- Email: [email protected]
paperboard packaging prepared with nano-modified overprint varnish
Applied Surface Science 266 (2013) 319–325
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Lotus-like paper/paperboard packaging prepared with nano-modified overprint varnish Wenting Chen a , Xinling Wang a,∗ , Qingsheng Tao b , Jinfang Wang b , Zhen Zheng a , Xiaoliang Wang b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, PR China Unilever R&D Shanghai, 66 Lin Xin Road, Shanghai 200335, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 14 November 2012 Accepted 5 December 2012 Available online 16 December 2012 Keywords: Paper and paperboard Packaging Varnish coating Nano-structured Water resistance Anti-frost
a b s t r a c t Paper/paperboard has great advantages over plastics as packaging material in terms of cost and sustainability. However, the application of paper/paperboard packaging is limited due to their inferior water resistance performance. Here we report a functional overprint varnish prepared with the aim to significantly abate the wettability of the printed packaging. The prominent water repellency of the varnish is rendered by its unique nano-structured morphology, a technology bio-mimicking lotus surface. The frost formation on the applied packaging is also inhibited. Moreover, the transparency and the application process of the varnish are retained. We expect such varnish has high potential for expanding the application field of paper/paperboard packaging, especially for uses requiring a strict standard of water resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Papers and paperboards are extensively used in various industries. The primary consumption of this kind of material is for producing various types of flexible, semi-rigid, or rigid packaging [1]. In addition to its effective cost and high application flexibility, paper/paperboard is also considered as the most environmentally friendly material for packaging, in view of its natural sourcing and easy recyclability. As a result, paper and paperboard are the largestused packaging material throughout the world, by weight. The global paper packaging market in 2011 is reported to be worth 236 billion US dollar [2]. Paper and paperboard are sheet materials comprising an interlaced network of cellulose fibers, which is intensely vulnerable to water or moisture owing to its hydrophilic nature. In many cases, an additional barrier material such as aluminum or plastic [3–5] is incorporated to improve their performance and function. However, this laminated paper/paperboard packaging is not only more expensive but also less recyclable than those made only by paper [1]. Besides the metallic or plastic film, over-print varnish is another widely used protection technology, which is very cost-effective and very easy to apply. However, varnish does not have as good water
∗ Corresponding author. Tel.: +86 21 5474 5817; fax: +86 21 5474 1297. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.018
repellency as film. In an effort to enhance the water-resistance, hydrophobic polymer has been utilized to produce the over-print varnish, or wax/silicone additive has been incorporated. Although the performance of such varnish is good, neither of the methods is able to render the printed paper packaging completely waterproof like a lotus leaf. The mechanism underlying the super-hydrophobic properties of the lotus leafs was unveiled more than two decades ago [6]. Since then, mimicking such natural phenomenon to prepare the lotuslike surface has aroused intense interest both in academia and in industry, as the surface has demonstrated self-cleaning [7], water repellency [8], stain repellency [9], anti-bacteria [10], and many other superior benefits [11,12]. However, most of the reported technologies of preparing such surfaces are highly complicated and very difficult to be applied in industrial scale. That is probably why only rarely have packaging materials, to the best of our knowledge, been reported to be modified to have the lotus-like surface. Nevertheless, if lotus-like superhydrophobic modification can be further applied to paper or paperboard packaging, to utilize its unique lotus-like properties, such as water and moisture-repellent, anti-frost formation, it will provide better packaging appearance benefits with less water/moisture/frost damage. In addition, it could also eliminate the usage of plastic coating on paper package and thus reduce environmental impact by yielding fully recyclable paper packages.
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Here we report a novel technical route to hydrophobically modify an over-print varnish with fumed silica. The optimized formulation capable of imparting the lotus-like property to paper and paperboard packaging has been identified.
2. Experimental 2.1. Materials Waterborne varnish (07-1449-LT705) was supplied by Weilburger® (Germany). Silica nano-particles (AEROSIL® R812S, average particle size 7 nm, specific surface area 220 m2 /g) were purchased from Evonik Degussa. Decamethylcyclopentasiloxane (D5) and polydimethylsiloxane (PDMS, BP-9400) were supplied by Dow Corning and Bluestar Silicones (Shanghai) Co., Ltd., respectively. Nonionic surfactant p-octyl polyethylene glycol phenyl ether (OP) was bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All these materials were used as received without any treatment.
