Value-added mechanicalgrades offer a potential alternative for heatset offset printing, as well as helping prolong the life of older paper machines

by Charles P Klass

New grades fill the gap

For many heatset offset printing applications such as inserts, catalogs, coupons and Sunday magazines, there is a need for paper grades in the gap between newsprint and what North Americans term No.5 lightweight coated (LWC). Although supercalendered (SC) papers can be printed using the heatset offset process, they are prone to problems with water sensitivity, dot gain and dusting or milking. And while blade-coated LWC can provide the required surface strength, ink receptivity and printability characteristics for heatset offset printing, it also requires a high percentage of expensive kraft pulp in the furnish, which drives up production costs. The printing problems associated with SC papers have led customers to look for papers that will run well on the heatset press and that cost less than LWC (Figure 1).
Achieving the ability to coat a sheet with a high mechanical content similar to newsprint is the key to making the new grades required. Fortunately, metering size presses have made this a practical target. In addition, on-machine finishing with soft nip calendering can provide the desired gloss levels. In fact, this combination is now being successfully applied on a sheet comprising 100% deinked old newspapers (ONP).
The new combination of metering size press coating and soft nip calendering meets the needs of growing market segments for new value-added groundwood grades. Not only that, but it also has the potential to save some of the older paper machines that are no longer competitive in commodity newsprint.

Evaluating demand

In North America, groundwood-containing publication papers have traditionally started with newsprint at the low end of the quality spectrum and LWC at the high quality end. In Europe, the market included filled SC magazine papers that were printed on rotogravure presses. For many years, newspapers were printed in black ink by letterpress. Use of color was limited to Sunday comics and magazines. The Sunday magazine was usually printed on rotogravure presses and was often referred to as the "Roto" section. In the past, North American newsprint was also unfilled - primarily due to the 3% maximum ash content specified in the customs classification to avoid higher duties for printing and writing papers.
About two decades ago, publishers began replacing letterpress with offset printing presses. As offset became the dominant printing process for newspapers, the need for a sheet with better surface strength and resistance to linting became important. Following the introduction of the USA Today publication in 1982, use of run-of-press color became common in daily newspapers. In turn, improvements were made to newsprint to produce a higher brightness sheet.
Technologies such as thermal gradient calendering and soft nip calendering were also developed, providing a smoother sheet. With the elimination of the 3% maximum ash content requirement for customs classification, some newsprint producers began to add filler to improve opacity and ink receptivity.
At the other end of the mechanical publishing papers spectrum, LWC papers were produced by blade coating and supercalendering. The blade coating operation requires a strong base sheet and this necessitates the use of high levels of expensive bleached kraft pulp (BKP) and limiting the filler content.

Rising demand

The 1980s witnessed an increased use of inserts, coupons and higher quality Sunday magazines in newspapers. At the same time, the market saw booming growth in direct mail and catalogs. Heatset offset became the dominant printing process for these applications. Although LWC paper performs well in heatset offset printing, it is relatively expensive for many of the emerging applications.

The European practice of producing magazine papers by filling and supercalendering was brought to a number of mills in North America in the 1980s. SC papers can be printed using heatset offset if special care is taken with ink tacks and press operating conditions. But as the filler content of an uncoated SC mechanical sheet is increased in an attempt to improve sheet density and print gloss, the strength loss from the bond-disrupting effect of the filler is so great that linting and dusting occur. In order to obtain adequate internal sheet strength, it is necessary to use relatively high levels of expensive BKP in uncoated SC mechanical papers. In addition, the pigment at the surface of SC papers is compacted by supercalendering, but there is no adhesive to hold the pigment particles together or bind them to the sheet (Figure 2).
Printers running heatset offset presses that alternate between LWC and SC papers often experience problems with SC papers such as:
• Inferior print dot resolution.
• Large dot gain.
• High water sensitivity, which affects runability.
• Milking and dusting.

