Fibrous Materials for Paper

Fibrous Materials


Paper production is mainly based on fibrous raw material which may consist of primary or secondary fibers. Primary fibers are obtained directly from plant raw materials, mainly from wood and annual non-wood plants. Industrially, mostly thinnings and sawmill wastes are used. Secondary fibers are produced from re¬covered paper. Rags are used only in very small amounts. Synthetic and mineral fibers do not play an important role.

As its name indicates, chemical pulp is obtained by chemical pulping from wood or annual non-wood plants, mechanical pulp is produced by mechanical defibra¬tion of wood. Worldwide, wood is 90 % of the raw material source. The use of annual non-wood plants is especially suitable for regions with low wood resources, larger areas that are not used for agriculture and larger surpluses of agricultural remains (e. g. straw). Fiber production from non-wood plants is more expensive and causes more environmental pollution than that from wood. Chemical pulp from jute, flax, hemp, sisal and bagasse are suitable for special paper grades (e. g. filtration papers, lightweight printing papers). In tropical and subtropical coun¬tries, fiber wood is grown mainly in plantations.

In chemical pulp production yields of 45 to 55 % are obtained. The yield in mechanical pulp production is about 80 to 95 %. Compared with chemical pulp, about twice as much mechanical pulp can be obtained from the same amount of wood. Additionally, dissolved substances in chemical pulping charge the water circuits and effluents much more and have to be processed with high expenses. Mechanical pulping is an energy intensive process. 1.3 to 3 MWh t–1 are con¬sumed, depending on the product (paper grade) and the process design. During chemical pulp production, energy is produced by burning black liquor. A modern kraft mill can deliver a power surplus of about 30 MW.

Before the invention of the mechanical and chemical pulping processes in the 19th century, rags (used textiles) were the only raw fiber source.  In the next ten years, the worldwide consumption of pulp fibers is expected to be 46 % for chemical pulp, 10 % for mechanical pulp and 44 % for recycled paper pulp.

Chemical Pulp

Chemical pulp is produced by chemical pulping of vegetable raw materials such as not only hardwood and softwood, but also from straw from different kinds of cereals, bagasse, reed, or esparto grass, and from other annual non-wood plants.
During chemical pulping, the most of the lignin is removed from the raw mate¬rial. The yield of the pulp is therefore only 45 to 55 %. Two main industrial proc¬esses of chemical pulping are used:
. • The sulfate process with sulfate pulp as product
. • The sulfite process with sulfite pulp as product.

The alkaline sulfate process uses sodium sulfide and sodium hydroxide as pulping chemicals. The acidic sulfite process is based on calcium, magnesium, and sodium or ammonium bisulfide. Worldwide, the sulfate process is used in up to 90 % of cases.
The lignin content that remains in the pulp after the pulping process can be further reduced by bleaching. The development of bleaching agents with zero chlorine or low chlorine content like oxygen, hydrogen peroxide or ozone leads to a higher residual lignin content in the so-called TCF (total chlorine free) and ECF (elementary chlorine free) pulps compared with the conventionally bleached pulps.

The quality of pulp depends on the kind of wood used. Softwood pulps have longer fibers, resulting in higher strength, whereas hardwood pulps with shorter fibers for instance produce a more uniform sheet. Paper containing only chemical pulp as the fiber component is called wood-free paper, i. e. it has only a low residual lignin content.

Sulfate pulps have distinctly higher strength than sulfite pulps, especially if they are unbleached. That is why unbleached sulfate pulps are mostly used in papers with high strength demand such as corrugated board, packaging papers, or techni¬cal papers. Highly bleached pulps are used in high-quality printing papers.

The dominant process of semi-chemical pulping is the neutral sulfite semi-chemical process (NSSC), with sodium or ammonium sulfite as the digestion chemicals. The pulp yield is in the range 70–80 % depending on the wood species, most commonly hardwood species like birch, beech, maple, oak and eucalyptus. This pulp is used for fluting production – often as the sole component in furnish.

Mechanical Pulp
Mechanical pulp is produced from wood by mechanical defibration. Different processes are in use:
. • Stone groundwood (SGW) is produced by the mechanical defibration of round wood logs by a grinding stone.
. • Refiner mechanical pulp (RMP) is obtained by mechanical defibration of wood-chips in a refiner and is carried out without pre-treatment. Defibration takes place under atmospheric pressure. The wood chips are defibrated at tempera¬tures of about 100 °C, mostly in two stages with consistencies of 20 to 30 % in the second stage.
. • In the refiner process with thermal pre-treatment (TMP), wood chips are treated with steam at 110 °C to 130 °C for 2 to 5 min before they are defibrated under high pressure.
. • In the refiner process with chemical pre-treatment (CTMP/CMP), wood chips are impregnated with chemicals before they are mechanically defibrated in the refiner under high pressure of about 3 bar or at atmospheric pressure, mainly in two stages.

