Guadua angustifolia bamboo fibers as reinforcement of polymeric matrices: An exploratory study
Introduction
Automotive, aerospace and construction industries have shown special interest in developing new polymeric composites due to its versatility of manufacture, and especially to the high cost of production of traditional materials. Developed countries use around 50% of polymers produced on the manufacture of composite materials [1].
Synthetic fibers such as carbon, aramid, glass and nylon are commonly used as reinforcement in composite polymeric materials. However, in the last two decades natural fibers have become an important resource in composite industry [2], [3], [4], [5], [6]. The average annual growth rate of global market for polymeric composites reinforced with natural fibers was 38% from 2003 to 2007 [7]. In the future, it is expected that the application of this kind of materials increases.
The use of natural fibers as reinforcement of polymeric matrices gives multiple benefits such as lower abrasion during production, possibility of recycling, higher mechanical properties per weight unit, and lower manufacturing cost per volume unit [8]. Nevertheless, the potential of natural fibers has not been completely profited in the polymeric composite industry due to its physicochemical incompatibility with commonly used hydrophobic matrices (polystyrene, polyester, polyethylene, among others). Natural fibers are composed by a high content of cellulose that confers its hydrophilic properties, resulting in a low compatibility between the reinforcement and the matrix, which are the composite phases. Low compatibility may bring poor stress transfer between phases and thus affect the mechanical properties of the final composite material, and in consequence, its use may be limited [2], [9], [10], [11], [12].
The literature reviewed presents the two commonly used ways to improve the compatibility between polymer matrices and natural fibers: the first is the modification of the physicochemical properties of the matrix, using reactive extrusion processes [13], [14], [15]. The second alternative is the modification of the physicochemical properties of natural fibers. This modification can be performed in three different manners: by using coupling agents [2], [9], [11], [16], [17], [18], [19], [20]; by a graft polymerization of monomers compatible with the polymer matrix [1]; and subjecting fibers to plasma treatments [21], [22], [23], [24], [25], [26], [27], [28].
At industrial level, modification of the physicochemical properties of natural fibers is the most used alternative to increase the compatibility between composite phases. The traditional method to improve the bonding between fiber and matrix employs coupling agents that react with the hydroxyl groups present in the amorphous region of fibers, leaving the cellulose structure exposed in order to bond the polymer matrix. The main disadvantage of using treatments of this type is that they generate a reconfiguration of proportions of the constituents (cellulose, hemicellulose and lignin), reducing the mechanical strength of fibers [29]. However, several studies [2], [9], [11], [16], [17], [18], [19], [20], suggest that the mechanical performance of the composite improves when the natural fibers employed have been previously treated with coupling agents.
The modification of surface properties of fiber using plasma is an innovative technique; the aim of this treatment is to increase the mechanical grip on fiber–matrix interface by modifying the surface properties of fibers without major changes in its molecular structure [28]. Some research has been made using natural fibers aiming to evaluate the efficiency of this technique, where different gases have been used, among which the most common were He and Ar. Different exposure times and electrical currents have also been used [21], [22], [24], [23], [25], [27], [26]. In order to establish the effectiveness of the treatment, various polymer matrices have been reinforced using untreated and plasma treated natural fibers [21], [22], [25], [27], [26]. The main conclusion of these researches is that the composite tends to be more resistant as a result of the increased bonding between fibers and the polymer matrix. Such resistance was attributed to the increasing surface roughness of the reinforcing material.
This paper presents the results obtained during an exploratory study of some techniques employed to modify the surface properties of Guadua angustifolia bamboo fibers. In order to do this, a common coupling agent such as sodium hydroxide (NaOH) and a novel plasma technique were used. The surface appearance was assessed using a scanning electron microscope, while the influence of each treatment on the mechanical properties was evaluated using tensile tests. These treatments were made in order to, in second stage, develop a polymeric composite material using as reinforcement Guadua angustifolia bamboo fibers, that could be used as non-structural material in buildings.
Section snippets
Fiber extraction
Guadua angustifolia bamboo fibers were extracted from the bottom part of culms, using a compression load similar to that described by [30]. Initially, bamboo raw samples were cut into segments of rectangular cross section with 10 cm of length, which were immersed in a solution containing sodium hydroxide (NaOH) at 10% during 96 h. After immersion, segments were carefully washed up using tap water. The rectangular segments were crushed, applying compression loads of about 100 kN. Finally, the
Fiber mechanical testing
Table 1 presents the results of the average tensile strength for Guadua angustifolia bamboo fibers. N values correspond to fibers without treatment; P to fibers with plasma treatment; H210 and H260 to fibers immersed in sodium hydroxide with a concentration of 2% for 10 and 60 min respectively; and H1010 and H1060 to the same treatment with a concentration of 10% for 10 and 60 min respectively.
Results showed that plasma treatment does not reduce the tensile resistance; similar results were found
Conclusions
It can be concluded that the treatment with the coupling agent (sodium hydroxide solutions) used in this research notably affects the tensile strength of Guadua angustifolia bamboo fibers. Although the coupling agent works as surface cleaner, it also affects the tensile strength because it removes the hemicellulose and lignin constituents. SEM images suggest that this treatment works as a surface cleaner, but for higher concentrations and prolonged times it results on the division of macrofiber.
Acknowledgements
The authors acknowledge the support given by the Universidad Nacional de Colombia, for the development of this work, through DIB proyect 18946.
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