Abstract
Recently, enhancing the performance of polyurethane (PU) coatings with cellulose nanomaterials (CNM) has been actively researched. Cellulose nanomaterials exhibit considerable potential to increase the mechanical strength of PU coatings due to their high aspect ratios and elastic moduli. In this study, PU reinforced with CNM was coated onto paper to enhance the paper’s mechanical strength and soiling resistance. To investigate the reinforcing effect, two different CNM, cellulose nanocrystals (CNC) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TOCN), were selected, and suspensions with different ratios of PU and CNM were prepared. After coating the paper with each of them, the mechanical properties of the paper, including tensile strength, folding endurance, and soiling resistance, were evaluated. The mechanical strength and anti-soiling performance of the PU-CNM coated papers were greatly enhanced. Especially, PU-TOCN had superior properties as a durable paper coating despite a low TOCN concentration, less than 2%, because the TOCN crosslinked with PU via polyaziridine. Furthermore, the PU-CNM coating protected the paper from being contaminated, which was confirmed by scanning electron microscopy and energy dispersive X-ray mapping. Consequently, durable paper exhibiting soiling resistance was fabricated by coating the paper with PU-TOCN suspensions.
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Mechanical Properties and Soiling Resistance of Paper with Polyurethane Coating Reinforced with Cellulose Nanomaterials
Sooim Goo,a Simyub Yook,a Shin Young Park,a Wanhee Im,a and Hye Jung Youn a,b,*
Recently, enhancing the performance of polyurethane (PU) coatings with cellulose nanomaterials (CNM) has been actively researched. Cellulose nanomaterials exhibit considerable potential to increase the mechanical strength of PU coatings due to their high aspect ratios and elastic moduli. In this study, PU reinforced with CNM was coated onto paper to enhance the paper’s mechanical strength and soiling resistance. To investigate the reinforcing effect, two different CNM, cellulose nanocrystals (CNC) and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TOCN), were selected, and suspensions with different ratios of PU and CNM were prepared. After coating the paper with each of them, the mechanical properties of the paper, including tensile strength, folding endurance, and soiling resistance, were evaluated. The mechanical strength and anti-soiling performance of the PU-CNM coated papers were greatly enhanced. Especially, PU-TOCN had superior properties as a durable paper coating despite a low TOCN concentration, less than 2%, because the TOCN crosslinked with PU via polyaziridine. Furthermore, the PU-CNM coating protected the paper from being contaminated, which was confirmed by scanning electron microscopy and energy dispersive X-ray mapping. Consequently, durable paper exhibiting soiling resistance was fabricated by coating the paper with PU-TOCN suspensions.
Keywords: Cellulose nanomaterials; Durable paper; Mechanical strength; Polyurethane coating; Soiling resistance
Contact information: a: Department of Forest Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea; b: Research Institute of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea;
* Corresponding author: page94@snu.ac.kr
INTRODUCTION
Polyurethanes (PU) have been used in various applications, such as adhesives, coatings, printing, and medicine, because they have versatile properties owing to the ratio of their hard and soft segments (Osman et al. 2003; Chattopadhyay and Raju 2007; Rahimi and Mashak 2013; Hung et al. 2014; Cataldi et al. 2017). Coatings are an important application because PU exhibit excellent mechanical strength and toughness; good chemical, abrasion, and corrosion resistance; and low-temperature flexibility (Akindoyo et al. 2016). Furthermore, PU coating has been used to ensure that fibrous materials are waterproof (Meng et al. 2009; Cataldi et al. 2019; Mazzon et al. 2019).
Recently, waterborne polyurethane (WPU) dispersions, which have low volatile organic compound emissions, have received attention as a type of PU coating, because awareness for the necessity of environmental protection is greater than ever (Fan et al. 2015; Zhou et al. 2015). Additionally, WPU exhibits desirable physical and chemical properties, such as low viscosity, which is helpful in the coating processes (Noble 1997). However, the performance of WPU coatings must be enhanced to obtain a similar mechanical strength to solvent-borne PU coatings (Cheng et al. 2016). UV-curable WPU coating is a method designed to improve the performance of the WPU coating. The WPU coatings cured under UV irradiation exhibit considerable mechanical strength, chemical resistance, and non-toxicity (Çanak and Serhatlı 2013; Noreen et al. 2016), even though they are less resistant to solvent and heat (Chuang et al. 2008). Hyper-branched polyurethane (HBPU) that contains a number of functional groups improved the thermal and physical performances of the coating because of its high crosslinking density (Florian et al. 2010). Several researchers have ensured the desirable properties of HPBU coating by controlling the ratio of isocyanate groups to hydroxyl groups (Petrović et al. 2002; Mishra et al. 2012). Recently, research has been active in reinforcing WPU with nano-sized fillers such as silica, ZnO, TiO2, Fe2O3, clay, and cellulose nanomaterials. These nanomaterials can affect not only the mechanical but also the rheological, anti-corrosion, and barrier properties (Rahman et al. 2008; Saha et al. 2011; Rashvand and Ranjbar 2013; Noreen et al. 2016).
