Indianlournal of Fibre & Textile Research Vol. 26, September 2001, pp. 302-307

Modification of natural polymers: Part IV--Graft copolymerization of methyl acrylate onto Cannabis indica fibre initiated by eerie ammonium nitrate

Ghanshyam S Chauhan" & Harinder Lal

Department of Chemistry, Himachal Pradesh University, Summer Hill, Shimla 171 D05, India

and

A S Singha & B S Kaith Department of Applied Sciences, Regional Engineering College, Hamirpur 177 D05, India

Received 16 February 2000; revised received and accepted 10 May 2000

Cannabis indica fibre has been graft copolymerized with methyl acrylate using ceric ammonium nitrate as initiator. The effect of monomer, initiator and HNO) concentrations, amount of water, reaction temperature and reaction time on per cent grafting has been studied. The swelling behaviour in DMF, n-butanol and water follows the pattern DMF > H20> n­butanol for the grafted fibres and H20> DMF > n-butanol for the ungrafted fibre. Both the ungrafted and grafted fibres degrade thermally in multiple and single stage respectively. Grafting has not been found to improve the thermal stability of the fibre. A plausible explanation to account for the trends in results has been provided.

Keywords: Cannabis indica, Ceric ammonium nitrate, Grafting, Graft copolymerization, Methyl acrylate, Thermal stability

1 Introduction

Cellulose and its graft copolymers find extensive applications in diversified fields. Grafting is known to improve the characteristic properties of cellulose, such as water repellancyl, thermal stabilit/, flame resistance3

, dyeabili ty4 and abrasion resistances. Cannabis fibre is a natural fibre obtained from the stem and branches of Cannabis indica, an annual herb abundantly found in the Himalayas. Cannabis fibre contains 70% cellulose, 5.8% moisture, lignin and water-soluble organic materials. In native form, it is used for making ropes, speciality footwear, etc. However, the fibre suffers from poor strength and low weather resistance as compared to the synthetic fibres . To improve its strength, weather resistance and thermal stability, the fibre has been graft copolymerized with methyl acrylate (MA). Grafting of MA onto cellulose has been studied earlier. Ide6

grafted various vinyl monomers including MA onto cellulose using ceric ammonium nitrate (CAN) as initiator. Sharma and Daruwalla7 reported grafting of MA individually and in binary mixtures onto cotton yarn by emulsion technique initiated by ceric ions. Poly(MA) grafted carbamoyl ethylated cotton showed

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reduced crystallinity and elastic recovery as compared to the original fibreS. Okiemen and Idehen9 grafted MA onto cellulose and thiolated holocellulose and found lower incorporation of graft in the latter. The molecular weight (Mw) of the grafted chains was reported greater than that of the homopolymer when MA was grafted onto alkali-treated and beaten cellulose lO

. MA shows lesser reactivity than ethyl acrylate, allyl chloride and acrylonitrile when it is grafted onto cellulose by ionic Xanthation method II . However, the order of grafting reactivity was found to be MA > AN when these monomers were graft copolymerized onto ferrous cellulose by Fe+2 and N­bromosuccinimide activator systemsl2. Different grafting frequencies using same reaction conditions have been observed for grafting of MA, ethyl acrylate and ethyl methacrylate onto carboxymethyl cellulose 13.

The modification of natural cellulosic fibres by graft copolymerization of methyl methacrylate (MMA) onto Cannabis indica l4

, rayon lS andjute l6 has recently been reported. In the present work, an attempt has been made to evaluate optimum grafting conditions for grafting of MA onto Cannabis indica fibre using Ce+4 ions as initiator and the graft copolymers have been characterized by swelling and thermal studies.

CHAUHAN et af. : MODIFICATION OF NATURAL POLYMERS: PART IV 303

2 Materials and Methods 2.1 Purification of Materials

The fibre isolated from the stem of Cannabis indica was immersed in water at 25°C for 24h and then dried in air oven at 50°C. Methyl acrylate (Merck) was washed with 5% NaOH and dried over anhydrous Na2S04. The dried monomer was distilled and the middle fraction was used in grafting reactions. Ceric ammonium nitrate (reagent grade) was used as such and the cellulose was estimated by modified sulphite method'8.

2.2 Graft Copolymerization Cannabis indica fibre (l.Og) was immersed in 100

mL of distilled water and the known amounts of monomer, initiator and HN03 were added to the reaction mixture. The contents were refluxed at 45°C for 2h. The optimum reaction conditions for maximum grafting were evaluated as a function of concentrations of initiator, monomer, HN03. amount of water, reaction temperature and the reaction time. Homopolymer was removed from the fibre by repeated solvent extractions using acetone until a constant weight was obtained. Graft copolymers were dried in an air oven at 50°C. Swelling studies of grafted and un grafted fibres were carried out in DMF, H20 and n-butanol as reported elsewhere'8. Per cent grafting (Pg) and per cent swelling (Ps) were calculated as reported earlier'4. Thermal properties of the fibres were studied on Shimadzu DT-40 (simultaneous TG/DT) thermal analyzer in air heating at a rate of 20°C/min.

