Macadamia Nutshells for Bio-Synthetic Polymer Composites: Comparison
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The global production of macadamia nuts has witnessed a significant increase, resulting in the accumulation of large quantities of discarded nutshells. These nutshells possess the properties of remarkable hardness and toughness, which are comparable to those of aluminum. Incorporating natural fillers to enhance the properties of composite materials for various applications, including light duty, structural, and semi-structural purposes, is a common practice. Given their inherent hardness and toughness, macadamia nutshells present an intriguing choice as fillers, provided that the manufacturing conditions are economically viable. Macadamia nutshell-filled polymeric composites hold promising potential for manufacturing various structural components, including sandwich composites, with prospective applications in infrastructure, aerospace, and automotive industries. Additionally, macadamia shells can be processed at high temperatures in a specialized low-oxygen environment to produce biochar, which finds applications in carbon filters, life-saving medical treatments, industrial nano-powders, and cosmetics.

  • macadamia nutshells
  • polymer composites

1. Bio-Synthetic Polymer Composites

Researchers have long been exploring opportunities to incorporate natural fibers into various industries, a trend that gained momentum with the emergence of polymers in the early 19th century. Synthetic polymers and composites have become ubiquitous worldwide; however, their production and recycling processes contribute to environmental pollution, prompting the need for alternative solutions utilizing natural fibers. Bio-synthetic composites, composed of natural materials and artificially prepared or synthesized polymers, offer strength and structural integrity to the final product. Cellulosic fibers derived from sources such as flax, alpaca, hemp, jute, and wood are biodegradable and commonly used as reinforcement in composites with different thermoplastic matrices. Bio-composite materials offer numerous advantages over conventional materials, including higher specific strength, stiffness, and fatigue resistance, thereby enabling more adaptable structural design. Additionally, bio-composites are biodegradable, possess superior tensile strength, exhibit low specific gravity, and are recyclable. Consequently, these materials find application in diverse product manufacturing and innovative fields. Green composites, compared to their synthetic counterparts, offer various advantages, including reduced tool wear [1] and biodegradability [2]. Furthermore, natural fibers exhibit higher specific strength than do glass fibers while maintaining a similar specific modulus [3]. Many of these fibers are obtained through the processing of agricultural, industrial, or consumer waste [4]. Natural fibers are widely accessible and find applications in a wide range of industries,

2. Macadamia Nutshell Reinforced Composites

2. Macadamia Nutshell Reinforced Composites

The unique characteristics of macadamia nutshells, such as their low density, high mechanical strength [5][6], biodegradability, and recyclability, make them highly suitable for a wide range of innovative product designs. Researchers have extensively studied the performance of composites composed of macadamia nutshells, pine cone wastes [7] and various polymers, including poly lactic acid (PLA) [8], polyethylene (PE), polyester, polybenzoxazine [9], polypropylene (PP) [10], as well as certain resins [11][12][13].

2.1. Composites with Poly Lactic Acid

Chensong et al. [5] conducted a study on the mechanical properties of bio-composites composed of macadamia nutshell powder and poly lactic acid (PLA). The strength and stiffness of the composites were found to be dependent on the weight content of macadamia nutshell particles. Specifically, a 40% weight content of nutshell powder resulted in a 9.8% increase in the elastic modulus. However, the hardness of the composites was not affected by the weight content, and an increase in powder content led to a decrease in both flexural and tensile strength. Rakesh et al. [14] investigated composites of PLA with the addition of a plasticizer, such as Triacetin. The inclusion of the plasticizer caused changes in the morphology of the composites. Among the tested compositions, the composite with 8% plasticizer exhibited a maximum tensile strength of approximately 11 MPa, along with satisfactory elongation at break.
In a study by Xiaohui et al. [15], the additive manufacturing of composites using poly lactic acid (PLA) and macadamia nutshell (MS) was explored. The macadamia shell samples were treated with alkali and silane, resulting in morphological changes. The PLA composite containing 10 wt% of the treated macadamia nutshell showed thermal and mechanical properties comparable to those of pure PLA, as well as promising characteristics for scaffold applications. The PLA composite with 10% treated macadamia nutshell exhibited the best performance, showing potential for use in lightweight and structural parts.
In another report, the authors studied such composites made of four kind of nutshells such as walnut, almond, macadamia (MSP) and wild almond [16]. The PLA/MSP composite was the most water-resistant regardless of surface treatment. Crystallization degree of these representatives also improved.

