Wet strength

Thewet strengthofpaperandpaperboardis 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 drytensile forceat break.^[1]

With combined board such ascorrugated fiberboardor withlaminations,wet strength also includes the ability to stay intact under humid or wet conditions. Wet strengthadhesivesare often needed.

Mechanism

Thecellulosefibreweb of paper is mainly held together byhydrogen 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.^[1]

Various techniques, such as refining of thepulpand wet pressing on thepaper machinecan 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.^[1]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.^[2]

Wet strength chemicals

Wet strength chemicals improve the tensile properties of the paper both in wet and dry state by crosslinking thecellulose fibreswithcovalent bondsthat do not break upon wetting.

Wet strength development should not be confused withsizing,the first representing the strength of the paper once wet, the latter being the speed and amount of water absorbed by the paper. Normal wet strength resins are:urea-formaldehyde(UF),melamine-formaldehyde(MF) andpolyamide-epichlorohydrin(PAE).

Neutral sizing agents arealkylketene dimers(AKD) andalkenylsuccinic anhydride(ASA).

Coatings

Sometimes wet strength can be obtained bycurtain coatinga polymer onto a paper or paperboard. Laminating aplastic filmis also a method of providing wet strength.

Applications

A wide range of paper materials are wet strengthened:

Environmental considerations

Recyclingof 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

^^a ^b ^cNeimo, Leo, ed. (1999). "13".Papermaking Chemistry.Papermaking Science and Technology. Vol. 4. Helsinki, Finland: Fapet OY. pp. 289–301.ISBN 952-5216-04-7.
^Hung, Duong Van; Nakano, Yusuke; Tanaka, Fumihiko; Hamanaka, Daisuke; Uchino, Toshitaka (30 November 2010). "Preserving the strength of corrugated cardboard under high humidity condition using nano-sized mists".Composites Science and Technology.70(40): 2123–2127.doi:10.1016/j pscitech.2010.08.011.

[wet-1] Neimo, Leo, ed. (1999). "13".Papermaking Chemistry.Papermaking Science and Technology. Vol. 4. Helsinki, Finland: Fapet OY. pp. 289–301.ISBN 952-5216-04-7.

[2] Hung, Duong Van; Nakano, Yusuke; Tanaka, Fumihiko; Hamanaka, Daisuke; Uchino, Toshitaka (30 November 2010). "Preserving the strength of corrugated cardboard under high humidity condition using nano-sized mists".Composites Science and Technology.70(40): 2123–2127.doi:10.1016/j pscitech.2010.08.011.

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