Integrated manufacturing of REciclable multi-material COmposites for the TRANSport sector

Abstract

The mechanical and anti-corrosive evaluation of a hybrid epoxy resin–SiO2 and graphene oxide (GO) are presented. Three composite materials were prepared with 0%, 0.1 wt% and 0.5 wt% GO concentrations. The hybrid material was prepared by the sol-gel process incorporating the silica particles in situ within the epoxy resin (ER) matrix and previously that ER was functionalized with carboxyl groups using abietic acid and labeled as functionalized epoxy resin. The deposition of the three hybrids in aluminum 6061 substrates was made by blade coating, measuring wet and dry film thickness. The study of mechanical properties involved adhesion, pencil scratch hardness, and abrasion test methods where the incorporation of 0.5 wt% of GO improved the mechanical properties considerably. The anti-corrosive properties of the coatings were evaluated through electrochemical impedance spectroscopy and accelerated corrosion using a salt spray chamber showing that GO forms an anti-corrosive barrier increasing the operation life of the coatings in corrosive environments. Anti-ice properties were related to the contact angle measurement from which the GO concentrations showed more hydrophobic behavior. All the tests were carried out according to ASTM standards. The incorporation of 0.5% of GO showed a significant improvement in the mechanical and anti-corrosive results, improving corrosion resistance up to 500 h. The abrasion tests had an increase in 35%, its hardness up to 9H, and the wear index improved by 29.14% compared with composites with 0.1 wt% of GO and without GO. The HREF1 and HREF5 materials do present an increase in the contact angle thanks to the incorporation of graphene oxide. The results of electrochemical impedance spectroscopy and the impedance curves show a better behavior for the HREF5 composite due to the difference in resistance over time.

Working on a manuscript? Avoid the common mistakes Introduction

Degradation can be defined as the deterioration of a material due to the action of the environment in which it is immersed (pressure, humidity, mechanical stress, pH of fluids, atmospheric chemical agents, etc.) causing the loss of physical and mechanical properties of the material. When speaking of corrosion, it is understood only as the chemical or electrochemical degradation of a material, usually a metal, by its environment.1,2 Organic coatings including epoxy primers have become a common solution to protect metal substrates against corrosion and environmental erosion, leading to a great development in the materials used to improve the properties of the coatings.3 Due to the outstanding features of this type of coating, nanofillers are being introduced to produce hybrid materials, combining organic and inorganic properties and enhancing the protective applications.4 For instance, Araki et al.5 have studied the effect of the viscoelasticity of epoxy/silica hybrids in terms of morphology. Also, silica has been used in polyimides6 and poly(tetramethylene oxide)7 organic coatings to study the resistance against the attack of atomic oxygen. Wang et al.8 tested tubes of a nano-silica/graphene oxide hybrid to study the flame retarding effect and thermal conductivity. Silica is not the only inorganic nanofiller. Zhao et al.9 modified the surface of ZrO2 nanoparticles with a styrene coupling agent and used them as an additive to improve corrosion protection. Most authors conclude that nanofillers help organic coatings because the microporosity diminishes blocking the way for corrosion.

Hybrid coatings also represent a promising solution in the aerospace industry to stop using anti-corrosive protective systems based on electrochemical salt baths to form chromates.2 Hexavalent chromium has good corrosion resistance due to its strong rust-preventing properties. However, it is a great enemy to the environment and highly toxic for humans.10 Furthermore, there are extensive investigations into the diversity of nanofillers for hybrid materials applied in the aeronautical sector to provide more benefits besides corrosion protection such as anti-icing/de-icing properties.11,12,13,14

Multiple works can be found where the improvement of clear epoxy resin with SiO2 nanoparticles or nanoclusters is analyzed in many mechanical, chemical, or thermal aspects15 and by different synthesis routes.16 Thus, the improvement of clear resin by adding silica nanoparticles is already a known fact and we elaborate in this paper on more technical details about the structure and the interesting and novel improvement on corrosion resistance.

Graphene oxide (GO) is of interest to this work because of its impermeability against diffusion of water, oxygen, and corrosive Cl- ions; therefore, it acts as a barrier in the coatings.17 Recently, Parhizkar et al.18 and Pourhashem et al.19 have studied the corrosion protection of epoxy coatings on steel substrates enhancing the interfacial adhesion with modified amino-functionalized GO. In addition, GO research also affects the thermal performance and stability.20 Ramezanzadeh’s group has investigated the interaction between silica particles and GO sheets through different publications, and recently covered the coating systems.21,22,23 Nevertheless, there is no reported information on using a functionalized epoxy matrix or electrochemical results for aeronautical aluminum as the coated substrate.

Thus, the objective of this work was to study the effect of GO on the mechanical and anti-corrosive properties within a functionalized epoxy-SiO2 hybrid, characterizing the material with different techniques and test methods to evaluate the properties as a coating material for aluminum 6061-T5.

Materials

Natural mineral graphite powder (90%) was generously donated by Mr. Alfredo Ayala, of the Centro de Física Aplicada y Tecnología Avanzada (CFATA). Chemical reagents for the synthesis of graphene oxide (GO), sulfuric acid (H2SO4, 98 wt%), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl), and potassium permanganate (KMnO4, 99.5 wt%) were acquired from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS, 98 wt%), ethanol (C2H5OH, 98 wt%), and sodium hydroxide (NaOH, 98%) were purchased from Sigma-Aldrich. Abietic acid (C20H30O2, 97%) was obtained from Sigma-Aldrich, and bisphenol/epichlorohydrin epoxy resin with Mn?=?700 was bought from Poliformas plásticas. The curing agent used to prepare the coatings was polyoxypropylendiamine “HD-307” with a specific gravity of 0.97?±?0.02 and viscosity of 3500 mPa.s at 25°C and was from Poliformas plásticas. Materials for the ASTM test methods included a set of standard pencils, weighted standard car for hardness, adhesion tape, and a 1-mm/2-mm metallic grid.

