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Final Technical Report

Contract n QLK5 - 2001 - 00619 Apr il 1

st

, 2002 - March 31

st

, 2005

OPTIMISATION OF SCREENING AND CLEANING TECHNOLOGYTO CONTROL DEINKING PULP CLEANLINESS

SCREENCLEAN

JACKSTDT GmbH

INSTYTUT CELULOZOWO-PAPIERNICZYPULP & PAPER RESEARCH INSTITUTE

Centre Technique du Papier (CTP), France Project Coordinator

Advanced Fibre Technologies Oy (AFT), Finland

Jackstdt GmbH (Avery Dennison), Germany

Instytut Celulozowo-Papierniczy (ICP), Poland

Papiertechnische Stifftung (PTS), Germany

Laboratoire des Ecoulements Gophysiques et Industriels (LEGI), France

October 2005


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SCREENCLEAN Final Technical Report CTP - AFT - ADJ - ICP - PTS - LEGI October 2005

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Contract details and partnership

Title: Optimisation of Screening and Cleaning Technology to Control Deinking Pulp Cleanliness

Acronym: S C R E E NC L E A N

Type of contract: Shared-cost RTD actionThematic priority: 1.1.1 / 5.3.2

Total project cost1 619 988

Contract number Duration EU contribution

QLK5-2001-00619 36 months 809 986

Commencement date

April 1

st

, 2002

Period covered by final report

1 April 2002 31 March 2005

Project coordinator CENTRE TECHNIQUE DU PAPIER

Name: Franois JULIEN SAINT AMAND Address: BP 251, Domaine Universitaire, 38044Grenoble Cedex 9, France

Telephone+33 4 76 15 40 25

Telefax+ 33 4 76 15 40 16

E-mail [email protected]

Key words: Paper Recycling, Deinking, Screening, Cleaning, Stickies

World wide web address: www.webCTP.com

List of participantsAuthors

Centre Technique du Papier (CTP), France CoordinatorFranois Julien Saint Amand, Bernard Perrin, Thierry Delagoutte

Advanced Fibre Technologies Oy (AFT), Finland Contractor (AC to CTP, ICP, PTS)Robert Gooding, Antti Huovinen

Jackstdt GmbH (Avery Dennison), Germany Contractor (AC to CTP, ICP, PTS)Peter Heederik, Andreas Pahl, Wolfgang Haar

Instytut Celulozowo-Papierniczy (ICP), Poland ContractorHenryk Gonera, Jozef Dabrowski, Tomasz Mik

Papiertechnische Stifftung (PTS), Germany ContractorLutz Hamann, Oliver Cordier

Institut National Polytechnique de Grenoble (INPG) / Laboratoire des Ecoulements Gophysiques etIndustriels (LEGI), France Contractor (AC to CTP)

Dariusz Asendrych, Michel Favre-Marinet


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Table of Content

1. Introduction....................................................................................................................................... 5

1.1. Background............................................................................................................................... 5

1.1.1. Recovered paper recycling................................................................................................ 51.1.2. Deinking............................................................................................................................. 61.1.3. Stickies............................................................................................................................... 8

1.2. Objectives ............................................................................................................................... 10

2. Project development and partnership ............................................................................................ 11

2.1. Project structure...................................................................................................................... 11

2.2. Development of the programme ............................................................................................. 13

3. Material and methods..................................................................................................................... 13

3.1. Equipment and raw materials ................................................................................................. 133.1.1. Research means and pilot equipment ............................................................................. 133.1.2. Paper raw materials......................................................................................................... 143.1.3. Adhesive raw materials.................................................................................................... 14

3.2. Stickies control methods......................................................................................................... 163.2.1. Laboratory screening methods (macro-stickies).............................................................. 163.2.2. Stickies size and shape analysis ..................................................................................... 173.2.3. Extraction methods (micro-stickies)................................................................................. 19

3.3. Adhesive rheological properties.............................................................................................. 203.3.1. Low-speed elongation tests............................................................................................. 203.3.2. High-speed compression tests ........................................................................................ 21

4. Results and discussions................................................................................................................. 23

4.1. Pulping .................................................................................................................................... 234.1.1. Background and objectives.............................................................................................. 234.1.2. Study of basic pulping parameters .................................................................................. 244.1.3. Comparison of drum and batch pulpers .......................................................................... 324.1.4. Development of a new pulping technology...................................................................... 354.1.5. Mill trials........................................................................................................................... 454.1.6. Conclusions and perspectives......................................................................................... 57

4.2. Pressure screening ................................................................................................................. 584.2.1. Background and objectives.............................................................................................. 584.2.2. Numerical simulation studies........................................................................................... 594.2.2.1. Numerical model of pressure screening.......................................................................... 594.2.2.2. Numerical flow simulation ................................................................................................ 63

4.2.2.3. Particle deformation analysis ........................................................................................... 664.2.2.4. Conclusions ..................................................................................................................... 694.2.3. Optimisation of stickies screening ................................................................................... 704.2.3.1. Stickies extrusion ............................................................................................................. 704.2.3.2. Optimisation of screen plate design ................................................................................ 774.2.3.3. High-consistency screening............................................................................................. 894.2.4. Simulation of screening systems ..................................................................................... 934.2.5. Conclusions and perspectives......................................................................................... 98

4.3. Centrifugal cleaning ................................................................................................................ 994.3.1. Background and objectives.............................................................................................. 994.3.2. Stickies density .............................................................................................................. 100

4.3.3. Hydrocyclone cleaners .................................................................................................. 1014.3.3.1. Stickies and cleaner operating parameters ................................................................... 1014.3.3.2. Cleaner design parameters ........................................................................................... 103


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4.3.3.3. Cleaning versus screening............................................................................................ 1064.3.4. Rotary cleaner................................................................................................................ 1084.3.5. Conclusions and perspectives ....................................................................................... 111

4.4. Flotation ................................................................................................................................ 1134.4.1. Background and objectives............................................................................................ 113

4.4.2. Basics of the flotation process....................................................................................... 1144.4.3. Deinking flotation lab and pilot equipment.................................................................. 1164.4.4. Study of basic stickies flotation parameters (lab flotation) ............................................ 1174.4.5. Pilot stickies flotation tests............................................................................................. 1314.4.6. Conclusions and perspectives ....................................................................................... 135

4.5. Pressure filtration .................................................................................................................. 1374.5.1. Background and objectives............................................................................................ 1374.5.2. Basics of pressure filtration process.............................................................................. 1384.5.3. Pressure filtration pilot equipment .............................................................................. 1394.5.4. Preparation of process waters....................................................................................... 1404.5.5. Pilot pressure filtration tests........................................................................................... 1424.5.6. Conclusions and perspectives ....................................................................................... 148

5. Conclusions.................................................................................................................................. 149

5.1. Removal of stickies in deinking lines .................................................................................... 1495.1.1. Optimisation of pulping to improve further stickies removal .......................................... 1495.1.2. Optimisation of macro-stickies removal......................................................................... 1525.1.2.1. Screening....................................................................................................................... 1525.1.2.2. Cleaning......................................................................................................................... 1565.1.2.3. Flotation ......................................................................................................................... 1575.1.2.4. Global macro-stickies removal process ......................................................................... 1585.1.3. Optimisation of micro-stickies removal .......................................................................... 1615.1.3.1. Flotation ......................................................................................................................... 1615.1.3.2. Process water treatment................................................................................................ 162

5.1.4. Conclusion ..................................................................................................................... 1625.2. Recycling friendly adhesives................................................................................................. 163

5.2.1. Pressure sensitive adhesives........................................................................................ 1635.2.1.1. What is a pressure sensitive adhesive? ........................................................................ 1635.2.1.2. What determines the tack and the adhesion of a PSA? ................................................ 1635.2.1.3. Emulsion adhesives ....................................................................................................... 1655.2.1.4. Hot melt pressure sensitive adhesives .......................................................................... 1695.2.2. Potential influence of adhesive components on the separation of stickies ................... 1705.2.3. Recommendations to improve PSAs ............................................................................ 171

6. Exploitation and dissemination of results ..................................................................................... 172

6.1. Exploitation............................................................................................................................ 172

6.2. Dissemination........................................................................................................................ 173

7. Policy related benefits .................................................................................................................. 174

7.1. Communities added value and contribution to EU policies................................................... 174

7.2. Contribution to Community social objectives........................................................................ 176

8. Literature cited.............................................................................................................................. 177

ANNEXE List and copies of the publications resulting from the project 182


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SCREENCLEAN PROJECT FINAL REPORT

1. Introduction

1.1. Background

1.1.1. Recovered paper recycling

Recycling can be considered as a very old tradition of the paper industry as the first papers weremanufactured from old rags and recovered papers have always been a source of fibres raw materials.Today, the European CEPI countries represent about 28 % of the total paper and board production,i.e. 95 millions tons out of 340 million tons produced worldwide in 2003 [1]. The average worldwiderecovered paper recycling rate is now close to 50% and even higher in Europe. Referring to the CEPIcountries considered as the European Declaration on Paper Recovery was launched in 2000, it seemsthat the target of 56 % fixed for 2005 should effectively be reached as the recycling rate increasedfrom about 50 % in 2000 to 53.9 % in 2003 [1]. This corresponds to an increase by 10 million tons of

the recovered paper utilisation rate over this 5 years period.

