Recycled fibre brought a problem into the paper mill that virgin pulp never had: a load of dissolved and colloidal anionic material that eats cationic additives before they reach the fibre. Retention aids fail, sizing does not hold, and the wet end becomes unstable. The industry’s answer is a low-viscosity cationic polyamine used as an anionic trash catcher.
This guide covers how polyamine (CAS 42751-79-1) works in the wet end: what anionic trash is, why the low-viscosity grades are the right tool, where to dose, how it compares with polyDADMAC, and what to measure.
What anionic trash is
Mechanical pulp, recycled fibre, coated broke and de-inking plants all release material into the process water:
| Source | Anionic species released | Consequence |
|---|---|---|
| Mechanical pulp | Lignin derivatives, resin acids, hemicellulose | Consumes cationic retention and sizing chemicals |
| Recycled fibre | Starch, coatings, adhesives, ink residues | High cationic demand, deposits, poor formation |
| Coated broke | Latex, dispersants, calcium carbonate fines | Foam, deposit build-up, pitch problems |
| De-inking plant | Surfactants, soaps, dispersants | Carry-over into the wet end |
The measurement that matters is cationic demand – how much cationic charge the stock consumes before the fibres themselves get any. If that demand is high and unmanaged, every downstream chemical is being spent on the wrong target.
Why the low-viscosity grades
Anionic trash capture is a pure charge problem. You want the maximum number of cationic sites per kilogram of product, delivered quickly and dispersed evenly into a very dilute suspension. That points straight at grades A and B:
| Grade | Viscosity | Fit for trash capture |
|---|---|---|
| A (50-200 mPa.s) | Lowest | Best – highest charge density, easiest to dilute and disperse |
| B (200-500) | Low | Good where some bridging is also useful |
| C and above | Medium to high | Not ideal – charge per kilogram drops and solution handling gets harder |
There is also a practical reason: a 50% solution at 5000 mPa.s is painful to dilute in a mill. Grade A flows, mixes and dilutes without special equipment.
Where to dose
| Dosing point | Purpose | Notes |
|---|---|---|
| Machine chest or blend chest | Neutralise trash before it reaches the wet end | Needs good mixing time; the standard location |
| Suction side of the fan pump | Fast neutralisation just ahead of the headbox | Only with reliable flow-proportional dosing |
| Thick stock before refining | Protect downstream additives | Higher consistency – dispersion matters more |
| Ahead of the size press or coater | Stabilise the surface chemistry | Application-specific |
The general rule: dose where the mixing energy is high and the residence time before the next additive is long enough for neutralisation to complete – typically 30-60 seconds.
Dose and control
Trash capture is controlled on cationic demand, not on a fixed dose. Typical product doses fall in the range of 0.5-5 kg per tonne of paper, but the number that matters is the residual: after dosing, the cationic demand of the stock should be low enough that the retention aid can work.
| Control method | What it gives you | Practicality |
|---|---|---|
| Colloid titration (PVSK or similar) | Direct charge demand | Accurate, needs a lab |
| Streaming current detector | Continuous charge signal | Best for automatic dosing control |
| Zeta potential | Surface charge state | Useful for diagnosis, slow for control |
| Retention / first-pass ash | Indirect – tells you if the programme works | Lagging indicator |
Polyamine versus polyDADMAC in the wet end
| Property | Polyamine (grade A) | PolyDADMAC |
|---|---|---|
| Charge density | Higher | Lower |
| Cost per unit charge | Lower | Higher |
| Molecular weight | Lower | Medium |
| Best role | Trash capture, charge neutralisation | Trash capture plus retention, more bridging |
| Behaviour at high conductivity | Charge still effective | Charge still effective |
Many mills run both: polyamine as the workhorse trash catcher, polyDADMAC where retention and bridging are also needed, or where the mill prefers a product with a longer track record in food-contact grades. The choice is often about total cost per unit of charge neutralised rather than about performance alone. See PolyDADMAC in papermaking for the other side of that comparison.
Where it goes wrong
| Symptom | Likely cause | Response |
|---|---|---|
| Retention aid stops working despite dosing | Trash demand exceeds the polyamine dose | Measure cationic demand; raise dose or add a second dosing point |
| Deposits and stickies on the wire | Overdosing – cationic polymer complexes with anionic trash into sticky material | Reduce dose; improve mixing; check dosing pump calibration |
| Foam in the white water | Dispersant carry-over, or polymer/surfactant interaction | Check de-inking carry-over; adjust defoamer separately |
| Sizing not holding | Cationic demand consuming the sizing promoter | Dose polyamine upstream of the size addition |
| Viscosity of the working solution creeping up | Dilution water hardness or microbial growth | Use softer dilution water; prepare working solution fresh |
Frequently asked questions
Can polyamine replace polyDADMAC completely? In many recycled-fibre mills it can take over the trash-capture duty; retention and drainage are usually better served by keeping a bridging polymer in the programme.
What dilution should I use? 0.1-1% working solution, prepared with good agitation and used within a shift or two. Neat dosing causes local overdosing and deposits.
Does it affect paper strength? At correct dose, no – it protects strength resins by removing the trash that would consume them. Overdosed, it can hurt formation and retention.
Is it suitable for food-contact paper? Grade suitability depends on the application and on national rules for food-contact paper; confirm the specific grade approval before use.
Contact PolyBlueChem for polyamine grade A samples and wet-end support.