WRA vs. MRA Corrugated Additives

Corrugated packaging can lose performance when it is exposed to high humidity, condensation, refrigeration, water, or other wet conditions. One part of designing for those environments is the adhesive used to bond the corrugated medium to the linerboard.

Two terms buyers may encounter are MRA, or Moisture Resistant Adhesive, and WRA, or Water Resistant Adhesive. Both describe enhanced moisture resistance in corrugating adhesive, but they are intended for different levels of exposure.

The important distinction is that MRA and WRA describe the water resistance of the adhesive bond. They do not by themselves make corrugated board waterproof, and they do not prevent paper fibers from losing strength when exposed to moisture.

Why Moisture Matters to Corrugated Packaging

Corrugated board is primarily made from paper-based linerboard and corrugating medium. Changes in humidity and direct moisture exposure can affect the physical properties of those materials.

Moisture can affect:

• Compression performance.
• Rigidity.
• Stacking performance.
• Dimensional stability.
• The bond between the liner and fluted medium.

Standard starch-based corrugating adhesives work well in ordinary environments. When greater resistance to moisture is required, resin-enhanced adhesive formulations may be used to help the glue bond remain intact under wetter conditions.

What Is MRA?

MRA stands for Moisture Resistant Adhesive.

Within the commonly used corrugated wet-strength hierarchy, MRA represents the lighter level of enhanced moisture resistance.

MRA may be considered when a corrugated package needs more moisture resistance than a conventional adhesive provides but is not expected to encounter severe or prolonged water exposure.

Examples may include:

• Humid warehouse environments.
• Seasonal humidity changes.
• Short periods of elevated moisture exposure.
• Applications where condensation is possible but sustained wetting is not expected.
• Packaging specifications that call for a moisture-resistant adhesive bond.

The actual suitability of MRA depends on the board specification and expected exposure rather than simply whether a shipment is described as “indoor” or “lightweight.”

What Is WRA?

WRA stands for Water Resistant Adhesive.

WRA represents a higher level of water resistance than MRA and is intended for applications where the corrugated adhesive bond may encounter more demanding moisture conditions.

WRA may be considered for:

• High-humidity distribution environments.
• Refrigerated or cold-chain applications where condensation may occur.
• Agricultural or produce packaging.
• Packaging exposed to changing climates during transportation.
• Applications involving intermittent or more substantial water exposure.
• Customer specifications requiring greater adhesive water resistance.

WRA improves the resistance of the adhesive bond to water. It should not be interpreted as meaning the entire corrugated package is waterproof.

MRA vs. WRA: Quick Comparison

What About WPA?

WPA is commonly used to describe the highest level of water-protection performance within the MRA/WRA/WPA adhesive framework.

The three levels are generally described as:

• MRA: moisture-resistant performance for lighter exposure.
• WRA: medium-level water-resistant performance.
• WPA: the highest level of water-protection performance within this adhesive classification.

Even the term WPA should not be interpreted to mean that an untreated corrugated box becomes permanently waterproof. Corrugated performance still depends on the paper, adhesive, coatings or treatments, box design, exposure time, and environmental conditions.

Does WRA Make Corrugated Stronger?

Not in the same way that increasing ECT, changing board grade, or moving to a different wall construction changes the dry structural specification.

The purpose of WRA or MRA is primarily to help the adhesive bond retain integrity during moisture exposure.

That distinction matters because moisture can affect two different parts of corrugated performance:

• The paperboard itself can lose stiffness and compression performance as moisture increases.
• The adhesive bond between liner and medium can weaken or separate if the adhesive is not sufficiently water resistant.

A water-resistant adhesive addresses the second problem. It does not completely eliminate the first.

How Is Water Resistance Tested?

Corrugated water resistance can be evaluated with laboratory methods that examine whether the liner and medium remain bonded after exposure to water.

One relevant method is TAPPI T 812, which evaluates wet ply separation of solid and corrugated fiberboard. It is intended primarily to distinguish board made with weather-resistant adhesive from board made with nonweather-resistant adhesive.

