- Email: [email protected]
Flame and thermal resistance of phosphorus-functionalized poly(methyl methacrylate) and polystyrene
Polymer Degradation and Stability 26 (1989) 305-331
Flame and Thermal Resistance of PhosphorusFunctionalized Poly(Methyl Methacrylate) and Polystyrene C. P. Reghunadhan Nair,* G. Clouet~ & Y. Guilbert Institut Charles Sadron (CRM EAHP), 6 rue Boussingault, 67083 Strasbourg-Cedex, France (Received 1 November 1988; revised version received 30 December 1988; accepted 9 January 1989)
ABSTRACT A series ofphosphorusfunctionalizedpoly(methyl methaco'late) ( P M M A ) and polyso, rene (PS) samples where the phosphorus functions are located at the chain ends, as pendant groups and as blocks in the middle of the polymer chain were evaluated for their .[lame and thermal resistance. The limiting oxygen index values (LOI) of the polymers revealed that the[tame retardant e~'ciency of the phosphorus was dependent on its nature and position in the polymer chain and the type of polymer rather than on its absolute concentration. On a comparative scale, polymers with phosphorus at chain ends and in the middle of the chain exhibited better flame retardant properties. The polymers were subjected to thermogravimetric analysis (TGA) in air and N 2 under dynamic conditions and the trends in thermal behaviour with change in structure and composition have been examined and compared with that ¢~fthe homopolymers. Except when located at chain ends, the phosphorus /'unctionalization adverseO' affbcted the initial thermal characteristics of the P M M A samples, but with an improvement at elevated temperatures. For PS, phosphorus functionalization as pendant groups reduced their thermal resistance whereas chain-end and mid chain functionalization either did not affk,ct or slightly improved their overall thermal "resistance. All phosphoruscontahTing poO,mers left some char residue at higher temperatures. The activation energies .for the ma/or step o/ thermal decomposition were * Permanent address: Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum-695022, India. ++To whom correspondence should be addressed. 305
Polymer Degradation and Stability 0141-3910/89/$03'50 ( ! 1989 Elsevier Science Publishers Lid, England. Printed in Great Britain
306
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert calculated from the thermograms. As the phosphorus content increased, the activation energy for a given degradation step was found to decrease except when the degradation was principally due to the phosphorus containing polymer block where a reverse trend was observed. The flame resistance of the polymers could be correlated to some extent with their thermal behaviour.
INTRODUCTION Phosphorus functionalization is considered to be one of the efficient means of conferring flame retardancy on polymeric systems. Polymers with phosphorus functions as an integral part of the chain exhibit better flame retardancy and overcome several drawbacks associated with physical blends. The phosphorus moiety confers flame resistance mainly by modifying the mode of thermal decomposition of the basic polymer during ignition. 1 Very few studies have been devoted to the understanding of the mechanism of flame retardancy of phosphorus containing polymers and correlation with their thermal behaviour. Generally, the mode of action and hence its efficiency depends upon the nature and position of the phosphorus function. 2 The thermal decomposition characteristics of P M M A and PS are well understood. The former decomposes completely to monomer and the latter to m o n o m e r and volatile oligomers at elevated temperatures. Impurities either in the chain or as additive are known to influence their decomposition pattern. 3-9 Phosphorylation is generally known to affect adversely the thermal resistance of polymeric systems especially at the onset of degradation.1 o- ~4 The aim of the present study is to examine the thermal and flame resistant behaviour of a variety of phosphorus functionalized P M M A and PS samples as a function of the nature, position and composition of the functional groups. An attempt has been made to correlate the thermal behaviour to the observed flame retardancy of the polymers. The kinetics of the major thermal decomposition steps are discussed.
EXPERIMENTAL The phosphorus containing P M M A and PS samples were synthesized as described earlier ~5-18 and had comparable molecular weights. Their structures are shown in Table 1. They were pre-dried at 60°C under vacuum for several hours before use. T G A measurements were carried out on a Mettler TC10A thermal analyzer coupled with a micro processor for data acquisition. Samples
Flame and thermal resistances o f P M M A and PS
307
(approx. 10mg) were subjected to dynamic T G A at a heating rate of 10°C/min in N2 or in air with a gas flow rate of 200ml/min. The Limiting Oxygen Index (LOI) values were measured on a Stanton Redcroft flame meter by a modified method: Powdered samples (500 mg) were placed in a glass cup (diameter, 25-6 mm; height, 4.5 mm) situated in the middle of the chimney (diameter, 75 mm; height, 450 mm) and the flame was applied from the top of the chimney for 10 s. The flow rate (N 2 + 02) was maintained at 17 litres/min. The percentage of oxygen in the 0 2 - N 2 mixture just sufficient to sustain the flame for 30s was taken as the LOI. The values never represent the absolute LOI (ASTM) of the samples but have been found to be quite reproducible. The error in the estimation is about 5%.
RESULTS A N D DISCUSSION
Flame retardancy (1) Copolymers o[" MMA and Styrene with phosphorylated acrylates and
methacrylates The copolymers were synthesized by copolymerization of the appropriate comonomers. Their LOI values are tabulated in Tables 2 and 3a. Obviously the values increased with phosphorus content. However, LOI values for c o m p o u n d I are not significantly high and are comparable to the literature values.19 On the other hand, copolymerization of M MA with a phosphorus containing acrylate m o n o m e r (sample series II) improves its LOI values over the phosphorus containing methacrylate. This may be attributed to the fact that acrylates generally depolymerize less easily than methacrylates. Certain P M M A flame retardant formulations incorporate 1(~20% methyl acrylate as c o m o n o m e r probably for the same reason. 2° It is found that when it is present in the PS backbone the phosphorylated acrylate monomer does not significantly contribute to its flame resistance, i.e. they are more efficient with PMMA. The flame retardant action in P M M A may also be arising from the contribution to anhydride formation (catalyzed by phosphorus), in addition to the possible formation of a protective char coating on the polymer surface. In PS the former mechanism is not operative. (2) Phosphorus in the middle of the chain These polymers (series IV and V) which have a multiblock structure of phosphonamide and the vinyl monomer, were synthesized by radical means. The LOI values of these poly(MMA-b-phosphonamides) and poly(styreneb-phosphonamides) of various compositions are shown in Tables 2 and 3a. It is found that, given the same phosphorus content, the phosphonamide
llI
II
Sample No.
[~
\ /
Ph/~ \
CO2CH3/~\
CO2CHa/~ \
~H3
\ 1
O
CO2CH2CH20~(OEt)2/, J.
O
CO2CH2CH20~(OEt)2/y].
lJ
CO2CH2CH2OP(OEt)zAJ, II O
CH3
Chemical structure
TABLE 1 Structure and Synthetic Details of the Polymers
Random copolymer
Random copolymer
Random copolymer
Nature of polymer
Free radical copolymerization
Free radical copolymerization
Free radical copolymerization
Mode of synthesis
16
16
15
Ref.
VII
VI
V
1V
Ph/~J.
/
II!1
ll\
CH s
O
II
(EtO)2---P N--CH2
O
II
CH2
II
C
S
Hll
Ph/.J.
I
/
\
/
\
~
\
S
C02CH3/n
II
I -
CH 3
S-I-CH2--C--------~S
/
[I
C
S
CH 3
b
\
1 Ph/.
I/ CH3
I
O
II
P--~OEt)2
O
II
N--P~(OEt)2
CH 3
I
CH3 CH2--CH2--N
CH 3
1
N
N - - C - - S I - C H 2 - - C H ] - S - - C - - N CH2--CH2
S
CH 3
q
(EtO)2--P--N--CH2--CH2--N
I
CH 3
Ph/p\
II1\
I
CO2CH3//p \
Linear PS end-capped with P-function
Linear PMMA end-capped with P-function
Multiblock copolymer
Multiblock copolymer
Free radical polymerization using P-containing thermal initiator
Free radical polymerization using P-containing thermal initiator
Through polymeric initiator
Through polymeric initiator
18
18
17
17
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
310
TABLE 2 LOI Values of Various Phosphorus Functionalized P M M A
Series I
Series H
Series I V
Series VI
P M M A blend
polyphosphonamide P% 2"7 5'3 7"6 8-9 10 10"6
LOI
P%
LOI
19 20"5 23 24 25 >25
1"4 2-6 3"5 4"7 5-0 6-5
22 24 25 28 30 36
P%
LOI
P%
LOI
P%
LOI
22 25 27 30
0"25 0'35 0"6 1"05
20 22"5 24 26"5
1 2 3 5
19 20 21-5 23
0'9 1"05 1'8 2"0
block is slightly more efficient in P M M A than in PS. In both these systems the phosphonamide can act as a blockage for the chain unzipping process. Complementary TGA studies suggest that the increased efficiency of the phosphonamide in P M M A is due to the induced decomposition of the polymer by the phosphorus function probably causing a protective char TABLE 3a LOI Values of Various Phosphorus Functionalized PS
Series III
Series V
Series VH
P%
LOI
P%
LOI
P%
LOI
2-6 4'0 5"2 6"5 7'7
22 23 23"5 25 27
0"5 1'0 1"96 2"84 3"1
25 28 34 35 36
0"22 0"27 0"93 1"33 1"48
22 26 29 29 32
TABLE 3b Efficacy Coefficient of the Phosphorus in Various Polymers
Polyrner type
I
II
III
IV
V
VI
VII
PMMA +
poly(phosphonamide) blend Efficacy coefficient
0"85
2 " 6 6 0 " 9 6 5 - 8 6 4.14
5"81 7"30
l'00
Flame and thermal resistances (~f P M M A and PS
311
coating at the onset of inflammation. In these cases, however, the flame retardant efficiency of the phosphorus element is considerably higher than that in the polymers with pendant phosphate groups. Comparison with the results of the physical blend of P M M A and polyphosphomanide (Table 2) shows that the efficiency of the phosphorus is higher when it forms part of the polymer chain. (3) Phosphorus at the chain ends" (compounds VI and VII) P M M A and PS samples end-capped with phosphoramide groups through a dithiocarbamate linkage were synthesized by vinyl polymerization using thiuram disulfide as catalyst. 18 It has previously been reported that P M M A and PS, when end-capped with a phosphorus function (anionically or through chain transfer agents), exhibit exceedingly good flame resistance. 21'21 However, the values reported in Tables 2 and 3a are not as high as expected, but are slightly improved over the LOI values of the multiblock copolymers, with the same phosphorus content. It can be seen that PS is rendered more flame retardant than the P M M A by end phosphorylation. Thermogravimetric analysis has revealed that the dithiocarbamate moieties are stable under thermolytic conditions, 23 but are known to dissociate into radicals under photolytic conditions, z'* The lower (than expected) flame retardancy in these cases may be partially attributed to possible photolytic chain end-scission during inflammation, which may lead to easy chain unzipping. The ALOI values for all types of polymers as a function of their phosphorus content are plotted in Fig. 1, applying the least square method for a first order equation. The slopes of the plots give the apparent flame retardant efficiencies of the particular p h o s p h o r u s function on a comparative scale. The efficacy coefficients, so determined, are presented in Table 3b. It can be seen that they are the highest in the end-functional polymers followed by the block copolymers.
