Indian Journal o f Chemi stry Vol. 44A. October 2005. pp. 2024-2029

Thermal behaviour and spectral studies on cotton cellulose modified with phosphorus, sulphur and metals

J B Dahiya' & Sushila Rana

Department of Che mi stry , Guru Jambheshwar Uni versi ty, Hisar 125 001 , India, Email: j bdic @yahoo.com

Received 25 April 2005: revised 28 July 2005

The potential !lame re tardancy o f cellulose benzyithiophosphate (C BTP) and its metal co mplexes has been investi gated by their the rmal degradation behaviour using TG and DSC techniques from ambient temperatu re to 600°C in air and nitrogen atmosphere. From the resulting data of TG curve using Broido method, the acti vatio n energies of CBT P and its metal complexes for deco mpositi on stage arc found to be in the range 3 1-45 kJ mo l- 1 and arc lower than that o f ce ll ulose ( 165.85 kJ mol-1

) in air atmosphere. Modified cellulose sampl es, when heated, give ri se to high char yiel ds. The values o f M I have also been fo und reduced and spread in wide range of temperature signi ficantl y by in troducing phosphonJs. sulphur and metals in cellul ose. The FriR spectra of chars of modified cellulose indicate tl1at dehydrati on takes place and co mpounds containing C=O. C=C, P-S-P and P=O groups arc formed . These and re lated observations suggG 1 tha t uch dcrivati sa ti on may gi ve ri se to novelll arne retardant treatments for ce ll ulosic materials .

IPC Code: Int. C l7 C08Ll /08; C09 K2 1/00; GOI N

Cotton textiles play a central ro le in modern life but concern about the contribution of this material to fire hazard s 1-

2 re mains an obstacle to their wider use. Many available fire retardant additives, typically based on chlorine, bromine, antimony, or vanadium are regarded as environmentally unacceptable3

.

Polymers intrinsica lly capable of producing large quantiti es o f carbonaceous char offer a route to fire retardant syste ms without these drawbacks. There is a strong corre lati on between char yie ld and fire resistance for not onl y does char formation occur at the expense of volatile combustible gases, but char acts as a barrier, sea ling the polymer against oxygen. Ou r ea rl ier study4-7 has shown that the inherent fl ammabi lity of any po lymer reduces as its char­fo rming tendency increases.

Cellul ose benzylthi ophosphate (CBTP) and its meta l compl exes have been studied for their internal degradative be hav iour and hence potential flame retardancy. CBTP compounds have been subjected to thermal degradati on from ambient temperature to 600°C using TG and DSC techniques. The chars of CBTP compounds have also been in vestigated by FI'U~ spectroscopy.

Materials and Methods

T he co mpound s studi ed duri ng the present stud ies arc: ce ll ulose (CD H Ind ia) (1), cellulose

benzylthiophosphate (CBTP) (II), Cr(III), Fe(ll) and C u(ll) complexes (111)-(V) of CBT P.

Benzyldichl orothiophosphate was prepared by dropwise addition of 1.0 mol ( 103.8 mL) be nzy l a lcohol to the solution o f 1.0 mol (10 l.S mL) PSCI3

and was refl uxed at atmosphe ric pressure with ag itation in presence of 0 .500 g of anhydrous MgCh plus 0.500 g of anhydrous Cu 2C b cata lyst. Benzyldichlorothiophosphate was d istilled and

collected at 167- l72°C/20 mm Hg. CBTP was prepared by treati ng 8. I 00 g ce llul ose

(0 .05 mol of an anhydrog lucose unit o f cellul ose) in 100 mL pyridine w ith 21.6 mL (0 . 15 mol)

benzyldichlorothiophosphate at 90°C for 6 h with constant stirring. The product obtained was filt ered, rinsed with pyridi ne, washed thoroughly wi th water and dried over P20 5 in vacuo.

Compounds (111)-(V) were prepared by treat ing 2-3 g of CBTP with 5% aqueous soluti on of chromium(III) sulphate , iron(Il) sulphate heptahydrate and copper(II) sulphate pentahydra te at room temperature for 72 h with stin·ing. Each product was fi ltered, washed with excess warm water unti l fi ltrate was free from matal sal t and dried in vacuo ove r P20 5.