Among several nanomaterials, cellulose nanomaterials (CNM), including cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF), are expected to have great potential as fillers due to their morphology, mechanical strength, non-toxicity, biodegradability, and good miscibility with hydrophilic polymer matrices (Abdul Khalil et al. 2012; Lei et al. 2019). Each CNM exhibits unique properties in terms of charge density and morphology such as the average width and aspect ratio. When compared with CNC, CNF has a higher aspect ratio, i.e., a higher ratio of length to width. With respect to the composite of PU and CNM, it has been reported that CNC with low loading levels showed good dispersion in a WPU matrix and significantly enhanced mechanical strength (Gao et al. 2012; Zhang et al. 2012). Another study showed that CNC enhanced the stiffness and thermal stability of the nanocomposites by interacting with both the soft and the hard segments of PU to form hydrogen bonds (Rueda et al. 2013). Furthermore, 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TOCN) have been investigated to improve the performance of PU-CNM composites. Compared with CNF, TOCN have a high negative charge density and high aspect ratio, which are critical for enhancing the mechanical properties of the PU coating (Cheng et al. 2016).
PU coating can be applied in the preparation of paper products that require resistance to abrasion and contamination. In addition, PU enhances the mechanical properties of paper including tensile strength and folding endurance (Guo et al. 2014). Some paper grades such as banknotes especially need superior physical properties and durability. These papers should not only have high mechanical strength but also be resistant to contaminants. Little research has been done on PU reinforced with CNM. Most studies on PU coatings have focused on abrasion resistance only (Zhang et al. 2012; Wang et al. 2015; Fan et al. 2016), and no attention has been devoted to soiling resistance (i.e., anti-contamination). In this study, PU was reinforced with CNM and used as a coating on paper. The effects of the CNM on the PU coating in terms of soiling resistance and mechanical strength were studied. To research how different types of CNM affect the reinforcing performance, CNC and TOCN were selected as CNM. Additionally, the soiling mechanism of the coated paper was studied to investigate how the PU-CNM coating protected the paper from contamination.
EXPERIMENTAL
Materials
A PU suspension (35 wt%) was supplied from T&L Co., Ltd. (Yongin, Korea). Polyurethane was synthesized with polycarbonate diols. Polyaziridine, as a crosslinking agent, was purchased from Stahl (Waalwijk, Netherlands). Two types of CNM, CNC, and TOCN, were used. Figure 1 shows the transmission electron microscope (TEM, Carl Zeiss, LIBRA 120, Oberkochen, Germany) image of the CNC and the scanning electron microscope (SEM, Carl Zeiss, Germany) image of the TOCN. CNC was purchased from CelluForce (Montreal, Canada) as a powder; the solid content was 94.5 wt%. The average width and length of the CNC were 6.3 nm and 73.9 nm (the standard deviations were 1.9 nm and 25.6 nm), respectively. The zeta potential of the CNC was -58.5 mV. The CNC were dispersed in deionized water to a concentration of 2 wt% before use. The TOCN was provided by Hansol (Seoul, Korea); the fibril width was 14 nm ± 4 nm. The solid content and zeta potential of the TOCN suspension were 0.95% and -56 mV, respectively. The average width and length of CNM were evaluated by measuring the length and width of several hundred nanofibers from the TEM or SEM images. The zeta potentials of the CNC and TOCN (0.1 wt% suspension) were measured using a Zetasizer (Nano Zs, Malvern Instruments, Ltd. UK) at 25°C. The base paper was made of cotton linter, supplied by KOMSCO (Daejeon, Korea). The properties of the base paper are presented in Table 1.
Table 1. Properties of the Base Paper
Fig. 1. The (a) TEM image of CNC and (b) SEM image of TOCN
Preparation of PU-CNM Suspensions
The PU suspension was mixed with the CNC and TOCN based on wet weight ratios. The ratio of the mixtures (PU:CNM) was varied as 100:0, 80:20, and 60:40. The mixed PU-CNM suspension was stirred at 600 rpm for 30 min. Polyaziridine was diluted to 50% and then added to the PU-CNM suspension at 4 wt%. The PU-CNM suspension with polyaziridine was stirred at 600 rpm for 1 h. The suspension was deaerated under vacuum before it was applied as a coating.