Thermogravimetric analysis The essential features of the thermal degradation of both P M M A and PS are the same, in the sense that both degrade completely to monomer (with oligomers in the case of polystyrene). The thermal behaviour of P M M A is sensitive to molecular characteristics like chain terminal structures, method of" preparation and molecular weight. 25 The presence of 'impurities' in the polymer chains usually decreases their thermal resistance by reducing the chain symmetry and crystallinity which decreases the intermolecular interactions. Exceptions are when the impurities are themselves capable of increasing these by way of dipolar interactions, H-bonding, etc. The
312
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert 20 |
15-
0
10
~
.
~
o
•
_.1 II
× ~
Nf"
5
0
I
I
I
I
I
2
4
6
8
10
P (*/*)
Fig. 1. ALOI versus P% for the systems. A: (+) MMA-DMP; B: (ll) MMA-ADP; C: ([3) styrene-ADP; D: (O) poly(MMA-b-phosphonamide); E: ("k) poly(styrene-b-
phosphonamide); F: (A) polyMMA series VI; G: (0): polystyrene series VII.
influence of the nature and concentration of comonomers in P M M A on its thermal behaviour has been the subject of immense investigation. 3-9 But the role of the phosphorus functions have not been much examined in this respect. In this study the functional polymers quoted in Table 1 were subjected to T G A in N 2 and air and their decomposition patterns were examined with respect to structure. The kinetics of the major decomposition processes were also studied.
Degradation kinetics The kinetic treatment of thermal degradation is mostly based on the relationship, d c / d t = kf(c), where c is the conversion at time t, k is the temperature-dependent rate constant and f(c) a temperature-independent function o f c. In h o m o g e n e o u s kinetics, conversion is defined as concentration and the conversion function is assumed to have the form f ( c ) = ( 1 - c)" where n is the order of the reaction, k is expressed in the Arrhenius form: k = A
e -E/RT
(E = activation energy)
Flame and thermal resistances o f P M M A and PS
3!3
therefore: d c / d t = A e - E / R T ( 1 -- C)"
dc/(1 - c)" = A e E/RT d t integrating:
f" dc/(1
-- c)" = A
To
fT
e E/RT d t
To
For the integration of the right-hand side of this equation, various assumptions and approximations have been used by different authors. Notable among them is that due to Coats and Redfern 26 which gives the final integrated equation as: -ln(I-c)=(RT2/E)[(1-2RT)/E]e
E/RT
forn=l
Plots of --ln(--ln(1 -- c ) / T 2) versus 1 / T give straight lines with slopes equal to E / R . In this study this equation was used to determine the activation energy, assuming first order kinetics. Representative plots are shown in Fig. 2 and the excellent linearity of the points confirms the assumed reaction order. 16.0
165 --
1 5 0 --
uL 14.5 c E ¢z 14C i
13'5
[
13.0 1.70
1.7..5
I 1.80 lIT
Fig. 2.
Coats-Redfern
I 1.85 x 103
p l o t for the d e t e r m i n a t i o n o f
EA
I 190
I 1-95
( h e a t i n g rate 1 0 C / m i n )
(V):
M M A - D M P . Nz, D M P = 80%, 240 300°C; (A): M M A ADP; N z, A D P = 56%, 240~ 32OC; (D): PMMA-Phosphonamide, air, phosphonamide = 6.7%; (Q): styrene-ADP, air, ADP = 49%, 260-300°C; ( . ) : polyADP, air, 220-290°C.
et%)
3-21 5"32 5"84 10'03 10-65 11-65
0"125 0"24 0"27 0"70 0"84 1"00
3.21 5'32 5.84 10.03 10.65 11.65
0-125 0.24 0-27 0.70 0.80 1.00
Composition (Mol.l?action of DMP)
P(%)
Composition (Mol fraction of D MP)
TABLE 4a
0-20 0-25 0-7 12.4 15.5 22.2
% residue at 600°C
TABLE 4b
270-350 230-350 22~350 220-315 220-300 220-290
Step I 355-430 350-420 355-450 320-340 300-330 295-315
Step 11 ---350-450 350-460 320480
Step IH
Temp. range for decomposition (°C)
284 269 241 250 264 254
T l o(°C)
0-0 0-5 0-0 11"4 12"5 12"6
% residue at 600°C
270-350 260-340 220-330 250-300 240-305 220-300
Step I
350-390 --305-315 305 310 300-315
Step II
Temp. range for decomposition (°C)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f Series I in Air
301 271 263 253 256 268
TI0(°C)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f Series I in N 2
100 43 36 137 166 192
Step H
---10 0 0
Step III
163 264 181 149 117 116
Step 1
40 82 115"5
54 --
Step H
EA for the decomposition stages (kJ/mol)
66 79 76 72 70 83
Step I
E Afor the decomposition stages (kJ/mol)
3
Flame and thermal resistances of PMMA and PS
315
Copolymers of M M A with D M P (series I) The synthesis of this series was affected by the copolymerization of diethyl2-(methacryloyloxy)ethyl phosphate (DMP) with MMA. The thermal decomposition characteristic of these copolymers of various compositions are quoted in Tables 4a and 4b, and representative thermograms in N 2 are shown in Figs 3 and 4. It is seen that the decomposition temperature decreases as the phosphorus content increases and that the decomposition pattern changes with composition. A two stage decomposition occurs at low phosphorus content (DMP ~- 10%). The first stage is a combination of DMP-induced and the vinyl end-initiated decomposition. To investigate the contribution of the vinyl end-initiated decomposition, TGA measurements were carried out for three polymers of the same composition, but fractionated to have different molecular weights. The thermograms were essentially superimposable, showing the insignificance of the terminal unsaturation on this step of degradation. (Otherwise the extent of the vinyl end-initiated, first stage decomposition would have increased with decrease in molecular weight.) The rest of the thermogram resembles that of pure PMMA. As the phosphorus content increases, both the first and second stages are shifted to lower temperatures, with the percentage weight 1 0 0 --
8C
\
-~ 6 0
/1
"~ 4 0 .
20-
0 100
I 200
300
Temperature
Fig. 3.
400 (°C)
Z7
500
. . . . . . . .
,
600
TGA of MMA DMP copolymer in air. DMP compositions (mol % ) ( ( - ) 70: ( ....... ) 80; ( - - . ) 100 ( p o l y D M P ) . Heating rate 10 C / r a i n .
) 24,
316
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
loss in the first stage increasing at the expense of that of the second. As the phosphorus content is increased further, a third stage makes its appearance. This stage arises from the residue undergoing a very slow decomposition. The amount of char remaining increases continuously as the phosphorus content increases and the thermograms tend to resemble that of poly(DMP) except for the residue. Figure 4 also includes the TGA of poly(DMP). Its T10 (temperature corresponding to 10% weight loss), which is at 254°C is considerably lower than that of PMMA. The phosphate pendant group cannot be expected to decrease the thermal stability of the P M M A backbone. Hence the apparent lower stability is due to the fact that the decomposition is triggered at the pendant groups themselves, the products of which can give rise to a variety of other structures, which undergo their characteristic decompositions. The crosslinked structures arising from the polymerization of the resulting vinyl bond or the intermolecular anhydride formation may account for the enhanced residue at higher temperatures. Four side group induced reactions are probable: (1) Elimination of phosphoric acid diethyl ester
CH 3 L ~--CH2--C--~ f
CH 3 I ~ - - C H 2 - - C - - ~ + OH--P--(OEt)2 I If °
o//C \O__CH~_CH 2
o
\
/----~ A
CH=CH 2
"O~P_OEt J OEt (2) Elimination of diethyl vinyl phosphate
CH3 ~--CH2--C--~
CH 3 I ~--CH2--C--~ I
C=O,,
dl\
/ CH2 ¢~CH O \ II OIP--OEt I OEt
f, O
+ CH2=CH--P~(OEt)2 I1
o OH
317
Flame and thermal resistances of P M M A and PS
(3) Elimination of ether TH2\
/CH2\ C H 2 \ C C I
I
H+"
/C
o,o I R
C H 2 \ /CH2\ /CH2\ C C I
o
I
C\o/C
+ R--O--R
O
I R
(4) Intra or intermolecular anhydride formation /CH2\ CH2\/CH2\ C C I
I
/ /c\ O
/ CH2 \ /CH2\ 5 H 2 \ C C >
c, O O I
f
I
O
I
+ HOH2C--CH2--P--(OEt)2
H CH 2--CH2--1P--(OEt) 2
fl
O
O CH 3 I
~CH2--C. I
o//C~o__ H 0% / O--H2C - CH 2--P--{OEt)2 II C O CH 3 F J ~CH2--C~ ~CH2--C~ I I CH3 C----O / ,
+ HOHzC--CHz--P--(OEt)2
O
\C=O f
~CH2--C~
11 O
J
CH3 Type (1) reaction is the thermal elimination reaction which is known to occur in organic phosphates. Alkyl acrylates and methacrylates are known to undergo reactions of type (2) and it has been shown that the phosphorus acid is capable of catalyzing the anhydride formation ofe~ter groups (type 3) in methacrylates 1° which accounts for the flame retardant action of the
318
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
phosphorus functional group in PMMA. Type (4) reaction is another possibility in these cases. Since no attempts were made to identify the volatile products in order to confirm the mechanism of decomposition, we postulate the above reactions as the only ones plausible. It is seen from the thermograms that a small amount of phosphorus is not at all advantageous in improving the thermal and flame characteristics, in the sense that thermal decomposition is easily triggered and practically no char residue is left which could otherwise have acted as a thermal protective barrier during inflammation. On the other hand, in air, all the polymers of this series underwent catastrophic degradation around 260°C and there was no appreciable change in the pattern of decomposition except that at high phosphorus content, a plateau region with stable char residue was seen (Fig. 3). The char content in air was lower than that in N 2 showing that thermo-oxidative degradation giving rise to volatile products is considerable. It may be concluded that the flame retardant action is due to the protective char covering, formed in presence of the phosphorus function. To form an appreciable amount of char, a large proportion of phosphorus-containing comonomer is required thus accounting for the reduced flame retardant efficiency of this copolymer system. 100
80
o
".'1
4o
20
o[ lOO
"' , . . . \
I 2oo
i
? Z - ~ 7 7 s 7 .........................
'\~ ..... 300 400 Temperature (*C)
• 500
.
I 600
Fig. 4. T G A of M M A - D M P copolymers in N2. D M P compositions (mol %) ( ) 12.5; (. . . . ) 24; ( ....... ) 70; ( - - . - - ) 80; ( - - - . ) 100 (polyDMP). Heating rate 10°C/min.
Flame and thermal resistances of P M M A and PS
319
The thermal decomposition characteristics of copolymers with various compositions are compiled in Tables 4a and 4b. The activation energy for a particular step of decomposition has a tendency to diminish with increase in phosphorus content for low phosphorus-loaded polymers. Reverse trends were observed when the decomposition steps correspond principally to the polymers themselves. To compare the trend in E A with composition, the decomposition steps falling roughly in the same temperature range were considered for polymers with the same range of composition.
