Glycol selection plays a foundational role in polyester resin synthesis, directly influencing processing behavior, coating performance, and long-term durability. While many formulators are familiar with industry-standard glycols such as neopentyl glycol (NPG), alternative glycol structures can unlock meaningful performance trade-offs when applied strategically. 

To better understand how glycol structure impacts resin and coating behavior, Eastman conducted a statistically designed triangle glycol study evaluating the three aliphatic and cycloaliphatic glycols commonly used in saturated polyester coatings, listed below. 

   

Using a designed mixture experiment, saturated polyester resins were prepared using varying ratios of these glycols as detailed in the chart below. To isolate the effect of the glycol structure, all resins were synthesized to similar acid numbers, hydroxyl numbers, and molecular weights. 

  

Impact of Glycol Structure on Polyester Resin Processing 

Glycol structure strongly influenced polyester resin synthesis behavior. The cycloaliphatic structure of Eastman™ CHDM provided the fastest processing times, attributed to reduced steric interference and improved accessibility of its primary hydroxyl groups. 

In contrast, Eastman TMPD™ processed the slowest due to steric shielding and the presence of a secondary hydroxyl group, which reduces reactivity. Eastman NPG™ fell between these two extremes, serving as a well-understood reference point for formulators. 

Several glycol blends demonstrated synergistic processing behavior, particularly combinations of CHDM and NPG, which delivered faster reaction times without a proportional increase in viscosity. 

The contour map below highlights the above findings, presenting a regression analysis that indicates faster processing times for blends containing higher amounts of CHDM. 

Cure Response and Film Formation 

Cure response trends reflected glycol-structure effects beyond resin synthesis. CHDM-containing polyesters demonstrated the fastest cure rates when cross-linked, as shown below, consistent with their increased reactivity during processing.

Despite slower processing, TMPD-based systems cured at rates comparable to NPG-based systems. Certain binary blends, including CHDM–TMPD compositions, exhibited faster-than-expected cure behavior, highlighting synergistic formulation effects.

VOC and Viscosity Trade-Offs 

VOC and viscosity trends emphasized inherent structure–property trade-offs. NPG- and TMPD-based coatings showed similar VOC levels under controlled test conditions, as shown below, though TMPD is often associated with lower VOC potential in commercial formulations. 

CHDM-based systems produced higher solution viscosities and required additional solvent to achieve application viscosity, resulting in higher measured VOCs. These effects stem from increased polymer-chain rigidity and intermolecular interactions. 

Mechanical Properties: Hardness and Flexibility 

CHDM-containing coatings delivered the highest film hardness due to increased glass transition temperature (Tg). This makes CHDM well suited for applications requiring abrasion or block resistance. 

Flexibility testing revealed TMPD as the least flexible glycol. NPG- and CHDM-containing systems offered improved flexibility, with certain blends providing a favorable hardness–flexibility balance through synergistic effects. 

Durability Performance in Aggressive Environments 

Durability testing further differentiated glycol structures. CHDM-based coatings demonstrated superior stain and detergent resistance, humidity resistance, and salt spray performance, driven by their higher Tg and tighter polymer packing. 

Across multiple exposure tests shown below, the cycloaliphatic structure of CHDM consistently enhanced long-term coating durability, albeit with formulation trade-offs related to viscosity and VOC. 

Conclusion 

Eastman’s polyester resin glycol portfolio provides formulators with a variety of powerful optimization levels, depending on end-use application. In general, NPG provides balanced, well-understood performance; TMPD enables high-solids/lower-VOC and stain-resistant formulations; and CHDM delivers superior durability and hardness. That said, rather than selecting a single glycol, many optimal formulations leverage strategic glycol blends to balance processing efficiency, mechanical performance, VOC targets, and environmental resistance. 

Interested in optimizing polyester coating formulations using “Eastman’s NPG, TMPD, or CHDM glycols? Click below to contact a member of our technical team to discuss formulation guidance, product selection, and samples.