Glossary of Paper Terms

Absorbency

The property of a material that causes it to take up a liquid with which it is in contact. Several measures of absorbency are: (a) the time required for the material to take up a specified volume of liquid; (b) the rate of rise of liquid along a vertical strip dipping into the liquid; (c) the area of a specimen wetted in a specified time; (d) the quantity of a liquid taken up by a completely saturated specimen. The method of measurement depends on the specific use of paper.

Absorbent Papers

Soft, loosely felted papers that readily absorb water solutions or liquid chemicals. They are not sized with water-repellent agents, but may be treated with materials that enhance their wet strength. They include blotting, filter, matrix, and toweling papers, and base paper for the manufacture of vegetable parchment, artificial leather, vulcanized fiber, and many other processed papers.

Acidity/Alkalinity

In paper, the condition that results in an acid or alkaline solution when the paper is treated or extracted with water. In testing paper acidity/alkalinity, the specimen is extracted with water at a definite temperature, and the extract is tested to determine its pH value or is titrated to determine the total amount of acid or alkali present.

Alpha Cellulose

The portion of a celulosic material that can be filtered out of a mixture of the fibrous material and 8% sodium hydroxide solution, after the fibers have been previously swollen with 17.5% sodium hydroxide solution. This determination is applicable primarily to pulps and to papers made from cotton or chemical wood fibers. For papers containing lignin, coatings, fillers, etc., certain corrections must be made.

Alpha Pulp

Chemically treated wood pulp having greater than 90 percent alpha cellulose, i.e. cellulose that is resistant to 17.5 percent sodium hydroxide solution at 25 degrees Celsius.

Apparent Density

The apparent weight per unit volume. It is often calculated by dividing the basis weight by the thickness, though it must be recognized that the numerical value thus obtained depends on the definition of the ream. Consistent numerical values can be obtained by using in every case the basis weight in metric units (gsm) and the thickness in millimeters.

Aqueous Coating

A water-based coating applied after paper production, either on-machine or off-machine. An aqueous coating usually gives a gloss, dull, or matte finish and helps prevent the ink from rubbing off.

Archival Paper

Paper that is alkaline and will not deteriorate over time. Archival papers must meet national standards for permanence: they must be acid-free and alkaline with a pH of 7.5 to 8.5; include 2% calcium carbonate as an alkaline reserve; and not contain any groundwood or unbleached wood fiber.

Ash

The inorganic residue after igniting a specimen of wood, pulp, or paper so as to remove combustible and volatile compounds.

Basic Size

The customary sheet size used to establish the basis weight of a ream (500 sheets) of a given grade of paper. Basic size vary by grade: Book is 25” x 38” while Cover is 20” x 26”.

Basis Weight

The weight in pounds, of a ream (500 sheets) of a paper cut to a given standard (basic size). Each paper grade such as cover, bond, or book has its own basic sheet size, which determines its basis weight.

Beater

A machine consisting of a tank or “tub” usually with a partition or “midfeather” and containing a heavy roll revolving against a bedplate. Both roll and bedplate may contain horizontal metal bars set on edge. The beater may be “furnished” by either (1) pumping stock slurry from a pulper or (2) adding pulp or wastepaper slowly with sufficient water so that the mass may circulate and pass between the roll and bedplate. The primary function of the beater is to initiate the development of the fiber by cutting, bruising, fibrillating and hydrating the fibers. Fillers, dyestuffs and sizing materials may be added to the beater and thus incorporated with the paper stock.

Binder

A material used to cause substances to bond or adhere. In the paper industry, binders are used widely to cause fibers to bond, coatings to adhere, or as laminates.

Bleach

An oxidizing or reducing agent used to remove color from pulp so that it has a higher brightness.

Bleaching

The process of chemically treating pulp fibers to reduce or remove coloring matter so that the pulp is improved in terms of whiteness or brightness.

