In the paper mill, the wet end is a constant chemical balancing act: fibers, fines, fillers and dissolved anionic substances all compete for retention, drainage and cleanliness. PolyDADMAC (polydiallyldimethylammonium chloride, CAS 26062-79-3) is one of the most widely used cationic polymers in papermaking — as an anionic trash catcher (ATC), a retention aid component, and a deposit control chemical. This guide covers how it works in the wet end and how to use it.
1. The Wet-End Problem: Anionic Trash
Paper furnish contains more than fibers. Dissolved and colloidal anionic substances (DCS) — from pulping, bleaching, recycled fiber (OCC/ONP), broke, and additives like dispersants — carry a negative charge. These “anionic trash” compounds:
- consume cationic additives (retention aids, sizing agents, dyes), wasting chemical;
- destabilize the wet end, causing retention loss and deposits;
- degrade drainage and increase white-water recirculation problems.
The standard countermeasure is a cationic polymer dosed as an anionic trash catcher — and PolyDADMAC is the classic choice because of its very high charge density.
2. How PolyDADMAC Works as an ATC
PolyDADMAC’s defining feature is its high, permanent cationic charge (one positive charge per repeat unit, independent of pH). When dosed to the thick stock or thin stock, it:
- Neutralizes anionic trash — the polymer binds the dissolved and colloidal anions, reducing the cationic demand of the furnish.
- Protects downstream additives — with the trash neutralized, expensive retention aids, sizes and dyes work as intended instead of being consumed by the DCS.
- Improves retention — fines and fillers, once charge-neutralized, aggregate with fibers and are retained on the sheet.
Because PolyDADMAC has low-to-medium molecular weight, it neutralizes charge without over-bridging — which is exactly what an ATC should do, leaving the bridging role to the high-molecular-weight retention aid.
3. Typical Dosing Points and Practice
| Parameter | Typical value |
|---|---|
| Dose (as ATC) | 0.05-1 kg/t of paper (50-1000 g/t) |
| Dosing point | Thick stock or thin stock, upstream of the retention aid |
| Order | ATC (PolyDADMAC) → retention aid (PAM or microparticle) |
| Monitoring | Cationic demand / zeta potential, retention, drainage |
Key practice points:
- Dose PolyDADMAC before the retention aid so the trash is neutralized first — dosing in the wrong order wastes both chemicals.
- Match the dose to the furnish’s cationic demand; measure it (e.g. by poly-DADMAC titration or streaming-current measurement) and dose to a target.
- When the furnish changes (more recycled fiber, more broke, seasonal wood changes), re-optimize the dose — anionic demand can shift dramatically.
4. PolyDADMAC vs Polyamine as ATC
Polyamine is the other common high-charge ATC; see our polyamine in papermaking guide. The practical differences:
- Charge density: PolyDADMAC’s charge is slightly higher and pH-independent; polyamine’s is also high but can vary with pH.
- Molecular weight: both are low-to-medium; PolyDADMAC grades span a similar range.
- Choice driver: water chemistry and mill trials usually decide — many mills use both in different positions or switch based on furnish.
5. Retention Systems with PolyDADMAC
PolyDADMAC is rarely the sole retention chemical. Typical modern systems:
- Single polymer: high-MW cationic PAM or polyamine-based retention aid, with PolyDADMAC as the ATC upstream.
- Dual polymer: PolyDADMAC (or polyamine) + anionic PAM — the anionic PAM bridges the charge-neutralized fines.
- Microparticle: PolyDADMAC + bentonite or silica microparticle — high shear tolerance, widely used on high-speed machines.
6. Deposit Control and Machine Cleanliness
By fixing colloidal pitch and stickies onto fibers rather than letting them deposit on wires, felts and rolls, PolyDADMAC contributes to machine cleanliness. Mills running high-recycled-fiber furnish often use PolyDADMAC-based programs to control stickies and reduce breaks and downtime.
7. FAQ
Is PolyDADMAC a retention aid?
It is an ATC and charge neutralizer that supports retention; the actual retention aid is usually a high-molecular-weight polymer or microparticle system. All are needed in a complete program.
How much PolyDADMAC should I add?
Typically 0.05-1 kg/t of paper. Dose to the measured cationic demand of your furnish rather than a fixed number.
Can I use PolyDADMAC with anionic PAM?
Yes — they are complementary. PolyDADMAC neutralizes the trash first, then anionic PAM bridges. Never mix them directly in the same tank (they precipitate).
Does PolyDADMAC affect paper strength or brightness?
At normal ATC doses it has minimal effect on strength or brightness; its main effect is a cleaner, more stable wet end.
8. References and Further Reading
- Peer-reviewed literature on wet-end chemistry and anionic trash control
- TAPPI technical guidance on retention and drainage systems
- Industrial literature from paper chemical suppliers
This article is published for industrial and technical reference by PolyblueChem. Doses are starting points — optimize by mill trial.
PolyblueChem supplies PolyDADMAC (CAS 26062-79-3) for papermaking and water treatment, with COA and MSDS. See also our PolyDADMAC guide and polyamine papermaking guide. Contact us for technical support.