Лако-красочные материалы — производство

Polyols for adhesive applications can be generally broken down into three main categories: (1) polyether polyols, (2) polyester polyols, and (3) and polyols based on polybutadiene. Polyether polyols are the most widely used polyols in urethane adhesives because of their combination of performance and economics. They are typically made from the ring­opening polymerization of ethylene, propylene, and butylene oxides, with active proton initiators in the presence of a strong base as shown in Fig. 12.

Polyether polyols are available in a variety of functionalities, molecular weights, and hydrophobicity, depending on the initiator, the amount of oxide fed, and the type of oxide. Capped products are commercially available as well as mixed-oxide feed polyols, as shown in Fig. 13. Polyether polyols typically have glass transitions in the — 60°C range, reflecting the ease of rotation about the backbone and little chain interaction. As one would expect from such low glass transition temperatures, they impart very good low-temperature performance. The polyether backbone is resistant to alkaline hydrolysis, which makes them useful for adhesives used on alkaline substrates such as concrete. They are typically very low in viscosity and exhibit excellent substrate wetting. In addition, their low cost and ready availability from a number of suppliers add to their attractiveness.

The more commonly used polyether polyols range in molecular weight from 500 to 2000 for diols and 250 to 3000 for triols. Lower-molecular-weight, higher-functionality polyols are traditionally used in rigid-foam applications but have also been used as cross-linkers for two-component, fast-curing urethane adhesives. Polytetramethylene gly­cols (PTMOs; see Fig. 14) can be considered a subset of polyether polyols. They offer

initiator alkylene oxide

Figure 12 Ring-opening polymerization to form polyether polyols.

HO—ЄСН — CH — CH — CH — 0->— H 2 2 2 2 n

Figure 14 Structure of polytetramethylene oxide.

polyester

improved physical properties compared to polyethers based on ethylene oxide, propylene oxide, or butylene oxide, combining high tensile strength (due to stress crystallization) with excellent tear resistance. They are also noted for their excellent resistance to hydro­lysis. They are typically priced at a premium to other polyols.

Polyester polyols are used widely in urethane adhesives because of their excellent adhesive and cohesive properties. Compared to polyether-based polyols, polyester-based polyol adhesives have higher tensile strengths and improved heat resistance. These benefits come at the sacrifice of hydrolytic resistance, low-temperature performance, and chemical resistance. One of the more important application areas for these products is in the solvent-borne thermoplastic adhesives used in shoe sole binding. These products are typi­cally made from adipic acid and various glycols (see Fig. 15).

Some glycerine or trimethylolpropane may be used to introduce branching structures within the polyester backbone. Phthalic anhydride may also be used to increase hardness and water resistance. Inexpensive terephthalic acid-based polyesters from recycled poly — ethyleneterephthalate (PET) resins have more recently become popular.

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HO-HCH^CO-J-R-t-OC-fCH-M-OH

25 п 25л

Figure 16 Structure of polycaprolactone diol.

Polycaprolactones (see Fig. 16), another type of polyester polyol, offer improve­ments in hydrolysis resistance and in tensile strength (can stress crystallize) over adipic acid-based polyester polyols. They are typically higher in viscosity and higher in cost than polyether polyols of comparable molecular weight. When moisture resistance is critical, urethane adhesives incorporating polybutadiene polyols are used. These products are hydroxy-terminated, liquid polybutadiene resins. The hydrocarbon backbone greatly decreases water absorption, imparting excellent hydrolytic stability. Polybutadiene compounds also have exceptional low-temperature properties, with glass transition tem­peratures being reported below — 70°C [26]. These products are priced at a 40 to 50% premium over comparable polyether polyols. The structure of polybutadiene polyols is shown in Fig. 17.