Лако-красочные материалы — производство

It is important to reproduce the particle size of any latex and also to be able to measure it. Particle sizes generally encountered range from less than one micron for a fine latex, to about three microns for a coarse one. The distribution as well as the average size are both important. Particle size can be measured by many techniques, which include:

• Scanning electron microscope (SEM)

• Ultra centrifuge

• Light transmission/scattering

• Soap titration

• ‘Nanosizer’

Each method gives a different value and can be considered relative. It is necessary to establish standards using one technique. As a general rule, the ‘bluer’ the latex appears, the finer the particle size for any given polymer system.

Early calibrations were made using the electron microscope which gave reproducible results with polystyrene emulsions. Sample preparation which involves drying and coating the particles with metal to render them opaque to electron beams can cause deformation and coagulation of particles. For the soft polymers often used for surface coatings, electron micrographs can give misleading results. In recent years a new
technique which gives a reliable weight average particle size has been developed. Polymer particles undergo Brownian movement; the energy due to thermal effects of any particle depends only on the temperature and a large number of particles have a Gaussian distribution of energies around this characteristic value. When a laser beam is passed into a diluted emulsion, light will be reflected by a particle which acts as a source of light. If the particle is moving towards or away from a detector, the wavelength of the light will change according to the Doppler principle. By measuring the change in wavelength the particle size can be computed. This method is quick, reproducible and does not involve errors of preparation. This is the principle of operation of the Nanosizer (from Coulter). There are many variations of this principle available today. Some use the relationship between Brownian motion and size and the interference or scattering with a laser beam to determine particle size. At one time, disc centrifuges were often used, but the determination could be tedious. The best suited instrumentation should be discussed with equipment suppliers.

The particle size of an emulsion is an important factor in its application and it is also a useful indicator of stability. A robust recipe will produce latices with a small spread of particle size. There will always be variations in weighing or metering reactants, in reaction temperatures and times as well as raw material variations. If these variations do not cause particle instability, good consistency of particle size will result. The first sign of instability is particle agglomeration, which will give an increase in particle size if moderate and emulsion breakdown if highly unstable.