Introduction
In the multilayer board manufacturing process, achieving strong ply bonding, good stiffness, and stable machine runnability is always a challenge—especially when using mixed waste paper furnishes. Spray starch plays a crucial role in solving these issues by acting as an internal bonding and strength-improving agent.
What is Spray Starch?
Spray starch is a specially modified starch (maize/corn or tapioca based) designed for spray application on the wet web of paper and multilayer board machines. Unlike surface sizing starch, spray starch is applied between plies to improve inter-layer bonding and internal strength.
It has:
- Low gelatinization temperature
- Good tackiness after drying
- High affinity for cellulose fibers
Uses of Spray Starch in Multilayer Board Machine
Spray starch is mainly used for:
- Improving ply bonding between different layers (top, middle, and back ply)
- Reducing delamination during converting, creasing, and printing
- Improving internal bond (Z-direction strength)
- Enhancing burst strength, RCT, and stiffness
- Improving sheet integrity during high-speed operation
- Supporting fiber bonding when low-quality furnish is used
Importance of Spray Starch in Board Production
Spray starch is important because:
- Multilayer boards depend heavily on inter-ply strength, not just fiber strength
- It allows cost optimization by improving strength without increasing GSM
- Helps maintain consistent quality when furnish quality fluctuates
- Improves machine runnability and reduces web breaks
- Acts as a cost-effective alternative to high dosages of dry-strength chemicals
How Spray Starch Works in a Multilayer Board Machine
In a multilayer board machine, board strength does not depend only on individual ply strength. The most critical factor is bonding between layers (ply bonding). Poor inter-layer bonding leads to delamination, low stiffness, and converting problems.
Spray starch works as an internal bonding agent that strengthens the bond between layers during sheet formation.
1. Location of Spray Starch Application in Multilayer Board Machine
Spray starch is applied between wet plies, not on the finished dry board.
Application locations:
- Between bottom ply and middle ply
- Between middle ply and top ply
- On the wet web before press section
“this location matters because: At this stage, fibers are still mobile and moist, allowing starch to penetrate and bond effectively.”
2. Spray Application on the Wet Web
At the spray point:
- Board moisture is typically 80–90%
- Spray showers apply fine starch droplets
- Droplets spread evenly across machine width
Application concept: Spray nozzles → fine starch droplets → wet fiber web
“Key function:High moisture helps starch move easily into the fiber structure instead of remaining on the surface.”
3. Penetration of Spray Starch into Fiber Network
Once sprayed:
- Starch penetrates the open pores between fibers
- Capillary action pulls starch into fiber contact zones
- Starch reaches the inter-ply interface
Application Concept: Two fiber layers with starch particles moving between them
This is important because:True ply bonding happens inside the sheet, not just on the surface.
4. Fiber Bonding Mechanism (Hydrogen Bonding)
Spray starch molecules contain hydroxyl (–OH) groups.
These groups:
- Form hydrogen bonds with cellulose fibers
- Act as a bridge between fibers of different plies
- Improve Z-direction (through-thickness) strength
Application concept: Fiber → starch → fiber (bridge formation)
Result: Multiple thin fiber layers start behaving like one solid board.
5. Heat Activation in the Dryer Section
When the sheet enters the dryer section:
- Heat causes starch to gelatinize
- Starch granules swell and become tacky
- Fibers are locked together permanently
Application concept: Wet board → dryer heat → gelatinized starch → strong bond
“Important point: Spray starch is designed with a low gelatinization temperature, so normal dryer heat is sufficient.“
6. Formation of a Strong Bonding Film
After drying:
- Starch forms a thin, continuous film at fiber junctions
- This film increases:
- Ply bond strength
- Board stiffness
- Resistance to delamination
Application concept: Top ply + starch film + middle ply + starch film + bottom ply
7. Board Structure: Before vs After Spray Starch
Without Spray Starch
- Weak physical contact between plies
- Easy layer separation
- Poor converting performance
With Spray Starch
- Strong internal bonding
- Improved stiffness and strength
- Stable board during printing and die-cutting
This comparison clearly shows why spray starch is essential in multilayer boards.
8. Key Factors Affecting Spray Starch Performance
For best results, the following must be controlled:
- Moisture at spray point – Too dry = poor penetration
- Uniform spray coverage – Prevents weak zones
- Correct dosage – Avoids wastage and water load
- Adequate dryer temperature – Ensures gelatinization
- Good retention system – Reduces starch loss to white water
Working Principle Summary (One-Line Diagram Flow)
Spraying → Penetration → Fiber bonding → Heat activation → Strong multilayer board
Advantages of Spray Starch
Key Benefits
- Excellent ply bonding – Strong adhesion between layers
- Low gelatinization temperature – Activates easily in dryer section
- Easy application – Can be sprayed directly on wet web
- Improves stiffness and spring-back – Better converting performance
- Cost effective – Lower cost compared to synthetic binders
- Improves retention of fines – Better sheet formation
Disadvantages of Spray Starch
Points to Consider
- Over-dosage can increase cost without proportional benefit
- Poor spray control may cause streaks or uneven bonding
- Low retention increases COD/BOD in white water
- Limited penetration if web moisture is too low
- Requires proper spray system maintenance
Application Points in Multilayer Board Machine
Spray starch is applied at strategic locations to maximize bonding efficiency:
Common Application Points
- Between bottom ply and middle ply
- Between middle ply and top ply
- On forming section / before couch roll
- After partial dewatering, where web still has sufficient moisture
Application Best Practices
- Maintain uniform spray coverage
- Typical dosage: 4–8 kg/ton (depends on furnish & board grade)
- Ensure correct solids concentration
- Synchronize spray with machine speed
- Use proper retention aid program to avoid starch loss
Spray Starch vs Surface Sizing Starch
| Parameter | Spray Starch | Surface Starch |
| Application | Wet web (between plies) | Size press |
| Main function | Ply bonding | Surface strength & printability |
| Penetration | Internal | Surface only |
| Board benefit | Delamination control | Pick resistance |
Major Spray Starch Suppliers in India
Here are trusted Indian manufacturers supplying spray starch for paper & board mills:
- Bharat Starch Industries
- Saurashtra Specialities Pvt. Ltd.
- Santosh Starch Limited
- RVP Starch Products Pvt. Ltd.
- Angel Starch & Food Pvt. Ltd.
- SPAC Starch Products India Pvt. Ltd.
- Varun Industries
- Paramesu Biotech
- Vijaynagar Starch
Conclusion
Spray starch is an essential wet-end additive in multilayer board machines, especially where ply bonding, stiffness, and delamination control are critical quality parameters. When applied at the right location, dosage, and solids, it significantly improves board performance while keeping production costs under control.
For best results, spray starch should be used along with a well-balanced retention and drainage program and properly maintained spray systems.
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