Q: I am new in powder coating and am working with a major powder coating supplier. I would like some information regarding gel time, such as the effect of increasing and decreasing and what factors may be involved with it.
A: This is a very broad topic, and I will try to answer it in general terms. First, let’s define gel time: This is the property of a thermosetting powder coating at which a melted powder stiffens due to the polymerization of reactive components.
The test is conducted by placing a small volume of powder coating onto a heated surface (hot plate). Typically, the hot plate is set at a constant temperature of 200°C (see ASTM D-4217 Standard Test Method for Gel Time of Thermosetting Coating Powder). The molten powder is stirred with a wooden splint or tongue depressor until it stiffens. Gel time is recorded as the duration between the onset of melt and the stiffening of the molten mass.
The gelation signifies the level of polymerization required to stiffen the binder, but not complete cure. Complete cure occurs following a much longer dwell time than what is considered the gel time. Regardless, gel time is a relative indication of cure rate.
Gel time is influenced by kinetics, which defines the reactivity of the resin and crosslinker. This reactivity is driven by a few factors, the most prominent being functionality (number of reactive groups per molecule) and catalysis. Secondary factors are stoichiometry and rheology (melt viscosity).
Resins with a high level of functionality react faster than those with lesser functionality. For example, a typical epoxy resin possesses about two reactive groups per molecule and reacts accordingly, while a novolac-modified epoxy will have 3-5 reactive groups per molecule and will react more rapidly due to a higher concentration of reactive groups.
A catalyst is defined as a compound that increases the speed of a reaction without being consumed by the reaction. In powder coatings, catalysts are very specific to a reaction. Therefore, a catalyst that increases the speed of an epoxy-carboxyl reaction will not affect a hydroxyl-isocyanate reaction.
Epoxies are catalyzed most commonly with basic compounds such as imidazoles. Ammonium salts such as benzyl tri-methyl ammonium chloride and choline chloride also accelerate epoxy cure. Common catalysts for hydroxyl-isocyanate reactions are based on tin compounds such as dibutyl tin dilaurate and stannous octoate. Concerns for toxicity have led researchers to develop non-tin catalysts based on bismuth, zinc, and zirconium chemistry.
Stoichiometry affects gel rate. A formulation having a balanced stoichiometry (1 to 1) of reactive groups generally reacts faster and more completely than a formula having reactive components outside of stoichiometry.
Finally, melt viscosity plays a role in gelation rate. Reactive groups are more mobile at low viscosity; hence, lower viscosity provides faster cure rate.
As for the powder formulator, raw material suppliers (particularly polyester suppliers), provide “standard” (180-200°C) cure and fast- or low-temperature cure (150-160°C) products. As you can imagine, the fast-cure products possess shorter gel times. The powder formulator can also introduce a catalyst to the formula to increase cure.
Increasing cure rate (decreasing gel time) through whatever means will afford certain performance features. Obviously, the powder will then have the potential to cure at a lower temperature or shorter dwell time. And depending on the catalysis, the powder may exhibit less flow and leveling, resulting in a more textured finish.
In addition, over-catalyzed formulas may experience “b-staging” in the extrusion process. This is an unwanted phenomenon in which the powder partially reacts during the compounding stage, resulting in higher viscosity (less flow and leveling) and possibly the creation of “bits” of localized granules of cured material. These bits can cause protrusion defects in the finished film.
Manipulating gel time is a somewhat complex proposition that should be carefully examined. I hope that this explanation helps.