Copolymers of MMA with 2-(Acryloyloxy)ethyl diethyl phosphate (ADP) Polyacrylates have a higher decomposition temperature than polymethacrylates, and the presence of a small amount of acrylate confers a slight increase in stability to P M M A and decreases its tendency to unzip. Polymethyl acrylate and polyethyl acrylate were found to have T10 values of 337°C with E a values of 100 and 102 kJ/mol, respectively, for a single step decomposition. The 2-(acryloyloxy) ethyl diethyl phosphate (ADP) monomer was incorporated into the PMMA chain in order to take advantage of this property as well as the flame retardant property of the phosphate group. However, copolymerization resulted in a decrease in the overall thermal stability. Typical thermograms in N 2 are shown in Fig. 5. The decomposition occurred in two significant stages. The length of the second stage decreased as the ADP content increased. This stage is therefore the decomposition of the P M M A backbone. The first stage must be due to 1
0
O-100 Fig. 5.
0
~
I 200
\
__.
ill,lli21 '
300 400 Temperature (~C)
I 500
_ill -~
600
T G A of M M A A D P copolymers in N 2. A D P compositions (mol %) ( ( ) 2 3 ; ( ....... ) 3 1 ; ( . --)56. Heating rate 10C/rain.
) 14:
TABLE 5a
1.30 3.52 5.27 6.46 9"35 12.30
0.05 0-14 0.23 0.31 0"56 1.00
285 278 290 283 269 248
Tlo(°C)
0"0 0"5 2.6 10.0 23.3 25.0
% residue at 600°C
TABLE 5b
250-340 260-305 240-320 240-320 220-310 200-255
Step I 34~400 320-430 321~350 32(~370 310-330 260-295
Step II
Major degradation steps (°C)
-350-440 370-440 330-400 307-320
Step III 70.8 59 90 67 63"6 81
Step 1
P(%)
1"30 3"52 5-27 6"46 9"35 12"30
Composition (Mol fraction , f ADP)
0-05 0"14 0"23 0"31 0"56 1.00
279 257 266 256 257 241
T ao(°C)
0"0 0"2 1"0 3"0 26-0 25-6
% residue at 600°C
250-330 245-300 240-300 235-315 220-300 220-290
Step 1
330-365 300-370 3130-380 318 340 310-330 300-315
Step II
Major degradation steps (°C)
---350-415 330-400 325-400
Step III
-72 58 -177
Step IH
238 159 155 81 72 48
Step I
37 28 37-5 53-5 150 161"5
Step II
24 ---
---
Step III
E a for the degradation steps (kJ/mol)
114 88 69 79-5 168 56
Step II
the degradation steps (kJ/tool)
E A for
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f M M A with A D P (series II) in Air
P(%)
Composition (Mol.[raction of A DP)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f M M A with A D P (series II) in N z
~"
0
Flame and thermal resistances o f P M M A and PS
321
the various side chain-induced reactions. As the A D P content is increased, an intermediate stage makes its appearance at 310-350°C which corresponds to the major decomposition stage of poly(ADP). This step, which increases with increasing A D P content, has an apparent activation energy tending towards that of pure poly(ADP). It is interesting to see the evolution of the third stage becoming slower as the phosphorus content is enhanced. In other words, phosphorus retards the last stage of decomposition and a considerable proportion of char is left. The temperature ranges for the various steps and their energies of activation are cited in Tables 5a and 5b. Since the char residue left is higher than theoretically expected, based on the content of ADP, it is to be presumed that A D P helps the formation of crosslinked structures, which can also account for the flame retardant action. The thermograms ofcopolymers of different composition can be seen in Fig. 5. The char residue at 600°C is plotted as a function of the phosphorus content in Fig. 6. The char content theoretically expected is also shown in the same figure. In air, the initial decomposition temperature (IDT) is considerably lowered. At small phosphorus content the thermal behaviour is similar to that of pure P M M A in that a single stage decomposition occurs (and no residue is left at higher temperature). When the phosphorus content exceeds about 6 ° a second sharp weight loss is seen at 300°C. This step is typical of the poly(ADP) blocks. At this composition the probability of finding a diad of A D P is 0.43. It is interesting to see that char residues 30
20
I"1
-~ 1 5 -
El
6 lo n 5
I 0
02
04 Mol fraction
06 of A D P
08
10
Fig. 6. Dependence of the char yield at 600°C o n the A D P content. (Q): M M A - A D P ~N2); (D): s t y r e n e - A D P (air); (A): M M A - A D P (air): (....... ): theoretical variation.
322
C. P. Reghunadhan Nair, G. Ctouet, Y. Guilbert
increase markedly above this concentration. It may be presumed that blocks of A D P are helpful in causing crosslinked structures. It may therefore be presumed that a critical concentration of A D P is required to achieve flame retardancy through protective char formation. In this case also, IDT decreases across the series. The E A for the major step decreases initially with increasing phosphorus-content but increases and tends towards the value for pure A D P as the A D P content increases further. However, that for the first step remains unaltered. (When referring to the tables for comparison of the decomposition steps and their EAS, the actual temperature range rather than the step number should be taken into consideration.)
Copolymer of styrene with ADP Polystyrene is much more thermally stable than P M M A and incorporation of styrene into P M M A actually improves the thermal characteristics of the latter. 3 As expected, copolymerization with A D P brings down its initial decomposition temperature. The thermograms show three well defined stages of degradation. The first stage, at low temperature, should be triggered from the side chains, whose proportion increases as the content of A D P increases with a corresponding diminution in T1o- The second stage is a small sharp h u m p at 310°C attributable, as in the case of ( M M A - A D P ) copolymer, to the skeletal decomposition of poly(ADP) blocks whose temperature remains unaltered with composition but whose proportion increases marginally as the A D P content increases. The third stage (330-390°C) is reminiscent of the random decomposition of PS. Its shape is tilted. As the phosphorus content increases, this stage of decomposition is facilitated giving rise to a stable char content, i.e. A D P plays some role in the decomposition of the PS backbone. The char residue increases much more than expected (on the basis of A D P content) showing that A D P is helpful in causing crosslinked residual structures. Although E a for the first step remains practically unaltered, that for the second stage tends towards that for poly(ADP) showing the same trend as in the case of M M A - A D P , decreasing in the beginning and then increasing. The EA for the PS backbone decomposition decreases as the phosphorus content increases. Poly(ADP) underwent essentially two stages of decomposition in air and in N 2. The second, due to backbone splitting, occurs around 310°C in N2 and 300°C in air. Representative thermograms for this system are shown in Fig. 7. In air, unlike the case of the M M A - A D P system, all the three stages observed in N 2 were exactly reproduced in the same pattern except that they occurred at a relatively lower temperature. A fourth stage due to the char residue was also to be seen. Unlike the case of DMP-containing PMMA, the char content of high ADP-loaded polymer in air was higher than that in N2.
Hame and thermal resistances of PMMA and PS
323
100
x?.~ 80
Z x
x
-~
/
20
'(
i----
o
I 100
200
I
\
300 400 T e m p e r a t u r e (°C)
"
I 500
I 600
Fig. 7. T G A of s t y r e n e - A D P c o p o l y m e r s in air. A D P c o m p o s i t i o n s (mol %) (......... ) 0 (polystyrene); (. . . . . ) 20; ( ....... ) 43; ( . --) 100 (polyADP). H e a t i n g rate 10°C/rain.
The thermal characteristics of the copolymers are cited in Tables 6a and 6b. The activation energies for the major decomposition stages (I and III) are given in the same table. The trend in air is exactly the same as in N 2 except that the absolute E a is slightly lower. The variation in the residual char at 600°C in air and in N 2 as a function of ADP content can be seen in Fig. 6. The theoretical char content, based solely on the content of ADP, is also shown. The shift between the two curves is a measure of the extent to which ADP causes crosslinked structures (or decrease in the zip length) in the PS backbone. In general, incorporation of ADP energetically facilitates the thermal degradation of copolymers unless the degradation steps are those involving the splitting of the polymer backbone enriched in ADP. This behaviour is graphically represented in Fig. 8 for the different degradation steps.
Poly(MMA-b-phosphonamide) (series IV) These multiblock copolymers were synthesized by free radical means. TGA of copolymers of various compositions (richer in MMA) showed that the polymers undergo a two stage decomposition with a Tlo around 290~C which is higher than that of AIBN initiated PMMA. The shape of the thermograms remains unaltered in N2 and in air. P M M A synthesized in the presence of free radical initiators usually undergoes a two stage decomposition in N 2. The first stage starts around 260~C and is known to be
4-64 5.65 6.41 7.95 8"61 12.30
0"20 0'26 0.3i 0.43 0.49 1.00
296 293 291 267 264 248
Tlo(°C)
5'7 9'3 11.9 17"6 19.7 25
% residue at 600°C
265-325 260-315 260-320 240-315 220-310 200-255
Step I 340-390 315 330 320-335 318-328 318-320 260-295
Step II
Major degradation steps (°C)
-330-410 335 390 350-390 340-390 307-320
Step III 67 72 81 48 60 81
Step I
P(%)
4"64 5-65 6"41 7"10 7"95 8-61 12"30
Composition (Mol fraction o[ A D P)
0"20 0-26 0"31 0"36 0"43 0"49 1"00
267 263 267 267 263 266 241
T 1o(°C)
7-4 15-0 17"7 22"4 20-2 27"7 25"6
% residue at 600°C
245-330 235 310 240-300 240-300 230-310 230-300 220-290
Step I
330-375 310-320 310-320 310-320 310-320 310-320 300-315
Step H
Major degradation steps (°C)
-350-375 350-380 350-375 345-390 340-370 315-400
Step III
-167 114 89 76 177
Step III
84 66 80 72 48 46 48
Step I
76 52 78 85 123 163 162
Step II
-98 125 98 68 59 --
Step IH
E A for the degradation steps (kJ/mol)
168 71 74 102 147 56
Step H
E Afor the degradation steps (kJ/mol)
N 2
TABLE 6b Thermal Decomposition Characteristics of Copolymers of Styrene with A D P (Series III) in Air
P(%)
Composition (Mol fraction of ADP)
TABLE 6a Thermal Decomposition Characteristics of Copolymers of Styrene with A D P (Series III) in
3"
~"
"~
t.~
325
Flame and thermal resistances of PMMA and PS 240
210
18C o
IE 15C
u~ 12C
Iq
9C
A
6C 0
x
I 02
I 0.4 Mol f r a c t i o n
I 06 of ADP
I 0.8
I 10
Fig. 8. Change of Ea with ADP content for different degradation steps. (A) MMA ADP, N 2, 2"a step involving ADP; (x) Styrene-ADP, N 2, 315-330°C involving ADP; (D) Styrene-ADP, N 2, major degradation step 330-390°C; (*) MMA ADP, air, P' stage 220-320°C. triggered by the vinyl end-initiated depolymerization which is not observed in anionically synthesized P M M A and those prepared in the presence of conventional chain transfer agents. In these block copolymers, vinyl ends are not expected to be present. Because of the synthetic method, the P M M A blocks are usually end-capped with dithiocarbamate groups which have been shown not to cause a two stage decomposition to P M M A . 23 Therefore it can be concluded that the first stage is an induced decomposition caused by the phosphonamide blocks. This is confirmed further by the fact that the two stages occur in both air and N 2. Further, the weight loss in the first stage increases linearly with phosphorus content as shown in Fig. 9. Usually the two stages observed in N 2 for radically prepared P M M A merge into a single stage in air, reducing the overall stability of the polymer in air. But this is not observed in the T G A (air) of these copolymers. The first stage is therefore undoubtedly not caused by the vinyl ends and the mechanism is u n k n o w n for the moment. Similar induced decompositions by phosphorus functions are known in oxygen-containing polymers. 1 It is interesting that despite the initial degradative behaviour, the polymer is apparently stabilized in air. It may hence be concluded that the initial degradation leads to some crosslinked structures, augmenting the thermal stability of the residue. This is further reflected in the increased flame retardancy of this series of polymers. It is a well known practice to add crosslinking agents to thermoplastic flame retardant formulations. 1 As the phosphorus content
326
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert 40 35 30
7 v 25 o 20 ..c
10
Fig. 9. Weight loss in the first step as a function of the phosphorus content for the polymer series IV.
