The application of self-cleaning coatings presents one of the most effective ways to protect the surfaces of the building materials. The effect of TiO2/kaolin based coatings applied to three types of substrates: non-porous, porous and highly porous, was investigated. Mechanical activation was applied for the impregnation of the active TiO2 component (in content of 3 and 10 wt. %) into the kaolin support. Surface properties (roughness, hydrophilicity and micro-hardness) and functional properties (photocatalytic activity and self-cleaning efficiency) were studied in order to define the optimal formulation of the applied coatings. The effect of the photocatalytic behavior of the coated substrates in terms of self-cleaning ability was assessed by the photodegradation of Rhodamine B, performed before and after durability tests. The results obtained in this paper showed that photocatalytic activity of the TiO2/kaolin composite coating generally depends on the procedure of TiO2 impregnation into the kaolin clay and the loaded TiO2 content.
The surfaces of building materials (glass, ceramic and roofing tiles) used for outdoor applications are exposed to various kinds of environmental pollution which has intensified nowadays. In order to prevent the change of the structure of materials, to lengthen their service life and to keep a long-term esthetic appearance, coatings with self-cleaning properties have been applied (
Clay minerals (natural and synthetic) are promising support materials because of their high specific surface area, high absorption capacity, large pore volumes, chemical stability and good mechanical properties. TiO2-clay nanocomposite enhances the decomposition of organic pollutants during photocatalytic degradation. In addition, it provides more active surface sites, reduces agglomerations and prevents nanoparticles from spreading in the environment. The immobilization of TiO2 particles on the silicate layer of clay minerals can have a significant influence on the adsorption properties of the photocatalyst which is in direct correlation with the surface properties of the applied clay mineral (
The TiO2/kaolin composite powders were obtained by the impregnation of TiO2 commercial suspension (in contents of 3 and 10wt.%) into a kaolin clay support. The commercial TiO2 suspension used for this purpose (80 wt.% anatase and 20 wt.% rutile; grain size < 100 nm; content of dry matter 30.0 ± 1.0 wt. % and pH 7) was obtained from Degussa company, Germany. The process of impregnation was carried out by mechanical activation in two different conditions of grinding. Firstly, the impregnation was performed in an attritor mill for 90 min at the speed of 1500 rpm. The second way of impregnation was carried out in a planetary mill during a period of 180 min at the speed of 200 rpm. The material/ball ratio was 1/5 in both procedures, while the pH value was in the range of 9 to 9.5. This pH value was maintained with the NaCO3 and NaOH solutions, with concentrations of 0.67M and 2.25M, respectively. The obtained powders were dried at 105oC for 24 hours.
According to the TiO2 content and the condition of impregnation, the obtained TiO2/kaolin composite powders were labeled as E0A; E1A; E2A; E0P; E1P; E2P. The initial letter E indicates the kaolin clay used as support, numbers 0, 1 and 2 indicate the content of TiO2 (0- without TiO2, 1- 3.wt%TiO2 and 2- 10wt.% TiO2) while the last letter A / P indicate the attritor/ planetary mill conditions, respectively.
The obtained TiO2/kaolin composite powders were used to make suspensions that later acted as protective coatings on mineral substrates. The composite suspension was formed by suspending 0.5g of TiO2/kaolin composite powder into demineralized water (100 ml) using di-ammonia-hydrogen citrate as a dispersing agent. The suspension was stirred at 300 rpm, for 1hour. Ultrasonic bath (30 min) was used in order to prevent possible agglomeration.
Spray technique was applied for the deposition of the suspension onto the surface of the substrates under the following conditions: spraying pressure 6.5MPa, distance of the spray device from the sample 90cm, angle of spraying 45o, and diameter of nozzle 1.3mm. The coated substrates were afterwards dried at RT/24h.
Three types of mineral substrates were used as the medium for the application of the obtained suspensions as protective coatings. The first one, considered a porous system, was a clay roofing tile (CRT) produced in industrial conditions (Company NEXE Group) by extrusion, the second substrate was produced in laboratory conditions by pressing (P= 73MPa) using 60 wt.% of kaolin clay (35 wt% kaolinite) and 40 wt.% of fly ash obtained from the thermal power plant REK Bitola, Republic of Macedonia. The used fly ash influenced the increase of porosity of the mineral substrate (
The particle size distribution of the TiO2/kaolin composite powders was determined by Malvern Instruments, zeta-nanoseries, NanoZS under the following conditions: refraction index of the investigated suspensions (n=1.55), light absorption (a=0.3) and pH = 9. Additionally, a Malvern Mastersizer 2000 instrument was used for a more precise determination of the micro-size distribution of the particles from 0.02 to 2000 mm. For this measurement, the sample particles were dispersed in water with sodium pyrophosphate added with agitation and sonication until a stable dispersion was obtained.
