Kraft paper or kraft is

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed ...

paperboard Paperboard is a thick paper-based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker (usually over 0.30 mm, 0.012 in, or 12 Inch#equivalences, points) than paper and has certain ...

(cardboard) produced from

chemical pulp Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemical or plant-based additives, pulp is the major raw mater ...

produced in the

kraft process 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 chip ...

. Sack kraft paper (or just sack paper) is a porous kraft paper with high

elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the cl ...

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 process 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 ...

es degrade

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

more, leading to weaker fibers, and mechanical pulping processes leave most of the

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity ...

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 In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

nature of lignin interferes with the formation of the

hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

between cellulose (and

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

) 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.

Manufacture

Wood pulp Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemical or plant-based additives, pulp is the major raw mate ...

for sack paper is made from

softwood file:Pinus sylvestris wood ray section 1 beentree.jpg, Scots Pine, a typical and well-known softwood Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. The main diff ...

by the

. 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 A paper machine (or paper-making machine) is an industrial machine which is used in the pulp and paper industry to create paper in large quantities at high speed. Modern paper-making machines are based on the principles of the Fourdrinier Machin ...

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. Machine direction elongation can be further improved by pressing between very elastic cylinders causing more microcrepping. The paper may be coated with

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...

(PE) to ensure an effective barrier against

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

, grease and

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, although recyclability is hindered.

Zein Zein is a class of prolamine protein found in maize (corn). It is usually manufactured as a powder from corn gluten meal. Zein is one of the best understood plant proteins.Momany, Frank A.; Sessa, David J.; Lawton, John C.; Selling, Gordon W.; H ...

coatings are also water resistant but allow better recyclability. 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

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 A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accep ...

, and non-wood species like

bamboo Bamboos are a diverse group of evergreen perennial flowering plants making up the subfamily Bambusoideae of the grass family Poaceae. Giant bamboos are the largest members of the grass family. The origin of the word "bamboo" is uncertain, bu ...

and

kenaf Kenaf tymology: Persian ''Hibiscus cannabinus'', is a plant in the family Malvaceae also called Deccan hemp and Java jute. ''Hibiscus cannabinus'' is in the genus ''Hibiscus'' and is native to Africa, though its exact origin is unknown. The name ...

can be used in the kraft process.

Qualities

* Normal kraft paper is strong and relatively coarse. It has high

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

. The

grammage Grammage and basis weight, in the pulp and paper industry, are the area density of a paper product, that is, its mass per unit of area. Two ways of expressing grammage are commonly used: * Expressed in grams (g) per square meter (g/m2), regardle ...

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 Kappa (uppercase Κ, lowercase κ or cursive ; el, κάππα, ''káppa'') is the 10th letter of the Greek alphabet, representing the voiceless velar plosive sound in Ancient and Modern Greek. In the system of Greek numerals, has a value o ...

hardwood Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...

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 paper machine. * Hunting cartridge paper is a kraft paper used in

shotgun shell A shotgun shell, shotshell or simply shell is a type of rimmed, cylindrical (straight-walled) cartridges used specifically in shotguns, and is typically loaded with numerous small, pellet-like spherical sub- projectiles called shot, fired thro ...

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/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 Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress, evolved with high speed rotary functionality, which can be used for printing on a ...

or offset printed. These papers require a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often

supercalendered A calender is a series of hard pressure rollers used to finish or smooth a sheet of material such as paper, textiles, rubber, or plastics. Calender rolls are also used to form some types of plastic films and to apply coatings. Some calende ...

Applications

* As a plastic-hazard-free paper in

paper sack A paper sack is a packaging type that can be constructed of one or several layers of high quality kraft paper, usually produced from virgin fibre. Paper sacks can also be referred to as industrial paper bags, industrial paper sacks or multi-wall ...

s for cement, food, chemicals, consumer goods, flour bags etc. * Electrical insulation in large oil-filled

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s * Paper grocery bags, multiwall sacks,

envelopes An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a shor ...

and other packaging * An inexpensive material for lining

particle board Particle board, also known as chipboard or low-density fiberboard, is an engineered wood product manufactured from wood chips and a synthetic resin or other suitable binder, which is pressed and extruded. Particle board is often confused with ori ...

s * The base paper for

sandpaper upright=1.35, Sheets of sandpaper with different grit sizes (40 (coarse), 80, 150, 240, 600 (fine)). Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to ...

* As wrapping for

flower bouquet A flower bouquet is a collection of flowers in a creative arrangement. Flower bouquets can be arranged for the decor of homes or public buildings, or may be handheld. Handheld bouquets are classified by several different popular shapes and style ...

s * In

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

, for fabricating

drop tank In aviation, a drop tank (external tank, wing tank or belly tank) is used to describe auxiliary fuel tanks externally carried by aircraft. A drop tank is expendable and often capable of being jettisoned. External tanks are commonplace on modern ...

s * As a wrapping for the French dish

Gigot bitume A (asphalt leg of lamb) is a leg of lamb prepared by wrapping the meat in kraft paper and placing in a bath of hot asphalt. This preparation method is used traditionally in France to celebrate the completion of the structural portion of constr ...

, which consists of a leg of lamb wrapped in Kraft paper and cooked in hot asphalt

References

* Yam, K. L., ''Encyclopedia of Packaging Technology'', John Wiley & Sons, 2009,

External links

*
What is The True Difference Between Kraft Paper and Butcher Paper?
{{DEFAULTSORT:Kraft paper Paper Packaging materials

Manufacture

Qualities

Applications

See also

References

External links