Rosin alum sizing

AKDs are waxy, water-insoluble solids with melting points around 50 °C, and ASAs are viscous water-insoluble liquids at room temperature. It is necessary to prepare them as stabilised emulsions by dispersion in a cationic polymer (normally cationic starch). Small amounts of retention aid and surfactants may also be present. Particle size distributions are around 1 fim, and addition levels around 0.1% (of pure AKD or ASA) by weight of dry fibre. This is an order of magnitude lower than the amount of rosin used in rosin-alum sizing (1-2%). Emulsions of AKD are more hydrolytically stable than ASA, and the latter must be emulsified on-site and used within a few hours. 

Rosin sizing usually involves the addition of dilute aqueous solutions or dispersions of rosin soap size and alum to a pulp slurry (44—46). Although beater addition of either coreactant is permissable, addition of both before final pulp refining is unwise because subsequently exposed ceUulose surfaces may not be properly sized. The size and alum should be added sufficiendy eady to provide uniform distribution in the slurry, and adequate time for the formation and retention of aluminum resinates, commonly referred to as size precipitate. Free rosin emulsion sizes, however, do not react to a significant degree with alum in the pulp slurry, and addition of a cationic starch or resin is recommended to maximize retention of size to fiber. Subsequent reaction with aluminum occurs principally in the machine drier sections (47). 

These rosin-based sizes, whether paste, Hquid, or emulsions, can be used to size all grades of paper that are produced at acid pH. The latter include bleached or unbleached kraft Hnerboard and bag paper, bleached printing and writing grades, and cylinder board. In addition, polyaluminum compounds have been used in place of alum, most notably, polyaluminum chloride (48), which can reduce barium deposits where these have been a problem. The barium chloride by-product is more water-soluble than barium sulfate. Other polyaluminum compounds such as polyhydroxylated forms of alum and polyaluminum siHcosulfate have been evaluated as alum replacements. 

Internal Sizing. The most widely used internal sizes are alkyl ketene dimers (AKD), alkenylsuccinic anhydrides (ASA), and rosin-based sizes that are used with papermaker s alum (aluminum sulfate with 14 waters of hydration), polyaluminum chloride (PAG), or polyaluminum siUcosulfate (PAS) (61). The rosin-based sizes are used under acidic conditions. Since the mid 1980 s there has been a steady conversion from acid to alkaline paper production, resulting in static to declining demand for the rosin-based sizing systems. Rosin is a complex mixture of compounds and consists primarily of monocarboxyhc acids with alkylated hydrophenan threne stmctures (62). A main constituent of wood rosin, gum rosin and taH-oil rosin is abietic acid. 

The neutral or alkaline pH of the paper machine system necessary for a high extract pH in the paper is not compatible with the rosin size-alum sizing system. To set rosin size properly on fiber, alum is required. Since total acidity is an approximate measure of the amount of alum in solution, it is often used as a control in connection with rosin sizing. The most effective size-alum precipitate forms at a papermaking pH between 4.2 and 4.8 and a total acidity between 90 and 150 ppm. The low pH of the process water, high total acidity, excess alum, and the size-alum precipitate all place a severe limit on the permanency properties of paper. 

If pre-treatment is not possible, then the only solution is to remove the rosin/alum reaction as far from the anionic trash as possible, i.e. pre-mix sizing or CRS, and reduce the contact time by late addition of size and alum. This, however, will not cure the problem completely, as the high surface area effect will still be present. 

Ito K., Isogai A., Onabe F., Retention behavior of size and aluminum components on handsheets in rosin-ester size alum... 

Kitaoka T, Isogai A., Onabe F, Endo T, Sizing mechanism of rosin emulsion size-alum systems. Part 4. Surface sizing by rosin emulsion size on alum-treated base paper, Nord. Pulp Pap. Res. 7,16(2), 2001,96-102. 

