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Appl. Sci. 2013, 3(2), 490-514; doi:10.3390/app3020490

Article
Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry
Mohamad Ali Tehfe , Fanny Louradour , Jacques Lalevée * and Jean-Pierre Fouassier
Institut de Science des Matériaux de Mulhouse IS2M–UMR 7361–UHA; 15, rue Jean Starcky, 68057 Mulhouse Cedex, France; E-Mails: tehfe_mohamadali@hotmail.com (M.A.T.); fanny.louradour@gmail.com (F.L.); jp.fouassier@uha.fr (J.-P.F.)
*
Author to whom correspondence should be addressed; E-Mail: jacques.lalevee@uha.fr; Tel.: +33-3-89-60-88-03; Fax: +33-3-89-60-87-99.
Received: 15 March 2013; in revised form: 28 March 2013 / Accepted: 1 April 2013 /
Published: 24 April 2013

Abstract

: The present paper reviews some aspects concerned with the development of green technologies in the photopolymerization area: use of visible light sources (Xe and Hg-Xe lamps, diode lasers), soft irradiation conditions (household lamps: halogen lamp, fluorescence bulbs, LED bulbs), sunlight exposure, development of very efficient photoinitiating systems and use of renewable monomers. The drawbacks/breakthroughs encountered when going on the way of a greener approach are discussed. Examples of recent achievements are presented.
Keywords:
cationic photopolymerization; radical photopolymerization; photoinitiators; visible light sources; soft irradiation conditions; sunlight exposure; LED; laser diodes; household lamps; halogen lamps; renewable monomers

1. Introduction

Photopolymerization reactions are commonly presented as belonging to a green technology characterized by low electrical power input and energy requirements, low temperature operation and no volatile organic compounds release (solvent-free systems) [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. In industrial sectors, such as radiation curing, imaging, microelectronics, medicine or optics (with various and very different applications, e.g., in coatings, varnishes, paints, adhesives, graphic arts, printing plates, stereolithography, photoresists, laser direct imaging, computer-to-plate technology, holographic optical elements or tooth repair), light-induced polymerization reactions have been very well-known for many years [19]. These reactions involve a polymerizable radical or cationic matrix and a more or less complex photoinitiating system (PIS) [19,20,21,22,23,24,25,26]. Mercury lamps are largely used as light sources together with doped Hg lamps or microwave powered lamps. The delivered lights in the 280–450 nm range often satisfactorily match the absorption of PIS. In the context of green chemistry, the avoidance of volatile organic compounds is also an important issue.

Free radical photopolymerization (FRP) is undoubtedly the most popular compared to cationic photopolymerization (CP). A PIS contains at least a photoinitiator (PI) and/or a photosensitizer (PS): PI (or PS) has to absorb the light [19]. Upon excitation, in FRP, PI becomes excited (PI*) and generates (1–4) a radical, R, either directly through cleavage or in the presence of an electron/hydrogen donor. When PS is used and excited, the excitation has to move from PS* to PI by energy (2) or electron transfer (3): the same R is formed or new ion radicals are created, respectively.

PI → PI* (hv) → radicals R●;
PS → PS* (hv) → PI* → R●;
PS → PS* (hv) → PS●;+ + PI●− →→→ radicals
R + radical monomer → polymer

In CP (5–8), onium salts (e.g., the iodonium salt referred to here as Ph2I+; several commercial derivatives that do not release benzene are known) are used as PI [19]. Their direct homolytic/heterolytic decomposition followed by hydrogen transfer reactions leads to a proton. Their photosensitized decomposition occurs according to energy (6) or electron transfer (7).

PI (Ph2I+) (hv) → PI* →→→ H+
PS → PS* (hv) → PI* →→→ H+
PS → PS* (hv) → PS●+ + PI●− (e.g., Ph2I)
H+ (or PS●+) + cationic monomer → polymer

In free radical promoted cationic polymerization (FRPCP) (9–11), a radical, R, is produced from a radical source (RS) (a PI or a PS can play such a role) and then oxidized by Ph2I+ to form Ph2I and a cation, R+, suitable for the ring opening reaction (ROP) of epoxides or the cationic polymerization of vinyl ethers (the Ph2I species readily decomposes into PhI and Ph) [19].

PI → PI* (hv) → R
R + Ph2I+ → R+ + Ph2I (→ R+ + PhI + Ph)
R+ + monomer → polymer

The PI, PS and RS have to be selected to absorb the irradiation wavelengths [19]. In FRP, the selection of near UV/visible photosensitive systems for industrial applications is quite easy (and almost feasible on laboratory scale experiments at any UV-visible wavelength). In CP, as the PIs mainly absorb in the UV, the search and the design of suitable PS compounds as energy or electron donors for visible light-induced polymerizations are necessary, but this appears as a rather complex task for the photocuring of coatings in industrial lines. Due to its versatility, FRPCP is certainly one of the most interesting and promising ways for a cationic polymerization under exposure at λ > 350 nm (up to 700 nm), but the occurrence of efficient reactions (10 and 11) is not so trivial, and the oxygen quenching of the radicals is detrimental.

