Paper chemicals designate a group of
chemicals
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...
that are used for
paper manufacturing, or modify the properties of
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 distribu ...
. These chemicals can be used to alter the paper in many ways, including changing its color and
brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminance, ...
, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.
Chemical usage is not only for imparting properties to paper but to handle the water cycles in the process, conditioning of fabrics, cleaning of equipment and several other applications.
Chemicals used in paper manufacturing
Pulping
Chemical pulping involves dissolving lignin in order to extract the cellulose from the wood fiber. The different processes of chemical pulping include 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 ...
, which uses caustic soda and sodium sulfide and is the most common; alternatively, the use of sulfurous acid is known as the
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 ...
, the
neutral sulfite semichemical is treated as a third process separate from sulfite, and
soda pulping
Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical. In the ''Soda-AQ'' process, anthraquinone (AQ) may be used as a pulping additive to decrease the carbohydrate degradation. The soda process gives ...
which is the least ecologically hazardous utilizing
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and al ...
or
anthraquinone
Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic organic compound with formula . Isomers include various quinone derivatives. The term anthraquinone however refers to the isomer, 9,10-anthraquinone (IUPAC: 9,10-dioxoan ...
.
Caustic soda
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
is added to increase the pH in the
pulping
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 mat ...
process of fibers. The higher pH of the paper-fiber solution causes the fibers to smoothen and swell, which is important for the grinding process of the fibers.
Bleaching
In the production of white paper, the wood pulp is bleached to remove any color from the trace amounts of lignin that was not extracted in the chemical pulping process. There are three predominant methods of bleaching:
# Elemental
chlorine bleaching uses
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
and
hypochlorite
In chemistry, hypochlorite is an anion with the chemical formula ClO−. It combines with a number of cations to form hypochlorite salts. Common examples include sodium hypochlorite (household bleach) and calcium hypochlorite (a component of ...
.
# Elemental chlorine-free bleaching is more environmentally friendly since it eliminates the use of hypochlorite and replaces chlorine with
chlorine dioxide
Chlorine dioxide is a chemical compound with the formula ClO2 that exists as yellowish-green gas above 11 °C, a reddish-brown liquid between 11 °C and −59 °C, and as bright orange crystals below −59 °C. It is usually ...
or
sodium chlorate
Sodium chlorate is an inorganic compound with the chemical formula Na ClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leaves sodium chloride. Sever ...
.
# Totally chlorine-free bleaching utilizes
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...
. This is the most environmentally friendly process since it eliminates all chlorinated pollutants.
Sizing
Most paper types must have some water-resistance to maintain a specific writing quality and printability. Until 1980, the typical manner of adding this resistance was by using a
rosin
Rosin (), also called colophony or Greek pitch ( la, links=no, pix graeca), is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene comp ...
in combination with
alum
An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the ...
. When the paper industry started using chalk instead of
china clay
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedra ...
as filler, the paper chemistry had to switch to a neutral process. At several places AKD (
alkyl ketene dimer
Alkyl ketene dimers (AKDs) are a family of organic compounds based on the 4-membered ring system of oxetan-2-one, which is also the central structural element of propiolactone and diketene. Attached to the oxetane ring of technically relevant a ...
) and ASA (
alkenyl succinic anhydride) are used. Latest development is to us
Surface Size which is applied using a size press. The advantage of surface sizing is that it does not interfere with the backend water chemistry.
Strengthening
Wet-strength
Wet-strength additives ensure that paper retains its strength when it gets wet. This is especially important in
tissue paper
Tissue paper or simply tissue is a lightweight paper or, light crêpe paper. Tissue can be made from recycled paper pulp on a paper machine.
Tissue paper is very versatile, and different kinds of tissue are made to best serve these purposes, w ...
. Chemicals typically used for this purpose include
epichlorohydrin
Epichlorohydrin (abbreviated ECH) is an organochlorine compound and an epoxide. Despite its name, it is not a halohydrin. It is a colorless liquid with a pungent, garlic-like odor, moderately soluble in water, but miscible with most polar organ ...
,
melamine
Melamine is an organic compound with the formula C3H6N6. This white solid is a trimer of cyanamide, with a 1,3,5-triazine skeleton. Like cyanamide, it contains 67% nitrogen by mass, and its derivatives have fire retardant properties due t ...
,
urea formaldehyde
Urea-formaldehyde (UF), also known as urea-methanal, so named for its common synthesis pathway and overall structure, is a nontransparent thermosetting resin or polymer. It is produced from urea and formaldehyde. These resins are used in adhesive ...
and
polyimines. These substances polymerize in the paper and result in the construction of a strengthening network.
![Cationic starch](https://upload.wikimedia.org/wikipedia/commons/2/2f/Cationic_starch.jpg)
To enhance the paper's strength,
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
ic starch is added to wet pulp in the manufacturing process. Starch has a similar chemical structure as the cellulose fibre of the pulp, and the surface of both the starch and fibre are negatively charged. By adding cationic (positive charged) starch, the fibre can bind with the starch and thus also increase the interconnections between the fibres. The positively charged portion of the starch is usually formed by
quaternary ammonium cation
In chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cation ...
s. Quaternary salts that are used include 2.3-epoxy propyl trimethyl ammoniumchloride (EPTAC, also known as or Glytac Quab, GMAC™) and (3-chloro-2-hydroxypropyl) trimethyl ammonium chloride (CHPTAC, also known as Quat 188, Quab 188, Reagens™).
