What is Paper?
Paper is an aqueous deposit of any vegetable fibre in sheet form. If you tear a piece of paper you will see a number of minute ‘whiskers’ sticking out from the line of the tear. These are the fibres, they are very small in size (not much more than a millimetre in length) and there are millions of them in any piece of paper.
Sources of Paper
Paper is made from cellulose, trees being the main source of cellulose fibre (or woodpulp). Besides wood pulp, paper can be made from other materials such as
Cotton
Flax
Esparto
Straw
Hemp
Manila
Jute
Some of the pulp properties depend upon the process used to separate the fibres from the timber. The main processes are called mechanical and chemical. (See Making Paper from Wood pulp). If the pulp mill is close to the paper mill, i.e. it is part of an integrated mill,the liquid pulp is pumped direct to the paper mill. Otherwise, after straining, the pulp is dried and packed into bales. The trees used in Spruce Wood Fibres papermaking are mainly grown in Scandinavia, North America and Canada. They are cone-bearing, for example, spruce, pine or firsoftwoods) and leaf-bearing trees such as birch, beech or eucalyptus (hardwoods).
The Paper Mill
The bales of dry pulp are broken up in a hydra pulper (a large circular metal tank in which a mixture of pulp and water circulates). The liquid pulp, looking a little like porridge, is then pumped to machines known as refiners. These modify the fibres by passing them between sets of fixed and rapidly moving metal bars.
As the fibres undergo this treatment they are chopped or bruised to varying extents. This operation, sometimes referred to by its older name of ‘beating’, is extremely important as it mainly controls the type of paper being made. The pulp is now known as stock. It is at this point that the size and colours are added. Size consists of starch or resin whose particles settle on the fibres, producing a surface that does not blur when inked.
Dyes are added to colour the paper. Even if the paper is to be white, some colouring is added, as without it the paper has a slightly yellowish look. Quite frequently mineral matter such as china clay is added to the stock, to make the paper less transparent.
Manufacturing Process
Now the actual process of changing stock into paper begins. A typical paper machine is approximately 100m long and 4m wide, and can cost over £200 million to build. The machine has a ‘wet end’ and a ‘dry end’. At the wet end, the stock is diluted to 200 parts water and 1 part fibre.
The diluted pulp, looking a little like milk, is pumped through a horizontal slit, from where itlands on a fast moving plastic mesh called the wire. The wire is made to vibrate and it has suction equipment below it so that water drains rapidly, leaving the fibres as a mat on top of the mesh. The wire can travel at speeds of up to 2000m/minute.
By the time it reaches the end of this section the fibres have become a sheet of paper, though very moist and having little strength. It then passes to the press section. This consists of a number of sets of heavy rollers through which the moist paper is conveyed, on thick felts. Even at this stage the paper ‘web’ is still weak and moist.
It then passes to the dry end which consists of a large number of steam heated drying cylinders, up to 100 or more on large paper machines. Part of the way down the bank of drying cylinders is the size press. It is here that a solution of water and starchcan be added to the sheet in order to further improve the surface.
Beyond the drying cylinders comes thecalender, which consists of a stack of polished iron rollers mounted one on top of the other. Its function is to polish or glaze the surface of the paper as it passes between the rollers.
Still travelling at up to 2000m/minute, the paper now comes off the machine ready for ‘kindling’ into large reels, each of which may contain up to 20 tonnes of paper. These reels are later either cut into sheets or slit into smaller reels according to the type of paper and the customer’s requirements.
A separate process is the coating of the paper with china clay and then polishing it through the calender. This makes art paper, the glossy kind used for pictures. There are 3 main kinds of modern coating processes: blade coating, air knife coating and roll coating.
Blade coating is normally done off the paper machine, as is air knife coating. But roll coating is done on the paper machine and can either be a complete coating or a first coat followed by ‘off-machine’ coating by the blade or air knife process.
Future of Paper
Some manufacturers have started using a new, significantly more environmentally friendly alternative to expanded plastic packaging made out of paper, known commercially as paper foam. The packaging has very similar mechanical properties to some expanded plastic packaging, but is biodegradable and can also be recycled with ordinary paper.
With increasing environmental concerns about synthetic coatings (such as PFOA) and the higher prices of hydrocarbon based petrochemicals, there is a focus on zein (corn protein) as a coating for paper in high grease applications such as popcorn bags. Also, synthetics such as Tyvek and Teslin have been introduced as printing media as a more durable material than paper. Types and Thickness The thickness of paper is often measured by caliper, which is typically given in thousandths of an inch. Paper may be between 0.07 millimetres (0.0028 in) and 0.18 millimetres (0.0071 in) thick. Paper is often characterized by weight. In the United States, the weight assigned to a paper is the weight of a ream, 500 sheets, of varying "basic sizes", before the paper is cut into the size it is sold to end customers. For example, a ream of 20 lb, 8.5 in × 11 in (216 mm × 279 mm) paper weighs 5 pounds, because it has been cut from a larger sheet into four pieces. In the United States, printing paper is generally 20 lb, 24 lb, or 32 lb at most. Cover stock is generally 68 lb, and 110 lb or more is considered card stock.
Conclusion
The production and use of paper has a number of adverse effects on the environment. Worldwide consumption of paper has risen by 400% in the past 40 years leading to increase in deforestation, with 35% of harvested trees being used for paper manufacture. Logging of old growth forests accounts for less than 10% of wood pulp, but is one of the most controversial issues.
