Paper coatings with multi-scale roughness evaluated at different sampling sizes
Graphical abstract
Research highlights
► Paper coating with micro- to nanoscale roughness pattern. ► Roughness measurements by atomic force microscopy and non-contact profilometry are complimentary. ► Correlation length and spectral densities can be linearly extrapolated between different sampling sizes.
Introduction
Paper substrates are widely used and preferred due to high strength, flexibility, low cost and recyclability. However, they have a complex structure with cellulosic units arranged at multiple levels including molecules, fibrils, fibers and fiber networks [1]: the two or three dimensional structures develop at few nanometers and proceed to several centimeters [2]. The hierarchical ordering forms during production in the pulp and paper mill, where the specific interactions within a paper network can be tuned by modifying pulp composition and paper machine operations. The individual fibers and network properties determine the final paper performance and may often be floculated [3] and/or anisotropically distributed [4], as reflected in its mechanical properties [5]. The anisotropy of deterministic features can be assessed by modern means [6], including microwaves [7], wavelet transforms [8], [9], ultrasonic systems [10] or laser diffraction [11]. The complexity is not only present in the paper bulk but also manifests at the surface, where it determines end user properties such as visual appearance (e.g., gloss, color, and flatness), printability (e.g., capillary action, ink drying and absorption) [12], processability (e.g., friction) [13], and barrier resistance (e.g., water absorption, gas penetration, and grease) [14], [15]. Monitoring paper surfaces is important for quality control in production sites [16] and provides insight on how to improve surface properties.
The paper qualities and interfacial properties depend on the surface topography, which has a multiscale structure that can be altered by coating or calendering [17]. The large surface features or waviness result from long-range fiber alignments during paper machine operations and originate from the base sheet. The fine length-scale phenomena of small and high-frequency surface irregularities or roughness are characteristic for coating pigments and pigment packing [18], [19]. The multilevel roughness of latex coatings with calcium carbonate pigments was evaluated by Wang et al. [20]. The uncoated papers show two slopes of the roughness depending on the sampling size, likely attributed to the effects of single fiber directions. The coated papers present a linear or bilinear function, depending on the rod- or blade-coating process. The following properties are defined by the surface roughness:
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The gloss depends on refractive index, coating color, particle packing, porosity and surface roughness [21], [22], [23], [24]. Xu et al. [25] found good agreement between gloss and root-mean-square roughness: measurements with a stylus profilometer provided somewhat better correlations for base papers, while atomic force microscopy gave best results for printed papers. However, the common roughness parameters often do not uniquely relate to gloss, as rough surfaces sometimes provide higher gloss than smooth ones [26]. Often dimensionless parameters should be used [27] or the effect of microroughness and pore diameter on gloss uniformity should be considered [28], [29], [30] in combination with a two-scale roughness model [31]. The latter provides an acceptable physical description, as the pigment packing and pore density influences the refractive index by entrapment of air in-between the roughness profile. It was verified that gloss relates to the macroroughness during the first calendering steps, while an additional reduction in microroughness further improves gloss [32]. For styrene–butadiene coatings, multiple coating steps improved the gloss in parallel with reduced surface roughness while prior smoothening of the base substrate by hot calendering improved gloss and reduced roughness [33], [34]. Recently, the influences of different roughness scales on gloss were studied for calcium carbonate and kaolin coatings, taking into account the lateral distribution of surface heights and critical roughness values that correlate with specular reflection [35], [36].
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The wettability depends on the surface topography and chemistry that need to be controlled for creating hydrophobic barrier layers [37]. The influences of chemical composition are described by the Young's equation for smooth surfaces [38]. The surface roughness mostly amplifies the wetting properties: i.e., an intrinsic hydrophilic material becomes more hydrophilic as the numbers of asperity contacts increase, while the hydrophobic properties of a hydrophobic material are favoured at higher roughness [39], [40]. Some examples of roughness-controlled hydrophobic plant leafs are known in nature: their surfaces are roughened by papillose epidermal cells that form asperities or papillae, and are supplementary covered with waxes of mixed hydrocarbon compounds that arrange onto submicron-sized asperities [41], [42], [43], [44]. Thus, water-repellent and self-cleaning surfaces can artificially be produced by mimicking the hierarchical surface roughness in combination with hydrophobic moieties [45], [46], [47], [48]. Wenzel [49] found that the contact angle of a liquid with a rough surface is different from that with a smooth surface. Cassie and Baxter [50] showed that air (or gas) pockets may be trapped in the cavities of a rough surface. The creation of multilevel surface roughness by the superposition of roughness patterns [51], [52], [53], and fractal roughness [54] may finally lead to superhydrophobicity.
The above discussion reveals the interest in fabricating micro- to nanoscale roughness patterns on papers for packaging. In this report, we present a multiscale coating morphology that spontaneously forms by drying of organic nanoparticles from aqueous dispersion. We previously observed that such a top-coating improves the hydrophobicity and printability of macroporous paper substrates [55], which requires better understanding and characterization of the surface morphology. Therefore, a quantitative study is required that monitors roughness at various length scales with complementary techniques.
Section snippets
Materials and sample preparation
Organic nanoparticles were synthesized by partial imidization of poly(styrene–maleic anhydride) or SMA that transforms into poly(styrene–maleimide) or SMI. A high-molecular weight SMA copolymer with relative molecular weight Mr = 60,000 and 26 mol% maleic anhydride was obtained from Polyscope (Geleen, The Netherlands) and imidized by Topchim N.V. (Wommelgem, Belgium) according to the reaction conditions given in a previous paper [56]. Briefly, 140 g SMA was charged with an equivalent amount
Scanning electron microscopy (SEM)
The morphology of uncoated and coated papers is shown in Fig. 1. The fibers and fillers of uncoated papers are fully covered by a microdomain structure of the coating. Different drying procedures were used such as air-drying, or drying in a circulating hot-air oven for 2 min at 70 °C, 2 min at 100 °C, 30 s at 120 °C, 1 min at 120 °C, 2 min at 120 °C and 3 min at 120 °C: they did not significantly influence the coating morphology. Also nanoparticles dried under similar conditions onto inert and smooth glass
Direct comparison between NCP and AFM data
The most direct comparison of data from both sampling techniques would include an analysis over similar surface areas. Due to the intrinsic limitations in sampling sizes and differences in sampling bandwidth, overlapping conditions for NCP and AFM measurements could not be included. Therefore, our main concern focusses on extrapolating data between NCP and AFM. In one way, roughness parameters from NCP images with smaller sizes L × L could be artificially obtained by a software-zoom on the
Conclusions
An organic nanoparticle coating effectively covers macroporous paper substrates and induces an unique surface morphology with multi-scale roughness. A microdomain structure develops during drying through aggregation and lack of viscous flow of nanoparticles with relatively high glass transition temperature.
We analysed the variation of surface roughness parameters over a broad range of sampling areas by combining two characterization techniques: non-contact profilometry (scanning 2000 μm × 2000 μm
Acknowledgements
Pieter Samyn and Jürgen Van Erps acknowledge the Research Foundation Flanders (F.W.O.) for a Postdoctoral Research Fellowship grant. Financial support was obtained from the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT-Vlaanderen). We thank Topchim N.V. (Wommelgem, Belgium) for delivering testing materials. Peter Mast (Department Applied Materials Science, Ghent University) performed scanning electron microscopy studies and AFM studies were done at the
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