I
05
I
I
10
I
1.5
2Ù
I
25
P (°/o)
increases the second stage decomposition is slowed down and shifted to a higher temperature. In Fig. 10 the thermograms (air) of PMMA prepared under different conditions are shown. The AIBN initiated polymer decomposes earlier than that end-capped with a dithiocarbamate group, each showing only single stage decomposition. The vinyl end initiated depolymerization of AIBN initiated PMMA is suppressed by oxygen at low temperature by trapping of the depropagating polymer radicals to form peroxides. However, at higher temperatures the peroxides undergo decomposition generating reactive radicals capable of inducing an easy random chain scission in the polymer backbone. The activation energies 100 .
.
.
.
.
\ 80
Z
~
,
\
4o
20-
.u_~ .........
I
100
200
300
Temperature
400 (°C)
I-.'~."~ 500
j
....
600
Fig. 10. TGA in air of ( ) polyMMA-AIBN initiated; ( - - - ) polyMMA-series VI; (....... ) poly(MMA-phosphonamide) (phosphonamide: 6.7%); ( - - . - - ) poly(MMA-bphosphonamide) (phosphonamide: 16%). Heating rate 10°C/min.
Flame and thermal resistances 0[" PMMA and PS
327
corresponding to the major degradation processes are presented in Table 7. It can be seen that both steps are energetically favoured as the polymer becomes enriched in phosphorus. Apart from the above suggestions, it is difficult to draw definitive conclusions regarding the exact flame retardant action of the phosphonamide from the thermograms. The char residue is not considered to act as a protective coating. The possibility of some vapour phase action from the products of induced decomposition cannot be ruled out.
Poly (styrene-b-phosphonamide) (series V) The T G A of this series of block copolymers shows an entirely different behaviour. The shape of the thermograms remains essentially unaltered, although an increase in thermal stability was observed especially in the later stages. As the phosphorus content is increased, the rate of degradation is lowered with a concomitant decrease in EA. However, the char content was not very high. When the phosphorus content exceeded 2% a small second decomposition stage was observed at 350-370°C, probably due to the self degradation of polyphosphonamide blocks. The thermograms in air showed a slight decrease in IDT, some char was also observed. The thermal characteristics of these systems can be found in Table 8. Figure 11 illustrates the thermogram in air of some of these PS copolymers. The phosphorylation results in an overall increase in thermal stability. In this case a good consistency between the flame and thermal behaviour is noted. The general trend for the E a for random scission of the vinyl block to decrease with phosphonamide content in their block copolymer shows that the phosphonamide is catalyzing the random degradation. The changes in E A are shown in Fig. 12.
End functional polymers It has been observed that end-capping of the P M M A with dithiocarbamate obviates vinyl end-initiated thermal degradation, 23 and the decomposition temperature is quite independent of molecular weight. But when polymers of types VI and VII (Table 1) were analyzed by TGA, the P M M A samples were found to degrade at lower temperatures than their non-phosphorus containing analogues. Similar thermal behaviour for phosphorus endfunctional P M M A has already been reported. 22 In this case, the role of phosphorus in inducing thermal decomposition is not clear. However, for end-functional PS with low phosphorus content, not much difference could be observed in the thermal behaviour compared with that of pure PS. At high phosphorus content, a small weight loss, probably arising
0.82 1.00 2.84 3"10
5.5 6.7 19.0 21.0
288 293 293 290
Air
0"6 0"5 0"5 0-6
N2
0 0 0"5 1"5
Air
Residue at 600°C
270-310 260-320 260~330 265 330
Step I
360-410 350-420 360-420 360-420
Step I1
Major degradation steps (air and N2)(°C)
88 123 138 115
Et
Air
140 114 140 102
EH
390 390 365 360
N2
TIo("C )
353 358 352 354
Air
1-0 1'0 1.7 5"0
N2
Residue at 600°C
2"2 2.5 5-0 8-6
Air
380-430 380-430 350-450* 350-450*
N2
320-400 335-400 32ff410 320~20
Air
Major degradation range (air and Nz)(°C)
172 162 150 109
EH
230 210 196 166
N2
200 159 134 156
Air
E A for the decomposition reaction (kJ/mol)
52 81 94 53
Et
N2
Activation energy (kJ/mol)
and in Air
TABLE 8 Thermal Decomposition Characteristics of Poly(styrene-b-phosphonamide) in N 2 and in Air
301 303 304 290
N2
T l o(° C)
N 2
* A small separate peak due to the self decomposition of the poly(phosphonamide) blocks could be seen at 350-370°C.
P(%)
0"37 0"86 1.00 2"40
P(%)
Weight % of phosphonamide
2"5 5'8 6-7 16"0
phosphonamide
of
Weight %
TABLE 7 Thermal Decomposition Characteristics of Poly(MMA-b-phosphonamide) in
3"
~-
to oo
Flame and thermal resistances of PMMA and PS
329
100,
80
~
I
60
', I
1] 401 I t \
20
L
i
0
100
I
I
200
300
I
400 (°C)
Temperature
Fig. I I.
......
500
I
600
TGA in air o£ poly(styrene-b-phosphonamide). Weight % of ghosphonamide: (
) = 0 (PST), (. . . . . ) = 55, ( . . . . . . . ) = 21.
240
22C
20C
~
D
18C
--×
g ~6c-
12C
10C
I
0
5
I
10 Phosphonamide
15
20
(wt-°/o)
Fig. 12. Influence o f the p h o s p h o n a m i d e content on the activation energy for the random scission of the vinyl block in A: (/~) p o l y ( M M A - b - p h o s p h o n a m i d e ) , air and B: ( x ) in N2; C: ( D ) poly(Styrene-b-phosphonamide), air and D: ( O ) in N 2.
330
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
from the phosphorus function, was observed around 200°C. The major decomposition was observed to occur in 2 steps unlike that in pure PS. The thermograms could not furnish any conclusive evidence for the mechanism of flame retardation. End group condensation has been suggested as a reasonable mechanism for flame retardation in this kind of polymer. 21'22
CONCLUSION Evaluation o f flame and thermal resistance of the various types of phosphorus functional P M M A and PS reveals that these properties are dependent on the nature and concentration of the particular function. Phosphorylation causes an easy thermal degradation of P M M A which accounts for its flame retardant action through formation o f a protective char barrier or interference of the degradation products in the vapour phase chain reaction. For polystyrene, only chain end and chain middle phosphorylation confer flame and thermal resistance.
REFERENCES 1. Weil, E. D., Flame-Retardant Polymeric Materials, Plenum Press, Vol. 2, Chap. 4. 2. Chiotis, A., Clouet, G. & Brossas, J., Polym. Bull., 6 (1982) 577. 3. McNeill, I. C., Eur. Polym. J., 4 (1968) 21. 4. Grassie, N., Chemical Reactions of Polymers, ed. E. M. Felles, Interscience, New York, 1964. 5. Grassie, N. & Grant, E. M., Eur. Polym. J., 2 (1966) 225. 6. Grassie, N. & Grant, E. M., Eur. Polym. J., 3 (1967) 619. 7. McNeill, I. C., Straiton, T. & Andersen, P., J. Polym. Sci., Polym. Chem. Ed., 18 (1982) 2085. 8. Camino, G., Costa, L., Devalle, G. & Guaita, M., Poly. Deg. andStab., 4 (1982) 459. 9. McNeill, I. C. & McGuiness, R. C., Poly. Deg. and Stab., 9 (1984) 167. 10. Camino, G., Grassie, N. & McNeill, I. C., J. Polym. Sci., Polym. Chem. Ed., 16 (1978) 95. 11. Brown, C. E., Wilkie, C. A., Smukalla, J., Cody, R. B., Jr & Kinsinger, J. A., J. Polym. Sci., Polym. Chem. Ed., 24 (1986) 1297. 12. Camino, G., Costa, L., Clouet, G., Chiotis, A., Brossas, J., Bert, M. & Guyot, A., Poly. Deg. and Stab., 6 (1984) 105; ibid, 6 (1984) 177. 13. Kallitis, J. & Tsolis, A. K., J. Appl. Polym. Sci., 31 (1986) 635. 14. Clouet, G. & Knipper, M., Poly. Deg. andStab., 17 (1987) 151. 15. Reghunadhan Nair, C. P., Clouet, G. & Brossas, J., J. Polym. Sci., Polym. Chem. Ed., 26, (1988) 1791. 16. Reghunadhan Nair, C. P. & Clouet, G., Eur. Polym. J., 25 (3) (1989) 251. 17. Reghunadhan Nair, C. P. & Clouet, G., Polym., 29 (1988) 1909.
Flame and thermal resistances of PMMA and PS
331
18. Reghunadhan Nair, C. P. & Clouet, G., Makromol. Chem., 190 (1989) 1243. 19. Bauer, R. G., Kavchok, R. W. & O'Connor, J. M., J. Fire Retard. Chem., 3 (1976) 99. 20. Jung, K. A., Jakob, K. H. & Wopker, W., US Pat. 4,585,818 (1986), R6hm Gmbh. 21. Chiotis, A., Ph.D. dissertation, Strasbourg, 1981. 22. Knipper, M., Ph.D. dissertation, Strasbourg, 1985. 23. Reghunadhan Nair, C. P., Clouet, G. & Chaumont, P., J. Polym. Sci., Polvm. Chem. Ed. 27 (1989) 1795. 24. Otsu, T. & Tazaki, T., Polym. Bull., 16 (1986) 277. 25. Kashiwagi, T., Inaba, A., Brown, J. E., Hatada, K., Kitayama, T. & Masuda, E. Macromolecules, 19 (1986) 2160. 26. Coats, A. W. & Redfern, J. P., Nature, 201 (1964) 68.
