Selection of Photoinitiators: TPO, ITX, and DETX

Introduction

Photoinitiators are compounds that initiate the polymerisation process when exposed to ultraviolet or light‐emitting diode light. In these curing systems, they decompose monomers into reactive species that subsequently form a polymer network. The selection of an appropriate photoinitiator is critical. Industries such as electronics, coatings and inks require a careful evaluation based on both the product formulation and the process conditions. A detailed comparison between TPO, ITX and DETX is provided below.

Classification

Photoinitiators are divided into two main types: Type I and Type II. Type I photoinitiators cleave to produce free radicals immediately when exposed to light. Diphenyl(2,4,6‐trimethylbenzoyl)phosphine oxide, often termed TPO, is an example. It operates rapidly under ultraviolet light exposure.
Type II photoinitiators generate radicals by abstracting hydrogen from a co‐initiator such as an amine. Isopropyl thioxanthone (ITX) and diethyl thioxanthone (DETX) are examples of this class. They require the presence of a co‐initiator and a longer exposure period to undergo curing.

Absorption Characteristics

The absorption behaviour of photoinitiators is an important parameter in material selection. TPO exhibits strong absorption in the near‐ultraviolet wavelength range. Consequently, it is well suited for systems utilising short‐wave ultraviolet light.
ITX and DETX absorb in the longer ultraviolet and visible light regions. Their absorption ranges ensure compatibility with many LED sources. Given that these ranges are effective for achieving greater cure depth, they may be preferred for surface curing where light penetration is limited.

Curing Performance

The curing performance of these photoinitiators is distinct. TPO cures quickly and is effective with a wide range of monomers. It also shows low sensitivity to oxygen, thereby reducing the risk of incomplete curing. In contrast, ITX and DETX exhibit a slower curing speed because they require a co‐initiator to generate radicals through hydrogen abstraction.
The reactivity with various monomers differs between these types. Oxygen inhibition can be significant and, consequently, additional additives may be required to achieve a complete polymerisation.

Colour Stability and Yellowing

Colour stability is an important criterion in coatings that are required to remain clear or white. TPO generally results in minimal yellowing and maintains a clean finish in clear and white formulations.
Isopropyl thioxanthone (ITX) and diethyl thioxanthone (DETX) can exhibit minor yellowing. This effect is observed in both pigmented and non‐pigmented systems where optical clarity is paramount. Although this slight yellowing may not affect the performance of the final product, it is a critical consideration for certain applications.

Application Suitability

Each photoinitiator is appropriate for specific applications. TPO is commonly used in electronics, 3D printing and clear coatings. It provides rapid curing and maintains optical clarity. ITX is employed in traditional ultraviolet curing systems and is effective in UV ink formulations and screen printing, given that its absorption at longer wavelengths supports the process conditions.
DETX is applied in LED‐curable inks, flexible packaging and overprint varnishes. Its compatibility with modern light sources is well documented.

Formulation Considerations

Several formulation parameters influence the choice of photoinitiator. Some systems require a co‐initiator, such as an amine, to improve the curing efficiency. It is necessary to ensure adequate solubility and formulation stability. Odour is also a factor; TPO typically exhibits a neutral odour, whereas ITX and DETX have a noticeable odour. Cost and availability further influence the selection process. Often, multiple formulation trials are conducted to balance curing speed, colour stability and overall performance. Experienced formulators consider these factors along with the required cure depth and the specific printing or coating process.

Comparison Table: TPO, ITX, and DETX

Conclusion

Photoinitiators are essential for initiating the polymerisation process under ultraviolet and LED light. A clear understanding of the differences between Type I and Type II initiators is necessary for accurate formulation design. TPO offers rapid curing and minimal colour alteration, which is why it is used in clear coatings and electronics. ITX and DETX are effective in systems employing longer wavelength sources, given that formulation adjustments are implemented. For further technical support, please consult Stanford Advanced Materials (SAM).

Frequently Asked Questions

F: What role do photoinitiators play in curing systems?
Q: They initiate the polymerisation process by generating free radicals under ultraviolet or LED light.

F: Why should one choose a specific type of photoinitiator?
Q: The selection is based on light absorption, curing speed, colour stability and process compatibility.

F: Do all photoinitiators require a co‐initiator?
Q: No, Type I initiators operate without co‐initiators, whereas many Type II initiators require co‐initiators such as amines.