Maleic anhydride-fortified rosin sizes are prepared by reacting gum, tall oil, or wood rosin with maleic anhydride in a heated reactor equipped with turbine agitation and nitrogen purge at temperatures typically between 180 °C and 220 °C for 2 h to 4 h, followed by rapid cooling to below 100 °C before emulsification with sodium or potassium hydroxide and a stabilizer in a high-shear mixer operating at 3000–5000 rpm or a colloid mill. The Diels-Alder adduct formed from levopimaric and abietic acids introduces a succinic anhydride-derived dicarboxylic acid moiety that raises the acid number of the fortified rosin and increases the anionic charge density of the emulsion. The resulting emulsion typically has a volume-median particle size between 0.3 µm and 2.0 µm, a Brookfield viscosity at 25 °C of 50–200 mPa·s at 35% solids, and a pH after neutralization between 8.0 and 10.5; these parameters are routinely monitored because particle size and charge density control the response to alum and cationic retention aids. When the fortified rosin is injected at the fan pump or thick-stock valve, the additional carboxylic acid groups compete with dissolved and colloidal anionic substances for cationic aluminum hydrolysis products. The charge limit of maleic anhydride-fortified rosin therefore cannot be separated from the anionic trash load, the degree of whitewater closure, the filler type, and the alum dosage. Production-scale observations on fourdrinier and twin-wire machines show that when the fortified rosin charge is pushed beyond the corresponding alum and cationic polymer demand, first-pass retention decreases sharply and the sheet entering the press section carries a higher proportion of unretained fines and aluminum rosinate flocs, which translates into Cobb variability and sheet defects. For typical bleached board and fine paper grades, the operating window for maleic anhydride-fortified rosin addition is commonly 0.15–0.60 wt% active rosin on oven-dry fiber, with alum dosages from 0.5–1.2 wt% Al₂(SO₄)₃·14H₂O on oven-dry fiber depending on furnish anionic demand. Published data for highly closed mills with anionic trash levels exceeding 1500 µeq/L are limited, but mill data indicate that the size charge limit is frequently reached before the intended Cobb target when the cationic demand of the thick stock exceeds 300–500 µeq/L and the whitewater conductivity rises above 2500 µS/cm.

Why Does Maleic Anhydride Fortification Shift Anionic Charge and Alum Demand?

At molecular level, the shift in anionic charge arises because each maleic anhydride molecule that forms a Diels-Alder adduct with levopimaric or abietic acid introduces two carboxylic acid groups, one of which is a succinic acid moiety and the other remains on the original resin acid, increasing the total carboxyl content from approximately 1.0–1.2 carboxyl equivalents per mole in unfortified rosin to as many as 3.0 carboxyl equivalents per mole for the adduct. This molecular change increases both the acid number and the charge density of the emulsified rosin particles; commercial fortified rosin emulsions typically carry an anionic charge characterized by a zeta potential between -30 mV and -60 mV at neutral to weakly alkaline pH before alum addition. The pKa values of the maleic anhydride-derived carboxyl groups are in the approximate range 3.5–5.5, which means that at a headbox pH of 4.5–5.2 only a fraction of the carboxyl groups are ionized, and the effective anionic charge is sensitive to small pH fluctuations. At the wet end, the size particles compete with fiber fines, dissolved hemicelluloses, lignin sulfonates, dispersants, and oxidized pitch for cationic aluminum species. The alum demand therefore rises non-linearly, because aluminum hydrolysis products must first neutralize the dissolved anionic demand of the whitewater, then destabilize the fortified rosin particles, and finally provide sufficient excess cationic charge to deposit the rosin onto the fiber surface. In mills that monitor cationic demand with a Mütek PCD-03 or equivalent streaming current detector, the transition from anionic to cationic demand occurs at a higher alum-to-size ratio for maleic anhydride-fortified rosin than for unfortified rosin, often by 20–40% at equal size solids. This means that charge limits are better expressed as the ratio of cationic demand in the thick stock to anionic charge contributed by the size, rather than as a percentage addition alone.

