DSpace at VNU: Flame-retarding behaviors of novel spirocyclic organo-phosphorus compounds based on pentaerythritol

Macromolecular Research DOI 10.1007/s13233-015-3085-2 www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673 Flame-Retarding Behaviors of Novel Spirocyclic Organo-Phosphorus Compounds Based on Pentaerythritol DongQuy Hoang1,2 and Jinhwan Kim*,1 Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Korea University of Science, Vietnam National University, Ho Chi Minh City, Vietnam Received July 26, 2014; Revised February 7, 2015; Accepted May 1, 2015 Abstract: In order to find effective flame retardant for charrable polycarbonate (PC) and non-charrable acrylonitrilebutadiene-styrene copolymer (ABS), a series of novel organo-phosphorus compounds derived from 4-(hydroxymethyl)1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (HPO) flame retardant (FR) were synthesized and their flame retardancies were investigated for the mixtures containing PC or ABS The successful synthesis of high purity FRs was verified by spectroscopic analysis, 1H and 31P nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR) In an attempt to provide a basis to understand the flame retardancy behaviors of synthesized FRs, various other techniques such as thermal analysis and micro-scale calorimetry were employed The flame retardancies were determined by UL-94 vertical test methods The results show that V-0 ratings are achieved at 3-5 wt% loadings of FR for PC and V-1 rating at 30 wt% for ABS This big difference is believed to be resulted from the fact that the main mechanism of flame retardancy is based on the condensed phase in the case of PC Nevertheless, effective gas phase acting FR is needed for ABS containing mixtures Both peak heat release rate obtained from micro-calorimeter experiments and the decomposition activation energy determined from differential scanning calorimetry (DSC) results are greatly reduced for the PC/FR mixtures, indicating that a stable insulating barrier is formed between fire and charrable PC containing substrate On the other hand, ABS is a non-charrable polymer and the flame retardant acting in the gas phase is more desirable The findings obtained in this study clearly implies that it would not be easy to find a promising phosphorus based FR which is good not only in flame retardancy but also in other properties such as hydrolytic and thermal stability for non-charrable polymer like ABS Keywords: organo-phosphorus, flame retardant, spirocyclic phosphorus compound, PC, ABS Introduction Polymeric materials are widely used as engineering plastics in a variety of applications and the quality of modern life has been improved by their utilization However, polymers are inherently easy to be decomposed when exposed to heat are highly flammable Therefore, their poor flame resistance should be enhanced by incorporating the Flame retardant (FR) in certain areas of applications.1,2 Polycarbonate (PC) has excellent mechanical properties like high impact strength and reasonably high thermal stability PC is known to be highly flame-retardant plastic and sometimes classified as a self-extinguishing polymer, exhibiting the UL-94 V-2 rating without addition of any FR due to its inherent char-forming ability.3 Contrary to PC, acrylonitrilebutadiene-styrene copolymer (ABS) is extremely flammable It has a low limiting oxygen index (LOI) value of 18.3 and burns completely in air, producing large quantities of dense *Corresponding Author E-mail: jhkim@skku.edu black smoke and leaving very little charred residue.4-6 No rating is recorded for neat when tested by the UL-94 In order to use ABS in the application areas requiring a high degree of flame retardancy, for an example, in electric and electronics applications, effective FR should be added to delay or even extinguish the burning of flame generated accidentally Recently, many literatures have been reported on the performances or the fire inhibition efficiencies of phosphorus FRs and FR systems combined with synergistic components in attempts to apply them to both PC and ABS.6,7-19 The efficiency of phosphorus-based FR depends upon not only the amount of the phosphorus (P) element existing in the compound but also the ability to form the charred residue The amount of P element is a direct index for the ability of flame retardancy since more phosphorus volatiles that act as active species in gas phase are generated during the decomposition of combusting polymeric material On the other hand, the species enabling the formation of stable residual char is more desirable for the polymer where the main mechanism is based on the gas phase mode of action As an example of promising FRs © The Polymer Society of Korea and Springer 2015 D Hoang and J Kim for this case, many researchers studied the flame retardancy of organo-phosphorus compound derived from 4-(hydroxymethyl)-1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane (referred as HPO afterwards) for poly(butylene terephthalate), polypropylene, and polyethylene and reported that these derivatives generate high residual char.20-23 The higher the amount of residual chars after combustion, the lower the amount of combustible material