Influence of Antimony Oxide on Epoxy Based Intumescent Flame

polymers

Article Inﬂuence of Antimony Oxide on Epoxy Based Intumescent Flame Retardation Coating System

Riyazuddin 1, Samrin Bano 2, Fohad Mabood Husain 1, Rais Ahmad Khan 3 , Ali Alsalme 3,* and Jamal Akhter Siddique 4,*

1 Department of Food Science and Nutrition, King Saud University, Riyadh-11451, Saudi Arabia; [email protected] (R.); [email protected] (F.M.H.) 2 Center of Chemical Sciences and Technology, Institute of Science and Technology, JNTU, Hyderabad 500085, Telangana State, India; [email protected] 3 Department of Chemistry, College of Science, King Saud University, Riyadh-11451, Saudi Arabia; [email protected] 4 Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India * Correspondence: [email protected] (A.A.); [email protected] (J.A.S.)

Received: 15 September 2020; Accepted: 11 November 2020; Published: 17 November 2020

Abstract: Ethylenediamine modified Ammonium polyphosphate (EDA-MAPP), and Charring-Foaming Agents (CFA) was prepared via a simple chemical approach and further utilizes for the preparation of Epoxy resin based intumescent flame retardation coatings. The ratio belongs to MAPP and CFA was fixed at 2:1 ratio. Comparative thermo gravimetric analysis TGA study of Modified Ammonium polyphosphate (MAPP) and Ammonium polyphosphate (APP) investigated. Sb2O3 was introduced into flame retardation coating formulation at various amounts to evaluate the synergistic action of Sb2O3 along with flame retardant coating system. The synergistic action of Sb2O3 on flame retardation coating formulation was studied by vertical burning test (UL-94V), thermo gravimetric analysis (TGA), Limited Oxygen Index (LOI), and Fourier Transform Infra-Red spectroscopy (FTIR). The UL-94V results indicated that adding Sb2O3 effectively increased flame retardancy and meets V-0 ratings at each concentration. The TGA results revealed that the amalgamation of Sb2O3 at each concentration effectively increased the thermal stability of the flame retardant coating system. Cone-calorimeter study results that Sb2O3 successfully minimized the combustion parameters like, Peak Heat Release Rate (PHRR), and Total Heat Release (THR). The FTIR result shows that Sb2O3 can react with MAPP and generates the dense-charred layer which prevents the transfer of heat and oxygen.

Keywords: modiﬁed ammonium polyphosphate; antimony oxide; synergist; Epoxy coatings

1. Introduction Wood has been using as primary structural raw material due to its valuable properties, such as complexion, low mass, high strength, and low-level insulation properties. Due to its high flammability, the application of wood is limited in some fields for the purpose of safety [1–3]. Polymer-fabricated materials are recognized now a days as the major engineering material that involve in aerospace, automobile, shipbuilding, construction, and many other multiple fields are linked with people’s routine lives. Such materials show strength and low weight ratio along with decent stability in terms of physical and chemical behavior, their resistance capacity in terms of corrosion, heat, chemical, etc. with self-lubrication and other superior properties is quite acceptable [4–6]. Flammability of polymers is a big challenge in implement and expands their uses; scientists have established many approaches to overcome with this problem, like as flame retardant filler technology, fire retardant coating technology, fire retardant solution soaking and chemical grafting flame retardant

Polymers 2020, 12, 2721; doi:10.3390/polym12112721 www.mdpi.com/journal/polymers Polymers 2020, 12, 2721 2 of 14 technology [7–9]. Flame retardant coating is one of the solutions to overcome with flammability, majorly divided into two categories: (1) Intumescent flame retardant coatings and (2) Non-intumescent flame retardant coatings. The most useful intumescent coatings are more preferred than non-intumescent coating because intumescent coatings forms a foam like structure on the material surface that restricts the flow heat and oxygen. So, the practical application of intumescent flame retardant (IFR) coating technology is recognized as more suitable for reducing the fire damages of wood [10]. Generally, IFR system consist three major ingredients: (1) acid source, (2) carbonizing agent, and (3) foaming agent [11]. For the period of heating, IFR coating mechanism can form the protective charred layer with foam to protect the wood substrate from burning process [12]. The char can be formed by cross-link and cyclization reaction of flame retardant molecules [13]. This charred layer slows down the shifting of oxygen and heat in between substrate and its surroundings [14]. Previous studies demonstrated that pentaerythritol (PER) and ammonium polyphosphate (APP) combination has shown impressive intumescence flame retardation [15–17]. Some other studies also reported that APP, PER, and Melamine were used in the composition of IFR system, but it was not given satisfactory outputs due to its low-level performance of anti-oxidation and fire retardancy [18]. PER act as carbonization source but it is moisture sensitive in nature. So it can easily absorb the water content from atmosphere and ooze out from the IFR system, leading to a terrible corrosion. Several studies have been done to develop and their practical application of different types of carbonizing agents [19]. In this regard, from the last decade researchers have been showing a great attention towards triazine derivate to synthesize highly stable charring-foaming agents [20]. Triazine derivative acts as a charring agent and also as foaming agent as they contain abundant amount of carbon and nitrogen atoms in its structure [21,22]. Different kinds of Triazine derivatives have been utilizes as foaming and charring agent in IFR systems and their effects were extensively investigated [23,24]. There are so many studies where they used APP and Triazine derivates as an intumescent flame retardant system. However, it has some limitations, because triazine derivatives produce the char which show poor efficiency. In order to improve char thickness and hardness, need to incorporate synergistic agents in IFR system. There are some studies in which they have added synergistic agents, such as zeolites [25], organoboran siloxanes [26], and some metal compounds and transition metal oxides. Such studies clearly explain that synergistic agents can greatly improve the thickness of the char by forming cross linkages. Antimony trioxide (Sb2O3) is a white colored powder, non-carcinogenic agent. It can be involved as a synergist to improve the performance of an intumescent flame retardant system. It is very cheap and environmentally friendly. It considered as a flame retardant during the condensation phase. While burning, it can react with hydroxyl groups of the ammonium polyphosphate causing to endothermic decomposition. The current study, an enthusiastically approaches, cost effective, and non-hazardous antimony trioxide selected as a synergistic agent to investigate its action on Epoxy/IFR coating system. The IFR method consisting of ethylenediamine modified ammonium polyphosphate and charring foaming agent (CFA). In this work, APP is modified with ethylenediamine because APP behaves as acid source and blowing agent, on other hand MAPP can show charring action too. CFA polymer was prepared from triazine molecule. It is hydrophobic in nature and has thermally stable triazine ring. Due to these reasons, CFA cannot be affected by atmospheric moisture and show great flame retardancy. For the first time, we attempted a trial to prepare a new IFR coating composition of MAPP and CFA to improve thermal strength and flame retardant performance of the coating composition. To this coating composition, an eco-friendly synergistic agent Sb2O3 is added to increase the thickness of the intumescent char. The limited oxygen index (LOI), vertical burning test (UL-94V), thermo-gravimetric analysis (TGA), Fourier transform infra-red spectroscopy (FT-IR), and cone calorimeter studies have been employed to investigate the synergism between antimony trioxide and Epoxy/IFR system. Polymers 2020, 12, 2721 3 of 14

