Recycling of waste paper with different density of liquid toner.

Mirkovic, Ivana Bolanca ; Majnaric, Igor ; Bolanca, Zdenka 等

1. INTRODUCTION

The effectiveness of the deinking process in paper recycling depends on numerous factors. The following ones have to be mentioned: the printing technique, the ink characteristics, the properties of the printing substrate and the components of the coating. The other influential factors are the kind and quantity of chemicals used in different phases of the process as well as the chemical and physical conditions of the system such as pH value, consistency of fiber suspension, defibering time and the hydrodynamic factors of the flotation process (Lassus, 2000).

The size, shape and the surface properties of the dispersed ink particles, the air volume and the size of the air bubbles have special importance for the effectiveness of deinking flotation. The effectiveness of the process depends on the ability of the printing ink adherence on air bubbles and the strength of this bond in order to prevent their detaching and rebinding to the cellulose fibers (Renner, 2009; Carre, 2008).

In the described problems, the majority of authors researched the hydrodynamic factors of the process and the influence of chemical and physical conditions of the system on the process effectiveness, while the influence of the printing techniques and the conditions in printing process haven't been studied much (Li et al., 2007, Zhao et al., 2004).

The investigation results of the liquid toner density of ElectroInk on the effectiveness of the print recycling have been presented. Deinking process mechanism in relation to the characteristics of the printing substrates used in printing is discussed. In scientific sense, the work is the contribution to the explanation of the print deinking mechanism in relation to the liquid toner concentration in combination with the coated and uncoated printing substrates.

The recent investigations have proved the bad effectiveness of print recycling made in the technique of digital offset printing. The investigations have to prove if it is possible to contribute to solve the mentioned problem by choosing the conditions in different phases of indirect electrophotographic printing (scorotrone voltage, laser strength, offset cylinder voltage, concentration of the liquid toner etc.). The investigation results can be applied in the production of the recycled papers, in formulation of new printing materials and in the design of graphic products taking into consideration the postulates of the sustainable development.

2. EXPERIMENTAL

The prints made on digital offset machine Turbo Stream HP Indigo were used for analysis. The printing form contained different printing elements: standard CMYK step wedge in the range from 10-100% tone value, standard ISO illustration for the visual control, textual positive and negative microelements, wedges for determination the greyness and the standard wedge with 378 patches for production of ICC profiles and 3D gamut.

Five different densities of liquid ElectroInk (D 1.20--2.00) were used in combination with the printing substrate as follows. The fine art paper (Symbol Gloss) and the wood free natural paper (Acroprint), all the products of the firm Fedrigoni, were used as the printing substrates. Papers do not differ in their basic chemical composition of the paper raw materials. The only difference is in the finishing process referring to coating.

For print recycling the method of alkaline chemical deinking flotation was used, which was described in details in the previous work (Bolanca & Bolanca, 2005). The handsheets were made using a laboratory sheet former, according to standard method T 205.

For determining ISO brightness of the recycled fibres the spectrophotometer DataColor, Elrepho 450X was used. Residual toner size, toner dirt particles number and toner area were assessed with image analysis software Spec*Scan, Apogee System. This system is utilizing scanner to digitize image. Threshold value (100), white level (75) and black level (65) were chosen after comparing computer images to handsheet.

3. RESULTS AND DISCUSSION

For observing and explaining the deinking flotation process, the image analysis method and the brightness of handsheet have been chosen. In figure 1 the count of dirt particles (toner particles) on handsheet made from the fibres obtained from print recycling with different liquid toner concentrations is presented.

The investigation results show the trend of dirt particles increase on handsheet after flotation in the function of the concentration increase of the ink carrier in ElectroInk when the uncoated paper was used for printing (628-1024). However, when the paper coated on both sides was used for printing the number of dirt particles of ElectroInk on handsheet varied and there is no expressed trend in relation to the toner concentration used in the printing phase (673-892). The difference in the total number of the dirt particles on handsheet in relation to the printing substrates used for prints intended for the deinking flotation can be seen. There is greater number of particles on handsheet obtained from the fibres of print recycling where the prints were made on paper coated on both sides (79,9-86,1 % in relation to D 1,20-2,00).

