Лако-красочные материалы — производство

The polymerization of acrylic adhesives via free radicals (chain reaction) allows acrylic adhesives to be dispensed and cured using unique techniques. Other types of adhesive products often cure by condensation polymerization reactions which dictate certain cure characteristics and handling techniques.

Different curing possibilities for acrylics follow from the fact that the free radicals, once formed, during acrylic adhesive cure will continue to propagate throughout the curing adhesive. This means that cure, even if begun at a very localized region, can proceed throughout the adhesive glue line without additional mixing. This is in contrast to epoxy and urethane types which require intimate mixing for full cure.

As a consequence, two distinctly unique methods of handling and curing acrylic adhesives follow from their cure chemistry. These are the so-called “accelerator lacquer’’ cure and the ‘‘no-mix’’ or ‘‘honeymoon’’ cure. These methods of handling acrylics are depicted in Figs. 5 and 6.

Figure 5 depicts the use of a peroxide-containing accelerator lacquer to cure the acrylic adhesive. The accelerator lacquer is usually in the form of a low viscosity liquid that is brush or spray applied to one or both of the substrates being bonded. Figure 5 shows the use of a solvent-carried peroxide-containing lacquer film former. When this accelerator lacquer is applied to one of the surfaces and dried, it represents a source of free radicals due to the presence of the peroxide incorporated into it. The peroxide in the accelerator lacquer will remain unchanged chemically, until adhesive is applied over the lacquer. At this point, amines in the adhesive react with the peroxide-containing lacquer to disassociate the peroxide, forming free radicals. These free radicals are then transported into the bulk of the adhesive and continue to propagate through the polymerizing metha­crylate monomers to give the final full cure.

The use of accelerator lacquers as a means to cure acrylics has been a very popular method since it allows the priming and storing of parts, the achievement of very rapid cures, and does not require expensive meter-mix-dispense equipment. It should be noted that this cure mechanism is not possible with condensation polymers, such as epoxies and urethanes, since they require full and intimate mixing of both components to achieve complete cures.

Figure 6 depicts another unique method of curing acrylics, the ‘‘honeymoon’’ or ‘‘no-mix’’ method. This technique also takes advantage of the ability of free radicals to

Substrate with Substrate with Substrates joined

component A component В and adhesive cures

propagate throughout the adhesive to complete the cure without the need for intimate mixing. In this approach, a bead of component A is applied to one surface, while a bead of component B is applied to the other. As in the case of the accelerator lacquer, no reaction occurs until the two surfaces are mated. Once the two surfaces are joined, a rapid gen­eration of free radicals occurs followed by monomer polymerization and full adhesive cure. As with the accelerator lacquer, this technique requires the use of only inexpensive dispensing equipment, but is not possible with adhesive types other than acrylics.