Dr Abraham Chemtob
email: abraham.chemtob@uha.fr

Agnès Rannée

Laurent Chalan

Diane Fischer

Sophie Bistac

Continuous Flow Reactor For Miniemulsion Chain Photopolymerization: Understanding Plugging Issue

Keywords: plugging, continuous process, photoreactor, photopolymerization, interfacial tension

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Introduction

In the polymer industry, there are currently considerable societal and legislative constraints to become compliant with more stringent environmental and health regulations: formulations with low or no volatile organic compounds (VOCs), less energy-consuming processes, greater integration of renewable energy source and monomer resources, and minimal waste production. At the forefront of this emerging sustainable polymer economy are two major VOC-free technologies: polymerization in dispersed media [START_REF] Asua | Polymer Reaction Engineering[END_REF] mostly emulsion and suspension polymerizationsand UV-vis photopolymerization [START_REF] Fouassier | Photoinitiators for Polymer Synthesis: Scope, Reactivy and Efficiency[END_REF] performed respectively in water and without solvent. By merging these two eco-efficient approaches, there is the potential for a synergistic photopolymerization in dispersed media, paving the way for a more energy-efficient and environmentally friendly way to synthesize polymer materials [START_REF] Jung | Visible-Light-Regulated Controlled/Living Radical Polymerization in Miniemulsion[END_REF].

Most exploratory experiments in this field routinely use batch operations with UVtransparent quartz glassware [START_REF] Capek | Photopolymerization of butyl acrylate microemulsion. Effect of reaction conditions and additives on fates of desorbed radicals[END_REF][START_REF] Durham | Suspension thiol-ene photopolymerization: Effect of stabilizing agents on particle size and stability[END_REF][START_REF] Jain | Photopolymerization of butyl acrylate-in-water microemulsions: Polymer molecular weight and end-groups[END_REF], far from the technological and ecological optimum.

Continuous operations, or flow operations, may be a much better way to conduct the photopolymerization of monomer emulsions. Two considerations primarily drive the development of continuous flow reactors for heterogeneous polymerizations:  Any wave-induced reaction, either based on photo-, sono-or microwave-chemistry, operates to best advantage when ensuring a close contact between energy source and reaction streams [START_REF] Wiles | Continuous process technology: a tool for sustainable production[END_REF]. Distinctive by their low optical path and efficient mixing, continuous photoreactors are thus best positioned to mitigate the significant loss of photon flux when the distance from the lamp is increased [START_REF] Anderson | Using Continuous Processes to Increase Production[END_REF]. In the specific case of a reactive dispersed medium, the issue of light attenuation is even exacerbated because scattering (droplet, particle) and absorption (photoinitiator, monomer) both limit the irradiated volume fraction within the reactor [START_REF] Jasinski | Photopolymerizable Monomer Miniemulsions: Why Does Droplet Size Matter?[END_REF].

 Polymerization in dispersed medium relies on the reaction of unsaturated monomers by chain-growth polymerization. The chain polymerizations of typical olefin monomers employed in emulsion polymerization are highly exothermic, thereby requiring a very efficient heat dissipation. For instance, the changes in enthalpy during the polymerization of vinyl acetate (-88 kJ/mol) or methyl acrylate (-78 kJ/mol) [10] are very close to the theoretical difference between the bond energies of the π-bond in an alkene and the σ-bond in an alkane. Due to the inherent high surface-to-volume ratio in continuous reactors, the removal of heat can be highly efficient, allowing for fine control of reaction temperatures [START_REF] Gilmore | Continuous Flow Photochemistry[END_REF].

To date, performing photochemical reactions in continuous reactors has been so far a very successful approach for purifying water via photo-oxidation of organic pollutants [START_REF] Mills | Water-purification by semiconductor photocatalysis[END_REF], growing algae exploited as non-fossil energy source [START_REF] Oren | Industrial and environmental applications of halophilic microorganisms[END_REF], or boosting the yield of specific organic reactions including photochlorination and singlet oxygen-mediated oxidation in the pharmaceutical industry [START_REF] Cassano | Photoreactor analysis and designfundamentals and applications[END_REF]. In the field of photopolymerization in dispersed media, there are much fewer examples and none of the known processes has been implemented industrially. In 2012, Tomovska et al. were the first to implement a quartz tubular reactor to synthesize a polyurethane-acrylate latex (20 % solids content) [START_REF] Daniloska | High performance pressure sensitive adhesives by miniemulsion photopolymerization in a continuous tubular reactor[END_REF][START_REF] Daniloska | Hybrid miniemulsion photopolymerization in a continuous tubular reactor-A way to expand the characteristics of polyurethane/acrylics[END_REF][START_REF] Tomovska | Miniemulsion Photo-Copolymerization of Styrene/Butyl Acrylate in a Continuous Tubular Reactor[END_REF]. Their continuous miniemulsion photopolymerization process involved a mean residence times, τ, of at least 4 min; and any decrease of τ did not afford enough time for sufficient reaction completion. In a variant emulsion-type process, Seeberger showed that styrene continuous photopolymerization (≈ 20 % solids content) could result in full conversion with a shorter residence time of 36 s [START_REF] Laurino | Snowballing Radical Generation Leads to Ultrahigh Molecular Weight Polymers[END_REF]. Recently, we reported the continuous photopolymerization of n-butyl acrylate (BA) miniemulsion (25 % solids content) in a compact helix minireactor prepared in poly(tetrafluoroethylene) (PTFE) coiled tubing, yielding 95% conversion after 27 s residence time [START_REF] Chemtob | Flash Latex Production in a Continuous Helical Photoreactor: Releasing the Brake Pedal on Acrylate Chain Radical Polymerization[END_REF].

Common to these studies are relatively low solids contents, and the fact any increase resulted in reactor fouling and plugging issue. Previous studies have hinted that reactor clogging may originate from the lower monomer/reactor wall interfacial tension compared to water/reactor wall interfacial tension, thus driving a progressive diffusion of monomer from the droplets to the reactor surface through the aqueous phase [START_REF] Daniloska | Designing tubular reactors to avoid clogging in high solids miniemulsion photopolymerization[END_REF].

Because the increase of solid contents is a critical issue for the commercial use of any continuous photopolymerization process, developing a better understanding of this mechanism is of prime importance. In this work, the photoinitiated miniemulsion photopolymerization of BA in a helix-type photoreactor forms the basis to identify some key parameterssurfactant concentration, solids content, reactor surface, inner tube diameterresponsible for the plugging. This set of results suggests that a threestep mechanism of plugging should take place. Under thermodynamic control, the "diffusional wetting" [START_REF] Rosen | Surfactants and Interfacial Phenomena[END_REF] of monomer droplets on reactor surface may be the trigger, resulting in the displacement of water by a thin layer of monomer film. Due to the difference of osmotic pressure between this low curvature monomer wetting film and the monomer nanodroplets, Ostwald ripening may contribute subsequently to higher film thickness. Finally, a polymerization-induced solid builds up on reactor surface because irradiation is optimal in the peripheric section of the tube, leading ultimately to clogging of the channels. Herein, only the initial step of adhesional wetting has been extensively investigated.