available and thus the greater flame retardancy is achieved.24 In this study, we aim at the synthesis of novel phosphorus FRs based on the derivative of HPO having high P contents and being expected to generate greater chars upon combustion and providing the efficient flame retardancy for PC and ABS The successful synthesis of high purity FRs was verified by spectroscopic analysis, 1H and 31P nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR) In an attempt to provide a basis to understand the flame retardancy behaviors of synthesized FRs, various other techniques such as thermal analysis and micro-scale calorimetry were employed The differences in the effects of flame retardancy on the formulations depending on PC and ABS are investigated Experimental Materials Dimethyl methylphosphonate, pentaerythritol, phosphorus oxychloride, phenylphosphonic dichloride, methanol, dichlorophenylphosphine, and methyl iodide were purchased from Aldrich Thionyl chloride, methylene chloride, pyridine, acetonitrile, hexane, methanol, chloroform, dioxane, triethylamine, and diethyl ether were purchased from Samchun Chemical Company, Korea Diethylphosphinic acid, PC, and ABS of commercial grades were provided by the Cheil Industries, Korea Methylphosphonic dichloride, diethylphosphinic chloride, phenyl methyl phosphinyl chloride, and methyl methylphosphonochloridate as the starting materials were synthesized in our laboratory Synthesis of Methylphosphonic Dichloride:25,26 Dimethyl methylphosphonate as a starting material and pyridine as a catalyst were introduced into a round-bottom flask equipped with a temperature controller, reflux condenser, and a stirrer Thionyl chloride was slowly added to the flask at room temperature The mixture was slowly heated to 110 oC and refluxed at that temperature for h Distillation of the reaction mixture gave methylphosphonic dichloride which has a melting point of 35-36 oC 1H NMR (CDCl3, ppm): δ=2.52 (d, 3H) 31P NMR (CDCl3, ppm): δ=45.00 (s) Synthesis of Diethylphosphinic Chloride:27 Thionyl chloride was added dropwise into diethylphosphinic acid placed in a round-bottom flask equipped with a temperature controller and reflux condenser with stirring The mixture was heated to 75 oC and refluxed at that temperature for h The reaction product was concentrated on a rotary evaporator The crude product was distilled to give the pure product 31P NMR (CDCl3, ppm): δ=76.6 (s) Synthesis of Methyl Methylphosphonochloridate: Methanol mixed with ether was slowly added to a mixture of methylphosphonic dichloride and triethylamine dissolved in ether at 0-5 oC under nitrogen atmosphere After refluxing for h, triethylamine hydrochloride was filtered and the clear filtrate was evaporated to obtain the crude product Further purification gave the pure product 1H NMR (CDCl3, ppm): δ=3.90 (d, 3H), 2.00 (d, 3H) 31P NMR (CDCl3, ppm): δ=47.775 (s) Synthesis of Phenyl Methyl Phosphinyl Chloride: [Step 1] Dimethyl Phenylphosphonite (1):28 Methanol in hexane was slowly added to a mixture of dichlorophenylphosphine and pyridine in hexane at 0-5 oC under nitrogen condition After stirring for h at room temperature, pyridine hydrochloride was filtered and the clear filtrate was evaporated to obtain 1H NMR (CDCl3, ppm): δ=7.45-7.55 (m, 5H), 3.61 (d, 6H) 31P NMR (CDCl3, ppm): δ=162.368 (s) [Step 2] Methyl Methylphenylphosphinate (2) was prepared according to the method of Korpium et al.29 1H NMR (CDCl3, ppm): δ=7.80-7.90 (m, 2H), 7.50-7.60 (m, 3H), 3.65 (d, 3H), 1.70 (d, 3H) 31P NMR (CDCl3, ppm): δ=49.623 (s) [Step 3] Phenyl Methyl Phosphinyl Chloride (3): A similar procedure used for preparing methylphosphonic dichloride was adopted to synthesize from the reaction of with thionyl chloride in presence of pyridine Distillation of the reaction mixture gave the pure product 1H NMR (CDCl3, ppm): δ= 7.85-7.91 (m, 2H), 7.60-7.65 (m, 1H), 7.52-7.58 (m, 2H), 2.22 (d, 3H) 31P NMR (CDCl3, ppm): δ=53.106 (s) Synthesis of Organo-Phosphorus Flame Retardants (FRs) Five different novel phosphorus FRs whose structures are shown in Table I were synthesized Schemes to synthesize these FRs are presented in Scheme I The success of synthesis was confirmed by 1H, 31P NMR, and differential scanning calorimetry (DSC) analysis which are presented in Figures and Their generic names and abbreviations which will be used afterwards are also given in Table I Synthesis of 4-(hydroxymethyl)-1-oxido-2,6,7-trioxa-1phosphabicyclo[2.2.2]octane (HPO) The synthesis of HPO was carried out by reacting phosphorus oxychloride with pentaerythritol following the procedure found in the literature.30,31 H NMR (DMSO-d6, ppm): δ=5.12 (s, 1H), 4.62 (d, 6H), Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol Table I Structures and Characteristics of FRs Synthesized in this Study FR Generic name Mp (oC) (Pentaerythritol phosphate alcohol) Phenyl Methyl Phosphinate PPM Phosphinate 19.47 70 192.9 (Pentaerythritol phosphate alcohol) Diethyl Phosphinate PDE Phosphinate 21.80 75 164.1 (Pentaerythritol phosphate alcohol) Methyl Methyl Phosphonate PMM Phosphonate 22.76 70 168.8 bis(Pentaerythritol phosphate alcohol) Phenyl Phosphonate bis-PP Phosphonate 19.27 65 317.9 bis(Pentaerythritol phosphate