Mechanical properties of acrylic coatings
Organic modification of nanoparticles’ surface is considered as one approach to improve the compatibility of nanoparticles with polymer matrix leading to enhancing some properties of polymer coatings. The efficiency of enhancement of mechanical properties for acrylic coatings depends on nature and content of TMSPM used to modify TiO2 nanoparticles. The acrylic coatings filled by 2 wt.% nano TiO2 (in comparison with solid part of acrylic resin), which was modified with different TMSPM content, were prepared as mentioned in experimental section. The TiO2 nanoparticles modified with 1, 3, 5, 10 and 20 wt.% TMSPM (in comparison with nano TiO2) were abbreviated as m-TiO2-1, m-TiO2-3, m-TiO2-5, m-TiO2-10 and m-TiO2-20. The abrasion resistance of acrylic coatings filled by m-TiO2 modified with various TMSPM content was displayed in Table 1.
As can be seen in Table 1, the unmodified and modified TiO2 nanoparticles improved the abrasion resistance of the acrylic coating. The acrylic coating filled by 2 wt. % TiO2 nanoparticles modified with 3 wt.% TMSPM had the highest abrasion resistance. However, the abrasion resistance value of acrylic coatings filled by TiO2 nanoparticles modified by higher TMSPM content was lower than that of the acrylic/m-TiO2-3 coating. These obtained results were similar with the results determining the abrasion resistance of the acrylic coatings filled by ZrO2 nanoparticles modified with TMPSM [31].
Thank to small size, the nanoparticle could be filled into defects in coating structure, consequently, the coating’s structure became more tightly [15, 17, 30]. Due to the difference of nature and structure, the nanoparticles and polymer matrix are poorly compatible, leading to the agglomeration of the nanoparticles in the polymer matrix [2]. Silane coupling agent grafted on the surface of the nanoparticles acted as a bridge between nanoparticle and polymer matrix [6]. Hence, the dispersion of the nanoparticles modified with silane coupling agent in polymer matrix was improved. However, the efficiency of the silane coupling agent grafted on the surface of the nanoparticles was reduced when using high silane coupling agent content [20, 31]. The reason was that in the modification process, the silane coupling agent reacted with the hydroxyl groups on the nanoparticles' surface to produce a layer of silane coupling agent. This new-formed layer covered the surface of nanoparticles and thus preventing silane coupling agent grafting continuously onto the TiO2 nanoparticles. The silane coupling agent residues could be polymerized and thus forming the third phase in coating [20, 31]. Hence, using the nanoparticles modified with high silane coupling agent content could reduce the regularity of the acrylic coatings. It means that the abrasion resistance of the acrylic coatings was lower in comparison with the coatings filled by TiO2 nanoparticles modified with TMSPM content over 3 wt.%. As from obtained results, the TiO2 nanoparticles modified with 3 wt.% TMSPM was chosen for further studies.
The abrasion resistance of investigated acrylic coatings depends on size and content of modified TiO2 nanoparticles. The acrylic coatings filled by different content of TiO2 nanoparticles modified with 3 wt.% TMSPM (m-TiO2-3) were prepared as mentioned in experimental section. The abrasion resistance of investigated coatings containing 0.5; 1; 2 and 4 wt.% m-TiO2-3 was performed in Table 2.
As can be seen from Table 2, the abrasion resistance of the acrylic coating grew up with increasing content of m-TiO2-3 nanoparticles. However, the acrylic coating filled by 4 wt.% m-TiO2-3 nanoparticles had a lower abrasion resistance value in comparison with the acrylic coating filled by 2 wt.% m-TiO2-3 nanoparticles. When the acrylic coating filled by the low m-TiO2-3 nanoparticles content, the silane coupling agent could establish a good phase interaction between the nanoparticles and polymer matrix. The m-TiO2-3 nanoparticles could fill into defects of acrylic coating leading to be tighter coating’s structure (as discussed above). In addition, the m-TiO2-3 nanoparticles also acted as a reinforcing agent. As a result, the mechanical properties of the acrylic coating were improved. However, using high content of m-TiO2-3 nanoparticles could lead to the agglomeration of the nanoparticles and the decrease in the phase interaction between the nanoparticles and polymer matrix [29]. Hence, the abrasion resistance of acrylic coating reduced. As can be seen from obtained result, the acrylic coating filled by 2 wt. % m-TiO2-3 nanoparticles reached the highest value of abrasion resistance. Therefore, using 2 wt. % m-TiO2-3 nanoparticles was suitable content for the acrylic coating. The acrylic coating filled by 2 wt. % m-TiO2-3 was selected for further studies.