Experimental Section 2.1 Materials

Technical grade monomers, n-butyl acrylate (BA, > 99 mol%), was supplied by Sigma-Aldrich and used as received (i.e. without distillation or any purification step). Sodium dodecyl sulfate (SDS) and hexadecane, acting respectively as surfactant and hydrophobic agent, were purchased from Sigma-Aldrich. Freshly distilled water was used for the preparation of all monomer miniemulsions. Darocur TPO, also referred to as diphenyl (2,4,6trimethylbenzoyl) phosphine oxide, is a α-cleavable water-insoluble radical photoinitiator, which was kindly donated by BASF. Panels of bare aluminum AR 2024 T3 (Labomat) and glass substrate (P80120 AN1 type, Brotlab) were used.

Miniemulsion continuous photopolymerization of BA

The helix minireactor was constructed by winding a PTFE tubing (Bola®, Germany) of dimensions 1.5 mm inner diameter (i.d.)  2.1 mm outer diameter (o.d.) around a quartz cylinder (diameter: 60 mm, length: 400 mm). 27 coils of PTFE tubing were assembled, leading to an internal volume of 9 mL. All photopolymerizations were performed using a 18 W black light fluorescent tube (Osram, diameter: 26 mm; length: 590 mm). The black light lamp was placed in the axis of the quartz cylinder. The distance between the lamp surface and the coiled PTFE tubing was 17 mm. At this distance, an irradiance of 2.6 mW/cm 2 was measured by radiometry (Ocean Optics HR4000 spectrometer) within the 350-410 nm range (maximum emission at 370 nm). The monomer miniemulsion was circulated in the helically coiled tube at a constant flow rate, and for simplicity processes was developed to be complete after a single passage through the reactor. A membrane pump (Ismatec Reglo-Z, with pump head Z-181) ensured the circulation of fluid from the reservoir to the minireactor, and the flow rate was initially determined and regularly checked to detect any deviation. In conventional experiments, the flow rate was fixed at 20 mL/min ensuring a mean residence time, τ, of 27 s (i.e. the average time needed for a droplet to pass through the reactor). The reactor was fed from a reservoir (250 mL Erlenmeyer flask) containing the as-emulsified miniemulsion (100 g) and stirred with a magnetic stirrer at ambient temperature. The reservoir was covered with aluminum foil to prevent adventitious polymerization caused by exposure to ambient light. Gravimetric measurements confirmed the photolatency of the monomer miniemulsion stored in the reservoir. The minireactor was initially filled with the monomer miniemulsion before exposure to UV light, and the initial time of the reaction was taken when the black light lamp (already switched on) was placed in the axis of the quartz cylinder.

Samples were collected at the outlet of the tube for different analyses (size measurement, conversion) described below. A 250 mL Pyrex beaker was used to collect the resulting latex product at the outlet tube. No temperature increase between the inlet and the outlet of the reactor was found in all runs. Samples were collected at different reaction times corresponding to multiples of the theoretical residence time in order to determine the number of residence times needed to reach the steady-state operation. To prevent post-polymerization, the samples were collected in an amber glass flask covered with aluminum foil to prevent light exposure. A droplet of 1 mol% hydroquinone solution was added immediately after collection to inhibit polymerization.

Except when stated in the text, the typical monomer miniemulsion had a monomer content of 25 wt%. Miniemulsions were prepared by mixing an organic reactive phase containing BA, hexadecane (4 wt% / monomer), photoinitiator (0.75 wt% / monomer) and an aqueous phase containing SDS (4 wt% / monomer). The two phases were mixed together using a magnetic stirrer during 10 minutes at 600 rpm. The resulting coarse emulsion was then emulsified under ultrasonification (Branson Sonifier, 450 W/L) for 7 minutes at 90 % amplitude while maintaining the stirring.

Characterization techniques

Droplet and particles diameters, respectively labeled as D d and D p , were determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instrument). Typically, the monomer miniemulsion or the resultant latex was diluted 125 times in filtered and distilled water before measurement. Monomer miniemulsion stability was evaluated by monitoring the temporal evolution of the backscattering signal (reflectance) at ambient temperature (temperature set point 25 °C) through a TurbiscanLAB equipement (Formulaction). A steady signal provides an indirect proof that a stable miniemulsion was formed. The number-average molecular weight ( ) and molecular weight distribution / , designated as PDI, were determined using size exclusion chromatography (SEC).

The analysis was performed on a Shimadzu LC-20AD liquid chromatograph equipped with two Varian PL gel 5 μm MIXED-C columns (column, injection and refractometer temperature: 30 °C; injection volume: 100 μL) and a refractive index detector (Shimadzu RID-10A). THF was used as the eluent at a flow rate of 1.0 mL/min. The values were determined relative to linear polystyrene calibration standards. Fouling was assessed on a cut tube internal surface by (single-reflection) attenuated total reflection infrared spectroscopy (ATR-IR, Bruker Vertex). Each ATR-IR spectrum was based on the averaged results of 50 scans at a resolution of 4 cm -1 . The monomer conversions were determined by a conventional gravimetric technique. Contact angle measurements were performed with a drop shape analyzer -Kruss DSA 100 with the software Drop Shape Analysis. The deposited volume was 0.2 µL with a 0.50 mm diameter syringe. The surface energy γ was calculated from Owens and Wend equation [START_REF] Owens | Estimation of the Surface Free Energy of Polymers[END_REF] from the contact angles of two polar liquids (water and glycerol) and one non-polar liquid (hexane). Surface tension of BA and interfacial tension of BA/water systems were measured using the pendant drop technique (Krüss DSA 100). The drop volume was equal to 11 mm 3 . The evolution of the interfacial tension was followed as a function of time and the value of surface tension was taken at an equilibrium time of 34 s.

Results and Discussion

A review of key parameters causing plugging

The macroflow photochemical set-up consists of an energy-saving black light lamp emitting a UV-A broadband emission centred at 370 nm (irradiance = 2.6 mW/cm 2 ).

As shown in Figure 1, the lamp was surrounded by a quartz cylinder (60 mm i.d.), around which a poly(tetrafluoroethylene) tube (1.5 mm i.d.) was coiled up allowing the irradiation of a total volume of 9 mL. The BA miniemulsion (25 % solids content, D d = 80 nm) containing a water-insoluble phosphine oxide-type photoinitiator (PI, 0.75 wt% with respect to monomer) was circulated at a controlled flow rate (20 mL/min, τ = 27 s). Our first study served to initially set up equipment and find optimal concentrations, irradiance, and flow rates [START_REF] Chemtob | Flash Latex Production in a Continuous Helical Photoreactor: Releasing the Brake Pedal on Acrylate Chain Radical Polymerization[END_REF]. For sake of simplicity, the process was developed to be almost complete (95 %) after one passage through the continuous minireactor, and to reach steady-state operation after only two residence times, limiting the portion of products being out of specification. Figure 2 shows monomer conversion as a function of the number of residence times up to 10. The output is an acrylate latex having an average particle diameter of 120 nm and =103 kDa. Above a threshold solids content of 30 wt% (run B-C), the minireactor systematically plugs after only 2-3 residence times. The clogging of channels is also manifested upon decreasing the amount of SDS from 2 wt% (run A) to 1 wt% (run D) or 0.7 wt% (run E). Interestingly, all these monomer miniemulsions have a similar average diameter (80 nm) and are stable against coalescence and Ostwald ripening as suggested by a relatively steady backscattering signal measured with Turbiscan® (Figure 3). In addition, there is no apparent destabilization resulting from the contact with the PTFE tubing as checked by a stable droplet size (DLS data) for miniemulsion staying up to 3 hours in the minireactor in the absence of UV irradiation (data not given). To rule out the hypothesis that emulsion instability may be a major cause of plugging; an instable miniemulsion has been deliberately produced by removal of hexadecane (run F).

