Silane Based Redox Initiating Systems: Toward a Safer Amine-Free

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Silane Based Redox Initiating Systems: Toward a Safer Amine-Free, Peroxide-Free, and Metal-Free Approach Dengxia Wang,†,‡,§ Patxi Garra,†,‡ Florian Szillat,∥ Jean Pierre Fouassier,† and Jacques Laleveé *,†,‡ †

Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France Université de Strasbourg, Strasbourg F-67081, France § Shandong Institute of Nonmetallic Materials, Jinan 250031, China ∥ Dentsply Sirona, De-Trey-Straße 1, Konstanz 78467, Germany Downloaded via IDAHO STATE UNIV on April 23, 2019 at 17:03:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Room temperature redox initiated free radical polymerization (RFRP) has always attracted high attention in the field of materials due to its advantages of energy saving, high efficiency, and easy operation. However, the current redox initiating systems are based on toxic aromatic amines and hazardous peroxides (e.g., dibenzoylperoxide). In the present paper, the redox two component (2K) initiating performances of silanes (as reducing agents) in combination with a highly stable iodonium salt (as oxidizing agent) were studied for the first time under mild conditions (RT, under air). Optical pyrometry measurements and DSC investigation results showed that the diphenylsilane (DPS) exhibited a unique initiating property for several (meth)acrylate monomers. Remarkably, thermal postcuring (B-stage) is also possible using this system. Based on electron spin resonance (ESR) experiments, the initiating chemical mechanisms of RFRP are established. Importantly, the new proposed initiating systems can be used for the preparation of tack-free glass fibers and carbon fibers composites at room temperature.

1. INTRODUCTION Free radical polymerization (FRP) for the preparation of polymers and composites under mild conditions (room temperature, under air) are of huge interest and importance.1−3 As robust under these conditions, two-component (2K) mixing of reducing and oxidizing agents in redox initiated polymerization has been widely performed in the materials area; nevertheless the current highly efficient reference redox initiating systems (RISs) are usually based on amine/ dibenzoylperoxide (BPO) systems4−6 which show several potential issues: high toxicity of amines, storage of the hazardous peroxides, and so on. Development of safer RIS to fit the requirements of room temperature polymerization has therefore been the pursuit of scientists.1,7−9 More recently, the development of peroxide-free and amine-free redox systems for ecofriendly and safer applications was proposed.10−15 These systems show high performances but still have some defects such as the presence of metals (cerium, manganese, copper, vanadium, ...). In this context, metal-free and peroxide-free organic compounds able to react in redox processes are highly researched. Particularly, only one study shows the possibility to use iodonium salts (safe/unexplosive oxidizing agents/ decomposition temperature >300 °C) in RFRP of methacrylates (room temperature, under air).16 One should mention the work of Crivello et al. that used of vapor state silanes in © XXXX American Chemical Society

redox cationic polymerization in combination with (i) onium salts and (ii) expensive Pt or Pd catalysts.17,18 It has also be found that silver(I) can react with silanes in a redox reaction but with low performances in RFRP (reaction was more efficient in cationic polymerization).19,20 To the best of our knowledge, the redox free radical polymerization from organosilanes and iodonium salts mixing was never reported although the use of simple and cheap silanes is highly attractive. The iodonium salts are now well reported as cationic photoinitiators for the curing of epoxy resins, they have also been used in multicomponent initiating systems for radical polymerization usually in combination with photoinitiator and amine.21 In this paper, simple, economical, and environmentally friendly organosilanes are explored in high-efficiency and mild redox initiators in combination with iodonium salts; that is, they can be used at room temperature under air. The redox initiating ability of different silanes in combination with iodonium salt will be reported; a structure/reactivity/efficiency relationship will be drawn. In this context, diphenylsilane is clearly found the best compound for mild polymerization conditions and for the curing of composites. Interestingly, Received: February 1, 2019 Revised: April 4, 2019

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DOI: 10.1021/acs.macromol.9b00233 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Chart 1. Chemical Structures of Silanes, Iodonium Salt (Iod), and Additives (Phosphines) Used in This Study

