External Crosslinkers, Explained

Finishing Chemistry 301 is now in session. February 26, 2005

In a recent post, someone mentioned using external crosslinkers with waterbased clear. What are external crosslinkers? What are their benefits?

Forum Responses
(Finishing Forum)
From contributor A:
Crosslinking is a technique used to improve the hardness or chemical resistance of a coating, or both. Crosslinking occurs when leather is tanned, or when blood clots to form a scab.

There are two common crosslinking agents used in waterbornes: aziridines and isocyanates. They can be very effective at enhancing the properties of a given wb coating, but they can't turn an inherently poor coating into a good one.

They are also very effective at being major health hazards. They are, in some ways, more toxic than the lacquer solvents they usually replace, and they pass right through a standard respirator (unless you use a special cartridge specifically made for your crosslinker). They are odorless, so you don't have the warning of typically toxic solvents. Aziridines, in particular, are extremely dangerous, and are generally applied wearing a full body suit with a supplied air respirator.

There is a third class of crosslinker which is non-hazardous and can be extremely effective if formulated in the right coating. It not only increases hardness and chemical resistance, it increases flexibility, which guarantees against cracking if a thick coating is applied. This type of crosslinker will soon be available for use with the new PianoLac OEM.

If you are thinking about using a crosslinker, get the msds and read it. Take it seriously.

There are also "self" crosslinking coatings. These don't need an external chemical to be added in order to activate their crosslinking mechanism. The manufacturer can achieve this in two ways: he can use an inherently self-crosslinking resin that crosslinks as soon as it starts to dry, or he can use a resin and an internal crosslinker in the manufacturing process that also crosslinks as it dries. In general, these systems are not toxic and dangerous, as are aziridines and isocyanates. There's no need to worry about pot life or adding too much or too little.

They are also not as effective as the external crosslinkers. They can even impair hardness while improving chemical resistance, or vice-versa.

If you want a really hard, chemical resistant, tough finish for heavy duty use (bar top), external crosslinking of a coating that has these properties is the way to go. Self crosslinkers may not be as good as a high quality, non-crosslinked coating. The only way to test the properties is to spray out panels and compare for hardness and chemical resistance. Manufacturer's data sheets are a rough guide, and often don't yield useful data, especially when comparing hardness and abrasion resistance numbers (which are very subjective). Relative chemical resistance can be more reliably evaluated.