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How to Properly Select UV Coating Photoinitiators
Release time:
2024-05-17 14:38
As an environmentally friendly and energy-saving new technology, one of the key elements of light-curing technology lies in the selection and application of photoinitiators. Also known as light-curing agents, photoinitiators possess unique properties: they can absorb specific wavelengths of energy within the ultraviolet (250–400 nm) or visible (400–800 nm) spectrum. After absorbing this energy, photoinitiators are converted into chemical energy, generating active species such as free radicals or cations. These active species then trigger the polymerization and crosslinking of monomers, thereby achieving material curing. Photoinitiators are indispensable core components of light-curable coatings; they have a decisive impact on the rate of light curing in these coatings and are crucial for ensuring fast and efficient curing.
In the formulation of UV-curable coatings, photoinitiators, as non-film-forming substances, are added in much lower amounts compared to oligomers and reactive diluents—typically ranging from 1% to 5%. If the amount of photoinitiator is insufficient, it may lead to incomplete curing of the coating, resulting in a tacky or sticky film. On the other hand, if the amount of photoinitiator is excessive, it can cause other problems, such as excessively rapid curing, poor leveling, foaming, and blistering. Therefore, both the photoinitiator itself and its photochemical reaction products must not compromise the chemical and physico-mechanical properties of the cured coating.
I. Classification of Commonly Used Photoinitiators
Photoinitiators can be classified into two categories based on their different abilities to absorb radiant energy: ultraviolet photoinitiators and visible-light photoinitiators. Ultraviolet photoinitiators primarily absorb energy from the ultraviolet region (wavelength range of 250–400 nm), whereas visible-light photoinitiators primarily absorb energy from the visible-light region (wavelength range of 400–800 nm).
According to their different initiation mechanisms, photoinitiators can be classified into free-radical polymerization photoinitiators and cationic photoinitiators, among which free-radical polymerization photoinitiators are the most widely used. Within these two categories, based on the differences in their mechanisms for generating free radicals, free-radical photoinitiators can further be subdivided into two types: cleavage-type photoinitiators and hydrogen-abstraction-type photoinitiators.
II. Key Considerations for Selecting Photoinitiators
When selecting a UV photoinitiator, the following factors are recommended for consideration:
1. Does the absorption spectrum of the photoinitiator match the emission spectrum of the light source? Spectral matching is a crucial principle in the photocuring process. It requires that the absorption spectrum of the photoinitiator closely align with the emission spectrum of the light source used, ensuring that the light energy can be efficiently absorbed by the photoinitiator and converted into the energy needed for the chemical reaction. By using several different photoinitiators simultaneously—each effective over a different wavelength range—it’s possible to initiate curing across a broader spectrum. Compared to using a single photoinitiator, this approach typically results in faster curing speeds.
2. The efficiency of photoinitiators. This includes the quantum yield of active intermediates generated and the reactivity of these active intermediates. Highly efficient photoinitiators can accelerate the photocuring reaction more rapidly, thereby improving production efficiency. In other words, a photoinitiator with high efficiency produces a high yield of active intermediates (free radicals or cations) and the generated active intermediates exhibit high reactivity.
3. For colored systems, since pigments exhibit varying absorption characteristics in the ultraviolet (UV) region, when selecting a photoinitiator, it is essential to take into account the pigment’s impact on UV light absorption. Dark-colored coatings are prone to issues where the deep layers of the coating fail to receive sufficient energy from the photoinitiator, thus hindering the effective initiation of the polymerization reaction. This can ultimately lead to poor curing performance in the deeper layers of the coating. This phenomenon is particularly pronounced in black coatings, often resulting in poor adhesion and even paint chipping during the cross-hatch test. By choosing a photoinitiator that is least affected by the pigment’s UV absorption, we can ensure the smooth progression of the photocuring reaction.
4. The selected photoinitiator should have good solubility and be fully soluble in both the active diluent and the oligomer. Good solubility helps ensure a uniform distribution of the photoinitiator throughout the system, thereby enhancing the efficiency of the photocuring process.
5. The photoinitiator selected must meet the requirements of low odor, low toxicity, and zero environmental pollution. In particular, the photolytic byproducts of the photoinitiator must also satisfy the criteria of low odor and low toxicity to ensure both safety and environmental friendliness during use.
6. Volatility and migratory properties of photoinitiators: Photoinitiators should exhibit stability with low volatility and be resistant to migration. Photoinitiators with low volatility and migratory properties can help reduce environmental pollution and health risks to humans.
7. After light curing, no yellowing should occur. This is particularly important for substrates that are white, light-colored, or colorless, as any color change can significantly affect the product’s appearance and quality. At the same time, the photoinitiator should not cause degradation of the polymer during aging, ensuring its durability and stability.
8. The photoinitiator must possess excellent thermal stability, which ensures the material’s reliability and durability under high-temperature conditions. Furthermore, the material must also demonstrate outstanding storage stability, meaning it is unlikely to decompose or undergo other undesirable side reactions during storage, thereby guaranteeing that the material can maintain its original performance and quality even after prolonged storage.
9. The ease of synthesis and cost of photoinitiators. The photoinitiators used should be reasonably priced and readily available. Photoinitiators that are easy to synthesize and have lower costs can help reduce production costs and enhance market competitiveness.
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