Thermal characterization of chemically treated coconut husk fibre

Transcription

1 Indian Journal of.fibre & Textile Research Vol. 37, March 2012, pp Thermal characterization of chemically treated coconut husk fibre G M Arifuzzaman Khan & Md. Shamsul Alam a Polymer Research Laboratory, Department of Applied Chemistry and Chemical Technology, Islamic University, Kushtia, Bangladesh and Minoru Terano School of Materials Science, Japan Advanced Institute of Science and Technology, Japan Received 2 December 2010; revised received and accepted 31 January 2011 Effect of chemical treatments using sodium chlorite, sodium hydroxide and acrylamide monomer on thermal behavior of coconut husk (coir) fibre has been studied by means of scanning electron microscopy (SEM) and thermogravimetric analyses (TG, DTG, DTA). A significant variation in fibre surface occurred by chemical treatments is clearly observed in SEM images. The TG and DTA curves show two-stages of decomposition for all the fibres; first below 100 C indicating the moisture loss and the second between 320 C and 360 C due to major degradation of fibre. Chemical treatment increases the thermal stability of fibre through physical and chemical changes. It is found that the temperature below 180 C marginally affects fibre properties, whereas temperature at 180 C significantly decreases tensile strength and degree of polymerization. Keywords: Coconut husk, Chemical treatment, Degree of polymerization, Thermal degradation, Tensile strength 1 Introduction Natural cellulose based fibres are increasingly gaining attention in view of their applications in engineering end uses such as building materials 1 and structural parts of motor vehicles 2,3, where reduction in weight is required. There are at least 1000 types of plants that bear commercially usable fibres 4. Agricultural waste is a prominent source of cellulose based fibres. These include coir, straw residue, coffee husk, bagasse and many more. The advantages of natural fibres arise from their renewability, low cost, easy availability and high specific strength and stiffness, when compared to synthetic fibres like glass and carbon, besides a minimum health hazard involved in the use of natural cellulose based fibres. However, one of the major drawbacks seems to be the limited thermal stability of these fibres, the first degradation occurs at temperature 180 C. This degradation behavior of cellulose based fibre is greatly influenced by its chemical structure. Nassar and Mackay 5 reported the thermal stability of cellulose based fibre in the order: lignin αcellulose hemicellulose. Contrary data was published a To whom all the correspondence should be addressed. dr.alamiu@yahoo.com by Ramiah 6 in 1970 showing the order: hemicellulose lignin α-cellulose by using the dilatometric method. Xiao et al. 7 analyzed the thermal characteristics of alkali soluble lignin, hemicellulose and cellulose from maize stem and rice straw. They showed that hemicellulose degraded first followed by lignin which showed less degradation and therefore its structure was more stable. Limited work has also been reported on the thermal degradation behavior of natural fibres by chemical treatments These studies represent the various changes occurred in fibres (splitting to filaments, crystallinity, elasticity, sorbency, fastness, thermal resistance etc.) that depends upon the condition as well as degree of chemical modifications. Although the chemical structure of cellulose from different natural fibres is the same, the degree of polymerization (DP) varies 13. The mechanical properties of a fibre are significantly related to DP. Coconut husk (coir) fibre, relatively inexpensive and abundant in Bangladesh, is found to be good for potential reinforcement in polymer matrices 14. In the present work, the coir fibre surface has been treated with sodium chlorite, sodium hydroxide and acrylamide monomer (AM) and their effects on the thermal stability of fibre are analyzed by SEM, TG, DTA and DTG measurements. The effect of heat