alcohol) bis-PM Phosphonate Methyl Phosphonate 22.11 60 279.2 Scheme I Synthesis scheme for organo-phosphorus flame retardants (FRs) employed in this study 3.30 (s, 2H) 31P NMR (DMSO-d6, ppm): one single peak δ = -0.82 Synthesis of PPM Phosphinate, bis-PP Phosphonate, and bis-PM Phosphonate Synthesis of PPM Phosphinate (2,6,7-trioxa-1-phosphabicyclo [2.2.2]oct-4-ylmethyl phenyl methylphosphinate P-oxide): A mixture of HPO (18.01 g, 0.10 mol) and pyridine (7.90 g, 0.10 mol) in 200 mL acetonitrile was placed into a 500 mL three-necked round-bottomed flask equipped with mechanical stirrer, a dropping funnel, and a condenser with a nitrogen inlet The mixture was stirred and cooled to 0-5 oC A solution of phenyl methyl phosphinyl chloride (19.20 g, 0.11 mol) in 50 mL acetonitrile was added dropwise After stirring for 15 at 0-5 oC, the reaction temperature was increased slowly to 80 oC and then the reaction was refluxed for 24 h The reaction mixture was concentrated by removing the solvent and washed with distilled water three times A pure solid of mp Macromol Res Abbreviated Name P Content (wt%) Yield (%) Figure DSC thermograms of five different FRs synthesized in this study 192.9 oC was obtained upon drying (70% yield) H NMR (DMSO-d6, ppm): δ=7.75-7.84 (m, 2H), 7.63-7.71 (m, 1H), 7.55-7.63 (m, 2H), 4.68 (d, 6H), 3.83-3.91 (m, 1H), 3.57-3.64 (m, 1H), 1.74 (d, 3H) 31P NMR (DMSO-d6, ppm): two single peaks; δ = -1.172, 50.392 Mp (by DSC)=192.9 oC Synthesis of bis-PP Phosphonate (Phosphonic acid, phenyl-, bis(2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-ylmethyl) ester, P,P'-dioxide): Similar procedure mentioned above was used to synthesize bis-PP Phosphonate It was prepared from the reaction of HPO (36.0 g, 0.2 mol) with phenylphosphonic dichloride (19.5 g, 0.1 mol) in the presence of pyridine (15.8 g, 0.2 mol) dissolved in 400 mL acetonitrile After isolating, a pure solid of mp 317.9 oC was obtained upon drying (65% yield) H NMR (DMSO-d6, ppm): δ=7.72-7.84 (m, 3H), 7.58-7.67 (m, 2H), 4.70 (d, 12H), 3.98-4.05 (m, 2H), 3.89-3.97 (m, 2H) 31 P NMR (DMSO-d6, ppm): two single peaks; δ=-5.421, 21.616 Mp (by DSC)=317.9 oC Synthesis of bis-PM Phosphonate (Phosphonic acid, D Hoang and J Kim Figure 1H and 31P NMR spectra of five different FRs synthesized in this study Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol methyl-, bis(2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-ylmethyl) ester, P,P'-dioxide): A similar procedure was used to synthesize bis-PM Phosphonate It was prepared from the reaction of HPO (36.0 g, 0.2 mol) with methylphosphonic dichloride (13.3 g, 0.1 mol) in the presence of pyridine (15.8 g, 0.2 mol) dissolved in 400 mL acetonitrile After isolating, a pure solid of mp 279.2 oC was obtained upon drying (60% yield) H NMR (DMSO-d6, ppm): δ=4.67 (t, 12H), 3.89-3.94 (m, 2H), 3.83-3.88 (m, 2H), 1.54 (d, 3H) 31P-NMR (DMSO-d6, ppm): two single peaks; δ = -5.408, 35.028 Mp (by DSC) = 279.2 oC Synthesis of PDE Phosphinate and PMM Phosphonate Synthesis of PDE Phosphinate (2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-ylmethyl diethylphosphinate P-oxide): A mixture of HPO (18.01 g, 0.10 mol) and triethylamine (10.10 g, 0.10 mol) in 200 mL dioxane was placed into a three-necked round-bottomed flask equipped with mechanical stirrer, a dropping funnel, and a condenser with a nitrogen inlet The mixture was stirred and cooled to 0-5 oC A solution of diethylphosphinic chloride (15.46 g, 0.11 mol) in 50 mL dioxane was added dropwise And the reaction temperature was increased slowly and refluxed for 24 h at 60 oC After cooling and standing, the filtrate was collected by filtration and solvent was evaporated to obtain solid product High purity product (75% yield) was obtained after purification by flash column chromatography H NMR (CDCl3, ppm): δ=4.62 (d, 6H), 3.85 (d, 2H), 1.721.80 (m, 4H), 1.13-1.20 (m, 6H) 31P NMR (CDCl3, ppm): two single peaks; δ = -2.187, 69 577 Mp (by DSC)=164.1 oC Synthesis of PMM Phosphonate (Phosphonic acid, methyl-, 2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-ylmethyl methyl ester, P-oxide): A similar procedure was used to synthesize PMM Phosphonate It was prepared from the reaction of HPO (18.01 g, 0.1 mol) with methyl methylphosphonochloridate (14.13 g, 0.11 mol) in the presence of triethylamine (10.10 g, 0.10 mol) dissolved in 250 mL acetonitrile High purity product (70% yield) was obtained after purification by flash column chromatography H NMR (CDCl3, ppm): δ=4.62 (d, 6H), 3.90-3.94 (m, 1H), 3.83-3.87 (m, 1H), 3.76 (d, 3H), 1.53 (d, 3H) 31P NMR (CDCl3, ppm): two single peaks; δ = -5.434, 35.180 Mp (by DSC)= 168.8 oC Measurements and Sample Preparation Spectroscopic Analysis: 1H and 31P NMR were performed on a Varian Unity Inova 500NB spectrometer by using CDCl3 and DMSO-d6 as solvents and tetramethylsilane (TMS) as a reference The chemical shift of 31P NMR spectra are relative to the external standard of 85% H3PO4 Infrared spectrum (IR) was obtained by using a Nicolet 380 FTIR spectrometer Thermal Analysis: Differential scanning calorimeter (DSC) was carried out on a TA 2910 DSC instrument at a heating rate of 10 oC/min under a flow of N2 gas Thermal gravimetric analysis (TGA) was