Chart 2. Chemical Structures of Methacrylate and Acrylate Monomer Blends Used in This Study

(DMAPDP) were purchased from TCI. Diphenylsilane (DPS; purity >97%) and 4-(diphenylphosphino)styrene (4DPPS) were purchased from Sigma-Aldrich. Di-tert-butyl-diphenyl iodonium hexafluorophosphate (Iod) was obtained from Lambson Ltd. (U.K.). The efficiency of different silane/Iod and silane/Iod/phosphine initiating systems were checked by using different (meth)acrylate monomers or blends; denoted as Resin 1: bisGMA-TEGDMA (bisphenol A-glycidyl methacrylate and triethylene glycol dimethacrylate) was composed of a bisGMA/TEGDMA blend (70/30% w/ w) obtained from Sigma-Aldrich; Resin 2: a model methacrylate resin was composed of UDMA (urethane dimethacrylate) /HPMA (hydroxypropyl methacrylate)/BDDMA(butanediol dimethacrylate) (33.3/33.3/33.3% w/w/w); Resin 3: a blend was composed of a UDMA/PEG200DMA/BDDMA (33.3/33.3/33.3% w/w/w); Resin

thermal postcuring (B-stage) is also possible using this system. Also, phosphine implementation in silane/Iod/phosphine three-component RIS allows tack-free polymerization of high-tech polymer/fibers composites (glass or carbon fibers).

2. EXPERIMENTAL SECTION 2.1. Chemical Compounds. All of the reactants were selected with high purity and used as received (Chart 1). Methyldiphenylsilane (MDPS), dimethylphenylsilane (DMPS), triphenylsilane (TPS), methyltriphenylsilane (MTPS), and methoxydimethylphenylsilane (MODMPS) were purchased from Alfa Aesar. Diphenylsilane (DPS; purity >98%), 2-(diphenylphosphino)benzoic acid (2DPPBA) and 4-(dimethylamino)phenyldiphenylphosphine B


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Figure 1. (A) Optical pyrometric measurements (sample T (°C) vs time) under air for Resin 1 in the presence of different RIS: (1−6) 1.0 wt % Iod with 1.0 wt % (1) DPS; (2) DMPS; (3) MDPS; (4) TPS; (5) MTPS; (6) MODMPS; (7) DPS alone; and (8) Iod alone; 4 mm thick samples after mixing. (B) Picture of mixing method of silane and Iod for the optical pyrometric measurements (see also the Experimental Section). 4: Ebecryl 605 (acrylate resin) was composed of a epoxyacrylate/ tripropylene glycoldiacrylate (75/25% w/w). All formulations (Chart 2) were prepared from the monomers from Sigma-Aldrich (except Ebecryl 605 from Cytec), and redox polymerizations were carried out at room temperature (RT; 21−25 °C) under air. The viscosities of Resins 1 and 2 are given in the Supporting Information (Table S1). 2.2. Definition of the Two Components Used for Redox Experiments. All redox formulations were prepared from the bulk resins in two separate cartridges: a first cartridge (cartridge 1) with the silane and the other one (cartridge 2) containing the iodonium salt (Iod; Figure 1B). A 1:1 Sulzer mixpac mixer was used to mix all components together at the beginning of each polymerization experiment. All polymerization experiments will be performed under mild conditions: at room temperature (RT) (21−23 °C) and under air. 2.3. Definition of the Three Components Used for Redox Experiments. All redox formulations were prepared from the bulk resins in two separate vials: a first vial (vial 1) with the silane and Iod, the other one (vial 2) containing the phosphine (Figure 5B). Then, the same volume of formulation from vials 1 and 2 was transferred to a plastic cup at the same time and was stirred about 10 s quickly at the beginning of each polymerization experiment. All polymerization experiments will be performed under mild conditions: at room temperature (RT; 21−23 °C) and under air. 2.4. Polymerization in Bulk Followed by Optical Pyrometry. The use of optical pyrometry to follow polymerization reactions was used here: temperature versus time profiles were followed using an Omega OS552-V1−6 infrared thermometer (Omega Engineering, Inc., Stamford, CT) having a sensitivity of ±1 °C for 2 g (sample thickness ∼4 mm). This setup was also used to monitor the T (°C) vs time upon irradiation with a LED@405 nm. 2.5. Differential Scanning Calorimetry (DSC) for Thermal Polymerization. A total of 10 mg of fresh formulations in the presence of RIS were inserted in an aluminum 100 μL crucible. Thermal polymerization was carried out from 0 to 300 °C at a heating rate of 10 °C/min under nitrogen flow (100 mL/min). A METTLERTOLEDO DSC differential scanning calorimeter was used. 2.6. Preparation of Composites. Sulzer mixpac mixer was used to inject the fresh formulation in the presence of RIS onto the carbon or glass fibers fabrics. The ratio of the resin to the fabric was controlled at about 50/50% (w/w) by a balance. After a few minutes, the fabric was completely impregnated by the resin formulation, and the prepregs started the curing process at room temperature, under air. 2.7. Electron Spin Resonance (ESR) Spin Trapping (ESR-ST). Electron spin resonance−spin trapping experiments were carried out using an X-band spectrometer (Magnettech MS400). The radicals were observed under nitrogen saturated media at room temperature. N-tert-Butyl nitrone (PBN) was used as a spin trap agent in tert-