The effect of Ag/Zn zeolite content on the abrasion resistance of the acrylic coating filled by 2 wt. % m-TiO2-3 nanoparticles was also evaluated. The abrasion resistance of the acrylic coating filled by 2 wt. % m-TiO2-3 nanoparticles combined with different Ag/Zn zeolite particles content was displayed in Table 3.
As can be seen from Table 3, the abrasion resistance of the acrylic coating increased in the presence of Ag/Zn zeolite particles in comparison with the neat acrylic coating. However, the Ag/Zn zeolite particles caused the reduction in the abrasion resistance of the acrylic coatings filled by 2 wt. % m-TiO2-3 nanoparticles. The abrasion resistance of the acrylic coating decreased as increasing Ag/Zn zeolite content particles. The Ag/Zn zeolite was inorganic compound and thus having higher hardness in comparison with the acrylic polymer. Therefore, the Ag/Zn zeolite particles could improve mechanical properties of acrylic coating. However, the zeolite has a porous structure, consequently, the Ag/Zn zeolite particles could influence on the structure of acrylic coating filled 2 wt.% m-TiO2-3 nanoparticles. As a result, the abrasion resistance of the acrylic coatings filled by both m-TiO2-3 nanoparticles and Ag/Zn zeolite particles was reduced.
Morphology of acrylic coatings
As mentioned above, the properties of the acrylic coatings were affected by size and content of m-TiO2-3 nanoparticles and Ag/Zn zeolite particles. Their size and the morphology of the acrylic coating can be observed in FESEM images of the coatings’ cross surface (Fig. 3).
Fig. 3 showed a poor dispersion of the unmodified TiO2 (u-TiO2) nanoparticles in the polymer matrix (Fig. 3 (a,b)). The agglomeration of the u-TiO2 nanoparticles could be seen obviously (Fig. 3.b) while the m-TiO2 nanoparticles could disperse more regularly in the polymer matrix. The size of the m-TiO2 nanoparticles in the acrylic coating was about 100 nm (Fig. 3.d) which was similar with the size of initial m-TiO2 nanoparticles (Fig. 2.b). The u-TiO2 nanoparticles tend to agglomerate together producing large clusters which are more stable status in comparison with single particles because of its high surface energy. In addition, TiO2 nanoparticles and acrylic polymer are different in nature leading to poor compatibility of nano TiO2 and polymer matrix. Furthermore, hydroxyl groups on the surface of the TiO2 nanoparticles could form hydrogen bond among TiO2 nanoparticles [16]. However, when the TiO2 nanoparticles were modified with TMSPM, amount of hydroxyl groups on the surface of m-TiO2 nanoparticles decreased. Moreover, the silane coupling agent could act as a bridge between TiO2 nanoparticles and polymer matrix. Consequently, compatibility of m-TiO2 nanoparticles with acrylic polymer matrix was improved [20, 31]. It means that dispersion of m-TiO2 nanoparticles in polymer matrix was enhanced. As a result, the properties of acrylic coating filled m-TiO2 nanoparticles were higher than those of the acrylic coating filled u-TiO2 nanoparticles.
The FESEM images of the cross-surface of the acrylic coating filled Ag/Zn zeolite particles and Ag/Zn zeolite particles combined with m-TiO2 nanoparticles indicated that these particles could disperse regularly in polymer matrix. The size of Ag/Zn zeolite particles dispersed in the acrylic coating was 200 nm.
Antibacterial activity of acrylic coatings
The antibacterial activity for Escherichia Coli and Staphylococcus Aureus of the acrylic coatings filled by 2 wt. % m-TiO2-3 nanoparticles and different content of Ag/Zn zeolite particles was presented in Table 4.
