Finishing or improving surface properties, such as chemical resistance to scratches, gloss and hardness of heat-sensitive materials, such as plastics, wood and certain types of glass, is a challenge.
These materials are adversely affected by surface finishing processes that produce heat. Heat damages the substrate and can cause discoloration and even deformation. For this reason many conventional methods of polymerization of coatings for protection, surface modification or adhesion, which require heat for film formation, cannot be used with materials such as plastics.
Cured with UV and EB energy
Two forms of polymerization of coatings, inks and adhesives, which does not generate heat that can affect thermosensitive substrates, are ultraviolet (UV) and electron beam (EB) curing. Curing is defined as the conversion of liquid into solid. UV and EB curing are special processes by which coatings, inks and adhesives are cured or dried on the substrate, using "energy" from UV and EB sources, instead of using conventional heat.
UV/EB curable coatings are usually composed of solvent-free liquids that can be applied to a substrate and turned into solid film in a fraction of a second by exposure to ultraviolet light or electron beam wavelengths. Unlike competing liquid technologies, no heat is required either to evaporate a vehicle solvent or to reticular a coating, which is an obvious advantage with heat-sensitive substrates.
UV/EB curable formulations
Both technologies involve the use of acrylate-based monomers and oligomers, which polymerize directly onto the substrate. Such polymerization occurs at high speed, when the coating, ink or adhesive is exposed to an ultraviolet light source or electron beam. A complete UV or EB formulation consists of a mixture or fusion of a base oligomer—which imparts most of the basic properties to the final material—monomers—which dilute the formulation to a convenient viscosity and help form cross-links between oligomer segments—and additives.
The latter, such as pigments, fillers, wetting agents, sliding aids or other materials, can be added to impart specific properties, aid in the process or improve performance. Photoiniciators, which absorb light and are directly involved in the production of free radicals, are necessary for UV curing but not for the EB process.
Formulations developed to improve the surface properties of heat-sensitive materials vary even better than the properties that need to be improved. Coatings, inks and adhesives for UV and EB curing are fully solid, which means that their formulation does not need either solvent or water. Heat-sensitive materials can be coated to improve durability or appearance. Scratch, chemical and weather resistance properties, to name a few, can be improved with the addition of UV or EB curing formulations. Adhesives can be used to adhere two heat-sensitive substrates either permanently or transiently, as may be required by some self-adhesive applications.
Application of a UV/EB formulation
Coating, ink, and adhesive formulations that have been developed to be cured with UV or EB energy can be applied using all conventional application methods—curtain coating, screen printing, or spraying to name a few—and cured using UV or EB instead of heat-generating methods. Once coated, substrate materials are transported through a UV light source or EB curing unit, although it can also be done with static UV curing.
The UV and EB curing equipment
The curing equipment of both systems is relatively easy to use and both can complement most conversion operations. Being radiant energy, they directly convert reactive liquid formulations into solids and do not depend on evaporation systems that generate heat and consume a large amount of energy in the removal of solvent or water. The ultraviolet light spectrum in a UV lamp and the directed electrons in an electron beam interact with acrylate chemicals specially formulated to cure materials.
EB technology, in very simple terms, uses electron beam accelerators, a power supply, an acceleration chamber, beam windows, and safety protection and power take-off barriers. UV equipment usually consists of a light source, which is usually a glass tube, an electrical supply, a reflector system to concentrate UV radiant energy on the white substrate and a conveyor belt system to move the coated substrate, although static curing is also possible.
UV and EB equipment have a lot in common, but there are also differences. The costs are different; EB equipment is higher in cost than UV equipment. EB curing methods are often able to operate at much higher line speeds, allow for greater penetration, and can cure opaque materials. Continuous and prolonged operating types are more useful for EB technology, while operations that require material changes favor UV technology.
Advantages of UV and EB curing
The advantages of both systems not only have to do with the use in thermosensitive materials and in the fact that they do not exert a negative impact on the substrate. The formulations offer a superior finish both aesthetically and in terms of performance. They also confer higher productivity and offer eco-friendly processing alternatives.
It is possible to produce aesthetically modified surfaces, the result of which is a greater gloss without yellowing or finishes. Pigments and fillers can be used to produce a variety of ink and coating finishes.
Scratch, chemical and water resistance are physical properties that can be produced to generate finishes that protect the substrate and allow materials to be used in applications that would not be possible without finishing.
The use of UV and EB curing also results in higher productivity and consequent better profitability, along with lower energy consumption. The ability to handle materials immediately after curing, a shorter exposure time that offers cleaner finishes with fewer defects and rejections, and energy efficiency are also factors that result in greater use of UV and EB curing. These finishing methods for heat-sensitive materials also favor the recyclability of the substrate and are ecological.
Future of curing in heat-sensitive materials
Going forward, the forces driving the commercial use of these technologies applied to heat-sensitive substrates are not only energy savings and the fact that the systems are generally solvent-free, but also high productivity, as an additional benefit. Other factors that have also driven the advancement of technology, in addition to the better appearance of the product, are the reduction of total costs and the greater productivity in printing, decoration, adhesion or coating processes, along with the almost total elimination of VOCs. In the future, greater use of UV or EB curing in the finishing of heat-sensitive materials is envisaged. UV or EB cured coatings, inks and adhesives are a durable, attractive and economical alternative to conventional coatings, which require heat for development or cross-linking.
The forces that initially drove the commercialization of these technologies were energy savings and the fact that the systems are generally solvent-free. High productivity and consequent higher profitability are additional benefits that will also benefit producers working with heat-sensitive materials.
Flexible plastics they use curing UV and EB include printed adhesive labels, vinyl films with decorative paper laminates, plastic tubes, shrink films, phonographic films, magnetic recording media, coated abrasive films, non-stick films, CD, DVD, credit cards and many more. DVD discs use UV-curable materials that are coated spinning and then cured.
The main advantages in the raw materials of monomers, oligomers and photoinitiators make UV and EB technologies suitable for coatings on automotive plastic parts for outdoor use.
Not only is the UV/EB curing process more effective for the coating of plastics, but also the selection of monomers and oligomers allow a solvent-free, VOC-free application with fast curing, higher productivity and lower energy consumption. Designed for use in heat-sensitive substrates and high-speed production of delicate printed images on sophisticated printed circuit boards, UV and EB curing produces a finish with superior corrosion resistance.
Plastic glass or lenses can be coated for scratch resistance and darkening or polarization. Cell phones, with plastic parts, thermosensitive, coated with UV or EB coatings and inks to protect from scratches and moisture, and with better appearance.
There are many applications where radiation curing is commercially viable and can be used to benefit the finisher and to improve product performance. These benefits can only be made visible when all parties involved in the process, including raw material suppliers, formulators, application and curing equipment suppliers, converters and finishers, as well as end users, are educated in the principles and processes of this technology.
*Sartomer Company
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