performed on to 10 mg samples under air and nitrogen at a heating rate 5, 10, 20, and 40 oC/min using a TGA 2050 thermogravimetric analyzer Macromol Res Sample Preparation for UL-94 Test: Mixture of a synthesized flame retardant with ABS or PC at the designated composition was processed in a Haake PolyDrive mixer with 60 rpm for at 230 oC for ABS and at 240 oC for PC UL-94 Measurement: Fire retardancy performance was evaluated according to the testing procedure of FMVSS 302/ZSO 3975 with test specimen bars of 127 mm in length, 12.7 mm in width, and about maximum up to 3.2 mm in thickness Micro Calorimeter Test: Samples were exposed to an FAA micro calorimeter instrument (FTT) according to ISO/TC 61/SC N1161 Specimen mass was 2-5 mg Heating rate was K/s The specimen temperature was 750 oC The combined flow rate was 100 cm3/min, the oxygen concentration in the combustor was 20% O2 v/v, and the combustor temperature was 900 oC The results presented were averaged from at least three experiments Scanning Electronic Microscopy: The morphology of char was investigated for the outer surface of residues obtained after UL-94 test by a JEOL 6700F SEM graphs of the residual char samples were recorded after gold coating surface treatment Results and Discussion Synthesis of Flame Retardants (FRs) Among five compounds synthesized in this study, synthetic routes for bis-PP Phosphonate and bis-PM Phosphonate are clearly stated in the literatures.21,23 However, PPM Phosphinate, PDE Phosphinate, and PMM Phosphonate are not found in the literature and are considered as novel materials if synthesized successfully For this reason, the synthesis of these FRs is discussed in the Experimental section in more detailed manners The success of synthesis was verified by the 1H and 31P NMR spectroscopic analysis and thermal analysis by DSC From 1H and 31P NMR results presented in Figures and 2, one can observe the appearance of very distinct signals of corresponding structures of synthesized FRs From these results, we confirm the successful synthesis of high purity FRs Thermal Degradation Behaviors and Modes of Action of FRs Thermal decomposition behaviors of synthesized FRs and their mixtures with ABS or PC were investigated by TGA carried out under air and nitrogen conditions from 50 to 700 oC at a heating rate of 20 oC/min The results are given in Figure The detailed TGA data including the initial decomposition temperature (Tonset), the temperature at which 10% mass loss (T10) occurs, and the fraction of charred residue remaining at 650-700 oC are summarized in Table II Both PC and ABS show one-step thermal decomposition under nitrogen (Figure 3(A)) and two-step degradation under air (Figure 3(B)) Under air, the first step of mass loss occurs at 400-540 oC and a slower second degradation related to the char oxidation is observed On the other hand, all pure FRs, that is, bis-PP Phosphonate, bis-PM Phosphonate, PPM phosphinate, PDE Phosphinate, and PMM Phosphonate show two-step D Hoang and J Kim Figure TGA thermograms of neat ABS, PC, and FRs under nitrogen (A) and air (B) (a: bis-PP Phosphonate, b: bis-PM Phosphonate, c: PPM Phosphinate, d: PDE Phosphinate, and e: PMM Phosphonate) Table II Summary of Thermal Stability Parameters for the Materials Employed in this Study Samples Tonset (oC) T10 (oC) Residue at 650 oC (%) N2 Air N2 Air N2 Air bis-PP Phosphonate 350.1 317.3 368.9 351.3 41.4 12.0 bis-PM Phosphonate 313.6 308.2 351.7 346.9 31.3 9.0 PPM Phosphinate 261.7 265.4 313.7 312.5 17.7 3.1 PDE Phosphinate 253.6 248.2 301.0 304.5 9.6 11.2 PMM Phosphonate 252.8 237.2 299.9 288.4 15.8 7.9 Neat ABS 360.4 351.0 402.0 396.0 0.0 0.0 ABS/30 wt% bis-PP Phosphonate 327.2 302.7 363.5 355.5 17.9 7.8 ABS/30 wt% bis-PM Phosphonate 322.7 311.8 360.7 357.1 16.0 7.3 ABS/30 wt% PPM Phosphinate 241.9 256.4 339.8 331.4 7.2 3.5 ABS/30 wt% PDE Phosphinate 269.9 264.5 330.6 331.3 6.8 3.0 ABS/30 wt% PMM Phosphonate 235.4 219.0 365.8 345.2 3.5 o Samples o Tonset ( C) 1.0 o T10 ( C) Residue at 700 C (%) N2 Air N2 Air N2 Air Neat PC 461.7 452.7 514.5 501.7 22.5 0.2 PC/5 wt% bis-PP Phosphonate 365.5 344.7 429.6 413.8 23.1 3.5 PC/4 wt% bis-PM Phosphonate 336.1 323.7 422.4 408.1 22.8 4.6 PC/4 wt% PPM Phosphinate 307.2 287.1 417.4 397.1 19.7 2.9 PC/4 wt% PDE Phosphinate 282.7 274.5 428.5 399.9 18.5 0.7 PC/3 wt% PMM Phosphonate 284.5 279.8 414.7 393.7 21.1 3.8 decomposition leaving high amount of residual char (9.641.4%) under nitrogen and relatively low amount (3.1-12.0%) under air The rapid weight loss is observed for the first step degradation of FRs at the temperature ranges from 250-370 oC followed by the slow second step degradation at about 370 oC These results indicate that HPO based FR compounds synthesized here possess the promising aspects as a FR such in the viewpoint of char-forming ability for flammable polymer containing system, especially for non-charrable polymer such as ABS There are differences in term of the thermal behavior of pure FRs under nitrogen and air conditions Pure FRs lose weight gradually and leave high solid residues at 700 oC under nitrogen while much lower amount of residues are observed when decomposed under air, especially in the case of bis-PP Phosphonate