In modern paper and board manufacturing, multi layer board machines (with Top Former) require close monitoring of wet-end conditions. One of the most critical elements for stable runnability and consistent quality is a well-designed Retention and Drainage Aid (RDA) programme.
What is an RDA Programme?
RDA (Retention and Drainage Aid) programme is a structured wet-end chemical and process strategy used to:
- Improve fiber, fines, and filler retention
- Control drainage rate on forming wires
- Maintain good sheet formation
- Reduce white water solids
- Stabilize dewatering across multiple wires
“In multi-layer board machines, drainage happens extremely fast and under high shear conditions. Without a proper RDA programme, retention losses and formation defects become unavoidable.“
Why RDA is Critical in Multi-Wire Board Machines
Compared to single-wire machines, multi-wire board machines operate with:
- Very high drainage speed
- High shear forces at fan pump, screens, and head box
- Greater fines and ash loss
- Higher sensitivity to over- or under-flocculation
Without optimized RDA control, mills often face:
- Poor ash retention
- Weak ply bonding
- Dirty white water systems
- Excessive vacuum load
- Wire choking and poor formation
Objectives of an Effective RDA Programme
An ideal RDA programme should deliver:
- High First Pass Retention (FPR)
- Controlled and uniform drainage on all wires
- Stable and fine sheet formation
- Optimized vacuum consumption
- Lower chemical cost per ton of board
- Cleaner wires and savealls
Chemistry Used in RDA Programme
1. Primary Retention Aid – CPAM
Cationic Polyacrylamide (CPAM) is the backbone of most RDA systems.
Functions:
- Initial flocculation of fibers and fines
- Improves fiber and filler retention
Typical dosage: 200–500 g/ton (depends on furnish, ash level, and shear)
2. Microparticle / Secondary Retention System
Multi-wire machines almost always require a microparticle system to survive high shear.
Common systems include:
- CPAM + Bentonite
- CPAM + Colloidal Silica
- Dual polymer systems
Functions:
- Re-flocculation after shear
- Improves drainage without damaging formation
Typical Bentonite dosage: 1.0–3.0 kg/ton
3. Fixing Agents (Wet-End Stabilizers)
Fixing agents are often underestimated but are extremely important.
Examples:
- Poly-DADMAC
- Low-dose PAC
Functions:
- Neutralize anionic trash
- Stabilize wet-end chemistry
- Improve overall RDA efficiency
These are usually added before the fan pump.
Correct Addition Points – The Key to Success
Correct chemical addition points determine whether the RDA programme succeeds or fails.
Recommended Addition Sequence for Multi-Wire Machines
- Fixing Agent – Thick stock or machine chest
- CPAM (Primary Retention Aid) – Before fan pump
- Bentonite / Silica (Microparticle) – After pressure screen or just before head box
This sequence ensures:
- Proper mixing of CPAM
- Controlled floc breakup under shear
- Re-flocculation at the former for better retention and drainage.
Key Process Parameters to Monitor
A stable RDA programme must be linked with process data, not just chemical dosage.
Retention Indicators
- First Pass Retention (FPR)
- Ash retention
- White water consistency
Drainage Indicators
- Vacuum levels (zone-wise)
- Sheet solids at couch
- Drainage length
Formation Indicators
- Formation index
- MD/CD variability
- Visual floc size
Common Problems and RDA Adjustments
Poor Retention
Possible causes:
- Low CPAM dosage
- Incorrect addition point
- High anionic trash
Corrections:
- Add or optimize fixing agent
- Adjust CPAM molecular weight
- Improve mixing efficiency
Fast Drainage but Poor Formation
Possible causes:
- Over-dosage of CPAM
- Excessively strong flocs
Corrections:
- Reduce CPAM dosage
- Strengthen microparticle system
- Adjust shear exposure
Slow Drainage or Sheet Sealing
Possible causes:
- Excess fines
- Over-dosing of bentonite
- Dirty or blinded wires
Corrections:
- Reduce microparticle dosage
- Improve furnish cleaning
- Clean and maintain forming wires
Typical RDA Programme – Example
| Chemical | Typical Dosage Range |
| Fixing agent | 0.5–1.5 kg/ton |
| CPAM | 250–400 g/ton |
| Bentonite | 1.5–2.5 kg/ton |
| White water solids | < 0.4% |
| Couch solids | 18–22% |
(Actual values must be optimized grade-wise and machine-wise.)
Best Practices for Long-Term RDA Stability
- Maintain grade-wise standard recipes
- Avoid frequent and sudden dosage changes
- Keep pH and conductivity stable
- Maintain clean wires and approach flow system
- Always optimize changes step-by-step
Conclusion
The Retention and Drainage (RDA) programme in a multi-layer board machine is not just about adding chemicals. It is a balanced interaction between chemistry, machine design, and operating conditions.
A properly designed and maintained RDA programme results in: 1. Better runnability. 2. Lower production cost. 3. Improved board quality. 4. Stable multi-wire operation
For modern high-speed board machines, RDA optimization is not optional—it is essential.