Although these conditions gave lower conversions (72 %) presumably due to the lower penetration depth of this larger size emulsion (size cannot be determined precisely by DLS), the system does not suffer any plugging. The last parameter triggering blockage of the stream is the use of a thinner tube diameter of 1 mm (run G), while larger tubes of 1.5 mm (run A) or 1.9 mm (run H) run smoothly. This result clearly points out the difficulty of implementing microreactor for photopolymerization in dispersed media. Several conclusions can be inferred from this series of scouting experiments.

 The use of dispersed phase nanodroplets, as individual reactors able to confine the solid products to these droplets, is not an efficient strategy to prevent channel clogging.

This suggests that monomer diffusion through the aqueous phase is possible even in the presence of costabilizer.

 The use of a low-energy and low-adhesive PTFE surface is in no circumstances a way to keep polyacrylate away from the tubing walls, and avoid clogged channels.

 Against all prediction, the relatively fast flow rates (20 mL/min) remains ineffective in protecting against the adsorption of polymer chains on the reactor wall.

 Although the reactor clogging upon solid formation has been a commonly cited concern of microreactor, it can also plague macroreactors (with an internal channel diameter ≥ 1 mm).

 Clogging can occur with highly stable monomer miniemulsions. Based on droplet size analysis by DLS, there is no sign of early destabilization in the reactor before the formation of solid (without UV irradiation). Therefore, a long-term kinetic stability of the initial miniemulsion makes no guarantee regarding plugging.

 The solid content (≥ 30 wt%), surfactant content (≤ 1 wt%) and tubing diameter (≤ 1 mm) are the most sensitive parameters as regards plugging. In contrast, droplet size, irradiance, photoinitiator concentration, flow rate have much less impact (see results

published in a spate study).

 There are several evidences indicating that channel clogging may be related to wetting phenomena, i.e. the displacement from a surface (reactor wall) of one fluid (water) by another (monomer). While, the change in SDS concentration has almost no effect on droplet size (runs A, D and E, Table 1), it can strongly affect the values of interfacial tensions. In addition, a decreased tube diameter causes a larger interfacial area for polymer adsorption, which can increase the surface free energy.

Visual inspection of the tubes after plugging (run B, Table 1) shows that the phenomenon is highly localized. Accordingly, ATR-FTIR analysis (not given) of the unplugged "clean" surface of the reactor wall surface did not reveal any coated polymer. To move forward, we have also studied the model miniemulsion (run A, Table 1) which does not show any sign of plugging. After several residence times, the entire internal tubing surface was systematically analyzed by SEM and optical microscopy on its whole length for presence of incipient signs of precipitates. As revealed in Figure 4, after 10 or even 1 residence time, the internal section of the PTFE tube shows, at very specific sites, that solid build-up occurs even though channel clogging has not manifested. These deposits were attributed to polymer adsorption despite the lack of conclusive evidence from EDX surface compositional analysis.

Plugging seems to originate from nucleation sites on channel surface, proceeding by subsequent accumulation of polymer.

Can plugging be driven by adhesional wetting of monomer droplets?

In a conventional spreading wetting, a liquid in contact with a solid substrate and another fluid (typically air) increases its surface area of contact with the substrate at the expense of this fluid. By contrast, our assumption is that an adhesional wetting may be effective in our case and account for the occurrence of plugging. In this case, a liquid (BA) not originally in contact with the substrate (PTFE), and in the form of droplet, makes contact with that surface by displacing another fluid (Water) and adhere to it [START_REF] Rosen | Surfactants and Interfacial Phenomena[END_REF]. Figure 5 is a graphical depiction of this process. Adhesional wetting can be also expressed in terms of decrease of surface free energy by the equation:

eq 1
Where a is the contact surface area gained by BA when it comes in contact with the substrate, is the interfacial tension of the PTFE substrate in equilibrium with BA, the interfacial tension at the PTFE-water interface, and the interfacial tension at the BAwater interface. or total decrease in surface free energy per unit area of the system due to adhesional wetting is negative, the system decreases in surface free energy, and the adhesion of BA can then occurs spontaneously, leading ultimately to the formation of a thin layer of BA. It is clear from this expression that a low-energy-surface, which is the case of PTFE (i.e. =

21.1 mN/m), does not influence adhesional wetting, the driving force of this type of wetting is rather the competition between the wetting of the two liquids (BA and water) on a given substrate reflected by the quantity . In this process, any decrease of the interfacial tension increases the tendency for adhesion to occur, while any reduction of the interfacial tension decreases the extent of adhesional wetting.

Our assumption is that the high cohesive energy of water should disfavor the wetting compared to a less cohesive organic liquid like BA [START_REF] Cabane | The physics of liquid water[END_REF], such difference should be manifested by a negative quantity .Consequently, can become a measure of the driving force behind the adhesional wetting process, but requires determination of interfacial tensions. A 3-step approach was conducted to evaluate for the BA/water/PTFE system without surfactant (3.2.1), with surfactant at different concentrations (3.2.2), and in the miniemulsion form (3.2.3). Obviously, continuous photopolymerization requires specific tubing both solvent-resistant and transparent to UV or visible light (with suitable photoinitiators). Nevertheless, the determination of using other substrates than PTFE can shed light on the influence of reactor wall material on plugging. Changing the PTFE for aluminum (the surface energy of cleaned aluminium is close to 29 mJ/m 2 [START_REF] Sharifi Golru | Effects of different surface cleaning procedures on the superficial morphology and the adhesive strength of epoxy coating on aluminium alloy 1050s[END_REF]) leads to a strong negative value of -49 mN/m, while a more hydrophilic surface such as glass yields -11 mN/m. The fluid to be displaced being water and not air, it is consistent to find that a more hydrophilic substrate can be less favorable to adhesional wetting. In summary, changing the tube composition does not appear as an efficient lever for action, as a way of limiting plugging. The values of contact angle summarized in Table 3 are relevant for demonstrating the better wetting of BA compared to water regardless of the substrate, this is because the high intermolecular attractive forces between water molecules tend to decrease its tendency to wet. The larger the discrepancy between the contact angles of the two liquids (water and BA), the stronger is the tendency for adhesion to occur. Figure 6 shows that an addition of surfactant, by reducing the water-BA and water-PTFE interfacial tensions, may cause the empirical factor to have a less negative value (maximum is -20 mN/m) and make adhesion wetting less thermodynamically favored.

Adhesional wetting of Water / BA / PTFE system

With a value of close to 0 (2 mN/m), there is little possibility to get a positive wetting coefficient. This suggests that inverse emulsion methods (employing for example water-soluble acrylamide or acrylic acid monomers dispersed in an inert hydrocarbon solvent) may operate without plugging.

