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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Paper physics > Papermaking potential characterisation of different cellulose fibres pulps           Update: February 26th 2007
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Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
(November 2006)
 
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"

III - Paper physics

III - 5 - Papermaking potential characterisation of different cellulose fibres pulps

Jean-Marie Serra-Tosio, Nadège Reverdy-Bruas, Yves Chave

Prevision of the pulp papermaking potential is of primary importance in paper industry. Indeed, all along the process, from the pulp preparation to the final product, the aim consists in establishing H-bonds between fibers to build a coherent network.
There are numerous methods to characterize the initial properties of wet pulps and fibers. There are also plenty of tests to study the physical properties of papers. On the opposite, there is a lack of methods to describe the evolution of physical properties of web and fibers during formation, pressing and drying. The drying unit operation is on the scope of this study.

It is also interesting to be able to characterise the paper properties regarding the kind of pulps they are made of and the process operating conditions. Consequently this work is focused on the relation between the pulp behavior submitted to different drying strategies and the obtained paper properties. An original method is developed to dry ten paper strips under restrain. The VARIDIM© apparatus is used [Figure 1]. It allows measuring shrinkage for different loads applied on one end of the sample while ambient temperature and relative humidity are maintained constants. Papers dried with these conditions are then tested in an extensometer to determine mechanical properties such as Young modulus.

Front view of the VARIDIM© apparatus   VARIDIM©
Figure 1 - Front view of the VARIDIM© apparatus
 
  Figure 2 - Example of typical results recorded during
a trialcontaining the ten strips to be tested in
the VARIDIM© apparatus – displacement versus time.

The main originality of this study lies in the possibility of classifying pulps – versus their level of beating – by the mean of their maximum specific work developed during drying.

In papermaking, shrinkage is sometimes required in order to obtain particular paper properties, and among them, a higher strain. In this study, we propose to introduce a new pulp characteristic and we thus define the specific tension [Equation 1] and the drying shrinkage specific energy [Equation 2]. The analysis of the shrinkage specific energy evolution versus the specific tension shows a maximum which is characteristic of each pulp.

 

The pulps studied are hardwood and softwood bleached chemical pulp submitted to a laboratory standard Valley beating. Schopper-Riegler degree is measured during the Valley beating and water retention value (W.R.V.) is determined with the standard method, i.e. centrifugation at 3000 g. Samples studied are laboratory handsheets and the useful strips length is 70 mm. They are then submitted to controlled conditions of drying that are maintained constant: a 20% relative humidity air flow and a 23°C temperature. Finally, when no more shrinkage is registered [Figure 2], the dried strips can be tested in an extensometer to determine some mechanical characteristics.
The mean time required to dry ten strips is roughly 30 minutes. During a trial; displacement, temperature and relative humidity are measured and plotted versus time.

For each level of beating, we thus measured the shrinkage decrease when the load increases. It was also shown that the higher the Schopper-Riegler degree, the higher the shrinkage at a given load. This is a well known result due to the higher aptitude to shrink of swollen fibres.
Regarding the good correlations obtained between drying shrinkage and water retention value, we focused attention on the definition of a correlation that should link drying shrinkage, load applied and W.R.V. Even though drying shrinkage seems to be well correlated with W.R.V. whatever the pulp is, we defined the following law [Equation 3] and adjusted the five parameters for hardwood and softwood pulp separately.

 

The law was chosen in order to satisfy the limit conditions: when no load is applied, drying shrinkage is a function of W.R.V., and when the load reaches a limit (function of W.R.V.) drying shrink-age is equal to zero. On [Figure 3], calculated shrinkage is plotted versus measured one. An excellent correlation is found.

Correlation between calculated and measured values of shrinkage
Figure 3 - Correlation between calculated
and measured values of shrinkage

Finally, the strips dried with different restrains were tested in an extensometer in order to determine some mechanical properties. On [Figure 4], the huge decrease of the Young Modulus when the drying shrinkage increases is depicted. This expected result is more pronounced for the more beaten pulps.

Evolution of Young modulus versus drying shrinkage
Figure 4 - Evolution of Young modulus
versus drying shrinkage

Other properties such as the rupture strain were tested because they are highly influenced by the drying shrinkage. As expected, this property highly increases when the drying shrinkage increases. This result can be easily understood since it is necessary to draw tight the paper (that had shrunk) before reaching the maximum strength of the network itself.

Evolution of shrinkage specific work versus specific tension.
Figure 5 - Evolution of shrinkage specific work
versus specific tension.

We are now working to improve the drying technique using the VARIDIM© apparatus. Indeed, it would be interesting to improve the accuracy of measurements by increasing the length of the paper strips, from 70 mm to 200 mm. Besides, other parameters should be considered to define a law for calculated drying shrinkage that would be better adjusted to the measured data. We could introduce the fibre length parameter for example.

Finally, in order to approach industrial papers, we plan to make in-plane orientated papers. Industrial interest would be reinforce because papermachine made papers all have a fibrous orientation that confers them a drying behaviour very different to a handsheet.

 
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