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.

Figure 1: Chemical reaction for a polyurea (top) and polyurethane (bottom)

This article provides an introduction to polyurea chemistry. It reviews the advantages and disadvantages of these interesting polymers in adhesive and sealant applications. Formulation principles are given to achieve adhesive and sealant products that have fast cure with certain properties that are superior to conventional polyurethane systems.

Polyurea Definition and Comparison to Polyurethanes

The Polyurea Development Association (PDA) is the official trade association for the polyurea industry. It is represented by three alliances: PDA Europe (Brussels, Belgium), PDA America (Kansas City, MO), and PDA China.

Polyurea is defined by the PDA as a material derived from the reaction product of an isocyanate component and a resin component. The isocyanate can be aromatic or aliphatic in nature. It can be a monomer, polymer, or any variant reaction of isocyanates, quasi-prepolymer, or a prepolymer. The prepolymer or quasi-prepolymer can be made of an amine terminated polymer resin or a hydroxy terminated polymer resin.

Polyurea elastomers have sometimes been characterized as a modified two-component polyurethane, but in essence they represent a separate and unique technology. Table 1 attempts to define the differences between a polyurea, polyurethane, and a polyurea/polyurethane hybrid.

The classic water / isocyanate reaction used with most moisture cured polyurethane adhesives and sealants also produce urea groups at the end of the process. However, this reaction should not be considered a polyurea reaction since the mechanism is a two-step process. It is a much slower reaction and produces carbon dioxide as a byproduct.

The properties of polyurea, polyurethane, and polyurea/polyurethane products can be made to have similar properties, since there is a great degree of freedom in formulation. However, the unique characteristics of polyurea systems are described in the next section.

The gel time of such polyurea compositions is typically on the order of a few minutes and it is consistent over a wide temperature range (-17°C to 60°C) since the polyurea polymer does not rely on a catalyst to complete the reaction. This unique "auto-catalytic nature is one of the most notable characteristics of polyurea products. Two component polyurea elastomeric coatings have been available for some time for construction coatings. Applications include tank linings, pipeline coatings, flooring and parking decks, bridge coatings, aquarium lining, road striping, oil platforms, potable water storage tanks, food processing facilities, and many others.

Adhesive and sealant applications are just now developing. Adhesion can be difficult especially for the fastest setting polyurea formulations. The fast set time does not allow sufficient wetting and penetration to occur to provide exceptionally high degrees of bond strength. However, set time can be managed with proper formulation. The high number of hydrogen atoms on the polyurea backbone (Figure 2) provides a significant opportunity for hydrogen bonding to occur.

Figure 2: Structure of the polyurea backbone structure. (SpecialChem Fig. Ref.: Polyurea backbone)

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Advantages and Disadvantages

Two component polyurea systems can be formulated to set in as little as 2-3 secs without the use of a catalyst (as is generally the case with polyurethanes). As a result, one of the first commercial applications for polyurea systems was in RIM applications in the automotive sector.

Furthermore, the cure characteristics are very uniform over a broad range of application temperature and relative humidity. Unlike polyurethanes, polyureas are unaffected by ambient moisture during application and cure. This makes polyureas ideal for the use in outdoor applications such as the building and construction sectors. Polyureas can cure at temperatures as low as -20°C.

One of the main advantages of modern two-component polyurea formulations is that the reactivity can be adjusted to allow it to cure at desired rates over a wide temperature range. This is important for adhesives and sealants as too fast a set time will result in inadequate penetration and wetting. Even with slower setting formulations, polyurea adhesives and sealants generally require the use of two component automatic metering and mixing equipment employing a static mixing tube. When they are used as sprayable coatings, polyurea systems typically do not contain any solvent of volatile organic components (VOCs).

Table 2 provides for a performance comparison of common polymers. The main disadvantages of polyurea in adhesive and sealants applications is that they are very fast and usually require automated metering and mixing equipment. The polyether backbone structure of the polyurea molecules is also not suited for environments containing strong oxidizing chemicals. This is especially true for the aromatic based polyurea products.

Compared to polyurethanes, polyureas can have superior physical properties such as high temperature performance, adhesion, oil and solvent resistance, and tear strength. Due to the very high reactivity, polyureas are known to impart a non-sagging characteristic once it is applied even though it is mixed in a low viscosity liquid state.

Polyurea sealants have been claimed to provide moisture resistance equivalent to most polyurethane sealants with superior UV and color stability. More importantly, they offer the option of low temperature cures with a wide latitude of gel times. Polyureas also offer low VOC and plasticizer free formulations making them an environmentally friendly alternative to certain other types of sealants.

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

Bayer's 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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