Adhesional wetting of BA miniemulsion / PTFE system in presence of surfactant

Table 5 gives estimations of and in the case of 3 miniemulsions of variable SDS concentrations: 2 wt% (run A, no plugging), 1 wt% (run D, plugging) and 0.7 wt% (run E, plugging). For the calculations, the emulsions were assumed to act as water phase. The excellent wetting of BA on PTFE ( ) does not permit to be positive. However, SDS addition has a significant effect on wetting coefficient, which at 2 wt% concentration is very close to the minimal value (-21.5 mN/m) achieved in the previous non dispersed system BA/Water/PTFE (section 2.2). As shown in Table 1, an increase of SDS concentration is not really accompanied by a decreased droplet size, meaning that this supplement of surfactant is more readily available to modify the surface tensions of the system, in particular to reduce . Nevertheless, the slight differences of between these 3 samples does not fully account for the fact that the miniemulsions prepared with a SDS concentration lower or equal to 1 wt% (runs D and E) plug, whereas the continuous process runs smoothly at higher concentration (run A). For now, two hypotheses can be put forward: on the one hand, plugging is likely to be simply retarded in run A (at [SDS] = 2 wt%), and may occur at a residence times higher than 10. On the other hand, plugging may never take place experimentally, even though it is thermodynamically favored. Whether a monomer droplet can adhere to reactor wall depends on both thermodynamic and kinetic considerations. The clogging will be impossible under any and all reaction conditions if it does not pass the test of thermodynamic feasibility. Clogging is possible only if the free energy difference ΔG for adhesional wetting is negative (Sec. 2.1). A negative ΔG does not, however, mean that plugging will be observed under a particular set of operating conditions in particular the optimization of flow rate. The ability to avoid a thermodynamically feasible plugging depends on its kinetic non feasibilityon whether the process proceeds at a low enough rate under a proposed set of reaction conditions.

Conclusions and perspectives

Continuous processes have become a popular approach to increase polymer productivity, yield high-quality products, and recently make optimal photopolymerization processes in dispersed media. The continuous flow photoreactor for miniemulsion photopolymerization of BA showed promising results, however, reactor plugging and its consequence as regards maintenance and operating costs may limit its widespread application. Our report provides a first insight into the parameters causing plugging, and the complex plugging mechanism. This study revealed that miniemulsion stability, size or flow rate have minimal impact, while solids content, surfactant concentration or tubing diameter are crucial elements to avoid channel clogging, and should be carefully selected. Plugging occurs on localized sites of the tubing, where polymer build-ups gradually. Although there is no consensus on the precise mechanism, adhesional wetting is presented as the main triggering event, mainly driven by the high cohesive energy of water. However, adhesional wetting accounts only for the formation of a thin BA film, and should be completed by a more complete plugging mechanism discussing the role of Ostwald ripening and light depth in the growth of this initial monomer layer.

Calculation of has been used as a quantitative marker to assess the plugging propensity. There was no possibility to find thermodynamically disfavored conditions ( > 0) preventing adhesional wetting. More severe plugging is expected when hydrophobic tubes are used, and efforts should be focused on increasing hydrophilicity through surface modification. However, optimization of operating conditions through tubing composition may be difficult because UV light transparency must be also maintained. In the future, synergistic interactions with different surfactants could be a solution to improve water wetting on reactor surface compared to BA. The best approach may be to further reduce and with the use of specific surfactants such aerosol OT (sodium bis(2-ethyl-1-hexyl) sulfosuccinate) or fluorosurfactant. Aerosol OT is known in surfactant science for its exceptional ability to decrease surface and interfacial tensions, and for this reason, it is widely used in microemulsion composition. In the search for an efficient solution to prevent plugging, continuous polymerization should also take advantage of the experience of other fields such membrane bioreactors used in waste water treatment [START_REF] Le-Clech | Fouling in membrane bioreactors used in wastewater treatment[END_REF] where intermittent bubbling, pulsing air or more complex reactor design have been implemented for more than 20 years to prevent risks of clogging.

Figure 1 .Figure 2 .

 12 Figure 1. Schematic representation of the photochemical polymerization set-up comprising a helix minireactor

Figure 3 .

 3 Figure 3. BA miniemulsion stability assessed by the reflectance measurements during ageing time in the middle of the sample vial (Turbiscan data). The miniemulsion comprises different concentrations in SDS: 2 wt% (D d = 80 nm, run A, ▲), 1 wt% (D d = 80 nm, run D, •), 0.7 wt% (D d = 80 nm, run E, ♦).

Figure 4 .

 4 Figure 4. Images of the inner section of the PTFE tube before reaction and after 1 or 10 residence times (τ = 27 s) in the helix minireactor taken by optical microscopy (1 rst line) and SEM (2 nd line).

Figure 5 .

 5 Figure 5. Adhesional wetting of BA droplet on a PTFE substrate in a water medium. θ is the contact angle of BA on this substrate surrounded by water

Figure 6 .

 6 Figure 6. Evolution of the adhesional wetting coefficient of BA/water/PTFE system as function of the SDS concentration in the aqueous phase.

Table 1 .

 1 Effect of solids content, tubing diameter, surfactant (SDS) and co-stabilizer (hexadecane) concentrations on plugging during continuous-flow photopolymerization of BA miniemulsion. Irradiance = 2.6 mW/cm², [PI] = 0.75 wt% / monomer, τ = 27 s, flow rate = 20 mL/min, number of residence times = 10.

Table 2

 2

		water
	BA
		PTFE substrate
		water
	θ BA
	a	PTFE substrate

gathers the values of the 3 interfacial tensions appearing in eq 1. They were determined by a series of conventional methods described in the experimental section: liquid/liquid interfacial tensions were measured by pendant drop method while the solid/liquid

Table 2 .

 2 Calculated interfacial tensions values involved in the Water / BA / PTFE system

	Medium 1 / Medium 2	γ
		mN/m
	Water/PTFE	40
	BA/PTFE	2
	Water/BA	25

Table 3 .

 3 Equilibrium contact angles of BA and water deposited on PTFE, aluminum and glass

	Liquid/Substrate	θ
		°
	Water/PTFE	105
	BA/PTFE	41
	Water/Glass	57
	BA/Glass	10
	Water/Aluminum	92
	BA/Aluminium	10

Table 4 .

 4 Effect of the addition of SDS on and

	Liquid/PTFE	θ	Liquid/Liquid
		° mN/m		mN/m
	Water/PTFE	105 40	Water/BuA	25
	Water+SDS 0,7% /PTFE 79 14	Water+SDS 0,7% / BA 17
	Water+SDS 1% /PTFE 75 11	Water+SDS 1% / BA 16
	Water+SDS 2% /PTFE 72 10	Water+SDS 2% / BA 14

Table 5 .

 5 Contact angle, solid/liquid interfacial tension and adhesional wetting coefficient for the BA miniemulsion prepared with different concentrations of SDS.

		Liquid/Substrate	θ°		ΔG/a a
				mN/m	mN/m
		MiniE 0,7%SDS /PTFE	79.0	13	-26.5
		MiniE 1%SDS /PTFE	74.8	11	-23.8
		MiniE 2%SDS /PTFE	70.7	8	-21.5
	a	was determined considering a constant value of	= 15 mN/m

Acknowledgements: Financial support by French National Research Agency (ANR, Programme Chimie Durable-Industries-Innovation under contract number ANR-2012-CDII-006-02) is gratefully acknowledged.


Dr Abraham Chemtob
email: abraham.chemtob@uha.fr

Emeline Lobry

Dr Agnès Rannée

Florent Jasinski

Dr Marta Penconi

Dr Esther Oliveros

Dr André M Braun

Dr Adrien Criqui

Flash Latex Production in a Continuous Helical Photoreactor: Releasing the Brake Pedal on Acrylate Chain Radical Polymerization a

The continuous photopolymerization of acrylate and methacrylate monomer miniemulsions (25 % solids content) was investigated at room temperature in a compact helix minireactor.

Using n-butyl acrylate, the process yields 95 % conversion after only 27 s residence time, and gel-free high-molecular-weight products. Under optimized conditions, a 25-fold increase in efficiency was obtained when compared to a batch photopolymerization. The reaction set-up offers a frugal process because of moderate irradiance (2.6 mW/cm 2 ), photoinitiator concentration (0.75 wt%), and low-power UV-A fluorescent lamp.

