3.1. Morphological characterization of the coated papers
SEM surface micrographs show presence of pores in the surface of uncoated paper C60 (Figure 5a and b). Surfaces of uncoated C80 and C120 sack kraft paper showed the similar attributes as C60, nevertheless, the pictures are not presented here. A reduction of pores was observed with the deposition of the coating layers (Figures 5c to h), especially in L2 and L3, in which the coatings led to a smooth and uniform layer over the paper substrate. On the other hand, SEM surface images of L5 (Figures 5g and h) showed a heterogeneous coating, with a porous and rough surface due to the large amount of coating material on the paper surface. Samples of the coated paper L4 are not shown here as the external surfaces are the same shown for L2 (Figures 5c and d).
SEM images of the papers cross section showed that spraying layers of CNFs and CNFs/NC on the paper surfaces increased the thickness and also creating a tortuous pathway for any substance to cross the paper. The uncoated paper C60 showed many micropores throughout the cross section, which was extended to all coated papers (Figure 6). The coated papers designated as L2, L3, L4 and L5 had more compact layers (external or internal) due to the coatings composed by CNFs or CNFs/NC.
3.2 Thickness and basis weight of the papers and films
Thin CNF layers are sufficient to change the surface properties of the coated sack kraft paper, while thicker and uniform layers are required to change barrier properties (Brodin et al. 2014). The CNF and CNF/NC layers were deposited on the sack kraft papers by the spraying method, and the increase of thickness and basis weight were proportional to the number of layers deposited. Each layer of CNFs or CNFs/NC increased the basis weight of the paper by about 10 g m-² - 20 g m-² (Table 1).
The films produced from pure CNFs and CNFs/NC presented similar basis weight than each coating layer deposited on the coated papers (L2, L3, L4, and L5 treatments).
3.3 Tensile strength of the multilayered papers
The coated papers had different values of basis weight, therefore the mechanical results were also divided by the basis weight value of the correspondent treatment (Table 2), in order to normalize the comparison among the properties of the papers. Table 3 presents the mechanical properties of the films produced with CNFs and CNFs/NC.
CNFs usually form a network in nanoscale and have more fibrils per gram of material when compared to fibers in microscale, and consequently can form more inter-fibril bonds (Brodin et al. 2014). All the papers decreased in tensile strength values as compared to C60 (uncoated), when analyzing the normalized values (divided by basis weight). Lavoine et al. (2014) studied the mechanical properties of CNF coated papers by two methods of coating, bar coating and size press. The two methods studied revealed higher tensile strength for uncoated paper than CNF coated papers. The authors associated this behavior to four factors: (1) non-uniformity of coating layers; (2) insufficient coat weight of CNFs to promote improvements on mechanical properties; (3) CNFs did not penetrate into paper structure; and (4) greater amount of water penetrated in the paper structure instead CNFs.
Sack kraft paper (C120 = 120 g m-²) is usually used for production of packages for grains and seeds weighing up to about 60 kg. Then, the present results observed for coated papers were comparable to C120 uncoated paper. There was an increase of around 66% in tensile strength for L2 and around 44% for L3, when compared to C120. The L4 coated paper showed the same average value of tensile strength presented by C120. On the other hand, normalized tensile strength of L5 was around 19% lower than C120. The coating process performed for preparation of L5 treatment subjected the substrate to the highest number of wetting/drying cycles during the coating application. Successive wetting/drying cycles may induce some hornification process to the paper fibers. Swelling occurs in the cell volume when cellulose fiber is placed in suspension with water, and when they are dried, their volume shrinks. However, when these fibers were re-suspended in water, the original volume is not recovered, decreasing the fibers capacity of water retention. Moreover, when water is removed from the fibers, part of cell wall collapse, increasing inter-fibrillar bonding, forming irreversible and partially reversible hydrogen bonds (Ferreira et al. 2017). Besides, the pore volume is reduced modifying the pore size distribution of the paper (Hubbe et al. 2007), which leads to the increase of stiffness of the paper fibers. Cao et al. (1999) compared papers produced from once dried fibers and never dried fibers, concluding that the once dried fibers was less conformable and, consequently the paper made from them resulted in lower strength. The same behavior was observed for the L5 coated paper of the present work.
Furthermore, the high standard deviation obtained for basis weight of the L5 paper suggests a heterogeneous coating that did not increase the tensile strength of the paper. Afra et al. (2016) reported an increase in tensile index for CNF coated papers proportional to the added CNF basis weight. Syverud and Stenius (2009) studied mechanical properties of CNF coated paper and reported 6% of decrease on tensile index with 2 g m-² of CNF addition compared to uncoated paper. However, the authors reported that application of thicker layers (4 and 8 g m-²) promoted 6% and 14% of tensile index increase, respectively. All coated papers showed an increase of Young’s modulus in relation to the uncoated papers. Similar result was reported in Afra et al. (2016), with CNF coating layers on paper sheets, relating that stiffness is proportional to the applied CNF basis weight. On the other hand, Lavoine et al. (2014) observed a decrease in Young’s modulus after CNF application on the paper. The authors observed a decrease of around 20 - 45% in Young’s modulus with the coating composed of 3 - 14 g m-² of CNFs. The values of Young’s modulus were divided by the basis weight to normalize the comparison among treatments. In this work, spraying a layer of 10 g m-² of CNFs (L2) and around 20 g m-² of CNFs/NC (L3) resulted in a great increase in Young’s modulus (56% for L2 and 28% for L3). In other cases, the application of greater basis weight of CNFs/NC (57 g m-² for L5 and 42 g m-² for L4), a considerable decrease in Young’s modulus (30% and 7% for L5 and L4, respectively) was observed, in comparison to C60. The elongation at break for the coated papers reduced after the coatings of around 60%, 50%, 50%, and 40% for L2, L3, L4 and L5, respectively, in relation to uncoated papers. Sack kraft paper is constituted by many micro wrinkles that confers extensibility properties to the paper. In contrast, CNFs and CNFs/NC form a layer that restrict the extensibility and decrease elongation of the paper. Agglomerated NC layers are responsible for stress concentrations in the paper and decreases the elongation at break (Gabr et al. 2013). Figures 7 and 8 show the evolution of stress-elongation of the samples during the tensile test. Stress x specific elongation (mm/mm) of uncoated papers C60, C80 and C120 showed maximum stress of around 35 MPa.
