Emulsion Photopolymerization: Development of Processes and Photoreactors – PHOTOEMULSION

The project breakthrough consists in the irradiation of stable monomer nanodroplets (minior
nanoemulsions) having diameters smaller than 100 nm. Maintaining small enough droplet
sizes provides two benefits: scattering impairing light penetration is attenuated, and the
radiation absorption required for polymerization is optimized. The replacement of existing
thermal processes by a photochemical route also relies on the implementation of
photoreactors traditionally employed in air or water depollution control, as well as organic
synthesis. Expected benefits relate to the fast generation rate of reactive species under UV
radiation, allowing reduced polymerization times of the order of several minutes versus
several hours thermally. In addition, the reactions are performed at room temperature and
following a continuous process, which complies with impeding European directives on the
reduction of energy consumption.

- Investigations shed light into the complex “Light-Emulsion” interplay, and elucidated the
influence of the optical properties of monomer nanodroplets on photopolymerization
progress.
- Short wavelength UV irradiation (< 300 nm) induced acrylate miniemulsion self-initiation
and photopolymerization without the aid of any external photoinitiator.
- Of high significance was a helix-type minireactor (internal tube diameter: 1.5 mm)
enabling a flash continuous conversion in less than 30 seconds.
- Novel polysulfide latex with semi-crystalline properties were synthesized via an
innovative step-growth radical photopolymerization process (thiol-ene).

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PHOTOEMULSION has already resulted in 8 publications in international journals, 12 oral
presentations and a patent application is under review. 6 dissemination actions dedicated to
a wide audience were also organized (film, interview, seminar, and hands out in Colleges).
Furthermore, the three temporary staffs (2 postdocs and 1 engineer) are now employed in
various R&D departments in France and abroad.

Emulsion radical polymerization processes have created considerable wealth in many industrial sectors, including coatings, adhesives, paints, additives for textiles and paper… Traditionally, waterborne polymer dispersions are produced in stirred tank reactors by the thermally-activated free-radical polymerization of a starting monomer emulsion. The objective of this project is to exploit UV radiation to promote the formation of high solid contents polymer latexes. The breakthrough is to generate highly concentrated dispersions of monomer nanodroplets (< 100 nm), that are subsequently photopolymerized in a specially designed photochemical reactor to produce polymer nanoparticles. Replacing a thermal process by a photochemical one has several advantages: (i) UV irradiation can induce very high rates of initiation that directly affect the overall rate of polymerization. In fact, the radical generation is known to be controlled by the incident photon flux and by the absorption conditions of the reaction system inside the photochemical reactor. Remaining within the nanometric (submicrometric) size range, the ratio of light absorption vs. scattering will be optimized in order to work under best conditions as far as light penetration is concerned. (ii) Photopolymerization will favour the implementation of continuous processes replacing the production limiting semi-batch operations used nowadays in industry. (iii) Photochemically initiated radical polymerizations are generally temperature independent and bear a much higher potential of application due to defined solubility, emulsion stabilization and controlled polymerization kinetics. (iv) A photochemical technology also complies with impeding European directives on solvent emissions and energy reductions.
A viable emulsion photopolymerization process can thus impart a wide range of attractive advantages of process intensification, but there is currently no mature technology in this field. To achieve this objective, the PHOTOEMULSION project will integrate contributions from two academic laboratories and an industrial partner: the Laboratoire de Photochimie et d'Ingénierie Macromoléculaires (LPIM, Université of Haute Alsace, Mulhouse), the Laboratoire des Interactions Moléculaires et Réactivité Chimique et Photochimique (IMRCP, Université Paul Sabatier, Toulouse III) and the MÄDER group. The LPIM (ex-Department of Phochemistry) is internationally recognized in the development of radiation-curing technology while the IMRCP has a complementary expertise in the domains of photochemical technology and engineering. A major change from the initial project submitted in 2010 is the involvement of MÄDER, responsible for planning and upscaling process innovations. MÄDER has a well-established activity in the sector of industrial and decorative coatings, and has been a pioneer company in the design and use of water-based paints and bio-sourced resins/monomers. As a result of the project, an innovative photopolymerization process in water will be introduced and applied in the highly technical market segment of bio-sourced latexes.