Introduction

Pushing chemical production processes to their limits with regard to efficiency, emission control, compactness, safety and energy consumption is a currently spreading trend. As a major route to commodity and high-performance plastic materials, the intensification of polymerization reactions is a subject of high technological significance. [1] At the forefront is the radical chain polymerization, that represents 40 % of all processes for the production of commercial polymers, including some of the most common plastics such as polystyrene, acrylic or vinyl polymers. It is of high interest that the propagation step governing the chain-growth may be very fast, given that the propagation rate coefficient (k p ) is for most monomers in the range of 10 2 -10 4 L mol -1 s -1 . Nevertheless, the paradoxical observation is that polymer chemists have always strived to limit reactivity. The highly exothermic nature of radical chain polymerizations, the high activation energies involved, and the tendency toward the gel effect combine to make heat dissipation and polymer architecture control very challenging on an industrial scale. To avoid uncontrolled acceleration of the polymerization rate that may cause disastrous "runaway" reactions, the conventional approach consists in limiting the concentration of free monomer within the reactor through semi-continuous operations, with obvious adverse consequences for productivity. [START_REF] Asua | [END_REF] With this limitation in mind, we have developed a novel eco-efficient, "flash", radical chain linear photopolymerization, [1] able to reduce reaction time scales from hours to seconds. Our synergetic approach integrates macroflow photochemistry and emulsion-type polymerization. The result is a practical "single pass" helix photochemical reactor for the continuous photopolymerization of n-butyl acrylate (BA) miniemulsions. We show that high conversions can be achieved within short residence times at ambient temperature, yielding a stable high molecular-weight (MW) acrylic polymer latex.

As sketched in Scheme 1, our flow photochemical system comprises a black light lamp, with a broadband emission centred at approx. 370 nm and a low irradiance (2.6 mW/cm 2 ). The lamp is positioned in the axis of a quartz cylinder (60 mm i.d.), around which a UV-transparent and solvent-resistant poly(tetrafluoroethylene) tube (1.5 mm i.d.) is coiled up (Peschl Ultraviolet, Mainz, Germany). A total volume of 9 mL was irradiated, but the tube length can be varied to accommodate higher or lower volume.

The monomer dispersion is introduced at a controlled flow rate (20 mL/min), and the polymerized latex is collected at the outlet of the tube. Such macroflow photoreactor design is a variant of the Birr/Booker-Milburn model released in 2005 that has been extensively used in preparative organic chemistry. [3,4] Although continuous emulsion polymerizations in micro- [5,6] and macro-reactors [7,8] (in particular tubular reactors [9,10] ) have always attracted significant attention, some recent results have emphasized the advantage of using photochemical processes and corresponding photoreactors. [11][12][13][14] Chemists have realized that improvements involve not only the distribution control of MW and particle size, [15,16] but also high MW values, [13] polymerization rates, [14] film properties, [17] energy efficiency, installation costs and size. Aiming at a rational approach for a smarter way of conducting linear radical chain photopolymerization, we chose a simple and highly efficient reaction setup. Several features are significant in this set-up with regard to the photoreactor (i.), the UV lamp (ii.) and the monomer emulsion (iii.).

i. The high surface-to-volume ratio of a helical minireactor provides optimal polymerization heat transfer capability. Furthermore, in a dispersed medium, the combined effect of scattering and absorption limiting light penetration may be mitigated by an efficient mixing within shallow channels. In an industrial context requiring productivity and minimal plugging issues, [11] a macrofluidic reactor with millimetric channels seems more suitable than a microreactor with i.d. < 500 µm.

ii. Though UV-curing heavily relies on medium-pressure Hg arcs, we believe that a low power UV-A black light fluorescent tube [START_REF]The blacklight lamp is a low-pressure mercury vapour lamp having an inner envelope coated with a fluorescent powder that emits long-wave UVA radiation[END_REF] may be better suited to linear photopolymerization. It features low price, good conversion of electrical energy to UV radiation (30 %), safer UV-A emission, and last but not least negligible heat release.

Therefore, radical polymerization can take place at ambient temperature without the need for a water cooled jacket, and with potentially less transfer reactions.

iii. Viscosity and solvent emission problems are avoided by the heterogeneous polymerization conditions in water. In addition, a monomer miniemulsion overcomes many of the disadvantages of a conventional macroemulsion. Smaller nanosized monomer droplets (< 100 nm) are instrumental in promoting a better through polymerization, while kinetic stability prevents major size changes between emulsification stage and polymer product.

Experimental Section

Materials

Technical grade monomers, n-butyl acrylate (BA, > 99 mol%), methyl methacrylate (99 mol%) and butyl methacrylate (99 mol%) were supplied by Sigma-Aldrich and used as received (i.e. without distillation or any purification step to remove the radical inhibitor).

Sodium dodecyl sulfate and hexadecane, acting respectively as surfactant and hydrophobic agent (against Ostwald ripening), were purchased from Sigma-Aldrich. Freshly distilled water was used for the preparation of all monomer miniemulsions. Darocur TPO, also referred to as diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, is a α-cleavable water-insoluble radical photoinitiator, which was kindly donated by BASF Specialty Chemicals. (E)-[1-(2,5-Dimethyl-3-furyl)ethylidene](isopropylidene)-succinic anhydride (DFIS, formerly known as Aberchrome 540, > 95 mol%) was supplied by Extrasynthèse, and used as chemical actinometer. Ethyl acetate (> 99.7 mol%) and tetrahydrofuran (THF, > 99 mol%) were purchased from Aldrich.

Miniemulsion polymerization in a continuous-flow helical minireactor

As sketched in Scheme 1, the helix minireactor was constructed by winding a poly(tetrafluoroethylene) (PTFE) tubing (Bola®, Germany) of dimensions 1.5 mm inner diameter (i.d.)  2.1 mm outer diameter (o.d.) around a quartz cylinder (diameter: 60 mm, length: 400 mm). 27 coils of PTFE tubing were assembled, leading to an internal volume of 9 mL. Except when stated, all reactions were performed using a 18 W black light fluorescent tube (Osram, diameter: 26 mm; length: 590 mm). The black light lamp was introduced into a Pyrex protecting tube placed in the axis of the quartz cylinder. The distance between the lamp surface and the coiled PTFE tubing was 17 mm. At this distance, the estimated irradiance was 2.6 mW/cm 2 within the 350-410 nm range. A plastic grid wrapped several times around the Pyrex sleeve enabled to vary the irradiance values. Irradiance measurement and emission spectrum of the black light lamp (Figure S1 in supporting information) were determined with an Ocean Optics HR4000 spectrometer. The UV spectral output shows a broadband FWHM spectral bandwidth of the 370 nm peak of about 20 nm. The tiny secondary peak at 404 nm leaking through the filter gives the lamp its purple glow. As the lamp does not emit heat, no external cooling jacket was installed. The monomer miniemulsion was circulated in the helically coiled tube at different flow rates (7 -40 mL/min). A membrane pump (Ismatec Reglo-Z, with pump head Z-181) ensured the circulation of fluid from the reservoir to the minireactor, and the flow rate was initially determined and regularly checked to detect any deviation. In conventional experiments, the flow rate was fixed at 20 mL/min ensuring an irradiation time (residence time) of 27 s. The reactor was fed from a reservoir (250 mL Erlenmeyer flask) containing the as-emulsified miniemulsion (100 g) and stirred with a magnetic stirrer at ambient temperature. The reservoir was covered with aluminum foil to prevent adventitious exposure to ambient light. Gravimetric measurements confirmed the photolatency of the monomer miniemulsion stored in the reservoir. Note that dissolved oxygen was not removed from the miniemulsion, and that all formulations were processed continuously in a single pass. The minireactor was initially filled with the monomer miniemulsion before exposure to UV light, and the initial time of the reaction was taken when the black light lamp (already switched on) was placed in the axis of the quartz cylinder.

