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coating solutions used here, cellulose content of 3% was selected as the most effective from a practical point of view. The cellulose solutions were obtained in a high efficiency laboratory-scale IKAVISC kneader type MKD 0.6-H60. The mixture of 50/50 water and NMMO with an addition of 3% of cellulosic material (based on NMMO weight) was heated at a temperature of 95 C and under low pressure (of about 6.7 kPa), and the excess of water was removed from the system. The process was continued until a homogeneous and transparent cellulose solution in NMMO was obtained.
adhesion to paper fibers. Afterwards, NMMO was washed out from the paper with water. The NMMO cellulose solution, containing dispersed kaolinite, was applied in research conducted by Se Young Yoon (2007). Such a system was used to form paper-based composite materials of improved strength properties. In the literature, one may find information into the properties of membranes, made of cellulose dissolved in NMMO, which afterwards were used to filtrate, e.g. gases (Jie et al. 2005). The above-mentioned patents and scientific articles did not include the experiments presented in this publication, i.e. in neither was the cellulose solution in NMMO used to coat the surface of paper. The method applied in this research is safe because the solvent is both non-toxic and non- flammable. Furthermore, NMMO is considered envi- ronmentally friendly. It can be reused with a percent recovery of 98% or even higher (Rosenau et al. 2001; Chen et al. 2015). This technology also significantly reduces problems related to post-consumer material recycling. The objective of the research was to determine the interactions between the NMMO cellulose solution and paper, whose surface had been coated by that solution. In particular, the impact on the structural and strength properties of paper was investigated. This study is part of a larger research project on fibrous material modification by means of a surface treatment operation, namely by coating the substrate with the solution containing cellulose dissolved in NMMO.
Base paper preparation
Commercial, bleached softwood pine kraft pulp (BSK) was used to prepare laboratory handsheets. Pulp parameters were as follows: initial moisture content 93.78%; a -cellulose content 86.6%; DP 1081; Schopper-Riegler value SR-12. Pulp samples were prepared according to standard ISO 5263-1:2004. Unbeaten pulp was examined and laboratory sheets of 70 g/m 2 were formed in Rapid-Ko¨then apparatus according to standard ISO 5259-2:2001. The obtained laboratory sheets were used as base paper for a subsequent coating. Unbeaten pulp was used in order to eliminate the influence of the beating process on the paper mechan- ical properties. Consequently, it was possible to evaluate the full development of strength properties, resulting only from the applied modifications.
Materials and methods
Coating
The application of cellulose-NMMO solution on the paper surface was conducted using the standard coating process. This operation was carried out with the use of an automatic coater (‘Control Coater’) of IPP/TUL, Poland, at the speed of 16 cm/s and with the use of a standard Mayer rod No. 3 (K-bar), which gave a wet film thickness of 24 l m and, hence, a dry coating thickness of about 0.8 l m. Prior to the coating process, the cellulose solutions were melted and kept at a temperature of 80 ± 2 C. Solidification of the coating occurred as a result of washing out the solvent (NMMO) and—afterwards—drying the coated papers.
Cellulose solution preparation
Bleached beech kraft pulp of Degree of Polymerisa- tion (DP) of 655 and a -cellulose content of 92.3% was used for the cellulose solution preparation. This type of pulp was selected because of its relatively low degree of polymerisation and short fibers that enabled the dissolution of such cellulose under mild condi- tions. According to the authors’ experience, it is very difficult to run a coating operation using solutions of higher than 5% cellulose concentration due to high solution viscosity, resulting in runnability problems. Conversely, the application of significantly lower concentration solutions results in insufficient deposi- tion of cellulose on the base paper. Therefore, for the
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