Spare a thought for the paper recycler.

The evolution of printing over recent years has resulted in new printing processes as well as the development of the old. The associated ink developments have presented and continue to present challenges to the recycling technologist because the quality of the recycled product depends largely

on the success of the deinking, that is, removal of old ink from the pulp. This in turn is influenced largely by the structure and composition of the surface coating applied. New formulations often pose fresh challenges to the deinking technologist. The original problem of deinking fibre printed with simple mineral oil.-based news inks has drastically increased with the introduction of non impact inks, water-based gravure and flexo, conventional UV-cured inks and coatings, cationic UV-cured inks and coatings including water-based flexo, gravure and screen process, heal set and cold-set web offset news inks. Modern recycling plants must be able to deink all types of inks and each category has its own deinking peculiarities. Mixed office waste may increasingly be expected to contain new inks including high density laser and photocopy, digital offset, coloured laser and photocopy, water-based gravure, UV-cured, thermal printing, and inkjet inks both solid and liquid.

The nature of the ink contaminants that may be present in fibre furnishes can be grouped into four categories:

(i) News and magazine inks. The problems presented by these have been largely resolved in conventional recycling plants. tim most recent innovation being cold-set colour news inks.

(ii) Radiation curing inks and coatings arising from magazine covers and packaging products. The problems created by these arise from their tough film-forming characteristics and they require special treatment. (iii) Non impact inks. These are predominantly fused toner systems which include digital offset inks and which. like UV-curing inks, are difficult to break down.

(iv) Water-based inks. Paradoxically, although considered to be environmentally friendly during application. these are difficult to deink and were once considered a 'looming menace' for the paper recycler.

Some ink components relevant to the deinking process are summarised in table 1.

The nature of the problem

In conventional deinking ink is detached from the paper surface by mechanically repulping under alkaline conditions (pH 9.5 to 11.0), in the presence of surfactants. The sodium hydroxide used swells the cellulose fibres and aided by the deinking surfactants, the ink separates. Resin-based binders may undergo saponification or hydrolysis which assists ink removal. The chemical technology of deinking has been fully reviewed elsewhere (1). Under mechanical agitation in the pulper, the ink detaches and breaks up into particles of varying size. The ease of detachment depends upon the chemical and physical properties of the binder and in some polymerising inks becomes more difficult with aging. The work of adhesion between binder and base paper, and binder and deinking surfactant is an important factor. The particle size ranges produced are influenced additionally by the printing process and the base paper (Table 2).

TABLE 2

INK PARTICLE SIZES (IN MICRONS) IN THE REPULPER

                      Uncoated Paper             Coated Paper

Letterpress           2-30 microns               10-100 microns
Offset Litho          2-30                       5-100
Flexo                 0.3-1                      0.7-2
Gravure               2-30                       5-30
Laser/Photocopier     40-400                     40 400
UV                    Large Continuous Sheets

Source: Ferguson LD Introduction to Printing Technology and Ink
Chemistry. TAPPI De-inking Seminar Notes, TAPPI Press, Atlanta GA
1992

It is most important to remove from the pulp slurry the detached ink particles since it is these that most degrade the optical properties of the finished sheet. Large particles which are visible to the naked eye cause 'speckiness' and particles smaller than 40 microns across cannot be resolved by the naked eye but instead contribute a greyness to the appearance of the paper. This in turn reduces the dynamic range attainable on the printed sheet as well as its optical brightness. In general, the success of the deinking process is usually measured by the ISO brightness of the final pulp. Of course, the final brightness of the pulp is also influenced by variations that arise from different fibre types present in the furnish. These can vary from the high brightness exhibited by the bleached kraft fibres used in printing and writing papers to the low brightness of thermomechanical pulps used in many magazines and newspapers.