On paper machines with high closure, the alum and cationic retention aid demand generated by maleic anhydride-fortified rosin must be tracked against the total cationic demand of the furnish rather than the size addition rate in isolation. Headbox pH is frequently controlled between 4.2 and 5.2 for rosin sizing, but the pH of the thick stock after alum addition can be as low as 3.8–4.0 in systems with high acid alum demand, and this depresses fiber charge and can dissolve calcium carbonate filler in short circulation. A practical charge limit is reached when the combined addition of alum, polyaluminum chloride, cationic starch, and retention polymer can no longer maintain a stable zeta potential near the iso-electric range or a slightly cationic streaming current at the headbox without causing drainage, formation, or retention problems. Mills using online charge analyzers have observed that an increase in fortified rosin dose from 0.3 wt% to 0.5 wt% can double the anionic charge in the headbox furnish when the whitewater conductivity is already elevated, resulting in a shift of headbox zeta potential from approximately -8 mV to -15 mV and a corresponding loss of first-pass retention from 75% to 60% on a fine paper furnish. The charge limit therefore is the point at which incremental size addition no longer improves sizing because retention collapses, not the point at which the size dispersion itself becomes unstable. On a production machine, this limit may be as low as 0.35 wt% active rosin on oven-dry fiber when the furnish contains high levels of peroxide-bleached mechanical pulp and recycled fiber, or as high as 0.80 wt% in a clean bleached kraft furnish with low anionic trash.

Charge Titration, Retention and Deposit Risk in Closed Whitewater Loops

Charge titration data from production wet ends show that maleic anhydride-fortified rosin contributes both colloidal charge and dissolved anionic species, but the colloidal component dominates when the emulsion is well prepared. In a typical measurement with a PCD-03, anionic demand is titrated against 0.001 N poly-DADMAC and cationic demand against 0.001 N PES-Na, with results expressed as microequivalents per litre or microequivalents per gram of solids. A fortified rosin emulsion at 35% solids may exhibit an anionic charge density of 200–400 µeq/g as received, compared with 100–250 µeq/g for unfortified rosin of equivalent solids, depending on emulsifier and neutralization. At the headbox, the interaction between aluminum hydrolysis products and the multi-carboxyl rosin surface produces a charge patch that can be either beneficial or detrimental depending on the ratio of cationic aluminum to anionic rosin. If the alum dose is too low, the rosin particles remain anionic and pass into the whitewater; if the alum dose is too high, charge reversal occurs and the rosin particles become cationic, but the excess aluminum hydroxides may aggregate into deposits on forming fabrics, press felts, and vacuum boxes. Production experience with closed whitewater loops indicates that sustained operation at an alum-to-size ratio above 3:1 on an active solids basis often increases the frequency of aluminium rosinate deposition and reduces wire life, while operation below 2:1 may not sufficiently retain the size. The charge limit is therefore constrained at both the lower and upper ends by retention and deposit formation, not only by the target Cobb value. In addition, the high shear in pressure screens and headbox slice flow can strip weakly attached rosin aggregates from fiber surfaces; the resulting free rosin accumulates in tray water and is measured as an increase in turbidity and chemical oxygen demand.