butylbenzene. ESR spectra simulations were carried out using WINSIM software.

3. RESULTS AND DISCUSSION 3.1. Redox Initiating Systems (RIS) based on Two Components Silane and Iodonium Salt Combination. 3.1.1. Initiating Effect Comparison of RIS from Different Silanes for Resin 1 at Room Temperature. In this work, optical pyrometric measurements were chosen as they offer a fast and reliable assessment of polymerization efficiency (through its exothermicity).22,23 From the curves (Figure 1), we can see that DPS/Iod RIS exhibited outstanding initiating property with a fast polymerization (gel time = 1.5 min corresponding to the time needed to reach the maximal temperature). Remarkably, combination of DPS and Iod leads to a high exothermicity of 105 °C, i.e., similar to amine/benzoylperoxide (100 °C)24 using the same resin and setup. Therefore, it can be concluded that the new (safer) RIS is at least as efficient as this reference. DMPS/Iod and MDPS/Iod RIS exhibited a much lower activity than DPS/Iod (same resin); that is, after 2.5 h mixing, a solid was found at the bottom of the beaker with a liquid layer on top of it. On the contrary, for TPS/Iod, MTPS/Iod, and MODMPS/ Iod RIS resin system, after 24 h, no polymerization at all was observed. No curing of the resin 1 was obtained using DPS or Iod alone showing that the 2K mixing of these compounds is clearly needed to trigger the DPS/Iod redox reaction. In RFRP, as in most of polymerization initiations, efficient initiating systems need to produce high enough initiating radicals per unit of time to counter inhibiting reactions (e.g., dynamic oxygen inhibition under air and phenolic stabilizers).25,26 Here, the low initiating activity of DMPS and MDPS may be ascribed to its higher steric hindrance on the Si−H functionality compared to DPS. For TPS the steric hindrance of the silane is quite high. Therefore, it is probably hard for Iod to be close enough to the Si−H bond (free radical cannot be generated). For MTPS and MODMPS, there is no hydrogen atom in the silane structure; accordingly no reduction reaction can occur, i.e., free radicals cannot be generated. 3.1.2. Effect of RIS Concentration on Initiating Efficiency for Resin 1 at Room Temperature. In order to optimize the material preparation and the reaction times, concentration of RIS has to be assessed. Indeed, gel times of 2−10 min are often C


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Macromolecules required in the industry in order to have sufficient workability.27 For example, the 2K mixed resin has to have sufficient time to soak the fibers or to fill the mold and then rapidly cure, avoiding premature curing during the first 1−2 min of the mix. The polymerization profiles of the methacrylates (Resin 1) under air are depicted in Figure 2. As shown, the high-

Table 1. Summarized Data of Polymerization based on Different Concentration of DPS and Iod for Resin 1 DPS concentration (wt %)