Most of the recovered papers and boards are still used to produce brown grade packaging papers andboards (figure 1) though a drastic increase of their use in the production of white paper grades hasbeen observed over the two last decades through the development of the deinking process.

0

10

20

30

40

50

60

70

80

90

100

Share of total paper & board production (%)

Utilisationrate(%

)

Mixed grades Corrugated and kraft Newspapers and magazines High grades

Newsprint

Other graphic grades

Case materials

Carton

Boards

rapp

ings,other

pack.p

aper

Hou

sehold&

sanitary

ers

Figure 1: Recovered paper utilisation by sector within the CEPI countries in 2003 [1]

Wood-containing recovered papers, i.e. old newspapers (ONP) and magazines (OMG) are mainlyused to produce newsprint, while wood-free recovered papers such as mixed office waste (MOW) aremainly used for the production of tissue household and sanitary papers. By contrast, the utilisation ofdeinked pulp in other graphic grades, such as super-calendered (SC) and light-weight coated (LWC)papers is currently limited to about 10 % on average in Europe, as shown in figure 1. Consequently

these paper grades show the highest potential towards increased recycling in the European pulp andpaper industry. A few mills already produce SC or LWC papers with up to 100 % DIP [2].


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1.1.2. Deinking

Deinking is indeed a way of recycling, which is used to produce high quality papers. White gradepapers can be produced from post-consumer or post-industrial recovered papers. This means that thecomponents which cause a reduction of brightness, mainly the inks, must be removed, but also that allthe additives used during printing, converting and using the paper must also be removed. From the

recycling point of view these additives are contaminants. They include various grades of adhesives(such as binding materials, labels, tapes), staples, plastic films, inks, varnishes, and all thecomponents of the pulp which cannot be used to produce paper. In some cases mineral fillers andcoating pigments must also be removed to produce paper grades such as tissue papers and to alesser extent LWC papers.

Figure 2: Typical deinking process for improved newsprint, SC and LWC [3]

A typical deinking line proposed for the production of improved newsprint, SC and LWC papers [3] isillustrated in figure 2. Such a deinking process with 2 process water loops or similar processes withoutwashing has become very common in Europe for the production of newsprint and graphic papers [2].The global deinking process includes a number of pulp treatment processes, i.e. pulping, dispersing,bleaching and refining in some cases, and particle separation processes, i.e. screening, centrifugalcleaning, froth flotation and washing [4], as well as pulp thickening processes with filters and pressesand process water treatment processes, including pressure filtration and dissolved air flotation.

Pulping

Recovered papers are separated into individual fibres while inks and all the additives added to thepaper during the printing and converting process are (or should be) detached from the fibres, duringthe pulping step. In the field of deinking, high-consistency batch pulpers or continuous drum pulpersare used to promote the action of the deinking chemicals (caustic soda, sodium silicate and soap) andbleaching chemicals (hydrogen peroxide) normally added in the pulper. Various studies were recentlydevoted to the pulping process and to the analysis of the defibering and ink detachment kinetics [5-8].The optimisation of the pulping step is a prerequisite for the optimisation of the subsequent deinkingprocess steps. Increasing pulping time, temperature and pH leads to excessive fragmentation andeven re-deposition of inks on the fibres, which is detrimental to their removal at the flotation step [5-8].The effects of the pulping conditions on the stickies size, shape and physical-chemical dispersion hadnot been investigated as much as for the inks, but the pulping conditions were assumed to have astrong influence on the stickies as well, which motivated the relatively large research effort devoted to

the optimisation of pulping in this project.


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Dispersion and bleaching

Hot dispersing with high-speed disperser of kneaders [9] is performed after pulp thickening at leastonce at the end of the first deinking loop. The main objective is to detach the inks still attached ontothe fibres in order to remove them in the second deinking loop. Bleaching is normally associated withdispersion to make use of the high consistency and temperature to promote the chemical reactions.

These processes also produce some effects on stickies, which were shown to become more round-shaped during kneading and easier to remove by subsequent screening, cleaning and flotation [10].However, as stickies should be removed before, in the first deinking loop, the dispersing and bleachingsteps appeared less relevant to the stickies issue.

Screening

Pressure screens are implemented after pulping (after the pulper screen or drum screening section) toremove the coarse contaminants. Screening is performed at high or medium consistency (2 to 4 %)with holes and/or with slots down to 0.20 mm. Low-consistency screening with typically 0.15 mm slotsis then performed after flotation to complete the removal of stickies. The screening process, where thefibres have to pass the screen plate while the contaminants should be retained, has been extensivelyinvestigated through theoretical and experimental studies. The probability screening theory canbasically be used for paper pulps as it applies to particles such as fibres and thin contaminants havingat least one dimension smaller than the slot width [11-15]. The very complex flow conditions whichgovern the hydrodynamic particle separation phenomena were investigated experimentally and withthe help of CFD simulation [16-21]. The effects of the screen operating and design parameters wereinvestigated on pilot scale, extensively but mainly in the field of mechanical pulp for fractionation andfor the removal of shives [22-27] as well as with flat-shaped model contaminants [28-30]. Stickiesscreening is more complex as soft stickies particles can be extruded through slots and more or lessfragmented in pressure screens [31-32]. A large part of the research effort has therefore been devotedto the understanding and optimisation of stickies screening, and also because screening is the mosteffective technology to remove contaminants in the macro-stickies size range, as illustrated in figure 3.

Figure 3: Unit operation removal efficiencies versus particle size range [33]

Cleaning

High-density (heavy-weight) contaminants such as sand, glass and metal particles are removed withforward cleaners (hydrocyclones), generally in two steps, first at high-consistency to remove heavyparticles and protect downstream equipment and then at low consistency (about 1%) to remove finesand and protect fine slotted screen baskets from excessive wearing. Special low-density contaminantcleaners, including reverse, through-flow and rotary cleaners have been developed [34]. Stickies areknown to be particularly difficult to remove because of their density close to neutral buoyancy.Comparative studies between different cleaners have shown that cleaners with high radial acceleration

(small static cleaners) and with high residence time (rotary cleaner) are more effective [34-36].


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Flotation

The froth flotation process is the key technology to remove inks with limited fillers and fines losses.Hydrophobic inks are removed in a large particle size range as far a they are detached from the fibres,which requires normally a second post-flotation step implemented in the second deinking loop afterhot-dispersing (figure 2). Macro-stickies are, on average compared to inks, less hydrophobic and also

too large to be removed efficiently under conventional flotation conditions. The use of flotation toremove macro-stickies from screening reject streams at particularly low consistency has been reportedin a deinking mill [37-38]. Micro-stickies should in principle be easier to remove according to figure 3.Considerable research work has been devoted to the physical-chemical aspects of the ink flotationprocess [39-44]. These aspects of stickies flotation were however not sufficiently understood regardingthe stickies surface properties and the role of surfactants, which led to additional investigation onthese aspects in the framework of this project.

