For the purpose of assuring the waterproofness of materials used for electrical appliances, some experiments have already been carried out by the author on wettability of solids to interpret it from practical and theoretical view-points. In this paper, observations are first made on the change in configuration of water droplet evaporating on organic polymer solids and on the relation of time required for complete evaporation to the contact angle between the droplet and the solid. Difference in wettability of polymers is then studied and discussed. Next, discussion is also given on measured changes in wettability of polymers in contact with water.
In general, there are two factors to be considered which are likely to influence the wettability of solid surface in contact with water, namely the penetration of water molecules into the solid and the change in orientation of polar groups in the solid. Because of permeable nature of water molecules, the penetration theory is adopted, and by assuming the wettability to be proportional to the amount of sorbed water, the following equations are derived for the time rate of wettability:
(
W-
W0)/(
Ws-
W0)=4/_??_·_??_
W=σι cosθ
where
D is the diffusion coefficient of water molecule,
d the thickness of the solid,
t the time, σι the surface tension of water, θ the contact angle, and
W0 and
Ws are the amounts of wettability
W at
t=0 and
t=∞ respectively.
To a certain extent, these equations gave a clue for discussing the observed data and reasonable values of diffusion coefficient were obtained.
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