MethodologySynthesis

Graphite oxide (GrO) was synthesized from natural mineral graphite by the modified Hummers method.24 This process consisted of dispersing 2 g of graphite powder in 46 mL of H2SO4 at ??5°C in an ice bath immersed flask under constant stirring; then, 6 g of KMnO4 was gradually added keeping the temperature below 20°C. Once homogenized, the mixture was removed from the ice bath, brought up to 35°C to initiate the reaction and maintained for 2 h. Then, 92 mL of distilled water was added drop-by-drop, maintaining the stirring for 15 min. The reaction was terminated by transferring the mixture to a vessel with 270 mL of distilled water and 10 mL of H2O2 (30% v/v). Subsequently, centrifugation washes were performed using 400 mL of an HCl solution (2.5% v/v) to remove metal ions and distilled water until obtaining a pH close to 7. Finally, the graphene oxide was obtained from the exfoliation of the stacked sheets in the graphite oxide, through the dispersion of batches of 300 mg of GrO in distilled water (10 mg/mL) and subjected to an ultrasonic bath treatment (42 kHz?±?6%) for 3 h. The product was then dried to obtain a black GO powder.

On the other hand, 40 g of epoxy resin (Mn?=?700) and 0.85 g (0.003 mol) of abietic acid were mixed with stirring for 20 min at a temperature between 95°C and 115°C to obtain functionalized epoxy resin (FER).

The hydrolysis sol-gel25 process was made with a TEOS: EtOH solution (13.75:5 v/v) mixed with a previous 30-min ultrasonic bathed water: EtOH:GO solution (5:20:x v/v/m) with constant stirring and at a temperature of 95°C, where x represents 0 g (0 wt%), 0.04 g (0.1 wt%), and 0.2 g (0.5 wt%) of GO powder for the three hybrids. After 30 min, 40 g of FER were added dropwise and the mixture was left under reflux for 1 h at 105°C, and the viscosity was monitored visually.

Preparation of aluminum substrates

For the adhesion and hardness mechanical characterization of the coating-substrate system, nine 8 × 8 cm2 aluminum samples were machined, and sanded with a ¼” orbital sander up to 360 sandpaper. For the abrasion test, nine 10 × 10 cm2 samples with rounded corners (r?=?1 cm) and a central hole of D?=?1/2” aluminum specimens were machined, sanded with a ¼” orbital sander up to 360 sandpaper. All samples were cleaned in a 15-min isopropanol ultrasonic bath and 15-min acetone ultrasonic bath.

Preparation of hybrid coatings

To study the mechanical properties as a coating, the hybrid epoxy coatings containing 0% (HREF), 0.1% GO (HREF1), and 0.5% GO (HREF5) were prepared with the polyamine curing method for epoxy polymers. The prepared solutions were mixed with the polyoxypropylendiamine HD-307 commercial hardener in a ratio of 100:15 m/m, followed by a 5-min ultrasonic bath to eliminate any air bubbles trapped. Then, the resin was kept for 10 min in a chamber at 30°C to evaporate any solvent remanent to evaporate due to the curing exothermic reaction. Finally, the hybrid coatings were carefully blade coated on the cleansed aluminum substrates and dried at room temperature for 5 h for the first dry, and then a second dry for 24 h for a total drying.26 The wet thickness of the coatings (about 45?±?10 µm) was established with the two micrometers in the blade-coating equipment.

Characterization

X-ray diffraction (XRD) analysis was used to verify the oxidation of mineral graphite and the exfoliation to obtain GO sheets. The equipment was a Rigaku Ultima IV diffractometer, equipped with an X-ray generator with a maximum power of 3 kW, voltage of 20–60 kV, and output current of 2–60 mA from 5° to 80°.

Raman dispersive spectra were carried out to confirm graphene oxide using a Bruker model Senterra with an optic microscope 20× objective, with a resolution of 9?15 cm-1, an integration time of 2 s, 8 scans, and power of 25 mW. The spectra were recorded in the 110?3400 cm-1.

Attenuated total reflectance–Fourier transformed infrared (ATR-FTIR) spectra measurements were achieved using a Thermo Scientific Model Nicolet 6700, with a scan speed of 0.32 s, a gain of 8.00, and a voltage (Vpp) of 5.73 V. The spectra were recorded in the wavenumber range between 4000 cm?1 and 400 cm?1 at a resolution of 4 cm?1.

Scanning electron microscopy (SEM) was used to observe the dispersion quality and the morphology of the hybrids, the equipment was a SU8230 Hitachi SEM/STEM CFE, and the samples were gold coated using an AMS Au-Coater. The sample for transmission electron microscopy (TEM) was prepared by dispersing GO in distilled water and placed and dried in the TEM grid. Afterward, it was observed with 60?80 kV in a JEOL TEM 1010 microscope.

To be considered in the aeronautical market, the ASTM D-2369-20 Standard Test Method for Volatile Content of Coatings27 was conducted.

Mechanical tests

The adhesion strength, hardness, and abrasion of the different hybrid material samples were studied by the ASTM Standard Methods:

• ASTM D3363-20 Standard Test Method for Film Hardness by Pencil Test

  

  » Publication Date: 27/10/2023

  » More Information

  « Go to Technological Watch