The phase composition of the TiO2/kaolin composite powders was determined by X-ray diffraction (Philips PW 1710). The investigations were done under the following conditions: monochromatic CuKα radiation with λ=1.5418 Ȧ in the 5-55o of 2θ range, scan rate 0.02o, 0.5 s per step.
The measurement of the contact angle values (Surface Energy Evaluation System, Advex Instruments, Brno, Czech Republic) was carried out with two experimental fluids: (distilled water and glycerol) before and after the water rinsing procedure and adhesion test. Droplets of the appropriate experimental fluid (cca 5 μl in volume) were gently deposited on the coated mineral substrate by micro syringe. The initial contact angle measurements (θci, after 1s) were performed at five different points for each of the three specimens of the investigated mineral substrates. Each droplet deposited onto the surface of the mineral substrate was measured five times.
The surface roughness (Surtronic 25, Taylor Hobson) of the reference mineral substrates and of the coated samples, before and after water rinsing procedure and adhesion test, was evaluated based on the Ra parameter which represents the average roughness values obtained with a 4mm linear probe length. The obtained data were calculated according to ISO 4287 standard.
Before and after the water rinsing procedure and adhesion test, the Vickers micro-hardness values of the reference and of the coated samples were measured by Vickers micro-hardness technique (Microhardness tester model HVS 1000A, ZZV Precision Toll Supply) applying 0.3kg load.
The water absorption values by capillarity of the reference and the coated substrates were evaluated according to standard SRPS U. M8.300, 1985 – EN (
where:
S is the area of the sample in contact with water.
The capillary water absorption coefficient (A) was defined as the slope of the linear section of the curve obtained by plotting the mass change per area (Qi) vs. the square root of time (ti1/2).
The photocatalytic activity of the synthetized powders (E0A; E1A; E2A; E0P; E1P; E2P) and that of the coated mineral substrates (before and after water rinsing and adhesion test) was investigated by monitoring the Rhodamine B (RhB) concentration change under the UV/VIS irradiation according to the procedures described in detail in (
where:
C0 presents the RhB solution concentration for the sample in the dark at defined time.
C presents the RhB solution concentration for the sample under UV/VIS light irradiation at defined time.
The RhB concentration was measured by UV-VIS spectrophotometer (EVOLUTION 600 spectrophotometer).
The assessment of the durability of the coated surfaces was carried out by using water rinsing test (as essential for self-cleaning of the coating) and tape adhesion test.
Water rinsing durability test was applied in order to examine the stability of three mineral substrates coated with the suspensions made of E2A / E2P nanocomposite powders in severe conditions (rain rinsing). Namely, the laboratory simulated rain rinsing procedure (
A modified adhesion test according to the procedure for assessment of the porous models (
The functional properties (photocatalytic activity and self-cleaning efficiency) of the coated samples were measured before and after the water rinsing and tape adhesion tests during UV/VIS irradiation. The Rhodamine B (10ppm) was used as the model pollutant forphotocatalytic activity assessment, while the results of the contact angle measurements were used for the estimation of self-cleaning properties.
The results of the photocatalytic activity of the obtained TiO2/kaolin composite powders (E0A; E1A; E2A; E0P; E1P and E2P) obtained by monitoring the photocatalytic degradation of RhB during the period of 210min by UV/VIS irradiation are presented in
Photocatalitic activity values of the TiO2/kaolin composite powders: (a) E0A; E1A and E2A (attritor mill) and (b) E0P; E1P and E2P (planetary mill).
It is evident that composite powder E2A has the highest photocatalytic activity value (65%), while the composite powder E2P has the lowest one (55%).
Regarding the TiO2 content impregnated into the kaolin clay support, the samples impregnated with 10% TiO2 showed significantly higher photocatalytic activity than the samples impregnated with 3% TiO2, while the samples treated under the same conditions without impregnated TiO2 showed no photocatalytic activity under the UV/VIS irradiation (
Depending on the type of mill, during the process of impregnation, the TiO2/kaolin composite powders obtained in an attritor mill showed higher photocatalytic activity (for 10%) than those impregnated in a planetary mill. Based on those preliminary photocatalytic activity results, the TiO2/kaolin composite powders E2A and E2P were additionally characterized and used for the creation of photocatalytic suspensions which were further applied on the chosen mineral supports (
Particle size distribution of the most photocatalytic active TiO2/kaolin composite powders i.e. E2A and E2P is presented in
Particle size distribution of the E2A and E2P powders, determined by a Malvern Instruments zeta-nanosizer.