As can be seen from Table 4, the acrylic coatings filled by m-TiO2-3 nanoparticles did not kill E. Coli bacteria. The obtained results seemed to be conflicted with other publication [15]. The reason was that TiO2 nanoparticles can kill bacteria because of their photo-catalytic properties. Under UV irradiation, TiO2 nanoparticles indicated the photo-catalytic properties and producing reactive oxygen species (ROS) such *OH, hydroperoxide, etc. [35]. The new-former ROS are able to kill bacteria. It means that the TiO2 nanoparticles showed antibacterial activity with light (UV) irradiation. Therefore, in this test, the acrylic coatings filled by m-TiO2-3 nanoparticles had insignificant antibacterial activity. The same result was reported by Ashrafi and his coworkers [38].
However, the acrylic coating filled by both m-TiO2-3 and Ag/Zn zeolite particles showed a high antibacterial activity. The antibacterial activity of acrylic coatings grew up with increasing Ag/Zn zeolite particles content. The acrylic coating filled by both m-TiO2-3 and 1 wt.% Ag/Zn zeolite particles could kill 99.99 % E. Coli bacteria after 24 hours of testing. It could be seen that there was no difference in antibacterial activity between the coating filled by 1 wt.% Ag/Zn zeolite particles and that by 2 wt.% Ag/Zn zeolite particles because most of all bacteria were killed. As the mechanism antibacterial of metal like Ag, Zn, etc., the metal could kill bacteria due to ion metal and ROS (which was produced by the oxidation-reduce of metal) [35]. Therefore, the metal content is enough to produce the necessary ROS and most bacteria will be killed.
The antibacterial activity of the acrylic coatings filled by 2 wt. % m-TiO2-3 nanoparticles and different content of Ag/Zn zeolite particles for S. Aureus was similar for E. Coli (Table 5). The Ag/Zn zeolite particles played a role of antibacterial agents in acrylic coatings. The antibacterial activity of investigated coatings grew up with rising Ag/Zn zeolite particles content. However, the antibacterial activity of acrylic coating filled by 2 wt.% Ag/Zn zeolite particles was insignificantly in comparison with that of the acrylic coating filled by 1 wt.% Ag/Zn zeolite particles. Hence, 1 wt.% Ag/Zn zeolite particles in acrylic coating was selected for further studies.
Thermo-oxidation stability of acrylic coatings
Thermogravimetric analysis (TGA) diagrams of the acrylic coating without TiO2 nanoparticles, with modified TiO2 nanoparticles, and modified TiO2 nanoparticles combined with Ag/Zn zeolite particles were presented in Fig. 4. The temperatures at which 5, 50 and 90 % weight of samples lost (T5, T50 and T90, respectively) were displayed in Table 6.
As could be seen from Fig. 4, the TGA diagrams of the acrylic coatings could be divided into three periods. In the first period, from ambient temperature to 240 oC, the weight of all acrylic coatings was fairly stable (the weight loss of the acrylic coatings was zero). However, from 240 oC, the weight of the acrylic coating filled by m-TiO2-3 nanoparticles started to reduce slightly. The weight loss also occurred in the neat acrylic coating and acrylic coatings filled by m-TiO2-3 nanoparticles combined with Ag/Zn zeolite particles at the temperature over 240 oC. The starting temperature of weight loss of the investigated acrylic coatings could be arranged as follow: Acrylic/m-TiO2-3 < Acrylic/m-TiO2-3/1 wt.% Ag/Zn zeolite < acrylic. The reason may be thermal transport increased during heating process of acrylic coatings in the presence of nanoparticles [36]. However, the acrylic coating filled by Ag/Zn zeolite particles which have the microporous structure, more or less, reduced the thermal transport. Although the weight loss of the neat acrylic coating occurred later in comparison with the acrylic/m-TiO2-3 coating, the weight of neat acrylic coating showed a faster reduction. The T5 (which is assigned with temperature in which 5 wt. % weight of materials lost) of acrylic/m-TiO2-3 coating was higher than that of the neat acrylic coating, 319.54 oC for the former and 318.16 oC for the later. The T5 of acrylic/m-TiO2-3/1 wt.% Ag/Zn zeolite coating was the lowest. It was explained that m-TiO2-3 nanoparticles could make acrylic coating structure become tighter and less defect (as mention in above) and limit permeation of oxygen into the coating. Thus, the T5 of acrylic coating filled m-TiO2-3 was improved. Besides, the microporous zeolite structure made coating structure became less tight, having more defects. Moreover, due to reducing T5 of acrylic coating [37].
