Polyethyleneimine (PEI) is a multifunctional fine chemical additive in the papermaking industry. Its core function lies in its high cationic density and high reactivity, effectively “capturing” anionic substances in the papermaking system and enhancing the bonding between fibers, thus playing a valuable role in several key processes. There is no universally accepted “optimal” molecular weight for polyethyleneimine (PEI); the key is to determine it based on the specific application scenario, the method of addition (intra-pulp addition or surface sizing), and the ultimate performance goals.

Ⅰ.Specifications and selection recommendations

Simply put, molecular weight determines the behavior of PEI: low molecular weight PEI has good permeability and excels at crosslinking; medium molecular weight PEI has excellent overall performance and is the most widely used; while high molecular weight PEI has stronger adsorption and flocculation capabilities.

Ⅱ.Molecular weight selection guide for different application scenarios

1. As a wet strength agent/reinforcing agent.

This is the core application of polyethyleneimine PEI. The molecular weight selection depends on the addition method and the final performance focus:

Surface sizing (pursuing ultimate strength): Recent studies show that molecular weights of 600 or 1,800 are most effective for surface treatment. Reinforcing agents modified with these molecular weights can significantly improve both dry and wet strength of paper, for example, dry strength can be increased by 227% and wet strength by 207%. If the goal is to maximize the paper’s hydrophobicity (water resistance), then a molecular weight of 1,800 would be a better choice.

Intra-pulp addition (pursuing comprehensive performance): The situation is different when PEI is added directly to the pulp. Studies have found that polyethyleneimine PEI with a molecular weight of 10,000 performs best in this stage, most effectively building a cross-linked network between fibers, thereby achieving the optimal reinforcing effect.

2. As a retention/filtration/stabilizing agent.

PEI plays a role in this field by “capturing” fine particles and interfering substances in the slurry.

In industrial practice, high molecular weight PEIs with a molecular weight of 70,000 (such as P-1050 and UN-1015) are the mainstream choice. Their extremely high cationic density can efficiently neutralize and flocculate anionic waste and fillers, thereby achieving excellent retention rates, white water purification effects, and resin control.

3. As a processing aid (e.g., anchoring coating agent)

When used as an adhesion promoter in paper-plastic (e.g., PE) lamination:

Medium-to-high molecular weight polyethyleneimine PEI, typically represented by a molecular weight of 70,000, provides just the right amount of film-forming properties and reactivity, ensuring the coating is firmly anchored to the substrate.

Unilong’s 75W molecular weight polymer can be used as an anchoring coating agent. When used as a base coat in the extrusion lamination of paper with polyethylene (PE), polypropylene (PP), or polyethylene terephthalate (PET) film, it significantly enhances the adhesion between paper and plastic. In the preparation of high-barrier packaging materials, it can be used as a gas-barrier anchoring coating agent to enhance the adhesion of barrier layers such as polyvinyl alcohol (PVA) or ethylene-vinyl alcohol copolymer (EVOH) to the substrate (such as paper or olefin films).

4. Special modification applications (e.g., preparation of novel reinforcing agents)

If polyethyleneimine needs to be used as a raw material to react with other chemicals (e.g., silane coupling agent ) to prepare more advanced functional additives:

The choice of molecular weight needs to be determined based on the performance and stability of the final product. For example, 600 and 1,800 are preferred for preparing novel reinforcing agents for surface sizing; while 10,000 is a better solution for preparing reinforcing agents for in-sizing.

Ⅲ.Unilong’s customized recommendations

Are you primarily focused on its application as a wet strength agent, or would you like to understand its effectiveness as a retention/filtration aid in waste paper treatment? If you could share more specific application details (such as the type of paper it’s used for and the current process challenges), I might be able to provide you with more targeted recommendations.