Лако-красочные материалы — производство

‘‘Conventional’’ or ‘‘first generation’’ acrylic adhesives are the earliest examples of this technology. They contain methacrylated monomers, a variety of polymers, an

initiator/stabilizer package, and methacrylic acid. Such formulations, a typical example of which is shown in Table 1, emerged in the early 1970s and found considerable utility in bonding thermoplastics.

Products classified as ‘‘first generation” are still on the market and are sold primarily for bonding thermoplastics. They provide excellent gap filling characteristics, rapid cure and adhesion, and are generally stronger than the substrates themselves.

Typical bond performance of adhesives of the first generation type is shown in Table 2. The adhesives have good adhesion to thermoplastics such as polystyrene, ABS, and poly(vinyl chloride), to wood and rubber, and to steel and aluminum provided the metals are clean. It can also be seen from this table that adhesion to galvanized steel is lower than to other metals. Bonding to galvanized steel was a problem with first genera­tion acrylic adhesives but it has been overcome with more recent embodiments of this technology covered later in this chapter.

It should be noted that if adhesives of the type shown in Table 2 are applied and cured using an accelerator lacquer, bond strengths equivalent to those achieved with mix-in peroxide pastes are obtained. In bonding ABS to itself, for example, the literature reports stock breaks in the range of 5.9 MPa when either a mix-in accelerator or an accelerator lacquer are used.

Data have also been reported in company literature claiming first generation acrylic adhesives will resist aggressive environments such as exposure to alcohols and hot water immersion. The patent literature also reports that when using mix-in accelerators, good bond strengths are retained for at least 35 days in aggressive environments (e. g., in condensing humidity cabinets). The failure mode of substrate failure in plastic substrates, such as ABS, is also reported.

The so-called ‘‘Dexter Hysol (DH) acrylic adhesives’’ are an example of one of the later generation products and are loosely an extension of the original technology. DH and other generations are often lumped together under a general term, ‘‘second generation’’ acrylic adhesives. These adhesives differ from the original offerings in the polymers used, the monomers used, and an increasing use of novel cure chemistry and specialty adhesion enhancing additives. In some cases the term ‘‘DH’’ is used interchangeably with ‘‘second generation,’’ although adhesive suppliers would argue that this is not accurate and the two terms represent quite different types of products.

Nevertheless, one obvious difference between these materials and earlier types is that the accelerator lacquers used with these newer systems often were oily, and peroxides other than benzoyl peroxide were often used. The oily accelerator lacquers that are available for curing DH acrylics are difficult to apply and once primed, parts are difficult to handle.

Second generation and DH acrylic adhesives were notably different from earlier systems in that they were the first type of acrylic adhesives that could be cured and good bonds to unprepared metals could be obtained. These systems showed the ability not only to bond to unprepared steel and aluminum surfaces, but also to aluminum and steel surfaces that still had varieties of different oils and drawing compounds on them. This ability marked a significant advancement in acrylic adhesives and established them as a unique family of adhesive materials for bonding oily and unprepared metals.

These types of acrylic adhesive systems enjoyed considerable popularity when they were first introduced. More recently, however, they have been replaced by other, more “user-friendly” adhesive formulations that do not require nondrying (oily) primers and accelerators. In addition, some of the key raw materials have been of limited availability, interfering with manufacturers’ ability to supply product consistently.