Selecting Natural Fibers for Bio-Based Materials with Conflicting Criteria

1Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia 2Laboratory of Bio-Composite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia 3Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia


Introduction
Proper material selection became crucial process in engineering to achieve both successful sustainable designs and customer satisfaction attributes (AL-Oqla and Alves et al., 2010;Pickering, 2008;Hayajneh et al., 2005). The implementation of new materials as well as bio-composites in a specific industrial sector is limited by several constrains and factors (AL-Oqla and Dweiri and Al-Oqla, 2006;Luz et al., 2010;Blennow et al., 2014;Sgroi et al., 2014a;Hayajneh et al., 2008;Asfar and AL-Oqla, 2003). Therefore, selecting the most suitable material type for a particular application is a complex matter where proper decisions have to be taken utilizing the pairwise comparisons, which is the base of decision making process in different engineering applications (Dweiri and Al-Oqla, 2006;AL-Oqla and Hayajneh, 2007;Al-Oqla and Omar, 2012;Al-Widyan and Al-Oqla, 2011;Dalalah et al., 2010;AL-Oqla et al., 2015a).
Several studies had investigated and addressed the competitiveness, as well as capabilities and suitability of natural fibers in polymeric matrices. Most of researches focused on the mechanical properties, manufacturing processes, chemical modifications to improve fiber/polymer compatibility, as well as other technical issues. Some tried to compare between different natural fiber composites regarding certain purposes and applications. Alves et al. (2010;Mir et al., 2013;Sarikanat, 2010) studied the properties of jute/plastic composites considering several investigated characteristics such as crystallinity, thermal stability, durability, fiber modification and weathering resistance. Moreover, the suitability of such composite to the automotive industry was also investigated throughout eco-design components. In addition, Pickering et al. (2007) extensively investigated PP composites reinforced with hemp fibers throughout the injection molding process. Authors studied fibers modifications and treatments in addition to the optimization of the hemp fiber quality. Besides, Taniguchi (2001) had utilized various cellulose fibers derived from natural sources like wood pulp, cotton, hemp and seaweed, etc. In that study rotating twin disks with shear stress was used to produce Micro-Fibrillated Cellulose (MFC). Moreover, natural fiber composites were examined for automotive components throughout eco-design assessment using bagasse/PP composites by (Luz et al., 2010). Furthermore, the appropriateness of the natural fiber composites was examined by both eco-design and parametric studies for industrial applications. It was reported that (Huda et al., 2008) performed impact tests to study the impact strength of the bamboo fiber/PLA composites. According to their results, the fibers treatment processes were able to improve the adhesion forces between the matrix and the fillers which leaded to an improvement in the impact strength. Such treatments also provided effective resistance to micro propagation during the impact. Besides, a comparison between biobased plastics with petrochemical plastics was conducted by (Shen et al., 2009). This was performed regarding their technical and market potentials. They have presented recommendations for better understanding of the potentials of bio-based plastics and petrochemical ones.
On the other hand and based on the literature, there were little studies regarding precise selections of the proper Natural Fiber Composites (NFCs) for a particular application. Sapuan et al. (2011) have implemented AHP method to select the proper material for automotive dashboard panel. The properties of materials considered in that study were limited to density, young modulus and tensile strength. Moreover, Cheung et al. (2009) have discussed the potential of natural fiber composites through mechanical and thermal properties of the animal based fiber composites. They have mentioned the potential of both plant and animal for the biomedical engineering applications. They also introduced some factors to be considered in screening general materials for that branch of applications including being bio-inert, bi-functional and bio-active.
The NFCs final products' performance and properties strongly depend on the properties of their individual constituencies as well as the polymer/filler interfacial characteristics. Based on literature; proper evaluation of NFCs composites for industrial applications is not enough discussed regarding wide range of desired criteria. On consequence, there is a lake of information regarding selecting the proper natural fiber composite materials for many applications. This is due to the large possibilities of generating new natural fiber composite materials with novel properties (AL-Oqla et al., 2015a; Kalia et al., 2011).
Therefore, further comparisons between the NFCs constituents (fillers and matrices) are still needed in an extensive manner regarding combined and integrated desired criteria and factors that may affect their selection in different applications to end up with proper consistent informative selection decisions about the natural fibers capabilities. This in order would dramatically help designers and decision makers to reach the best choice of NFC materials according to their design criteria and limitations. Consequently, this study aims to elaborate more desired characteristics that designers have to take into consideration regarding selecting NFC materials and their constituents to achieve better performance to their designs. Also, to conduct some pairwise comparisons between different fiber types regarding some selective criteria to emphasize the need for better NFCs evaluations regarding further integrated criteria.