2.2. Functionalization of waterborne varnish The modifying agent for the varnish was prepared by mixing four components (D5, R812S, BP-9400 and OP), with the mass ratio of D5/R812S/BP-9400/OP at 57:3:9:1. The final waterborne varnish was prepared through adding the modifying agent into commercial varnish with various mixing proportions, under vigorous stirring. The detailed preparation procedure as used was as follows. Firstly R812S nanoparticles were dispersed into D5 silicone oil, and then BP-9400 was added into this mixture, which was then blended with the commercial varnish. Finally, the modified varnish was obtained after OP was added dropwise with a buret and then stirring for a further 10 min.
2.3. Application of the varnish onto printed paperboard Before usage, the modified varnish was homogenized evenly by a 10-min ultra-sonication. Afterward, it was applied onto an offset printed paperboard (15 cm × 15 cm) with an anilox roller.
2.4. Characterization ATR (attenuated total reflectance) spectra were recorded on a Nicolet 6700 FT-IR spectrometer of Thermo Electron Corporation. The observation of the morphology of the samples was carried out with a field emission scanning electron microscope (FESEM, HITACHI S-4800). Rheological behavior of the modified varnish was investigated with an Anton Paar Physica MCR301 instrument. The static water contact angles were measured on a KRÜSS DSA100 Drop Shape Analyzer (KRÜSS, Germany) using a 5 L triply distilled water droplet at ambient temperature. 2.5. Performance test Three aqueous liquids, including distilled water, mud slurry and ink, were dripped onto the paper surfaces laid on a slope with a 20-degree angle, to test the water resistance. The durability of the performance was examined with a peristaltic pump dripping water, from 10 cm above, for 2 h at a constant rate of 1 drop per second. The anti-frosting experiment was conducted in a cabinet which automatically opened and closed its door according to a pre-set program. Two pieces of paper (20 cm × 16 cm) coated with the original and the modified varnish, respectively, were stuck onto the inner wall of the cabinet, which was set to be closed for 300 s and then open for 10 s before closed again, simulating real using conditions. In order to fully frost the paper, the cabinet ran the open-close loop program for 5 h before remaining closed at −15 ◦ C for another 19 h. 3. Results and discussion The fabrication process of the lotus-like printed paper packaging is illustrated in Fig. 1. Hydrophobic nanoparticles (R812S) and silicone polymer (PDMS, BP9400) are homo-genously dispersed in a solvent (D5 silicone oil) to prepare the modifying agent [13], and subsequently the dispersion is emulsified into a waterborne overprint varnish. During a normal printing process, the particles and the PDMS are deposited onto the paper packaging, so modifying the surface to be superhydrophobic. A very important feature of a lotus-like, e.g. superhydrophobic, surface is the micro-/nano-structured morphology. Therefore, the process to fabricate such kind of morphology is of critical importance for preparing the superhydrophobic surface. A widely used
Fig. 1. Schematic illustration of the fabrication process of the lotus-like paper packaging via modifying the overprint varnish with hydrophobic nanoparticles and silicone polymer.
W. Chen et al. / Applied Surface Science 266 (2013) 319–325
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Fig. 2. FESEM images of the modified varnish comprised of (a) 0 wt%, (b) 10 wt%, (c) 20 wt%, (d) 40 wt%, (e) 70 wt% and (f) 100 wt% modifying agent on the paper packaging.
technical route for this purpose is by depositing a layer of nanosized particles [14,15]. In this work, the surface morphologies of the paper packaging with modified varnish are shown in Fig. 2. We can find more and more nanoparticles on the paper surface,
along with the increasing concentration of the modifying agent. The hydrophobic silica nanoparticles uniformly deposit and cover up the entire paper surface when the proportion of the modifying agent is over 40 wt% (image d). As shown in the high resolution
Fig. 3. ATR spectra of original varnish, modified varnish comprised of 20 wt%, 40 wt%, 70 wt% and 100 wt% modifying agent.
Fig. 4. The logarithm of intensity ratio of peaks 1030 cm−1 and 1730 cm−1 as dependent on the modifying agent content in the varnish.
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Fig. 5. The static water contact angles on the surfaces coated with modified varnish comprised of various proportion of modifying agent. Insets show the shapes of water droplets (5 L) on surfaces of paper packaging coated with modified varnish.
