Papermaking terminology


- Virgin fibre:

Fibre produced directly by processing wood (pulpwood or chips).

- Recycled fibre:

Fibre produced by processing used paper (= recycled cellulose fibre).

Paper pulp:

- Mechanical pulp:

Pulp obtained by mechanical separation of wood fibres.

- Thermo Mechanical Pulp: (Thermo Mechanical Pulp) :

This pulp is obtained after mechanical processing of wood chips under steam pressure and at a high temperature.

- Chemical pulp:

In this case the substances binding the fibres together inside the wood chips are dissolved using chemicals. The main chemical pulp production processes are the Kraft process and the Bisulfite process; they basically differ in the chemicals used.

- De-inked pulp :

Pulp obtained by processing paper collected from the graphic industries (printers, binders, routers) and households. The paper is dispersed in the presence of water and chemicals, any undesirable elements (staples, glue, etc.) are extracted, then the ink comes to the surface of the eliminated fibres.

- Bleaching:

The pulp generally obtained by simple treatment of wood or recycled fibres is not usually sufficiently white to be used for production of printing or writing paper. The pulp generally needs to be bleached by treating with chemical products such as peroxides.

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Paper surface treatment

Smooth :

The paper is pressed between two rollers to eliminate the main surface unevenness.

This operation is performed at the end of the paper making machine.

Calendering :

The sheet of paper is pressed between different rollers. This operation, generally performed outside the paper making machine, smoothes the surface of the paper more intensively than a simple smoothing operation.

Calendering improves print rendering. Machines with a high number of rollers, known as supercalenders, are used to produce supercalendered (SC) paper.

Calendering is also used to increase paper gloss.

Coating :

The sheet of paper is pressed between different rollers.

Operation consisting of depositing a solution of mineral fillers (called pigments: kaolin, calcium carbonate, etc.) on the surface of the paper, which is then dried. Coating gives the paper a uniform surface and therefore improves its printability. The quantity of coating varies according to paper categories, ranging from a few grammes per m2 and per side for pigmented papers to over 25 g per m2 and per side for traditional coated papers.

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Printing processes

The offset process :

This printing process makes use of the physical properties of certain materials with the characteristic of having a hydrophobic surface (repelling water) in some cases and a hydrophilic surface (attracting water) in others.

The printing form consists of a metal plate prepared in such a way that the hydrophobic compound is found in the areas to be printed and the hydrophilic compound in the unprinted areas.

During printing, an emulsion of ink and a solution called “wetting solution” (water mixed with chemical compounds) is deposited on the plate; naturally, the ink is deposited on the hydrophobic sections and the wetting solution on the hydrophilic sections.

Surfaces of offset plates are particularly sensitive to wear and run the risk of rapid deterioration in direct contact with paper. The ink is therefore first transferred onto a cylinder covered with a blanket (a synthetic layer with varying degrees of compressibility, with a non abrasive surface).

The ink is then transferred from the blanket to the paper in a second stage.

Offset printing may be done on a sheet fed machine or on a web fed rotary press, with the paper in rolls.

Some offset rotary machines are fitted with a dryer propelling hot air onto the paper, rapidly drying the inks on the paper. Rotary machines without dryers are used solely for printing paper with a fairly open surface, such as standard or improved newsprint paper.

The rotogravure process :

This process uses a grooved printing form. A copper and/or chrome cylinder is engraved with ink cells whose size depends on the volume of ink to be deposited on the paper.

Low viscosity inks are used, to facilitate transfer of the ink from the cells to the paper.

The flexography process:

In flexography, the printing form is a flexible polymer relief plate. The printing area corresponds to the raised section of the plate. A film of ink is deposited on it, then directly transferred to the paper by contact.

Flexography is mainly used for printing packaging, rarely for publications (apart from certain newspapers in the UK and Italy).

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Technical characteristics of paper

Structure properties :

- Grammage :

Weight of a square metre of paper, expressed in g/m2.

- Bulk:

This is the volume of a gramme of paper, calculated as the ratio between the thickness and the grammage of the paper, expressed in cm3/g. By extension, bulk characterises the “feel” of the paper’s strength to the touch.

Optical properties :

- Whiteness:

Index used to compare the visual aspect of the paper with a benchmark white. A light ray with a given wavelength is shone on the paper in precisely defined conditions, and the whiteness is calculated based on analysis of the light reflected by the paper. There are several methods for measuring whiteness (ISO, CIE, etc.); since the measurement methods are different, the results obtained are not identical.

- Opacity :

Evaluation of paper show-through. Opacity depends on the fibrous raw materials used, the quantity and quality of the mineral fillers contained in the paper or deposited on its surface, and of course the grammage.

- Gloss:

Paper gloss is obtained by more or less intensive calendering. Here again, there are different types of measurement: Hunter, Gardner, etc.

Surface properties :

- Surface condition :

The surface condition of paper can be characterised by its roughness or smoothness. There are several methods of measurement, using the air flow between the paper and a ring placed on its surface to characterise the surface condition. The Bekk smoothness method measures the time taken for a given volume of air to seep through, expressed in seconds. The Bendtsen and PPS roughness methods measure the air flow and are expressed respectively in ml/min and µm.

Mechanical properties :

- Tensile strength :

This involves measuring the force required to break a sample of paper of given dimensions. For use on a rotary press, the tensile strength is mainly measured in the production direction, called the machine direction, as opposed to the cross direction.

- Breaking length : Length of paper beyond which the paper breaks under its own weight, calculated from the tensile strength and grammage of the paper.

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Paper classification

Paper with wood:

Paper produced using a minimum 40% of mechanical pulp.

Low wood content paper:

Paper produced using a mechanical pulp content of between 5% and 40%.

Woodfree paper:

Paper produced using at least 95% of chemical pulp.

Newsprint paper:

Paper that has only undergone smoothing at the end of the paper making machine. The main surface unevenness has therefore been eliminated. The shade of newsprint paper has been standardised by the IFRA (an international press research body) and its whiteness is between 58% and 60%.

Improved newsprint paper:

Newsprint paper with whiteness between 60% and 80%.

Offset paper: Paper treated with a light surface coating of starch for improved printability. Offset papers have a matte appearance.

Pigmented paper:

A coating of mineral fillers of less than 8g/m2 per side is deposited on the surface of pigmented papers. This treatment smoothes out any unevenness in the paper surface and therefore improves print rendering. The mineral filler is deposited at the end of the paper making machine. Pigmented papers have a matte appearance, or a silk appearance obtained after light calendering.

MFC (Machine Finished Coated) paper:

Pigmented paper with a mineral filler deposited by equipment installed directly on the paper making machine.

FCO (Film Coated Offset) paper:

Paper coated using a technology depositing the mineral filler as a film.

Glazed, supercalendered paper (SC):

Paper whose surface has been smoothed by intensive calendering: supercalendering. The surface obtained gives good print rendering. It always has a gloss finish.

LWC (Light Weight Coated) paper:

Paper with a light coating of mineral fillers, from 8 to 10 g/m2 per side, and low grammage of 35 to 70 g/m2. Most LWC papers have a gloss finish; matte and semi-matte versions also exist.

MWC (Medium Weight Coated) paper:

Coated paper with a minimum grammage of 75g/m2.

Modern coated paper:

Paper with a mineral filler coating of between 10 and 25 g/m2 per side. These papers exist in gloss, semi-matte and matte versions.

Traditional coated paper:

Woodfree paper, with a surface coating weight of over 25 g/m2 per side.

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