Analysis of Kraft Paper Production Process

Kraft paper is renowned for its high strength and abrasion resistance, with its production process balancing fiber structure and performance optimization. Below is a detailed explanation of the core production flow from raw material processing to finished product formation:
1.   Raw Material Selection and Pulping
Raw Material Screening
Mainly uses wood fibers from coniferous trees (e.g., pine, spruce), whose long fiber structure gives paper high strength.   Some recycled kraft paper uses waste pulp (e.g., old kraft paper bags, cardboard).
Raw materials must be debarked, impurities removed, and cut into wood chips for use.
Pulping Process
Chemical pulping: Wood chips are cooked with caustic soda (NaOH) or sulfate (e.g., sodium sulfide) to dissolve lignin, separating pure fibers to form "kraft pulp."  This process causes minimal fiber damage and yields high-strength paper.
Mechanical pulping: Wood is ground directly by a grinder, producing short fibers with more lignin residue.   It is often used for low-strength kraft paper or mixed with recycled pulp.

2.   Beating and Ingredients
pulping process
Pulp is sent to a beater, where mechanical action cuts and rubs fibers to fibrillate them, increasing bonding between fibers.   The beating degree (an index to measure fiber fibrillation) is typically controlled at 30–50°SR;   a higher degree enhances paper tightness and strength.

Additive Addition
Sizing agents: Such as rosin or synthetic latex, improve water resistance and prevent ink penetration during printing.
Fillers: Talc, calcium carbonate, etc.   Small additions improve smoothness, but excess reduces strength.
Dyes: Added as needed to produce natural (brown-yellow), white, or colored kraft paper.

3.   Paper Forming
Paper Machine Manufacturing
Pulp is diluted into a low-concentration suspension (about 0.5–1% concentration), then evenly spread on the wire mesh of a long wire paper machine via a headbox.
Fibers dehydrate and form on the wire section, excess water is squeezed out by press rolls, and the paper is dried by drying cylinders (steam-heated) to reduce moisture content to 6–8%.

Special Processes
Size press: Applying sizing to the paper surface during drying to enhance water resistance and printability, common in packaging kraft paper.
Calendering treatment: Improving paper smoothness and gloss via calender rolls (e.g., single-sided kraft paper), though excessive calendering may reduce strength.

4.   Slitting and Post-Treatment
Slitting and Rewinding
The dried paper web is slit into flat sheets or rewound into rolls according to specifications for easy storage and transportation.
Special Processing
Waterproof treatment: Coating with polyethylene (PE) or wax layers for moisture-proof packaging.
Rust-proof treatment: Adding rust inhibitors (e.g., volatile corrosion inhibitors) for metal product packaging.
Lamination processing: Laminating with plastic film or aluminum foil to enhance barrier properties for food packaging, etc.

5.   Key Points of Quality Control
Strength indicators: Tensile strength, burst strength, and tear strength are core parameters, affected by fiber length, beating degree, and drying temperature.
Surface performance: Roughness affects printing effects;   fluffing and dusting must be controlled (improved via sizing and fiber impurity screening).
Environmental requirements: Modern processes increasingly use chlorine-free bleaching (e.g., oxygen bleaching, ozone treatment) to reduce pollution, with recycled kraft paper achieving a waste paper recovery rate of over 80%.