It is evident that E2A powder (10wt.% TiO2/attritor mill) possesses a three modal particle size distribution with the following maximal particle diameters: 50nm, 255nm and 1700nm, while the E2P powder (10wt.% TiO2/ planetary mill) has bimodal particle size distribution with maximal particle diameters of 217 nm and 1138nm. The small fraction of the particles with the average diameter size smaller than 100 nm were obtained only in the case of the impregnation in an attritor mill, which is evidently the reason for the better photocatalytic behavior of this powder in comparison with the powder obtained by the impregnation in a planetary mill. Also, it was noticed that both powders had particles distributed in micro-size range,
Particle size distribution of the E2A and E2P powders, determined by a Malvern Mastersizer 2000 instrument.
The phase composition of both E2A and E2P powders are shown in
XRD patterns of composite powders E2A and E2P.
The values of average capillary water absorption coefficient presented in
Average capillary water absorption coefficient (A) for the reference mineral substrates and for the coated substrates
Mineral substrate | A [kg/m2min1/2] |
---|---|
Clay roofing tile, CRT | |
CRT-R | 0.73 |
CRT-E2A | 0.59 |
CRT-E2P | 0.63 |
Clay-fly ash composite, CFA | |
CFA-R | 8.22 |
CFA-E2A | 6.43 |
CFA-E2P | 6.45 |
Window glass, WG | |
WG-R | 1.04x10-6 |
WG-E2A | 1.03x10-6 |
WG-E2P | 1.03x10-6 |
After the deposition of the coating on to the mineral substrates, the average capillary water absorption coefficient decreased for all the examined substrate samples (
The comparison of the surface properties (surface roughness and Vickers micro-hardness), before and after the application of both coatings (E2A / E2P), provided the opportunity to investigate the surface changes which evidently had a great influence on the functional properties of the coated samples.
Surface roughness values (Ra parameter) were used for evaluating the surface roughness of the reference and coated substrates. The values obtained for Ra are presented in
Surface roughness values (Ra parameter)
Mineral substrate | Ra [μm] | ||
---|---|---|---|
Reference and coated substrates | Coated substrates after tape adhesion test | Coated substrates after water rinsing test | |
Clay roofing tile, CRT | |||
CRT-R | 2.45 | - | - |
CRT-E2A | 2.50 | 2.47 | 2.45 |
CRT-E2P | 2.55 | 2.50 | 2.45 |
Clay-fly ash composite, CFA | |||
CFA-R | 12.31 | - | - |
CFA-E2A | 12.24 | 12.25 | 12.27 |
CFA-E2P | 12.26 | 12.28 | 12.30 |
Window glass, WG | |||
WG-R | 0.09 | - | - |
WG-E2A | 0.21 | 0.18 | 0.09 |
WG-E2P | 0.27 | 0.24 | 0.09 |
The results of the Vickers micro-hardness (HV) test presented in
Vickers micro-hardness (HV) values
Mineral substrate | Vickers micro-hardness, HV | ||
---|---|---|---|
Reference and coated substrates | Coated substrates after tape adhesion test | Coated substrates after water rinsing test | |
Clay roofing tile, CRT | |||
CRT-R | 45.0 | - | - |
CRT-E2A | 45.8 | 45.5 | 45.5 |
CRT-E2P | 45.5 | 45.3 | 45.3 |
Clay-fly ash composite, CFA | |||
CFA-R | 13.3 | - | - |
CFA-E2A | 13.7 | 13.5 | 13.7 |
CFA-E2P | 13.4 | 13.4 | 13.4 |
Window glass, WG | |||
WG-R | 453.0 | - | - |
WG-E2A | 456.3 | 453.8 | 453.0 |
WG-E2P | 455.7 | 453.5 | 453.0 |
The results of the average initial contact values qci of the reference substrates (CRT-R, CFA-R and WG-R) and of the coated substrates (CRT-E2A, CRT-E2P, CFA-E2A, CFA-E2P, WG-E2A and WG-E2P) are presented in
Average initial contact angle values qci
Mineral substrate | Average initial contact angle θci (o) |
---|---|
Clay roofing tile, CRT | Water as a working liquid |
CRT-R | 54.7 |
CRT-E2A | 36.99 |
CRT-E2P | 38.97 |
Clay-fly ash composite, CFA | Water as a working liquid |
CFA-R | 52.56 |
CFA-E2A | 35.29 |
CFA-E2P | 37.92 |
Window glass, WG | Glycerol as a working liquid |
WG-R | 67.02 |
WG-E2A | 27.40 |
WG-E2P | 28.73 |
The self-cleaning properties of the coated substrates assessed by the measurement of the initial contact angle qci are presented in
Initial contact angle of E2A and E2P coatings applied to: (a) CRT substrate, (b) CFA substrate and (c) WG substrate as the function of UV/VIS irradiation time.
