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3.6.4.2.2 Synthetic Sizing Materials
These were only developed in the second half of the last century. Driving forces have been the increasing use of the cost and performance effective calcium carbonate as filler and coating pigment and also the need to produce paper with high and permanent paper strength. To enable its use the paper has to be produced under alkaline conditions. Other paper grades must have very hard sizing in order to resist edge wicking in liquid packaging applications or photographic base paper, or to exhibit hot water resistance for gypsum board. Many mills also seek an alter¬native sizing procedure to the wet-end application, e. g. in the form of a surface treatment in the drying section, to obtain higher efficiency.

. • Alkyl Ketene Dimer (AKD) was the first synthetic size and appeared in the patent literature in 1953. Structurally, alkyl ketene dimers are unsaturated lactones. In the manufacturing process a frequently used synthesis route is: the acid chloride of a carboxylic acid is prepared (fatty acids, C16–22 homologs treated with, e. g. phosphorus trichloride, phosphorus pentachloride, thionyl chloride or phos¬gene), followed by intermolecular lactone ring condensation via a labile inter¬mediate carboxylic acid obtained by dehydrohalogenation of the acid chloride with triethanolamine in an organic solvent. Newly developed processes work without any solvent. To handle and to use AKD in a paper mill, the wax must be converted into tiny particles (0.5–2 mm) dispersed in water. The emulsification is usually effected in a hot (75–90 °C) solution containing the cationic starch stabi¬lizer (alternatively a cationic polymer e. g. polydadmac, polyvinylamine) and a small amount of surfactant, e. g. sodium lignin sulfonate, to which are fed AKD flakes. After melting the AKD, the mixture is forced through a microfluidizer and cooled. Small amounts of a promoter (cationic polymer with low molecular weight and high charge density) and biocide can also be included. The solid content of the commercial products varies between 6 and 30 %. Some scientists believe that AKD reacts with the hydroxy groups of cellulose to form b-keto esters.

. • Alkenyl Succinic Anhydride (ASA), as a synthetic sizing agent for paper, was first synthesized in 1974. It is composed of an unsaturated alkenic hydrocarbon back¬bone coupled to succinic anhydride. It is usually manufactured in two stages: First an unsaturated linear or branched olefin, e. g. a 1-alkene, is isomerized by moving its double bond randomly from its a-position. This will yield an ASA product that is liquid at room temperature, a condition important for easy emul¬sification of the size on application in the paper mill. In the second step, the mixture of isomerized alkenes is reacted with maleic anhydride to produce the ASA raw material. Normally ASA is a yellowish oily product. This 100 % active substance can be stored as such for a long time but must be well protected from water or humidity.
. • Polymeric Sizing Agents (PSA) are mainly based on styrene-acrylates, polyure¬thanes or styrene-maleic acid anhydride. They are either water-based dispersions or aqueous solutions. These products were developed around 1960. Since that time an increasing number of size-presses for strength improvement with starch have been installed. The styrene-maleinate polymers are anionic, aqueous solutions with a solid con¬tent of 20–30 % and an alkaline pH (mainly containing the ammonium salt). They are specially designed for surface application. Their advantages are very good compatibility with all other anionic chemicals (e. g. native and anionic starches, CMC, optical brighteners, anionic polyacrylamides) and high stability in the size-press or in water doctors. The disadvantages of styrene-maleinate polymers are that the sizing effect depends on a certain amount of alum in the base paper, the improvement of printability on wood-free white papers is lim¬ited, and these products are not suitable for wet-end addition (internal sizing). In respect of this, the cationic styrene-acrylate dispersions are much more com¬monly used. They possess high paper stock affinity and the sizing effect is largely independent of the alum content and pH in the stock suspension. There¬fore they can be used for nearly all paper grades, depending on the cationic density of the polymer. With increasing cationic density a reduction in the opti¬cal brightening and the paper brightness will occur. Most products of this class are suitable for wet-end and surface application. The sizing effect is developed within the paper machine. The sizing is improved by intense contact drying on hot metal surfaces (e. g. drying cylinders). The sizing obtained is resistant to acids and alkalis. The surface properties of the paper are improved, especially pick strength and printability and linting and dusting are also reduced.

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