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Winter 2009 - Article 4

From The Lab
How Cross-Linking Works

Floor finishes are very interesting creations. They must remain stable and in liquid form until they are applied to a floor. Once, on a floor, they must dry and cure in such a fashion as to impart protection while simultaneously enhancing the appearance of a floor. This must be done under the toughest of conditions – abusive human foot traffic. One of the key reasons that floor finishes perform so well under these abusive conditions is the fact that the polymers used are able to join together using something we call a cross-linking agent. Without a cross-linking agent, a floor finish lacks durability and toughness – leading to premature wear, excessive soiling and black marking. This can be illustrated as follows:

Non-Cross-Linked Floor Finish

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Cross-Linked Floor Finish

The illustrations show the difference in how interconnected the polymers of a floor finish become once cross-linked. These tight, complex connections are what give a floor finish film its toughnes and soil and detergent resistance.

Most floor finishes are cross-linked with a zinc-containing material. The zinc acts as an attraction point for the polymers, bringing them together. While in liquid form, the zinc is in a mixture or complex that includes volatile materials. This complex allows the zinc to coexist with the polymers – keeping them from reacting. As the finish is applied to a floor and begins to dry, the volatiles escape, leaving zinc in its elemental form (Zn++). The two positive charges on the zinc begin to look for negative charged molecules. As you may have surmised, the polymers found in finishes contain a negative charge (usually a hydroxyl group – OH-). At this point the zinc acts as an attraction point, bringing the polymers together into a tight, interconnected network. When completely dried and cured, this networking is the primary mechanism at work, giving finishes their toughness and durability.