Лако-красочные материалы — производство

Most UV/EB adhesives are based on an addition polymerization curing mechanism. They consist of acrylic esters of various types or combinations of acrylates with aliphatic or aromatic epoxy resins, urethanes, polyesters, or polyethers. Although the epoxy­based systems have higher tensile strengths, their elongations are less than those of the urethane-based systems. In addition, the urethane-based systems have better abrasion resistance. UV/EB adhesives that undergo cationic polymerization are based on epoxies with reactive diluents and cyclic monomers. The main advantage of UV/EB-curable adhesives is rapid curing at room temperature, so they can be used to bond heat — sensitive substrates, such as polyfvinvl chloride). In addition, the rapid cure often eliminates the need to fix parts and greatly increases production rates.

UV/EB-cured adhesives have been used to replace solvent-base adhesives because of the increasing cost of properly recovering and disposing of solvents. Most of these adhesives are single-component materials that require no mixing and produce little waste.

The cross-linked nature of UV/EB-cured adhesives results in good chemical, heat, and abrasion resistance; toughness; dimensional stability; and adhesion to many sub­strates. Unlike thermal curing, EB or UV curing requires pure raw material grades. The use of dual-curing systems allows opaque substrates to be cured.

The mechanical properties of UV-curable structural adhesives are dependent on polymer molecular mass and cross-linking density. These factors are related to the prepolymer, degree of stiffness or flexibility, and functionality. The overall adhesive strength is affected by:

— Adequate UV transmission through the bond line

— Adhesive thickness

— UV intensity

— Postcuring (by heat or exclusion of oxygen, as in anaerobics)

The main components of UV/EB-cured adhesives are reactive oligomers which contribute adhesion, toughness, and flexibility to the overall properties of UV/EB adhesives. Typical reactive oligomers include acrylated epoxy resins and aromatic urethanes.

Monomeric diluents are low molecular mass monofunctional molecules that reduce the viscosity of liquid oligomers. Some, such as methacrylates, increase the toughness and adhesion. Various types of acrylates are used as reactive monomers.

Cross-linking monomers such as 1,3-butylene glycol dimethacrylate, tripropylene glycol diacrylate, and pentaerythritol tetracrylate are also used in UV/EB adhesive formulations.

Free-radical initiators trigger the cross-linking reaction. In EB-cured adhesives, the electrons act as free-radical initiators for addition polymerization. Therefore, no chem­ical initiator additives are needed. In UV-cured adhesives, photoinitiators, which release free radicals when exposed to UV radiation, are required to initiate addition polym­erization. The most recent UV — and EB-curing systems involve cationic polymerization mechanisms.

Typical UV-curable adhesive applications include the electronics, automotive, med­ical, optics, and packaging industries, as well as tapes and labels. EB-curable adhesives are used in magnetic tapes and floppy disks, where magnetic particles are bonded to films, as well as in packaging, tapes, and labels.

Laminating adhesives have the largest share of the UV/EB-curable adhesives market. Adhesives are applied and then cured in-line. The lack of solvent in these adhesives eliminates the drying step required when using solvent-based adhesives. The high degree of cross-linking in UV/EB-cured laminates gives high bond strength, good heat resistance, and good chemical resistance. Substrates that are typically laminated in­clude: film to paper, foil, fabric, glass, film, or wood; paper to foil, wood, or paper; wood to wood; and glass to glass or metal.

The pressure-sensitive sector of the UV/EB-curable adhesives market is not nearly as large as that of the laminating-adhesives segment. Because slight changes in exposure time greatly affect peel strength and tack, the formulation of these adhesives is critical. Currently, there are three approaches used to make pressure-sensitive adhesives. The first is based on a UV-curable system employing conventional tackifiers blended with UV-reactive moieties. The process must be closely controlled to attain repeatable tack and peel strength. The second approach is based on an EB-curable system that is actually a hot-melt adhesive. EB-cross-linkable thermoplastic rubbers are usually used in these adhesives and result in hot-melt pressure-sensitive adhesives with better heat resistance than conventional elastomeric hot-melt adhesives. The third approach in­volves the development of inherently tacky oligomers for pressure-sensitive adhesives.

There are several limitations to the use of UV/EB-cured adhesives. EB equipment is expensive. UV equipment is less expensive, but the materials themselves are usually more costly because of the need for photoinitiators. To cure adhesives properly, one substrate must be transparent to UV radiation. However, the necessity of having a transparent substrate has been removed by the introduction of dual-curing adhesives.

These adhesives are quickly set by a UV cure and are more hilly cured by a second mechanism involving the introduction of heat or moisture or the elimination of oxygen (anaerobics). In EB-curable adhesive, the depth of EB penetration is limited by the density of the material, rather than its opacity.

The cure time of UV adhesives is usually less than 60 s and depends on:

— Bond-line joint thickness: As the thickness increases, UV radiation loses its ability to penetrate totally, necessitating a second cure.

— Type of substrate: A transparent substrate such as glass with a small gap may take as little as 5 s to cure. Opaque and darker substrates require longer curing times.

— Light intensity: The more intense the UV light, the faster the cure.

The curing time of EB adhesives is comparable to that of UV adhesives. In general, EB radiation allows adhesive curing to be achieved at greater depths than is possible with UV radiation. Electrons can pass through substrates that are opaque to UV light. In addition, the area of exposure and the depth of penetration can be controlled by means of the EB conditions.