Internal sizing develops resistance to penetration of aqueous liquids throughout the sheet. Internal sizing is accomplished by adding materials to the stock before the headbox to retard water penetration into the fifinal paper. Water penetration is retarded by the nonpolar portions of the size molecule. A reactive portion of the size molecule anchors it to the surface of the fifiber. Surface sizing works by a different mechanism and occurs at the size press where an application of starch (or other material) fifills the capillaries of paper, making water penetration much more diffificult. Starch is not hydrophobic as are internal sizing agents. Rosin sizing with alum, used since the early 1800s, is only effective below pH of 7 or so. Alkaline sizing was developed in the 1940s and 1950s. It has recently enjoyed widespread use in the United States in printing papers fifilled with calcium carbonate that must be used at pH of 7 or higher because in acid conditions it decomposes to carbon dioxide gas that causes pitting of the sheet. Paper may be hard-sized (high resistance to liquid penetration, such as many printing and packaging papers), slack-sized (low resistance to liquid penetration, such as newsprint), or no-sized paper (toweling, blotting, and sanitary papers). There are two common methods used in internal sizing: rosin sizing, which must be carried out at pH 4e6, and alkaline sizing, which is carried out at pH 8 or higher with alkenyl succinic anhydride (ASA) or alkyl ketene dimer (AKD). There are several important advantages to alkaline papermaking. The papers have longevity as no residual acid is present to degrade carbohydrates; calcium carbonate (an inexpensive, bright fifiller) can be used; paper is stronger and less brittle; there is less corrosion on the paper machine; and there are fewer problems using recycled fifiber containing calcium carbonate fifiller. Thus easier to size than groundwood pulps; semibleached pulps are easier to size than bleached pulps; and pulps with moderate levels of a-cellulose are easier to size than pulps that are almost pure alpha cellulose. The explanation centers around the number of carboxylate groups as anchor points in sizing. It is known that rosin sizing decreases the strength of refifined pulp for papermaking.while less than 20% of printing papers were manufactured under alkaline conditions just a few years ago, the percentage is expected to be more than 80% by 1995. With most internal sizing methods (rosin or alkaline), hardwood pulps are much easier to size than softwood pulps; sulfate pulps are easier to size than sulfifite pulps, which are both easier to size than thermomechanical pulps, which are easier to size than groundwood pulps; semibleached pulps are easier to size than bleached pulps; and pulps with moderate levels of a-cellulose are easier to size than pulps that are almost pure alpha cellulose. The explanation centers around the number of carboxylate groups as anchor points in sizing. It is known that rosin sizing decreases the strength of refifined pulp for papermaking.
Internal Sizing With Rosin
The most common internal size traditionally is rosin at 2e9 kg/t (4e20 lb/ton) rosin solids on pulp, precipitated with alum, Al2(SO4)3, a process developed by Illig in 1807 when it was called engine sizing because the chemicals were added to the stock at the beating engine. Abietic acid and homologues are now forti- fified by addition of maleic anhydride to give a tricarboxylic acid via the DielseAlder reaction shown in Fig. 4.3. This process was invented by Wilson and Duston (1943). Rosin may be used in the salt form as a solution at pH 10e11 (soap size), as a solid salt form with 20%e30% water (paste rosin size) or in the free acid form as an emulsion (emulsion size), which is effective at slightly higher headbox pH than the others. There are several TAPPI standards for rosin characterization. Normally the rosin is added before the alum. If alum is added before rosin, the process is called reverse sizing. Reverse sizing might be useful, for example, in water containing high amounts of calcium which might precipitate the rosin before it could react with alum. Reverse sizing is also recommended in alkaline systems.
Internal Sizing With Alkenyl Succinic Anhydride or Alkyl ketene Dimer
Paper manufactured under alkaline conditions uses synthetic sizing agents (Fig. 4.4) such as AKD (one trade name is Aquapel) or ASA, which were developed in the 1930s and becameavailable in the 1950s, at the rate of about 0.5e1.5 kg/t (1e3 lb/ton). AKD is prepared by dimerization of the acyl chlorides of fatty acids. ASA is made by reacting mixtures of C-16 to C-20 olefifins with maleic anhydride. Unlike forti- fified rosins, the anhydride functionality must not be hydrolyzed with water or else the size will be ineffective and cause pitch problems. Like other anhydrides and esters, these agents are hydrolyzed more quickly with increasing alkalinity and temperature, so emulsions of ASA should be stored for short periods of time under slightly acidic conditions (pH 3.5e4 for ASA) at relatively low temperatures. Hydrolysis is also acid catalyzed, but to a much smaller extent. AKD is much more stable and can be stored as a dilute emulsion for several weeks to 3 months (although occasional inspection by Fourier Transform Infrared Spectroscopy (FTIR) analysis may be useful to verify this). Hydrolysis of ASA and AKD produces carboxylic acids that decrease the pH
These agents are not water soluble and must be used as emulsions. Cationic starch is used to stabilize these emulsions. According to Markillie (1989), the cationic starch solution used to emulsify ASA must be cooled below 38 C (100 F) and have a pH of 4e4.5 to minimize ASA hydrolysis. If the starch viscosity is excessively high, poor emulsifification may result, leading to poor ASA retention, but generally low starch viscosity leads to poor sizing with ASA. A low starch solution pH for emulsifification of AKD can interfere with the process, so the pH should be around 5e7. The starch intrinsic viscosity (4% starch solution at 150 F) should be low, around 1.0 to 1.1, when emulsifying AKD to give maximum sizing, which is the opposite of ASA. Quaternary ammonium starches work better than tertiary amino starches for alkaline sizing. Generally, 3:1 cationic starch (about 0.30% N) to ASA or AKD is used for emulsifification, with an additional 8 lb/ton cationic starch for papermaking.
The anhydride functionality reacts with the hydroxyl groups of cellulose, which is catalyzed by relatively high pH and temperature to give sizing by the formation of an ester linkage, although many argue that these agents are actually held to the fifiber by weak bonds. Certainly only a small portion of the size molecules (<30%) form the covalent ester linkage; on the order of 0.01%e0.03% size on pulp must be retained for sizing. Cationic polymers such as cationic starch (degree of substitution [DS] w 0.03 cationic amines) help retention and sizing with these materials. Some synthetic sizes incorporate cationic groups within the size. ASA is used with 4e5 kg/t (8e10 lb/ton) alum and has rapid on-machine curing. There is literature indicating that alum has a detrimental effect on AKD sizing, and AKD sizing fully develops off the machine over a period of 1e2 weeks. Sizing with any of these synthetic sizes is not well understood, and there are numerous problems in the mills using these agents. Papers tend to be slippery (lower coeffificient of friction), especially with AKD sized paper using excess AKD
to develop suffificient sizing for size press holdout; forming fabrics may have a decreased life using calcium carbonate fifillers at high levels; there is increased picking at the press section; and there may be poorer sizing at the size press with alkaline sizes. Because water drains more quickly from calcium carbonate than clay, calcium carbonate fifilled papers may have poorer formation, and some of the foils may be removed from the wet end to help formation for these grades.