Corrugated board is one of the most popular packaging materials in Germany - its market share is currently over 67 percent. No wonder, because corrugated board is light and at the same time stable due to its special lightweight construction. But what about the moisture resistance of corrugated board and how can this property be further improved? We offer you an overview.

For which industries and products does wetness resistance play a role?

Hardly any company leaves its products out in the rain - so why should packaging be made of moisture-resistant materials? The reason here is usually air humidity, which either intentionally or unintentionally affects the products. The following industries are strongly affected:

• Fresh and chilled products: To keep frozen and refrigerated food fresh, is a high humidity ideal. In deep-freeze stores, experts therefore recommend increasing the humidity to up to 95 percent.

  Fruit and vegetables stay fresh for a particularly long time at two degrees Celsius and a relative humidity of 80 percent. The problem: Cardboard boxes and packaging tolerate the high humidity less well because they absorb moisture from the environment like a sponge.

• Longer storage times: Products that can be stored well (for example, plastic or glass) can also be kept for longer periods in non-climate-controlled warehouses. This practice is sometimes necessary when seasonal goods need to be stored until next year. Difficult: Cartons are less able to withstand a storage period in the wet and cold season than some plastic products, unless they are made of moisture-resistant corrugated board.

• export industry or overseas transport: If goods are shipped by shipping container, it is essential to ensure that the packaging is resistant to water. Although the container protects the cargo against rain and splashing water, it does not protect it from the extreme climatic conditions that prevail at sea.

If there are high temperature differences between day and night, condensation can form in the container and drip from the ceiling onto the cargo. If corrugated board cartons are not resistant to moisture here, damp cartons can be dented by the weight of the stacks and, in the worst case, tear open.

Our blog offers you even more information about resilient, resistant and sustainable packaging!

Cardboard as hygroscopic material

Chemical pulp, the basic material of corrugated board, has hygroscopic properties. This means that the fibers absorb moisture from the ambient air or release moisture into the ambient air until an equilibrium is reached.

In more humid environments, cardboard also retains more water and loses stiffness.

You are probably familiar with this phenomenon from everyday experiments: a damp or wet sheet of paper can no longer be folded into stable constructions and is much easier to tear than a dry sheet.

Under what conditions do you test the properties of corrugated board?

Paper, cardboard and paperboard are tested under standard climatic conditions so that the results are comparable. Specifically, a temperature of 23 degrees Celsius and a humidity of 50 percent must prevail in the environment.

These conditions apply internationally - even in tropical regions. While cardboard tolerates higher temperatures without problems, deviations in humidity quickly create problems in processing. The paperboard begins to stretch, curl and form warps.

Is the wetness resistance dependent on the flute?

In general, the stability and load capacity of corrugated board depend on the flute, more precisely: on the number of flutes.

What are the types of corrugated board?

To produce corrugated board, the industry takes advantage of the hygroscopic properties of paper. A smooth paper web is exposed to moisture and high temperatures to form it into a corrugated web over fluted rolls.

This produces waves from 0.3 mm in height and 1.3 mm in length (graphic wave) to 5 mm in height and 10 mm in length (emperor wave). Incidentally, experts refer to the length of a flute as the "wave pitch". In combination with a smooth top layer, various types of corrugated board are then produced:

1. Simplex (single-ply corrugated board): Here the smooth inner and outer covers enclose a corrugated sheet.
2. Duplex (two-ply corrugated board): A smooth outer and inner cover surround two corrugated sheets separated by a smooth intermediate cover. The corrugated sheets can be of the same corrugation type in terms of corrugation height and corrugation pitch or two different corrugation types, for example fine and medium corrugation.
3. Triplex (three-ply corrugated board): Here, the smooth outer and inner covers enclose three corrugated sheets, which in turn are separated by two smooth intermediate covers. Combinations of different corrugation types are also common here, for example, coarse corrugation - medium corrugation - coarse corrugation.

Which type of corrugated board is particularly stable?

In addition to the glue and paper used, the construction method determines the stability and wet strength of a corrugated board. Generally speaking:

The higher the flute, the more weight the board can carry. The more corrugated layers have been processed, the higher the surface load can increase. A single-ply corrugated board with 1.5 mm microflute can withstand a surface load of 10 kg, while a two-ply board made of fine flute (3 mm) and coarse flute (4 mm) can withstand a surface load of up to 40 kg.