Blotting Paper

An unsized paper used wherever absorption is the required characteristic or where soft spongy paper is needed, even though the absorption is of secondary importance. It is often made from rag, cotton linters, chemical or mechanical wood pulp, or mixtures of these. The paper is porous, bulky, of low finish, and tends to possess little strength. The normal basis weight ranges from 60-140 pounds (19”x24”/500). Some grades are made with a smooth machine finish, which makes them suitable for printing with coarse-screen halftones.

Brightness

The reflectivity of pulp, paper, or paperboard for specified blue light measured under standardized conditions on a particular instrument designed and calibrated for this purpose. If a paper lacks brightness, it will absorb too much light, and little light will reflect back through the ink.

Broke

Paper that has been discarded anywhere in the process of manufacture. “Wet broke” is paper taken off the wet press of a paper machine; “dry broke” is made when paper is spoiled in going over the dryers or through the calender, trimmed off in the rewinding of rolls, trimmed from sheets being prepared for shipping, or discarded for manufacturing defects. It is usually returned to a repulping unit for reprocessing.

Bursting Strength

A measure of the ability of a sheet to resist rupture when pressure is applied to one of its sides by a specified instrument, under specific conditions. It is largely determined by the tensile strength and extensibility of the paper or paperboard. Testing for bursting strength is very common although its value, except for limited, specific purposes is questionable.

C1S

Paper that is coated on one side only; C2S – coated on both sides.

Calcium Carbonate

A chemical compound (CaCo3), occurring in nature usually from sea deposition, or obtained commercially by chemical precipitation. Calcite and aragonite are the two principal crystalline types with calcite being the thermodynamically stable form. Chalk is a naturally occurring form used only to a limited extent in papermaking because of impurities present. The precipitated carbonate is preferred due to its obvious higher purity and smaller particle size than the natural product. This carbonate may be produced by precipitation of milk of lime with carbon dioxide gas or sodium carbonate, or precipitation from calcium chloride-sodium carbonate reactors. Calcium carbonate is used both as a filler and as a coating pigment.

Calender

A set or “stack” of horizontal cast-iron rolls with chilled, hardened surfaces, resting one on the other in a vertical bank at the end of the paper machine. The paper is passed between all or part of these rolls to increase the smoothness and gloss of its surface.

Calendering

The process of finishing a sheet of dried paper by pressing it between highly polished metal cylinders of a calender stack.

Caliper

The thickness of a single sheet of paper measured by a micrometer and expressed in thousandths of an inch.

Cellulose

The main solid constituent of woody plants, occurring widely elsewhere in the vegetable kingdom. Chemically it is a linear polysaccharide of high molecular weight. Wood cellulose is the material remaining after a large portion of the lignin and certain carbohydrates have been removed by pulping and bleaching.

Cellulose Fiber

Main component of the walls of all plant cells; cellulose gives plants their structural support and makes plant material fibrous.

Chlorination

In pulping, the treatment of wet pulp, with a compound containing available chlorine, as a step in removing unwanted non-cellulosic matter and bleaching the pulp.

Clay

A natural, earthy, fine-grained, substance used as both a filler and a coating ingredient to improve smoothness, brightness, and opacity.

Cleaner

Equipment used to remove impurities in the slurry before it makes it to the paper machine.

Close Formation

The formation of a sheet that is uniform and free from a wild or porous appearance when viewed by transmitted light.

Cobb Test

A method for measuring the water absorptiveness of sized paper and paperboard, by determining the weight of water absorbed through one surface under a definite pressure.

Color Fastness

The property of a paper, dye, or dyed paper to retain its color in normal storage or use or to resist changes in color when exposed to light, heat, or other deleterious influences.

Color Specification

The quantitative description of color. The color specification consists of a dominant wavelength, purity, and luminous reflectivity under standardized conditions.

Commercial Match

The duplication of a paper in a mill run which does not exactly match the sample but which is close enough to be considered acceptable.

