Лако-красочные материалы — производство

Typical plant for the commercial manufacture of acrylic resins for surface coatings applications consists of a large stainless steel reactor, usually with a working capacity of between 5m3 and 20m3, although 30m3 vessels are becoming more common. The reactor is usually designed for a normal working pressure of up to 1-2 bar and working temperatures up to 160-170°C.

Heating is often by steam or pressurised hot water, but other methods such as thermal fluid or induction heating are also frequently employed. It is normal to zone the heating coils to protect the exposed areas of the reactor walls from localised overheating.

The reactor is normally equipped for external cooling via coils or jacket, and also with internal cooling coils for quenching the reaction in an emergency, and to assist in controlling the temperature against the reaction exotherm.

Agitation of the reaction mass is usually by means of a multi-bladed stirrer, preferably driven via a variable speed gear box.

The vessel is equipped with a reflux condenser, water separator (for reactions involving water removal such as methylolation of acrylamide) and a vacuum receiver. The reactor top is normally fitted with an illuminated sight glass and entry ways for monomer, solvent and initiator.

An emergency outlet protected by a pressure rated venting disc and routed to a dump tank is included as a precautionary measure. Once reaction is complete, the product is adjusted for solids specification by addition of solvent. It is usual to carry out the dilution with solvent in a blending vessel equipped with an agitator, condenser and a means of heating and cooling. The use of a blending vessel frees the reactor for the next manufacturing cycle. Surface coatings require material that is essentially free of solid particles and foreign matter. Thus, it is essential that the acrylic polymer solutions are filtered to a high standard, and the equipment utilised must be capable of filtering out particles down to a size of below 10 microns.

The manufacturing process usually consists of a controlled addition of monomer and initiator to solvent under reflux in the reaction vessel. The monomer mixture is fed to the reactor from a measuring tank (e. g. on load cells), either by gravity feed via a flow meter, or else is pumped using a metering pump. The initiator is normally added at a controlled rate from a separate measuring tank or via a metering pump. The monomer and initiator addition takes place at a controlled rate over periods varying from 1-6 hours, whilst maintaining the temperature at the required level. Further additions of initiator (“spikes” or “boosters”) are often made, on completion of the main additions, to achieve full conversion of monomer to polymer. The reaction is most commonly carried out in a quantity of solvent slightly less than the specification to allow some latitude in achieving the final solids content and viscosity specifications.

Due to the chain transfer effect of some solvents where higher molecular weight polymers are required, the polymerisation is carried out at high solids content (80-85%) and diluted with further solvent when the polymerisation is completed.

The quantity of initiator employed varies between 0.2% — 4.0%, and may be of any of the types described earlier. The total process time varies between 8 and 30 hours with the reaction end point being greater than 99% conversion of monomer, as determined by

Acrylic monomer and solvent vapours will form explosive mixtures with air. Thus, if a reactor is charged and heated to reaction temperature, then in all probability the contents will pass through a composition where the atmosphere forms an explosive mixture. In order to counter this, a vacuum-nitrogen cycle should be undertaken immediately after charging the initial solvent. This is done by evacuating the reactor, and then releasing the vacuum to nitrogen, and the nitrogen increased to build up a slight positive pressure in the reactor. The cycle is repeated to ensure that no air remains in the reactor. A gentle through batch flow of nitrogen is maintained during reaction, and this is supplemented by a surface flow during cooldown following completion of the polymerisation.

The exclusion of air is also desirable due to the fact that oxygen is an effective polymerisation retarder, and it will have a significant effect on product consistency if not countered.