Polyamidoamine epichlorohydrin resins are water-soluble cationic thermosetting polymers formed by condensation of adipic acid with diethylenetriamine followed by controlled reaction with epichlorohydrin. The azetidinium functional group and pendant secondary amines impart a cationic charge density commonly measured between 1.0 meq/g and 3.5 meq/g at pH 4–9, while the molar mass distribution is more often specified by intrinsic viscosity than by absolute molecular weight because the resin is a highly branched polyelectrolyte and absolute molar mass determination is poorly reproducible. Selection for a given papermaking furnish begins not with the dry tensile response but with the colloidal charge state of the wet end, because the resin must first be retained on anionic fibre fines, fibre surfaces and filler particles before the sheet enters the dryer section. Charge reversal occurs when the cumulative cationic demand of the furnish minus the dissolved and colloidal anionic material has been exceeded by the addition of cationic polyelectrolytes, yielding a streaming potential or zeta potential value that crosses from negative to positive. The wet tensile strength obtained after curing is measured by TAPPI T 456 or ISO 3781, but the wet tensile result alone does not reveal whether a resin dosage is close to the charge reversal boundary. The charge density of the resin itself is determined by polyelectrolyte titration against potassium polyvinyl sulfate or sodium polyethylenesulfonate using a particle charge detector, with the endpoint signalled by a change from positive to negative or from negative to positive depending on the titrant. On paper machines with high-shear gap formers, a positive headbox charge above +5 mV has been associated in operational reports with deposit formation on forming fabric surfaces, increased white water turbidity, unstable first-pass retention and felt filling. The charge reversal limit is therefore not a single resin property; it is a system boundary that depends on anionic trash load, conductivity, pH, temperature, filler surface chemistry, cationic starch dosage, retention aid type and the point of PAE addition.

Why Does the Point of Zero Charge Shift with Conductivity and Sulfate Ion Concentration?

Electrokinetic measurements of furnish charge are strongly influenced by the electrical double layer, and the point of zero charge derived from streaming potential or zeta potential is not a fixed millivolt reading when conductivity changes. In closed water loops, conductivity values above 2000 µS/cm compress the diffuse double layer and suppress the magnitude of the streaming potential, so a furnish that appears to be near neutral by zeta potential may still possess significant anionic demand when titrated with a particle charge detector. Sulfate ion introduced from aluminium sulfate or oxidative treatment can compete with anionic dissolved and colloidal material for cationic sites on the PAE molecule, shifting the apparent charge demand by 50–150 µeq/L in some recycled furnishes; published data for specific mill configurations is limited, but the direction of the effect is consistent with electrokinetic theory. The particle charge detector is less affected by double-layer compression than streaming potential instruments, but the detector response becomes sluggish when conductivity exceeds 3000 µS/cm because the titration endpoint is broadened by the high ionic background. The practical consequence is that a PAE resin selected solely on the basis of zeta potential may be overdosed in high-conductivity furnish, because the instrument underestimates the remaining anionic demand. Conversely, a furnish with low conductivity below 800 µS/cm can show a sharp zeta potential transition and may exhibit charge reversal at a lower PAE dosage than expected from the particle charge detector alone. Temperature also shifts the point of zero charge because the azetidinium ring opens faster above 50°C, generating secondary chlorohydrin and diol structures with lower cationic charge; a resin solution held at 30°C for more than 8 h can lose enough charge to require an upward dosage adjustment. For these reasons, charge reversal limits are normally specified as a filtrate charge window measured by polyelectrolyte titration rather than as a single zeta potential value.

Table 1 presents a representative data set generated under controlled laboratory conditions for a bleached softwood kraft furnish at pH 7.0, conductivity 1200 µS/cm, and stock consistency 0.8%. The first-pass retention values are laboratory mass-balance data from a Britt dynamic drainage jar and are not a substitute for machine-specific retention measurement.