Elemental analysis E lementa l analyses of phosphorus by

co lori metrically, chiorine and sul phur by gravi metrically and nitrogen by Kjeldahl method were

DAHIYA & RANA THERMAL BEHAVIOUR A D SPECTRAL STUDIES 0 MODIFIED COTTON CEL L LOSE 2025

Tahl e I - Analytical data and descripti on of DSC curves of cellu lose, CBTP and metal complexes

No. Compound UV visible peak/(nrn) Temperature/° C Nature of peak D. /-1/(Jg- 1)

Initi atio n Maximum Final ( I) Ce ll ulose 237

272 3 10

(II ) Ce ll ulose ben zylthi o- 265 phosphate (CBTP) 275 IP'if=3 18. S%=2. 1) 285

305 ( III ) Cr(l ll ) complex ofC BTP 470*

IP'if=3. 18. S'7n=2. 1. 630* Cr'7r=6 99)

(IV ) Fe(ll ) complex oiTBTP 485* (P'if=3. 18. S%=2. 1. Fe%=4.28)

(VJ Cu(ll ) complex ofCBTP 630* (P%=3. 18. S%=2 .1, Cu 'if=3. 10)

*Peak in addition to CBTP

carried out. The transition metal (Cr, Fe and Cu) anal ys is was carried out by flame-atomic absorption spectrophotometry using Analytik Jena make Vario 6 spectrophotometer. The results are given in Table 1.

V-visible spectral analysis

Reflectance UV -visible spectra of cellulose, CBTP and metal compl exes of CBP were recorded usmg mode l 330, UV-S- IR, Hitachi , Japan .

Thermal analysis

DSC and TG thermograms of all compounds were carried out using Rheometric DSP-SP, TGA-1500 thermal analyzer. The DSC and TG thermograms were recorded in static air and nitrogen from ambient temperature tO 6QQ°C at a heating rate Of l0°C min- 1

•

FTIR spectroscopy

FTIR spectra of compounds and chars were recorded b:' the KBr pellet technique usmg a Shimadzu FTIR-8001 PC, Kyoto, Japan.

Char-acterization of compounds

When ce llulose was reacted with benzyldichlorothiophosphate, CBTP and chlorodeoxyce llulose we re formed . Owing to the method e mpl oyed, the pyridinium complex was also formed in very small amount ( ===1 %)8 CBTP was charac teri zed by its FTTR spectrum and metal comp lexes of the CBTP by their re fl ec tance UV -vi s spectra.

110 160 210 En do 275 3 15 332 En do 332 345 420 Exo 420 482 520 Exo

7.65 101 660 1025 .20 39.30 52.83 15.22

110 236 260 En do 260 292 360 Exo 360 438 495 Exo

11 5 23 1 256 En do 256 289 308 Exo

17 l.-+2 13.82 2008 30 .86 82.62 93.82 24.28 72.82 15320 28 .32 20.2 1

308 320 335 Exo 335 348 445 Exo 137 2 14 225 En do 225 253 275 Exo 275 28 1 289 Exo 289 297 425 Exo 124 220 240 En do 240 276 335 Exo 335 349 443 Exo

The Cr(III) complex of the CBTP showed two absorption bands around 630 and 470 nm which may be due to 4A2g(F) ~ 4T2g(v 1) and 4A2g(F) ~ 4

T1gCv2) transitions, respectively , suggesting an octahedral structure of the complex. The Fe(Il) complex showed one absorption band at 485 nm which may be due to the spin-allowed transition 5T2g ~ 5Eg for octahedral complex. The Cu(II) complex showed a sharp band at 630 nm due to 2Eg ~ 2T2g. This band is typi ca l of Cu(IJ) ions in a tetragonally distorted octahedron

9

Results and Discussion TG thermograms of compounds (1-V) were recorded

in static air and nitrogen atmosphere from ambient temperature to 600°C with a heating rate of l0°C min-:. DSC thermograms were recorded only in static air in the same temperature range and with same heating rate and only representative thermograms for three compounds (1). (II ) and (V) are given (Figs 1-3).

The various endotherms and exotherms of DSC with the values of enthalpy change, !1H, are given in Table l. An endotherm be low 100°C due to evolution of sorbed moisture in each compou nd is not g iven in Table l. The DSC curve of ce llulose shows an endothermic peak at 160°C corresponding to the dehydration and depolymerization of cellulose . Thi . endotherm is followed by two exotherm wit h maxima at 315 and 345°C with M-1 values 7.6 an 1016.6 J/g, respectively, which may be attributed W

decomposition of cellulose leading to the formati on

2026 INDIAN J CHEM, SEC A, OCTOBER 2005

'0.0

0 >< Ol

t ! a: 0

~fi' t- E ..:~

!ij

~ l.O

0 ~ z: Ol

TEMPERATURE (1C)

Fig. l - DSC and TG curves for cellulose in static air (- ) and nitrogen (---).

