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Cellulose (2018) 25:3595–3607
was not continuous. As a consequence, the initial contact angle was lower (about 45 ), and its decrease in time was faster, reaching a constant value equal to approximately 4 . On the basis of the measurements conducted, it can be stated that the papers with a layer of regenerated cellulose exhibited higher contact angle as compared to the samples not treated with the cellulose solution. Apparently, the observed changes might have been affected by various factors, i.e. different hydrophobicity of regenerated cellulose, surface roughness and capillary structure of the material. Therefore, full explanation is difficult and requires further investigations.
Interesting results were obtained while studying changes in the elongation of the samples of uncoated paper and of paper with regenerated cellulose coating (Fig. 8). The initial elongation of uncoated paper amounted to 1.86%. The application of cellulose coating (without heat treatment) caused an increase in this parameter to 3.5%. Heat treatment caused a further increase in elongation to approximately 5% (after 5 min of heat treatment), and the maximum value was reached after 20 min of heating (approxi- mately 5.8%). Longer heat treatment times caused a decrease in this parameter. It is worth pointing out that the obtained maximum elongation was comparable with values typical of beaten cellulose pulps. An increase in both tensile strength and elongation of paper coated with cellulose indicate the high binding properties of the regenerated cellulose from the solution in NMMO. The relatively long heat treatment time (measured in minutes) required for the elongation increase was the shortcoming of the process. Figure 9 presents the changes in bursting strength of the studied samples of paper. The results indicate that cellulose coating itself is rather weak. The bursting strength of base paper and coated paper without heat treatment was almost the same. The heat treatment applied caused a large and quick increase in bursting strength. The highest value of this parameter was obtained after 10 min of heat treatment, and then it decreased (up to 20 min). After that time, bursting strength remained almost unchanged. These results confirm earlier observations that an increase in paper strength properties results mainly from the interac- tions of the regenerated cellulose with the entire matrix of the fibrous structure of paper. Longer heating times causeed the deeper penetration of a cellulosic solution into a paper structure. As a result, fibers became surrounded by a layer of cellulose, which contributes to an increased bonded area of fibrous network (this effect can be seen in photographs— Fig. 2c, d). The double folds number, the only fatigue test conducted on paper, was the last property studied. This parameter is particularly important for materials exposed to multiple deformations, such as banknote paper, map paper or book cover paper. The number of double folds is frequently identified—not quite cor- rectly—with paper fracture resistance while folded. The results obtained in this research are presented in Fig. 10. It is well known that the number of double
Mechanical properties of paper coated with a cellulose solution
Tensile strength and tear resistance were measured as the key functional parameters of paper materials. The classic method of improving papermaking ability applied on an industrial scale (process of beating) causes an increase in tensile strength and, simultane- ously, a decrease in tear resistance. For this, both of these parameters are often presented together. Fig- ure 7 shows a comparison of changes in tensile strength as a function of tear resistance. A single red rhombus point represents the properties of uncoated paper. On the basis of the obtained results, it is possible to conclude that a continuous cellulose layer causes an increase in the tear resistance of paper (from 7.14 to 8.8 mN m 2 /g), and also a small increase in tensile strength (by approximately 3.6 N m/g). Addi- tional heating operation applied to coated papers resulted not only in penetration of the paper structure by the cellulose solution but also could have caused changes of the paper structure, since the hot NMMO- water system (containing about 16% of water) has an ability to swell or partially dissolve cellulose fibers. The structural changes of the samples resulted in quick and significant increase in tensile strength. Just after 5 min of heating operation, tensile strength increased by more than 40 N m/g in comparison to uncoated, reference paper. The observed increase of the tensile strength correlated with the decreasing value of tear resistance when the heat treatment time of the coating was prolonged. These results were the basis for quantifying the optimum time of coating heat treat- ment as no longer than 10 min—exceeding this time resulted in excessive loss of paper tear resistance.
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