Date-Palm Fiber as a Reinforcement Filler in Polymer Composites

Natural fibers offer a great advantage of being used as a reinforcement in polymer matrix composites because of the many advantages natural fibers offer over conventional reinforcement fillers. Date palm fiber is one of the most available natural fibrous materials in the Middle Eastern region to be exploited as a fiber reinforcement in polymers. In the present work, the fibers extracted from the date palm tree trunk, branches, and leaves were used for the reinforcement of the polypropylene matrix. Electron microscopic images show excellent bonding between the fiber and matrix as no fiber pullout is observed. The thermal (heat deflection temperature) and mechanical properties (Izod impact, tensile and flexural modulus) of the composites increased with an increase in the fiber loading from 20% to 60%, which in turn resulted in excellent mechanical properties in the final product. The work has immense significance in using date palm as an easily available natural resource for a useful product.

MAHgPP, MAH-SEBS) is commonly used as a coupling agent for natural fiber-based composites. These coupling agents form covalent/hydrogen bonds with the hydroxyl groups on the fiber surface and thus enhance the muchneeded bonding between the fiber and the matrix [14][15][16]. Impact modifiers, usually elastomeric or olefinic based are also used to improve impact resistance/toughness of the composites [17]. Studies were carried out using coupling agents at high loading levels and using date-palm fiber from tree branches. The fibers were chemically treated (which removes lignin from the sample and retain cellulosic fiber material only). Then it was washed and dried in an oven. Polypropylene is one of the most common polymers used in making composites because of its exceptional properties like high strength, stiffness, good chemical resistance, and weldability, which render it applicable in several corrosion-resistant structures [18]. The compound was formulated and hand mixed. Lumps were produced by an extruder. It was then granulated and test specimens were injection molded. Adhesion between the fiber and matrix was reported as good [19][20].
The good bonding between the matrix and fiber helps in stress transfer from the matrix to fibers to achieve favorable properties in the composites. If only naturally compatible fibers and matrices, such as cellulose fiber and cellulose ester polymers are used then there is not much need for any compatibilizers and coupling agents. The objective of the current study is to use date-palm fiber from a whole tree, not just selected parts of the tree, as against the previously reported studies. This makes a hundred percent material available contrary to thirty percent or even less when specific parts of the tree or only cellulosic material were segregated and used. Using fiber from the whole tree not only saves material and elevates supply chain position but also saves time, which is required to prepare the usable fiber. The whole tree is fed to a shredder and fibers are obtained. The very large or very small size of the fibers was removed through screening. The remaining fibers, 4-10mm long were collected. Impinged dust in the material from the desert environment over the years are removed by simply washing with tap water. No chemical treatment was used therefore, it is safer for the environment and no lignin/ cellulose separations occur either, thus, utilizing a hundred percent of the much scarce resources of the deserts, water, and wood. We have also tried higher fiber loading than what appeared in the literature, to increase the feasibility of the product. An optimal level of different ingredients was used in the formulation. The end product was characterized by mechanical, thermal, and matrix-fiber bonding. In the future, other functional additives can be incorporated for color, weathering resistance, flammability resistance, etc. The composite is useful in making a fence, facades, lumber, wood-like decorative articles, etc. Thus, recycling and conserving natural resources, which is an indispensable necessity of the dessert region, can be realized.

Experimental
A homo polypropylene (high melt flow: MFR = 47 g/10 min @ 230°C & 2.16kg load) was used as a matrix, maleic anhydride grafted PP was used as a coupling agent. Impact modifier, lubricant, thermal and processing aids were also used in the formulation before compounding in a twin-screw extruder. 4 -10 mm long date-palm fibers (see Figure 1) were prepared by shredding the whole date palm tree, screened for bigger particles, and finally washed with water and dried. The dried date palm fibers are used as such for compounding. The fibers were then introduced into the polymer at a suitable location upstream in the extruder via a side feeder, which in turn was fed via gravimetric feeder. The rest of the ingredients were tumble-mixed and fed in the extruder's throat via gravimetric feeder. The extruder processing temperature was kept between 180-200°C. Composite strands coming out of the extruder were cooled in a water bath and pelletized into pellets. The pellets were injection molded to obtain ISO test specimens (Figure 2). Thermal and mechanical properties and electron microscopy were carried out on the samples to characterize and evaluate the composite.