The higher efficiency of fortified rosin sizes is beHeved to result from the semihydrophilic nature of the rosin adduct molecules, which results in a more dispersed system of particles during size precipitation by alum. Consequendy, there is a more uniform distribution of somewhat smaller particles on the sized fibers. This dispersing effect may result from the strong tendency of aluminum to coordinate with organic anions. 

The pulp and paper industry and potable and wastewater treatment industry are the principal markets for aluminum sulfate. Over half of the U.S. aluminum sulfate produced is employed by the pulp and paper industry. About 37% is used to precipitate and fix rosin size on paper fibers, set dyes, and control slurry pH. Another 16% is utilized to clarify process waters. The alum sold for these purposes is usually Hquid alum. It is frequendy acidic as a result of a slight excess of H2SO4. Aluminum sulfate consumption by the pulp and paper industry is projected to remain constant or decline slightly in the near term because of more efficient use of the alum and an increased use of alkaline sizing processes (13). 

Rosin is commonly modified with maleic or fumaric acid to improve efficiency. Since the 1970s, dispersions of unsaponifted rosin have become more popular as a result of their improved sizing efficiency, lower alum requirements, and reduced pH sensitivity vs saponified rosins. Cationic dispersed rosin size, which can be effective at near-neutral and neutral papermaking conditions, is also available (63—65). Commercially available rosin sizes include Pexol, Neuphor, and Hi-pHase (Hercules Inc.), Plasmine and NeuRos (Plasmine), Stafor (Westvaco), Novaplus, and Novasize (Georgia Pacific), and NeuRos and Roscol (Akzo Nobel). 

The normal sequence of addition ia the dyeiag process is pulps, filler, dyestuffs, rosia size, and alum. The dyestuffs are either taken up by the fiber because of their affinity or they must be fixed on the fiber in a finely divided form by suitable fixing agents. Alum, which is required to precipitate the rosin size, has a strong precipitating and fixing effect on dyestuffs. 

In the manufacture of colored papers, it is best to add the dyestuffs before addition of rosin size and alum. This is not always possible in continuous dyeing procedures where dyestuffs must be added to stock containing size and/or alum, and this may cause premature laking of the dyestuffs and subsequent loss of tinctorial strength and/or dullness of shade. The proper selection of dyestuffs can help to reduce these disadvantages. 

The trend towards increased calcium carbonate usage and therefore neutral pH paper making has meant that there has been a steady decline in the use of rosin and alum and a concomitant increase in the use of sizes which are effective at higher pH. Commercially the most important in this group are the alkenyl succinic anhydrides (ASA) and the alkyl ketene dimers (AKD). These are able to esterify fibre surfaces directly and are more effective at neutral to high pH. 

Since the early days of machine made paper in the first half of the nineteenth century, the most widely applied method of Internal sizing has been the use of naturally occurring resinous materials ("rosins") in conjunction with an aluminium salt, usually aluminium sulphate (called "alum" by paper-makers). Various forms of rosin sizes (rosin soaps, rosin emulsions, fortified rosins) have been developed over the years to improve the process, but these variants still involve the use of alum as a means of ensuring that fibres retain a layer of size. 

The important point is that the cellulose in these alum/rosin sized papers is susceptible to acid hydrolysis, which results in a lowering of the degree of polymerisation and, eventually, to a serious reduction in the strength of fibres and to complete embrittlement of the paper. Some recent work in the writer s laboratory suggests that when alum/rosin papers are made, the hydroxonium ions which lead to the degradation are adsorbed independently of aluminium ionic species (4). 

One of the most industrially important characteristics of papers is their chemical stability, which enables them to withstand degradation with its consequential loss of tensile and tear strength and fold endurance under normal conditions of use. However, this stability is not absolute. Cellulose is susceptible to oxidation and the glycosidic linkage is susceptible to hydrolysis. In order to protect book papers from acid degradation, they must not be exposed to acid. Acids are generated from the alum-rosin size as well as from such... 

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