SEM image (inset in Fig. 2d), a large amount of nanoparticles exist on the paper surface, which is a clear contrast to the bald surface of un-modified varnish (inset in Fig. 2a). Besides the visual evidence of SEM images, the surface modification of the paper packaging can also be characterized via the variation of its outmost chemical composition, which is indicated in their ATR spectra (see Fig. 3). The characteristic absorption peaks at 1030 cm−1 correspond to the Si O stretching vibration [16,17], and the peaks at 1730 cm−1 (C O) can be assigned to the carbonyl stretching vibration [18,19], which is assumed to be solely due to the original varnish as the peak disappears when the coating is totally composed of the modifying agent. For this reason, the intensities of the peaks at 1030 cm−1 and 1730 cm−1 can indicate the amount of the modifying agent and the original varnish at the surface of paper packaging, respectively. The greatly enhanced intensities of 1030 cm−1 peaks, and the declining intensities of 1730 cm−1 peaks indicate that the outermost surface is gradually occupied by the modifying agent when increasing its amount in
Fig. 6. Photographs of water droplets (10 L) on the surface of paper packaging coated with (a) original varnish; (b) modified varnish comprised of 40 wt% modifying agent.
the formulation. Moreover, the ratio of the intensities of the peaks at 1030 cm−1 and 1730 cm−1 , I1030 /I1730 , can quantitatively correspond to the content of modifying agent left on the paper surface. Surprisingly, the result displayed an excellent linear (R = 0.977) relationship between the proportion of the modifying agent and log(I1030 /I1730 ), as shown in Fig. 4. Here we should note that the D5 silicone oil in the modifying agent has evaporated away after the
Fig. 7. Photographs of liquid repellency test on the paper surface coated with original varnish (upper row) and modified varnish comprised of 40 wt% modifying agent (lower row); liquid a-water; b-mud slurry; c-ink.
W. Chen et al. / Applied Surface Science 266 (2013) 319–325
Fig. 8. Photographs of waterproof durability test (continuously hit by water droplets for 2 h) on the paper surface coated with original varnish and modified varnish comprised of 40 wt% modifying agent.
curing of coating. The Si-containing substance on the modified surface is mostly composed of R812S silica nanoparticles and BP-9400 silicone. Based on the results of SEM and ATR, it is clear that a nanostructured surface comprising hydrophobic silica nanoparticles (R812S) and silicone oil (BP-9400) is successfully fabricated on a piece of paper packaging after coating it with the modified varnish. Such kind of surface has been extensively reported to exhibit a superhydrophobic property [14,20,21]. To measure the wettability of the modified surfaces, their water contact angles (CA) are obtained with a 5 L droplet as the indicator. As shown in Fig. 5, the water contact angles reflect an escalating trend with the increase of the modifying agent. The water contact angles of the paper packaging reach 150◦ or even higher when the component of modifying agent in the varnish is over 30 wt%, clearly indicating superhydrophobic surfaces are obtained as expected. As a comparison, the water contact angles on the smooth paper packaging coated with unmodified varnish and modified varnish with 20 wt% modifying agent are 99◦ and 110◦ , respectively.
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Fig. 9. Photographs of anti-frosting test on the paper surface coated with original varnish and modified varnish comprised of 40 wt% modifying agent.
The macroscopic wettability difference between the original varnish and the modified varnish is also revealed by the photographs shown in Fig. 6. The water droplets sitting on the superhydrophobic paper packaging maintain their round shape, while those on normal packaging spread over the surface with a much larger contact area. In other words, the paper packaging with the modified varnish is difficult to be wetted by water. These photos also demonstrate that the modified varnish has maintained good transparency. The superhydrophobic property has drawn a lot of attentions due to its huge application potential in various fields. One of the most distinct functions of this property is that it allows water roll over quickly, binging in enormous benefits of anti-stain [22] and self-cleaning [12,23]. In order to estimate whether the modified varnish also has those advantages, we dripped three types of aqueous drops onto the paper surfaces on a 20-degree slope. Distilled water traces are observed on the paper surface with original varnish (Fig. 7a1) while water drops roll down entirely without any traces left on the paper surface with modified varnish (Fig. 7a2). Similar phenomena are observed when distilled water is replaced
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behavior of this kind of modified varnish. The apparent viscosities, , of the original varnish and modified varnish are shown in Fig. 10. At low shear rate (<10 s−1 ), the viscosity of the modified varnish is as ten times as that with the original varnish. Actually, the modified varnish presents gel-like appearance at low shear rate, especially in static state. Nevertheless, this varnish shows a dramatic shear thinning effect [28], and at high shear rate its viscosity is nearly identical to that of the original varnish. All these results are in accordance with the reported observations of polymer nanocomposites [29], and the theoretical expectations describing similar shear thinning phenomena of dispersions containing nanomaterials [30]. 4. Conclusion
Fig. 10. Rheological behavior curves of original varnish and modified varnish comprised of 40 wt% modifying agent.
