Cellulose (2018) 25:3595–3607
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‘0’ means that no heat treatment was applied (sample NH), and that the solvent was removed immediately after the coating was applied. A single red rhombus point represents the value of the discussed parameter for the reference uncoated paper. The reference papers were not heated either. The data in Fig. 4 indicate that the apparent density of the studied samples of paper with a cellulose coating was higher than the apparent density of the base paper itself. In the case of the constant thickness of a coating and, thus, the same amount of deposited cellulose, it was expected that the average apparent density of the coated paper would be similar, regardless of the post-treatment method. It was revealed, however, that apparent density was increasing when heat treatment duration was prolonged. These results could suggest that heat treatment of cellulose coating modifies the internal structure of the entire material. This is evidenced as well by the results of the measurement of air permeance (Table 1 and images presented in Fig. 5). A close view of the heat- treated samples reveals the lack of continuous layer on the surface. Moreover, the fiber network is tightly surrounded by the regenerated cellulose. The lack of fibrils suggests that the small particles (e.g. fibrils, fines) could have been partially dissolved and absorbed by the cellulose-NMMO solution. This could result in higher permeance of the whole structure. The range of changes of apparent density depended on the method of sample post-treatment. The most significant changes were already observed for short heat treatment times. After 5 min of heat treatment, the apparent density of the tested sample increased by approximately 0.5 g/cm 3 , whereas change by barely 0.006 g/cm 3 in apparent density was observed when heating time was increased from 5 to 40 min. Evidently, the major changes in the structure of the paper occur during the first 5–10 min of heating. Further data analysis demonstrated that the change of apparent density was caused by changes in the thickness of the samples. This implies that the increase of apparent density results from the increased shrink- age of the whole structure. A similar observation can be found in the literature (Ferreira et al. 2015). The hydrophobic properties of applied coatings were tested for four cases: reference paper without a coating, paper with a continuous coating (NH), and papers with a coating heated for 10 and 40 min before solvent removal. It is commonly accepted that native
cellulosic fibers are mainly hydrophilic, and a paper structure is of a capillary-porous nature. The contact angle for such materials is usually equal to 0 (i.e. water immediately penetrates the paper structure if no hydrophobic additive has been used). In the case of the applied cellulose solution in NMMO, cellulose dissolution and regeneration cause the reorganisation of the intra- and inter-molecular hydrogen bonds. Other scientists (O’Sullivan 1997; Zimmermann et al. 2016) determined that conversion from cellulose I to cellulose II occurs during the cellulose regeneration process. Furthermore, Biganska and Navard (2009) found also that the regeneration process may influence the final properties of cellulose. Cellulose regenerated in a water bath after the crystallisation of solutions exhib- ited a uniform and compact structure. Conversely, regeneration from the molten solution resulted in a porous structure surrounded by dense ‘‘skin’’. Depend- ing on the final organisation of the structure, part of the –OH groups may not be available for interactions with liquid located at a cellulosic surface (Hubbe et al. 2015). Yamane et al. (2006) proved that regenerated cellulose can exhibit increased hydrophobic proper- ties. The results of measurements presented in Fig. 6 demonstrated that paper without a cellulosic coating had a contact angle equal to 0. It was already noticed that, during the solvent removal process, the presence of cellulose coating increased the samples’ resistance to water. A sample of paper without a coating, immersed in a water bath, quickly disintegrated into individual fibers. The coated samples easily survived a 2-min immersion in water, preserving their structure. This proved that the cellu- lose introduced into the paper bonded the fibers and made the structure more hydrophobic. Measurements of the contact angle for a continuous regenerated cellulose coating showed an increase in the initial contact angle to more than 50 . This effect, however, disappeared quickly and after approximately 6 s, the contact angle reached the constant value of circa 10 . The final value corresponds to the value of contact angle for micro-crystalline cellulose found in works by Yamane et al. (2006). For the sample of paper coated with cellulose and heated for 10 min, the initial contact angle was about 50 as well but it decreased more rapidly to the same value as that for continuous regenerated cellulose coating. For longer times of heat treatment (40 min), the layer of cellulose
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