Compressibility of porous TiO2 nanoparticle coating on paperboard
© Stepien et al.; licensee Springer. 2013
Received: 19 August 2013
Accepted: 14 October 2013
Published: 25 October 2013
Compressibility of liquid flame spray-deposited porous TiO2 nanoparticle coating was studied on paperboard samples using a traditional calendering technique in which the paperboard is compressed between a metal and polymer roll. Surface superhydrophobicity is lost due to a smoothening effect when the number of successive calendering cycles is increased. Field emission scanning electron microscope surface and cross‒sectional images support the atomic force microscope roughness analysis that shows a significant compressibility of the deposited TiO2 nanoparticle coating with decrease in the surface roughness and nanoscale porosity under external pressure.
61.46.-w; 68.08.Bc; 81.07.-b
KeywordsNanoparticles Compressibility Wettability Liquid flame spray process
Nanoparticles exhibit extraordinary electronic, optical, and mechanical properties compared to bulk materials. This is due to two facts: first, nanoparticles have a large surface-to-volume ratio, i.e., a large number of atoms are located on the surface with distinct contribution to the free energy; second, quantum confinement manifests in small scale. For example, the color of nanoparticles can be varied over the whole visible spectrum simply by controlling the size and morphology of silver nanosphere lithography  or the size of semiconductor quantum dots such as CdS . Nanosized TiO2 particles have been applied in various industries ranging from sunscreen cosmetics  and whitening paint pigments  to catalyst supports , dye-sensitized solar cells , and self-cleaning surfaces via photocatalytic activity . TiO2 can be found in four different crystalline forms: anatase, rutile, brookite, and akaogiite - a dense, high-pressure phase of TiO2[8–10]. The crystalline structure of TiO2 particles plays a crucial role, for example, in dye-sensitized solar cells, which require anatase phase [11, 12].
We have recently demonstrated controlled wettability from superhydrophobic to highly hydrophilic surfaces on TiO2 nanoparticle-coated paperboard by liquid flame spray (LFS) deposition . It is noteworthy that superhydrophobicity is only observed on paper and paperboard whereas TiO2 nanoparticle deposition by LFS on aluminum foil resulted in a slightly hydrophilic surface . Superhydrophobicity on a paperboard originates from the nanoscale roughness with the organic components, typically binders used in papermaking that will evaporate during the LFS deposition with surface temperatures of 100°C to 300°C on paperboard surface 0.5 m after the flame . These volatile organic compounds condense into a thin carbonaceous layer on deposited TiO2 nanoparticles.
Flame-based methods for nanoparticle deposition have been investigated since the 1980s [16–21]. In the LFS process, a liquid precursor is fed into a high-temperature flame in which the precursor is atomized into small droplets that evaporate in the flame. The precursor material gas decomposes and nucleates forming nanoparticles that can be collected on a moving web. LFS is suitable for deposition of various metal and metal oxide nanoparticles with a relatively narrow and controllable size distribution of nanoparticles with diameters from 2 to 200 nm . The morphology of the deposited nanoparticles can be controlled via process parameters including gas and precursor feed rates, precursor concentration, distance of the substrate from the burner, and deposition time (web speed) .
In this article, we investigate the compressibility of such LFS-deposited TiO2 nanoparticle coating on paperboard by calendering. Calendering is a traditional surface finishing technique widely used in the paper industry to give the paper surface a smoother and glossier look . In calendering nip, paperboard web is compressed between rolls with controllable temperature, pressure, nip time (web speed), and nip roll materials. Compressibility of the nanoparticle coating will affect surface properties such as wettability. Individual nanoparticle compressibility has been studied [24–26] under high-pressure by X-ray diffraction. However, as far as the authors know, a systematic study of porous nanoparticle coating compressibility has not been presented until now.
The compressibility of TiO2 nanoparticle-coated paperboard surfaces was investigated by calendering in which the paperboard is compressed between two rolls as shown in Figure 1b. Calendering is a well-known surface finishing technique widely used in papermaking. In our case, we use a soft roll/hard roll calender (DT Laboratory Calender, DT Paper Science Oy, Turku, Finland) with a lineload of 104 kN/m and a temperature of 60°C. The samples were treated with the same parameters in successive calendering nips with the nanoparticle-coated surface always facing the steel roll to prevent nanoparticle adhesion to the polymer roll. A schematic illustration of the calender is presented in Figure 1b.
Surface chemistry was studied with water contact angle measurements performed using the commercial contact angle goniometer KSV CAM 200 (KSV Instruments Ltd., Helsinki, Finland) with an automatic dispenser and motorized stage. The images of the droplets were captured by a digital CCD camera with a 55-mm-zoom microscope lens with a blue LED light source and analyzed with the KSV CAM software. The standard deviation of the contact angle (CA) measurements was approximately ±3°. Contact angles of the Milli-Q (Millipore, Billerica, MA, USA, resistivity 18.2 MΩ) purified water was measured in air in ambient conditions (room temperature 23°C ± 1°C and relative humidity 30% ± 5%) after 2 s of the droplet application. The volume of the droplets was approximately 2.0 μL, and the reported CA values are mean values of three individual measurements.
