Free radical initiators, which are the most typically used with acrylate functional resins can be described as either ” hydrogen abstraction type and alpha cleavage type . Hydrogen abstraction initiators have their specific uses, especially in three or four way photoinitiator blends. The alpha cleavage initiators have a generally higher efficiency, due to their generation of free radicals via a uni-molecular process, while hydrogen abstraction initiator require a hydrogen donating source in order to generate the free radicals .
Norrish Type I photo-initiators.
Alpha–cleavage photo-initiators are classified based on their mechanism to generate radical species and categorized as Norrish Type I photo-initiators. This type of photo-initiators will absorb ultraviolet (UV) light and undergo hemolytic cleavage within their molecules to produce free radicals and subsequently initiate the polymerization process. Norish type I initiators are typically compounds containing benzoyl groups. The carbonyl group of the initiator absorbs a photon and is transformed into an excited state. Subsequent homolytic cleavage of the excited α-carbon bond produces two radical fragments. For instance, the cleavage of 2,2-dimethoxy-1,2-diphenyl-ethan-1-one largely yields a methoxybenzyl and benzoyl radical. The benzoyl radical initiate free radical polymerization whereas the methoxybenzyl radical decomposes to give the more stable methyl radical and methyl benzoate.
Flychem’s range of photoinitiators covers a wide variety of requirements
| FLYCURE PI-184 | a-Hydroxyketone | 1-Hydroxy-cyclohexyl- phenyl-ketone | White to off- white crystalline powder | Recommended for clear wood, plastic, metal and UV-stabilized coatings Suitable for overprint varnishes |
| FLYCURE PI-1173 | a-Hydroxyketone | 2-Hydroxy-2-methyl-1- phenyl-propan-1-one | Colorless to slightly yellow liquid | Recommended for clear wood, plastic, metal and UV-stabilized coatings Suitable for overprint varnishes |
| FLYCURE PI-907 | a-Aminoketone | 2-Methyl-1 [4-(methylthio)phenyl]- 2-morpholinopropan-1-one | White to slightly beige powder | Recommended for inks |
| FLYCURE PI-TPO | Mono acyl phosphine (MAPO) | Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide | Yellowish crystal powder | Recommended for UV-stabilized clear coatingsSuitable for pigmented wood, plastic and metal coatings and inks |
| FLYCURE PI-819 | Bis acyl phosphine (BAPO) | Phenylbis (2,4,6-trimethylbenzoyl) phosphineoxide | Pale yellow crystal | Recommended for pigmented wood, plastic, metal and powder coatingsSuitable for UV-stabilized clear coatings and inks |
Norrish Type I photo-initiators.
Norish type II initiators absorb UV light to form excited molecules which then abstract an electron or hydrogen atom from a donor molecule (synergist). The donor molecule then reacts with a monomer to initiate polymerization. Common type II photoinitiator systems include benzophenone and its derivative and isopropyl thioxanthone in combination with a synergist such as tertiary amines. The amines function as active hydrogen donors for the excited photoinitiators. The abstraction of hydrogen produces very reactive alkyl-amino radicals which subsequently initiate polymerization.
Flychem’s range of photoinitiators covers a wide variety of requirements
| FLYCURE PI-BP | Benzophenone | White to off- white crystalline powder | Recommended for clear wood, plastic, metal and UV-stabilized coatings . Suitable for overprint varnishes |
| FLYCURE PI-DETX | 2,4-DIETHYLTHIOXANTHONE | Light Pale colour crystalline powder | Recommended for inks |