, USA) with a probe for direct entry. Mass selective detector operated in electron impact ionization mode with the ionization energy of 70 eV. The samples for the MS analysis were prepared as methanol solution in the concentration of 0.01 wt.%. 

Preparation and characterization of latexes
Latexes of functionalized core-shell particles bearing in the structure a flame retardant were synthesized by the semi-continuous non-seeded emulsion polymerization comprising a variable content of acrylic monomers (see Table 1). The prepared HACTP was shown to be readily soluble in the utilized acrylic monomers and was incorporated into core, shell and core-shell structure of latex particles, respectively. The core/shell weight ratio of latex particles was 1/1, which means a shell thickness about 10 % of the particle diameter. The nature of acrylic monomers forming core and shell phases was chosen to achieve a calculated Tg (using the Fox equation [[endnoteRef:28]]) of approximately 12 °C and 3 °C, respectively. If HACTP was not applied in the monomer composition of core phase, AMA was utilized instead to achieve a slight cross-linking with the aim to maintain mechanical properties and chemical resistance of resulting polymer materials comparable with the polymers containing HACTP in the core of latex particles. The shell layer of all latex particles included a constant amount of HEMA repeat units to provide hydroxyl functionalities for interfacial crosslinking by the reaction with MF resin added during latex coating formulation. To improve the colloidal stability of latexes and to ensure the acid catalysis of the MF-crosslinking reaction, carboxyl functionalities were introduced into the structure of core and shell layers by copolymerization with a constant amount of MAA into all the prepared emulsion copolymers.   [28: . Fox T.G., Flory P.J.: (1950) 2nd-Order transition temperatures and related properties of polystyrene. 1. Influence of molecular weight. J Appl Phys 21:581591] 

The latexes were produced in a 700 ml glass reactor by under nitrogen atmosphere at 85 °C. The reactor charge was put into the reactor and heated to the polymerization temperature. Then the monomer emulsion was fed into the stirred reactor at feeding rate about 2 ml/min in two steps (1. core preparation, 2. shell preparation). After that, during 2 hours of hold period the polymerization was completed. The recipe of emulsion polymerization is shown in Table 2. The pH was adjusted to 8.5 with 20 wt.% N,N´-dimethylethanolamine solution. The solids content of final latexes was about 45 wt.%.
The average particle sizes of latex particles in the water phase were obtained from dynamic light scattering experiments performed using a Coulter N4 Plus instrument (Coulter Corp., UK). All the DLS measurements were conducted at 25 °C. The concentration of the measured polymer dispersion was approximately 0.05 wt.% of solids. For the glass transition temperature (Tg) and the gel content measurements, specimen were prepared by pouring the latexes into a silicone mould. Films were formed by water evaporation at room temperature for a month. The Tg of the dried latex copolymers was determined by means of differential scanning calorimetry (DSC) using a Pyris 1 DSC instrument (Perkin-Elmer, USA). The measurements were carried out at the heating rate of 10 °C.min-1 with N2 atmosphere. The testing temperature range was -80 to 120 °C. The gel content of microgel copolymers was determined according to CSN EN ISO 6427 using a 24-h extraction with THF in a Soxhlet extractor. Around 1 g of the dried latex sample was put into the thimble. After the extraction, the thimble was dried in an oven at 75 °C for 6 h, cooled in a desiccator overnight, and the gel content was calculated from the initial and final weights, assuming that the gelled material remained in the thimble. The presence of bonded HACTP in latex particles was investigated by 31P(H) NMR by testing the gelled polymer material that remained after the 24-h extraction with THF. The microgels LP0.4-0 (synthesized in the presence of 0.4 wt.% of HACPT in monomer feeds forming the core of latex particles) and the microgels L0-P0.2 (synthesized in the presence of 0.2 wt.% of HACTP in monomer feeds forming the shell structure of latex particles) were the subject of the analysis. 31P(H) NMR spectra (δ, ppm; J, Hz) were recorded using a Bruker Advance III(tm) spectrometer (Bruker, Corp., Germany) at the frequency of 31P: 500 MHz equipped with MAS VTN 500SB BL4 N-P/F-H probe. The samples were filled into 4 mm ZrO2 rotor with the rotation speed 10 kHz. The hydrogen atoms splitting were not observed because of using zgig - inverse-gated decoupling program.