Iod concentration (wt %)

reaction starting time (min)

max. temperature peak time (min)

peak temperature (°C)

0.1 0.2 0.3 0.4 0.5 0.8 1.0 0.2 0.4

0.1 0.2 0.3 0.4 0.5 0.8 1.0 0.4 0.2

>25 15 5.5 3 2.7 1.3 1.3 6 6.5

18 8 4.2 3.1 1.6 1.5 7.1 7.8

53 63 74 109 106 104 74 73

Figure 3. The RIS exhibited excellent initiating ability of (i) ebecryl resin (Resin 4), (ii) the mixing resin of Resins 1 and 2,

Figure 2. Optical pyrometric measurements (sample T (°C) vs time under air of Resin 1 in the presence of RIS, for: (1) 0.1 wt % DPS + 0.1 wt % Iod; (2) 0.2 wt % DPS + 0.2 wt % Iod; (3) 0.3 wt % DPS + 0.3 wt % Iod; (4) 0.4 wt % DPS + 0.4 wt % Iod; (5) 0.5 wt % DPS + 0.5 wt % Iod; (6) 0.8 wt % DPS + 0.8 wt % Iod; (7) 1.0 wt % DPS + 1.0 wt % Iod; (8) 0.2 wt % DPS + 0.4 wt % Iod; and (9) 0.4 wt % DPS + 0.2 wt % Iod; 4 mm thick samples.

efficiency RIS developed here for room temperature exactly meets this requirement, and the curing time is controlled within 2−15 min. There is a positive effect of an increase of the content of silane and Iod to decrease the gel time (i.e., when more DPS and iodonium salt was added, a short time of polymerization is expected). The polymerization process is very fast if the concentration of the DPS and Iod are higher than 0.8 wt % (maximum temperature T > 100 °C; starting reaction time 50 °C; Figure 2, curves 2−5, 8, and 9). Therefore, it has high potential to control the gel time if the concentration of the DPS and Iod was tuned between 0.2 and 0.8 wt %. The data of polymerization based on different concentration of DPS and Iod are summarized in Table 1; excellent tuning of the reaction times (up to 15 min) is made possible. In order to certify the role of the silane, we compared the DPS from other supplier (TCI) to that of from Sigma-Aldrich. A same perfect initiating efficiency was obtained, indicating the robustness of newly developed DPS/Iod system (see further optical pyrometric measurements in Supporting Information, Figure S1). At the same time, in order to examine whether or not the initiator is sensitive to the light, we have compared the polymerization profiles with and without irradiation. The results showed that this kind of RIS is not sensitive to light (see optical pyrometric measurements in Supporting Information, Figure S2). 3.1.3. Versatile Initiating Efficiency of DPS/Iod RIS for Broad Range of Resins. We have checked the initiating ability of DPS/Iod RIS for other (meth)acrylate resins, as shown in

Figure 3. Optical pyrometric measurements (sample T (°C) vs time) under air for different resins in the presence of 0.5 wt % DPS + 0.5 wt % Iod, for resin (1) Resin 4; (2) Resin1/Resin2 (50/50 w/w); (3) Resin1/Resin3 (50/50 w/w); 4 mm thick samples.

or (iii) Resin 3 with Resin 1. For the mixing resin, the polymerization finished within only 5 min in the presence of 0.5 wt % RIS. Interestingly, for Resin 4, it is possible to have comfortable gel times of more than 35 min (Figure 3, curve 1) with excellent efficiency (120 °C exothermicity) which is not possible with amine/peroxide reference RIS under air at room temperature (see the literature).28 For Resin 2, no polymerization occurs for the proposed 2K system; this can be ascribed to a strong oxygen inhibition for the polymerization under air of this low viscosity resin. Indeed, the lower the viscosity of the resin, the stronger the oxygen diffusion in the monomer and therefore the associated oxygen inhibition.25,26 Resin 2 is a low viscosity resin (0.053 Pa.s) compared to Resin 1 (5.7 Pa.s) in agreement with higher oxygen inhibition for Resin 2 vs Resin 1. 3.1.4. Thermal Initiating Efficiency of DPS/Iod RIS. In order to (i) exclude thermal decomposition of separate Iod and DPS and (ii) show potential thermal B-stage curing, we explored the initiating ability of DPS/Iod RIS upon heating by DSC experiment right after 2K mixing (Figure 4). The results showed that DPS/Iod resin system was very sensitive to heat even for a relatively low initiator concentration. For the system with a 0.2 wt % RIS concentration, the polymerization process D