Another method, TAPPI T 845, measures wet pin adhesion. In that test, corrugated board is immersed in water and the force required to separate the liner from the medium is measured.

The appropriate test and acceptance criteria should come from the package specification, customer requirement, or applicable technical standard rather than from an informal pass/fail rule applied to every corrugated box.

When Should You Consider MRA?

MRA may make sense when the package is expected to encounter some moisture or humidity beyond normal dry conditions but is not being designed for substantial water exposure.

Questions to ask include:

• Will the carton move between temperature-controlled and humid environments?
• Is condensation possible?
• Will it spend time in a humid warehouse?
• Does the customer specify moisture-resistant adhesive?
• How long will the exposure last?

MRA should be selected because it meets the exposure requirement—not simply because the board is lightweight or the application is indoors.

When Should You Consider WRA?

WRA is generally considered when the expected environment creates a greater risk to the adhesive bond.

Potential examples include:

• High-humidity distribution.
• Refrigerated products.
• Produce and agricultural applications.
• Condensation during temperature changes.
• Outdoor or partially exposed handling.
• Transportation through changing climates.
• Specifications that explicitly require water-resistant corrugating adhesive.

The exposure itself should drive the requirement. A heavier corrugated box does not automatically require WRA, and a lightweight box is not automatically limited to MRA.

Moisture-Resistant Adhesive Is Only Part of the Package

If moisture exposure is significant, adhesive selection should be considered along with the rest of the corrugated specification.

Other factors may include:

• Containerboard properties.
• Wet-strength paper where specified.
• Coatings or treatments.
• Board grade.
• Single-wall or multi-wall construction.
• Box design.
• Ventilation requirements.
• Product moisture or refrigeration.
• Storage time.
• Stacking requirements.
• Actual temperature and humidity conditions.

For example, maintaining the glue bond does not guarantee that a carton will retain its original dry compression strength after long exposure to high humidity.

What Should Buyers Put on a Corrugated Specification?

If moisture resistance is important, avoid relying on a vague instruction such as “make it water resistant.”

A stronger specification identifies:

• The expected environmental exposure.
• Required board construction and strength.
• Whether MRA, WRA, or another adhesive performance level is required.
• Any required wet-strength or water-resistance test.
• Acceptance criteria when applicable.
• Any coating or treatment requirement.
• Storage, refrigeration, or end-use conditions that affect performance.

If the requirement comes from a customer, regulatory program, military specification, or another formal standard, use the exact requirement rather than substituting a general WRA or MRA description.

MRA vs. WRA: Which Should You Choose?

Choose based on the level and duration of expected moisture exposure.

MRA may be appropriate when:

• The package needs enhanced resistance to humidity or limited moisture exposure.
• Severe or sustained wetting is not expected.
• The customer specification calls for a moisture-resistant adhesive level.

WRA may be appropriate when:

• The package may encounter more demanding moisture conditions.
• High humidity, condensation, refrigeration, or wet handling are realistic risks.
• A customer or technical specification requires water-resistant adhesive performance.

Neither should be chosen solely because one is cheaper, the board is heavier, or the shipment travels farther.

The Bottom Line

MRA and WRA are different levels of moisture resistance in corrugating adhesive.

MRA is generally used for lighter moisture-resistance requirements. WRA provides a higher level of resistance for more demanding wet or humid conditions.

Their job is to help preserve the bond between corrugated liner and medium when moisture is present. They do not make all corrugated board waterproof, and they do not eliminate the effect humidity can have on the paper itself.

For demanding applications, moisture resistance should be specified as part of the complete package design—including board grade, wall construction, coatings or treatments, product, storage, stacking, and distribution environment.

Need Help Specifying Corrugated for Humid or Wet Conditions?

Browse corrugated packaging or Contact Packaging HERO if you need help evaluating corrugated board, moisture-resistant adhesive requirements, wall construction, or other packaging specifications for your application.

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