Polyurea adhesives and sealants offer fast-curing systems with very interesting properties. Polyurea chemistry has been well developed for high performance maintenance coatings. However, polyureas are not well known as adhesives or sealants except in applications that require exceptionally fast cure, such as high volume assembly of parts, construction sealants when inclement weather might adversely affect a slower curing product, or as adhesives and sealants for automobile manufacture.

Polyureas are similar to polyurethanes but they are based on reacting an isocyanate with a multifunctional amine rather than with a polyol (Figure 1). The polyurea chemical reaction occurs quickly without added catalysts and can provide good bonding and physical properties within a few minutes at room temperature (although fully optimized properties may require several hours). Polyureas tend to be more expensive than polyurethanes, but their fast cure rates are often worth the extra cost.

Applications

Polyurea is being successfully used as a multi-purpose joint fill, caulking, and sealant material. It can provide a flexible, formable, weather tight, and traffic resistant seal for all types of building joints. These include expansion joints and control joints in masonry floors, perimeter joints, panels and doors, water reservoirs, etc. The polyurea sealant has excellent crack bridging properties with high elongation and tensile strength. The fast cure time and impermeability to moisture allows for a quicker installation with a wider application window. Proper surface preparation and substrate pretreatment is always necessary.

Polyurea adhesives have been developed primarily for non-sagging products that can be conveniently manipulated in a low viscosity liquid form but then applied to vertical surfaces as a thick non-running material.

The auto industry provides a good example of the use of polyurea adhesives. During automotive manufacturing, structural adhesives can be subjected to temperatures of about 200°C. At these temperatures, many of the more common adhesives (e.g., polyurethanes) lose their integrity by foaming, cracking, and/or softening with a consequential loss in physical and adhesive properties. Polyureas can be mixed and applied via static mixers, cure quickly, and resist these adverse conditions.

Nolax (Switzerland) has also found that fast reacting polyurea adhesive bonds very well to soft and hard woods. They provide high green strength in seconds and full bond strength after 1-2 days of cure at room temperature. These adhesives are resistant to temperature of greater than 100°C and provide D4/C4 water resistance according to EN 204/205.²

Formulation Development

As with other adhesives and sealants, one should not consider the chemical class (e.g., "polyurea") a specific material. Rather, it is a description of a technology that represents a variety of formulation possibilities to achieve desired performance. This is done through the selection of various raw materials, much like polyurethane chemistry. The selection of the proper raw materials can be a very complex procedure, which is described elsewhere.³

As indicated in the earlier discussion, polyurea polymers are derived from the reaction product of an isocyanate component and a resin blend component. Similar to polyurethanes, the polyurea molecular backbone is composed of hard and soft segments. The soft segment of the polyurea polymer is formed from multi-functional, high molecular weight amine-terminated polyether polyols, whereas, lower molecular weight aromatic diamine chain extenders are responsible for the hard segment.

Much of the more development formulation for polyurea adhesives and sealants has focused on methods to moderate the fast cure reaction. This allows greater freedom in application processes (e.g., longer working life) as well as better wetting and penetration into porous substrates. Several processes have been developed to accomplish this task.

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The slower reaction kinetics of secondary, alkythiolated, and hindered primary aromatic diamine chain extenders, allow for a more controlled viscosity rise in the reaction mixture. This minimizes the initial viscosity of the reaction to permit more time for thorough mixing of the resin and isocyanate components before the mixture begins to gel. The slower reaction is not achieved by incorporation of hydroxyl-terminated resins, but through careful formulating of amine-terminated chemicals to achieve the slower rates.

By modifying the traditional diamine structure and increasing molecular weight through formation of an oligomer, reactivity can be modified to the point were adhesive applications become feasible while still maintaining the well-known superior properties of the polyurea system. This can be accomplished by what can be considered to be the condensation of p-amino-benzoic acid with a difunctional diol with the formation of an amine functional telomer. The reactivity of the p-amino group with isocyanates is decreased by the carboxyl functionality.⁴

Bayers aspartic esters technology uses Desophen NH1420 solvent-free amine functional resin in combination with Desmodur E-210 isocyanate to create a polyurea sealant that allows reactivity to be adjusted from approximately two to 30 minutes. The freedom to specify a fast or slow reactivity makes two-component polyurea a versatile solution for a wide spectrum of applications.

Polyurea sealants have also been formulated with specially engineered MDI-based isocyanates, which contribute to extending pot life and helping create polyurea polymers to meet specific performance requirements.

Polyurea properties can range over a significant degree depending on the formulation employed. For example, typical polyurea sealant properties range in hardness from 20-95 Shore A, 180-1600 psi tensile strength, 150-1000% elongation, and 100-180 pli tear strength. Table 3 shows the properties of two optimized polyurea formulations.

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