7.0

0 ><l

6 .0 50.0 .... t .... a:

.!. s.o .... = "' ;;:; 4 .0 ~

0

~~ .... . <~ .... = l 0 ~ z: ....

10..0

~0 ~~0

0 .0 100 200 JOO -'00 soo 800

TEMPERATURE ( 1 C)

Fi g. 2. - DSC and TG curves for cellulose benzylthiophosphate (CBTP) in stati c air (- ) and nitrogen (-- - ).

and volati li ?.ation of levoglucosan and other volatile products 10

. After this, there is another exothermic

sharp peak at 482°C (~H= 1025 .2 Jig), which may be attributed to the oxidation of volatile and charred products.

The DSC thermogram of CBTP is quite d ifferent in character from that of cel lu lose. In CBTP, an e ndotherm ith max tmum at 236°C wi th

39.30 Jig e nthalpy change. ~H, is ass igned to

1Q.O

0 :.< ... t

l ~ 0

c:~ .... = .... . ...:~

"' .... ;:; ::0 ~ ~

0

2 ... z.o

0.0 100 :lOO aGO ~ 100

TEMPERATURE ('C) ·

Fig. 3 - DSC and TG curves for Cu(ll ) comp lex o r CBTP in static air(-) and nitrogen (-- - ).

dethiophosphorylation, dehydrobalogenation and dehydration reactions. This endotherm is fo llowed by two exotherms w ith peak maxima at 292 and 438°C

with 52.83 and 15 .22 Jig va lues of ~H representing the decompos ition lead ing to the format ion of ta r products and the oxidation of volati le prod ucts and char residue, respectively. In DSC therrnograms of metal complexes of CBTP, the endotherms with maxima in the temperature range 214-231°C represent the release of metal thiophosphate in addit ion to dehydration and dehalogenation . The Cr(III) , Fe( II ) and Cu(II) complexes show first ex otherms at 289, 253 and 276 due to decompositi on as well as oxidative degradation; followed by exotherms at 320(348), 281(297), and 349, respect[ve ly, due to oxidation of volatile products and charred residues.

Table 1 shows that the decomposition and oxidation processes of modified ce llu lose are found occurring at lower temperatures tha n tha t of pure cellulose. The values of ~H are a lso found reduced and spread tn w id e range of temperature s ignif ica ntl y by introducing phosphorus, su lphur and metals in ce ll ulose .

TG thermograms of cellu lose and its deri vatives show three sigu! ficant areas of wei ght loss , termed as three stages of thermal degradat ion (Table 2). T he kinetic parameters are calculated for decompos ition stage from TG curve using Bro ido method 11

• T he equation used in this method is :

DAHIY A & RANA : THERMAL BEHAVIOUR AND SPECTRAL STUDIES ON MODIFIED COTTON CELLULOSE 2027

Table2-DescriptionofTGcurvesofcellulose. C BTPandmetal complexeso fCBTP in ai r a nd N2

No. Compound Stage Temperature range (° C) Weight Loss(%) Till (O C) Char yie ld (%)

Air Nz Air Nz Air Nz Air

( I) Cellulose First 17G-300 175- 300 5.5 68 330 335 6.2 8.3 Second 30G-355 30G-360 69.2 72.8 Third 355-580 36G-585 18.0 11.7

(II ) Cellulose benzyl- First IIG-250 145-290 9.3 12.5 300 305 28.5 29 .6 thiophosphate (C BTP) Second 25G-330 29(}-350 37.7 22.4

Third 33G-560 35G-565 21.3 32.5 (III ) Cr(lll ) complex of First 13G-220 135-2 10 5.0 9. 1 255 260 3 I. I 3 14

C BTP Second 22G-300 2 1G-305 37 2 29.3 Third 30(}-500 305-500 24 .1 29.5

(IV) Fe(ll ) compl ex of First IOG-210 IIG-215 7.6 8.6 250 255 30 2 30.7 CBTP Second 21G-280 215-290 46.0 41.5

Third 28(}-500 29G-500 14.9 19.9 (V) Cu(ll ) compl ex of First IIG-210 IIG-195 3.7 13 5 255 255 35 .0 35 .4