and bis-PM Phosphonate To investigate the Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol Figure FTIR spectra of neat bis-PP Phosphonate (a) and bisPM Phosphonate (c) and of residual chars of bis-PP Phosphonate (b) and bis-PM Phosphonate (d) collected after the first step of thermal decomposition under nitrogen chemical nature of bis-PP Phosphonate and bis-PM Phosphonate left over degradation, FRs are undergone the thermal decomposition on a TGA instrument under nitrogen and the charred residues were collected after the first step of degradation and analyzed by FTIR, whose results are given in Figure Very distinct characteristic peaks of cyclic and bicyclic structures appear at 678, 756, and 840 cm-1 before decomposition but, after decomposition, decrease significantly Furthermore, the O=PO3 appearing at 1314 cm-1 also disappears after decomposition in the case of in bicyclic compound Simultaneously, new bands which are believed to be characteristics of a P-OH group appear at 2350, 1632, and 986 cm-1 The appearance of P-OH group shows that the decomposition of P-O-C bond takes place The results also reveal that the strong absorption bands of polyphosphoric acid appear at 2756-2850, 2350, 1636, 1127, 986, and 889 cm-1 due to the generation of P-O-P Therefore, the second step decomposition is related to the reaction among the products formed from the first step decomposition, that is, the liberation of phosphoric acid that condensates further to polyphosphoric acid The thermogravimetric curves of mixtures of different FR with PC or ABS are presented in Figures and Tonset of PC/FR and ABS/FR mixtures are quite lower than those of neat PC and ABS This may be due to the evaporation of FR or the decomposition products generated from interactive reaction between polymer and FR in the earlier degradation of FR These results indicate that the presence of the FR decreases the onset temperature of degradation and consequently leads to slower degradation of a matrix polymer This is more dominant in the PC case (Figure 5) It should be noted that the thermal degradation of neat ABS shows only one-step decomposition and very little amount of residue is remaining However, the presence of FR significantly contributes to charring at 650 oC under nitrogen (3.5-18.0%) and under air (1.0-8.0%) The Macromol Res Figure TGA thermograms of neat PC and various PC/FR mixtures under nitrogen (A) and air (B) charring is enhanced very distinctively at 500 oC under air (18-35%) (Figure 6) On the other hand, no significant increase in charring amount is found for the mixtures with PC The kinetic parameters of thermal degradation can be used to evaluate the thermal stability The activation energy (E) is calculated using dynamic TGA experiments measured at various heating rates by adopting the modified Ozawa’s method as follows.32,33 d  logr  E = –0.4567 d  1/T  R where r is the heating rate, T corresponds to the temperature giving the same heat loss at different heating rate, E is the activation energy of the decomposition reaction, and R is the gas constant According to above equation, the activation energy can then be determined from the plot of log r vs 1/T at a given mass loss TGA experiments at four different heat rates of 5, 10, 20, and 40 oC/min were carried out and the experimental results and the kinetic parameters obtained therein are given in Figures and and summarized in Table III It is found that the E values of PC/FR mixtures are lower than that of neat PC at low mass loss and higher at high mass loss This suggests that the thermal degradation of PC is accelerated by the presence of FR at earlier stage of degradation and then hindered by the char formed at later stage of degradation D Hoang and J Kim Table III Activation Energy of Neat PC and PC/FR Mixtures at Different Heating Rates Under Air Mass Loss Activation Energy (kJ/mol) Neat PC bis-PP bis-PM PPM PDE PMM 0.05 120.1 100.8 115.7 89.1 72.4 84.0 0.20 146.3 74.9 87.6 88.9 87.5 91.5 0.30 159.8 64.4 70.6 86.5 98.9 105.3 0.50 203.0 101.0 127.5 141.7 152.3 159.0 Table IV UL-94 Results for Various PC/FR Mixtures FR wt% of P in FR PC/FR (wt/wt) UL-94 Rating wt% of P in PC/FR Mixture bis-PP Phosphonate 19.27 96.0 / 4.0 95.0 / 5.0 V-1 V-0 0.77 0.96 bis-PM Phosphonate 22.11 97.0 / 3.0 96.0 / 4.0 V-1 V-0 0.66 0.88 PPM Phosphinate 19.47 97.0 / 3.0 96.0 / 4.0 V-1 V-0 0.58 0.78 PDE Phosphinate 21.80 97.0 / 3.0 96.0 / 4.0 V-1 V-0 0.65 0.87 PMM Phosphonate 22.76 97.5 / 2.5 97.0 / 3.0 No rating V-0 0.57 0.68 Table V UL-94 Results for Various ABS/PC Mixtures FR bis-PP Phosphonate wt% of P in FR 19.27 ABS/FR (wt / wt) UL-94 Rating Dripping wt% of P in ABS/FR Mixture 70 / 30 No rating No 5.78 bis-PM Phosphonate 22.11 70 / 30 No rating No 6.63 PPM Phosphinate 19.47 70 / 30 V-1 No 5.84 PDE Phosphinate 21.80 70 / 30 1st: 0s, 2nd: burn Yes 6.54 PMM Phosphonate 22.76 70 / 30 V-1 No 6.83 Flame Retardancies of FRs UL-94 vertical test results for various FR containing mixtures are presented in Tables IV and V present the The amount of FR loading varies from to 30 wt% In comparison between PC and ABS, remarkably much lower amount of FR is needed to impart flame retardancy for PC This is easily understandable when