Etude de l'enchevetrement et de l'efficacite de l'oxyde de polyethylene utilise comme agent de retention sur des machines a papier

The retention of the fines particles on the paper sheet is a crucial parameter in papermaking, because fines improve the optical and mechanical properties of the paper. Moreover, a constant fines retention value contributes to the stabilization of the wet-end and to the runnability of the paper machine. Polyethylene oxide (PEO), a synthetic neutral polymer with a high molecular weight, is used as a retention aid with a cofactor. While PEO is efficient in contaminated systems, it disentangles under the effect of shear, which decreases its ability to flocculate fines. An experimental sensor that measures the pressure drop of a PEO solution passing through a capillary has been developed and gives an indirect indication of the size of the PEO entanglements. Experiments on a laboratory twin-wire former have shown a good correlation between PEO pressure drop and fines retention.;One of the objectives of the study was to investigate the effect of PEO state of entanglement on fines retention on two types of paper machines, a slow pilot paper machine and a fast industrial machine, and to compare it with the influence of other important wet-end variables. The other goal was to monitor the evolution of the PEO state of entanglement along its industrial makedown process in order to optimize the PEO state of entanglement when it is injected to the pulp.;Trials have been performed on the Fourdrinier-type pilot paper machine of the Centre Specialise en Pates et Papiers (Trois-Rivieres, QC). Steps changes have been made on 3 wet-end variables (PEO state of entanglement, PEO dosage and CF-PEO ratio). Headbox and whitewater consistencies have been measured, along with optical and mechanical properties of the paper. Reproducible results have shown that the fines retention is about 15% higher with entangled PEO than with disentangled PEO and that increasing the CF-PEO ratio leads to an increase of fines retention without harming significantly the sheet formation. Dynamic variations of the whitewater consistency have been recorded and identified with first order transfer functions. Whitewater consistencies obtained using a dynamic simulation based on mass balances fit well with real values.;Subsequently, the variability of the PEO pressure drop prior to its injection to the pulp in a newsprint mill has been recorded online with the PEO pressure drop sensor. Fluctuations with a 20% amplitude have been observed and might be due to PEO entanglement inhomogeneities in the storage tank. A part of these variations have then been compared with those of the broke flow and the polymer flow. A dynamic numerical model that describes the variations of the whitewater consistency has been created and validated. The obtained transfer functions have shown that the effect of PEO entanglement on whitewater consistency is quite small compared to the influence of the other variables (about 10%) but cannot be neglected.;Finally, PEO samples have been taken at several points of its industrial makedown process and have been measured with the PEO pressure drop sensor. Four series of tests have been performed at several periods of the year (at different temperatures and different batches of PEO granules). It has been observed that the PEO pressure drop decreased significantly between the dissolution of the PEO granules and the PEO injection into the pulp. A detailed investigation of the effect of the various process units on PEO disentanglement have shown that the injection pump and, to a lesser extent, the transfer pump, are the main factors of PEO disentanglement. On the other hand, the mixing time in the dissolution tank and the residence time in the storage tank don't seem to affect the PEO state of entanglement in a significant way.