Thermal degradation studies of cellulose phosphates and cellulose thiophosphates

Die Angewandte Makromolekulare Chemie 141 (1987) 157- 183 (Nr. 2379)

Department of Chemistry, Kurukshetra University, Kurukshetra 1321 19, India

Thermal Degradation Studies of Cellulose Phosphates and Cellulose Thiophosphates

Baljinder Kaur, Ishwar S. Gur, and Hari L. Bhatnagarl

(Received 7 April 1986)

SUMMARY: The thermal degradation of cellulose, cellulose phosphates, and cellulose thio-

phosphates was studied by differential thermal analysis, dynamic thermogravimetry, and derivative thermogravimetry from ambient temperature up to 75OOC. Various thermodynamic parameters for different stages of thermal degradation of cellulose and its derivatives have been obtained following the methods of Broido, and Freeman and Carroll. Infrared spectra of thermally degraded samples were obtained. The data were analyzed in an effort to obtain more information concerning the flame-retardant mechanisms of cellulose phosphates and cellulose thiophosphates. Lower values of decomposition temperatures and activation energies of decomposition and higher char yields of cellulose phosphates as compared to cellulose lead to the conclusion that cellulose phosphorus esters are good flame retardants, and this property i s retained even when these esters were subjected to ion exchange by Na+ and K+ ions. However, with the introduction of sulphur atoms, there was some decrease in this property.

ZUSAMMENFASSUNG: Der thermische Abbau von Cellulose, Cellulosephosphaten und Cellulosethiophos-

phaten wurde mit Hilfe der Differentialthermoanalyse, der Thermogravimetrie und der Differentialthermogravimetrie, ausgehend von der Umgebungstemperatur bis 750 "C, untersucht. Verschiedene thermodynamische Parameter wurden fiir die ver- schiedenen Abbaustufen von Cellulose und ihren Derivaten nach den Methoden von Broido und Freeman und Carroll erhalten. Von den thermisch abgebauten Proben wurden IR-Spektren aufgenommen. Die Daten wurden analysiert, um mehr Informa- tionen fiber den flammhemmenden Mechanismus von Cellulosephosphaten und -thiophosphaten zu erhalten. Kleinere Werte fur die Abbautemperaturen und die Ak- tivierungsenergien des Abbaus sowie hahere Kohleausbeuten ftihren zu dem SchluB, dal3 Cellulosephosphorsilureester gute Flammhemmer sind und da8 diese Eigenschaft auch dann erhalten bleibt, wenn die Polymeren dem Ionenaustausch mit Na+-

* Correspondence author.

0 1987 Hiithig & Wepf Verlag, Basel OOO3-3146/87/$03.00 157

B. Kaur, I. S. Gur, and H. L. Bhatnagar

und Kf -1onen ausgesetzt werden. Durch die Einfuhrung von Schwefelatomen macht sich jedoch eine AbschwPchung der flammhemmenden Wirkung bemerkbar.

Introduction

Cellulose, one of the major textile materials, is intrinsically flammable. Flaming combustion by definition is a gas-phase reaction. In the flaming combustion of cellulose, cellulose first breaks down into smaller volatile fuel compounds, which then volatilize, mix with air, and burn. Laevoglucosan has been seen to be the primary volatile fuel compound'*2 formed during combustion of cellulose. However, cellulose can be rendered flame resistant with the application of proper condensed phase-active flame retardants, which generally alter the mechanism of degradation of cellulose in a way that the formation of laevoglucosan and other flammable volatile gases is minimized and, correspondingly, char formation is increased3. In the present case, cellulose has been treated with various phosphorylating agents to obtain certain flame-retardant compounds. Studies on thermal degradation of these samples have been concentrated on decomposition temperatures, obtained by DTA curves, physical changes such as weight loss, char yield, etc. and thermodynamic parameters such as energy of activation, entropy of activation, free energy of activation, etc., obtained by TG curves.

Experimental

Samples

The following samples of cellulose, cellulose phosphates, and cellulose thiophosphates were selected for the present work.

Sample (i): cellulose (from Schleicher and Schull, Dassel, West Germany) dried to constant weight i. vac. over P4010 at 60 "C; sample (ii): cellulose phosphate, prepared4 by treating cellulose (3.24 g; 0.02 mol of anhydroglucose units) with phosphorus oxychloride (9.20 g; 0.06 mol) in pyridine at 115 "C for 2 h. The product was washed with water, filtered, and dried first in air and then i.vac. over P4010; sample (iii): cellulose ammonium phosphate, prepared by treating cellulose (4.86 g; 0.03 mol of anhydroglucose units) with a mixture of phosporus oxychloride (6.89 g; 0.045 mol) and urea (7.20 g; 0.12 mol) in pyridine at I 1 5 "C for 24 h. The product was filtered, washed with water, dried in air, and then dried i.vac. over P.,Olo; samples (iv-v): cellulose sodium phosphate and cellulose potassium phosphate, prepared by treating 2.0 g of cellulose ammonium phosphate with 1N NaOH or KOH, respectively, for 72 h at room temperature. The product was washed thoroughly with water, filtered,

158

Thermal Degradation of Cellulose Derivatives

and dried i. vac. over P,O,,; samples (vi-ix): cellulose thiophosphate, cellulose ammonium thiophosphate, cellulose sodium thiophosphate, and cellulose potassium thiophosphate, prepared as their corresponding cellulose phosphate samples (ii-v) by using phosphorus thiochloride in place of phosphorus oxychloride.

