Papier is produced in the shape of a continuous web which contains vegetable fibres, mainly wood based. Other types of fibres can be used as well – straw, cotton and various sorts of synthetic fibres – but wood is the principal raw material. Different types of wood have fibres with different structures. Fibres from coniferous trees like spruce and pine, for instance, are considerably longer than those from deciduous trees like birch and beech. Before it can be used for the production of paper, however, all wood needs special treatment to free – unlock- the fibres from the surrounding tissue. This can be done either : - mechanically - or chemically.
Mechanical unlocking is a process that uses grinders in which wood chips are pressed unto a rotating, rough stone under the addition of large amounts of water, producing a mixture of fibre fragments and water, the so-called wood pulp. Depending on the intended use this pulp can be relatively coarse or fine and in some cases, it is bleached before going into production All natural components of the wood are retained in this unlocking process, including lignin, which explains the yellowing tendency of paper produced from mechanical pulp. Papers produced from this type of pulp are known as mechanical – or wood containing - papers. Mechanical unlocking in a magazine grinder
Chemical unlocking can either be done on the basis of acids or by way of alkaline reactions In both cases, wood chips of approx. 2 x 2 x 1.5 cm are boiled under pressure in chemical solutions in order to dissolve two of the basic wood components – resin and lignin. The boiling process also serves to soften the wood and allows for the subsequent removal of individual fibres. The resulting product, known as cellulose, is then rinsed to remove remaining chemicals, sorted, bleached where appropriate, and dried. In acid pulping, a solution is used consisting of magnesium bisulphite and free floating acids. The resulting product is known as sulphite cellulose. In both acid and alkaline pulping, all waste water is collected and re-used in energy generation and raw material processing. This way, over 95% of all chemicals are fully recycled. In alkaline pulping, caustic soda and sodium sulphate are used, The resulting product, known as sulphate cellulose, has the advantage of excellent strength properties. Papers made exclusively from cellulose pulp, are known as wood-free.
Key processing steps in sulphite cellulose production and waste water recycling
Several additives are mixed in with the actual raw materials, the fibres, to ensure that the paper will have the desired characteristics. Examples of these additives are: Fillers, clay, alum, pigments and optical brighteners. Many different minerals can be used as fillers. Such as: Silicates : magnesium silicate (talc), kaolin, aluminium silicate. Sulphates: calcium sulphate luminate (satin white), barium sulphate (blanc-fix, baryt), calcium sulfate, aniline. Carbonates: calcium carbonate (limestone, chalk sludge), magnesium carbonate, magnesite (patent white). Oxides: titan dioxide (titan white).
Depending on particle size and structure, these fillers each have their own particular properties and purpose: <ul><li>Filling the gaps in the paper‘s fibre compound. </li></ul><ul><li>Evening out the paper surface (which has to be closed to be suitable for writing and printing). </li></ul><ul><li>Increasing the opacity of the paper (making it less transparent). </li></ul><ul><li>Increasing the whiteness of the paper. </li></ul><ul><li>Enhancing softness and foldability of the paper. </li></ul><ul><li>In general, by their greater specific weight, fillers increase the paper‘s substance (g/m²). </li></ul>
Paper is a more or less porous plaiting, capable of absorbing significant quantities of moisture. As the paper absorbs this moisture, the fibres expand and the degree to which they do so, determines the dimensional stability of a given paper grade. However, with the exception of blotting and filter papers, most papers are expected to retain applied moisture in clear, crisp outlines. Glues strongly inhibit a paper‘s capability to absorb water and watery solutions..
Glues most commonly used in paper recipes are either natural resin based – like colophonium, which is made from the resin of coniferous trees – or synthetic compounds. Resin glues, in the form of diluted resin soap or resin dispersion, are mixed in with the fibre/water suspension, together with alum (aluminium sulphate) to help bind the individual glue particles to the fibres. Synthetic glues have reactive properties which allow for direct binding to the paper substance – without the addition of binding agents such as alum. When the paper is dried, the glue particles stick together, forming a water rejecting layer in the paper pores which constitutes the actual glueing effect. Glueing only marginally increases the paper‘s strength.
Pigments – obvious ingredients in coloured paper recipes – are also used in the production of white papers. No paper is purely white. All papers have a certain colour tone, either in the red, blue, green or yellow dimension. Since raw materials and additives interact differently with pigments as well, refiner agents must be used to achieve an even paper shade. Apart from pigments, optical brighteners are added to increase the paper‘s visual whiteness by highlighting waves from the invisible part of the spectrum – much the same as in most detergents.
The actual properties of a paper, are not exclusively determined by the characteristics of its ingredients – the fibres, which can be relatively short or relatively long, and the various additives such as fillers and glues. Another important factor is refining. Fibres are refined – beaten - in a watery solution which contains approx. 4 - 7% fibre material. The principal reason for refining is to increase the paper‘s strength. When the refining knives are close together, the fibres are cut rather than crushed, retaining their original structure. The paper substance, containing these short fibre fragments, has a gritty, brittle touch. This is called free beating As the paper begins to take shape on the paper machine, the porous fibres are positioned loosely on top of each other, creating small pockets in the paper substance, The result is a soft, voluminous, highly absorbent paper with little strength. Examples are mechanical book papers and blotting paper.
Refining aggregates: Conical refiner Disk refiner
When the refining knives are slightly wider apart, the fibres are crushed, splitting up into small single strings – the so-callled fibrils – and losing their original structure. This is called wet beating. The result is a rather greasy, slithery mass. During paper formation, the fibrils become closely entangled, resulting in a very dense, little absorbent paper with low opacity and high strength. Examples are translucent tracing paper and glassine.
