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Wet strength

Wet strength

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Encyclopedia
Wet strength of paper
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....

 is a measure of how well the fibreweb is holding together upon a force of rupture. Wet strength is routinely expressed as the ratio of wet to dry tensile force
Tensile strength
Ultimate 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...

 at break.

Mechanism


The 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....

 fibreweb of paper is mainly held together by hydrogen bonds. These are dependent on physical contact between the fibres and can be broken by wetting of the fibres. The residual strength of a wetted paper is less than 10 %.

Various techniques, such as refining of the pulp
Pulp (paper)
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

 and wet pressing on the paper machine can be used to mechanically somewhat reduce the strength loss of the paper upon wetting. To improve the wet strength it is common to use chemicals. The use of chemicals can retain as much as 10 - 30 % of the original dry strength of the paper. The wet strength chemicals improves the dry strength as well.

Wet strength chemicals



Wet strength chemicals improve the tensile properties of the paper both in wet and dry state by crosslinking the cellulose fibres
Pulp (paper)
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...

 with covalent bonds that do not break upon wetting.

Different wet strength chemicals are in use, like wet strength resins or neutral sizing agents. Normal wet strength resins are: urea-formaldehyde
Urea-formaldehyde
Urea-formaldehyde, also known as urea-methanal, named so for its common synthesis pathway and overall structure, is a non-transparent thermosetting resin or plastic, made from urea and formaldehyde heated in the presence of a mild base such as ammonia or pyridine...

 (UF), melamine-formaldehyde
Melamine resin
Melamine resin or melamine formaldehyde is a hard, thermosetting plastic material made from melamine and formaldehyde by polymerization. In its butylated form, it is dissolved in n-butanol and xylene. It is then used to cross-link with alkyd, epoxy, acrylic and polyester resins, used in surface...

 (MF) and polyamide-epichlorohydrin (PEA). Neutral sizing agents are alkyl ketene dimer
Diketene
Diketene is an organic compound formed by dimerization of ketene. Diketene is a member of the oxetane family. It is used as a chemical reagent in organic chemistry. It is a colorless liquid and heating regenerates the ketene monomer...

 (AKD) and alkyl succinic anhydride (ASA).

Applications


A wide range of paper materials are wet strengthened:
  • Tissue paper
    Tissue paper
    Tissue paper is a lightweight paper or, light crêpe paper. Tissue can be made both from virgin and recycled paper pulp.-Properties:Key properties are: strength, absorbency, basis weight, thickness , brightness, stretch, appearance and comfort....

  • Filter paper
    Filter paper
    Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

  • Liquid packaging board
    Liquid packaging board
    Liquid packaging board is a multi-ply paperboard with high stiffness, strong wet sizing and a high barrier coating. Only virgin paper fibers are used. The barrier coating must hold the liquid and prevent migration of air and flavors through the paperboard....

  • Paper bag
    Paper bag
    A 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....

  • Paper chemicals
    Paper chemicals
    -Optical brightening agent:Optical brightener is used to make paper appear more white. Optical brightening agents use fluorescence to absorb invisible radiation from the ultraviolet part of the light spectrum and re-emit the radiation as light in the visible blue range. The optical brightening...


Environmental considerations


Recycling of wet strengthened paper requires higher intensity reworking conditions in the deinking
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 action and chemical means. In Europe the most common...

process to break the fibre web. Some wet strength papers are not considered recyclable.