Are you looking for a sustainable packaging solution that offers a pleasant feel and ideal conditions for brilliant color printing, while being extremely tear-resistant and robust? Then pulp board is the right choice. Here you can find out everything you need to know about the raw material pulp and how it is obtained, as well as the advantages of the packaging made from it.

The basic material: What is pulp and from what can it be obtained?

Chemical pulp is a fibrous mass produced by the chemical and mechanical processing of plant fibers. It consists primarily of cellulose and, along with groundwood pulp, is an important starting material for paper production.

Approximately 90 percent of the pulp produced around the globe is made from wood. The use of straw, bagasse (fibrous, ground plant residues from sugar production), kenaf (Hibiscus cannabinus) and bamboo (Bambusoideae) is becoming increasingly less important worldwide.

Depending on the region, industrial residual wood or plantation wood serve as raw material for pulp production. In Europe and North America, sawmill residues (mainly coniferous wood) are mostly used, while in South Africa, South America and Australia, hardwood from forest plantations (eucalyptus) is frequently used. The wood is debarked and then processed into chips. These are chemically digested.

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Pulp extraction: the processes at a glance

Two chemical processes are used in pulp production:

• the alkaline sulfate process and
• the acid sulfite process.

The sulfate process was developed by Carl Ferdinand Dahl (Gdansk) from a method of pulping wood using sodium hydroxide solution devised by Hugh Burgess and Charles Watt in England in 1851 and introduced in the USA in 1854.

In their approach, a high sodium hydroxide requirement made evaporation, burning, and "recausticizing" an expensive proposition. Dahl introduced sodium sulfate directly into the recovery system.

In the process, he noticed that the sodium sulfite formed by reduction brought about a significantly improved fiber quality. In 1884, he received a patent for his process modification.

The alkaline conditions produced very strong fibers, which were, however, dark brown in color. For a long time, therefore, they were used only for the production of unbleached paper products such as paper sacks and cardboard packaging.

Sulfite digestion was patented in the USA in 1867 for the American chemist Benjamin Chew Tilghman. His process used a solution of calcium bisulfite and free sulfur dioxide. From the 1870s, Carl Daniel Ekman in Sweden and Alexander Mitscherlich in Germany further developed the method to industrial maturity. Mitscherlich's acid sulfite boiling produces light-colored fibers. If wood chips from well debarked wood are used, a whiteness of almost 70 percent (ISO) can be achieved. Further lightening is possible by chlorine bleaching with chlorinated lime.

How does pulp production work?

The starting material for pulp production is wood chips that are as uniformly shaped as possible. Mechanical comminution enables homogeneous impregnation of the wood with the respective pulping solution. If the chips are too coarse, the impregnation is incomplete.

This leads to unopened residues. Chips that are too small, on the other hand, have the disadvantage that the fibers turn out too short. Sawmill waste (splinters) is therefore just as unsuitable for pulp production as chipped wood.

In the sulfate process, the wood chips are cooked for three to six hours at a pressure of 7 to 10 bar in caustic soda, sodium sulfate and other alkaline liquids. At a temperature of up to 170 °C, hemicelluloses, lignins and other wood building blocks are dissolved alkaline.

The lignin is cleaved by a nucleophilic attack of the sulfide anion and passes into soluble alkali lignin, the so-called black liquor. This is separated from the remaining pulp with the aid of cellulose filters.

By using a small amount of sulfur, this process can be used to produce pulp with good paper properties from low-grade, resin-rich wood species such as spruce, pine, or pine.

However, only about 45 to 55 percent of the raw material used can be recovered in the form of cellulose during pulping. Since selectivity decreases sharply with increasing chemical input or longer cooking times or temperatures, the degree of digestion cannot be increased indefinitely.

Approximately 95 percent of the world's pulp is produced by the sulfate process.

In the sulfite process, wood with a low wood content and well debarked (spruce, beech, fir, poplar) is digested in an acidic or neutral environment. Calcium, magnesium, sodium and/or ammonium ions are used as cations.

The lignin is subjected to sulfonation, converting it into a water-soluble salt of lignosulfonic acid that can be easily removed from the fiber.

Depending on the respective pH value, the hemicelluloses present in the wood remain in the fiber with the cellulose or are converted into sugars by acid hydrolysis and pass into the aqueous phase.

The very light-colored pulps produced in this way are used today almost exclusively in chemical cellulose or paper production. To produce 1,000 kilograms of pulp using the sulfite process, about five solid cubic meters of wood and 90 kg of sulfur are required.

