Mechanical Properties of Polypropylene Composites Reinforced with Macadamia Nutshell Fibers

: The use of natural fibers as an additive in polymeric matrices has attracted interest of the automotive industries, for its low cost, mechanical properties, biodegradability and lightness. However, the hydrophilic nature of the fiber makes polymer compatibility difficult. Fiber surface treatments can be used to enhance the fiber/matrix interface. In the present work, polypropylene (PP) composites reinforced with fibers from macadamia nutshell were obtained and characterized. Macadamia nutshell fibers were treated by an alkaline treatment with sodium hydroxide (NaOH 4%) to improve adhesion between fibers and matrix. Fibers were characterized by techniques of Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The PP was mixed with the macadamia fibers (treated and untreated), in proportions of 5 and 10% (wt/wt) using a thermokinetic mixer. Furthermore tensile, flexural and impact specimens (Pure PP and composites) were prepared by an injection molding process and tested for evaluation of mechanical properties. The results showed that the insertion of treated fibers in the PP matrix increased the stiffness of the composites. However, the composites reinforced with untreated fibers presented higher impact energy absorption when compared to composites reinforced with treated fibers.


Introduction
In recent years, natural fibers such pineapple, flax, jute, coconut, sugarcane bagasse, palm, and others have been extensively studied as an alternative to reinforce polymer matrix due to their advantages such as sustainable, low cost, low density and biodegradable features [1][2][3]. However, the incompatibility with nonpolar matrices, moisture adsorption and vulnerability to thermal degradation are the main disadvantages for the use of natural fibers in composites [4][5][6][7]. Thus, to improve interfacial contact is necessary a superficial change. Many treatments have been used in order to improve compatibility of natural fibers, which can be physical or chemical, with the goal of modifying the surface of the fibers [8][9][10][11].
The alkaline treatment applied to natural fibers has been used. This treatment decreases the amount of hemicellulose, lignin, waxes, and oils soluble in alkaline solution, and therefore reduces the level of fiber aggregation, making a surface rougher. In others words, the alkaline treatment of natural fibers changes the orientation of highly packed crystalline cellulose and forms an amorphous region, which makes the interior of the fiber more accessible for chemical action. Furthemore, treatment reduces the diameter and increases the aspect ratio of the fiber [12][13][14].
Natural fibers from macadamia nutshell (Macadamia integrifolia) are a cellulosic material readily available and can be used as reinforcement in a thermoplastic-based composite [15]. Macadamias are native to Australia's rainforests. Its almond is highly valued, and can be consumed raw or toasted, being widely used in confectionery and ice creams. The kilo of macadamia nut varies from US$ 0.91 to US$ 1.17 to the producer and gives a good economic return when the productivities are superior to three tons per hectare [17].
The technological development of macadamia crops occurred in the Hawaii Agricultural Experiment Station (HAES), where the main cultivars planted in the world were selected. Due to its climatic conditions, Brazil is among the countries with the highest potential for macadamia nut production in the world. Currently in Brazil, macadamia cultivation is concentrated in regions that traditionally produce coffee [17][18][19][20].
The great production of macadamia has to do with its high international market value, thus generating a lot of waste. For each ton of macadamia, 70 to 77% of its grains produced are considered residues [21].
As an alternative for the redirection of residues from a factory near the region where this research is being developed, we chose the bark fiber as reinforcement in the composite. In this work it was evaluated the potential of macadamia nutshell fibers as reinforce in composites using polypropylene (PP) as matrix and also an alternative surface treatment, in order to improve interfacial fiber/matrix adhesion.

Materials
Macadamia nutshell fibers were obtained by Tribeca, located at Fazenda Santa Marta in Piraí-RJ. Polypropylene (PP) obtained from Braskem was used as matrix.

Treatment of the Macadamia Nutshell Fibers
First, the fibers were ground, oven dried at 100ºC to constant weight, then sieved in a 35 mesh sieve. Fibers were washed with warm water at 100ºC until the wash water did not display any further color change. Fibers were again oven dried for 24 hours at 100°C. Subsequently the fibers were submitted to alkaline treatment with NaOH (4% w/v) for an hour under constant stirring at 70°C. After that process, the fibers were washed with distilled water until they reached neutral pH, and dried. The chemical modification provides more dimensional stability, reduce water absorption capacity and give resistance to fiber against fungal decay [23].

Fibers Characterization
Micrographs were obtained in a scanning electron microscope HITACHI with tungsten filament operating at 10kV, employing low-vacuum technique and secondary electron detector. Samples were dispersed on a brass support and fixed with a double face 3M tape. Macadamia nutshell fibers untreated and treated were characterized by X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). X-ray diffractograms were obtained in a Shimadzu diffractometer, model XDR-6100. The measuring conditions were: CuKa radiation with graphite monochromator, 30 kV voltage and 40 mA electric current. The patterns were obtained in 10-502 θ angular intervals with 0.05 step and 1s of counting time. The crystallinity index (CI) was calculated by Segal's empirical method, by Eq. (1): (1)

Composites Processing
The treated and in untreated macadamia nutshell fibers were mixed the polypropylene (PP) in a thermokinetic mixer model MH-50H, with the speed rate kept at 5250 rpm, in which fibers were responsible for 5 and 10 wt% of the composition. After mixing, composites were ground and injected directly in a mold with specific dimensions for tensile, flexural and impact specimens.

