Enhancement of the Mechanical Behavior of Starch-Palm Fiber Composites

Abstract

This study discusses the fabrication of starchbased hybrid composite reinforced with chopped randomly oriented flax, sisal, and date palm fibers. The tensile properties, before and after chemical treatment, as well as the morphology of the fibers were evaluated. The hybrid composites were fabricated using hot compaction technique at 5MPa and 160°C for 30min. Fracture surface investigations using field emission scanning microscopy showed a good adhesion between fibers and matrix. The fracture surface revealed the presence of matrix micro cracks as well as fibers fracture and pullout. Hybrid composites containing 20 vf % sisal, and 5 vf % flax at 25 vf % date palm as well as 35vf% sisal, and 5 vf % flax at 10 vf % date palm had the optimum mechanical properties and consequently can serve as competitive ecofriendly candidates for various applications. A finite element (FE) approach was developed to simplify the treatment of random orientation of chopped fibers and predict elastic modulus using Embedded Element technique. Analyses based on rule of hybrid composite (ROHM), COX rule, and Leowenstein rule are presented to validate both experimental and FE numerical results. The FE results compared favorably with the experimental results. Introduction The construction of natural fiber bio-composite may have very good applications in the automotive and transportation industry such as car door panels which may save up to 45% from door panel carrier weight, bio-based cushions, the driver’s seat back rest, etc. Moreover, reducing cost of bio-composites will be more desirable to industrial economic development [1]. Biodegradable composite materials based on natural fibers and starch had attracted attention over the past several years. Starch is one of polysaccharide matrices. Owing to its low cost, availability as a renewable resource, biodegradable and nontoxic degradation products, it is one of the important raw materials used for packaging, biomedical applications, and in some By-Products of Palm Trees and Their Applications Materials Research Forum LLC Materials Research Proceedings 11 (2019) 201-210 doi: https://doi.org/10.21741/9781644900178-15 202 automotive parts. Starch, however, has some drawbacks such as poor melting process ability, high water solubility, difficulty of processing, and brittleness. Gelatinization process converts starch to thermoplastic starch (TPS) and improves those draw backs [2-3]. Date-Palm fiber (DPF) is a low cost material with mechanical properties that depend on the place of extraction. DPF can be considered one of the best types of fibers regarding several evaluation criteria such as specific strength to cost ratio if compared to other fiber types [4]. Sisal fiber (SF) is known by its high strength but it has some limitations such as high cost and is not cultivated in Egypt [5]. Flax fiber (FF) has mechanical properties near to SF; however, the cultivation of Flax has been diminished in Egypt as it can be replaced by other imported materials [1, 6]. Several fiber types are incorporated into hybrid composites and such composites can be tailored to meet various design requirements in a more economical way than conventional composites. Their behavior depends on the characteristics and the mechanical properties of the incorporated fibers [7]. Several factors will affect the composite mechanical properties such as fiber type, length, orientation, characterization, resin type, and volume fraction of the reinforcements [8]. The objective of the present work is to study the behavior of starchbased hybrid composites containing three types of fibers, namely, DPF, FF, and SF, and to compare the mechanical properties obtained to flax/date palm hybrid composite at 1:1 matrix/ fiber volumetric ratio. The present work involves both experimental and numerical investigations. Composite preparation stage was performed by using different mixtures of fibers with different volume fraction as shown in Table 1. The composite analysis first stage was based on measuring mechanical properties, examining the fracture surfaces, and applying the morphological characterization of the materials. Finally the finite element analysis (FEA) stage; where different models were implemented using ABAQUS software. Mixed FE-analytical approaches are suggested for the prediction of the Young’s Modulus of reinforced composite having randomly oriented chopped fibers. An attempt is suggested to overcome the difficulty of representing random orientation of chopped fiber across composite in finite element representation. The attempt is based on unidirectional fibers having 33.3% volume fraction to represent the actual fiber volume fraction of randomly oriented chopped fiber in the composite. Experimental Work Materials Preparation, Characterization, and Mechanical Testing: Corn Starch was purchased from Aro Sheri Company in Egypt with an average particle size of 16μm. Glycerin with 99.7% purity was used as a plasticizer. Gelatinization process of starch following Ref. [3] methodology is used to form TPS by mixing native starch with 30Wt. % glycerin and 20Wt. % distilled water in temperature range from 60–80C. Adding glycerin improves process ability and reduces embrittlement by inhibiting the retro gradation process. The TPS was kept in polyethylene bags over night to enhance its flow properties before being used. Flax and sisal strands were donated by the Egyptian Industrial Center E.I.C. DPFs were extracted from the stem of date palm trees at the American University in Cairo. Sodium hydroxide (NaOH) with molecular weight 40g/mol. was used for alkaline treatment of fibers. The three fibers (DPF, FF, and SF) were chemically treated using the following procedure: 1) Dipping in 5% NaOH for 3 hours at room temperature. 2) Rinsing the treated fibers in cold water. 3) Dipping the fibers in 5% acetic acid to remove any excess NaOH from fibers surface. 4) Rinsing in cold water and oven drying at 120°c for 3hrs. 5) The treated fibers were cut By-Products of Palm Trees and Their Applications Materials Research Forum LLC Materials Research Proceedings 11 (2019) 201-210 doi: https://doi.org/10.21741/9781644900178-15 203 manually into short fibers with average length varies from 15 to 30mm according to the aspect ratio. Characterization and testing were performed using the following procedures: 1) Measuring fibers diameter before and after chemical treatment by Leica stereoscopic microscope using 10 samples with a μm divisions scale lens. 2) Measuring density of TPS and fibers using the Mittler Toledo densitometer for 10 samples (Xylene was used as the immersing liquid with relative density is 0.86). 3) Tensile testing using Instron 3382 universal testing machine at 50% RH, 18oC and strain rate of 0.01per min. with a gauge length of 50mm at strain rate of 0.01/min. 4) Fracture surface study of fibers Using ZEISS scanning electron microscope (SEM) operated at a vacuum pressure 1e-4 mbar and 8KV. Hybrid Random Composite Preparation: The different composites were prepared using 1:1 fiber to matrix volume fraction according to Eqs. 14. The DPF was used with 50vf% to 20vf% of fiber at different SF and FF volume fraction percentages. The fiber cutting length was based on fiber aspect ratios. Stearic acid with concentration 98% was used as a mold coating releasing agent. The fiber mixture was uniformly distributed in a die cavity (120X80X10mm) to form ten different fiber volume fractions of hybrid composites as shown in Table 1.The emulsified TPS was poured on the random mixed fibers. The mixture was then pre-heated at 140±3°c for 30min to remove excess water from the mixture. This was followed by hot pressing at 5MPa and 160°c for another 30min then cooling at a rate of about 2°C/min. VT = Vf + Vm = WWf ρf + WWm ρm = ( 4 ∗ df 2 ∗ lf ∗ nf ) + (Wm ∗ lm ∗ hm ) (1)