Лако-красочные материалы — производство

A. General Principles of Manufacture

A typical phenolic resin is made in batches, in a jacketed, stainless steel reactor equipped with an anchor-type or turbine-blade agitator, a reflux condenser, vaccum equipment, and heating and cooling facilities. Molten phenol and formalin (containing 37 to 42% for­maldehyde or paraformaldehyde), in molar proportions between 1:1.1 and 1:2, along with water, and methanol are charged into the reactor and mechanical stirring is begun. To make a resol-type resin (such as those used in wood adhesives manufacture), an alkaline catalyst such as sodium hydroxide is added to the batch, which is then heated to 80 to 100°C. Reaction temperatures are kept under 95 to 100°C by applying vacuum to the reactor, or by cooling water in the reactor jacket. Reaction times vary between 1 and 8 h according to the pH, the phenol/formaldehyde ratio, the presence or absence of reaction retarders (such as alcohols), and the temperature of the reaction.

Since a resol can gel in the reactor, dehydration temperatures are kept well below 100°C, by applying vacuum. Tests have to be done to determine first, the degree of advancement of the resin, and second, when the batch should be discharged. Examples of methods of such tests are the measurement of the gel time of a resin in a 150°C hot plate or at 100°C in a water bath. Another method is measuring the turbidity point, that is, precipitating the resin in water or solutions of a certain concentration.

Resins that are water soluble and have a low molecular weight are finished at as low a temperature as possible, usually around 40 to 60°C. It is important that the liquid, water — soluble resols retain their ability to mix with water easily when they are used as wood adhesives. Resols based on phenol are considered to be stable for 3 to 9 months. Properties of a typical resin are a viscosity of 100 to 200 cP at 20°C, a solids content of 55 to 60%, a water mixibility of a minimum of 2500%, and a pH of 7 to 13, according to the application for which the resin is destined.

Phenol-formaldehyde (PF) resins present lower reactivity at a pH of about 4. The accepted effect of the pH and of the phenol/formaldehyde molar ratio on the rate of polymerization and rate of hardening of phenolic resols is shown in Fig. 2. Recently, however [9], the concepts expressed in the graph have been found to be only partially correct, at least with regard to the dependence of the PF adhesive rate of curing as a function of pH. The expected asympthotic acceleration expected over pH 7 to 8 and due to the formation of phenate ions has been proven not to be the only effect present. At first acceleration occurs, but after a pH of approximately 8 to 9, the rate of hardening of the resin slows down considerably [9], as shown in Fig. 3, contrary to accepted wisdom. There are several reasons for this behavior [9], the easier of these to accept being the formation of a ring involving phenol, the methylol group, and Na ions, which was postulated already 50 years ago [7]. The existence of this ring has been shown to be untrue [9] and the persistence of the concept is due to the ease with which the behavior shown in Fig. 3 can be explained. The reason for the acceleration, however, was ascribed to and proved to be due to the existence of and equilibria pertaining to quinone methides [9,20]. The structure of the elusive oligomeric quinone methides in PF resins has also been elucidated [21] (see page 549).

The probable reason why the behavior in Fig. 3 was not noticed earlier appears to be due to the slow gel times of PF resins, which makes it very tedious to check reactivity effectively.