considering that PC is a charrable polymer but ABS is a highly noncharrable polymer PC is a charrable and self-extinguishing polymer and by itself shows a V-2 rating in UL94 test.3 Thus, addition of very small amount of FR shows remarkable increase in the flame retardancy for the mixtures of PC On the other hand, ABS is highly combustible and fails in the UL-94 test This polymer does not leave any char upon combustion; therefore, gas phase action is believed to be the main fire retardant mechanism for obtaining effective retardancy.6,8,34 To impart effective flame retardancy for non-charrable ABS, a greater amount of FR should be added to the compound V-0 rating cannot be achieved even at 30 wt% FR loading for the mix- ture of ABS Only V-1 rating was obtained at 30 wt% loading of PPM Phosphinate and PMM Phosphonate It was reported that, with increasing the oxidation state of the phosphorus, additional charring is observed and the release of phosphorus-containing volatiles diminishes.35-37 It is also reported that relative amount of phosphorus-containing volatiles escaped from combusting medium is abundant in an order: phosphate < phosphonate < phosphinate, suggesting that gas phase action increases with decreasing the oxidation state of the phosphorus Our results shown in Figure 3(A) are exactly agreed with this assertion Very large amounts of charred residues are observed for bis-PP phosphonate and bis-PM phosphonate, lower amounts are observed for PMM phosphonate and PPM phosphinate, and lowest residue is noticeable for PDE phosphinate When 30 wt% of FR is incorporated into ABS, no rating is recorded for bis-PP phosphonate and bisPM phosphonate containing mixtures but V-1 rating is obtained in the case of PPM phosphinate and PMM phosphonate containMacromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol ing mixtures The P content in neat FR is 19.27 and 22.11 wt% for bis-PP phosphonate and bis-PM phosphonate, respectively, while this value is 19.47 and 22.76 wt% for PPM phosphinate and PMM phosphonate, respectively The P content is almost the same for bis-PP phosphonate and PPM phosphinate but quite different UL-94 test results were obtained Considering the chemical structure, PPM phosphonate is one-ring containing phosphate-base compound but bis-PP phosphonate is two-ring containing phosphonate-base compound The same assertion can be addressed for bis-PM phosphonate vs PMM phosphonate containing mixtures These results clearly show that the flame retardancy relying on gas phase action increases with decreasing the oxidation state of the phosphorus It was reported that the fire retarding efficiency of organophosphorus FR having -CH3 group exhibits the best fire retarding performance on both ABS and EVA.38 In this study, both PPM phosphinate and PMM phosphonate have -CH3 group directly attached to -P(O) and their flame retardancies are better than PDE phosphinate The UL-94 test results for the mixtures of FRs with PC are given in Table IV and are in good agreements with above assertion To obtain V-0 rating, wt% loading is needed for PMM Phosphonate, wt% for bis-PM Phosphonate, PDE Phosphinate, or PPM Phosphinate, and Figure TGA thermograms of neat ABS and various ABS/FR mixtures under nitrogen (A) and air (B) Macromol Res Figure TGA thermograms of neat PC under air at different heating rates wt% for bis-PP Phosphonate Although the presence of above FRs contributes to the charring for ABS, char layers formed after combustion are not stable enough to endure further thermo-oxidative decomposition and consequently to protect the intact portion of material exposed upon combustion Balabanovich39 reported that HPO decomposes exothermally in the temperature range of 296-340 oC and this can negate the thermal insulating characteristic of the char and additionally warm up the bulk of the polymer containing mixture HPO compounds studied in this work not show exothermal phenomena at 296-340 oC Nonetheless, no exothermic peaks are observed for FRs studied here One can note the DSC results presented in Figure 1, where the maximum recorded temperature is 350 oC Probably inherent exothermal effect reported in the lieterature is less significant or the exothermal decomposition may be shifted higher than 350 oC Another point to be noted is that ABS/bis-PP Phosphonate and ABS/bis-PM Phosphonate mixtures leave the considerable amount of charred residue while very little charred residues are observed for ABS/PMM Phosphonate and ABS/PPM Phosphinate mixtures (Figure and Table II) No rating is recorded for the former and V-1 ratings are observed for the latter These finding again fortify our assertion that these FRs generating the char layers can work predominantly only in the condensed phase and that FRs having high P contents show the good flame retardancy on charrable PC but are not effective on ABS at which gas phase action is the main mechanism for imparting the fire retardancy From all the results presented above, it can be concluded that the flame retarding effect of a FR is strongly dependent on not only the P content of FR incorporated but also the chemical structure of FR Micro-calorimeter (MC) is a bench-scale instrument used to investigate the