with mud slurry (Fig. 7b1 and b2). Furthermore, the experiments also demonstrate the unique function of inhibiting contamination, as shown in Fig. 7c1 and c2. Deep black ink is left on the paper surface coated with original varnish (Fig. 7c1), and the paper was totally stained. In contrast, traces are barely perceivable on the surface with the modified varnish (Fig. 7c2). The three results clearly indicate that the modified varnish we prepared in this work is able to deliver an extremely water resistant property to the paper packaging. In addition to the transitory water-impermeability as proven above, we have also investigated the durability of the superhydrophobic surface under long-lasting impact of water drops. The test is conducted by continuously dripping water, at 1 drop per second for 2 h, onto the paper surface laid on a 20-degree slope. As shown in Fig. 8, the paper coated with original varnish was wetted, whereas the one with the modified varnish maintains its dryness as well as the surface superhydrophobicity. Both the pieces of paper were weighed before and after the test to quantify the difference of their water adsorption. There was 0.030 g water being taken into the paper with original varnish, while the weight gain of the modified paper is undetectable with the same electronic balance. This experiment demonstrates that the waterproof surface rendered by the modified varnish can endure contacting or even impacting of water for at least 2 h. In some areas, the transportation and storage of frozen products may result in frost formation on their packaging, which may significantly damage the artwork and quality. In those cases, a frostinhibiting packaging is highly desirable. It has been reported that a superhydrophobic surface has an inherent property of delaying and inhibiting frosting process [24–26], implying the modified varnish we prepared may also functionalize the paper packaging as anti-frosting. After staying in a cabinet for 24 h (with controlled occasional door opening and closing to simulate real conditions), the paper surface with original varnish is covered with much more frost than the paper surface with modified varnish (Fig. 9). Liu et al. has discovered that contact angle affects the frost formation process by influencing vapor condensation on the cold surface. Increasing the contact angle of a surface will improve restraining the frost crystal nucleation [26,27]. This mechanism explains why our lotus-like superhydrophobic paper surface, of a contact angle over 150◦ , has a much stronger ability to restrain frost formation than the ordinary surface coated with original varnish. An essential requirement of a practical varnish is that its viscosity has to be suitable in a real printing condition. In order to clarify the industrial perspective, we characterize the rheological
In the present study, we have successfully prepared lotus-like superhydrophobic paper packaging by coating paper with our functionalized varnish modified with R812S silica nanoparticles and PDMS silicone oil. From the SEM images and the ATR spectra, it is revealed that a nano-structured surface morphology has been fabricated on the packaging. Moreover, macroscopically the modification does not bring a perceivable influence on the varnish transparency. With increase of the modifying agent, the prepared varnish shows enhanced capability in preventing the applied paper from being wetted by water. Especially when the component of modifying agent in the varnish is above 30 wt%, the static water contact angle on the packaging surface can reach over 150◦ , which is as super-hydrophobic of a lotus leaf. The applied packaging demonstrates an outstanding performance of inhibiting various aqueous contaminations and enduring water impact for hours. Furthermore, the lotus-like property significantly restrains the frost formation on the packaging surface. The modified varnish we prepared in this study displays a substantial shear thinning effect, which makes it suitable for a variety of printing applications. Such a transparent overprint varnish promises great potential applications on a wide variety of paper/paperboard packaging, with waterproof, easy cleaning, anti-frost, and other valuable functions. Nevertheless, the industrial value of this material also highly depends on its bio-safety, i.e. whether or not the nanoparticles will be disassociated from the varnish, and then inhaled by human. The study on nano-toxicity has been very intensive in recent years, while there is no decisive conclusion yet on the potential risk of the exposure to nano-sized materials [31,32]. The next target of this project is to explore the possibility and investigate the process of nanoparticles leaving the varnish matrix, during the production, transportation, storage and usage periods of the superhydrophobic packaging. With the data, we will then decide whether or how to apply the technology onto a commercialized product. Acknowledgements This work is supported by the Master Student Program of Unilever. And we thank the AMDM (Advanced Measurement and Data-Modeling), Department of Unilever for SEM and ATR measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.apsusc.2012.12.018. References [1] M.J. Kirwan, Paper and paperboard packaging technology, in: M.J. Kirwan (Ed.), Paper and Paperboard – Raw Materials, Processing and Properties, Blackwell Publishing, London, UK, 2005, pp. 1–5.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Lotus-like paper/paperboard packaging prepared with nano-modified overprint varnish Wenting Chen a , Xinling Wang a,∗ , Qingsheng Tao b , Jinfang Wang b , Zhen Zheng a , Xiaoliang Wang b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, PR China Unilever R&D Shanghai, 66 Lin Xin Road, Shanghai 200335, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 14 November 2012 Accepted 5 December 2012 Available online 16 December 2012 Keywords: Paper and paperboard Packaging Varnish coating Nano-structured Water resistance Anti-frost
