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Cellulose (2018) 25:3595–3607
coating, a continuous coating layer is obtained (Fig. 2b). When the heat treatment was applied, the contin- uous layer disappeared (Fig. 2c, d). This effect suggests that the cellulose solution penetrated the structure of the material when the samples were heated. As a result, not only the surface but also the structure of paper was modified. Based on the property measurements and SEM images it could be concluded that, as heat treatment time was extended, the degree of penetration of the paper structure by the cellulose solution increased. Therefore, modification not only of the paper surface, but also of the entire structure of the paper was possible. The results presented below show to what extent the modifications influence the most important properties of paper. The surface roughness values are shown in Fig. 3. The lowest roughness level was found for paper with a continuous regenerated cellulose layer (NH—without further heating). When heat treatment was applied, roughness increased quickly up to the value comparable with that for uncoated paper. For the longer heat treatment times (over 20 min), a certain decline in the roughness of the surface was observed. This decrease might be explained by possible partial swelling and/or
The most important structural and mechanical param- eters of all paper samples were determined. The results are presented in Table 1, and also shown in Figs. 2, 3, 4, 5, 6, 7, 8 and 9. The basic statistical analysis is presented as a coefficient of variation (given in Table 1) and as error bars, based on standard deviation values (given in figures).
Structural and surface properties
The microscopic observations of the surface of paper samples constituted the first stage of the research. Figure 2 presents: (a) surface of reference paper without coating, (b) surface coated with cellulose dissolved in NMMO, where solvent was removed immediately after a coating operation, (c) surface with coating, where solvent was removed after 10 min of heat treatment of the coated paper, (d) coated paper surface, where solvent was removed after 40 min of heat treatment. The presented pictures show that, depending on the applied methodology of modifying a surface by means of the same kind of cellulose solution, it is possible to obtain different coating structures. If the solvent is removed immediately after
Table 1 Structural and mechanical properties of investigated papers (COV, %—coefficient of Variation is given inside the parentheses)
Bursting strength index (kPa m 2 /g)
Surface roughness (ml/min)
Tensile index (Nm/g)
Elongation (%)
Double folds number
Tear resistance index (mNm 2 /g)
Air permeance (ml/min)
Heat treatment time (min)
Apparent density (g/cm 3 )
0
0.555 (2.66%) 0.593 (1.03%) 0.594 (1.09%) 0.595 (1.72%) 0.599 (2.41%) 0.617 (2.59%) 0.521 (3.13%)
289 (9.9%) 605 (9.2%) 623 (9.1%) 634 (8.6%) 587 (8.5%) 563 (8.7%) 611 (5.4%)
23.81 (3.21%) 61.28 (6.73%) 61.09 (5.09%) 59.77 (4.17%) 64.47 (3.91%) 58.93 (4.21%) 20.21 (3.52%)
3.50 (8.72%) 4.96 (9.82%) 5.48 (3.12%) 5.76 (8.83%) 5.48 (9.03%) 4.57 (8.12%) 1.86 (8.33%)
37 (7.3%) 1527 (8.1%) 1228 (9.0%) 1176 (4.4%) 1248 (8.1%) 1251 (6.4%) 7 (7.31%)
8.80 (4.58%) 5.94 (5.93%) 5.80 (4.32%) 3.34 (5.0%) 2.07 (3.68%) 2.05 (4.12%) 7.14 (6.18%)
0.80 (5.32%) 4.45 (6.41%) 6.22 (6.29%) 4.65 (4.90%) 4.83 (5.19%) 5.28 (6.28%) 0.79 (4.78%)
181 (9.3%) 4900 (4.6%) 4980 (2.9%) [ 5000 (3.3%) [ 5000 (2.7%) [ 5000 (3.1%) 4500 (10.4%)
5
10
20
40
60
Uncoated
(reference) paper
123
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