flammability parameters of materials on small-scale conditions.40,41 Heat release rate (HRR) and peak heat release rate (PHRR) are important parameters to evaluate fire safety.42 For the PC/FR mixtures along with neat PC, D Hoang and J Kim Figure The plots of log r vs 1/T for neat PC and various PC/FR mixtures at different mass losses (W: mass loss) Table VI PHRR of Neat PC and PC/FR Mixtures Figure Heat release rate (HRR) of neat PC and various PC/FR mixtures samples obtained V-0 ratings are evaluated employing MC and the detailed PHRR data are shown in Figure and summarized in Table VI Pure PC shows a sharp PHRR at 419.1 W/g while the PHRR value is reduced significantly for PC/FR mixtures PHRR is reduced by 35.1, 33.3, 27.2, 25.4, and 23.7% for the mixture of PC with wt% PDE Phosphinate, wt% Sample PHRR (W/g) Neat PC 469.1 PC/5 wt% bis-PP Phosphonate 313.1 PC/4 wt% bis-PM Phosphonate 357.8 PC/4 wt% PPM Phosphinate 341.6 PC/4 wt% PDE Phosphinate 304.6 PC/4 wt% PPM Phosphinate 341.6 bis-PP Phosphonate, wt% PPM Phosphinate, wt% PMM Phosphonate, and wt% bis-PM Phosphonate, respectively The reduction in PHRR indicates that the addition of FRs enhance the char layer formation during combustion FTIR Spectra and SEM Analysis of the Charred Residue To understand how the formation of char affects the flame retardancy on ABS and PC, the chemical structure of charred residue left after UL-94 test was evaluated by FTIR and the morphologies were investigated with SEM FTIR spectra of charred residues of ABS/FR mixtures after UL-94 tests are Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol Figure 11 SEM micrographs of residual chars of ABS/FR mixtures after UL-94 tests: (A) PDE Phosphinate, (B) PPM phosphinate, (C) PMM Phosphonate, (D) bis-PM Phosphonate, and (E) bis-PP Phosphonate Figure 10 FTIR spectra of residual chars of various ABS/FR (A) and PC/FR mixtures (B) after UL-94 tests given in Figure 10(A) The peaks at 1207, 996, and 913 cm-1 are attributed to P=O, P-OH, and P-O-P, respectively.43 The FTIR spectra of charred residues of PC and PC/FR after UL-94 test are shown in Figure 10(B) The bands corresponding to P=O and P-OH may overlap with the bands of pure PC and are not clearly identified However, the new band appears at 942 cm-1, which is a characteristic of P-O-P These results clearly indicate that the polyphosphoric acids exist in the charred residues Balabanovich39 reported that the thermal decomposition of bis(pentaerythritol phosphate alcohol) (HPO) liberates phosphoric acid condensing to a polyphosphoric acid Some derivatives of HPO producing intumescent char on heating are known to be promising components of intumescent flame retardants (IFRs) for polypropylene and polyethylene.20-23,39 Oue experimental results presented here support again that the formation of polyphosphoric acid existing in the char layer is a critical factor to achieve V-0 rating with at 3-5 wt% loadings of FRs for PC where the condensed phase action of flame redardancy is dominating SEM micrographs presented in Figures 11 and 12 show the Macromol Res Figure 12 SEM micrographs of residual chars of PC/FR mixtures after UL-94 tests: (A) PDE Phosphinate, (B) PPM phosphinate, (C) PMM Phosphonate, (D) bis-PM Phosphonate, and (E) bis-PP Phosphonate D Hoang and J Kim morphology of charred residues collected after UL-94 tests From the SEM micrographs, one can observe the presence of cavities that become pathways of gas fragments generated from the combustion and heat evolved during burning process At the same time, one observe the compact charred layers which can exert as protective layers to inhibite the transmission and diffusion of heat during contacting fire.44,45 Furthermore, the char layers show many swollen bubbles which remain unbroken on the surface These foam structures produced from instumescent decomposition of polymer/FR mixtures would be expected to provide an effective insulating barrier between the substrate and heat source Conclusions The novel organo-phosphorus flame retardants mainly derived from reactions between pentaerythritol and phosphorus containing starting species were synthesized and their flame retardancy for PC and ABS were investigated Synthesized FRs show good efficiency as flame retardant on PC but lesser efficiency on ABS UL-94 V-0 ratings are achieved with 3-5 wt% loadings of FRs for PC and V-1 rating at 30 wt% loading for ABS Both PHHR obtained from micro-calorimeter experiments and the decomposition activation energy determined from DSC results are greatly reduced for the PC/FR mixtures TGA, FTIR, and SEM results also reveal that the addition of FR enhances the char layer formation during combustion, indicating that stable insulating barrier is formed between fire and charrable PC containing substrate and thus increasing the flame retardancy On the other hand, ABS is a non-charrable polymer and the flame retardant acting in the gas phase is more desirable The FRs synthesized in this study is believed to act mainly in the condensed phase Therefore, insufficient effect as a powerful FR is achieved for ABS Even though FRs themselves leave high residual chars (18-35 wt%) at 500 oC, no UL-94 V-0 rating is recorded for ABS/FR mixtures