The TiO2 nanoparticle-coated paperboard surface was exposed to UVA light (Bluepoint 4 ecocure, Hönle UV Technology, Gräfelfing, Germany) with a central wavelength of 365 nm using a filter for 320 to 390 nm. A constant intensity of 50 mW/cm2 was applied for 30 min that converted the initially superhydrophobic surface to a highly hydrophilic one.
The scanning electron microscopy (SEM) imaging of the samples was performed using a field emission scanning electron microscope (FE-SEM; SU 6600, Hitachi, Chiyoda-ku, Tokyo, Japan) with an in-lens detector. All samples were carbon-coated to obtain conductivity. The secondary electron (SE) imaging mode was used for topographical imaging with a magnification of ×50,000 and ×5,000 with an accelerating voltage of 2.70 kV and a working distance of 4 to 5 mm. Cross sections of the TiO2 nanoparticle-coated samples were prepared using an Ilion+ Advantage-Precision Cross-Section System (Model 693, Gatan Inc., Pleasanton, CA, USA). One cross section was milled for each calendered sample with an argon broad ion beam using an accelerating voltage of 5 kV for 150 min. The paper samples were platinum-coated before the cutting to improve heat exchange and to reduce heat damage at the cutting area.
An atomic force microscope (AFM; NT-MDT NTEGRA Prima, Moscow, Russia) was used for further surface characterization. Gold-coated, reflective probes (NSG10) were used with an intermediate spring constant k = 11.5 N/m, a maximum tip radius of curvature of 10 nm, and a resonance frequency of 190 to 325 kHz (Europe MicroMasch, Tallinn, Estonia). Images were captured using the tapping mode at ambient conditions (room temperature 24°C ± 1°C and relative humidity 38% ± 5%). After landing with tip on the sample surface, a damping ratio (Asp/A0) of 0.5 to 0.6 and a line frequency of 0.25 to 0.6 Hz were optimized for imaging. The AFM was placed on a vibration isolation table (TS-150, Table Stable, Zwillikon, Switzerland) to eliminate external vibrational noise. Image processing and root-mean-square (RMS) roughness Sq calculations were carried out using the scanning probe image processor program (SPIP™, Image Metrology A/S, Hørsholm, Denmark). Before calculation, images were plane-corrected and the ISO 11562 Gaussian profile filter was implemented.
Results and discussion
It is well known that superhydrophobicity can only be observed on rough surfaces, i.e., both chemical and physical effects contribute to superhydrophobicity. Classical theories by Wenzel  and Cassie and Baxter  have been used to explain observed contact angles on rough substrates: on rough, hydrophobic surfaces, the water droplet resides mostly on air and thus exhibits very high contact angles. Shibuichi et al. [29, 30] presented an elegant analysis of how apparent contact angles are affected by the surface roughness compared to a smooth surface. Here, in our study, the bulk compressibility of the reference paperboard has a minor effect on water contact angles whereas superhydrophobic TiO2 nanoparticle-coated paperboard supports the analysis by Shibuichi et al. [29, 30]: increasing the number of calendering nips results in a decrease of the water contact angles on the hydrophobic side and increase on the hydrophilic side after the ultraviolet treatment in Figure 2. This is expected as adding the number of successive calendering nips will reduce surface roughness. The water contact angle is approximately 130° and 25° after 15 calendering nips for TiO2 nanoparticle-coated samples without and with UV treatment, respectively. This indicates that the TiO2 nanoparticles do not adhere to the steel calender roll but rather remain on the paperboard surface. Removal of the nanoparticles from the surface would bring the contact angles closer to those values of the reference paperboard in which the water contact angles are almost independent of both the number of calendering nips and the UV treatment.
In summary, we have investigated the compressibility of TiO2 nanoparticle coatings on paperboard. Our analysis shows that the morphology of deposited nanoparticle coating undergoes a significant transition even in a single calendering cycle. The surface roughness values are reduced as expected, and nanoparticle coating shows a higher sensitivity for the compression than the reference paperboard. The compression will reduce superhydrophobicity as air pockets collapse in nanoporous TiO2 coating under compression as clearly observed from the SEM cross-sectional images. We believe that LFS-deposited nanoparticle coatings will find many applications in the future from controlled wettability to enhanced sensing in surface-enhanced Raman scattering. Understanding the stability of such nanoparticle coatings is crucial for reproducible and reliable performance of the functional coatings.
Atomic force microscopy
Field emission scanning electron microscope
Isopropanol (or isopropyl alcohol)
Liquid flame spray
Scanning electron microscopy
Time-of-flight secondary ion mass spectrometry
X-ray photoelectron spectroscopy.
This work was supported by the Finnish Funding Agency for Technology and Innovation (Tekes) under the project 'Liquid flame spray nanocoating for flexible roll-to-roll webmaterials’ (grant no. 40095/11). JJS wishes to thank the Academy of Finland (grant no. 250 122) for the financial support.
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