[background]
Epoxy resin is one of the most important functional monomers for the preparation of coatings, adhesives and composites. It is usually crosslinked by heat curing or light curing to form thermosetting epoxy resin. However, the two curing methods have obvious disadvantages in application. For example, the epoxy resin monomer (especially bisphenol-A type) needs to be mixed with curing agent evenly and then cured for a long time at a high temperature, which has a large energy consumption. During the curing process, the light radiation can only penetrate a very thin layer of resin on the surface, resulting in the current epoxy prepared by light curing Only for coating neighborhood. Another common disadvantage is that when the resin and curing agent are mixed, they are expected to polymerize slowly at room temperature and cannot be stored for a long time; at the same time, when fillers are added to prepare composite materials, high content fillers will lead to a sharp increase in the viscosity of the system, which is not conducive to processing, and the composite materials prepared will have more defects and affect the mechanical properties. Therefore, it is of great significance to develop a new processing and molding method / process for the preparation of high-performance epoxy matrix composites.
[research results]
Based on this, the team of Professor Robert Liska of Vienna University of technology adopts the free radical induced cationic front-end polymerization method to drive the epoxy polymerization (curing) with the heat generated in the polymerization process without external heating, greatly simplifying the production process and saving energy consumption. Using this method, epoxy composite with high filler content can be prepared, such as glass microspheres with a maximum content of 74 vol%, graphite with a maximum content of 28.7 vol%, carbon fiber staple with a maximum content of 39.6 vol%, mica with a maximum content of 23.4 vol%, and mica with a maximum content of 29.7 Vol At the same time, the carbon fiber textile / epoxy composite prepared by this method has similar mechanical properties to the composite prepared by traditional heat curing, which can be widely used in practical applications. The research results were published in "Composites Part A: Applied Science and manufacturing" in a paper entitled "radical induced cationic front polymerization for preparation of epoxy composites".
What is "free radical induced cationic front-end polymerization"? Students who have studied polymer know what is free radical polymerization and cationic polymerization, and a few know what is front-end polymerization, but what kind of polymerization mechanism is it to combine these three polymerization methods together? Don't worry. Let's learn about the front-end polymerization. It's the first time that many students have heard about this kind of polymerization. The front-end polymerization (FP) is a kind of reaction mode which transforms the polymer monomer into polymer through the movement of local reaction region in the polymer monomer. The front-end polymerization is mainly used in exothermic reaction. It only needs to be heated for a short time at the initial stage of the reaction, and then stop heating. With the help of the thermal autocatalytic effect of exothermic reaction, the polymerization of monomers can be completed. The whole reaction process does not need stirring and is continuous and easy to control. Therefore, the advantages of energy-saving, time-saving and controllable process of front-end polymerization provide opportunities for rapid preparation of new materials, and also provide an effective way to prepare high-performance materials in the extreme service environment with new reaction mode. The front-end polymerization has been used in the curing of epoxy resin for a long time, but its further development is hindered by one disadvantage of the front-end polymerization: the heat released in the previous step escapes greatly, resulting in the remaining heat is not enough to maintain the next step of polymerization. Based on this, Professor Robert Liska, the author of this paper, developed a new front-end polymerization mode, free radical induced cationic front-end polymerization (ricfp) (polymer chemistry, 2015, 6 (47): 8161-8167). There are three basic substances in ricfp system: epoxy monomer, photoinitiator and thermal free radical initiator. The reaction flow chart and preparation diagram are as shown in Fig. 1 and Fig. 2: firstly, the prepolymer containing various reagents is exposed to ultraviolet light, which causes the photoinitiator to decompose to generate cations, and the cations will capture a proton from the monomer or solvent to form a strong acid, which will initiate the epoxy polymerization and release a large amount of heat, and a large amount of heat will cause the thermal free radical initiator to decompose to generate free radicals, and the From the base, more cations can be produced by splitting photoinitiator, and then acid, polymerization and heat can be produced A cycle (as shown in Figure 2) is formed to ensure that there is enough heat in the polymerization curing process, so that the polymerization process continues to spread from the surface to the interior until all epoxy monomers are polymerized. This method does not need external heating to promote and maintain curing. At the same time, after the first step of UV decomposition of photoinitiator to generate cations, the UV lamp can be turned off, and the subsequent cations are generated by free radicals. Figure 1. Ricfp reaction flow chart (picture source: polymer chemistry, 2015, 6 (47): 8161-8167) Figure 2. Schematic diagram of ricfp preparation of epoxy (picture source: Composites Part A: Applied