a b s t r a c t Paper/paperboard has great advantages over plastics as packaging material in terms of cost and sustainability. However, the application of paper/paperboard packaging is limited due to their inferior water resistance performance. Here we report a functional overprint varnish prepared with the aim to significantly abate the wettability of the printed packaging. The prominent water repellency of the varnish is rendered by its unique nano-structured morphology, a technology bio-mimicking lotus surface. The frost formation on the applied packaging is also inhibited. Moreover, the transparency and the application process of the varnish are retained. We expect such varnish has high potential for expanding the application field of paper/paperboard packaging, especially for uses requiring a strict standard of water resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Papers and paperboards are extensively used in various industries. The primary consumption of this kind of material is for producing various types of flexible, semi-rigid, or rigid packaging [1]. In addition to its effective cost and high application flexibility, paper/paperboard is also considered as the most environmentally friendly material for packaging, in view of its natural sourcing and easy recyclability. As a result, paper and paperboard are the largestused packaging material throughout the world, by weight. The global paper packaging market in 2011 is reported to be worth 236 billion US dollar [2]. Paper and paperboard are sheet materials comprising an interlaced network of cellulose fibers, which is intensely vulnerable to water or moisture owing to its hydrophilic nature. In many cases, an additional barrier material such as aluminum or plastic [3–5] is incorporated to improve their performance and function. However, this laminated paper/paperboard packaging is not only more expensive but also less recyclable than those made only by paper [1]. Besides the metallic or plastic film, over-print varnish is another widely used protection technology, which is very cost-effective and very easy to apply. However, varnish does not have as good water
∗ Corresponding author. Tel.: +86 21 5474 5817; fax: +86 21 5474 1297. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.12.018
repellency as film. In an effort to enhance the water-resistance, hydrophobic polymer has been utilized to produce the over-print varnish, or wax/silicone additive has been incorporated. Although the performance of such varnish is good, neither of the methods is able to render the printed paper packaging completely waterproof like a lotus leaf. The mechanism underlying the super-hydrophobic properties of the lotus leafs was unveiled more than two decades ago [6]. Since then, mimicking such natural phenomenon to prepare the lotuslike surface has aroused intense interest both in academia and in industry, as the surface has demonstrated self-cleaning [7], water repellency [8], stain repellency [9], anti-bacteria [10], and many other superior benefits [11,12]. However, most of the reported technologies of preparing such surfaces are highly complicated and very difficult to be applied in industrial scale. That is probably why only rarely have packaging materials, to the best of our knowledge, been reported to be modified to have the lotus-like surface. Nevertheless, if lotus-like superhydrophobic modification can be further applied to paper or paperboard packaging, to utilize its unique lotus-like properties, such as water and moisture-repellent, anti-frost formation, it will provide better packaging appearance benefits with less water/moisture/frost damage. In addition, it could also eliminate the usage of plastic coating on paper package and thus reduce environmental impact by yielding fully recyclable paper packages.
320
W. Chen et al. / Applied Surface Science 266 (2013) 319–325
Here we report a novel technical route to hydrophobically modify an over-print varnish with fumed silica. The optimized formulation capable of imparting the lotus-like property to paper and paperboard packaging has been identified.
2. Experimental 2.1. Materials Waterborne varnish (07-1449-LT705) was supplied by Weilburger® (Germany). Silica nano-particles (AEROSIL® R812S, average particle size 7 nm, specific surface area 220 m2 /g) were purchased from Evonik Degussa. Decamethylcyclopentasiloxane (D5) and polydimethylsiloxane (PDMS, BP-9400) were supplied by Dow Corning and Bluestar Silicones (Shanghai) Co., Ltd., respectively. Nonionic surfactant p-octyl polyethylene glycol phenyl ether (OP) was bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All these materials were used as received without any treatment.
