UDK 66	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 46(1)19(2012)
STABILIZATION OF RUTILE TiO2 NANOPARTICLES WITH GLYMO IN POLYACRYLIC CLEAR COATING
STABILIZACIJA RUTIL TiO2 NANODELCEV Z GLYMO V POLIAKRILNEM TRANSPARENTNEM PREMAZU
Jerneja Godnjavec1, Bogdan Znoj1,2, Jelica Vince2, Miha Steinbucher2, Andrej @nidar{i~3, Peter Venturini1,2
1Center of Excellence for Polymer Materials and Technologies, Tehnološki park 24, 1000 Ljubljana, Slovenia 2Helios Group, Količevo 2, 1230 Domžale, Slovenia 3Kolektor Nanotesla Institut Ljubljana, Stegne 29, 1521 Ljubljana, Slovenia jerneja.godnjavec@polimat.si
Prejem rokopisa - received: 2011-02-15; sprejem za objavo - accepted for publication: 2011-07-06
Titanium dioxide (TiO2), in the crystal form of rutile with high refractive index, 2.7, is one of the most important inorganic pigments. In the recent years nano TiO2 has attracted interest as an inorganic material for nanocomposites. It is used for the development of photocatalysts12, semiconductors in solar cells34 and for the protection of materials from UV damage5-8. To achieve high UV absorbing efficiency of TiO2 nanoparticles into polymer matrix and to yield a better compatibility with polymeric materials surface treatment of TiO2 nanoparticles with 3-glycidyloxypropyltrimethoxysilane (GLYMO) in a polyacrylic clear coating was investigated. The grafting of GLYMO on the TiO2 nanoparticles surface was characterized with TGA and FTIR techniques. The stability of TiO2 nanoparticles in a acrylic clear coating was evaluated by zeta potential measurement. Microstructural analysis of nanoparticles was done by SEM and the influence of surface treatment on stability of nanoparticles in dispersion and acrylic coating was analysed. Finally the UV absorption effectivness of acrylic clear coating with treated nanoparticles of rutile TiO2 was measured to determine the effect of improved dispersion. The performance of nanocomposite coatings was demonstrated trough accelerated weathering by gloss measurement. The results showed that surface treatment of TiO2 nanoparticles with GLYMO improves nanoparticles dispersion and preserves UV protection of the acrylic clear coating.
Keywords: titanium dioxide, nanoparticles, polyacrylic clear coating, surface treatment, GLYMO, UV protection
Titanov dioksid (TiO2), v rutilni kristalni obliki z visokim lomnim količnikom, 2.7, je eden najbolj pomembnih anorganskih pigmentov. V zadnjih letih je nano TiO2 pritegnil pozornost kot anorganski material za nanokompozite. Uporablja se za razvoj fotokatalizatorjev1,2, polprevodnikov in soalrnih celic34 in za UV zaščito materialov5-8.
Da bi dosegli ustrezno dispergiranje TiO2 nanodelcev v polimerni matrici in boljšo kompatibilnost s polimernimi materiali smo preiskovali površinsko obdelavo TiO2 nanodelcev v poliakrilnem transparentnem premazu. Za optimizacijo dispergiranja je bila površina nanodelcev obdelana z 3-glicidiloksipropiltrimetoksisilanom (GLYMO). Pripenjanje GLYMO na površino TiO2 nanodelcev smo ovrednotili z termogravimetrično analizo (TGA) in s FTIR spektroskopsko analizo. Stabilnosti TiO2 nanodelcev v akrilnem transparentnem premazu smo analizirali z meritvijo zeta potenciala. Mikrostrukturno analizo nanodelcev izvedli s SEM. Na koncu smo analizirali učinkovitost UV absorpcije akrilnega transparentnega premaza z površinsko obdelanimi rutil TiO2 nanodelci, da bi določili efekt izboljšane disperzibilnosti. Z izpostavo pospešenega staranja smo določili funkcionalnost nanokompozitnih premazov na osnovi meritve sijaja. Rezultati so pokazali da povšinska obdelava TiO2 nanodelcev z GLYMO izboljša disperziblnost nanodelcev in ohrani UV zaščito akrilnega transparentnega premaza.
