Abstract
Bamboo (Arundinaria clarkei) short fibers were treated with 6% conc. NaoH solution for five different durations viz. 3 h, 6 h, 9 h, 12 h and 24 h. Effect on physical properties like diameter and density were studied. Fibers were examined using Scanning Electron Microscope (SEM) to study the effect of alkali treatment on its surface characteristics. Single fiber tensile tests were conducted to assess the tensile strength of the fibers. Comparative study of the SEM micrographs indicated removal of outer layer with the surface irregularities becoming more prominent with increasing duration. The fiber surface was scanned using an Atomic Force Microscope (AFM) to quantify the increase in surface roughness with alkali treatment. Tests results showed positive effect on the strength till 6 h beyond which the tensile strength was observed to reduce. Fourier Transform Infrared (FTIR) spectroscopy results indicated reduction in content of holocellulose along with hemicellulose, lignin and cellulose which explains the variation in tensile strength with increasing duration of treatment.
Introduction
With the advent of green lens in technical advancement, an increased attempt has been made to replace more and more synthetic products with organic materials. Synthetic fibers possess threat not only to the environment but also to humans and hence are being replaced by natural fibers wherever possible. Natural fiber reinforced synthetic polymers are the products of this attempt. These materials retain the natural element without compromising much on the strength of the materials and can be increasingly used in the industry and real world. They are available in abundance, renewable, biodegradable at the end of its useful life, light in weight, possess acceptable properties and are less expensive. Energy requirement in producing natural fibers especially vegetable fibers is less and for several applications natural fibers have potential to replace synthetic fibers [1, 2]. Several types of natural fibers have been tried by researchers worldwide which include fibers like flax, banana, hemp, areca, sisal, okra, kenaf, ramie, jute, borassus, etc. but bamboo is the one which has a great potential among these due to its superior mechanical properties and also due to the fact that it grows rapidly thereby catering to ever increasing demand for naturally obtained reinforcement [3, 4].
A major disadvantage encountered while using natural fibers like bamboo is the presence of natural impurities and dead cells which are scaled, degraded and do not provide good surface for the action of the synthetic matrix materials of composites [5]. For this purpose, the fibers are initially treated with mild bases to provide an enhanced surface. There is a lot of debate over what composition and time is optimum for treatment of natural fibers [6].
From review of relevant literatures, it is observed that the concentration on NaOH varied from 1 to 10% by weight, duration up to 24 h, and with or without application of heat [7, 8, 9, 10]. Researchers have also attempted combination of alkali and acetylene which resulted in reduced water absorption, improved thermal stability, surface roughness but reduced tensile properties [11]. Treatment methods other than alkalization tried by researchers are salinization, acetylation, benzoylation, malenization, isocyanate treatment, peroxide treatment, enzymatic treatment, corona and cold plasma treatment [12, 13, 14, 15, 16, 17, 18].
This work involves treating bamboo fibers over varied periods of time to choose the iteration best suited for its use in composite materials. Fibers were treated with 6% NaOH for 3 hours, 6 hours, 9 hours, 12 hours and 24 hours respectively. These samples were tested for structural and physical integrity through various tests. An FTIR analysis was also conducted for all the samples to compare the relative composition at various steps.
Materials and methodology
Material
Fibers from bamboo plant (Arundinaria clarkei) which is native to north eastern region of India were obtained from Manipur, India. From the supplier it was learnt that the fibers were extracted by retting process from the culm of mature bamboo shoot. The strips were cut and soaked in water for 4 days. This allows fiber separation from the flesh of the culm. The flesh was combed out mechanically and washed with distilled water. The fibers were then dried, cut into 50 mm length (approx.) and subjected to chemical treatments.
Chemical treatment
The fibers were divided into 6 batches, based on the duration of the treatment-untreated (UT), 3 hours (3 h), 6 hours (6 h), 9 hours (9 h), 12 hours (12 h) and 24 hours (24 h). Each batch, consisting of about 30 g of fibers, was treated with 6% by weight of NaOH. The fibers were then washed thoroughly with distilled water and oven dried at 50C for 3 hours. The dried fibers were then tested for their mechanical and physiological properties.
