Retention Aid Adjustments for Bamboo and Wheat-Straw Short-Fibre Pulp Lines

Bamboo and wheat straw have earned a firm place in furnishes that need to move away from long virgin fibre, but they are rarely plug-and-play on the wet end. A retention aid programme that performs well on softwood or recycled pulp often needs rethinking when short non-wood fibres dominate. The fines load is higher, the anionic charge is less predictable, and the gap between good drainage and good formation becomes uncomfortably narrow. What follows is a practical way to approach those adjustments, based on the questions we hear most often from papermakers running these furnishes.

▶ Why Short Non-Wood Fibres Behave Differently

Bamboo and wheat straw are not simply shorter versions of wood fibre. They carry parenchyma cells, pith, epidermal fragments and fines that raise the specific surface area of the furnish. More surface means more adsorption sites for polymer, so the same retention aid dose can behave very differently from one furnish to the next.

The charge picture is equally important. Wheat straw tends to bring silica and ash into the process, which increases the anionic load, particularly when white water is closed up. Bamboo brings extractives and hemicellulose that can consume cationic sites and contribute to deposits. Because fibre length is short, mechanical entanglement contributes less to retention, so the polymer has to do more of the bridging work. Over-flocculate in response, and sheet formation suffers.

Seasonal and process variability makes this harder. Crop year, cooking conditions, bleaching sequence and washing efficiency all move the charge demand. A recipe that works in one delivery may need a small correction in the next.

▶ Begin with Charge Demand, Not with Dosage

When retention falls, the instinct is to add more polymer. On short non-wood furnishes that often makes things worse: overdosing can re-stabilise fines, generate foam and leave sticky deposits in the wire pit and press section. Before touching the dose, measure the cationic demand of the headbox furnish and the white water, together with pH, conductivity, consistency and ash. These numbers tell you whether the retention aid is being consumed by dissolved and colloidal material before it ever reaches the fibres.

If charge demand is high, a coagulant or fixative ahead of the retention aid is usually a more economical answer than raising the retention polymer dose. A low molecular weight cationic coagulant can neutralise anionic trash and leave the high molecular weight retention aid free to bridge fibres and fines. Feed the coagulant where mixing is good and the residence time is sufficient; feed the retention aid after the last high-shear stage and before the headbox. Make-down water quality also matters, so dilute the polymer to a low working concentration, use fresh solution and avoid over-shearing it in the transfer line.

▶ Matching Chemistry to Bamboo and Wheat-Straw Furnish

The table below summarises the tendencies we see most often. It is a starting point for a trial, not a substitute for a furnish-specific charge titration and jar test.

▶ A Practical Adjustment Sequence

Changing several variables at once makes it impossible to know what worked. We prefer a stepwise sequence, with each step held long enough to produce a stable trend.

1. Baseline the furnish: charge demand, pH, conductivity, consistency, ash and current first-pass retention;
2. Review the retention aid chemistry: molecular weight, charge density and ionic form should match the dominant fines and the anionic load;
3. Fix the make-down routine before changing the dose: dilution, water quality, solution age and mixing energy;
4. Confirm the addition point: the polymer should have enough time and turbulence to mix, but not so much shear that the flocs are destroyed;
5. Add a coagulant or fixative if charge demand is high, and re-baseline after it is stable;
6. Trial a dual-polymer or microparticle system if single-polymer retention cannot deliver both drainage and formation;
7. Verify with first-pass retention, wire drainage, white-water consistency, sheet formation and strength, then change only one variable at a time.

▶ Dual-Polymer and Microparticle Options

Short non-wood furnishes often respond well to a two-component approach because it separates charge neutralisation from bridging. The options below are worth evaluating in a laboratory or pilot trial before a full machine change.

• Coagulant plus retention aid: a low molecular weight cationic coagulant reduces anionic trash, followed by a high molecular weight retention aid for bridging;
• Cationic then anionic polymer: a dual-polymer sequence can improve retention where the furnish has mixed charge character, but the addition points and dose balance need careful tuning;
• Microparticle with cationic retention aid: bentonite or silica microparticles can widen the operating window, giving better drainage without collapsing formation;
• Dispersion support: short fibres can clump, so a dispersant may help the furnish present a more uniform sheet to the wire.

More chemical is not automatically better. Keep the total dose disciplined, dilute each component properly, and check that the polymers are not competing for the same adsorption sites.

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▶ Troubleshooting the Wet End

Most retention problems on bamboo and wheat-straw lines show up as one of a few familiar patterns. The cause is not always the retention aid itself.

• Drainage is slow but retention is acceptable: the floc may be too large or too shear-sensitive. Review polymer molecular weight and the addition point;
• Retention is poor and white water is cloudy: charge demand is probably high. Run a charge titration before increasing the retention aid dose;
• Formation is poor despite good retention: over-flocculation or poor fibre dispersion may be the issue. Consider a lower dose, a different charge density or a dispersant;
• Foam and deposits appear: overdosing, degraded polymer solution or high extractives can all contribute. Check solution age and make-down water;
• Performance drifts between deliveries: raw material variability is likely. Build a quick charge and ash check into incoming furnish approval.

Working with a Polymer Partner

Short non-wood pulp lines reward patient optimisation rather than a single off-the-shelf answer. It helps to work with a supplier who can run laboratory tests on your actual furnish, discuss the charge data with your wet-end team and support a trial on the machine. Our paper industry applications cover retention and dispersion chemistry, and our technical team can review your furnish and white-water samples before recommending a starting point.

Paper Making Dispersant for Fiber Dispersion and UniformityPolyacrylamide dispersant for fiber and filler dispersion, supporting formation and tissue uniformity at low dosage in short non-wood pulp optimisation.View Product →

We manufacture polyacrylamide in powder and emulsion form and supply papermakers in many regions. If you are planning a change, contact our technical team with your furnish details and current retention data, and we will help you design a sensible trial rather than a wholesale switch.

Bamboo and wheat straw do not demand a completely different retention strategy, but they do demand a more careful one. Start with charge and fines, choose the polymer chemistry to match, control the make-down and addition point, and change one thing at a time. The reward is a wet end that drains steadily, forms a saleable sheet and uses less chemical than a dose-led approach would suggest.