The development of photopolymerization reactions towards a green technology can be found in five directions:

(1)

search for new PI or PS being able (i) to absorb the visible lights that are very often lost when employing conventional mercury lamps and PISs and/or (ii) to move the system towards a UV-free exposure (doped Hg lamps, Xe-Hg lamps, Xe lamps).

(2)

use of newly developed high intensity LED or laser diodes operating at well-defined near UV/visible wavelengths avoiding the use of Hg-based lamps and the presence of more energetic UV wavelengths (254, 313 nm). Today, in industrial applications, LED technology allows highly packed arrays operating at 365 or 395 nm, together with a low heat generation, low energy consumption, low cost and low maintenance; the development of laser diode arrangements ensures high intensity monochromatic irradiations from the blue to the red part of the spectrum.

(3)

development of PISs for soft irradiation conditions and use of low visible light intensity sources, e.g., household devices: halogen lamp, fluorescent bulbs and LED bulbs.

(4)

use of sunlight, which is a cheap and inexhaustible energy source (but strongly affected by the weather and location) that might be of interest for (i) particular outdoor applications (e.g., for paint drying) and (ii) the possibility of curing large dimensioned pieces or surfaces without requiring any irradiation device.

(5)

search of natural products or renewable monomers (the plant oil derivatives present attractive features, such as versatility, biodegradability and low cost).

In a general way, the questions that have to be solved for getting a high polymerization efficiency concern the PISs and the starting monomers, as well as their adaptation to the available light sources. In the present paper, we will (i) discuss the drawbacks/breakthroughs encountered when going on the particular way of a greener approach for photopolymerization reactions, (ii) define the key points for the design of a high performance PIS in such conditions and (iii) show, as examples, some of our new or recent achievements using soft illumination conditions (e.g., household lamps and sunlight exposure; typically ~2–10 mW/cm2), visible light irradiation (400 nm < λ < 800 nm), use of renewable monomers, etc.

2. Drawbacks/Breakthroughs on the Way to Greener Photopolymerization Reactions

2.1. The Photopolymerization Reactions

In photopolymerization reactions [19], the matching of the PIS absorption spectrum with the emission spectrum of the light source, as well as the number of available incident photons, I0, is crucial. The absorption properties of PI, PS and PIS (ground state spectra and molar extinction coefficients, ε) play a decisive role, as the polymerization rate, Rp, is directly connected with the amount of light absorbed (Iabs): Iabs = I0 (1-10εcl) where I0, ε, c and l stand for the incident light intensity, the molar extinction coefficient, the photoinitiator concentration and the sample thickness, respectively. The delivered flux of photons can be very high with Hg lamps (Hg arc lamp, doped Hg lamps, electrodeless Hg lamps; typically > 1–2 W/cm²), highly packed arrays of light emitting diodes (LED) at 365 or 395 nm (a few W/cm²), Hg-Xe or Xe lamps and quite low with household devices (halogen lamps, fluorescent bulbs and white or blue LED bulbs; <10 mW/cm²), diode lasers (10–100 mW/cm²) or sun (2 mW/cm²). Typical examples of emission spectra are given in Figure 1 for various sources.

Applsci 03 00490 g001 1024
Figure 1. Emission spectra of various light sources: Hg-Xe lamp (A), Xe lamp (B), household lamps (C) (fluorescent bulb (FB) (a) and blue (b) or white (c) LED bulb), Halogen lamp (D). Laser diodes can operate, e.g., at 405, 457, 473, 532, 635 and 808 nm.

Click here to enlarge figure

Figure 1. Emission spectra of various light sources: Hg-Xe lamp (A), Xe lamp (B), household lamps (C) (fluorescent bulb (FB) (a) and blue (b) or white (c) LED bulb), Halogen lamp (D). Laser diodes can operate, e.g., at 405, 457, 473, 532, 635 and 808 nm.
Applsci 03 00490 g001 1024

2.2. Reactions under High Intensity Sources Emitting Visible Lights

The FRP in the radiation curing area is largely and easily achieved upon irradiation with UV, near UV/visible light high intensity sources (various Hg lamps and, more recently, LED arrays) under air [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18], as many efficient (commercially or laboratory available) PISs can operate in the 280–450 nm wavelength range [19]. This is exemplified in Figure 2 for the photopolymerization of an epoxy-acrylate matrix under air upon exposure to a laboratory Xe-Hg lamp. On industrial lines for coating applications, the exposure time is obviously much shorter and the attained cure speeds are really high.

The same holds true in CP as, in addition, oxygen inhibition does not occur. Fast curing speeds are reached under light exposure below 400 nm.

The situation is more complicated in FRPCP, as the usual photoinitiating systems are naturally less efficient and sensitive to the presence of oxygen, but new PISs have led to promising developments (see below).

Going to longer wavelength exposures (450–700 nm) can also be achieved in FRP, CP and FRPCP using appropriate conventional PISs, provided that relatively high intensity light sources and viscous media are used. A real progress, however, has been realized in recent works and many PISs that meet this challenge (even with low intensity lights and low viscosity media) have been proposed in the last five years (see, e.g., [19,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76] and references therein).

