The influence of prints ageing on Number and size of ink particles in the Paper recycling process.

Bolanca Mirkovic, Ivana ; Dragojevic, Maja

Abstract: Recycled fibres have become an important source in the paper production. In the deinking proces, the ink should first detach from the fibers, then ink and fibers separate from each other by flotation according to different surface properties. In this work the research results of the influence of the prints ageing on number and size of ink particles and the area they occupy on handsheet in different phases of the paper recycling process have been presented. Results are discussed in relation to prints from different printing techniques made with inks of different formulation. The research results confirm the prints ageing influence on the size and the number of ink particles on handsheet from different phases of the paper recycling process in the function of the printing technique and the ink composition in the framework of experimental conditions. Weaker recycling effectiveness of the aged offset prints with the ink with the increased part of the renewable raw material has been observed in relation to other samples, which is explained by the oxidative processes.

Key words: offset prints, digital prints, ageing of prints, deinking flotation, image analysis, brightness

1. Introduction

Recycled fibres have become an important source in the paper production. In the deinking process, the ink should first detach from the fibers, then ink and fibers separate from each other by flotation according to the different surface properties.

The efficiency of the deinking process depends on type and amount of chemicals used in the processing stages, physical and chemical operating conditions such as: temperature, pH, pulp consistency, gas type and gas flow rate [Theander & Pugh, 2004]. These process variables can lead to changes in the secondary processes such as bubble-particle capture mechanism and stability of the froth [Rao & Stenius, 1998; Heindel & Garner, 1999]. Other important parameters are: hardness of water, dissolved and colloid substances, size of ink particles, degree of hydrophobicity of ink particles and hydrodynamic parameters of the flotation process. [Lassus, 2000; Thompson, 1997].

The main problems of such complex process are the effectiveness of the printing ink or toner detaching from the cellulose fibers, removal of ink particles from suspension, as well as purification of the waste water. One of the factors of the paper recycling process effectiveness is the print age.

Generally, ageing of paper or prints could be in fact defined as a sum of all irreversible physical and chemical processes which happen in the material during time.

Deterioration in quality of an aged paper can manifest itself in chemical permanence and the decrease in mechanical durability [Strlic, 1998]. The permanence of paper or prints depends on the chemical resistance of its components and of the influence of external factors [El-Saied et.al 2000]. It includes lightfastness and points at resistivity of the printing ink against fading and colour change after exposition to light [Proksch, 1999].

The durability depends mainly on the physical and mechanical characteristics of the raw materials, impact of microclimatic factors such as heat, humidity or radiation, on contamination by ions and gas from the environment and action of micro organisms [Johansson, 2000; Johansson, 2000; Bukovsky, 2000]. Exposure of paper to very short wavelength ultraviolet (254nm) radiation is induced post-radiation effect, which are influenced internal and external factors [Lee et al. 1989]. Result of exposure to visible and ultraviolet radiation of paper is its discoloration effects during and after exposure. The aim of Bukovsky's report is to consider the share of the short-term action daylight with partly reduced UV radiation induced oxidation degradation of grounwood paper in the presence of various amounts of secondary chromophores, formed in the paper itself [Bukovsky, 2003]. The adsorption of sulphur dioxyd by paper as the atmospheric polutant, depends on many factors [Johansson & Lennholm, 2000]. Temperature affects S[O.sub.2] absorption. The increase of S[O.sub.2] absorption with increasing temperature is not high, based on chemical reaction kinetics. Transition metal ions present as soluble salts may increase the sulphur dioxide absorption too. The influence of papermaking additives on S[O.sub.2] absorption by paper is noticed. Papers containing rosin, alum size, calcium carbonate or mechanical pulp pick up more sulphur dioxide. Atmospheric ozone treatment can enhance the strength properties of mechanical pulp.

Natural ageing process of paper and prints causes the degradation of cellulose. The presence of moisture, oxidative agents and micro organisms is important in this process and especially the presence of acidic substances. The results in this case are the hydrolysis of cellulose that appears in shortening of its chain along with changes in content of crystalline form [Emsley & Ali, 2001]. Short-term irradiation of paper initiates light induced oxidation reactions, which continue even after paper is stored in the dark [Bukovsky, 2000].

Discolouration of a paper may be caused by the formation of chromophores upon ageing as the result of exposure among other light and volatile gases [Carter et al. 2000].

Acid catalyzed hydrolysis of cellulose was recognized to be the primary reaction of the accelerated deterioration of paper. Acid hydrolysis of cellulose and related carbohydrates is one of the most important factors responsible for the degradation of paper during ageing. It is well know that strength loss of paper during accelerated ageing increased linearly with the partial pressure of oxygen [Kolar et al. 1999]

Other processes which lead to paper degradation are alkaline hydrolysis, thermolysis, and physical-mechanical damage and attack of micro-organism and mould. For study of accelerated ageing of paper new methods are being developed and recently a mathematical model was presented for temperatures from Rychlyet at al. [Rychly et al. 2002].