At a recycled linerboard mill processing old corrugated containers with a stock consistency of 0.8–1.2% in the machine chest, the dissolved and colloidal anionic material load can exceed 800–1200 µeq/L depending on the furnish mix, the age of the recycled fibre and the efficiency of the fibre recovery system. The PAE resin is commonly added after the machine chest but before the mixing pump at 0.15–0.45% on bone-dry fibre to avoid excessive exposure to high-shear zones and anionic trash, because early addition consumes the resin on dissolved anionic material rather than on fibre surfaces. When the PAE dosage exceeds the total anionic demand, the furnish flips to a positive apparent charge, causing fibre flocs to disperse, retention aid efficiency to collapse and pressure screen rejects to rise. The charge reversal limit in this application is not a fixed dosage but a moving boundary that follows the furnish demand; operators titrate headbox filtrate with a particle charge detector and hold the net charge between −50 µeq/L and 0 µeq/L. On production-scale linerboard machines, a positive headbox charge of +25 µeq/L has been reported to coincide with Uhle box vacuum loss, felt filling and reduced sheet release at the press section. The use of alum at 0.5–1.5% on bone-dry fibre can increase the tolerance to PAE overdosing by adding anionic sulfate and aluminium species that re-establish a negative furnish charge, but alum also raises conductivity and can destabilize the PAE if the pH drops below 4.5. Batch-to-batch variance in resin epichlorohydrin content can alter the wet end charge demand by 10–20%, requiring a titration of each delivered resin batch before setting the dosage ramp.

Positioning PAE Addition Relative to Cationic Starch and Microparticle Retention Programs

Cationic starch and PAE compete for the same anionic fibre surfaces and dissolved anionic material, but cationic starch has a lower charge density, typically 0.2–0.5 meq/g, and a higher molecular weight than PAE. The common sequence in packaging and fine paper grades is to add cationic starch at the suction side of the fan pump and PAE after the pressure screen or immediately before the headbox, because PAE has faster adsorption kinetics and can be shear-degraded if exposed to the fan pump impeller. In high-shear twin-wire gap formers producing woodfree fine paper, PAE residence times of 3–8 s between the injection point and the headbox are frequently sufficient for adsorption when the furnish charge is negative, but the adsorption efficiency drops sharply if the furnish has already been charge-reversed by cationic starch or polyamine. Microparticle retention programs using colloidal silica or bentonite can buffer charge because the microparticles have high anionic surface charge and can capture excess cationic PAE, but at a cost of reduced wet strength development because some resin is removed with the fines in the white water loop. The charge reversal limit therefore shifts when the microparticle dosage is changed; a decrease in colloidal silica from 0.3% to 0.1% on bone-dry fibre can bring the furnish closer to positive charge at the same PAE dosage. Operators using a particle charge detector should set an upper limit of 0 µeq/L to +10 µeq/L in the headbox only when a microparticle system is present, because the microparticle will re-anionize the white water before the sheet is formed. Without a microparticle system, the headbox target is normally maintained below 0 µeq/L.

Because tissue production combines high-speed crescent formers with creping adhesive chemistry, the wet end charge window is often narrower than in packaging or printing papers. PAE is used at 0.2–1.0% on fibre to provide temporary or permanent wet strength for towel and tissue grades, and the resin is usually added after the furnish dilution but before the headbox to minimize contact with the creping adhesive. The creping adhesive package applied to the Yankee dryer is often cationic when based on polyamide-amine epichlorohydrin or anionic when based on acrylic acid copolymers, and the interaction between the sheet surface charge and the adhesive layer can affect adhesion, blade wear and sheet handling. A positive headbox charge above +2 mV on a crescent former running at 900–1200 m/min has been associated with fibre pickout, edge trim moisture variability and increased doctor blade load because the sheet releases with a higher adhesive residue. The charge reversal limit in tissue is further constrained by the low basis weight, because a small excess of cationic polymer represents a larger number of charges per unit fibre mass than in a heavier board sheet. Wet end charge is monitored by particle charge detector titration of the approach flow filtrate, and the accepted operating range is frequently −30 µeq/L to −10 µeq/L for towel and −20 µeq/L to 0 µeq/L for napkin grades. Published data for specific tissue configurations is limited because each mill uses different Yankee coating chemistry, but the operational correlation between positive furnish charge and creping blade wear is widely reported in industry.