The CTMP and CMP processes differ in the intensity of the chemical treatment and the yield. The yield of softwood CTMP is about 91 % to 96 %. CMP is more heavily chemically treated and therefore has more the character of a chemical pulp,
i. e. higher strength properties at a cost of lower light-scattering coefficient (see Table 2.2). For hardwood, the yield of CMP can decrease to 80 %.

During mechanical defibration, lignin is plasticized and remains in the pulp. This is the reason for the lower strength properties but higher light-scattering coefficient and opacity compared to chemical pulp. The residual lignin in mechan¬ical pulps results in a poorer and less stable brightness level. Mechanical pulps cannot be brightened by oxidative and reductive bleaching to the same extent as chemical pulps as only negligible delignification takes place compared to the bleaching of chemical pulp.
Paper grades that contain pulp produced by mechanical or chemimechanical processes are referred to as wood-containing papers. Mechanical pulps are mainly used in short-life printing papers, hygienic papers and board.

A comparison of some of the main characteristics of chemical and mechanical pulps: yield, BOD (biological oxygen demand), grinding or refining energy (in the case of chemical pulp for final pulp preparation), freeness, tensile index and light scattering coefficient, is given in Table 2.2. The yield decreases from 97 % for SGW to 40 % for chemical pulp (kraft type) and the effluent impact increases i. e. the biological oxygen demand increases. Specific energy consumption is lower for conventional chemical pulps than for chemimechanical pulps but chemical consumption is higher.

Freeness is measured as about 100 ml for (fines-containing) stone groundwood (SGW) and 780 ml for chemical pulp with high long-fiber content.

Chemical pulps have a higher strength potential (tensile index) but lower light scattering ability.
Light scattering values are given for the pulp before undergoing additional me¬chanical treatment. Post-refining of mechanical pulp increases the light scattering coefficient whereas refining of chemical pulp decreases this property.

Recovered Paper, Recycled Fibers
Hans-Joachim Putz Role of Recovered Paper in the Paper and Board Industry
In the past chemical pulp was the most important raw material for paper produc¬tion, but now this has been replaced by recovered paper. 158 million tons of this raw material were used globally in 2002. This volume exceeds the total volume of woodpulp, i. e. chemical pulp (117 million tons) and mechanical pulp (36 million tons)

[1]. These figures show that recycled fibers play a very important role today in the global paper industry as a substitute for virgin fiber pulps.
Figure 2.1 shows the global increase in recovered paper usage, compared to paper production since 1961. Globally the use of recovered paper increased by approximately 5.8 % annually whereas annual paper production growth was only
3.6 %.

The paper industry is the exclusive relevant user of recovered paper as a secon¬dary raw material – at least in terms of material recycling. Various recovered paper processing systems with mechanical and chemical unit processes prepare recycled fibers for the production of paper and board grades. These processing systems use different recovered paper grades that contain either chemical or mechanical fibers or, mainly, an undefined mixture of both. Some paper and board grades can be made from recycled fibers exclusively. This includes paper grades such as corrugat¬ing medium and testliner or newsprint in Europe. For newsprint and other grades blends of recycled and virgin fibers are also used. The proportion of recycled fibers in the raw material furnish can vary from about 5 % for fine papers to 100 % depending on the paper grade or geographic region.

Figure 2.2 shows the largest recovered paper consuming countries. They used in total 122 million tons in 2002 corresponding to 77 % of the global recovered paper consumption. Main Definitions for Statistics

Consideration of recovered paper use and recovery of used paper products requires definitions. Statistical definitions can be related to the world, a continent, a country or a certain region (e. g. Europe) or to a certain category of paper products (e. g. graphic papers or newsprint). Differentiation is given by three statistical parame¬ters:
. • Recovered paper utilization rate, in percent, is the amount of recovered paper used as raw material in the paper industry, in tons, divided by paper production, in tons, on an annual basis, multiplied by 100.
. • Recovery or collection rate, in percent, is the amount of collected recover

ed paper for material recycling, in tons, divided by paper consumption, in tons, on an annual basis, multiplied by 100.
. • Recycling rate, in percent, is the amount of recovered paper used as raw material in the paper industry, in tons, divided by paper consumption, in tons, on an annual basis, multiplied by 100.

From these definitions it becomes obvious that the utilization rate is related to recovered paper usage in paper production, whereas the recovery rate is related to the amount of collected paper related to paper consumption. Both rates can be affected directly either by the paper industry by the use of more or less recovered paper in paper production or by the waste management industry collecting more or less recovered paper from the paper consumed. The recycling rate is a more theoretical value putting recovered paper usage in relation to paper consumption. A high level of recovered paper usage in paper production combined with strong net paper export rates will result in a high recycling rate, which has to be satisfied by recovered paper imports from abroad.

