Introduction
Internal sizing plays a crucial role in the performance and quality of paperboard produced on a multilayer board machine. It directly influences water resistance, printability, coating performance, and dimensional stability of the board. In grades like FBB, CBB, Duplex, and Triplex board, proper internal sizing is essential to achieve consistent Cobb values and trouble‑free converting.
What is Internal Sizing?
Internal sizing is the addition of hydrophobic chemicals in the Wet end of the paper or board machine to reduce liquid penetration into the sheet. These chemicals attach to fibers and fillers, making the paper structure resistant to water and other liquids.
“Unlike surface sizing, internal sizing works throughout the fiber matrix, providing base‑level moisture resistance.”
Importance of Internal Sizing in Multilayer Board
Multilayer board consists of different layers, each serving a specific function. Internal sizing must be optimized accordingly.
Why internal sizing is important:
- Controls Cobb value
- Reduces edge wicking
- Improves dimensional stability
- Prevents blistering during coating
- Supports printing and converting performance
Layer‑wise role of internal sizing:
- Top layer: Supports coating holdout and print quality
- Middle layer: Controls moisture pickup and stiffness
- Back layer: Reduces edge absorption and warping
Types of Internal Sizing Agents Used in Board Machines
Rosin Sizing
Rosin sizing is a traditional internal sizing method used in the wet end of pulp and board machines to make the finished board resistant to water and liquids by deposition hydrophobic resin in the fiber matrix. The classic system is based on rosin + alum chemistry.
“Particularly common in packaging boards and multilayer boards where moisture behavior is critical.”
How Rosin Sizing Works
The mechanism is a precipitation and fusion process:
- Rosin (or rosin emulsion) is added into the wet pulp slurry.
- Alum (aluminium sulphate or PAC) is added later — it reacts with rosin to form aluminium resinate complexes, which are water-insoluble. These complexes precipitate on fiber surfaces.
- During drying, the resinates melt/fuse to fibers, forming a hydrophobic barrier that increases resistance to liquid penetration.
“Free rosin + sodium resinate + aluminum ions → aluminum rosinate deposit.
→ Drying & heat profiling is critical — proper fusion ensures sizing effect.”
Advantages:
- Well-established, cost-effective for many board grades
- Rapid sizing effect — sizing often “locked in” as soon as the board leaves the machine
- Works with variety of pulps and fillers
- Improves printability and surface properties
Limitations:
- Requires acidic conditions — can cause fiber degradation over long term
- Not ideal for alkaline processes (where calcium carbonate fillers are used)
- Sensitive to water hardness and pH fluctuations
Key Process Parameters:
A. pH Control
- Optimum 5.5–6.5 pH in traditional rosin systems.
- At higher pH, rosin holds poorly; at lower pH, alum demand increases & retention suffers.
B. Order of Addition
- Rosin emulsion added after refining.
- Alum (or PAC) added soon after to form resinates.
- Retention aids help hold the rosin particles on fibers.
“Order matters — incorrect sequencing can drastically reduce sizing effectiveness.”
C. Temperature
- Higher stock temperatures aid distribution but can destabilize rosin if too high.
- Low temperature can reduce reaction kinetics.
D. Fillers & Hardness
- Calcium/Magnesium interfere with rosin sizing — can precipitate rosin prematurely.
- Highly filled sheets need extra sizing or retention aids.
E. Machine Section
- Most rosin reactions finish during drying/fusing section — drying profile affects final size.
Dosage & Practical Setup
Dosage Ranges
These vary by furnish, machine speed, board grade, and target sizing degree.
- Rosin size: 2–8 kg/ton pulp solids
- Alum: 8 – 25 kg/ton pulp solids (ratio varies)
Emulsion vs. Solid
- Use dispersed rosin emulsions for easier control and retention.
- Emulsions help stabilize rosin against premature coagulation in the wet end.
Retention Aids
- Cationic starch or polymers can assist rosin particle retention.