Applsci 03 00490 g002 1024
Figure 2. Polymerization kinetic of an epoxy-acrylate (Ebecryl 605 from Cytec) upon Hg-Xe lamp exposure (60 mW/cm²); photoinitiator: 2-2'-dimethoxy-2-phenylacetophenone. Under air; the polymerization only starts with the irradiation.

Click here to enlarge figure

Figure 2. Polymerization kinetic of an epoxy-acrylate (Ebecryl 605 from Cytec) upon Hg-Xe lamp exposure (60 mW/cm²); photoinitiator: 2-2'-dimethoxy-2-phenylacetophenone. Under air; the polymerization only starts with the irradiation.
Applsci 03 00490 g002 1024

2.3. The Oxygen Inhibition

In FRP and FRPCP, a well-known drawback [19] concerns the oxygen inhibition (12–14), which is due to the excited triplet state quenching (3PI or 3PS) by O2 and the scavenging of the initiating R and propagating RMn radicals by O2 (a nearly diffusion controlled reaction; highly stable peroxyl radicals are formed). The polymerization starts in the film as soon as oxygen is consumed. The practical effects of this phenomenon strongly depend on the experimental conditions. In highly viscous or thick samples (e.g., epoxy acrylate matrices), the re-oxygenation process is slow, which leads to an efficient polymerization after an inhibition period. The top layer in contact with air is easily polymerized, provided that a high PI concentration and a high light intensity are used: this is easily feasible in thin samples; it might be more complicated in thick samples. On the opposite, in very low viscosity media (e.g., di- or tri-functional monomers, such as trimethylol-propane triacrylate (TMPTA) or 3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX), the re-oxygenation remains efficient, thereby reducing the monomer conversions. In addition, when the light intensity is attenuated, the oxygen inhibition has a dramatic effect on the polymerization profile, due to (i) the lowering of the initial O2 consumption process and, as a consequence, (ii) the decrease of the initiating radical concentration (as a higher amount of these radicals are trapped by O2). As is known, decreasing the oxygen inhibition effect can be achieved through various strategies (see a review in [19]). The recent introduction of a novel approach has led to successful results (see below).

PI → 1PI* (hv) → 3PI* → R → → → RMn
3PI* + O2 → quenching
R (RMn) + O2 → R-O2 (RMn-O2)

2.4. The Soft Irradiation Conditions

The above mentioned considerations explain why the FRP is difficult when using visible light and low intensity sources under air. For example, the development of sunlight photosensitive formulations (see, e.g., [77,78,79,80,81,82,83,84,85] and references therein; see also the patent literature aiming at industrial applications) for the drying of paints for crack-bridging applications, anti-soiling properties, the manufacture of interpenetrating polymer networks (IPN) usable as protective coatings and glues, the fabrication of glass fiber reinforced composites, hard and rigidified four layer glass cloth laminate, clearcoats and polymer–clay composites has been realized in the past, but these systems, except some of them (e.g., those described in [79,80,84]) suffer from oxygen inhibition and a relatively low photosensitivity.

As stated above, except some colored systems (e.g., the ferrocenium salts), the usually employed cationic PIs (onium salts) for CP absorb in the UV. Even in academic laboratories, efficient photosensitization reactions of cationic PISs upon visible light is rather limited, as the possible efficient electron donor/onium salt couples are in a very limited number, despite careful research [19].

In FRPCP, the main problem concerns the choice of PIs, leading to an efficient R+ initiating cation (3): few examples were known; most systems operated in the near UV; the efficiency/reactivity was not so high [86,87,88,89,90,91,92,93,94,95]. Interesting systems have been shown to work under sunlight, but in laminated conditions [96,97,98], they have, however, opened up promising perspectives. Through the very recent development of efficient visible light sensitive systems, FRPCP has known a substantial progress (see below).

2.5. The Development of New Photosensitive Systems

It clearly appeared that the development of PIS should proceed through new concepts, ensuring an increase of their photochemical/chemical reactivity. In this direction, a noticeable improvement was noted with the introduction of the silyl chemistry into PISs [99,100]. The silane (e.g., tris-(trimethylsilyl)silane (TTMSS)) becomes a magic additive, which renders more feasible the photopolymerization reactions in aerated conditions. In a silane containing PIS for FRP, initiating silyl radicals are generated: (i) they consume oxygen (15); (ii) scavenge the peroxyls (16 and 17) and (iii) regenerate new silyls. As a consequence, the oxygen inhibition is reduced, and the total amount of interesting R3Si increases, so that oxygen becomes a mediator in the initiating radical production.

R3Si + O2 → R3Si-O2
R3Si-O2 + R3Si-H → R3Si-O2H + R3Si
RMn-O2 + R3Si-H → RMn-O2H + R3Si

The same holds true in FRPCP, which is usually affected by the presence of oxygen. In a silane containing PIS, the oxygen inhibition is dramatically decreased as resulting from (15–17). Moreover, the addition of the iodonium salt allows an oxidation of the silyl radical (18): a R+ cation is formed and can serve as a very efficient initiating species (19). In such PIS, an interesting feature relates to the possibility of forming the same cationic species, R+, whatever the starting absorbing radical source (RS) (contrary to reaction 10, where the nature of the cation is dependent on the starting PI). RS can be a usual PI or PS (but also any other compound) being able to form silyls by cleavage of, e.g., a C–Si or a Si–Si bond (20), and an electron/proton transfer with, e.g., ketones or dyes (21).