Recovered paper utilization rates globally and nationally may be used for com¬parison. Such statistics can give, however, a false impression of the recycling activ¬ities in different countries as the following discussion explains. The range of na¬tional utilization rates extends globally from 5 % to more than 100 %. Figure 2.3 shows the 12 largest papermaking countries, accounting for almost 80 % of the global paper production in 2002 of 331 million tons. Simultaneously, they con¬sumed about three quarters of the world’s recovered paper volume. Heading the list are South Korea, Germany, Indonesia and Japan with utilization rates above 61 %. The lowest utilization rates are for Finland and Sweden. This is because more than 80 % of their paper production is exported due to their low population figures and related low level of national paper consumption. Utilization Rates for Different Paper Grades

Utilization rates for different countries should not be compared without further comment. It is important to know the structure of the production program of the different national paper industries in the main product categories of packaging papers and board, graphic papers, household and hygiene papers as well as spe¬cialty papers because the utilization rates for these product segments differ sig¬nificantly.

In Fig. 2.5 for the CEPI countries the recovered paper utilization by the main paper categories is presented and additionally the utilization rates are given. The x-axis is a summarizing ordinate representing the total paper production in the CEPI countries of 91 million tons, subdivided into the production of the various paper categories. The broader a single paper category, the larger the paper produc¬tion (for example, the biggest category, with 34 million tons, is other graphic pa¬pers). For each paper product category (e. g. newsprint) the used recovered paper volume is shown on the y-axis. In the circles the corresponding recovered paper utilization rate is given. It becomes obvious that the highest volume of recovered paper is used in the production of case materials (19.2 million tons). The second highest recovered paper utilization rate of 73 % is in newsprint with a volume of 7.4 million tons. In the other paper categories between 2.0 and 4.4 million tons ofrecovered paper are used, resulting in recovered paper utilization rates between 9 % and 61 %.

Traditionally, packaging papers and board have the highest recovered paper utili¬zation rate. In the CEPI countries this ratio has reached almost 75 %. Demanding quality specifications have to be fulfilled by these recycled fiber-based paper and board grades to ensure trouble-free converting, for example, to corrugated board boxes or folding boxes and adequate functional characteristics of the finished prod¬ucts. Occupying second place is the utilization rate of household and hygiene papers (61 %). Due to high yield losses in the flotation and washing steps, the proportion of recycled fibers in hygiene paper is not higher than 40 % on average. The figure of 40 % deinked pulp comes from about 60 % yield of about 65 % utiliza¬tion rate.

The group of other papers, comprising gypsum liners, special papers for waxing, insulating and roofing, achieve a 51 % utilization rate in the CEPI coun¬tries, but there are also many specialty paper grades such as cigarette paper, filter paper, or banknote paper which never can use recycled fibers.
Finally, the level of the utilization rate for graphic papers is only 23 %. Due to the wide spectrum of the paper grades produced, a distinction is necessary for this product segment because newsprint has reached a level of 73 %. The utilization rate of the other graphic papers averages only 9 %. Included among these other graphic papers are wood-containing and woodfree papers that are coated or un¬coated. Some use 100 % recycled fiber furnish, e. g. recycling copy papers. Resources of Recovered Paper

Before using recovered paper for paper production its recovery is necessary. Re¬sources of recovered paper are in general collection from industrial enterprises
(e. g. printing houses), business operations (e.g supermarkets) or offices and from private households and small commercial enterprises. Whereas the recovery of used paper from industry, business operations and offices has a long history and can be easily managed, recovery of used paper from private households is more complex. Additional recovered paper resources cannot be expected in the first cate¬gory, because these paper grades mainly belong to the so-called “pre-consumer” grades, are of high quality and are already almost 100 % recovered. Collected paper from private households is “post-consumer” recovered paper which is usually mixed and always requires a sorting process.

As far as national recycling activities are concerned, an assessment of the paper recovery rates of different countries is more relevant than a comparison between national recovered paper utilization rates. Figure 2.6 shows the collection rates vs. the utilization rates of recovered paper in the CEPI countries. The bubble size is equivalent to the tonnage of recovered paper used in each country. Due to in¬tensified collection in each country the average rose from 41 % in 1991 to 56 %, corresponding to a total volume of recovered paper of 46 million tons in 2002. It becomes obvious that the highest collection rates in the CEPI countries are by Germany, Switzerland, the Netherlands, Norway, Sweden and Austria, whereas the highest utilization rates are reached in Ireland, Denmark, Spain, Greece, the United Kingdom and Hungary. The largest tonnages of recovered paper are used in Germany, France, Italy, the United Kingdom and Spain.

