Absorbing water
Why do newspapers absorb water quickly when compared with other papers?
G. THILAGAVATHI
Tamil Nadu
Paper is an aggregation of fibrous plant cellulose tissues which are bonded by pulp of finer fibres and bonding material. Specifically the surface of the paper is glazed by this finer pulp so that we have the smooth writing comfort. This renders such paper more expensive as compared to the news print paper devoid of the glazing layer.
The aggregated web of the fibrous particles leaves large number of fine gaps or pores between them which support absorption of liquids like water by capillarity action.
Capillarity action is a result of surface tension of the liquid in contact with the solid surface. According to this, finer the gaps, more will they be filled by the absorbed liquid. In news print paper the fine capillary pores come in contact with the water which gets ‘sucked’ or absorbed. And, in the other variety of (or glazed) paper, the pores are not open for the water to enter.
In fact, when a piece of paper is put in water, the top layer of the glazing pulp gets soaked and swells somewhat blocking the pores between the solid fibres of the paper. When such soaked paper is dried the glaze-pulp cracks and opens up some of the pores.
That is why, although a ‘good’ paper does not absorb water and does not blot ink while writing on it, it will do so if it is wetted for some time and then dried. Similarly, a good paper will start blotting ink if it is ironed at some temperature when the glaze-pulp cracks and opens up the pores for the capillarity to absorb.
Dr. H. K. SAHU
IGCAR
Kalpakkam, Tamil Nadu
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