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phosphines as additives. Especially, we selected Resin 2 as it has a very low viscosity (∼0.05 Pa.s) which leads to extremely high oxygen inhibition: the DPS/Iod alone was essentially inefficient in that resin (see above and Figure 5, curve 6). For FRP, oxygen inhibition is always an issue. Indeed, if the rate of initiating radical production (νfree radical generation) is higher than the oxygen diffusion (that controls the reoxygenation of the resin during the polymerization), an efficient polymerization process is expected. On the contrary, if νfree radical generation is lower than the reoxygenation, all of the initiating radicals will be consumed by oxygen and cross-linking cannot be observed. Therefore, the resin properties (e.g., viscosity) play a key role on the polymerization properties under air.25,26 The different polymerization profiles in the presence of phosphine additives are reported in Figure 5. Phosphines are well-known for their ability to overcome oxygen inhibition leading to a better polymerization efficiency.21,26 It can be seen that the RIS with DMAPDP (Chart 1) as an additive exhibited highest initiating efficiency (with only 1.0 wt % concentration, the polymerization reaction started within 2 min, Figure 5, curve 1). Remarkably, in Resin 2 (a resin that is difficult to polymerize due to low viscosity),29 the threecomponent RIS exhibited excellent initiating ability. RIS with 2DPPBA or 4DPPS as an additive also exhibited perfect initiating efficiency (with 2.0 wt % concentration, the polymerization reaction started within 5 min, Figure 5, curves 3 and 5). There is a positive effect of decreasing the content of silane and Iod to extend the induction time (i.e., when less DPS, Iod, and phosphine was added, a longer time is expected for the polymerization). Therefore, the proposed upgraded DPS/Iod/phosphine RIS can be considered as a good way to cure radical monomers upon mild room temperature conditions. 3.2.2. Thermal Initiating Efficiency of DPS/Iod/Phosphine RIS. The initiating ability of DPS/Iod/phosphine RIS upon heating was also investigated by DSC experiment (Figure 6). The results showed that DPS/Iod/phosphine thermal initiator and resin system was very sensitive to heat. For the system with a 1.0 wt % DPS/Iod/2DPPBA RIS concentration, the

Figure 4. Thermal polymerization experiments using DPS/Iod as RIS (enthalpy vs heating temperature), heating rate: 10 K/min, under N2, in Resin 1. For RIS: (1) 0.2 wt % DPS + 0.2 wt % Iod; (2) 0.1 wt % DPS + 0.1 wt % Iod; (3) 1.0 wt % DPS alone; (4) 1.0 wt % Iod alone.

started at 25 °C, which is consistent with the optical pyrometry results (polymerizations without heating). For the system with a 0.1 wt % RIS concentration, the polymerization process started at 52 °C. For this latter system the polymerization did not occur within 25 min at room temperature according to the optical pyrometric measurement results (see above). For the sample containing DPS alone, the polymerization began when the resin was heated above 150 °C (and with poor efficiency). For the system containing Iod alone, the polymerization did not occur below 300 °C showing clearly that the presence of both DPS and Iod is required for curing at low temperature. Therefore, DSC results further indicated the synergic initiating effect of DPS and Iod in a redox reaction. 3.2. Redox Initiating Systems (RISs) from Upgraded Three Components: Silane/Iodonium Salt/Phosphine. 3.2.1. Initiating Efficiency of RIS from DPS/Iod and Different Phosphine Additives. As a next step, we investigated the initiating efficiency of DPS/Iod in the presence of different

