Paper and Board Manufacturing ::
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Overview
As papermaking is quite a complicated technical process, computer controls monitor it from start to finish. The computers make adjustments to ensure such things as evenness, appropriate moisture content, proper mixing and proportioning of pulp and chemicals, sheet (of paper) formation, thickness and grammage, are controlled.
Manufacturers use mixtures of chemical wood pulp, mechanical wood pulp and other additives to make paper with very specific characteristics. Paper can vary in thickness as well as in strength and printability, i.e. how well the paper performs in a printing machine. Manufacturers may also apply a coating to the paper to improve its colour and brightness as well its printability.
World paper demand is projected to grow by about three percent a year through the next decade. That means global consumption will rise by almost 50 percent by 2010!
[Source: www.cppa.org]
The Processes
Wet processes
Stock preparation
The material to be used for making paper is first converted into pulp (a concentrated mixture of fibres held in liquid). As many of these fibres come from natural sources, e.g. plants, this process often requires many stages of separation and washing. Once the fibres have been taken out, they may also be bleached or dyed to alter the appearance of the final product.
Figure 1 :: Stock preparation equipment
Layering/Bonding
The pulp mixture is diluted with water resulting in a very thin slurry (gooey liquid). This dilute slurry is drained of water through a fine mesh/moving screen. Water is also removed through gravity, vacuum action, pressure and heat. A watermark may be impressed into the paper at this stage of the process (a distinguishing mark pressed onto the paper during manufacture; visible when paper is held up to the light.). As water is removed, cellulose fibres bond to form a web of paper (cellulose fibres are fibres that come from plant cell walls and contain glucose, which is a polymer).
Figure 2 :: Kraft paper layering
Forming
The moving web is then pressed and dried into a continuous sheet of paper. Most mass produced paper is made using a continuous process to form a reel or web.
Figure 3 :: Photomicrograph of Paper Surface
Coating
Coating, the next stage, is the application of a suspension (layer) to the paper surface. It may be performed on the paper machine or on a separate off-machine coater. Coating may be on one or both sides and is followed by drying.
Raw paper that contains only pressed and dried pulp is very absorbent (soaks up water, like a sponge) and does not provide a good surface upon which to write or print. Thus a huge variety of additives (chemicals) are employed to add desired properties (characteristics) to the paper. These are applied in a coating called the size. Sizing agents are often polymers designed to provide a better printing surface. The most commonly used type of polymer is starch (a natural polymer), but there are as many types of polymer employed as there are types of paper.
Coatings can also be applied to the top layer of paper to improve the printing surface by making it smoother. The texture of raw paper is rough, and so to achieve greater smoothness, coatings consisting of latex or other binders and fillers such as kaolin or calcium carbonate (types of clay) are used. Glossy, silk or matt finish papers such as magazine paper are made in this way. Additionally glossy or textured surfaces can be applied using a process called supercalendering, in which the fresh rolled paper is run through an additional series of rollers to imprint the desired texture. The glossy effect is achieved at the end of the printing process, by adding a clear layer over the printing.
Figure 4 :: Paper coater equipment
Dry Processes
Finishing
This covers the finishing of pulp, paper and paperboard articles. In this process, the reeled up paper is converted (trimmed, transferred to smaller reels, cut into sheets, etc.) for sale or further processing.
Figure 5 :: Reels of Paper in Finishing Room
Common types of paper
| Product Type | World production in Tonnes (A) | Pulp / fibre type | Additives |
|---|---|---|---|
| Newsprint | 32,404,000 (14%) | mostly mechanical and/or recycled with some Kraft or sulphite | |
| Printing and Writing | 70,039,000 (31%) | bleached wood free chemical, some unbleached (C)TMP | china clay (gloss), chalk (matt), titanium dioxide, size |
| Household and Sanitary | 13,125,000 (6%) | softwood sulphite, some Kraft and recycled | synthetic resins, Urea, melamine formaldehyde |
| Packaging | 98,983,000 (43%) | Kraft | wax, plastic, metal foil |
| Other | 13,858,000 (6%) |
(A) Food and Agriculture Organisation (FAO) Forest Products Yearbook 1991
(B) Rice, T.(ed), Out of the Woods, Friends of the Earth, 1995
Kraft papers
These are noted for their strength, are made predominantly from wood pulp and used primarily as packaging material. They can be converted into a wide variety of products such as grocery bags, envelopes etc. Their industrial uses include tapes, laminates for desks and countertops, and box coverings.
Special papers
Greaseproof and glassine papers are generally made from mechanical wood pulp and are ideal for wrapping foods. Glassine papers are supercalendered (explained above) and translucent or opaque.
Sanitary papers
These include any type of paper used for sanitary disposable purposes. Generally these papers are absorbent, bulky and have a soft texture.
