ALTERNATIVE COCONUT FIBER SURFACE TREATMENTS METHODS FOR THE DEVELOPMENT OF POLYMERIC MATRIX COMPOSITES
F. O. Yamane*, M. L. A. da Silva, D. I. França, M. V. Mantuvanelli, D. Thomazini, M. V. Gelfuso
Instituto de Engenharia Mecânica, Universidade Federal de Itajubá - UNIFEI,
Av. BPS, 1303, CEP 37500-903, Itajubá, MG, Brazil
*email:
Universidade Federal de Itajubá - UNIFEI
ABSTRACT
The coconut fibers are an abundant source of natural materials, renewable, high-availability, accessibility and biodegradable. This great interest in developing and using materials made from renewable sources occurred mainly in response to the global attention for the use of natural products and the preservation of the environment. In this paper, coconut fibers received different surface treatments, environmentally friendly, based on high values of temperature and pressure. The coconut fibers were characterized by FTIR, SEM, water absorption, weight loss and wetting angle. The results indicated that the methods were effective, changing the chemical composition of the fiber as well as their surface morphology and raising the hydrophobic character. Thus, the compatibility between the fiber and the polymeric matrix should be increased, providing the application of these new fibers in the development of composite materials.
Keywords: coconut fiber, surface treatment, hydrophobic character.
INTRODUCTION
Recently, the green coconut fiber (GCF) application is growing attention due to its easy availability. The coconut husk is available in large quantities as residue from coconut production in many areas, which is yielding the coarse coir fiber. Because of its hard-wearing quality, durability and other advantages, it is used for making a wide variety of floor furnishing materials, yarn, rope etc. GCF is the most interesting products as it has the lowest thermal conductivity and bulk density as compared to other natural fibers (1).
Nowadays, there is an increasing environmental consciousness and awareness of the need for sustainable development, which has raised interest in using natural fibers as reinforcements in polymer composites to replace synthetic fibers(2). The advantages of natural fibers include low cost, low density, unlimited and sustainable availability, and low abrasive wear of processing machinery(3).
Natural fibers are hydrophilic in nature as they are derived from lignocellulose, which contain strongly polarized hydroxyl groups. These fibers, therefore, are inherently incompatible with hydrophobic thermoplastics, such as polyolefins. The major limitations of using these fibers as reinforcements in such matrices include poor interfacial adhesion between polar-hydrophilic fiber and non-polar-hydrophobic matrix, and difficulties in mixing due to poor wetting of the fiber with the matrix. This in turn would lead to composites with weak interface (4).
The synthesis of polymer composites containing lignocellulosic fibers will often result in fibers physically dispersed in the polymeric matrix. In the majority of cases, poor adhesion and consequently poor mechanical properties result. Hence, surface treatment of the fibers is essential. Generally surface modification of lignocellulosic fibers is not necessary to improve the bonding for the preparation of biodegradable composites, in view of the similar chemical nature of both the fiber and matrix, which have a hydrophilic nature, unlike the situation with synthetic polymers, which tend to be hydrophobic(5).
However investigations carried out so far (6-8) have shown that coir fiber are not an effective reinforcement for polymer matrix composites. The water adsorbed into the lignocellulosic surface of the hydrophilic coir fiber apparently prevents an efficient adhesion to the hydrophobic polymer matrix, which also happens in other natural fiber composites (9,10). As a consequence the incorporation of coir fiber tends to decrease the mechanical strength of polyester composites for any volume fraction of fiber (8). In principle, there are ways to reverse this decreasing mechanical properties condition. Therefore, Monteiro and co-authors (11) proposed, a strong alkali treatment of coir fiber improves the adhesion to the polyester matrix and thus increases the composites strength by approximately 50 % for a volume fraction of 30 % of coir fiber.
In this paper were examined ecofriendly surface treatments,avoiding the use of acids, bases and organic solvents. These treatments, based on heating and pressure, are important to improve the adhesion between the coconut fiber and the hydrophobic polymer matrix.
EXPERIMENTAL
Green Coconut Fibers - GCF
GCF are a by-product of empty coconuts waste, available in large quantities in the coastal regions of Brazil, after drinking green coconut water. Coir mesh matting supplied by EMBRAPA (Brazilian Enterprise for Agricultural Research), Fortaleza, Brazil) is produced from a process based on drying, grinding, and sorting of fibers starting from green coconut shells.
