Moisture-Cure Urethanes

Because of their curing mechanism, moisture-cure urethanes are tolerant of damp surfaces. Too much moisture on the substrate surface is, of course, detrimental, because isocyanate reacts more easily with water rather than with reactive hydrogen on the substrate surface, leading to adhesion problems. Another factor that limits how much water can be tolerated on the substrate surface is carbon dioxide (CO2). CO2 is a product of isocyanate s reaction with water. Too rapid CO2 production can lead to bubbling, pinholes, or voids in the coating [9]. 

Pigmenting moisture-cure polyurethanes is not easy because, like all additives, pigments must be free from moisture [9]. The color range is therefore somewhat limited compared with the color range of other types of coatings. 

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OxazoHdines, formed by reaction of alkanolamines with aldehydes, ate useful as leather tanning agents and ate effective curing agents for proteins, phenoHc resins, moisture-cure urethanes, etc. They also find use as antimicrobial agents. 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

EVA-based Rubber-based Moisture-cure urethane... 

To improve on penetration and heat resistance (properties normally in opposition when formulating hot melts), reactive hot melt systems are being developed. Moisture curing urethanes have already gained wide acceptance in a number of wood and automotive applications. UV cure is slowly gaining acceptance in PSA, labeling, and other markets. 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

The basis of most one-component, moisture-curing urethanes is the synthesis of an isocyanate-terminated prepolymer, as shown ... 

Catalysts serve a dual purpose in one-component moisture-curing urethanes. The first purpose is to accelerate the prepolymer synthesis. The second purpose is to catalyze the curing reaction of the adhesive with moisture. The most common catalysts used to promote both prepolymer formation (NCO/OH) and later the adhesive curing reaction (NCO/H2O) are dibutyltin dilaurate and DMDEE ((tertiary amine. A stabilizer such as 2,5-pentanedione is sometimes added when tin is used, but this specific stabilizer has fallen from favor in recent years, due to toxicity concerns. DMDEE is commonly used in many one-component moisture-curing urethanes. DMDEE is one of the few tertiary amines with a low alkalinity and a low vapor pressure. The latter... 

Two-pack epoxies Coal tar epoxies Two-pack urethanes Moisture-cured urethanes... 

Moisture-cured urethanes These are the only paints in this class of materials that are single pack. The curing is provided by moisture from the atmosphere. This has the advantage that the material can tolerate a degree of dampness in the atmosphere and on the surface to be coated. Its disadvantage is that the film thickness per coat is limited and that, once opened, the entire container must be used immediately. Conversely, if the humidity is very low the cure may be lengthened or stopped. 

Seals Seals are normally transparent, quick-drying, highly penetrating materials that are used to stop cement dusting and contamination by oils, etc. The moisture-cured urethanes are particularly suitable, since they penetrate to produce a hard-wearing surface. However, if safety factors preclude their use there are also epoxy and even oleo-resinous products available. 

Science and Technology The functional group idea has the highest score because the technology is well developed. Information regarding the use of functional group such as moisture-cured urethane is readily available. 

Environmental and safety Water scavenger such as zeolite is non-toxic. On the contrary, other ideas cause harmful effects to human and the environment. For example, the use of functional group, such as moisture-cured urethane, involves diisocyanate. 

Demand for n-butanol as n-butyl acetate accounts for about 13 percent of domestic consumption of the alcohol. This acetate ester is a general purpose coatings solvent with particular application in nitrocellulose lacquers and moisture cured urethane coatings formulations. n-Butyl acetate is the traditional medium boiling solvent for nitrocellulose lacquers and, as such, is the standard by which other solvents are compared. 

Organic zinc-rich primers are based on epoxy/polyamides, high-molecular-weight linear epoxies, moisture-cured urethanes, high-styrene resins, chlorinated rubbers and epoxy esters. In this type of primer zinc is covered with a monomolecular layer of the binder which facilitates the adhesion of the film to the substrate. 

Polyurethane 541-D-101 Valspar Moisture-cured urethane intermediate coat... 

One-Package Moisture-Cure Urethane Coatings (ASTM Type 2) Single-Package Blocked Adduct Urethane Coatings (ASTM Type 3)... 

One-Package Moisture-Cure Urethane Coatings (ASTM Type 2). One-package moisture-cure urethane coatings consist of isocyanate-terminated polymers cured by atmospheric moisture. The rate of cure depends upon the humidity in the air and the presence of certain tertiary amine catalysts that accelerate the N(X)-water reaction. A major difficulty with this type of coating system is pigmentation. 

A relative new type of moisture-cure urethane coating consists in the formation of terminal oxazolidine group containing systems... 

The presence of water will lead to the formation of ureas and evolve CO2 as shown in Fig. 5. This mechanism is thought to proceed through the formation of an unstable intermediate, carbamic acid, which then decomposes to give CO2 and an aromatic amine. The amine will then react further with another isocyanate to give a urea linkage. All common moisture-cured urethanes give off CO2 upon curing, which can pose problems... 

Chem. Descrip. Propylene glycol-based diol Uses Polyol in moisture-cured urethane coatings and filled elastomers Properties R-Co 50 max. clear liq. hyd. no. 4 500 0.5% max. water Storage Store under diy, inert gas blanket such as nitrogen to minimize contamination from contact with air and water Jeffol PPG-400 [Huntsman]... 

The most common textile adhesives are available as solutions in water or solvent, as dispersions in water, or as solids that melt under the appUcation of heat. However, 100% reactive liquid adhesives such as epoxies and moisture-cured urethanes are also used. 

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