Particularly stable are three-layer corrugated cardboard, which is why they are used in overseas transport and even for the construction of furniture.

How to measure the stability of corrugated board?

Three properties are decisive here:

1. The burst strength

   For this purpose, corrugated board is clamped in a circular holder and subjected to a uniformly increasing pressure. The amount of force required to cause the cardboard to burst is specified in kilopascals (kPa).

2. The piercing work

   In this test, a standardized pyramid-shaped body pierces a clamped layer of corrugated board. The amount of force required to pierce is measured in joules (J).

3. The edge crush resistance

   This unit indicates how much force it takes to press in a layer of corrugated board when the force hits the cardboard layer at a 90-degree angle. The specification in kilonewtons per meter (Kn/m) reveals how well packages made of the corrugated board in question can be stacked on top of each other.

Wet strength corrugated board - What is it and what is the use?

Rain, high humidity, condensation in the shipping container - during transport, corrugated board boxes are often exposed to extreme environmental conditions. To ensure that they retain their stability, do not deform or even separate into their individual layers, they should ideally be made of wet-strength corrugated board.

Wet strength - what does it mean?

A corrugated board is wet-strengthened if , due to wet-strengthened gluing of the layers and/or the use of wet-strengthened papers, the board does not disintegrate even after 24 hours of soaking.

You want to know more about the wet strength and resilience of our products? Just contact us, we are looking forward to your inquiry.

How to make cardboard wet-strengthened?

In order to protect corrugated board against the influence of moisture, the paper industry has two adjusting screws at its disposal:

1. The gluing

   Smooth top webs and corrugated webs are glued in the production of corrugated board with starch glue based on wheat, corn or potato starch. In this process, the glue is applied to the corrugated web and glued under the action of heat, first to the inner cover and then to the outer cover.

   The glue is given waterproof properties by the addition of additives (resins and alkaline agents). Thus, the individual paper webs of the corrugated board do not separate from each other even in humid environmental conditions.

2. The paper

   The more resilient the paper grade from which the individual layers of corrugated board are made, the more resilient and wet-strength the packaging will ultimately be.

   In general, corrugated webs are always made from recycled paper, which, with its shorter pulp fibers, is less tear-resistant and durable than virgin fiber paper.

Wet-strength papers are not easy to recycle because they cannot be dissolved when water is added. To ensure that they can also be recycled as waste paper, they should only be added to the recycling process in small fractions and should not be excessively contaminated.

For the cover layers, on the other hand, it depends on the quality grade: They consist of either Schrenz (recycled waste paper), testliner (higher-quality recycled paper) or kraftliner (paper made from at least 80 percent virgin fibers).

If the inner and outer covers are made of kraftliner, the highest stability is achieved. In addition, paper can be used to whose pulp fibers so-called wet strength agents (special resins) have been added during production.

Possibility of finishing with foils and others

In addition to wet-strength gluing and the processing of wet-strength papers, there is another option for making corrugated board more resistant to moisture: Coatings with polyethylene (PE). These are either on the outer cover or in the intermediate layers of the board.

On the outer lid, the coating not only has the advantage of repelling water, but also makes it easier to print on the packaging. However, plastic coatings on paper packaging have a distinct disadvantage: they make smooth recycling virtually impossible. In the interests of a sustainable circular economy, producers and retailers should prefer packaging made of wet-strength glued corrugated board, which can be easily recycled under certain conditions.

Did you know?

Corrugated board can be finished with coatings. 

Corrugated board doesn't always have to have the natural brown look - there are many variants with a top layer of high-quality white paper. They are particularly suitable for colored printed primary packaging (single wall) or for high-quality designed shipping packaging (double wall).

Conclusion: Even without the rating "wet-strength", corrugated board is one of the most stable packaging solutions

From fine graphic corrugated board to heavy-duty cardboard for overseas transport and furniture construction - probably no other material in the packaging industry shows itself to be so versatile. It's no wonder that corrugated board, at over 67 percent, is the most chosen packaging solution of the present day.

For more stability, you should choose a multi-layered cardboard, depending on your needs. If the packaging is exposed to high humidity and condensation, stable paper grades and a wet-strength bonding are essential. Important for a sustainable recycling economy: Combined materials of corrugated board and film prove to be unnecessary in practice and make recycling more difficult.