The results of the evaluation of photocatalytic activity based on the RhB degradation efficiency, with UV/VIS irradiation, are presented in
Photocatalytic activity of the coating composed of E2A and E2P composite powder and applied to: (a) clay roofing tile (CRT), (b) composite material (CFA) and (c) window glass (WG).
The results of the modified tape adhesion test presented in
The relative mass loss (Δm/S) after the application of modified tape test
Mineral substrate | Sample | μm/S [μg/cm2] |
---|---|---|
CRT | CRT-R |
77.7 |
CRT-E2A | 64.5 | |
CRT-E2P | 65.7 | |
CFA | CFA-R |
262 |
CFA-E2A | 248 | |
CFA-E2P | 255 | |
WG | WG-R |
18.2 |
WG-E2A | 26.8 | |
WG-E2P | 30.4 |
The tape takes off only the deposited powder (dust) from the atmosphere
The comparisons of the surface roughness values, Ra parameter, for the three types of samples before and after the application of the tape adhesion test and water rinsing durability test are presented in
The results of the θci of the coated CRT, as the function of UV/VIS irradiation time before and after durability (water rinsing and tape adhesion) tests are presented in
Self-cleaning efficiency assessment: a. CRT-E2A, b. CRT-E2P, c. WG-E2A, d. WG-E2P, (E2A and E2P – substrates with coating; AWRT- after water rinsing test; ATAT- after tape adhesion tests).
(a and b). It is evident that there was a significant increase of θci for both E2A and E2P coatings, applied on CRT substrate, after the durability test. The increase of θci was more pronounced after the water rinsing procedure for both coatings had been applied to the CRT substrate, having its maximum at the start of the experiment (without UV/Vis irradiation).
As a result of very low resistance towards rinsing with water of both E2A and E2P coatings applied on WG substrate, (
The increase of the θci value of both CRT and WG coated substrates after the applied durability tests (without the influence of UV/VIS irradiation) could be the result of the physical removal of the deposited coatings and of the changes of the surface morphology during the performing durability tests. The existence of a decreasing trend line of the θci value as the function of UV/VIS irradiation after the durability tests of the analyzed coatings suggests that a considerable self-cleaning efficiency still exists.
The results of photocatalytic degradation efficiency of Rhodamine B with UV/VIS irradiation before and after the appropriate durability tests of the coated CRT and WG substrates are presented in
Photocatalytic activity assessment: a. CRT-E2A, b. CRT-E2P, c. WG-E2A, d. WG-E2P (E2A and E2P – substrates with coating; AWRT- after water rinsing test, ATAT- after tape adhesion tests).
According to the presented results, it can be concluded that both coatings exhibited relatively good durability when they were applied to CRT and WG substrates. The coating composed of E2A powder and applied on WG substrate showed better photoactivity after durability tests, suggesting good compatibility and stability with this substrate, which is in good correlation with other reported studies (
The research showed that photocatalytic activity results of the obtained TiO2/supported kaolin powders greatly depend on the TiO2 impregnation into the kaolin clay procedure and on the content of the TiO2 loading.
Based on comparative investigation, it was evident that the samples impregnated by mechanical activation in an attritor and planetary mill possessed mainly micro-sized particle distribution. The impregnation of the TiO2 by mechanical activation in an attritor mill leads to the formation of small fractions of nano-sized particles with an average diameter of 50 nm, which resulted in a more pronounced photocatalytic activity of these composites.
Similar trend was also noticed for the deposited coatings. The coating obtained from the powder impregnated with the highest amount of TiO2 in an attritor mill also had the highest hydrophilicity (lower contact angle) and was potentially more efficient in the removal of the pollutant from the studied substrates (window glass and industrial clay roofing tile) than the coating prepared with the powder containing the same amount of TiO2, but impregnated in a planetary mill. This fact is in correlation with the values of particle size distribution and their stability.
The procedure for the preparation of the TiO2/kaolin composite photocatalyst presented in this study, the achieved durability and the negligible influence of the deposited photocatalytic TiO2/kaolin composite coating on surface properties (surface roughness, Vickers micro-hardness and contact angle values) present an advantageous and very important argument for further investigation in this field.
The financial support from Serbian Ministry of Education, Science and Technological Development (Contract No. III45008) is gratefully acknowledged.