Advantageously, fluorescent lamps require only a brief warm-up period (30 -60 s after being switched on) to achieve a steady and full light output. During this short period, the lamp was placed in a UV-opaque box. Samples were collected at the outlet of the tube for different analyses described below. A 250 mL Pyrex beaker was used to collect the resulting latex product at the outlet tube. No temperature increase between the inlet and the outlet of the reactor was found in all runs. Samples were collected at different reaction times corresponding to multiples of the theoretical residence time in order to determine the number of residence times needed to reach the steady-state flow (steady conversion and constant properties, in particular constant particle size of the polymer obtained). To prevent postpolymerization, the samples were collected in an amber glass flask covered with aluminum foil to prevent light exposure. A droplet of 1 mol% hydroquinone solution was added immediately after collection to inhibit polymerization.

Note that some specific experiments were performed with two other UV lamps: a second black light (5 W, Osram, I = 5.2 mW/cm 2 in the 350-410 nm range) and a medium-pressure Hg arc (TQ150, Heraeus Noblelight, 150 W, effective arc length: 44 mm). In contrast to the black light, this lamp provides a polychromatic light, with the most powerful UV output within the spectral range of absorption of the photoinitiator at 367 and 313 nm. The measured irradiance within the 350-410 nm region was 310 mW/cm 2 . Miniemulsion polymerization in a conventional immersion-type photochemical reactor (photo-batch) and in a stirred tank batch macroreactor (thermal-batch) is described in supporting information.

Except when stated in the manuscript text, the monomer miniemulsion had a monomer content of 25 wt%, and its typical composition is given in Table 1. Miniemulsions were prepared by mixing an organic reactive phase (monomer, hexadecane, photoinitiator) and an aqueous phase (sodium dodecyl sulfate, distilled water). First, the two phases were mixed together using a magnetic stirrer during 10 minutes at 600 rpm. The resulting coarse emulsion was then emulsified under ultrasonification (Branson Sonifier, 450 W/L) for 7 minutes at 90 % amplitude while maintaining the stirring. These conditions were optimized to achieve the steady-state droplet size distribution for a miniemulsion of 100 mL. To achieve larger droplets, the surfactant concentration was adapted (1.5 wt% / monomer) without affecting the stability.

Characterization techniques

Droplet and particles diameters, respectively labeled as D d and D p , were determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instrument). Typically, the monomer miniemulsion or the resultant latex was diluted 125 times in filtered and distilled water before measurement. Monomer miniemulsion stability was evaluated by monitoring the temporal evolution of the backscattering signal (reflectance) at ambient temperature (temperature set point 25°C) through a TurbiscanLAB equipement (Formulaction). A steady signal provides an indirect proof that a stable miniemulsion was formed. The number-average molecular weight ( ) and molecular weight distribution / , designated as PDI, were determined using size exclusion chromatography (SEC). The experimental procedure for actinometric measurements (using (E)-[1-(2,5-dimethyl-3furyl)ethylidene]-(isopropylidene)-succinic anhydride (DFIS), formerly commercialized as Aberchrome 540) was published elsewhere. For these specific measurements, the typical monomer/PI/hexadecane organic phase was replaced by ethyl acetate/DFIS/hexadecane for the preparation of the non polymerizable miniemulsions. The organic phase content was 30 wt% and 0.08, 0.15, 0.5 and 1.5 wt% SDS (with respect to the organic phase content) was employed to produce a series of miniemulsions of variable average diameter. By applying the same emulsification methodology described above, miniemulsion droplets with an average size of 150 nm, 100 nm, 80 nm and 50 nm were prepared (DLS data). To respond to the need of actinometric experiments, the sample was diluted about 7 times to get a 4 wt% organic phase content miniemulsion. The resultant system was passed several times in the helix minireactor with a residence time of 27 s at a flow rate of 20 mL/min (similar conditions as those of photopolymerization).

The analysis was performed on a

Results and Discussion

We initially chose to investigate the photopolymerization of a BA miniemulsion (25 % solids content) containing a water-insoluble phosphine oxide-type photoinitiator (PI, 0.75 wt% with respect to monomer). Diphenyl-(bis(2,4,6-trimethylbenzoyl) phosphine oxide was chosen for its UV-A absorption spectrum being consistent with the emission spectrum of the light source (see Figure S1 of supporting information), as well as for its photobleaching that enables depth polymerization. Furthermore, BA is an ubiquitous monomer of waterborne acrylic dispersions, mass produced for applications as coatings and adhesives. This monomer is amenable to a continuous polymerization process at room temperature because of fast propagation (k p = 1.3 × 10 4 L mol -1 s -1 at 20 °C), [[19]] highly exothermic polymerization (ΔH = -78 kJ/mol) [START_REF] Brandrup | Polymer Handbook[END_REF] difficult to control in a macrobatch reactor, and strong tendency to chain transfer. [START_REF] Ahmad | [END_REF] We used SDS and hexadecane, a conventional surfactant/hydrophobe pair, to stabilize the BA miniemulsion. After emulsification provided by an ultrasonifier, an average droplet diameter (D d ) of 80 nm was determined by DLS. Minor changes of the backscattering signal during 4 h indicated that the as-produced miniemulsion was relatively stable against coalescence and Ostwald ripening (see Turbiscan data in Figure S2 of supporting information). The DLS data nonetheless showed a small size drift from 80 nm to 110 nm during the same period. Prior to UV irradiation, we also checked that the type of container (glass vial or Teflon tube), as well as several passes in the minireactor, did not affect the stability.

Figure 1 shows the kinetic results of a continuous single pass photopolymerization carried out at a residence time of 27 s (flow rate 20 mL/min). Under these conditions, a steady-state flow at 95% conversion was reached very fast after only 2 residence times, yielding a latex product with an average particle diameter of 120 nm. The reactor construction enabled reproducible results (5 replicates), and no temperature increase at simple spectroscopic set-up to assess light penetration. At 365 nm, the extinction (sum of absorption and scattering) coefficient of a BA miniemulsion (80 nm) was measured to be 150 cm -1 . As a result, at the beginning of the irradiation time, only 3 % of the incident photons are transmitted beyond 0.1 mm of the optical path within the reactor tube (1.5 mm i.d.).

Consequently, most of the reactor volume receives little (or none) of the actinically relevant radiation. The main difference is that the "dark" volume accounts for 85 % in the minireactor, as compared to more than 98 % in the immersion macroreactor. Yet, increasing the diameter of the reactor tube (e.g. to 1.9 mm) or the droplet size (e.g. to 160 nm) did not diminish conversion (> 90 % still achieved after 2 residence times). This result suggests that mass transfer could, to some extent, compensate for a limited depth of light penetration.

Accordingly, several studies reported the increased mixing properties of helical minireactors compared to straight tubular reactors, and their ability to approach plug flow. [24] ii.