Differential Thermal Analysis (DTA)

The differential thermal analysis curves were obtained from Bhabha Atomic Research Centre, Bombay. The DTA curves were run under static air atmosphere at a heating rate of 8 "C/min. Calcined alumina was taken as reference material.

Thermogravimetric (TG) and Derivative Thermogravimetric Analyses (DTG)

The TG and DTG thermograms were obtained using a DuPont 951 TG attached to a DuPont 1090 thermal analyzer. The thermograms were run under static air atmosphere at a heating rate of 10"C/min.

Infrared Studies

For IR studies, KBr discs containing 2% charred products of cellulose and its derivatives were prepared and analyzed using a Beckman spectrophotometer IR-20, USA. The charred samples were prepared in a DTA cell. Heating was stopped at the desired temperature and after cooling the residue was quickly transferred to a sample container.

Characterization of the Samples

Cellulose Phosphate

Cellulose on reaction with phosphorus oxychloride5 and subsequent hydrolysis with water forms predominantly cellulose phosphate and chlorodeoxycellulose. The pyridinium complex is also formed in very small amounts (N < 1'70). The reaction products were identified by IR spectral studies. Based on literature reports6.' of the formation of intermediates during the reactions of phosporus oxychloride in pyridine, a proposed mechanism of formation of the above mentioned products is given in Scheme 1.

159


Scheme 1

0

...

0 0 II

H H ...

(11) (111)

C H , 4 - ! - 0 & CH*-O-P-OH

..., &+-+ ...q!p& ..*, o\... L...

H H o H H H o H H Ho H (IV) (VI) ( V l l )

+

Cellulose Ammonium Phosphate

Cellulose on reaction with phosphorus oxychloride and urea in pyridine followed by hydrolysis with water, forms cellulose ammonium phosphate (compound VII) as shown in Scheme 2, (a). This type of compounds has also been observed by Katsuura and Inagaki*. During this reaction cellulose carbamate is also formed as a side product, as evidenced by a peak at 1720 cm-' due to C = O groups in the IR spectrum. Low chlorine content (Tab. I) shows that chlorodeoxycellulose is formed in a very small amount.

Cellulose Sodium and Potassium Phosphates

Cellulose ammonium phosphate on treatment with 1N NaOH and KOH exchanges ions as illustrated in Scheme 2, @).:The ion exchange is evidenced by a decrease in

160


Scheme 2a A N

NH.CONH2 - 111 -NHI COH

(1)

0 0 It CH,+P-OCN It

CI - K.. ... o\ ...

(11) (111) (IV)

0 It e

eO!H. -co2 ... -

H o\ ... H H o H

Scheme 2b 0 0 0

... HO 4...

(VII) (X) (XI) Where A = Na or K

nitrogen content in cellulose sodium and potassium phosphate compared to that of cellulose ammonium phosphate as shown in Tab. 1 . Cellulose carbamate, which is also present along with cellulose ammonium phosphate, gets hydrolyzed under

161


alkaline conditions and the resulting compound then undergoes ion exchange to form compound XI11 (Scheme 2, b). This is evidenced by disappearance of IR peaks at 1720 cm- ' due to C = 0 groups in cellulose sodium and potassium phosphates.

Cellulose Thiophosphates

Cellulose thiophosphate, cellulose ammonium, sodium, and potassium thiophosphates have structures similar to their corresponding cellulose phosphates.

Results and Discussion

The DTA thermograms of samples i-ix were obtained in static air medium and are shown in Fig. 1 - 3. The peak temperatures for various endotherms and exotherms are given in Tab. 1. To support the interpretations involved in the discussion of various processes taking place during thermal degradation of cellulose and its derivatives, these samples were subjected to thermal degradation in air in the temperature range 150 - 400 "C and the IR spectra of the residual products were recorded. The IR spectra of chars of cellulose phosphate, cellulose ammonium phosphate, cellulose sodium phosphate, cellulose thiophosphate, and cellulose ammonium thiophosphate are shown here in Fig. 4 - 8. The spectra of the rest of samples and their chars being similar to those of their corresponding ammonium derivatives, have not been shown.

The DTA curve of pure cellulose shows a small endotherm in the temperature range 70 - 80 "C, followed by two exotherms with their respective peak maxima at 345 and 363°C. The endotherm is due to the loss of sorbed moisture. Shafizadeh and coworkers9-" have shown that thermal degradation of cellulose takes place through two alternative pathways. The first one dominating below 300 "C involves dehydration, rearrangement, formation of carboxyl and carbonyl groups, evolution of carbon monoxide and carbon dioxide, and formation of carbonaceous residue. The second one, which starts at about 300 "C and overshadows the first one at higher temperatures, involves cleavage of glycosyl units by intramolecular transglycosylation and formation of a tarry mixture containing laevoglucosan, other anhydro- sugars, and some randomly linked low molecular weight sugar derivatives, which then evaporate. At about 400°C the char, formed after complete disappearance of glycosyl units, is relatively stable and contains - 30% aliphatic and 70% aromatic components. On further heating, the less stable aliphatic groups are preferentially decomposed through homolytic cleavage

162


Temperature ("C) 100 300 500 700 900

100 1 DTG

100 300 500 700 900

DTG

1oc -

Temperature ("c)

Fig. 1 . Thermal analysis of cellulose in static air.

of C-C and C-H bonds. Combination of some of the resultant free radi- cals give a highly condensed and crosslinked carbonaceous material com- posed mainly of polycyclic aromatic groups. So, the exotherms in the DTA curve represent the above mentioned dehydration, rearrangement, and decomposition reactions. Since the thermal degradation in the present work is carried out in static air medium, it can be mentioned that oxygen in the atmosphere is consumed before combustion is complete and afterwards the remainder decomposes in inert medium. Last exotherm represents crosslinking and aromatization of the char in inert medium.