AOX Limits and Food Contact Compliance Constrain PAE Grade Selection

PAE resins intended for food contact paper and board are regulated under FDA 21 CFR 176.170 in the United States and under BfR Recommendation XXXVI in Germany, both of which limit the starting materials and the residual epichlorohydrin, 1,3-dichloro-2-propanol and 3-chloro-1,2-propanediol content of the finished paper. Regulation (EC) No 1935/2004 requires that no constituents migrate in quantities endangering human health, and for PAE-treated paper the migration assessment is performed with cold-water and hot-water extracts according to DIN EN 645 and DIN EN 647. The total organically bound chlorine of the resin or the mill effluent is determined by ISO 9562, and low-AOX PAE grades typically contain 0.5–1.0% total organically bound chlorine on dry resin. The constraint of a low-AOX grade is that it is often produced with a lower epichlorohydrin ratio and longer hydrolysis, which reduces the azetidinium content and the cationic charge density; the resulting PAE may be less efficient at charge neutralization and may require a higher dosage to achieve the same wet tensile strength. This higher dosage can push the furnish closer to the charge reversal boundary, so compliance-driven resin substitution is not neutral from a wet end stability perspective. Mills switching from a conventional PAE to a low-AOX grade should re-titrate the furnish and reset the headbox charge target before increasing the wet strength dosage. The use of BfR XXXVI-compliant resins does not eliminate the need for mill-specific migration testing because the coating, retention aid system and drying conditions affect the extractable residue.

Alkaline fine paper furnishes containing precipitated calcium carbonate at 15–25% filler load present a different charge environment because the calcium carbonate surface has a weak positive charge at neutral pH while the wood fibre remains anionic. PAE can adsorb onto calcium carbonate and be lost to the filler rather than to the fibre surface, shifting the charge reversal boundary because the filler consumes cationic resin without contributing to wet strength development. The dosing point is moved after the filler addition to avoid pre-adsorption; otherwise the wet tensile retention measured by ISO 3781 falls by 15–30% compared with the same resin added after filler blending. The presence of calcium carbonate also buffers the system pH at 7.5–8.5, which accelerates azetidinium ring opening and reduces the resin charge density over the residence time in the approach system. In high-speed alkaline fine paper production on gap formers, the headbox zeta potential target is commonly −10 mV to −5 mV, and the filtrate charge measured by particle charge detector is held between −40 µeq/L and −20 µeq/L. If the filtrate becomes positive, the first-pass retention of precipitated calcium carbonate drops sharply because the filler particles carry a positive surface charge and electrostatic repulsion increases; the result is a visible white water cloud and a shift in sheet ash distribution toward the wire side.

When Wet Strength Loss During Broke Repulping Exceeds Operational Tolerance

Wet strength loss in broke repulping becomes the limiting variable when a PAE grade is selected for heavy wet strength applications such as sack paper, wallpaper base or filter paper. A resin with high azetidinium content and high cationic charge density develops strong wet tensile strength, but the cured network is resistant to mechanical disintegration in the repulper and requires chemical oxidation or enzymatic treatment to recover fibre. The charge reversal condition creates an additional complication because deposits of agglomerated PAE and fines can form on the extraction plate and reduce repulper throughput. In mills operating broke repulpers at 60–70°C, the use of oxidative agents such as sodium hypochlorite at 0.5–2.0% on broke fibre is common for conventional PAE wet strength, but overdosing of hypochlorite can generate chlorinated organic byproducts and shift the post-bleach furnish to a more anionic state, which then demands higher fresh PAE in the paper machine wet end. The charge reversal limit in the repulper is observed as a positive filtrate charge on the repulper discharge screen, and it indicates that the amide crosslinks are not being hydrolyzed at the expected rate. Selecting a PAE with a lower epichlorohydrin ratio reduces the repulping burden but also lowers wet tensile retention, so the grade choice is a compromise between the wet strength target and the recovered fibre quality. Drainability of the repulped stock is checked by ISO 5267-1, and a drop in Schopper-Riegler value below the machine requirement indicates that the repulper charge and temperature conditions are insufficient to reduce the wet strength resin network.