{"id":14579,"date":"2026-08-24T05:29:24","date_gmt":"2026-08-24T05:29:24","guid":{"rendered":"https:\/\/bonitopak.com\/?p=14579"},"modified":"2026-08-24T05:29:26","modified_gmt":"2026-08-24T05:29:26","slug":"can-carrier-wet-strength-what-happens-in-ice-and-humidity","status":"publish","type":"post","link":"https:\/\/bonitopak.com\/fr\/can-carrier-wet-strength-what-happens-in-ice-and-humidity\/","title":{"rendered":"R\u00e9sistance \u00e0 l'humidit\u00e9 du support : que se passe-t-il en cas de gel et d'humidit\u00e9 ?"},"content":{"rendered":"

Par Leo Chan<\/strong>\u00a0 | R\u00e9dacteur, BonitoPak | Plus de 20 ans d'exp\u00e9rience dans le domaine des emballages en p\u00e2te moul\u00e9e<\/p>\n\n\n\n

Published: 24 August 2026 \u00a0 | \u00a0 Last reviewed: 24 August 2026<\/em><\/p>\n\n\n\n

Fiber loses strength when wet because water breaks the hydrogen bonds holding fibers together. Two separate chemistries address this: sizing slows water getting in, and wet strength agents create bonds that survive wetting. Most specifications only ask about one of them.<\/p>\n\n\n\n

Can carrier wet strength is the first objection anyone raises about a fiber carrier, and it deserves a technical answer rather than reassurance. Cans come out of refrigeration, they meet warm humid air, condensation forms, and the pack has to keep holding several kilograms.<\/p>\n\n\n\n

This article explains why fiber weakens when wet, the two different ways that gets addressed, what the published numbers actually show, and what to ask a supplier.<\/p>\n\n\n\n

Do Paper Can Carriers Get Soggy? The Mechanism<\/strong><\/h2>\n\n\n\n

The mechanism is specific and worth understanding, because it explains why some fixes work and others do not.<\/p>\n\n\n\n

In dry paper, hydrogen bonding between fibers is the dominant adhesive force, and it operates over very short distances. Those bonds are what hold a molded part together. They are also water sensitive.<\/p>\n\n\n\n

When water reaches the fiber network, water molecules form hydrogen bonds with the hydroxyl groups on the cellulose. That competition cleaves the bonds between fibers, and the structure loses strength rapidly. The fibers themselves are largely undamaged. What fails is the bonding between them.<\/p>\n\n\n\n

Swelling compounds it. When fibers take up water they expand, and pore volume within the sheet can increase substantially, opening larger passages for more water to flow through. The process accelerates itself.<\/p>\n\n\n\n

Two Different Fixes That Get Confused<\/strong><\/h2>\n\n\n\n

This is the distinction that matters most, and buyers regularly ask about one while meaning the other.<\/p>\n\n\n\n

<\/th>Sizing<\/strong><\/th>Wet strength agent<\/strong><\/th><\/tr><\/thead>
Typical chemistry<\/td>Alkyl ketene dimer, rosin<\/td>Polyamide-epichlorohydrin and similar<\/td><\/tr>
What it does<\/td>Makes fiber water repellent<\/td>Creates water resistant bonds<\/td><\/tr>
Effect<\/td>Water penetrates more slowly<\/td>Structure survives when water arrives<\/td><\/tr>
Analogy<\/td>A raincoat<\/td>Reinforcement inside the material<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Wet strength agents work through two routes. Protection means building a rigid cross linked network around the fibers that stops them swelling. Reinforcement means forming new, water insensitive covalent bonds that hold when hydrogen bonds fail.<\/p>\n\n\n\n

There is also a distinction between temporary and permanent agents. Temporary chemistries form bonds that break again on rewetting. Permanent ones form covalent bonds that do not. For a carrier facing repeated condensation cycles rather than a single exposure, that difference is the whole question.<\/p>\n\n\n\n

What Wet Strength Actually Means as a Number<\/strong><\/h2>\n\n\n\n

The industry definition is more modest than the phrase suggests.<\/p>\n\n\n\n

A material is generally considered to have wet strength if the ratio of wet to dry tensile strength exceeds 0.1. In other words, retaining more than ten percent of dry strength when wet qualifies. That is a low bar, and it is worth knowing when a supplier describes a part as having wet strength without giving a figure.<\/p>\n\n\n\n

The useful question is not whether a carrier has wet strength but how much it retains, measured how, and after how long.<\/p>\n\n\n\n

Molded Fiber Water Resistance: The Sizing Numbers<\/strong><\/h2>\n\n\n\n

Published research on internal sizing of recycled fiber gives concrete figures rather than adjectives.<\/p>\n\n\n\n

Propri\u00e9t\u00e9<\/strong><\/th>Unsized<\/strong><\/th>With approximately 1% AKD<\/strong><\/th><\/tr><\/thead>
Water take up<\/td>174 g\/m2<\/td>About 24 g\/m2<\/td><\/tr>
Water contact angle<\/td>35.2 degrees<\/td>109.1 degrees<\/td><\/tr>
Dry tensile index<\/td>Baseline<\/td>Increased slightly<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Water uptake falling to roughly one seventh of the untreated figure, with contact angle moving from wetting to strongly repellent, is a large effect from around one percent addition. The contact angle crossing 90 degrees is the point where water beads rather than spreading.<\/p>\n\n\n\n

A useful secondary finding is that sizing did not damage dry strength. Comparative work found alkyl ketene dimer sizing had no significant effect on dry strength, matching practical papermaking experience, although rosin sizing under the same test conditions did reduce dry strength. The choice of sizing chemistry is not neutral.<\/p>\n\n\n\n

The Numbers on Molded Pulp Specifically<\/strong><\/h2>\n\n\n\n

Work on pulp molded products made from bleached bagasse, published in ACS Omega<\/a>, is closer to a carrier than handsheet testing is. It found wet strength rising sharply to around 3 kN per metre at a wet end addition of roughly 0.2 percent polyamide-epichlorohydrin, then largely levelling off. Contact angle reached 114 degrees with water proofing agents present.<\/p>\n\n\n\n

The levelling off matters commercially. Beyond a modest dosage, adding more of the agent stops buying meaningful additional strength, so a supplier proposing heavy treatment is adding cost and downstream problems without proportionate benefit.<\/p>\n\n\n\n

Can Carrier Wet Strength Improves When Both Are Used<\/strong><\/h2>\n\n\n\n

The most useful finding for carrier design is that sizing and wet strength chemistry combine better than either alone. Research at VTT published in BioResources<\/a> found that internal sizing increased the wet strength of already wet strengthened paper, with the effect lasting through soaking for up to a month.<\/p>\n\n\n\n

The striking part is the ratio. Low and moderate wet strength agent additions combined with sizing produced higher wet strength than the highest agent addition used alone. The combination outperformed the heavier single treatment. The result was confirmed at pilot machine scale rather than only in handsheets.<\/p>\n\n\n\n

The practical reading is that a carrier specified with moderate wet strength chemistry plus proper sizing should outperform one loaded with wet strength agent alone, at lower dosage and lower cost.<\/p>\n\n\n\n

The Honest Downsides<\/strong><\/h2>\n\n\n\n

Molded fiber water resistance is not free of consequences, and a supplier who does not raise them is not being complete.<\/p>\n\n\n\n