Cationic Frontal Polymerization Research Articles

A Novel Route to Glass Fiber‐Reinforced Epoxy Matrix Composites: Visible Light Activated Radical Induced Cationic Frontal Polymerization

AbstractIn the current study, a novel radical‐induced cationic frontal polymerization (RICFP) concept capable of rapid curing at room temperature via visible light irradiation is represented. Initially, the optimal formulation, which can be most effectively cured with visible light irradiation, is determined based on thickness, hardness, curing speed, and mechanical properties using FT‐IR, DSC, TGA, and flexural test methods. Subsequently, the viability of the method is illustrated by fabricating glass fiber‐reinforced composites through the hand lay‐up technique, employing the optimized formulation and glass fibers in various forms (chopped strand mat and biaxial). Mechanical properties of the obtained composites, including bending, tensile, and shear tests, are carried out according to relevant international standards and compared with reference composites thermally cured with amine‐based hardener by conventional method.

Synthesis and characterization of foams generated via gel state frontal polymerization

AbstractIn conventional foams, anisotropy within the microcellular structure can only be achieved through confinement in one or more directions during the foaming process. Herein, we present a novel, versatile, and energy efficient method to create unique high‐performance foams where the anisotropy of the microcellular and the microcellular structure itself is not dictated by external confinement during the foaming process. To generate such foams, an initial crosslinked gel is first created where the crosslink density of the gel can be tuned by UV intensity and cure time. Next, the gel can be foamed while simultaneously creating a second network via radically induced cationic frontal polymerization (RICFP). In this case, the anisotropy in the foam is dictated by propagation front during the RICFP and the microcellular morphology and cell surface area are dictated by the crosslink density of the gel. These two unique qualities lead to rigid foams that can be generated from a gel without the need for confinement where the microcellular size and shape can be tuned by the crosslink density of the gel precursor. A one‐pot liquid system composed of miscible epoxies, acrylates, and chemical blowing agents (CBAs), allows for control over the foam physical, mechanical, and thermal properties with glass transition temperatures ranging from 74.7 to 125.8°C. Additionally, by patterning the crosslink density of the initial gel through controlled exposure of UV, complex microcellular structures can be formed that are not possible in conventional foaming processes. This positions gel frontal polymerization and foaming as an advanced technique to produce high performing anisotropic foams with a wide array of tunable physical, mechanical, and thermal properties.

Redox cationic frontal polymerization: a rapid curing approach for carbon fiber-reinforced composites with high fiber content

Conventional frontal polymerization processes for epoxy-based composites rely on cations and radicals generated by a short (and local) light or heat stimulus in the presence of an iodonium salt and a radical thermal initiator. However, due to heat losses, the propagation of the exothermic curing front is often limited by sample geometry and filler concentration. Redox cationic frontal polymerization (RCFP) is a promising approach to radically expand the composition and design options of frontally cured epoxy-based composites. By adding stannous octoate as reducing agent, a higher number of radicals and cations are generated at lower temperature, which yields highly cured composite even at elevated filler content. In the current study, RCFP was used to cure standard unidirectional carbon fiber-reinforced composites based on a commercially available epoxy resin and the properties were compared with its anhydride hardener-cured counterpart. Cure degree and thermal properties of the resins were determined by ATR FT-IR spectroscopy and DMA analysis. Subsequently, unidirectional composites with a fiber volume content of ~ 60% were produced via vacuum infusion and subjected to DMA, tensile, compression, and inter-laminar shear tests. The results showed a remarkable similarity between mechanical properties of RCFP and anhydride hardener-cured composites. The RCFP-cured composites exhibited even a higher damping resistance and compression strength than anhydride hardener-cured composites. The results show that RCFP allows for a significant reduction in the curing time (from several hours to 60 min), while it yields composites with properties comparable to classic anhydride-cured systems.Graphical abstract

Novel thermal initiator systems for radical induced cationic frontal polymerization

Frontal polymerization is an extremely efficient and rapid method for producing bulk polymers, characterized by a self-sustaining curing front that propagates throughout the resin.

Free-Standing 3D Printing of Epoxy-Vinyl Ether Structures Using Radical-Induced Cationic Frontal Polymerization.

