Moisture absorption characteristics and effects on mechanical properties of Kolon/epoxy composites
Graphical abstract
Introduction
The fabrics produced from para-aramid (p-aramid, poly (p-phenylene terephthalamide)) fibers (Kevlar, Twaron, Kolon) are commonly used for soft body armor in ballistic protection, lightweight vehicle-armor structures, aerospace and automotive industry for the manufacture of parts, boat hulls and sporting goods. These light fibers have high tenacity, strength to weight ratio, thermal stability, impact resistance and great flexibility. The aramid fibers present the synthetic fibers with high thermal and strength properties since the chain molecules in these fibers are mainly placed along the fiber axis. Their extraordinary properties also offer strong resistance to abrasion and organic solvents; they are non-conductive, low-flammable and their degradation starts at 500 °C [[1], [2], [3]]. The mechanical properties of these aramid microfibers are further enhanced by integration into matrices, which results in high performance composites. The most widely used matrices are epoxy resins due to their ease of processing, high mechanical and adhesion characteristics, chemical resistance, corrosion resistance and relatively low shrinkage during curing [4]. Besides, this resin is not expensive. Epoxy resins are among the most significant thermosetting polymers and they are generally applied in coating, high-performance adhesives, encapsulation of electronic devices and composite matrix. Owing to their high mechanical strength, epoxy composites are used as structural parts in aerospace, automobile and civil engineering [5,6].
Silicon carbide (SiC) particles present a very desirable ceramic material that can be applied as reinforcement in different polymer matrices due to their excellent properties such as high mechanical strength and hardness, high thermal conductivity and low thermal expansion. Silicon carbide is a rigid crystalline material that has been used for grinding wheels, steel, cast iron, plastics and rubber [7]. It is a commonly used material for anti-wear applications, as well [8]. The application of inorganic fillers as reinforcement in epoxy resins has become a common practice. The fabrication of nanoparticle-filled polymers for improving tribological and mechanical properties of composites has become a big challenge in recent years [[9], [10], [11]]. An extremely high surface area of nanoparticles is one of their most attractive characteristics since it creates a large interface in a composite [12]. Due to this fact, much work has completed the fabrication of the SiC nanostructures, such as nanofibers, nanowires and nanowhiskers. The remarkable mechanical properties of the SiC nanofibers nominate them the promising candidates for the reinforcement in polymer-matrix and metal-matrix composites [13]. Some studies have reported the enhanced mechanical properties of epoxy composites by using the β-SiC nanoparticles [14], SiC nanowires [15] and SiC nanofibers [16] as reinforcement.
Despite their good mechanical strength it is well known that the mechanical properties of epoxy resins are considerably altered by moisture absorption, especially at higher temperatures. In spite of that, the mechanical properties of the fiber reinforced composites, controlled by the properties of the matrix and fiber/matrix interfacial bonding, can be severely affected by the environmental parameters, such as temperature, humidity and mechanical stress. A certain number of high performance structures are always exposed to some changeable high temperature and moisture conditions throughout their entire service lifetime. This kind of continuous exposure may cause the deterioration in mechanical behavior of the aircraft structural components, even leading to their failure [17].
The fiber/epoxy matrix interfaces play a very significant role in the stress transfer in a composite. However, these interfaces are sensitive to the chemical reactions of moisture. A strong matrix plasticization is induced by the non-uniform moisture absorption in composites [18]. Moisture degrades some mechanical properties of polymer-matrix composites considerably. It is supposed that moisture causes matrix plasticization or degradation of the fiber/matrix interface in most composites. Some data in literature confirmed that the aramid-reinforced composites can react to moisture because moisture may penetrate along the interface and hygroscopic aramid filaments. Thus, moisture may change the filament-matrix bond and filament properties as well. It is a well-known fact that the absorbed moisture has little influence on the tensile properties of the aramid filaments or the composite [19,20]. However, the interlaminar shear strength of the aramid/epoxy composites can be diminished by the moisture absorption [21].
This paper aims for the preparation and mechanical characterization of the SiC/epoxy composites with the SiC particles and nanofibers in order to distinguish the properties related to their shape. It has been noted that not enough research of the moisture properties has been conducted into the epoxy composites containing the SiC nanofibers. In this study the comparison of the tensile and impact strength results of the water/moisture exposed and unexposed Kolon fabric specimens is presented. The changes in the mechanical properties of the Kolon specimens after the water absorption in duration of 72 h were analyzed. In addition, the tensile test simulation of the epoxy composite by using software Abaqus® was carried out.
Section snippets
Materials and methods
The multiaxial p-aramid fabrics (Martin Ballistic Mat, Ultratex, Serbia) with the p-aramid fiber type Kolon (Heracron, Kolon Industries, Inc., Korea) were used for the manufacturing of the composites. Each fabric layer consisted of the fiber knitting layers with four respective angle orientations: +45°, 0°, 90° and −45°. The fibers were coated with the polyurethane (PU) film inside (Desmopan, Bayer) and stitched together with a light-weight polyester thread (Korteks, Turkey) [2]. The neat Kolon
Water absorption results
The immersion of the specimens in water and exposure to high humidity (70%) were performed according to the ISO 62 standard and the square specimens were immersed in distilled water or moisture exposed at the temperature of 23 °C for 72 h. The specimens were removed from the water and wiped with a clean cloth before their weight was measured at times. The percentage of the water absorption was determined by the weight difference using the following equation:where is the
Conclusion
The effect of moisture absorption on composite performance has been discussed in terms of changes in mechanical properties. In this study the tensile and impact properties of the untreated Kolon/epoxy specimens were compared with the ones that underwent the water absorption in duration of 72 h (immersion or high humidity) followed by desorption.
The tensile test results confirmed that the SiC nanofibers had a great influence in the tensile properties as they were used as reinforcement. The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Contract Nos. 451-03-9/2021-14/200287 and 451-03-9/2021-14/200325) and by Czech Science Foundation (GA 18-10907S). The authors would also like to acknowledge the contribution of the COST Action CA18120.
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