Diameter measurement
Diameter measurements of the fibers were taken for untreated and treated fibers from randomly selected fibers from each batch using a Tool Makers Microscope (Make: Mitutoyo). Each fiber was measured at five different points evenly spaced along the fiber length, along 4 different orientations. An average of the total 20 readings for each fiber was considered as the fiber’s average diameter.
Density test
Density readings of the fibers were obtained using a Digital Density Balance (Make: Contech) which would give the density reading with respect to the volume of the liquid displaced due to the addition of fibers, knowing the fiber mass. The fibers were immersed in Hexane (C
Tensile test
About 5 samples, from UT, 3 h, 6 h and 9 h and 12 h batches, were tested for tensile strength. Tensile test for 24 h fibers were not possible due to excessive fiber deterioration by alkali action. The test was conducted on an Instron Universal Testing Machine (UTM) (Model: 3366) at the rate of 1 mm/min loading, according to ASTM D3822 [19]. The gauge length was 20 mm for fibers of all batches. For better gripping of fibers in the UTM jaws, the fiber ends were paper tabbed using adhesive.
Scanning electron microscopy
A Scanning Electron Microscope (SEM) (Make: Zeiss) was used to view and record images of the surface of fibers. Images of untreated fibers and treated fibers were recorded to compare the variations in the surface morphology as a result of the chemical treatment. Roughly 10 mm length of the fiber was cut (from various regions of the fiber) and was mounted on a carbon tape stub. The fibers were observed for surface variations such as degraded external surface, formation of spots and patches.
Surface roughness
In order to get a better understanding of the surface morphology of the fibers and also to quantify the surface texture as a result of the chemical treatments, an Atomic Force Microscope (AFM) (Make: Bruker) was used. Fibers of approximate 10 mm length were mounted on a carbon tape stub and viewed under AFM. Surface readings were taken at various locations of the fiber and the surface roughness values (Ra value) were recorded. The length considered here was 10
Fourier transform infra-red spectroscopy (FTIR)
FTIR analysis was conducted for the samples of untreated and treated fibers. A routine of 25 scans with a resolution of 4 [1/cm] was adopted. The samples were ground with Potassium Iodide for the testing. An FTIR spectrometer (Make: SHIMADZU Corporation) was employed.
Results and discussion
Surface roughness
Surface roughness plays an important role in the binding ability of the fiber with the matrix. The optimum treatment required for an effective fiber-matrix system should result in high roughness value as well as good tensile strength. Variation in surface roughness with increasing duration of treatment is shown in Fig. 1. It’s observed that the Ra (average roughness value) and Rq (root mean squared value) increases as the treatment is continued beyond 3 h. They reach a peak value with maximum roughness value at 6 h (upon comparing Ra values) and 9 h (upon comparing Rq values) and it starts falling again. This is probably due to the action of alkaline treatment which wears out the surface fibers. The treatment erodes the surface, exposing fresh rougher layer underneath. When treated beyond 9 h, the surface texture smoothened out due to prolonged erosion. The abnormal increase in the roughness value of 24 h due to the extensive deterioration of the fiber.
Diameter measurement
Minor differences in the variation of the diameter was observed upon treatment (Table 1). The decrease in diameter of the treated fiber (in comparison to the untreated fiber) is due to the removal of surface layers. The small increments in the diameter over the duration of treatment is probably due to inherent difference in the diameters of the natural fibers in their raw state, deposition of salts during treatment or unravelling of fiber sub-strands.
Variation in diameter of bamboo fibers
Variation in diameter of bamboo fibers
Variation in density of bamboo fibers
Variation in surface roughness.
It is seen that density of the treated fiber is slightly greater the untreated fiber. The density values are presented in Table 2. This is supported by the results from the diameter measurement which varied due to the combined effect of removal of hemicellulose, pectin, etc., and deposition of the salts and loosening of fiber sub-strands. The dip in the density of the fiber after 9 h of treatment is due the major mass loss from removal of outer fiber layers.
Variation in tensile strength.
Untreated fibers.