The application of frontal polymerization to additive manufacturing has advantages in energy consumption and speed of printing. Additionally, with frontal polymerization, it is possible to print free-standing structures that require no supports. A resin was developed using a mixture of epoxies and vinyl ether with an iodonium salt and peroxide initiating system that frontally polymerizes through radical-induced cationic frontal polymerization. The formulation, which was optimized for reactivity, physical properties, and rheology, allowed the printing of free-standing structures. Increasing ratios of vinyl ether and reactive cycloaliphatic epoxide were found to increase the front velocity. Addition of carbon nanofibers increased the front velocity more than the addition of milled carbon fibers. The resin filled with carbon nanofibers and fumed silica exhibited shear-thinning behavior and was suitable for extrusion-based printing at a weight fraction of 4 wt %. A desktop 3D printer was modified to control resin extrusion and deposition with a digital syringe dispenser. Flexural properties of molded and 3D-printed specimens showed that specimens printed in the transverse direction exhibited the lowest strength, likely due to the presence of voids, adhesion issues between filaments, and preferential carbon nanofiber alignment along the filaments. Finally, free-standing printing of single, angled filaments and helical geometries was successfully demonstrated by coordinating ultraviolet-based reaction initiation, low air pressure for resin extrusion, and printing speed to match front velocity.

Thermally initiated cationic frontal polymerization of epoxies and vinyl ethers through a lone onium salt

AbstractFrontal polymerization (FP) of epoxy monomers is typically achieved with a radical‐induced cationic frontal polymerization (RICFP) process that combines a thermal radical initiator with an onium salt superacid generator. In this paper, we show that both thermal and UV‐initiated cationic frontal polymerizations are possible for common epoxy and vinyl ether monomers with only an iodonium superacid generator in the absence of a standalone thermal radical initiator. Increasing superacid generator concentration resulted in an increase in front velocity, as did the addition of vinyl ether to epoxies. The front velocity is reduced by the addition of 4‐methoxyphenol (MeHQ), indicating free‐radicals play a significant role.

High performance foams and their nanocomposites generated via liquid state frontal polymerization

AbstractAnisotropy in naturally occurring or synthetic microcellular structures is an important feature for the development of materials with high specific stiffness and strength, in addition to creating materials with unique physical properties. Polymeric foams constitute a broad class of materials that are widely used for their advantages of low density, high specific mechanical properties and high insulative properties. Traditional synthetic routes are slow, energy demanding processes that employ the use of high temperature ovens, freezers, or high‐pressure equipment. Herein we present a convenient and energy efficient method to produce anisotropic high performance polymeric foams via rapid radically induced cationic frontal polymerization coupled with chemical blowing agents. The degree of pore orientation and degree of anisotropy are a result of the propagating front working in concert with the foam volume expansion. This paper presents results into FP foam formation to illustrate how changes in boundary conditions and front initiation position affect both the microcellular structure and their resulting physical and mechanical properties. Additionally, results are presented to show how changes in resin formulation, such as the addition of nanoparticles affect both properties as well as the microcellular structure and anisotropy.

Elaboration of epoxy foam by radical induced cationic frontal polymerization (RICFP): A proof of concept

The radical induced cationic frontal polymerization (RICFP) was used for the first time to elaborate epoxy foam from a mixture of a polyglycerol polyglycidyl ether (Denacol® EX-512) and 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate (CE) in 9/1 wt ratio. The initiating system was composed of 3.5 wt% of p-(octyloxyphenyl) phenyl iodonium hexafluoroantimonate (I-Sb) used as acid photogenerator, and 5 wt% of 1,1,2,2-tetraphenyl ethanediol (TPED) acted as thermal radical initiator. Only 19 s of UV irradiation were necessary to generate a propagation velocity of the polymerization front of 24.0 cm/min. A maximum temperature of 201 °C was measured at the polymerization front. In the presence of 1 wt% of xylene playing the rule of physical foaming agent, the matrix was expanded by a factor 5. The prepared foams exhibited cell diameters ranging from 1.4 to 2.2 mm and an apparent density of 220 kg/m3.