Washing

The very small microscopic particles tend to follow the flow split in the pulp thickening and washingprocesses. Washing is a very effective deinking process to remove small inks, the equipment beingspecially designed to avoid the retention of such small particles by the fibres retained on the filteringelement in order to increase their removal with the filtrate [4]. High wash-deinking efficiency is alwaysassociated with high solid losses as mineral fillers, pigments and micro-stickies are also removed withthe ink. Washing is mainly used for tissue papers where almost complete deashing is required, as wellas for the production of market DIP and LWC paper grades.

Process water treatments

Deinking process waters are normally clarified by dissolved air flotation (or micro-flotation) in order toremove the inks from the water circuits. The DAF process is however not selective as all suspendedsolids are removed together with the inks. Considerable research work has been devoted to thecharacterisation of the dissolved and colloidal materials in the process waters, their destabilisationleading to the formation of secondary stickies and the impacts on deposits and paper quality [45-48].

The presence of micro-stickies in deinking process waters has been reported and pressure filtrationhas been proposed to remove them selectively [49]. Pressure filtration is based on the same techniquethan pressure screening, except that the apertures (holes down to 0.1 mm diameter or ultra-fine slots)are so small that fibres are normally retained.

1.1.3. Stickies

It is well known that, among the difficulties encountered in the field of deinking to maintain or increase

the paper quality, the development of various adhesive material to be found in the recovered papers is

one of the most important problems, if not the most crucial in some mills. Such adhesives lead to

numerous stickies problems including deposits on the paper machine, visual defects in the paper and

problems in the printing machines due to residual sticky specks. Despite considerable progress indeinking technology the stickies problems are far from being solved:

- On one hand the amount of adhesive material in deinking furnish is currently growing even faster

for various reasons such as the development of advertising inserts and product samples glued in

magazines and newspapers.

- On the other hand, the increasing use of deinking pulp in high quality graphic papers, a necessity

to further increase the recycling rate up to 56 % set by the EC for 2005 (European Declaration on

Paper Recovery), requires almost a complete removal of stickies to meet todays quality standards

of super calendered (SC) and light weight coated (LWC) papers.

Among the various adhesives and hot melt glues recovered with wood-containing deinking furnish,

newspapers (ONP) and magazines (OMG) from different sources including household collection,

as well as with wood-free deinking furnish, mainly mixed office waste (MOW), the pressure sensitiveadhesives (PSA) used for adhesive labels and tapes are of considerable concern.


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Indeed, due to their inherent physical properties, the solid adhesive particles, or primary stickies

produced during the re-pulping of the recovered papers are very difficult to remove from the pulp even

with the latest deinking technology. In addition some components of the adhesive material are soluble

under conventional deinking conditions and contribute to increase the load of dissolved and colloidal

material (DCM) in the process water, at the origin of the formation of secondary stickies.

-Adhesives are soft and tend to be broken down into small particles during the pulping step.

- Small particles are difficult to remove by pressure screening, even with fine slots, since adhesives

may be extruded through the slots.

- Adhesives are also difficult to remove by centrifugal cleaning, since their density is generally very

close to the density of the pulp.

- Deinking flotation is not very effective in removing primary stickies since adhesives, especially

water-based adhesive products, have normally no particular hydrophobic character.

- Microscopic stickies and associated dissolved and colloidal material can be separated from the

pulp by washing, and may be partly removed by process water treatments such as micro-flotation,

but not selectively, i.e. at the expenses of increased rejects and chemical costs.

Figure 5 shows a rough outline of these processes prioritised according to application and classifiedaccording to pulp suspension and process water [50]. The outline also contains the currentlyconventional classification of particle sizes and the associated definition of the three important stickiesfractions into macro-stickies, micro-stickies and potential secondary stickies.

fillers, fines, short fibers, long fibers

salts, colours, lignin, toner, binder particles orig., resin, adhesive particles orig., fragments of binder- and adhesive films

creation of secondary stickies

colloidal and dissolved material filtrable dispersed solids (stock consistency)

molecular colloidal finely dispersed coarsly dispersed

macrostickiesmicrostickiesmol./colloidal substances (potential secondary stickies)

proce

sswater

stocksuspensio

n

deinking-flotation

mikroflotation (DAF)

water cleaning I fiber recovery

particle-filtration

fiber recovery

membrane filtrationrev. osm. I nanofiltration I ultrafiltration I microfiltration

screening

biological treatment, ozonisation, evaporation

cleaning

L. Hamann, P TS Heidenau

fixation

0,001 0,01 0,1 1 10 100 1000m

washing / thickening

sedimentation

water cleaning I fiber recovery

Figure 5: Classification of stickies and separation processes in pulps and process waters [50]


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Indeed, there is no precise definition of the term "stickies" which is used to describe various materialsand a lot of problems in the recycling and papermaking processes: Tacky particles (or particles able todevelop tackiness in particular conditions) present in the paper and in deposits in various places(paper machine wires, drying cylinders, etc.), represent various forms of stickies. All these problematicmaterials originate from various sources, mainly additives used during papermaking and converting.Nevertheless, adhesive products probably represent one of the main sources.

According to the terminology drawn up in a worksheet by the Zellcheming Committee of Experts onStickies/Recycling Criteria of Recovered Paper Utilisation in 2003, stickies can be subdivided intomacro-stickies, micro-stickies and potential secondary stickies, depending on their tendency to formdeposits and on their screening behaviour under defined separation criteria. The dissolved andcolloidal substances are termed a potential to form secondary stickies.

- Macro-stickies and micro-stickies are distinguished by their separation behaviour under standard

testing conditions (such as the INGEDE method n4), which is usually determined using laboratory

screening with 0.10 mm slots in the case of deinking pulps. Stickies found in the screening residue

are macro-stickies whereas stickies in the screening accepts are referred to as micro-stickies.

Macro and micro-stickies are defined as filterable particles (suspended solids).

- Primary stickies are introduced with the raw material and show an adhesive effect under standard

testing conditions (e.g. INGEDE method n4).

- Secondary stickies are produced by physical-chemical processes during the recycling treatment

and show an adhesive character under standard testing conditions.

- Colloidal and dissolved substances are not referred to as micro-stickies. They are considered as

potentially (secondary) sticky forming substances if they have or assume an adhesive character.

At present there is as yet no sharp distinction in particle side between micro-stickies and potential

secondary stickies. The particle size that can be separated using a filter sheet for pulp consistency

determination, usually about 5 m, might in future become a possible alternative for definition [50].

It is now agreed that recycling friendly adhesives should, as far as possible, be designed in such a

way to be removable as solid particles in the recycling process, and should not be soluble, since the

potential secondary stickies problems are believed to be more difficult to manage.

1.2. Objectives

The general objectives of the project was to develop new solutions to solve recycling problems andimprove the quality of paper products from recovered papers. Practically, the following targets wereset at the beginning of the project:

- To identify the most relevant adhesives materials, which have to be investigated to achieve the

project objectives, i.e. widely used adhesives causing stickies problems in the field of deinking,

and to develop new methods to improve the characterisation of the adhesive particles in the pulp.

- To investigate the influence of all the pulping parameters on the size and shape distribution of theadhesives particles, and to develop new pulping conditions to promote their removal in the

subsequent deinking steps, especially regarding their screening ability.

- To study the mechanisms of pressure screening, with special emphasis placed on the behaviour

of soft visco-elastic particles, in order to develop the understanding of the behaviour of stickies in

screens, to optimise the screening conditions and to develop more efficient screening technology.

- To investigate the centrifugal cleaning parameters, in order to optimise the stickies removal

efficiency regarding the specific properties of pressure sensitive adhesives, and to evaluate the

possibilities of new cleaning and rotation cleaning technology.

- To evaluate the contribution of conventional deinking flotation, a process essentially used for the

removal of inks, to the overall removal of stickies.

- To investigate new ways for the selective removal of stickies from process waters, with specialeffort placed on the optimisation of pressure filtration.


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- To define the best strategies for the production of stickies free deinked pulp, and to establish

guidelines for the implementation in current deinking lines.

- To evaluate the impact of the rheological properties of the adhesives on their removal ability, in

order to establish guidelines for the development of new recycling friendly adhesive products for

the pulp and paper industry.