Ključne besede: titanov dioksid, nanodelci, poliakrilni transparentni premaz, površinska obdelava, GLYMO, UV zaščita
1 INTRODUCTION
Second, for producing suitable nanocomposites, it is necessary to disperse the nano-particles without agglo-
Architectural coatings are usually used to enhance	meration in organic binders11,12. Due to their extremly
the durability of wood in exterior environment. Inorganic	large surface-area/particle-size ratio, nano-particles have UV absorbers are often used in
coatings formulations,	high tendency of agglomeration13. Many efforts have
since they increase polymer stability. TiO2 possesses a	been taken in order to overcome the problem of agglo-
lot of attracting advantages, like thermal stability and	meration. Polymeric dispersants containing different
long-term life time, compared to traditional organic UV	functional groups are usuallly used in order to prevent
absorbers9,10.	the high tendency of nanoparticles to form aggregates in
There are two limitations of using TiO2 as UV	the wet state of coating and in the dry paint film.
absorber. First, TiO2 particularly in anatase crystale form	Polyacrylic acid, polyacrylamide and their copolymers
and less in rutile form exhibits strong photocatalytic	are widely used to disperse inorganic particles in
behavior when absorbing UV-rays, which is harmful for	aqueous film14. Recently, to suppress the photocatalytic
the photostability of polymer materials5. As a result,	property of TiO2, usually silica15-17 or silane18-20 were
TiO2 nanoparticles as catalyst can create , which can	coated onto TiO2 cores. Kang et al. suppressed the
produce highly reactive free radicals and exert strong	photocatalytic property of a TiO2 by coating them with
oxidizing power.	the SiO2 shell in chloroform17. Toni et. al. coated dense
SiO2 shells onto TiOi particles by seeded sol-gel process of tetraethyl silicate (TEOS) in ethanol16. The use of different coupling agents such as trialkoxy silanes for surface modification of nanoparticles is recommended. For example, Ukaji et. al. coated thin aminoethylamino-propyltrimethoxysilane layers onto TiO2 particles in ethanol by adding silane during ball-milling dispersing procedure8. Shafi et. al. coated octadecyltrihydrosilane layers on TiO2 surfaces in heptane by ultrasonic irradi-ation21. M. Sabzi et. al. showed that surface treatment of TiO2 with aminopropyl trimethoxysilane improves nano-particles dispersion and UV protection of the urethane clear coating22.
The purpose of this study was to investigate the influence of TiO2 nanoparticles surface modifiction by different wt. % of 3-glycidyloxypropyltrimethoxysilane on transparency and UV absorbing efficiency of poly-acrylic coating.
2 EXPERIMENTAL WORK
Sample name	GLYMO/TiO2 (/)
A	0/1
B	0.1/1
C	1/1
D	2/1
TiO2 nanoparticles with rutile crystal structure were prepared by co-precipitation method which is described elsewhere23. Deagglomerated rutile nanoparticles were prepared as dispersion in water and propilenglycol in high-energy horizontal attrition mill.
2.2 Surface modification of TiO2 nanoparticles in dispersion with silane coupling agent
As silane coupling agent we used 3-glycidyloxy-propyltrimethoxysilane (GLYMO) with chemical for-
CH2-CH-CH20-(CH2)3Si(0CH3)3
which is a bifunctional organosilane, possessing reactive organic epoxide and hydrolyzable inorganic metoxysilyl groups. It bounds chemically to both inorganic material and organic polymers. According to the producer in the presence of water the methoxy groups hydrolyze to form reactive silanol groups which bound to inorganic substance24.
We prepared dispersions of TiO2 nanoparticles grafted with GLYMO according to the following
Table 1: Weight ratio of GLYMO and nano TiO2, dispersed in water Tabela 1: Utežno razmerje GLYMO in nano TiO2, dispergiranih v
vodi
Sample name
A
D
GLYMO/TiO9 (/)
0/1
0 1/1
1/1
2/1
procedure. First, TiO2 nanopowder in dispersion with wetting and defoaming agent was milled for 1,5 hour at 25 °C with organic surface active agent with zirconia balls 1 mm diameter. Than silane coupling agent of different concentrations was added as shown in Table 1 and the dispersion was milled additionally for 1 hour at the same conditions.
2.3	Preparation of clearcoat with integrated rutile
crystalline nanoparticles of TiO2
We prepared clearcoat in the laboratory as described elsewhere23. Water based dispersions of TiO 2 nanoparticles were than added to clearcoat of 0,6 wt. %, stirred at approximatly 1000 rpm for 20 minutes and prepared for the testing.
