Improve UV Curables with Cellulose Ester Additives

Higher productivity is achieved due to near-instantaneous UV curing, making coating and printing processes more economical. Furthermore, the lack of heat in UV curing is ideal for preventing damage to heat-sensitive materials, such as plastics. Lastly, the absence of organic solvents in UV-curable systems contributes to significantly reduced VOCs.

There are some drawbacks that one should consider when formulating a UV-curable coating or ink. The absence of a solvent and fast-curing speed can lead to poor wetting, leveling, and flow properties. Due to the high cross-link density that forms in UV coatings and inks, shrinkage can also occur upon UV curing, causing adhesion issues. Ultimately, these issues can lead to a coating or ink with suboptimal appearance and performance. Cellulose esters, like cellulose acetate butyrate, can be used as additives to mitigate these issues and protect the value of UV-curable coatings and inks.

Eastman cellulose acetate butyrate (CAB) additives with high butyryl content are compatible with and dissolve in common UV-curable acrylate and styrene monomers. Below is a chart outlining a few recommended CAB grades and their solubility in these monomers.

Case Study: UV-Curable Overprint Varnishes

When formulated into UV-curable applications, such as inks and overprint varnishes, cellulose acetate butyrates function as wetting and leveling additives. This is important, especially given that substrates used in these applications include low–surface energy plastics, such as TPO. Below are formulations comparing an overprint varnish with and without Solus™ 2100.

For a liquid to achieve good wetting on a surface, it must have a surface tension close to or lower than that of the surface. As observed above, the liquid surface tension of the formulation containing the Solus™ 2100 additive is lower than the control and can provide better wetting. However, the cured-film surface tension of the formulations highlights much more of a difference. The formulation containing Solus™ 2100 has a surface tension much closer to a level to that of TPO (27.6 dynes/cm). Generally, better wetting increases film formation capabilities, reduces the chance for visual defects, and can improve adhesion and mitigate shrinkage. Thus, using Solus™ 2100 is especially important for reducing and eliminating issues pertaining to film formation that can occur with curtain coating and high-speed roll coating applications.

The effect of the increased wetting provided by Solus™ 2100 can be seen further below by examining the drop spread of the overprint varnish formulations applied to TPO.

Here, the formulations were measured over two minutes and compared for relative drop spread, where higher relative drop spreads indicate better wetting of the system relative to one another. The control formulation without Solus™ 2100 applied to TPO had no drop spread, meaning the formulation had poor to no wetting. On the other hand, the formulation with Solus™ 2100 began to show better wetting almost immediately after application.

These formulations were also visually inspected upon application on TPO and after 150 seconds. Images of this inspection, shown below, reveal that the control formulation receded over the 150-second time frame due to its relatively high surface tension. This recession indicates poor wetting and provides a visual example of the film-formation challenges a formulator may face without incorporating a cellulose ester, such as Solus™ 2100.

For applications where overprint varnishes or inks are applied to food packaging, Eastman offers cellulose ester additives that are approved for both direct and indirect food contact. Eastman manages this by owning cGMP compliance for its cellulose ester manufacturing process.

Using Cellulose Esters in UV-Curable Systems

As mentioned, cellulose acetate butyrates are typically used as additives that are dissolved in monomers and added into UV-curable coatings and inks. The table below lists Brookfield viscosities of recommended grades when used at a 5% concentration in common monomers. Cellulose esters with lower molecular weights can be used in applications where viscosity is a limitation.
 

Legend
Value = Brookfield viscosity at 24° C
I = Insoluble

Summary

UV-curable inks and coatings provide many benefits over solvent- and water-based inks but may suffer from visual defects and performance issues. Eastman's cellulose acetate butyrate (CAB) additives reduce or eliminate film-formation, wetting, and leveling issues that occur with UV-curable inks and coatings. Click below to request more information and start working with a sample for your formulation.

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