2.2. Composites with Polyethylene and Polyester

Sevda et al. [17] conducted a study on high-density polyethylene (HDPE) composites reinforced with microcrystalline cellulose (MCC) and nutshell fiber (N). The researchers also incorporated polyethylene graft maleic anhydride (PE-g-MA) to enhance the interface between the components. The prepared samples were subjected to accelerated weathering for 672 h in total, during which changes in morphology, weathering, mechanical properties, and chemical composition were analyzed.
Exposure to weathering conditions led to a decrease in flexural strength and an increase in the modulus of elasticity of 62%. Color changes and a loss of gloss were predominantly observed in the MCC/nutshell reinforced composites, along with an increase in surface roughness.
Laert et al. [18] conducted a study on the mechanical and thermal properties of low-density polyethylene (LDPE) composites incorporating Macadamia integrifolia residue. Various fiber contents (0%, 5%, 10%, and 20% by weight) were investigated, and it was found that the composites with a 20% fiber content performed the best. The inclusion of fibers increased the stiffness of the composites compared to neat LDPE, but this led to a reduction in toughness and resilience, resulting in lower impact energy absorption. Chensong et al. [19] investigated the flexural properties of polyester composites reinforced with macadamia nutshell particles at four weight fractions: 10%, 20%, 30%, and 40%. The presence of voids in the composites was observed to decrease the flexural strength. The authors also reported that the flexural strength of the polyester did not improve with the addition of macadamia nutshell particles.

2.3. Composites with Polypropylene (PP)

Lucas et al. [20] studied composites made of macadamia nutshell residues (MR) and polypropylene (PP) composites using different MR contents (5, 10, 15, 20, 25, and 30%wt). Characterizations were conducted mainly focusing on the effect of moisture retention, and additionally, life cycle assessment (LCA) was obtained. The presence of MR content allows thermal stability. Meanwhile, it creates cracks and voids in the interface although it does not affect mechanical performance substantially. TGA and DTG curves, and the nature of moisture retention during the 7 days of the representatives were obtained. LCA revealed higher MR contents (30%) to promote lower environmental impacts than does the classical handling of nutshells (Figures 7 and 8 of reference no. [20]).
Nycolle et al. [21] investigated the effect of an alkaline treatment and coupling agent on the thermal and mechanical properties of macadamia nutshell residue (5 to 30% wt)-based PP composites. Such a treatment allows interfacial adhesion between the fiber and matrix. The FTIR spectra and X-ray diffraction pattern (XRD) obtained from the representatives before and after the treatment present how effectively functional groups were changed and crystallinity was transformed. Thermal degradation and mechanical properties were studied along with morphological analysis to observe the performance of treatments of the fibers and how they interfaced with the matrix. An addition of 30% wt treated fiber to the PP exhibited an enhancement of 67.5% in the tensile modulus. However, it was established that a higher fiber content being added to the PP enhanced the stiffness, and consequently reduced the impact strength of the materials (Figures 2, 4 and 7 from reference [21]).