The project is focused on the optimisation of the removal of primary PSA stickies in the deinking lines,and more particularly on the macro-stickies as the objective is to remove as much as possible stickiesbefore dispersion into microscopic particles or colloidal and dissolved components. The issues relatedto secondary stickies and more generally to colloids in process waters are out of the scope of thisproject, since extensive research, including the European project Colloid control [51], has alreadybeen devoted to this topic. Dissolved and colloidal stickies components were however considered forthe relevant deinking process steps treated in this project.

2. Project development and partnership

2.1. Project structure

As the project is essentially devoted to the improvement of the recycling techniques, which are themost effective in removing the pressure sensitive adhesives, a partnership has been elaborated toprovide the basic scientific knowledge and the test facilities required for the study of these techniques.It includes three Paper Research Institutes (CTP, ICP and PTS) which were in charge of all thepapermaking trials, one adhesive suppliers (Jackstdt, now Avery Dennison) for the supply of differenttypes of adhesive materials, one screening equipment supplier (CAE, now AFT) for the design and themanufacture of screen plates, and one University (LEGI) for the numerical simulation of screeningphenomena. A deinking mill was involved for the evaluation of a new pulping process on mill scale.

The first part of the programme was devoted to the selection and characterisation of the adhesivematerial to be investigated and to the development of test methods. New analytical techniques to

quantify the amount of adhesive in the pulp were further developed in the course of the project as wellas new adhesive products to be tested (WP1).

The second part is the most important part of the project since the key recycling technologies wereinvestigated simultaneously by the different partners. This part was devoted to the optimisation ofpulping to improve the size and shape of adhesive particles (WP2), to pressure screening (WP3) andcentrifugal cleaning (WP4) to improve the removal of the adhesives and to deinking flotation (WP5)and pressure filtration (WP6) to further complete the stickies removal.

Most of the research effort has been placed on the optimisation of pulping, screening and to a lesserextend cleaning, since the basic idea was to avoid as far as possible the fragmentation of theadhesives in the process. Screening is the most effective technology to remove impurities in the visiblesize range. Optimised pulping is a prerequisite to produce large adhesive particles removable by

screening. Centrifugal cleaning under optimised conditions should be very effective for the removal ofhigh-density or low-density adhesives.

The efficiency of deinking flotation and the potential of pressure filtration were also investigated.Indeed, microscopic adhesive particles were expected to be produced, at least with some adhesives,even under optimised pulping conditions. Such microscopic particles can only be removed significantlyfrom the pulp by flotation, as far as the adhesive material is sufficiently hydrophobic. Microscopicparticles tend to accumulate in the process water, if no efficient means are provided to remove them.Pressure filtration was investigated regarding the possibility to remove stickies from the process water.

In this second step of the project, the fundamental mechanical and hydrodynamic aspects, such asnumerical flow simulation for the optimisation of screen plate design, were investigated first, to developa better understanding of the specific behaviour of stickies with respect to the properties of the

adhesive material, and further to develop new and improved technology.


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In the third part of the project, all the results obtained in the previous steps were drawn together andanalysed in order define optimised stickies removal strategies and guidelines for the development ofnew recycling friendly adhesive products (WP7).

The general structure of the project and the interactions between the different work-packages as wellas the related co-operations between the project partners are illustrated in figure 6.

WP1: Raw Materials and Methods

CTPAFT - ADJ - ICP - PTS

1.1: Papers and adhesives

1.2: Stickies control methods

WP2: Pulping

ICPCTP

WP3: Screening

CTPAFT - LEGI

WP4: Cleaning

PTSCTP

WP5: Flotation

PTSCTP

2.1: Basic parameters

2.2: Drum pulper

2.3: New technology

2.4: Mill Trials

3.1: CFD simulation3.2: Screen plate design

3.3: Screening model

3.4: High cons. screen

4.1: Conv. cleaners4.2: New cleaners

4.3: Rotary cleaner

5.1: Lab. flotation5.2: Pilot flotation

WP6: Filtration

PTS

WP7: Stickies Removal Strategies

CTPAFT - ADJ - ICP - PTS

6.1: Process waters

6.2: Pilot tests

7.1: Removal of stickies in deinking lines

7.2: Recycling friendly adhesives

Figure 6: Project management structure and partners involved in the different workpackages

The main contributions and responsibilities of the project partners were as follows:

- CTP - Centre Technique du Papier (France) was responsible for the co-ordination of the project,for WP1 and WP7, which include most of the partners and for WP3 about pressure screening.

- AFT - Advanced Fiber Technologies, formerly CAE (Finland) was in charge of screening relatedwork in WP1 and more particularly in WP3, as a subcontractor of CTP.

- ADJ - Avery Dennison, Jackstdt GmbH (Germany) was in charge of adhesive product relatedwork in WP1 and WP7, as a subcontractor of CTP, ICP and PTS.

- ICP- Instytut Celulozowo Papierniczy (Poland) was responsible for WP2 about pulping. A Polishdeinking mill was involved in this workpackage, as a sub-contractor of ICP, for the mill trials.

- PTS - Papiertechnische Stiftung (Germany) was responsible for the workpackages WP4 aboutcentrifugal cleaning, WP5 about flotation and WP6 about pressure filtration.

- LEGI - Laboratoire des Ecoulements Gophysiques et Industriels (France) was in charge of thenumerical simulation in WP3 about screening as CTP subcontractor.

The project structure in figure 6 will be followed in this document to report on materials and methodsused in the project (WP1) and on the main project results (WP2 to WP7).


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3.1.2. Paper raw materials

Background

Data about the average composition of deinking raw material in Europe has been collected in theframework of INGEDE projects [2]. Most of the recovered paper grades were ONP (Old NewsPapers)and OMG (Old MaGazines) generally mixed (household collection) or delivered separately, as well asMOW (Mixed Office Waste). The data collected from the participating mills gave:

- 50 % ONP, 45 % OMG and less than 3% MOW for newsprint,

- more OMG (about 60 %) for SC papers,

- slightly less OMG and more MOW (about 5 %) for LWC papers

The main raw material for the production of wood-free market DIP is MOW.

Reference paper mixtures

These data and other sources were used to define typical European wood-free and wood-containingdeinking raw material to be used (or simulated) by the partners in this project:

- Wood-containing furnish: a mixture of 50% newsprint (ONP) and 50 % magazines (OMG)

- Wood-free furnish: mixed office waste (MOW)

Reference re-pulping conditions were based on the standard INGEDE deinking chemistry (table 1).

NaOH 0.6 % (100%)

Sodium silicate 1.8 % (1.3-1.4 g/cm3)

Oleic acid 0.8 % (extra pure)

H2O

2 0.7 % (100%)

Table 1: Deinking formulation (related to oven-dry paper) according to INGEDE method n11

The option to use unprinted papers as model papers was selected in some cases as new stickiescontrol methods based on the coloration of the adhesive were investigated, since ink from printedpapers tends to cover the adhesive and thus to produce grey particles (see section 2). The use ofunprinted papers also improved the evaluation of stickies in the case of handsheet image analysismethods, since the background is brighter and more even. Bleached chemical fibres (hardwood andsoftwood pulp mixtures) were also used for screening and cleaning tests, as far as the most relevantdeinking raw material characteristics, such as fibre composition and ash content, were respected.

3.1.3. Adhesive raw materials

Background

Adhesives enter the paper recycling chain with deinking raw materials in the form of self adhesiveslabels, tapes and envelopes, as well as glues for advertiser samples in magazines and book bindings.Water-based adhesives play a major role as they account for about 80 % of the total PSA market forlabel applications in Europe, which represent a total consumption of 551,000 t pressure sensitivelaminates, including [54]:

- 267,000 t of face material (label paper) average: 80 g/m

- 67,000 t of adhesive average: 20 g/m

- 217,000 t of backing paper (75% glassine type) average: 65 g/m


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These pressure sensitive adhesives are usually water-based acrylates modified with tackifier resins,the typical formulation of a water-based adhesive being as follows [54]:

- Acrylic polymer (2-ethylhexylacrylate or butylacrylate copolymers) 60 to 100 %

- Tackifier resin (rosin acid, rosin ester, hydrocarbon base resins) 0 to 40 %

- Additives (Ammonia, defoamers, wetting agents, fungistatica) 1 to 5 %

Others categories of pressure sensitive adhesives include, besides water-based acrylic adhesives:

- Water-based rubber adhesives including Styrene Butadiene Rubber (SBR) and Styrene IsopreneStyrene (SIS) types, the later being the most common

- Hot-melt based rubber adhesives

Among these rubber adhesives, hot-melts show a growing market and are becoming more commonthan solvent based PSA [55].

