Photoinitiator 651 (Benzil Dimethyl Ketal / BDK) | CAS 24650-42-8 | High-Purity UV Curing Agent for Coatings, Inks, Adhesives & 3D Printing Resin

651
Photoinitiator 651 (Benzil Dimethyl Ketal, BDK, CAS 24650-42-8) is a highly efficient Type I (Norrish I) free-radical photoinitiator widely used in UV-curable systems. It initiates rapid photopolymerization of unsaturated polyesters and acrylates upon UV exposure, delivering fast cure speed, low odor, low volatility, and excellent compatibility with various monomers and oligomers. Applications include UV coatings, inks, adhesives, 3D printing resins, optical films, and dental materials. Available in high purity (≥99.0%) as white crystalline powder.
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1. Product Overview

Photoinitiator 651 (also known as Benzil Dimethyl Ketal, BDK, 2,2-Dimethoxy-1,2-diphenylethanone, Irgacure® 651) is a highly efficient Type I (Norrish I) free-radical photoinitiator. Upon exposure to ultraviolet (UV) light, it undergoes photochemical cleavage to generate active benzoyl and dimethoxybenzyl radicals, which initiate the rapid polymerization and crosslinking of unsaturated prepolymers and monomers.
 
Property
Detail
Chemical Name
2,2-Dimethoxy-1,2-diphenylethanone
CAS Number
24650-42-8
Molecular Formula
C₁₆H₁₆O₃
Molecular Weight
256.30 g/mol
MDL Number
MFCD00008475
EC Number
246-386-6
Appearance
White to off-white crystalline powder

2. Quality Specifications

Test Item
Specification
Test Method
Assay (GC)
≥ 99.0% (typical ≥ 99.5%)
Gas Chromatography
Appearance
White crystalline powder
Visual Inspection
Melting Point
64 – 67 °C (lit. 67–70 °C)
Capillary Method
Volatiles (Loss on Drying)
≤ 0.30% (typical ≤ 0.2%)
105 °C, 2 h
Ash Content
≤ 0.10%
800 °C Ignition
Color (APHA)
≤ 20
Platinum-Cobalt Method

3. Physicochemical Properties

Property
Value
Property
Value
Physical State
White crystalline solid/powder
Boiling Point
169 °C / 7 mmHg (approx. 380 °C estimated)
Flash Point
> 110 °C (closed cup); 190 °C
Density
~1.13–1.18 g/cm³ (estimated)
Vapor Pressure (25 °C)
0.002 Pa
Water Solubility
66.32 mg/L at 25 °C; practically insoluble
Refractive Index
~1.559 (estimated)
Log P (octanol/water)
2.73–2.95 (estimated)
Solubility
Soluble in acetone, ethyl acetate, methanol, toluene, acrylate monomers, dichloromethane; insoluble in water
Stability
Stable but light-sensitive. Incompatible with strong acids and strong oxidizing agents

4. UV Absorption Characteristics

Photoinitiator 651 exhibits strong UV absorption in the following wavelength ranges:
λmax (in methanol): 250 nm and 340 nm
Absorption range: 254 – 390 nm
Highly sensitive to medium-pressure mercury lamp outputs (254 nm / 365 nm), enabling fast cure speeds.

5. Mechanism of Action

Photoinitiator 651 is a Type I (Norrish I) photoinitiator. Upon UV irradiation, it undergoes α-cleavage to generate:
Benzoyl(benzoyl radical)
Dimethoxybenzyl radical (which can further decompose to methyl radical and methyl benzoate)
These active free radicals initiate the polymerization and crosslinking of unsaturated prepolymers (e.g., acrylates, unsaturated polyesters) and mono-/multifunctional monomers.

6. Applications

Photoinitiator 651 is widely used across multiple UV-curable systems:
Application Area
Description
UV Coatings
Wood coatings, paper varnishes, metal and plastic coatings. Compatible with various acrylate systems; delivers fast cure speed and excellent film appearance.
UV Inks
Offset, letterpress, flexographic, and digital printing inks. High initiation efficiency, low yellowing, suitable for transparent and white ink systems.
UV Adhesives
Pressure-sensitive adhesives, structural adhesives, and sealants; particularly suitable for transparent substrate bonding.
Optical Films / Electronics
LCD optical film adhesives, PCB solder masks, semiconductor packaging materials, and optical waveguide materials.
3D Printing Resins
SLA/DLP photopolymer resins; high resolution, low shrinkage, suitable for precision molding.
Dental Materials
Dental composite resins, light-cured filling materials, and denture base materials.

7. Performance Features

High purity (≥ 99.0%) ensures high initiation efficiency and low residual odor, making it suitable for odor-sensitive coatings and food packaging printing applications.
Multiple UV absorption peaks covering 250–390 nm range, highly sensitive to medium-pressure mercury lamp outputs.
Norrish Type I photofragmentation mechanism generates active benzoyl and dimethoxybenzyl radicals upon exposure, delivering excellent initiation efficiency.
Excellent compatibility with acrylate monomers and oligomers (epoxy acrylates, urethane acrylates, polyester acrylates).
Can be co-initiated with synergists such as DETX (diethylthioxanthone) and benzophenone to broaden spectral response and enhance deep-cure performance.
Crystalline solid form facilitates weighing and compounding; low dust generation, good flowability, suitable for industrial production.
Low yellowing index, excellent cured film appearance, suitable for clear varnishes and white systems.

8. Recommended Dosage

Application System
Recommended Addition (wt%)
Remarks
UV Clear Coatings
1.0 – 3.0%
Can be combined with benzophenone for improved surface cure
UV White / Pigmented Coatings
2.0 – 4.0%
Higher dosage recommended for highly pigmented systems
UV Inks
2.0 – 5.0%
Recommend combination with ITX or DETX
UV Adhesives
1.0 – 3.0%
Adjust according to substrate transparency
3D Printing Resins
0.5 – 2.0%
Low dosage sufficient for efficient initiation in SLA/DLP systems