Kinetic and Chemical Effects of Clays and Other Fillers in the Preparation of Epoxy–Vinyl Ether Composites Using Radical-Induced Cationic Frontal Polymerization

Addition of fillers to formulations can generate composites with improved mechanical properties and lower the overall cost through a reduction of chemicals needed. In this study, fillers were added to resin systems consisting of epoxies and vinyl ethers that frontally polymerized through a radical-induced cationic frontal polymerization (RICFP) mechanism. Different clays, along with inert fumed silica, were added to increase the viscosity and reduce the convection, results of which did not follow many trends present in free-radical frontal polymerization. The clays were found to reduce the front velocity of RICFP systems overall compared to systems with only fumed silica. It is hypothesized that chemical effects and water content produce this reduction when clays are added to the cationic system. Mechanical and thermal properties of composites were studied, along with filler dispersion in the cured material. Drying the clays in an oven increased the front velocity. Comparing thermally insulating wood flour to thermally conducting carbon fibers, we observed that the carbon fibers resulted in an increase in front velocity, while the wood flour reduced the front velocity. Finally, it was shown that acid-treated montmorillonite K10 polymerizes RICFP systems containing vinyl ether even in the absence of an initiator, resulting in a short pot life.

Effect of Excessive Iodonium Salts on the Properties of Radical-Induced Cationic Frontal Polymerization (RICFP) of Epoxy Resin

Effect of Excessive Iodonium Salts on the Properties of Radical-Induced Cationic Frontal Polymerization (RICFP) of Epoxy Resin

Redox cationic frontal polymerization: a new strategy towards fast and efficient curing of defect-free fiber reinforced polymer composites.

Frontal polymerization of epoxy-based thermosets is a promising curing technique for the production of carbon fiber reinforced composites (CFRCs). It exploits the exothermicity of polymerization reactions to convert liquid monomers to a solid 3D network. A self-sustaining curing reaction is triggered by heat or UV-radiation, resulting in a localized thermal reaction zone that propagates through the resin formulation. To date, frontal polymerization is limited to CFRCs with a low fiber volume percent as heat losses compromise on the propagation of the heat front, which is crucial for this autocatalytic curing mechanism. In addition, the choice of suitable epoxy monomers and thermal radical initiators is limited, as highly reactive cycloaliphatic epoxies as well as peroxides decarboxylate during radical induced cationic frontal polymerization. The resulting networks suffer from high defect rates and inferior mechanical properties. Herein, we overcome these shortcomings by introducing redox cationic frontal polymerization (RCFP) as a new frontal curing concept. In the first part of this study, the influence of stannous octoate (reducing agent) was studied on a frontally cured bisphenol A diglycidyl ether resin and mechanical and thermal properties were compared to a conventional anhydride cured counterpart. In a subsequent step, a quasi-isotropic CFRC with a fiber volume of >50 vol%, was successfully cured via RCFP. The composite exhibited a glass transition temperature > 100 °C and a low number of defects. Finally, it was demonstrated that the redox agent effectively prevents decarboxylation during frontal polymerization of a cycloaliphatic epoxy, demonstrating the versatility of RCFP in future applications.

Iodonium Borate Initiators for Cationic Photopolymerization and Their Application in Radical-Induced Cationic Frontal Polymerization

Iodonium antimonates and borates are well established initiators in the field of cationic photopolymerization and preferred substances for radical-induced cationic frontal polymerization (RICFP). Extensive research in the fields of ionic liquids and lithium ion battery electrolytes made many superacids based on the borate anions available. Cyanide-ligated borane B2 has lower molecular weight compared to state-of-the-art borates and is less expensive to synthesize from commercial starting materials. By comparing reactivity in the epoxy-based monomer bisphenol-A-diglycidylether, B2 outperforms all commercial systems in a broad range of temperatures with excellent epoxy group conversions of up to 95%. Besides its good performance as a cationic initiator, B2 is also capable of thermally and photochemically induced RICFP. Exceptional are transparent B2 specimens in contrast to the tested commercial systems, which are black. High reactivity, good frontal parameters, easy synthetic access, and sufficient pot life of formulations make cyanide-ligated iodonium borates a compelling alternative to current state-of-the-art initiators.