The DTA curves of cellulose phosphate and cellulose ammonium, sodium, potassium phosphates (samples ii-v) show one endotherm followed by three

163

B. Kaur, 1. S, Gur, and H. L. Bhatnagar

Temperature ("CI 100 300 500 700 900

DTA8

O r . T G Y

Temperature ("C)

Fig. 2. Thermal analysis of cellulose phosphate (- ), cellulose ammonium phosphate ( - - -), cellulose sodium phosphate ( - - -), and cellulose potassium phosphate ( * * ) in static air.

exotherms. The endotherm with peak maxima at 112,98,117 and 117 "C for the respective samples, in addition to the loss of sorbed moisture, represents dephosphorylation, dehydrohalogenation, and acid catalyzed dehydration of cellulose. This is supported by larger weight loss (12% in cellulose phosphate) than that of cellulose (1.5%) up to 180 "C in the TG curves of these samples, and in the IR spectra of their chars at 150 "C (Fig. 4 - 6), showing a decrease in the intensities of bands due to P = 0 and P-0 stretching at 1230 and 830 cm-' (dephosphorylation process), C-C1 stretching at 680 cm-' (dehydrohalogenation process, only in cellulose phosphate), and hydroxyl

164

Thermal Degradation of Cellulme Derivatives

Fig. 3. Therm

Temperature ("CI 100 300 500 700 900

1 . ' " " '

100 300 500 700 900 Temperature ("C)

1 analysis of cellulose thiophosphate (- ), cellulose ammonium thiophosphate ( - - -), cellulose sodium thiophosphate ( - - -), and cellulose potassium thiophosphate ( - * ) in static air.

stretching and bending at 3300, 3400, 1320, and 1380 cm-' (dehydration process). Because chlorodeoxycellulose is formed in very small amounts in cellulose ammonium, sodium and potassium phosphates, the peak at 680 cm-' (C-C1) is not observable in the IR spectra of these samples. The next peak, an exotherm, with maxima for samples ii-v at 256, 326,324, and 330 "C, respectively, represents a decomposition process. The reaction mechanism dominating atJower temperatures (c 300 "C) in thermal degradation of pure cellulose, which involves formation of carboxyl and carbonyl groups, evolution of CO and C02, and formation of carbonaceous residues

165

Tab.

I, Peak te

mpe

ratu

res

in th

e D

TA th

erm

ogra

ms

for

cellu

lose

, cel

lulo

se p

hosp

hate

s, a

nd c

ellu

lose

thio

phos

phat

es in

air.

E

Sam

ple

Com

poun

d El

emen

tal a

naly

sis

Peak

tem

pera

ture

(“C

) N

atur

e of

the

DTA

no

. pe

ak

P c1

N

S in

itiat

ion

max

imum

ter

mi-

(QO)

na

tion

-

-

-

-

-

80

145

endo

, sm

all

i C

ellu

lose

32

6 34

5 35

0 ex

o, la

rge

350

363

421

exo,

larg

e

228

256

270

exo,

sm

all

270

340

604

exo,

larg

e 60

4 80

0 -

exo,

larg

e

ii C

ellu

lose

pho

spha

te 7

.0

8.5

0.8

-

-

112

228

endo

, lar

ge

-

iii

Cel

lulo

se a

mm

on-

9.2

0.9

10.8

-

ium

pho

spha

te

246

336

617 -

iv

Cel

lulo

se so

dium

8.

5 0.

3 5.

2 -

phos

phat

e 26

4 32

7 59

0

phos

phat

e 25

8 34

0

-

V C

ellu

lose

pot

assi

um 8

.3

0.4

4.4

-

98

326

430

830

117

324

362

732

117

330

363

222

336

617

264

327

590

258

340

600 -

-

endo

, la

rge

exo,

sm

all

exo,

larg

e ex

o, la

rge

endo

, lar

ge

exo,

sm

all

exo,

larg

e ex

o, la

rge

endo

, lar

ge

exo,

sm

all

exo,

larg

e

Q

“E D,

=I a x r

600

778

-

exo,

larg

e

Tab.

1.

Con

tinue

d.

Elem

enta

l ana

lysi

s Pe

ak te

mpe

ratu

re (

"C)

Nat

ure

of t

he D

TA

Sam

ple

Com

poun

d no

. pe

ak

P c1

N

S in

itiat

ion

max

imum

ter

mi-

(VO)

natio

n Y

*

z vi

vii

I

viii

ix

Cel

lulo

se

7.6

5.5

thio

phos

phat

e

Cel

lulo

se a

mm

on-

6.4

0.5

ium

thio

phos

phat

e

Cel

lulo

se s

odiu

m

5.8

0.2

thio

phos

phat

e

Cel

lulo

se p

otas

sium

5.6

0.2

thio

phos

phat

e

0.9

7.9

-

220

264

426

260

285

385

809

270

416

556

795

251

392

508

7.5

6.7

-

4.8

6.0

-

3.9

5.8

-

110

248

338

475

I 20

283

345

516

828

103

342

441

608

842

103

330

428

603

220

264

426

620

198

285

385

809

200

416

556

734

212

392

508

780

-

-

endo

, lar

ge

2 ex

o, s

mal

l G

exo,

larg

e 4

exo,

larg

e a % a.

endo

, lar

ge

exo,

sm

all

3

exo,

larg

e ex

o, la

rge

8 z ex

o, s

mal

l en

do, l

arge

m 9

exo,

larg

e ex

o, la

rge

2.

exo,

larg

e 3 ti

exo,

sm

all

endo

, lar

ge

exo,

larg

e ex

o, la

rge

exo,

larg

e

F

P 9


COO0 3000 2000 1500 1000 500 Wove number (ern-')

Fig. 4. IR spectra of (a) cellulose phosphate and (b-e) chars of cellulose phosphate at 150°, 250°, 30O0, and 400°C, respectively.