Further more detailed information on adhesives is reported in section 4.6.2.

Reference adhesives

For various reasons, including the time needed to come to a clear conclusion about the distribution ofadhesive types in European deinking raw material (a prerequisite to define any standard adhesivemixture) and the difficulty to distinguish different types of adhesives mixed in the same pulp sample, itwas decided to focus on the most common adhesives. Two reference adhesives were selected:

- Water-based acrylic adhesives: ADJ Reference E 115

- Hot-melt based rubber adhesives: ADJ Reference D 170

The compositions of the Avery Dennison Jackstdt (ADJ) reference adhesives are given in table 2.The E 115 adhesive is coated as dispersion in water and the D170 adhesive is coated as hot-melt.

Adhesive -Component

Product description function

1 Copolyacrylate Backbone polymer

2 Terpenphenolic resin Tackifier

3 Wetting agent Wetting of adhesivewhen coating

E 115

4 Antifouling agent Avoids fungusformation

1 Styrene Isoprene block copolymer Backbone polymer

2 Styrene Butadiene block copolymer Backbone polymer

3 Hydrocarbon tackifying resins Tackifiers

4 Polyethylene powder Avoids migration

D 170

5 Antioxidant Avoids thermal andUV degradation

Table 2: Composition of the E 115 and D170 reference adhesives


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Adhesive samples

The reference adhesives were supplied by ADJ to CTP, ICP and PTS in the following forms:

- Standard labels with 20 g/m2adhesive (in the form of rolls delivered on request) to be used as raw

material in the deinking process optimisation tests (WP2 to WP5). Special samples with theadhesive films between 2 silicone release papers, were also delivered in order to produce

adhesive particles for the stickies extrusion tests reported in section 2.4.

- Adhesive material in the form of liquid dispersion (about 50 % dry content) for the E 115 adhesiveand in the form of solid samples for the D170 adhesive, to be used for manufacturing the adhesivetest samples required for the different rheological tests.

3.2. Stickies control methods

The aim was to define common methods and to develop new methods for the efficiency assessmentsof the different deinking process steps in order to generate consistent input data for the optimisation ofthe stickies removal strategy and to compare the results obtained by the project partners. A clear needfor improved and standardised stickies control methods also exists at European industry level. Oneimportant point considered in the development of improved and standardised stickies control methodswas that the project needs were somewhat different from the objectives in European standardisation:

- In this project project, the pulp samples were generally over-contaminated with adhesives, andefforts in terms of control costs could be placed on complete and accurate characterisation of thestickies properties, especially particle size and shape.

- Standard stickies control methods are primarily intended for use in deinking mills, which refers tolow adhesive contents in the pulp and implies easy and low time consuming control procedures.

3.2.1. Laboratory screening methods (macro-stickies)

Background

The most common primary stickies control methods are based on the use of laboratory screening withfine slots to remove stickies and contaminants from the fibre suspension in order to be able to analysethe sticky particles with to various methods.

- Macro-stickies generally refer to particles retained on the laboratory screen plate

- Micro-stickies generally refer to particles passing the slots of the laboratory screen plate

These definitions are widely accepted though their are somewhat confusing since macro-particlesnormally refer to particles in the visible size range while micro-particles refers to particle sizes underthe visibility limit of the human eye, i.e. about 100 m and less for high contrasted particles. With labscreens using slots widths around this size limit, it is clear that large flat-shaped particles in the visiblesize range cannot be completely removed if particle thickness is significantly lower than the slot width.

Some preliminary tests, clearly confirmed that the effectiveness of the laboratory screening methods inextracting the macro-stickies from the pulp strongly depends on particle shape and that a largeamount of adhesives in the visible size range passed the slots. Consequently, it was clear that labscreening accepts should also be controlled to analyse both visible and microscopic micro-stickies.

Different lab screening conditions are used. The Haindl Fractionator, the Somerville screen and thePulmac Masterscreen are the most common, though the Brecht-Holl and Weverk screens are also used.

- CTP used the Somerville screen equipped with 0.10 mm slots (0.08 mm slots before the project)for deinking pulps. The Weverk and Brecht-Holl screens were also available.

- PTS used the Pulmac Masterscreen instead of previously the Haindl Fractionator, both devicesbeing equipped with 0.10 mm slots for deinked pulps.

- ICP used the Brecht-Holl apparatus equipped with 0.10 mm slots (0.15 mm before the project).


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Standard methods

The INGEDE method n4 for the analysis of macro-stickies [56], which recommends 0.10 mm slots fordeinking pulps (or 0.15mm slots for long fibre recycled pulps), is the most commonly used in Europe.A modified method which allows to use 0.10 mm for all types of recycled pulps after enzyme digestionhas also been proposed [57]. Consequently, it was decided to use 0.10 mm slots in this project for themacro-stickies, keeping in mind that the micro-stickies should also be analysed.

Concerning the equipment is was decided to use the Somerville screen, the Haindl Fractionator or theBrecht-Holl apparatus, and not the automatic Pulmac Masterscreen, which worked on a different way,i.e. with a rotor instead of a membrane to generate the pressure pulses. A prerequisite to obtaincomparable adhesive control results was first to agree on the slot width, i.e. 0.10 mm, and then to usethe same detailed slot design. Laboratory screen plates were supplied by AFT to CTP, ICP and PTS(deliverable D2) to ensure slot accuracy and uniformity. The slots were milled. The internal controldata at AFT gave the following slot characteristics:

- Nominal slot width: 0.10 mm measured slots: min. 0.10 mm, max. 0.11 mm

- Nominal slot length: 45 mm measured length: min. 44.6 mm, max. 44.8 mm

3.2.2. Stickies size and shape analysis

Standard methods

Among the key parameters which govern the efficiency of the stickies removal process steps, particleshape is almost as important as particle size. Macro-stickies from self-adhesive labels are initially flat-shaped (adhesive film thickness of typically 20 m) and can become long-shaped or round-shaped,depending on pulping and hot dispersing conditions.

With usual laboratory control methods of macro-stickies, such as INGEDE method n4, stickies size ismeasured after heating and pressing, which increases the apparent size of the particles. If stickies areanalysed in handsheets the apparent size is also increased but to a lesser extent because of the lowertemperature and pressure (handsheet formers using vacuum drying). These usual stickies controlmethods do not give any information about the thickness of the particles.

With on-line measuring systems, such as the Simpatic on-line speck analyser (Techpap), the projectedsize of contrasted particles is measured directly in the pulp flow. Again, there is no information on theshape of the stickies shape especially with flat-shaped particles, since the measured size dependsstrongly on the orientation of the particle in the pulp flow.

Development of direct measuring methods

With usual laboratory stickies control methods, the size distribution of the adhesive particles afterheating and pressing is measured by image analysis. Most of the image analysers include various

functions, which can be used to evaluate the shape of objects (spread out stickies) in two dimensions.

- 2D particle shape analysis:

ICP developed a new 2D image analysis methods to evaluate the shape of unaltered adhesiveparticles in two dimensions. In this macro-stickies control method the adhesives are removed from thepulp by lab screening and analysed directly (without heating and pressing). The method also allowedto distinguish the two reference adhesives through colouration. To obtain coloured stickies in the pulpthe adhesives labels were first stuck onto the coloured surface of paper which had been printed withone-colour toner in a xerox copying machine. The coloured macro-stickies recovered by lab screeningwere then picked up with a preparation needle and gently placed on a transparent adhesive tape in apreparation box (figure 8) for further analysis. The different stickies types were discriminated by colourimage analysis. The system also allowed to determine particle size and shape factor and to distinguish

the black ink particles. Figure 8 shows some coloured long-shaped stickies and toner inks obtainedaccording to this method.


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Figure 8: Method for the particle shape analysis and distinction by colour of two model stickies

This method gives a good evaluation of particle shape in the case of round or long-shaped particles,

but does not allow to evaluate the thickness of the particles in the case of flat-shaped adhesives.

