Understanding wet-strength paper
What are wet-strength papers and what do they do?
Papers are sometimes called ‘high wet-strength’ or ‘full-wet strength’ if the wet tensile value is 15% to 25% of the dry value, or even higher. One common use of high wet-strength papers is for beer bottle labels in countries such as Germany where the labels must be washed off in caustic solution without dissolving. The wet-strength design delays the labels from dissolving in the caustic solution. For this reason, these papers are sometimes called ‘caustic insoluble’ even though they would eventually dissolve.
‘Medium wet-strength’ or ‘partial wet-strength’ papers normally have wet tensile of about 10-15% and commonly have a minimum and maximum specification for wet-strength or caustic solubility. This is because some breweries, specifically in Canada, wash the labels from returned bottles but they require the labels to fully dissolve or pulp during the process. The labels cannot remain intact as they do in German breweries and many others around the world. These labels have enough wet-strength to resist damage during the labeling and packaging at the brewery and to resist damage in use by consumers, including ice chests, but they do not have enough wet-strength to withstand the German bottle washing process.
None of our papers are intended to be ‘waterproof’ or water resistant such that water cannot penetrate; water will penetrate our papers whether from the non-metallized side or the metallized side. Penetration is slower from the metallized side, however.