Alkyd resin based nanocomposite coatings filled with different content of silicon oxide (SiO₂) and titanium oxide (TiO₂) nanoparticles (2 and 4^wt/_wt%) were prepared by solvent casting method. Characterization of the prepared nanocomposite coatings was performed by Differential Scanning Calorimetry (DSC) and Scanning Electron Microscope (SEM). The nanocomposite coatings were also tested for their Chemical stability, Mechanical properties, Abrasion and Impact resistance. DSC analysis shown that all the characteristic temperatures (glass transition temperature-T_g, reaction temperature-T_r) were shifted to lower values by increasing the nanoparticle content. Mechanical characterization confirmed good elasticity with bending of f=2cm and impact resistance of 1 kg/cm². Increasing the nanoparticle content, the wear resistance was changed as well. Better abrasion resistance was registered for nanocomposite coatings with SiO₂ nanoparticles compared to nanocomposites with the same content of TiO₂.

Keywords: nanocomposite coatings, alkyd resin, TiO₂, SiO₂

Design and development of new types of polymer based nanocomposites (NC) is rapidly emerging as a multidisciplinary research activity whose results could broaden the applications of polymers to the great benefit of many different industries: automotive, construction, aerospace, electrical and electronics, food packaging.^1‒4 Polymers, resins and plastics containing additives such as clays and various nanostructures have recently become widely used as replacements for conventional materials of steel and other metal products, or their coatings, especially in the field of automotive industry. The application of the polymer NC coatings for the corrosion protection of steels was reviewed by Toshiaki Ohtsuka⁵ he reported various systems mainly based on conductive polymer matrix. The incorporation of layered silicates into polymer matrix was a research focus for many years, since the results of the Toyota research team showed that the incorporation of 2 vol% of exfoliated clay into Naylon-6 contributes to about 90^oC increase in glass transition temperature and double Young’s modulus.⁴ Traditionally, alkyd resins as oil-modified polyesters, have been placed amongst the most widely used resins in exterior solvent-based inexpensive coating applications because of their high gloss and durability. Alkyd-based nanocomposite coatings were reported to exhibit better properties compared to the traditional ones.² It was shown that water vapour permeability of nanocomposite coatings remarkably decreased with respect to neat coating. For example, a decrease of water vapour permeability by a factor of 15 has been measured.¹ Inorganic–organic composites based on organo-alkoxysilanes and other alkoxides have demonstrated their usefulness even for hard coatings on eye-glass lenses.⁶ It has been shown that the addition of nanoparticles, especially in combination with epoxy silanes, which act as an inorganic as well as an organic crosslinking agent, leads to a substantial increase of the abrasion resistance of such systems without losing any transparency. Therefore, due to the incorporation of nanoparticles into the certain polymer matrix, high abrasion resistance can be obtained.^1,5 Avella et al.⁷ have shown that the presence of only 2% nanoparticles strongly improves the abrasion resistance, as well as the wear of Poly(Methyl methacrylate) (PMMA). ⁶ The nanocomposites showed an average weight loss about half (2,5% against 5% of removal material) with respect to that of neat polymer.

The addition of SiO₂ nanoparticles into acrylic-based polyurethane coatings apparently improved the micro and macro hardness as well as abrasion resistance. Another type of nano meter SiO₂ is the colloid SiO₂, which was also used to improve hardness, wear resistance and scratch resistance of the coatings. Literature survey also reported the improved performances of epoxy/nano-SiO₂ and methacrylate coating on polycarbonate substrate.² Yang et al.⁸ have studied the influence of the nature and the size of TiO₂ and SiO₂ nanoparticles on the optical and electrical properties of Poly(p-phenylene vinylene) based nanocomposites.⁸ Jiratumnukul et al.⁹ have studied nanocomposite alkyd coatings prepared with organoclay, by FTIR and XRD analyses.⁹ They have found that the X-ray profiles showed an exfoliated structure for the organoclay alkyd coating. The application of the organoclay to alkyd-based clear coatings showed a significant enhancement of the hardness and impact strength of the coating film compared with those of unmodified montmorillonite equally incorporated into the coating formulations.⁹ Mironi-Narpaz et al.¹⁰ have worked also on nanocomposite systems based on unsaturated polyester (UP)-alkyd resin (without styrene) mixed with different organo-clays.^10,11 Processing parameters such as mixing modes, applied shearing levels, clay contents, and mixing-temperatures were investigated. Prolonged high shear levels promoted the intercalation and exfoliation of the silicate layers, resulting in a better dispersion of clay particles.