- 3D particle shape analysis:

The analysis of the 3 dimensions of a particle is in principle possible if the particle is observed underdifferent angles. Image analysis methods based on the use of several video cameras could bedeveloped to evaluate the shape of particles viewed in a pulp flow. The feasibility of such 3D on-lineanalysis of particle size and shape has been evaluated at CTP. Technical solutions were defined butshowed to be too expensive to develop.

Development of indirect measuring methods

A stickies size and shape analysis method has been evaluated at CTP, based on the assumption that

laboratory screens have a higher potential in separating particles according to their smallest dimension(particle thickness) than pressure screen, though the largest dimensions have also a significant effect(higher lab screening efficiency with large adhesive films compared to small films at given thickness).The size distribution of the adhesive particles in the accepts of laboratory screens gives an indicationabout the shape of the adhesive particles. Large particles passing the slots correspond to flat-shapedadhesives (with lower thickness than the slot width). A small proportion of small particles in the screenaccepts corresponds to round-shaped particle (with higher thickness or diameter than the slot width).The 3-stage laboratory screening method illustrated in figure 9 includes two steps.

Step 1: Pulp samples are first treated with a 2-stage laboratory screening arrangement, the first stagebeing equipped with 0.15 mm slots and the second one with the standard 0.10 mm slots. The acceptsof the second stage are collected on a 200 mesh wire screen. The fraction passing the wire screenshould not contain micro-stickies in the visible size range. Indeed, the retention of small adhesive

particles is much higher with a thickening device equipped with a wire screen with 75 m wireopening, compared to a laboratory screen equipped with slots with 50 m slot width. In this first stepthe fractions retained on the screen plates (R 0.15 and R 0.10) are analysed for the evaluation ofstickies size and shape. The water fraction passing the wire screen can also be analysed to controlsmall micro-stickies and evaluate the amount of these small micro-stickies, which should be found inmill process water circuits.

Step 2: The pulp fraction passing the 0.10 mm slots is then submitted to enzyme treatment afterthickening on the 200 mesh screen. The enzymatic stickies control method [57] has been developed atCTP in order to facilitate the use of finer slots (0.08 mm slots instead of usually 0.10 or 0.15 mm slots).The method enables the use of even finer slots, i.e. 0.05 mm slots. Enzyme digestion degrades thefibres and consequently reduces the amount of fibres retained on the screen. It improves particlecount with the INGEDE method n4 since residual fibres tend to cover stickies which should normally

be counted. Both fractions retained and passing the slots (R 0.05 and P 0.05) are analysed.


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0.15 mm

0.10 mm

200 mesh(75 m) 0.05 mm

Enzymedigestion

R 0.15

R 0.10

R 0.05

P 200 P 0.05

Step 1: 2 laboratory

screens in cascade

Step 2: laboratory

screening after

enzyme digestion

Figure 9: Three-stage laboratory screening method for indirect stickies size and shape assessment

In this indirect method for the evaluation of the size and shape of the stickies, the size analyses of the

fractions retained on the screen plate (R 0.15, R 0.10 and R 0.05) can be performed by image analysiseither on standard handsheets or with the INGEDE method. The last fraction passing the slots (P 0.05)has to be analysed on handsheets. The evaluation of the shape of the particles requires to establish,for each of the 3 fractions retained on the screen plates, correlation between particle size distributionsmeasured directly on the particles (ICP method in figure 8) and after pressing and drying (INGEDEmethod n4 or handsheet analysis). This methods however is very time consuming.

3.2.3. Extraction methods (micro-stickies)

Micro-stickies in the form of finely dispersed adhesive particles and potential stickies in the form ofcolloidal and molecular mater contribute strongly to the stickies problems observed in mills [52, 53]

Extraction methods are currently the only possibilities to control these categories of stickies as theyare not separable by laboratory screening, even with very fine slots.

Conventional DCM Extraction Method

The conventional extraction method using dichloromethane (DCM) as the solvent is used by severalresearch institutes to extract and quantify characteristic component of adhesive material. This methodhas gained widespread acceptance to evaluate micro-stickies and colloidal stickies in pulp samples,though the method is not selective, since other materials such as coating binders are extracted andquantified together with PSA material.

CTP used and developed this method for targeted stickies analyses performed in this project. Themain problem of this method at the present time is its accuracy. Indeed, the rate of extract is quantified

by weight after total solvent evaporation. Unfortunately, the weight measured may be sometimes verylow (some mg) which makes it difficult to get accurate result.

Development of a new DMF extraction method

PTS has developed a more targeted extraction method using DMF (dimethylformamide) which permitscomplete, selective measurements of sticky loads by means of solid-liquid extraction. The descriptionof the protocol and advantages of this new method have been reported (in the Annex 6 of report D1).

The recognition rate of coating binders achievable with DMF extraction showed to be nearly 50 %,which is enough to clearly identify the individual binder loads of different pulps. The reproducibility ofthe extraction results was therefore regarded as excellent. The recognition rates of further potential

stickies such as adhesives were also investigated to make sure that all stickies types present weredetected by the measurements.


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The comparative results of the DCM and DMF extraction are shown in figure 10 in the case of binders.The binder content of the paper samples increased towards the sample comprised of 100 % offsetpapers. The DMF extraction results reflected the trend towards increasing binder contents, with asufficient degree of detail or differentiation, while the results of the DCM extraction were significantlyless satisfying. The absolute extract contents determined by DMF extraction were on average abouttwice as high as those obtained by DCM extraction, i.e. the binder detection by DMF extraction was

significantly more complete.

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

100 % newsprint

100 % coated gravure

papers

50% newsprint 30%

coated gravure 20%

coated offset

100 % coated offset

papers

Extract yield [ % ]

Extraction inturbulent flow - DMF

Conventinal SoxhletExtraction - DCM

growing content

of coating binders

Figure 10: DCM/DMF extraction in comparison for the evaluation of coating binders

3.3. Adhesive rheological properties

The analysis of the forces applied on stickies during pulping and screening indicated that adhesiveparticles should be submitted to high-speed deformation, namely during contacts with pulper or screenrotor elements, as rotor velocities are generally in the range of 10 to 20 m/s. In the case of screening,the analysis of possible single step extrusion of adhesives through slots showed that both strain andstrain rate would be very high with particles much larger than the slots, if the particle effectively passesthe slot during the screening phase. With integral calculation of the average strain at high deformation

(= ln d0/d = ln d0/w) and for a particle squeezed by a factor 3 in 10 ms, the strain rate (/t) would beof the order of 10

2s

-1. Results about the dynamic moduli, G and G, of adhesive products have been

reported but the measurements were performed in dry state at much lower strain rate [58].

The analysis of the dynamic behaviour of adhesives was necessary to provide input data for the

numerical simulation work performed at LEGI about stickies screening. The work was subcontracted toLEMTA (Laboratoire dEnergtique et de Mcanique Thorique et Aplique, INPL/CNRS, Nancy, France)where expertise was available in this field and for the analysis of friction factors of soft materials [59].The equipment at LEMTA had however to be adapted to the very soft adhesive materials to be tested.Preliminary low-speed elongation tests were done at CTP to provide some first simulation input data.

3.3.1. Low-speed elongation tests

Preliminary tests were performed with paper testing equipment at elongation velocities up to 1 m/min,which gave a maximum strain rate of 0.4 s

-1 in the narrow part of the adhesive test samples (40 mm

length, 15 mm width and 2 mm thickness). The dimensions of the test samples were not exactly thosedefined for standard rubber testing procedure because of the difficulties encountered in preparingsamples with very tacky material (figure 11).


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Adhesive sheets were produced from the two reference adhesive raw materials, cut in water andtested after soaking. Both adhesives recovered almost their initial length in a few minutes afterelongation by a factor 10. Figure 11 shows pictures of an adhesive sample back to its initial positionafter a first elongation and submitted to a second elongation.