Surface Treatments
The GCF were treated with two different innovative methods. The first treatment involves keep the fibers under pressure of 2 atm during two hours in contact with water at 120º C, was defined as “pressure”. The second treatment is to make the fibers keep in contact between ceramic plates at 200º C during 10 seconds, was defined as “plates”.
Characterizations
The natural and the two kinds of treated GCF were analyzed by the following techniques: scanning electron microscopy, SEM, at Federal University of São Carlos - UFSCAR, model JEOL 5800 LV;infrared spectroscopy with technique of attenuated total reflectance, FTIR-ATR from Perkin Elmer,which requires an easy sample preparation:the sample must be in direct contact with the ATR crystal, andpotassium bromide (KBr) is not necessary. Finally, tests of water absorption, contact angle and weight loss were realized to evaluate the effects on fibers surface.
RESULTS AND DISCUSSION
SEM
The Fig.1 illustrates that the surface of the GCF is covered with a layer of substances, these may include pectin, lignin, waxes and impurities. The surface was not smooth, spread with nodes and irregular stripes(12), showed fibrils that constitute the fiber were covered with a layer of a cuticle which has been identified as aliphatic wax (13). The surface appearance of the treated fibers is more homogeneous than the natural fiber.
Figure 1 - Micrography of natural GCF (A), treated with pressure (B) and treated with plates (C)
FTIR/ATR
All the spectra of GCF, Fig. 4, 5 and 6,reveal a broad and intense peak at 3340 cm-1 suggesting hydrogen-bonded ν (O-H) stretching vibration from the cellulose and lignin structure of the fiber.The carbonyl (C=O) signal at 1728cm-1, is from lignin and hemicellulose; the characteristic band is attenuated in the treated fibers.The band at 1241 cm-1 is related to the vibration ν (C-O) of esters, ethers and phenols groups attributed mainly to a presence of waxes in the epidermal tissue and the attenuation of this band in the treated fibers results from the removal of those waxes(14).
Figure 4 - Infrared spectra of natural GCF
Figure 5 - Infrared spectra of GCF treated with pressure
Figure 6 - Infrared spectra of GCF treated with plates
Water absorption, contact angle and weight loss
These parameters are important to analyze the influence of surface treatments on the hydrophilic character of the materials. The Tab. 1 shows the obtained values for the natural and treated GCF.
Table 1 - Contact angle, maximum water absorption and weight loss for the natural and treated GCF.
Samples / ContactAngle / Maximum Water
Absorption / Weight
Loss
Natural / 38,9 ± 4,9 / 156 %
Pressure / 53,1 ± 3,4 / 149 % / 8 %
Plates / 36,5 ± 2,2 / 94 % / 0,1 %
Comparing the maximum water absorption of GCF treated with pressure, it is observed a reduction of 4,5 % in the value obtained from the natural GCF and an increase about 39 % in the contact angle.The weight loss of 8 % confirms the material removal of the superficial layer of this fibers.
Analyzing the results for GCF treated with plates, it is observed a reduction of 39,7 % in the maximum water absorption and an reduction about 6 % in the contact angle. The lower value found for water absorption (94 %) can be explained by modifying the morphology of the fibers with the surface treatment, according to the micrograph of Fig. 3. The protrusions presents on the natural GCF are not present at the same intensity in the fiber treated with plates, generating a smooth surface, hindering the water absorption. The weight loss value of 0,1 % is not significant, indicating that the surface material was not efficiently removed. This treatment only provides a redistribution of the surface material, affecting the morphology of the fiber.
As expected, greater the angle, lower the percentage of water absorption, consequently the material presents a less hydrophilic character. That’s exactly what happened with the GCF treated with pressure, indicating that the fiber becomes more compatible with the hydrophobic polymeric resin.
CONCLUSIONS
The results indicated that the proposed methods were effective, changing the chemical composition of the fiber as well as their surface morphology and raising the hydrophobic character of GCF. The FTIR technique shows that the bands related to the functional groups present on lignin molecules were attenuated with these ecofriendly treatments, based on heating and pressure.
The obtained values of contact angle for GCF treated with pressure increased around 39 % comparing to the natural fibers, indicating that the hydrophobic character of the fiber increased significantly. Thus, the compatibility between the fiber and the polymeric matrix should be increased, by using ecofriendly methods, providing the application of these fibers in the development of new composite materials.
ACKNOWLEDGEMENTS
The authors thank FAPEMIG (Minas Gerais State Foundation for Research Development) for the financial support. We are also indebted to Natalia Zanardi, From Federal University of São Carlos for performing the scanning electron microscopy.
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