Contaminants

A general papermaking term applicable to extraneous and usually harmful matter in pulp or non-fibrous raw materials. The term is more specifically applied to such things as adhesives, wet-strength resins, inks, dirt, coatings, asphalt, plastics, rubber, etc. found in recyclable waste papers

Couch Roll

A paper machine roll primarily involved in dewatering and picking off, or couching, of the newly formed paper web from the wire on which it was formed and partially dewatered and in the transfer of the web to the wet press felt for further dewatering. On a fourdrinier machine either a suction couch roll or a pressure couch is used. The suction couch roll consists of a heavy metal shell drilled with many small holes through which a suction box inside this shell can apply a high vacuum for rapid removal of water from the sheet as it is carried by the wire over this roll immediately prior to its transfer from the wire to a felt for passage through the wet presses. The pressure couch consists of a pair of rolls forming a pressure nip through which the wire and partially dewatered sheet pass for further water removal by pressure immediately prior to transfer of the sheet from the wire to the wet press felt. The two rolls involved are termed top couch roll and bottom couch roll.

Cross Direction

The direction of the paper at right angles to the Machine direction.

Curl

The curvature developed when one side of a paper specimen is wetted. It was formerly used as a measure of the degree of sizing.

Dandy Roll

Used on the wet end of the paper machine to smooth the formation, reduce bubbles, and to impress a pattern if desired; the raised areas on the wire covered dandy can give a watermark or laid pattern.

Delignification

The process of removing lignin from wood or other cellulosic material by means of chemicals, leaving a residue of cellulose, hemicelluloses, and other carbohydrate materials.

Dimensional Stability

The property of a sheet of paper that relates to the constancy of its dimensions, especially as they are affected by changes in moisture content, with compressive or tensile stresses, or with time under stable ambient conditions.

Dirt

Any foreign matter embodied in a sheet of paper, paperboard, or pulp which has a marked contrasting color to the rest of the material when viewed by reflected or transmitted light. In paper it is generally determined by reference to a standard dirt chart.

Driers

A series of steam-heated metal cylinders, 30-60 inches in diameter, varying in number up to 130 or more, and arranged in two or more tiers. The cylinders are gear driven, and the wet paper passes over and under successive cylinders. The temperature of the cylinders, their number, and their speed determine the drying capacity of the paper machine.

Dry End

The mill term for the drying section of the paper machine, consisting mainly of the driers, calenders, reels, and slitters.

Embossed

A raised pattern or design mechanically pressed into the paper surface.

Felts

A wide loop of fabric that draws moisture from the web of paper and supports the paper as it travels through the press section of the paper machine.

Finish

The aesthetic and physical characteristics of the surface of paper, often characterized by its smoothness:

• Super Vellum – a very toothy rough surface
• Super Smooth – an extremely smooth surface achieved with calendaring.

Formation

The orientation and distribution of fiber in paper. Uniform distribution results in superior formation.

Grain

The machine direction of paper. The direction in which most fibers lie in a sheet of paper.

Groundwood Pulp

A mechanical wood pulp produced by pressing a barked log against a pulpstone and reducing the wood to a mass of relatively short fibers.

Hardwood

Wood obtained from a class of trees known as Angiosperms, such as birch, maple, oak, gum, eucalyptus, and poplar. These trees are characterized by broad leaves and are usually deciduous in the temperate zones.

Hardwood Pulp

Any pulp made from a hardwood or mixture of hardwoods by either a chemical or mechanical process.

Headbox

On fourdrinier machines: A large flow control chamber which received the dilute paper stock or furnish from the stock preparation system and by means of baffles and other flow evening devices, maintains sufficient agitation of the mixture to prevent flocculation of the fibers, spreads the flow evenly to the full width of the paper machine and provides delivery of stock to the fourdrinier wire uniformly across its full width. The height of the liquid in an open headbox or the air pressure in a closed headbox provide the requisite speed of flow of the stock onto the fourdrinier wire.

Holdout

The extent to which a paper or board surface resists penetration by aqueous or nonaqueous fluids. Where the fluid involved is water or water vapor this property is usually termed Sizing. Nonaqueous fluids of concern include printing inks, lacquers, and various oils or waxes.

Hydrophilic

Readily wetted by water.

Hydrophobic

Water repellent; not wetted by water.

Hydropulper

Equipment used to reconstitute and blend fiber, water, pigments and chemistry preparing the “slurry” for the paper machine.

