Ahmad Y. Al-Maharma

This study investigates the effect of silylation and cellulose loading on the mechanical properties of epoxy composites. We use the hydrolyzed 3-Aminopropyltriethoxysilane (KH550) as a crosslinker for epoxy and as a coupling agent for cellulose. The mechanical properties of the epoxy composites are evaluated using molecular dynamics simulations. The improvement in the interfacial adhesion between epoxy and cellulose, achieved by using KH550, is demonstrated through the pulling out of cellulose from the epoxy composites. The results indicate that the nanocovalent bonds formed by KH550 at the epoxy/cellulose interface have a higher enhancement effect on the pulling force compared to increasing the cellulose content. For instance, the force needed for pulling 44.1 wt.% of raw cellulose is 93 ± 5 (kcal/mol)/Å, while the one required to pull the 28.1 wt.% of silylated cellulose is 97 ± 4 (kcal/mol)/Å. The silylated cellulose at 28.1 wt.% enhances the tensile modulus, shear modulus, and strength of the epoxy-KH550 composite by 14.55%, 15.65%, and 15.64%, respectively, compared to its counterpart reinforced with raw cellulose. Using the silylation treatment on cellulose that reinforces epoxy-KH550 at 43.9 wt.% improves the elastic modulus, shear modulus, and tensile strength of the epoxy composite by 4.23%, 4.64%, and 18.07%, respectively.

This study investigates the effect of silylation and cellulose loading on the mechanical properties of epoxy composites. We use the hydrolyzed 3-Aminopropyltriethoxysilane (KH550) as a crosslinker for epoxy and as a coupling agent for cellulose. The mechanical properties of the epoxy composites are evaluated using molecular dynamics simulations. The improvement in the interfacial adhesion between epoxy and cellulose, achieved by using KH550, is demonstrated through the pulling out of cellulose from the epoxy composites. The results indicate that the nanocovalent bonds formed by KH550 at the epoxy/cellulose interface have a higher enhancement effect on the pulling force compared to increasing the cellulose content. For instance, the force needed for pulling 44.1 wt.% of raw cellulose is 92.5±4.5 (kcal/mol)/Å, while the one required to pull the 28.1 wt.% of silylated cellulose is 97.1±3.6 (kcal/mol)/Å. The silylated cellulose at 28.1 wt.% enhances the tensile modulus, shear modulus, and strength of the epoxy-KH550 composite by 14.55%, 15.65%, and 15.64%, respectively, compared to its counterpart reinforced with raw cellulose. Using the silylation treatment on cellulose that reinforces epoxy-KH550 at 43.9 wt.% improves the elastic modulus, shear modulus, and tensile strength of the epoxy composite by 4.23%, 4.64%, and 18.07%, respectively.

Silylation treatment improves the hydrophobicity of cellulose by reducing the number of hydroxyl groups in the cellulose chains that are available to react with moisture in the surrounding environment. Additionally, silylation increases stress transfer from cellulose to synthetic nanofillers by forming covalent bonds between the hydroxyl groups of cellulose and the oxidized surface of these nanofillers. This study investigates the impact of silane coupling agents on the tensile properties of cellulose nanocomposites. The cellulose nanocomposites are reinforced with four types of C/SiC-based nanofillers: carbon nanotubes, graphene nanoplatelets, silicon carbide nanotubes, and silicon carbide nanoplatelets. Subsequently, the nanofillers are subjected to surface treatment using the silane coupling agent KH550. The mechanical properties of the cellulose nanocomposites are evaluated by molecular dynamics simulations based on the polymer’s consistent forcefield. The results indicate that the reinforcements of silylated silicon carbide nanotubes and carbon nanotubes increase the tensile modulus of cellulose by 18.03% and 24.58%, respectively, compared to their untreated counterparts. Furthermore, the application of silylation treatment on the surface of C/SiC nanofillers increases the yield strength and ultimate tensile strength of cellulose nanocomposites due to enhanced load transfer between cellulose and these reinforcements.