When Maleic Anhydride Charge Exceeds the Rosin–Alum Stoichiometric Window

If maleic anhydride charge is increased beyond the point at which aluminum hydrolysis products can provide adequate cationic counter-charge at a given pH, three simultaneous failure modes appear. First, the rosin particles do not fully retain, and the concentration of free rosin in the whitewater increases, causing foam, pitch-like deposits, and variability in Cobb values across the reel. Second, the added alum required to neutralize the excess charge depresses the headbox pH below the effective rosin-sizing window, shifting the aluminum equilibrium from cationic polynuclear species toward soluble Al³⁺ or neutral precipitates depending on the pH trajectory. At pH values below 4.0, aluminum exists predominantly as Al³⁺, which can neutralize sulfonic and carboxylic acid groups on fiber surfaces but does not provide the optimal charged patch for rosin retention; at pH values above 5.5, the concentration of cationic aluminum hydrolysis products declines, and insoluble Al(OH)₃ forms, which scavenges rosin but does not orient it on the fiber. Third, the excess sulfate introduced with alum accumulates in closed loops and raises the whitewater conductivity, compressing the electrical double layer and making colloidal charge measurement less reliable. The practical consequence is that incremental maleic anhydride-fortified rosin addition above the charge limit produces a non-monotonic Cobb response: initial additions improve water resistance, but further additions can increase Cobb values because the size is not retained and the wet end becomes over-cationized or destabilized. Industrial troubleshooting reports indicate that a fortified rosin dose increase from 0.4 wt% to 0.7 wt% without proportional alum adjustment can raise the 60-s Cobb value from approximately 22 g/m² to 31 g/m², while also increasing headbox pH variability from ±0.15 to ±0.35 pH units. When such reversions occur, the corrective action is not necessarily to add more size but to reduce the maleic anhydride charge, optimize alum and pH, or introduce a low-charge cationic fixative upstream of the size addition.

At the dry end, Cobb testing is performed according to ISO 535:2023 or TAPPI T 441 om-20 with a test area of 100 cm² and a standard water contact time of 60 s for most packaging grades, although 30 s or 120 s may be specified for lightweight or highly sized grades. The Cobb value is the mass of water absorbed per square metre under a 1 cm water head, and for rosin-sized acid fine papers it typically falls between 20 g/m² and 35 g/m² depending on basis weight, size dose, alum retention, and test water temperature. Temperature of the test water is held at 23 °C unless otherwise specified; a deviation of even 3 °C can shift Cobb values by 5–10% because water viscosity changes the penetration rate. Control of Cobb is therefore not a single-variable function of maleic anhydride charge; it is a convolution of size retention, size spreading, fiber surface coverage, drying history, and sheet formation. When the charge limit is exceeded, the resulting loss of retention increases the amount of unretained rosin that cycles back through the saveall, where it can interact with cationic polymers and alum to form tacky agglomerates that deposit on press rolls. The most reliable online indicator of impending Cobb instability is a rise in headbox cationic demand followed by a drop in first-pass retention, not a direct increase in size addition. Table 2 summarizes the standard methods and typical control ranges used to audit such failures.

Cobb Value Response Follows Retention More Than Size Dose

The relationship between maleic anhydride charge and Cobb is indirect and retention-mediated. A well-retained fortified rosin particle at 0.25 wt% addition can produce a lower Cobb value than a poorly retained dose of 0.50 wt%, especially when the sheet is formed on a high-speed gap former where drainage forces may strip loosely attached rosin from fines. The sizing reaction itself requires the rosin to spread over the fiber surface and orient the hydrophilic carboxyl groups toward the fiber, leaving the hydrophobic hydrophenanthrene nucleus exposed; this orientation depends on the aluminum species bridging the carboxyl groups and the fiber carboxylate sites. If the charge limit is crossed, excess anionic rosin remains in the water phase, and the sheet entering the drying section has less surface-adsorbed rosin plus higher concentrations of sulfate and aluminum hydroxide that can hydrolyse the already retained size during drying. Therefore, Cobb control is best achieved by maintaining the rosin-alum charge ratio within the retention window and by using a charge titration or zeta potential measurement as the primary control variable. The upper charge limit of maleic anhydride-fortified rosin is most effectively defined as the dose at which the headbox cationic demand no longer decreases with incremental alum addition or the point at which first-pass retention falls below the machine-specific target for a given furnish. In many acid systems, this corresponds to a fortified rosin dose of 0.4–0.6 wt% on oven-dry fiber and an alum dose of 0.8–1.2 wt%, with headbox pH controlled at 4.3–4.8; beyond this range, additional size charge produces diminishing or negative returns. Operations using filler-free bleached kraft with low anionic trash may extend the limit to 0.8 wt% if pH and alum are tightly controlled, while operations with high calcium carbonate content cannot tolerate the low pH required for rosin-alum sizing and should avoid maleic anhydride-fortified rosin in favour of alkyl ketene dimer or alkenyl succinic anhydride.