Principles for the Application of Free-Radical Photoinitiators

Excerpted from: “Photopolymerization Technology and Materials”

As one of the key raw materials in photopolymerization formulations, photoinitiators require adherence to several common principles during formulation design and application, such as: Principles of compatibility with light sources, with pigments, with coating thickness, dosage principles, and other principles (solubility principle, combination principle, safety principle, price principle) And so on. Regardless of the matching principle employed, the ultimate goal remains the same: to develop cost‑effective formulation products. The requirements for photoinitiators vary widely across different formulations; the specific choice, dosage, and combination of photoinitiators must be determined through empirical experimentation—especially as personalized, custom‑made products become increasingly prevalent, with each distinct performance profile demanding a corresponding tailored photoinitiator.

Principle of matching with the light source: Currently, mercury lamps are the predominant light source in the photopolymerization industry. The main spectral lines of conventional medium-pressure mercury lamps are listed in Table 3, and Figure 10 shows the ultraviolet emission spectrum of such a lamp. As can be seen from Table 3 and Figure 10, mercury lamps emit radiation across a broad wavelength range, from 220 nm to 1300 nm, with varying intensities. Metal‑halide lamps are a type of mercury lamp that can enhance the intensity at specific wavelengths by incorporating different metal additives into the lamp’s arc, thereby tailoring its emission spectrum. In practice, they are often used in conjunction with conventional medium-pressure mercury lamps. Therefore, when formulating photopolymerization systems, it is essential first to consider the type of light source and select photoinitiators whose absorption peaks match the source’s emission profile, in order to maximize the efficiency of photoinitiator utilization. For example, α‑hydroxyketone photoinitiators have relatively short absorption wavelengths and can be effectively activated by standard medium-pressure mercury lamps. In contrast, acylphosphine oxide and thioxanthone photoinitiators exhibit longer absorption maxima, typically around 370–400 nm; if an iron‑doped lamp—designed to boost the 370–390 nm band—is employed, the resulting polymerization performance can be superior compared to that achieved with conventional medium-pressure mercury lamps.

Today, UV‑LED light‑source technology is becoming increasingly mature. In particular, the commercialization costs of 365 nm, 385 nm, 395 nm, and 405 nm wavelength sources are steadily declining. Compared with mercury‑lamp sources, UV‑LEDs offer numerous advantages—such as energy efficiency, environmental friendliness, high efficacy, health safety, and long service life—prompting greater investment in formulation research and development tailored to UV‑LED technologies. However, since UV‑LEDs emit monochromatic light, the range of available photoinitiators is significantly narrower than that for mercury lamps. Consequently, selecting an appropriate photoinitiator for UV‑LED systems requires careful consideration of compatibility. When formulating UV‑curable systems for UV‑LEDs proves challenging, employing a hybrid approach that combines UV‑LED and mercury‑lamp irradiation can, to varying degrees, help achieve energy‑saving and environmentally friendly outcomes.

Principle of color matching: The principle of matching photoinitiators with color primarily means that the photoinitiator’s UV absorption peak should align with the color’s transmission window. The so‑called transmission window refers to the spectral range in which the pigment or dye absorbs light relatively weakly, allowing UV radiation to pass through more effectively and thereby enhancing its interaction with the photoinitiator. If the photoinitiator’s UV absorption peak does not match the pigment/dye’s transmission window, the pigment or dye will compete with the photoinitiator for absorption of the corresponding UV wavelengths, reducing photoinitiation efficiency. Coupled with oxygen inhibition, this can even prevent polymerization altogether. In practical applications, the selection of a photoinitiator must also be coordinated with the pigment’s opacity, dosage, particle size, and other factors. For example, pigments with strong opacity tend to absorb more light; therefore, a photoinitiator with high absorbance at the same concentration should be chosen, and its dosage may need to be increased accordingly. When the pigment loading is higher, the photoinitiator dosage should likewise be adjusted upward. Additionally, larger pigment particles hinder light penetration, so when selecting a photoinitiator, one should opt for products with high absorbance at the same concentration or increase the photoinitiator dosage as needed.

Principle of matching with coating thickness: In practical applications, variations in coating thickness are unavoidable. When selecting photoinitiators for thick coatings, the guiding principle is to ensure adequate curing both in the deep layers and at the surface; this can be achieved by combining long‑wavelength photoinitiators with relatively short‑wavelength ones. The total dosage of the initiator blend should also be adjusted according to the final coating thickness. For thin coatings, special attention must be paid to oxygen inhibition. In choosing photoinitiators, it is advisable to favor hydrogen‑abstraction‑type initiators that offer some resistance to oxygen inhibition, used in conjunction with cleavage‑type initiators, while appropriately increasing their loading. A typical combination is 184 plus BP; however, the addition level should not be excessive, as overly high concentrations may lead to light shielding.

Dosage principles: Whether using a mercury‑lamp source or a UV‑LED source, in practical applications photoinitiators must be selected not only to match the light source but also to account for factors such as absorbance and dosage. The optimal dosage is determined by the need to achieve adequate polymerization; highly active photoinitiators can be used at lower levels, while less active ones may require higher loadings. Alternatively, combining high‑activity and low‑activity photoinitiators can both meet polymerization requirements and help balance formulation costs. Increasing the photoinitiator loading does accelerate curing, but there is an upper limit: excessive amounts can lead to numerous issues, including light‑shielding effects, increased radical coupling, excessively high instantaneous temperatures that deform heat‑sensitive substrates, overly rapid polymerization that adversely affects adhesion, heightened volumetric shrinkage causing deformation, reduced final molecular weight, diminished overall mechanical properties, higher raw‑material costs, compromised aging resistance, and accelerated yellowing of the finished product. Conversely, reducing the photoinitiator dosage may result in incomplete polymerization, increased energy consumption, and substandard final‑product performance.

Bernhard Steyrer and colleagues used a 405 nm DLP 3D printer to compare the effects of three photoinitiators—ivocerin (bis(4‑methoxybenzoyl)diethylgermanium, BAPO, and TPO‑L—whose UV absorption spectra are shown in Figure 11—on the final product’s performance. Under identical conditions, both ivocerin and BAPO exhibited higher absorbance at 405 nm than TPO‑L. However, the results indicated that the overall superior performance was not achieved by ivocerin or BAPO, which showed relatively high absorbance at 405 nm, but rather by TPO‑L, which displayed comparatively weaker absorption at this wavelength. At low concentrations, ivocerin and BAPO demonstrated high photoinitiator activity; yet, as their loading was increased, they increasingly manifested pronounced light‑shielding effects, thereby adversely impacting the final product’s properties.