Results and discussions
The processing temperature during compounding and molding was kept below 200°C throughout the processing.
When the processing temperature during injection molding was raised above 200°C, the fiber undergoes degradation and burning which was obvious from the smell and the shift in color (see Figure 3). The degradation of the fibers was also verified by Thermogravimetric analysis (TGA) and it was found that the degradation and burning start around 230°C (Figure 4). Therefore, the melt conversion process should be kept well below 200°C. No effect of date-palm fiber loading was observed on %crystallinity of the PP, which was between 34-38% according to the DSC test ( Figure 5). Therefore, it appears that the modulus of the composite depends on fiber loading percentage and adhesion of fiber to the polymer matrix.  After molding, the fibers in the finished product were characterized by SEM (see Table 2). The final fiber length after extrusion and injection molding is between 3 to 4mm, which is a typical length of reinforcements in the composite. Thus, it points that the extrusion and injection molding were done at optimal conditions. Further evaluation by SEM shows that the fiber has a rough surface, the diameters vary from 95 to 500 micron, and that the fibers are not distributed uniformly ( Figure 6, Figure 7). Optimizing the extruder screw design will further help in improving the dispersive mixing of fiber in the PP matrix.    As shown in Figure 10, the composite exhibits better impact properties for all the fiber loading tested. As the loading of fiber increase in the composite the impact increases almost linearly. At all loading of date-palm fiber, the notched impact increased. The percent change in the notched IZOD test is indicative of good bonding between the fiber and the polymer matrix. At 60% loading of the date-palm fiber, the percentage change is 170%, which is highly favorable.
Another advantage observed by the reinforcement of PP by date palm fiber is the increase in the modulus as a function of percent loading of the fiber (Figure 11). Especially at 60% loading of the fiber, the percent change is almost doubled. Thus, good mechanical integrity in the composite was achieved which is apparent from the Flexural modus data below.  In the case of tensile modulus, an increase was observed only at a high loading of fiber that is at 60% ( Figure  12). These could be the result of the impact modifier used. Impact modifiers are known to lower modulus and increase the toughness, which was observed earlier. It could also be due to the wide variation in the aspect ratio of the natural date-palm fibers used (see Figure 7).
Heat deflection temperature (HDT) measured at 1.8 and 0.45MPa ( Figure 13, Figure 14) for the composite samples shows that the HDT increases gradually with increases in the fiber loading from 20% to 60% compared to the base resin.
Properties represented in the previous sections show that with higher fiber loading, the properties of the composite are improved to a great extent. This increase in the properties is attributed to the good adhesion between the fiber and the matrix as was concluded earlier by SEM.

Conclusion
An optimized PP composite formulation with different loading of date-palm fiber obtained from the whole tree along with different functional additives was used in the reported work. Without segregation of the fiber based on part of the tree, cellulose or lignin content, exact sizes, and chemical treatments, improved fiber yield drastically. Excellent bonding between fiber and matrix was achieved, which in turn resulted in good mechanical properties in the product. Additives were used at an optimum level, which kept the properties and prices in check. Thus, a viable composite was created for commercial use. Other additives for various additional functionality can be incorporated, such as flame-retardants, and UV additives for weather resistance, etc. Similarly, using PP with high molecular weight or other polymer-based resins the properties of the composite can be tailored as required. No chemical treatment on date-palm fiber was used, which saves money, time, and the environment. A wide range of fiber lengths can be used, which further saves the cost of screening and rejecting unwanted sizes, which drops percent yield, but we can still optimize shredding to obtain uniform fibers to improve the properties. An optimum process was used to obtain a viable product and to show that the composite can be produced on a commercial scale rather than just a laboratory endeavor.