2. Materials and Experimental The Epoxy resin and amine hardener used in this study were obtained from Sigma Aldrich (Munich, Germany). Ammonium polyphosphate (APP, Crystalline form II, n 1000) and Ethanol ≥ (95% pure) were obtained from Samchun pure chemicals, South-korea. Ethylenediamine (EDA, Purity: 99.5%), acetone (Pure: 99%), ethanolamine (ETA, Pure: 99%), and antimony tri-oxide (powder, 5 µm, ReagentPlus®, 99%) were analytical grade and obtained from Sigma Aldrich. Cyanuric chloride (Purity-99%) was procured by Sigma Aldrich. All the chemicals and solvents were used as received.

2.1. Preparation of MAPP The clean and dry three-neck round bottom flask of 1000 mL which is equipped along a magnetic stirrer, a mixer of ethanol and water (400 mL:15 mL) was poured. At 30 min later, 9 g of ethylenediamine was poured into the flask and stirred at 200 rpm for 20 min. Then 50 g of APP was added slowly into the above solution. The prepared solution was heated around 80 ◦C for 6 h. After the completion of reaction, the obtained mixture gets cooled at room temperature. The resulted white solid was filtered, washed with ethanol and water. The obtained white solids were kept in hot air oven at 50 ◦C for next 20 h to remove moisture content.

2.2. Preparation CFA The derivative of triazine-containing macromolecule that was designated as charring-foaming agent (CFA) synthesized via nucleophillic substitution reaction. In the initial step, 1 mol of cyanuric chloride and 500 mL of acetone were poured into a dry and clean three-neck 2000 mL, flask already fitted with a magnetic stirrer which was immersed in ice water bath. Then 1 mol of each ethanolamine and NaOH were completely dissolved in distilled water, and the mixture was slowly added into the flask for a period of 2 h. This step was maintained at 0–10 ◦C for 4 h. The second step of the reaction takes 0.5 mol of ethylenediamine and 1 mol of NaOH were dissolved in distilled water and the solution was added drop wise to the above reaction mixture. Then the reaction temperature gets increased to 50–60 ◦C. This reaction was permissible to continue for 4 h. In the third step of chemical reaction, again a mixture of 1 mol of NaOH and 0.5 mol of ethylenediamine were dissolved in distilled water; this aqueous solution was added slowly into the reaction flask. Then the reaction temperature was upraised to 75 ◦C to evaporate acetone solvent. This step was allowed to proceed for 4 h. when the reaction was completed; the obtained mixture was cooled at room temperature and filtered. After filtration, the collected particles of white solid were washed with distilled water and dried in hot air oven at 60 ◦C for 3 h.

2.3. Sample Preparation

Intumescent ﬂame retardant (IFR) coating system contained of MAPP, CFA and a Sb2O3 as a synergist. The proportion of MAPP to CFA was set at 2:1 ratio and the incorporation of Sb2O3 changes from 0 to 4 wt% respectively. The intumescent ﬂame retardant coating formulations are represented in Table1. All the constituents were homogeneously mixed using high-speed disperse mixer to prepare IFR coating formulations. The prepared coatings were implied on particular sized plywood pieces manually by using a paintbrush at room temperature. This process was repeated to make 1.5 2 mm ± coating thickness. Once coating has been done the samples were dried for 48 h at room temperature. Polymers 2020, 12, 2721 4 of 14

Table 1. Composition of the intumescent ﬂame retardation coatings.

Sample Resin (%) MAPP (%) CFA (%) Sb2O3 Pure Epoxy Resin 100 0 0 0 Epoxy/intumescent ﬂame retardant (IFR) 70 20 10 0 Epoxy/IFR/Sb2O3-2% 68 20 10 2 Epoxy/IFR/Sb2O3-4% 66 20 10 4

2.4. Characterization Methods

2.4.1. FT-IR Test A Nicolet Avator 360 spectrometer instrument (Nicolet, Madison, WI, USA) was involved for 1 FTIR analysis in the range of 4000–500 cm− . For the FT-IR analysis, the samples were pressed into pellet forms with KBr powder.

2.4.2. X-Ray Diﬀraction Study D8 X-ray Diﬀractometer model with Cu Kα radiation were used for the XRD study for the structural analysis of samples. The scanning rate of 0.02◦ per second with the 2 degrees and the scanning range was 5–50◦.