R3Si + Ph2I+ → R3Si+ + Ph2I
R3Si+ + monomer → Polymer
RS (hv) → R3Si + counter radical
RS + R3SiH (hv) → R3Si + RSH

Therefore, the novel introduction of this silyl chemistry opens a new way to cure coatings under UV and visible lights. Interestingly, this also allows photocuring under soft conditions (visible light using exposure to Xe lamps, household halogen lamps, diode lasers (405, 457, 532, 635 nm), LED bulbs and sun; low intensity sources), under air, using relatively low viscosity matrices [101]. Figure 3 shows the role of the silane in the typical FRP and FRPCP of aerated curable formulations under near UV/visible lights. The germyl [102] and boryl [103] chemistries can play a similar role. In the same way, N-vinyl carbazole (NVK) was recently advantageously introduced into a formulation instead of the silane [104]; NVK is a cheap and efficient alternative to tris(trimethylsilyl)silane. Using other PISs and monomers (e.g., divinyl ethers), higher final conversions can be reached.

Applsci 03 00490 g003 1024
Figure 3. Photopolymerization kinetics of (A) an acrylate monomer (trimethylol-propane triacrylate (TMPTA) from Cytec) using a phosphine oxide as the photoinitiator in the absence (1) or in the presence (2) of tris-(trimethylsilyl)silane (TTMSS) (3% w/w); (B) an epoxy monomer ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX), Uvacure 1500 from Cytec) using an hydroxyl alkyl acetophenone/iodonium salt (1%/1% w/w) as the photoinitiating system in the absence (1) or in the presence (2) of TTMSS (3% w/w); under air; Xe lamp exposure.

Click here to enlarge figure

Figure 3. Photopolymerization kinetics of (A) an acrylate monomer (trimethylol-propane triacrylate (TMPTA) from Cytec) using a phosphine oxide as the photoinitiator in the absence (1) or in the presence (2) of tris-(trimethylsilyl)silane (TTMSS) (3% w/w); (B) an epoxy monomer ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX), Uvacure 1500 from Cytec) using an hydroxyl alkyl acetophenone/iodonium salt (1%/1% w/w) as the photoinitiating system in the absence (1) or in the presence (2) of TTMSS (3% w/w); under air; Xe lamp exposure.
Applsci 03 00490 g003 1024

The current efforts result in an amazing series of proposals of new PISs [27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]. For example, we have recently introduced PISs ([51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,99,100,101,102,103,104,105,106,107]) exhibiting really novel absorption properties (red-shift absorptions, multicolor absorptions, enhanced molar extinction coefficients, ε): e.g., colored substituted or functionalized ketones [60], modified organometallic derivatives [70,71,72,108,109,110,111,112,113,114,115,116,117] (ruthenium-, iridium-, platinum-, zirconium- and zinc-based complexes, titanocene derivatives…), various series of dye-based skeletons [61,62,63,65,66,67,68,74,75,76,68] (e.g., phenylenediamine, polystilbene, polyazine, violanthrone, acridinedione, 2,7-di-tert-butyldimethyldihydropyrene, bodipy, boranyl, thiophene, perylene bis-dicarboximide, hydrocarbons, pyrromethene, pyridinium salt…), di- and tri-functional architectures of photo initiators [64], light harvesting compounds [57,58] (where a strong molecular orbital coupling occurs, leading to ε huge values) and push-pull and multicolor photoinitiators (novel chromophores; donor-π-acceptor arrangements; unusual broad absorptions from the blue to the red wavelengths…).

2.6. The Photoredox Catalysis

The further introduction of the silyl chemistry into photoredox cycles (as those known in organic synthesis purposes using photocatalysts (PC) [108,109,110,111,112,113,114,115,116,117]) has recently led to interesting possibilities of FRP and FRPCP reactions under soft conditions in aerated media [75,76,105,106,107]. Novel PIs working as PCs through an oxidation cycle (metal complexes or organic metal-free compounds) in combination with a silane and an iodonium salt have been designed (22–25); they allow successful excitations of cationic or radical matrices up to 635 nm under air. Other systems involving a PC, an amine AH and an alkyl halide R-Br operate through a reduction cycle (26 and 27). When PC stands for a PI that is regenerated in these PISs, the photoinitiator becomes a photoinitiator catalyst (PIC).

