Лако-красочные материалы — производство

Extensive development has led to the successful use of permanently tacky, nonsetting pressure-sensitive adhesives in aircraft and bodywork construction, as well as in other fields [1]. Pressure-sensitive adhesives are high-viscosity polymer systems that at least partly retain the properties of a liquid in their final state. This allows them to

Table 4.1 Classification of the different types of adhesive according to their mode of setting.
Mode of curing	Type of adhesive	Typical base resin (example) Characteristic properties
No curing Pressure-sensitive adhesives EVA, acryle resins • rapid processing • not harmful to the environment Physical curing Contact adhesives Neoprene rubber • easy processing • low price • solvent-containing Adhesive dispersions Similar to contact adhesives • easy processing • long flash-off time • very low solvent content Heat-sealing adhesives PVC copolymers • rapid processing • activation energy needed • solvent-containing • dispersion in water possible Hot-melt adhesives Polyamide • very rapid processing • solvent-free Physical and chemical curing Pressure-sensitive adhesives with postcure Silane crosslinked polyester Acryle resins • under development Contact adhesives with addition of a curing agent Neoprene rubber with isocyanate curing agent • easy processing • low price • solvent-containing

EVA = ethylene vinyl acetate; PUR = polyurethane resin; PVC = poly (vinyl chloride); UP = unsaturated polyesters.

Adhesives

Polyaddition

adhesives

Polyconden-
sation adhesives

Figure 4.1 Classification of adhesives according to their mode of setting.

completely adapt to the roughness-induced surface contour of the adherents, thereby forming adhesive bonds.

Pressure-sensitive adhesives are not only used in the form of adhesive tapes coated with binders on one or both sides, but also as transfer systems which are applied to the solid material, together with a base material which is later stripped off. Further­more, pressure-sensitive adhesives can be applied in the molten state at temperatures of approximately 80 °C, allowing them to be solvent-free from the start of the application. Another possibility is to apply them to the adherent as aqueous disper­sion, the aqueous component evaporating later. Finally, at present, other pressure — sensitive adhesives are available which are applied to the surfaces as solvent-free or solvent-containing, low-viscosity systems and later are shortly irradiated with ultra­violet (UV) light or heated to develop adhesive capacity (see Section 5.7.2.5).

Pressure-sensitive adhesives are generally applied to one of the surfaces to be bonded. A good bond is only obtained by uniform short pressing of the adherent surfaces, which must be exactly positioned because the bond has a relatively high strength immediately after the pressing. In generally, an increase in strength by 50-100% of the initial value is observed during the course of several hours or days.

Pressure-sensitive adhesives have very good peel strength and, in comparison to chemically setting systems, a relatively low shear strength. Tack does not necessarily have a direct connection with peel strength occurring later; indeed, systems with a relatively low tack may have very high peel strength in the final state, or vice versa. Compared with various other adhesive systems (chemically setting high-perfor­mance adhesives included), pressure-sensitive adhesives develop very good aging resistance, often even on surfaces which are difficult to bond. The reasons for this behavior have been discovered only recently [2].

In general, pressure-sensitive adhesives are solvent-free and almost monomer free, they are easy to process, and have relatively low susceptibility to processing flaws. Processing aids are rarely needed, and any processing speed is possible. After joining, the bond is immediately ready for mechanical loading.

4.2