2.5. Flame Retardancy Test The LOI values of all coating formulations were measured as per ASTM- D2863 standard procedure on HC-2C Oxygen Index instrument (Jiangning County Analytical Instrument Factory, Jiangning, China). The coating formulations were applied on the plywood sheets of 130 mm 6.5 mm 3 mm × × dimension to perform the LOI test. The prepared intumescent ﬂame retardant coatings were applied on the surface of all sides of plywood pieces of the dimension of 127 mm 27 mm 3 mm. Then the coated plywood pieces were × × dried at room temperature for 48 h. The UL-94V test was performed as per ASTM-D3801 standard method. The coated plywood sheets were ignited for 30 s to calculate the burning rate, and ﬂame retardation times of each sample.

2.6. Thermo Gravimetric Analysis Test Thermal stability of the all coating formulations was performed by using SDT Q600 V20.9 Build-20 instrument (TA Instruments, New Castle, DE, USA) with 20 ◦C/min of heating rate at a temperature range of 30–800 ◦C under nitrogen atmosphere at the ﬂow rate of 20 mL/min. The weight of each coating formulation was taken approximately 4–5 mg.

2.7. Combustion Test The combustion parameters of all coating formulations were performed on cone calorimeter 2 in accordance with ISO 5660–2002 standard method by exposing 50 kW m− of external heat ﬂux. All samples were vertically laid on sample holder.

3. Results and Discussion

3.1. FTIR of MAPP

1 The Figure1 shows, FTIR spectra of MAPP and APP. The peak attributed at 3407 cm − is because + of the NH4 ion asymmetric stretching vibrations [27]. From this data, it can be seen that MAPP 1 spectrum contains peaks at 2915, 2864, and 1535 cm− , which speciﬁcally attributed for the characteristic + stretching absorption peaks of -CH2-CH2- and NH3 ion. The appeared peaks which are corresponding + (+)( ) to -CH2-CH2- and NH3 ion in MAPP proved that NH3 − O-P bond is formed successfully. PolymersPolymers 20202018,, 1210,, 2721x FOR PEER REVIEW 5 of 14 Polymers 2018, 10, x FOR PEER REVIEW 5 of 14

Figure 1. FT-IR spectra of Modified Ammonium polyphosphate (MAPP) and Ammonium Figure 1. 1. FTFT-IR-IR spectra spectra of of Modified Modified Ammonium polyphosphate (MAPP) and Ammonium Ammonium polyphosphate (APP). polyphosphate (APP).(APP). 3.2.3.2. XRD of MAPP 3.2. XRD of MAPP Figure2 2 shows shows the the XRD XRD spectraspectra ofof APPAPP andand MAPP.MAPP. The The didiffractionffraction peakspeaks ofof APPAPP and MAPP are Figure 2 shows the XRD spectra of APP and MAPP. The diffraction peaks of APP and MAPP are appeared atat samesame position, position, but but a newa new peak peak at 11.92at 11.92◦ is obtained° is obtained for MAPP. for MAPP. This specific This specific peak in peak MAPP in appeared at same position, but a new peak at 11.92° is obtained for MAPP. This specific peak in isMAPP attributed is attributed for the presence for the presenceof (NH4) 5 ofP3 O (NH10 H4)25O.P3O This10·H2 resultO. This showing result that showing the crystalline that the cr structuresystalline MAPP is attributed for the presence of (NH· 4)5P3O10·H2O. This result showing that the crystalline ofstructures APP are of unchanged APP are unchanged by the reaction by the between reaction APP between and ethylenediamine. APP and ethylenediamine. structures of APP are unchanged by the reaction between APP and ethylenediamine.

Figure 2. The XRD curves of APP and MAPP. Figure 2. The XRD curves of APP and MAPP. Figure 2. The XRD curves of APP and MAPP. 3.3. TGA Graph of MAPP and APP 3.3. TGA Graph of MAPP and APP 3.3. TGAThe Graph thermal of MAPP degradation and APP curves of APP and MAPP shows under N2 atmosphere in Figure3. TheirThe degradation thermal behaviorsdegradation had curves apparent of APP diﬀerences. and MAPP For APP, shows there under were N2 two atmosphere decomposing in processesFigure 3. The thermal degradation curves of APP and MAPP shows under N2 atmosphere in Figure 3. whichTheir locateddegradation at about behaviors 315 C and had 640 apparentC, respectively. differences. However, For APP, MAPP there shown were only two one decomposing peak during Their degradation behaviors◦ had apparent◦ differences. For APP, there were two decomposing itsprocesses degradation which process. located at The about thermal 315 °C degradation and 640 °C, process respectively. of APP However, could be dividedMAPP shown into two only steps. one processes which located at about 315 °C and 640 °C, respectively. However, MAPP shown only one Thepeak ﬁrst during step its was degradation in the range process. of 200–410 The C,thermal it might degradation be due to theprocess elimination of APP of could ammonia be divided and water into peak during its degradation process. The◦ thermal degradation process of APP could be divided into contenttwo steps. in The the thermalfirst step degradation was in the range process of 20 of0– polyphosphate.410 °C, it might be The due second to the step elimination was beyond of ammonia 450 C, two steps. The first step was in the range of 200–410 °C, it might be due to the elimination of ammonia◦ and water content in the thermal degradation process of polyphosphate. The second step was beyond and water content in the thermal degradation process of polyphosphate. The second step was beyond

