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Monday, 28 March 2011 20:15

Sheet Production and Converting: Market Pulp, Paper, Paperboard

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End products of pulp and paper mills depend on the pulping process, and may include market pulp and various types of paper or paperboard products. For example, the relatively weak mechanical pulp is converted into single-use products such as newspapers and tissue. Kraft pulp is converted into multi-use paper products such as high-quality writing paper, books and grocery bags. Sulphite pulp, which is primarily cellulose, can be used in a series of diverse end-products including specialty paper, rayon, photographic film, TNT, plastics, adhesives, and even ice cream and cake mixes. Chemi-mechanical pulps are exceptionally stiff, ideal for the structural support needed for corrugated container board. The fibres in pulp from recycled paper are usually shorter, less flexible and less water permeable, and can therefore not be used for high-quality paper products. Recycled paper is therefore mainly used for the production of soft paper products like tissue paper, toilet paper, paper towelling and napkins.

To produce market pulp, the pulp slurry is usually screened once more and its consistency adjusted (4 to 10%) before it is ready for the pulp machine. The pulp is then spread onto a travelling metal screen or plastic mesh (known as the “wire”) at the “wet end” of the pulp machine, where the operator monitors the speed of the moving wire and the water content of the pulp (figure 1; the presses and the cover of the drier can be seen in the upper left; in modern mills, operators spend a great deal of time in control rooms). Water and filtrate are drawn through the wire, leaving a web of fibres. The pulp sheet is passed through a series of rotating rolls (“presses”) that squeeze out water and air until the fibre consistency is 40 to 45%. The sheet is then floated through a multi-storey sequence of hot-air dryers until the consistency is 90 to 95%. Finally, the continuous pulp sheet is cut into pieces and stacked into bales. The pulp bales are compressed, wrapped and packaged into bundles for storage and transport.

Figure 1. Wet end of pulp machine showing fibre mat on the wire.

PPI070F1

Canfor Library

Although similar in principle to making pulp sheets, paper making is considerably more complex. Some mills use a variety of different pulps to optimize paper quality (e.g., a mix of hardwood, softwood, kraft, sulphite, mechanical or recycled pulps). Depending on the type of pulp used, a series of steps is necessary prior to forming the paper sheet. Generally, dried market pulp is rehydrated, while high-consistency pulp from storage is diluted. Pulp fibres may be beaten to increase the fibre-bonding area and thereby improve paper sheet strength. The pulp is then blended with “wet-end” additives (table 1) and passed through a final set of screens and cleaners. The pulp is then ready for the paper machine.

Table 1. Papermaking additives

Additive

Location applied

Purpose and/or examples of specific agents

Most commonly used additives

Talc

Wet end

Pitch control (prevent deposition and accumulation
of pitch)
Filler (make brighter, smoother, more opaque)

Titanium dioxide

Wet end

Pigment (brighten sheet, improve printing)
Filler (make brighter, smoother, more opaque)

“Alum”(Al2(SO4)3)

Wet end

Precipitates rosin sizing onto fibres
Retention aid (fix additives to fibres, improve pulp
fibre retention)

Rosin

Wet end

Internal sizing (resist liquid penetration)

Clay (kaolin)

Wet/dry

Filler (make brighter, smoother, more opaque)
Pigment or surface coating (impart colour)

Starch

Wet/dry

Surface sizing (resist liquid penetration)
Dry strength additive (increase strength, reduce
surface lint)
Retention aid (bind additives to paper, improve
pulp fibre retention)

Dyes and
pigments

Wet/dry

e.g., acid, basic or direct dyes,colour lakes,
CaCO3, may also include solvent vehicles

Latex

Dry end

Adhesive (reinforce sheet, bind additives to paper,
fill pores)
Waterproofing (resist liquid penetration)

Other additives

Slimicides

Wet end

e.g., thiones, thiazoles, thiocyanates, hiocarbamates, thiols, isothiazolinones,
formaldehyde, glutaraldehyde, glycols, naphthol,
chlorinated and brominated organics, organic
mercury compounds

Defoamers

Wet end

e.g., pine oil, fuel oil, recycled oils, silicones, alcohols

Wire treatment
agents

Wet end

e.g., imidazoles, butyl diglycol, acetone, turpentine,
phosphoric acid

Wet and dry
strength additives

Wet end

e.g., formaldehyde resins, epichlorohydrin, glyoxal,
gums, polyamines, phenolics,
polyacrylamides, polyamids, cellulose derivatives

Coatings,
adhesives and
plasticizers

Dry end

e.g., aluminium hydroxide, polyvinyl acetate,
acrylics, linseed oil, gums, protein glues, wax
emulsions, azite, glyoxal, stearates, solvents,
polyethylene, cellulose derivatives, foil, rubber
derivatives, polyamines, polyesters,
butadiene-styrene polymers

Others

Wet/dry

Corrosion inhibitors, dispersants, flameproofing,
antitarnish agents, drainage aids, deflocculants, pH
control agents, preservatives

 

The flow spreader and headbox distribute a thin suspension (1 to 3%) of refined pulp onto a moving wire (similar to a pulp machine, only at a much higher speed, sometimes in excess of 55 km/h) which forms the fibres into a thin felted sheet. The sheet moves through a series of press rolls to the dryer section, where a series of steam-heated rolls evaporate most of the remaining water. Hydrogen bonds between the fibres have fully developed at this stage. Finally, the paper is calendered and reeled. Calendering is the process by which the paper surface is ironed smooth and its thickness reduced. The dried, calendered paper sheet is wound onto a reel, labelled and transported to the warehouse (figure 2; note waste paper under reel, and unenclosed operator control panel). “Dry-end” additives can be added before calendering on the paper machine or in separate “off-machine” coating operations in the converting sector of the industry.

Figure 2. Dry end of a paper machine showing full paper reel and operator using air slitter to cut end.

PPI070F2

George Astrakianakis

A variety of chemicals are used in the papermaking process to provide the paper with specific surface characteristics and sheet properties. The most commonly used additives (table 1) are typically used at the per cent level, though some such as clay and talc may contribute as much as 40% to the dry weight of certain papers. Table 1 also indicates the diversity of chemical additives which may be used for specific production purposes and products; some of these are used at very low concentrations (e.g., slimicides are added to process water in parts per million).

The process of making paperboard is similar to that of making paper or pulp. A suspension of pulp and water is dispersed onto a travelling wire, the water is removed, and the sheet dried and stored as a roll. The process differs in the way that the sheet is formed to give thickness, in the combining of multiple layers, and in the drying process. Board can be made from single or multi-layered sheets with or without a core. The sheets are usually high-quality kraft pulp (or kraft and CTMP blend), while the core is made from either a blend of semi-chemical and low-cost recycled pulp or from entirely recycled pulp and other waste material. Coatings, vapour barriers and multiple layers are added according to the end use to protect the contents from water and physical damage.

 

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