Paper Wet End: Additive Order & Compatibility
TL;DR In the wet end the ORDER you add chemicals matters as much as the dose, because most of them carry a charge and compete for the same fibre surface. The workable rule: fix the anionic charge (charge demand) first, then add cationic additives — dry strength, sizing promoter — then the retention polymer last, close to the headbox. Adding a cationic strength or sizing aid before the charge is under control wastes it, and dosing two high-charge products together makes them precipitate each other instead of working.
Why order matters at all
Nearly every wet-end additive is charged: retention aids, dry strength polymers, sizing promoters, coagulants. They all adsorb onto the same negatively charged fibre and filler surface, so they compete. Add them in the wrong order and a cationic additive is consumed by anionic trash before it reaches the fibre, or two oppositely charged products neutralise each other into a useless floc. The dose can be perfect and still do nothing.
Charge demand comes first
Charge demand — the anionic "trash" dissolved and colloidal in the stock — is the thing to measure and fix before anything else. If it is high, a cationic retention aid or dry strength agent is partly eaten before it can work. Neutralise it first with a fixative/coagulant, then the additives that follow do their intended job at a lower dose.
A workable addition sequence
The retention polymer goes in last, as late as possible before the headbox, because it works by bridging flocs and that structure must survive into the sheet — add it too early and shear tears it apart. A documented example of order effect: with an anionic system, dry strength added after the flocculant gives higher dry and wet tensile than adding it before. Small sequence change, measurable result.
What fights what
| Pairing | Problem | What to do |
|---|---|---|
| Cationic additive + high charge demand | Additive consumed by anionic trash | Fix charge first, then dose |
| Two high-charge opposites together | Mutual precipitation, useless floc | Separate addition points; leave contact time |
| Retention aid added too early | Shear breaks the flocs before the sheet | Dose late, near the headbox |
| AKD size + poor retention | Size not held on fibre, slow cure lost | Sort retention before blaming the size |
| Sizing + excess cationic starch/alum | Charge reversal, sizing loss | Balance total cationic demand |
Common mistakes
- Dosing to a fixed recipe instead of to the measured charge demand of today's furnish — especially on recycled stock, where it swings.
- Adding everything at one point — additives need contact time and their own injection points, not a single manifold.
- Blaming the product — weak sizing or strength is often a retention or charge problem upstream, not the size or the strength agent itself.
- Ignoring the whitewater loop — recirculated additives change the charge balance you dosed for.
Key takeaways
- Order is a lever, not a detail — the same doses in a different sequence give different paper.
- Measure and fix charge demand first; dose cationic additives after.
- Retention aid goes in last, near the headbox.
- Give products their own injection points and contact time; don't co-dose opposites.
Watch
Frequently asked questions
Does the order of adding wet-end chemicals matter?
Yes, as much as the dose. Almost all wet-end additives are charged and compete for the same fibre and filler surface, so the sequence decides how much of each actually adsorbs and works. The same doses added in a different order produce measurably different paper — for example, dry strength added after the flocculant gives higher tensile than adding it before.
What is charge demand in papermaking?
Charge demand is the amount of anionic 'trash' — dissolved and colloidal anionic material — in the stock. It matters because it consumes cationic additives (retention aids, dry strength, sizing promoters) before they reach the fibre. Measuring and neutralising charge demand first, with a fixative or coagulant, lets the additives that follow work at a lower dose.
What is the correct wet-end addition sequence?
A workable order is: measure charge demand, add a fixative/coagulant to neutralise it, then dry strength, then sizing with its promoter, and finally the retention aid as late as possible before the headbox. The retention polymer goes last because its flocs must survive into the sheet; added too early, machine shear tears them apart.
Which wet-end additives are incompatible?
The problem is usually charge, not the specific products. Two high-charge additives of opposite sign dosed together precipitate each other into a useless floc, so they need separate injection points and contact time. A cationic additive dosed into high charge demand is wasted. And AKD sizing fails without good retention — often mistaken for a bad size when it is really a retention problem.
Does VCYCLETECH supply wet-end paper chemicals?
Yes. VCYCLETECH manufactures the full wet-end set — retention aids, dry strength agents, AKD/ASA/rosin sizing, coagulants and fixatives — in China, factory-direct, with a batch Certificate of Analysis on every lot, ISO 9001/14001/45001 certification, free samples for mill trials and OEM/ODM service, plus application support on dosing and sequence. Email sales@vcycletech.com.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment and process chemicals — paper chemicals, surfactants, biocides, coagulants and flocculants, phosphonates and dispersants — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
- Papermaking — Wikipedia
- Paper chemicals — Wikipedia
- Zeta potential (charge) — Wikipedia
- Opportunities in Papermaking Wet-end Chemistry — NC State (Hubbe)
Related: Internal sizing agents · Dry strength agents · Retention aids · Polyacrylamide (PAM) · Paper chemicals