Effect of a Dicycloaliphatic Epoxide on the Thermo‐Mechanical Properties of Alkyl, Aryl Epoxide Monomers Cured via UV‐Induced Cationic Frontal Polymerization

AbstractRadical induced cationic frontal polymerization (RICFP) is a promising route to achieve rapid curing of epoxy‐based thermosets, requiring only a localized exposure with UV light. In the presence of a diaryliodonium‐based photoinitiator and a thermal radical initiator, a self‐sustaining hot front cures epoxide monomer via a cationic mechanism. However, the cationic polymerization of diglycidyl ether derivatives is slow (in comparison with other epoxides with higher reactivity) and, as a consequence, frontal polymerization is sluggish because the heat loss is not compensated by the rate of heat release. Cycloaliphatic epoxies possess a higher ring strain than diglycidyl ether derivatives and can be blended with the latter to increase its rate of frontal polymerization. In the current work, a comprehensive study on the influence of 3,4 epoxycyclohexylmethyl 3,4‐epoxycyclohexane carboxylate (CE) on cure kinetics, viscosity, front velocity, mechanical, and thermo‐mechanical properties of frontally cured bisphenol A diglycidyl ether derivatives is presented. The results show a direct relationship between frontal velocity and amount of reactive diluent while an inverse relationship with the storage viscosity is observed. It is found that increasing the content of cycloaliphatic epoxide reduces the glass transition but increases mechanical properties of frontally cured bisphenol A diglycidyl ether derivatives.

An Epoxy Adhesive Crosslinked through Radical‐Induced Cationic Frontal Polymerization

AbstractUV‐initiated cationic frontal polymerization is exploited as a solvent‐free, extremely fast, and low‐temperature technique to obtain epoxy‐based adhesives. Epoxy formulations are prepared by blending commercial resins at different weight ratios and adding photo and thermal initiators at different percentages. In addition, the influence of other critical parameters, including the joint thickness, the nature of the adherends, and the temperature, is studied. As the reaction front is thermally sustained, the boundary conditions play a key role during the curing process and heat dissipation through the adherends in particular. The thermal properties of the epoxy formulation are studied through differential scanning calorimetry analysis, and the joint strengths are investigated by carrying out single lap off‐set shear tests under compression. The results demonstrate the feasibility of obtaining joints by means of the radical induced cationic frontal polymerization of the epoxy adhesives, which exhibit comparable epoxy group conversion and mechanical performances to the ones cured by traditional energy‐intensive techniques.

Bismuthonium‐ and pyrylium‐based radical induced cationic frontal polymerization of epoxides

AbstractCationic polymerization is a powerful tool when it comes to adhesive, coating, composite, and bulk material production. A variety of monomers is suitable for this technique, however epoxy‐based systems are used in many applications, due to their high reactivity and versatility. With the introduction of the radical induced cationic frontal polymerization (RICFP), an even more efficient pathway to cure epoxides and with it a variety of new applications was obtained. The prevailing initiator class for RICFP applications is the iodonium salt. Decades of research and fine‐tuning of the formulations lead to a highly effective and versatile initiator system. With the introduction of bismuth‐ and oxygen‐based onium salts for frontal polymerization, the well‐known iodonium salt is challenged. Bismuthonium hexafluoroantimonates shows high rate of polymerization and conversions of 84% in an epoxy system. The advantage of bismuthonium‐based systems is the pot life of the formulations, which can be cured at the press of a button. In this study, iodonium salts last around 3 days before a significant viscosity increase is measured, while the bismuthonium initiator lasts over 10 times as long with no significant drop in reactivity or frontal velocity.

Front velocity dependence on vinyl ether and initiator concentration in radical‐induced cationic frontal polymerization of epoxies

AbstractFormulations containing vinyl ethers and epoxy were successfully polymerized through a radical‐induced cationic frontal polymerization mechanism, using an iodonium salt superacid generator with a peroxide thermal radical initiator and fumed silica as a filler. It was found that an increase of vinyl ether content resulted in higher front velocities for divinyl ethers in formulations with trimethylolpropane triglycidyl ether. However, increased hydroxymonovinyl ether either decreased the front velocity or suppressed frontal polymerization. The kinetic effects of the superacid generator and thermal radical initiator with varying vinyl ether content were also studied. It was observed that increasing concentrations of initiators increased the front velocity, with the system exhibiting higher sensitivity to the superacid generator concentration.