The higher shear levels effects were achieved by vigorous mechanical mixing and they were intensified by adding large amounts of clay and optimized matrix viscosity.¹¹ Lin et al.¹² have worked on preparation and morphologies of the biodegradable glycerol-based alkyd resins/layered silicates nanocomposites, as well as their thermal stabilities and mechanical properties.¹² Although the mechanical properties of these glycerol-anhydride alkyd resins/clay nanocomposites were greatly improved, Lin et al. still had a problem how to minimize the formation of aggregates in the case of mica or talk within the alkyd resin matrix. The properties changes of the nanocomposite coatings were attributed to the modification of coating structures and different diffusion behavior in the nanocomposite coatings compared to the conventional coatings. The most of the studies related to the nanocomposite coatings were focused on mechanical, optical or anticorrosion properties. In this work, we have examined the chemical, thermal and abrasion resistance properties of the hybrid materials using the same alkyd resin in order to check the influence of the inorganic nanoparticles on the behavior of the obtained nanocomposites. SiO₂ and TiO₂ nanoparticles of different content were selected and incorporated into alkyd resin to obtain nanocomposite coatings. However, in order to transfer the nanocoating technology from research and lab scale level into the industrial production of low-cost nanocoating materials, further efforts are needed.

Alkyd resin with solid content of 44.6% and viscosity and density of μ₂₀=51s and r₂₀=886kg/m³ respectively was used as received. The TiO₂ nanoparticles were Degussa P25, ca. 80% anatase, 20% rutile; with particle size around 21 nm. SiO₂ nanoparticles (Aerosil 300, consists of spherically shaped primary particles, with a mean diameter of 7nm and specific surface area of 300m²/g) also from Degussa, Germany were used.

Nanocomposite coatings preparation

Nanocomposites were prepared by incorporation of TiO₂ and SiO₂ nanoparticles in alkyd based coating formulation with a high-speed disperser at a speed of 400 rpm for 30 min at room temperature. In order to solve the dispersion problem of the nanoparticles in alkyd resin based coatings, previously the alkyd resin was mixed with dispersant agent. Two types of nanoparticles, TiO2 and SiO₂, have been selected and used in concentration of 2 and 4^wt/_wt %. Coating films with thickness of ~60μm were obtained on Teflon sheet and allowed to dry at room temperature for 5 days.

Characterization

Characterization protocol of the prepared nanocomposite coatings includes Differential Scanning Calorimetry (DSC), Scanning Electron Microscope (SEM), Nanoidentation and Atomic Force Microscopy (AFM), chemical stability, abrasion and flexibility test.

Differential scanning calorimetry

Thermal properties of the coatings were analyzed using differential scanning calorimeter (Perkin Elmer DSC -7 Analyzer), from -30^oC to 260^oC with the heating rate of 20K/min. The measurements were performed under nitrogen.

Scanning Electron Microscopy

The morphology of the samples was followed by Scanning Electron Microscope using the JEOL system. Vacuum Au/Pd alloy deposition of the samples in a Polaron Sputtering apparatus was performed previously.

Chemical stability

Nanocomposite coating samples (1x1cm) were cut and they were tested in various acid and alkaline media, for different times of exposure (2, 3, and 5days) up to the moment of dissolving the samples.

Nanoidentation

The roughness was measured by Profilometer 500. The average value was obtained from 5scannings, each of them 500 μm in contact mode with 10 mg applied contact force. The Hardness (H) was determined from nanoindentation using Berkovich Diamond indentor, with 10mN applied force (load time=15s, holding time=10s, unloading time=15s). The average value from minimum 10 measurements was done.

Abrasion resistance

Abrasion resistance was followed by means of TABER Abraser machine under load of 10N. The abrasion resistance was reported as number of cycles up to the coating crack as well as the change in mass of the test specimen per number of performed cycles. The twist flexibility was determined using a standard method DIN 53211 by twisting a panel with a coating film over a metallic stick of defined diameter. During the specified time the coated film should not crack and separate from the panel.

The incorporation of nanoparticles in appropriate polymer matrix or resin could significantly affect the thermal, barrier, mechanical and wear properties, as well as the morphology of the composites coatings.^2-9 The characteristic thermal parameters of the studied nanocomposite coatings obtained by DSC analysis are presented in Table 1. The presence of TiO₂ and SiO₂ nanoparticles contributed to the shifting of the characteristic temperatures: glass transition temperature (T_g) and curing reaction temperature (Tr) to the lower temperatures when compared to the characteristic temperatures of the pure alkyd resin. The glass transition temperature shifting to lower temperatures could be an indication for better flexibility of the obtained nanocomposites coatings, with higher nanoparticles content. Remarkable effect was registered in alkyd resin filled with 4 wt % TiO₂ nanoparticles, were the T_g value was lower than the T_g of alkyd resin filled with the same quantity of SiO₂ (T_g was shifted for 10 degrees). The obtained values for T_g of the alkyd-based nanocomposites lower than 30^oC (T_g<25-30^oC), suggest higher durability of the obtained nanocoatings which is in agreement with the practical data. The noteble T_r shifting to the lower temperatures in the nanocomposite systems indicate on the catalytic effect of the nanoparticles in the alkyd reaction. The curing reaction temperature decreases as compared to the curing temperature of neat coating, also the heat of curing value increases (DH_r). This indicate that the interaction between TiO₂ and SiO₂ nano-particles and alkyd matrix improves the curing of nanocomposite coatings. Similar results were obtained alkyd-based systems modified with other resins or nanoparticles such as ZnO.⁶

