Improved Barrier Properties of Paper by Surface Coating using Microcrystalline and Microfibrillated Cellulose

Sammanfattning

Packaging paper must have functional barrier properties in order to effectively protect the packaged goods and to withstand transport and storage. A variety of coating systems exist to reduce the permeability of air, water vapor, or grease. In recent years, one class of materials that has gained increasing attention for such applications is cellulose-based additives, particularly microcrystalline cellulose (MCC) and microfibrillar cellulose (MFC). This study aims to explore the impact and possibility of combining MCC and MFC in a single coating formulation. The investigation focuses on how this combination influences the physical behaviour of the coating mixture, as well as the resulting barrier and mechanical properties of the coated paper. Another key objective was to determine whether it is possible to increase the solids content of these coatings without compromising their processability. Initial laboratory-scale trials were conducted, in which coatings containing various MCC/MFC ratios and solids contents were prepared and coated onto a paper substrate (substrate A) using a laboratory coater. Carboxymethyl cellulose (CMC) was also added to increase water retention and runnability. Based on the results of these trials, two specific formulations were selected for further evaluation in pilot-scale coating trials at UMV Coating Systems. These included a blend of 80/20/5 (w/w/w%) MCC/MFC/CMC with a final solids content of 12.5-%, and a formulation containing 100/0 (w/w/w%) MCC/MFC/CMC with a solids content of 17.9-%. In the pilot-scale application, the coating mixtures 80/20/5 and 100/0/5 MCC/MFC/CMC had adjusted solids contents of 8.5% and 13.6%, respectively. Following the coating trials, several key material properties were evaluated, including air permeability, water vapor transmission rate (WVTR), water resistance (Cobb 60), oil resistance, as well as tensile strength and tensile stiffness. All samples demonstrated a clear improvement in barrier properties against air permeability and increased resistance to water vapor transmission. However, the barrier against water absorption decreased, and no visible improvement was observed in oil resistance. The coating containing MFC and MCC showed a slight enhancement in tensile strength in the machine direction. In addition to the originally planned experiments, the effect of hot calendaring was also investigated. These results demonstrating a significant improvement in air permeability.