Лако-красочные материалы — производство

Addition polymerization is exothermic, and one of the major constraints to high produc­tion rates is the problems associated with heat removal. In processes using ethylene, the pressure of the gas determines solubility in the liquid phases (i. e., water and vinyl acetate monomer droplets) and in the polymer particles. This concentration of ethylene at the point of polymerization determines the ethylene content of the final polymer. Use of high pressures in such systems eliminates refluxing of the vinyl acetate, losing a very effective heat removal mechanism available to simple batch-process PVA production. Refluxing, however, gives condenser vapor losses. Great care has to be taken to ensure that the condenser is adequate to deal with the volume of vapor to be condensed. Returning condensate from an inverted condenser is also a very effective cooling agent, as it is often very much below the reaction temperature. The sensible heat removal adds to the evaporative cooling.

Care also has to be taken not to overcool the reaction. If so, a slowdown in the polymerization rate may occur, with excess free monomer, leading to an exotherm fol­lowed by foaming or an overloading of the condenser. A reduction in the monomer feed rate at this time is essential, but again care has to be exercised, as a sudden loss of cooling from the incoming monomer stream coupled with a drop-off in the reflux rate can give an uncontrollable exotherm.

The batch process uses a kettle fitted with an agitator. Other features are tempera­ture probes and a cooling jacket. Simple processes operate at atmospheric pressure and use a condenser. If no condenser is fitted or ethylene gas is to be used, the reactor must be pressurized. On larger kettles, an external heat exchanger may be employed. This system is attractive, as it avoids the need to manage reflux, and the total cooling capability has flexibility through variable rates of pumping through the heat exchanger. Its disadvantage is the need to clean and maintain the heat exchanger. and manufacture of grades of adhesive with poor mechanical stability, high viscosity, or a tendency to foul surfaces can be difficult or uneconomic.

The batch process starts with filling the reactor with most of the water, much or all of the stabilizer [frequently poly(vinyl alcohol) in adhesives], and a small proportion of the monomer. On agitation and raising the temperature to above 65°C, addition of a water — soluble free-radical generator such as ammonium persulfate initiates polymerization. This establishes the number of particles and the average particle size of the emulsion polymer. A continuous stream of vinyl acetate is run or pumped in with additional initiator until the required concentration of polymer is obtained, this coinciding with the maximum working volume of the kettle. It follows that in this process there is a wide spread of residence times within the reactor. The initial polymer is present from the outset, but shells of fresh polymer built around the early particles have a relatively short period within the reactor. The water and the stabilizers are also present at the beginning, which gives maximum time for degradation and grafting reactions. It is, however, energetically inefficient to agitate such viscous solutions over the full period of the process.

One alternative is the Loop process [1-3]. This employs a rather simple principle. A small volume of reaction mixture is recirculated, while streams of monomer and water phase [a stabilizer solution such as aqueous poly(vinyl alcohol)] are pumped into the reactor in the correct proportions. The reactor is fully filled and a balancing volume of product is released through a pressure-sustaining valve. Any unreacted monomer remaining in the outlet stream polymerizes on the way to the cooling tank or over a few hours, prior to packing. The volume of this type of reactor is only 40 to 80 L compared to 3000 to 100,000 L for a batch reactor.

The two types of reactor may be compared. The Loop reactor is more efficient energetically as the volume of reaction mixture to be agitated is so much less. It should be said, however, that the savings are not proportional to the volumes involved, as the diameter of the Loop pipes give greater frictional losses. As with many calculations involving viscosity in emulsion polymer production, complications arise due not only to pseudoplasticity of reaction mixtures, but also because of different behavior at different shear rates and temperatures.

One of the greatest contrasts between the processes is the residence time within the reactor. It has already been noted that the poly(vinyl alcohol) in the batch process is usually present from the start. The residence time is therefore several hours. In contrast, the mean residence time of materials in the Loop process is around 2 to 10min. This has obvious advantages in terms of minimizing degradation of colloids but will also restrict grafting between colloid and monomer.

Aside from process comparisons, the main contrast between the systems is that of size, weight, and cost, especially for pressurized systems. Construction of batch reactors for use with ethylene at pressures of 1000psi (70atm) and upward has to be massive. The simple construction of the Loop process—just pumps and pipework—lends itself to use at high pressures. Apart from cost and weight, the small volume of the Loop reactor has obvious safety advantages. Despite these attractions, the Loop reactor system has so far been used successfully only for low-pressure systems such as poly(vinyl acetate) homo­polymer for adhesives and copolymers for paint. Large-scale production of ethylene-vinyl acetate copolymers has yet to be demonstrated.