16 Dec Innovation in photopolymerization – Toward innovative materials and coatings

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DATE

December 2021

These last decades, photopolymerization has seen an important development, especially in adhesive and coating applications [1] and with the emergence of the 3D printing field. This process relies on the formation of a crosslinked material from monomers/oligomers in liquid state, initiated by a photo-initiator (PI) generating an active center (radical, cation, anion) upon UV irradiation. Considered as an environmentally friendly process, the photopolymerization reactions are characterized by a solvent-free, low energy consumption and rapid through cure process, enabling a precise spatial and temporal control of the reaction as the reactions occur only in the illuminated areas [2].

Generalities on photopolymerization

Photopolymerizations can occur according to two different mechanisms: free-radical and ionic (in general cationic) polymerization.

Free-radical polymerization is the most common reaction described in the literature. It is initiated by a radical, formed by photo-excitation of a PI. It has been largely employed for the development of materials and coatings as it enables very fast cure. Three main classes of molecules can undergo UV free-radical polymerization: unsaturated polyester/styrenes, acrylates and thiol-polyenes. The two first reactions are based on a free-radical chain growth polymerization while the thiol-polyene relies on free-radical additions. However, this polymerization suffers from oxygen inhibition and due to a high reactivity, generally leads to shrinking and cracking.

The free-radical UV polymerization mechanism is detailed below. The first step corresponds to the transition of a PI from a stable state to an excited state after light absorption, leading afterwards to the generation of a radical (step 1). The as-formed radicals initiate the reaction according to the same mechanisms than for a classic free-radical polymerization (step 4).[6] Photosensitizers can also be introduced to speed up the polymerization of low reactive monomers or if PI exhibit low UV-absorbance. PS absorb the energy of light to form an excited state which transfer their energy to the PI. Two routes are possible to transfer the excitation from PS to PI, either by energy (step 2) or electron transfer (step 3).

PI → PI*(hv) → radicals R^•(1)

PS → PS*(hv) → PI* → R^•(2)

PS → PS*(hv) → PS^• + PI^•→ radicals (3)

R^• → radical monomer → (4)

The other class of UV-curing mechanism is the cationic-initiated photopolymerization. Cationic photopolymerizations are initiated by onium salts, that upon UV irradiation, lead to the formation of Bronsted acids which then initiate the cationic polymerization. Cationic photopolymerization presents the significant advantages of not being sensitive to oxygen and to conduct to low film shrinkage and thus better adhesion on the substrate. Moreover, unlike free-radical reaction, cationic photopolymerization once initiated can proceed without light [3]. However, the main drawbacks of cationic photopolymerization are its high sensitivity towards water and its significantly lower kinetic rate compared to free-radical photo-polymerizations.

UV-cationic photopolymerization is mostly applied for epoxy resins but multiple reactive functions can be involved in this reaction as presented in the figure below.

A large range of UV-curable molecules are commercially available. SPECIFIC POLYMERS proposes alternative monomers and polymers bearing additional functionalities conferring diverse properties to the materials such as hydrophilicity, hydrophobicity, oleophobicity, optical, anticorrosive or fireproofing [4] properties. The UV-curable monomers can be adapted to free-radical or cationic photopolymerization depending on the requirements of the application.

Last advances on photopolymerization

Recently, much progress has been performed to overcome problems associated with the efficiency, wavelength flexibility, environmental and safety issues for the various existing photoinitiating systems [5]. Among these axis of research, one specific challenge has retained our attention and consists in using photopolymerization for elaborating composite materials. As a matter of fact, the introduction of fillers conducts to light scattering phenomena that prevent an efficient light penetration into the material and limit the complete curing of the material. Different approaches have thus been investigated to answer this problematic such as the use of low optical density PI, photo-bleachers or visible light system. In parallel to optic strategies, multicomponent photo-initiating systems, latent species or dual cure have been tested. [6]

