With the development of modern science and technology, the range of paper applications has become increasingly broad. Some types of paper require the use of water or need to be processed in water, such as nautical charts, specialty map papers, banknote paper, advertising posters, medical cotton paper, tablecloth paper, industrial filter paper, photographic base paper, and tea filter paper.
For these specific types of paper, it is necessary to impart certain wet strength properties, meaning that wet strength agents must be added during the papermaking or post-processing stages to give the paper certain wet strength.
1. Key Concepts Related to Wet Strength
- Wet Strength: After the finished paper is saturated with water, it usually loses most of its strength. The remaining strength is called wet strength. Wet strength is often expressed as the wet-to-dry strength ratio, indicating the ratio of wet strength to dry strength, typically represented as a percentage. Wet strength can be categorized into initial wet strength and rewet wet strength. Initial wet strength refers to the strength of the paper after being wetted once on the paper machine. Rewet wet strength refers to the strength of the paper after it has been rewetted multiple times; usually, the term “wet strength” refers to rewet wet strength.
- Wet Strength Paper: Wet strength agents are typically defined as those that allow paper to retain at least 15% of its original dry strength after being thoroughly wetted. If the wet strength of paper is above 15% of its dry strength, it is considered wet strength paper. Wet strength paper can be divided into “temporary” wet strength paper and “permanent” wet strength paper based on how long the wet strength lasts.
2. Mechanisms of Wet Strength
The strength of paper depends on the strength of the fibers themselves, the strength of the bonding between fibers, and the arrangement and distribution of fibers in the paper. The most important factor is the bonding between fibers.
This bonding force primarily comes from hydrogen bonds. The ability of cellulose fibers to form hydrogen bonds is due to the hydroxyl groups present in the cellulose. The hydrogen bonding between fibers allows the fibers to stick together and provide strength to the paper without the use of adhesives.
The mechanism of wet strength enhancement by wet strength agents is generally considered to involve two mechanisms:
- “Protection” Mechanism: In this mechanism, wet strength agents create a crosslinked network structure around the fibers, preventing the fibers from swelling and absorbing water, thus protecting the existing hydrogen bonds between fibers. This mechanism is known as the “concentric cross-linking” mechanism.
- “Reinforcement” Mechanism: In this mechanism, wet strength agents form new, water-resistant bonds between fibers. This bonding structure, formed by crosslinking between the fibers and wet strength agents, persists even after the natural bonds are broken by water. These new, water-resistant bonds, such as covalent bonds and hydrogen bonds, help to increase the paper’s wet strength. This mechanism is also known as the “reinforced new bonds or interlocked bonding” mechanism.
When wet strength agents are added during papermaking, the pulp, which contains interlocked fibers and wet strength agent molecules, undergoes chemical reactions during drying, forming bonds between fibers and the additives. When the paper is wetted again, the wet strength agents stay between the fibers, preventing water from entering the fibers and damaging the bonding. This creates the wet strength.
3. Common Wet Strength Agents
In the early stages of wet strength paper production, water-resistant agents like paraffin and polyvinylidene chloride were added to create a protective water-resistant layer on the paper. However, this method did not significantly improve the paper’s wet strength.
In the 1930s, people discovered that adding certain water-soluble synthetic resins to the papermaking pulp, which then cured on the paper machine, could significantly improve the paper’s wet strength. Since then, the use of wet strength agents has developed rapidly.
Common types of wet strength agents include:
- Formaldehyde Resins: Such as urea-formaldehyde resins (UF resins) and melamine-formaldehyde resins (MF resins).
- Polyamide-Polyamine Epichlorohydrin Resin (PAE Resins): One of the most widely used wet strength agents today.
- Polyethyleneimine (PEI): Used for certain specialty papers due to its good wet strength properties.
- Dihydroxy Starch: An eco-friendly wet strength agent commonly used in premium paper production.
- Polyacrylamide: Sometimes used in specific conditions, particularly for specialty applications.
4. Selection and Application of Wet Strength Agents
When selecting wet strength agents, factors such as the final paper application, the price, and the properties of the wet strength agent must be considered. Wet strength agents typically need to have the following characteristics:
- They must be high polymers with sufficient mechanical strength to protect the bonding between fibers from swelling and damage.
- They must be cationic, capable of adsorbing onto negatively charged fibers, and able to retain quickly and effectively.
- They must be water-soluble or water-dispersible to ensure uniform distribution in the pulp.
- They must form a chemical network structure that is heat-curable, providing resistance to water swelling.
The use of wet strength agents is crucial for producing specialty papers, especially those made from recycled fibers or straw pulp. Wet strength agents effectively enhance the paper’s wet strength, making it suitable for various applications. As the use of wet strength agents continues to evolve, particularly with the development of more environmentally friendly agents, papermaking will focus more on sustainability and eco-friendly production processes.