Newsprint
The term refers to paper between specific grammages that is destined for newspapers. Most newsprint is used to print daily and weekly newspapers. The other major uses are inserts and flyers, newspaper supplements and directories.
In the last 15 years, world demand for newsprint has grown at an average annual rate of 2.1 percent. Newsprint demand, however, is extremely sensitive to economic cycles, and in the short run, it is not uncommon to observe large changes in demand and supply.
Figure 7 :: Newsprint sheet
Paperboard
Definition
Paperboard, within the commercial papermaking industry, is simply the term used to describe a thick sheet of paper. Because of this simple definition, there is some confusion in the industry as to what qualifies as paperboard. Thickness (caliper) and bulk are significant aspects in paperboard manufacture, as the end product must be able to withstand the bending forces of packaging, load carrying, shipping etc., while still maintaining their shape. Consequently, these paperboard products are usually comprised of two or more plies to provide the required stiffness. Plies is the term used to describe layers of paper.
Within the industry, paperboard is referred to by weight ( (16 ounces) pounds per square foot or grams per square metre), by thickness (by thousandths of an inch, or 'points' - i.e. 0.009" thick paperboard is also know as 9-point) or by name (foodboard, corrugating medium, boxboard etc.).
Figure 8 :: A cross section of paperboard showing flutin that is required for stiffness
Categories
Paperboard is a generic term, mainly associated with boxboard (multiply board), but can also apply to corrugated board.
Corrugated board comprises two main classifications: linerboard and corrugating medium, used almost entirely in the manufacture of shipping cases. Linerboard, which is hard, strong and firm is manufactured in two forms: kraft liner (produced from kraft pulp) and recycled liner (made from recycled fibres). Flutin is the wave-like material used as the middle ply in corrugated shipping cases. The combination of linerboard on the outside and flutin in the middle results in a light and strong finished product, known as corrugated medium.
Boxboard may be uniform in substance and quality throughout, or it may emerge from the mill in a single sheet composed of layers made from different types of pulp. Boxboard is of three types: folding, set-up and miscellaneous boxboard.
Folding boxboard is made into products such as detergent boxes and milk or juice containers. For containers that willl carry products to be ingested, other processes may be required such as the addition of aluminium foil to block out light and oxygen, in order to keep the contents fresh.
Set up boxboard is used when a more rigid container is required , for example a jewellery presentation box.
Figure 9 :: Miscellaneous boxboard includes many kinds of paperboard used for hundreds of products including paper plates.
Other issues and Regulations
Environment
[See also the page on Recycling and Waste Paper]
Detailed information concerning the environmental impact, from an eco-friendly viewpoint, of pulp and paper manufacturing can be found at www.foe.co.uk
Employment in Europe
The industry provides direct employment for around 250,000 people, and indirect employment - through the paper and forest cluster - for a total of 3.5 million people.
The European pulp and paper industry is a vital part of the paper and forest economic cluster that generates an annual turnover of more than EUR 400 billion.
CEPI member countries produce in excess of 90 million tonnes of paper and board each year. The industry is made up of more than 1,260 pulp and paper mills.
See also: www.paperonline.org
Regulations and guidelines for the industry
The IPPC Directive (96/61/EC) is being introduced across Europe to improve environmental protection standards. IPPC stands for Integrated Pollution Prevention and Control. In essence, the IPPC Directive is about minimising pollution from various sources throughout the European Union.
The Directive requires integrated permits to be obtained from various industry sectors including pulp and paper. "Integrated" means that the permits must take into account the whole environmental performance of the plant, i.e. emissions to air, water and land, generation of waste, use of raw materials, energy efficiency, noise, prevention of accidents, risk management, etc.
Together with the IPPC, something called Best Available Techniques (BAT) have been introduced to help give an idea of how industries can improve. In environmental protection law, BAT means the most effective and advanced techniques that can be practically adopted to prevent harmful emissions and other environmental impacts, or reduce them to acceptable limits.
By May 2004 all 25 Member States had fulfilled this basic obligation. In point of fact, in the UK the paper industry was one of the first industrial sectors in the UK to adopt the IPPC.
Additional information
IPPC S6.01: Sector Guidance: Pulp and Paper - Applies to the UK directly, but also applies across all EU countries that have adopted the IPPC.
Available from: Environment Agency 2000
ISBN 0 11 310171 6
Cover Price £25.00 (Hard copy available from the Stationery Office)
For more information see :: www.environment-agency.gov.uk
Finnish site that deals with the same issues :: www.environment.fi
Further Information
One book that covers all types of paper and paperboard packaging is:
'Paper and Paperboard Packaging Technology' by Mark J. Kirwan
Available from :: store.blackwell-professional.com
Price $239.99 :: ISBN: 1405125039
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