In the recycling effectiveness research area waste paper exposed to aging during the summer months, defined as a summer effect have been studied [Haynes, 1998]. It is conclusion that the summer effect is due to the ageing or thermal drying of printing inks. This drying will lead to increased ink fragmentation (more ink to remove) and ink attachment (ink can not be separated from the fibre chemical or flotation mechanism).

In this work the research results of the prints ageing process on size of ink particles and the area they occupy on handsheet in different phases of paper recycling process have been presented. The results have been discussed in relation to the prints from different printing techniques made by differently formulated inks. Generally, the smaller number of researchers was occupied by the researches in this domain in relation to the ones who studied physical and chemical deinking conditions or the hydrodynamic factors of the flotation process.

In the scientific sense the work is the contribution to the explanation of the influence of the ageing process on the detaching mechanism of the ink particles, their size and detaching successfulness from the system. Except that it is interested in relation to the result application in the production of the recycled papers.

2. Experimental

The samples of colour prints are obtained by the digital offset printing with liquid toner ElectroInk (Indigo E-Print 1000+ printing machine), digital printing with dry toner (Xerox DC 50) and offset printing with different inks composition (Heidelberg sheet fed machine).

In the conventional offset printing process the model ink with the composition presented in table 1 has been used.

The prints were made on the coated paper. The unique test form was used in printing. One print series was accelerated aged in the microclimatic chamber at temperature of 800C and the relative humidity of 65% in the period of 10 i 30 days without radiation. Samples prepared in this way were used in the process of the alkaline deinking flotation.

In the phase of sample soaking, deinking chemicals (1% hydrogen peroxide, 0,4 % surfactant, 0,2% DTPA, 1% sodium hydroxide and 1% sodium silicate) were added. The consistency is 10% in regard to the dry substance. A good mixing action was achieved. The disintegration stage was continued for 45 minutes. Suspension was diluted to 0,6 % pulp consistency. An optimum level of hardness was maintained in the flotation cell from 200 ppm CaC[O.sub.3]. The flotation time was eight minutes. The handsheets were made using a laboratory sheet former, according to standard method T 205. Brightness was determined by using the standard method ISO 2469. Residual ink size (dirt particles) ink number and ink areas 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

The study of the paper recycling process is the challenge in the context of science and development, because for the explanation of the mechanisms, it demands the complex approach and the monitoring of each process phase. Only one segment of the broad researches is presented in this work, which contains the influence of the ageing process on the mechanism of the particle detaching from the cellulose fibers, their size and influence on the flotation effectiveness and the characteristics of the recycled fibers.

In figure 1 the division of the particle size and their surface on handsheet after disintegration for previously described samples are given.

[FIGURE 1 OMITTED]

Research results show that the ageing process of prints influences the number of the ink particle size on handsheet after the disintegration process, with all samples. The smallest influence was noticed in processing the digital offset Indigo prints in the described experimental conditions. In this case the additional characteristic was estimated, which refers to great number of particles belonging to higher classes in relation to other printing techniques. By processing the non aged print the ink particles are distributed in all 26 classes. In the lowest class 0,001-0,006 [mm.sup.2] there are 562 particles present and in the highest one >5,0 [mm.sup.2] there are 5 ones. For the aged print, these numbers are somewhat greater and they are 674 for the lowest class and 8 for the highest one. In other digital printing technique which is also on the basis of electrophotography, but which uses in this case the dry toner, 703 particles are in the class 0,001-0,006 [mm.sup.2] for the non aged print and 1226 particles are for the aged print. In this case the particles are distributed including the class 0,15-0,2 [mm.sup.2], to which one particle of the non aged print and 3 particles of the aged print belong. The obtained results are explained by the principles of the mentioned printing techniques, in the way of toner adhering to the printing substrate, of toner interaction with the printing substrate, drying process of prints and the chemistry of the toner itself. The greatest influence of the ageing process on the increase of the ink particle number on handsheet after disintegration was noticed in processing the offset print by ink with greater share of renewable raw material. In this case in the framework of the described conditions, the number of ink particles after ageing is increased to 63,54%, while the surface coverage of the handsheet by the ink particles is increased for 75,6%. The obtained results can be explained by the increased share of the vegetable oil in ink, which is submitted to oxidation reactions by a free radical mechanism which includes the appearance of hydroperoxid, the propagation of the radicals and their termination with the forming of cross-links. It has to be added here that the presence of oil modified alkyds in ink causes networking, which with time, can induce the covalent bonds between ink and substrate over the oxidative polymerisation. Further researches showed that the ink particles which were formed during disintegration phase influence by their size, form and surface characteristics their detaching successfulness from the cellulose fibres within the suspension in the flotation phase. In table 2 only some of the results of the image analysis of handsheed before and after flotation are presented as well as brightness.