3 Results and Discussion Ceric ion is known to form complex with cellulose

at C2 and C) glycolic positions. The complex decays to generate active sites where the monomer gets incorporated to yield graft copolymer. Monomer is also initiated by Ce+4 ions via a complex formation. The grafting process is given below:

Ce+4 Decompoi ' Cell -OH ~ Complex (I) Cell +Ce+3 + H+

M+Ce+l-,Complex Decom poj M+Ce+3

M+ nM-. (Mlo- M

(M)n.M + Cell-.CeIHM)n- M (Graft)

Cell-(M)n-M'+Ce+4-'Cell -(M)n+I +Ce+4

(M)n-M +(M),,-M -.(Mh n+2 (Homopolymer)

(I )

... (2)

... (3)

.. . (4)

... (5)

... (6)

.. . (7)

Predominance of process shown in Eq. (6) over that in Eq. (7) will result in higher graft yield at the cost of homopolymer formation. It is also understandable form the above scheme that the concentrations of Ce +4 ions and monomer affect yields to the substantial extents. The radicals produced from the complexes of Ce+4 ions with monomer and cellulose initiate the grafting reactions.

3.1 Effect of Monomer, Initiator and Acid Concentrations It is evident form Figs 1 and 2 that the grafting

increases with the increase in both monomer and initiator concentrations. While there is leveling off in Pg after maximum graft yield with the further increase in monomer concentration, the P g shows a steep decrease when the initiator concentration is increased beyond 10.0 x 10-3 moleslL. Such variation in graft

80

n,C7)40

OL-~~~~~ __ ~~~ 0' 11-10 16.65 22·20 ZT.15 3~

(MAl X 'O-~ moles/L

Fig.l-Effect of monomer concentration on grafting [Cannabis fibre, Ig; [CAN] x 103, moleslL; [HN03] x H)3, 17.4 moles!l ; temperature, 25°C; time, 120 min; and H20, 100 mL]

80

60 o

rI!' 40

20

Fig.2-Effect of initi ator concentrat ion on grafting [Cannabis fibre, Ig; [MA] x l02

, 22.4 moleslL; [HN03] x103, 17.4 moleslL;

temperature, 25°C; time, 120 min; and H20. 100 mL]

304 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2001

yie ld is expected. During the initial stages of reaction, many more act ive sites are generated on backbone and many more monomer radicals are produced at the higher monomer and initiator concentrations, leading to the increase in P g. However, at their higher concentrations, the homopolymer formati on predominates due to the increased mutual annihilation of grow ing monomer macroradicals. The variation in HN03 concentration affects g raft yield, affording maximum P g (79.5) at 17.44 x 10-3 moles/L of acid concentrati on in the reaction medium. However, the graft yield falls drastically beyond the optimum concentrations of the acid. At the lower concentration , HNOJ is known to catalyze grafting reactions. However, at higher concentrations due to its ox idizing nature there are more s ide reactions, leading to the wastage of monomer (Fig. 3) .

3_2 Effect of Reaction Time and Temperature

The per cent grafting fall s sharply when reaction tempera ture is increased from 25°C to 55°C at an in terval of 10°C (Fig. 4 ). Termination processes are frequent at higher temperatures and least grafting is expected. However, in the present case, it appears that the complexes formed by the reaction of initiator with monomer and cellulose fibre are not stable even at moderate temperatures. Lower popUlation of these complexes will evidently lead to the creation of lesser act ive sites on the backbone and lesser initiation of the monomer. At higher temperatures, the fibre lose water-soluble components and thus a decrease in apparent graft yield is witnessed. Similarly, Pg was studied as a function of reaction time and it was initially found to increase with the increase in reaction time. At higher reaction time, Pg decreases due to the predominance of the homopolymer formation at the cost of grafting reactions, though the variation is not so sharp as observed in the cases of CAN and monomer concentrations. Grafting between 55.9% and 79 .5% was obtained when the reaction time changed from 90 min to I SO min (Fig. 5).

