Лако-красочные материалы — производство

One of the advantages that a formulator has using a polyurethane adhesive is the ability to tailor the adhesive properties to match the substrate. Flexible substrates such as rubber or plastic are obvious matches for polyurethane adhesives because a tough elastomeric pro­duct can easily be produced. Polyurethanes derive much of their toughness from their morphology.

Polyurethanes are made up of long polyol chains that are tied together by shorter hard segments formed by the diisocyanate and chain extenders if present. This is depicted schematically in Fig. 10. The polyol chains (typically referred to as soft segments) impart

low-temperature flexibility and room-temperature elastomeric properties. Typically, the lower-molecular-weight polyols give the best adhesive properties, with most adhesives being based on products of molecular weight less than 2000. Generally, the higher the soft segment concentration, the lower will be modulus, tensile strength, hardness, and tear strength, while elongation will increase. Varying degrees of chemical resistance and heat resistance can be designed by proper choice of the polyol.

Short-chain diols or diamines are typically used as chain extenders. These molecules allow several diisocyanate molecules to link forming longer-segment hard chains with higher glass transition temperatures. The longer-segment hard chains will aggregate together because of similarities in polarity and hydrogen bonding to form a pseudo­cross-linked network structure. These hard domains affect modulus, hardness, and tear

strength and also serve to increase resistance to compression and extension. The hard segments will yield under high shear forces or temperature and in fact determine the upper use temperature of the product. Once the temperature or shear stress is reduced, the domains will re-form.

The presence of both hard segment and soft segment domains for polyurethanes gives rise to several glass transition temperatures, one below — 30°C which is usually associated with the soft segment, transitions in the range 80 to 150°C, and transitions above 150° C. Transitions in the range 80 to 150° C are associated with the breakup of urethane hydrogen bonds in either the soft segment or the hard segment. Transitions higher than 150°C are associated with the breakdown of hard segment crystallites or aggregates. Linear polyurethane segmented prepolymers can act as thermoplastic adhe­sives which are heat activated. A typical use for this type of product is in the footwear industry.

By proper choice of either the isocyanate or the polyol, actual chemical cross-links can be introduced in either the hard or soft segments that may be beneficial to some properties. The effectiveness of these cross-links is offset by a disruption of the hydrogen bonding between polymer chains. Highly cross-linked polyurethanes are essentially amor­phous in character exhibiting high modulus, hardness, and few elastomeric properties. Many adhesives fall into this category.