Science and manufacturing, 2020: 105855) 2. The optimization of the components of ricfp polymerization precursor due to the high viscosity of the bisphenol A epoxy resin (badge), which is not conducive to curing process, the author first explored the influence of different diluents on ricfp polymerization. In this paper, the effects of different diluents (CE, npdge, hddge, EOM) on the viscosity, activity and mechanical properties of ricfp were studied with bis (4-tert-butylphenyl) tetraiodide (perfluoro-tert-butyl) aluminate (i-al) as photoinitiator and phenylpinacol (tped) as thermal radical initiator. The results show that all systems containing diluent (20 mol%) have lower viscosity and higher curing rate than those without diluent. It was found that among the bifunctional epoxy diluents, aliphatic epoxy diluents (npdge, especially hddge) were more effective than cycloaliphatic epoxy diluents (CE) in viscosity reduction due to their low molecular weight and viscosity, and could slowly improve the curing rate. However, EOM with monohydroxy group can significantly improve the curing rate (~ 7cm / min). In addition, DSC was used to investigate the effect of different diluents on the curing of the system. It was found that the temperature required for polymerization of the system containing EOM was the lowest and the reaction rate was the fastest. Then DMA and nano indentation tests were carried out to investigate the influence of diluent on TG value and polymer homogeneity. It is found that the storage moduli of several systems before and after TG transformation are almost the same, but the TG values of npdge and hddge systems are relatively low, while the hardness values of each system are similar without much difference. In general, EOM is a diluent with the best performance, so it is selected as diluent in the subsequent research. Figure 3. Effect of different diluents on ricfp (picture source: Composites Part A: Applied Science and manufacturing, 2020: 105855)3. At present, the use of ricfp in the preparation of epoxy matrix composite is very wide, but the preparation of epoxy matrix composite with high filler content has always been a problem in the industry, so the author selected a series of common fillers (mica, graphite, carbon fiber staple, glass microspheres and aluminum powder) to explore the limit amount of fillers in the preparation of epoxy matrix composite by ricfp. It is found that the reaction rate and temperature of ricfp are greatly influenced by the fillers with different properties and contents: with the increase of fillers, the reaction rate and temperature will gradually decrease. This is mainly because the increase of fillers will absorb part of the heat, while the proportion of monomers will decrease, thus reducing the total heat release, resulting in low reaction rate. The thermal conductivity of the material will have a greater impact on this reduction trend. When carbon fiber, graphene and aluminum with high thermal conductivity are used as fillers, the polymerization rate decreases slowly with the increase of filler content, and the system temperature can reach 220 ℃ even at some content, slightly higher than that of the system without filler. However, the ultimate limit content experiment shows that the maximum addition of glass microsphere, graphite, carbon fiber staple, mica and aluminum powder can reach 74 vol%, 28.7 vol%, 39.6 vol%, 23.4 vol% and 23.4 vol% respectively, which shows that ricfp is an excellent method to prepare epoxy composite with high filler content. At last, the mechanical properties of the epoxy / carbon fiber composite reinforced by carbon fiber felt show that the mechanical properties of ricfp are similar to those of the traditional heat curing epoxy composite, which shows that ricfp has great practical application potential. Figure 4. Effect of different kinds and contents of fillers on polymerization rate and system temperature when ricfp solidifies epoxy (picture source: Composites Part A: Applied Science and manufacturing, 2020: 105855) Figure 5. Comparison of mechanical properties of epoxy / carbon fiber composite prepared by ricfp method and traditional heat curing method (picture source: Composites Part A: Applied Science and manufacturing, 2020: 105855) [summary] the author explored the effect of different reactive diluents on ricfp cured bisphenol A epoxy, and found that EOM is the best reactive diluent with the best comprehensive effect, which can effectively reduce the viscosity of the system and improve the reactivity. Then ricfp method is applied to prepare epoxy matrix composite. It is found that this method can greatly improve the content of filler, and maintain the excellent mechanical properties similar to the traditional thermal curing method. It will be widely used as a very energy-saving curing method in the future. References: 1. Bomze D, knaack P, Liska R. successful radial induced cationic frontal polymerization of epoxy based Mooners by C – C laboratory compounds [J]. Polymer chemistry, 2015, 6 (47): 8161-8167.2. Tran a D, Koch T, knaack P, et al. Radical induced cationic frontal polymerization for preparation of epoxy composites[J]. Composites Part A: Applied Science and Manufacturing, 2020: 105855. At the forefront of polymer science, communication groups such as "plastics" and "coating adhesive" have been established, and small editors have been added as friends (micro signal: polymer Xiang, please note: name unit Title Research direction), and invited to join the group.
Source: polymer science frontier
Statement: only on behalf of the author's personal point of view, the author's level is limited, if there is any unscientific, please leave a message below for correction!
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