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Daily newspapers have to develop a range of new techniques to ensure their survival in the long term against the rapid advance of alternative media, including the Internet. One idea that has been a great success in markets as far apart as Japan and Germany is to include advertising inserts with daily newspapers. This distribution channel offers a relatively cheap marketing vehicle by helping to offset production and distribution costs.
The main paper grades used for product advertising tend to be intermediate grades, i.e. grades located between standard newsprint and LWC (lightweight coated) papers in terms of quality. Paper used for inserts can include recovered fiber. And with the process technology available today, it is now possible to improve the recovered paper that is typically used for standard newsprint qualities to reflect the higher requirements of insert papers. This article reviews key sheet properties affected by inclusion of recycled fiber and methods to address these issues.
FIGURE 1: Content of deinked pulp in wood-containing printing papers.
EUROPEAN PERSPECTIVES. Standard newsprint consumption in Germany has hardly changed since 1991, running at around 2.4 million tons/yr. However, production has risen substantially, particularly from 1992 to 1995 when new several new newsprint lines were installed. These included Haindl Papier’s Schwedt unit, Papierfabrik Palm's Eltmann mill and Sachsen Papier’s Eilenburg plant. Although standard newsprint production has significantly increased, Germany remains one of the main importers of standard newsprint among European Union (EU) countries. Over the same period, there has also been a considerable rise in the production of uncoated magazine papers, including both SC (supercalendered) and improved newsprint grades.
There are several reasons for the stagnation in standard newsprint consumption. It is primarily attributable to declining circulation figures among the leading newspapers. Indeed, the decline of the larger newspapers would have led to a slump in newsprint consumption were it not for the revival in regional advertising. Today, far more local titles are now distributed free of charge on a door-to-door basis, financed by advertising content.
As large daily newspapers have had to develop ways of protecting themselves from the onslaught of electronic media, they are increasingly turning to advertising inserts—an idea that is now practiced with great success in a number of areas, including Japan. Including advertising inserts with daily newspapers significantly reduces the negative impact of high distribution costs. Similar trends have occurred and continue in the U.S.
In Germany, advertising inserts are also becoming big business, but German publishers still lag far behind their Japanese counterparts. While Japanese newspapers may include up to 30 inserts, only about five are used in Germany. In other words, the weight ratio of inserts to total weight in Japan can be as high as 70%. In Germany, the same weight ratio of inserts is about 20%.
In the future, the substantial market that already exists for inserts will become far more significant. In view of this, the paper industry will need to meet the corresponding product demands for inserts.
FIGURE 2: Relationship between brightness potential and stock pre-paration losses for processing household collection 50% ONP 50% OMG
INSERT PAPER TRENDS, FIBER REQUIREMENTS. Since inserts are mainly used for product advertising, high print quality with good color reproduction is generally required. In some applications, the inserts are printed rotogravure and thus must meet standards for this print process.
Comparing the price/quality relationship between the various mechanical printing paper grades, it becomes apparent that there are three main groups —standard newsprint, supercalendered (SC) papers, and lightweight coated (LWC) grades. But there are also a number of intermediate products, such as improved newsprint and super-improved newsprint, which are positioned between standard newsprint and SC grades. Similarly, there are grades positioned between SC grades and LWC grades, such as surface treated improved newsprint and machine finished coated papers. The coated papers group includes LWC, medium-weight coated, and heavyweight coated grades.
The major products used for inserts are the intermediate grades between standard newsprint and LWC papers. As a result, producers need to know to what extent these grades can be produced using recovered paper, and what requirements are then placed on fiber stocks.
Currently, the use of deinked fiber in groundwood printing papers varies widely depending on the specific grade (Figure 1). The area below the line in the figure represents the proportion of recovered paper used. For example, standard newsprint is often comprised of 100% deinked pulp, as is improved newsprint. Secondary fiber content in SC grades printed rotogravure is around 30% at present, and content ranges up to 20% for LWC grades. The content of deinked pulp in medium-weight and heavy-weight coated grades is somewhat higher.
