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The influence of TiO2 nano-particles (nano-TiO2) was studied relative to the mechanical and optical properties of CMYK printed paperboard after coating with a UV-curable varnish. Commercial duplex paperboard (glazed grayback paperboard, 230 g/m 2) was printed with a CMYK offset printing process. Board samples were coated with a nano-TiO2 modified U...
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... printing, specifically cyan, magenta, yellow, and “key” (black) (Almutawa and Moon 1999; Bergman 2005). Mechanical properties related to creasing and folding processes are crucial for the performance of a paperboard in the packaging industries. In the process of converting paperboards, making the corner is a critical phase. The straightness of a corner is also critical for the strength of the paperboard boxes. The corner must be sharp and form no cracks if folded by 90 degrees (Beldie 2001; Borbély 2010). Cracks on the fold-line in the paperboard box must be avoided, and this is a critical subject in box making processes (Beex and Peerlings 2009). In some cases, UV-curable coatings have negative effects on mechanical behaviors, especially the brittleness of fold-lines. Increasing brittleness has resulted in some operational drawbacks including reducing foldability, breaking in the fold-line after a die- cutting process, and peeling in the coater, as well as limiting their aesthetic appeal (Coffin et al. 2011). In previous work by the authors (Soltani et al. 2014), it was found that the tear and fold resistances of printed paperboard were reduced through a coating process with the UV-cure coater. Papermaking has, in effect, been practicing nanotechnology for a long time (Reitzer 2007). In recent years, there has been a great increase in the variety of additives used in papermaking and also the coating processes. Nanoparticulate additives are used, for instance, to improve printability, to increase surface strength, as crosslinking agents, for antistatic effects, as flame retardant treatments, as anti-slip coating materials, as microbiocides, and as miscellaneous chemicals for repelling or attracting various substances (Hajas et al. 2005; Mohiedin et al. 2010; Kharisov and Kharisova 2010; Shen et al. 2010). The use of nanoparticles is a convenient way to enhance the coatings properties, as their aspect ratio is high and their efficiency also can be high (Cristea et al. 2011). For stabilization of color under indoor conditions, nanoparticles can be dispersed in a continuous medium down to a primary particle size well below 50 nm. Such nanoparticles can be used in various coating applications without the need for additives (Hajas et al. 2005). Recently, inorganic nano-materials such as nano-TiO 2 and nano-ZnO have received much attention since their application was found to improve the optical properties and endowed other special functions to coatings (Shi et al. 2008; Sow et al. 2010, 2011). The present work considered the use of nano-TiO 2 to enhance the properties of paper, wood, and other lignocellulosic materials (Blanchard and Blanchet 2011; Sun et al. 2010; Wang et al. 2012). The goal was to evaluate the effects of nano-TiO 2 in the coating formulation on the optical and mechanical properties of UV-curable coated-printed paperboard. Commercial glazed duplex paperboards were printed using a CMYK offset method (based on the paint-pattern shown in Fig. 1) and coated with a commercial UV-curable glossy varnish. The processes are described in Part 1 of this series (Soltani et al. ...
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Citations
... Surface damages on coated papers and paperboards occurred during the folding process are one of the most frequently encountered problems in the printing industry. Since the folding process is one of the most commonly used operations in the graphical post production it could cause severe financial losses and significant environmentaly adverse effects (Holik, 2013;Soltani et al., 2016;FOGRA, 2020). To reduce the negative effects of folding process, the fold cracking resistance was gaining importance and has become an active field of research. ...
The folding process is one of the most commonly used print finishing operations in graphical production. Therefore, surface damages on coated papers and paperboards, which are frequently appearing during this process, can have significant negative economic and environmental impacts. To reduce the adverse effects, fold crack resistance has become an active field of research. In the last decades, there were several computer-aided visual assessment methods introduced for fold-crack evaluation. These techniques were based on similar concepts of digital image analysis to quantitatively characterise the surface damage, but they had differences in utilisation as well as in the used image feature. In this study, fold crack distribution has been introduced as a new digital image feature for quality assessment. Fold crack distribution can be determined as a measure of crack scattering over the folding line. In this paper descriptive statistics, mean value, standard deviation and coefficient of variance have been used for qualitative characterisation of surface damage. The obtained results for mean value demonstrated increasing tendency by increasing the basis weight and had slightly lower value for samples in machine than cross direction. These results confirm the basic assumptions that on thicker substrates (i.e. on samples with higher basis weight) the cracks are larger, longer or grouped and that the folding process generates higher surface destruction is cross than machine direction. In the case of standard deviation, results suggest that in a case of varying mean values, crack distribution should be defined via coefficient of variation. Based on this analysis, the proposed approach to the calculation of crack distribution can serve as a new image characteristic for the qualitative measurement of the fold-crack resistance of coated papers.
