Why use additives?
Additives are materials that allow for the fine tuning of a formulation. These materials are used to impart properties or cosmetic effects that cannot be achieved with only oligomers and monomers. Examples of these properties include:
As you can see, there are several opportunities to use additives in your next formula!
Forms of Additives
Additives can fall into several categories: reactive additives, non-reactive additives, liquids, and powders. Some additives dissolve into the system, while others will need to be dispersed. Due to there being various types of additives that impart different properties, there are many ways that additives need to be handled. Some can be added directly to the blend of oligomers and monomers, while others will need to be dissolved or dispersed into lower viscosity ingredients like monomers, before being added to the blend.
Differences from Evaporative Systems (100% solids vs. evaporative)
Unlike evaporative systems, most UV curable coatings do not contain volatile solvents that will evaporate. In an evaporative system, when water or solvents evaporate, the film thickness is reduced allowing suspended solids to concentrate towards the surface. The amount of particles does not change as solvent evaporates, but as the film thickness reduces, the particles are now dispersed in only the solid portion of the coating. Therefore, in a 100% solids system, more additives or additives with a larger particle size need to be used to achieve the same effect as seen in an evaporative system.
Additives for Outdoor Exposure
To protect a substrate or part from the sun, UV absorbers and hindered amines are typically added to the UV formula. Although these are common ways to protect a coating, they interfere with UV light transmission through the film. If there are issues with through cure, the UV package could be the culprit. This can be remedied by changing the wavelength of the cure unit to produce longer wavelengths that are closer to the visible region. In this range, the UV package will be more transparent.
Reactive additives are used to reduce or eliminate migration or leaching post-cure. Migration and leaching many times occur with silicone additives. Silicones are added to reduce the coefficient of friction (CoF) of the system. After curing, they will slowly exude from the coating, producing slip. However, the silicones can be removed by handling or cleaning. Slightly higher concentrations of reactive silicones are required because of how they react into the coating and do not migrate. These higher concentrations allow for the additive to more homogenous in the film and as the coating wears, the silicone concentration will not be reduced.
Oligomers and Monomers as Additives
By adding small amounts of highly functional oligomers or monomers, cure speed increases and oxygen inhibition is reduced. Care must be taken as not to make undesirable characteristic changes based on other oligomer or monomer properties. Higher functional oligomers impart hard, less flexible properties and if added to a soft and flexible formula, may make it too hard and brittle. Sometimes, the added crosslink density can increase the tensile strength, creating stronger film with higher elongation.
This is just a sampling of the many types of additives that can be used in free-radical curing systems. Learn more by downloading the Bomar Jaylink additive sell sheet.