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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Papermaking and environment engineering > Influence of wetting phenomena upon penetration of water into paper at short contact times, in relation with paper coating           Update: July 20, 2011
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Researchers of the LGP2
(May 2011)
 
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Laboratory of Pulp and Paper Science and Graphic Arts

I - Papermaking and Environment Engineering

I - 3 - Influence of wetting phenomena upon penetration of water into paper at short contact times, in relation with paper coating
Martine Bouchon, Martine Rueff, Véronique Morin

In the paper industry, many paper grades are coated in order to improve their surface properties. The coating process consists in applying an excess of coating colour on the surface, metering the amount of coating colour and then drying the coated paper. A coating colour is mainly made of pigments and binders dispersed in water and, as soon as it is put in contact with the paper, some water may flow into the paper. This coating colour dewatering takes place in a very short period of time, i.e. less than 100 milliseconds, plays a role in the quality of the end product and, therefore, should be well controlled. Thus, it is of major importance to get a good understanding of the phenomena that take place.

During a previous work carried out at the laboratory (PhD thesis by Hélène Bernard - 2001), an experimental set-up capable of reproducing some aspects of the industrial process, i.e. application and metering, was developed [Figure 1]. This study showed that it was necessary to consider a contact angle very close to 90° in order to be able to simulate the kinetics of penetration of water into a hydrophilic paper using a model based on the Szekely’s equation and considering the porous structure of paper as an assembly of parallel, tortuous capillaries with various diameters. This result raises two questions : the first one is related to the dynamic wetting of paper and the second one to paper structure modelling.

Experimental set-up developed at the laboratory
Figure 1 - Experimental set-up developed at the laboratory

We have continued this experimental study, focusing on the influence of the wetting phenomena upon the penetration of water into paper. We developed an experimental set-up, based on a conventional concept, to be able to monitor the spreading of liquid drops on at a high frequency, i.e. at least 1000 frames per second. This study enabled us to show the following points :

Contact angle and drop diameter as a function of time
Figure 2 - Contact angle and drop diameter as a function of time

The presence of a contact angle with a high value does not necessarily mean absence of wetting: these high values come from the dynamics of spreading. On the other hand, the interactions between the liquid and the substrate are instantaneous. If we apply this result to the penetration of a liquid into a paper, we can conclude that the concept of a wetting delay is valid even if humidification takes place instantaneously. This could explain the low penetration rates of some liquids into some supports. The dependence of the value of the contact angle on the velocity of the contact line could also explain the low penetration rates observed, even in the absence of a wetting delay.

volume of liquid transferred to paper per unit surface area in the pilot 
   coater (left) and the laboratory set-up (right)   volume of liquid transferred to paper per unit surface area in the pilot 
   coater (left) and the laboratory set-up (right)
Figure 3 - Volume of liquid transferred to paper per unit surface area in the pilot coater (left) and the laboratory set-up (right)

The experiments performed on the Centre Technique du Papier (CTP) pilot coater and on our laboratory set-up demonstrated that [Figure 3]:

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