In the second period, the weight of acrylic coating reduced sharply. Most of the acrylic coatings lost weight in this period due to breaking and thermo-oxidation degradation of the polymer chains. The dTGA diagrams of the acrylic coatings indicated the temperature corresponding to maximum degradation rate were 350.8 oC, 372.01 oC and 364.15 oC for the neat acrylic, acrylic coating filled by m-TiO2-3 nanoparticles and acrylic coating by filled m-TiO2-3 nanoparticles combined with Ag/Zn zeolite particles, respectively. The weight loss was caused by the degradation of organic parts in acrylic coating. If heating temperature was higher than 400 oC, the weight of samples was unchanged.
Weathering durability of acrylic coatings
Infrared spectroscopy analysis
In aging process, polymer coatings are exposed to UV irradiation, heat and high humidity. Polymer degradation can be obtained in chemical changes firstly before other symptoms. Therefore, monitoring chemical changes of functional groups of polymer upon accelerated weathering test is very necessary to propose the mechanism of polymer degradation. Infrared (IR) spectroscopy is a sensitive method which can identify and quantify functional groups in the polymer coating. The IR spectra of investigated acrylic coatings before and after accelerated weathering test (36 cycles) were shown in Fig. 5. It was obvious that the absorptions in the IR spectra of aged acrylic coatings were clearly significant changed in comparison with IR spectra of the initial acrylic coatings. For instance, the absorption located at the wavenumber of 3440 cm-1 (corresponding to hydroxyl stretching) was bigger for the acrylic coating filled by m-TiO2-3 nanoparticles, Ag/Zn zeolite particles and TiO2 nanoparticles combined with Ag/Zn zeolite particles. It could also be observed that shoulder peak located at 1780 cm-1 appeared during aging process for all of acrylic coatings. While the absorptions located at 2925 and 1450 cm-1 (which were assigned to C-H of alkane stretching and bending) and absorption located at 1150 cm-1 (which was characteristic of C-O of ester group) indicated in reversion. The change in the wavenumber of some functional groups in acrylic coatings were displayed in Table 7.
Carbonyl index (CI) and photo-oxidation index (PI) are usually used to evaluate the degradation degree of polymer coatings [1, 34]. The CI and PI of acrylic coatings were calculated on the basis of absorptions’ intensity which was corresponding to hydroxyl groups, carbonyl groups, and C-H (of alkane groups) as below formulas [34]:
PI = A/B (1) CI = C/B (2)
There: A, B, C represents of hydroxyl, C-H (alkane) and carbonyl peak heights, respectively.
The CI and PI of acrylic coatings with different compositions during the accelerated weathering test were presented in Fig. 6 and Fig. 7.
As can be seen from Fig. 6, the CI of the acrylic coatings grew up upon aging test. The neat acrylic coating had the highest increase in CI among investigated acrylic coatings. Their CI of the samples could be arranged as neat coating > acrylic/(Ag/Zn zeolite) > acrylic/m-TiO2-3/(Ag/Zn zeolite) > acrylic/m-TiO2-3. It means that the weather degradation of acrylic coatings filled by m-TiO2-3 nanoparticles was the lowest among investigated coatings. In other word, the weather resistance of acrylic coating was improved in the presence of m-TiO2-3 nanoparticles.
Fig. 7 indicated that the PI of investigated acrylic coatings rose during aging process. The acrylic/(Ag/Zn zeolite) coating had the highest increase of PI. This value of the neat acrylic coating slightly increased in the first periods of aging process which was followed by sharply increasing. After 36 cycles (432 hours) of the artificial aging test, the increase in PI of the investigated acrylic coatings was in order: neat coating < acrylic/m-TiO2-3/(Ag/Zn zeolite) ~ acrylic/m-TiO2-3 < acrylic/(Ag/Zn zeolite). However, the CI of the neat acrylic coating illustrated the highest increase due to the difference of degradation mechanism of neat acrylic coating in comparison with acrylic composite coatings. This is needed further studies to propose mechanism degradation for the acrylic coating.
