Covestro Bayhydur 304 water-based PU curing agent


Based on Hydrophilic modified aliphatic polyisocyanate of hexamethylene diisocyanateHDI span>

Supply form

None Solvent type


Use As a curing agent or base material for high-quality water-based polymer dispersions; it can also be used as an additive to improve the performance of water-based coatings and adhesives

Product Specifications



Measurement units

Test method


18.2 ±0.5


DIN EN ISO 11 909



DIN 53 217/3



DIN EN 22 719

*The above values ​​are general information and are not part of product specifications.

Bayhydur 304 generally has good compatibility with the following solvents: esters, ether esters, xylene and solvent naphtha 100. However, the solubility/compatibility of the prepared solution must be tested. Only use polyurethane grade solvents (water content <0.05%).

Due to the hydrophilic nature of Bayhydur 304, it is easily emulsified in aqueous media. Mainly used as a cross-linking agent for water-based hydroxyl polyols (Bayhydrol®), and can also be used as a cross-linking agent for most water-based polymer dispersions with neutral pH values, including polyurethane dispersions (Dispercoll® U), polyacetic acid Vinyl ester, polyacrylate and synthetic rubber dispersions. After adding Bayhydur 304, the prepared two-component system must be used up within the pot life. The pot life not only depends on the polymer content, but also is affected by other components such as resins, thickeners, plasticizers, etc. In many dispersions, the pot life is usually several hours, but the end of the pot life cannot be judged by the gel. In polymer dispersions with a content of 40 – 60%, it is recommended to add 3-10% of Bayhydur 304 as a cross-linker with the adhesive dispersion. In order to emulsify Bayhydur 304 in adhesive dispersions, effective mechanical stirring is required. Manual stirring is not sufficient to ensure uniform emulsification of the system, especially in low-viscosity adhesive dispersions. Insufficient dispersion can lead to particle deposition in the adhesive.
If Bayhydur 304 needs to add modification additives, the additives must not contain groups that react with isocyanate, nor can they contain secondary components that can react with isocyanate. It is particularly noteworthy that any additives added must not be alkaline or contain secondary components that have a catalytic effect on isocyanate groups. Even low concentrations of these components can cause viscosity to rise and even solidify.

This product is sensitive to moisture and should be stored in the original sealed container at a temperature not exceeding 30°C. When stored under appropriate conditions, this product has a shelf life of at least 6 months. During storage, viscosity will increase slightly.

Safety: Hazard Identification
This product contains isocyanate, which may cause allergies in contact with skin. The safety data sheet should be read carefully. It includes information on labeling, transportation and storage, as well as information on product use, product safety and ecology.