Лако-красочные материалы — производство

In this example the use of a Redox initiator to start the polymerisation at a low temperature is illustrated. Previous examples have described isothermal polymerisations, but this reaction pattern is commonly used in industry as it reduces the need for cooling. On a large reactor, the heat required to raise the temperature would be provided by polymerisation. The technique has been termed ‘shotgun polymerisation’. The small particle size and an increase in particle hydration due to the acid groups limit the non-volatile content to 35%. Formulation 2-6 is a laboratory scale formulation.

FORMULATION 2-6

Procedure

1. 740g de-ionised water, 12g sodium lauryl sulphate and 150g of a monomer mixture (comprised by 190g methyl acrylate, 190g 2-ethylhexyl acrylate and 20g acrylic acid), are added to a two litre flask and purged with nitrogen.

2. After 30 minutes purging, 4g each of ammonium persulphate and sodium bisulphite are added and slow heating applied.

3. When the temperqture reaches 30°C, the remainder of the monomer mixture is added over a period of 105 minutes. During the monomer addition, the temperature is gradually increased to 60°C, and then it is raised rapidly to 90°C and held for 30 minutes before cooling.

Properties

pH 3.9

Non-volatile content 35+1%

Viscosity 450cPs at 25°C

Particle size 0.13 pm

6. Preparation of a Water Soluble Acrylic Copolymer

In this example, the emulsion polymerisation technique is used to prepare a high molecular weight acrylic copolymer in latex form. The latex is then destabilised and the pendant acid groups on the acrylic polymer backbone are used to solubilise the polymer in water. Care must be taken during the latex destabilisation process to avoid the formation of water insoluble particles.

FORMULATION 2-7

Procedure

1. Prepare a pre-emulsion by adding a solution of stabilising colloid in water to a mixture of the acrylic monomers and surfactant.

2. Charge the reaction vessel and heat to 80°C.

3. Add the pre-emulsion to the reactor over 2 hours at 80°C. Then hold a further 2 hours at 80-85°C after completion of the pre-emulsion addition.

4. Once conversion is complete, cool to 50°C.

5. Add 30 parts of water to 100 parts of acrylic latex in the reaction vessel.

6. Adjust the pH to above 7.5 with tri-methylamine stirring the mixture during the pH adjustment.

1. Once the pH is stable at 7.6-7.7 adjust the temperature to ca 40°C and discharge via a vibrating screen filter.

In this form the polymer is completely soluble in water, and latex stability is no longer a factor to be considered in the preparation of any surface coating from this polymer.

This is an example of preparing a water reducible resin using an emulsion polymerisation technique. In the next section, the preparation of water reducible resins using a solution polymerisation technique is considered.