2.2. Functionalization of waterborne varnish The modifying agent for the varnish was prepared by mixing four components (D5, R812S, BP-9400 and OP), with the mass ratio of D5/R812S/BP-9400/OP at 57:3:9:1. The final waterborne varnish was prepared through adding the modifying agent into commercial varnish with various mixing proportions, under vigorous stirring. The detailed preparation procedure as used was as follows. Firstly R812S nanoparticles were dispersed into D5 silicone oil, and then BP-9400 was added into this mixture, which was then blended with the commercial varnish. Finally, the modified varnish was obtained after OP was added dropwise with a buret and then stirring for a further 10 min.
2.3. Application of the varnish onto printed paperboard Before usage, the modified varnish was homogenized evenly by a 10-min ultra-sonication. Afterward, it was applied onto an offset printed paperboard (15 cm × 15 cm) with an anilox roller.
2.4. Characterization ATR (attenuated total reflectance) spectra were recorded on a Nicolet 6700 FT-IR spectrometer of Thermo Electron Corporation. The observation of the morphology of the samples was carried out with a field emission scanning electron microscope (FESEM, HITACHI S-4800). Rheological behavior of the modified varnish was investigated with an Anton Paar Physica MCR301 instrument. The static water contact angles were measured on a KRÜSS DSA100 Drop Shape Analyzer (KRÜSS, Germany) using a 5 L triply distilled water droplet at ambient temperature. 2.5. Performance test Three aqueous liquids, including distilled water, mud slurry and ink, were dripped onto the paper surfaces laid on a slope with a 20-degree angle, to test the water resistance. The durability of the performance was examined with a peristaltic pump dripping water, from 10 cm above, for 2 h at a constant rate of 1 drop per second. The anti-frosting experiment was conducted in a cabinet which automatically opened and closed its door according to a pre-set program. Two pieces of paper (20 cm × 16 cm) coated with the original and the modified varnish, respectively, were stuck onto the inner wall of the cabinet, which was set to be closed for 300 s and then open for 10 s before closed again, simulating real using conditions. In order to fully frost the paper, the cabinet ran the open-close loop program for 5 h before remaining closed at −15 ◦ C for another 19 h. 3. Results and discussion The fabrication process of the lotus-like printed paper packaging is illustrated in Fig. 1. Hydrophobic nanoparticles (R812S) and silicone polymer (PDMS, BP9400) are homo-genously dispersed in a solvent (D5 silicone oil) to prepare the modifying agent [13], and subsequently the dispersion is emulsified into a waterborne overprint varnish. During a normal printing process, the particles and the PDMS are deposited onto the paper packaging, so modifying the surface to be superhydrophobic. A very important feature of a lotus-like, e.g. superhydrophobic, surface is the micro-/nano-structured morphology. Therefore, the process to fabricate such kind of morphology is of critical importance for preparing the superhydrophobic surface. A widely used
Fig. 1. Schematic illustration of the fabrication process of the lotus-like paper packaging via modifying the overprint varnish with hydrophobic nanoparticles and silicone polymer.
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Fig. 2. FESEM images of the modified varnish comprised of (a) 0 wt%, (b) 10 wt%, (c) 20 wt%, (d) 40 wt%, (e) 70 wt% and (f) 100 wt% modifying agent on the paper packaging.
technical route for this purpose is by depositing a layer of nanosized particles [14,15]. In this work, the surface morphologies of the paper packaging with modified varnish are shown in Fig. 2. We can find more and more nanoparticles on the paper surface,
along with the increasing concentration of the modifying agent. The hydrophobic silica nanoparticles uniformly deposit and cover up the entire paper surface when the proportion of the modifying agent is over 40 wt% (image d). As shown in the high resolution
Fig. 3. ATR spectra of original varnish, modified varnish comprised of 20 wt%, 40 wt%, 70 wt% and 100 wt% modifying agent.
Fig. 4. The logarithm of intensity ratio of peaks 1030 cm−1 and 1730 cm−1 as dependent on the modifying agent content in the varnish.
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Fig. 5. The static water contact angles on the surfaces coated with modified varnish comprised of various proportion of modifying agent. Insets show the shapes of water droplets (5 L) on surfaces of paper packaging coated with modified varnish.
