Chemical-Cure Urethanes

Chemical-cure urethanes are two-component coatings, with a limited pot life after mixing. The reactants in chemical-cure urethanes are  

A substance bearing free or latent active hydrogen-containing groups (i.e., hydroxyl or amino groups) [8] 

The first reactant acts as the curing agent. Five major monomeric diisocyanates are commercially available [10]  

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They are formed by isocyanate (R-N=C=0) reactions, typically with hydroxyl groups, amines, or water. Some typical reactions are shown in Figure 2.6. Polyurethanes are classified into two types, depending on their curing mechanisms moisture-cure urethanes and chemical-cure urethanes [1]. These are described in more detail in subsequent sections. Both moisture-cure and chemical-cure polyurethanes can be made from either aliphatic or aromatic isocyanates. 

An interesting variation of urethane technology is that of the blocked polyisocyanates. These are used when chemical-cure urethane chemistry is desired but, for technical or economical reasons, a two-pack coating is not an option. Heat is needed for deblocking the isocyanate, so these coatings are suitable for use in workshops... 

Uniroyal Chemical, MDI-Prepolymer Systems - Viable Alternatives to MOCA Cured Urethanes. April 18, 1978. 

The elastomeric type sealants are chemically curing systems based on a limited variety of polymers. The best known of these being polysulfide, silicone (see Silicones structures), Polyurethane and the modified silicones, which are urethane prepolymers with reactive silane end groups. All these are available as single- or two-part systems. 

Two-part sealants, such as the chemically curing two-part polysulfide, silicone, epoxy or urethane resilient sealants, are applied on-site by mixing two parts the polymer base part and the catalyst together and within the pot-life of hardening, which is usually one hour, the sealant is obtained. Chemically curing thermosets have much greater service lives than the others, and they usually need adhesion additives in order to achieve a proper bond to a surface. 

The effects of solvent exposure on the viscoelastic properties of several vinyl ester resins (phenohc-novolac epoxy, propoxylated bisphenol-A fu-marate, urethane and bisphenol-A epoxy based) and various unsaturated polyester resins (terephthalic or isophthaUc acid with a standard glycol based) containing 10wt% glass fiber were studied [130]. The results of dynamic mechanical analysis showed that the influence of exposure time to the solvent as well as the influence of temperature depended on the styrene content and chemical composition of the studied resins, while the amount of cobalt octoate used for the synthesis as the accelerator had no influence on the viscoelastic properties of the prepared materials after solvent exposure. It was also found that not fully cured urethane vinyl ester and the terephthahc acid-based unsaturated polyester resins showed excellent resistance to sulfuric acid exposure. However, interactions between the tested resins and petroleum could possibly occur through intermolecular bonding between the non-polar chains of the cured resins and the solvent. 

Urethanes (Chemically cured) The isocyanate (-N=C=0) group is used to cross-link the resin (curing type) cured PU available in two separate containers chemically cured PU has good water and chemical resistance aliphatic urethanes have good gloss and color retention. 

Thus, for using chemically curing adhesives only a few systems are applicable such as two-part systems consisting of a resin and a hardener, for example, epoxy resins. Another appropriate approach is the application of, for example, urethanes and silicones that cure by humidity and do not need complex hardware. However, the bi est disadvantage of using these systems is the expanded necessary curing time. 

Moisture-curable urethane systems (one-pack) can be considered as two-component systems which use atmospheric moisture as the second component. One-pack urethane coatings can be produced that are similar in physical properties to the two-pack systems for almost all applications. These highly complex systems can have a great deal of flexibility. Claimed advantages are a one-pack system, rapid cure, even at low temperatures, excellent chemical and abrasion resistance and good flexibility. Although these systems have been available for some time in other countries of Europe, they are only recently beginning to be of interest in the UK. 

Materials. The adhesives and primers used in this study were model and commercial materials that were cured according to conditions appropriate for the specific adhesive chemistry. Adhesives A and B were conventional epoxy/Versamid and epoxy/dlcyandiamlde adhesives, respectively. Adhesives C and D were commercial urethane and epoxy/polyamlde adhesives, respectively. Adhesive E was a conventional two-part epoxy/amldoamlne adhesive. Adhesive F was a vinyl plastlsol adhesive. The adhesive primers used in this study were a urethane crosslinked epoxy electrocoat primer and spray primers based on tall oil modified epoxy ester, and polyesterpolyol/isocyan-ate chemistry. Dicyandlamlde was obtained from Aldrich Chemical Company. Epon 828 was obtained from Shell Chemical Company. 

Figure 3. Infrared spectra of acrylic-urethane coating. Spectrum A denotes cured coating B denotes coating exposed to condensing humidity C denotes coating exposed in QUV weathering chamber and D denotes coating exposed in a dry QUV. (Reproduced from ref. 14. Copyright 1986 American Chemical Society.)...

Millable urethanes are processed on standard rubber-processing machinery. They may be either peroxide or sulfur cured. The sulfur-cured varieties have some chemicals added to enable sulfur curing to take place. These polyurethanes also have the properties of castable polyurethanes but need to be processed on standard rubber machinery. 

Commonly used adhesives include epoxy, rubber, acrylic or vinyl emulsions, and urethanes. Epoxies are high-priced, but they have better chemical resistance and durability than the others, and they have dominated the market in outdoor applications. Significant advantages of the epoxy-based adhesives are that they have no solvents and, therefore, exhibit little shrinkage. They cure relatively fast and, therefore, are not as exposed to inclement... 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

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