The maximum paper recovery rate in an industrialized country with a well-developed infrastructure is about 80 % [7]. This theoretical limit of the recovery rate occurs since approximately 20 % of the used paper products are not collectible, because they are contaminated, long living products or are destroyed by disposal or burning. This includes hygiene paper, specialty paper products, long-life paper products, papers used in industrial applications, and papers used for other pur¬poses in private homes such as fuel or compost material. Considering the theoret¬ical recovery rate of 80 %, the recovery rate of 75 % already achieved in 2002 in Germany is equivalent to a paper collection efficiency of about 94 %. Collecting additional used paper is probably not economically feasible.

At the global level, no data are available for the different sources of collection systems of recovered paper by tonnage. In Europe, CEPI has conducted recovered paper surveys which found, for 2002, the following five sources of recovered pa¬per:
. • Recovered paper from households: 38 %
. • Recovered paper from trade and industry: 33 %
. • Recovered paper from converting and printing: 15 %
. • Recovered paper from offices: 10 %
. • Recovered paper from unsold newspapers and magazines: 4 %.

The recycling rate was introduced by CEPI for the first time as the relation between recovered paper utilization and paper and board consumption. Related to the con¬sumption of paper and board the figure indicates the internal material recycling of recovered paper in a country or a region, so long as no net import of recovered paper occurs. Nevertheless, recycling rates close to 100 % indicate that either a large amount of recovered paper is imported or a high volume of produced paper and board is exported. Figure 2.7 shows the development of the recycling rates in the CEPI countries from 1991 with an increase of 2.2 % p. a. until 2002, when it was approaching 52.7 % and it should reach 56 % in 2005, far from the theoretical or practical limit. Lists for Recovered Paper Grades
Globally no common list of recovered paper grades exists. A European Standard was established in 2001 by CEN (European Committee for Standardization) as EN 643 for Europe [8]. All recovered paper grades are classified into the following five groups:
. • Ordinary grades
. • Medium grades
. • High grades
. • Kraft grades
 • Special grades.
 Recovered paper grades are generally not defined by physically verifiable quality characteristics. Instead, the naming of recovered paper in the list uses one of the following criteria:
. • Places of occurrence, such as supermarket corrugated paper and board (1.04)
. • Former paper grades of which the recovered paper consists, such as mixed mag¬azines and newspapers (1.10)
. • Mixed categories such as mixed sorted papers and boards (1.02).

Globally and in Europe, the use of deinked pulp (DIP) in newsprint dominates (55 %–65 %), followed by hygiene papers (20 %–15 %) and printing and writing papers (about 12 %). Therefore, another very important paper grade in Europe describes a wood-containing recovered paper mix, commonly used for deinking. It is called “Sorted graphic paper for deinking” (1.11) and originates primarily from household collections.
The complete list of recovered paper grades is published in the Annex of Volume I and is available also on the website of GesPaRec (
Recovered paper grade lists are not comparable between Europe, USA and Asia. Therefore recovered paper grades are summarized very often in the following four groups:
. • Mixed grades: comprising always a mixture of various paper and board grades used for the production of packaging paper and board.
. • Corrugated and kraft grades: comprising predominantly packaging paper and board grades used for the production of packaging paper and board. The term OCC is also very often used for this recovered paper grade and stands for “old corrugated containers”.
. • Deinking grades: comprising exclusively graphic paper grades used – after a deinking process – predominantly for the production of graphic and tissue pa¬pers.
. • High grades: comprising predominantly white lightly printed graphic papers and board (very often woodfree), recovered by separate collection and therefore relatively clean. Use of Recovered Paper Grades

Figure 2.8 shows that the use of recovered paper in paper production in the CEPI countries varies greatly by grade of recovered paper. In 2002, the consumption of recovered paper of all CEPI countries was as follows:
. • 41 % OCC grades
. • 26 % Deinking grades
. • 20 % Mixed grades
. • 13 % High grades.

Whereas mixed grades and OCC grades (91 % by weight) primarily find use in the production of packaging papers and board, the majority of all deinking grades (80 % by weight) go to graphic paper production. These recovered paper grades are also used for household and sanitary papers with the addition of nearly two-third high grades. The production of other papers requires various proportions of all four recovered paper grade groups.
The total amount of 43 million tons of recovered paper was used:
. • 45 % for the production of case materials
. • 17 % for newsprint production
. • 10 % for wrappings and other packaging papers
. • 9 % for carton boards
. • 8 % for household and sanitary papers
. • 7 % for other graphic papers
. • 4 % for other paper grades.

The individual paper grades need specific recovered paper grades as raw materials. Thus collection and sorting of these resources has to be done in such a way as to satisfy the needs of the paper industry.