SEM image (inset in Fig. 2d), a large amount of nanoparticles exist on the paper surface, which is a clear contrast to the bald surface of un-modified varnish (inset in Fig. 2a). Besides the visual evidence of SEM images, the surface modification of the paper packaging can also be characterized via the variation of its outmost chemical composition, which is indicated in their ATR spectra (see Fig. 3). The characteristic absorption peaks at 1030 cm−1 correspond to the Si O stretching vibration [16,17], and the peaks at 1730 cm−1 (C O) can be assigned to the carbonyl stretching vibration [18,19], which is assumed to be solely due to the original varnish as the peak disappears when the coating is totally composed of the modifying agent. For this reason, the intensities of the peaks at 1030 cm−1 and 1730 cm−1 can indicate the amount of the modifying agent and the original varnish at the surface of paper packaging, respectively. The greatly enhanced intensities of 1030 cm−1 peaks, and the declining intensities of 1730 cm−1 peaks indicate that the outermost surface is gradually occupied by the modifying agent when increasing its amount in
Fig. 6. Photographs of water droplets (10 L) on the surface of paper packaging coated with (a) original varnish; (b) modified varnish comprised of 40 wt% modifying agent.
the formulation. Moreover, the ratio of the intensities of the peaks at 1030 cm−1 and 1730 cm−1 , I1030 /I1730 , can quantitatively correspond to the content of modifying agent left on the paper surface. Surprisingly, the result displayed an excellent linear (R = 0.977) relationship between the proportion of the modifying agent and log(I1030 /I1730 ), as shown in Fig. 4. Here we should note that the D5 silicone oil in the modifying agent has evaporated away after the
Fig. 7. Photographs of liquid repellency test on the paper surface coated with original varnish (upper row) and modified varnish comprised of 40 wt% modifying agent (lower row); liquid a-water; b-mud slurry; c-ink.
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Fig. 8. Photographs of waterproof durability test (continuously hit by water droplets for 2 h) on the paper surface coated with original varnish and modified varnish comprised of 40 wt% modifying agent.
curing of coating. The Si-containing substance on the modified surface is mostly composed of R812S silica nanoparticles and BP-9400 silicone. Based on the results of SEM and ATR, it is clear that a nanostructured surface comprising hydrophobic silica nanoparticles (R812S) and silicone oil (BP-9400) is successfully fabricated on a piece of paper packaging after coating it with the modified varnish. Such kind of surface has been extensively reported to exhibit a superhydrophobic property [14,20,21]. To measure the wettability of the modified surfaces, their water contact angles (CA) are obtained with a 5 L droplet as the indicator. As shown in Fig. 5, the water contact angles reflect an escalating trend with the increase of the modifying agent. The water contact angles of the paper packaging reach 150◦ or even higher when the component of modifying agent in the varnish is over 30 wt%, clearly indicating superhydrophobic surfaces are obtained as expected. As a comparison, the water contact angles on the smooth paper packaging coated with unmodified varnish and modified varnish with 20 wt% modifying agent are 99◦ and 110◦ , respectively.
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Fig. 9. Photographs of anti-frosting test on the paper surface coated with original varnish and modified varnish comprised of 40 wt% modifying agent.
The macroscopic wettability difference between the original varnish and the modified varnish is also revealed by the photographs shown in Fig. 6. The water droplets sitting on the superhydrophobic paper packaging maintain their round shape, while those on normal packaging spread over the surface with a much larger contact area. In other words, the paper packaging with the modified varnish is difficult to be wetted by water. These photos also demonstrate that the modified varnish has maintained good transparency. The superhydrophobic property has drawn a lot of attentions due to its huge application potential in various fields. One of the most distinct functions of this property is that it allows water roll over quickly, binging in enormous benefits of anti-stain [22] and self-cleaning [12,23]. In order to estimate whether the modified varnish also has those advantages, we dripped three types of aqueous drops onto the paper surfaces on a 20-degree slope. Distilled water traces are observed on the paper surface with original varnish (Fig. 7a1) while water drops roll down entirely without any traces left on the paper surface with modified varnish (Fig. 7a2). Similar phenomena are observed when distilled water is replaced
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behavior of this kind of modified varnish. The apparent viscosities, , of the original varnish and modified varnish are shown in Fig. 10. At low shear rate (<10 s−1 ), the viscosity of the modified varnish is as ten times as that with the original varnish. Actually, the modified varnish presents gel-like appearance at low shear rate, especially in static state. Nevertheless, this varnish shows a dramatic shear thinning effect [28], and at high shear rate its viscosity is nearly identical to that of the original varnish. All these results are in accordance with the reported observations of polymer nanocomposites [29], and the theoretical expectations describing similar shear thinning phenomena of dispersions containing nanomaterials [30]. 4. Conclusion
Fig. 10. Rheological behavior curves of original varnish and modified varnish comprised of 40 wt% modifying agent.