PC + Ph2I+ (hv) → PC●+ + Ph + Ph-I
Ph + R3Si-H → Ph-H + R3Si
R3Si + Ph2I+ → R3Si+ + Ph + Ph-I
PC●+ + R3Si → PC + R3Si+
PC + AH → PC●− + AH●+ (eventually → PC-H + AH(-H))
PC●− + R-Br → PC + R + Br

2.7. Renewable Monomers and Oligomers

Renewable monomers/oligomers have been proposed and studied; e.g., (i) acrylates: acrylated vegetable oils [118], natural or naturally derived products (photocrosslinkable polylactides [119], ε-caprolactone [120,121], poly (lactide-co-ethylene oxide-co-fumarate) [122], poly(caprolactone-co-lactic acid) [123], methacrylate based gelatine derivatives [124], acrylate modified starch [125] and itaconic acid based photocurable polyesters [126]; (ii) epoxides: epoxidized sunflower [127,128], epoxidized soybean oil (ESO), linseed oil, vernonia oil or castor oil (see in [129]), limonene dioxide (LDO) [130] (limonene is a liquid terpene found in various volatile oils, such as cardamom, nutmeg and turpentine; LDO can be formed through oxidation of limonene by peracids), epoxidized natural rubbers [131], vegetable oils [132] and epoxidized fatty acid (EFA); or (iii) resins based on vegetable oil [133,134], soybean [135], rosin ester [136], tung [137] and palm stearin [138,139] and castor oil. The photopolymerization of such monomers is more or less efficient as a function of the chemical structure, the multifunctional character or the irradiation conditions. Some typical renewable monomer compounds are shown in Scheme 1.

Applsci 03 00490 g013 1024
Scheme 1. Investigated Renewable Monomers.

Click here to enlarge figure

Scheme 1. Investigated Renewable Monomers.
Applsci 03 00490 g013 1024

3. Greener Photopolymerization Reactions: Attained Performance in Recent Laboratory Scale Experiments

3.1. 3a/ Soft or Eco-Friendly Photopolymerization of Synthetic Monomers

In this part, we will show some examples (extracted from our own work [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]), which illustrate today’s green character of the photopolymerization reactions of synthetic monomers (other experiments using renewable monomers will be presented below with more details). TMPTA (trimethylol propane acrylate) and EPOX ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate) will be used as representative low viscosity monomers. Divinyl ethers (e.g., triethylene glycol divinyl ether (DVE-3)) can also be photopolymerized. All the formed coatings are tack-free.

3.1.1. Design of New PIS Allowing a UV-Free Exposure and Ensuring the Use of Visible Light

Figure 4 shows some polymerization profiles of EPOX using typical visible light absorbing PISs. A Xe lamp ensures fast CP and FRPCP processes. The FRP of acrylates is also feasible under such irradiation conditions. Therefore, visible photons can be successfully used and Hg lamps avoided. The recent development of di-and tri-functional architectures of PIs, light harvesting PIs and push-pull and multicolor PIs opens a route towards highly absorbing PIs in the 400–800 nm range [57,58,64,73].

3.1.2. Use of Newly Developed LEDs and Laser Diodes Avoiding Hg-Based Lamps

Excellent conversion vs. time curves can be recorded upon excitation with a laboratory LED device at 365 nm (Figure 5) [64]. Commercial highly packed LED systems lead to the cure speeds attained with Hg lamps. According to the usual absorption spectra of PIs in the UV, many PIs work in these conditions [19]. A smaller number of systems can operate at 395 nm. Recently developed PISs operating in the near-UV/visible range (e.g., [57,58]) noticeably extend the scope of the existing structures and should be efficient upon a 395 nm LED irradiation.

Applsci 03 00490 g004 1024
Figure 4. Photopolymerization profiles of 3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) under air upon Xenon lamp exposure (~60 mW/cm2) in the presence of (1) bis(acyl)phosphine oxide (BAPO)/iodonium salt (1%/1% w/w); (2) BAPO/iodonium salt/TTMSS (1%/1%/3% w/w); (3) BAPO/iodonium salt/tetraphenyldisilane (1%/1%/3% w/w). Instead of BAPO, more colored structures, such as titanocenes and other dyes, can be used.

Click here to enlarge figure

Figure 4. Photopolymerization profiles of 3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX) under air upon Xenon lamp exposure (~60 mW/cm2) in the presence of (1) bis(acyl)phosphine oxide (BAPO)/iodonium salt (1%/1% w/w); (2) BAPO/iodonium salt/TTMSS (1%/1%/3% w/w); (3) BAPO/iodonium salt/tetraphenyldisilane (1%/1%/3% w/w). Instead of BAPO, more colored structures, such as titanocenes and other dyes, can be used.
Applsci 03 00490 g004 1024
Applsci 03 00490 g005 1024
Figure 5. Photopolymerization profiles of EPOX under air upon 365 nm LED exposure (~50 mW/cm2) in the presence of (a) triazine-pyrene/TTMSS/iodonium salt (1%/3%/2% w/w); (b) pyrene/(TTMSS/iodonium salt (1%/3%/2% w/w); (c) iodonium salt (2% w/w).

