A Complete Guide to Water-Based Polyurethane Adhesives

Water-based polyurethane adhesives are adhesives formulated using water-based polyurethane as the binder. Based on their appearance and particle size, water-based polyurethanes can be divided into three categories: polyurethane aqueous solutions (particle size < 0.001 μm, transparent appearance), polyurethane dispersions (particle size 0.001–0.1 μm, translucent appearance), and polyurethane emulsions (particle size > 0.1 μm, milky white appearance). However, in the relevant literature, the latter two categories are conventionally referred to collectively as polyurethane emulsions or polyurethane dispersions, and the distinction is not strictly enforced. In practical applications, polyurethane emulsions or dispersions are more common than aqueous solutions.
Compared to solvent-based polyurethane adhesives, water-based polyurethane adhesives offer advantages such as being solvent-free, odourless, and non-polluting.

Classification of Water-Based Polyurethane Adhesives

Classification by the Properties of Hydrophilic Groups

Depending on whether the polyurethane molecule contains ionic groups in its side chains or main chain and on the type of these groups, water-based polyurethane adhesives can be classified as anionic, cationic, or nonionic.

Classification by Product Appearance

Water-based polyurethane adhesives can be classified into polyurethane emulsions, polyurethane dispersions, and polyurethane aqueous solutions. Polyurethane emulsions and polyurethane dispersions are the most widely used in practice.

Classification by Application Form

Water-based polyurethane adhesives can be classified into two categories based on their application form: single-component and two-component.

Classification by Polyurethane Raw Materials

Based on the type of primary polyol oligomer, they can be classified into polyether-based, polyester-based, and polyolefin-based types. Based on the isocyanate raw material used in the polyurethane, they can be classified into aromatic isocyanate-based, aliphatic isocyanate-based, and alicyclic isocyanate-based types. They can be further subdivided based on specific raw materials, such as toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), and diphenylmethane diisocyanate (MDI).

Formulation Composition of Water-Based Polyurethane Adhesives

Common Oligomeric Polyols

Poly(tetrahydrofuran) CAS 25190-06-1

1,4-Butanediol CAS 110-63-4

1,6-Hexanediol CAS 629-11-8

Trimethylolpropane (TMP) CAS 77-99-6

Commonly Used Diisocyanate Monomers

Chain Extenders

Chain extenders are commonly used in the preparation of water-based polyurethane adhesives. There are various hydrophilic chain extenders that can introduce ionic groups; in addition to these speciality chain extenders, substances such as 1,4-butanediol, ethylene glycol, diethylene glycol monomer, hexanediol, ethylenediamine, and diethylenetriamine are also frequently used. Since amines are more reactive with isocyanates than water, diamine chain extenders can be mixed with water or converted into ketimines to allow the chain extension reaction to occur simultaneously with emulsification and dispersion.

Ionizing Agents

Ionising agents are key raw materials that impart good water dispersibility or self-emulsifying properties to water-based polyurethane adhesives, and are used to reduce or eliminate the need for added surfactants.