L4 and L5 coated papers presented curves with a second mechanical performance due to the presence of the deposited layers. During the tensile loading, the break of different layers do not occur at the same time. The layers composed by CNFs and CNFs/NC, which are stiffer, are responsible for the initial mechanical performance (first rupture), and then the paper substrate is finally mechanically broken (Figure 9).
3.3 Contact angle of the papers and films
Measuring the contact angle of a water drop on the paper substrate surface gives an insight into the hydrophobicity of the surface, which can be defined as the tendency of a water drop not to spread on the substrate. CNF coating layers induced the decrease of the contact angle (87°) for L2 coated papers, in comparison to uncoated paper C60 (111°). However, this difference was not observed for the L3 treatment that was coated with CNFs and CNFs/NC (110°) (Figure 10). Greater basis weight did not indicate lower hydrophobicity for the papers C80 and C120.
The film produced from CNFs/NC showed greater contact angle in relation to the CNF film, confirming that CNF/NC coating seems to be more effective for decreasing hydrophobicity. The hydrophilic nature of CNFs, especially after the cell wall fibrillation process is a determining factor in its application as paper coating. NC are formed by layers of tetrahedral silica sheets and alumina octahedral sheets containing Ca2+ or Na+ that have the capacity to adsorb water (Paiva et al. 2008; Silva and Ferreira, 2008), whereas when adequately dispersed into the polymeric matrices, the nanoclays presents good barrier properties (Gabr et al. 2013; Gaikwad and Seonghyuk 2015). It was observed that the L5 coated paper resulted in different contact angles, when evaluating the internal and external faces of the coated paper, even if the layer composition is the same on both sides. This may have occurred due to some heterogeneity of the layers deposition on the paper surface, with regions with excessive deposition of coating, and others with insufficient coating. The surface roughness of the different deposited layers may also have some influence on the hydrophobicity. Although L3 and L5 have a rougher surface compared to L2, they tended to have greater contact angles and this was probably due to the presence of NC, which sometimes caused agglomerations and surface granularity. Low values of contact angles (<90°) indicate that the material have lower hydrophobicity, while high contact angles (>90o) indicate low hydrophobicity (Yuan and Lee 2013).
3.4 Water vapor transmission rate of the films and papers
Films produced with CNFs/NC showed less vapor transmission compared to films produced with only CNFs (Figure 11). The addition of NC to the composition of the CNF films resulted in reduction of around 11% on the WVTR evaluated in the study. Generally, the incorporation of NC into CNFs is expected to retard the degradation process, due to lower moisture present due to the formation of silicate layers as a protective barrier, and/or via inducing obstacles that could delay the passage of water vapor (Leszczyńska et al. 2007). Literature data shows WVTR values ranging between 234 – 332 g m-² day-1 for films produced with CNFs/NC (Rodionova et al. 2011; Lu et al. 2015).
When successfully dispersed in polymeric matrix, NC can hinder the diffusion trough the matrix due the tortoise path created by exfoliated plates that could improve barrier properties (Gaikwad and Seonghyuk 2015). The NC grains adsorb water, which expand and form barrier layers against vapor transmission (Vartiainen et al. 2010). As previously discussed, CNFs are hydrophilic materials with high aspect ratio and strong capacity to form close networks (Dufresne 2013) that may promote the decrease of WVTR. CNF/NC coating layers led to lower WVTR than pure CNF coating layers. SEM micrographs (Figure 5a to h) demonstrated that the CNF and CNF/NC layers reduced the porosity of the paper substrate, contributing to WVTR reduction. Reduction of WVTR was observed for all multilayered papers when compared to uncoated samples (C60, C80 and C120). Analyzing the uncoated papers with different basis weight, it is noted that greater basis weights did not result in lower WVTR, indicating that the real influence on this property is exerted by the composition of the paper/film and coating agent. The coating with 10 g m-² of CNFs on 60 g m-² sack kraft paper (L2 coated paper) promoted a decrease of around 4% in WVTR in relation to C60, leading to a homogeneous less porous surface that may be further coated with other hydrophobic and barrier agents in the future.
L3, L4 and L5 presented lower WVTR compared to L2 and uncoated papers (C60, C80 and C120). In this case, the greater number of layers deposited on the paper surface may have created more obstacles against the vapor passage. Results showed that L3 coated paper led to a reduction of around 17% for WVTR in relation to the 60 g m-² uncoated paper (C60). Additionally, L3 led to the greater contact angle among all the coated papers (see Fig. 15), which means a better combination of layers for this treatment composition, including for mechanical properties. The coated papers L4 and L5 presented a reduction of around 14% for WVTR in comparison to the reference uncoated paper (C60).