Corrugated Board Adhesives: Starch-Based Glues and How They Affect Performance

The adhesive that bonds linerboard to corrugating medium is one of the most critical — and often overlooked — components in corrugated packaging. A corrugated box might use premium containerboard and flawless die-cutting, but if the adhesive bond fails, the box fails. Delamination, zipper joints, poor flat crush, and warp are all connected to adhesive performance.

The vast majority of corrugated board is manufactured using starch-based adhesives, a technology that dates to the earliest days of the corrugated industry but continues to evolve. Understanding how these adhesives work — their chemistry, application, and failure modes — is fundamental to producing quality corrugated board.

The Starch-Based Adhesive System

Why Starch?

Corn starch (in North America; wheat or tapioca starch in other regions) is the predominant adhesive for corrugated manufacturing for several compelling reasons:

• Cost: Corn starch is inexpensive and widely available, making it the most economical adhesive option by a wide margin.
• Performance: When properly formulated and applied, starch adhesives create bonds that exceed the tear strength of the paper itself — meaning the paper fails before the bond does.
• Sustainability: Starch is a natural, renewable, biodegradable material that does not interfere with corrugated recycling. It dissolves during repulping without contaminating the recycled fiber.
• Speed: Starch adhesives can gel and set rapidly enough to support corrugator speeds up to 1,200 ft/min.
• Food safety: Starch-based adhesives are food-contact safe, an important consideration for packaging that may contact food products.

The Stein-Hall Formula

The most widely used adhesive formulation in the corrugated industry is the Stein-Hall adhesive, developed in the 1930s and still the foundation of most corrugator adhesives today. It is a two-part system:

Carrier portion (pre-gelatinized starch):

• A small percentage of the total starch (typically 15-20%) is cooked (gelatinized) to form a viscous paste
• This carrier provides the adhesive's body and viscosity at room temperature
• It acts as a suspending medium for the raw starch portion

Raw starch portion:

• The majority of the starch (80-85%) is added raw (uncooked) to the carrier
• Raw starch granules are suspended in the carrier, contributing to the adhesive's solids content
• These granules remain ungelled until they reach the gel point on the corrugator

Additional components:

• Caustic soda (sodium hydroxide): Lowers the gel point of the raw starch by swelling the granules, making them gel at a lower temperature. Typical caustic levels are 2.5-3.5% of starch weight.
• Borax (sodium tetraborate): Crosslinks the starch molecules, increasing viscosity, tack, and bond strength. Typical levels are 1-3% of starch weight.
• Water: The primary carrier fluid. Typical solids content is 20-28% for most applications.

The Gel Point

The gel point — the temperature at which the raw starch granules swell and gelatinize, forming a continuous adhesive film — is the most critical parameter in starch adhesive performance.

How it works on the corrugator:

1. The adhesive is applied to the flute tips of the corrugating medium at room temperature (or slightly above)
2. The medium with adhesive contacts the linerboard at the pressure roll (singlefacer) or hot plates (double backer)
3. Heat from the corrugator transfers through the paper into the adhesive
4. When the adhesive reaches the gel point, the raw starch swells rapidly, absorbing water and forming a gel
5. The gel creates an initial "green bond" that holds the components together
6. As the bond line continues to heat, water evaporates and the starch film dries to form a strong, permanent bond

Gel point ranges:

Lower gel points enable faster bonding at lower temperatures, which allows higher corrugator speeds. However, an excessively low gel point risks premature gelation on the glue rolls, causing uneven application and defects.

Adhesive Preparation

Mixing Process

Modern corrugated plants use automated adhesive kitchens that batch starch adhesive to precise specifications. A typical batch sequence:

1. Carrier cook: A measured amount of starch is added to water in the cooking tank, heated to 150-170°F to fully gelatinize the carrier portion.
2. Cool and transfer: The carrier is cooled slightly and transferred to the main mixing tank.
3. Raw starch addition: The raw starch portion is slurried in water and added to the carrier with mixing.
4. Chemical addition: Caustic soda and borax are added in controlled amounts.
5. Final mixing: The batch is mixed until uniform, then checked for viscosity, gel point, and solids content.
6. Transfer to corrugator: The finished adhesive is pumped to holding tanks at the corrugator, then metered to the glue stations.

Quality Parameters

Adhesive quality is monitored continuously through several key measurements:

Deviations from target ranges can cause a cascade of quality problems on the corrugator, from poor bonding to warp and washboarding.

Adhesive Application on the Corrugator

Singlefacer Glue Station

The singlefacer glue station applies adhesive to the flute tips of freshly formed corrugating medium before it bonds to the first liner.

Key components:

• Glue pan: Holds the adhesive supply
• Glue roll: Rotates through the glue pan, picking up adhesive
• Doctor roll/blade: Meters the adhesive to a controlled film thickness on the glue roll
• Pressure roll: Presses the medium flute tips against the glue roll to transfer adhesive

The glue film thickness must be precisely controlled:

• Too much adhesive: Causes wet bond, slow set, washboarding, and wasted starch
• Too little adhesive: Causes poor bond, delamination, and reduced flat crush resistance

Typical glue film weight at the singlefacer is 4-8 grams per square meter (dry weight).