Flame and Thermal Resistance of PhosphorusFunctionalized Poly(Methyl Methacrylate) and Polystyrene C. P. Reghunadhan Nair,* G. Clouet~ & Y. Guilbert Institut Charles Sadron (CRM EAHP), 6 rue Boussingault, 67083 Strasbourg-Cedex, France (Received 1 November 1988; revised version received 30 December 1988; accepted 9 January 1989)
ABSTRACT A series ofphosphorusfunctionalizedpoly(methyl methaco'late) ( P M M A ) and polyso, rene (PS) samples where the phosphorus functions are located at the chain ends, as pendant groups and as blocks in the middle of the polymer chain were evaluated for their .[lame and thermal resistance. The limiting oxygen index values (LOI) of the polymers revealed that the[tame retardant e~'ciency of the phosphorus was dependent on its nature and position in the polymer chain and the type of polymer rather than on its absolute concentration. On a comparative scale, polymers with phosphorus at chain ends and in the middle of the chain exhibited better flame retardant properties. The polymers were subjected to thermogravimetric analysis (TGA) in air and N 2 under dynamic conditions and the trends in thermal behaviour with change in structure and composition have been examined and compared with that ¢~fthe homopolymers. Except when located at chain ends, the phosphorus /'unctionalization adverseO' affbcted the initial thermal characteristics of the P M M A samples, but with an improvement at elevated temperatures. For PS, phosphorus functionalization as pendant groups reduced their thermal resistance whereas chain-end and mid chain functionalization either did not affk,ct or slightly improved their overall thermal "resistance. All phosphoruscontahTing poO,mers left some char residue at higher temperatures. The activation energies .for the ma/or step o/ thermal decomposition were * Permanent address: Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum-695022, India. ++To whom correspondence should be addressed. 305
Polymer Degradation and Stability 0141-3910/89/$03'50 ( ! 1989 Elsevier Science Publishers Lid, England. Printed in Great Britain
306
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert calculated from the thermograms. As the phosphorus content increased, the activation energy for a given degradation step was found to decrease except when the degradation was principally due to the phosphorus containing polymer block where a reverse trend was observed. The flame resistance of the polymers could be correlated to some extent with their thermal behaviour.
INTRODUCTION Phosphorus functionalization is considered to be one of the efficient means of conferring flame retardancy on polymeric systems. Polymers with phosphorus functions as an integral part of the chain exhibit better flame retardancy and overcome several drawbacks associated with physical blends. The phosphorus moiety confers flame resistance mainly by modifying the mode of thermal decomposition of the basic polymer during ignition. 1 Very few studies have been devoted to the understanding of the mechanism of flame retardancy of phosphorus containing polymers and correlation with their thermal behaviour. Generally, the mode of action and hence its efficiency depends upon the nature and position of the phosphorus function. 2 The thermal decomposition characteristics of P M M A and PS are well understood. The former decomposes completely to monomer and the latter to m o n o m e r and volatile oligomers at elevated temperatures. Impurities either in the chain or as additive are known to influence their decomposition pattern. 3-9 Phosphorylation is generally known to affect adversely the thermal resistance of polymeric systems especially at the onset of degradation.1 o- ~4 The aim of the present study is to examine the thermal and flame resistant behaviour of a variety of phosphorus functionalized P M M A and PS samples as a function of the nature, position and composition of the functional groups. An attempt has been made to correlate the thermal behaviour to the observed flame retardancy of the polymers. The kinetics of the major thermal decomposition steps are discussed.
EXPERIMENTAL The phosphorus containing P M M A and PS samples were synthesized as described earlier ~5-18 and had comparable molecular weights. Their structures are shown in Table 1. They were pre-dried at 60°C under vacuum for several hours before use. T G A measurements were carried out on a Mettler TC10A thermal analyzer coupled with a micro processor for data acquisition. Samples
Flame and thermal resistances o f P M M A and PS
307
(approx. 10mg) were subjected to dynamic T G A at a heating rate of 10°C/min in N2 or in air with a gas flow rate of 200ml/min. The Limiting Oxygen Index (LOI) values were measured on a Stanton Redcroft flame meter by a modified method: Powdered samples (500 mg) were placed in a glass cup (diameter, 25-6 mm; height, 4.5 mm) situated in the middle of the chimney (diameter, 75 mm; height, 450 mm) and the flame was applied from the top of the chimney for 10 s. The flow rate (N 2 + 02) was maintained at 17 litres/min. The percentage of oxygen in the 0 2 - N 2 mixture just sufficient to sustain the flame for 30s was taken as the LOI. The values never represent the absolute LOI (ASTM) of the samples but have been found to be quite reproducible. The error in the estimation is about 5%.
RESULTS A N D DISCUSSION
Flame retardancy (1) Copolymers o[" MMA and Styrene with phosphorylated acrylates and
methacrylates The copolymers were synthesized by copolymerization of the appropriate comonomers. Their LOI values are tabulated in Tables 2 and 3a. Obviously the values increased with phosphorus content. However, LOI values for c o m p o u n d I are not significantly high and are comparable to the literature values.19 On the other hand, copolymerization of M MA with a phosphorus containing acrylate m o n o m e r (sample series II) improves its LOI values over the phosphorus containing methacrylate. This may be attributed to the fact that acrylates generally depolymerize less easily than methacrylates. Certain P M M A flame retardant formulations incorporate 1(~20% methyl acrylate as c o m o n o m e r probably for the same reason. 2° It is found that when it is present in the PS backbone the phosphorylated acrylate monomer does not significantly contribute to its flame resistance, i.e. they are more efficient with PMMA. The flame retardant action in P M M A may also be arising from the contribution to anhydride formation (catalyzed by phosphorus), in addition to the possible formation of a protective char coating on the polymer surface. In PS the former mechanism is not operative. (2) Phosphorus in the middle of the chain These polymers (series IV and V) which have a multiblock structure of phosphonamide and the vinyl monomer, were synthesized by radical means. The LOI values of these poly(MMA-b-phosphonamides) and poly(styreneb-phosphonamides) of various compositions are shown in Tables 2 and 3a. It is found that, given the same phosphorus content, the phosphonamide
llI
II
Sample No.
[~
\ /
Ph/~ \
CO2CH3/~\
CO2CHa/~ \
~H3
\ 1
O
CO2CH2CH20~(OEt)2/, J.
O
CO2CH2CH20~(OEt)2/y].
lJ
CO2CH2CH2OP(OEt)zAJ, II O
CH3
Chemical structure
TABLE 1 Structure and Synthetic Details of the Polymers
Random copolymer
Random copolymer
Random copolymer
Nature of polymer
Free radical copolymerization
Free radical copolymerization
Free radical copolymerization
Mode of synthesis
16
16
15
Ref.
VII
VI
V
1V
Ph/~J.
/
II!1
ll\
CH s
O
II
(EtO)2---P N--CH2
O
II
CH2
II
C
S
Hll
Ph/.J.
I
/
\
/
\
~
\
S
C02CH3/n
II
I -
CH 3
S-I-CH2--C--------~S
/
[I
C
S
CH 3
b
\
1 Ph/.
I/ CH3
I
O
II
P--~OEt)2
O
II
N--P~(OEt)2
CH 3
I
CH3 CH2--CH2--N
CH 3
1
N
N - - C - - S I - C H 2 - - C H ] - S - - C - - N CH2--CH2
S
CH 3
q
(EtO)2--P--N--CH2--CH2--N
I
CH 3
Ph/p\
II1\
I
CO2CH3//p \
Linear PS end-capped with P-function
Linear PMMA end-capped with P-function
Multiblock copolymer
Multiblock copolymer
Free radical polymerization using P-containing thermal initiator
Free radical polymerization using P-containing thermal initiator
Through polymeric initiator
Through polymeric initiator
18
18
17
17
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
310
TABLE 2 LOI Values of Various Phosphorus Functionalized P M M A
Series I
Series H
Series I V
Series VI
P M M A blend
polyphosphonamide P% 2"7 5'3 7"6 8-9 10 10"6
LOI
P%
LOI
19 20"5 23 24 25 >25
1"4 2-6 3"5 4"7 5-0 6-5
22 24 25 28 30 36
P%
LOI
P%
LOI
P%
LOI
22 25 27 30
0"25 0'35 0"6 1"05
20 22"5 24 26"5
1 2 3 5
19 20 21-5 23
0'9 1"05 1'8 2"0
block is slightly more efficient in P M M A than in PS. In both these systems the phosphonamide can act as a blockage for the chain unzipping process. Complementary TGA studies suggest that the increased efficiency of the phosphonamide in P M M A is due to the induced decomposition of the polymer by the phosphorus function probably causing a protective char TABLE 3a LOI Values of Various Phosphorus Functionalized PS
Series III
Series V
Series VH
P%
LOI
P%
LOI
P%
LOI
2-6 4'0 5"2 6"5 7'7
22 23 23"5 25 27
0"5 1'0 1"96 2"84 3"1
25 28 34 35 36
0"22 0"27 0"93 1"33 1"48
22 26 29 29 32
TABLE 3b Efficacy Coefficient of the Phosphorus in Various Polymers
Polyrner type
I
II
III
IV
V
VI
VII
PMMA +
poly(phosphonamide) blend Efficacy coefficient
0"85
2 " 6 6 0 " 9 6 5 - 8 6 4.14
5"81 7"30
l'00
Flame and thermal resistances (~f P M M A and PS
311
coating at the onset of inflammation. In these cases, however, the flame retardant efficiency of the phosphorus element is considerably higher than that in the polymers with pendant phosphate groups. Comparison with the results of the physical blend of P M M A and polyphosphomanide (Table 2) shows that the efficiency of the phosphorus is higher when it forms part of the polymer chain. (3) Phosphorus at the chain ends" (compounds VI and VII) P M M A and PS samples end-capped with phosphoramide groups through a dithiocarbamate linkage were synthesized by vinyl polymerization using thiuram disulfide as catalyst. 18 It has previously been reported that P M M A and PS, when end-capped with a phosphorus function (anionically or through chain transfer agents), exhibit exceedingly good flame resistance. 21'21 However, the values reported in Tables 2 and 3a are not as high as expected, but are slightly improved over the LOI values of the multiblock copolymers, with the same phosphorus content. It can be seen that PS is rendered more flame retardant than the P M M A by end phosphorylation. Thermogravimetric analysis has revealed that the dithiocarbamate moieties are stable under thermolytic conditions, 23 but are known to dissociate into radicals under photolytic conditions, z'* The lower (than expected) flame retardancy in these cases may be partially attributed to possible photolytic chain end-scission during inflammation, which may lead to easy chain unzipping. The ALOI values for all types of polymers as a function of their phosphorus content are plotted in Fig. 1, applying the least square method for a first order equation. The slopes of the plots give the apparent flame retardant efficiencies of the particular p h o s p h o r u s function on a comparative scale. The efficacy coefficients, so determined, are presented in Table 3b. It can be seen that they are the highest in the end-functional polymers followed by the block copolymers.