PolymersPolymers 20182020, ,1012, ,x 2721 FOR PEER REVIEW 66 of of 14 14 Polymers 2018, 10, x FOR PEER REVIEW 6 of 14 450 °C, this weight loss was attributed to the release of phosphoric acid, polyphosphoric acid, with this weight loss was attributed to the release of phosphoric acid, polyphosphoric acid, with APP APP450 ° decomposition.C, this weight loss The was thermal attributed degradation to the releaseof MAPP of isphosphoric same as thermal acid, polyphosphoric degradation of acid,APP, withand decomposition. The thermal degradation of MAPP is same as thermal degradation of APP, and also alsoAPP included decomposition. the thermal The thermal behavior degradation of EDA salt. of After MAPP incorporating is same as thermal the EDA, degradation the residue of ofAPP, MAPP and included the thermal behavior of EDA salt. After incorporating the EDA, the residue of MAPP was wasalso muchincluded higher the thermalthan that behavior of APP betweenof EDA salt.300 andAfter 400 incorporating °C. The formation the EDA, of morethe residue char residue of MAPP at much higher than that of APP between 300 and 400 C. The formation of more char residue at this thiswas stage much contributed higher than to that the followingof APP between formation 300 ofand intumescent ◦400 °C. The char formation layer, so of it morecan be char observed residue that at stage contributed to the following formation of intumescent char layer, so it can be observed that the thethis introduction stage contributed of EDA to thein APP following effectively formation enhanced of intumescent the flame retardancy char layer, inso the it can combustion be observed tests that. introduction of EDA in APP eﬀectively enhanced the ﬂame retardancy in the combustion tests. the introduction of EDA in APP effectively enhanced the flame retardancy in the combustion tests.

Figure 3. TGA curve of APP and MAPP. Figure 3. TGA curve of APP and MAPP. Figure 3. TGA curve of APP and MAPP. 3.4.3.4. FTIR FTIR of of CFA CFA 3.4. FTIR of CFA TheThe FTIR FTIR spectrum spectrum of of CFA CFA is is shown in Figure 44.. This spectrumspectrum consist the broadbroad adsorptionadsorption −11 peakspeaksThe between between FTIR spectrumthe the range range of of 3300CFA 3300–3450– is3450 shown cm cm− inareare Figure because because 4. Thisof of symmetrical symmetrical spectrum consiststretching stretching the vibrationsbroad vibrations adsorption of of O O-H-H −1−1 1 −1 1 andpeaksand N N-H -betweenH bonds; bonds; the and and range the the peaks of peaks 3300 at at–28503450 2850 cm cm cmand −areand 2927 because 2927 cm cmofcan symmetrical− becan allocated be allocated stretching to νC-H to inν -vibrationsC-HCH2in-CH -CH2- ofgroup.2-CH O-H2 - −1−1 1 −1 Theandgroup. peaksN-H The bonds; at peaks 1580, and at1358, 1580,the 1159 peaks 1358, and at 1159 10602850 and cmcm 1060canand cmbe 2927 −assignedcan cm becan assignedto bethe allocated νC = to N ν thetr-N, toν νC Cν-=NC -andNH inνtr-N, -νCHC-Oν,2 respectively.-C-NCHand2- group.νC-O , −1 −1 1 Moreover,Therespectively. peaks at this 1580, Moreover, spectrum 1358, this1159 does spectrum and not 1060 show doescm the notcan peakshowbe assigned atthe 850 peak to cm the at 850 νthatC = cm N ν is−tr - correspondingN,that νC-N is and corresponding νC-O, respectively. to the to C- theCl stretchingMoreover,C-Cl stretching vibration this vibrationspectrum and it and indicates does it indicatesnot that show CFA that the is CFA peak formed is at formed 850successfully. cm successfully.−1 that is corresponding to the C-Cl stretching vibration and it indicates that CFA is formed successfully.

Figure 4. FT-IR curve of and charring foaming agent (CFA). Figure 4. FT-IR curve of and charring foaming agent (CFA). Figure 4. FT-IR curve of and charring foaming agent (CFA).

Polymers 2018, 10, x FOR PEER REVIEW 7 of 14 Polymers 2020, 12, 2721 7 of 14 Polymers 2018, 10, x FOR PEER REVIEW 7 of 14 3.5. Thermo-Gravimetric Analysis (TGA)of CFA 3.5. Thermo-Gravimetric Analysis (TGA)of CFA 3.5. ThermoThe losses-Gravim in massetric ofAnalysis sample (TGA) with ofrespect CFA to increase in temperature can be evaluated by thermo- gravimetricThe losses losses analysis in in mass mass and of ofsample it sample gives with the with respect information respect to increase to increasethat in the temperature in char temperature have can been be evaluated can formed; be evaluated by this thermo is the by- gravimetricthermo-gravimetricdegradation analysisbehavior analysis andand thermal it and gives it stability gives the information the of information the sample. that that the the char char have have been been formed; formed; this this is is the degradationThe Dif ferential behavior Thermal and thermal Analysis stability (DTA of) theand sample. TGA curves of the CFA are represented in Figures 5 andThe 6 with D Diifﬀferentialerential Table 2. ThermalThermal The CFA AnalysisAnalysis has shown (DTA)(DTA a) and greatand TGA TGA amount curves curves of of charof the the at CFA CFA 700 are areand represented represented 800 °C are in in38.20 Figures Figure and5s 5and34.80 and6 , withrespectively.6 with Table Table2. The The2. The result CFA CFA hasstates has shown thatshown the a greatasynthesized great amount amount CFA of of char is char thermall at at 700 700y and more and 800 800stable ◦°CC and are admirable 38.20 and 34.8034.80,charring, respectively. -foaming agent The result at high states temperature that the synthesized because of CFAits stable is thermall thermally triaziney more ring structure. stable and In admirable the DTG charringcharring-foamingcurve, it -canfoaming be clearly agent seen at high that temperature the CFA decomposition because of its has stable three triazine main ringsteps. structure. The initial In stepthe DTG take curve,place in itit canthecan berangebe clearly clearly of 25 seen seen0–310 that that °C the theand CFA CFA the decomposition peakdecomposition at 287 °C has ishas three due three mainto the main steps. evaporation steps. The initialThe of initial stepammonia takestep place takeand placeinnitrogen the in range thegas ofrangefrom 250–310 theof 25 CFA.◦0C–310 and The °C the secondand peak the atdegradation peak 287 ◦ atC is287 due range°C to is the dueappears evaporation to the at evaporationthe range of ammonia from of ammonia310 and to nitrogen 370 and °C, nitrogengasthis from has gas a the peak from CFA. at the The 353 CFA. second°C Theand degradation second it is attributed degradation range for appears range the partial atappears the bond range at the breakage. from range 310 from toThe 370 310 third◦C, to this370step has° C, of thisadecomposition peak has at a 353 peak◦C occurs and at 353 it at is °C370 attributed and to 432 it is°C for attributedtemperature the partial for bond range. the breakage. partial At this bond step The , breakage.the third weight step The ofloss decomposition thirdhas occurred step of decompositionoccursdue to atthe 370 thermal to occurs 432 decomposition◦C temperatureat 370 to 432 macromolecular range.°C temperature At this step, backbonerange. the weight At structure.this loss step has, the occurred weight dueloss tohas the occurred thermal duedecomposition to the thermal macromolecular decomposition backbone macromolecular structure. backbone structure.

