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What you should know about filled adhesives and coatings

Adhesives and coatings sometimes contain fillers, because such ingredients can provide functional properties for certain applications. Examples of typical fillers are zinc oxide, titanium dioxide, antimony oxide, calcium carbonate, clay, etc.

These illustrations and the other common fillers are particulate materials that frequently do not dissolve readily in common solvents or disperse easily in water. That is why the handling of filled adhesives and coatings by converters can require some special considerations.

There are many reasons to use a filled material. The filler may provide a specific property, such as the fire retardancy provided by antimony oxide. Another function of a filler could be to provide color. Titanium dioxide or zinc dioxide will impart a degree of whiteness. If opacity is a desirable characteristic for the product, calcium carbonate might be the filler of choice.

A complex use for a filler would be to provide a controlled degree of adhesion. Adding clay to an adhesive or coating will decrease the amount of adhesion to a particular surface, because the clay particles at the surface displace the active ingredient. The inert clay does not bond. The smaller quantity of adhesive at the surface reduces the resulting bond.

All the examples cited for use of filler require that the material has uniform dispersion throughout the mass of the product to obtain the desired characteristics. This leads to the cardinal rule when using a filled product: Make certain the filler is properly mixed into the product.

The filler particles are generally heavier than the adhesive or coating containing them. With time, the fillers will therefore settle to the bottom of a container. It is very important that the user mix the particles back into the product uniformly before application. Without such uniformity, there will be a considerable variation in the desired property, whether it is fire retardancy, color, or adhesion.

Completely mixing a filled product before use can also eliminate another potential problem due to agglomeration of filler particles with time. Agglomerates can cause difficulties on a coating line. A common example is the trapping of an agglomerate under the knife of a knife-over-roll coater. Until the agglomerate eventually becomes dislodged, there will be a streak in the coating at the point on the web corresponding to the trapped item under the knife. Obviously, the deformation of the coating makes that portion of the product useless.

Converters occasionally like to purchase an adhesive or coating and then formulate it themselves to achieve special characteristics, sometimes by adding fillers.

To make the task of adding the filler easier and to try to ensure dispersion uniformity, they may use a filler that is a concentrated paste or dispersion. Although these usually mix readily into a liquid adhesive or coating, they can present a hidden problem. The choice of the liquid to prepare the paste or dispersion is crucial.

A plasticizer is a common material to prepare a paste. One might therefore add a material that is 80% filler, such as zinc oxide, and 20% plasticizer, such as dioctyl phthalate, to an adhesive or coating. In this instance, adding one part of the required filler adds one quarter of a part of another ingredient - the plasticizer. Is this amount of new material in the final system going to cause any problem?

In many cases, it might. Extensive testing would be necessary to determine the suitability and applicability. One technique to avoid introducing a new material is to prepare the paste using the adhesive or coating itself as the liquid. In the example cited, the paste would then be 80% zinc oxide and 20% original adhesive or coating. In such a case, one is only adding filler to the final product.

Fillers can be a common ingredient in certain adhesives and coatings. They perform useful and necessary functions. Never use such products without understanding how the fillers work and without having complete knowledge of their potential influence on the application. Otherwise, an ingredient as seemingly innocuous as a filler can cause serious problems.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at 505/299-6871.

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