Coatings Clinic cure.

Many coatings failures and problems such as poor scratch and abrasion resistance, a tendency to be easily torn or gouged, water sensitivity, and poor solvent resistance tend to be blamed on insufficient cure, often called undercure. The coating is unable to resist mechanical or chemical stresses. Cracking and chipping are associated with overcure, as are difficulties with adhesion of repair coats and topcoats. The coating is too hard and rigid and too resistant to bite-in by the solvents in subsequent coats.

But what is cure? Are we talking about crosslink density? Something else? A dictionary gives a very broad definition: "a process or method whereby a product is preserved, perfected or readied for use." A good paint-related definition of cure is "the removal of solvents and achieving of sufficient molecular weight to give properties that meet the user's needs." That says nothing about crosslinking, which may not even be necessary. The idea is to produce a tough, resistant film. This can be done by beginning with a high molecular weight material (as in lacquers) or by chain extending and/or crosslinking low and medium molecular weight resins to produce the needed molecular weight. Most high-solids coatings need lots of crosslinker, first to chain extend the low molecular weight oligomers, then to crosslink them into tough films. Baking has a double purpose of removing solvent and driving the crosslinking reaction. Depending on the end use and film properties required, the level of crosslinking needed may be high or low.

Thermoplastics such as nitrocellulose, poly(methylmethacrylate), and the acrylic copolymers that were used in automotive lacquers for many years do not crosslink. They already have sufficient molecular weight. They just need to have solvent driven off. Thermosetting resins such as acrylic- and polyester-melamines and the more reactive epoxy-amines, polyol-isocyanates, and alkyds (oxidative cure) build molecular weight through crosslinking. Except for oxidative cure, the curing process takes energy, which usually is heat, but can be UV, electron beam, or microwave radiation. The degree of cure depends on the amount of energy, although catalysts can increase the rate of reaction.

The glass transition temperature ([T.sub.g]) is important to the cure process. Cure stops when the film becomes glassy. Most bake temperatures exceed the [T.sub.g]s of cured coatings, so most films do not vitrify (become glassy) during the bake. However, air dry cure can be arrested if the film becomes glassy or approaches that state. Raising the temperature of the film (say by the sun) will soften the coating allowing additional cure to occur.

Cure can be monitored and evaluated in the laboratory by a variety of techniques, including dynamic mechanical analysis (DMA), thermal mechanical analysis (TMA or indentometer), differential scanning calorimetry (DSC), oscillatory rheological measurements, tensile and elongation measurements, Fourier transform infrared spectroscopy (FTIR), dielectric spectroscopy, and measurement of extractables/gel content. A few of these techniques, particularly IR, have been applied to follow or test degree of cure on-line or next to it, but we usually must turn to less esoteric methods for measurement of cure. Examples are solvent rubs (ASTM D 5402), pencil hardness (ASTM D 3363), indentation hardness (ASTM D 1474), and rocker or pendulum hardness (Sward--ASTM D 2134; Konig and Persoz--ISO 1522). Unfortunately, plant and field techniques are subjective, have poor precision, and are difficult to relate to material properties or performance. However, if used carefully, they can give rapid estimations of whether the coating has adequate properties for its end use.

If tests show that a coating is undercured, one response is to apply more energy such as heat lamps for an air dry coating, a higher temperature or longer time in the oven for a baked coating (but be careful of yellowing and other color changes that may occur), and stronger lamps or more time of exposure for UV cure. More crosslinker or a more reactive one may be needed. An overcure situation usually means pushing the customer to lower the oven temperature, although the problem may be due to a high frequency of line stoppages so that work pieces often are stuck in the oven. It may be necessary to reformulate with less crosslinker or a less reactive one to produce a film that is not so brittle, but this may hurt other properties.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].