3.3 ElTect of Amount of Water

Fig. 6 shows that the Pg varies form IS to 79.5 when the amount of water is increased from SO mL to 100mL. However, further increase in the amount of water leads to drast ic decrease in Pg• Water helps in swelli ng of the fibre and thus enhances monomer accessibility to the swo llen backbone which gets reduced with the higher amount of water in the reaction medium. 1t is also explained by the fact that

80

60

40

20

o~ __ ~ ____ ~ __ ~~ __ ~ o 8.7 17.4 26 .1 34.S

~N03J XH5~ moles/L

Fig.3-Effect of nitric acid concentrati on on g rafting [Cannabis fi bre. Ig; [MA] x 102

, 22.4 moles/L; [CANI x IOJ, 1.0 mo les/L;

temperature. 25°C; time, 120 min; and H20 , 100 mL]

80

60 o

20

OL-____ ~ ____ ~ __ ~~ ____ ~ o 25 35 45 55

Temp.;C

Fig.4--Effect of temperature o n grafti ng [Cannabis fibre. Ig ; [MA] X 102

, 22.4 moles/L; [CAN] x IOJ, 1.0 mo les/L; [HNO, ]

x lO" 17.4 moles/L; time. 120 min ; and H20,IOO mL]

80

d!'

90 120 150

React ion Ti mt' ,min

180

Fig.5--Effect of reaction time on grafting [Cannabis fibre, Ig; [MAl X 102

, 22.4 moles/L; [CAN] x 10" 1.0 moles/L; [HN03J x 103

, 17.4 mo les/L; temperature, 25°C; and H20,100 mL]

CHAU HAN el at. : MODIFICATION OF NATURAL POLYMERS: PART IV 305

60

50

rP' 40

20

0 0 50 100 150 200

Amount of Water, ml

Fig. 6-Effect of amount of water on grafting [Cannabis fibre , 1 g; [M A] X 102

, 22.4 molelL, [CAN] x 103, 1.0 molelL; [HN03] x 103

,

17 .4 moles/L, temperature, 25°C; and time, 120 min]

both the growing macroradical chains and backbone are insoluble in water.

3.4 Characterization of Graft Copolymers

3.4.1 Swelling Behaviour

The swelling behaviour of the grafted and

ungrafted fibres in DMF, n-butanol and H20 is shown

in Fig. 7. The trend of per cent swelling (Ps) for the

grafted fibres fo llows the pattern : DMF > H20> Il­

butanol and that for the ungrafted fibre : H20> DMF

> Il-butanol. A similar trend has been reported earlier

for the swelling of Cannabis indica and its graft

copolymers with MMAI4. Swelling in DMF increases

with the increase in grafting and then decreases after

reaching at optimum Ps of 150. However, Ps

decreases steadily in the case of water but remains

somewhat constant in the case of n-butanol. Thi s is

understandable from the fact that the water affects the

changes in the hydrogen bonding of cell ulose by

loosing it and penetrates deeper into the matrix . These resu lts are expected as DMF can solvate poly(MA)

moieti es. Further, grafting opens up cellu lose matrix

so that more solvcnt can diffuse. Since water and Il­

butanol do not have affin ity for poly(MA) grafted on

the fib re, the lower Ps is observed in these solvents . ]n

case of ungrafted fibre, water can associate with the

hydroxyl groups of fibre cellulose, and can penetrate

and diffuse ill the backbone matri x as compared to

other solvents.

160

. DMF

120 o

0.. .. 80 o ·

n-butanoL x 0

0 20 40 60 80 100 Pg

Fig. 7 - Per cent swelling as a functi on of per cent grafting [graft copolymer, Ig ; solvent, 10 mL; and time, 1440 min]

100 150

235

75 cannabis 100 42

233

Cann-9- poly (MA ) 75

$ 50 0 -'

~ 50

~ 2S

25

430 0

16 24 32 0 5 10 15 20 25 Tim. ,min

Fi g. 8--TG curves of Ca/lnabis and Cann-g-poly(MA) fibres

3.4.2 Thermal Analysis Primary thermograms (TGA and DT A) of

ungrafted and grafted fibres are shown in Figs . 8 and 9.

3.4.2.1 Thermogravimetric Analysis

Generally, cellulose and its derivatives are known to degrade by thermo-oxidation, dehydration, depolymerizati on and glycosan formation. Further, cellu los ics degrade to smaller units by depoly­merization followed by pyrolysis. The pyrolys is may produce products which combine to form char and residue. The dehydration of anhydroglucose uni t. leading to the fo rmat ion of anhydride, occurs in the temperature range of 180°-280°C. It is then followed by depolymerization above 280°C at 1,4 glycosidi c linkage - the weakest link in the chain . The rapid pyrolysis usually occurs at 3 10° - 350°C. Grafting of acry loni trile has been reported to have no appreciable effect on the course of thermo-oxidation and dehydration of cellulose . The cellulosics degrade