2 KHAN et al.: THERMAL CHARACTERIZATION OF CHEMICALLY TREATED COCONUT HUSK FIBRE 21 treatment on the physical properties of fibre has also been observed. 2 Materials and Methods 2.1 Materials The coconut husk (coir) fibre produced in Bangladesh was retted into water for one month to separate the fibre from cementing and gummy materials. The extracted fibres were scoured in a solution of 3.5 g Na 2 CO 3 (Merck, German) and 5 g soap flake (Lever Brothers Bangladesh Ltd.) per liter of water at 70 C in a large beaker. The fibre-to-liquor ratio was maintained at 1:50. Finally, the fibres were washed, dried and stored in desiccator. All the chemicals used in this investigation were of laboratory reagent (LR) grade. 2.2 Methods The untreated fibres were treated with 0.7% NaClO 2 (BDH, England) solution (buffered to ph 4) at C for 90 min. The concentration (weight %) of NaClO 2 was maintained at 35% with respect to fibre weight. After treatment, excess chlorite was neutralized with 0.2% sodium metabisulphite (Merck, German) solution for 15 min and then washed thoroughly with distilled water. The untreated fibres were soaked in 250% NaOH (BDH, England) solution with respect to fibre weight in a water bath where the temperature was maintained at 30 C for 10 h. The fibre-to-liquor ratio was maintained at 1:50. The alkali treated fibres were neutralized with dilute CH 3 COOH (BDH, England), washed several times and finally dried at room temperature. AM monomer (Merck, German) was dissolved in cold distilled water. The reaction bath was prepared by 80% AM monomer, 1% K 2 S 2 O 8 (initiator) (BDH, England) and 1% FeSO 4 (catalyst) (BDH, England) based on weight of fibre. Grafting reaction was carried out at 60 C maintaining the fibre-to-liquor ratio at 1:50 for 90 min. After treatment, the grafted fibres were thoroughly washed with hot distilled water to remove the loosely attached suspended homopolymer 15. The major constituents, such as cellulose, hemicellulose and lignin as well as minor constituents such as pectin, fatty and waxy materials of fibres were isolated according to TAPPI standard 16. Untreated and treated fibres were cut into equal length of 20 cm and placed in an electric oven for heat treatment in presence of air. The physical properties of heat treated fibres were compared by varying time and temperature. 2.3 Measurements The thermogravimetric analyses of untreated and treated fibres were conducted by thermal gravimetric analyzer (TGA-Model TG 50, supplied by TA Instrument). 20 mg sample of each type of fibre was taken for the analysis. The samples were heated up steadily at a rate of 20 K/min from 25 C to 500 C in a nitrogen atmosphere. To get perfection, analysis was done twice for each sample. The samples were used to determine the tensile properties (tensile strength and elongation at break) with the help of an Instron tensile tester (Model 1011). The gauge length was 1.0 cm, and the crosshead speed was 2.5 mm/min. Ten samples were investigated with 15% standard deviation. Coir fibre (5 g) was dissolved in phosphoric acid at 55±2 C in a 250 ml conical flask with continuous stirring till the solution became clear. It was then cooled suddenly to 30 C in a cold-water bath and finally filtered on a Buchner funnel. The prepared solution was taken quickly in a viscometer upto the mark. The viscometer was then placed in a thermostatically controlled water bath at 30±0.2 C. To attain the temperature, viscometer was kept for 5 min in water bath. Then viscosity was determined by measuring the flow time. The viscosity of phosphoric acid was also measured at 30±0.2 C. The relative viscosity (η r ) and specific viscosity (η sp ) of the cellulose in phosphoric acid solution were calculated using following formula: η r = η solution / η solvent η sp = η r 1 The degree of polymerization (DP) was calculated using the following equation 17 : DP= 200 η sp /C( η sp ) where C is the concentration (g/100ml) of fibre in solution. SEM micrographs of fibre surface and crosssections of untreated and treated fibres were taken using a scanning electron microscope (Philips XL-30). Prior to SEM observation, the samples were coated with gold in a plasma sputtering apparatus. 3 Results and Discussion 3.1 SEM Analysis SEM images of untreated and treated coconut husk (coir) fibres surfaces are shown in Fig. 1. The image

3 22 INDIAN J. FIBRE TEXT. RES., MARCH 2012 Fig. 1 SEM images of (a) untreated fibre, (b) sodium chlorite treated fibre, (c) sodium hydroxide treated fibre and (d) AM treated fibre (magnification 100) Fibre Table 1 Structural characteristics and mechanical properties of untreated and treated fibres Cellulose Hemicellulose Lignin Pectin Waxy matters DP Tensile strength % % % % % MPa of untreated fibre is shown in horizontal orientation. The intercellular gaps, in the form of shallow longitudinal cavities, can be clearly marked as cellular units and are partially exposed. The intercellular space is filled up by the binder lignin and fatty substances that hold the individual cells firmly in the fibre. So, the surface of untreated fibre looks relatively smooth as compared to the rough surface due to voids and absence of wax on the surface of the