Cellulose, chemical pulp
| Infobox on Cellulose, chemical pulp | |
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| Example of Cellulose, chemical pulp |  |
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| Origin | This table shows only a selection of the most important countries of origin and should not be thought of as exhaustive.
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| Ventilation | Recommended ventilation conditions: - for dry pulp: air exchange rate: 6 changes/hour (airing) - for wet pulp: air exchange rate: 10 - 20 changes/hour (airing) See text for more particulars |
| Risk factors | Chemical pulp is highly flammable, so protect from sparks, cigarette ends, fire and naked lights. Smoldering fires are also a risk.(see also text) |
Cellulose, chemical pulp
Contents
Description
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or Waste Paper. Wood pulp is the most common raw material in papermaking.
There are three main chemical pulping processes.
- The sulfite process dates back to the 1840's, and it was the dominant process before the second world war.
- The kraft process, invented in the 1870's and first used in the 1890's, is now the most commonly practiced strategy.
- Soda pulping is a specialty process used to pulp straws, bagasse, and hardwoods with high silicate content.
Mechanical pulping
There are two major mechanical pulps, thermo mechanical pulp (TMP) and groundwood pulp (GW).
- In the TMP process, wood is chipped and then fed into large steam-heated refiners where the chips are squeezed and made into fibres between two steel discs.
- In the groundwood process, debarked logs are fed into grinders where they are pressed against rotating stones and made into fibres. Mechanical pulping does not remove the lignin, so the yield is very high, >95%, but also causes paper made from this pulp to yellow and become brittle over time. Mechanical pulps have rather short fibre lengths and produce weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than chemical pulp.
Deinked pulp
Paper recycling processes can use either chemical or mechanical pulp. By mixing with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibres separated again. Most recycled paper contains a proportion of virgin fibre in the interests of quality. Generally deinked pulp is of the same quality or lower than the collected paper it was made from.
There are three main classifications of recycled fibre:
- Mill broke or internal mill waste – this incorporates any substandard or grade-change paper made within the paper mill which then goes back into the manufacturing system to be re-pulped back into paper. Such out-of-specification paper is not sold and is therefore often not classified as genuine reclaimed recycled fibre. However, most paper mills have been recycling their own waste fibre for many years, long before recycling became popular.
- Preconsumer waste – this is offcuts and processing waste, such as guillotine trims and envelope blank waste. This waste is generated outside the paper mill and could potentially go to landfill, and is a genuine recycled fibre source. Also includes de-inked preconsumer (recycled material that has been printed but did not reach its intended end use, such as waste from printers and unsold publications).
- Postconsumer waste – this is fibre from paper which has been used for its intended end use and would include office waste, magazine papers and newsprint. As the vast majority of this paper has been printed (either digitally or by more conventional means such as lithography or rotogravure), it will either be recycled as printed paper or go through a deinking process first.
Recycled papers can be made from 100% recycled materials or blended with virgin pulp. They are (generally) not as strong nor as bright as papers made from virgin pulp.
Besides the fibres, pulps may contain fillers such as chalk or China Clay, which improve the characteristics of the paper for printing or writing. Additives for sizing purposes may be mixed into the pulp and/or applied to the paper web later in the manufacturing process. The purpose of sizing is to establish the correct level of surface absorbency to suit the ink or paint.
Whiteness, strength and cleanness are the most important features in papermaking pulps.
Applications
Paper can be produced with a wide variety of properties, depending on its intended use.
For representing value: paper money, bank note, cheque, security, voucher and ticket.
For storing information: book, notebook, magazine, newspaper, art, zine, letter.
For personal use: diary, note to remind oneself, etc.; for temporary personal use: scratch paper.
For communication: between individuals and/or groups of people.
For packaging: corrugated box, paper bag, envelope, wrapping tissue, Charta emporetica and wallpaper.
For cleaning: toilet paper, handkerchiefs, paper towels, facial tissue and cat litter.
For construction: papier-mâché, origami, paper planes, quilling, paper honeycomb, used as a core material in composite materials, paper engineering, construction paper and paper clothing.
For other uses: emery paper, sandpaper, blotting paper, litmus paper, universal indicator paper, paper chromatography, electrical insulation paper and filter paper.
Shipment/storage
Woodpulp is shipped in (unitized) pressed bales (or rolls) and can be either 'dry' or 'wet'.
Wet pulp should not be stowed with goods liable to be damaged by drainage. Contact with water is liable to rust-stain the wrappers, but should not necessarily damage the contents other than causing possible bursting of the covering through expansion. Wrappers are usually of kraft paper and bound with wire.
The produce is subject to varying moisture content, which should be taken into account on assessment of any weight differences.
Contamination by any fibrous material may cause loss in value.
Wetting by salt water renders pulp unacceptable for use in the viscose process and any bale so wetted are mostly rejected, but may yet be used in papermaking.
Chemical pulp should be duly ventilated, to dissipate the production gases. Due to substantial water vapour release, wet pulp should be ventilated immediately at the beginning of the voyage. Where wet pulp is transported in standard containers, condensation will deposit on the internal container 'skins'.
Risk factors
- Self-heating / Spontaneous combustion
- Odor
- Contamination
- Mechanical influences
- Toxicity / Hazards to health
- Shrinkage/Shortage
- Insect infestation / Diseases