What Is Spray Starch? How It Works in Paper & Board Mills
SEVAGRAM EXCIPIENTS
Walk through any kraft paper or duplex board mill and you will likely find a spray bar mounted somewhere on the size press section, fine mist of starch slurry settling onto the sheet as it passes underneath. That is spray starch at work — one of the simplest, most direct ways to add strength to paper after it has already formed.
Unlike starch blended into the pulp slurry before sheet formation, spray-applied starch is added on the surface, late in the process, which gives mills a different kind of control over where the strength actually ends up.
What Is Spray Starch?
Spray starch is a modified starch — usually oxidized or enzyme-converted to lower its viscosity — that is cooked into a thin slurry and applied directly onto the surface of paper or board through spray nozzles, rather than being mixed into the pulp before the sheet forms.
Because the starch sits mostly on or near the surface rather than throughout the fiber mat, mills get a concentrated strength boost exactly where the sheet tends to need it most: the outer plies of multi-ply board, or the top side of kraft paper that will later be printed or glued.
How Spray Starch Differs From Other Starch Treatments
There are three common ways mills get starch into paper, and each behaves differently:
- Wet-end addition — starch is mixed into the pulp slurry before the sheet forms. It is cost-effective for bulk strength but a portion is lost to the white water system and never ends up doing useful work.
- Size press application — starch solution is applied in a pond or film between two rolls as the sheet passes through. This is common for surface sizing but spreads starch fairly evenly across the surface.
- Spray application — starch slurry is applied through nozzles, which allows more targeted placement, profile control across the sheet width, and easier retrofitting onto an existing machine without major rebuild work.
Spray starch is generally chosen when a mill needs surface strength without the capital cost of installing or rebuilding a size press, or when precise cross-machine profile control matters more than blanket coverage.
Why Paper Mills Choose Spray-Applied Starch
Spray application earns its place on the machine for a few practical reasons. It tends to require lower capital investment than installing a size press, since the spray bar can often be fitted onto existing equipment. It also gives operators the ability to adjust application rate across the sheet width in real time, which helps correct profile problems that a flooded nest press cannot easily fix.
For board mills, spraying starch onto the top liner before the sheet is fully dry can improve printability and reduce dusting, both of which matter to converters further down the supply chain.
Spray Starch Application: What Happens on the Machine
In practice, spray starch arrives as a powder and is cooked on-site in a jet cooker or batch cooker, typically diluted to a low solids percentage so it flows easily through spray nozzles without clogging. The slurry is held at an elevated temperature to keep viscosity stable, then pumped to a spray bar positioned over the sheet — commonly just before or after the size press, or ahead of a calendar stack.
Getting consistent results depends on a few variables staying in a tight range: cooking temperature, dilution, and the particle size distribution of the starch itself. A starch that cooks unevenly or settles out of slurry will clog nozzles and create streaks across the sheet, which is why mills tend to stick with a supplier once they find one whose particle size and cook-out behaviour stays consistent batch to batch.
Frequently Asked Questions
Is spray starch the same as size press starch?
No. Size press starch is applied through a film or pond between rollers, while spray starch is applied through nozzles, which allows for more targeted placement and profile control.
What type of starch is used for spray application?
Oxidized and enzyme-converted modified starches are the most common choices, since they need lower cooking viscosity to flow cleanly through spray nozzles.
Can spray starch be added to an existing paper machine without a rebuild?
In most cases, yes. A spray bar can usually be retrofitted onto an existing machine section without the capital cost of installing a full size press.
Does spray starch affect paper printability?
Yes, when applied to the top surface it can reduce dusting and improve surface bond strength, both of which affect how well the sheet prints or laminates downstream.