Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process. Sack kraft paper, or just sack paper, is a porous kraft paper with high elasticity and high tear resistance, designed for packaging products with high demands for strength and durability.
Pulp produced by the kraft process is stronger than that made by other pulping processes; acidic sulfite processes degrade cellulose more, leading to weaker fibers, and mechanical pulping processes leave most of the lignin with the fibers, whereas kraft pulping removes most of the lignin present originally in the wood. Low lignin is important to the resulting strength of the paper, as the hydrophobic nature of lignin interferes with the formation of the hydrogen bonds between cellulose (and hemicellulose) in the fibers.Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high quality paper where strength, whiteness, and resistance to yellowing are important.
Natural or unbleached kraft paper has light brown color, while bleached kraft paper is white. Natural kraft paper is the strongest of the common packaging papers and is used when maximum strength is required, such as in industrial bags, grocery bags, inner plies of multiwall sacks, or plain wrapping paper. Kraft fibers are also used to produce liner of corrugated board. Bleached kraft paper is the strongest white paper. It is used where strength, printability, and appearance are important, such as flour and sugar bags, labels, and envelopes. Bleached kraft fibers are also used in production of solid bleached sulfate (SBS), a high quality paperboard.
Kraft paper can be coated or laminated to improve its barrier properties and provide additional strength. Polyamide or polyamine resin can be added into kraft fiber and cured under heat to produce wet strength kraft paper. When wet, this type of paper can retain a third of its dry strength and can be used in high humidity conditions.
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