sodium chlorite treated fibre [Fig. 1(b)]. Micrographs of the 5% sodium hydroxide treated fibre are shown in Fig. 1(c). The pictures show a large number of holes or pits on the fibre surface. These are most possibly arising from the removal of fatty deposit tyloses on the surface. Given the large number of these pits, it can be considered that a large number of the tyloses, globular fatty deposits, lay hidden inside the surfaces of the untreated fibre. Table 1 shows that the tensile strength of 5% sodium hydroxide treated fibre Elongation-at-break % Untreated Sodium chlorite treated Sodium hydroxide treated AM treated ( MPa) is much less than that of the untreated fibre ( MPa). The reduction in tensile strength might be due to the formation of a large number of pits on the fibre surface. Similar results were also observed by Rout et al. 18 SEM image of AM grafted fibre is shown in Fig. 1(d). The image shows PAM grafts deposited in the intercellular gaps and on the surface of the cellular units. A comparison of Fig. 1(d) with Fig. 1(a) clarifies this point-the intercellular gaps have been reduced because of impregnation of PAM grafts, thereby causing the surface to become more or less uniform and smooth. 3.2 Thermal Properties The TG curves of untreated, sodium chlorite treated, sodium hydroxide treated and AM treated fibres are shown in Figs 2-5 and the corresponding values of weight loss are listed in Table 2 as a

4 KHAN et al.: THERMAL CHARACTERIZATION OF CHEMICALLY TREATED COCONUT HUSK FIBRE 23 Fibre Untreated Sodium chlorite treated Sodium hydroxide treated AM treated Table 2 TG of untreated and treated fibres Temperature range C Weight loss % Residual char, % Fig. 2 TG, DTA and DTG curves of untreated fibre Fig. 3 TG, DTA and DTG curves of sodium chlorite treated fibre function of temperature. In all the cases, initial peak between 30 C and 150 C indicates removal of moisture from the fibre with a temperature maximum at 57.7 C. The weight loss below 300 C is negligible; above that temperature the fibres begin to degrade fast and at 400 C, only residual char is obtained due to loss of hydroxyl groups and depolymerization of cellulose to anhydroglucose units. In case of untreated fibre, the peak corresponding to moisture loss appears at 57 C with 4.8% degradation. The main degradation peak occurs between 300 C and 360ºC with 70% degradation. Above the main degradation stage, all the volatile Fig. 4 TG, DTA and DTG curves of sodium hydroxide treated fibre materials are driven off from the sample resulting in the residual char. In sodium chlorite treated fibre, the weight loss (7.9%) due to moisture is more than that in untreated fibre because sodium chlorite treated fibre contains more pores, which is also confirmed by SEM image [Fig. 1(b)]. However, the main degradation peak of sodium chlorite treated fibre occurs at a higher temperature than untreated fibre. About 63% weight loss is found in the temperature range C. In sodium hydroxide treated fibre, the moisture loss peak is not found to have any considerable change compared to the untreated one, but the weight loss at 100 C is found to be slightly higher in the untreated fibre. This may be due to removal of fatty-waxy materials; pectins as well as hemicelluloses fibre became more hydrophilic in nature. So, moisture loss is easier and weight loss is higher below 100 C compared to untreated one. The

5 24 INDIAN J. FIBRE TEXT. RES., MARCH 2012 main degradation peak temperature is shifted to higher temperature region. About 19 C increase in the degradation peak temperature and higher percentage of residue indicates better thermal stability of sodium hydroxide treated fibre. Saha et al. 19 reported that alkali treatment reduces hemicellulose in the fibre, thereby making the fibre thermally more stable. Ray et al. 11 also reported the increased thermal stability of alkali treated jute fibre. The TG graph of AM grafted fibre (Fig. 5) is quite different to untreated and other treated fibres. The weight loss below 330 C is very small due to the presence of less moisture and other volatile mater. However, in the temperature range C the fibre shows 78% weight losses which are greater than those of untreated and treated fibres. This may be explained on the basis of distribution in the crystal lattice of cellulosic fibre; on graft copolymerization fibre becomes more amorphous 20. DTG of untreated and treated fibres are studied as a function of rate of weight loss (mg/min) versus temperature (Table 3). In the case of untreated fibre the two decomposition peaks are found at 298 C and C at the rate of and mg/min weight