Kraft paper

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A roll of kraft paper Papelparamolde.jpg
A roll of kraft paper

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

Contents

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. [1]

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. [2]

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.

Name

Kraft paper, and the kraft process used to produce it, are named from the German word kraft, meaning strength, in reference to its mechanical properties. This shares an origin with the English word "craft", leading to linguistic confusion with paper intended for artistic uses and paper craft.

Manufacture

Woodchips used for kraft paper production Woodchips for paper production.jpg
Woodchips used for kraft paper production

Wood pulp for sack paper is made from softwood by the kraft process. The long fibers provide the paper its strength and wet strength chemicals are added to even further improve the strength. Both white and brown grades are made. Sack paper is then produced on a paper machine from the wood pulp. The paper is microcrepped to give porosity and elasticity. Microcrepping is done by drying with loose draws allowing it to shrink. This causes the paper to elongate 4% in the machine direction and 10% in the cross direction without breaking. [1] Machine direction elongation can be further improved by pressing between very elastic cylinders causing more microcrepping. [1] The paper may be coated with polyethylene (PE) to ensure an effective barrier against moisture, grease and bacteria, although recyclability is hindered. Zein coatings are also water resistant but allow better recyclability. [3]

A paper sack can be made of several layers of sack paper depending on the toughness needed.

Kraft paper is produced on paper machines with moderate machine speeds. The raw material is normally softwood pulp from the kraft process.

Maintaining a high effective sulfur ratio or sulfidity is important for the highest possible strength using the kraft process.

The kraft process can use a wider range of fiber sources than most other pulping processes. All types of wood, including very resinous types like southern pine, [4] and non-wood species like bamboo and kenaf can be used in the kraft process. [5]

Qualities

Applications

Kraft paper bags 9698Baliuag, Bulacan Proper 01.jpg
Kraft paper bags

See also

Related Research Articles

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<span class="mw-page-title-main">Newsprint</span> Cheap paper used to print newspapers

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<span class="mw-page-title-main">Filter paper</span> Semi-permeable paper barrier

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<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Paperboard</span> Thick paper-based material

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<span class="mw-page-title-main">Pulpwood</span> Timber intended for processing into wood pulp for paper production

Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered products.

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<span class="mw-page-title-main">Lamination</span> Technique of fusing layers of material

Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materials, such as plastic. A laminate is a layered object or material assembled using heat, pressure, welding, or adhesives. Various coating machines, machine presses and calendering equipment are used.

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Paper bag</span> Open-end flexible container made of paper

A paper bag is a bag made of paper, usually kraft paper. Paper bags can be made either with virgin or recycled fibres to meet customers' demands. Paper bags are commonly used as shopping carrier bags and for packaging of some consumer goods. They carry a wide range of products from groceries, glass bottles, clothing, books, toiletries, electronics and various other goods and can also function as means of transport in day-to-day activities.

<span class="mw-page-title-main">Pulp mill</span> Facility which pulps wood or plant fibre

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or non-bleached, depending on the customer requirements.

<span class="mw-page-title-main">Nonwoven fabric</span> Sheet of fibers

Nonwoven fabric or non-woven fabric is a fabric-like material made from staple fibre (short) and long fibres, bonded together by chemical, mechanical, heat or solvent treatment. The term is used in the textile manufacturing industry to denote fabrics, such as felt, which are neither woven nor knitted. Some non-woven materials lack sufficient strength unless densified or reinforced by a backing. In recent years, non-wovens have become an alternative to polyurethane foam.

<span class="mw-page-title-main">Coated paper</span> Paper coated with a compound or polymer

Coated paper is paper that has been coated by a mixture of materials or a polymer to impart certain qualities to the paper, including weight, surface gloss, smoothness, or reduced ink absorbency. Various materials, including kaolinite, calcium carbonate, bentonite, and talc, can be used to coat paper for high-quality printing used in the packaging industry and in magazines.

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

<span class="mw-page-title-main">Paper</span> Material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses, herbivore dung, or other vegetable sources in water. Once the water is drained through a fine mesh leaving the fibre evenly distributed on the surface, it can be pressed and dried.

A variety of tests are used to determine ink and paper and paperboard quality, and to measure their interactions. They are necessary to balance print quality, cost, and wear on the press. Some of the important paper and ink tests are listed here:

Woodfree uncoated paper (WFU), uncoated woodfree paper (UWF) or uncoated fine papers are manufactured using wood that has been processed into a chemical pulp that removes the lignin from the wood fibers and may also contain 5–25% fillers. Both softwood and hardwood chemical pulps are used and a minor part of mechanical pulp might be added. These paper grades are calendered.

Fluff pulp is a type of chemical pulp made from long fibre softwoods. Important parameters for fluff pulp are bulk and water absorbency.

Hemp paper is a paper variety consisting exclusively or to a large extent from pulp obtained from fibers of industrial hemp. The products are mainly specialty papers such as cigarette paper, banknotes and technical filter papers. Compared to wood pulp, hemp pulp offers a four to five times longer fibre, a significantly lower lignin fraction as well as a higher tear resistance and tensile strength. Because the paper industry's processes have been optimized for wood as the feedstock, production costs currently are much higher than for paper from wood.

Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

References

  1. 1 2 3 Paulapuro, Hannu (2000). "5". Paper and Board grades. Papermaking Science and Technology. Vol. 18. Finland: Fapet Oy. pp. 121–122. ISBN   952-5216-18-7.
  2. Hubbe, Martin A.; Lucia, Lucian A. (2007). "The "Love-Hate" Relationship Present in Lignocellulosic Materials". BioResources. 2 (4): 534–535. doi: 10.15376/biores.2.4.534-535 . Retrieved 2015-02-02.
  3. Parris, N (2002). "Recyclable zein-coated kraft paper and linerboard" (PDF). Progress in Paper Recycling. 11 (3): 24–29. Retrieved 4 November 2019.
  4. "The Southern Pines" (PDF). US Department of Agriculture. 1985. Archived from the original (PDF) on 2007-07-09. Retrieved 2007-09-13.
  5. John, Anton. "Hardwood grades explained" . Retrieved 26 October 2022.