2.4	Characterization of the samples
Surface morphology of coated sample was studied by scanning electron microscopy (SEM, ZEISS Gemini Supra 35 VP), with a maximum resolution up to 5 nm. Nanoparticles were dried on brass holders by adhesive carbon band with thin layer of gold. The samples were placed in the vacuum chamber of the instrument and then were examined at various magnifications. Grafting of GLYMO on TiO2 nanoparticles was analysed by FT-IR. The infrared spectra of original and modified TiO2 were conducted using a FT-IR spectrometer (PERKIN ELMER Spectrum 100) and thermogravimetrical analysis by TGA/DSC 1, METTLER TOLEDO.
Zeta potential was obtained to investigate the surface character of original TiO2 and TiO2 modified with GLYMO. The electro-phoretic mobility data, measured also by Zetasizer Nano series HT by of the dispersions were transformed into ^-potential according to25 (eq. 1):
" Arniu
C =
(1)
where e is a dielectric constant of the dispersing medium and ^ the solvent viscosity. pH was measured by pH meter (Mettler Toledo).
UV-VIS transmittance of modified samples was measured for estimating UV-shielding ability and transparency. 0.6 wt. % of surface treated TiO2 nanoparticles was integrated into acrylic clear coating and 200 ^m films were prepared. The different free film systems (coating-UV absorber) were analysed by UV-VIS spectrophotometer Varian Cary 100. The film transmittance was measured in the wavelength range 280 to 720 nm.
At the end, wood blocks measuring 15x7 cm2 (longitudinal X tangential) x 0,5 cm width were cut from air dried boards from the specie pine. Two layers of clear coat of thickness of 200 pm were applied on pine blocks. Coated wood plates were used to assess weathering exposure degradation (QUV accelerated weathering tester, Q - PANEL LAB PRODUCTS). Simulation of exterior use was done by six weeks weathering by an

r
optimised cycle defined: 4h at (60 ± 3) °C and 4h water shower at (50 ± 3) °C. Only light of the solar type was activated on the QUV with sources type UVA-340 nm26. Gloss at 60° was measured by Micro-TRI-gloss (Byk Gardner).
3 RESULTS AND DISCUSSION
3.1	SEM characterization
The size of the nanoparticles, grafted with GLYMO was analysed by scanning electron microsope (SEM). SEM images at two different magnifiations of sample C are shown on Figure 1. Agglomeration tendency of TiO2 nanoparticles in the dispersion cannot be determined because of the preparation of the sample, however we assume, that some agglomerates are probably present as seen on SEM image. Individual particles can be identified, which appears to be polydisperse in size but below 100 nm.
3.2	TGA analysis
To estimate the amount of GLYMO grafted on nanoparticles, the various percentage of GLYMO -grafted nano-TiO2 particles were analyzed by TGA technique. Figure 2 shows the TGA curves of untreated TiO2 nanoparticle, GLYMO alone and treated TiO2 nanoparticles with different percentages of GLYMO. For untreated nanoparticles (sample A), the weight loss from 120 till 600 °C is almost negligible and is probably due to desorption of physisorbed water27. For GLYMO alone the weight loss begins at 120 °C and ends at 200 °C. As can be seen for sample B, C and D, the various weight percentages of GLYMO grafted nanoparticles show sharp weight loss, beginning near 220 °C, continues till 620 °C, which is a consequence of oxidative thermal decomposition of grafted GLYMO as quantitatively
Figure 2: TGA curves of untreated TiO2 nano-particle (sample A) and treated TiO2 nano-particle (sample B-D). The composition of all samples is described in Table 1.
Slika 2: TGA krivulje površinsko neobdelanih (vzorec A) ter površinsko obdealnih (vzorci B-D) nanodelcev TiO2. Sestava vseh vzorcev je podana v Tabeli 1.
shown in Table 2. The largest amount of grafted GLYMO was in the case of sample C.
Table 2: TGA-based weight losses caused by grafting of GLYMO and the amount grafted/added GLYMO of various GLYMO - grafted nanoparticles TiO2. The composition of all samples is described in Table 1.