- Helps especially on multilayer boards with fines and fillers.
ASA (Alkenyl Succinic Anhydride)
ASA is preferred where fast sizing development is required.
Working principle: ASA reacts rapidly with fibers but must be freshly emulsified before use.
Typical dosage: 0.5–1.5 kg/ton
Advantages:
- Immediate sizing effect
- Suitable for high‑speed machines
Disadvantages:
- Emulsion stability issues
- Higher risk of hydrolysis
- Greater deposit tendency
AKD (Alkyl Ketene Dimer)
AKD is the most widely used internal sizing agent in modern multilayer board machines.
Working principle: AKD reacts with cellulose hydroxyl groups to form a hydrophobic ester bond, providing permanent sizing.
Operating conditions:
- pH range: 7.0–8.5
- Compatible with calcium carbonate fillers
Typical dosage: 0.8–2.5 kg/ton
Advantages:
- Long‑term sizing stability
- Suitable for alkaline systems
- Good aging resistance
Disadvantages:
- Slow curing
- Slipperiness at high dosage
- Deposit risk with poor fixation
Wet‑End Chemistry Requirements for Effective Internal Sizing
Maintaining stable wet‑end conditions is critical for sizing efficiency.
Recommended operating parameters:
- pH: 7.2–8.2
- Stock temperature: 35–45°C
- Stable conductivity
- Controlled filler content
“Sudden changes in pH or conductivity can drastically reduce sizing performance.“
Fixation and Retention – The Key to Internal Sizing
Internal sizing agents must be properly fixed onto fibers to be effective.
Common fixation chemicals:
- Cationic starch
- PAC (Polyaluminum Chloride)
- Polyamines / PolyDADMAC
Retention aids:
- CPAM
- Microparticle systems (bentonite or colloidal silica)
Recommended chemical addition sequence:
- Internal sizing agent (AKD / ASA)
- Fixative
- Cationic starch
- Retention aid
Layer‑Wise Internal Sizing Strategy
Top Layer
- Lower internal sizing dosage
- Focus on coating and print performance
- Avoid over‑sizing
Middle Layer
- Moderate sizing dosage
- Improves moisture resistance and stiffness
Back Layer
- Higher sizing if required
- Controls edge wicking and converting issues
Common Internal Sizing Problems and Solutions
High Cobb Value
Causes:
- Poor fixation
- Low drying temperature
- Unstable pH
- Insufficient dosage
Solutions:
- Improve fixation chemistry
- Optimize drying
- Maintain stable pH
Blistering During Coating
Causes:
- Excessive internal sizing in top layer
- Poor moisture release
Solutions:
- Reduce internal sizing in top layer
- Optimize surface sizing formulation
Deposits and Stickiness
Causes:
- Overdosing of AKD or ASA
- Poor retention
- Unstable emulsions
Solutions:
- Improve retention system
- Optimize dosage
- Use fresh, stable emulsions
Interaction of Internal Sizing with Surface Sizing and Coating
Internal sizing should complement, not replace, surface sizing and coating.
Balanced approach:
- Internal sizing for moisture resistance
- Surface sizing for surface strength
- Coating for printability
Typical Internal Sizing Targets for Multilayer Board
| Property | Typical Target |
| Cobb60 (Top) | 22–28 g/m² |
| Cobb60 (Back) | 30–40 g/m² |
| Edge wicking | Minimal |
| Curl | Nil |
Best Practices for Internal Sizing in Board Machines
- Stable pH is more critical than high dosage
- Proper fixation improves sizing efficiency
- Avoid over‑sizing of top layer
- Always evaluate sizing together with retention
Conclusion
Internal sizing is a foundation step in producing high‑quality multilayer board. By selecting the right sizing agent, maintaining stable wet‑end chemistry, and applying a layer‑wise strategy, mills can achieve consistent quality, lower chemical costs, and improved runnability.
Optimizing internal sizing in coordination with surface sizing and coating ensures superior board performance for printing and converting applications.