In the corrugated packaging industry, moisture resistance is crucial for maintaining box integrity during storage, shipping, and handling. Generally, the industry has accepted three levels of "wet strength" adhesive: Moisture Resistant (MRA) for the lightest possible applications, Water Resistant (WRA) for medium level performance, and Waterproof (WPA) for highest level performance. This comparison focuses on the two most commonly used additives: Water Resistant Adhesive (WRA) and Moisture Resistant Adhesive (MRA).

Understanding the Fundamentals

MRA/WRA/WPA are labels given to the performance of the starch bond, generally indicated by the amount of wet-strength resin in the starch adhesive. These resins do not directly contribute to box strength, they simply allow the starch to keep doing its job when in high humidity. Both additives serve the same fundamental purpose: preventing adhesive failure when corrugated boxes encounter moisture or high humidity conditions.

Standard corrugated board uses starch-based adhesives that perform well under normal conditions but can fail when exposed to moisture. As humidity rises, the stacking strength of corrugated boxes will diminish and the standard starch adhesives can break down due to their lack of moisture resistance capability. This is where WRA and MRA additives become essential.

Chemical Composition and Formulation

MRA (Moisture Resistant Adhesive)

MRA most closely resembles domestic adhesive in formulation and application. A typical MRA starch will contain approximately 0.5% to 1% liquid resin relative to the liquid volume of the batch. For a typical 650- to 750-gallon batch, MRA requires 4 to 5 gallons of wet-strength resin.

WRA (Water Resistant Adhesive)

Water Resistant Adhesive will have between 1% and 1-1/2% resin relative to the liquid volume of the batch. This higher concentration of wet-strength resin provides enhanced moisture protection compared to MRA formulations.

Performance Characteristics

Moisture Protection Levels

MRA provides basic moisture resistance suitable for lightweight applications where minimal humidity exposure is expected. This level of resin would typically be used in a lightweight board combination where no exposure to increased humidity may occur.

WRA offers superior moisture protection and represents the middle ground between basic moisture resistance and waterproof applications. WRA probably has the widest range of applications due to the fact that most corrugated box plants only have the ability to make one type of "wet strength" adhesive.

Testing Standards and Performance

No TAPPI test method is recognized to evaluate MRA level of "wet strength." Generally, a one-hour soak test using a similar procedure to TAPPI method T-812 will be sufficient. If the board does not float apart, the box meets the minimum level of moisture resistance.

The typical test method for WRA starch is TAPPI T-812, commonly called the 24-Hour Soak Test. An additional test which is suitable for heavier test board (over 250-pound C Flute) is the wet pin adhesion test described in TAPPI T-821, with a generally accepted norm that the wet pin adhesion value should be at least 4 pounds per 2 lineal feet of glue line.

Manufacturing Requirements and Processing

Batch Solids Requirements

Fresh starch with at least 25% batch solids for MRA and 28% solids for WRA are recommended. The higher solids requirement for WRA reflects its more demanding formulation and processing needs.

Application Settings

MRA can be applied using standard domestic starch settings, making it easier to integrate into existing production lines. WRA adhesive formula can be used on lightweight boxes using normal or domestic starch settings. When running heavyweight boards, 250-pound test and above with greater end use demand, it is necessary to open the glue gap settings, at least an additional 0.004", especially at the single facer.

Curing Requirements

Both additives require proper curing for optimal performance. A minimum stack cure time of 4 hours is essential for optimum results with WRA. This extended curing time is critical for achieving the full moisture resistance benefits.

Applications and Use Cases

MRA Applications

• Lightweight corrugated boxes
• Indoor storage applications
• Minimal humidity exposure environments
• Cost-sensitive projects where basic moisture protection suffices
• Applications similar to domestic adhesive requirements

WRA Applications

• Medium to heavy-weight corrugated boxes
• Boxes that contend with a variety of moisture conditions including high humidity, tropical climates, and top ice packaging
• Agricultural and food packaging
• Extended storage periods
• Transportation through varying climate conditions
• Military specifications applications, with W5c and V3c corrugated meeting military specifications

Cost Considerations

MRA typically offers lower material costs due to its reduced resin content (0.5-1% vs 1-1.5% for WRA). However, the cost differential must be weighed against performance requirements and potential losses from moisture-related failures.

WRA requires higher initial investment in materials and may demand modified processing parameters, but provides superior protection that can prevent costly product damage and returns in moisture-prone environments.

Quality Control and Testing

Field Testing Methods

A simple test to tell the difference is to soak sample (flutes vertical) for 24 hours in 72 degree water. Untreated starch adhesive comes apart in 20 minutes but MRA should not come apart (liner off the flute tips) by more than 0.25" after 24 hours. With WRA levels of resin, there should be some tack remaining at the starch bond as you pull it apart.

Production Guidelines

Use freshly resinated adhesive, not more than 6 hours old. Apply the same glue line in light applications and approximately 50% more for 250-pound test and above. Allow the board to cure in its original stack for a minimum of 4 hours.

Making the Right Choice

The decision between WRA and MRA should be based on several factors:

Choose MRA when:

• Operating in controlled, low-humidity environments
• Processing lightweight boards with minimal moisture exposure risk
• Cost optimization is the primary concern
• Applications mirror standard domestic adhesive requirements

Choose WRA when:

• Processing medium to heavy-weight boards
• Expecting exposure to high humidity or moisture conditions
• Requiring compliance with military or specialized packaging standards
• Long-term storage or transportation through varying climates is expected
• Customer specifications demand enhanced moisture resistance

Environmental and Storage Considerations

Traditional corrugate starts with wood fibers, which provide a degree of moisture resistance. Water-Resistant Adhesive (WRA) enhances durability under high humidity and prolonged exposure to wet conditions. Both additives help maintain package integrity, but WRA provides significantly better protection in challenging environmental conditions.

Proper storage of materials and finished products remains crucial regardless of the additive chosen. Climate-controlled environments optimize the performance of both MRA and WRA formulations.

Quick Reference Comparison Chart

While both WRA and MRA serve the critical function of moisture resistance in corrugated packaging, they address different performance levels and applications. MRA provides cost-effective basic moisture protection for controlled environments, while WRA delivers robust performance for demanding applications. Understanding the specific requirements of your packaging application, environmental conditions, and performance standards will guide the optimal choice between these two important corrugated additives.

The investment in appropriate moisture resistance technology ultimately protects product integrity, reduces waste, and ensures customer satisfaction across the supply chain.