Fig. 5. IR spectra of (a) cellulose ammonium phosphate and (b-e) chars of cellulose ammonium phosphate at 150°, 250°, 30O0, and 400°C, respectively.

168

Wove number (crn-'1

Fig. 6. IR spectra of (a) cellulose sodium phosphate and (b-e) chars of cellulose sodium phosphate at I S O O , 250°, 30O0, and 400°C, respectively.

I .

LOO0 3000 2000 1500 1000 500 Wove number Icm-'1

Fig. 7. IR spectra of (a) cellulose thiophosphate and (b-e) chars of cellulose thiophosphate at ISO", 250°, 3W0, and 400°C, respectively.

169

B. Kaur, I. S. Gur, andH. L. Bhatnagar

I . . . . I . . . . I . I I I 0 . . I . I . # * . I t ,

1000 3000 2000 1500 1000 500 Wave number (cm-'1

Fig. 8. IR spectra of (a) cellulose ammonium thiophosphate and (b-e) chars of cellulose ammonium thiophosphate at 150 O , 250 O , 300 O , and 400 OC, respectively.

is accelerated here due to the catalytic effect of acid, released in the first step of thermal degradation of cellulose phosphate. Further, the decomposition temperature had considerably lowered down in cellulose phosphate (256 "C) from that of pure cellulose (345 "C). Although rate of combustion and temperature of decomposition are not always related, yet it is usually observed that samples which decompose below 300 "C produce less laevoglucosan and thus lower the availability of fuel for flame propagation. This results in a decreased rate of combustion. However, the decomposition temperatures of samples iii-v are much higher than that of cellulose phosphate, but they are still lower than that of pure cellulose. The next exotherm in samples ii-v with peak maxima at 340,430,362, and 363 "C, respectively, represents crosslinking, aromatization, etc., i. e. reactions in inert medium, due to exhaustion of oxygen during the first step of thermal degradation. The phosphoric acid or its salts, released in the first step, polymerize to form polyphosphates. The last exotherm in the samples ii-v, with peak maxima in the range 732 - 830 "C is probably due to decomposition of these polyphosphates. The interpretations explaining the three exotherms are also supported by IR

170


spectra of chars of these samples (Fig. 4 - 6). At 250 "C, in the case of cellulose phosphate, the bands at 2900 (C-H str), 1490 (C = C and C = N vibrations of pyridine), 1160 (C-0-C str), 1125 and both 1060 and 1035 cm-' (skeletal vibrations involving C-0 str, etc.) show a decrease in intensity, indicating that the decomposition process is taking place. New bands appear at 1700 (C=O), 1100, 900 and 750 cm-'. The band at 1100 cm-' is due to ionic phosphate (Pe4-) and the bands at 900 and 750 cm-' are due to P-0-P str and P-0 str of polyphosphate, respectively. Also, there is a shift from 1630 (conjugated C=C) to 1600 cm-' explaining skeletal rearrangement. At 400°C, all the normal bands due to cellulose disappear completely and intense bands at 1700 (C=O), 1600 (C=C), 1230 (P=O), 1100 (PO4-), 900 (P-0-P), and 750 cm-' (P-0) remain. The IR spectra of cellulose ammonium phosphate show that at 250°C the band at 1720 cm-I becomes less intense, which explains the decomposition of cellulose carbamate. At 300 "C, the band at 1700 cm-' (C = 0) appears, but the intensity of this band is much lower than in case of cellulose phosphate. This indicates that in cellulose ammonium phosphate, carbonyl groups are formed in comparatively less amounts.

But in cellulose sodium and potassium phosphates, fairly intense bands at 1700 (C = 0) and 1600 cm-' (C = C) are observed at high temperatures.

The nature of DTA curves for cellulose thiophosphates (samples vi-ix) is somewhat similar to that of cellulose phosphates except that an additional exotherm is obtained in the former. The peak maxima of the additional exotherm for the former are at 475, 516, 608, and 603 "C, respectively, for the various samples. In cellulose thiophosphates the transformation of structure I into structure 11, as shown in Scheme 3, takes place at low temperatures, and then the mixture of I and I1 degrades at higher temperatures8. This is

Scheme 3

S 0 ...\ow- II ...A q$$- II - HO 0 \. .. o \ ...

HO H O H H H H

supported by the appearance of an IR peak at 1240 cm-' (P = 0), in addition to the band at 780 cm-' (P=S) in the chars of cellulose thiophosphate

( I 1 (11)

171


samples at 250 "C (Fig. 7 - 8). Hence, in the DTA curves of samples vi-ix, the endotherms represent dethiophosphorylation, dehydrohalogenation, and acid catalyzed dehydration reactions, and the three exotherms denote decomposition reactions of structures I and I1 (Scheme 3) leading to char formation, and crosslinking of char in air and inert medium, respectively. The last exotherm with maxima at 828 and 842°C in cellulose ammonium and sodium thiophosphate, respectively, represents decomposition of polythiophosphate. However, this peak is missing in cellulose thiophosphate and cellulose potassium thiophosphate, representing that decomposition of polythiophosphate hethese cases is taking place at higher temperature. These interpretations are supported by IR spectra of chars of samples vi-ix. The changes being similar to those in cellulose phosphates, are not discussed here. It is clear from Tab. 1 that the decomposition temperatures of cellulose thiophosphate (248 "C) and cellulose ammonium thiophosphate (283 "C) are considerably lower than that of cellulose (345 "C), but those of cellulose sodium and potassium thiophosphate (342 and 330 "C) are not much less.