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The investigation results show that greater handsheet area is covered with toner particles when the coated paper was used for printing (27,0-77,3 % in relation to D 1,20 do 2,00). The obtained results show that there is the need for particle size analysis according to classes in order to understand the process.

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The investigation results on handsheet before print flotation obtained from the uncoated paper show greater number of particles of greater classes than 0,04[mm.sup.2], i.e. the particles visible with naked eye in relation to the handsheet obtained from the fibres of the prints printed on coated paper (34,6-68,4 % for D from 1.20-2,00).

One of the conditions for the removal of the toner particles in the flotation process is the fragmentation of toner particles as well as their size. The number of particles on handsheet before flotation decreases at 77,3-90,0% in relation to the density of toner used for print preparation when the uncoated paper was used for printing. By using the paper coated on both sides the removal of particles by flotation, in the described experimental conditions, decreases drastically (the decrease is about 10 times). The brightness of handsheet is in accordance with the presented results, so higher values were obtained for the recycled fibres of prints made on uncoated paper in accordance with the concentration of ElectroInk used in printing (D 1,20-2.00 uncoated paper, brightness 91,0-86,8, coated paper, brightness 86,4-81.0)

The removal of ink from the substrate, in deinking process depends on characteristics of the printing substrate. With prints made on coated paper there is no contact between ink and fibers. The paper coating disintegrates as the recovered paper is pulped and fragments of the ink film are released. Opposite to the uncoated paper, the adhesion of ink on paper depends on the surface structure, ashes content, fiber types and the mechanism of ink drying in the printing process.

4. CONCLUSION

Based on the investigation results described in the experimental conditions, the following conclusions can be made. By the application of different concentration of ink carrier in liquid ElectroInk, in the process of indirect electrophotographic printing, the number and the size of toner particles on handsheet made from the fibres of the deinking fotation phase can be influenced. The described phenomenon, the type of the substrate, i.e. the coating on paper used in printing is under greater influence.

The results justify the creation of statistic models including multifactorial analysis and the determination of the essential conditions in different phases of the indirect electrophotographic printing (voltage of scorotrone, laser strength, voltage of the offset cylinder, concentration of the liquid toner etc.) which leads to the optimization of the deinking flotation process and to obtaining fibres with better characteristics. In this sense, the continuation of these investigations goes on.

5. REFERENCES

Bolanca, I. & Bolanca Z., (2005). The Optical Properties of Deinked

Pulp, Journal of Imaginig Science and Technology, Vol. 49, No. 3, 284-292, ISSN: 1062-3701

Carre, B., (2008). Overview of the Deiking Technology, Final Conference of COST Action E46, Bordeaux, October 2008

Lassus, A. (2000). Deinking Chemistry, In: Recycled Fiber and Deinking, Gottsching, L., Pakarinen, H. (Eds.), 241-266, Fapet Oy, ISBN 952-5216-07-1, Jyvaskyla

Li, G., Xie, X.F., Wang, A.L. & Chen, K.F., (2009). Application of Hydrodynamics in Design of Flotation Deinking Cell, In New Trends in Fluid Mechanics Research, Zhuang F.G., Li J.C., (Eds.), pp.459, ISBN 978-3-540-75994-2, Springer, Berlin

Renner, K., (2000). Deinking Chemistry, In: Recycled Fiber and Deinking, Gottsching, L., Pakarinen, H. (Eds.), 241266, Fapet Oy, ISBN 952-5216-07-1, Jyvaskyla

Zhao, Y., Deng, Y., Zhu, J.Y., (2004). Roles of Surfactants in Flotation Deinking, Progres in Paper Recycling, Vol 14, No1, (November, 2004) pp 41-45