The research results show that the greatest number of ink particles was detached in the flotation process of the non aged offset prints made by the ink with smaller share of the renewable raw material. The total number of ink particles on handsheet is smaller for 92,8% when comparing the handsheet before and after flotation. The smallest effectiveness of ink separation by flotation is in recycling the digital offset Indigo prints which can be explained by the size and form of ElectroInk particles which appeared in the disintegration phase as the function of the printing principle, ink chemistry and its interaction with the substrate. As the indicator of the deinking flotation successfulness in combination with the image analysis, it is good to use brightness gain which gives the difference in brightness for handsheet before and after flotation. The greatest value in this case is obtained in processing the non aged offset print with smaller share of the renewable raw material which is 7,1, and by ageing the prints is it decreased to 4,9. The flotation effectiveness is smaller in using the offset print with greater share of the renewable raw material for the non aged print and it is 5,4, and for the aged print it is 3,1. The difference in brightness gain for the non aged prints and the aged print is smaller when compared all the samples. For processing the Indigo prints it is 0,6 which shows bad recycling of these samples. Regarding the size of the particles present on handsheet obtained by processing the Indigo prints and optic non homogeneity which they cause in valorisation of the recycling effectiveness it is necessary to use the results of the image analysis in combination with the brightness results [Bolanca & Bolanca, 2005].

4. Conclusion

Research results confirm the influence of the print ageing process on size and number of ink particles on handsheet from different phase of the paper recycling process in the function of the printing technique and ink composition. Weaker recycling effectiveness was noticed with aged offset prints made with ink with greater share of the renewable raw material which is explained by the oxidative processes. The oil modified alkyds can induce by tine the covalent bonds between ink and fibres over the oxidative polymerization. The presence of interactions of such type makes the most important segment in the investigation strategy of process successfulness of deinking flotation of aged prints, and the further researches go in this direction.

5. References

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Thompson E. V., (1997). "Review of Flotation Research by the Cooperative Recycled Fiber Studies Program, Department of Chemical Engineering, University of Maine" in: Doshi M. R., Jeffrey M. D., (Eds.), Paper Recycling Challenge Vol. II, Doshi & Associates Inc., Appleton, 31-68

Strlic, M.; Kolar, B.;. Novak, G; Pihar, B.; (1998). Ageing and Stabilization of Alkaline Paper, J. Pulp Pap. Sci. 24, 89-94

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Proksch, A.; Hofer, H. H.; Le, P. C.; Knopp, W.; (1999). Alterungbestandigkeit gestrchener Papiere, Proceedings of PTS Symposium, Vortragsband, Munchen, 17-22

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This Publication has to be referred as: Bolanca Mirkovic, I. & Dragojevic, M. (2006). The Influence of Prints Ageing on Number and Size of Ink Particles in the Paper Recycling Process, Chapter 06 in DAAAM International Scientific Book 2006, B. Katalinic (Ed.), Published by DAAAM International, ISBN 3-901509-47-X, ISSN 1726-9687, Vienna, Austria

DOI: 10.2507/daaam.scibook.2006.06

Authors' data: Bolanca Mirkovic I.[vana], Dragojevic M.[aja], Faculty of Graphic Arts, University of Zagreb, grfibolanca@yahoo.com, majadra@yahoo.com.

Table 1. Composition of the model ink

 Composition of the Composition of the
 model ink 1 (%) model ink 2 (%)

Alkyd resin 10,0 10,0
Hard resin 0 21,0
Pigment 17,0 17,0
Additives 13,0 13,0
Mineral oil 26,5 0
Vegetable oil 5,0 39,0
Modified colophony 28,5 0
resin

Table 2. Results of the image analysis and handsheet brightness before
and after flotation

 Total
 Total area of
 Phase of number of particles Brightness
Sample recycling particles [mm.sup.2] %

Indigo, non aged Before flotation 1011 169,326 83,2
 After flotation 731 112,046 85,9

Indigo, aged Before flotation 1313 211,773 80,8
 After flotation 1063 151,18 82,9

Xerox, non aged Before flotation 1526 21,485 76,0
 After flotation 345 4,156 79,1

Xerox, aged Before flotation 2916 47,977 74,7
 After flotation 1537 22,485 77,8

Offset, with Before flotation 4362 37,803 76,5
more renewable After flotation 1479 12,221 81,9
material, non aged

Offset, with Before flotation 11964 154,892 74,0
more renewable After flotation 5529 46,781 78,1
material, aged

Offset, with Before flotation 4330 36,803 78,3
less renewable After flotation 313 2,487 85,4
material, non aged

Offset, with Before flotation 5534 39,874 78,2
less renewable After flotation 629 3,526 83,1
material, aged