The solvent-free formulation of this technology leads to the non-emission of volatile organic components.
By Painting Zone
A reddish-brown fog settled in recent weeks over the skies of New York, the scene of more than one apocalyptic movie. The scene was accompanied by the return of N95 face masks to deal with a climate crisis that has lingered since before the pandemic, this time exacerbated by Canada's wildfires.
However, this was just a sample before the main reflectors of the Capital of the World of the photochemical smog with which millions of people on the planet must deal, especially India, considered the most populous country today. According to researchers, six of the ten cities with the highest air pollution are part of this nation that at the end of 2019, when Covid-19 infections began, saw 1.6 million people die from their exposure to high levels of particulate matter 2.5.
This type of smog, first identified in 1943 in the city of Los Angeles, is typical of urban areas with a lot of industrial activity. The main primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOCs), which are destroyers of the ozone layer and lead to a secondary pollutant such as ground-level ozone (tropospheric ozone), which is harmful to the health of all species.
As we explained in the first edition of Zona de Pinturas this year, although some of these compounds are of natural origin, most come from human action when using organic solvents. Thus, conventional, solvent-based paints comprise one of the main emission sources.
Faced with this, the industry has advanced in different sustainable alternatives such as water-based liquid coatings, with little or no solvent, as well as thermosetting powder paint. All of these can be cured through ultraviolet (UV) technology.
In 2004, a Clean Air Technology Center (CATC) technical bulletin on ultraviolet and electron beam cured coatings, inks and adhesives highlighted the increasing increase in their use over the past decade. Since then, this Pollution Prevention (P2) technique has expanded into a variety of applications: from coating, printing and exclusive products.
UV curing ink, for example, has proved very attractive not only for its almost instantaneous drying properties, but for stopping emitting VOCs. The development of ultraviolet light technology dates back to the mid-60s and becomes an advanced method to cure printing inks in the 80s, extending its use to the area of varnishes, adhesives and different types of coatings.
This is considered the most effective system for the rapid transformation, at room temperature, of a solvent-free liquid resin into a polymeric product with high strength.
UV curing results in photoinitiating the polymerization of monomers and multifunctional polymers to become a three-dimensional structure.
"The monomers are molecules of very low molecular weight that will allow precisely to have low viscosity, and with this the viscosity levels in the system are adjusted to those that are appropriate," explains the expert technician in paints and coatings of First Quality Chemicals, Oliver Peña.
The viscoelastic and mechanical properties of UV-cured polymers are precisely controlled from the chemical structure and functionality of the monomer to be used, together with the curing conditions, according to the determined application.
As Peña points out, monomers help modify the properties of the film due to their different functionalities. This is used as a diluent reagent, outside of its role as a viscosity adjuster.
In general, the hardening of organic coatings by means of light occurs from a polymerization reaction with radical mechanism, such as acrylic resins, or through a cationic mechanism, such as epoxy resins.
However, the photoinitiator is essential not only to control the degree of initiation of the reaction, but also the penetration of the incident light, and with it the depth of curing.
"The photoinitiator is the one who will dictate the limitations for the arrangement of the curing equipment, since there are photoinitiators that have absorption peaks in 400 namómetros, while there are some that have absorption peaks in 200," says Peña, who points out that having one of the wavelengths in which the enhancer absorbs is vital to optimize the formulation and the finished product.
Likewise, he comments that there are different types of photoinitiators: cationic, anionic, and radical polymerization. These consist of aromatic ketones that generate free radicals when exposed to ultraviolet light; either by opening the double bonds C – C or subtraction of hydrogen from the donor molecule.
And well, how do you cure it by exposure to ultraviolet radiation? According to Peña, polymerization and curing follow the three stages of conventional polymerization: initiation, propagation and termination.
As he clarifies, a high film thickness forces a greater curing requirement with long wavelength, "and they will begin to generate what we know as oxygen inhibition because the free radical that is on the surface of the coating will begin to generate hydroperoxides and peroxides, which lead us down other paths that we do not require."
At the same time, it suggests not making pigmentation uniform with any type of pigment. "Reds will heal faster than yellows, yellows than blues, and blues than blacks; This is due to the difference in light absorption, reflection and transmission," he says.
Benefits of technology
Among the benefits of this technology, Oliver Peña highlights the energy savings compared to baked systems by allowing high production speeds, so it is not necessary to spend more time for the delivery of the product.
"It has a lower storage requirement and ultraviolet curing equipment can be not as complex, and processing is possible almost immediately after curing," he says.
However, it emphasizes the ecological benefits of having a formulation free of volatile organic compounds, which allows reducing emissions through the process. To this is added the performance. "You have low heating of substrate, which helps because you do not have an aggressive cure with the substrate as such," he points out and remarks that it can generate products of high durability and versatility, as well as chemical and mechanical resistance against solvents.
UV technology for coil coatings
Earlier this year, Beckers, a supplier of coil coatings and industrial paints, developed the first commercial paint formulations for dual ultraviolet and electron beam curing in the coil coatings market.
The innovative technologies, which build on Beckers' 18 years of pioneering work, can significantly reduce VOC and CO2 emissions and improve efficiency in the industry by providing up to twice the surface coverage per kilogram of paint.
The European Coil Coating Association (ECCA) considers ultraviolet (UV) and electron beam (EB) curing technology to be the most important game changer in terms of decarbonization of the coil coating industry.
Until now, the only available industrial application of radiation curing in coil coating has been electron beam (EB) irradiation used for packaging material.
"UV/EB curing allows us to use up to 100 percent solvent-free solid formulations that are more efficient and provide up to twice the surface coverage per kilogram of paint," said Beckers CTO Gavin Bown.
Solvent-free solid UV/EB paint formulations are more sustainable, requiring approximately half the paint volume and significantly reducing volatile organic compounds (VOCs).
"Compared to conventional gas curing, the UV/EB cold curing process also uses much less energy, facilitates the transition from natural gas to renewable electricity, avoids the use of expensive gas, and does not require water for cooling," Bown added.
"Additional benefits of UV/EB curing include a lower transport carbon footprint, as about 50 percent less paint is required per m2 of surface coverage, and the instant curing process offers the potential for coating," he said.
Beckers is currently working with customers who are completely replacing their conventional coil coating lines with UV/EB curing or modernizing existing lines. Others are adding additional coating steps or partially switching to UV/EB on their existing lines.