Hygroexpansivity

The change in dimension of paper that results from a change in the ambient relative humidity. It is commonly expressed as a percentage and is usually several times higher for the cross direction than for the machine direction. This property is of great importance in applications where the dimensions of paper sheets and cards or construction board are critical.

Integrated Mill

A paper or board mill that produces substantially all its own pulp. A partially integrated mill is one that produces some but not all of its pulp. A non-integrated mill is one that has the luxury to purchase quality pulps in the open market.

Internal Bond

The force with which fibers are bonded to each other within a sheet of paper or paperboard.

Kraft Pulp

Pulp produced by a process where the active cooking agent is a mixture of sodium hydroxide and sodium sulfide. The term “kraft” is commonly used interchangeably with “sulfate” and is derived from a German word that means “strong”.

Lignin

The noncarbohydrate portion of the cell wall of plant material; it is usually determined as the reside after hydrolysis with strong acid of the plant material, after removal of waxes, tannins, and other extractives. Lignin is amorphous, has high molecular weight, and is predominantly aromatic in structure. It is not one compound, but varies in composition with the method of isolation and with the species, age, growing conditions, etc., of the plant. It is more or less completely removed during chemical pulping, but it is not removed by mechanical pulping. Bleaching of the pulp further removes or modifies any remaining lignin. Left in pulp, lignin causes yellowing over time.

M Weight

The weight in pounds of 1000 sheets of paper.

Machine Direction

The direction of paper parallel with the direction of movement on the paper machine. It is also called the grain direction. The direction at right angles to the machine direction is called the cross-machine direction or simply cross direction.

Machine Finish

Any finished obtained on a paper machine. It may be that of the sheet as it leaves the last drier or as it leaves the calender stack. It may also be a dry or water finish.

Machine Glazed

The finish produced on a Yankee machine, where the paper is pressed against a large steam-heated, highly polished revolving cylinder, which dries the sheet and imparts a highly glazed surface on the side next to the cylinder, leaving the other side rough – i.e., with the texture of the felt used on the machine.

Mechanical Wood Pulp

Any wood pulp manufactured wholly or in part by a mechanical process, including stone-ground wood, chemi-groundwood and chip mechanical pulp. Uses include newsprint printing papers, specialty papers, tissue, toweling, paperboard, and wallboard.

Mercerization

The process of treating vegetable fibers with an alkaline reagent, with or without tension, so as to increase their diameter, density, strength, luster, and receptiveness to dyes. Increases porosity as well.

Moisture Content

The percentage by weight of water in sawdust, pulp, pulpboard, paper, or paperboard.

Mullen

Bursting strength. So called from the name of the instrument used in the test.

Nip

The “line” of contact between two rolls, such as press, calender, or supercalender rolls. Owing to the compressibility of the felt and/or the web of paper, the “line” of contact is actually a narrow zone. Wet nip refers to those at the presses; dry nip usually refers to those at the calendars.

Opacity

The property of a sheet that obstructs the passage of light and prevents seeing through the sheet objects on the opposite side. This property is especially important for printing papers.

pH

The measure of acidity or alkalinity of a material: paper with a pH below 7.0 is considered acidic; paper with a pH above 7.0 is considered alkaline. An expression of the hydrogen-ion concentration, and thus the acidity or alkalinity, of an aqueous solution. The pH value is the negative logarithm, to the base ten, of the hydrogen-ion concentration. A pH of 7 represents a neutral solution; decreasing pH values below 7 represent increasing acidity, and increasing values above 7 represent increasing alkalinity. The pH values of hot or cold aqueous extracts are empirically correlated with properties of paper such as its permanence, its reaction with the fountain etch in offset printing, and others.

Point

One thousandth of an inch. It is used in expressing the thickness of paper or board.

Porosity

The property of having connected pores or minute interstices through which fluids may pass. It is dependent on the number of pores and their distribution in size, shape, and orientation. The porosity of paper is commonly evaluated by measuring its air permeability.

Press

In a paper machine a pair of rolls between which the paper web is passed for one of the following reasons: (1) Water removal at the Wet press; (2) Smoothing and leveling of the sheet surface at the Smoothing press; (3) Application of surface treatments to the sheet at the Size Press.