CBTP Second 21G-310 195-315 41.9 20.3 Third 31G-500 315-500 16.5 30.7

Table 3 - Activation energies and frequency factors of decomposition stage of cellulose, CBTP and metal co mpl exes in a ir and N2

Sr. No . Compound Temperature range (0 C)

Air N2

(I ) Cellulose 30G-355 30(}-360 (II ) Cellulose benzyl- 25G-330 29G-350

thiophosphate (C BTP) (III ) Cr(lll) complex 22G-300 21(}-305

ofCBTP (IV) Fe(! I) complex 21G-280 215-290

ofCBTP

(V ) Cu(ll ) complex 21(}-310 195-315 ofC BTP

[ [ / ]] Ea I [ R Z 2 l In In - = - --+ In --T m

y R T Ea f3 where y is the fraction of number of initial molecules not yet decomposed, f3 is the heating rate, Z is the frequency factor and Tm is the temperature at maxi mum reac tion rate. Us ing Broido equation , plots of In lin( 1/y)J versus liT for decomposition stage of thermal degradation of cellulose, CBTP and meta l complexes are plotted. The activation energies and frequency factors calculated from the slopes and intercepts , respectively, of these plots are given in Table 3.

The wei ght loss up to 1 00°C due to sorbed moisture in all compounds was not considered fo r degradation studies. In the first stage of thermal degradation of cellulose (I) the weight loss of 5.5 (6.8)% in the temperature range 170-300 ( 175-300) °C in air (nitrogen) atmosphere was due to dehydration of anhydroglucopyranose chain

£, (kJ mor' ) Z (s- 1)

Air Nz Air Nz

165.85 172.09 2.63x l0 12 1.28x I 0 1.1

31.20 34.57 3.37x lo·' 4.06x to· '

36.86 38.58 3.25x l0 1 4.83x t0 1

43 .63 55 .42 3.72x l02 2.97x to'

4409 42.00 5.12x 102 6.66x t0 1

segments. For CBTP (II) and metal complexes (111-V), the weight loss of 3.7-9.3 (8.6-13.5)% in the temperature range 100-250 ( 110-290) °C in air (nitrogen) atmosphere was probabl y the consequences of acid catalyzed dehydration.

The second stage is primaril y due to chain decomposition process and ox idati ve degradation. Cellulose showed a weight loss of 69 .2 (72.8)% in the temperature range 300-355 (300 -360 ) °C wi th activation energy 165 .85 ( 172.09 ) kJ mol - 1 in air ( nitroge n) a tm os ph e re 7 For CBTP and metal complexes, the corresponding weight loss was 37.2-46 .0 (20.3-41.5)% in th e te mp e rature ran ge 210-330 (195-350) °C with activation e ne rg) in the rang e 31-45 (34-56) kJ mol - 1 in ai1 ( nitrogen ) atmosphere. Tabl e 2 shows th e temperature (Trn) of cellulose compounds at the ir maximum weight loss rate . The weight loss in thi s stage corresponded to the first exothenn in the DSC thermograms.

2028 INDIAN J CHEM , SEC A. OCTOB ER 2005

3600 12111 800 .wo

WAVENUMBER(ca-1)

Fi g. 4 - FTIR spectra of (a) CBTP and chars of CBTP obtained at (h) 200 °C. (c) 225 °C , (d) 25 0°C, (c) 300°C, and (f) 35 0 °C, respectively.

Third stage of degradation, covering the temperature range of 355-580 (360 -585 ) °C for cellulose, showed a 18 .0 (11.7) % weight loss and the weight loss for modified ce llulose compounds was 14.9-24 . 1 ( 19.9-32 .5) % in th e te mperature ra nge 28 0-560 (290-565) °C in air (nitrogen) at mos phere and a ttributed to oxidation of vo lat il e compounds, crosslinking and aromatization of charred re s idu es.