The technological requirements for the use of deinked pulp in these paper grades includes consideration of the impact on the following sheet properties:
- Optical characteristics, including brightness, dirt speck area, and neutral color
- Strength, including tensile and tear strength
- Surface quality characteristics, including shive content and calendering profiles
- Contaminants, including cationic demand, chemical oxygen demand (COD) requirements, and stickies content.
In terms of optical characteristics, deinked pulp needs to have a brightness of 65% to 72% ISO and a dirt speck area of between 20 to 50 mm2/m2. The pulp should also have a neutral color. Strength characteristics should at least match those of a mechanical pulp stock. Good surface qualities require shive-free deinked pulp with less than 5% coarse fiber fraction (Bauer McNett 14-mesh screen residue fraction). Contaminants must also be limited to a COD value of less than 1,000 mg/l, with a stickies content of less than 200 mm2/kg. Furthermore, cationic demand must be less than 100 mg/l.
To comply with these requirements, producers must use the appropriate stock preparation measures, some of which are described below. (The reference furnish is a 50:50 mix of recovered newspapers and magazines.)
OPTICAL CHARACTERISTICS. From the standpoint of optical characteristics, it is clear that by using the latest two-loop systems deinked pulp used for standard newsprint production in central Europe can be brightened to 58% to 60% ISO. By optimizing the bleaching process, brightness can be boosted to 64% ISO. This limit on bleaching potential is attributable to residual ink particles attached to fines and fillers. Studies at Voith Sulzer's technology center showed that as more “grayed” fillers and fines are removed, the greater the improvement in bleachability.
Tests on deinked pulp used in standard newsprint proved that post-washing alone is not enough to improve brightness. This can only be attained by subsequent bleaching. With an additional 10% post-washing losses, a further increase of almost 4% ISO brightness can be achieved. And if 15% post-washing losses can be accepted, brightness improvement is approximately 5% ISO. By incurring a level of 30% total losses, the brightness requirements for SC and LWC grades are clearly attainable (Figure 2).
CLEANLINESS CHARACTERISTICS. The next factor that has to be considered when using recycled fiber is cleanliness. Experience shows that the required cleanliness of 50 ppm can only be attained by higher energy input at the dispersion stage. Typical relationships between dispersion energy input and dirt speck area reduction indicate specific energy inputs of 40 to 70 kWh/ton currently used for standard newsprint production are not enough to fulfill cleanliness requirements. By increasing the energy input to 100 to 130 kWh/ton, however, the required cleanliness can be attained without sacrificing tear strength. This indicates that high specific dispersion energy does not reduce fiber length.
FIGURE 3: Possibilities of adapting fiber quality for improved newsprint and SC papers.
Of course, higher process losses clearly have a negative effect on stock preparation economics. The issue then is to decide which is more cost-effective: to use deinked pulp under these conditions or to resort to primary furnishes such as thermomechanical pulp (TMP) or groundwood pulp. This question cannot be answered without exact data on operating costs and raw material price trends. Deinked pulp costs largely depend on recovered paper prices. However, for TMP or groundwood, energy costs are the primary factor that determines manufacturing costs.
After the liberalization of power tariffs in 1999, charges are expected to stabilize at around the same level throughout Europe—roughly $0.18 to $0.21/kWh. The price of recovered paper used for deinked pulp production greatly depends on market supply and demand. The Asian crisis, for example, significantly reduced recycled fiber demand, which took prices well below $50/ton during 1999.
Comparing the cost of deinked pulp and TMP as a function of end brightness is essential for producers. For example, to obtain a higher brightness with TMP, only more intensive bleaching is required. But with deinked pulp, high-end brightness involves greater substance losses. At the lower end of the brightness scale, between 60% to 66% ISO, it is preferable to use deinked pulp. With realistic recovered paper prices around $50/ton and power tariffs at $0.21/kWh, deinked pulp remains slightly more cost-effective, even at a brightness level of 70% ISO.
PAPER STRENGTH. Another element to be considered when using recycled fiber is the comparative strength of the deinked, TMP and stone groundwood pulps used for improved newsprint grades. Compared to a bleached kraft pine stock refined to 470 CSF used as strengthening material for an improved grade, groundwood-based paper stock has the lowest tear strength, followed by TMP. The strength of deinked pulp lies between the TMP and pine kraft values.
