Two-component Urethane

Two-component urethane adhesives 7.1. Structural urethane adhesives... 

The two-component urethane structural adhesives are among the most difficult to characterize, simply because of the widely varying properties that are possible. These adhesives may be rigid plastics similar in modulus to standard epoxy adhesives, with glass transition temperatures of the cured adhesive being approximately 60°C. 

Two-component urethane adhesives are used to bond sheet molding compound (SMC) panels for automotive OEM and aftermarket applications. Two-part urethanes are used as laminating adhesives in the RV industry. 

With the exception of fillers, the raw materials used in two-component urethanes are all liquids. The two components have an isocyanate side and a polyol side. The raw materials are combined in various ways in order to produce with liquids that are combined at a 1 1 volume ratio, preferably, thus keeping the dispensing equipment as simple as possible. Fixed ratios of 3 2 or 1 2 are also commonly used. Ratios other than these are possible, but require the use of a... 

It would not be possible to discuss the infinite variations of two-component urethane adhesives in this writing. In order to demonstrate the extremes possible with two-component urethanes, two simplified adhesives are shown below with widely differing properties ... 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

The two-component waterborne urethanes are similar in nature to the one-component waterborne urethanes. In fact, many one-component PUD s may benefit from the addition of a crosslinker. The two-component urethanes may have higher levels of carboxylic acid salt stabilizer built into the backbone than is actually needed to stabilize the urethane in water. As a result, if these two-component urethane dispersions were to be used as one-component adhesives by themselves (without crosslinker), they would show very poor moisture resistance. When these two-component urethane dispersions are used in conjunction with the crosslinkers listed in Fig. 8, the crosslinkers will react with the carboxylic pendant groups built into the urethane, as previously shown in the one-component waterborne urethane section. This accomplishes two tasks at the same time (1) when the crosslinker reacts with the carboxylic acid salt, it eliminates much of the hydrophilicity associated with urethane dispersion, and (2) it crosslinks the dispersion, which imparts solvent and moisture resistance to the urethane adhesive (see phase V in Fig. 5). As a result of crosslinking, the physical properties may be modified. For example, the results may be an increase in tensile properties and a decrease in elongation. Depending upon the level of crosslinking, the dispersion may lose the ability to be repositionable. (Many of the one-component PUD s may... 

The term modified two-component urethanes refers to systems in which one or more of the reactants in a two-component system are temporarily blocked to prevent reaction. The modified two-component system does not have the long shelf life of a standard one-component blocked system. 

Thus far, the discussion has dealt with the versatility of urethane adhesives, the wide variety of substrates urethanes can bond, and the infinite variations of one-component and two-component urethanes. Some substrates have posed particular challenges for urethanes. Many adhesion problems have been discussed, but not all can be covered in this brief synopsis. However, two of the more common problems, with possible solutions, will be mentioned. 

Topcoats over primer are often applied by airless spray. Trends to higher standards of exterior durability have encouraged the use of methacrylated alkyds and two-component urethane finishes. 

In more recent years one- and two-component urethane coatings based on polyethers have made a significant penetration of the urethane coatings market. Due to low cost and wide latitude in physical properties ranging from very flexible to hard, tough and solvent-... 

Similarly, excellent two-component urethane coatings were obtained from the adduct of trimethylolpropane and 4-bis(isocya-natomethyl)cyclohexane (BDI), which was developed by Suntech (Sun Oil Co.) ( ). 

Adrub RTV Molding Rubber. [Adhesive Prods.] Two-component urethane used for molds, gaskets, sealants, caoDdng, embossing rollers. 

Calthane. [Cal Polymers] Two-component urethane dastomer for flexible and rigid castings, prosthetics, underwater sports equipment, optied lenses, art objects, mech. devices, flexible windshields, industrial tubing or sheeting, toys, adhesives. 

Permacor. [Arnco] Two-component urethane liq. urethane for repair of pneumatic tires. 

Superflex. [Amco] Two-component urethane polymer flaq>ioofing system for low-pmsure tires. 

The second major classification of common polyurethane adhesives is the two-component system. Two-component polyurethane adhesives are widely used where fast cure speeds are critical, as on OEM (original equipment manufacturers) assembly lines that require quick fixture of parts, especially at ambient or low bake temperatures. Two-component urethanes are required in laminating applications where no substrate moisture is available or where moisture cannot penetrate through to the adhesive bond. Two-component urethanes are also useful where CO2 (generated by a one-component moisture cure) or a volatile blocking agent would interfere with the adhesive properties. 

One- and two-component urethane systems are typically 100% solids with little or no VOC emissions. However, solvents can sometimes be used as carriers in non-structural versions, which can release VOCs at around < 25 g/L. The isocyanate content and the potential for off-gassing (if mixing off-ratio) must also be considered. 

Chem. Descrip. Acrylic polymer in naptha, toluene Uses Leveling agent, anticratering agent for solvent-based systems, thermoset high-solids coatings, two-component urethanes, food-contact coatings... 

Uses Polyol in two-component urethane and coil coatings, epoxy maintenance, general industrial, and leather coatings Feabffes Offers better pot life with isocyanate and melamine crosslinkers bestflow/leveling... 

Chem. Descrip. Bismuth carboxylate in 2-ethylhexanoic acid Uses Catalyst for blocked isocyanate and two-component urethane coatings, coil coatings, aerospace, automotive, urethane coatings for plastics... 

Chem. Descrip. Zirconium chelate in 2,4-pentanedione Uses Catalyst for two-component urethane coatings (automotive topcoats, high performance maintenance coatings, aerospace coatings, coatings for structural plastics)... 

Chem. Descrip. Sat., aliphatic, linear, hydroxyl-terminated polyester diol Uses Polyester for sol n. coatings (both one- and two-component), urethane thermoplastics, and prepolymer and one-shot castables Features PUs exhibit weatherability, aging, high strength and abrasion resistance, and high hardness... 

Uses Surfactant, slip agent, mar resist, aid for coatings, two-component urethanes, polyester-melamine, epoxies, alkyd/melamine, selfcrosslinking acrylics, etc. 

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