Table 2. Mixing Ratios of PU and CNM
Coating the PU-CNM Suspension on the Paper
The PU-CNM suspensions were coated on the base paper using a laboratory bar coater (Automatic Film Applicator, TQC Sheen, Capelle aan den IJssel, Netherlands). The coating speed was 70 mm/s, and the wire thickness was 4 µm. The coated paper was dried in a hot-air dryer at 120 °C for 2.5 min, after which it was passed through a drum dryer at 100 °C for curing. The coating on the back side of the paper was prepared the same way. The coated papers were conditioned at 23 °C ± 1 °C and 50% ± 2% relative humidity for 1 day.
Characterization
Properties of the suspension
To evaluate the dispersion stability of the mixed PU-CNM suspension, the change in light transmittance along the height of a vial was measured for 12 h using a Turbiscan LAB stability analyzer (Formulaction, Toulouse, France). Low-shear viscosity was measured by a Brookfield DV2TLV viscometer (Brookfield Engineering Laboratories, Inc., Middleboro, MA, USA) using the no. 4 spindle at 100 rpm for 30 s.
Mechanical strength
The tensile strength of the coated paper was measured in the machine direction according to ISO 1924-2:2 (2008). The folding endurance was measured in the cross-machine direction with ISO 5626 (1993) (MIT test for folding endurance), because banknotes require high folding endurance in the cross-machine direction.
Soiling resistance
Wet and dry soiling resistance tests were conducted. To evaluate the soiling resistance under the wet condition, the coated paper samples were exposed to contaminants composed of ceramic beads, colored powder, artificial sweat, and lanolin. The specimens were fixed with a wooden holder and shaken with the contaminants in a bottle for 10 min, 15 min, and 20 min. The color difference (∆E) after soiling, compared with that before soiling, was measured to determine the anti-soiling performance of the paper. Another test was conducted under the dry condition, and the contaminants consisted of 0.4 mL of oil, 0.3 g of clay, and 0.4 mL of ethanol. The contaminants and glass beads were mixed together in a tubular shaker, and then paper specimens were put into the shaker and mixed with the contaminants for 30 min. The specimens were then removed and wiped with a wet towel followed by a dry towel. Anti-soiling performance under the dry condition was measured by ∆E and ISO brightness. The CIELAB color and ISO brightness were measured using an Elrepho spectrophotometer (AB Lorentzen & Wettre, Stockholm, Sweden).
SEM and energy dispersive spectrometer (EDS) analysis
The surface of the paper before and after the soiling test was observed by SEM. The specimens were air dried and sputter coated with platinum. The SEM images of the specimens were obtained by a SUPRA 55-VP. The presence of the PU-CNM coating and dirt on the paper specimen was confirmed by detecting nitrogen, aluminum, and silicon by EDS (XFlash Detector 4010, Bruker, USA) with a working distance of 8.5 mm and an accelerating voltage of 5 kV.
RESULTS AND DISCUSSION
Characteristics of PU-CNM Suspension
The dispersion stability of the PU-CNM suspension was quantitatively evaluated over time. As the first transmission (T) was greater than 0.2%, turbidity was evaluated based on the transmission values. Figure 2 shows the change in the transmission of PU8C2, PU6C4 PU8T2, and PU6T4 suspensions over 12 h. For PU8C2 and PU6C4, there was a very small variation, less than 5%, of the transmission rate over time. Therefore, it was confirmed that the CNC were well dispersed and maintained stability in the PU suspension.
Fig. 2. Profile of T (%) over time: (a) PU8C2 suspension, (b) PU6C4 suspension, (c) PU8T2 suspension, and (d) PU6T4 suspension
In contrast, the transmission rate at the bottom of the PU-TOCN suspension vial quickly changed after 7 h, indicating that phase separation between the TOCN and the PU occurred. Even though phase separation was observed, it is expected that the PU-TOCN suspension is sufficiently stable to apply as a coating because no aggregation was visible within 7 h.
Figure 3 presents the low-shear viscosity measurements of the suspensions depending on the mixing ratio of CNM. The viscosity of the PU-TOCN increased exponentially with the addition of TOCN, whereas the viscosity of the PU-CNC was not affected by the addition of CNC. This was likely the result of TOCN’s much greater viscosity compared to that of the CNC. The viscosity of the CNC (2%) and TOCN (0.95%) were 52 cPs and 1174 cPs, respectively. Despite TOCN’s high viscosity, suspensions with the selected mixing ratios had relatively low viscosities, sufficient for application in paper coatings. The PU8T2 and PU6T4 suspensions showed viscosities of 114 cPs and 292 cPs, respectively, which were similar with those of the PU8C2 and PU6C4 suspensions (134 cPs and 152 cPs).