Click here to enlarge figure

Figure 5. Photopolymerization profiles of EPOX under air upon 365 nm LED exposure (~50 mW/cm2) in the presence of (a) triazine-pyrene/TTMSS/iodonium salt (1%/3%/2% w/w); (b) pyrene/(TTMSS/iodonium salt (1%/3%/2% w/w); (c) iodonium salt (2% w/w).
Applsci 03 00490 g005 1024

Laser diodes also lead to efficient FRP, CP and FRPCP [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]. New PIS exhibit an absorption that satisfactorily matches the emission of the sources from the blue to the red: this is exemplified in Figure 6, where three kinds of PI can be used with four examples of LED bulbs; TMPTA, as well as EPOX can be polymerized. Laser diode arrays obviously allow faster curing speeds.

Applsci 03 00490 g006 1024
Figure 6. (A) Polymerization profiles of TMPTA upon Xe-Hg lamp irradiation (λ > 390 nm) in laminated conditions in the presence of (1) Napht (0.5% w/w); (2) Napht/Ethyl-dimethylaminobenzoate (EDB) (0.5%/4.5% w/w); (3) Napht/EDB/phenacyl bromide (0.5%/4.5%/3% w/w). (B) Compared polymerization profiles of EPOX under air upon a red LED bulb irradiation in the presence of: (1) Pent/Ph2I+ (0.5%/ 2% w/w) and (2) Pent/TTMSS/Ph2I+ (0.5%/3%/ 2% w/w). Insert: emission spectra of the used LED bulbs (2–12 mW/cm²); different photoinitiators recently proposed [75,76].

Click here to enlarge figure

Figure 6. (A) Polymerization profiles of TMPTA upon Xe-Hg lamp irradiation (λ > 390 nm) in laminated conditions in the presence of (1) Napht (0.5% w/w); (2) Napht/Ethyl-dimethylaminobenzoate (EDB) (0.5%/4.5% w/w); (3) Napht/EDB/phenacyl bromide (0.5%/4.5%/3% w/w). (B) Compared polymerization profiles of EPOX under air upon a red LED bulb irradiation in the presence of: (1) Pent/Ph2I+ (0.5%/ 2% w/w) and (2) Pent/TTMSS/Ph2I+ (0.5%/3%/ 2% w/w). Insert: emission spectra of the used LED bulbs (2–12 mW/cm²); different photoinitiators recently proposed [75,76].
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3.1.3. Development of PISs for Soft Irradiation Conditions

Household devices, such as halogen lamps, fluorescent bulbs and LED bulbs, deliver low intensity visible light and are used in organic synthesis. They have been recently introduced for the photopolymerization of low viscosity monomers under air [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76] (see, again, the polymerization profiles under a red LED bulb exposure in Figure 6). Today, many PIS allow FRP, CP and FRPCP in these irradiations conditions: e.g., Figure 7 shows an efficient polymerization of EPOX under halogen lamp exposure. It is obvious that all the work on the design of PISs carried out in this area should be very helpful for potential and promising applications with more energetic light sources in both laboratory scale devices and industrial lines.

Metal-based, as well as metal-free photoinitiator catalysts operating on the basis on a photoredox catalysis can efficiently initiate a radical or a cationic photopolymerization or a radical/cationic hybrid curing [75,76,105]. Figure 8 shows the achieved performance when using a Ru complex as a photocatalyst for the FRPCP of EPOX under a household fluorescent bulb exposure. Ir-, Pt- or Zn-based complexes also lead to interesting results under soft visible irradiation; interpenetrated radical/cationic networks can be formed (see, e.g., [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,105]).

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Figure 7. Compared polymerization profiles of EPOX under air upon halogen lamp irradiation (~12 mW/cm²) in the presence of: (1) BAPO/Ph2I+ (1%/2% w/w); (2) NVK/Ph2I+ (1%/2% w/w); and (3) BAPO/N-vinylcarbazole (NVK)/Ph2I+ (1%/3%/2% w/w). Other PIs, such as bodipy, boranyl, violanthrone, pyrromethene dyes, etc., can operate [61,62,63,65,66,67,68,74,75].

Click here to enlarge figure

Figure 7. Compared polymerization profiles of EPOX under air upon halogen lamp irradiation (~12 mW/cm²) in the presence of: (1) BAPO/Ph2I+ (1%/2% w/w); (2) NVK/Ph2I+ (1%/2% w/w); and (3) BAPO/N-vinylcarbazole (NVK)/Ph2I+ (1%/3%/2% w/w). Other PIs, such as bodipy, boranyl, violanthrone, pyrromethene dyes, etc., can operate [61,62,63,65,66,67,68,74,75].
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Figure 8. IR spectra recorded in the course of a photopolymerization of EPOX; initiating system: Ru(bpy)2+/Ph2I+/(TTMSS (0.2%/2%/3% w/w) upon fluorescence bulb irradiation (~5–12 mW/cm²). A final conversion of 60% is obtained after 4 min.

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Figure 8. IR spectra recorded in the course of a photopolymerization of EPOX; initiating system: Ru(bpy)2+/Ph2I+/(TTMSS (0.2%/2%/3% w/w) upon fluorescence bulb irradiation (~5–12 mW/cm²). A final conversion of 60% is obtained after 4 min.
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3.1.4. Use of Sunlight Irradiation

Sun is the lowest intensity source used in this paper (2 mW/cm²). Efficient photopolymerization reactions still appear as relatively extremely hard. The FRP was mainly restricted to complex paint formulations (see the patent literature) or acrylates dispersed in a solid matrix [80,92]. On the opposite side, CP and FRPCP were reported as possible [96,97,98].

