Humidity Effect on Paper Flatness Using 3D Profilometry

Humidity, the amount of water vapor in the air, plays an important role in the performance of instruments in different industries. For example, high humidity may cause malfunction of electronic devices. The condensation on the circuit boards results in short circuit inside the instrument. High humidity also reduces the capacity of chemical plants and refineries where furnaces are used in the process. Humidity control is critical in the paper and pulp industry. Composed of cellulose fibers, paper is hygroscopic in nature. It absorbs moisture from a humid atmosphere, leading to changes in paper dimensions, flatness and strength.

Flatness is critical to the proper performance of printing paper. It communicates functional characteristics and makes an impression of the paper quality. A better understanding of the effect of humidity on paper flatness, texture and consistency allows optimizing the processing and control measures to obtain the best product. Quantifiable, precise and reliable surface inspection of the paper in different humid environments is needed to simulate the use of paper in the realistic application. The NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology with unique capability to precisely measure the paper surface. A humidity controller provides precise control of the humidity in a sealed chamber where the test sample is exposed to the moisture.

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The printing paper was cut and placed in the humidity chamber, with the relative humidity (RH) value controlled by a humidity controller attached to the chamber. The 3D surface morphology of the paper was first measured in an atmosphere with an RH value of 50%, which is close to that of the ambient atmosphere. The RH value in the chamber was then raised to 90%, and the printing paper was kept in such environment for 2 h. A 20-second 3D surface measurement at this 90% RH was performed. The moisture was discharged and the humidity chamber maintained an RH value of 10% for another 2 h, followed by the last 3D surface measurement of the paper.

The 3D view and the false color view of the printing paper at different humidity are shown and compared in Fig. 2 and Fig. 3, respectively. The Flatness and Roughness values of the paper at different humidity are listed in Table 1. The flatness value was directly calculated from the 3D surface profile as measured, and the roughness value was obtained after the form removal was performed in the analysis software. It can be observed that the printing paper is relatively flat before its exposure to excessive humidity. The FLTq value is 19.3 µm. As the RH value increases to 90%, the high level of moisture causes swelling and distortion of the paper, leading to formation of a ridge at the center and significantly increased FLTq value of 375 µm. When the moisture in the paper is removed by being exposed to the atmosphere with an RH value of 10%, the FLTq value reduces to 55.3 µm.

The humidity changes the dimension of individual cellulose fibers in the paper. A high moisture content in paper increases the paper dimension. The variation in the environmental moisture content and the directional cellulose fiber alignment result in a change of paper flatness. Depending on the percentage of fibers present in the paper and the degree of bonding within the paper structure, papers react to the humidity change in different ways. The NANOVEA non-contact profilometer equipped with a humidity chamber and a humidity controller provides an ideal tool to precisely measure the 3D profile of paper samples in a humid environment.

In this application, we have showcased that the NANOVEA ST400 3D Non-Contact Profilometer equipped with a line sensor and a humidity chamber is an ideal tool for analyzing the surface morphology of samples exposed to a humid environment.

The moisture content in air plays an important role in the flatness of the printing paper. Undesirable appearance and quality defects like buckles occur when the papers are exposed to the environment with a varied humidity. In this study, the paper significantly distorted as the relative humidity value increased from 50% to 90%. Such a distortion is reduced when the paper is exposed to a dry environment of RH value 10%, as the moisture content in the paper is discharged.

The data shown here represents only a portion of the calculations available in the analysis software. NANOVEA Profilometers measure virtually any surface in fields including Semiconductor, Microelectronics, Solar, Fiber Optics, Automotive, Aerospace, Metallurgy, Machining, Coatings, Pharmaceutical, Biomedical, Environmental and many others.