Effect of Mercerization and Benzoyl Peroxide Treatment on Morphology, Thermal Stability and Crystallinity of Sisal Fibers

Studies on the use of natural fibers as replacement to man-made fiber in fiber-reinforced composites have increased and opened up further industrial possibilities. Natural fibers have the advantages of low density, low cost, and biodegradability. However, the main d isadvantages of natural fibers in composites are the poor compatibility between fiber and matrix and the relative high moisture sorption. Therefore, chemical treatments are considered in modify ing the fiber surface properties. In this study, Sisal fibers were modified using alkali and benzoyl peroxide solution of different concentration for different time intervals. Morphological changes, thermal stability and crystallinity of fibers were investigated using scanning electron microscope (SEM), TGA and XRD technique. Thermal stability of sisal fibers were decreased on mercerization. Whereas, sisal fibers treated with benzoyl peroxide the enhanced thermal stability. In case of XRD studies, sisal fibers show enhanced crystallin ity.


Introduction
Effect o f chemical t reat ments on surface mo rphology, thermal behavior and structure of natural fibers was reported by various authors [1][2][3][4]. Chemical treat ments remove the lign in fro m su rface o f natu ral fibers and fib er su rface becomes rough. Chemical treat ments also reduce the number of free hydroxy l groups of the cellu lose, which results in the reduct ion o f the polarity o f the cellu lose mo lecu les and enh ance th e co mpatib ility with hyd ropho b ic po ly mer matrices [5]. A lkaline t reated sisal fibers were d ipped in permanganate solution at concentrations of 0.033, 0.0625, and 0.125% in aceton e fo r 1 minut e [6]. As a result of permanganate t reat ment, the hydroph ilic tendency of the fibers was reduced. Pero xide t reat ments include different concentration of benzoyl pero xide or dicu my l pero xide in acetone solut ion fo r about d ifferent minutes after alkali pretreatment [7][8][9]. Pre-treat ments of the fiber can clean the fib er su rface, chemically mo d ify the su rface, stop the mo is tu re abso rp tion p rocess and increas e th e s u rface roughness [10]. As the natural fibers bear hydro xyl groups fro m cellulose and lignin, therefore, they are amenable to modification. The hydro xy l groups may be involved in the hydrogen bonding within the cellulose mo lecules there by reducing the activity towards the matrix. Chemical modifications may activate these groups or can introduce new moiet ies that can effectively interlock with the matrix. Mercerization, isocyanate treatment, acrylation, permanganate treatment, acetylation, silane treat ment and peroxide treat ment with various coupling agents and other pre-treat ments of natural fibers have achieved various levels of success in improving fiber strength [3]. The interest in using natural flax fibers as reinforcement in b iocomposites has increased dramatically and also represents one of the most important uses.
Cellu losic fibers are hygroscopic in nature; moisture absorption can result in swelling o f the fibers which may lead to mic ro-cracking of the composite and degradation of mechanical p roperties. This problem can be overco me by treating these fibers with suitable chemicals to decrease the hydroxyl groups which may be involved in the hydrogen bonding within the cellu lose mo lecules. A nu mber of fiber surface treatments like silane treat ment, benzoylation and peroxide treat ment were carried out which may result in improved mechanical performance of the fiber and composite [2,3]. The possibility of forming mechanical and chemical bonding at the fiber surface is main ly dependent on the surface morphology and chemical co mposition of the fibers. Therefore, the microscopic analysis of fiber surface topology and morphology is of utmost importance in fibrous composites M orphology, Thermal Stability and Crystallinity of Sisal Fibers In the present paper, we have reported the mercerizat ion and peroxide treat ments of sisal fibers using sodium hydroxide and benzoyl pero xide of different concentration and for d ifferent time interval. Effect of these treatments on morphology, crystallinity and thermal stability of sisal fibers is meagerly reported in literature.

Materials
Sisal fibers were obtained fro m sisal research station, Bamra -Sarnbalpur, Orissa. Benzoyl pero xide, NaOH and acetone of 99.5% purity were supplied by S.D. Fine, India. Before chemical treat ment sisal fiber were previously cut in definite size, washed with distill water and dried at 60℃ followed by so xhlet extract ion of sisal fibers with acetone for 72 hours and purified fibers were d ried at roo m temperature.

Scanning Electron Microscopy (S EM)
Scanning electron microscopic studies of sisal fibers and all its chemically t reated fibers were carried out on Electron Microscopy Machine (LEO 435 VP, UK). Since cellu lose has a non conducting behavior so it was gold plated to have an impact.

Thermo Gravi metric Analysis (TGA)
Thermal analysis was carried out in nitrogen at mosphere at a heating rate of 10℃ / min by (Pyris Diamond) thermal analyzer (Perkin Elmer, USA) Nitrogen is supplied at a rate of 200 ml per minute.

