Dear Joe,
In polyester/TGIC systems, the Tg is sensitive. Every weight percentage of TGIC reduces the Tg by approximately 1.5 to 2ºC. Why does TGIC reduce the Tg of the coatings

Dear Joe,

In polyester/TGIC systems, the Tg is sensitive. Every weight percentage of TGIC reduces the Tg by approximately 1.5 to 2ºC. Why does TGIC reduce the Tg of the coatings? My idea is that TGIC has a very low equivalent weight, and therefore a low crosslinking density. Generally, low equivalent weight hardeners reduce the Tg. The same thing happens with polyester/Powderlink 1174, which also has a low equivalent weight. But this effect does not occur with polyester/hydroxyalkyl amide systems. Why?

Shiva

Dear Shiva,

Before I answer your question, I would like to define Tg for our readers. Tg refers to “glass transition temperature,” which is reported in degrees Celsius. It is typically measured by exposing a sample to an increasing temperature and measuring the heat flow of the material versus a blank sample. The most common method uses a thermoanalytical test known as differential scanning calorimetry, or DSC. The Tg is relevant because it identifies the temperature at which the material, a powder coating in this case, undergoes a change from a rubbery to a glassy state. In other words, it’s a fancy melt point temperature test.

In polyester powder coating formulas, the addition of triglycidyl isocyanurate (TGIC) depresses the Tg of the powder coating. For every percent addition of TGIC, the powder Tg is reduced approximately 1.5 to 2.0ºC because the TGIC is soluble in the polyester resin. As you state, TGIC has a low molecular weight (297 g/mol) and equivalent weight (101 g/mol). However, it is the solubility that causes the depression in Tg.

Cytec’s Powderlink 1174 (tetramethoxy methyl glycoluril) also depresses the Tg of polyester powder coatings due to its solubility in the resin. Its molecular weight is 350 g/mol. Formulations using Powderlink 1174 typically contain higher than normal Tg polyester resins to compensate for the Tg reduction caused by the Powderlink.

Primid crosslinkers (hydroxy alkyl amides, or HAAs) possess limited solubility in the polyester resins typically used to formulate powder coatings. Because of this effect, it is important to achieve adequate dispersion of the HAA in the polyester resin during extrusion as a powder is manufactured. Accordingly, HAA crosslinkers do not depress the Tg of the powder coating. Interestingly, the most common HAA product is Primid XL-552, and its molecualar weight is 320 g/mol.

So Tg depression is caused by the solubility of a material in the base resin, instead of by the molecular or equivalent weight.

Thanks for your interesting question, and good luck formulating powder coatings.

Dear Joe,

We recently stripped an oval dining table about 17 ft long and approximately 4 ft wide. We are unable to find two-part wood bleach and were advised that it has been put into a more dangerous category and will no longer be available. Why? Is there any other bleaching product available for wood refinishing?

Louis Baum

Hello Louis,

It sounds like you have a difficult project on your hands. Basically there are three types of wood bleaching systems:

• Two-part peroxide
• Chlorine
• Oxalic acid

They all work well if used correctly. The two-part peroxide system has a limited shelf life because the mixture is neutralized with time. It doesn’t remove the stronger dye stains. Chlorine works well, but you have to make sure the concentration is high enough. Regular laundry bleach is too weak. The oxalic acid system works uniquely. It kills stains caused by the reaction of iron with natural wood tannins. It also lightens graying due to weathering of wood surfaces.

Good luck with your project.



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