Further confirmation of these interpretations is obtained from the thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses of samples i-ix (Fig. 1 - 3). The TG curves of cellulose phosphates and cellulose thiophosphates show three significant areas of weight loss which have been discussed here in terms of three stages of thermal degradation. The first stage occurring below 200"C, is due to the loss of sorbed water from the samples, dephosphorylation, dehydrohalogenation, and acid catalyzed dehydration processes and corresponds to the endotherm in DTA curves. The second, which involves major weight loss, occurs in the temperature range 200 - 360 "C and represents the decomposition process as discussed earlier. This stage corresponds to the first exotherm in the DTA curves. And the last stage involving smooth weight loss in the temperature range 360 - 580 "C and corresponding to the second (second and third in cellulose thiophosphates) exotherm in DTA thermograms, represents crosslinking and aromatization of char in inert medium. From TG curves, the kinetic parameters for these stages of thermal degradation were determined using the method described by Broido'' and Freeman and C a r r ~ l l ' ~ . The equation involved in Broido method is

and that employed in Freeman and Carroll method is

172


= --.- E, A(1/T) Alog W 2.303 R Alog W '

the rate of decomp~sition'~ given by

Z - R --exp dW dT T - R H

--- (3)

where y is the fraction of number of initial molecules not yet decomposed, T, is the temperature of maximum reaction velocity, W is the weight fraction of the material undergoing degradation at time t, RH is the rate of heating ("C/min) and Z is the frequency factor.

1 From the slopes of TG curves in Fig. 1 - 3, plots of In In - vs. r for

various stages of thermal degradation were drawn. Linear plots were obtained in each case. As an illustration, plots for the second stage of thermal degradation of samples i-ix are shown in Fig. 9. The activation energies, E,, and the frequency factors, Z, determined from the slopes and intercepts of these plots, respectively, are given in Tab. 2-4. Using Freeman and Carroll method, plots of A log RT/A logW vs. A(l/T)/AlogW for the second stage of thermal degradation of these samples were drawn and are given in Fig. 10. The activation energies, E,, and the order of reaction, n, from the slopes and intercepts of these plots, respectively, are given in Tab. 3. These parameters were evaluated using least squares method. Using transition state the01-y'~ other thermodynamic parameters such as entropy of activation and free energy of activation were calculated and are given in Tab. 5 . The char yields (in wt.-07o) for these samples at 975 K are given in Tab. 5 .

( :>

First Stage of Thermal Degradation

Tab. 2 presents the activation energies and frequency factors for the first stage of thermal degradation. In cellulose the small weight loss of I .5% up to 120"C, due to the loss of sorbed moisture, was neglected. In cellulose phosphate and cellulose ammonium, sodium, potassium phosphate, weight loss of 8.6, 8.2, 9.3, and 8.5070, respectively, occurs in the temperature range of 80 - 180°C with the respective activation energies of 32.5, 29.8, 32.7 and 31.9 kJ mol- ' , primarily due to dephosphorylation, dehydrohalogenation and acid catalyzed dehydration. In cellulose thiophosphate also 10.4%

173

B. Kaur, I. S. Cur, and H. L. Bhatnagar

1.6

0.8

- 0

5 -0.8

x \ - - C -

-1.6

-2.L

0.8

0

-0.8

-3.2

4 . 0

4 . 8 1.62 1.70 1.78 1.86 1.9L

lo3 .T-' (K-')

Fig. 9. Plotsofln In- vs. Id *T-' using Broido Eq. for (a) cellulose (0),

cellulose phosphate (A), cellulose ammonium phosphate (a), cellulose sodium phosphate ( o) , and cellulose potassium phosphate (A); (b) cellulose thiophosphate (A), cellulose ammonium thiophosphate (OJ, cellulose sodium thiophosphate (.), and cellulose potassium thiophosphate (A) for second stage of thermal degradation in static air.

( 3

weight is lost up to 180 "C with an activation energy of 30.4 kJ mol-' . But in cellulose ammonium, sodium, potassium thiophosphates only 2 - 4% weight is lost up to 120°C, representing loss of sorbed water, and no more weight

174


Tab. 2. Activation energies, I&, and frequency factors, Z, using Broido method for the first stage of thermal degradation of cellulose, cellulose phosphates, and cellulose thiophosphates in air.

Sample Compound Temperature E, (kJ mol-l) Z (s-') no. range ("C)

