Pulp and Paper Production

Paper Production

• Paper is a thin material made by pressing together cellulose fibers from wood, rags, grasses, and drying them.
• The biggest market share in packaging is 50%.
• The first documented papermaking process was in China in AD 105.
• Paper is derived from papyrus, the ancient Greek name for Cyperus papyrus plants.
• Paper is composed of pulp (mechanical, chemical, or recovered), additives (bleaching, fillers, sizing agents), and colourants (dyes, inks).

Pulp

• Pulp is a lignocellulosic fibrous material.
• It's prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags.
• Wood pulp contains cellulose (40-50%), lignin (15-30%), and hemicellulose (15-25%).
• Cellulose is the desired component for papermaking.
• Lignin, the substance that makes wood tough, is removed to improve paper properties.

Wood Pulp Types

• Softwood pulp (35%) is derived from coniferous trees (e.g., pine, spruce) and is long and thin, generally more preferable.
• Hardwood pulp (65%) is derived from broadleaf trees (e.g., birch, maple) and is short and thick, often more economical.
• Lignin content is less in hardwood pulp than softwood.

Paper Properties

• Softwood pulp produces paper that is flexible, strong, and good for printing.
• Hardwood pulp produces paper that is absorbent, opaque, and thick.

Softwood vs Hardwood Pulp

• Softwood pulp is more expensive but offers better flexibility, strength, and printability.
• Hardwood pulp is less expensive but offers better absorbency and opaqueness.

Papermaking Processes

• Mechanical pulping processes don't remove lignin, resulting in a yellowing and brittleness of the paper over time.
• Chemical pulping processes separate lignin from cellulose, preserving fiber length, and avoiding lignin-related deterioration.
  • Types of chemical pulp include Kraft, Sulfite, and Soda pulps.
• Recycled pulp can be used to create new paper products. The process of removing inks and impurities is called de-inking.

Paper Additives

• Fillers (e.g., clay, chalk) increase paper mass and strength.
• Sizing agents improve the paper's resistance to water absorption.

Paper Characteristics

• Internal sizing is applied directly to pulp during papermaking.
• External sizing (also known as surface or tub sizing) is added after paper is formed, pressed, and dried; it results in improved wet strength.
• Wet strength is added to prevent the paper from getting weak when it's getting wet.
• Colourants (e.g., dyes, inks) provide colour.

Paper Properties - Measurements

• Grammage: Paper weight per square meter (gsm).
• Caliper: Paper thickness (micron or points).
• Bulk: Ratio of caliper to weight.
• ISO 216: Standard for paper sizes (e.g., A4, A3).
• ANSI: North American paper sizes (e.g., letter, legal).

Paper Properties - Testing

• Tensile strength tests the resistance of paper to tearing.
• Stiffness shows the ability of paper to resist bending.
• Paper's ability to hold up to moisture or curling is essential to avoid problems in paper production.
• Opacity measures the degree of light transmission through paper.
• Brightness and whiteness are measured at a specific wavelength.

Paper Coating and Finishes

• Coated paper has a smooth surface created at the paper mill.
• Coating finishes (e.g., matte, silk, glossy) affect print quality and appearance.

Printing Inks

• Inks are typically liquid or paste solutions; they hold pigments (colorants), and provide workability and drying properties.