Standard rubber tes t sample Adhes ive sample and tensi le tes t (2nd

elongation)

NF T 46-006

Type H1 testsample shape &

dimensions(narrow portion)

33 mm length

6 mm width

2 mm thickness

Figure 11: Low-speed adhesive elongation tests

The typical stress strain relations are shown in figure 12. Both adhesives showed a first elastic partfollowed by a drop of the modulus with lower strength at the second elongation, especially with thehot-melt rubber product. The first pass elastic modulus was about one order of magnitude higher with

the hot-melt rubber than with the acrylic adhesive, which gave G 20 kPa at 1m/min after soaking inhot water. The elastic modulus showed to increase with increasing elongation velocity and to decreasewith increasing temperature.

L-L0 / L0

F E 115

0 1 5 10

1st

2nd

L-L0 / L0

F E 115

0 1 5 10

1st

2nd

L-L0 / L0

F D 170

0 1 5 10

1st

2nd

L-L0 / L0

F D 170

0 1 5 10

1st

2nd

Water-based acrylic adhesive Hot-melt rubber adhesive

Figure 12: Typical low-speed elongation stress strain curves

3.3.2. High-speed compression tests

The experimental equipment and method illustrated in figure 13 were developed to record stress strainrelations applied on adhesive materials at high strain rates:

- a cylindrical adhesive sample (18 mm diameter and 28 mm height) is placed on a bottom plateequipped with a load sensor,

- a weight is dropped from different heights on the top plate above the adhesive sample,

- the compression of the adhesive sample is measured with a laser recording the height of thebottom plate.


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h

F(t)

(t)

Vma x = (2gh)1/2

Laser

Sample Lubricant

Mass

h

F(t)

(t)

Vma x = (2gh)1/2

Laser

Sample Lubricant

Mass

Figure 13:High-speed adhesive compression tests

The initial strain rate are controlled by the drop height (0.15 to 0.9 m, e.g. 0.8 m gives a drop velocityof 4 m/s and an initial strain rate of 150 s

-1) and the stress level by the weight. The samples are

covered with talcum to allow adhesive material to slip between the plates and avoid barrelling of thecylinder during the compression. The recorded data (figure 14 left) are converted into stresses ofCauchy and true axial strains.

0

0,2

0,4

0,6

0,8

1

0 0,05 0,1 0,15Time (s)

Sensoroutput(V).

Distance to top plate (laser)

Load on bottom plate

0

1

2

3

0 0,1 0,2 0,3 0,4 0,5Strain

Stress

Experimental

Model

Figure 14:Dynamic test: recorded load and compression and experimental results versus model

The dynamic compression test is distinguished from the usual quasi-static tensile test by the fact thatthe strain rate drastically decreases during compression, towards zero at the end of the tests. Theresidual stress due to the weight at the end of the test is considerably lower than the one due to thehigh deceleration of the weight at the impact. The strain level reached with adhesive material can berelatively significant, up to 1. Figure 14 (right) shows a typical example of stress strain curve obtainedwith the dynamic compression test, at non-constant strain rate.

In principle, a large number of tests should be performed to determine, as a function of the strain rate,the stress strain relations, which are required for the simulation of particle extrusion. The developmentof a model describing the dynamic behaviour of viscoelastic material will enable to determine theserelations from the experimental data. The theoretical curve plotted in figure 14 represents theestimated result obtained from a viscoelastic model with two relaxation times. Different relaxationstimes generally refer to the components of the generalised Maxwell model where infinite viscosity ofone component gives the solid behaviour. The new theoretical model under development is based onthe thermodynamics of irreversible processes produced by the stresses and strains applied to thepolymers [60-61]. Indeed, polymer chains exhibit the highest disorder, i.e. maximum entropy, at steadyrelaxed state, while stresses tend to stabilise. The complex behaviour of adhesive material dependson the relaxation times spectrum which is related to the micro-structural reorganisations observed atdifferent scales. Further analyses of adhesive rheological properties are in progress to determine more

completely the dynamic behaviour of the adhesive material at high strain rates.


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4. Results and discussions

4.1. Pulping

4.1.1. Background and objectives

Pulping is the first step of the deinking process. Defibering, ink and adhesive detachment, and particlecomminution in the pulper are achieved by forces imposed on the recovered paper through the actionof the rotor. And therefore the pulping step strongly affects the quality of the pulp to be treated in thesubsequent steps of the deinking process, and as such it is decisive for good deinking. Most of theresearch work about pulping has been devoted to the optimisation of the recovered paper re-pulpingand deinking processes, namely in terms of deflaking and ink detachment kinetics [5-8].

However, and despite of its considerable role, little work has been done to characterise the pulpingphenomena with respect to stickies and other contaminants besides inks. The contaminant size andshape distributions are also governed by the pulping conditions, as understood in terms of mechanical

stresses and physical chemistry. The latter is determined by the deinking bath, with a possibleadmixtures, as well as by pulping temperature. The development of new optimised pulping conditionsaimed at creating the quite large particles of the contaminants is a prerequisite for their efficientremoval in the subsequent process step, especially fine slot screening.

The objectives in the research programme devoted to pulping in this project were more particularly:

1. to study the influence of basic pulping parameters on both stickies and inks,

2. to evaluate state of the art pulping technology, i.e. batch and drum pulpers,

3. to develop a new pulping technology, which is based on the agglomeration of stickies, and

4. to test, after the lab pilot scale optimisation, the new pulping technology on mill scale.

Except for the second task, which was performed at CTP where relevant pilot pulping equipment was

available, all the pulping studies were done at ICP. The scientific approach towards the developmentof a new pulping process was based on the experience gained by ICP with the agglomeration ofprinting ink particles, which was acquired in the time when newspapers were printed by letterpresswith inks being dispersion of carbon black in dyed mineral oil, and drying process of the newsprint inkwas mainly by absorption [62]. By setting proper conditions (hydrodynamic and physico-chemical)during pulping of newspapers it was possible to achieve agglomeration of such inks. Spherical-shapedparticle were obtained after 20-45 minutes pulping and were large enough to be removed by cleaners.Further investigations also allowed to agglomerate successfully tiny particles of xerographic toner inks.

Figure 15: Agglomerates removed from recovered newspaper re-pulped with deinking chemicals


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4.1.2. Study of basic pulping parameters

Deinking of pulp fibres is essentially a laundering or cleaning process in which the ink is considered tobe the dirt. To dislodge the ink particles from the fibres, chemicals are used in the pulping step, alongwith heat and mechanical energy. The detached ink particles are then removed from the stock in

subsequent steps of the deinking process. Both deinking lines and deinking technologies are designedto remove the ink particles, and therefore they are usually not suitable for efficient separation from thestock such specific contaminants as the sticky particles (stickies) are. The latter are created during thepulping step by comminution of various adhesive materials to be found in recovered papers, amongthem the pressure sensitive adhesives (PSA) used for adhesive labels and tapes, also as adhesivelayers applied to attach samples to paper, are of considerable concern. The printing ink, which usuallyis fixed on paper in the form of small solid dots, consists of pigments and dyes, dispersed in vehiclesor binders, to which some other ingredients are added. During the pulping step these dots behave likebrittle solids and they are divided into smaller elements which are close in their size to the size of thepigment particles used in given ink. However, the layers of PSA materials applied on paper are such acontinuous film of elastomers in which there is a lack of such frontiers as perceived in printing ink dots(between pigment particles and binders), so the comminution of the PSA layers is more chaotic in itscharacter. Moreover, ink particles are usually more hydrophobic than sticky particles, and therefore the

ink particles may be important constituent of the co-agglomerates with stickies, under favourableconditions of the pulping process. The ICP studies carried out in a frame of this project, were aimed atapplying the forces imposed on recovered paper in the pulper not only for defibering, ink and adhesivedetachment, and particle comminution, but also for agglomeration of adhesive impurities and for theirco-agglomeration with ink (and other) particles. In such an approach the typical activities of the pulperare perceived as the first step which is necessarily required for successful accomplishment of suchagglomeration and co-agglomeration processes in which ink particles, and perhaps some otherparticles too, may be useful components for such enlarging of the co-agglomerates, which is neededfor their successful removal in the subsequent steps of the deinking process, especially in fine slotscreening. The studies, in the work package devoted to the pulping technology (WP 2: Pulping), wereaimed at combining together two different processes, namely: de-inking and de-sticking, therefore.According to the results previously gained by ICP, the agglomeration of printing ink particles (andtoner particles too) is promoted by adding to the pulping step such properly selected substances whichare able to combine the particles together; however, such agglomeration is possible under the laminarregime flow of slurry in the pulper. A very distinctive feature of such flow is the continuity of externallayer of the slurry in the pulper, and the laminar regime of flow occurs at different consistencies,depending on construction of the pulper and its rotor as well.