306 INDIAN J. FIBRE TEXT. RES .. SEPTEMBER 2001

0 X w

<J

0 0 Z W

7

6

5

4

3

2

8

7

6

5

4

3

2

Cannabis fibre

Cann-g-Poly (MA)

370.0 354.0

429.2

Fig. 9--DT A curves of Cannabis and Cann-g-poly(MA) fibres

thermally in two or more stages but their graft copolymers degrade in single degradation stage. These differences arise mainly due to the opening up of cellulose matrix and consequent loss of crystallinity during the grafting process. Consumption of OH groups in the grafting reaction, leading to retardation of anhydroglucose formation process, and distinct thermal behaviour of the functional groups incorporated in the grafting process contribute to different thermal degradation patterns. In the present

study, the details of the degradation processes are deduced from the thermograms (Fig. 8).

TG analysis of original Cannabis fibre and Canl1-g-poly(MA) was carried out in air atmosphere at a heating rate of 20°C/min. The decomposition of both the fibres was studied as a function of reaction time (Fig. 8). Degradation pattern of Cannabis indica fibre has already been reported 14. Cannabis indica fibre shows three inflections in the temperature range of 30°C-150°C, 383°-420°C and 473c-518°C. The first inflection is due to the loss of water content in the fibre and it accounts for about 11 % weight loss. Further, about 2% weight loss results from the volatilization of some incorporated organic compounds. Initially, the decomposition begins from 236°C and continues up to 383°C. The rate of degradation is very fast in the temperature range just above the initial decomposition temperature (IDT). This degradation pattern initially corresponds to dehydration and anhydroglucose formation and then depolymerization sets in due to the breakage of 1-4 glycosidic linkages. It takes about 3 min to complete degradation in this temperature range accounting for about 66% weight loss. In the temperature range of 383°-420°C, the rate of degradation is slower as volatile products are formed. The rate of degradation again becomes faster in the temperature range of 420°-473°C. The slower rate of degradation in the temperature range of 473°-518°C is due to the char formation. No residue is left as the fibre decomposes to 100%. On the other hand, the TG curve of Call1l-g­poly(MA) shows very small infl ection between 42°C and 107°C which is due to the loss of moisture and the initial decomposition of grafted fibre begins almost at the same temperature (233°C) as that of the ungrafted fibre (236°C) after 22.4 min of heating. The difference in the degradation patterns of the two fibres in the initial stages before reaching the IDT is strikingly clear. The loss of moisture in the graft copolymers is very less. It is expected since grafted poly(MA) chains are hydrophobic. Further, organic compounds present in the fibre matrix of cellulose are lost in the grafting process. These two reasons account for the most flat nature of first inflection in the graft copolymer. Further, the graft copolymer is degraded in single step as observed earlier l 4

. The decomposition of the graft fibre continues steadily up to 430°C at which final decomposition starts and the complete decomposition occurs at 450°C. The decomposition process is very fast as it completes

CHAUHAN et al.: MODIFICATION OF NATURAL POLYMERS: PART IV 307

within about 3 min. The whole decomposition of the grafted fibre takes place within 32.25 min while that of the ungrafted fibre takes place within 36 min. From these observations, it appears that the grafting of MA onto cellulose has di sturbed the overall structure of the cellulose. The region between 473°C and 518°C in the TG curve of the ungrafted fibre is missing in th~ TG curve of grafted fibre, suggesting that the grafted chains have di sturbed the crystallinity of the cellulose.

3.4.2.2 Differential Thermal Analysis

Differential thermal analysis (DT A) curves of the Cannabis indica and Cann-g-poly(MA) fibres are shown in Fig. 9. It is observed that Cannabis indica fibre shows two major exothermic peaks at 382°C and 419°C while the grafted fibre shows a single exothermic peak at 429°C. The exothermic peak starting at 370°C and shooting to 382°C relates to the temperature region between 383°C and 420°C. It corresponds to the release of energy in volatilization process. The single exothermic peak at 429°C may be due to the complete decomposition of the grafted poly(MA) chains in addition to the decomposition of the cellulose content of the fibre. Thermal degradation products of poly(MA) are known to be obtained between 292°C and 399°CI9.20.

4 Conclusion

Grafting of vinyl monomers onto cellulosic backbones opens up cellulose matri x in a considerable way and it is man ifested in the observed changes in its swelling and thermal behaviour. Cannabis indica fibre and its graft copolymer degrade thermally by different patterns. It is observed that the grafting does not improve the thermal stabi lity of Cannabis indica fibre.

Acknowledgement Two of the authors (ASS and BSK) are grateful to

the Principal, Regional Engineering College, Hamirpur, for providing facilities to carry out thi s work. References

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