Fig. 3. Low-shear viscosities of the PU-CNM suspensions depending on the mixing ratio of CNM
Mechanical Strength of the Coated Paper
Figure 4 shows the coat weight of the PU-CNM on papers depending on different mixing ratios. The coat weight decreased when increasing the mixing ratio of CNC and TOCN, which resulted from the decrease of the solid content of the suspension.
The mechanical strength of the paper as coated with PU-CNC and PU-TOCN is presented in Fig. 5. The tensile strength of the base paper was enhanced greatly by coating it with all mixing conditions of the PU-CNM suspensions (Fig. 5(a)) because PU coating positively affected on mechanical strength (García-Pacios et al. 2011). In particular, paper coated with the PU-TOCN suspensions exhibited the greatest tensile strength even though the dry weight ratio of the TOCN based on PU was less than 2% in both mixing ratios. The tensile strength of the PU-TOCN coated papers was enhanced by increasing the mixing ratio of the TOCN. In contrast, the PU-CNC coated papers showed a slight decrease in the tensile strength with the addition of CNC. There are two reasons for these results. The first reason is based on the characteristics of TOCN, which has a greater aspect ratio than that of CNC (Isogai et al. 2011). Therefore, TOCN might have a greater impact on enhancing tensile strength. This can also be confirmed by the fact that the PU6T4-coated paper had greater tensile strength than that of the PU8T2-coated paper. The second reason is likely due to crosslinking between the PU and CNM via polyaziridine.
Fig. 4. Coat weight of the coated papers depending on the mixing ratio of the CNM. The error bar represents the standard deviation.
Fig. 5. The (a) tensile strength, (b) strain at break, and (c) folding endurance of the coated paper depending on the mixing conditions of the PU-CNM suspensions. The error bar represents the standard deviation.
Polyaziridine is known for reacting with the carboxyl groups present in PU at ambient temperature to crosslink the PU network (Ollé et al. 2008; Xia and Larock 2011), as shown in Fig. 6. The carboxyl groups of PU make aziridine rings open to form ester groups. Therefore, TOCN containing carboxylic acid could be expected to bond with carboxylic acid of PU strongly via aziridine. In contrast, CNC would be less crosslinked with PU because CNC has mainly sulfonate functional groups. Thus, the decrease in tensile strength with increasing mixing ratio of CNC can be explained by weaker crosslinking via aziridine as well as decreased coat weight.
Fig. 6. Crosslinking reaction scheme of PU via polyaziridine (Ollé et al. 2008)
Both coatings, PU-TOCN and PU-CNC, resulted in a significant improvement in the folding endurance compared with that of the base paper. This is similar to the result obtained in a previous study that reported that PU considerably improved the folding endurance of paper (Guo et al. 2014). Folding endurance can be affected by flexibility and crosslinking density of the coating layer (Zhu et al. 2016). By the formation of flexible PU-CNM coating layers on the paper, folding endurance could be significantly enhanced. Unlike tensile strength, the PU-CNC coated papers showed a similar level of folding endurance to the PU-TOCN coated papers, despite PU-CNC’s weaker bonding force. This result might be caused by PU-CNC showing higher strain than PU-TOCN (Fig. 5(b)), which indicates that folding endurance is the result of the synergistic effect between flexibility and crosslinking between the PU and the CNM.
Anti-soiling Performance of Coated Paper
Anti-soiling tests were performed under wet and dry soiling conditions. For PU-CNC, the wet soiling test was only conducted with a mixing ratio of 80:20, which exhibited better mechanical strength. Figure 7 shows photographs of the contaminated papers after the wet soiling test. The PU-CNM coated papers appeared brighter than the base paper. To evaluate the anti-soiling performance quantitatively, the color difference (∆E) from before and after the wet soiling test was ascertained (Fig. 8). There was a notable reduction in ∆E for all the coating conditions, including a 50% reduction when compared with the base paper for the 10 min test. The remarkable reduction of ∆E in the PU-CNM coated papers was because the PU used in this study was synthesized by polycarbonate diols. Such PU is known for great mechanical strength and weathering resistance because the polycarbonates are characterized by strong carbonate bonds (García-Pacios et al. 2011). The PU-CNC coated paper had a lower ∆E value in the 10 min test than the PU-TOCN coated papers; however, the PU-TOCN coated papers showed lower values in tests longer than 10 min. This means the PU-TOCN coating is better resistance to long-term contamination.
