Figure 3 Classification of photoinitiator.
2.1. UV photoinitiator
The earliest species of PIs, mainly absorb UV light to achieve
initiation, have been widely employed in numerous fields of UV
photopolymerization for many years. Firstly, the cleavage type (Norrish
Ⅰ), refers to formation of radicals by homolytic reaction of weak
covalent bond in molecular, includes these species of benzils, benzoins,
oxime esters and so on.[4-5, 10, 25-26] Figure 4
presents several structures of commonly-used cleavable UV PIs such as
2-hydroxy-2-methylpropiophenone (1173), 1-hydroxycyclohexyl phenylketone
(184), 2,2-dimethoxy-2-phenylacetophenone (DMPA),
2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (2959), methyl
benzoylformate (MBF), benzil, diphenyliodonium (Iod),
2-benzyl-2-(dimethylamino)-4’-morpholinobutyrophenone (369),
phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO),
diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), ethyl
(2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L), oxime esters OXE-10
and OXE-02, and cleavage mechanism of PI 1173.[27]Most UV PIs of cleavage type undergo one-step cleavage to polymerize the
monomers except oxime esters which requires two-step cleavage.
Figure 4 Structures of commonly-used cleavable UV PIs, and cleavage
mechanism of 1173.
Different from cleavage type, the hydrogen abstraction type PIs produce
radicals by interacting with hydrogen donors, then triggers
photopolymerization reaction.[23, 28] In some
cases, hydrogen abstraction type PIs with outstanding light absorption
can also sensitize other PIs with weak light absorption, they can be
called photosensitizer. Figure 5 records some structures of
commonly-used hydrogen abstraction type UV PIs and photosensitizers such
as benzophenone (BP), 4,4’-bis(diethylamino) benzophenone (EMK),
isopropyl thioxanthone (ITX), 9,10-dibutoxyanthracene (DBA) , and
initiation mechanism of BP and donor photoinitiating
system.[27] In addition, several commonly-used
hydrogen donors are also exhibited in Figure 5, such as N-phenylglycine
(NPG), ethyl 4-dimethylaminobenzoate (EDAB), triethanolamine (TEOA),
N-methyldiethanolamine (MDEOA), 2-mercaptobenzothiazole (2-MBTA),
2-mercaptobenzimidazole (2-MBMA) and 2-mercaptobenzoxazole (2-MBXA).
Figure 5 Structures of commonly-used UV PIs of hydrogen abstraction type
and several hydrogen donors, initiation mechanism of BP/doner system.
Both types of PIs possess their own characteristics, for cleavage type,
rapid initiation and one-component are advantages in practical
application,[29] for hydrogen abstraction type,
two-component PIS can effectively relieve the problem of oxygen
inhibition, and is generally multifunctional.
2.2. LED photoinitiator
The traditional high-pressure mercury lamp, as the light source of UV
photopolymerization, has some shortcomings such as generation of ozone,
toxic mercury, short service life and large consumption of energy, which
lead to its limitations in practical application. Recently, there has a
tendency that the light source of photopolymerization has been moving
from UV toward near-UV or visible light using light-emitting-diodes
(LEDs).[5] LEDs possess many merits such as
higher-operating efficiency, long service life, low cost, safety, and
environmentally friendly,[3, 6-7, 11, 30-43] as a
result, LED photopolymerization has boomed in recent years. Whereas the
absorption wavelengths of most traditional UV PI usually are shorter
than 365 nm, and do not match with the emission wavelengths (usually
higher than 365 nm) of LEDs, so most of the commercially available UV PI
cannot be used for LED-induced
photopolymerization.[1, 30] Therefore, the
development of PI suitable for LED light sources is necessary.
At present, the LED PI of cleavage type reported mainly include TPO,
BAPO, ITX, coumarin-based oxime esters,[1, 16, 43]naphthalimide sulfur ether,[23, 29]glyoxylates,[3, 31, 44-45] and
acylgermanes,[46-48] and so on, the relevant
structural formulas are shown in Figure 6 – Figure 9. LED PI of
hydrogen abstraction or photosensitizers reported mainly include
anthraquinone derivatives,[35-36, 49]cyclohexanone derivatives,[37, 50]cinnamoylformate derivatives,[7] porphyrins
derivatives,[51-52] and so on, the relevant
structural formulas are shown in Figure 10 – Figure 13.
Figure 6 Several chemical structures of coumarin-based oxime esters.
Figure 7 Several chemical structures of naphthalimide sulfur ether.
Figure 8 Several chemical structures of glyoxylates derivatives.
Figure 9 Several chemical structures of acylgermane derivatives.
Figure 10 Several chemical structures of anthraquinone derivatives.
Figure 11 Several chemical structures of cyclohexanone derivatives.
Figure 12 Several chemical structures of cinnamoylformate derivatives.
Figure 13 Several chemical structures of porphyrins derivatives.
2.3. NIR photopolymerization
Compared with Ultraviolet (UV) light, Near-Infrared (NIR) light
possesses a lower scattering coefficient which is beneficial for a
deeper penetration,[53] and the release of heat is
useful for the photopolymerization.[54] Cyanines
are good near infrared light absorbers, can be tailor made by changing
the length of methane chain and substitution pattern which can cover a
large absorption wavelength region between 700 nm and 1000
nm.[55] Cyanines can react with iodonium salts to
generate free radicals and conjugate acid which can efficiently initiate
polymerization of acrylate monomers, epoxides and vinyl ether
monomers.[54, 56-57] Oxime esters can interact
with cyanines to initiate photopolymerizaiton of radical
monomers.[58] Alternatively, up-conversion
nanoparticles can absorb the NIR laser to emit visible light and UV
light which can be absorbed by traditional photoinitiators to trigger
photopolymerization.[59-60] NIR
photopolymerization can be applied in digital imaging in Computer to
Plate (CtP),[61] powder
coating,[62] 3D
printing,[63] etc.