Studies on behaviour of different PSA labels during pulping

The mechanical strength of the self-adhesive layer is decisive for its susceptibility to comminution;however, in technology of the pressure sensitive adhesives (PSA) two different strengths arerecognized: adhesive strength and cohesive strength. Experiments performed by ICP, clearly showedsuch a different behaviour of the same adhesive layer applied on different base materials (figure 16).

Figure 16: The PSA labels on plastic foils after pulping (45 minutes) with the model recovered paper(the rectangles are equal to an initial surface of the label)


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Almost all studied labels on plastic foils (only the Acetat foil was an exception) survived the pulping ofrecovered paper without their dividing into smaller parts, and with an inconsiderable comminution ofthe adhesive layer. Therefore, the experiments proved that the forces applied to the self-adhesivelayer during the pulping step were weaker than the adhesive strength between the adhesive layer andthe plastic foil.

However, the labels with a pressure sensitive adhesive applied on paper base are such a completelydifferent case in which the paper base is disappearing during the pulping step, and this fact is quicklyfollowed by comminution of the adhesive layer. In such a special case, which is of utmost significancefor the industrial practice, the cohesive strength of the adhesive layer is decisive for its susceptibility tocomminution. Size of the small granules, being created from the adhesive layer during flow of slurry inthe pulper, is determined by the velocity gradient between the slurry layers in which ends of theadhesive layer are anchored, as well as by an initial length of the adhesive layer. Such a specificcourse of the phenomenon should not be expected during the plug flow of slurry in the pulper,therefore. In pulping experiments with non-printed wooden paper and with an admixture of the paperlabels coated with the PSA layer, amounted to about 300 mg per kg of the oven dried pulp, thereduction of stickies size progressed during the pulping run under the laminar regime of flow, and withdeinking chemistry according to the INGEDE method n11. The acrylic adhesives were moresusceptible to comminution than the hot-melt rubber adhesives. Particles of the latter, however,

became spherical in shape during such pulping, so they could be easily separated on slotted screens.Nevertheless, under such conditions there was a lack of agglomeration of the sticky particles. Theirquite hydrophilic character was an obstacle in the way of the agglomeration process. To that end anadmixture of the agglomerant is needed, and the presence of such contaminants in the slurry as ink ortoner particles would be very helpful.

Effect of pulping intensity and addition of the agglomerants

In this project two PSA paper labels, both of the Avery Dennison Jackstdt (ADJ), were selected asthe reference adhesives, namely: the water-based acrylic adhesive (denoted as E 115), and the hot-melt rubber adhesive (denoted as D 170). Before pulping experiments, the rubber adhesive (D 170)was stained yellowand the acrylic adhesive (E 115) was stained cyan. After pulping experiments, theparticles rejected on the slotted screen (with 0.10 mm slots) were collected on white filter paper.Together with the paper, and still in wet state, they were placed between two sheets of the transparentpolyethylene foil. Such kind of sandwich was introduced into the scanner. In that procedure adetachment of the particles from surface of filter paper is avoided, and the scanner glass is kept clean.The particles of both adhesives, as well as ink or toner particles, were finally identified during thecomputer-aided image analysis (using the Spec*Scan System, byApogee System Inc.), according tothe grey scale value range (GSV) established for each kind of particles. Therefore the D 170 particleswere detected within the GSV range of 220-240, and the E 115 particles were detected within the GSVrange of 135-190. The black particles of ink or toner were detected for the GSV below 100. Such amethod, which we will refer to as the ICP method, gives an insight into the size and shape of differentparticles, which are separately perceived in image analysis. Their shape was additionallycharacterised as an eccentricity(Ecc), according to the Spec*Scan System, byApogee System Inc.;for perfectly round particle its Ecc=1, and the more oblong the particle is the higher its Ecc.

Possibilities of such successful co-agglomeration of the stickies with toner particles were shown in thepulping experiments carried out with wood-free xerographic paper (denoted as OWP) had been non-impact printed on its entire surface (one side only) with black toner. Among the studied agglomerants,added in quantities from 1.2% to 1.5% (in relation to oven dried paper), 1-octadecanol (denoted OD)was quite efficient in promoting such co-agglomeration during the pulping run under the laminarregime of flow; with deinking chemistry according to the INGEDE method n

o 11 (without peroxide,

however). The ICP method made possible the visualisation of the agglomeration and co-agglomeration processes during such pulping. An example is presented in figure 17, showingagglomerates of the sticky particles and their co-agglomerates with toner particles.


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From figure 17 it is clearly visible that the comminution of toner and adhesive particles is followed bytheir agglomeration and co-agglomeration, during the pulping step run under the laminar regime offlow and with an admixture of the agglomerant. Due to such co-agglomeration of adhesive particleswith toner particles, the co-agglomerates were created with a prevailing effect of the black colour, andtherefore with grey scale values below 100; it means they were classified as the toner agglomerates,in the computer aided image analysis, besides particles and agglomerates detected within the GSV

ranges characteristic of both coloured adhesives. The area of toner agglomerates, it means theircontent, was much larger than contents of the sticky particles from both studied PSA materials. It isthe proof that such successful co-agglomeration of toner particles with properly disintegrated debris ofthe PSA layer is possible during pulping run under the laminar regime of flow, and with an admixture ofthe agglomerant, i.e. such hydrophobic surface active agent which is able to combine togetherparticles with different surface activities. All of the studied contaminants had practically the same modevalue of their eccentricity coefficient (Ecc). It seems to be a proof that under conditions of the pulpingstep favouring agglomeration and co-agglomeration processes there is a strong tendency to createmore homogenous particles of the contaminants. More intensive pulping, run at higher rotation speedsof the rotor as well as at higher pulp consistencies (to keep the laminar flow of the slurry), resulted in aviolent growth in amount of the co-agglomerates created by toner and sticky particles. It may beunderstood, that the co-agglomeration process is promoted both by proper comminution of theparticles and by better dispersion of the agglomerant, during such laminar pulping of recovered paper

non-impact printed with toner, in the presence of the PSA materials. There is a need to adjust properlytemperature during pulping to the melted point of the agglomerant.

An influence of the energy parameters and their joint action with physico-chemicalparameters upon the alterations in size and shape of the particles

At the beginning of the ICP investigations, carried out in a frame of the WP 2 Pulping, it was assumedthat thorough knowledge of the average size Aav [mm

2] of the macro-stickies and their shape

expressed as an eccentricity Ecc would be sufficient to characterise their properties. The abovediscussed experiments showed, however, that such values as the amount of macro-stickies SA[mm

2/kg] and their number SN[n

o/kg] used alone were not fully proper in investigating the phenomena

connected with creating stickies and their common agglomeration and/or their co-agglomeration withother particles, while pulping recovered paper. In order to accurately describe the phenomenagoverning the creation of the stickies and their agglomerates and/or co-agglomerates, the knowledgenot only of the average values of such properties but also of their distributions is required.

Nevertheless, even in such pilot plant experiments there is no possible to analyse the entirepopulation of stickies, owing to the uncontrolled participation of sticky particles of the PSA materials increating the deposits on pulper surfaces, see figure 18. Moreover, the statistical analysis is not usedfor the entire population of the stickies created during experiments, but only for a part of it, collected inthe screen (with 0.10 mm slots), i.e. without the stickies with a surface size smaller than 0.014 mm

2,

which corresponds to the value of the lower threshold of size, as established for the particles countedin the computer-aided image analysis.

Figure 18: Deposits of organic matters on pulper surfaces


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In attempts to describe the size A [mm2] distribution of the macro-stickies, the parameters of the

following distributions were taken into consideration, namely: normal distribution, lognormaldistribution, and dislocated lognormal distribution. It would be much more correct to refer to thedislocated lognormal distribution to accurately describe the distribution of the macro-stickiesproperties. Additionally, a general evaluation of the comm

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