The sizes of ink particles that occur in a deinked pulp are important because of the significant affect they have on its optical properties. Leighton and Miranda (2) have shown that reducing the particle size at the same mass concentration of ink decreases reflectance. Small particles such as those produced by flexographic inks (less than 1 micron) absorb light uniformly over their entire surface area. On the other hand, large platelets of the order of 200 microns or more (such as those produced by laser and UV-cured inks) demonstrate a non-uniformity of absorption. This is thought to be because more light is absorbed by the top side of a particle than by its underside, an affect which is accentuated by large fiat particles (3). The combined affect on reflectance of fibre type and ink particle size distribution is such that high reflectance pulps experience greater reflectance loss than low reflectance pulps for a given amount of residual ink (4).

The problem then is one of removing the detached ink particles from the recycled fibre slurry, and the importance of rendering them into collectible size ranges cannot be overstated. The difficulty is that different particle size ranges require different methods of removal and a typical pulp can have a wide range. The papermaking technologies suitable for the removal of so called 'contraries' are summarised in figure 1.

Slotted screens remove particles from 200 microns in size upwards. These would include very large, bulky, ink particles as well as paper clips and other large contraries. Vortex cleaners employ a centrifugal action to separate fibre from non-fibre but their relative density differences must be large for this to succeed. They will then remove particles in the size range 100 to 350 microns. The most important process and one by which the majority of particles are removed is flotation which removes particles in the size range 20 to 150 microns.

In flotation deinking air bubbles are introduced into the pulp together with collector chemicals (surfactants) which increase the hydrophobic nature of the ink particles and therefore their reluctance to associate with water. The air-water boundary of the bubble is hydrophobic and the ink particle must penetrate it in order to attach itself. Hydrophilic particles such as those formed by water-based inks will remain with the water environment. Hence flotation deinking will not work for water-based inks. This is an important consequence for water-based systems in general.

Ink particles which do attach themselves to bubbles may acquire sufficient buoyancy to float them to the surface whence they are removed. Particles which are less than 20 microns across have insufficient inertia, on impact, to penetrate to the hydrophobic air-water interface of the bubble and do not become attached. Particles which are larger than about 150 microns are too large to gain sufficient buoyancy to float to the surface. The particle size range collectible by flotation must therefore lie within 20 to 150 microns for efficient removal. Ink particles detached in the pulper must therefore lie in this range. This is crucial for flotation deinking.

Reference to table 2 shows that flotation is suited to letterpress, litho and solvent-based gravure inks but not to water-based flexo. Moreover, only a small proportion of laser, photocopier and UV ink particles can be removed without further size reduction.

Finally, washing deinking is only affective with particles less than about 10 microns in size, because larger particles and especially those which are still attached to fibre clumps may become trapped in the fibre slurry. Wash deinking chemicals are necessary to prevent re-deposition of ink particles. Extensive washing. however is uneconomical and requires environmentally demanding clarification procedures. It is therefore only used to remove the smallest particles and then in moderation.

Overall then, flotation is the preferred option but the importance of particle size distribution can now be seen. The deinking strategy then becomes one not only of detaching ink from fibre but one of producing the best particle size distribution for collection. Inks which are difficult to deink are those which not only may be difficult to remove from the fibre but which do not produce easily collectible particles.

Difficult Ink categories

Water-based flexo and gravure inks

Water-based inks are difficult to separate by flotation for two reasons. Firstly, the particle sizes produced after pulping are at best colloidal and too small to flotate (table 2). Secondly, the resins employed are necessarily hydrophilic, making it difficult to separate them from the aqueous environment of the pulp slurry, other than by copious washing.

Although cellulosic packaging waste is recycled, the biggest threat from waterbased inks is currently felt in the recycling of newsprint. Water-based flexographic newspaper printing now offers greatly improved quality and is already well established in the printing of certain newspapers: production is set to increase with the installation of further flexo presses. The future will probably also see a growth in the use of water-based gravure inks for magazine printing. The problem with water-based newsprint inks lies in their composition and application. They are usually formulated using acrylic resins which are soluble under alkaline conditions where they exist in the water soluble ionic form but insoluble in the carboxylic acid form. The ink is applied in alkaline solution and then saponified to the insoluble carboxylic acid form either by making contact with the acidic newsprint or if the alkaline ammonium salt is used, by heating; the ink thus sets. The reaction is unfortunately irreversible and these inks show notoriously poor alkali resistance. This is not a problem for newspaper printing, but since the resin resolubilises in the alkaline environment of the repulper, the ink can only be removed by prohibitively extensive washing and not at all by flotation. The nature of the problem is such that recycling plants will not accept 'returns' and 'overs' from flexo newsprint sources. Nevertheless, small quantities inevitably enter the waste system from consumer sources.