The char layers are not stable enough to endure the thermo-oxidative decomposition of highly combusting ABS at the elevated temperatures This clearly implies that it would not be easy to find a promising phosphorus based FR which is good not only in flame retardancy but also in other properties such as hydrolytic and thermal stability for non-charrable polymer like ABS Acknowledgments The authors wish to thank the Cheil Industries for the financial support and D Hoang appreciates the financial support from Vietnam National University-Ho Chi Minh City (VNU-HCM) (No C2013-18-04) References (1) D Price, G Anthony, and P Carty, in Fire Retardant Materials, A R Horrocks and D Price, Eds., Woodhead Publishing Ltd., England, 2001, pp 1-30 (2) Y Hu, F You, and L Song, in Fire Hazard Analysis and Evaluation of Polymeric Materials, Chemical industry press, Beijing, 2007 (3) S Liu, H Ye, Y Zhou, J He, Z Jiang, J Zhao, and X Huang, Polym Degrad Stab., 91, 1808 (2006) (4) P Carty and S White, Polymer, 35, 343 (1994) (5) J Troitzsch, in International Plastics Flammability Handbook, 2nd ed., Hanser Publications, New York, 1990, p 20 (6) D Hoang and J Kim, Polym Degrad Stab., 93, 36 (2008) (7) J Kim, K Lee, K Lee, J Bae, J Yang, and S Hong, Polym Degrad Stab., 79, 201 (2003) (8) D Hoang, B N Jang, and J Kim, Polym Degrad Stab., 93, 2042 (2008) (9) S V Levchik, Proceedings of the 17th Annual BCC Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Stamford, USA, 2006 (10) E D Weil and S V Levchik, J Fire Sci., 25, 241 (2007) (11) S V Levchik and E D Weil, Polym Int., 54, 11 (2005) (12) S V Levchik and E D Weil, Polym Int., 54, 981 (2005) (13) S V Levchik and E D Weil, J Fire Sci., 24, 345 (2006) (14) G R Grubbs, M E Kleppick, and J H Magill, J Appl Polym Sci., 27, 601 (1982) (15) Y Wang, B Yi, B Wu, B Yang, and Y Liu, J Appl Polym Sci., 89, 882 (2003) (16) D W Allen and E C Anderton, Polym Degrad Stab., 45, 399 (1994) (17) K H Pawlowski and B Schartel, Polym Degrad Stab., 93, 657 (2008) (18) W Zhou and H Yang, Thermochim Acta, 452, 43 (2007) (19) H Ma, L Tong, Z Xu, Z Fang, Y Jin, and F Lu, Polym Degrad Stab., 92, 720 (2007) (20) G Fontaine, S Bourbigot, and S Duquesne, Polym Degrad Stab., 93, 68 (2008) (21) H Peng, Q Zhou, D Wang, L Chen, and Y Wang, J Ind Eng Chem., 14, 589 (2008) (22) F Xie, Y Wang, B Yang, and Y Liu, Macromol Mater Eng., 291, 247 (2006) (23) J E Telschow, Phosphorus Sulfur Silicon Relat Elem., 144146, 33 (1999) (24) P Joseph and J R Ebdon, in Fire Retardant Materials: Recent Developments in Flame-Retarding Thermoplastics and Thermosets, A R Horrocks and D Price, Eds., Woodhead Publishing Ltd., England, 2001, pp 220-63 (25) Purnanand, P D Shakya, and S Saxena, Phosphorus Sulfur Silicon Relat Elem., 177, 1093 (2002) (26) L Maier, Phosphorus Sulfur Silicon Relat Elem, 47, 465 (1990) (27) J P H Martin and M A Stephen, J Chem Soc Perkin Trans., 1, 736 (1981) (28) H J Harwood and D W Grisley, J Am Chem Soc., 82, 423 (1960) (29) O Korpiun, R A Lewis, J Chickos, and K Mislow, J Am Chem Soc., 90, 4842 (1968) (30) Y Halpern, US Patent 4,454,064 (1979) (31) F J Vyverberg and R W Chapman, US Patent 6,455,722 (2002) (32) T Hatakeyama, in Handbook of Thermal Analysis, Z Liu, Ed., Chemical Industry Press, Beijing, 1994, pp 47-48 (33) T Ozawa, Bull Chem Soc Jpn., 38, 1881 (1965) Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol (34) B N Jang and C A Wilkie, Thermochim Acta, 433, (2005) (35) A Lorenzetti, M Modesti, S Besco, D Hrelja, and S Donadi, Polym Degrad Stab., 96, 1455 (2011) (36) U Braun, A I Balabanovich, B Schartel, U Knoll, J Artner, M Ciesielski, M Doring, R Perez, J K.W Sandler, V Altstadt, T Hoffmann, and D Pospiech, Polymer, 47, 8495 (2006) (37) D Price, L K Cunliffe, K J Bullett, T R Hull, G J Milnes, J R Ebdon, B J Hunt, and P Joseph, Polym Degrad Stab., 92, 1101 (2007) (38) C Nguyen, M Lee, and J Kim, Polym Adv Technol., 22, 512 (2011) (39) A I Balabanovich, Thermochim Acta, 435, 188 (2005) (40) ASTM Standard D7309, Standard Test Method for Determin- Macromol Res (41) (42) (43) (44) (45) ing Flammability Characteristics of Plastics and Other Solid Materials Using Microscale Combustion Calorimetry, ASTM International, DOI: 10.1520/D7309-13 (2011) R E Lyon and R N Walters, J Anal Appl Pyrolysis, 71, 27 (2004) G Gallina, E Bravin, C Badalucco, G Audisio, M Armanini, A D Chirico, and F Provasoli, Fire Mater., 22, 15 (1998) G Camino, G Martinasso, L Costa, and R Gobetto, Polym Degrad Stab., 28, 17 (1990) P Lv, Z Wang, K Hu, and W Fan, Polym Degrad Stab., 90, 523 (2005) G Chen, B Yang, and Y Wang, J Appl Polym Sci., 102, 4978 (2006)  ... especially in the case of bis-PP Phosphonate and bis-PM Phosphonate To investigate the Macromol Res Flame-Retarding Behaviors of Novel Spirocyclic Organo-phosphorus Compounds Based on Pentaerythritol. .. the earlier degradation of FR These results indicate that the presence of the FR decreases the onset temperature of degradation and consequently leads to slower degradation of a matrix polymer This... phosphonate and bisPM phosphonate containing mixtures but V-1 rating is obtained in the case of PPM phosphinate and PMM phosphonate containMacromol Res Flame-Retarding Behaviors of Novel Spirocyclic