Figure 2 shows the variation in tensile strength of the bamboo fibers with variation in duration of treatment. The tensile strength improves due to treatment till duration of 6 h beyond which a drop in tensile strength is seen. With 3 h of treatment the tensile strength increased by about 45% which increased further by about 48% or an overall increase by 115% when compared with the strength of untreated fibers. With increased duration of treatment a drop in tensile strength was seen which, was an indication of fiber weakening signifying that, around 6 h of treatment with NaOH of concentration 6% by weight is optimum. Though after 9 h of treatment the average tensile strength dropped, large variation in tensile strength was observed and one fiber among them showed a tensile strength of around 605 MPa. This suggests that there is a possibility of improvement in tensile strength beyond 6 h and close to 9 h which can be explored further in future.
Scanning electron microscopy
SEM images of untreated and treated bamboo fibers are shown in Figs 3–8. Surface of the UT fibers (Fig. 3) appears to be smooth mainly due to presence of a waxy/oily sheath that conceals the ridges on the fiber surface. 3 h NaOH treated fiber (Fig. 4) shows partial degradation of outer surface with impurities still adhering onto the surface. The degradation is a result of reaction between NaOH, lignin and hemicellulose present in the outermost layer of the fiber. 6 h and 9 h treated fibers (Figs 5 and 6) show prominent ridges indicating further removal of the lignin and hemicellulose layers. 12 h and 24 h treated fibers (Figs 7 and 8) are observed to have degraded to the extent that micro-coils are now visible especially in 12 h samples, indicating that the alkali treatment has completely degraded the outer layer of the fiber. 24 h fiber samples show ridges which are prominent and have few micro-coils indicating excessive removal of material making the fiber very weak.
Fibers treated for 3 h.
Fibers treated for 6 h.
Fibers treated for 9 h.
Fibers treated for 12 h.
Fibers treated for 24 h.
FTIR graph of untreated to 24 h (bottom to top).
The SEM images suggest that, the 6 h–9 h duration seems to be the optimum range for 6% NaOH treatment on the bamboo fibers. 3 h treatment is not sufficient enough to remove lignin and hemicellulose layer. 12 h and beyond proved to be harmful for the structure of the fiber.
FTIR plots of all the fibers are shown in Fig. 9. Hemicellulose, Holocellulose, Lignin and Cellulose are the organic compounds present in the fibers which contribute to the structural integrity of bamboo fibers [8, 20, 21]. In the FTIR analysis, wavenumber 2357 corresponds to the acetyl groups present in Holocellulose which comprises about 65% of the bamboo fibers. The results for samples of untreated fibers, bamboo treated with 6% NaOH over 3 h, 6 h and 9 h show distinctive peaks of equal intensity for Holocellulose. There is a change of trend on further treatment for 12 h and 24 h. Holocellulose degradation begins at 12 h of treatment and the peak decreases in intensity. We observe a huge shift in the peak intensity, suggesting 70–85% degradation in the structure of Holocellulose.
Wavenumber 1053 corresponds to C-O stretching in Cellulosic component of the fibers. These peaks appear distinctively in treatment analysis of 12 and 24 h of NaOH treatment. The lower orders of treatment do not show any stretching in Cellulosic groups.
2922 wavenumber appears in all the samples analyzed. The intensity of the peak increases progressively from 3 h to 24 h of treatment. In aliphatic carbon compounds C-H stretching, both symmetric and asymmetric is denoted by this wavenumber. The glucose units present in the cellulose chains are the aliphatic chains displaying this trend over successive treatment periods with 6% NaOH. The increase in trend is almost uniform.
Conclusion
From the results it’s observed that surface treatment provided to the bamboo fibers had a positive effect on its tensile strength as an improvement was seen till 6 h duration beyond which it dropped. A large variation in diameter and density as a result of treatment was not seen which can be attributed to combined effect of removal of fiber elements, deposition of salts and loosening of fiber sub-strands. SEM micrographs suggests that duration somewhere between 6 h and 9 h is optimal for removal of impurities, waxy layers which otherwise would make the fiber-matrx interface weak in a composite. FTIR spectroscopy shows removal of holocellulose and thereby strengthens and supports the tensile strength results.