Molecular Attraction
Primary mechanism of strength development in paper and board where hydroxyl groups on adjacent cellulose fibres form weak chemical bonds as the web dries. This inter-fibre hydrogen bonding creates a cohesive network that gives the sheet its tensile strength and structural integrity. Bond density varies.
The number of these attractions depends on the surface area of the fibres and the degree of refining they have undergone.
Strength Development
Mechanical treatment of the pulp in a refiner increases the amount of fibrils available to interact on a molecular level. Increased inter-fibre hydrogen bonding leads to a denser sheet with higher burst strength and internal bond strength. If the refining is too aggressive, the fibres may become too short, which reduces the overall tearing resistance of the final product.
Surface sizing agents also interact with these bonds to modify the absorption properties of the paper without disrupting the core strength.
Moisture Sensitivity
Presence of water molecules can easily disrupt these attractions because the water forms its own bonds with the cellulose hydroxyl groups. When paper becomes wet, the inter-fibre hydrogen bonding is weakened, causing the sheet to lose its stiffness and strength. This phenomenon is why uncoated paper is not naturally resistant to humidity and requires coatings or internal sizing for moisture sensitive applications.
Finished sheets regain strength only after complete dehydration.
