Structure Isocyanate-polyol coatings

Urethane resins are synthesized from isocyanates and chemical compounds with hydroxyl or urethane groups (—N—C—O—) including water, polyesters, epoxies, and acrylics [8,9]. The chemical structure of urethane coating is shown in Fig. 13.4. Polyester and epoxy have better barrier resistance to moisture and chemical attack than the acrylic polyol. Ahphatic isocyanate-based coatings are resistant to UV hght and have excellent gloss and color retention. 

The catalyst selection for blocked isocyanates depends on the nature of the blocking agent, the desired cure temperature, the polyol structure, and the application. Mono-, di- and trialkyltin compounds are effective catalysts for many blocked isocyanate reactions with hydroxyl compounds. In most solvent-borne coatings, DBTDL is used as a catalyst. 

An important class of waterborne coatings is the family of polyurethane dispersions (PUDs). lonizable groups (usually carboxyl) incorporated to the PU backbone allow the polymer to be dispersed in water after neutralization with amines. The properties can be varied substantially depending on the structure of the polyol and diisocyanate used to prepare them. Aliphatic and cycloaliphatic isocyanates are preferred because of their low reactivity with water and... 

The typical structure of urethane acrylate oligomers is shown in Figure 11.3. The chemistry of urethane acrylate is very versatile. Essentially, the chemistry and the final properties of urethane acrylate coatings are governed by the chemical structures of isocyanate and polyol and functionality. 

Nowadays, the trend of research in this field is toward new reaction routes to establish the linking process among polyol and isocyanates obtained from biopolymers to synthesize hybrid PUs and, thus, reach to overcome thermal and mechanical properties of these materials. Also other important parameters such as functionality of diols, isocyanate type, and chain extenders are relevant in PU synthesis [13]. In these materials, the molecular structure varies from rigid cross-linked to linear or highly extensible elastomers hence, their applications are versatile, such as coating or membranes in medical devices, materials in construction engineering, automotive parts, foams and adhesives. 

Two-package polyol urethane coatings (ASTM type 5). Here, the curing involves reaction between isocyanate-terminated adducts and di- or poly-functional hydroxyl-containing prepolymers having different backbone structures. 

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