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

Flame-Retarding Behaviors of Novel Spirocyclic Organo-Phosphorus CompoundsBased on Pentaerythritol DongQuy Hoang 1,2 and Jinhwan Kim* ,1 1 Department of Polymer Science and Engineering,

Flame-Retarding Behaviors of Novel Spirocyclic Organo-Phosphorus Compounds

Based on Pentaerythritol

DongQuy Hoang 1,2 and Jinhwan Kim* ,1

1 Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Korea

2 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

acrylonitrile-butadiene-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

some-times 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,

acrylonitrile-butadiene-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

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 com-busting 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

*Corresponding Author E-mail: jhkim@skku.edu

for this case, many researchers studied the flame retardancy

of organo-phosphorus compound derived from

4-(hydroxy-methyl)-1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane

(referred as HPO afterwards) for poly(butylene

terephthal-ate), 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

combus-tion 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

reso-nance spectroscopy (NMR) and Fourier transform infrared

spec-troscopy (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

cal-orimetry 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,

metha-nol, dichlorophenylphosphine, and methyl iodide were

pur-chased from Aldrich Thionyl chloride, methylene chloride,

pyridine, acetonitrile, hexane, methanol, chloroform, dioxane,

triethylamine, and diethyl ether were purchased from

Sam-chun Chemical Company, Korea Diethylphosphinic acid,

PC, and ABS of commercial grades were provided by the Cheil

Industries, Korea Methylphosphonic dichloride,

diethylphos-phinic 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 110oC and

refluxed at that temperature for 6 h Distillation of the reaction

mixture gave methylphosphonic dichloride which has a

melt-ing point of 35-36oC 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 tem-perature controller and reflux condenser with stirring The mixture was heated to 75oC and refluxed at that tempera-ture for 4 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 0-5oC under nitrogen atmosphere After refluxing for 6 h, triethylamine hydrochloride was filtered and the clear filtrate was evaporated to obtain the crude product Further purifi-cation 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 0-5oC under nitrogen condition

After stirring for 6 h at room temperature, pyridine hydro-chloride was filtered and the clear filtrate was evaporated to

obtain 1 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 3 from the reaction of 2 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 calorim-etry (DSC) analysis which are presented in Figures 1 and 2

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-1-phosphabicyclo[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

1H NMR (DMSO-d6, ppm): δ=5.12 (s, 1H), 4.62 (d, 6H),

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

mechani-cal stirrer, a dropping funnel, and a condenser with a nitrogen

inlet The mixture was stirred and cooled to 0-5oC A solution

of phenyl methyl phosphinyl chloride (19.20 g, 0.11 mol) in

50 mL acetonitrile was added dropwise After stirring for 15 min

at 0-5oC, the reaction temperature was increased slowly to

80oC and then the reaction was refluxed for 24 h The

reac-tion mixture was concentrated by removing the solvent and

washed with distilled water three times A pure solid of mp

192.9oC was obtained upon drying (70% yield)

1H 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)=192.9oC

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 phenylphos-phonic dichloride (19.5 g, 0.1 mol) in the presence of pyri-dine (15.8 g, 0.2 mol) dissolved in 400 mL acetonitrile After isolating, a pure solid of mp 317.9oC was obtained upon drying (65% yield)

1H 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)

31P 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,

Table I Structures and Characteristics of FRs Synthesized in this Study

(Pentaerythritol phosphate alcohol) Phenyl Methyl Phosphinate PPM Phosphinate 19.47 70 192.9 (Pentaerythritol phosphate alcohol)

(Pentaerythritol phosphate alcohol) Methyl Methyl Phosphonate PMM Phosphonate 22.76 70 168.8 bis(Pentaerythritol phosphate alcohol)

bis(Pentaerythritol phosphate alcohol)

Scheme I Synthesis scheme for organo-phosphorus flame

retar-dants (FRs) employed in this study

Figure 1 DSC thermograms of five different FRs synthesized in

this study

Figure 2 1H and 31P NMR spectra of five different FRs synthesized in this study.

methyl-, bis(2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-ylmethyl)

ester, P,P'-dioxide): A similar procedure was used to

syn-thesize 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 279.2oC was obtained upon drying (60% yield)

1H 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) =

279.2oC

Synthesis of PDE Phosphinate and PMM Phosphonate.

Synthesis of PDE Phosphinate

(2,6,7-trioxa-1-phosphabicy-clo[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 0-5oC A solution of diethylphosphinic

chloride (15.46 g, 0.11 mol) in 50 mL dioxane was added

drop-wise And the reaction temperature was increased slowly and

refluxed for 24 h at 60oC After cooling and standing, the

fil-trate 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

1H NMR (CDCl3, ppm): δ=4.62 (d, 6H), 3.85 (d, 2H),

1.72-1.80 (m, 4H), 1.13-1.20 (m, 6H) 31P NMR (CDCl3, ppm):

two single peaks; δ = -2.187, 69 577 Mp (by DSC)=164.1oC

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

1H 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)=

168.8oC

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 10oC/min under a flow of N2 gas Thermal gravimetric

