No abstract

We introduce a method to measure paper wetting based on the measurement of monochromatic (670 nm) light transmission through paper. The samples were 49.8–51.7 g m−2 handsheets, the hydrophobicity of which was tuned by the fibre content of the furnish. In addition, one of the samples was internally sized. Impact wetting with isopropanol and water mist pulses was applied. The duration of a pulse was 100 ms, and the amount of liquid per pulse was 12.6 ± 1.2 g m−2. Optical changes in a paper sample were detected by measuring the average intensity of transmitted light through a selected area. The results showed that different wetting processes (pure capillary and combined capillary and diffusion wetting) could be distinguished from each other. With the presented method, differences in the individual behaviour of the samples during wetting could also be seen.

Understanding of wetting processes for papers of various qualities is important for the analysis and the adjustment of paper-production processes as well as for designing properties of paper for printing. In this investigation, papers of one kind with three different sizing-content values were examined by time-dependent attenuation of ultrasound transmission in mixtures of isopropanol and water. The attenuation depends on the stage of absorption of liquid into the paper sample, which furnishes the signature of the paper type and quality. Papers that have contact angles of 40° (without size), 70° (low size content) and 110° (high size content) were tested and a measurement procedure was developed. The measurement results showed that the development of attenuation with time differs at different frequencies, indicating the possibility of detecting various phases of wetting phenomena.

In the first part of this work ultrasonic thickness resonances were measured with offset paper samples, 80-240 g m-2 in grammage. The resonance frequency was found to be linearly dependent on the grammage of the samples. In the second part of this investigation 240 g m-2 offset paper samples were wetted with a mist pulse of either water or isopropanol. This pulse, 61±4 ms in duration, was generated by a high-power ultrasonic actuator. The change in resonance frequency, initially 660±5 kHz, was monitored during the wetting of the paper sample. An ultrasonic burst, frequency swept from 500 to 900, 600 to 1000 and 1300 to 1700 kHz, was transmitted through the sample at a 10 Hz repetition rate. This made it possible to follow the wetting of the sample at several frequencies by the same measurement. It was also possible to follow the temporal development of the apparent absorption coefficient of the sample. The shape of the transmission curves suggests that two different processes, capillary wetting and diffusion wetting, can be distinguished.