Лако-красочные материалы — производство

TABLE 1-18: FORMULATION FOR AN ACRYLIC COPOLYMER IN MEK

Procedure

1. Benzoyl peroxide is dissolved in a mixture of methyl methacrylate and ethyl acrylate.

2. 40% of the monomer mixture and methyl ethyl ketone are charged to a reactor set for reflux.

3. The reactor is heated to 9(fC and ten minutes after reflux starts, the balance of the monomer mixture is added at a constant rate over a period of two and a half hours.

4. Reflux is maintained for a further two hours when a polymer with a solids content of 58% is obtained, corresponding to a conversion of 98.5%. If the non-volatile content is less than 58%, then additional “shots" of benzoyl peroxide are added.

The use of a delayed addition of part of the monomer is very common in industrial practice. In this instance it allows a relatively constant monomer concentration to be maintained. The addition of initiator over the same two and a half hour period gives a slight increase in the rate of radical production, which will increase the rate of polymerisation whilst reducing the molecular weight. The reactivity ratios of 2.0 and

0. 26, for methyl methacrylate and ethyl acrylate respectively, means that the initial copolymer will be rich in methyl methacrylate. The delayed addition of monomer mixture means that the range of copolymer distribution will be narrower than would be the case if all the monomer were added initially. The initial reflux temperature is about 85°C and will rise slightly to about 90°C towards the end of the polymerisation.

The addition of initiator to a monomer premix is commonly practised in industry, but is potentially hazardous if the storage of the premix is not carefully monitored. The generation of free radicals in the premix, however caused, (possibly warming the premix or external contamination) can have catastrophic results.

If a monomer conversion curve was being constructed, then sampling of the non-volatile content would commence at stage 3. The non-volatile content is normally monitored at hourly intervals during this stage. Extra initiator may be added if the course of the polymerisation does not follow the predicted time/non-volatile profile (as illustrated in a previous section of this book).

Procedure

1. Charge all solvents and 10% of the premixed monomers and 10% of initiator to the reactor with stirring.

2. Heat to 80°C — the polymerisation is exothermic — so control the temperature with cooling as necessary to below 10(f C.

3. When the initial exotherm is complete add the remaining monomers and initiator as a premix over a three hour period at 90-10СҐC.

4. Add the final 10% of the initiator and reflux for two hours.

Sample for non-volatile content, if below 50% continue heating under reflux, possibly with the addition of a booster shot of peroxide until non-volatile content reaches 50%.

Cool and discharge through a suitable sized filter.