Kraft paper is manufactured through raw material preparation, kraft pulping, pulp washing, stock preparation, sheet forming, pressing, drying, and finishing.
What Is Kraft Paper and Why Is It Used in Packaging
Kraft paper is a strong paper grade manufactured primarily from kraft pulp, which contains cellulose fibers separated from wood through chemical pulping. The word “kraft” comes from the German term for strength and reflects the material’s mechanical performance.
Kraft pulping preserves fiber length and strength more effectively than many high-yield pulping methods. The resulting paper provides the tensile strength, tear resistance, and durability needed for demanding packaging applications.
Kraft paper is not the same as corrugated cardboard. Kraft linerboard is used as the outer or inner facing of corrugated board, while the finished board is produced later by bonding linerboard to a fluting medium.
Properties Required for Packaging
Packaging performance depends on several interconnected paper properties. Their required levels vary according to the package structure, converting process, and transportation conditions.
- Tensile strength helps the paper resist breaking during converting, filling, and handling.
- Tear and burst resistance limit crack growth and protect packages from internal or external pressure.
- Stiffness and dimensional stability support stacking, shape retention, and consistent processing.
- Basis weight and thickness influence strength, cushioning, material use, and package weight.
- Smoothness, porosity, and sizing affect printing, glue adhesion, liquid absorption, and moisture resistance.
Raw Materials for Kraft Paper Production
Softwood and Hardwood Fiber
Softwood kraft pulp is commonly produced from pine, spruce, and fir. Its relatively long fibers create a strong network with good tensile strength, tear resistance, and durability.
Hardwood pulp comes from species such as eucalyptus, birch, and aspen. Its shorter fibers improve formation, surface smoothness, and printability by filling spaces between the longer softwood fibers.
Most packaging grades use controlled softwood and hardwood blends to balance strength with surface quality. The selected ratio depends on the paper grade, raw material supply, and required converting performance.
Virgin Pulp and Recycled Fiber Blends
Virgin kraft pulp provides consistent fiber length, bonding potential, and mechanical strength. It is commonly used in high-strength products such as sack kraft paper and performance-grade linerboard.
Recycled kraft fiber reduces virgin material demand and may lower raw material costs. However, repeated recycling shortens fibers, reduces bonding potential, and increases the need for effective contaminant removal.
Mills adjust the virgin-to-recycled fiber ratio according to product specifications and available furnish. A controlled blend can reduce costs while maintaining acceptable strength, formation, and machine runnability.
The Kraft Pulping Process
Wood Preparation and Chip Screening
Wood must be converted into clean, consistently sized chips before kraft cooking. Proper preparation supports uniform chemical penetration and reduces the risk of undercooked or overcooked material.
- Debarking removes bark, soil, and abrasive contaminants that can reduce pulp quality or damage equipment.
- Chipping cuts the debarked wood into pieces suitable for controlled digester cooking.
- Screening removes oversized chips, fines, bark fragments, and other unsuitable material.
Cooking Wood Chips into Kraft Pulp
Screened wood chips are cooked with white liquor in batch or continuous digesters under controlled temperature and pressure. White liquor mainly contains sodium hydroxide and sodium sulfide dissolved in water.
The cooking chemicals penetrate the wood structure and dissolve much of the lignin binding the cellulose fibers together. Cooking conditions must match the wood species, chip dimensions, digester type, and required pulp properties.
Lignin Removal during Kraft Cooking
Lignin provides structural rigidity in wood but must be removed sufficiently to release individual cellulose fibers. Sodium hydroxide and sodium sulfide break down and dissolve much of this material during cooking.
The dissolved lignin combines with extractives, hemicellulose fragments, and spent chemicals to form black liquor. The degree of lignin removal affects pulp yield, fiber strength, paper color, and later processing requirements.
Unbleached kraft pulp retains some residual lignin, which contributes to its characteristic brown appearance. Additional bleaching stages remove more residual lignin when white or high-brightness kraft paper is required.
Pulp Washing and Stock Preparation
Washing, Screening, and Cleaning the Pulp
Cooked pulp still contains black liquor, residual chemicals, knots, bark fragments, sand, and uncooked wood particles. Washing separates the black liquor from the fibers before screening and further stock preparation.
Countercurrent washing places the cleanest wash water in contact with the cleanest pulp. This arrangement improves black liquor removal while limiting fresh water consumption.