Press Section

A series of rolls pressing against each other through which the paper travels, along with felts, that removes additional moisture/water from the paper web.

Pulper

A machine designed to break up, defiber, and disperse dry pulps, mill process broke, commercial waste papers, or other fibrous materials into slush form preparatory to further processing and conversion into paper or paperboard. It normally consists of a tank or chest with suitable agitation to accomplish the dispersion with a minimum consumption of power. It may also be used for blending various materials with pulp.

Refiner

A machine used to rub, macerate, bruise, and cut fibrous material, usually cellulose, in water suspension to convert the raw fiber into a form suitable for formation into a web or desired characteristics on a paper machine. The many types of refiners differ in size and design features but most can be classified as either jordans or disk refiners. Beaters are not usually referred to as refiners although in a broad sense they serve a similar function.

Retention

The amount of filler or other material which remains in the finished paper expressed as a percentage of that added to the furnish before sheet formation.

Rewinder

See Winder. The term rewinder is often used for the winder in the finishing room, distinguishing it from the winder that follows the slitter at the end of the paper machine.

Sheeter/Winder

Equipment to further convert paper into sheets or rolls.

Sheffield

A test used to measure the smoothness of paper by measuring the rate of airflow over the surface of the sheet. The lower the number, the smoother the sheet.

Size

Any material used in the internal sizing or surface sizing of paper and paperboard. Typical sizes are rosin, glue, gelatin, starch, modified celluloses, synthetic resins, lattices, and waxes.

Sizing

(1) A property of paper resulting from an alteration of fiber surface characteristics. Internal sizing is a measure of the resistance to the penetration of water and various liquids. Surface sizing relates to the increase of such properties as water resistance, abrasion resistance, abrasiveness, creasability, finish, smoothness, surface bonding strength, and printability, and the decrease in porosity and surface fizz; (2) the addition of materials to a papermaking furnish or the application of materials to the surface of paper and board to provide resistance to liquid penetration and, in the case of surface sizing, to affect one or more of the properties listed in (1).

Smoothness

The property of a surface determined by the degree to which it is free of irregularities. In printing, the smoothness of the paper in the printing nip is important and is referred to as printing smoothness. Smoothness improves as the paper is compressed and locally deformed under mechanical pressure.

Softwood

Wood from coniferous trees whose leaves are needlelike such as pine, spruce, or hemlock or scale-like as cedar.

Softwood Pulp

A pulp made from softwood or coniferous wood species.

Speck

A particle of contrasting appearance in pulp or paper.

Starch

A white, odorless carbohydrate found in various plants. When extracted and purified, primarily from tapioca, corn, potatoes, and wheat, it is used in paper as an adhesive or sizing agent.

Stiffness

The ability to resist deformation under stress. Resistance to a force causing the specimen to bend is termed bending or flexural stiffness.

Stock

(1) Pulp which has been beaten and refined, treated with sizing, color, filler, etc. and which after dilution is ready to be formed into a sheet of paper; (2) Wet pulp of any type at any stage in the manufacturing process; (3) Paper on inventory or in storage; (4) Paper or other material to be printed, especially the paper for a particular piece of work; (5) A paper suitable for the indicated use, such as coating raw stock, milk-carton stock, tag stock, towel stock, etc.

Stock Preparation

A term for several operations that occur between pulping (or bleaching) and formation of the web on a paper machine. It may include, for example, repulping, beating, refining, cleaning, etc.

Supercalender

A calender stack used to increase density, smoothness, and gloss of paper. It is constructed on the same general principle as the calender, except that alternate chilled cast-iron and soft rolls are used in the supercalender. The soft rolls are constructed of highly compressed cotton or paper. It is not an integral part of the paper machine, whereas the calender is.

TAPPI

An acronym for the Technical Association of the Pulp and Paper Industry which is concerned with the establishment of testing standards, etc. for the pulp and paper industry.