loss respectively. The first peak is for degradation of hemicelulose and second peak for cellulose and lignin. However, the decomposition rate of hemicellulose is more prominent in case of untreated fibre than in the cases of sodium chlorite treated and sodium hydroxide treated fibres. The decompositions of cellulose and lignin of sodium chlorite treated, sodium hydroxide treated and AM treated fibres are observed at 360, and C with corresponding rate of weight loss 0.633, and mg/min respectively. Thus, it can be concluded from the DTG studies that rate of thermal decomposition of polyacrylamide (PAM) grafted chain in AM treated fibres is higher than that of cellulose and lignin. DTA curves are found to have exothermic peaks coinciding with regions of weight loss observed in TG as shown in Figs 2-5 and Table 3 for untreated and treated fibres. All types of fibres show an endothermic peak within the temperature range C due to loss of moisture. The degradation is significant above 330 C. A large endothermic peak between 330 C and 370 C represents the complete loss of OH groups of monomer unit of cellulose and depolymerization and volatilization of cellulose followed by exothermic decomposition of lignin 21. Sodium chlorite treated fibre shows broader exothermic peak at 359 C ( uv) for degradation of hemicellulose. This may be due to the presence of more hemicellulosic matter in sodium chlorite treated fibre than in other fibres. In case of AM treated fibre, the endothermic peak becomes broader and a shoulder peak appears at 365 C due to PAM grafted chain addition with fibre (Fig. 5). These results represent the degradation process of both the lignocellulose and the grafted PAM chain. 3.3 Effect of Heat Treatment on Physical Properties of Untreated and Treated Coir Fibres The degradation behavior of untreated and treated fibre is investigated by varying time and temperature (Tables 4 and 5). It is observed that the DP of fibre decreases with the increase in temperature and time. At low temperature, the rate of degradation is lower but at higher temperature degradation occurs very rapidly. The intrinsic viscosity of fibre decreases with Fig. 5 TG, DTA and DTG curves of AM treated fibre Table 3 DTG and DTA of untreated and treated fibres Fibre DTG DTA Temperature Rate of Temperature Peak C weight loss mg/min C Untreated Endothermic Endothermic Exothermic Endothermic Endothermic Sodium chlorite treated Exothermic Sodium Endothermic hydroxide Endothermic treated Exothermic AM treated Endothermic Endothermic Exothermic

6 KHAN et al.: THERMAL CHARACTERIZATION OF CHEMICALLY TREATED COCONUT HUSK FIBRE 25 Table 4 Effect of temperature on tensile strength (TS) and degree of polymerization (DP) of untreated, sodium chlorite treated, sodium hydroxide treated and AM treated fibres for 4 h Temp., C Untreated coir fibre Sodium chlorite treated coir fibre Sodium hydroxide treated coir fibre AM treated coir fibre TS, MPa DP TS, MPa DP TS, MPa DP TS, MPa DP Room temp Table 5 Effect of time on tensile strength (TS) and degree of polymerization (DP) of untreated, sodium chlorite treated, sodium hydroxide treated and AM treated fibres at 180 C Contact time, h Untreated fibre Sodium chlorite treated fibre Sodium hydroxide treated fibre AM treated fibre TS, MPa DP TS, MPa DP TS, MPa DP TS, MPa DP the progress of time, which corresponds to the decrease in DP. The degradation of fibre up to 180 C has been studied and it is found that at higher temperature DP decreases to a great extent. After heating for 4 h at 180 C, loss of DP value is 19.4% for untreated fibre, 11.5% for sodium chlorite treated fibre and 10.4% for sodium hydroxide treated fibre. But the degradation of fibre below 140 C is very low. Time also affects the degradation process. In this study after heating the fibre at 180 C for 1 h, it shows decrease in DP of 3.2% for untreated fibre, 3.1% for sodium chlorite treated fibre and 2.6% for sodium hydroxide treated fibre of its original molecular weight. But when it is heated at the same temperature for 5 h DP decreases to 23.6% for untreated fibre, 15.0% for sodium chlorite treated fibre and 12.1% for sodium hydroxide treated fibre. It may be explained that chemical reaction at elevated temperature falls into two groups, namely reaction proceeds with rupture of the main chain and the reaction proceeds without rupture of the main chain. When fibre is heated, owing to thermal fluctuation, the energy of thermal motion at some point of the system becomes commensurable with the energy of chemical bond and this results in the rupture of chemical bond. The C-C bond is one of the