It is about a fibre's absorption of gaseous water molecules. Attached find a section from the book 'Physical properties of textile fibres'. Assuming the fibre is exposed to a humid (e.g. RH 50%) environment. The water molecules in the air are in gaseous phase. When the indirect molecules attach to the direct molecules, what phase are they in (once they are attached)? - Theoretically, there are a number of options, but so far I could not find a satisfying answer in the literature, they always call it moisture absorption, but do not specify...
I would say that as soon as the H2O molecules adsorb to the polymer surface they will no longer be in the gaseous phase but become liquid, hence it is called condensation. In a gas such as water vapor the molecules are farther away from each other and do not attach to each other as in the figure.
This reaction is adsorption and shares some characteristics with traditional condensation. It is condensation in the broad sense (gas transitioning to a condensed phase). It differs from traditional condensation in these ways
1. it release more heat, potentially significantly more depending on how strongly it binds to the fibers.
2. It happens at temperatures well above the dewpoint.
3. It does not form a liquid phase, but rather hydrates a solid
4. It's rate and mechanism strongly depend on the chemical characteristics of the surface with which it reacts
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