Materials and Methods
To develop and enhance better evaluations of the natural fibers and their capabilities, necessary factors for their selection in the bio-based composites were determined. These factors were then integrated and utilized to express the potential capabilities of the available natural fibers through a combined evaluation scheme. The considered evaluation criteria included physical, chemical, mechanical as well as economic characteristics. To expand the future sustainable design possibilities, categories of criteria were utilized to evaluate fibers' capabilities to enhance achieving better decisions regarding selecting the most appropriate fiber type for polymeric based composites. To illustrate the effectiveness of such desired keen evaluations on the selection process, pairwise comparisons have been conducted between fibers regarding both single and integrated criteria. For each considered factor, an average value was used for the comparison purposes. Each comparison was interpreted in a proper separate figure to facilitate addressing the variations in fiber's capabilities.

Results
The complete NFCs attributes and capabilities depend on the physical and chemical composition of the inherent materials. Therefore, NFCs are to be investigated regarding many different features and properties before being considered to be used in any particular application. Such considerable criteria of selecting the natural fiber composite products can be according to Natural Fiber Properties (NFP), Polymer Base Properties (PBP), Composite Characteristics (CC) and Composite Performance (CP) (AL-Oqla and , where different attributes and characteristics have to be investigated and taken into account to reach better evaluations of the NFC materials like physical, mechanical, chemical, biological and environmental properties as well as toxicity, bio-stability, life cycle, durability, biodegradability, weather resistance, occupational health and safety. Criteria that affect selection of natural fibers are shown in Fig. 1. Such criteria can be the key driver for the industry to enhance selection the most appropriate NFC materials for a given application through better selection of the composite constituents. Comparing various natural fiber types considering wide desired criteria required reliable date from keen experimental works (AL-Oqla and Pickering, 2008;AL-Oqla et al., 2014a;Pilla, 2011). Various properties of the available natural fibers are shown in Table 1. In such comparisons, the average values of various properties were utilized.
Comparisons between different natural fiber types namely coir, date palm, hemp, sisal, flax and jute are conducted regarding their specific modulus of elasticity and specific strength as shown in Fig. 2. and 3 respectively. It can be clearly shown that natural fibers vary in their physical and mechanical properties. These comparisons actually compounded fibers' modulus of elasticity and specific strength relative to their densities to give more relative information about different fiber types.

Discussion
It is clear that flax, hemp and jute are the best in both cases of specific strength and specific modulus. But further investigations regarding wider criteria rather than physical and mechanical properties are needed to make more realistic informative decisions regarding the best fiber type. That is; further criteria like cost will change the priority of the previous three fiber types as seen in Fig. 4 where the specific strength to the cost ratio comparisons were conducted. It is obvious that date palm fibers are the best type among all other types if further economic criteria are considered. It can be seen that date Palm fiber is about four times better than jute.
On the other hand, a chemical composition investigation between commonly used fibers in addition to the date palm one regarding all of cellulose, lignin and hemicellulose contents reveals the effect of cellulose content in the mechanical properties of such fibers. Such comparisons are illustrated in Fig. 5. It can be deduced that fibers with higher cellulose contents possess better mechanical properties. That is; sisal, hemp, flax and jute have relative higher values of specific moduli and specific strengths than coir and date palm fibers ( Fig. 2 and 3) as they have higher cellulose contents (Fig. 5).
On the other side, the amount of cellulose content has a negative effect on other beneficial characteristics of the natural fibers, as that of the elongation to break one. A comparison between various fibers regarding the elongation to break property (Fig. 6) reveals that coir and date palm have much better elongation to break characteristics than other fibers because of their lower contents of cellulose. This in order make the selection of the best natural fiber type for polymeric composites is a complex matter with conflicting criteria problem. Therefore, extensive comparisons of fibers are still needed to be conducted regarding wide beneficial criteria to establish a date base for more proper evaluations of the available natural fibers' capabilities. This in order would dramatically lead to expand their implementations in various green products to enhance environmental as well as industrial sustainability. Moreover, proper comparative studies between fibers' capabilities would enhance finding new potential fiber types with optimum conflicting desired characteristics that may improve the overall desired characteristics of the bio-based materials like that of date palm fibers.

Conclusion
Selecting an appropriate natural fiber type to form NFCs is affected by several criteria and considered as a multi-criteria decision making problem. Better evaluations of natural fibers regarding wide range of criteria have several advantages. This can lead to better informative decisions regarding selecting the suitable NFCs for industrial applications on one hand, and enhance achieving better performance on the other. Evaluations of NFCs' constituents should consider combined economic and environmental characteristics as well as technical ones to be able to achieve more reasonable confident decisions. Moreover, new potential fiber types can be discovered and utilized through better evaluations using wider desired criteria.