Preparation and evaluation of latex thermosetting coatings
To produce thermosetting coating systems based on latex particles bearing a flame retardant, MF resin CYMEL 303 LF in the amount corresponding to the weight ratio of the emulsion polymer/MF crosslinker of 5/1 (based on total solids), was added to the latex with agitation. No coalescing solvents were used. The coating films with a wet thickness 120 m were cast on glass and metallic panels by drawing the thermosetting coating systems using a blade applicator. The film-formation process was performed firstly by leaving the films at room temperature (23 °C) for 24 h to achieve water evaporation and sufficient coalescence of latex particles, secondly by curing the films at 130 °C for 3 h in a hot-air oven to reach a high level of conversion of the crosslinking reaction between HEMA functional groups and MF hardener.
The resulting coating films based on latex particles containing variable amount of HACTP in their core, shell and core-shell structure, respectively, were evaluated for their gloss, hardness, adhesion, impact resistance, chemical resistance and flame stability. The dry film thickness was determined using a three-point instrument (BYK-Gardner, Germany) in the case of films prepared on glass panel, thickness of specimen on metallic panels was measured by the Sauter TE 1250-0.1 F Digital Coating Gauche (Sauter, Germany). The gloss of coatings was measured by a micro TRI-gloss µ instrument (BYK-Gardner, Germany) using a gloss-measuring geometry 60°. The hardness of test films was measured by the pendulum hardness tester “Perzos” pendulum (BYK-Gardner, Germany) following the CSN EN ISO 1522. The adhesion was determined using the cross-cut tester (Elcometer Instruments, UK) following the CSN ISO 2409. The impact resistance was evaluated according to CSN EN ISO 6272 using the Elcometer 1615 Variable Impact Tester (Elcometer Instruments, UK) and the chemical resistance was determined by methyl ethyl ketone (MEK) rubbing following ASTM D 4752. All experiments were carried out at room temperature (23  1 °C).
For evaluating the flame stability using the dual cone calorimeter (Fire Testing Technology, UK), specimens of the approximate dimensions 80 x 50 x 4 mm3 were prepared by pouring the thermosetting coating systems into a silicone mould. Films were air-dried at room temperature (23 °C) for 5 days and then cured at 130 °C for 3 h in a hot-air oven. The measurements were performed in a sample holder suitable for testing of thermally thin materials. The centre of a measured sample was situated 6 cm from the lowest part of the cone heater. The heat release rate was calibrated by burning methane. The cone radiancy 25 kWm-2 corresponds to a cone temperature 680 °C. It was set from the calibration diagram for a distance of the sample from the cone edge 6 cm [endnoteRef:29]. For each sample, three specimens were tested and averaged values of the results were collected.  [29: . Rychlý J., Hudáková M., Rychlá R.: (2014) Burning of thermally thin polyethylene mixtures. J Therm Anal Calorim 115:527-535] 


Results and discussion
Characterization of synthesized phosphazene derivative
The prepared hexaallylamino-cyclo-triphosphazene was characterized by means of elemental analysis; the elements C, H, N present in the compound were determined. The determination of chlorine was made by combustion in an oxygen atmosphere to verify the full substitution of Cl atoms. The amount of phosphorus elements was calculated additionally. The obtained data are listed in Table 3. It is evident that the theoretical contents of individual elements C, H, N, P, Cl in the synthesized HACTP molecule are consistent with the experimental results, which confirms the theoretical structure of the prepared derivative. Further, the synthesized derivative was characterized by means of 31P(H) NMR analysis. The full substitution of HCCTP was demonstrated by a singlet in the 31P(H) NMR spectrum at a chemical shift δ (S) = 18.79 ppm, as shown in Fig. 2. 
The synthesized HACTP was also characterized by FT-IR spectroscopy. FT-IR spectrum is shown in Fig. 3. The P‒N cycle vibration is located at 1213 and 1190 cm-1, the vibration for the NH bond correspond to 3273 and 1607 cm-1. Valence vibrations (υas and υs) of CHx groups (x = 1–3) were found between 2849 and 2932 cm-1. Bands of deformation vibrations δas of CHx groups (x = 1–3) occurred in the region between 1508 to 1553 cm-1. Vibrations (υas) at 551 cm-1 corresponding to PCl groups are missing, which confirms the full nucleophilic substitution of HCCTP. The successfulness of the synthesis of HACTP was monitored by mass spectrometry as well. The MS spectrum is presented in Fig.4. The theoretical molar mass of HACTP is 471.45 g/mol. MS results revealed that almost the identical value of 471.9 g/mol was obtained, which correspond to a fully substituted derivative.