Weight loss of acrylic coatings
Under effect of aging process, the chemical functional groups of the acrylic coatings will be changed. The polymer chain scissions were caused by photo-degradation reaction, hydrolysis and erosion, etc. to produce low weight molecules. As a result, the weight (thickness) of acrylic coatings was decreased. It was reported that thickness loss of polymer coating was 5 – 25 μm/year upon natural aging test [33]. Hence, monitoring weight change of polymer coating during accelerated weathering test is one of simple methods to evaluate polymer degradation degree [2, 9, 10, 15, 16]. The weight change of the investigated acrylic coatings with different compositions upon the aging process was illustrated in Fig. 8. It was clear that the weight of neat acrylic coating reduced upon aging test. For the rest investigated acrylic coatings, the weight of the composite acrylic coatings increased in the first 72 hours of the aging which was followed by decreasing in the weight of investigated acrylic coatings. After 36 cycles of the aging test (equal 432 hours), the weight loss of investigated acrylic coatings could be arranged as follow: Neat acrylic coating > Acrylic coating filled Ag/Zn zeolite > Acrylic coating filled m-TiO2 > Acrylic filled m-TiO2.
The reason of different weight changes among investigated acrylic coatings was degradation mechanism. For acrylic polymer, the acrylic polymer chain was degraded to form free radicals as Norrish type I and type II (as scheme 1) upon UV irradiation. And then, these free radicals continuously attacked to the polymer chain to produce chain propagation [1]. Consequently, scissions of the polymer chain were occurred to form low molecular weight products [15]. In addition, the acrylic polymer was hydrolyzed at the condensation periods [9, 16]. As a result, the molecular weight of neat acrylic coating was reduced. However, in the presence of TiO2 nanoparticles, the degradation mechanism of acrylic composite coating had a little difference in comparison with the degradation mechanism of neat acrylic coating. The TiO2 nanoparticles produced charge particles (such as electron and holes containing positive charge) under UV irradiation effect. After that, oxygen and water molecules were attacked by electro-particles to produce active free radicals and charged particles. These active parts attached onto the acrylic polymer chains. This was the reason of the investigated coatings’ weight increase in the beginning period of aging process [32].
In comparison with the neat acrylic coating, the weight loss of the acrylic coatings filled by both m-TiO2 nanoparticles and Ag/Zn zeolite particles was lower. It was explained by inorganic nanoparticles which made the coating structure becoming tighter (as discussion mentioned above). In addition, the inorganic nanoparticles could absorb UV irradiation and thus decreasing amount of UV rays exposing to polymer chains. In other word, the inorganic particles acted as a photo-stabilizer. Therefore, this was, more or less, one of the causes of the low weight loss of acrylic composite coatings.
Gloss changes
The surface properties of investigated acrylic coatings will be changed under the effect of the accelerated aging test. The gloss is one of the characteristic properties of the acrylic coating surface. The weathering degradation of the acrylic coating can be evaluated through the gloss retention of investigated samples [2, 6]. The gloss retention of investigated acrylic coatings having different compositions during the accelerated weathering test were demonstrated in Fig. 9. It is clear that the gloss retention at 60o angle of the acrylic coatings reduced during the accelerated aging process. After 36 cycles (432 hours) of the accelerated aging test, the gloss retention of the investigated coating could be arranged as Neat acrylic ~ Acrylic/(Ag/Zn zeolite) < Acrylic/m-TiO2/(Ag/Zn zeolite) < Acrylic/m-TiO2.
Under effect of the accelerated aging process, the surface of the investigated polymer coating became rougher due to the erosion of aging process [2, 6]. Therefore, the gloss of the investigated coatings was decreased in comparison with that of the initial coatings. On the base of the gloss retention, the acrylic/m-TiO2 coatings showed the highest weathering resistance (As above discussion). However, in presence of the Ag/Zn zeolite particles which had size range of 200-500 nm, the structure of acrylic coating became less tight in comparison with the acrylic coating filled m-TiO2 nanoparticles.