Thermogravimetric analysis The essential features of the thermal degradation of both P M M A and PS are the same, in the sense that both degrade completely to monomer (with oligomers in the case of polystyrene). The thermal behaviour of P M M A is sensitive to molecular characteristics like chain terminal structures, method of" preparation and molecular weight. 25 The presence of 'impurities' in the polymer chains usually decreases their thermal resistance by reducing the chain symmetry and crystallinity which decreases the intermolecular interactions. Exceptions are when the impurities are themselves capable of increasing these by way of dipolar interactions, H-bonding, etc. The
312
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert 20 |
15-
0
10
~
.
~
o
•
_.1 II
× ~
Nf"
5
0
I
I
I
I
I
2
4
6
8
10
P (*/*)
Fig. 1. ALOI versus P% for the systems. A: (+) MMA-DMP; B: (ll) MMA-ADP; C: ([3) styrene-ADP; D: (O) poly(MMA-b-phosphonamide); E: ("k) poly(styrene-b-
phosphonamide); F: (A) polyMMA series VI; G: (0): polystyrene series VII.
influence of the nature and concentration of comonomers in P M M A on its thermal behaviour has been the subject of immense investigation. 3-9 But the role of the phosphorus functions have not been much examined in this respect. In this study the functional polymers quoted in Table 1 were subjected to T G A in N 2 and air and their decomposition patterns were examined with respect to structure. The kinetics of the major decomposition processes were also studied.
Degradation kinetics The kinetic treatment of thermal degradation is mostly based on the relationship, d c / d t = kf(c), where c is the conversion at time t, k is the temperature-dependent rate constant and f(c) a temperature-independent function o f c. In h o m o g e n e o u s kinetics, conversion is defined as concentration and the conversion function is assumed to have the form f ( c ) = ( 1 - c)" where n is the order of the reaction, k is expressed in the Arrhenius form: k = A
e -E/RT
(E = activation energy)
Flame and thermal resistances o f P M M A and PS
3!3
therefore: d c / d t = A e - E / R T ( 1 -- C)"
dc/(1 - c)" = A e E/RT d t integrating:
f" dc/(1
-- c)" = A
To
fT
e E/RT d t
To
For the integration of the right-hand side of this equation, various assumptions and approximations have been used by different authors. Notable among them is that due to Coats and Redfern 26 which gives the final integrated equation as: -ln(I-c)=(RT2/E)[(1-2RT)/E]e
E/RT
forn=l
Plots of --ln(--ln(1 -- c ) / T 2) versus 1 / T give straight lines with slopes equal to E / R . In this study this equation was used to determine the activation energy, assuming first order kinetics. Representative plots are shown in Fig. 2 and the excellent linearity of the points confirms the assumed reaction order. 16.0
165 --
1 5 0 --
uL 14.5 c E ¢z 14C i
13'5
[
13.0 1.70
1.7..5
I 1.80 lIT
Fig. 2.
Coats-Redfern
I 1.85 x 103
p l o t for the d e t e r m i n a t i o n o f
EA
I 190
I 1-95
( h e a t i n g rate 1 0 C / m i n )
(V):
M M A - D M P . Nz, D M P = 80%, 240 300°C; (A): M M A ADP; N z, A D P = 56%, 240~ 32OC; (D): PMMA-Phosphonamide, air, phosphonamide = 6.7%; (Q): styrene-ADP, air, ADP = 49%, 260-300°C; ( . ) : polyADP, air, 220-290°C.
et%)
3-21 5"32 5"84 10'03 10-65 11-65
0"125 0"24 0"27 0"70 0"84 1"00
3.21 5'32 5.84 10.03 10.65 11.65
0-125 0.24 0-27 0.70 0.80 1.00
Composition (Mol.l?action of DMP)
P(%)
Composition (Mol fraction of D MP)
TABLE 4a
0-20 0-25 0-7 12.4 15.5 22.2
% residue at 600°C
TABLE 4b
270-350 230-350 22~350 220-315 220-300 220-290
Step I 355-430 350-420 355-450 320-340 300-330 295-315
Step 11 ---350-450 350-460 320480
Step IH
Temp. range for decomposition (°C)
284 269 241 250 264 254
T l o(°C)
0-0 0-5 0-0 11"4 12"5 12"6
% residue at 600°C
270-350 260-340 220-330 250-300 240-305 220-300
Step I
350-390 --305-315 305 310 300-315
Step II
Temp. range for decomposition (°C)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f Series I in Air
301 271 263 253 256 268
TI0(°C)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f Series I in N 2
100 43 36 137 166 192
Step H
---10 0 0
Step III
163 264 181 149 117 116
Step 1
40 82 115"5
54 --
Step H
EA for the decomposition stages (kJ/mol)
66 79 76 72 70 83
Step I
E Afor the decomposition stages (kJ/mol)
3
Flame and thermal resistances of PMMA and PS
315
Copolymers of M M A with D M P (series I) The synthesis of this series was affected by the copolymerization of diethyl2-(methacryloyloxy)ethyl phosphate (DMP) with MMA. The thermal decomposition characteristic of these copolymers of various compositions are quoted in Tables 4a and 4b, and representative thermograms in N 2 are shown in Figs 3 and 4. It is seen that the decomposition temperature decreases as the phosphorus content increases and that the decomposition pattern changes with composition. A two stage decomposition occurs at low phosphorus content (DMP ~- 10%). The first stage is a combination of DMP-induced and the vinyl end-initiated decomposition. To investigate the contribution of the vinyl end-initiated decomposition, TGA measurements were carried out for three polymers of the same composition, but fractionated to have different molecular weights. The thermograms were essentially superimposable, showing the insignificance of the terminal unsaturation on this step of degradation. (Otherwise the extent of the vinyl end-initiated, first stage decomposition would have increased with decrease in molecular weight.) The rest of the thermogram resembles that of pure PMMA. As the phosphorus content increases, both the first and second stages are shifted to lower temperatures, with the percentage weight 1 0 0 --
8C
\
-~ 6 0
/1
"~ 4 0 .
20-
0 100
I 200
300
Temperature
Fig. 3.
400 (°C)
Z7
500
. . . . . . . .
,
600
TGA of MMA DMP copolymer in air. DMP compositions (mol % ) ( ( - ) 70: ( ....... ) 80; ( - - . ) 100 ( p o l y D M P ) . Heating rate 10 C / r a i n .
) 24,
316
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
loss in the first stage increasing at the expense of that of the second. As the phosphorus content is increased further, a third stage makes its appearance. This stage arises from the residue undergoing a very slow decomposition. The amount of char remaining increases continuously as the phosphorus content increases and the thermograms tend to resemble that of poly(DMP) except for the residue. Figure 4 also includes the TGA of poly(DMP). Its T10 (temperature corresponding to 10% weight loss), which is at 254°C is considerably lower than that of PMMA. The phosphate pendant group cannot be expected to decrease the thermal stability of the P M M A backbone. Hence the apparent lower stability is due to the fact that the decomposition is triggered at the pendant groups themselves, the products of which can give rise to a variety of other structures, which undergo their characteristic decompositions. The crosslinked structures arising from the polymerization of the resulting vinyl bond or the intermolecular anhydride formation may account for the enhanced residue at higher temperatures. Four side group induced reactions are probable: (1) Elimination of phosphoric acid diethyl ester
CH 3 L ~--CH2--C--~ f
CH 3 I ~ - - C H 2 - - C - - ~ + OH--P--(OEt)2 I If °
o//C \O__CH~_CH 2
o
\
/----~ A
CH=CH 2
"O~P_OEt J OEt (2) Elimination of diethyl vinyl phosphate
CH3 ~--CH2--C--~
CH 3 I ~--CH2--C--~ I
C=O,,
dl\
/ CH2 ¢~CH O \ II OIP--OEt I OEt
f, O
+ CH2=CH--P~(OEt)2 I1
o OH
317
Flame and thermal resistances of P M M A and PS
(3) Elimination of ether TH2\
/CH2\ C H 2 \ C C I
I
H+"
/C
o,o I R
C H 2 \ /CH2\ /CH2\ C C I
o
I
C\o/C
+ R--O--R
O
I R
(4) Intra or intermolecular anhydride formation /CH2\ CH2\/CH2\ C C I
I
/ /c\ O
/ CH2 \ /CH2\ 5 H 2 \ C C >
c, O O I
f
I
O
I
+ HOH2C--CH2--P--(OEt)2
H CH 2--CH2--1P--(OEt) 2
fl
O
O CH 3 I
~CH2--C. I
o//C~o__ H 0% / O--H2C - CH 2--P--{OEt)2 II C O CH 3 F J ~CH2--C~ ~CH2--C~ I I CH3 C----O / ,
+ HOHzC--CHz--P--(OEt)2
O
\C=O f
~CH2--C~
11 O
J
CH3 Type (1) reaction is the thermal elimination reaction which is known to occur in organic phosphates. Alkyl acrylates and methacrylates are known to undergo reactions of type (2) and it has been shown that the phosphorus acid is capable of catalyzing the anhydride formation ofe~ter groups (type 3) in methacrylates 1° which accounts for the flame retardant action of the
318
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
phosphorus functional group in PMMA. Type (4) reaction is another possibility in these cases. Since no attempts were made to identify the volatile products in order to confirm the mechanism of decomposition, we postulate the above reactions as the only ones plausible. It is seen from the thermograms that a small amount of phosphorus is not at all advantageous in improving the thermal and flame characteristics, in the sense that thermal decomposition is easily triggered and practically no char residue is left which could otherwise have acted as a thermal protective barrier during inflammation. On the other hand, in air, all the polymers of this series underwent catastrophic degradation around 260°C and there was no appreciable change in the pattern of decomposition except that at high phosphorus content, a plateau region with stable char residue was seen (Fig. 3). The char content in air was lower than that in N 2 showing that thermo-oxidative degradation giving rise to volatile products is considerable. It may be concluded that the flame retardant action is due to the protective char covering, formed in presence of the phosphorus function. To form an appreciable amount of char, a large proportion of phosphorus-containing comonomer is required thus accounting for the reduced flame retardant efficiency of this copolymer system. 100
80
o
".'1
4o
20
o[ lOO
"' , . . . \
I 2oo
i
? Z - ~ 7 7 s 7 .........................
'\~ ..... 300 400 Temperature (*C)
• 500
.
I 600
Fig. 4. T G A of M M A - D M P copolymers in N2. D M P compositions (mol %) ( ) 12.5; (. . . . ) 24; ( ....... ) 70; ( - - . - - ) 80; ( - - - . ) 100 (polyDMP). Heating rate 10°C/min.
Flame and thermal resistances of P M M A and PS
319
The thermal decomposition characteristics of copolymers with various compositions are compiled in Tables 4a and 4b. The activation energy for a particular step of decomposition has a tendency to diminish with increase in phosphorus content for low phosphorus-loaded polymers. Reverse trends were observed when the decomposition steps correspond principally to the polymers themselves. To compare the trend in E A with composition, the decomposition steps falling roughly in the same temperature range were considered for polymers with the same range of composition.