... The optical characterization of the samples focused on obtaining spectral data (Soltani et al. 2016). For the optical properties, the CIELab (L*, a*, and b* values) color parameters were measured on handsheets using a Technibrite Micro TB-1C spectrophotometer (Elrepho Co., Stockholm Sweden) using TAPPI T1215 sp-03 (2007). ...
The optical and mechanical properties of paper sheets were compared following treatment with biopolymers using two different methods, i.e., internal functionalization and surface modification. Industrial chemi-mechanical pulp was provided, and two groups of paper handsheets were made. The first group of paper sheet samples were prepared with incorporated chitosan in a mixed-in-pulp slurry (at three treatment levels, i.e., 0%, 1%, and 2%), and in the second group, untreated handsheets were spray-coated with either 2% chitosan, 2% nanocellulose, 0.5% DTPA, or a mixed solution (0.5% DTPA + 2% chitosan + 2% nanocellulose). Then, the optical and mechanical properties of the prepared paper sheets were analyzed. The brightness, opacity, and greenness/redness of the samples were measured spectrophotometrically using the CIELab system. Additional properties of the paper, i.e., the water absorption, air permeability, and tear, tensile, and burst resistance, of the treated samples were also measured. The results indicated that the treatments had a considerable effect on the relative mechanical and optical parameters, which resulted in an increase in the mechanical properties, especially when the nano-particles were applied to the mixed-in-slurry process. Inversely, results from the spray-coating method indicated better results for the optical specifications.
... During the folding process substrates are subjected to high tensile stress on the outer side and compression stress on the inner side of folding line, which, in the case of coated papers, usually lead to surface damages, so called fold cracks. Fold cracking can decrease the visual appearance or functionality of the printed product and package, but with more serious damage it can lead to extreme reduction or complete loss of overall strength and the finished products can fall apart immediately or after short usage (Barbier et al. 2003, Fellers 2009, Holik 2013, Soltani et al. 2016, FOGRA 2020a. Since coated papers and boards suit the highest requirements with regard to printability, they are widely used in high-volume printing and packaging industry. ...
In this paper, white pixel percentage (WPP) value, as an overall measure of fold crack damages, has been analysed with respect to selected parameters of sample preparation and digitalization process, as well as the results of residual tensile strength. The WPP values were derived by an automated image processing algorithm, developed earlier, based on extensive comparative analysis of the existing computer-aided methods. Results indicate that WPP values correlate well with the extent of fold cracks on the coated samples, as far as the used parameters of sample preparation and digitalization are concerned. In the case of correlation with residual tensile strength, results for samples folded in cross direction revealed that the extent of the visually registered fold cracks agree well with the actual damage, while for samples folded in machine direction, the overall strength losses weren’t alarming, although the fold cracks were detected correctly. In addition, results pointed out that the simplest sample placement position (inner angle of 180°) is not applicable for realistic sample representation. Furthermore, scanners could provide a superior image quality in lab conditions, but for industry application, a camera-based solution would be more purposeful, while micrographs are more suitable for traditional visual analysis.
The influence of TiO2 nano-particles (nano-TiO2) was studied relative to the mechanical and optical properties of CMYK printed paperboard after coating with a UV-curable varnish. Commercial duplex paperboard (glazed grayback paperboard, 230 g/m 2) was printed with a CMYK offset printing process. Board samples were coated with a nano-TiO2 modified UV-curable varnish at four treatment levels (0, 0.2, 0.5, and 1%) using an industrial screen-coating machine. The samples were then dried using a UV lamp in an industrial UV drying machine. Sample discoloration was measured spectrophotometrically using CIELab parameters (L*, a*, b*, and ΔE) before and after coating. The whiteness, brightness, fold, and tear resistance of the ink films were also measured. The nano-treatment had a significant effect on the relative optical parameters, which resulted in increasing the lightness of the treated samples. Color change (ΔE) was recorded for all tested samples, and an unperceivable change was observed in case of the nano-treatment with 0.2% as the end value. The weakly perceivable changes were found in the cases of treatment with 0.5 and 1% nano-intensities. The nano-TiO2 treatment significantly improved the fold and tear resistance of the samples.