Double Backer Glue Station

The double backer glue station applies adhesive to the flute tips on the open side of the single-face web, then bonds the second liner. The double backer operates differently from the singlefacer:

• Adhesive is applied to moving web rather than to individual sheets
• The bond is formed at the hot plate section, where steam-heated plates provide the heat to gel and dry the adhesive
• Contact time on the hot plates is longer (several seconds vs. fractions of a second at the singlefacer)
• Precise temperature and speed control are critical to ensure the bond gels without overheating or underdrying

Specialty Adhesives

While standard Stein-Hall starch adhesive handles the vast majority of applications, several specialty adhesive formulations address specific performance requirements.

Waterproof and Water-Resistant Adhesives

Standard starch bonds soften and fail when exposed to prolonged moisture. For applications requiring moisture resistance — cold chain logistics, outdoor storage, agricultural packaging — modified adhesives are available:

• Resin-fortified starch: Starch adhesive blended with synthetic resins (urea-formaldehyde, polyamide, or polyvinyl acetate) that crosslink during heating, creating a water-resistant bond.
• Synthetic adhesives: Polyvinyl acetate (PVA) or hot-melt adhesives that provide inherent water resistance without starch.
• Silicate-based adhesives: Sodium silicate adhesives provide good water resistance and are used in some wax-alternative applications.

Water-resistant adhesives cost 2-5 times more than standard starch and may require higher corrugator temperatures or slower speeds for proper bonding.

High-Speed Adhesives

For corrugators running at maximum speeds (1,000+ ft/min), adhesive formulations are optimized for rapid gelation:

• Lower gel points (through higher caustic levels or modified starches)
• Higher solids for faster drying
• Modified starch bases (oxidized, acid-modified, or enzymatically modified) that gel more rapidly
• Specialty borax replacements that provide faster tack development

Low-Temperature Adhesives

Some applications require adhesive bonding at lower temperatures than standard corrugator heat sections provide:

• Cold-set adhesives that bond at or near room temperature using PVA or EVA emulsions
• Used primarily in hand-assembly operations, specialty converting, and small corrugators that lack high-temperature capability

Bio-Based Alternatives

Research continues into adhesive formulations based on alternative bio-based feedstocks:

• Soy-based adhesives: Modified soy protein adhesives have been tested but have not achieved cost or performance parity with corn starch
• Cellulose-based adhesives: Nanocellulose and modified cellulose adhesives are in development stages
• Lignin-based adhesives: Kraft lignin from pulping byproducts is being explored as an adhesive component

Adhesive Troubleshooting

Common Bond Failures

The Pin Adhesion Test

The pin adhesion test (TAPPI T821) is the primary method for evaluating bond quality in finished corrugated board. A series of pins are pushed through the flute tips to measure the force required to separate the liner from the medium.

Target values:

• Minimum acceptable: 40 lbs/ft of bond line
• Good bond: 60-100 lbs/ft
• Excellent bond (paper tears before bond fails): 100+ lbs/ft

Quality control programs in box plants routinely test pin adhesion from every corrugator run to verify bond integrity.

The Impact on Box Performance

Adhesive quality directly affects multiple box performance characteristics:

• Flat crush resistance: A weak bond allows flutes to separate from liners under flat crush load, dramatically reducing flat crush test values.
• Edge crush (ECT): While ECT is primarily a function of containerboard properties, delaminated flute tips reduce the effective ECT by preventing the medium from fully contributing to the edge crush column.
• Box compression: Ultimate box compression test (BCT) values depend on consistent edge crush across all panels, which requires uniform bonding.
• Moisture resistance: The bond line is often the first failure point in humid environments. Water-resistant adhesives improve wet performance.
• Printing quality: Adhesive-related washboarding creates an uneven printing surface that reduces print quality.

Adhesive Cost Management

Starch adhesive is a relatively small component of total corrugated cost — typically 1-3% of the total box cost — but it has an outsized impact on quality and productivity. Cost management strategies include:

1. Optimize solids content: Running at the minimum solids level that provides adequate bonding reduces starch consumption per square foot.
2. Control gel point precisely: A correctly set gel point enables the corrugator to run at maximum speed without bonding defects, improving overall productivity.
3. Minimize waste: Leftover adhesive from batch changes and line flushes represents waste. Right-sizing batches to match production schedules reduces this waste.
4. Maintain equipment: Worn glue rolls and applicator components apply adhesive unevenly, requiring higher overall glue weights to ensure adequate bonding at the thinnest points.
5. Monitor containerboard quality: Board characteristics (porosity, moisture content, surface treatment) affect adhesive absorption and bonding. Adjusting adhesive parameters to match board properties optimizes consumption.