Copolymers of MMA with 2-(Acryloyloxy)ethyl diethyl phosphate (ADP) Polyacrylates have a higher decomposition temperature than polymethacrylates, and the presence of a small amount of acrylate confers a slight increase in stability to P M M A and decreases its tendency to unzip. Polymethyl acrylate and polyethyl acrylate were found to have T10 values of 337°C with E a values of 100 and 102 kJ/mol, respectively, for a single step decomposition. The 2-(acryloyloxy) ethyl diethyl phosphate (ADP) monomer was incorporated into the PMMA chain in order to take advantage of this property as well as the flame retardant property of the phosphate group. However, copolymerization resulted in a decrease in the overall thermal stability. Typical thermograms in N 2 are shown in Fig. 5. The decomposition occurred in two significant stages. The length of the second stage decreased as the ADP content increased. This stage is therefore the decomposition of the P M M A backbone. The first stage must be due to 1
0
O-100 Fig. 5.
0
~
I 200
\
__.
ill,lli21 '
300 400 Temperature (~C)
I 500
_ill -~
600
T G A of M M A A D P copolymers in N 2. A D P compositions (mol %) ( ( ) 2 3 ; ( ....... ) 3 1 ; ( . --)56. Heating rate 10C/rain.
) 14:
TABLE 5a
1.30 3.52 5.27 6.46 9"35 12.30
0.05 0-14 0.23 0.31 0"56 1.00
285 278 290 283 269 248
Tlo(°C)
0"0 0"5 2.6 10.0 23.3 25.0
% residue at 600°C
TABLE 5b
250-340 260-305 240-320 240-320 220-310 200-255
Step I 34~400 320-430 321~350 32(~370 310-330 260-295
Step II
Major degradation steps (°C)
-350-440 370-440 330-400 307-320
Step III 70.8 59 90 67 63"6 81
Step 1
P(%)
1"30 3"52 5-27 6"46 9"35 12"30
Composition (Mol fraction , f ADP)
0-05 0"14 0"23 0"31 0"56 1.00
279 257 266 256 257 241
T ao(°C)
0"0 0"2 1"0 3"0 26-0 25-6
% residue at 600°C
250-330 245-300 240-300 235-315 220-300 220-290
Step 1
330-365 300-370 3130-380 318 340 310-330 300-315
Step II
Major degradation steps (°C)
---350-415 330-400 325-400
Step III
-72 58 -177
Step IH
238 159 155 81 72 48
Step I
37 28 37-5 53-5 150 161"5
Step II
24 ---
---
Step III
E a for the degradation steps (kJ/mol)
114 88 69 79-5 168 56
Step II
the degradation steps (kJ/tool)
E A for
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f M M A with A D P (series II) in Air
P(%)
Composition (Mol.[raction of A DP)
Thermal D e c o m p o s i t i o n Characteristics o f C o p o l y m e r s o f M M A with A D P (series II) in N z
~"
0
Flame and thermal resistances o f P M M A and PS
321
the various side chain-induced reactions. As the A D P content is increased, an intermediate stage makes its appearance at 310-350°C which corresponds to the major decomposition stage of poly(ADP). This step, which increases with increasing A D P content, has an apparent activation energy tending towards that of pure poly(ADP). It is interesting to see the evolution of the third stage becoming slower as the phosphorus content is enhanced. In other words, phosphorus retards the last stage of decomposition and a considerable proportion of char is left. The temperature ranges for the various steps and their energies of activation are cited in Tables 5a and 5b. Since the char residue left is higher than theoretically expected, based on the content of ADP, it is to be presumed that A D P helps the formation of crosslinked structures, which can also account for the flame retardant action. The thermograms ofcopolymers of different composition can be seen in Fig. 5. The char residue at 600°C is plotted as a function of the phosphorus content in Fig. 6. The char content theoretically expected is also shown in the same figure. In air, the initial decomposition temperature (IDT) is considerably lowered. At small phosphorus content the thermal behaviour is similar to that of pure P M M A in that a single stage decomposition occurs (and no residue is left at higher temperature). When the phosphorus content exceeds about 6 ° a second sharp weight loss is seen at 300°C. This step is typical of the poly(ADP) blocks. At this composition the probability of finding a diad of A D P is 0.43. It is interesting to see that char residues 30
20
I"1
-~ 1 5 -
El
6 lo n 5
I 0
02
04 Mol fraction
06 of A D P
08
10
Fig. 6. Dependence of the char yield at 600°C o n the A D P content. (Q): M M A - A D P ~N2); (D): s t y r e n e - A D P (air); (A): M M A - A D P (air): (....... ): theoretical variation.
322
C. P. Reghunadhan Nair, G. Ctouet, Y. Guilbert
increase markedly above this concentration. It may be presumed that blocks of A D P are helpful in causing crosslinked structures. It may therefore be presumed that a critical concentration of A D P is required to achieve flame retardancy through protective char formation. In this case also, IDT decreases across the series. The E A for the major step decreases initially with increasing phosphorus-content but increases and tends towards the value for pure A D P as the A D P content increases further. However, that for the first step remains unaltered. (When referring to the tables for comparison of the decomposition steps and their EAS, the actual temperature range rather than the step number should be taken into consideration.)
Copolymer of styrene with ADP Polystyrene is much more thermally stable than P M M A and incorporation of styrene into P M M A actually improves the thermal characteristics of the latter. 3 As expected, copolymerization with A D P brings down its initial decomposition temperature. The thermograms show three well defined stages of degradation. The first stage, at low temperature, should be triggered from the side chains, whose proportion increases as the content of A D P increases with a corresponding diminution in T1o- The second stage is a small sharp h u m p at 310°C attributable, as in the case of ( M M A - A D P ) copolymer, to the skeletal decomposition of poly(ADP) blocks whose temperature remains unaltered with composition but whose proportion increases marginally as the A D P content increases. The third stage (330-390°C) is reminiscent of the random decomposition of PS. Its shape is tilted. As the phosphorus content increases, this stage of decomposition is facilitated giving rise to a stable char content, i.e. A D P plays some role in the decomposition of the PS backbone. The char residue increases much more than expected (on the basis of A D P content) showing that A D P is helpful in causing crosslinked residual structures. Although E a for the first step remains practically unaltered, that for the second stage tends towards that for poly(ADP) showing the same trend as in the case of M M A - A D P , decreasing in the beginning and then increasing. The EA for the PS backbone decomposition decreases as the phosphorus content increases. Poly(ADP) underwent essentially two stages of decomposition in air and in N 2. The second, due to backbone splitting, occurs around 310°C in N2 and 300°C in air. Representative thermograms for this system are shown in Fig. 7. In air, unlike the case of the M M A - A D P system, all the three stages observed in N 2 were exactly reproduced in the same pattern except that they occurred at a relatively lower temperature. A fourth stage due to the char residue was also to be seen. Unlike the case of DMP-containing PMMA, the char content of high ADP-loaded polymer in air was higher than that in N2.
Hame and thermal resistances of PMMA and PS
323
100
x?.~ 80
Z x
x
-~
/
20
'(
i----
o
I 100
200
I
\
300 400 T e m p e r a t u r e (°C)
"
I 500
I 600
Fig. 7. T G A of s t y r e n e - A D P c o p o l y m e r s in air. A D P c o m p o s i t i o n s (mol %) (......... ) 0 (polystyrene); (. . . . . ) 20; ( ....... ) 43; ( . --) 100 (polyADP). H e a t i n g rate 10°C/rain.
The thermal characteristics of the copolymers are cited in Tables 6a and 6b. The activation energies for the major decomposition stages (I and III) are given in the same table. The trend in air is exactly the same as in N 2 except that the absolute E a is slightly lower. The variation in the residual char at 600°C in air and in N 2 as a function of ADP content can be seen in Fig. 6. The theoretical char content, based solely on the content of ADP, is also shown. The shift between the two curves is a measure of the extent to which ADP causes crosslinked structures (or decrease in the zip length) in the PS backbone. In general, incorporation of ADP energetically facilitates the thermal degradation of copolymers unless the degradation steps are those involving the splitting of the polymer backbone enriched in ADP. This behaviour is graphically represented in Fig. 8 for the different degradation steps.
Poly(MMA-b-phosphonamide) (series IV) These multiblock copolymers were synthesized by free radical means. TGA of copolymers of various compositions (richer in MMA) showed that the polymers undergo a two stage decomposition with a Tlo around 290~C which is higher than that of AIBN initiated PMMA. The shape of the thermograms remains unaltered in N2 and in air. P M M A synthesized in the presence of free radical initiators usually undergoes a two stage decomposition in N 2. The first stage starts around 260~C and is known to be
4-64 5.65 6.41 7.95 8"61 12.30
0"20 0'26 0.3i 0.43 0.49 1.00
296 293 291 267 264 248
Tlo(°C)
5'7 9'3 11.9 17"6 19.7 25
% residue at 600°C
265-325 260-315 260-320 240-315 220-310 200-255
Step I 340-390 315 330 320-335 318-328 318-320 260-295
Step II
Major degradation steps (°C)
-330-410 335 390 350-390 340-390 307-320
Step III 67 72 81 48 60 81
Step I
P(%)
4"64 5-65 6"41 7"10 7"95 8-61 12"30
Composition (Mol fraction o[ A D P)
0"20 0-26 0"31 0"36 0"43 0"49 1"00
267 263 267 267 263 266 241
T 1o(°C)
7-4 15-0 17"7 22"4 20-2 27"7 25"6
% residue at 600°C
245-330 235 310 240-300 240-300 230-310 230-300 220-290
Step I
330-375 310-320 310-320 310-320 310-320 310-320 300-315
Step H
Major degradation steps (°C)
-350-375 350-380 350-375 345-390 340-370 315-400
Step III
-167 114 89 76 177
Step III
84 66 80 72 48 46 48
Step I
76 52 78 85 123 163 162
Step II
-98 125 98 68 59 --
Step IH
E A for the degradation steps (kJ/mol)
168 71 74 102 147 56
Step H
E Afor the degradation steps (kJ/mol)
N 2
TABLE 6b Thermal Decomposition Characteristics of Copolymers of Styrene with A D P (Series III) in Air
P(%)
Composition (Mol fraction of ADP)
TABLE 6a Thermal Decomposition Characteristics of Copolymers of Styrene with A D P (Series III) in
3"
~"
"~
t.~
325
Flame and thermal resistances of PMMA and PS 240
210
18C o
IE 15C
u~ 12C
Iq
9C
A
6C 0
x
I 02
I 0.4 Mol f r a c t i o n
I 06 of ADP
I 0.8
I 10
Fig. 8. Change of Ea with ADP content for different degradation steps. (A) MMA ADP, N 2, 2"a step involving ADP; (x) Styrene-ADP, N 2, 315-330°C involving ADP; (D) Styrene-ADP, N 2, major degradation step 330-390°C; (*) MMA ADP, air, P' stage 220-320°C. triggered by the vinyl end-initiated depolymerization which is not observed in anionically synthesized P M M A and those prepared in the presence of conventional chain transfer agents. In these block copolymers, vinyl ends are not expected to be present. Because of the synthetic method, the P M M A blocks are usually end-capped with dithiocarbamate groups which have been shown not to cause a two stage decomposition to P M M A . 23 Therefore it can be concluded that the first stage is an induced decomposition caused by the phosphonamide blocks. This is confirmed further by the fact that the two stages occur in both air and N 2. Further, the weight loss in the first stage increases linearly with phosphorus content as shown in Fig. 9. Usually the two stages observed in N 2 for radically prepared P M M A merge into a single stage in air, reducing the overall stability of the polymer in air. But this is not observed in the T G A (air) of these copolymers. The first stage is therefore undoubtedly not caused by the vinyl ends and the mechanism is u n k n o w n for the moment. Similar induced decompositions by phosphorus functions are known in oxygen-containing polymers. 1 It is interesting that despite the initial degradative behaviour, the polymer is apparently stabilized in air. It may hence be concluded that the initial degradation leads to some crosslinked structures, augmenting the thermal stability of the residue. This is further reflected in the increased flame retardancy of this series of polymers. It is a well known practice to add crosslinking agents to thermoplastic flame retardant formulations. 1 As the phosphorus content
326
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert 40 35 30
7 v 25 o 20 ..c
10
Fig. 9. Weight loss in the first step as a function of the phosphorus content for the polymer series IV.
