How It's Made
Paper is made from prepared pulp stock on a paper machine.
Preparation of stock may include addition of 'sizing' agents (to give the paper a smooth, even surface), pigments and other colouring agents, and materials to impart special properties (e.g. waterproofing).
Most paper machines are developed from the original Fourdrinier pioneered machine and the process is similar on all variants. Screened pulp is spread from a tank or headbox on to a horizontal wire mesh, constructed as an endless belt. As the belt rotates, water mixed with the pulp is drained off with the assistance of some suction. This water may include fillers, fines and even dyes, is cloudy and is called 'white water'. The remaining matt quickly consolidates to a stage where it can be led off via the couch roll (which draws off more water and lifts the paper mat into the pressing section), through roller presses to remove more water. The endless pulp mat continues through a series of steam-heated rollers that complete the drying process, and finally passes over smooth calendar rolls that give the paper a smooth finish. Paper feeds from the dryers on to large rollers where it is held for further processing.
Because the Fourdrinier draws water away below the belt, the paper tends to be one-sided (i.e. the upper surface has more fillers and is smoother while the lower surface loses more of its fines). Accordingly, a modern development has been the development of twin-wire formers, where pulp stock is injected between the wires and water is drawn off both above and below. De-watering is quicker, the paper produced is more uniform and the actual paper sheet formation can be completed very rapidly.
The final properties of paper depend on:
• type of pulp used,
• amount of bleaching,
• thickness of pulp mat created,
• pressure applied during paper manufacture,
• chemical additives,
• beating,
• coating.
Additives include alum, starches, clays and sizing agents, all of which are intended to increase the density of the paper, and provide a good printing surface. Other chemicals may be added to enhance the effectiveness of dyes.
Coatings are often applied to make the paper suitable for modern, high speed printing processes. While several processes are used, all involve the application of a thin coat of clay or calcium carbonate pigments, and binders such as starch or poly-vinyl alcohol. The coat is dried to give a very uniform and smooth surface.
'Beating' is the term used to describe a process where pulp is mechanically agitated while in solution. The wet pulp is passed between rotors with bars or knives which compress the individual cells. Generally, chemical pulps respond to beating more easily with cells becoming more flexible and compressing, while pulps with higher lignin content require significantly more treatment.
Paper made from unbeaten pulp is bulky, porous and has less tensile strength than from beaten pulps. This is because unbeaten pulp fibres tend to be stiff and springy, and do not collapse as well when the paper mat is pressed.
Paper Products
There are probably several hundred types of paper and paperboard, but they can be grouped as follows:
Newsprint
Made from mechanical pulps, although chemical pulp may be added in small quantities for basic strength. The paper is used pretty much as it comes off the machine and has no mineral (e.g. sizing) added. It is made at high speed (600-700 m per minute) on wide machines and is relatively cheap paper. Blue dye may be used to enhance its 'whiteness'.
Writing
These vary significantly in quality, but generally contain significant proportions of bleached sulphite pulp, alum and a rosin/starch size. High brightness is also a common factor.
Highest quality bond papers may include components of cotton/linen stock and alpha pulp and a starch or rosin size. But they are now being replaced by modern coated papers.
Greaseproof
These papers are semi-transparent, very dense and resist the penetration of greases and oils. They are produced from high cellulose content pulps with significant beating of the stock to aid compaction. Paper is formed at slower speed to aid formation and moisture content is retained at relatively high levels. This helps the resistance to greases, although greaseproof papers are not waterproof.
Wrapping and (brown) Bag
Most of these papers are made from unbleached kraft pulps because the first requirement is strength. This is achieved by using pulp with long, well formed fibres evenly laid and criss-crossed. These papers must be:
• flexible and have as little grain as possible (i.e. not tear across the grain
more readily than with it),
• have some stretch so they do not rupture too easily,
• smooth on one surface so printing and labelling are possible,
• well sized so that some resistance to wetting exists,
• reasonably dense to protect the contents,
• easy to glue.
Tissue
Tissue papers are usually made of well bleached sulphate or sulphite pulp with special treatment to enhance softness, absorbency and strength. Some forms are also have wet strength properties.
Paperboards
Paperboards come in many different forms. They may be made on a moving wire screen to give a board of single thickness (controlled by the thickness of the pulp layer and pressure), or using a second headbox so that the board has two layers (which may be of different consistency). Many other variants exist depending on whether it is to have a printed surface, strength requirements and so on.
Corrugated Board
Has a central corrugated layer which is unbleached and unsized, but is now mostly made from kraft pulp to maximise strength and resistance to crushing. Outer layers will depend on other requirements such as colour and printable surface. However, the strongest brown boxes are made from kraft pulp throughout.