Tearing Resistance

The force required to tear a specimen under standardized conditions. Two methods of measurement are in common use: (1) Internal tearing resistance, wherein the edge of the specimen is cut before the actual test; and (2) Edge tearing resistance.

Tensile Strength

The maximum tensile stress developed in a specimen before rupture under prescribed conditions. It is usually expressed as force per unit width of the specimen.

Titanium Dioxide

The white oxide of titanium, TiO2. There are two crystalline forms useful to the paper industry: the anatase form employed primarily as a filler pigment and the rutile form used primarily in pigmented coatings. Both types are particularly useful because of their white color, high brightness, and high refractive index (2.52-2.76) that make them highly effective for improving both brightness and opacity. Commercial grades are usually treated to facilitate use in the many papermaking and coating applications and to provide particle size for optimum optical behavior.

Tooth

Refers to a paper’s surface roughness.

Tub-Size Press

Generally a unit of a paper machine, designed for relatively heavy applications of sizing agents to paper of paperboard, usually located between two drier sections, comprising a tub or vat for holding the liquid sizing material, and a set of vertically oriented press rolls the bottom unit of which is usually partially submerged in the sizing material. In the customary operation of a tub-size press, the moving web enters the tub under a dip roll, is totally submerged in the liquid sizing material for a brief period, and then follows the contour of the bottom press roll into the nip of the press in reverse fashion. As it leaves the nip, it follows the contour of the top press roll, and then continues its forward travel into a second drier section. The tub or vat is generally constructed of wood or metal, while the size press unit is usually, but not always, made up of a pair of rolls of differing hardness and composition. Tub-size press units also include such auxiliary elements as pumps, piping, doctor blades, liquid level devices, thermostats, viscosity controllers, spreading rolls, and the like. In addition to tub-size presses on paper machines, such units may also be used with converting or processing machines such as air-driers, impregnators, etc.

Two-Sidedness

The property of having appreciable difference in color or texture between the wire and felt sides. The term is commonly applied to dyed papers, where the felt side is usually darker. It may occur in paper prepared from a mixed furnish of long- and short-fibered stock, the latter being more evident on the felt side, or in filled sheets, where more pigment is retained on the felt side.

Uniformity

The quality of being uniform in some property, such as color, finish, or especially formation and evenness of fiber distribution.

Unsized

Not having been treated with size – either during or after manufacture. Water absorbent.

Vellum Finish

An uncoated paper finish that is fairly even but not quite as even as a smooth finish.

Virgin Stock

Pulp that has not previously been used in the papermaking process. It is to be distinguished from secondary stock.

Wax Pick Test

Test to determine the resistance of the surface layer of a sheet to the breakaway of surface fragments, when the sheet is separated from the inked plate or blanket in the printing process.

Web

The sheet of paper coming from the paper machine in its full width or from a roll of paper in any converting operation.

Weight

The relative weight of paper.

• M Weight is the weight of 1,000 sheets of paper
• Basis Weight is the fixed weight of a designated number of sheets (typically 500) of paper in the basic size.
• Basic Size is an established standard sheet size for a specific type of paper grade. For example, the Basic Size of text weight paper is 28×38.

Wet End

That portion of the paper machine between the headbox and the drier sextion.

Wet Mullen

The Mullen bursting strength of paper or paperboard after complete saturation with water.

Wet Tensile Strength

The tensile strength of paper after it has been wetted with water under specified conditions.

Wire

On a Fourdrinier paper machine, the slurry is deposited onto a wire, a circular fabric mesh conveyor belt in the “forming” section of the paper machine. This is where water is drained and surface characteristics are established for a paper web.

Wire

On a Fourdrinier paper machine, the slurry is deposited onto a wire, a circular fabric mesh conveyor belt in the “forming” section of the paper machine. This is where water is drained and surface characteristics are established for a paper web.

Yield

The amount of a substance usually expressed as a percentage of starting material that remains after a processing action. In papermaking, examples are the pulp obtained from pulpwood, the amount of paper from pulp, the amount of shipped paper from manufactured paper, etc.

Z-Direction Tensile Strength

The tensile strength perpendicular to the plane of the sheet. It is used as a measure of bonding strength.