Words of cartonboard:
water absorption Wick and Cobb tests

The degree of water absorption by the cartonboard is measured through two different tests, Wick Test and Cobb Test

Among the key features of the cartonboard, in addition to weight, thickness and rigidity, we also find impermeability, a feature of great importance especially when the packaging is intended for the distribution of frozen and deep frozen foods.  

In fact, frozen and refrigerated foods require high performances from the cartonboard that contains them, especially due to their storage conditions. 

Storage temperatures: the crucial one for choosing the cartonboard  

Regarding frozen foods, the packaging must maintain its features even at a temperature of -20°C and must ensure that printing and painting do not crack at this temperature.  

For refrigerated foodstuffs, on the other hand, the storage temperature is between 0 and +3°C. At this temperature, the greatest risk is that linked to the humidity in the air which is very high and requires low water absorption by the cartonboard.  

The degree of water absorption by the cartonboard is measured through two different tests, one which evaluates the absorption of water along the edges, and the other the absorption of water into the surface. 

Wick test: water absorption along the edges 

For a wide range of applications in wet conditions, including frozen product packaging, a high degree of water resistance is required. Even though the packaging of these products is often internally laminated, the edges are usually not protected.  

The Wick test is performed to evaluate the water absorption along the edges.  

This process measures the capillary effect, which obviously decreases when the inner layers of the cartonboard are superglued (total gluing of all layers of the cartonboard).  

Since this test only affects the edges of the cartonboard, the internal sizing of the cartonboard and the type of fibers used are of the highest importance. 

Wick test procedure  

The sample to be examined is covered on both sides with a waterproof adhesive tape and cut to certain dimensions.  

The sample is weighed and dipped in water having a temperature of 80°C.  

Water can only be absorbed by the edges. After 20 minutes, the sample is taken out and weighed again. Weight gain is quoted in kg/m2. Wick test is used for the control of solid bleached board. 


Cobb test: water absorption into the surface  

The Cobb index measures the amount of water that is absorbed by the surface of the cartonboard during the Cobb test.  

Offset printing uses water. For this reason, cartonboard must be water-repellent to some extent. 

 The requirement of impermeability to water is, also in this case, particularly important for packages of frozen foods and refrigerated products.  

For coated surfaces, water absorption depends to a large extent on the composition of the coating. For uncoated surfaces, the determining factors are the internal gluing of the fibers and the composition of the glue used on the surface. 

The internal gluing of the fibers is extremely important, to slow down the absorption of water from the lower layers. 

Cobb test procedure 

The cartonboard sample is weighed and then a cylinder with a section of 1 dm2 is placed on the sample itself. 100 ml of water are poured into the cylinder and, after a minute, the cylinder is emptied and the excess water is dried from the surface of the cartonboard. Weight gain is quoted in g/m2 as a Cobb index in a set period of time of one minute.  

The test can be used for the outer surfaces and inner layers of a multilayer cartonboard. The inner layers are tested after delaminating the outer layers.  

For glued wood pulp board packaging, whose inner layers have undergone additional treatment to achieve high water resistance, the test duration is generally increased to 3 minutes. These containers are used for packaging frozen and refrigerated foods.

The test is carried out both on the inner layers and on the outer layers. 

Our most water resistant cartonboards 

Within the ProntoPack portfolio, we offer cartonboards that have successfully passed the Wick and Cobb tests and are suitable for freezing and refrigeration.  

Among these, we suggest you the Metsä Board Classic FBB, GC2 cartonboard with organoleptic properties and resistance to humidity that lead it to be widely used in the food sector.

Metsä Board Classic FBB is characterized by the lowest structural tolerances in its range of products and has, for all grammages, a water absorption index ≤40 gr/m2 from front and edge and from the back ≤50 gr/m2, all measured with a 3-minute Cobb test.