NIR-sensitized cationic frontal polymerization of vinyl ether and epoxy monomers

Radical Induced Cationic frontal polymerization (RICFP) of vinyl ethers and epoxides with Near-Infrared (NIR) sensitizer has been scarcely reported in the literature. In this paper, a two-component system including cyanine as NIR sensitizer and iodonium salt as co-initiator was employed to initiate the polymerization of cationic polymerizable monomers upon 808 nm NIR laser exposure. The activities of cationic polymerizable monomers like viny ethers, epoxides, and oxetanes were studied using the NIR initiating system aforementioned. The aryl glycidyl ethers (E51) and vinyl ethers exhibited higher conversion than cycloaliphatic epoxide (6110) and oxetane (GR43). Additionally, a three-component system comprising NIR sensitizer, iodonium salt and thermal initiator was investigated on RICFP. Vinyl ethers showed the highest reactivity, whose velocity can reach to 2.5 cm s−1 compared to other kinds of cationic polymerizable monomers. Compared with other thermal initiators, AIBN could promote the RICFP of vinyl ethers. But when the monomers turned to E51 and GR43, AIBN could not work on RICFP. Meanwhile, another thermal initiator (1, 1, 2, 2- tetraphenylethanediol) carrying a C–C labile showed acceptable reactivity on RICFP.

Radical photoinduced cationic frontal polymerization in porous media

AbstractTwo different interpenetrating phase composites were produced using a radical photoinduced cationic frontal polymerization process. The composites were based on polyurethane (PU) and aluminium open‐cell foams impregnated with a formulation of a cycloaliphatic epoxy with different concentrations of a cationic photoinitiator and a thermal initiator. The influence of both types of initiators on the frontal polymerization features was systematically evaluated for the PU foam. It was found to occur only when the concentration of both initiators was greater than 0.5 wt%, leading to full conversion of the epoxy in the whole volume of the 15 mm thick composite samples within less than 100 s. The maximum temperature reached by the propagation front was in the range 275–305 °C depending on the type of formulation, leading to pores in the epoxy phase and extensive degradation of the PU phase. In the case of the opaque aluminium foam, an additional layer of pure resin was required on the UV‐exposed surface, which corresponded to a critical mass of a few grams to ensure sufficient heat generation and trigger the front propagation. © 2020 Society of Chemical Industry

Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers.

Ultraviolet (UV)-induced cationic frontal polymerization has emerged as a novel technique that allows rapid curing of various epoxy monomers upon UV irradiation within a few seconds. In the presence of a diaryliodonium salt photoinitiator together with a thermal radical initiator, the cationic ring opening polymerization of an epoxide monomer is auto-accelerated in the form of a self-propagating front upon UV irradiation. This hot propagating front generates the required enthalpy to sustain curing reaction throughout the resin formulation without further need for UV irradiation. This unique reaction pathway makes the cationic frontal polymerization a promising route towards the efficient curing of epoxy-based thermosetting resins and related composite structures. This review represents a comprehensive overview of the mechanism and progress of UV-induced cationic frontal polymerization of epoxy monomers that have been reported so far in literature. At the same time, this review covers important aspects on the frontal polymerization of various epoxide monomers involving the chemistry of the initiators, the effect of appropriate sensitizers, diluents and fillers.

Radical induced cationic frontal polymerization for preparation of epoxy composites

Epoxy composites are usually prepared from two component formulations that have limited storage stability and require time and energy consuming post curing. Radical induced cationic frontal polymerization (RICFP) circumvents these drawbacks and allows the fast on demand curing of these materials. In this study, the most suitable reactive diluents for preparing composites based on highly viscous bisphenol-A-diglycidylether were identified. The results revealed that 1,6-hexanediol diglycidylether (HDDGE) and (3-ethyloxetan-3-yl)methanol (EOM) were most effective reactive diluents regarding viscosity and reactivity. Composite production with various types of fillers were successfully evaluated regarding their frontal polymerization parameters. It was possible to show that high filler contents of up to e.g. 74 vol% glass microspheres and 40 vol% short carbon fibres were able to be processed. In comparison with thermal curing techniques, the epoxy-based fibre reinforced polymer composites prepared by RICFP showed similar values in tensile measurement, but the manufacturing process was facilitated significantly.