Figure 5. Optical pyrometric measurements (sample T (°C) vs time) under air of Resin 2 for (A) (1) 1.0 wt % DPS+ 1.0 wt % Iod +1.0 wt % DMAPDP; (2) 0.5 wt % DPS+ 0.5 wt % Iod +0.5 wt % DMAPDP; (3) 2.0 wt % DPS + 2.0 wt % Iod +2.0 wt % 2DPPBA; (4) 1.0 wt % DPS + 1.0 wt % Iod +1.0 wt % 2DPPBA; (5) 2.0 wt % DPS + 2.0 wt % Iod +2.0 wt % 4DPPS; (6) 2.0 wt % DPS + 2.0 wt % Iod; 4 mm thick samples. (B) Picture of mixing method of DPS/Iod and phosphine additives for the optical pyrometric measurements. E


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Figure 7. Macrotopography of fabric prepregs (F1−F4) and composites (C1−C4) cured in the presence of DPS/Iod RIS at room temperature, under air. Access to prepregs F1−F2 using 0.1 wt % silane/0.2 wt % Iod in (F1) 50/50 (w/w) Resin 1/glass fibers; (F2) 50/50 (w/w) Resin 1/carbon fibers. Access to prepregs F3−F4 using 0.2 wt % DPS/0.2 wt % Iod in (F3) 50/50 (w/w) Resin 1/glass fibers; (F4) 50/50 (w/w) Resin 1/carbon fibers. Access to composites (C1−C4): at room temperature keeping for 10 min and heated 1 h at 100 °C under air.

Figure 6. Thermal polymerization experiments using DPS/Iod/ 2DPPBA RIS (Enthalpy vs heating temperature), heating rate: 10 K/ min, under N2, for Resin 2, For RIS: (1) 1.0 wt % DPS + 1.0 wt % Iod; (2) 0.8 wt % DPS + 0.8 wt % Iod +0.8 wt % 2DPPBA; (3) 1.0 wt % DPS + 1.0 wt % Iod +1.0 wt % 2DPPBA.

phosphine RIS for the synthesis of methacrylate/glass fibers and methacrylate/carbon fibers composites at room temperature. From the results of optical pyrometric measurements, initiator concentration of 0.5 wt % DPS/0.5 wt % Iod/0.5 wt % DMAPDP and 1.0 wt % DPS/1.0 wt % Iod/1.0 wt % 2DPPBA were selected to prepare composites. First of all, glass fibers and carbon fibers were impregnated by fresh formulations in the presence of three-component RIS, and prepregs were obtained (Figure 8, F1−F4). For DPS/Iod/DMAPDP resin

polymerization process started below 40 °C, which is consistent to the optical pyrometric measurement result (heat not required). For the system contained DPS/Iod without additive (in Resin 2), the polymerization began when the resin was heated near 100 °C, and polymerization is very intensive. Therefore, DSC results further indicated the synergic initiating effect of DPS, Iod and phosphine at a mild temperature even for a low reactivity monomer blend (Resin 2). 3.3. New RIS for Preparation of Composites. 3.3.1. Composites Using Two Components-DPS/Iod as RIS. The main advantages of composite materials are their high mechanical properties, relatively low weight and corrosion resistance. Indeed, smart combination of fibers (or fillers) and polymeric resins can be performed. Here, the new proposed DPS/Iod system was used as a RIS for the synthesis of methacrylate/glass fibers or methacrylate/carbon fibers composites at room temperature. From the results of optical pyrometric measurements, we optimized initiator concentration of 0.4 wt % DPS/0.4 wt % Iod RIS and 0.2 wt % DPS/ 0.4 wt % Iod as good choice to prepare composites (i.e., efficient systems but with an adapted gel time to ensure a good impregnation of the fibers). With the selected inducing time (3 and 6 min), this allows the resin to be soaked fully into the fibers. First of all, glass fibers or carbon fibers were impregnated by methacrylate resin embedded with RIS. After about 5−10 min, the obtained prepregs (Figure 7: F1−F4) became solid. Interestingly, the presence of glass fibers or carbon fibers does not delay the gel time of free radical polymerization. The surface and bottom of the composites looked a little tacky (probably due to the oxygen inhibition). However, after they were heated about 1 h at 100 °C under air, the small residue of starchy resin became fully solid (Figure 7, C1−C4). Here, one can see the usefulness of this system being able to cure through thermal B-stage (Figure 4). Also, the robustness of redox polymerization enables access to carbon fibers composites which is generally not possible using light induced polymerizations.30 3.3.2. Composites Using Three Components DPS/Iod/ Phosphine as RIS. Next, the preparation of composites was performed using the highly robust three-component DPS/Iod/

