Лако-красочные материалы — производство

During the early 1940s, with the major breakthrough in the development of versatile phenolic resin adhesives for the bonding of various materials, it was possible for the first time to use these adhesives for the bonding of metals. De Bruyne [23] added relatively large amounts (75%) of high-polymer polyvinyl formal (PVF) to brittle phenolic resin as early as in the resol state. Although at room temperature PVF is a solid material, it melts when the mixture is allowed to cure at elevated temperatures (ca. 170 °C), generating a high-performance adhesive with excellent deformation properties known as ‘REDUX’. This adhesive was a marker in the history of adhesive technology, and indeed is still used today, for example in some branches of aircraft manufacture (see Section 8.2.1).

Although the steric hindrance of resol condensation was recognized from the outset as one factor responsible for the plasticizing effect of PVF, another — not fully understood — factor soon became apparent. Important was that when PVF was added to resol as a coarse powder and allowed to cure, it formed domains rather than becoming homogeneously mixed. In the solidified state this led to limitations and control of crack propagation under load, as in the case of ABS plastics. If the resol reacts too much with PVF, or if the resol and PVF are homogeneously mixed in a mixture of ethanol and dichloromethane, the plasticizing effect is lost for the most part.

In the noncured condition, the volatile components of resol-containing adhesives may give cause for attention. Furthermore, due to the relatively large amount of thermoplastic PVF required, the resistance to heat of the cured adhesive was limited to approximately 60 °C, despite the phenolic component having high thermal stability.

In order to identify other possibilities of modification, novolaks (which are free from volatile components) were modified with butadiene nitrile elastomers. If a vulcanization agent (e. g. sulfur), a vulcanization aid (e. g. zinc oxide) and other vulcanization accelerators are added to the adhesive, these elastomers vulcanize when the novolak cures. These so-called ‘nitrile-phenolic adhesives’ have a Tg of approximately 0 °C due to their high nitrile content (up to 50%) and low content in phenolic components (ca. 20%). Once in service, however, they act as elastomers

and barely alter their properties up to a temperature of 200 °C, applied for a short period. Hence, these materials are widely used for the bonding of brake and clutch linings.

5.4.3