For example, based on a 100 g/m2 sheet, a tear strength of 800 mN would require a 45% pine kraft pulp content to be mixed into the groundwood base stock. With TMP, 40% kraft is required, while with deinked pulp, only 25% kraft has to be added.
SURFACE QUALITY. Higher sheet printability also requires better surface quality. The main criterion for surface quality is the coarseness of the fiber structure. Comparing the fiber length distribution of SC paper stock without deinked pulp with that of deinked pulp from recovered household paper shows that while the distribution of the short fiber fractions is almost the same, the R14 fraction shows considerable deviation. The high Rl4 content in deinked pulp is attributable to the coarse TMP fibers from the recovered newsprint content.
Scanning electron microscope photos of calendered Rapid-Köthen test sheets show the differences between groundwood pulp used for SC papers, TMP pulp used for newsprint, and deinked pulp. The groundwood pulp clearly has the least degree of coarseness, while the coarsest is TMP pulp. The coarse fibers are unsuitable for improved newsprint or SC papers, since these can create calendering problems and affect printability in the form of image losses and missing dots. The coarseness of deinked pulp for these grades therefore has to be reduced.
There are various ways of adapting fiber coarseness to meet the corresponding requirements (Figure 3). One method already used successfully comprises full stream refining in the low consistency range. Another possibility that originated from TMP production is separation and selective refining of the coarse fiber fraction. An economically interesting solution here is to separate out the coarse fiber fraction for use in standard newsprint. This approach assumes that a standard newsprint machine is available in addition to a paper machine producing improved grades or SC papers.
CONTAMINATION ISSUES. In simple terms, it is clear that there are several charges introduced into the approach flow system of a paper machine by the various stock components. Deinked pulp is generally anionic, as are TMP and chemical pulp, although the charge density of the latter is significantly lower. Wet broke charge levels usually depend on the retention system.
Today, cationic retention agents are mainly used in the production of mechanical printing papers. But before adding these agents, the stock component charges first have to be neutralized. The actual retention effect is determined by the amount of retention agent used over and above the neutralization point. The formation of neutralization reaction products cannot be excluded.
To ensure a consistent retention effect with the least possible use of chemicals, the stock components should have a uniform and low charge. This is obtained by thickening the stock components. The actively charged colloidal substances are thus pressed out and retained in the de-inking plant or TMP plant water loop, where they can be removed in concentrated form.
As an example, look at typical COD patterns for a two-loop deinking system and the paper machine water loop. At a specific effluent content of 5 l/kg of produced paper, a COD load of 5,300 mg/l can be expected in the first deinking loop. Another 3,300 mg/l is likely to be found in the second loop of the deinking cycle. If stock is fed to the paper machine at 12% consistency (Figure 4), the COD value in the whitewater loop will be 1,900 mg/l. But if stock feed consistency is increased to 30%, the COD level in the paper machine loop will be significantly reduced. COD values will stabilize at well below 1,000 mg/l.
Meeting required stickies limits is largely ensured by the use of fine screening. Today, most plants will utilize slot widths of 0.15 mm or less. By avoiding excessive shear forces and designing an intelligent screen arrangement, removal efficiencies of 80% to 90% are attainable.
FIGURE 4: COD in relation to Water Loop Separation for a newsprint system.
A deinked stock preparation flow diagram for improved news-print, SC, and LWC papers might at first sight seem very similar to the two-loop deinked pulp system used for standard newsprint production. But closer examination reveals significant differences. These can be summarized as follows:
- Additional medium consistency slotted screening
- High energy, high consistency dispersion
- An additional washing stage for the removal of grayed fillers and fines
- A refining stage for adapting fiber quality.
Today, with these additional modules and sufficient operational knowledge, deinked pulp quality can be improved to meet requirements for optical characteristics, strength, and surface qualities as well as colloidal contaminant levels needed for improved newsprint and SC papers.
HARALD SELDER is with Voith Sulzer Stock Preparation, Ravensburg, Germany.
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