{{Short description|Type of paper or paperboard}} {{redirect-distinguish|Craft paper|paper craft}} {{More citations needed|date=October 2011}} [[File:Papelparamolde.jpg|thumb|A roll of kraft paper]]
'''Kraft paper''' or '''kraft''', also known as '''brown paper''', 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 (physics)|elasticity]] and high tear resistance, designed for packaging products with high demands for strength and durability.
== Description == Pulp produced by the kraft process is stronger than that made by other [[wood pulp#Wood pulp|pulping processes]]; acidic [[sulfite process]]es 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.<ref>{{cite journal |last1=Hubbe |first1=Martin A. |first2=Lucian A. |last2=Lucia |year=2007 |title=The "Love-Hate" Relationship Present in Lignocellulosic Materials |journal=BioResources |volume=2 |issue=4 |pages=534–535 |doi=10.15376/biores.2.4.534-535 |url=https://ojs.cnr.ncsu.edu/index.php/BioRes/article/view/BioRes_2_4_534_535_Hubbe_Lucia_Love_and_Hate_Lignocellulosics |access-date= 2015-02-02 |doi-access=free }}</ref>
Kraft pulp is darker than other wood pulps, but it can be [[Bleaching of wood pulp|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.
''Sack kraft paper'' is a porous kraft paper with high [[Elasticity (physics)|elasticity]] and high tear resistance, designed for packaging products with high demands for strength and durability.<ref name="sack">{{cite book |last=Paulapuro |first=Hannu |title=Paper and Board grades |publisher=Fapet Oy |year=2000 |isbn=952-5216-18-7 |series=Papermaking Science and Technology |volume=18 |location=Finland |pages=121–122 |chapter=5}}</ref>
== Name == Kraft paper, and the [[kraft process]] used to produce it, are named from the German word {{Lang|de|kraft}}, meaning strength, in reference to its mechanical properties. This shares an origin with the English word ''craft'', leading to [[Mondegreen|linguistic confusion]] with paper intended for artistic uses and [[paper craft]].
==Manufacture== [[File:Woodchips for paper production.jpg|thumb|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|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 [[Crêpe paper|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.<ref name=sack/> Machine direction elongation can be further improved by pressing between very elastic cylinders causing more microcrepping.<ref name=sack/> The paper may be coated with [[polyethylene]] (PE) to ensure an effective barrier against [[moisture]], [[petroleum|grease]] and [[bacteria]], although recyclability is hindered. [[Zein]] coatings are also water resistant but allow better recyclability.<ref>{{cite journal | last =Parris | first =N | title =Recyclable zein-coated kraft paper and linerboard | journal =Progress in Paper Recycling | volume =11 | issue =3 | pages =24–29 | date =2002 | url =https://www.fpl.fs.fed.us/documnts/pdf2002/parri02a.pdf | archive-url =https://web.archive.org/web/20211104172056/https://www.fpl.fs.fed.us/documnts/pdf2002/parri02a.pdf | archive-date =2021-11-04 | url-status =dead | access-date =4 November 2019 }}</ref>
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]],<ref>{{cite web |url=http://www.fpl.fs.fed.us/documnts/usda/amwood/256spine.pdf |title=The Southern Pines |access-date=2007-09-13 |year=1985 |publisher=US Department of Agriculture |archive-date=2007-07-09 |archive-url=https://web.archive.org/web/20070709235422/http://www.fpl.fs.fed.us/documnts/usda/amwood/256spine.pdf |url-status=dead }}</ref> and non-wood species like [[bamboo]] and [[kenaf]] can be used in the kraft process.<ref>{{cite news |last1=John |first1=Anton |title=Hardwood grades explained |url=https://www.reallycheapfloors.com/blog/hardwood-lumber-grades-101-oak-natural-grades-explained/ |access-date=26 October 2022}}</ref>
==Qualities== * Normal kraft paper is strong and relatively coarse. It has high [[tensile strength]]. The [[grammage]] is normally 40–135 g/m<sup>2</sup>. * ''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 number|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/m<sup>2</sup>). This paper requires the best possible machine direction strength and cross machine elongation. This is done by high fiber orientation on the paper machine. * ''Hunting cartridge paper'' is a kraft paper used in [[shotgun shell]]s. 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/m<sup>2</sup>, which is further covered by an outer sheet of 60–80 g/m<sup>2</sup> with color and printing. * ''Candy wrapping paper'' and ''twisting paper'' are thin 30–40 g/m<sup>2</sup> kraft papers and are mostly [[flexo]] or [[Offset printing|offset printed]]. These papers require a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often [[supercalendered]].
==Applications== [[File:9698Baliuag, Bulacan Proper 01.jpg|thumb|Kraft paper bags]] * As a plastic-hazard-free paper in [[paper sack]]s for cement, food, chemicals, consumer goods, flour bags etc. * Electrical insulation in large oil-filled [[transformer]]s * [[Paper bag|Paper grocery bags]], [[Paper sack|multiwall sacks]], gift bags,<ref>{{Cite web |title=Kraft Paper Bags {{!}} Kraft Bags {{!}} BBCB |url=https://bigbrowncarrierbag.co.uk/collections/kraft-paper-bags |access-date=2025-11-07 |website=Big Brown Carrier Bag |language=en}}</ref> [[envelopes]], [[ring binder]]s,<ref>{{cite web |title=Collection Level |website=Library Preservation and Conservation Tutorial |publisher=Cornell University Library |url=https://chinapreservationtutorial.library.cornell.edu/content/collection-level/ |archive-url=https://web.archive.org/web/20240420005244/https://chinapreservationtutorial.library.cornell.edu/content/collection-level/ |archive-date=2024-04-20 |url-status=live |access-date=2026-01-22 |quote=The backbone is lined with kraft or wrapping paper.}}</ref> and other packaging * An inexpensive material for lining [[particle board]]s * The base paper for [[sandpaper]] * As wrapping for [[flower bouquet]]s * In [[World War II]], for fabricating [[drop tank]]s * As a wrapping for the French dish [[Gigot bitume]], which consists of a leg of lamb wrapped in Kraft paper and cooked in hot asphalt
==See also== * [[Butcher paper]] * [[Corrugated fiberboard]] * [[Gift wrapping]] * [[Manila paper]] * [[Paper bag]] * [[Paper sack]] * [[Parchment paper]]
==References== {{Reflist}} * Yam, K. L., ''Encyclopedia of Packaging Technology'', John Wiley & Sons, 2009, {{ISBN|978-0-470-08704-6}}
==External links== * {{Commons category-inline}} {{Paper}}
{{DEFAULTSORT:Kraft paper}} [[Category:Paper]] [[Category:Packaging materials]]