From Cargo Handbook - the world's largest cargo transport guidelines website
Infobox on Woodpulp
Example of Woodpulp
Origin -
Stowage factor (in m3/t)
  • 1,56/1,64 m3/t (dry-bales)
  • 1,50/1,56 (wet-bales)
Humidity / moisture -
Ventilation -
Risk factors See text


see also Cellulose, chemical pulp

Description / Application / Shipment and Storage / Risk factors

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or Waste Paper. Wood provides about 90 % of the basis for pulp production, while about 10 % originates from annual plants. Pulp is one of the most abundant raw materials world wide. It is most commonly used as raw material in papermaking, but is also used in textiles, food, pharmaceutical and many other industries as well.

There are two basic procedures used for separating cellulose fibres from timber. The first is a purely mechanical process whereby logs are stripped of bark, knotted and ground, using water as a coolant and transport medium for the fibres produced. The slurry of fibres is passed through screens and strainers to remove over-sized material which is returned to the grinders. It then passes over a cylinder board machine to convert it into sheet form. The sheets then pass through hydraulic presses to remove excess water. The sheets of pulp may be baled at that stage, but, for over-seas trade, are normally further dried to a moisture content of about 10%, before baling in hydraulic presses and banding.

There are various grades of mechanical wood pulp which are used for the manufacture of different types of paper or board.

The second process for the production of paper pulp from wood involves stripping and knotting as described earlier. After this, the timber is cut into wood chips. The wood chips are the raw materials for chemical treatment process.

The most important of these is an alkaline digestion process known as the sulphate processes which produces sulphate or ‘kraft’ paper pulp. The other major process involves digestion with sodium bi-sulphite to produce ‘sulphite’ pulp.

There are various refinements to both the sulphate and sulphite processes which are designed to produce pulps with different properties, such as mechanical strength and softness. Any type of pulp may be bleached to varying degrees to produce white pulps for paper or board manufacture. Ocean shipments of paper pulp usually involve the carriage of bleached material.

The bales are banded under compression using special equipment and if bands are broken it is not possible to restore the bales. This is of particular significance because modern paper/board-making processes rely on bales being in sound condition up to the time the pulp sheets are fed into a re-pulping machine. The pulping machines (hydro-pulpers) are essentially tall cylinders with conical bottoms in which there are horizontally rotating blades. The bales are conveyed to the tops of the cylinders. At this point the bands are cut and removed and then the whole bales are dumped into the hydro-pulpers (which are partially filled with water). For this reason the same paper pulp which forms the sheets within the bales is used to form the protective outer wrapping. Shippers/receivers often claim that the outer ‘protective’ wrappers form a part of the contents. By such arguments we believe that the bales are unprotected and are thus insufficiently packed.

If bales become seriously wetted the cellulose will absorb water like blotting paper and swell, breaking the bands, with consequent problems. Prolonged wetting, such as would occur if bales were partly immersed in water, can also affect the strength of cellulose fibres. High quality pulps which have been wetted are sometimes considered unsuitable for their original use and the pulp is sold for manufacturing a different product at a reduced price. Comparatively minor wetting can result in rusting of certain types of bands. Obviously any resulting rust staining produces localized spots of discolouration on finished white papers. Any obvious spots are unacceptable. This type of wetting can result from inappropriate ventilation of cold cargoes. It must be remembered that much of the wood pulp traded around the world is shipped from countries which experience very cold winters. Masters must therefore ventilate cargoes and record their adopted ventilation regime according to established good practice.

In theory, localized wetting of paper pulp can result in mould growth on the surface. However, there is normally sufficient moisture transfer through a bale to prevent this occurring particularly as cellulose does not provide adequate nutrition for most mould species.

There have been occasions when the swelling of seriously wetted bales has resulted in structural damage to the ship. This is a remote possibility but the consequences can be catastrophic.

Although paper machines are fitted with strainers, magnetic screens and similar devices, soiling of the outside of bales can result in particles of foreign material being incorporated in finished paper or board. Soiled bales, particularly where the soiling consists of particulate material such as grain or plastics granules, can be unacceptable to receivers. They can overcome the problem by tearing off the outer wrappers, but this not only results in loss of material but also is labour intensive. In an industry which is largely mechanised, providing suitable labour can be difficult and is costly.

Regenerated cellulose which is used to produce viscose rayon textiles and cellophane film is produced from very high quality bleached cellulose pulp. Because this process involves ejecting a solution of the cellulose through fine dies, any particulate matter in the solution can completely ruin the product. Pulp sold for this end use must be kept in scrupulously clean condition.

Cellulose will absorb odour and become tainted, although many taints can be removed in the paper-making operation. Because massive amounts of water are used in paper making the water is recycled. Paper makers are particularly wary of introducing tainting materials into the water because in some instances the taint may be absorbed by the finished product. Such tainting would not be acceptable in products to be used in the food or other sensitive industries.

Paper pulp will burn. During handling, abrasion between bales can produce significant quantities of cellulose fluff that is particularly inflammable. It is evident that when a fire in a paper product gains hold, a massive amount of heat is produced. The heat is sufficient to cause structural damage to a ship. Extinguishing a fire at this stage is a major operation almost certainly requiring flooding of a hold.

Masters are advised that it is imperative that no smoking is allowed in or near a cargo of paper pulp and stringent precautions must be taken to avoid sparks from any source entering cargo holds. Bales of paper must not be contaminated with oil, particularly vegetable oil. Cellulose has a large surface area such that atmospheric oxidation of the contaminating oil can result in self-heating to the point of combustion.

Mechanical damage
Although this is less of a problem with paper pulp than with paper reels, bad handling may result in the breaking of bands or puncturing and contamination, which can cause difficulties as previously described.

Sources used (which should be mentioned on cargohandbook.com)
The above article was produced by the Carefully to Carry Committee – the UK P&I Club’s advisory committee on cargo matters.