Troubleshooting Wet-End Chemistry: Foaming, Deposits & Poor Drainage in Paper Mills

The wet end of a paper machine is where chemistry, physics, and mechanical engineering converge under relentless time pressure. It is also where the majority of runnability problems originate. Foaming in the headbox, sticky deposits on forming fabrics and press felts, and sluggish drainage on the wire — these three issues account for a disproportionate share of unplanned downtime, sheet breaks, and off-quality production in mills worldwide. Each has a distinct root cause profile, and each demands a targeted diagnostic approach rather than a reflexive increase in chemical dosage. This guide walks through the mechanisms behind all three failure modes and provides practical troubleshooting frameworks grounded in wet-end chemistry principles.

▶Foaming: Sources, Mechanisms, and Chemical Control Strategies

Foam in the wet end is not a single problem — it is a symptom of surface-active material accumulating faster than the system can dissipate it. The primary sources of foam-generating surfactants in modern papermaking include wood extractives (fatty acids, resin acids, sterols), recycled fiber contaminants, broke reintegration, process water recirculation, and excess or improperly dissolved polymer additives. When these surface-active compounds concentrate at the air-water interface, they stabilize air bubbles into persistent foam structures that disrupt slice flow uniformity, cause barring in the sheet, and introduce air entrapment that weakens fiber bonding.

A critical and frequently overlooked contribution to wet-end foaming is the overdosing or improper dissolution of polyacrylamide-based retention and drainage aids. When PAM powder is added to the system without adequate pre-dissolution — particularly if the solution concentration exceeds 0.3% or the dissolution water temperature is too low — undissolved gel particles and partially hydrolyzed polymer fragments can increase the surface viscosity of the air-water interface. This stabilizes foam instead of suppressing it. The correct preparation protocol for PAM-based additives is a 0.1–0.2% aqueous solution, dissolved in clean water at 20–40°C with gentle agitation for a minimum of 60 minutes before dosing.

Troubleshooting foaming requires isolating whether the source is chemical (surfactant loading) or mechanical (air ingestion through pump seals, vortexed in the machine chest, or insufficient deaeration in the headbox). A practical first step is the Ross-Miles foam test using samples drawn from the fan pump suction and the headbox approach system. If foam persistence increases sharply between these two points, air ingestion in the approach system is a primary contributor. If foam levels are already high at the machine chest, the problem lies upstream in broke handling, recirculation, or additive chemistry.

Foam Control: Defoamer Compatibility with PAM Systems

Mineral oil and silicone-based defoamers are effective at breaking established foam, but their addition point and dosage rate must be carefully managed in mills also using PAM retention aids. Defoamer overdosage — particularly with oil-based products — can deposit hydrophobic material onto forming fabrics, reducing drainage rates and creating a secondary problem. The most robust approach is to address the root cause by controlling surfactant loading through white water clarification, broke washing, and periodic system cleanouts, using defoamer only as a trim tool rather than a primary control mechanism. When PAM is properly selected and dosed, its bridging and flocculation action can actually reduce free surfactant concentration in the white water by co-flocculating surface-active colloids with fiber fines, contributing indirectly to foam reduction.

▶Pitch and Sticky Deposits: Diagnosing the Chemistry Behind Fabric Blinding

Deposit problems at the wet end manifest in two broadly different forms: inorganic scale (calcium carbonate, calcium sulfate, silica) and organic stickies or pitch. Both can blind forming fabrics and felt pores, reduce drainage, cause sheet defects, and in severe cases trigger uncontrolled sheet breaks. The distinction matters because the chemistry required to address each is fundamentally different.

Inorganic Scale Formation

Inorganic scale forms when the concentration of sparingly soluble salts exceeds their solubility product in the white water loop. In alkaline papermaking — the dominant system globally since the transition from acid to neutral/alkaline sizing — calcium carbonate scaling is the most common inorganic deposit. It is promoted by high system closure (reduced fresh water dilution), elevated temperatures, and CO₂ stripping from white water, all of which shift the CaCO₃ equilibrium toward precipitation. Silica scaling is a secondary concern in mills using silicate-containing process water or broke containing sodium silicate from recycled packaging.

The first diagnostic step for suspected inorganic scale is ignition loss testing on fabric or felt deposits: inorganic deposits leave a substantial ash residue, while organic stickies burn cleanly. Identifying the specific ion species via ICP analysis of dissolved solids in white water samples directs the choice of scale inhibitor chemistry. Anionic polyacrylamide at very low molecular weight (below 500,000 g/mol) can function as a crystal growth modifier that prevents CaCO₃ crystals from reaching the critical size needed for surface adhesion — a distinct function from its role as a high-MW flocculation aid. Selecting the wrong molecular weight grade of APAM for scaling control is a common error that leads to ineffective treatment and wasted chemical spend.

Organic Stickies and Pitch Control

Organic stickies originate from two sources: primary pitch from wood resin (esterified fatty acids and resin acids released during mechanical pulping and high-temperature refining) and secondary stickies from recycled fiber contaminants (pressure-sensitive adhesives, hot-melt adhesives, latex coatings, wax, and ink residues). Both become problematic when the colloidal stability of the white water system is disrupted — typically during changes in pH, temperature, conductivity, or additive program — causing previously dispersed colloidal pitch particles to agglomerate and deposit on hydrophobic surfaces.

