A photoinitiator is a compound that undergoes a photoreaction on absorption of light, producing reactive species. These are capable of initiating or catalyzing chemical reactions that result in significant changes in the solubility and physical properties of suitable formulations. Hence, the photoinitiator is a compound that can transform the physical energy of light into suitable chemical energy in the form of reactive intermediates.
These changes are most commonly achieved by polymerization or polycondensation reactions. The process set off by a photoinitiator and light is called photopolymerization or radiation curing. It transforms a soluble liquid formulation into a hard and insoluble crosslinked polymer network. The cured coating is chemically and physically resistant and is used both to protect and decorate substrates such as plastics, wood and metal.
Radical polymerization of acrylate- or styrene-based formulations is the most widespread application so far, and a broad variety of radical photoinitiators has been developed. Most radiation curing is performed using near UV light (300-400 nm range), but initiators that extend into the visible, up to the infrared (IR) range, or on the blue side to the deep UV range are also available today.
Cationic photoinitiators that produce either a Brönsted or Lewis acid are used as initiators for cationically polymerizing materials (e.g., epoxies) or for resins capable of undergoing crosslinking via polycondensation reactions. The use of light to trigger the initiation process is the key to the outstanding advantages of radiation curing technology.
This is an environmentally progressive process because the formulations used contain only a minimal amount of solvent, if any. All components are copolymerized or entangled in the cured resin, and the amount of volatile organic compounds (VOC) is therefore very small.
Low-temperature curing: Since the initiation step is triggered by light, materials that would not withstand thermal treatment can be cured at low temperatures.
Space saving: Radiation curing is a much faster process than thermal treatment, allowing higher belt speeds and requiring less space for stoving. Cool-off zones are limited, if needed at all, and objects can be stacked immediately.
Control of time of the curing process: In the absence of light, the photoinitiator is a chemically and thermally stable compound. Formulations can be prepared and stored for long periods without any changes. Application of a suitable light source allows initiation of rapid curing "on demand".
Control of the geometry of the curing process: Light is easily applied under controlled spatial conditions. A photomask enables a formulation to be polymerized in a well-defined geometrical area, and the parts not exposed to light remain liquid. A suitable development process, which removes the non-polymerized part of the formulation, allows relief images to be produced.
The formulation is a blend of different ingredients designed to meet the requirements of the application. The photoinitiator is an additive typically used in concentrations of 0.5 - 6 % by weight. The uncured formulation requires:
Viscosity: This must be in accordance with the application process (e.g., rolling, dipping, spraying, etc.) and is mainly adjusted by the reactive diluent. Suitable additives, such as slipping agents, can further improve application properties.
Reactivity: The photoinitiator is the key component as regards the sensitivity of the formulation to light. Suitable selection of this additive is therefore of primary importance. For example, it must absorb light in a spectral range where the other components are sufficiently transparent.
A model for a urethane acrylate formulation
Cure speed is also influenced by the type of reactive diluent and oligomer used and their reaction during polymerization.
The cured coating is used either to protect the underlying material or to enhance esthetics. The composition of the formulation must be carefully balanced to meet the requirements of the cured film.
All components of the formulation influence the properties of the cured coating, including physical and chemical stability, flexibility, hardness, long-term durability, adhesion and appearance. The oligomer backbone determines properties such as flexibility and hardness. Polyfunctional monomers control the network density and thereby the mechanical characteristics. Additives are used to improve long-term stability or properties such as adhesion to the supporting material. Fillers and pigments adjust mechanical and physical properties and add color. Light stabilizers and pigments absorb light in the UV or visible range and might therefore compete with the photoinitiator for incidental light during the first step of the photopolymerization process. Careful selection of the photoinitiator is thus crucial for optimum curing efficiency: a-hydroxy ketone photoinitiators are highly versatile in clearcoat applications, while the recently developed bisacylphosphine photoinitiators allow curing of pigmented lacquers. The photoinitiator is mostly consumed during the curing step, and its cleavage products remain as end groups on the polymer network.
In coating applications, the formulation is applied as a thin layer (approximately 5-100 micron) on a supporting material. This layer is cured by brief exposure to a suitable light source. UV light is most frequently used in these applications. The coated material is exposed to light on a conveyor belt under a lamp. Belt speeds vary from several meters per minute (e.g., heavy wood panel coatings) to several hundred (e.g., protective coatings on paper). This cure speed, which is much higher than that of conventional curing processes, allows very high production throughput.