The fibres used for the production of paper are usually supplied in bales, which are then water dissolved in pulpers – installations that are somewhat like large food processors. Pumps then transport the fibre/water suspension, which at this point contains 4 - 7% fibres, to the first processing stations within the paper mill, where it is first ground, then mixed with additives to form the process suspension or stock. Next, this suspension is strongly diluted with water, to about 1% of the original density, after which it goes through several cleaning processes (in cleaners, vertical screeners and a lump catcher) where irregularities like sand, splinters and plastic are removed. It then goes onto the actual paper machine.
Headbox Wire section Press section Pre-drying section Size press Calender section Pope roller Post-drying section Diagram of a Fourdrinier paper machine
A paper machine consist of several components, or sectons. The stock goes through these sections one by one: Headbox Wiresectio Press section Pre-drying section Size press Post-drying section Calender section Winder In the headbox the stock is evenly spread out. Shuffling motions help the stock to spread and stimulate fibre matting. Then, through a narrow slit at the full width of the paper machine, the stock passes on to the wire, which consists of a continuous, circular motion metal or synthetic webbing. The wire, supported by rolls or frames, allows water to drain away downwards, while vacuum boxes, suction rolls and a top wire take care of further de-watering. Now containing 20% solid material, the paper web then passes on to the press section.
Development of fibre orientation in the wire section of the paper machine In the wire section, while water drains away through the screen, the fibres orient themselves in the wire‘s running directrion. This first phase of paper formation and de-watering on the wire determines two paper properties which are critical to the later printing characteristics: Fibre orientation and two-sidedness .
The stock flow at the moment of leaving the headbox causes the fibres to orient themselves in the wire’s running direction. Because the fibres, when water is absorbed or drained away, swell or shrink sideways up to seven times more than they do along their length; fibre orientation has a significant influence on the dimensional stability of the final paper. Hence the importance of the following two concepts: Machine direction (MD) and Cross direction (CD ) . The terms machine direction , grain direction, fibre orientation describe the working direction of the paper machine and the predominant orientation of fibres in the paper.
Capillary hygroscopic effects – Fibres easily absorb moisture, swelling up in the process. The expansion is most pronounced across the width of the fibres. Fibre orientation Fibre expansion
The next phase of de-watering takes place in the press section, where the paper stock, moving forward on a continuous felt, passes between several consecutive pairs of pressure (above) and suction rolls (below). At the end of the press section the stock has reached a dry content level of approx. 40 - 45%, at which point further mechanical drying is no longer possible. Further de-watering can only be achieved by means of evaporation, which takes place in the drying section, where drying wires press the paper stock onto steam heated cilinders.
In many cases, the drying process is followed by an inline finishing treatment, in which – still on the paper machine itself – a solution of synthetic resin or starch is applied to the paper surface. This is done in order to close the surface, preventing picking and dust forming in later stages. In the process, moisture is again added to the paper, which once again needs drying, this time up to a point where no further changes occur in the paper at a temperature of around 20°C and 55% relative humidity – the paper no longer absorbs water from, nor does it emit water into the air At this point, the paper is ready to be reeled.
For high-grade surface quality, »coating « is applied to the paper. The added opacity of the closed, homogenous surface of a matt coated grade or the shiny surface of a glossy coated paper makes full-colour print stand out much more clearly, while the higher whiteness of coated papers allow for a more exact colour reproduction. True to life full-colour print is only possible on coated papers. Coatings consist of a mixture of high-quality pigments and organic or synthetic binding agents.
The quality of a coated paper is a factor of method and volume of application. Obviously, base paper quality is important as well. These are the essential requirements in terms of paper and process quality: - Even sheet formation - Uniform absorption capability - No dots or pin-holes - Adequate stiffnes in MD and CD - Mechanical and climatic dimensional stability - Even moisture profile in the coating process
The simplest form of finishing is surface treatment in the size press, which is integrated in the paper machine. There are, however, limits to the amount of pigment that can be applied here. The more advanced film presses allow for higher concentrations of sizing and pigments to be applied. Inline coated papers are also known as „machine coated“, although the term equally applies to papers coated in offline (separate) machine coaters. Here as well, the quality of the finished paper is determined by method and volume of application. There are no clear boundaries between the traditional categories of machine coated, art and cast coated paper grades, the differences, in most cases, being gradual.
With reels, the following properties are important: - Good mill joins - Perfect cutting edges - Straight sides (no „telescoping“) - Even winding hardness - Good cores - Proper packaging With sheets, these are the principal requirements: - Exact size - Exact square cut - Clean cutting edges - Uniform quality - Proper, protective packaging The finished paper is delivered to the customer on reels or as sheets.
Cutting the reel into sheets can either be done lenghtwise (long) or across (short). Hence the terms Long grain and short grain
Machine direction Paper machine width 1000 x 700 mm 700 x 1000 mm Short grain Long grain
Testing machine direction 1. Fingernail test Pressing tightly, the fingernail traces along both edges of the paper, creating waves of different intensity. 3. Moisture test A piece of paper taken from the test sheet, is moisturized on one side. As the paper expands, it rcurls around MD. 2. Strip test Two identical strips are cut from the long and short side of the paper, then held horizontally, one on top of the other. The strip that bends most, is from CD. 4. Tear test When tearing the paper in both directions, the cleaner tearing edge indicates MD.
1. The wavy side is at straight angles with MD. 2. The stiffer strip is in MD. 4. The cleaner tearing edge is parallel to MD. 3. The paper curls around MD.