Chemical stability

From practical point of view, for the nanocomposite coatings a very important practical characteristic is their chemical stability in various media. The results obtained after dipping the samples in various (neutral, acid and alkaline media) for different times of exposure are collected in Table 2. The obtained results shown that the prepared nanocomposite alkyd resin/TiO₂ and alkyd resin/SiO₂ coatings are stable in water. In organic solvents, such as acetone and chloroform, both systems swelled. After 96 h of exposure in organic solvent, the nanocomposite sample with 4% TiO₂ was broken, while the sample with 4 % SiO₂ has lost the colour and become a transparent. In the presence of various acids, both nanocomposite coatings swelled and changed the colour to dark brown. In alkali media, both coatings were deformed and rolled, as well as changed their colour and porosity, they had yellow colour and they were porous (Table 3).

Mechanical properties

The results of the mechanical properties testing shown that all coatings, obtained with both nanoparticles, exhibited very high flexibility. The obtained value of flexibility was 2cm for all studied systems, without crack registered on the nanocomposite coatings due to the characteristics of alkyd resin. Both nanoparticles have higher aspect ratio which could contribute in the higher strength and mechanical resistance of the prepared nanocomposite coatings. The obtained data from the nanoidentation testing for the surface roughness and the hardness as well as the Young’s modulus are presented on Table 3. Nanoidentation data have shown that for both types of nanoparticles, the roughness of the obtained nanocomposite coatings remarkably increased, while the hardness decreased by increasing the nanopaticle content. Comparing the effect of the nanoparticle type, higher values for the hardness and the Young’s modulus were obtained in the nanocomposite coatings filled with TiO₂ nanoparticles (Table 3). Characteristic AFM surface profiles for the studied nanocomposite coatings are shown in Figure 1 –for Alkyd/TiO₂ system and Figure 2 for Alkyd/SiO₂ system. These images confirmed that the addition of both nanoparticles increased the roughness of the surface. This indicates that nanocomposite coatings with higher amount of nanoparticles, specially TiO₂, should have increased surface area resulting in enhanced adhesion¹³ (Figure 1 & 2). The abrasion resistance of the solid body is defined as its ability to withstand the progressive removal of materials from its surface as a result of mechanical action of a rubbing, scraping or erosion.⁶ Namely, the abrasion reduces the serviceability of the affected body and low abrasion resistance is a very important drawback that modern nanocoatings must overcome. The abrasion resistance of the nanocomposite coatings has been studied to evaluate if the presence of inorganic particles such as SiO₂ and TiO₂ could improve also the scratch resistance of the neat alkyd resin. Results for the abrasion resistance of the fabricated nanocomposite coatings are presented in Table 4. Better abrasion resistance was registered for the nanocomposites filled with SiO₂ compared to nanocomposites with TiO_2, for the mass changes after the certain number of cycles, (Dm _2%SiO2=0,0136g; SiO₂ Dm _2%TiO₂=0,0521g). According to the literature, this finding could be attributed to the fact that the nanoparticles support part of the applied load, thus the penetration into the polymer surface is reduced and only micro plugging and/or micro cutting phenomena can be generated, as can be seen in Figure 3B & Figure 4B (Table 4).

Figure 1 AFM surface profile for the Alkyd/TiO₂ nanocomposite coatings.

Figure 2 AFM surface profile for the Alkyd/SiO₂ nanocomposite coatings.

Nanocomposite morphology

Morphology changes before and after abrasion test measurements were followed by SEM. The obtained microphotographs for Alkyd/SiO₂ nanocomposite coatings are shown on Figure 3A, while for the system Alkyd/TiO₂ nanocomposites coatings, are shown on Fig. 4a. The resulting abrasion marks and surface micro ploughing effects were registered for both investigated systems (Figure 3B & Figure 4B). Under the attack of the rough abrasive paper, the grains penetrated deeply into the surface of the polymer, removing materials from the surface by extensive micro ploughing process. During this phase, the polymer matrix is highly plastically deformed due to the micro cutting and wear debris was formed.

Figure 3 Morphology of Alkyd/SiO₂ nanocomposites coatings before (A) and after (B) abrasion test measurements.

Figure 4 Morphology of Alkyd/TiO₂ nanocomposites coatings before (A) and after (B) abrasion test measurements.

Using TiO₂ and SiO₂ nanoparticles, two types of nanocomposites coatings based on alkyd resin were prepared. They were characterized in terms of their thermal and mechanical behaviour. The presence of both nanoparticles, TiO₂ and SiO₂, contributed to the shifting of the characteristic thermal parameters T_g and T_r to the lower temperatures indicating improved flexibility of the nanocomposite coatings. Nanoidentation data has shown that for both type of nanoparticles, the roughness of the obtained nanocomposite coatings remarkably increased, while the hardness decrease by increasing the nanopaticle content. From the studied systems, better abrasion resistance was registered for the nanocomposites with SiO₂ compared to nanocomposites with TiO₂.

The authors declare no conflict of interest.

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