Figure 5. TGA curve of CFA. Figure 5. TGA curve of CFA. Figure 5. TGA curve of CFA.

Figure 6. Diﬀerential Thermal Analysis (DTG) curve of CFA. Figure 6. Differential Thermal Analysis (DTG) curve of CFA. Figure 6. Differential Thermal Analysis (DTG) curve of CFA.

Polymers 2020, 12, 2721 8 of 14 Polymers 2018, 10, x FOR PEER REVIEW 8 of 14 Polymers 2018, 10, x FOR PEER REVIEW 8 of 14 Table 2.2. Thermal degradation data of CFA. Table 2. Thermal degradation data of CFA. Char Residue (%) Sample Char Residue (%) Sample Char700 °ResidueC 800 (%) °C Sample 700 C 800 C CFA 70038.20 °◦C 80034.80 °C ◦ CFACFA 38.20 38.20 34.80 34.80 3.6. Thermo Gravimetric Analysis of Coatings 3.6.3.6. Thermo Thermo Gravimetric Gravimetric Analysis Analysis of of Coatings Coatings Figures 7 and 8 and Table 3 show the TGA results of the IFR coatings with various amounts of Sb2OFigureFigures3 unders 77nitrogen andand8 8 and and atmosphere. Table Table3 3show show In thecomparison, the TGA TGA results results the of TGAof the the IFRcurves IFR coatings coatings of the with Epoxy/IFR/Sbwith various various amounts amounts2O3-2% ofand of Sb2O3 under nitrogen atmosphere. In comparison, the TGA curves of the Epoxy/IFR/Sb2O3-2% and SbEpoxy/IFR/Sb2O3 under nitrogen2O3-4% are atmosphere. showing high In comparison, thermal stability. the TGA From curves the data of the, it Epoxy can be/IFR observed/Sb2O3 -2%that and the Epoxy/IFR/Sb2O3-4% are showing high thermal stability. From the data, it can be observed that the Epoxypure resin/IFR decompose/Sb2O3-4% areat 192 showing °C (5% high Tonset) thermal with stability.a maximum From temperatur the data,e it of can 359 be °C observed where it thathas 15.5 the purepureMLR%/min. resin resin decompose decompose Pure resin at at coating192 192 °C◦ C(5% has (5% Tonset) shown Tonset) with6.6% with achar maximum a maximum residue temperatur at temperature above 500e of °C359 of and 359°C whereit◦C is whereburnt it has itup 15.5 has to MLR%/min.15.593.5%. MLR% / min.Pure Pureresin resincoating coating has shown has shown 6.6% 6.6% char charresidue residue at above at above 500 500°C and◦C and it is it burnt is burnt up upto 93.5%.to 93.5%.

Figure 7. TGA curves of coatings. FigureFigure 7. 7. TGATGA curves curves of of coatings coatings..

Figure 8. DTG curves of coatings. Figure 8. DTG curves of coatings. Figure 8. DTG curves of coatings.

Polymers 2020, 12, 2721 9 of 14

Table 3. Thermo gravimetric results of the coatings under nitrogen atmosphere.

Sample TOnset (◦C) MLR%/min Char Residue (%) at 800 ◦C Pure Epoxy Resin 192 15.2 6.6 Epoxy/IFR 187 12.3 18.5 Epoxy/IFR/Sb2O3-2% 165 11.2 23.7 Epoxy/IFR/Sb2O3-4% 122 8.3 27.5

The TOnset 5% temperature of pure Epoxy, Epoxy/IFR, Epoxy/IFR/Sb2O3-2% and Epoxy/IFR/Sb2O3-4% coatings are 192, 187, 165 and 122 ◦C respectively. The char residue percentage at 800 ◦C of the Epoxy, Epoxy/IFR, Epoxy/IFR/Sb2O3-2% and Epoxy/IFR/Sb2O3-4% coatings are 6.6, 18.5, 23.7, and 27.5% with MLR%/min values of 15.2, 12.3, 11.1, and 8.3 respectively. From this data it can be found that Epoxy/IFR is good thermal stable than pure Epoxy coating because during burning MAPP and CFA undergo esterification reaction to form a char which protects the flow of oxygen and heat to the coating material therefore it will be more stable. The Table3 data clearly indicates that Epoxy /IFR/Sb2O3-2% and Epoxy/IFR/Sb2O3-4% coatings have less thermal initial decomposition temperature, less MLR%/min and high char residue percentages. The reason is due to that the Sb2O3 can catalyze the esterification reaction between MAPP and CFA to form a thick protective char layer. This CFA is gently involved in esterification reaction because it is a polyhydroxyl triazine polymer. From these data, it is clear that addition of Sb2O3 can upsurge the thermal stability of the coating formulations.

