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Figure 2. FTIR spectra of different fibers prepared by A) Carbonate Hydrolysis and B) Mild Kraft.
phenol compounds. [39] The peak corresponding to 1730 cm − 1 is from unconjugated C O stretching which is due to vibration of aliphatic carboxylic acids and ketones of hemicellulose or lignin groups and that near 1650 cm − 1 is due to conjugated car- bonyl found in hemicellulose and lignin groups. [40] Moreover, CH symmetric and asymmetric stretching bands appeared at 2900 cm − 1 and hydrogen bonded stretching bands of OH groups at 3400 cm − 1 are characteristics peaks of hydroxyl groups of cellulose. [39] All the fibers, including the hemp hurds, had similar FTIR spectra since they had similar lignocellulosic com- positions as shown in Table 1 and none of the pulping resulted in significant change in the chemical structure of the fibers.
sample determined gravimetrically using a TGA microbalance (110 ° C until weight did not change). X-Ray Diffraction (XRD) Analysis : A Rigaku SmartLab X-Ray Diffractometer, operated at 45 kV and 40 mA with a Ni-filtered CuK radiation, was used to determine the crystallinity index of the samples. X-ray diffractograms were recorded at 0.02 ° s-1 over a 2 scan in the range 5 ° –60 ° . Segal Peak height method was used to calculate the crystallinity index (CI) of the samples using the following equation:
002 AM ) ( ) = − × I I (
(2)
Crystallinity Index CI
100/
I
002
Scanning Electron Microscopy (SEM) : Handsheets of the dif- ferent fibers produced using carbonate and mild kraft pulping process were made at a target basis weight of ≈ 40 g m − 2 as per the TAPPI T205 method with a light weight ( ≈ 0.15 kg) foam roller instead of a standard heavy (13 kg) brass roller, without any pressing, and dried twice on a drum dryer at 220 ° F. The morphological characterization of samples was carried out using Hitachi S3200N variable pressure scanning electron microscope (VPSEM). Samples were sputter coated with AuPd coating for 10 minutes and images were taken at 10kV. 3. Results and Discussions 3.1. Fourier Transform Infra-Red Spectroscopic Analysis The chemical structure of the different fibers prepared by car- bonate hydrolysis and kraft process were compared in Figure 2 to examine the effect of different pulping on the fiber struc- tures. The appearance of peak in the region of 1030–1170 cm − 1 is due to C O C and C O stretch of primary and secondary hydroxide groups of the carbohydrates and hydroxyphenyl, guaiacyl, and syringyl groups of lignin. [38] The peaks at 1281, 1370, and 1427 cm − 1 may be due to aromatic esters, ether, and
3.2. Pulp Yield
The yield of the carbonate and kraft pulps (total pulp wt% obtained after defibration) were calculated and found to vary from 49%–75% depending on the source of fibers and pulping, which were comparatively higher than the pulp yield produced by conventional methods as shown in Figure 3 A. The yield of carbonate hemp pulps was 71.3%, which was slightly less than the carbonate eucalyptus and bamboo, but almost the same as carbonate hardwood and softwood pulps. However, the yield of mild kraft hemp was similar to eucalyptus but higher than the other kraft pulps. This might be due to the morphology of the hemp hurds allowing easier penetration of pulping chemi- cals compared to hardwood and softwood. [16] The pulp yield was higher for the carbonate than the kraft for all the fiber types because carbonate was operated at lower pH. Higher pH resulted in higher hydroxyl concentration which controls the intensity of fiber defibration and consequently causes random chain scission in cellulose and hemicelluloses. This promotes peeling and results in lower pulp yield because the pulp defibration rate is proportional to the OH concentra- tion. [16,41,42] Thus, kraft pulping facilitated removal of higher
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