Cellulose is a biopolymer frequently implemented as fibers and membranes to fabricate high-performance and multi-functional composites. In this study, the molecular dynamics simulation is used to investigate the effect of CNTs on elastic modulus, ultimate strength, and toughness properties of silicon carbide nanotubes (SiCNTs) reinforced cellulose nanocomposite. Based on the simulation results, the temperature of the surrounding environment controls the stress transfer at the nanotube-cellulose interface. As the cellulose expands with increasing temperature creating more void regions at the filler-matrix interface, the load transfer from cellulose to nanotubes is reduced, leading to a noticeable reduction in tensile properties of cellulose composite. This behavior critically affects the tensile strength of cellulose composite reinforced with multi-walled CNTs (MWCNTs). The single-walled CNT with a high aspect ratio increases the cellulose composite's tensile strength and toughness, while the MWCNT introduces the most significant increase in cellulose stiffness. The difference in polarity between MWCNT and cellulose, along with the low aspect ratio of the external nanotube, would limit the capability of MWCNT in improving the tensile strength and toughness of cellulose. The single-walled SiCNTs are structurally unstable, and their stiffness degrades rapidly at high temperatures. Therefore, they are hybridized with CNTs to improve their stiffness and structural stability. SiCNT, which is hybridized with two CNTs, exhibits the best structural performance at 100, 300, and 500 K temperatures.

The weight of sandwich composite structures with foam cores mainly depends on the volume fraction and the distribution of air voids in its porous structure. A considerable reduction of the weight can be achieved at the expense of mechanical and strength properties by increasing the fraction of air voids in the sandwich composite's core. Therefore, high stiffness nano‐fillers such as graphene nano‐platelets (GNPs) are frequently used to improve the specific stiffness and mechanical strength of sandwich composites without sacrificing from the lightweight requirements. In this contribution, the effect of the square‐ and circle‐shaped distributions of air‐voids and GNPs on damage tolerance properties of thermoplastic polyurethane (TPU) resin is investigated. TPU foams with randomly oriented air voids and GNPs are used as a baseline to evaluate the improvement of the properties mentioned above. For that purpose, finite element based micro‐mechanical models for representative volume elements (RVEs) are used to homogenize the properties of multi‐scale models of TPU foam. The analysis results show that the air pores increase the ductility of brittle TPU resin significantly when the pores are non‐linearly distributed in circular functionally graded (FG) shape. Besides, it is concluded that Young's modulus degradation associated with the use of linearly distributed air‐voids composed of the square‐shaped FG foam can be compensated by using GNPs. The results of the study suggest that the functional grading design of TPU foam has excellent potential for applications, which require meeting the challenging weight, strength, stiffness and toughness requirements at the same time.

In the present review, the effect of porosity on the mechanical properties of the fabricated parts, which are additively manufactured by powder bed fusion and filament extrusion-based technologies, are discussed in detail. Usually, additive manufacturing (AM) processes based on these techniques produce the components with a significant amount of pores. The porosity in these parts typically takes two forms: pores with irregular shapes (called keyholes) and uniform (spherical) pores. These pores are present at different locations, such as surface, sub-surface, interior bulk material, between the deposited layers and at filler/matrix interface, which critically affect the corrosion resistance, fatigue strength, stiffness, mechanical strength, and fracture toughness properties, respectively. Therefore, it is essential to study and understand the influence of pores on the mechanical properties of AM fabricated parts. The technologies of AM can be employed in the manufacturing of components with the desired porous structure through the topology optimization process of scaffolds and lattices to improve their toughness under a specific load. The undesirable effect of pores can be eliminated by using defects-free raw materials, optimizing the processing parameters, and implementing suitable post-processing treatment. The current review grants a more comprehensive understanding of the effect of porous defects on mechanical performance and provides a mechanistic basis for reliable applications of additively manufactured components.

Natural composites can be fabricated through reinforcing either synthetic or bio-based polymers with hydrophilic natural fibers. Ultimate moisture absorption resistance at the fiber–matrix interface can be achieved when hydrophilic natural fibers are used to reinforce biopolymers due to the high degree of compatibility between them. However, the cost of biopolymers is several times higher than that of their synthetic counterparts, which hinders their dissemination in various industries. In order to produce economically feasible natural composites, synthetic resins are frequently reinforced with hydrophilic fibers, which increases the incompatibility issues such as the creation of voids and delamination at fiber–matrix interfaces. Therefore, applying chemical and/or physical treatments to eliminate the aforementioned drawbacks is of primary importance. However, it is demonstrated through this review study that these treatments do not guarantee a sufficient improvement of the moisture absorption properties of natural composites, and the moisture treatments should be applied under the consideration of the following parameters: (i) type of hosting matrix; (ii) type of natural fiber; (iii) loading of natural fiber; (iv) the hybridization of natural fibers with mineral/synthetic counterparts; (v) implantation of nanofillers. Complete discussion about each of these parameters is developed through this study.