|This article does not cite any references or sources. (April 2012)|
Paper chemicals are a group of chemicals that are used to influence paper properties.
For the production of chemical pulp from wood chips, following chemicals can be used:
- Caustic soda, Sodium sulfide for the Kraft process
- Sulfurous acid for the Sulfite process
- Caustic soda, Anthraquinone for the Soda pulping
Caustic soda is added to increase the pH in the pulping process of fibers. The higher pH of the paper fiber solution causes the fibers to smoothen and swell, which is important for the grinding process of the fibers.
Internal sizing of paper, most paper types must have some water-resistance in order to have a specific writing quality and / or printability. Until 1980 mainly a rosin in combination with alum was used. When the paper industry started using chalk instead of china clay as filler, the paper chemistry had to switch to a neutral process. Today mainly AKD (alkyl ketene dimer) and ASA (alkenyl succinic anhydride) are used.
Wet strength additive
Wet strength additives ensure that when the paper becomes wet, it retains its strength. This is especially important in a tissue paper. Typical chemicals used are as epichlorohydrin, melamine, urea formaldehyde and polyimines. These substances polymerize in the paper and result in construction of a strengthening network.
Dry strength additive
Dry strength additives, or dry strengthening agents are chemicals that improve paper strength of normal or not wet condition. Those strength including compression strength, bursting strength, tensile breaking strength, delamination resistance etc. Typical chemicals used are as cationic starch and polyacrylamide (PAM) derivatives. These substances act as binder of fibers, often under aid of aluminum ion in paper sheet.
To enhance the paper strength, cationic starch is added to wet pulp in the manufacturing process. Starch has a similar chemical structure as the cellulose fibre of the pulp, and the surface of both the starch and fibre are negatively charged. By adding cationic (positive charged) starch, the fibre can bind with the starch and thus also increase the interconnections between the fibres. The positively-charged portion of the starch is usually formed by quaternary ammonium cations. Quaternary salts that are used include 2.3-epoxy propyl trimethyl ammoniumchloride (EPTAC, also known as or Glytac Quab) and (3-chloro-2-hydroxypropyl) trimethyl ammonium chloride (CHPTAC, also known as Quat 188).
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 agent thus generates blue light that is added to the reflected light. The additional blue light offsets the yellowish tinge that would otherwise exist in the reflected light characteristics. It thus increases the brightness of the material (when the illumination includes ultraviolet radiation).
Pigments that absorb in the yellow and red part of the visible spectrum can be added. As the dye absorbs light, the brightness of the paper will decrease, unlike the effect of an optical brightening agent. To increase whiteness, a combination of pigments and optical brightening agent are often used. The most commonly used pigments are blue and violet dyes.
Retention agent is added to bind fillers to the paper. Fillers, such as calcium carbonate, usually have a weak charged surface. The retention agent is a polymer with high cationic, positively charged groups. An additional feature of a retention means to accelerate the dewatering in the wire section of the paper machine. Polyethyleneimine and polyacrylamide are examples of chemicals used.
Styrene butadiene latex, Styrene acrylic, dextrin, oxidized starch are used in coatings to bind the filler to the paper.
Co-Binders, natural products such as starch and also CMC (Carboxymethyl cellulose), are used along with the synthetic binders, like styrene acrylic or styrene butadiene. Co-binders are used to reduce the cost of the synthetic binder and improve the water retention and rheology of the coating.