The most effective chemistry-based approach to pitch and stickies control is fixation: using a cationic polymer to adsorb onto the negatively charged colloidal pitch particles, reversing their charge and attaching them to the fiber surface before they can deposit on fabrics. This is where cationic polyacrylamide plays a decisive role. Hengfeng's cationic PAM products for papermaking are specifically formulated with controlled charge density and molecular weight profiles to achieve simultaneous pitch fixation, fiber fines retention, and drainage improvement — avoiding the trade-off between stickies control and drainage performance that often occurs when using generic cationic polymers not optimized for pulp systems.

Key diagnostic steps when stickies deposits are suspected:

• Measure zeta potential of the white water at the fan pump — a value more negative than -15 mV indicates insufficient cationic demand coverage and high colloidal pitch mobility;
• Perform a cationic demand titration (colloid titration) on white water samples to quantify the anionic charge that must be neutralized by cationic additives;
• Check additive addition sequence — cationic PAM must be added downstream of anionic trash (anionic dispersants, starch, CMC) to prevent premature charge neutralization and polymer precipitation before it contacts the fiber mat;
• Inspect fabric conditioning programs — deposits already present on forming fabrics require enzymatic or alkaline cleaning before chemistry changes can restore drainage performance.

▶Poor Drainage: Systematic Diagnosis Beyond Simply Adding More Polymer

Poor drainage is the most consequential wet-end problem because its effects cascade directly into drying energy costs, machine speed limitations, and moisture profile non-uniformity in the finished sheet. When drainage deteriorates, the instinctive response in many mills is to increase PAM retention aid dosage — but this frequently makes the problem worse. Understanding why requires a clear model of what drainage PAM actually does and what it cannot do.

Drainage rate on the forming wire is governed by three resistances: the resistance of the fiber mat itself, the resistance of the drainage fabric, and the hydrodynamic resistance of the water being displaced through both. Retention aids — including PAM — primarily influence the first factor by aggregating fiber fines and fillers into larger floc structures that are less prone to migrating into and blocking fabric pores. However, if the root cause of poor drainage is an already-blinded fabric, an overloaded white water system with excessive fines concentration, or a pulp furnish with excessive freeness-reducing secondary fiber, adding more PAM will not resolve the underlying issue and may worsen mat formation by creating over-retention of fines that further increase mat resistance.

Step-by-Step Drainage Troubleshooting Protocol

A structured approach to drainage troubleshooting should begin with measurement, not chemistry adjustment. The Schopper-Riegler (SR) or Canadian Standard Freeness (CSF) values of the incoming stock provide the baseline freeness without any chemical treatment. If freeness has declined compared to historical benchmarks at the same furnish composition, the cause is either a change in fiber quality (degree of refining, secondary fiber ratio, fiber length distribution) or a change in white water chemistry (conductivity, pH, colloidal charge load). Both must be quantified before chemistry is modified.

The drainage contribution of the PAM program can be isolated using a dynamic drainage jar (DDJ) or a Britt jar test: run samples with and without current polymer additions at the current addition points, then test the sequence effect by varying the order of cationic and anionic components. In a properly functioning micro-particle or dual-polymer retention system, drainage improvement of 10–25% SR units relative to the untreated baseline is achievable. If jar tests show no measurable drainage response to PAM additions, the problem lies outside the chemistry program — in fabric condition, system closure, or stock preparation.

Hengfeng's dispersant PAM products for paper mills are designed to lower pulp slurry viscosity and improve fiber dispersion uniformity as a prerequisite step that allows retention and drainage aids to function more efficiently. By reducing fiber aggregation in the approach system, dispersant PAM creates a more homogeneous furnish that forms a more uniform, less resistant mat on the wire — directly improving drainage rate without increasing retention aid dosage. This is a particularly effective strategy in mills running highly refined or high-freeness-loss secondary fiber furnishes.

Common drainage problem scenarios and their primary causes:

• Gradual drainage decline over several weeks: typically fabric blinding by deposits — address with fabric cleaning before chemistry adjustment;
• Sudden drainage loss following a furnish change or broke reintegration surge: colloidal charge imbalance — measure cationic demand and zeta potential before adjusting PAM dosage;
• Drainage improvement that reverses within hours of PAM dosage increase: over-retention causing mat densification — reduce PAM dosage and evaluate molecular weight grade;
• Poor drainage at startup after extended shutdown: system chemistry imbalance from broke reintegration — flush and rebalance white water before running at speed;
• Seasonal drainage decline correlating with water temperature changes: viscosity effects on drainage rate — consider temperature-compensated dosage programs.

▶Integrating PAM Chemistry Into a Stable Wet-End Control Program

The three wet-end problems described above — foaming, deposits, and poor drainage — are interconnected through the colloidal chemistry of the white water system. A mill that manages its system charge balance (zeta potential), anionic trash load, and polymer addition sequence rigorously will experience all three problems less frequently and resolve them more quickly when they occur. The common thread is that PAM-based chemistry is most effective when it is applied to a well-characterized system, not used reactively to mask symptoms of deeper imbalances.

Jiangsu Hengfeng supplies a complete range of papermaking PAM products — including retention aids, drainage aids, dispersants, and cationic fixation agents — with technical support services designed to help mills build stable, measurement-based wet-end programs. For mills experiencing persistent foaming, deposit, or drainage challenges, Hengfeng's application engineers can conduct on-site water analysis, jar testing, and additive sequence optimization to identify the minimum effective chemistry program for your specific furnish and machine configuration. Contact our team with your white water analysis data and current additive program for a no-obligation technical assessment.