Why does wet paper on drying or ironing develop wrinkles or uneven surfaces?
Paper is an aggregate of pieces of fibrous vegetable tissue bonded together by a pulp of finer particles of similar material.
Infact, the different types of paper differ in the proportion of the fibres and the pulp. In general, the pulp is used, along with a mild glue-like substance, in order to achieve a smooth texture of the paper as the finer particles fill the gaps between the coarser fibres near the surface of the paper.
During paper making, the wet assemblage is run over or between a set of smooth and hot rollers. During this process, the glue component sets irreversibly. The glazed look is the result of this processing step.
In highly glazed paper, a special type of glue is used in larger quantities. Significant amount of this substance resides on the surface of the paper and prevents such papers getting wet easily.
However, when the paper is wetted in water, the ‘glaze’ pulp material sags while the glue remains set. And, on subsequent drying or ironing, uniform adhesion of the pulp to the fibrous material underneath is not ensured.
The fibrous material is not only rough, but is also stiffer. Thus, the texture of the new material is decided mainly by the fibrous bulk of the paper and not the ‘glaze’ stuff. Infact, often one can observe the stiff fibres projecting out of the wetted and dried paper at random at various locations. These effects create wrinkled and rough appearance to the surface of the paper. In high gloss paper, the surface glue restricts the release and popping out of the fibres even after wetting and drying, thus retaining the glossy texture substantially.