What is water resistance?
Water resistance is one of the most widely-sought properties of a box coating. But it’s important to understand at the outset that we’re not necessarily talking about waterproof, the total elimination of water penetration over a nearly infinite period of time. Some coatings, such as Coating X300™, have such high levels of water resistance that they may seem to be waterproof, but they’re actually water resistant. Also, when we talk about water resistance, we’re talking only about H2O in a liquid state. Moisture resistance—water in a gaseous state—is another topic entirely.

Why is water resistance important?
Water causes boxes to get wet and soggy, resulting in the loss of stacking strength and protective capabilities. For many industries, water-resistant boxes are absolutely essential. Produce boxes, for example, need to be protected both inside and out from water—water from damp fields, water from wet product and water from shipping ice. Meat boxes mostly need protection from wetness within the box, although some exterior protection can be needed, too, in a wet meat packing environment. And don’t forget building products...they can get wet in transit to or at the job site, where they might be stored in the open for days or weeks.

How is water resistance measured?
Water resistance is measured by the Cobb Sizing Test (TAPPI T-441). This test subjects the coated paper (and, ideally, an uncoated control) to a specific amount of water for a defined but changeable period of time. Cobb tests generally are performed for 2-, 15- or 30-minute time periods. At the end of the period, the sample is weighed precisely and compared to its original weight. Any extra weight is the amount of water absorbed.

Michelman Water-Resistant Coatings
Listed below are those Michelman coatings which are commonly used or recommended for water-resistant applications. Many of these coatings have multiple properties and will appear on other cross references as well.