Q: I do not know how to control the glass transition temperature (Tg) of a powder coating. I know that TGIC (a curing agent) reduces the Tg value by 1.5°C. But what about other ingredients, like additives (flow agents, CAB, texture agents, waxes, fillers, pigments, etc.)?
In the summer, my customers have problems with sintering. The Tg value of resins that I use is high enough, but I think the additives may be lowering the powder coating Tg. Do you have any suggestions about this?
A: This is a very good question, especially if the powder coating will be exposed to relatively high ambient temperatures. The Tg refers to the point at which a material undergoes a change from a rubbery to a glassy stage. It’s measured analytically by differential scanning calorimetry (DSC), which measures the flow of heat from a material as it experiences a prescribed increase in temperature (usually at a rate of 10°C per minute).
As a material melts, it absorbs heat (endothermic). When a chemical reaction occurs, heat is released (exothermic). You probably know this already.
In layperson’s terms, Tg is best considered a sophisticated method to measure softening or melt point. The Tg of a powder coating before undergoing cure is a good indication of melt point and correlates to physical storage and handling stability. The Tg of a cured powder coating is a good indication of degree of cure, hardness, chemical resistance, and overall coating performance.
Therefore, your question relates to the effect formulation components have on the Tg of a powder coating, in this case a TGIC polyester. The Tg of a polyester powder coating is mainly influenced by the Tg of the polyester resin used. Nevertheless, other components affect the overall Tg of the powder coating, especially TGIC, which as you mentioned depresses the Tg about 1.5°C. Other components in the formulation that can decrease Tg include waxes, cellulose acetate butyrate (CAB), and, to a lesser extent, flow agents.
Pigments and extenders do not affect Tg, as they are inorganic and inert; they do not melt or affect the melt point of the resinous components. They may affect the melt viscosity, but this will have little (if any) effect on the Tg.
As mentioned, the Tg of the polyester has the greatest effect on Tg. Commercially available polyester resins span a range of Tg from about 52-73°C. If you are attempting to increase the Tg of a powder coating, you should investigate formulating with a higher Tg polyester. If your formula contains organic additives such as wax, CAB, or plasticizer, you should evaluate formulas with reduced levels or the elimination of these additives.
Another issue to consider is the quality of the polyester resin. Low-quality polyesters may have low-molecular-weight impurities that can reduce Tg and compromise storage stability.
In addition, I recommend that you evaluate the effect of every organic additive present in the formula on the powder coating Tg. This includes waxes, flow agents, plasticizers (e.g., CAB), UV absorbers, light stabilizers, catalysts, and antioxidants. Pay close attention to the melt point of each of these materials; eliminate the bad actors and/or replace them with similar materials possessing higher melt points.
DSC is the best technique to measure the Tg of powder coatings. The method is simple to run, and the data generated is unambiguous. If you do not have access to a DSC instrument, you can use a more empirical method such as storage stability in a temperature-controlled water bath. The temperature of the water bath should reflect the ambient conditions the powder may encounter in manufacture, transport, storage, and application. The powder sample should be placed in a watertight container to ensure that the water bath does not contaminate the sample. Powder samples should be evaluated for clumping and/or sintering after 24 hours and one week exposure.