Even though PI absorption contributes to non-uniform irradiation, it remains necessary to trigger the polymerization process. In a recent publication, [25] we proposed a method to evaluate the efficiency of light absorption in highly scattering systems such as miniemulsions, using non-polymerizable (diluted) ethyl acetate miniemulsions containing a chemical actinometer (DFIS) to replace the oil-soluble PI. A chemical actinometer undergoes a wellcharacterized photochemical reaction and allows the estimation of the incident and absorbed photon flux. [START_REF] Kuhn | Chemical actinometry IUPAC technical report[END_REF] We could reach the interesting conclusion that, despite a rather large variation in the scattering properties of the miniemulsions, the efficiency of light absorption was hardly affected. However, the actinometric experiments were performed in quartz cells of 1 cm optical path length (irradiated at 367 nm under stirring). Therefore, considering the drastic differences in photoreactor geometry and operating conditions, control experiments were carried out to verify that the conclusions reached in reference 25 still hold for a continuous helical photoreactor where the miniemulsion is pumped through a coiled tubing of 1.5 mm diameter. Figure 2 shows the evolution of the accumulated number of absorbed photons after several passes (residence time: 27 s) in the minireactor. Each pass caused a steady increase of the number of absorbed photons by the actinometer solubilized within the hydrophobic droplets. Similarly to experiments performed in quartz cells, varying droplet size in the nanoscale range (50-150 nm) did not give rise to any significant change despite very large differences in extinction (scattering + absorption) coefficients (0.15 -2.20 cm -1 at 365 nm).

This result is also consistent with the observed minimal effect of the larger size of the BA miniemulsion droplets (D d = 160 nm) on conversion (> 95 %) and polymer characteristics ( =110 kDa, PDI = 4.5). Additionally, it is important to mention that no problem of plugging was observed during the tests carried out up to 18 residences times. In addition, the absence of fouling was assessed by scanning the internal tube wall with ATR-IR spectroscopy.

Nevertheless, two limitations should be mentioned: i. Runs carried out with higher solids contents (40 wt% and 50 wt%) led to plugging. As recently emphasized, [27] the origin of clogging may be sought in the lower monomer/reactor wall interfacial tension compared to the aqueous phase/reactor wall interfacial tension. Under these conditions, there is displacement of the aqueous phase by monomer on the reactor wall, referred to as adhesional wetting. This issue and possible solutions will be investigated in a separate study. ii. The described positive effect on reaction kinetics and control of polydispersity was much less pronounced for monomers exhibiting a slower rate of addition of macroradicals to monomer units, and polymers with much higher glass transition temperature (glass effect) were obtained, as it is the case for methacrylic acid esters. For example methyl methacrylate (k p = 6.5 × 10 2 L mol -1 s -1 at 50 °C) [START_REF] Beuermann | [END_REF] and nbutyl methacrylate (k p = 7.6 × 10 2 L mol -1 s -1 at 50 °C) [START_REF] Beuermann | [END_REF] exhibited only a limiting conversion of 40 % for a residence time of 27 s (see Table 3 for complete characterization data). These early findings emphasize that the added value of the minireactor presented in this work is probably most important for highly exothermic and rapid polymerization reactions. S3 of ESI). However, the improvement in molecular weight distribution control is rather limited compared to previous results, using the same monomer [15] but working with a microreactor and in a homogeneous reaction medium. Interestingly, an oil-soluble PI is not inconsistent with a flash polymerization, although the initiating (phosphinoyl and benzoyl) radicals are generated in pairs within the confined monomer droplets with a high probability for fast bimolecular termination. We believe that the formation of high MW polymers can only be accounted for by a large radical desorption ability from particles. [28] In order to investigate the effect of irradiance (runs D-G in Table 4), a grid wrapped several times around the quartz cylinder was used to lower gradually the irradiance. A two-fold decrease of the irradiance (1.3 mW/cm 2 , run E) caused the conversion to drop from 95 % to 60 %, while a very low irradiance of 0.6 mW/cm 2 (run D) still ensured 20 % conversion. Conversely, a stronger irradiance of 8.2 mW/cm 2 (run F) provided by another black light lamp did not affect the kinetic profiles. More surprisingly, a 40fold increase in irradiance (310 mW/cm 2 , run G) provided by a medium-pressure Hg arc resulted in a drop of the conversion (50 %). Irradiance is known to control the rate of producing primary radicals with direct consequence on kinetic chains. This means that molecular weights can be modulated somehow (see values in Table 4) without the necessity of changing the formulation (e.g. initiator concentration). With regard to kinetics, limited conversions occur if primary radicals are produced at too low concentrations (irradiance ≤ 1.3 mW/cm 2 ). Conversely, at too high concentrations (high irradiance), the termination mode may change from the normal bimolecular termination between propagating radicals to primary termination that involves propagating radicals reacting with primary radicals. [29] It is most interesting to note that the medium-pressure Hg lamp, which is the established workhorse for carrying out UV-curing, is therefore of little relevance for linear photopolymerization using flow processes. A high radiant power is mostly justified for open-air polymerization of high surface-to-volume coatings adversely affected by the presence of oxygen. Obviously, the closed reaction medium ensured by the minireactor prevents oxygen permeation, thereby decreasing the risk of excited states quenching and radicals scavenging.

As summarized in

Furthermore, the fast reaction rates were achieved without nitrogen purging or monomer purification. Therefore, a too high irradiance leading to a high radical count is not only unnecessary, but even detrimental to reaction progress. Interestingly, a low priced actinic black light used traditionally for fluorescence excitation in nondestructive material inspection or clinical diagnosis can become a valuable tool in designing a more efficient photopolymerization process.

Finally, Table 4 gathers also the impact of changing the residence time (13 -73 s) through flow rate variation. Residence times longer than 27 s (runs H-I) did not affect the final conversion (95 %), but decreased MW values ( < 100 kDa) and broadened MW distribution, indicating that the PI may be not totally consumed in a single pass, thereby producing more radicals under longer UV exposure. This assumption could be validated by gas or liquid chromatography measurements. Conversely, the lower initiating radical count produced at shorter residence times (17 s in run J and 13 s in run K) did not afford the same high level of conversion. However, higher MW polymer species were formed with a narrower MW distribution.

Conclusions

Latex is commonly referred to as a "product by process", meaning that even details in the processing conditions (operation, temperature, etc.) can exert a strong influence upon polymerization kinetics and polymer architecture. Inspired by the research in the field of thick UV-curable coatings, [START_REF] Green | Industrial Photoinitiators: A Technical Guide[END_REF] our strategy retains its key features including long-wave UV irradiation, and a photobleaching PI employed at low concentration. We believe that our mild intensification process may lay the foundation for a novel, eco-efficient, "flash" chain radical polymerization approach. Limited dependence on droplet size and channel thickness, less transfer reactions, moderate MW distribution, low power consumption and low priced equipment represent the main assets. In future developments, the issue of scalability could be addressed through the increase of tubing diameter and/or length (and therefore of photoreactor volume). These two options are easy to implement within such a macroflow photochemical reactor, in contrast to a microreactor. The corresponding flow rate is 20 mL/min, the reactor volume of 9 mL was unchanged

Table of contents entry

A novel continuous radical photopolymerization process enables to convert acrylate monomer emulsion with surprisingly high efficiency. The combination of a photochemical minireactor, the resulting very short polymerization times, low costs of production and the simplicity of the equipment might be of interest to change the way emulsion polymerization is performed both at research and industrial levels.

