Kraft paper
Encyclopedia
Kraft paper or kraft is paper
or paperboard
(cardboard) produced from chemical pulp produced in the kraft process
.
Pulp produced by the kraft process is stronger than that made by other 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.
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.
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
, and non-wood species like bamboo
and kenaf
can be used in the kraft process.
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
or paperboard
Paperboard
Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...
(cardboard) produced from chemical pulp produced in the kraft process
Kraft process
The kraft process describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers...
.
Pulp produced by the kraft process is stronger than that made by other pulping processes; acidic sulfite process
Sulfite process
The sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites , or bisulfites , depending on the pH...
es degrade cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
more, leading to weaker fibers, and mechanical pulping processes leave most of the lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
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.
Kraft pulp is darker than other wood pulps, but it can be bleached
Bleaching of wood pulp
Bleaching of wood pulp is the chemical processing carried out on various types of wood pulp to decrease the color of the pulp, so that it becomes whiter. The main use of wood pulp is to make paper where whiteness is an important characteristic...
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
Kraft paper is produced on paper machines with moderate machine speeds. The raw material is normally softwoodSoftwood
The term softwood is used to describe wood from trees that are known as gymnosperms.Conifers are an example. It may also be used to describe trees, which tend to be evergreen, notable exceptions being bald cypress and the larches....
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
Pine
Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...
, and non-wood species like bamboo
Bamboo
Bamboo is a group of perennial evergreens in the true grass family Poaceae, subfamily Bambusoideae, tribe Bambuseae. Giant bamboos are the largest members of the grass family....
and kenaf
Kenaf
Kenaf [Etymology: Persian], Hibiscus cannabinus, is a plant in the Malvaceae family. Hibiscus cannabinus is in the genus Hibiscus and is probably native to southern Asia, though its exact natural origin is unknown. The name also applies to the fibre obtained from this plant...
can be used in the kraft process.
Qualities
- Normal kraft paper is strong and relatively coarse. It has high tensile strengthTensile strengthUltimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
. The grammage is normally from 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 kappaKappa numberThe Kappa number is an indication of the residual lignin content or bleachability of wood pulp by a standarised analysis method.-Measuring method:...
hardwoodHardwoodHardwood is wood from angiosperm trees . It may also be used for those trees themselves: these are usually broad-leaved; in temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen.Hardwood contrasts with softwood...
kraft and has to have a good uniformity and formation.
- Spinning kraft paper is an especially strong about 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 shellShotgun shellA shotgun shell is a self-contained cartridge loaded with lead shot or shotgun slug designed to be fired from a shotgun....
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 or twisting paper are thin 30–40 g/m2 kraft papers and is mostly flexo or offset printedOffset printingOffset printing is a commonly used printing technique in which the inked image is transferred from a plate to a rubber blanket, then to the printing surface...
. These papers requires a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often supercalendered.
Applications
- Kraft papers are used in paper grocery bagsPaper bagA paper bag or paper sack is a preformed container made of paper, usually with an opening on one side. It can be one layer of paper or multiple layers of paper and other flexible materials. A bag is used for packaging and/or carrying items....
, multiwall sacks, envelopes and other packaging.
- Kraft paper is a cheap material for lining particle boardParticle boardParticle board, or particleboard , is an engineered wood product manufactured from wood particles, such as wood chips, sawmill shavings, or even saw dust, and a synthetic resin or other suitable binder, which is pressed and extruded...
s.
- The base paper for sandpaperSandpaperSandpaper, also known as glasspaper, is a heavy paper with abrasive material attached to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material...
is normally kraft paper.
See also
- Sack paperSack paperSack 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.-Manufacturing:...
- Manila paperManila paperManila paper is a type of paper originally made from Manila hemp or abacá. It is buff in color and the fibers are usually visible to the naked eye. Because manila paper is generally inexpensive, it is commonly given to children for making child art....
- Corrugated fiberboard
- Kraft rolls
- Wrapping PaperGift wrappingGift wrapping refers to the act of enclosing a gift in some sort of material. Wrapping paper is a kind of paper designed for gift wrapping. Gifts may also be wrapped in a box...
- Paper bagPaper bagA paper bag or paper sack is a preformed container made of paper, usually with an opening on one side. It can be one layer of paper or multiple layers of paper and other flexible materials. A bag is used for packaging and/or carrying items....