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AS Edexcel Graphics > Materials And Components > Flashcards

Flashcards in Materials And Components Deck (14):
1

Methods of producing wood pulp
Mechanical pulping
Disadvantages

• lower strength characteristics than softwood chemical pulps.
• paper can 'yellow' when exposed to bright lights due to high lignin content.

2

Methods of producing wood pulp
Chemical pulping
Advantages

• higher quality wood pulp produced with longer, stronger fibres that contain fewer impurities.
• produces 'chlorine-free' disposable products.
• waste lignin from the process can be burnt as a fuel oil substitute, often supplying power to the national grid or steam to local domestic heating plants.

3

Methods of producing wood pulp
Chemical pulping
Disadvantages

• lower yield than mechanical methods as the lignin is complete dissolved and separated from the fibres.
• no chemical pulp is produced in the UK, therefore it must be imported.

4

Methods of producing wood pulp
Waste pulping
Advantages

• makes use of recycled papers, which are a sustainable resource.
• well suited for 'bulk' grades of paper, i.e. Newsprint, tissue and packaging boards.

5

Methods of producing wood pulp
Waste pulping
Disadvantages

• cannot be recycled indefinitely as pulp loses quality - virgin pulp needs to be added.
• does not save any more energy in processing than other methods.
• requires considerable processing and additives to produce good-quality paper.

6

Fourdrinier process
What are the names (in order) of the four step process in producing paper from wood pulp?

Wet end
Press section
Dryer section
Calendar section

7

Fourdrinier process
Talk about the first step in the Fourdrinier process (wet end)

The wet end starts with the wood pulp diluted to 99 per cent water and 1 percent fibre to form a slurry that is held in the head box. A continuous stream of slurry is pumped from the head box through an adjustable slit (called the slice) onto a moving gauze wire belt that vibrates to drain off some of the water and allow the fibres to interweave. Raised patterns formed in the gauze create the watermark - a feature in many high-quality writing papers.

8

Fourdrinier process
Talk about the second step in the Fourdrinier process (press section)

The press section uses a system of nip presses or rollers that wrings out the majority of excess water from the pulp and stretches it out into a rough paper. It is as this stage that the thickness of the paper/board is determined. The gap between the press rollers sadhus yes to allow for differing thicknesses such as cars. The term card usually refers to paper which has a density greater than 160gsm (grams per square metre).

9

Fourdrinier process
Talk about the third step in the Fourdrinier process (dryer section)

The dryer section dries the paper using a series of steam-heated rollers by removing the moisture (just like ironing clothes). The resulting paper has a water content of 4-6 per cent and sizing agents, starches and resins can be added to enhance the papers properties.

10

Fourdrinier process
Talk about the fourth and final step in the Fourdrinier process (calendar section)

The calendar section comprises a series of rollers through which the paper is fed in order to smooth it out and give it a uniform thickness. The pressure applied to the paper by these rollers am determined the finish of the paper.

11

Methods of producing wood pulp
Mechanical pulping
Advantages

• provides a 90% yield from the pulpwood as it uses the whole of the log except for the bark.
• investment costs for mechanical pulp mills are relatively low in comparison with other types of mill.
• well suited for 'bulk' grades of paper, I.e. Newsprint and packaging boards.
• can be bleached to produce higher value-added products.

12

Talk about the mechanical process of wood pulp

The logs of coniferous trees are saturated with water and de-barked. The wood is ground down, which softens the lignin, and the mechanised forces separate the fibres to form 'ground wood pulp'. This pulp is screened to accept 1-2mm pieces, with larger pieces being re-circulated for additional screening. The resulting pulp can only be used for low-grade paper such as newspaper, so the pulp is bleached with peroxide or sodium hydroxide. This is the most widely used method in the UK for producing wood pulp.

13

Talk about the chemical process of wood pulp

After de-barking, the hard- and softwood logs are cut into 2cm chips along the grain. These are pounded into fragments and screened. The resulting pulp is stored and treated with either an acid or an alkali to break down the lignin. Most chemical pulp is made by the alkaline Kraft process, or sulphate process, which uses caustic soda and sodium sulphate to 'cool' the wood pulp. The amount of fibre produced is lower than with mechanical methods, but the fibres are longer, stronger and contain fewer impurities.

14

Talk about the waste process of wood pulp

Recycled paper and board used for waste pulp is often used for lower grades of paper, as it strength, durability and colour are not as good as virgin fibres (produced using mechanical and chemical methods). Waste pulp is often mixed with virgin fibres to produce better quality papers as fibres become shorter and weaker and lose their paper making qualities.