Detection of Moisture Adsorption on Joining Surface Using Static Electricity Distribution
Authors: Kikunaga Kazuya , Terasaki Nao 
Published: February 2020

Condition of a material surface is an important factor in strength of bond for adhesion. Especially, penetration of water into the adhesive joint part have negative effects on the strength of bond such as degradation of a polymer, corrosion degradation of an adherend, and degradation of a mechanical properties in the interfacial surface. Technologies of some surface analyses have been developed in order to evaluate the moisture adsorption on the material surface. The technologies, they are necessary to use specific materials or vacuum environments, are limited to tools usage. However, various adhesion between different materials have carried out in the air at production site, so the existing techniques cannot be applied as the surface analysis and evaluation at the production site. Therefore development of a new surface analysis technique adaptable a variety of materials in air, non-contact and non-destructive in order to evaluate the moisture adsorption on the surface is demanded.

So, we have focused on a relationship between static electricity and moisture as a means of observing the state of the material surface. Static electricity is less likely to occur with high humidity, also static electricity on the insulator surface is inhomogeneously charged. If it is possible to observe the two-dimensional distribution of static electricity with high spatial resolution, it may be possible to detect indirectly the influence of moisture adsorbed locally to the surface. So far, static electricity measurement technologies using targeted electrostatic fields include the static induction type, vibrating reed induction type fieldmeters and chopper stabilized instruments. Their sensors using the technologies are often used at the production site. However, when the sensors are used to measure electrostatic charge distributions within an object surface, it is necessary to narrow the sensing area for high spatial resolution and scan point by point over the whole surface. As a result, measurements of the charge distributions are quite costly in terms of time. State of the charge distribution on the surface change over time, it is important key to measure the static electricity distribution in a short period of time.

We developed a sensor that can visualize static electricity with a spatial resolution at 1 mm. Moreover, technology of measuring static electricity distribution with high spatial resolution in a short period of time using the 30 parallel sensor. In this study, it was examined on detection of a moisture adsorption by survey changes of the charge distribution on the object surface using the developed measurement technology of the static electricity distribution. First, static electricity distribution in an area of 30 mm × 30 mm on an acrylic surface was evaluated when the acrylic and a dried nylon cloth was friction. As a result, irregularly charged distribution of surface potential in the range of -1000 V ~ +1000 V was observed. Although absolute amount of the surface potential distribution was slightly changed, the shape of the distribution even after three minutes of the friction was unchanged. This result indicates that the electrostatic charges are unable to move within the same surface by reason of the insulating acrylic surface. Next, the part of the same acrylic surface and the wet nylon cloth was contacted lightly, it was completely dry the sample surface by natural evaporation. The charge distribution on the acrylic surface in the state was evaluated. As the result, the charges of the part on the surface contact with the wet nylon was lost. These results imply that the electrostatic charges flow as surface current through the formed water film on the sample surface, and the charges leaks in the air. Thus, it was revealed that the influence of moisture adsorption on the surface was able to detect indirectly by utilizing static electricity distribution. The measurement technique of static electricity distribution is possible to adapt to various materials in air, non-contact and non-destructive. Therefore, it was suggested the possibility of simply evaluating the influence of moisture adsorption on joining surface at production site of the adhesion.