From Tree to Paper: The Journey of Cellulose

Posted on Aug 30, 2024 in Technology

What is Cellulose?

Cellulose is the main component of cell walls in trees and other plants. This vegetable fiber, when observed under a microscope, resembles a human hair in structure. Its length and thickness vary depending on the type of tree or plant. For example, cotton fibers measure 20-25 mm, pine fibers 2-3 mm, and eucalyptus fibers 0.6-0.8 mm. Similarly, the cellulose content varies across different tree and plant species.

Currently, pulp mills in Chile extract fiber from pine and eucalyptus wood, separating it from other wood components like lignin and hemicellulose. For centuries, this fiber has served as the raw material for manufacturing various everyday objects, with paper production being the most significant.

Trees are the primary source of natural fibers, contributing to over 90% of global pulp production. The remaining 10% comes from other plants such as grasses, bamboo, sugarcane bagasse, cotton, flax, hemp, and others.

How Does Cellulose Form in Trees?

Trees, plants, and algae produce substances for growth through photosynthesis (photo = light / synthesis = to put together). In trees and green plants, this involves a chemical reaction in the leaves, aided by chlorophyll (the green pigment that absorbs sunlight and converts it into food). This process combines sunlight, carbon dioxide from the air, and water absorbed from the soil.

Through photosynthesis, the tree obtains nourishment in the form of sugars like sucrose and maltose. This process culminates in the deposition of glucose in the cambium layer (located between the bark and wood), where it is synthesized into cellulose.

The initial phase of photosynthesis involves the decomposition of water (H₂O) into oxygen, released into the atmosphere through stomata, and hydrogen. This process requires direct sunlight. Subsequently, hydrogen combines with carbon and oxygen from carbon dioxide (CO₂) to form a series of increasingly complex compounds, ultimately resulting in a stable organic compound known as glucose (C₆H₁₂O₆) and water. This phase utilizes stored energy and can occur in the dark.

Glucose reaches the cambium through the phloem, a component of the plant’s circulatory system. The cambium is a specific tissue found in woody plants, situated between the bark and wood. It typically consists of a single layer of embryonic cells responsible for plant growth. In trees, the cambium layer generates two new layers of cells annually. The first, formed inward, is wood or sapwood, scientifically known as xylem. These growth rings are visible when a tree is cut.

The Parts of a Tree

External Bark: The outermost layer of the trunk, protecting the tree from insects, diseases, extreme temperatures, and other damage.
Phloem: The inner bark, responsible for transporting carbohydrates produced in the leaves down to other parts of the tree, providing nourishment for growth.
Cambium: A thin layer of embryonic cells, located inside the inner bark, where tree growth occurs. Each year, the cambium produces cells that form the xylem and phloem.
Xylem: Also known as sapwood, it conducts water and minerals from the roots to the leaves. Sapwood is composed of long cellulose molecules that provide the tree with its strength.
Heartwood: In older trees, as new growth rings form, the inner rings of sapwood become clogged with resin and transform into heartwood. Heartwood cannot transport fluids but provides structural support to the tree.

Diagram explaining the parts of a tree

Cellulose constitutes approximately 50% of the dry weight of wood (after water extraction). The strong bonds between glucose molecules make cellulose highly resistant, contributing to the strength of wood. Lateral connections between cellulose molecules also contribute to their strength, causing them to clump together and form filaments. These filaments further intertwine to create thicker, rope-like structures called microfibrils.

Types of Cellulose

From a technical and commercial perspective, cellulose is categorized based on the process used to separate cellulose fibers from other wood components:

Chemical Cellulose: Derived from cooking wood particles (chips) with various chemicals at high temperatures and pressures.
Mechanical Cellulose: Also known as mechanical pulp, obtained by shredding wood at high temperatures and pressures.
Chemo-Thermo-Mechanical Cellulose (CTMP): Utilizes a combination of the previous processes.

Cellulose produced through these processes has a paste-like consistency (high water content) and retains a significant amount of lignin, giving it a brown hue similar to natural wood.

Since a major application of cellulose is in producing white paper, the pulp undergoes bleaching with chemicals to remove lignin, resins, metal ions, and other substances that could hinder the paper production process.

End Uses of Cellulose

The importance of cellulose is evident in its diverse applications, categorized based on the type of cellulose:

Mechanical Pulp: Primarily used for manufacturing newsprint, telephone directory paper, and flyers. This high-yield pulp retains a high percentage of lignin and other wood substances, making it relatively inexpensive.
Chemo-Thermo-Mechanical Pulp (CTMP): Possesses properties between mechanical and kraft pulp. Used in producing newsprint, packaging cardboard, printing and writing papers.
Unbleached Kraft Pulp: Primarily produced from pine using the kraft or sulfate process. Used in manufacturing packaging papers, kraft linerboard for corrugated boxes, kraft paper sacks, and cardboard.
Bleached Long-Fiber Kraft Pulp: Produced from pine wood, subjected to the kraft process and bleaching. Used for manufacturing white paper, packaging paperboard, and as fiber reinforcement in various paper types.
Bleached Short-Fiber Kraft Pulp: Produced from eucalyptus, acacia, or other hardwoods. Used in producing high-quality tissue papers, fine printing and writing papers, photocopy paper, and coated papers.

What is Paper?

Paper is a structure formed from interwoven cellulose plant fibers, creating a resilient and flexible sheet. These fibers originate from trees and are classified as long-fiber (around 3 millimeters, typically from pine or other conifers) or short-fiber (1 to 2 mm, mainly from eucalyptus).

Paper’s Origins

The term “paper” derives from the Latin word “papyrus,” referring to the Egyptian plant Cyperus papyrus. The Egyptians, Greeks, and Romans used papyrus leaves as a writing surface from 3000 BC to the 5th century AD.

Chinese Pioneers in Papermaking

In 105 AD, Cai Lun, an official serving the Chinese emperor He Di, invented paper as we know it today. He created a vegetable-based paste from bamboo, mulberry, and other plant fibers. The Chinese kept this technique secret for 500 years.

The Paper Production Process

The fibers required for papermaking are mixed in a pulper, creating a slurry. This slurry is then spread onto a moving wire mesh called a Fourdrinier, where the fibers intertwine to form a sheet.

As the mesh progresses, water drains from the slurry, leaving a thin film of fibers that forms the wet paper sheet. The paper’s weight or thickness can be adjusted by adding more fiber to the pulp.

The paper then passes through presses and steam-heated drying cylinders. Some papers, known as machine-glazed papers, pass through a single large cylinder, resulting in a smoother and glossier surface.

For papers requiring a very smooth and shiny surface, a thin layer of coating is applied. Finally, the paper is wound onto a reel, ready for use or cutting into various sizes.

Paper Machine

Headbox: The slurry containing fibers falls onto a moving wire mesh, forming the paper sheet.
Wire: Excess water drains from the slurry through the wire mesh.
Presses: The paper sheet passes through presses, removing more water.
Dryers: The wet paper sheet passes through heated drying cylinders.
Calender: A large cylinder that smooths and polishes the paper surface.
Size Press: Applies a starch coating to the paper.
Reel: The finished paper is wound onto a reel.