Tabela 2: Izguba mase vzorcev nanodelcev TiO2 ter razmerje vezan/dodan GLYMO površinsko obdelanih in neobdelanih z GLYMO glede na TGA analizo. Sestava vseh vzorcev je podana v Tabeli 1.
Sample	GLYMO:TiO2 (/)	Weight loss caused by grafting of GLYMO	The amount grafted/added GLYMO (%)
A	0	/	/
B	0,1	1,00	67
C	1	11,40	76
D	2	20,20	67
GLYMO	/	/	/
Figure 1: SEM image of sample C with inserted image at higher magnification
Slika 1: SEM posnetek vzorca C z vstavljenim posnetkom pri višji povečavi
3.3 FTIR spectroscopy
The hydroxyl groups on the surface of the TiO2 nanoparticles (TiOH) are reactive sites for the reaction with alkoxy groups of silane compounds, however corresponding bands are not present in spectrum of unmodified TiO2, since it was dried at 130 °C for 24h. The efficiency of silane grafting on TiO2 nano-particles was determined by Fourier transform infrared spectro-scopy (FTIR). Figure 3 shows normalised FTIR spectra of unmodified TiO2 nanoparticles (sample A), grafted TiO2 nanoparticles with GLYMO (sample B-D), GLYMO alone and GLYMO with addition of water. In the spectra of all TiO2 nano-particles the broad band between 400 and 800 cm-1 correspond to Ti-O-Ti network. GLYMO posseses two functional groups: epoxi and metoxysilyl, which both hydrolize and condensate. Epoxy band in FTIR spectra is preserved, while the intensity of Si-O-Me band is decreased. Also two bands of hydroxyl groups appear at ~3300 and ~1640 cm-1 because of hydrolysis of Si-O-Me groups. In spectra of
GLYMO with addition of water peak at 1050 cm1 appears, which we can assign to formation of Si-O-Si groups. Compared with the spectrum of non-modified TiOa, FTIR spectrum of GLYMO modified sample exhibits some new characteristic absorption peaks. Peak at ~1200 and 1093 cm-1 which belongs to Si-O-Me groups28 29, was observed in the spectra of samples C and D, however not in the spectruum of sample B, which indicates that only in the case of sample B complete hydrolysis and condensation of GLYMO takes place. In contrast to GLYMO spectruum, we cannot observe in GLYMO modified TiO2 spectra peak at 914 and 1254 cm-1 which corresponds to epoxi group30-34. We presume that the epoxi group opens and reacts with -OH groups which were formed by the hydrolysis of metoxysilyl groups. The broad bend at ~1050 cm-1 represent the Si-O-Si bond is observed which indicates the formation of Si-matrix35. The small peak in spectra of samples B -D at around 930 cm-1 reconfirms condensation reaction between methoxysilyl groups of GLYMO and the TiO2 surface hydroxyl groups36 8.
To sum up, Si-O-Me groups of GLYMO have reacted completley only in the case of sample B, in samples C and D are still present. Epoxi groups of GLYMO have reacted completely in all GLYMO modified TiO2 nanoparticles samples. The results of FTIR analysis confirm that Si-O-Si network has been formed probably around the TiO2 nanoparticles and in small amount Si-atoms of GLYMO are bound to TiO2 surface. Similarly, F. Bauer and coauthors reported about a polymerization-active siloxane shell formed around the nanoparticles Al2O3, when surface treated by GLYMO37. The reaction of GLYMO with TiO2 nanoparticles leads to different kind of stabilization of nanoparticles in the
Figure 3: FTIR spectra of untreated nano-TiO2 (sample A), grafted TiO2 nano-particles with GLYMO (sample B-D), pure GLYMO and GLYMO with addition of water. The composition of all samples is described in Table 1.
Slika 3: FTIR spektri neobdelanega nano-TiO2 (vzorec A), površinsko obdelanih nanodelcev TiO2 z GLYMO (vzorec B-D), samo GLYMO, GLYMO z dodatkom vode. Sestava vseh vzorcev je podana v Tabeli 1.
dispersion as suggested by the producers of GLYMO, since they claim that in the presence of water the methoxysilyl groups hydrolyze to form reactive silanol groups which bound to inorganic substance24.