Washed pulp passes through coarse and fine pressure screens to remove oversized contaminants. Centrifugal cleaners then separate dense particles such as sand and metal, reducing defects and protecting downstream equipment.
Refining, Blending, and Consistency Control
Refining applies controlled mechanical energy to make fibers more flexible and increase their bonding area. Proper refining improves tensile strength, burst strength, density, and sheet formation.
Insufficient refining produces weak bonding, while excessive treatment can shorten fibers, reduce tear strength, and slow water drainage. The refining level must therefore match the fiber furnish and target paper grade.
Refined pulp is blended with water, sizing agents, and any required strength additives. Consistency control maintains a stable fiber concentration before the stock enters the paper machine headbox.
Kraft Paper Machine Process
Forming the Wet Paper Sheet
In a modern kraft paper machine, the headbox distributes diluted stock uniformly across the forming section. Stable flow is necessary for consistent basis weight, fiber distribution, and cross-machine sheet quality.
Drainage elements, forming rolls, and vacuum boxes remove water as the fiber network develops. Drainage speed influences fiber orientation, formation, and the strength balance between the machine and cross-machine directions.
Pressing and Drying Kraft Paper
The wet sheet leaving the forming section contains a large amount of water. The press section removes part of this water mechanically by passing the sheet through roll nips supported by press felts.
Pressing consolidates the fiber network, improves bonding, and reduces the evaporation load in the dryer section. Excessive pressure can create an overly dense sheet and reduce bulk or surface quality.
Steam-heated cylinders then reduce the sheet to its target moisture level. Controlled drying limits curl, shrinkage, brittleness, and moisture variation across the paper width.
Surface Sizing, Calendering, and Reeling
Surface sizing applies starch or other treatment chemicals to control porosity, liquid absorption, and surface strength. The required treatment depends on the printing, gluing, moisture resistance, and converting needs of the paper grade.
Calendering improves smoothness and thickness uniformity by passing the paper through controlled roll nips. Excessive calendering must be avoided because it can reduce bulk and stiffness.
The finished sheet is wound into large parent rolls before slitting and rewinding. Final roll dimensions are selected according to the customer’s converting equipment and production requirements.
How Manufacturing Affects Packaging Performance
Strength, Basis Weight, and Sheet Uniformity
Tensile strength depends on fiber length, bonding, refining intensity, and fiber orientation. Long softwood fibers generally improve tensile and tear performance, while excessive refining may reduce tear resistance.
Basis weight affects material thickness and strength at a given sheet density. Increasing basis weight can improve performance, but it also raises material consumption and finished package weight.
Uniform fiber distribution prevents weak areas across and along the paper sheet. Stable headbox flow, furnish consistency, drainage, and machine control are therefore essential for predictable converting performance.
Surface Quality, Printability, and Glue Absorption
Surface smoothness influences printing detail, visual appearance, and glue-line uniformity. Porosity determines how quickly air, ink, adhesive, or other liquids can enter the sheet.
Kraft linerboard must accept adhesive without excessive penetration during corrugated board production. Sack kraft paper requires controlled air permeability without losing the strength needed for filling and handling.
Moisture content also affects stiffness, dimensional stability, folding, and creasing. Excessive moisture can cause slackness and curl, while insufficient moisture may lead to brittleness and cracking.
Common Kraft Paper Grades for Modern Packaging
Unbleached, Bleached, and Recycled Kraft Paper
These kraft paper categories differ mainly in fiber source, color, surface requirements, and mechanical performance. Mills adjust pulping, bleaching, stock preparation, and finishing according to the intended packaging use.
- Unbleached kraft paper retains its natural brown color and is commonly selected for industrial packaging, linerboard, and sacks requiring high strength.
- Bleached kraft paper undergoes additional treatment to create a brighter surface for printing, retail presentation, or selected food-packaging applications.
- Recycled kraft paper contains recovered fibers from old corrugated containers, kraft bags, or other suitable paper streams.
Sack Kraft Paper and Kraft Linerboard
Sack kraft paper is designed for industrial sacks, retail bags, and flexible packaging exposed to repeated handling. It requires high tear resistance, puncture resistance, energy absorption, and controlled porosity.
Virgin softwood pulp is commonly used to preserve the long fibers needed for demanding sack applications. Refining must remain controlled so that bonding improves without excessively reducing fiber length or sheet porosity.
Kraft linerboard forms the outer or inner facing of corrugated board. Its compression strength, stiffness, surface quality, and glue acceptance help the finished box withstand stacking, transport, printing, and converting.