3.7. Flame Retardancy Test Table4 represents the values of LOI of the Epoxy /IFR systems with varied ratio of MAPP and CFA. The data clearly show that by increasing addition of MAPP, the LOI values first remarkably increased, then gradually decreased. When the ratio of MAPP and CFA is 2:1, it reached to maximum value at 31.2 and this one pass the burning test with V-0 rating. Because a condensation reaction take place in between MAPP and CFA to form a dense, protective char layer which can inhibit the flow of oxygen and heat in between burning material and its surroundings. In the case of less content of MAPP, a crumbled charred layer is formed which is ineffective to restrict the flow of heat and oxygen. In the other hand, when the amount of MAPP reached maximum level, it could then produce an excess amount of gasses, such as ammonia and nitrogen, which resulted in broken of the stable charred layer. Hence, the combination of MAPP and CFA has shown effective action at optimal ratio of 2:1.

Table 4. Flame retardance results of Epoxy/IFR with diﬀerent ratio of MAPP/CFA.

Epoxy (%) Ratio of MAPP/CFA LOI (%) UL-94 100 0 25.2 No rating 70 1:0 26.5 V-1 70 1:1 27.1 V-1 70 1.5:1 28.3 V-0 70 2:1 31.2 V-0 70 3:1 29.6 V-2 70 4:1 28.4 V-2

Table5 represents the LOI values and UL-94V ratings of Epoxy /IFR and Epoxy/IFR/Sb2O3 coating systems. The data clearly reveal that the addition of Sb2O3 eﬀectively increases the LOI value of Epoxy/IFR system. The addition of Sb2O3 at 2 wt%, 4 wt%, remarkably increased the LOI values from 31.2 to 31.9 and 32.6, respectively. The reason behind to increase the LOI values is that Sb2O3 can involve in a chemical reaction with IFR system. In addition to this, Sb2O3 can also act as catalyst for the condensation reaction between MAPP and CFA. Polymers 2018, 10, x FOR PEER REVIEW 10 of 14 Polymers 2020, 12, 2721 10 of 14 Table 5. Limited Oxygen Index (LOI) and UL-94V results of the coatings.

Table 5. Limited OxygenSample Index (LOI) andLOI UL-94V (%) resultsUL-94 of the coatings. Pure EpoxySample Resin LOI25.2 (%) No UL-94 rating Epoxy/IFR 31.2 V-0 Pure Epoxy Resin 25.2 No rating Epoxy/IFREpoxy/Sb2O3/IFR-2% 31.231.9 V-0V-0 Epoxy/IFREpoxy/IFR/Sb2O3/Sb2O3-2%-4% 31.932.6 V-0V-0 Epoxy/IFR/Sb2O3-4% 32.6 V-0 From the Table 5 data, it can be known that Epoxy/IFR and Epoxy/IFR/Sb2O3 systems pass the UL-94V test and can reach V-0 rating. The IFR system form the intumescent char on burning which From the Table5 data, it can be known that Epoxy /IFR and Epoxy/IFR/Sb2O3 systems pass the isUL-94V responsible test and to slow can reach down V-0 the rating. burning The process IFR system and finally form the results intumescent in the extinguish char on burning of fire. Thus, which all is theresponsible coating formulation to slow down can the reach burning V-0 processrating. and finally results in the extinguish of fire. Thus, all the coating formulation can reach V-0 rating. 3.8. Compositions of Charred Layers 3.8. Compositions of Charred Layers Previous studies reveal that upon burning, intumescent flame retardant system forms a protectivePrevious charred studies layer reveal on the that surface upon burning, of the material intumescent [28–30 flame]. The retardant charred layer system can forms inhibit a protective the flow ofcharred heat and layer oxygen on the around surface the of thematerial material and [ 28surroundings.–30]. The charred The IFR layer system can inhibit also releases the flow the of non heat- flammableand oxygen gases around and the moisture material to andthe surroundings surroundings. to The dilute IFR the system heat alsoand oxygen releases around the non-flammable the burning material.gases and Consequently, moisture to the the surroundings char layer can to stop dilute the the burning heat and process oxygen andaround increase the the burning flame resistance material. ofConsequently, material. the char layer can stop the burning process and increase the flame resistance of material. The charred layerslayers ofof allall coating coating formulation formulation formed formed by by constant constant heating heating in in the the mu muffleffle furnace furnace for for20 min20 min at 500 at 500◦C. °C. Figure9 shows 9 shows the FT-IR the spectrum FT-IR of spectrum Epoxy /IFR, of Epoxy Epoxy/IFR,/IFR/Sb 2O Epox3-2 wt%,y/IFR and/Sb Epoxy2O3-2 /IFR wt%,/Sb 2O and3-4 wt%Epoxy/IFR charred/Sb layers.2O3-4 wt% charred layers.

Figure 9. Figure 9. FTIR curves of EpoxyEpoxy/IFR,/IFR, EpoxyEpoxy/IFR/Sb/IFR/Sb22O33--2%,2%, Epoxy/IFR/Sb Epoxy/IFR/Sb2O33--4%4% coatings coatings..

From the Figure 10 data, it is clear that the absorption peaks in the range of 2900–2950 cm 1 From the Figure 10 data, it is clear that the absorption peaks in the range of 2900–2950 cm−−1 corresponding to the symmetric and asymmetric stretching vibrations of C-H bonds in CH2 group. corresponding to the symmetric and asymmetric stretching vibrations of C-H bonds in CH2 group.

