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Type II Photoinitiators

Type II photoinitiators are an important class of compounds widely used in UV and visible light–curing systems. Unlike Type I photoinitiators, which undergo direct bond cleavage upon light absorption, Type II photoinitiators initiate polymerization through a bimolecular process. They typically require a co-initiator, such as an amine or hydrogen donor, to generate reactive radicals. This distinct initiation mechanism gives Type II photoinitiators unique advantages in terms of formulation flexibility, cost efficiency, and curing performance, making them highly attractive for industrial applications.

Photochemical Initiation Mechanism

The initiation process of Type II photoinitiators is based on an excited-state interaction. Upon irradiation, the photoinitiator absorbs light and reaches an excited singlet or triplet state. In this energized form, it interacts with a co-initiator, most commonly an amine, through hydrogen abstraction or electron transfer. This reaction generates free radicals, which initiate the polymerization of unsaturated monomers such as acrylates and methacrylates. Because a co-initiator is required, chemists can finely tune the system's reactivity and curing speed by selecting appropriate electron-donating molecules.

Representative Examples

Typical Type II photoinitiators mainly include benzophenone derivatives and thioxanthones. Benzophenone derivatives are widely used because of their strong UV absorption and efficient hydrogen abstraction when combined with amine co-initiators, enabling reliable free-radical generation. Thioxanthones absorb at longer wavelengths than benzophenone, making them particularly suitable for pigmented, filled, or thick formulations where deeper light penetration is required, and they exhibit high initiation efficiency under both UV and visible light irradiation.

Performance Advantages

Type II photoinitiators offer high formulation flexibility because the light-absorbing photoinitiator and the co-initiator can be independently selected to tailor curing speed and performance. They are particularly effective in pigmented, filled, or thick systems where light penetration is limited, ensuring more uniform curing. In addition, Type II photoinitiators often exhibit lower odor and better storage stability compared to some highly reactive Type I systems, which is beneficial for consumer-facing products.

Our Products

Type II	Catalog Number	CAS	Description	Reactivity	Surface Cure	Depth Cure	Yellowing	Odor	LED Cure
Benzophenone Derivatives	ALCM-PI-0012	119-61-9	Very cost effective crystaline solid.	•••	••••	•	Low	High	No
	ALCM-PI-0013	134 -84-9	Effective white crystalline flake.	•••	••••	••	Low	Medium	No
	ALCM-PI-0014	2128-93-0	White to off white powder, very fast photoinitiator with good surface cure.	••••	••••	••	Medium	Medium	No
	ALCM-PI-0015	606- 28-0	White crystalline, low yellowing.	••	••	••	Low	Low	No
	ALCM-PI-0016	90-93-7	Light yellow powder photoinitiator for LED curing. Does not require amines. Ideal for boost curing of black inks.	••••	•••	••••	High	Low	Yes
Thioxanthones	ALCM-PI-0017	5495-84-1	Light yellow crystalline powder photoinitiator for LED curing. Use with α-amino ketone for the best results and improve surface curing.	••••	•••	••••	High	Low	Yes
Thioxanthones	ALCM-PI-0018	82799-44-8	Pale yellow solid photoinitiator for LED curing. Use with α-amino ketone for the best results and improve surface curing.	••••	•••	••••	High	Low	Yes
Aminobenzoate Synergists	ALCM-PI-0019	10287-53-3	White crystalline synergist. Used in combination with Type II photoinitiators to increase reaction speed and improve surface curing.	Strong synergist	••••	•	Medium	Medium	With thioxanthones
Aminobenzoate Synergists	ALCM-PI-0020	21245-02-3	Pale yellow liquid synergist. Used in combination with Type II photoinitiators to increase reaction speed and improve surface curing.	Strong synergist	••••	•	Medium	Medium	With thioxanthones
Visible Light PI	ALCM-PI-0021	125051-32-3	Brown powder photoinitiator highly sensitive to visible and LED wavelength.	••••	••••	••••	High	Low	Yes

If you are interested in our products or would like more detailed technical information, please feel free to contact us. Our team will be pleased to discuss your specific application requirements and provide tailored solutions.

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