2020 年 74 巻 5 号 p. 530-534
Almost all paper is produced for further converting, it means it may undergo gluing, coating and/or printing. Therefore, normally certain specifications are agreed between the supplier and converter based on standard testing methods like Cobb, Gurley, Bendtsen, HST or Smoothness Beck etcetera and these specifications are binding. The problem is that sometimes there are still problems in the converting although all the specifications are met. In this case, nobody knows why these problems have occurred. We must look at the rough principles of the three typical converting processes like gluing, printing and coating. It is important to understand that nowadays all these processes are very fast, and things happen within milliseconds or just a few seconds. Because these processes are so fast, the surface of the paper or board that is converted plays a very important role. If we look more detailed at the gluing process, we can basically split it into 3 phases. In the First PHASE, a liquid film is applied on the paper or board surface by a pressure impulse within very short time. Here we talk about microseconds or milliseconds. The result depends on the rheology of the liquid, the strengths of the pressure impulse （which depends on the machine speed and settings） and the surface porosity of the substrate. Here, most important for the anchoring of the glue is the surface pore structure which is the relevant parameter to characterize. In PHASE 2, the water from the glue that has been applied will be absorbed. This is mainly controlled by the surface sizing/hydrophobicity of the substrate. Here the surface sizing or surface hydrophobicity is very important for the dewatering of the glue and to increase its viscosity to reach an optimal bonding. So, the surface hydrophobicity is the relevant parameter to characterize in this phase. PHASE 3 is the drying and hardening of the glue film which takes place after several minutes and hours. Because converting processes are very fast without phase 3, fast evaluation method is preferable. In this paper we introduce PDA/EST to evaluate penetration and predic behavior of paper and board surface for converting processes.
