Guidelines for Selecting Photoinitiators in UV Coating Formulations

Overview of Photoinitiators

– In light-curing products, photoinitiators serve as a critical component. They are substances capable of absorbing radiant energy and undergoing chemical reactions to generate reactive intermediates (free radicals or cations) with polymerization-initiating capabilities.

– In practical production, radical photoinitiators that generate free radicals are predominantly used, while cationic photoinitiators producing cations are extremely rare. This article focuses on radical photoinitiators.

Classification of Photoinitiators

• Radical photoinitiators are primarily categorized into two types based on their mechanism of generating active radicals: cleavage-type radical photoinitiators (also known as Type I photoinitiators) and hydrogen-scavenging radical photoinitiators (also known as Type II photoinitiators).
• Common cleaving-type photoinitiators are structurally predominantly arylalkyl ketone compounds. Commonly available grades include: 184, 2959, 651, 907, 369, 1173, 819, TPO, MBF, 754, etc.
• Common hydrogen-scavenging photoinitiators are structurally derived from benzophenones or heterocyclic ketones. Commonly available grades include: BP, ITX, 2-EA. Additionally, hydrogen-scavenging photoinitiators require co-initiators for activation. Currently, the primary co-initiators used are reactive amines and tertiary amine-type benzoic esters.

Selection of Photoinitiators

The effectiveness of photoinitiators in triggering polymerization reactions within light-curable products—and ultimately achieving the desired performance—depends on the harmonious interaction between the photoinitiation system, irradiation conditions, and product components. Therefore, selecting appropriate photoinitiators based on specific production processes and product formulations is particularly crucial.

The following sections will detail photoinitiator screening methods by examining their properties and illustrating them through specific case studies.

The absorption spectrum of the photoinitiator must match the emission spectrum of the light source.

Common light sources on the market include mercury lamps, LED lamps, induction lamps, and metal halide lamps. Among these, mercury lamps are the most widely used, emitting a spectrum between 200-450nm and serving as a general-purpose option. LED lamps are widely used in low-energy curing applications, with emission wavelengths concentrated around 365/375/385/395/405 nm.

When selecting a photoinitiator, choose one that exhibits significant absorption characteristics in the corresponding wavelength band of the light source’s emission spectrum.

Case Study:

In gel polish formulations, photoinitiator selection is heavily constrained by the light source. Common nail lamps use two types of tubes: fluorescent and LED. Fluorescent tubes emit within 370-420nm, while LED tubes emit around 365nm/395nm. Both emit in the long-wave region, requiring initiators that absorb longer wavelengths.

Table 1 lists the absorption peaks of various common photoinitiators. To achieve optimal initiation, photoinitiators with absorption peaks above 365nm should be selected, such as TPO and 819. Although 784 has a longer absorption peak wavelength, its high cost limits its market adoption.

In practical testing, TPO and 819 demonstrated the best performance among all photoinitiators, consistent with the predicted results.

Selection of Photoinitiators for Deep Curing in Colored Systems

• In colored systems, particularly dark-colored ones, pigments themselves absorb a portion of UV energy, preventing UV light from penetrating the paint film. This prevents deep-layer photoinitiators from absorbing sufficient energy to initiate polymerization, ultimately resulting in inadequate deep curing. Mild cases may exhibit reduced adhesion, while severe cases can cause surface wrinkling, compromising both the paint film’s appearance and its physical and chemical properties.
• Within the UV spectrum, longer wavelengths exhibit superior penetration capabilities, enabling them to reach deeper layers of the coating film more effectively. Conversely, shorter wavelengths struggle to penetrate to these depths. Consequently, without long-wave photoinitiators to absorb the energy from these longer wavelengths in the deeper layers, polymerization initiation becomes challenging. Therefore, deep-penetrating photoinitiators are indispensable in pigmented systems. Referring to Table 1, combining long-wave photoinitiators like TPO/819/651 with short-wave photoinitiators such as 184/1173 yields favorable results.

Case Study:

In UV single-coat color systems, black formulations often exhibit poor adhesion and crosshatch adhesion failure. Adding 1.5% 819 to the formulation significantly improved film adhesion, demonstrating 819’s role in promoting deep-cure.

Additionally, in black/white systems, the combinations 907/ITX + 184 and 369/ITX + 184 delivered outstanding results.

Selection of Photoinitiators for Systems with Yellowing Requirements

In certain varnishes and white systems, yellowing resistance is a critical indicator for evaluating coating film performance. Beyond selecting resins and monomers with excellent yellowing resistance, the yellowing tendency of photoinitiators should also be minimized. Photopolymerization initiators containing substituents like N-dimethylamino in their conjugated structures generally exhibit higher irradiation-induced yellowing tendencies. Similarly, the presence of such substituents in reactive amine structures also exacerbates yellowing.

Table 2 presents the yellowing indices of various photopolymerization initiators, using propyl oxide pentaerythritol triacrylate as the base material and without any initiator as the blank reference.

As shown in the table above, 184, 1173, 754, and MBF are all photoinitiators with minimal yellowing, making them the optimal choice for varnish and white system formulations.

Good solubility in active diluents and oligomers

Excellent solubility is a crucial prerequisite for incorporating photoinitiators into systems; superior compatibility ensures greater formulation stability.

The following table shows the solubility of selected photoinitiators in common solvents and monomers.

In recent years, water-based coatings have become increasingly prevalent, with water-based UV coatings also gaining significant attention. Currently, products with high water solubility on the market are few and far between. Commercially available options include: KIPEM, 819DW, BTC, BPQ, QTX, etc. 2959 achieves a water solubility of 1.7% and can also be used in water-based UV products.

Other Properties

When selecting photoinitiators, prioritize those with low odor, low toxicity, good thermal stability, and minimal volatility or migration. Ensure the selected photoinitiator components comply with local laws and regulations.

 

Conclusion

In summary, the selection of photoinitiators is not an isolated task but must be coordinated with the entire system and even the application process. It requires comprehensive consideration of the light source, other components in the system, and the performance requirements of the light-cured product to choose a photoinitiator that is both economical and highly effective.