analysis (TGA) was performed on 2 to 10 mg samples under air

and nitrogen at a heating rate 5, 10, 20, and 40oC/min using

a TGA 2050 thermogravimetric analyzer

Sample Preparation for UL-94 Test: Mixture of a synthesized

flame retardant with ABS or PC at the designated composi-tion was processed in a Haake PolyDrive mixer with 60 rpm for 7 min at 230oC for ABS and at 240oC 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 4 N1161 Specimen mass was 2-5 mg Heating rate was 1 K/s The specimen temperature was 750oC The combined flow rate was 100 cm3/min, the oxygen concentration in the combustor was 20% O2 v/v, and the combustor temperature was 900oC 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

com-pounds 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 Phosphi-nate, 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 1 and 2, one can observe the appear-ance 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 700oC at a heating rate of 20oC/min The results are given

in Figure 3 The detailed TGA data including the initial

decomposition temperature (Tonset), the temperature at which 10% mass loss (T10) occurs, and the fraction of charred resi-due 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 400-540oC 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 phosph-inate, PDE Phosphphosph-inate, and PMM Phosphonate show two-step

decomposition leaving high amount of residual char

(9.6-41.4%) 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 370oC

These results indicate that HPO based FR compounds

syn-thesized 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 700oC 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

Figure 3 TGA thermograms of neat ABS, PC, and FRs under nitrogen (A) and air (B) (a: bis-PP Phosphonate, b: bis-PM

Phospho-nate, c: PPM PhosphiPhospho-nate, d: PDE PhosphiPhospho-nate, and e: PMM Phosphonate)

Table II Summary of Thermal Stability Parameters for the Materials Employed in this Study

chemical nature of bis-PP Phosphonate and bis-PM

Phos-phonate 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 4

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

poly-phosphoric 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 5 and 6 T onset 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

domi-nant 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 650oC

under nitrogen (3.5-18.0%) and under air (1.0-8.0%) The

charring is enhanced very distinctively at 500oC 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

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 7 and 8 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 sug-gests that the thermal degradation of PC is accelerated by the presence of FR at earlier stage of degradation and then hin-dered by the char formed at later stage of degradation

d logr 

d 1/T 

- 0.4567E

R

-–

=

Figure 4 FTIR spectra of neat PP Phosphonate (a) and

bis-PM 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

Figure 5 TGA thermograms of neat PC and various PC/FR

mix-tures under nitrogen (A) and air (B)

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 3 to

30 wt% In comparison between PC and ABS, remarkably

much lower amount of FR is needed to impart flame

retar-dancy for PC This is easily understandable when

consider-ing that PC is a charrable polymer but ABS is a highly

non-charrable polymer PC is a non-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 vol-atiles 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 phospho-nate, 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 PP phosphonate and

bis-PM phosphonate containing mixtures but V-1 rating is obtained

in the case of PPM phosphinate and PMM phosphonate

contain-Table III Activation Energy of Neat PC and PC/FR Mixtures at Different Heating Rates Under Air

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

Table V UL-94 Results for Various ABS/PC Mixtures

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

phosphi-nate and PMM phosphophosphi-nate, respectively The P content is

almost the same for bis-PP phosphonate and PPM

phosphi-nate but quite different UL-94 test results were obtained

Consid-ering 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

organo-phosphorus 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

mix-tures of FRs with PC are given in Table IV and are in good

agreements with above assertion To obtain V-0 rating, 3 wt%

loading is needed for PMM Phosphonate, 4 wt% for bis-PM

Phosphonate, PDE Phosphinate, or PPM Phosphinate, and

5 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 decompo-sition and consequently to protect the intact portion of mate-rial exposed upon combustion Balabanovich39 reported that HPO decomposes exothermally in the temperature range of 296-340oC and this can negate the thermal insulating char-acteristic of the char and additionally warm up the bulk of the polymer containing mixture HPO compounds studied in this work do not show exothermal phenomena at 296-340oC Nonetheless, no exothermic peaks are observed for FRs studied here One can note the DSC results presented in Fig-ure 1, where the maximum recorded temperatFig-ure is 350oC Probably inherent exothermal effect reported in the lietera-ture 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 consider-able amount of charred residue while very little charred res-idues are observed for ABS/PMM Phosphonate and ABS/PPM Phosphinate mixtures (Figure 6 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 evalu-ate fire safety.42 For the PC/FR mixtures along with neat PC,

Figure 6 TGA thermograms of neat ABS and various ABS/FR

mixtures under nitrogen (A) and air (B)

Figure 7 TGA thermograms of neat PC under air at different

heating rates

samples obtained V-0 ratings are evaluated employing MC

and the detailed PHRR data are shown in Figure 9 and

sum-marized 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 4 wt% PDE Phosphinate, 5 wt%

bis-PP Phosphonate, 4 wt% PPM Phosphinate, 3 wt% PMM Phosphonate, and 4 wt% bis-PM Phosphonate, respectively

The reduction in PHRR indicates that the addition of FRs do 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

Figure 9 Heat release rate (HRR) of neat PC and various PC/FR

mixtures

Figure 8 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

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