Dual cure has led to interesting results according to the literature. The main strategies are the photoactivated redox polymerization and the thermal induced polymerization.
Redox polymerization is a two-component approach where a reducing agent is mixed with an oxidizing agent, generating active species that allow the polymerization of the surrounding media. Photo-redox enables the formation of free-radicals under very soft irradiation conditions and can be employed in ring opening and free radical polymerizations. The advantages are quite important as there is almost no initiator consumption and since both radical and ionic initiating species can be created. It was also demonstrated that, by using this combined approach (redox and photochemical activation), the oxygen inhibition at the surface could be overcome to a certain extent. Indeed, top surface conversion remains often low in redox system while excellent curing in-depth is achieved by photoactivated redox polymerization. [7]

Moreover, high irradiance and the nature of system can conduct to important local temperature increase which accelerates the polymerization process. Based on this phenomenon, the method of frontal polymerization has been implemented. After local initiation by proper stimulus, the reaction can proceed to adjacent zones by the movement of a reaction front. In photo-induced thermal frontal polymerization, the mechanism relies on the fact that the reaction heat produced by the polymerization conducts to the decomposition of thermal initiators. New radicals are thus formed, and the polymerization reaction can continue according to a self-sustaining reaction. This reaction can be applied for both free-radical and cationic polymerization and enables the curing of thick samples.

As mentioned in the introduction, photopolymerization [8] has opened new opportunities in the additive manufacturing field, allowing the preparation of tailor-made pieces with three-dimensional complex shape. Also called 3D printing, this technology is particularly appealing for aerospace, defence, medical and dental industries. Recently, composite materials have been obtained by combining the last advances in additive manufacturing and photopolymerization.

A sustainable approach for photopolymerization

Important efforts are currently made to include this promising technology in a more sustainable approach by the elaboration of biobased, biodegradable and recyclable liquid monomers and photo- polymers.

Vegetable oils are particularly promising precursors as they are non-toxic, low-cost, abundant and can be epoxidized or functionalized with acrylates. Through a careful selection of vegetable oils with specific fatty acid compositions, SPECIFIC POLYMERS has developed a range of epoxidized vegetable oils (EVOs) characterized by various epoxy content according to a green synthesis process. It has been evaluated by the University of Politecnico di Torino that all these EVOs have the ability to crosslink under cationic photopolymerization. . A wide range of Tg, from -20°C to 80°C, was then obtained [9].

Epoxidized castor oil

SP-3S-30-005

Epoxidized vegetable oil

SP-3S-30-006

Similar studies were performed on cardanol-based epoxies [10] and other products from SPECIFIC POLYMER’s portfolio such as the Diglycidyl ether of Vanillyl alcohol (DGEVA – SP-9S-5-005) and the Triglycidyl ether of phloroglucinol (PHTE – SP-9S-5-003) [11]. Here again very interesting results were obtained making UV-cationic photopolymerization an efficient and quite versatile technic for the preparation of epoxy-based thermoset materials.

Other biobased epoxies available in our catalog are listed here.

PHTE

Epoxy index (meq/g) : 7.2-8.5

SP-9S-5-003

DGEI

Epoxy index (meq/g) : 7.0 – 7.7

SP-9S-5-004

DGEVA

Epoxy index (meq/g) : 6.8 – 7.5

SP-9S-5-005

DGEMHY

Epoxy index (meq/g) : 6.5 – 7.9

SP-9S-5-006

DGEVAC

Epoxy index (meq/g) : 6.4 – 7.2

SP-9S-5-007

RTGE

Epoxy index (meq/g) : 6.8 – 7.6

SP-9S-5-009

Bio-based photopolymers were also obtained from succinic acid, itaconic acid, lignin or cyclodextrin leading to high quality 3D printed products. UV-curable biodegradable polymers are also outgrowing for biomedical applications. Hydrogels obtained from poly(lactide-co(glycolide) (PLGA) acrylates were proved to present different degradation kinetics depending on the monomers’ ratio. More and more, the fillers are also selected from renewable resources such as cellulose nanocrystals or chitin nanowhiskers. At last, the recyclability is a hot topic for thermosets and photopolymers are also concerned by this requirement. Dynamic networks were for instance, obtained from bisphenol A glycerolate diacrylate and hydroxyphenoxypropyl acrylate, by transesterification between the ester and hydroxyl moieties.