Characterization of latex copolymers
Latexes with negligible amount of coagulum (0.2 – 0.7 %) were synthesized by the semi-continuous non-seeded emulsion polymerization process with varying levels and location of HACTP molecules in core, shell and core-shell compositions and were stable for over 5 months. The presence of the copolymerized HACTP in the synthesized emulsion copolymers was tested using 31P(H) NMR analysis of two representative copolymers, namely L0-P0.2 (containing theoretically 0.2 wt.% of phosphazene derivative in the shell structure of latex particles) and LP0.4-0 (bearing theoretically 0.4 wt.% of HACTP in the core structure of latex particles). Before the analysis, THF extraction of the copolymer samples was performed and both the high-molar mass polymer material (gel fraction) and the low-molar mass soluble material (sol fraction) were analyzed. It was found that the spectra of both samples of the extracted low-molar mass molecules revealed the absence of phosphazene derivative. In the case of gel fractions of both investigated emulsion copolymers, almost the identical 31P NMR spectra were obtained; the representative spectrum of the LP0.4-0 gel fraction is shown in Fig. 5. It can be seen clearly that a singlet at a chemical shift δ (S) = 21.63 ppm occurred in the spectrum of the high-molar mass polymer fraction, which indicates the presence of HACTP molecules. Hence, it can be stated that HACTP was incorporated into the macromolecular structure of acrylic polymers via the emulsion polymerization technique proceeding at standard conditions. 
The prepared latex copolymers were studied from the point of view of particle size in water phase, glass transition temperature and gel content with respect to HACTP concentration and location inside the latex particles. These characteristic properties are listed in Table 1. For proving the core-shell morphology of latex particles, the diameters of core particles (samples taken after finishing the core polymerization step) and resulting core-shell particles were determined.  Diameters of core particles varied from 92 to 112 nm, whereas particle sizes of core-shell particles were between 120 – 137 nm, which is in a good accordance with the calculated dimension of shell thickness about 10 % of the particle diameter. The DLS results showed further that the latex particle size was not affected significantly by both the amount and the location of HACTP in emulsion copolymers. Nevertheless, Tg and gel content values were shown to be influenced markedly by the content of HACTP in latex particles. Although the pendant allyl double bonds of HACTP were supposed to remain partly unreacted (because of steric hindrance effects and lower reactivity in comparison to vinyl groups of acrylic monomers) and therefore the crosslinking ability of the phosphazene derivative was believed to be suppressed, the glass transition temperature and gel content of emulsion copolymers were found to be increased with the growing content of HACTP. In the case of emulsion copolymers containing 0.4 wt.% of HACTP in the core structure, a slightly lower gel content was determined in comparison with copolymers comprising the core crosslinked using 1.5 wt.% of AMA. Taking into account that AMA (bearing one more reactive vinyl double bond and one less reactive allyl double bond) was used approximately in five-fold excess related to HACTP (containing six allyl double bonds), this phenomenon is not surprising. The relatively high gel content of 86.2 % determined for emulsion copolymers LP0.4-0 (containing HACTP-crosslinked core structure) can be explained by two ways; firstly, the HACTP-generated network and secondly, chain transfer reactions occurring at radical polymerizations of alkyl acrylates which are responsible for the formation of branched or even crosslinked macromolecules. It was found further that the gel content was increased with the growing content of HACTP in shell structures of latex particles having AMA-crosslinked and HACTP-crosslinked core. This effect clearly reveals the formation of crosslinks and leads us to conclude that in the process of emulsion polymerization of common acrylic monomers, HACTP was proved to act as an efficient crosslinker leading to the formation of latex particles of microgel structure.

