Wet strength

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The wet strength of paper 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 at break. [1]

Contents

With combined board such as corrugated fiberboard or with laminations, wet strength also includes the ability to stay intact under humid or wet conditions. Wet strength adhesives are often needed.

Mechanism

The cellulose 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. [1]

Various techniques, such as refining of the pulp and wet pressing on the paper machine 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. [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 the cellulose fibres 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 (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. [3]

Coatings

Sometimes wet strength can be obtained by curtain coating a polymer onto a paper or paperboard. Laminating a plastic film is also a method of providing wet strength.

Applications

A wide range of paper materials are wet strengthened:

Environmental considerations

Recycling 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.

Related Research Articles

Corrugated fiberboard

Corrugated fiberboard is a material consisting of a fluted corrugated sheet and one or two flat linerboards. It is made on "flute lamination machines" or "corrugators" and is used for making cardboard boxes. The corrugated medium sheet and the linerboard(s) are made of kraft containerboard, a paperboard material usually over 0.01 inches (0.25 mm) thick. Corrugated fiberboard is sometimes called corrugated cardboard, although cardboard might be any heavy paper-pulp based board.

Papermaking Economic sector

Papermaking is the manufacture of paper and cardboard, which are used widely for printing, writing, and packaging, among many other purposes. Today almost all paper is made using industrial machinery, while handmade paper survives as a specialized craft and a medium for artistic expression.

Pulp (paper)

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemical or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

Medium-density fibreboard

Medium-density fibreboard (MDF) is an engineered wood product made by breaking down hardwood or softwood residuals into wood fibres, often in a defibrator, combining it with wax and a resin binder, and forming it into panels by applying high temperature and pressure. MDF is generally more dense than plywood. It is made up of separated fibres, but can be used as a building material similar in application to plywood. It is stronger, and more dense, than particle board.

Paperboard

Paperboard is a thick paper-based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper and has certain superior attributes such as foldability and rigidity. According to ISO standards, paperboard is a paper with a grammage above 250 g/m2, but there are exceptions. Paperboard can be single- or multi-ply.

Wood fibres are usually cellulosic elements that are extracted from trees and used to make materials including paper.

Paper machine

A paper 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 Machine, which uses a moving woven mesh to create a continuous paper web by filtering out the fibres held in a paper stock and producing a continuously moving wet mat of fibre. This is dried in the machine to produce a strong paper web.

Carton

A carton is a box or container usually made of paperboard and sometimes of corrugated fiberboard. Many types of cartons are used in packaging. Sometimes a carton is also called a box.

Cardboard box

Cardboard boxes are industrially prefabricated boxes, primarily used for packaging goods and materials and can also be recycled. Specialists in industry seldom use the term cardboard because it does not denote a specific material.

Kraft paper

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

Fiberboard

Fiberboard or fibreboard is a type of engineered wood product that is made out of wood fibers. Types of fiberboard include particle board or low-density fiberboard (LDF), medium-density fiberboard (MDF), and hardboard.

Tracing paper

Tracing paper is paper made to have low opacity, allowing light to pass through. It was originally developed for architects and design engineers to create drawings which could be copied precisely using the diazo copy process; it then found many other uses. The original use for drawing and tracing was largely superseded by technologies which do not require diazo copying or manual copying of drawings.

Paper Thin material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically and/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 distributed on the surface, followed by pressing and drying. Although paper was originally made in single sheets by hand, almost all is now made on large machines—some making reels 10 metres wide, running at 2,000 metres per minute and up to 600,000 tonnes a year. It is a versatile material with many uses, including printing, packaging, decorating, writing, cleaning, filter paper, wallpaper, book endpaper, conservation paper, laminated worktops, toilet tissue, currency and security paper and a number of industrial and construction processes.

Vulcanized fibre

Vulcanized fibre or red fibre is a laminated plastic composed of only cellulose. The material is a tough, resilient, hornlike material that is lighter than aluminium, tougher than leather, and stiffer than most thermoplastics. The newer wood-laminating grade of vulcanized fibre is used to strengthen wood laminations used in skis, skateboards, support beams and as a sub-laminate under thin wood veneers.

Deinking is the industrial process of removing printing ink from paperfibers of recycled paper to make deinked pulp.

A detackifier is a process chemical that is used for reducing tackiness of other substances. Spraying paint and paper making are processes where inputs may need to be detackified or "denatured".

Cardboard is a generic term for heavy-duty paper-based products having greater thickness and superior durability or other specific mechanical attributes to paper; such as foldability, rigidity and impact resistance. The construction can range from a thick sheet known as paperboard to corrugated fiberboard which is made of multiple corrugated and flat layers.

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 its strength and resistance to water.

Fibre modification is a research field in which researchers aim at developing and applying technologies to impart new properties to natural fibres such as those in paper, in order to increase their functionality. Research areas in this field include many different technologies, amongst which the chemical modifications of fibres are widely used. One important sector of application of the chemical modifications is the treatment of wood for giving it enhanced properties such as higher mechanical properties, water impermeability, less hygroscopicity, bacterial and fungal resistance. Transferring and adapting the technical knowledge on fibre modification available for the wood sector to the recycled paper sector is an innovative use of these chemical treatments which has been the subject of studies that have been carried out within an EU co-funded project called Fibre+.

Hemp paper means paper varieties consisting exclusively or to a large extent from pulp obtained from fibers of industrial hemp. The products are mainly specialty papers such as cigarette paper, banknotes and technical filter papers. Compared to wood pulp, hemp pulp offers a four to five times longer fibre, a significantly lower lignin fraction as well as a higher tear resistance and tensile strength. Because the paper industry's processes have been optimized for wood as the feedstock, production costs currently are much higher than for paper from wood.

References

  1. 1 2 3 Neimo, Leo, ed. (1999). "13". Papermaking Chemistry. Papermaking Science and Technology. 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.compscitech.2010.08.011.
  3. ↑ "Development of wet strength additives from wheat gluten".
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