The wet strength 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 distrib ...

and paperboard is a measure of how well the web of fibers holding the paper together can resist a force of rupture when the paper is wet. Wet strength is routinely expressed as the ratio of wet to dry

tensile force In physics, tension is described as the pulling force transmitted axially by the means of a string, a rope, chain, or similar object, or by each end of a rod, truss member, or similar three-dimensional object; tension might also be described as t ...

at break. With combined board such as corrugated fiberboard or with

lamination Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materia ...

s, wet strength also includes the ability to stay intact under humid or wet conditions. Wet strength adhesives are often needed.

Mechanism

The

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...

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 can be less than 10% of the original strength. Various techniques, such as refining of the

pulp Pulp may refer to: * Pulp (fruit), the inner flesh of fruit Engineering * Dissolving pulp, highly purified cellulose used in fibre and film manufacture * Pulp (paper), the fibrous material used to make paper * Molded pulp, a packaging material ...

and wet pressing on the

paper machine A paper machine (or paper-making machine) is an industrial machine which is used in the pulp and paper industry to create paper in large quantities at high speed. Modern paper-making machines are based on the principles of the Fourdrinier Machin ...

can be used to 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% to 30% of the original dry strength of the paper. The wet strength chemicals may improve the dry strength of the paper as well. A Japanese research team found that the effect of humidity on cardboard differs depending on how it is dispersed. Ultrasonic mist reduces the wet strength of cardboard more so than nano-mist.

Wet strength chemicals

Wet strength chemicals improve the tensile properties of the paper both in wet and dry state by crosslinking the cellulose fibres with

covalent bonds A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

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 (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 adhesiv ...

(UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PAE). Neutral sizing agents are alkylketene dimers (AKD) and alkenylsuccinic anhydride (ASA). In recent years, efforts have been devoted to develop environmentally friendly wet strength resins from renewable resources.

Coatings

Sometimes wet strength can be obtained by curtain coating a polymer onto a paper or paperboard. Laminating a

plastic film Plastic film is a thin continuous polymeric material. Thicker plastic material is often called a "sheet". These thin plastic membranes are used to separate areas or volumes, to hold items, to act as barriers, or as printable surfaces. Plas ...

is also a method of providing wet strength.

Applications

A wide range of paper materials are wet strengthened: *

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 ...

Filter paper Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solid particles from liquids or gases. The raw materials are different Pulp (paper), paper pulps. The pulp may be made from soft ...

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

Paper bag A paper bag is a bag made of paper, usually kraft paper. Paper bags can be made either with virgin or recycled fibres to meet customers’ demands. Paper bags are commonly used as shopping carrier bags and for packaging of some consumer go ...

Paper chemicals Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing i ...

Environmental considerations

Recycling Recycling is the process of converting waste materials into new materials and objects. The recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability to reacquire the p ...

of wet strengthened paper requires higher intensity reworking conditions in the re-pulping process to break the fibre web. Some wet strength papers are not considered recyclable.

References

{{DEFAULTSORT:Wet Strength Papermaking