2.4. Composites with Some Resins

Wechsler et al. [22] conducted a study comparing particleboards made from macadamia nutshells with resin derived from castor oil to conventional wood fiber/urea formaldehyde particleboards. The macadamia nutshell particleboards exhibited a 43% higher density, lower moisture retention, and reduced swelling. The internal bond strength was similar, but the modulus of rupture and modulus of elasticity were slightly lower compared to those of the conventional particleboards. Furthermore, the particleboards made with castor oil resin emitted less than 5% formaldehyde compared to traditional urea formaldehyde particleboards.
Derrick et al. [23] investigated the physico-mechanical properties of composite particleboards made from macadamia nutshells and Gum Arabic. The samples containing 50% Gum Arabic and 50% macadamia nutshells demonstrated favorable results, including the lowest average values of water absorption and swelling after submersion in distilled water, as well as the highest density, modulus of rupture, modulus of elasticity, internal bond strength, and compressive strength. A comprehensive study was conducted to determine the influence on the physico-mechanical properties of particleboards fabricated with particles of Eucalyptus saligna and macadamia nutshells [24]. Urea formaldehyde (UF)-based resin, an ammonium sulfate catalyst, and a paraffin emulsion were used in the fabrication process. The results indicated that particleboards with a high proportion of macadamia nutshell particles exhibited lower mechanical strength and dimensional stability. This was attributed to the thicker geometry of the macadamia nutshell particles, which limited their interaction with the adhesive.
Omid et al. [25] also investigated the use of synthesized phosphorous-based deep eutectic solvents or phosphorylated macadamia nutshell powder (p-wood) as a reinforcement in epoxy resin composites. The composites containing 20% p-wood exhibited a V-1 rating in the UL 94 Classification of the plastics flammability standard, along with significant reductions of 74% in the peak heat release rate and 344°C in the maximum smoke temperature compared to those of neat epoxy resin. The composites also showed an increase in char yield and limiting oxygen index value.
A study focused on the thermal behavior of benzoxazine composites reinforced with macadamia biomass was conducted [26]. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) were employed to investigate the properties of the composites. The results showed that the addition of 10% (v/v) of macadamia biomass in its natural form had minimal impact on the glass transition temperature, modulus of elasticity, and linear thermal expansion coefficient of the benzoxazine matrix.

2.5. Composites from Waste Macadamia Nutshells and Automotive Waste Plastic

A study focused on the production of wood plastic composite (WPC) panels via a combination of waste automotive plastics with macadamia nutshells as a matrix material was conducted [27]. The study examined the density, mechanical properties, microstructure, and thermal properties of the produced WPC panels and compared them with those of panels made solely from 100% automotive plastic. The investigation revealed that the addition of macadamia shells to the automotive waste plastic helped improve the modulus of elasticity under compression loading. The comprehensive modulus for the automotive waste plastic was 253 MPa. Since wood has a higher modulus than plastic does, the modulus of the fabricated panels increased with an increase in the content of macadamia shells.
The comprehensive modulus of the panel board increased as the proportion of macadamia shells increased, reaching a value of 548 MPa for a board containing a 75% macadamia shell mixture. However, in the flexural test, the incorporation of macadamia shell did not exhibit a reinforcing effect and led to a slight decrease in flexural strength. The report suggests that this result may be attributed to the poor interaction between the automotive waste plastics and macadamia shells or the immiscibility of the plastics. Overall, the study demonstrates that the addition of macadamia shells significantly increased the comprehensive modulus by 548 MPa in a WPC panel containing 75% macadamia shell.
Microstructure analysis of the fractured surfaces under compression revealed that the addition of macadamia shells partially transformed the brittle failure of the waste automotive plastic into a ductile failure. In terms of thermal properties, the WPC panel exhibited favorable flame-retardant properties compared to panels made solely from 100% plastic.

2.6. Macadamia Nutshell Fillers Studied for Purposes Other Than Composites

Jun et al. [28] developed carbon composites using macadamia nut shells, phenolic resin, and carbon fibers for their application as solid adsorbents in coal-fired power stations for the post-combustion capture of CO2. The newly developed composites exhibited a performance improvement of over 30% compared to that of their previously developed adsorbents. The introduction of phenolic resin resulted in an enhanced efficiency of CO2 adsorption. Yingge et al. [29] utilized macadamia nut shells as a precursor to prepare porous carbon material, which was subsequently used in the fabrication of sulfur–carbon composite material as the sulfur storage matrix for lithium–sulfur batteries. The study investigated the effect of temperature on the microstructure and electrochemical performance of the porous carbon material. The activation process at a temperature of 900 °C resulted in the desired pore structure of the carbon material. The material exhibited a super high specific surface area (3552.7 m2/g), larger pore volume (2.2 cm3/g), and higher mesoporous content (23.85%), providing significant technical advantages. Macadamia biomass was effectively used as carbon resource in cleaner production of iron [6] and materials for additive manufacturing [30].

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