Pulp in Paper Production: Foundation for High-Quality Paper Products

What is Pulp Made of?

Pulp consists mainly of cellulose, which is obtained through the chemical processing of plant fibers. About 90% of globally produced pulp is made from wood. The composition varies depending on the source material and manufacturing process:

• Cellulose: The main component that forms the basic structure of the fibers
• Residual lignin: Depending on the pulping process, unbleached pulp still contains about 2-3% lignin
• Hemicelluloses: Short-chain carbohydrates that remain in small amounts
• Mineral components: Traces of minerals from the source material

The quality of pulp is largely determined by fiber length. While softwood pulp has long, strong fibers averaging 3.3 mm in length and provides paper with good reinforcement, hardwood pulp is characterized by shorter fibers of about 1 mm in length, which are particularly suitable for smooth papers such as writing and copy paper, as well as tissue papers.

Pulp Manufacturing Processes

Pulp is produced through various chemical pulping processes, with two main methods dominating:

The Sulfate Process

The sulfate process, also called the Kraft process, is currently the predominant method with over 95% of world production. The process works as follows:

• The wood is processed into wood chips
• The wood chips are impregnated with cooking liquor (sodium hydroxide and sodium sulfide)
• At temperatures up to 170°C, wood components such as hemicelluloses and lignins are dissolved in an alkaline solution
• The lignin is depolymerized through nucleophilic ether cleavage by sulfite ions
• The process is stopped before all lignin is dissolved to protect the cellulose fibers

The sulfate process can be used for both hardwood and softwood and produces particularly strong fibers, which are initially dark in color and need to be bleached for many applications.

The Sulfite Process

The sulfite process now accounts for only about 2-3% of global pulp production:

• Sulfur dioxide is processed with water to form an acidic solution
• The acidic sulfite process dissolves the bond between lignin and cellulose through sulfonation and ether cleavage of lignin
• The sulfonated lignin becomes water-soluble and can be removed from the fiber mass
• The fiber strength is lower than with sulfate pulp, but the pulp is brighter

The sulfite process is particularly suitable for spruce, beech, or eucalyptus wood and has found a stable market niche in the production of dissolving pulp.

The yields per dry mass of wood are 40-45% for the sulfate process and 45-50% for the sulfite process. The remaining wood mass is converted into dissolved substances, which either become wastewater or are used in modern facilities for chemical recovery and energy generation.

Applications of Pulp in Paper Production

Pulp is suitable for different types of paper in varying degrees:

For Wood-Free Papers

Wood-free papers consist mainly of pulp and contain hardly any lignin. They are particularly durable and don't yellow as quickly as wood-containing papers. Typical applications include:

• High-quality printing and writing papers
• Office papers and copy papers
• Books with long shelf life
• Document papers

For Wood-Containing Papers

In wood-containing papers, pulp is added as reinforcement pulp to improve paper strength:

• Magazine and catalog papers
• Newsprint (in combination with recycled paper)
• Inexpensive printing paper

For Specialty Papers

Pulp also forms the basis for numerous specialty papers:

• Filter papers (e.g., coffee filters)
• Security papers
• Paper bags and robust packaging
• Hygiene papers such as tissues, toilet paper, and kitchen rolls

Importance of Pulp in Paper Production

Pulp is an essential raw material in the paper industry for several reasons:

Quality Characteristics

The quality of the pulp significantly determines the properties of the finished paper:

• Strength: Especially softwood pulp with long fibers gives paper high tear resistance
• Surface: Hardwood pulp with short fibers enables smooth, even paper surfaces
• Whiteness: Bleached pulp achieves whiteness degrees of 88-92% ISO
• Stability: High-quality pulp ensures long-lasting paper products
  
  

Economic Importance

Pulp is a globally traded raw material with significant economic importance:

• World production of bleached pulp exceeds 80 million tons annually
• In Germany, two large pulp mills operate using the sulfate process with a total capacity of over 1 million tons per year
• Pulp trading is an important economic factor in the global paper market

Is There Paper Without Pulp?

Yes, there are paper products that are manufactured without or with a very small proportion of fresh pulp:

Recycled Paper

Recycled paper is made from waste paper and can significantly reduce the use of fresh pulp:

• The waste paper recycling rate in Germany is about 79%
• Compared to virgin fiber paper, recycled paper saves on average 78% water, 68% energy, and 15% CO2 emissions
• High-quality recycled papers with the Blue Angel eco-label consist of 100% waste paper
  
  

Alternative Fiber Materials

Besides wood and waste paper, other fiber materials can also be used for paper production:

• Annual plants such as straw, bagasse (sugar cane residue), kenaf, or bamboo
• Cotton linters and textile scraps
• Hemp and flax fibers for specialty papers

However, these alternative materials play a subordinate role in global paper production compared to wood pulp and waste paper.

Conclusion: Pulp as a Key Raw Material in the Paper Industry

Pulp is and remains a central raw material in paper production, which through its versatile properties forms the basis for numerous paper products. The different qualities of softwood and hardwood pulp enable the production of papers with specific properties for a wide range of applications.

The environmental impacts of pulp production have been significantly reduced through modern methods of chemical recovery and energy optimization. Nevertheless, sustainable forest management and a high proportion of waste paper remain crucial for environmentally friendly paper production.

For the future of the paper industry, it will be important to further optimize the use of pulp, increase recycling rates, and develop innovative processes that further reduce resource consumption and environmental impact. Pulp will maintain its importance as a key raw material, while at the same time the circular economy in the paper industry will be further strengthened.