A technique to combat the problem has been devised by Galland and Vernac (5) who added a primary acidic flotation stage in which resin is precipitated and coalesced into flotatable size ranges. This is followed by conventional alkaline treatment and a secondary flotation stage in which other ink types are removed.

Dispersion Stability

More stable inks have been devised for packaging applications. In traditional emulsion polymer vehicles, glycol ethers are built into the backbone of insoluble styrene acrylate resins by condensing their hydroxyl groups with carboxylic acid groups in the resin. The glycol ether confers its hydrophilicity to the resin thus giving it dispersion stability in water. The resins used are alkali resistant and they cannot easily be resolubilised. The inks 'set' by coalescence of the resins into continuous films when the disperse phase (water) is removed. Indeed this has proven to be a disadvantage, leading to clogging of cells in anilox rollers and gravure cylinders. Even if they do not revert to their original colloidal sizes, the thin films used produce very small particles in the repulper which combined with the hydrophilic properties imparted by the glycol ethers, makes them difficult to remove by flotation. The new generations of graft copolymers also contain hydrophilic polymer segments to make them hydrophilic and although they have yet to be evaluated for deinkability, they are likely to be as difficult to remove as other emulsion types.

The current solution to the problem seems to lie in the development of flotation deinking additives such as agglomerants and collector surfactants rather than in making modifications to what are already delicate ink formulations. Much research has been conducted to make flotation deinking feasible by changing the flotation cell chemistry to render the ink particles hydrophobic so that they may be coagulated into collectible sizes. However, this requires that recyclers modify their flotation cells and chemical systems.

UV-cured inks and coatings

UV-curing inks and coatings find widespread application in the packaging industry where low taint and good adhesion to nonabsorbent substrates are important. Although flexible polymer substrates have no place in paper recycling, cellulosic ones do. However, the main source of UV-cured inks and coatings in recycled furnishes is magazine covers; magazines provide a necessary source of higher quality fibre in newsprint furnishes (typically 70 per cent newsprint to 30 per cent magazine) These are currently UV-curing systems based on conventional epoxypolyacrylate binders. They are produced by free radical polymerisation induced by an initiator activated by UV light. A more recent innovation has been that of cationic UV-cured inks and coatings which undergo an autocatalytic reaction once triggered by UV light. These are now available as water-based gravure, flexo and screen printing inks and as yet have still to be fully evaluated for deinkability.

UV-curing inks and coatings form tough, crosslinked continuous films which bond very firmly to the paper surface. Where penetration of the substrate occurs the ink may key to the fibres. The films are difficult to break down in the pulper because they form an impermeable layer which resists the alkaline environment. UV-cured films therefore tend to detach in large pieces, and even as sheets or strips of ink solid. The largest pieces may be removed by screening but because they are flat and platelike, the large specks are able to deform and escape through the slotted screens along with the fibre.

Centrifugal Cleaning

Furthermore, since their relative density is fairly close to that of water (1.25 to 1.35), centrifugal cleaning may not be very efficient. Their cross-linked surfaces are also resistant to attachment by the collector chemicals necessary for successful flotation, lowering its efficiency.

To reduce the ink flakes to particles of flotatable size, new plant has been introduced into the recycling process. Dispersers or dispergers mechanically break the particles by a rubbing or kneading action. To reduce fibre damage, high slurry consistencies (30 to 35 per cent) are employed and particle size reductions from the 150 to 400 down to the 40 to 150 micron ranges are achieved.

Research at the London College of Printing on thick screen process UV-cured films has shown that these can be broken down to flotatable sizes in a controlled way by applying intense ultrasound fields. The collapsing cavitation bubbles produced by the 20kHz ultrasound fields generate localised temperatures of 5500K and micro jet streams travelling at up to 170 [ms.sup.-1]. These impact onto surfaces at pressures of the order of 1,000 N[m.sup.-2], breaking down and dispersing particles without the use of chemicals.

Dispersion occurs because the localised forces in the ultrasound field are much greater than those supplied by mechanical action in the pulper. The large platelets of cured ink can be tailored down from sizes greatly in excess of 400 microns to those within flotatable range. Furthermore, it has been found that fibre cutting is substantially less with ultrasound fields than with mechanical repulping, an important consideration for sustained recycling. Work on 30 micron films of cationic UV-cured waterbased screen process inks suggests that although broken down into flotatable sizes, the ink particles are difficult to detach from the fibre.

Work elsewhere using conventional repulping has found similar detachment problems for cationic UV-cured water-based flexographic inks. With these films however, the particles are too small for flotation and the presence of attached fibres makes them difficult to separate by washing. Failure to remove the particles results in an effective 'dyeing' of the stock.

Non-impact inks

Mixed office waste, a less costly source of good quality chemical wood fibre, contains a high proportion of non-impact printed material, much of which is difficult to deink. Although inkjet printed waste comprising water/solvent-based dye, pigmented, or solid phase wax inks are to be found in mixed office waste, it is that from electrostatic printing systems that is predominant and most likely to affect waste paper recycling. New inks include high density laser and photocopier toners, digital offset inks, and increasingly, coloured laser and photocopier toners.

Toner compositions vary with machine manufacturer but generally consist of pigment, base binder resin, modifier resin and charge control agent. Carbon black pigments, coloured pigments or copolymer dyes which may be of the azo (yellow), quinacridone/xanthene (magenta) and phthalocyanine types are the likely colourants.

The base binder resin is heat fusible and is selected according to the press fuser characteristics. These resins make blends of styrene-acrylate, polyester or epoxy type materials. The modifier resin may be natural rosin which is added to modify the paper surface and thereby improve toner transfer from heated drum to paper. The charge control agents may be iron oxide, Cr(III) or Co(III) complexes of azo dyes, salicylates, nigrosines, or cetylpyridinium chloride, depending on whether the toners are negative or positive working, coloured or plain black. Other additives include slip agents such as polypropylene wax or silicones.

Surface energy studies indicate that ink composition influences deinking efficiency and that different compositions present different degrees of difficulty. Moreover, the chemical characteristics of both the paper surface and the toner play a major role in toner removal by flotation (6), (7). Overall, deinking efficiency is influenced by the surface energy relationships between toner, deinking surfactant and base paper and more than one surfactant is needed to deal with all types of toner; uncoated papers are more difficult to deink than coated.

New inks have been developed for the digital offset printing processes although these are still based on electrostatic technology. The Electro-inks developed for the Indigo digital offset presses 'set' from a liquid toner by a polymerisation process that progresses over a period of days. These polymers are thought to soften at the temperatures used in the recycling plant and may cause 'stickie' as well as deinking problems. The Xeicon digital offset press uses a heat-fusible thermoplastic polymer loaded with 'microtoner' capable of producing high resolution images. The 7 to 8 micron diameter microtoner particles are triboelectrically attached to ferric carrier particles to reduce the tendency to smoke and to assist transport in the press.

Laser and electrostatic inks generally are applied to the paper surface by a combination of heat and pressure. During fusion the glass transition temperature of the toner particles is exceeded, allowing them to flow together and also onto or into the paper surface, giving a very stable copy. Toner may be removed from the fibre surface by re-pulping under alkaline conditions but the detached particles are large, fiat and platelike, ranging in size from 40 to several hundred microns resulting in inefficient flotation.