Recently, the FRP of a viscous matrix under air (e.g., an epoxy-acrylate having a viscosity of ~14,000 cP) has been carried out using efficient PISs based on silyl radical chemistry (50% conversion within 20 s and a final conversion of 75% at t = 8 mn using a bis(acyl)phosphine oxide (BAPO) and a silane) [77]. CP and FRPCP now appear as relatively easily feasible (see e.g., Figure 9); once again, the use of a three-component photoinitiating system based on a photoinitiator, an iodonium salt and a silane (or N-vinylcarbazole) allows an efficient curing of a usual difunctional epoxide matrix under air (see, e.g., in [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]).

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Figure 9. Compared polymerization profiles of EPOX under air upon sunlight (Mulhouse-France, 2 mW/cm²); in the presence of: (1) BAPO/Ph2I+ (1%/2% w/w); (2) NVK/Ph2I+ (1%/2% w/w); and (3) BAPO/NVK/Ph2I+ (1%/3%/2% w/w).

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Figure 9. Compared polymerization profiles of EPOX under air upon sunlight (Mulhouse-France, 2 mW/cm²); in the presence of: (1) BAPO/Ph2I+ (1%/2% w/w); (2) NVK/Ph2I+ (1%/2% w/w); and (3) BAPO/NVK/Ph2I+ (1%/3%/2% w/w).
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3.2. 3b-Photopolymerization of Renewable Monomers

Some typical examples of photopolymerization profiles of renewable epoxy resins upon visible light exposure (Xe lamp) are displayed in Figure 10 (see also Table 1). Among the different compounds depicted in Scheme 1, LDO is the most reactive monomer. This is in agreement with the cyclohexyl epoxy structure, where the ring opening process is highly favorable [19]. The polymerization is slower with ESO, ELO (epoxidized linseed oil) and EFA, but quite good final monomer conversions can be reached (40%–60%; Figure 10B) using a combination of the photoinitiator with an iodonium salt and a silane; moreover, tack-free coatings are formed. In any case, a decrease of the band at ~790 cm1 (due to the epoxy ring) is monitored, whereas an increase of the IR absorption band of the polyether network is observed in the 1050–1150 cm1 range. The photoinitiating system is important for getting a high reactivity, as exemplified by Figure 10A, Figure 11, where different photoinitiating systems lead to very different polymerization profiles. Photoinitiating systems based on bis-acylphosphine-oxides (BAPO) are very efficient (Table 2).

Extremely soft irradiation conditions can also be used. Figure 12 shows the epoxide consumption and the formation of the polyether network under a household fluorescent bulb or sunlight exposure under air. In outdoor conditions, tack-free coatings are obtained with LDO, ELO and ESO (Table 2). As before, the polymerization profiles of these monomers are also clearly improved by the presence of a silane, i.e., for ELO, a tack-free coating is obtained within only 9 min in the presence of a silane (TTMSS) vs. 50 min in the absence of the silane.

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Figure 10. Photopolymerization kinetics of (A) an epoxy monomer, limonene dioxide (LDO) using different photoinitiating systems: (a) benzophenone-sulfonyl ketone (BPSK)/Ph2I+/TTMSS (1%/2%/3% w/w); (b) BAPO/Ph2I+/TTMSS (1%/2%/3% w/w); (c) 3,3-carbonylbis-(7-methoxycoumarin)/Ph2I+/TTMSS (1%/2%/3% w/w); (B) different epoxy monomers: (a) LDO; (b) epoxidized soybean oil (ESO); (c) epoxidized fatty acid (EFA); (d) ELO using an initiating systems: BPSK/Ph2I+/TTMSS (1%/2%/3% w/w). Xenon lamp exposure (λ > 400 nm; (~60 mW/cm²)); under air; BPSK is a benzophenone-sulfonyl ketone difunctional photoinitiator [19].

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Figure 10. Photopolymerization kinetics of (A) an epoxy monomer, limonene dioxide (LDO) using different photoinitiating systems: (a) benzophenone-sulfonyl ketone (BPSK)/Ph2I+/TTMSS (1%/2%/3% w/w); (b) BAPO/Ph2I+/TTMSS (1%/2%/3% w/w); (c) 3,3-carbonylbis-(7-methoxycoumarin)/Ph2I+/TTMSS (1%/2%/3% w/w); (B) different epoxy monomers: (a) LDO; (b) epoxidized soybean oil (ESO); (c) epoxidized fatty acid (EFA); (d) ELO using an initiating systems: BPSK/Ph2I+/TTMSS (1%/2%/3% w/w). Xenon lamp exposure (λ > 400 nm; (~60 mW/cm²)); under air; BPSK is a benzophenone-sulfonyl ketone difunctional photoinitiator [19].
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Figure 11. Photopolymerization profile of ELO using an initiating system: BAPO/Ph2I+/TTMSS (1%/2%/3% w/w) upon Xenon lamp exposure; under air.