3.4 pH and ^-potencial measurement
pH and ^-potential measurement were used to quantify the conditions leading to the stability of TiO2 dispersions. Relevant values of the pH - measurements and ^-potential of unmodified and modified TiO2 with GLYMO are collected in Table 3 and in Figure 4. The pH variation from 4,6 to 5,9 upon adsorption of GLYMO was detected. An increase of the pH from 5,6 up to 5,94 when increasing GLYMO/TiO2 from 0 to 1 (sample A, B and C) was accompanied with an increase of ^-potential from ~-33 to around —39 mV, with subsequent increase in colloid stability. We assume that TiO2 surface is probably mostly covered with modifier in monolayer. As ratio GLYMO/TiO2 increases to 2, the pH decreases to 5,43, resulting in decrease in zeta potential to -35,5 mV. We predict that the reason for decrease in pH and ^-potential is the excess amount of GLYMO or formatation of multilayer on TiO2 surface as observed by E. Ukaji et. al.8. It is supposed that thicker layer could be formed due to subsequent growth of layer to dimers, oligomers, and polymers.
Table 3: pH values for TiO2 dispersions, in the presence of different amounts of GLYMO (sample A-D). The composition of all samples is described in Table 1.
Tabela 3: pH vrednosti disperzij TiO2 v prisotnosti različnih deležev GLYMO (vzorec A-D). Sestava vseh vzorcev je podana v Tabeli 1.
Sample name	pH
A	5,60
B	5,78
C	5,94
D	5,43
3.5 Ageing behaviour
Artificial weathering results in surface degradation of the coatings, which affect the appearance of the coating,
GLYMOmOl {D
Figure 4: Zeta potential measurement vs. ratio GLYMO/TiO2
Slika 4: Diagram meritev zeta potenciala v odvisnosti od razmerja
GLYMO/TiO2
2	3	4
runa of exposum (number of weeitej
Figure 5: Gloss measurement vs. irradiation time in a QUV apparatus for the exterior use clearcotings with different UV absorbers, samples A-D, with their composition described in Table 1 Slika 5: Meritve sijaja v odvisnosti od časa izpostave v UV komori za premaz za zunanjo uporabo z različnimi UV absorberji, vzorci A-D, njhova sestava je podana v Tabeli 1
is the information on photostabilisation performances of UV absorbers8,38,39. M. Beyer and C. Jobos investigated the use of nano-scale light absorbers in water based glaze for outdoor applications. They showed that TiO2 proved to be effective additives6 in concentration 0,25 - 4 wt. % TiO29.
For outdoor weathering simulation of clearcoating gloss measurement during the exposure in QUV chambre for six weeks are displayed in Figure 5. Figure 5 shows gloss 60° vs. time of exposure in QUV chambre for clearcoating with modified TiO2 nanoparticles of different wt. ratios of GLYMO/TiO2. The results illustrated that addition of GLYMO decreases the gloss 60°. During accelerated weathering the gloss changes are strongly correlated with the degradation level of the surface coating. In some composite materials the polymer around the filler particles will degrade due to the particle catalytic effect40. The accelerated weathering with water spray induced the washing out of degradation products on the coatings surface and consequently a fresh surface was further exposed. During the accelerated weathering a pronounced loss of gloss was observed for coatings formulated with nanoparticle UV absorber41. We can conclude that the surface treatemtnof TiO2 nanoparticles with GLYMO reduces the gloss 60° of acrylic coating before exposure to accelerated weathering. The gloss change after six week exposure to accelerated weathering is the smalest in the case of sample C, which shows that the UV efficiency was improved the most, when ratio GLYMO/TiO2 was 1.
4 CONCLUSIONS
Surface modification and characterization of TiO2 nano-particles with GLYMO at different wt. percentages as an additive in a polyacrylic clear coating were investigated. TGA and FTIR analysis show that grafting of GLYMO on the nano-particles has occurred successfully. According to C-potential measurements the most
stable TiO2 nanoparticle dispersions are with ratio GLYMO:TiO2 0,1 and 1. At the end, artificial weathering results confirm that surface treatment of TiO2 nano-particles with GLYMO with ratio GLYMO:TiO2 = 1 improves nanoparticles dispersion and consequently its transparency the most and improves UV protection of acrylic clear coating.
Acknowledgement
The authors acknowledge the financial support from the Ministry of Higher Education, Science and Technology of the Republic of Slovenia through the contract No. 3211-10-000057 (Center of Excellence Polymer Materials and Technologies).
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