i ii iii

iv

V

vi vii

viii

ix

~~~

Cellulose - Cellulose phosphate 80 - 180 Cellulose ammonium 70 - 180 phosphate

phosphate Cellulose potassium 80 - 180 phosphate Cellulose thiophosphate 80 - 170 Cellulose ammonium - thiophosphate Cellulose sodium - thiophosphate Cellulose potassium - thiophosphate

Cellulose sodium 70 - 180

- 32.5 29.8

1.25 * 16 5.32 * Id

32.7

31.9

30.4 -

1.25 * 16

9.68 * 10'

6.99 * Id -

loss occurs up to 180 "C. This suggests that dethiophosphorylation in these samples is taking place at higher temperatures.

Second Stage of Thermal Degradation

The major weight loss in TG curves of samples i-ix, which is mainly due to decomposition process, takes place in two steps. In cellulose 4.1 '7'0 weight is lost in the temperature range of 120 - 300"C, representing a dehydration process, and the major weight loss commences at 300"C, which increases rapidly to 78.3% at 350°C (an overall 72.7% in the temperature range 300 - 350 "C), representing a decomposition process leading to char and tar formation. In cellulose phosphate and cellulose thiophosphate (samples ii, vi), there is initially sharp weight loss of 13.5 (240 - 270°C) and 10.8% (230 - 260 "C), respectively, followed by gradual weight loss of 16.5 (270 - 350°C) and 14.0% (260 - 320 "C). But in cellulose ammonium, sodium, and potassium phosphates and cellulose ammonium, sodium, and

175

CL

2 Ta

b. 3.

Act

ivat

ion

ener

gies

, E$,

and

freq

uenc

y fa

ctor

s, Z

, usi

ng d

iffer

ent m

etho

ds fo

r the

sec

ond

stag

e of t

herm

al d

egra

datio

n of

ce

llulo

se, c

ellu

lose

pho

spha

tes,

and

cel

lulo

se th

ioph

osph

ates

in

air.

Sam

ple

Com

poun

d no

. D

TG

Tem

pera

ture

E,

(kJ m

ol-')

z (

s-1)

n

max

ima

rang

e ("

C)

("C

) B

roid

o Fr

eem

an

Boi

do

Free

man

m

etho

d an

d m

etho

d an

d C

arro

ll C

arro

ll m

etho

d m

etho

d

i ii iii

iv

V vi

vii

viii

ix

Cel

lulo

se

-

334

Cel

lulo

se p

hosp

hate

262

300

Cel

lulo

se a

mm

oniu

m

-

phos

phat

e 325

Cel

lulo

se so

dium

pho

spha

te -

328

Cel

lulo

se p

otas

sium

-

phos

phat

e 325

Cel

lulo

se th

ioph

osph

ate

254

300

Cel

lulo

se a

mm

oniu

m

-

thio

phos

phat

e 257

Cel

lulo

se so

dium

-

thio

phos

phat

e 335

Cel

lulo

se p

otas

sium

-

thio

phos

phat

e 332

250 - 3

00

300 -

350

240 - 270

285 - 350

250 - 300

300 - 340

250 - 305

305 - 350

250 - 295

300 - 360

230 - 260

260 - 320

200 - 240

240 - 270

270-315

. 315 -

360

270 - 310

310 - 360

124.9

-

l.o6

*los

-

-

252.9

261.6

7.14 *

2.95 *

102'

1.10

197.0

1%.7

1.23-ld7

4.35*1d8

0.96

100.7

-

7.47*1(P

-

-

88.7

-

3.08.16

-

164.7

149.7

4.58 * 10"

4.47 10''

0.95

91.8

-

4.43.16

-

-

155.7

151.2

5.44.10''

5.66.10''

1.08

90.6

-

2.64.16

-

-

162.9

155.1

2.30 - 10''

1.01 -

0.82

215.7

217.1

1.53 *

1f19

8.77 * 102'

1.03

160.5

153.1

5.31 * 10"

3.12.

1.17

62.8

-

3.11 *id

-

-

213.4

216.6

1.50.10'9

9.70.102'

0.99

88.0

-

1.39.16

-

-

224.5

217.8

3.78 * 10"

3.52 * 10"

1.10

91.6

-

3.98.16

-

-

222.4

215.6

3.52. Id7

2.98 *

10'' 1.16

-

m P E P 3 a

r


Tab. 4. Activation energies, &, and frequency factors, Z, using Broido method for third stage of thermal degradation of cellulose, cellulose phosphates, and cellulose thiophosphates in air.

Sample Compound no.

Temperature & (kJ mol-') Z (s-') range ("C)

i ii iii

iv

V

vi vii

viii

ix

Cellulose Cellulose phosphate Cellulose ammonium phosphate Cellulose sodium phosphate Cellulose potassium phosphate Cellulose thiophosphate Cellulose ammonium thiophosphate Cellulose sodium thiophosphate Cellulose potassium thiophosphate

390 - 500 420 - 540 430 - 580

430 - 580

430 - 560

420 - 560 410 - 530

420 - 500

410 - 490

89.2 57.8 56.2

47.6

53.4

68.9 83.1

83.6

90.2

8.17 * Id 2.33 * 10' 1.59*10'

3.54

1.09.10'

1.23*1@ 1.43 - Id

3.32 * Id

1.26 * Id

potassium thiophosphates (samples iii-v, vii-ix), there is initially slow weight loss of 4.9,4.7,3.2,3.6,7.0, and 7.0% (250 - 300 "C), respectively, followed by a sharp weight loss of 18.5, 19.5, 22.8, 30.0, 35.7, and 35.8% (300 - 360 "C). The difference in this behaviour is due to the release of phosphoric acid and thiophosphoric acid by cellulose phosphate and cellulose thiophosphate, respectively, during the dephosphorylation step, which then catalyze dehydration and decomposition processes. But in cellulose ammonium, sodium, potassium phosphates and thiophosphates, salts of the respective acids are produced. These salts then combine with the water released due to dehydration, resulting in diminution of .the acidic character. Therefore, acid catalyzed reactions in these compounds would be minimized. Derivative thermogravimetric plots also indicate that the maximum weight loss in samples ii-ix occurs at 262, 325, 328, 325, 254, 257, 335, and 332 "C, respectively (Tab. 3). In cellulose phosphate and cellulose thiophosphate, there is an additional DTG maximum at 300 "C, representing a gradual weight loss following the sharp one.

