UNDERSTANDING WET STRENGTH: WHAT ARE MRA, WRA, AND WPA?

To determine the "wet strength" needs of a corrugated box plant, we must first define the level of "wet strength" needed. Generally, our industry has accepted three levels of "wet strength" adhesive.

1. Moisture Resistant (MRA) – used for the lightest possible applications.

2. Water Resistant (WRA) – medium level of performance.

3. Waterproof (WPA) – highest level of performance.

We will attempt to identify the key characteristics of each type of "wet strength" adhesive and when it would be used. We will also discuss proper test methods to determine the level of water resistance of a particular box.