I
05
I
I
10
I
1.5
2Ù
I
25
P (°/o)
increases the second stage decomposition is slowed down and shifted to a higher temperature. In Fig. 10 the thermograms (air) of PMMA prepared under different conditions are shown. The AIBN initiated polymer decomposes earlier than that end-capped with a dithiocarbamate group, each showing only single stage decomposition. The vinyl end initiated depolymerization of AIBN initiated PMMA is suppressed by oxygen at low temperature by trapping of the depropagating polymer radicals to form peroxides. However, at higher temperatures the peroxides undergo decomposition generating reactive radicals capable of inducing an easy random chain scission in the polymer backbone. The activation energies 100 .
.
.
.
.
\ 80
Z
~
,
\
4o
20-
.u_~ .........
I
100
200
300
Temperature
400 (°C)
I-.'~."~ 500
j
....
600
Fig. 10. TGA in air of ( ) polyMMA-AIBN initiated; ( - - - ) polyMMA-series VI; (....... ) poly(MMA-phosphonamide) (phosphonamide: 6.7%); ( - - . - - ) poly(MMA-bphosphonamide) (phosphonamide: 16%). Heating rate 10°C/min.
Flame and thermal resistances 0[" PMMA and PS
327
corresponding to the major degradation processes are presented in Table 7. It can be seen that both steps are energetically favoured as the polymer becomes enriched in phosphorus. Apart from the above suggestions, it is difficult to draw definitive conclusions regarding the exact flame retardant action of the phosphonamide from the thermograms. The char residue is not considered to act as a protective coating. The possibility of some vapour phase action from the products of induced decomposition cannot be ruled out.
Poly (styrene-b-phosphonamide) (series V) The T G A of this series of block copolymers shows an entirely different behaviour. The shape of the thermograms remains essentially unaltered, although an increase in thermal stability was observed especially in the later stages. As the phosphorus content is increased, the rate of degradation is lowered with a concomitant decrease in EA. However, the char content was not very high. When the phosphorus content exceeded 2% a small second decomposition stage was observed at 350-370°C, probably due to the self degradation of polyphosphonamide blocks. The thermograms in air showed a slight decrease in IDT, some char was also observed. The thermal characteristics of these systems can be found in Table 8. Figure 11 illustrates the thermogram in air of some of these PS copolymers. The phosphorylation results in an overall increase in thermal stability. In this case a good consistency between the flame and thermal behaviour is noted. The general trend for the E a for random scission of the vinyl block to decrease with phosphonamide content in their block copolymer shows that the phosphonamide is catalyzing the random degradation. The changes in E A are shown in Fig. 12.
End functional polymers It has been observed that end-capping of the P M M A with dithiocarbamate obviates vinyl end-initiated thermal degradation, 23 and the decomposition temperature is quite independent of molecular weight. But when polymers of types VI and VII (Table 1) were analyzed by TGA, the P M M A samples were found to degrade at lower temperatures than their non-phosphorus containing analogues. Similar thermal behaviour for phosphorus endfunctional P M M A has already been reported. 22 In this case, the role of phosphorus in inducing thermal decomposition is not clear. However, for end-functional PS with low phosphorus content, not much difference could be observed in the thermal behaviour compared with that of pure PS. At high phosphorus content, a small weight loss, probably arising
0.82 1.00 2.84 3"10
5.5 6.7 19.0 21.0
288 293 293 290
Air
0"6 0"5 0"5 0-6
N2
0 0 0"5 1"5
Air
Residue at 600°C
270-310 260-320 260~330 265 330
Step I
360-410 350-420 360-420 360-420
Step I1
Major degradation steps (air and N2)(°C)
88 123 138 115
Et
Air
140 114 140 102
EH
390 390 365 360
N2
TIo("C )
353 358 352 354
Air
1-0 1'0 1.7 5"0
N2
Residue at 600°C
2"2 2.5 5-0 8-6
Air
380-430 380-430 350-450* 350-450*
N2
320-400 335-400 32ff410 320~20
Air
Major degradation range (air and Nz)(°C)
172 162 150 109
EH
230 210 196 166
N2
200 159 134 156
Air
E A for the decomposition reaction (kJ/mol)
52 81 94 53
Et
N2
Activation energy (kJ/mol)
and in Air
TABLE 8 Thermal Decomposition Characteristics of Poly(styrene-b-phosphonamide) in N 2 and in Air
301 303 304 290
N2
T l o(° C)
N 2
* A small separate peak due to the self decomposition of the poly(phosphonamide) blocks could be seen at 350-370°C.
P(%)
0"37 0"86 1.00 2"40
P(%)
Weight % of phosphonamide
2"5 5'8 6-7 16"0
phosphonamide
of
Weight %
TABLE 7 Thermal Decomposition Characteristics of Poly(MMA-b-phosphonamide) in
3"
~-
to oo
Flame and thermal resistances of PMMA and PS
329
100,
80
~
I
60
', I
1] 401 I t \
20
L
i
0
100
I
I
200
300
I
400 (°C)
Temperature
Fig. I I.
......
500
I
600
TGA in air o£ poly(styrene-b-phosphonamide). Weight % of ghosphonamide: (
) = 0 (PST), (. . . . . ) = 55, ( . . . . . . . ) = 21.
240
22C
20C
~
D
18C
--×
g ~6c-
12C
10C
I
0
5
I
10 Phosphonamide
15
20
(wt-°/o)
Fig. 12. Influence o f the p h o s p h o n a m i d e content on the activation energy for the random scission of the vinyl block in A: (/~) p o l y ( M M A - b - p h o s p h o n a m i d e ) , air and B: ( x ) in N2; C: ( D ) poly(Styrene-b-phosphonamide), air and D: ( O ) in N 2.
330
C. P. Reghunadhan Nair, G. Clouet, Y. Guilbert
from the phosphorus function, was observed around 200°C. The major decomposition was observed to occur in 2 steps unlike that in pure PS. The thermograms could not furnish any conclusive evidence for the mechanism of flame retardation. End group condensation has been suggested as a reasonable mechanism for flame retardation in this kind of polymer. 21'22
CONCLUSION Evaluation o f flame and thermal resistance of the various types of phosphorus functional P M M A and PS reveals that these properties are dependent on the nature and concentration of the particular function. Phosphorylation causes an easy thermal degradation of P M M A which accounts for its flame retardant action through formation o f a protective char barrier or interference of the degradation products in the vapour phase chain reaction. For polystyrene, only chain end and chain middle phosphorylation confer flame and thermal resistance.
REFERENCES 1. Weil, E. D., Flame-Retardant Polymeric Materials, Plenum Press, Vol. 2, Chap. 4. 2. Chiotis, A., Clouet, G. & Brossas, J., Polym. Bull., 6 (1982) 577. 3. McNeill, I. C., Eur. Polym. J., 4 (1968) 21. 4. Grassie, N., Chemical Reactions of Polymers, ed. E. M. Felles, Interscience, New York, 1964. 5. Grassie, N. & Grant, E. M., Eur. Polym. J., 2 (1966) 225. 6. Grassie, N. & Grant, E. M., Eur. Polym. J., 3 (1967) 619. 7. McNeill, I. C., Straiton, T. & Andersen, P., J. Polym. Sci., Polym. Chem. Ed., 18 (1982) 2085. 8. Camino, G., Costa, L., Devalle, G. & Guaita, M., Poly. Deg. andStab., 4 (1982) 459. 9. McNeill, I. C. & McGuiness, R. C., Poly. Deg. and Stab., 9 (1984) 167. 10. Camino, G., Grassie, N. & McNeill, I. C., J. Polym. Sci., Polym. Chem. Ed., 16 (1978) 95. 11. Brown, C. E., Wilkie, C. A., Smukalla, J., Cody, R. B., Jr & Kinsinger, J. A., J. Polym. Sci., Polym. Chem. Ed., 24 (1986) 1297. 12. Camino, G., Costa, L., Clouet, G., Chiotis, A., Brossas, J., Bert, M. & Guyot, A., Poly. Deg. and Stab., 6 (1984) 105; ibid, 6 (1984) 177. 13. Kallitis, J. & Tsolis, A. K., J. Appl. Polym. Sci., 31 (1986) 635. 14. Clouet, G. & Knipper, M., Poly. Deg. andStab., 17 (1987) 151. 15. Reghunadhan Nair, C. P., Clouet, G. & Brossas, J., J. Polym. Sci., Polym. Chem. Ed., 26, (1988) 1791. 16. Reghunadhan Nair, C. P. & Clouet, G., Eur. Polym. J., 25 (3) (1989) 251. 17. Reghunadhan Nair, C. P. & Clouet, G., Polym., 29 (1988) 1909.
Flame and thermal resistances of PMMA and PS
331
18. Reghunadhan Nair, C. P. & Clouet, G., Makromol. Chem., 190 (1989) 1243. 19. Bauer, R. G., Kavchok, R. W. & O'Connor, J. M., J. Fire Retard. Chem., 3 (1976) 99. 20. Jung, K. A., Jakob, K. H. & Wopker, W., US Pat. 4,585,818 (1986), R6hm Gmbh. 21. Chiotis, A., Ph.D. dissertation, Strasbourg, 1981. 22. Knipper, M., Ph.D. dissertation, Strasbourg, 1985. 23. Reghunadhan Nair, C. P., Clouet, G. & Chaumont, P., J. Polym. Sci., Polvm. Chem. Ed. 27 (1989) 1795. 24. Otsu, T. & Tazaki, T., Polym. Bull., 16 (1986) 277. 25. Kashiwagi, T., Inaba, A., Brown, J. E., Hatada, K., Kitayama, T. & Masuda, E. Macromolecules, 19 (1986) 2160. 26. Coats, A. W. & Redfern, J. P., Nature, 201 (1964) 68.