PolymersPolymers 20182020,, 1012,, x 2721 FOR PEER REVIEW 1111 of of 14 14

Figure 10. Peak heat release rate (PHRR) curves of coatings Figure 10. Peak heat release rate (PHRR) curves of coatings

The increasing intensity order of these peaks is Epoxy/IFR Epoxy/IFR/Sb2O3-2 wt% The increasing intensity order of these peaks is Epoxy/IFR≤≤ Epoxy/IFR/Sb2O3-2 wt% ≤ Epoxy/IFR/Sb2O3-4 wt%. This order of absorption peak intensity demonstrates that the Sb2O3 Epoxy/IFR/Sbcan develop cross-linkages2O3-4 wt%. This with order IFR system of absorption and effectively peak intensity protect the demonstrates material form that the fire.the Sb The2O peaks3 can develop cross-linkages1 with IFR system and effectively protect the material form the fire. The peaks at 920–1088 cm− are assigned to the P-O-C, P-O-P, and P-O bonds stretching vibrations inside at 920–1088 cm−1 are assigned to the P-O-C, P-O-P, and P-O bonds stretching vibrations inside the the phospho-carbonaceous complex. These peaks of Epoxy/IFR/Sb2O3-2 wt%, Epoxy/IFR/Sb2O3-4 phospho-carbonaceous complex. These peaks of Epoxy/IFR/Sb2O3-2 wt%, Epoxy/IFR/Sb2O3-4 wt%, wt%, are showing high intensity and broadness than Epoxy/IFR/Sb2O3-0 wt%. It indicates that it arebehaves showing as ahigh catalyst, intensity and and show broadness synergistic than action Epoxy/IFR/Sb on the establishment2O3-0 wt%. It indicates of a dense that intumescent it behaves as a catalyst, and show synergistic action on the establishment of a1 dense intumescent phsospho- phsospho-carbonaceous char. The peak of absorption at 642 cm− is preferentially endorsed the carbonaceous char. The peak of absorption at 642 cm−1 is preferentially endorsed the stretching stretching vibration of Sb-O bond which indicates that Sb2O3 can form cross-linkages with IFR system. vibration of Sb-O bond which indicates that Sb2O3 can form cross-linkages with IFR system. The The intensity of this peak increased as Sb2O3 amount increases. These results suggested that Sb2O3 intensitycan promote of this the peak thickness increased of the as char. Sb2O3 amount increases. These results suggested that Sb2O3 can promote the thickness of the char. 3.9. Cone-Calorimetric Analysis 3.9. Cone-Calorimetric Analysis The combustion study of a sample can be performed by cone calorimeter based on oxygen consumptionThe combustion principle. study Cone of calorimetric a sample can analysis be performed provides the by information cone calorimeter for time based to ignition on oxygen (TTI), consumptionPeak heat release principle. rate (PHRR), Cone calorimetric total heat release analysis (THR), provides and the total information smoke production for time (TSP).to ignition The PHRR(TTI), Peakvalues heat are release regarded rate as (PHRR), important total indicator heat release to characterize (THR), and the total flammability smoke production of a material (TSP). in fire The situation. PHRR valuesThe PHRR are curves regarded of all as coating important compositions indicator are to shown characterize in Figure the 10 flammability and Table6. Figure of a material 11 and Table in fire6 situation.indicates thatThe purePHRR Epoxy curves coating of all coating has shown compositions a single and are sharp shown HRR in Figure peak with 10 and 756 kWTable/m 6.2 at Figure 177 s, 1whereas1 and Table the Epoxy6 indicates/IFR, Epoxythat pure/IFR Epoxy/Sb2O3 coating-2 wt%, has and shown Epoxy /aIFR single/Sb2 Oand3-4 sharp wt%, coatingsHRR peak have with PHRR 756 2 2 kW/mpeaks atat 260, 177 207, s, whereas 97.8 kW/ m the, respectively. Epoxy/IFR, When Epoxy/IFR/Sb the IFR and2O3-2 Sb wt%,2O3 are and introduced Epoxy/IFR into/Sb the2O3 coatings,-4 wt%, coatingsa great decrease have PHRR in PHRR peaks values at 260, is observed. 207, 97.8 kW/mThis is2, because,respectively. during When the combustion the IFR and process, Sb2O3 are the introduIFR systemced into and Sbthe2 Ocoatings,3 are involved a great in decrease the development in PHRR ofvalues a dense is observed. char layer This that inhibitsis because the, during flow of theoxygen combustion and heat process, to the material, the IFR system thus lowing and Sb the2O intensity3 are involve of thed in pyrolysis the development process and of a reducing dense char the layerheat release that inhibits quantity. the flow of oxygen and heat to the material, thus lowing the intensity of the pyrolysis process and reducing the heat release quantity.

PolymersPolymers 20202018,, 1210,, 2721x FOR PEER REVIEW 1212 ofof 1414

Table 6. Cone calorimeter results of the coatings. Table 6. Cone calorimeter results of the coatings. Sample TTI (S) PHRR (kW m−2) THR (MJ m−2) 2 2 Pure EpoxySample Resin TTI60 (S) PHRR (kW756 m− ) THR (MJ82 m − ) PureEpoxy/IFR Epoxy Resin 6055 756260 8263 Epoxy/IFR/Sb2O3Epoxy/IFR-2% 5543 260207 6342 Epoxy/IFR/Sb O -2% 43 207 42 Epoxy/IFR/Sb2O32 3 -4% 43 97 25 Epoxy/IFR/Sb2O3-4% 43 97 25

