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

Abstract

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 coeffi cient ( 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 semicontinuous operations, with obvious adverse consequences for productivity. [ 2 ] With this limitation in mind, we have developed a novel eco-effi cient, "fl ash," radical chain linear photopolymerization, [ 1 ] able to reduce reaction time scales from hours to seconds. Our synergetic approach integrates macrofl ow photochemistry and emulsion-type polymerization . The result is a practical "single pass" helix photochemical reactor for the continuous photopolymerization of The continuous photopolymerization of acrylate and methacrylate monomer miniemulsions (25% solids content) is 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 effi ciency is 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 fl uorescent lamp.

Continuous flow reactor for miniemulsion chain photopolymerization: Understanding plugging issue

“…As shown in s). Our first study served to initially set up equipment and find optimal concentrations, irradiance, and flow rates [19]. 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.…”

“…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 [18]. 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 [19].…”

Photopolymerization in dispersed systems

“…6) made up of UV-transparent fluoropolymer tubing wrapped at a defined distance around a radiation source of low electrical power (e.g. "black light" fluorescent tube) and with flow rates usually > 1 mL min -1 [62]. However, a transfer from batch to continuous process mode is by no means trivial.…”

“…The inset shows a magnification of the helically coiled tubing constructed by winding a PTFE tubing (1.5 mm inner diameter) around a quartz cylinder. This figure has been reproduced from [62] with permission of the copyright owners.…”

“…Additionally, radiation penetration can be significantly improved by decreasing the droplet size below a threshold of 100-150 nm due to a significant decrease of scattering when using smaller droplets. A set of photoreactors displaying different sizes (micro- [60], minireactor [167]) and geometries were built and assessed for emulsion radical photopolymerization of acrylate monomers. Notably the helixtype minireactors (internal tube diameter: 1.5 mm) enabled in continuous operation a flash conversion within less than 30 s of residence time.…”