X-Ray Di ffraction (XRD) Studies
X-ray diffraction studies were performed under amb ient condition on X-ray diffracto meter (D8 Advance, Brucker, AXS, Germany) using Cu Kα (1.5418Ǻ) radiat ion, Ni filter and scintillation counter as 40 kV and 30 mA on rotation between 5˚ to 50˚ at 2θ scale at 1 second step size. Percentage crystallinity and crystallinity index was calculated as follo wed Where I 22 and I 18 are the crystalline and amorphous intensities at 2θ scale close to 22˚ and 18˚, respectively.

Alkali Treatment
Sisal fiber were soaked in 5%, 10% and 15% (By Weight) solution of sodium hydroxide for 4 hr. at roo m temperature. After treat ment, fibers were thoroughly washed with distilled water and dried in hot air oven at 80 ℃ for 24 hours. Reaction is shown in figure 1.

Peroxi de Treatment
The pero xide treat ment was carried out on an alkali pretreated sisal fiber using benzoyl pero xide at a fixed concentration for different time intervals, reaction shown in figure 2. Sisal fibers were treated with 5% benzoyl pero xide in acetone for 30 and 45 minutes. After treatments, fibers were thoroughly washed with distilled water and dried in hot air oven at 80℃ for 24 hours

S EM Analysis
Scanning electron microscopic (SEM) p rovide an excellent technique for the study of surface morphology of raw and chemically mod ified sisal fibers. It has been observed that surface of raw sisal fibers differs in smoothness and roughness than chemically treated sisal fibers. These micrographs clearly showed the difference in their surface morphology. The raw fiber (Fig. 3a) surface is very smooth in comparison to treated fibers. Figure 3b-3d showing mercerized fibers while figure 4e-4f showing BP treated fibers.

Thermal Analysis (TGA)
TGA of raw fiber and chemical treated sisal fibers were carried out at a rate of 10℃/ min in n itrogen as a function of percentage weight loss versus temperature. The init ial and final deco mposition temperatures of raw fiber were 249℃ and 370℃ respectively with percentage weight loss of 9.7% and 70.5% respectively. The IDT for alkali, and benzoyl peroxide treated fibers are 220, 230, 225, 250 and 250℃ respectively and final decomposition temperatures are 329, 324, 317, 372 and 350℃ respectively as shown Table 1.
First stage decomposition is due to primary changes i.e. breakdown of hemicellulose, glycosidic linkage and loss of M orphology, Thermal Stability and Crystallinity of Sisal Fibers mo isture whereas the second stage of decomposition is due to cellulosic and lignin degradation. It is shown in Figure 5 and 6. Decrease in thermal stability in treated sisal fiber indicate that after chemical treat ments surface of fibers beco me rough and fiber beco me more amo rphous. The final deco mposition temperature decreases after alkali treat ments. The decrease in final stage decomposition temperature indicates that after treatments, there is much loss in cellulosic and lignin degradation as observed by several researchers [6]. The counter reading at peak intensity at 22˚ is said to represent the crystalline material and the peak intensity at 18˚ corresponds to the amorphous material in cellu lose [11][12]. Percentage Crystallin ity [13] and crystalline index [14][15] were calculated as follow: Where I 22 and I 18 are the crystalline and amorphous intensities at 2θ scale close to 22˚ and 18˚, respectively. A poor crystallinity index in case of alkali treated sisal fibers means poor order of cellulose crystals to the fiber axis during treatments, indicated by the lower crystallin ity index. Thus clearly indicate that cellulose crystals are better oriented in sisal fibers followed by alkali treated sisal fibers. The graph of raw and chemical treated fiber is shown in figure 7 and 8. X-ray results for pero xide treat ments which show an increase in the 'crystallinity' index indicate improvement in the order of the crystallites as the cell wall thickens upon peroxide treat ment. These treatments were reported to reduce the proportion of crystalline material present in plant fibers, as observed by several researchers [17][18].
The increase of crystallin ity index in pero xide treated sisal fibers ind icated that the chemical treat ments induced the crystallinity and it increase due to the removal of amo rphous materials like hemicellulose, lignin, and some other non-cellulosic material.

Conclusions
It is believed that the increase in the crystallinity index obtained by X-ray diffract ion is in actual fact an increase of the order of the crystallite packing rather than in increase in the intrinsic crystallinity. Morphology of sisal fiber was changed by chemical treat ments. The surface of sisal fiber becomes rougher after treat ments in co mparison with s mooth and clear surface of raw sisal fibers. The removal of surface impurities on plant fibers may be an advantage for fiber to matrix adhesion as it may facilitate both mechanical interlocking and the bonding reaction due to the exposure of the hydroxyl groups to chemicals such as resins and dyes.