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Figure 11. Photopolymerization profile of ELO using an initiating system: BAPO/Ph2I+/TTMSS (1%/2%/3% w/w) upon Xenon lamp exposure; under air.
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As the polymerization efficiency in the presence of EPOX, LDO or ESO in the same experimental conditions are relatively close, it is obvious that renewable monomers can be successfully used in photocurable formulations operating in a large range of excitation wavelengths delivered by polychromatic (Xe lamps, household lamps) and (quasi) monochromatic (LED and laser diodes) light sources and sun. Some high performance PISs developed in the last year (see, e.g., the 2012 and 2013 references in [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76]) should ensure a faster formation of tack-free coatings upon sunlight exposure under air: work is under progress.

Table Table 1. Polymerization rates and final conversions of different renewable epoxides using BAPO/Ph2I+ (1%/2% w/w) as the photoinitiating system in the absence (a) or in the presence (b) of tris(trimethylsilyl)silane (TTMSS) (3% w/w); under air; Xenon lamp irradiation (λ > 400 nm).

Click here to display table

Table 1. Polymerization rates and final conversions of different renewable epoxides using BAPO/Ph2I+ (1%/2% w/w) as the photoinitiating system in the absence (a) or in the presence (b) of tris(trimethylsilyl)silane (TTMSS) (3% w/w); under air; Xenon lamp irradiation (λ > 400 nm).
MonomersRp/[M0] c (s1) Conversion
LDO a0.01671.4%
ESO a0.00221.5%
ELO a0.00027.0%
EFA a0.0019.0%
LDO b0.05 81.4%
ESO b0.006 43.1%
ELO b0.0006 29.7%
EFA b0.001 29.0%

a in absence of silane; b in presence of silane; c [M0] is the initial monomer concentration.

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Figure 12. IR spectra recorded in the course of a photopolymerization of ELO; initiating system: BPSK/Ph2I+/TTMSS (1%/2%/3% w/w): (A) fluorescent bulb irradiation (from t = 0 to 30 min) and (B) sunlight exposure (from t = 0 to 30 min); under air.

Click here to enlarge figure

Figure 12. IR spectra recorded in the course of a photopolymerization of ELO; initiating system: BPSK/Ph2I+/TTMSS (1%/2%/3% w/w): (A) fluorescent bulb irradiation (from t = 0 to 30 min) and (B) sunlight exposure (from t = 0 to 30 min); under air.
Applsci 03 00490 g012 1024
Table Table 2. Irradiation times to get tack-free coatings using BAPO/Ph2I+ (1%/2% w/w) in the absence (a) or in the presence (b) of tris(trimethylsilyl)silane (TTMSS) (3% w/w) upon sunlight exposure; under air.

Click here to display table

Table 2. Irradiation times to get tack-free coatings using BAPO/Ph2I+ (1%/2% w/w) in the absence (a) or in the presence (b) of tris(trimethylsilyl)silane (TTMSS) (3% w/w) upon sunlight exposure; under air.
Monomer Tackfree
LDO a48 min
ESO a*
ELO a50 min
EFA a*
LDO b12 min
ESO b45 min
ELO b9 min
EFA b*

a in absence of silane; b in presence of silane; *: no tack-free coating after 1 h.

4. Conclusions

This paper has reviewed some aspects concerned with the development of green technologies in the photopolymerization area. Interesting visible light irradiation sources (Xe lamps, diode lasers, LEDs, household lamps and, obviously, sun) today allow large possibilities of excitation from the near-UV to the near-infra-red. The development of very efficient PISs sensitive in the blue-to-red wavelength range for radical and cationic polymerization reactions undoubtedly opens new opportunities of polymerization reactions. Working in the absence of UV lights under air is on the right path today. Harmful Hg lamps can be avoided. Applications where (i) low light intensities are available (e.g., with sunlight) or required or (ii) quite low viscosity monomers (particular acrylates or cationic monomers) or thin films have to be employed become possible. Using sunlight, which has been a dream for a long time, might be within reach. The photopolymerization of renewable monomers is quite feasible. However, such monomers have to be designed as a function of the applications and the desired final material properties.

Many new additional works have to likely be proposed, for example, in the radiation curing area. It seems difficult today to find renewable acrylates exhibiting a performance close to that of the usual synthetic di- and tri-functional monomer/oligomers. The situation is different with the renewable epoxides, i.e., the compared performance of LDO and artificial epoxides are close in terms of polymerization rates and conversions, and the fabrication of glass fiber-reinforced composites with epoxidized vegetable oils has already been reported. Important questions may appear, e.g., about the physical/mechanical/surface, etc., properties of the cured material when starting from a conventional synthetic monomer or a modified natural raw compound. All the work described here was conducted in organic media: the use of water-borne formulations is noticeably less developed and the investigation of the photopolymerization of water-reducible, as well as water-based dispersions upon visible light exposure under air might also deserve to be carried out.

In the different topics discussed throughout this paper, much has been done, but much still remains to be done. The efforts deployed during the last thirty years to develop green aspects of the photopolymerization area begin to change, however, what was a challenge into a reality.

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