Optimization of Al2O3 particle modification and UHMWPE fiber oxidation of EVA based hybrid composites: Compatibility, morphological and mechanical properties
Introduction
Fiber reinforced polymer (FRP) composites consist of thermosetting or thermoplastic resins as a matrix and fiber/fabric as reinforcement. The role of the matrix is to transfer loads to reinforcement, while the fibers provide a load-bearing effect [1]. Among the many other polymers, material which possesses exceptional properties such as high impact resistance, high strength, high tensile strength, good chemical and wear resistance, high cut and abrasion tolerance, light weight, etc. –is ultra high molecular weight polyethylene (UHMWPE) [2,3].
UHMWPE has become one of the most modified polymers in the industry, replacing the existing conventional polymers due to its characteristics which make it an excellent choice as reinforcement in the polymer matrix composites [1,4]. In comparison with other reinforcing fibers the density of UHMWPE fibers is low and it is an additional advantage in their application in the hybrid composites [5]. The properties are interfaced by the UHMWPE fibers contact with polymer matrix. However, the application of UHMWPE fibers as reinforcing material in hybrid composites have some limitations as a consequence of fiber non-polarity, high chemical inertness, high anisotropic property, poor creeps and heat resistances, low surface energy and weak adhesion to polymer matrix which leads to poor transmission of mechanical stresses through the composite [3,[5], [6], [7], [8], [9], [10]].
Other factors that may affect the characteristics of the hybrid composite refer to the fillers size, geometry, their amount and dispersion in the matrix. Improved interfacial adhesion for hybrid composite material can be achieved by applying different techniques of fibers, matrix and fillers modifications [9]. In order to improve UHMWPE fiber/polymer matrix bonding, fibers should be subject to the appropriate surface treatment. A variety of surface treatment methods including fast atom beams, laser irradiation, nitrogen ion implantation, oxygen-plasma treatment, chemical treatments (acid etching methods etc.), have been utilized to improve the surface energy of UHMWPE fiber [3,8,[10], [11], [12]]. These fiber surface treatments can improve the fiber/matrix adhesion and interface wetting to some degree but, at the same time, due to structural damage of the fibers, the mechanical properties could be decreased [11]. In order to enhance the surface roughness of the fiber, nanofilers are used in order to improve the frictional component of adhesion and toughening [12].
Poly (ethylene-co-vinyl acetate) (EVA) copolymer has application in many fields such as corrosion protection, packaging, electrical insulation. In polymer material science this copolymer is often used as adhesive for different substrates like wood, metals and polymers [13,14]. Compared to the UHMWPE fibers, the mechanical properties of the EVA as the matrix are inferior. As additional aspect of possible ways to improve composite mechanical characteristics is incorporating inorganic fillers like carbon nanotubes, alumina, silica, calcium carbonate, zinc oxide, gold, or other metals [15]. The incorporation of inorganic fillers into the matrix can have significant effects on morphological and thermal properties, and improve tensile strength, hardness, Young's modulus or stiffness [16,17].
Among the different inorganic fillers, alumina (Al2O3) has a numerous useful properties, including high strength and stiffness, mechanical strength, high adsorption, thermal stability etc. However, the recognized obstacle that limits interfacial interaction between fillers and the polymer matrix is particles hydrophilic surface with high chemical activity and tendency to aggregate in order to minimize the surface energies [18]. The incompatibility with polymers can be tackled with changing the surface properties by chemical modification [19]. One of the strategies comprises coating with coupling agents containing functional groups. As a result of functionalization, the organic chains from coupling agent are bonded or adsorbed on the surface of the fillers. Due that reaction, the forming of oxygen bridge bonds and agglomeration between particles is hindered [18].
According to the literature, it is necessary to find a way to increase the interfacial adhesion between matrix and fibers without degrading the fibers characteristics, to predict the best amounts of the fillers in order to obtain hybrid composite fiber with enhanced mechanical properties [11]. Modification of the constituents of hybrid system and material processing of is a challenge to obtain compatibility/improved properties of obtained composites.
In this work different alumina particles, Al2O3: commercial nanoparticles (n-Al2O3), whiskers (w-Al2O3) and synthesized Al2O3 (μ-Al2O3) and the one doped with iron oxide (Fe-Al2O3), were used as reinforcing fillers and UHMWPE fibers (PE) were used as a reinforcement in prepared composites. Hydrolysis of EVA and oxidation of PE produce EVAOH and ox-PE polymers, respectively, able to establish higher extent intermolecular interactions at interfacial surface contributing to improvement of mechanical properties. In that sense, the specific objectives were related to the study of: 1) the influence of the type and extent of alumina surface modification on the mechanical properties of composites, 2) native and modified EVA matrix and PE/ox-PE reinforcing fiber on the mechanical properties, and 3) overall contribution of interfacial interactions on the compatibility and performance of all produced composites.
Section snippets
Materials
Commercial EVA produced by DuPont™ (Elvax® 410; 18% vinyl acetate (VA), MI (melt flow index) = 500 g/10 min, melting point (DSC) = 73 °C) was used as polymer matrix. UHWMPE fibers (DSM Dyneema, Netherlands, SK75, filament diameter 20 μm, density 0.970 g/cm3) were selected as fiber reinforcement. The alumina spherical nanoparticles (<50 nm particle size, n-Al2O3) and alumina whiskers (diam. 2–4 nm × 400 nm, w-Al2O3) were supplied from Sigma Aldrich. Aluminum chlorohydrate (Locron L) in the
Synthesis of alumina particles
Synthesis of alumina particles was performed as reported previously and the details are presented in Supporting information S3.1 [20]. Obtained particles (Fe-Al2O3) were of submicron dimensions [20]. Particles without the addition of iron oxide were also prepared by the same procedure and were denoted as μ-Al2O3.
Modification of the particles with GLYMO
Preparing the samples with modified particles with GLYMO was conducted using the procedure proposed in literature and the details are presented in Supporting information S3.2 [21].
Modification of the particles with hydrolyzed GLYMO
GLYMO
Results and discussion
The presented results and measurements were performed in order to determine the influence of performed modifications to mechanical properties of hybrid composites.
Conclusion
In this paper, preparation of EVA and EVAOH based hybrid composites reinforced with neat PE and ox-PE fibers, with addition of 1, 3 and 5 wt% of fillers: n-Al2O3, w-Al2O3, μ-Al2O3 and Fe-Al2O3, and modified ones Al2O3/S and Al2O3/HS were presented. FT-IR, Raman and NMR methods were used for detailed characterization of used reinforcing fillers and fibers. Optical microscopy indicates highest extent of interfacial interactions at Al2O3/HS particles/EVAOH interface. Morphologies of hybrid
Acknowledgements
This research was financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia as a part of the project TR34011.
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