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.