As a result of these problems, printers have expressed a desire for papers that will run well on the heatset press to provide good quality process color printing that costs less than LWC. Since BKP is the most expensive raw material used in mechanical publication papers, the key to producing economical grades for the growing number of heatset offset applications is to keep the level of BKP in the furnish as low as is practical. To resist stresses on the sheet surface caused by offset printing, pigment can be applied to the surface in the form of coating, with the binder holding the pigment particles together and anchoring them to the sheet. In theory, the economical way to produce such a grade would be to coat newsprint.

Another solution

The development of the metering size press has facilitated numerous new surface treatment applications for base sheets that could not run successfully through a conventional size press or blade coater. Surface treatment with starch, pigmented starch or coating can improve the surface and optical properties of publication papers. So much so, that even a small quantity of starch on the surface can reduce linting by up to 50%.
In addition, the application of small amounts of coating can increase brightness, improve opacity and boost paper and print gloss. The potential of the brightness increase is of special interest to mills running furnishes that include large amounts of low brightness deinked pulp. Good sheet surface properties can also be obtained by on-machine soft nip calendering, while still maintaining opacity and bulk.
Recent entrants in the value-added mechanical grades spectrum include surface treated newsprint and SC papers, machine finished pigmented (MFP) papers, machine finished coated (MFC) papers and film coated offset (FCO) papers.
For some years, starch-treated newsprint grades have been standard in Japan and to some extent on the west coasts of North America. Treating the surface of newsprint with starch can reduce linting on the offset press by up to 50%. Small amounts of starch (0.1-0.2 g/m) have been applied on-machine with gate roll coaters to reduce linting, but runability problems and heavy wear on the crowned rolls have often resulted in high maintenance costs. The metering size press has created an alternative to these coaters that can improve efficiency and reduce costs.

Pigmenting papers

Starch treated newsprint is calendered with either a soft nip or a standard machine calender. More recently, similar applications have been made on SC papers to reduce milking and dusting.

Pigment can also be added to the starch surface size to produce machine finished pigmented (MFP) papers. These "slightly coated papers" were developed in Japan, where they are called Bitokoshi. MFP grades are sold as a low cost LWC substitute and manufactured in a basis weight range of 49-54 g/m.
At first, these grades were made by coating the sheet with a gate roll coater before running the web through a standard machine calender. However, quality problems due to film split pattern and high maintenance costs have occurred when using gate roll coaters. As a result, more recent installations use metering size presses and soft nip calenders.
MFP grades have gained broad acceptance in Asia, especially in China and developing countries. In fact, when a Chinese papermaker refers to LWC paper, he usually means MFP rather than a No.5 blade coated and supercalendered LWC paper.
Machine finished coated (MFC) papers were developed in Finland in the 1980s as a way to make value-added grades on narrow rebuilt newsprint machines. Since 1986, four machines in Finland and one in Canada have been converted to MFC production (Table 1).
Table 1 - Machines Converted to MFC Production Since 1986

Coats off

One of the other new grades developed in recent years is film coated offset (FCO) paper. In the mid-1980s, the metering size press was introduced as a device to overcome the speed limitations of conventional size presses on uncoated woodfree machines. Over the past decade, it has become the state-of-the-art film transfer coater and the key to new value-added groundwood paper grades.

The metering size press provides a contour-type coating with excellent fiber coverage. Thanks to a gentler metering system, good runability and high machine efficiency can be achieved even at low basis weights with high filler content. Unlike a blade coater, no stationary metering elements are in contact with the paper.
The metering size press has facilitated the development of the new FCO grade, which is more like LWC than MFC. FCO is manufactured in basis weights of 51-60 g/m. The soft nip calendered paper is not as smooth or glossy as LWC, which is supercalendered. But FCO does have a higher gloss and better smoothness than MFC. If four soft nips are used for finishing, it is possible to produce FCO with gloss and smoothness levels approaching those of LWC. FCO has become an economical alternative to LWC, especially at low basis weights. It provides high opacity and bulk without the need to use expensive titanium dioxide pigment.

