Лако-красочные материалы — производство

The properties of a resin which can be determined by simple test methods are shown in Table 18. The solids content of a resin usually is determined by the so-called dish method at 120°C for 2h [different times and sometimes lower temperatures (105°C) are often used as several variations of this method exist]. Even if it is a rather simple test, some deviations in the results might occur because not only does all water present as solvent in the liquid adhesive resin evaporate, but also a further condensation reaction with further water elimination takes place. Both liberate condensation water and this additional water is evaporated as well. The more severe the conditions during drying, the lower the solids content measured. Also some details of the test, such as the type of oven, the number of dishes in the oven at the same time, or recirculation of air or not, can influence the results of the test. The refractive index can be used as a quick method for the determination of solids content, however, the correlation between these two characteristic resin values is sometimes rather poor and not the same for all resins. The density is only important when using volumetric adhesive dosing systems, but not as a quality parameter of the adhesive.

One of the most important characteristics is the reactivity of the adhesive resin. With some methods also the start and the end point of the gelling process, the duration of its time span, the behavior of the resin during the test as well as the shape and strength of the gelled plug obtained are essential features of the gel time test. Gelling can occur within one or two seconds (as is usual for UFs) or gelation can span ten or more seconds (as is usual for melamine fortified resins). A long gel time can indicate a slow generation of cohesive bond­ing strength in the actual application of the resin. The behavior of the resin in the test tube (e. g., foaming) and also the consistency and strength of the gelled plug can be evaluated.

The temperature used for the gel time test should always be adjusted to the tem­perature of application of the resin. If the maximum temperature in a glue line during pressing is not higher than 70°C, then the gel test should be performed at such a tempera­ture and not at water boiling point. This is recommended in order to better interpret the behavior of the resin or the resin glue mix under its conditions of industrial application.