Dry-strength
Dry-strength additives, or dry-strengthening agents, are chemicals that improve paper strength normal conditions. These improve the paper's
compression strength
In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compres ...
,
bursting strength,
tensile breaking strength, and
delamination resistance. Typical chemicals used include cationic
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
and
polyacrylamide
Polyacrylamide (abbreviated as PAM) is a polymer with the formula (-CH2CHCONH2-). It has a linear-chain structure. PAM is highly water-absorbent, forming a soft gel when hydrated. In 2008, an estimated 750,000,000 kg were produced, mainly fo ...
(PAM) derivatives. These substances work by binding fibers, often under the aid of
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
ions in paper sheet.
Binders
Binders promote the binding of pigment particles between themselves and the coating layer of the paper.
Binders are spherical particles less than 1 µm in diameter. Common binders are
styrene maleic anhydride
Styrene maleic anhydride (SMA or SMAnh) is a synthetic polymer that is built-up of styrene and maleic anhydride monomers. The monomers can be almost perfectly alternating, making it an alternating copolymer, but (random) copolymerisation with less ...
copolymer or styrene-acrylate copolymer.
The surface chemical composition is differentiated by the
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
of
acrylic acid
Acrylic acid (IUPAC: propenoic acid) is an organic compound with the formula CH2=CHCOOH. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a ...
or an anionic
surfactant
Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...
, both of which are used for stabilization of the dispersion in water.
Co-binders, or thickeners, are generally water-soluble polymers that influence the paper's color viscosity, water retention,
sizing
Sizing or size is a substance that is applied to, or incorporated into, other materials—especially papers and textiles—to act as a protective filler or glaze. Sizing is used in papermaking and textile manufacturing to change the absorptio ...
, and gloss. Some common examples are
carboxymethyl cellulose
Carboxymethyl cellulose (CMC) or cellulose gum is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodiu ...
(CMC), cationic and anionic
hydroxyethyl cellulose (EHEC),
modified starch
Modified starch, also called starch derivatives, are prepared by physically, enzymatically, or chemically treating native starch to change its properties. Modified starches are used in practically all starch applications, such as in food produ ...
, and
dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch and glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds.
Dextrins can be produced from ...
.
Styrene butadiene latex, Styrene acrylic,
dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch and glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds.
Dextrins can be produced from ...
,
oxidized starch are used in coatings to bind the filler to the paper. Co-binders are natural products such as starch and CMC (
Carboxymethyl cellulose
Carboxymethyl cellulose (CMC) or cellulose gum is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodiu ...
), that are used along with the synthetic binders, like styrene acrylic or styrene butadiene. Co-binders are used to reduce the cost of the synthetic binder and improve the
water retention and
rheology
Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an ap ...
of the coating.
Fillers
Mineral fillers are used to lower the consumption of more expensive binder material or to improve some properties of the paper.
China clay
Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedra ...
, calcium carbonate,
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolu ...
, and
talc
Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent a ...
are common mineral fillers used in paper production.
Retention
A Retention agent is added to bind
fillers
In processed animal foods, a filler is an ingredient added to provide dietary fiber, bulk or some other non-nutritive purpose.
Products like corncobs, feathers, soy, cottonseed hulls, peanut hulls, citrus pulp, screening, weeds, straw, and cere ...
to the paper. Fillers, such as
calcium carbonate, usually have a weak surface charge. The retention agent is a polymer with high cationic, positively charged groups. An additional feature of a retention agent is to accelerate the dewatering in the wire section of the
paper machine.
Polyethyleneimine
Polyethylenimine (PEI) or polyaziridine is a polymer with repeating units composed of the amine group and two carbon aliphatic ''CHCH'' spacers. Linear polyethyleneimines contain all secondary amines, in contrast to branched PEIs which contain pr ...
and polyacrylamide are examples of chemicals used in this process.
Coating
Pigments
Pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s that absorb in the yellow and red part of the visible spectrum can be added. As the
dye
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
absorbs light, the brightness of the paper will decrease, unlike the effect of an optical-brightening agent. To increase whiteness, a combination of pigments and an optical-brightening agent are often used. The most commonly used pigments are blue and violet dyes.
Optical-brightening agent
Optical brightener is used to make paper appear whiter. Optical-brightening agents use
fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
to absorb invisible radiation from the
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
part of the light spectrum and re-emit the radiation as light in the visible blue range. The optical-brightening agent thus generates blue light that is added to the reflected light. The additional blue light offsets the yellowish tinge that would otherwise exist in the reflected light characteristics. It thus increases the brightness of the material (when the illumination includes ultraviolet radiation).
See also
*
Deinking
Deinking is the industrial process of removing printing ink from paperfibers of recycled paper to make deinked pulp.
The key in the deinking process is the ability to detach ink from the fibers. This is achieved by a combination of mechanical ac ...
recycled paper
*
Surface chemistry of paper The surface chemistry of paper is responsible for many important paper properties, such as gloss, waterproofing, and printability. Many components are used in the paper-making process that affect the surface.
Pigment and dispersion medium
Coating c ...
*
Organosolv (pulping technique)
References
External links
List of chemicals used in pulp and paper making
{{Authority control
Papermaking
Chemical processes
Cellulose
Pulp and paper industry