Cellulose (2018) 25:3595–3607
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Conclusion
On the basis of the conducted research, it can be concluded that it is possible to use regenerated cellulose from an N -Methylmorpholine N -oxide solu- tion to coat paper materials. The obtained results showed that regenerated cellulose-based coatings changed the surface, structural and strength charac- teristics of paper. Additional heat treatment enabled the modification of those properties even more signif- icantly. Heat treatment proved to be most effective in terms of strength properties improvement when applied for 5–10 min. Coated papers subjected to that optimal ‘‘thermal conditioning’’ exhibited an increase in tensile index, elongation, bursting strength index and double folds number. It is believed that heat treatment enhanced the penetration of the NMMO cellulose solution into the paper structure resulting in the higher number of contacts between paper fibers and regenerated cellulose chains. Therefore, the bonded area within the fibrous matrix was increased, which reinforced the coated papers. When heat treatment was not applied, penetration of the coating solution into the paper structure was limited and, thus, regenerated cellulose interacted mostly with surface fibers, covering them with a continuous coating film. As a result, additional bonding did not form deep inside the paper fiber network and the internal structure of the material was not mechanically strengthened. Such surface modification, however, contributed to the highest smoothness and hydropho- bicity out of all the examined papers. The measure- ments of the contact angle for continuous regenerated cellulose coating showed an initial contact angle of about 50 . This value, however, decreased quickly and, after approximately 6 s, the contact angle reached the constant value of about 10 , which corresponds to the value of contact angle for micro-crystalline cellulose. In general, the contact angle decreased as heat treatment time increased. It should be mentioned that the necessity of subjecting a coated paper to the process of solvent removal stage (washing out) is a shortcoming of this method, which limits its wet-end applications in the classical process of paper produc- tion. An increase in strength properties, however, and also (as previously reported in the literature—Yoon 2007) the possible use of various substances as additives to such coating solutions, may lead to the development of innovative products. As a result, the
Fig. 10 Effect of heat treatment time on the double folds number of coated papers (the value for base paper is given as a reference)
folds depends mainly on the length of fibers, but also on the degree of their mutual contact area (i.e. bounded area) in the paper structure. It is also for this reason that paper made of unbeaten pulp, quickly breaks while folded. In the discussed case, the reference paper (without coating) demonstrated a number of double folds amounting to 7. Cellulose coating applied on the surface of the paper increased the value of this parameter to 37. In turn, the relatively short heat treatment time applied (5 min) caused an increase in the number of double folds to more than 1500. Longer heating time resulted in a decrease of this parameter to the constant value of approximately 1200. This means that cellulose coating itself is not very resistant to folding. Yet again, these results indicate the significant bonding abilities of regener- ated cellulose. Considering the changes in strength properties of all coated papers it can be concluded that the highest values of these properties were found when coated papers were subjected to heat treatment for about 5–10 min. Longer treatment caused decrease in these properties. This allows the argument that this negative effect could be caused by exceedingly long NMMO- fibrous material interactions at high temperatures, which may have resulted in the excessive dissolution of fiber structural elements (e.g. cellulose, hemicellu- lose) within the paper. Consequently, it could have led to partial damage of the fibrous network and overall loss in material mechanical strength. This effect will be further investigated.
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