Figure 11. Total heat release (THR) curves of coatings. Figure 11. Total heat release (THR) curves of coatings. From the Figure 10 and Table6 data, it can be seen that Epoxy /IFR, Epoxy/IFR/Sb2O3-2 wt% and EpoxyFrom/IFR /theSb2 FigO3-4ure wt% 10 and coatings Table have 6 data cut, it the can ignition be seen timethat Epoxy/IFR, which are signiﬁcantly Epoxy/IFR/Sb endorsed2O3-2 wt% to and the formationEpoxy/IFR/Sb of protective2O3-4 wt% char coatings layer. have At the cut initial the ignition stage of time burning, which the are temperature significantly of endorsed these coatings to the risesformation gently of because protective of the char char layer. formation; At the therefore, initial stage it results of burning, in shortening the temperature of ignition of time. these The coatings Sb2O3 playsrises gently a remarkable because role of tothe improve char formation; the power therefore of the char, it results layer, protectionin shortening of the of charignition from time. damage The andSb2O cracking3 plays a toremarkable get low HRR role values. to improve the power of the char layer, protection of the char from damageThe and THR cracking values of to all get coatings low HRR are displayedvalues. in Figure 11 and Table6. From the data, the di ﬀerence in pureThe Epoxy THR coating values and of all Epoxy coatings/IFR, are Epoxy displayed/IFR/Sb2 Oin3 -2Figure wt%, 1 Epoxy1 and/ IFR Table/Sb 26.O 3 From-4 wt%, the coatings data, the is seendifference after 50s.in pure The Epoxy THR value coating of Epoxyand Epoxy/IFR, is 82 MJ/ mEpoxy/IFR/Sb2 at 300s, whereas2O3-2 wt%, the THR Epoxy/IFR value of/Sb Epoxy2O3-4 wt%,/IFR, 22 2 2 Epoxycoatings/IFR is/ Sbseen2O 3after-2 wt%, 50s. EpoxyThe THR/IFR /valueSb2O 3of-4 Epoxy wt%, areis 82 63 MJ/m MJ/m at, 42 300s MJ,/ mwhereas, 24 MJ the/m THRrespectively. value of 2 2 2 TheseEpoxy/IFR, results Epoxy/IFR/Sb specify that addition2O3-2 wt%, of IFR Epoxy/IFR/Sb and Sb2O3 2greatlyO3-4 wt%, decreases are 63 the MJ/m heat, release 42 MJ/m rate., 24 It isMJ/m due torespectively. this that the These MAPP results and CFAspecify agents that formaddition a dense of IFR protective and Sb2O char3 greatly layer decreases in the presence the heat of release Sb2O3 synergisticrate. It is due agent. to this The that formed the MAPP char and layer CFA lowers agents the form heat releasinga dense protective from the material.char layer in the presence

of SbFrom2O3 synergistic this data agent. it is proved The formed that Sb char2O3 enhanceslayer lowers the the thickness heat releasing and power from of the the material. char layer and improvesFrom the this combustion data it is proved properties that of Sb the2O3 coatings. enhances the thickness and power of the char layer and improves the combustion properties of the coatings. 4. Conclusions 4. Conclusions Ethylenediamine modified ammonium polyphosphate and N-ethanolamine triazine ethylenediamineEthylenediamine copolymer modified (CFA) is ammonium synthesized successfully polyphosphate and theirand properties N-ethanolamine were thoroughly triazine investigatedethylenediamine and copolymer compared. (CFA) Comparative is synthesized TGA successfully study of MAPPand their and properties APP also were investigated. thoroughly Theinvestigated synergistic and eff ectcompared. of antimony Comparative trioxide on TGA Epoxy study/IFR of coating MAPP system and APP is evaluated. also investigated. The UL-94V The andsynergistic LOI results effect indicate of antimony that Sb trioxide2O3 has significantlyon Epoxy/IFR increased coating thesystem flame is retardancyevaluated. ofThe coating UL-94V system. and AsLOI the results Sb2O indicate3 content that in theSb2 IFRO3 has coating significantly system, increased the LOI values the flame also retardancy increased remarkably. of coating system. The TGA As resultsthe Sb2 provideO3 content hints in the about IFR the coating addition system, of Sb the2O 3LOIgreatly values increasing also increased the thermal remarkably. steadiness The of TGA the coatingresults provide systemand hints the about interaction the addition between of MAPP Sb2O3 andgreatly Sb2 increasO3 is welling demonstratedthe thermal steadiness by FTIR results. of the coating system and the interaction between MAPP and Sb2O3 is well demonstrated by FTIR results.

Polymers 2020, 12, 2721 13 of 14

The study of synergistic effect of Sb2O3 on Epoxy/IFR coating system revealed the formation of stable protective charred layer. All the above results conclude that Sb2O3 is very effective synergistic agent in the Epoxy/IFR coating systems and helps in reducing the HRR emission remarkable. The study shows the good impact of Sb2O3 on epoxy based intumescent flame retardation coating system; still, further morphologically and mechanical study would give more information and open its role for further applications.

Author Contributions: Data curation, Formal analysis, and Writing—original draft, R.; Formal analysis, S.B.; Writing—review & editing, Project administration, F.M.H.; writing review and editing with provided resources for the revision, R.A.K.; Funding acquisition, A.A.; Conceptualization, visualization, Investigation, Methodology, Supervision, Writing—review & editing; J.A.S. All authors have read and agreed to the published version of the manuscript. Funding: The Deputyship for Research & Innovation, “Ministry of Education” in Saudi Arabia provides funding for this research work through project no- IFKSURG-1438-006. Acknowledgments: The authors extend their appreciation to the Deputyship for Research & Innovation, “Ministry of Education” in Saudi Arabia for funding this research work through project no- IFKSURG-1438-006. Conﬂicts of Interest: There are no conﬂict between authors.

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