Monosized Polystyrene Antiblock
The expertise we now apply in other areas is derived primarily from what we have learnt in decades of experience in the photographic field. One problem that used to arise with photographic films was that opposing surfaces tended to stick together when in intimate contact such as when rolled up or stacked, particularly under high humidity conditions. At the minimum this would lead to difficulty in handling and separating the materials, and in extreme cases the sheets bonded together into a solid block, giving rise to the term blocking. It was also found that an electrostatic discharge could occur when the surfaces were separated, producing a spark that would expose the film to give a dark spot or characteristic featherlike mark after development. Such marks were often of sufficient density to render the processed material useless.
It has long been the practice to overcome this difficulty by coating a so-called anti- sticking layer comprising an antiblocking agent or antiblock on one or both sides of the material. The antiblock provides a slightly rough surface on the film to prevent close contact with another surface. This aids slip, and helps avoid electrostatic discharges and sticking of the materials. Small particles of a mineral filler (in particular silica) or of a hydrophobic polymer (polystyrene or PMMA) are often used in such layers, but a number of disadvantages are encountered when using conventionally prepared materials. For instance, in most cases particle size distribution is rather broad which means that a high concentration is required as only the larger particles provide the antiblocking effect. In addition, the presence of numerous smaller particles can lead to a "milky" appearance, which is unacceptable for a photographic negative.
HARMAN technology Limited has developed a highly advanced method of preparing monosized polystyrene beads using a controlled dispersion polymerisation technique that provides spherical particles a few microns in diameter(µm, micrometres, or millionths of a metre), the ideal size for use as antiblocking agents.
Particles of this size are not only too small to observe with the naked eye, but can also be used in photographic negatives as they are invisible when the negative is printed on an enlarger. The Scanning Electron Microscope images below show spherical polystyrene beads prepared with this technique: the average particle size is 4 µm, but more importantly, all the particles are of approximately the same size, and thus all will play their part in achieving the desired effect. The antiblock can be produced as a dispersion in water or as a dry powder.
In order to exert its anti-blocking effect the dispersion of polystyrene beads is mixed with a binder (gelatin in photographic materials) and other components, and coated as a thin upper layer. The accompanying SEM image of the surface of a camera film shows how the low density of polystyrene helps the particles remain proud of the surface for maximum efficiency. Of course the antiblock could also be on the back of the material, and is also useful for other imaging materials such as ink jet recording materials.
The last picture is the surface of a film that has been in contact with another surface. The slight flattening of the antiblock particles after contact will be noted.
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