177

B. Kaur, I. S. Cur, and H. L. Bhatnagar

1O3.AT-'/Alog W (K-') 0 0.8 1.6 2.1 3.2 1.0 1.8 5.6 6.1

2.0

-6.0 3 cn - 0 -11.0 a \

a' -22.0 cn 0 -

-30.0 a

-38.0

-2.0

3 -10.0 cn 0

\ z -18.0

n' cn -26.0 z 0

-31.0

-12.0 0 0.8 1.6 2.4 3.2 1.0 1.8 5.6 6.1

103*AT-'/Alog W (K-')

Fig. 10. Plots of Alog R,/Alog W vs. (Id - A T-*/Alog W) using Freeman and Carroll Eq. for (a) cellulose (O), cellulose phosphate (A), cellulose ammonium phosphate (O), cellulose sodium phosphate ( .), and cellulose potassium phosphate (A); (b) cellulose thiophosphate (A), cellulose ammonium thiosphosphate (a), cellulose sodium thiophosphate ( ), and cellulose potassium thiophosphate (A) for second stage of thermal degradation in static air.

Tab. 3 gives the activation energies, the frequency factors and the order of the reaction for the second stage of thermal degradation. The values of the reaction order (Freeman and Carroll method)I3 were found to be about unity in all cases. In the case of cellulose phosphate the initial sharp and then gradual weight loss occurs with the respective energies of activation of 197.0 and 100.7 kJ mol-I. In the case of cellulose ammonium, sodium, and potassium phosphate the initial small and then sharp weight loss occurs with

178


activation energies ranging from 88 - 92 and 155 - 164 kJ mol-', respectively. The values determined by the method of Freeman and Carr011'~ are also in the same range. These values are much less than those of pure cellulose, where the two steps proceed with 124.9 and 252.9 kJ mol-' as respective activation energies. The values for the two steps of decomposition of cellulose thiophosphate and cellulose ammonium, sodium, potassium thiophosphate are 215.7, 62.8, 88.0, 91.6, and 160.5, 213.4, 224.5, and 222.4 kJ mol-I, respectively. These data show that the activation energies for the decomposition step of cellulose thiophosphates (213 - 225 kJ mol-') are higher than those of cellulose phosphates (155 - 197 kJ mol-I), in fact they are fairly close to that of pure cellulose (252.9 kJ mol-I).

Third Stage of Thermal Degradation

For samples i-ix, there is a gradual loss of weight in the temperature range 390 - 580 "C due to crosslinking and aromatization of the char in inert medium. Tab. 4 shows that the activation energies for these samples lie in the range 47 -91 kJ mol-'.

Tab. 5 lists the entropies and free energies of activation of samples i-ix, calculated at the average peak temperature of DTG curves at 580 K. The data show that there is a considerable variation in the values of entropy of activation, which becomes negative in some cases. But this variation is compensated by increased values of enthalpy of activation leading to almost similar values of free energy of activation (146-178 kJ mol-'). This is a very significant thing as it points out that the basic mechanism of thermal degradation of all the samples is the same. Tab. 5 also lists the char yields (in wt.-Yo) of the samples i-ix obtained from the TG curves at 975 K.

From DTA, DTG curves, and thermodynamic parameters calculated from the TG curves, it is evident that in cellulose phosphate there is (i) lowering of the decomposition temperature, (ii) lowering of the temperature at which maximum weight loss occurs, (iii) decrease in activation energy for decomposition, and (iv) increase in char yield as compared to that of pure cellulose. Therefore, it can be considered a good flame retardant, as the main role3 of a flame retardant is to lower the decomposition temperature, suppress the production of combustible volatiles, and spread the evolution of these materials over a wider temperature range, resulting in formation of higher amounts of char. In cellulose ammonium phosphate although the decomposition temperature is high, yet it can be seen from its low activation energy for decomposition and high char yield that it is still a good flame retardant.

179

Tab.

5.

Entro

pies

, A S

*, an

d fr

ee en

ergi

es, A

G*,

of a

ctiv

atio

n fo

r the

sec

ond

stag

e of t

herm

al d

egra

datio

n of

cel

lulo

se, c

ellu

lose

ph

osph

ates

, and

cel

lulo

se th

ioph

osph

ates

in a

ir.

Sam

ple No.

Com

poun

d A

S*

at 5

80 K (J

-K-'

.m

ol-')

Bro

ido

Free

man

and

B

roid

o Fr

eem

an a

nd

(wt.-

Vo)

met

hod

Car

roll

met

hod

met

hod

Car

roll

met

hod

A G* at

580

K (k

J.m

o1-')

C

har

yiel

d at

97

5 K

i ii iii

iv

V vi

vii

viii

ix

Cel

lulo

se

Cel

lulo

se p

hosp

hate

C

ellu

lose

amm

oniu

m

phos

phat

e C

ellu

lose

sodi

um p

hosp

hate

C

ellu

lose

pot

assi

um

phos

phat

e C

ellu

lose

thio

phos

phat

e C

ellu

lose

am

mon

ium

th

ioph

osph

ate

Cel

lulo

se so

dium

th

ioph

osph

ate

Cel

lulo

se p

otas

sium

th

ioph

osph

ate

121.

3 68

.4

- 16

.3

- 34.

2 - 22

.1

108.

5 10

8.4

77.7

77.2

171.

4 98

.1

- 1

6.6

- 1

4.6

- 9

.8

142.

2 14

3.1

%.3

94.9

177.

8 15

2.5

169.

4

170.

7 17

0.9

147.

2 14

5.8

174.

6

172.

8

157.

3 13

5.1

154.

5

154.