  Shimadzu LC-20AD liquid chromatograph equipped with two Varian PL gel 5 μm MIXED-C columns (column, injection and refractometer temperature: 30 °C; injection volume: 100 μL) and a refractive index detector (Shimadzu RID-10A). THF was used as the eluent at a flow rate of 1.0 mL/min. The values were determined relative to linear polystyrene calibration standards. Fouling was assessed on a cut tube internal surface by (single-reflection) attenuated total reflection infrared spectroscopy (ATR-IR, Bruker Vertex). Each ATR-IR spectrum was based on the averaged results of 50 scans at a resolution of 4 cm -1 . The monomer conversions were determined by a conventional gravimetric technique. Macroscopic gel content was determined gravimetrically by means of a Soxhlet extraction under THF reflux during 12 h. UV-Visible spectroscopy was conducted with a Varian Cary 4000 double-beam spectrophotometer. Its wavelength range is typically comprised between 200 and 800 nm. With regard to the as-formed BA miniemulsions, the transmittance (absorbance) measurements were performed using a quartz cell of 0.1 mm optical path. For the determination of the PI absorption spectrum (in acetonitrile), a 1 mm quartz cell was used.

  Using a simple reactor comprised of a helically coiled Teflon tubing (1.5 mm i.d., 9 mL volume), a UV-A black light-driven continuous photopolymerization of a BA miniemulsion led to a steady conversion of 95 % after 2 residence times of 27 s. This result represents a 25fold increase in efficiency as compared to a corresponding batch operation. In addition, high MW gel-free polymer colloids ( > 100 kDa) with a moderate polydispersity (PDI = 4.1) were collected. No clogging or fouling was observed when the solids content was kept below 25 %. Nevertheless, it should be noted that the described positive effect on the overall reaction kinetics has been only validated for very fast propagating monomers such as acrylates. Despite their versatility, the need remains for a new generation of highly reactive, functional, and affordable monomers that may be polymerized by a mechanism adapted to the demanding conditions of aqueous emulsions.

Figure 1 .

 1 Figure 1. Evolution of conversion (left y axis) and average droplet diameter (right y axis) versus the number of residence times for the BA miniemulsion polymerization (residence time: 27 s, flow rate: 20 mL/min, D d = 80 nm).

Figure 2 .

 2 Figure 2. Number of absorbed photons versus the number of passes (each lasting 27 s) in the helix minireactor for an ethyl acetate miniemulsion containing a water-insoluble chemical actinometer (DFIS) with different droplet sizes and extinction coefficients (0.15 -2.20 cm -1 at 365 nm) (4 wt% φ org , [DFIS] = 1.8•10 -4 mol•L -1 ).

A

  . Chemtob,* E. Lobry, A. Rannée, F. Jasinski, M. Penconi, E. Oliveros, A.M. Braun, A. Criqui Flash Latex Production in a Continuous Helical Photoreactor: Releasing the Brake Pedal on Acrylate Chain Radical Polymerization

Table 4 ,

 4 the BA photopolymerization was optimized by varying

	PI concentration, irradiance and flow rate. Keeping the residence time unchanged (27
	s), a decrease of the PI concentration (from 0.75 to 0.25 wt%, runs A-C) resulted in a
	lower polydispersity (3.3) without affecting significantly conversion (90 %) (see size
	exclusion chromatography traces in Figure

Table 1 .

 1 Composition of BA miniemulsion (25 % solids content, D d = 80 nm).

	Monomer wt% / total	Water wt% / total	Hexadecane wt% / monomer	Sodium dodecyl sulfate wt% / monomer	Photoinitiator wt% / monomer
	25.00	75.00	4.00	2.00	0.75

Table 2 .

 2 Effect of operation modes on BA miniemulsion polymerization (25 wt% solids content, D d = 80 nm)

	Reactor D p	Conversion		PDI	Productivity
		nm	%	kDa	/	g/h
	Photo-	120	95 (reaction	103	4.2	300
	flow		time of 27s)
	Photo-	140	85 (in 2h)	300	2.3	12
	batch
	Thermal-	100	95 (in 1h)	79	4.9	35
	batch

Table 3 .

 3 Effect of the monomer structure on flow photopolymerization in the helix minireactor (25 wt% solids content)

	Monomer	D d	D p	Conversion		PDI
		nm	nm	%	kDa	/
	Butyl acrylate (BA)	80	120 95	103 4.2
	Methyl methacrylate 70	75	40	32	4.5
	Butyl methacrylate	120 165 40	39	6.1

Table 4 .

 4 Effect of irradiance, flow rate and PI concentration on continuous-flow photopolymerization of BA miniemulsion (25 wt% solids content, D d = 80 nm) Irradiance values are provided in the 350-410 nm region b

	Run	Irradiance a	Residence	[PI]	Dp	Conv		PDI
		mW/cm²	time s	wt%	nm	%	kDa
	A	2.6	27 b	0.75	120	95	103	4.2
	B	2.6	27	0.5	135	85	107	4.3
	C	2.6	27	0.25	170	85	161	3.3
	D	0.3	27	0.75	135	17	214	2.8
	E	1.3	27	0.75	120	65	146	3.8
	F	8.2	27	0.75	130	95	65	6.0
	G	310	27	0.75	100	48	-	-
	H	2.6	73	0.75	130	95	70	5.1
	I	2.6	35	0.75	135	95	80	5.2
	J	2.6	17	0.75	145	50	122	3.8
	K	2.6	13.5	0.75	145	30	126	4.1

a

Acknowledgements: Financial support by French National Research Agency (ANR, Programme Chimie Durable-Industries-Innovation under contract number ANR-2012-CDII-006-02) is gratefully acknowledged. Keywords: photopolymerization, colloids, nanoparticles, photochemistry, radical polymerization

the reactor outlet was noted. This reflects the good heat transfer properties provided by both high surface-to-volume nanodroplets and minireactor. In comparison, for a batch process in an immersion well photoreactor of 100 mL, 2 hours at room temperature were required to reach 90 % conversion, as shown in Table 2. Featuring a productivity of 300 g of polymer/h, the flow set-up employed thus enables an approx. 25-fold increase of reaction efficiency compared to a conventional photochemical batch process.

The rate difference is less pronounced for a thermally induced batch polymerization (35 g/h) in a stirred tank reactor of 100 mL, but the reactor types are too different to make really meaningful comparisons. Another key difference between photochemical flow, photochemical batch and thermal batch processes is related to polymer architecture (Table 2). In contrast to the thermal polymerization, the UV processes yielded gel-free latexes. Our hypothesis is that chain transfer to polymer leading to gel in acrylate polymerization may be suppressed at ambient temperature because its activation energy is higher than that of propagation. [12] As a result, higher number average MW values ( > 100 kDa) can be obtained in the photochemical processes compared to a thermal one ( =79 kDa) with even a slight improvement in the MW distribution: polydispersity index (PDI) of 4.2 versus 4.9 for a thermal batch.

The reason for the highly increased productivity of the continuous photochemical process was addressed by examining both (i.) the extent of irradiated volume; and (ii.) the PI absorption within the helix minireactor.

i.

Previous investigations demonstrated that below a threshold average diameter of ca.

150 nm, any decrease of droplet size caused a significant decrease of scattering, thus improving light penetration and reaction rates. [22] Scattering and absorption coefficients of acrylate miniemulsions were also determined in diluted [22] and concentrated [23] media using an integration sphere or the Kubelka-Munk theory respectively. In this work, we employed a

Supporting Information

Supporting Information is available from the Wiley Online Library Received: Month XX, XXXX; Revised: Month XX, XXXX; Published online: