Mills employ chemical additive tools to develop differentiated products for ever-changing ink jet, laser paper markets

Tapping High-Growth Digital Imaging Market Can Boost Mill Profits

Differentiation creates challenges for the papermaker who is charged with producing the new, specialized grades. Demands usually include tight specifications and short time lines, all for relatively small production quantities in the beginning, a frustrating scenario. Raw material suppliers have proven quite helpful in these situations. It may be useful to partner with them as early as possible. This article covers some of the needs and means by which they might be addressed.

Product differentiation today usually means targeting the fast-growing markets for laser and ink jet printing papers. Conventional printing technologies evolve slowly, allowing significant time to adjust paper grades. However, the new digital printing technologies are changing rapidly, requiring faster action. As a result, paper companies are focusing more on understanding customer needs as early as possible, analyzing them for opportunities, and responding quickly with new grades of paper.

Indeed, the first and perhaps the most important step is to understand the market: what’s needed, by whom, when? Therefore, many paper companies have expanded their marketing departments to interface with end users, digital printing equipment suppliers, ink or toner suppliers, and independent industry experts. This increases the likelihood that development of any new digital printing technology or significant change in existing technology will be identified and understood early. The first to offer a new grade of paper satisfying new demands is likely to succeed in true product differentiation. In addition, after a new grade has been introduced, the marketing staff or technical service personnel must stay in frequent contact with customers, if not equipment producers and end-users, to determine areas for further grade improvement based on printing experience.

Paper Requirements. During new grade development, there are certain basic requirements for most grades of uncoated paper that should be maintained. This is especially important as we respond to market demands created by new printing technologies involving toners or dilute water-borne ink-jet inks. Specific requirements often call for dramatic changes in paper design versus offset or rotogravure printing papers. The basic paper requirements for any printing process include runnability on the intended printing equipment, smoothness, brightness, and opacity.

What’s changing and likely to change in the future? More and more color printing will continue to drive improvements in paper quality. Faster, wider, and more complex digital printing equipment will require paper which provides improved runnability. The office world will convert more and more to color printing, especially ink jet. Yes, ink jet is already taking market share from laser in the commercial office. This will demand high quality ink jet printing on uncoated or lightly size-press-treated paper, especially as printing speeds increase. The ideal papers are not yet available but a great deal of development work is currently underway.

Commercial digital printers range from quick printers who primarily use office-type laser printers, to commercial printers using large, high speed printing machines, sometimes referred to as digital presses, many of which use paper rolls rather than sheets. Current models are based on electro- photography, the same basic process used in office laser printers. These include the Indigo machines that use liquid toners and the Xeikon machines that use dry toners (also sold by AGFA-Chromapress, IBM, and Xerox). Future models will be based on ink jet and new printing technologies, such as ink coagulation, as well as further improvements based on electro- photography. Paper requirements will continue to change, demanding fast response.

Improving Uncoated Digital Printing Paper. The ideal uncoated paper for any type of digital printing, office or commercial, is woodfree, alkaline, uniformly bright and blue-white in shade, smooth, stiff, opaque, strong, and clean, that is free from surface debris and slitter dust. This is not easy to achieve. For example, heavy calendering can provide smoothness, but at the expense of stiffness and optical properties, especially brightness and opacity. Process control and filler choice are both critical to maintaining smoothness while maximizing brightness, bulk for opacity, and stiffness for runnability.

The first step is fiber preparation, including de-inking of reclaimed fiber, and bleaching. Though no details will be included here, changes are taking place, primarily due to increasing regulatory demands to protect the environment.

Nearly all cut-size papers for office and home printing are now alkaline and filled at increasing levels with bright-white pigments, primarily precipitated calcium carbonate (PCC). Why PCC? It is known for very high brightness, due to removal of impurities during its production. But, there’s more. The synthetic process can be varied significantly and several particle shapes are available. Also, particle size, particle size distribution, and specific surface area can be carefully controlled. These are key factors in sizing efficiency, control of paper machine deposits, and final paper properties such as smoothness, bulk, opacity, stiffness, coefficient of friction, and strength.

Blue-white brightness is desired for digital printing papers, especially when most of the page is to be left unprinted and visible, such as for text and graphics. A fluorescent whitening agent (FWA), commonly called an optical brightener, is often used as a furnish or size press additive to make the finished product appear more blue-white and bright. It works by absorbing ultraviolet radiation and re-emitting light primarily at the blue wave-length.

The alkaline revolution has resulted in changes in internal sizing and retention aids. ASA (alkyl succinic anhydride) and microparticle retention aids have increased market share very significantly as a result of the alkaline revolution and the increasing use of both fillers and secondary fibers. Microparticles account for at least 35% of the entire paper and paperboard retention aid market. They include colloidal silica, bentonite, and synthetic polymers. The microparticle systems are especially popular in producing uncoated woodfree papers. In addition, as paper machines increase in speed, optimizing both filler retention and paper formation becomes more difficult. Papermakers are confronting this dilemma by making greater use of microparticle retention aids.

The use of strength additives is increasing, especially to make up for the loss in sheet strength that is incurred as the recycled fiber content is increased in certain grades. Starch continues to hold the major share of the dry strength market compared to synthetic resins because of its lower cost. However, one drawback to the use of starch is that its presence in mill effluent adds to the biological oxygen demand (BOD). This is encouraging some papermakers to turn to alternative synthetic dry-strength resins. These include polyacrylamide (PAM) and derivatized gums like carboxylmethylcellulose (CMC) and guar.

For digital printing, evidence has been reported indicating that uncoated ink jet papers sized with AKD exhibit better ink jet print density and resistance to wicking vs. ASA sized papers; however, ink drying is faster with ASA. Of course, there are many routes to optimizing overall paper performance beyond simple choice of internal sizing chemical.

Laser Printing Papers. One of the most significant current needs in the cut-size market is cleaner digital printing papers. High-speed office laser printers are not cleaned nearly as often as conventional printing presses. Dust and debris build up inside the printer, interfering with print performance. The user is forced to call in the printer service representative who frequently simply cleans the printer. Very clean paper is also needed for the large, fast commercial digital printing machines referred to earlier.

How can we provide cleaner paper? Paper edge dusting can be minimized by using non-abrasive filler pigments, such as precipitated calcium carbonate, and by keeping knife edges sharpened. Surface debris can be minimized by using filler pigments of narrow particle size distribution, avoiding large particles, and by using adequate level and proper type of starch sizing. It helps to use metering size presses for surface treatment because conventional size presses can drive the starch into the paper. Surface dust can also result from two-sidedness where filler may be driven from the wire side toward the top side during paper formation and drying. Top-formers on the paper machine can minimize or prevent this.

The cleanest paper creates a certain amount of dust when slit and cut to size. It may become necessary to install brush and vacuum equipment following the cutting operation, just prior to ream wrapping. This problem must be solved before filler levels can be increased to maximize value in cut-size office papers, especially for the faster and more complex office printing equipment of the future, whether laser or ink jet.

As filler levels are raised in the paper, coefficient of friction may increase beyond specification levels. This may call for the addition of a lubricant, such as calcium stearate or synthetic wax emulsion, to the size press. It may also be possible to accomplish this by replacing a portion of the starch with a starch copolymer.

Ink Jet Printing Papers. Improved ink jet print-ability of uncoated papers has been accomplished by treating the paper surface with a hydrophobic polymer to provide a barrier to ink absorption. The polymer is often combined with size press starch used for surface strength. A common hydrophobic polymer used for this purpose is SMA (styrene maleic anhydride) or an ester derivative. Indeed, trials by Hewlett-Packard showed the ester salt of SMA to provide the best balance of ink jet printing properties. The base paper contained 1.5 kg/mton AKD internal size and 16% scalenohedral PCC filler. SMA is not the only choice for hydrophobic surface treatment. Others include styrene-acrylic emulsions.

Holding the ink on the surface improves ink jet print density. However, the hydrophobic treatment level must be low to avoid excessive ink holdout which can promote spreading and feathering of the image and slow drying of the print. It has proven impossible to optimize uncoated ink jet paper for both print density and print definition, or lack of spreading and feathering. This is why ink jet printability of uncoated ink jet papers does not approach that of coated ink jet papers. Also, because absorbency control is critical to the hydrophobic barrier approach, the paper furnish plays a crucial role. It must be consistent so that the surface treated paper provides consistent ink jet printability. Certain materials promote performance consistency, a benefit claimed for ASA internal sizing, for example.

Trials byHewlett-Packard have shown the ester salt of SMA to provide the best balance of ink jet paper printing properties. Shown: HP Deskjet 1000C.

It may be possible to use a cationic additive such as cationic starch, at least as part of a surface treatment, to provide some attraction for the anionic dyes or pigments from the ink jet inks. However, most of the available hydrophobic polymers are anionic and would not tolerate a cationic additive. An other approach is to use an ionic crosslinker, such as ammonium zirconium carbonate or potassium zirconium carbonate to avoid ammonia odor. These crosslink with the hydrophobic polymers and, perhaps, with the paper fiber. They may also bond with cationic carriers commonly used in ink jet inks. The net effect is improved print density and show through, as well as improved color-to-color bleed resistance (often measured by wicking).

The increased use of ink jet printing in the commercial office environment will expand the demand for low cost, high performance uncoated ink jet paper, approaching the print characteristics of fully coated ink jet paper. That paper should either look and feel like laser paper or be printable by both ink jet and laser printer. It should also be printable on both sides. To achieve this very significant improvement, a specially designed pigment is required, just as in high-quality coated ink jet papers. Though size press formulations are offered which combine silica pigments with fully-hydrolyzed polyvinyl alcohol binder, cost is high and both solids and rheology are less than ideal. One silica pigment supplier promotes the use of 30 to 60 parts precipitated silica with 100 parts starch or polyvinyl alcohol and 5 to 8 parts SMA for size press application to achieve 5 to 10 gsm coat weight. Solids are 10% to 15% and Brookfield viscosity is as high as 350 cps.

Commercial success may very well involve a new precipitated calcium carbonate (PCC) pigment specifically designed for this purpose, with low levels of starch and additives, applied by metering size press to achieve very uniform coverage at low coat weight. Again, the base paper must be designed in concert with the pigmented size press treatment. Consistent base sheet quality will be critical.

For the fast, web-fed commercial full-color ink jet printers of the future (to be introduced first by Scitex in 2000), paper is under development to provide good 4-color ink jet print quality at low cost. Again, ultimate optimization may very well involve the size-press application of a pigmented surface treatment using new pigments and binders. The paper surface must separate the ink jet color from the water and hold it on the surface, just as with high-priced coated ink jet paper today.

Are Paper Additives Always the Answer? Contrary to what additive supplier’s may say, it is not always best to use additives to address an opportunity for improvement. Some paper furnishes have been modified so much that the list of additives used is very long. Indeed, it may be beneficial to periodically review the furnish to determine if all additives are really needed. Paper machine optimization may be a better approach, in some cases, and should be considered.

A recent example is duplex image deletion, a problem that received a great deal of attention in the past, but is now under control by most paper mills. Duplex image deletion is caused during laser printing by changes in the paper as it is subjected to the high heat of toner fusion when the first side is printed. This can cause the paper to lose moisture and cockle or curl. Then, when the second side is printed, some areas of the paper may not contact the imaging drum properly and toner is not picked up, leaving partial or missing images. Some of the newer high speed copying and printing machines do not have the conventional toner transfer backing roll or blade, placing even more demand on the paper in this regard.

Contrary to earlier speculation, recent work indicates that there is no correlation with specific types of filler or other furnish additives to duplex image deletion. It is minimized or prevented by paper machine adjustments to carefully control critical paper properties including directionality, formation, and moisture content. Of course, after such adjustment it is important to be sure that other paper properties have been maintained. This raises another point which, though perhaps obvious to most, may not always be practiced until a problem arises. Any time a change in the furnish is made, including filler and additive type or level, the paper machine should be re-optimized to balance all of the important paper properties, including the basic requirements reviewed earlier.

In addition, a size press additive may be more advisable than a furnish additive. For example, most modern cut-size office papers designed for laser printing will take an ink jet image, as well. The 5-fold increase in the volume of synthetic surface sizing materials during the past decade is an indication of the tremendous growth in this market segment.

Conclusion. The fast pace of digital printing tech-nological change is forcing the papermaker to stay abreast of those trends, look for opportunities, and respond with new or modified uncoated papers quickly. Additive suppliers can help the papermaker achieve the properties needed. It may be advisable to partner with several of them to address opportunities through a team approach in order to minimize frustration and expedite success. The choice might be based not only on their current product offerings, but both their ability and willingness to modify their products in response to new requirements.

Donald I. Lunde is managing director, Market Opportunity Resources, Inc.