8 15

6.1

129.

8 12

8.9

157.

2

155.

8

10.0

35

.0

44.8

41.5

41

.5

35.0

14

.8

24.0

22.0

m e

F

W a

a 3 r


And this property is retained even when ammonium ions of cellulose ammonium phosphate are exchanged with sodium and potassium ions. Results for cellulose thiophosphates show that their decomposition temperatures are lower than those of cellulose, and are comparable to their corresponding phosphates, but the decomposition activation energies are higher and the char yields lower. Therefore, they do not seem to be so effective flame retardants as phosphates tend to be.

Mechanisms of Flame Retardancy for Cellulose Phosphates and Thiophosphates

The generally accepted mechanism for phosphorus flame retardancy is considered to occur primarily in the condensed phase. The phosphoric acid, thermally generated in the first step of thermal degradation of cellulose phosphate, accelerates dehydroxylation of cellulose and formation of conjugated double bonds in the chains8. Since the carbonized chains are thermally stable, the evolution of flammable gases is retarded and cellulose becomes self extinguishing. Phosphoric acid on heating gets polymerized to form polyphosphate which is seen to be a more effective catalyst for dehydroxylation reactions. The mechanism has been discussed in detail in our previous papers16-18. It is assumed that polyphosphoric acid forms a glassy film on the surface of the matrix. This film acts as a shield between the matrix and oxygen and hence flame propagation is stopped. Since cellulose phosphate also contains chlorodeoxycellulose, so chlorine also affects flame propagation. Halogens are believed to retard the flame by acting as free- radical scavangers in the gas phase.

The basic steps of thermal degradation of cellulose ammonium, sodium and potassium phosphates would be the same as those of cellulose phosphate, as dephosphorylation in these cases produces ammonium, sodium and potassium phosphates instead of phosphoric acid obtained in the case of cellulose phosphate. Ammonium phosphate on heating decomposes to ammonia and phosphoric acid which then catalyzes dehydration and decomposition reactions of cellulose, as in the case of cellulose phosphate. The NH, facilitates decomposition by reacting with intermediate carbonyl compounds to form glycosylamines or Schiff bases that are readily dehydrated by a f3-elimination p r o c e ~ s ~ ~ ~ ' ~ . However, in cellulose sodium and potassium phosphates, the cations Na+ and K', being neutral, do not catalyze any reaction, so the flame retardant property is weakened in these cases as compared to that of cellulose ammonium phosphates.

181


The thermal degradation of cellulose thiophosphates starts with the S II I

generation of thiophosphoric acid (HO-P-OH) and HCl; thiophosphoric

OH acid gets polymerized on heating, to form polythiophosphate of the form:

The catalytic effect of polythiophosphoric acid remains similar to that of polyphosphoric acid. As shown in Scheme 3, during the initial stages of thermal degradation, cellulose thiophosphate gets transformed into structure I1 and the mixture of these two then degrades at higher temperatures. The introduction of sulphur atoms (structure 11) retards the formation of conjugated double bonds by p-elimination of phosphorus ester groups2', explaining the decrease in flame retardant properties of cellulose thiophosphates.

One of the authors (B. K.) is thankful to the Council of Scientific and Industrial Research, New Delhi, for providing a research fellowship.

S. L. Madorsky, V. E. Hart, S. Straus, J. Res. Natl. Bur. Stand 56 (1956) 343 D. Gardiner, J. Chem. SOC. C, 1966, 1473 R. M. Perkins, G. L. Drake, Jr., W. A. Reeves, J. Appl. Polym. Sci. 10 (1966) 1041 J. D. Reid, L. W. Mazzeno, Jr., Ind. Eng. Chem. 41 (1949) 2828 S. H. Zeronian, S. Adams, K. Alger, A. E. Lipska, J. Appl. Polym. Sci. 25 (1980) 519 H. H. Bosshard, H. Zollinger, Helv. Chim. Acta 42 (1959) 1659 G. J. Martin, S. Poignant, M. L. Filleux, M. T. Quemeneur, Tetrahedron Lett. 58 (1970) 5061

* K. Katsuura, N. Inagaki, J. Appl. Polym. Sci. 22 (1978) 679 F. Shafizadeh, A. G. W. Bradbury, J. Appl. Polym. Sci. 23 (1979) 1431 F. Shafizadeh, J. Anal. Appl. Pyrolysis 3 (1982) 283

1 a2


I 1 Y. Sekiguchi, F. Shafizadeh, J. Appl. Polym. Sci. 29 (1984) 1267 A. Broido, J. Polym. Sci., Part A-2 7 (1969) 1761

l 3 E. S. Freeman, B. Carroll, J. Phys. Chem. 62 (1958) 394 I 4 L. Reich, S. Stivala, Elements of Polymer Degradation, McGraw-Hill, New York

1971, p. 102 Is K. J. Laidler, Chemical Kinetics, Tata McGraw-Hill, New Delhi 1976, p. 86 I6 R. K. Jain, K. Lal, H. L. Bhatnagar, J. Appl. Polym. Sci. 30 (1985) 897

B. Kaur, I. S. Gur, H. L. Bhatnagar, J. Appl. Polym. Sci., in press Is B. Kaur, R. K. Jain, I. S. Gur, H. L. Bhatnagar, H.-R. Schulten, J. Anal. Appl.

Pyrolysis, in press I 9 F. Shafizadeh, G. D. McGinnis, R. A. Susott, M. H. Meshreki, Carbohydr. Res.

33 (1974) 191 K. Katsuura, N. Inagaki, Text. Res. J. 45 (1975) 103

183

Documents

Thermal degradation studies of cellulose phosphates and cellulose thiophosphates