Char yield In order to understand the flame-retardant

properties, the char yie ld s (weight%) for cellulose, CBTP and metal complexes were calculated from the TG thermograms at 600°C where ce llulose ceases to show any significant weight loss. Table 2 shows that the char yields of modified cellulose compounds lie in the range 28-35 (29-36%), which ~as much higher than that of pure cellulose 6.2(8.3%) in air (nitrogen) atmosphere indicating a reduction in the amounts of low molecular weight volatile products formed during thermal degradation , thus suppress ing the combustion process. Table 2 also indicates that the formation of char is brought about at expense of a decrease in decomposition temperature of relevant exothermic peak. Further the char yields of metal complexes are

found slightiy higher than that of CBTP indica ting the quantitative effect of metals on char yie ld over CBTPI 2-13_

FTIR studies The FfiR spectra of the chars of CBTP obta ined at

different temperatures (up to 3500C) were recorded (Fig. 4). For chars produced at 200°C, compared to the FTIR spectra of the ori g inal material. the intensiti es of bands at 3495 (0 -H str. ), 1305 (0- H bending), 1230 (P-OH deformation) , 795 (P=S str. ) and 692 cm-1 (C-CI str. ) decreased and new bands at 1640 cm-1 (C=C str. ) emerged. T hese obse rvations show that dehydration and dcthiophosphorylation reactions take place in the initial stage of degradation.

At 225°C, the bands at 1560. 1455 (C=C, C=N str. ), 1234 (P-OH deformati on), 795 (P=S str.) and 692 c m- 1 (C-CI str. ) a lmost va ni sh and a new band at 1704 cm-1 (C=O str.) appea rs. At 250°C, characteristic bands of cellu lose di sappeared . At 300°C, new band at 1235 (P=O str. ), 1030 (P- 0 - P str. ) and 758 (P-0 str.) emerge suggesting that the decomposition of compound and pol ymerizati on of re leased benzylthiophosphoni c ac id occu rred at thi s stage 14. At thi s temperature, the band at 1662 (C=C str. ) shifts to 1605 cm- 1 (C=C conj ugation).

Thi s confirms the transformat ion o f struc tu re Cei i-0-P(=S)(-OH)(-OCH2C6H5) to struc ture Ce ii - S­P( =0)( - OH)( -OCHzC6Hs).

At 350°C, the bands at 1704 (C==O str. ), 1605 (C=C str.) 1235 (P=O str. ), 79:5 ( P=S s tr .), 75 8 (P-0 s tr .) a nd 510 cm- 1 (P-S-P str. ) remain indicating the formation of compounds having C=O, C=C, P=O and P-S- P groups.

Mechanism of degradation of CBTP and metal complexes Thermal degradation mechani sm of ce llul ose

benzylthiophosphate (CBTP) starts wi th the generation of benzylthiophosphoric ac id which alters the decomposition of the substrate to such an extent that the primary decomposition prod ucts are changed by dehydration from levogiucosan and other flammable products to carbonaceous char. At high temperature, benzylthi ophosphoric ac id polymeri zed, which is more effective in catalyz ing the dehydration . It can react with cellulose moiety which then breaks down to give water, benzy lthiophosphoric acid and unsaturated cellulose analogue and eventually char by repetition of these steps. Chlorodeoxyce llulose is al so formed together with CBTP, consequentl y the mechanism of degradation is a lso affected by

DAHIYA & RANA: THERMAL BEI-IAV IOUR A D SPECTRAL STUDIES ON MODIFIED COHO CELLULOSE 2029

chlorine. The released HCI then catalyzes a se ries of heterolytic and homol ytic cleavage of the substituent s on the carbon chain of the condensation products which give a carbonaceous char13

.

When metal complexes are heated, these decompose to give a metal benzylthiophosphate and benzylthiophosphoric acid. These products can polymerize to form meta l polythiophosphate and polybenzylthiophosphoric acid. which further catalyze the dehydration. There is also possibility that the released HCI may react with metal ions to form metal ch lorides. The effect of meta l chlorides as Lewis acids is si milar to that of HCI in promoting the dehydation,

d . d h . . JS- J(J con ensa t1on an c arnng reactions · .

Conclusions DSC thennograms show that the high va lues of

enthalpy of reaction for decompos ition and oxidation reactions corresponding to last two exotherms in cellulose are reduced to great ex tent and spread in wide range of temperature significantl y in mod ified cellulose sa mples. TG thermograms show that the formation of char is brought about at expense of a decrease in decomposition temperature of relevant exothermic peak indicating thiophosphorylati on and ex hibiting typical condensed phase fl ame-retardant activity due to presence of phosphorus, sulphur and metals. From these studies, it may be conc luded that

the cellulose benzylthiophosphate and it s meta l complexes system is a good flame retardant in that it reduces the heat of reactions and flammabl e volatiles and corresponding increase char yie ld.

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