most resistant to thermal influences. The presence of hydrogen atom in the polymer molecule greatly decreases the energy of C-C bond and that is why high molecular mass hydrocarbon like fibre possesses comparatively lower stability and hence is easily degraded by heating. The untreated fibre contains higher amount of lignin than sodium chlorite treated and sodium hydroxide treated fibres. Hence, the degradation is more prominent in untreated fibre. Table 4 shows that tensile strength values of untreated, sodium chlorite treated, sodium hydroxide treated and AM treated fibres decrease with the increase in temperature. It is probably due to the removal of absorbed moisture by the amorphous region of fibre with the increase in temperature. After heating for 4 h at 180 C the loss in tensile strength is about 28.6% for untreated fibre, 20.8% for sodium chlorite treated fibre, 18.4% for sodium hydroxide treated fibre and 16.4% for AM treated fibre. Again the loss of tensile strength of fibre below 120 C is very low. The melting point of lignin is 120 C. So, when fibre is heated at 120 C or above, the lignin molecules begin to melt with the rupture of the incrustants and hence the loss in tensile strength becomes higher 22. In general, a higher temperature or a longer duration of exposure lead to an increase in depolymerization and drop in tensile strength. Table 6 shows decrease in tensile strength with the increase in heating time at 180 C. After 5 h heating in oven, the loss of tensile strength becomes 36.4% for untreated fibre, 27.5% for sodium chlorite treated fibre, 24.6% for sodium hydroxide treated fibre and 23.2% for AM treated fibre. After 5 h heating, all the lignin of fibre is

7 26 INDIAN J. FIBRE TEXT. RES., MARCH 2012 oxidized. The untreated fibre contains larger amount of lignin. Thus, the loss of tensile strength is higher in untreated fibre than those in sodium chlorite, sodium hydroxide and AM treated fibres. The tensile strength of AM treated fibre is higher than that of untreated and other treated fibres. This is caused by the grafting of fibre with AM. The monomer might be mechanically binding the cellulosic chains or micro-fibrils conferring additional strength to fibre. Similar observation has also been made for AN grafted jute fibre Conclusion This study signifies that all the treatments improve the thermal stability of coconut husk fibre by lower weight loss and shifting of degradation peak to higher temperature. A series of experiments have been conducted to study the effect of heat on physical properties of untreated, sodium chlorite treated, sodium hydroxide treated and AM treated fibres. In all cases, fibres show none or only a slight decrease in tensile strength and DP at temperature below 180 C. For maximum exposure condition used with 180 C for 5 h, the drop of tensile strength is found to be roughly 36.4% for untreated fibre, 27.5% for sodium chlorite treated fibre, 24.6% for sodium hydroxide treated fibre and 23.2% for AM treated fibre. Acknowledgement The authors are thankful to the Director, MME Department, BUET, Dhaka, Bangladesh and Director, BCSIR, Dhaka, Bangladesh for TG, DTG, DTA, SEM and tensile strength measurements. References 1 Shafizadeh F, Cellulose Chemistry and Its Application, edited by T P Nevell and S H Zeronian (Ellis Horwood, New York), 1985, Sharma H S S, J Text Inst, 87 (1996) Nada A M A & Hassan M L, Polym Degrad Stab, 67 (2000) Singh R, Arora S & Lal K, Thermochim Acta, 289 (1996) 9. 5 Nassar M N & MacKay G D M, Wood Fibre Sci, 16(3) (1984) Ramiah M V, J Appl Polym Sci, 14 (1970) Xiao B, Sun X F & Sun R, Polym Degrad Stab, 74 (2001) Samal R K & Ray M C, J Polym Mater, 15 (1998) Basak R K, Saha S G, Sarkar A K, Saha M, Das N N & Mukharjee A K, Text Res J, 53 (1993) Rana A K, Basak R K, Mitra B C, Lawther M & Banerjee A N, J Appl Polym Sci, 64 (1997) Ray D, Sarkar B K, Basak R K & Rana A K, J Appl Polym Sci, 85 (2002) Khan G M A, Shaheruzzaman M, Rahman M H, Razzaque S M A, Islam M S & Alam M S, Fibre Polym, 10(1) (2009) Alam M S, Khan G M A & Razzaque S M A, J Natural Fibre, 6 (2009) Haque M M, Hasan M, Islam M S & Ali M E, Bioresource Technol, 100 (2009) Sikdar B, Basak R K & Mitra B C, J Appl Polym Sci, 55(12) (1995) TAPPI Standard Mathods (360, Lexington Avenue, NY), Schefer W, Textilveredelung, 4 (1969) Rout J, Tripathy S S, Nayak S K, Misra M & Mohanty A K, J Appl Polym Sci, 79 (2001) Saha S C, Ray P K, Pandey S N & Goswamy K, J Polym Sci, 42 (1991) Kaith V S & Kalia S, express Polym Letters, 2(2) (2008) Khan F, Macromol Biosci, 5 (2005) Mondal M I H, Alam R & Sayeed M A, J Appl Polym Sci, 92 (2004) 3622.