with mud slurry (Fig. 7b1 and b2). Furthermore, the experiments also demonstrate the unique function of inhibiting contamination, as shown in Fig. 7c1 and c2. Deep black ink is left on the paper surface coated with original varnish (Fig. 7c1), and the paper was totally stained. In contrast, traces are barely perceivable on the surface with the modified varnish (Fig. 7c2). The three results clearly indicate that the modified varnish we prepared in this work is able to deliver an extremely water resistant property to the paper packaging. In addition to the transitory water-impermeability as proven above, we have also investigated the durability of the superhydrophobic surface under long-lasting impact of water drops. The test is conducted by continuously dripping water, at 1 drop per second for 2 h, onto the paper surface laid on a 20-degree slope. As shown in Fig. 8, the paper coated with original varnish was wetted, whereas the one with the modified varnish maintains its dryness as well as the surface superhydrophobicity. Both the pieces of paper were weighed before and after the test to quantify the difference of their water adsorption. There was 0.030 g water being taken into the paper with original varnish, while the weight gain of the modified paper is undetectable with the same electronic balance. This experiment demonstrates that the waterproof surface rendered by the modified varnish can endure contacting or even impacting of water for at least 2 h. In some areas, the transportation and storage of frozen products may result in frost formation on their packaging, which may significantly damage the artwork and quality. In those cases, a frostinhibiting packaging is highly desirable. It has been reported that a superhydrophobic surface has an inherent property of delaying and inhibiting frosting process [24–26], implying the modified varnish we prepared may also functionalize the paper packaging as anti-frosting. After staying in a cabinet for 24 h (with controlled occasional door opening and closing to simulate real conditions), the paper surface with original varnish is covered with much more frost than the paper surface with modified varnish (Fig. 9). Liu et al. has discovered that contact angle affects the frost formation process by influencing vapor condensation on the cold surface. Increasing the contact angle of a surface will improve restraining the frost crystal nucleation [26,27]. This mechanism explains why our lotus-like superhydrophobic paper surface, of a contact angle over 150◦ , has a much stronger ability to restrain frost formation than the ordinary surface coated with original varnish. An essential requirement of a practical varnish is that its viscosity has to be suitable in a real printing condition. In order to clarify the industrial perspective, we characterize the rheological
In the present study, we have successfully prepared lotus-like superhydrophobic paper packaging by coating paper with our functionalized varnish modified with R812S silica nanoparticles and PDMS silicone oil. From the SEM images and the ATR spectra, it is revealed that a nano-structured surface morphology has been fabricated on the packaging. Moreover, macroscopically the modification does not bring a perceivable influence on the varnish transparency. With increase of the modifying agent, the prepared varnish shows enhanced capability in preventing the applied paper from being wetted by water. Especially when the component of modifying agent in the varnish is above 30 wt%, the static water contact angle on the packaging surface can reach over 150◦ , which is as super-hydrophobic of a lotus leaf. The applied packaging demonstrates an outstanding performance of inhibiting various aqueous contaminations and enduring water impact for hours. Furthermore, the lotus-like property significantly restrains the frost formation on the packaging surface. The modified varnish we prepared in this study displays a substantial shear thinning effect, which makes it suitable for a variety of printing applications. Such a transparent overprint varnish promises great potential applications on a wide variety of paper/paperboard packaging, with waterproof, easy cleaning, anti-frost, and other valuable functions. Nevertheless, the industrial value of this material also highly depends on its bio-safety, i.e. whether or not the nanoparticles will be disassociated from the varnish, and then inhaled by human. The study on nano-toxicity has been very intensive in recent years, while there is no decisive conclusion yet on the potential risk of the exposure to nano-sized materials [31,32]. The next target of this project is to explore the possibility and investigate the process of nanoparticles leaving the varnish matrix, during the production, transportation, storage and usage periods of the superhydrophobic packaging. With the data, we will then decide whether or how to apply the technology onto a commercialized product. Acknowledgements This work is supported by the Master Student Program of Unilever. And we thank the AMDM (Advanced Measurement and Data-Modeling), Department of Unilever for SEM and ATR measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.apsusc.2012.12.018. References [1] M.J. Kirwan, Paper and paperboard packaging technology, in: M.J. Kirwan (Ed.), Paper and Paperboard – Raw Materials, Processing and Properties, Blackwell Publishing, London, UK, 2005, pp. 1–5.
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