A variant of the sulfite method is the calcium bisulfite process. In this process, the wood processed into chips is cooked in pressure stoves with calcium bisulfite for seven to 15 hours. During this process, the wood chips become so soft that they can be easily crushed.

After cooking, they are shredded into fibers two to four millimeters long, washed, bleached if necessary, and then dried in the form of thick paperboard. The resulting pulps are particularly pure and are therefore preferably used for chemical applications.

The disadvantage of this process is that the chemicals cannot be recovered. As a result, it is not very economical and is hardly ever used for sulfite pulp production.

Another variation of the sulfite method is the magnesium bisulfite method. This works in a similar way to the calcium bisulfite variant. In contrast to the latter, however, it can be used continuously and offers good chemical recovery. It is mainly used for chemical pulp production from spruce wood.

What does the quality of the pulp depend on?

There are qualitative differences depending on whether the pulp is made from hardwood or softwood. Softwood pulp has long, strong fibers with an average length of 3.3 mm. As a result, it gives paper a reinforcement.

Hardwood pulp has fibers only about 1 mm long. It is most suitable for the production of smooth papers (writing, copying papers) or for tissue (sanitary paper) production. In addition to the basic properties of the raw materials, the quality of the cleaning and blending processes carried out during processing plays a decisive role in product quality.

Of particular relevance are the pulp cooking in caustic soda used in the sulfate process, the subsequent filtration and washing of the cooked stock, and bleaching, in which the pulp is treated with various oxidants.

Process quality in pulp production is ensured by numerous pH, conductivity, temperature and pressure measuring points. In this way, it can be ensured that the optimum parameters prevail throughout production.

Quality losses are mainly caused by too intensive pulping, too much fiber shortening or errors during pulp bleaching. Quality values of the mechanical strength test are the tear length and the tear strength.

There is also a correlation between the degree of polymerization of the pulp and fiber strength. Indications of the degree of fiber damage can be obtained by analyzing the viscosity of dissolved fiber samples.

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How is pulp turned into paperboard?

In the first step, the pulp is broken down into its fibers and ground into an aqueous pulp. From this, individual paper webs are produced simultaneously with several wire sections on Fourdrinier machines. These are pressed onto each other (calendered) when wet, during which they become entangled with each other.

By means of smoothing cylinders, the surface is leveled and the sheet structure is compacted. A starch solution applied in the size press ensures pick resistance and freedom from dust. Finally, depending on the product, a coating compound containing pigment, binder or chalk is applied to one or both sides.

What are the advantages of pulp board?

When using cartonboard as a material and packaging material, the following advantages basically arise:

• Environmentally friendly: cardboard is made of renewable materials and can be fully recycled.
• Flexibility: Thanks to different material qualities, optimal solutions can be found for every conceivable need.
• Protection: Corrugated cardboard boxes are extremely strong and protect their contents throughout the logistics process.
• Printability: Cartons are very suitable for individual printing.

Pulp board differs from other types of board in that it is completely wood-free. It is therefore acid-free and is characterized by high age resistance. It also has very good strength properties and is resistant to bending and splitting.

In order to ensure a high-quality printing result, paperboard made of pulp must have sufficient pick resistance. Otherwise, the solvents contained in the printing ink would penetrate the paper during printing, which would increase the adhesive strength of the ink.

The application of starch in the size press increases the pick resistance of the front and reverse sides and at the same time prevents dust formation. Coated and uncoated pulp board is suitable for all common finishing and printing processes. It can be easily die-cut, scored and grooved and is both printable and suitable for varnishing and bronzing.

Pulp board is one of the most important packaging materials in the food and non-food consumer goods sector.

Conclusion

Made from renewable raw materials and integrated into a virtually closed material cycle, cellulose board is ideal for anyone who values sustainable products. With its white, smooth surface, it offers the best conditions for printing, painting, foiling, film coating, varnishing or laminating.

FAQ

1. What is the difference between pulp board and other printable boards?

   Pulp board is a two- or three-ply board made of pure pulp. In the case of other printing boards (for example chromo board), the liner can also be made of bleached wood pulp and/or recovered paper.

2. Is cellulose board suitable for packaging food?

   Pulp board certified according to the recommendation of the German Federal Institute for Risk Assessment (BfR) is suitable for direct contact with dry, non-greasy foods.

3. Is pulp board compostable?

   Cellulose-rich materials can be degraded by microorganisms to humus, carbon dioxide and mineral compounds. Cardboard made from chemically unmodified cellulose can therefore be composted without any problems. However, for ecological reasons, you should send heavily printed cellulose cardboard to waste paper recycling.