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Kraft paper
Feb 07, 2018

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.[2]

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.[3]

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.


Qualities

  • Normal kraft paper is strong and relatively coarse. It has high tensile strength. The grammage is normally 40–135 g/m2.

  • 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.

  • Absorbent kraft paper is made with controlled absorbency, i.e., a high degree of porosity. It is made of clean low kappa hardwood kraft and has to have a good uniformity and formation.

  • Spinning kraft paper is an especially strong type of kraft paper with relatively low grammage (40 g/m2). This paper requires the best possible machine direction strength and cross machine elongation. This is done by high fiber orientation on the papermachine.

  • Hunting cartridge paper is a kraft paper used in shotgun shells. This paper needs a high tensile strength in the machine direction, which is the axial direction of the cartridges. In the cross direction, the cartridge is supported by the gun-pipe, but a sufficient elongation is needed. The body of the cartridge is wound of a kraft paper of 80–120 g/m2, which is further covered by an outer sheet of 60–80 g/m2 with colour and printing.

  • Candy wrapping paper and twisting paper are thin 30–40 g/m2 kraft papers and are mostly flexo or offset printed. These papers require a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often supercalendered.

Applications

Main articles: Paper bag and Paper sack


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