Mechanical Tests
The tensile and flexure tests were performed on an EMIC brand equipment with a load cell of 5 kN. The dimensions of tensile and flexural test specimens were in accordance with ASTM D 638-14 and ASTM D 790-15, respectively. Impact tests were performed on an EMIC brand Izod type equipment with a 4 Joules pendulum.
Composites were analyzed in an EMIC testing machine (model DL2000), equipped with pneumatic claws with a load cell of 5 kN. In the tensile tests, five specimens of composites were analyzed, with dimensions in agreement with the ASTM D 638 standard. In the flexural tests, a load was applied on the specimen at 1.3 mm min -1 crosshead motion rate. Five specimens were analyzed with dimensions in agreement with the ASTM D 790 standard. Impact tests were performed on an EMIC brand Izod type equipment with a 4J pendulum.

Fibers Characterization
The micrographs of the untreated macadamia nutshell fibers presented a homogeneous surface ( Fig.  1(a)), similar surfaces were observed on palm fibers and sugarcane bagasse [24,25]. On the other hand, the treated fibers were less agglomerated and rough ( Fig. 1(b)). It was observed a rough surface along with the presence of pores and pits (which carry water and nutrients to different plant locations) [26,27].  Fig. 2 shows the XRD of macadamia nutshell fibers in natural condition and after the alkaline treatment. It can be seen a diffraction pattern of lignocellulosic fibers with 2 theta peaks at 10 o and 22 o relayed to (101) and (002) lattice planes, respectively. Apparently, the diffractograms of both fibers are similar. However, when the crystallinity degree of each fiber was calculated, it was noticed that there was a difference, as shown in the Tab. 1. This fact can be explained due to partial removal of amorphous components, such as lignin and hemicelluloses, causing an increasing in the crystalline degree [26].

Mechanical Tests
The tensile strength and modulus of untreated and treated macadamia nutshell polypropylene composites are shown in Tab. 2. It was observed a tensile modulus increase for treated fiber when compared to untreated fiber composite. Alkaline treatment improves fiber/matrix adhesion due to the removal of impurities. Thus, mechanical properties of natural fibers depend on its surface. When the fibers are treated, their length and diameter decrease [28]. The same material in different granulometry has distinct densities and mechanical properties, as noted in a study considering pine fibers sieved in a 28 and 35 mesh sieve [29]. Costa et al. observed that untreated fiber with larger size has higher density, but decreasing its size and adding the alkaline surface treatment, the density decreases, contributing to the lightness of the composite [29].
Moreover, the use of short fibers favors the processing of the composite (in the case of this work by mixing in homogenizer and subsequent injection in molds), and avoids the length breaks due to processing, as seen in jute fibers in a PP matrix [30]. Therefore, many papers choose to use a short granulometry, including with fibers sieved in 35 mesh sieves, in order to obtain a better interaction fiber/matrix [31]. Because of this, in this work was used macadamia nutshell fibers sieved in 35 mesh sieve as reinforce in PP matrix composite. Composites reinforced with treated fibers presented an increase of 42% in stiffness when compared to the pure PP and 47% compared to the composites reinforced with untreated fibers (Tab. 2). The superficial treatment fibers and the percentage of fibers inserted in the matrix influenced directly in the mechanical properties of the composites. A similar behavior was studied with rice husk fibers [32].
The flexural strength and modulus of the composites were influenced by concentration fibers inserted in the matrix when compared to pure PP (Tab. 3). It was observed that flexural strength of the treated fiber composites was higher than untreated fiber composites because alkali treatment of the macadamia nutshell fibers helped to reduce the hydrophilic character of the fiber's surface resulting in increasing interfacial bonding between the fiber and polypropylene matrix in the composites [34]. Through the impact test it was shown that the insertion of macadamia fibers into the matrix improved the strength of the composites was due to the increase of energy absorbed in the impact, it was observed an increase of 103% (CP 10%) when compared to the pure polymer. Results similar were observed with hemp fibers [28,29]. The amount of fibers in the matrix also contributes to improve impact strength. The treatment performed on the fibers improved the adhesion between fiber/matrix, facilitating the transference of energy from the matrix impact, which is one of the factors influencing this property. The insertion of fibers in the matrix also increased the impact resistance due to the mechanism of energy dissipation. The fibers dissipated energy during the mechanical friction process

Conclusion
Alkaline treatment with sodium hydroxide alters the fiber morphology and crystallinity degree. This resulted in an increase in the mechanical properties in composites. It was observed also that the percentage of fiber directly interfered in the properties of the composites when compared to PP pure. This fact can be explained due to reaction between hydroxyl groups of macadamia nutshell fiber and the chemical agents causing an increase of the interfacial bonding fiber/matrix in the composites.