In the dry soiling tests, the ∆E values of the PU8T2 and PU6T4 coated papers were reduced by 27% and 23%, respectively, as compared with the base paper (Fig. 9). They were also lower than that of the PU-CNC coated paper. Furthermore, the PU-TOCN coated papers had greater ISO brightness values than the PU-CNC coated paper after contamination. This result also confirms that crosslinking between the PU and TOCN might improve soiling resistance. Comparing PU-only coated papers and PU-TOCN coated papers also revealed a greater dry soiling resistance in the PU-TOCN coated papers. Therefore, it can be concluded that adding a small amount of TOCN enhanced the dry soiling resistance performance of the PU coating.
Fig. 7. Photographs of the coated paper after the wet soiling test depending on the PU-CNM mixing conditions and soiling test duration; bp – base paper
Fig. 8. Color difference (∆E) depending on the duration of the wet soiling test; bp – base paper
Fig. 9. The (a) color difference (∆E) and (b) brightness values of the coated papers with different PU-CNM suspensions in the dry soiling test; bp – base paper
SEM and EDS Analysis
Figure 10 shows the SEM images of the paper coated with the PU8C2 suspension before and after the wet soiling test for 15 min. Before the wet soiling test, the PU-CNM coating layer completely covered the surface of the paper to form a smooth surface. However, after the soiling test, the fibers were exposed again because the PU-CNM coating layer peeled off due to physical impact during the soiling test. This behavior was also observed in the PU and PU-TOCN coated papers.
Fig. 10. SEM images of PU8C2-coated paper (a) before and (b) after the wet soiling test
Fig. 11. The (a) SEM image and (b)-(d) EDS mapping analysis of the surface of the PU6T4-coated paper after the wet soiling test: (b) Na-mapping, (c) Al-mapping, and (d) Si-mapping
The EDS analysis of the PU6T4-coated paper after the wet soiling test for 15 min is shown in Fig. 11. The EDS mapping was performed to observe the distribution of contaminants and the PU-CNM coating. A clear boundary can be observed between a part of the coating that peeled off and a part that remained from the centers of the images. The distribution of nitrogen was different around this line; on the right side, less nitrogen was present, showing that the fibers were revealed after the PU-CNM coating peeled off. Furthermore, Figs. 11(c) and 11(d) show the distributions of aluminum and silicon derived from vermiculite, that were part of the colored powder of the contaminants used in the wet soiling test. They were more concentrated on the exposed fibers found on the right sides of the images. The presence of aluminum and silicon was clearly divided at the boundary, indicating that the PU-CNM coating protected the paper from being worn out and contaminated. Accordingly, the key factor behind the anti-soiling property of the coated paper was the formation and adhesion of the PU-CNM coating layer on the surface. The papers coated with PU-TOCN exhibited stronger bonding between the PU and TOCN, resulting in good adhesion on the surface and the best anti-soiling effect.
CONCLUSIONS
- When mixing the polyurethane (PU) and cellulose nanomaterial (CNM), the viscosity of PU-TOCN increased with the addition of TEMPO-oxidized cellulose nanomaterial (TOCN), whereas the viscosity of PU-CNC was not affected by the incorporation of cellulose nanocrystals (CNC). Evaluating the miscibility between the PU and CNM, the PU-TOCN suspension was less stable than the PU-CNC suspension. However, no negative effects were found when applying the PU-TOCN suspension in coatings within 7 h.
- The PU-CNM coated papers exhibited significantly greater mechanical strength and improved soiling resistance in comparison to the base paper. In particular, the PU-TOCN coated papers exhibited greater mechanical strength and soiling resistance than those exhibited by the PU-CNC coated papers. This result was not only because TOCN has a greater aspect ratio than CNC but also because crosslinking via polyaziridine between the PU and TOCN was more active than with the CNC, resulting in stronger inter-bonding.
- The scanning electron micrograph (SEM) images and energy dispersive spectrometer (EDS) mapping analysis confirmed that fibers were exposed after the PU-CNM coating layer was removed in the soiling tests. Aluminum and silicon, which were components of contaminants, were adsorbed onto the exposed fibers. Therefore, the formation and adhesion of the PU-CNM coating layer was critical in fabricating durable, anti-soiling paper.
ACKNOWLEDGMENTS
This work was supported by the Technology Innovation Program (No. 10067241, Development of application technology of nanocellulose for manufacture of specialty paper with high security and durability) and was funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). The authors express their appreciation to T&L Co., Stahl, Hansol, and KOMSCO for providing the materials used in this study.
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Article submitted: July 18, 2019; Peer review completed: September 3, 2019; Revised version received and accepted: September 23, 2019; Published: September 25, 2019.
DOI: 10.15376/biores.14.4.8973-8986