Rebuilding to add value

There are numerous relatively narrow, slow newsprint machines that are no longer competitive with high capacity commodity machines. A good alternative to shutting these machines down is to rebuild and upgrade the unit to produce value-added surface treated and/or coated grades. However, there is more to a successful rebuild than simply installing a coater (Table 2).
Table 2 - Production/Printing Considerations for New Value-Added Grades

Filling a gap

Hybrid formers or fourdriniers are the most common configurations on many smaller newsprint machines. However, low drainage capacity at high speeds and a lack of flexibility in the fourdrinier section limits its use for the production of base paper for coated publication papers. In view of this, it is usually recommended that a fourdrinier is rebuilt into a hybrid former or gap former.
Even dewatering on both sides of the web in the press section is important to prevent differential coating penetration. The reduction in basis weight when changing to coated papers can have a positive influence on outgoing dryness from the press section. The addition of a steam box to improve dewatering and sheet consolidation even further should also be considered. In addition, a profiling steam box can improve CD moisture profile.
In preparation for the rebuild, the existing dryer section should be thoroughly evaluated and it may be necessary to remove some dryers to make room for the coater. Among the options available to make up for any shortage of drying capacity are the installation of spoiler bars and pocket ventilation systems. In addition, consideration should be given to slalom felting of the first dryer section to improve runability and decrease sheet flutter.
The installation of a metering size press is a key element in rebuilding a machine for value-added grades. A metering size press used to coat simultaneously both sides of the web demands a non-contact turning device after the coater. In addition, to achieve a compact installation with high evaporation rates, infrared dryers are often used for a major part of the evaporation before cylinder dryers.

A combination of air foil, infrared dryer and cylinder dryers is used on machines designed for high coat weights and high machine speeds. A surface dryness of 72-75% on the coated web is necessary before contacting the first afterdryer cylinder to avoid picking. In addition, the first two afterdryer cylinders should have non-stick covers and individual temperature controls. Equally, they should not be felted.
Soft nip calendering is a well-established technique for on-machine finishing that should be considered when rebuilding to produce value-added grades. The differences in gloss and smoothness between coated supercalendered and soft nip calendered grades are already diminishing. With soft nip calendering, steel roll surface temperatures of 200C and nip loads up to 350 kN/m are used to obtain gloss and smoothness. Caliper control with induction heaters can be used on soft nip calenders, but the response is much less than on a conventional machine calender.
Gloss levels of 50-52% and roughness of 1.5 to 1.7 mu PPS-10 are being achieved in FCO production using two soft nips. Gloss and smoothness values very close to those of LWC (54-57% gloss and 1.1 to1.2 mu PPS-10) have been achieved in FCO in pilot scale using four nips. Bulk is normally 5% to 10% higher with a soft nip calendered sheet compared with a sheet supercalendered to the same gloss. However, soft nip calendering provides superior ink snap when compared with supercalendering.

In the kitchen

A coating kitchen is required to make the coating color. FCO coatings may use either starch-latex or latex-CMC (carboxymethylcellulose) binder systems. Since pigments normally include kaolin clays and calcium carbonate, they are usually received in slurry form. In addition, if starch is used in the binder system, a crosslinker is required and a lubricant is normally included in the coating formulation as well. The coating kitchen may be either batch or continuous in design. It should be computer controlled and may not require an operator.
Machine efficiency depends on many factors and varies from mill to mill. Relatively new LWC machines with on-machine blade coating and off-machine supercalendering have overall efficiency rates of between 73% and 82%. MFC machines normally have a total efficiency of 77-83%.
Although the experience with FCO production is limited, it appears that overall efficiency is about 1-2% higher than for machines making MFC, primarily due to the more forgiving nature of the metering size press. As a result a typical startup curve for a machine rebuilt to make LWC would show 76% efficiency in the first year, 79% efficiency in the second and 83% efficiency in the third year. n

Charles P Klass is president of Klass Associates, based in Radnor, Pennsylvania, USA