About Latex (3)

Pigmenting of Latex

Commercial latex pigmenting liquid

Latex is usually pigmented by preparing a pigment dispersion and then mixing this with the latex to provide the final colored product. By using the same procedure as in master-batching; i.e., taking a crude dispersion of the pigment, adding this slurry to the latex and homogenizing the two materials, a phase transfer can be achieved in which the pigment is removed from the aqueous phase and deposited inside the latex particles. 

This provides a more intense color, improves washability and fade resistance and, generally, improves the physical characteristics of the paint. This process has made possible the use of the acrylic artists' watercolors. Without homogenization it would not be possible to achieve the brilliance and depth of color.

Oil Extending Latex

Pearl pigment for latex

The use of mineral oils in the compounding of rubber has been standard for many years to improve the working characteristics of the rubber and reduce the cost of the final compound. Oil can be introduced into the latex as an emulsion and mixed with the latex emulsions or, simply, as oil mixed with the latex emulsion, but none of these provide a true, uniform, stable mixture of oil and latex. Mixing oil and latex together and then homogenizing will produce a uniform product having superior physical characteristics.

Plasticizing of Latices

Many of the synthetic resin latices are extremely hard and brittle, but for a number of applications they must be modified to have greater flexibility and improved adhesion. Plasticizers normally used with these synthetic resins are commonly mixed with the latices under continuous agitation, heat, and pressure in an autoclave for one and one-half hours or longer to cause the plasticizer to migrate into the latex particle. By use of the

homogenizer both the time and temperature may be materially reduced in this operation.

Reduction of Agglomerates

In the emulsion polymerization of latices, many of the small polymer particles tend to loosely join together forming agglomerates. In some polymerization reactions the total quantity of agglomerates can reach 10% of the batch. If the latex is to be used for paint or fine-coating applications, these must be filtered out prior to use. By homogenizing, 90% of this agglomerated material can be separated back to the original particle size, reducing the load on the filter to 1% and saving up to 9% of the latex that would formerly have been lost.

Equipment and Process

For latex processing the APV homogenizers are operated in the pressure range of 2000 psig (13.8 MPa) to 8000 psig (55.2 MPa). These machines should have special wearing parts, when they are used to disperse solids or pigments.

Testing

The methods and type of equipment used cover approximately everything used in the paint and rubber fields. The microscope, however, is extremely useful to analyze solid dispersions, and a viscometer would be used for thread or for growing of latex particle operations.

References:

• Bennett, D. A., British Patent 976,212 (1964).
• Bennett, D. A., British Patent 976,213 (1964).
• Bennett, D. A. and K. G. Burridge, British Patent 976,214 (1964).
• Bennett, D. A., U.S. Patent 3,573,243 (1971).
• Burke, Jr.; O. W., U.S. Patent 4,344,859 (1982).
• Calvert, K. O., Ed., Polymer Latices and Their Applications (London: Applied Science Publishers, 1982)
• Halper, W. M. and F. D. Moss, British Patent 1,124,418 (1968).
• Kraus, G. Reinforcement of Elastomers [New York: Interscience Publishers (John Wiley), 1965]

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