Evaluation of coatings
Coating films cast from thermosetting two package aqueous coating systems based on the HACTP-modified emulsion microgels and MF crosslinker were prepared and their properties were tested. All the coating systems were able to form high-quality transparent films. Dry film thickness of the coatings was approximately 60 µm. The effect of HACTP on final coating properties is presented in Table 4. A slight enhancement of hardness of coating films was observed with the increasing content of HACTP in the shell layer of microgel particles. This fact can be attributed to increased network density of resulting polymer materials, which is consistent with Tg elevation of the corresponding emulsion copolymers (see Table 4). It was shown further that all the tested coatings exhibited high gloss and superior adhesion to glass and steel substrates. Also the impact resistance and chemical resistance of the resulting coatings were shown to reach maximum values suggesting the formation of crosslinked and elastic polymer materials in which sufficient coalescence of latex particles had been achieved before the MF crosslinking took place. It was found as well that HACTP amount or site inside latex particles did not deteriorate all the above-mentioned coating characteristics in comparison with the HACTP-free coatings. 
As our system of interest was focused on the evaluation of flame retardancy of the prepared phosphazene derivative in the resulting coatings, the effect of HACTP amount and location inside latex particles on combustion of MF-cured coatings was studied as well. The results obtained from measurements using dual cone calorimeter are presented in Table 5. It was found that the coatings comprising HACTP exhibited lower values of mean heat release rate, mean effective heat of combustion and total heat release. This phenomenon is more pronounced with increasing HACTP content, which indicates a slower flame spread due to incorporated HACTP. Decreased values of total smoke release during combustion of samples containing HACTP may reflect a more efficient oxidation of hydrocarbon chains in the presence of the phosphorus compound. It is assumed that the phosphorus atom reacts in the gas phase where the PO• radical is playing the main role [endnoteRef:30]. In the case of values of total oxygen consumed, no significant effect of HACTP presence was proved. Nevertheless, one of the most important criterions of the flammability evaluation is the maximum average rate of heat emission (MARHE) that decreased markedly by the growing content of HACTP in the tested coating materials. These results lead us to conclude that HACTP did act as a flame retardant in the investigated coating materials. The flame stability was influenced mainly by the HACTP content while the HACTP location inside emulsion microgels was not proved to be of great importance. [30: . Schartel S.: (2010) Phosphorus-based flame retardancy mechanisms—Old hat or a starting point for future development? Materials 3:4710-4745

Table 1: Composition and characteristics of latexes based on core-shell particles containing hexaallylamino-cyclo-triphosphazene (HACTP)
 
Table 2: Recipe of emulsion polymerization


Table 3: Results of elemental analysis of hexaallylamino-cyclo-triphosphazene (HACTP)


Table 4: Comparison of final properties of coating films based on latex particles differing in the amount and location of phosphazene derivative 

* 	Maximum evaluative value (representing the best property)

Table 5: Results of combustion in a cone calorimeter for MF-cured coatings based on latex particles differing in the amount and location of HACTP 

* All evaluated parameters are related to the initial mass of a tested sample



Fig. 1


Fig. 2


Fig. 3


Fig. 4


Fig. 5] 


Conclusions
HACTP was successfully prepared by nucleophilic substitution of HCCTP with allyl amine using the one-step synthesis. The structure of HACTP was verified using 31P(H) NMR, elemental analysis, mass spectrometry and FT-IR spectroscopy. The prepared phosphazene derivative was incorporated into the macromolecular structure of acrylic polymers via the emulsion polymerization technique proceeding at standard conditions, which was evidenced by 31P(H) NMR. During the emulsion polymerization of acrylic comonomers, HACTP performed the function of an efficient crosslinker leading to the formation of latex particles of microgel structure. In addition to that, the basic objective of this work was to evaluate the flame retardancy of the prepared phosphazene derivative in the thermosetting aqueous coatings based on the HACTP-modified emulsion microgels and MF crosslinker. The presence of HACTP was found to cause decreased amount of produced smoke and a slower flame spread during the material combustion without affecting transparency, flexibility, toughness and adhesive properties of resulting coatings. Thus, it can be concluded that HACTP acted as a flame retardant and low VOC thermosetting transparent coating systems with increased flame stability were developed, having a potential application as primer as well as topcoats. 

Acknowledgements
The Technological Agency of the Czech Republic (TE02000011) is gratefully acknowledged for supporting this work. 

References
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