Figure 8. Macrotopography of fabric prepregs (F1−F4) and composites (C1−C4) cured in the presence of DPS/Iod/phosphine RIS at room temperature, under air. Access to prepregs F1−F2 using 0.25 wt % DPS/0.25 wt % Iod/0.25 wt % DMAPDP in (F1) 50/50 (w/w) Resin 2/glass fibers; (F2) 50/50 (w/w) Resin 2/carbon fibers. Access to prepregs F3−F4 using 0.5 wt % DPS/0.5 wt % Iod/0.5 wt % 2DPPBA in (F3) 50/50 (w/w) Resin 2/glass fibers; (F4) 50/50 (w/ w) Resin 2/glass fibers. Access to composites (C1−C4): at room temperature keeping for (C1−C2) 10 min; (C3−C4) 90 min.

system, after about 5 min, the composite became solid. For carbon fibers reinforced composites, both surface and bottom are tacky-free (Figure 8, C1−C2). If tack-free polymers or composites were obtained from resin 2, it indicated that the conversion of the monomer is quite high (measured here >80%). Furthermore, it exhibited excellent network homogeneity and mechanical properties.31 It is a perfect method to F


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Macromolecules prepare carbon fibers composites at room temperature using DPS/Iod/DMAPDP as FRP initiator. For the DPS/Iod/ 2DPPBA resin system, after about 90 min fibers impregnation by the resin, the system became solid (Figure 8, C3−C4). 3.4. Chemical Mechanisms. For a better understanding of the DPS/Iod interaction, ESR-spin trapping experiments were carried out (Figure 9). In presence of PBN as spin trap agent,

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Figure S1. Optical pyrometric measurements of resin 1 in the presence of RIS, a comparison of DPS from Sigma-Aldrich and TCI-europe; Figure S2. Optical pyrometric measurements of resin 1 in the presence of RIS, a comparison of with and without irradiation; Table S1. viscosity of the resins (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jacques Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by China Scholarship Council (CSC) 201805290009. Authors wish to thank Gauthier Schrodj for the DSC acquisitions.

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Figure 9. ESR spectra obtained for ESR-spin trapping experiment using [PBN] = 10−2 M (as spin trap agent); [DPS] = 10−1 M and [Iod] = 10−2 M in tert-butylbenzene; under N2: simulated (red) and experimental (black) spectra.

some radicals are directly detected after mixing of DPS and Iod in tert-butylbenzene under N2. Two PBN/radical adducts are detected: PBN/Ar• characterized by aN = 14.1G; aN = 2.1G and PBN/H• characterized by aN = 14.5G; aN (2) = 7.1G in agreement with literature data.32,33 This suggests the redox reaction (r1) between DPS and Iod as the iodonium salt is reduced leading to aryl radicals. DPS + Ar2I+ → → Ar • + H•

(r1)

The radicals generated in (r1) can be assumed as the initiating species for the FRP processes observed above.

4. CONCLUSIONS The initiating performance of several redox free radical polymerization initiators (at room temperature) from twocomponent silane/iodonium salt or three-component silane/ iodonium salt/phosphine used were investigated. Initiator from DPS/Iod/phosphine exhibited unique RFRP initiating properties for several (meth)acrylate resins with excellent polymerization profiles (gel time and exothermicity). Importantly, these safer redox initiating systems enabled the preparation of tack-free glass fibers and carbon fibers composites at room temperature easy to implement. Several perspective applications can be achieved for the manufacturing of composites (safer and environmentally friendly). Other eco-friendly redox (photo)initiating systems are under investigation.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00233. G


Article

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Silane Based Redox Initiating Systems: Toward a Safer Amine-Free

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