Moreover, the process is not very efficient in detaching toner from the fibres which therefore tend to form ink-fibre aggregates. Removal of detached toner is difficult because:

(i) the fiat particles can flex through the screens.

(ii) the density of the separated material is close to that of water, making centrifugal separation difficult.

(iii) many particles are too small to be removed by cleaners and screens and too large to be removed by flotation.

Solutions to the problem have taken two approaches. Either to reduce the size of particles to ranges that can be collected by flotation or to increase the particle size to facilitate separation by screening.

The latter makes use of agglomerants to transform the flat platelets into spherical particles having dimensions that assist screening. This is achieved using a two component chemical system. The first component lowers the glass transition temperature of the toner to below 60 [degrees] C so that it softens and becomes tacky under normal recycling conditions. The second acts as a collector which promotes coagulation into spherical particles.

Reducing particle sizes in the range 40 to 300 micron down to 40 microns has been achieved by installing dispergers in the recycling plant. Steam explosion technology has also been developed whereby the thickened pulp is heated at 200 [degrees] C, under pressure, and discharged through a blow valve (8), (9). Rapid expansion through the valve breaks up the softened particles and disperses them in the washable size range.

The Materials Research Group at The London College of Printing has successfully applied high intensity ultrasound fields to both the removal and size reduction of fused toners from mixed office waste (10). The before and after micrographs depicted in figure 2 show that fibre detachment and particle size reduction is achieved. Particles can be tailored down to flotatable sizes by controlling the ultrasound exposure.

Summary

The search continues to find better methods of deinking, dispersing and collecting inks during fibre reclamation. Meanwhile, new printing technologies and inks are being developed and these will bring new challenges to the paper recycling technologist. The costs of solving and combating the problems of deinking are considerable, involving the development and application of chemical systems and mechanical plant, and introducing added capital and running costs.

The interests of the ink manufacturer and paper recycler are divergent. For the first the goal is to provide durable films which stick tenaciously to paper; for the second, it is to remove them. Rather than accept that some waste is not worth recovering, why not encourage cooperation between ink manufacturers and paper recycling technologists. For a start, consultations between the developers of new toner inks, the suppliers of deinking chemicals and paper makers could reduce future problems for non impact waste.

The adhesives industry is already cooperating on research initiatives to reduce the impact of the 'stickie' on the paper recycling process. Perhaps the time is ripe for the ink industry to consider ways of doing the same even if only to advise on what may be going up the barrel.

References

1) R.C. Thompson. The Technology of Paper Recycling; Professional Printer vol 41 No3 pp 11-21 (1997).

2) W. G. Leighton and J. F. Miranda. Tappi Image Analysis Workshop, Cincinnati (1992).

3) M. R. W. Walmsley, L. Silvera and J. B. Chapman. Appita Conference Proceedings Melbourne: p 571 (1994).

4) J. B. Chapman, M. R. W. Walmsley. Appita '95 pp 435-422 (1995).

5) G. Gallan, Y. Vernac and B. Carre The Advantages of Combining Neutral and Alkaline Deinking. 40th EUCEPA Symposium 1995. p 49 Manchester (October 1995).

6) K. Cathie, G. Moore. Understanding why Laser Printed Papers Deink Differently. Tappi Recycling Symposium (1993).

7) K Cathie, M. Mallouris. Use of Surface Energy Measurements and Other Parameters to Predict the Deinkability of Laser Printed Papers. 2nd Research Forum on Recycling (1993).

8) P. Seifert. Recent Innovations in Paper Recycling. Tappi Journal vol 77 No 2 pp 149-152 (1994).

9) A.K. Sharma, W. K. Forester, and E. H. Shriver. Physical and Optical Properties of Steam-Exploded Laser-Printed Paper. Tappi Journal vol 79 No 5 pp 211-221 (1996).

10) J. Lane, A. Manning, R. C. Thompson. Physics in Modern Papermaking - Ultrasonic Deinking of Mixed Office Waste. Institute of Physics, Printing, Packaging and Papermaking Group Symposium, held at Aylesford Newsprint, (Nov 1997).