Can epoxy be cured without heating to make composite materials?

Epoxy resin is one of the most important functional monobodies for the preparation of coatings, adhesives and composites, usually cross-linked by thermo-curing or photo-curing to form thermo-solid epoxy resin. However, the application of these two curing methods at the same time there are more obvious disadvantages, such as the thermal curing needs to mix epoxy monosome (especially bisphenol A type) and curing agent evenly after a long time curing at a high temperature, energy consumption is large; Another common disadvantage is that when the resin and curing agent mixed with the formation of the expected slow polymerization at room temperature, can not be stored for a long time, at the same time, when adding fillers to prepare composite materials, high content fillers will lead to a sharp increase in system viscosity, not conducive to processing, preparation of composite materials will also have more defects and impactTherefore, the development of a new process molding method/process is of great significance for the preparation of high-performance epoxy-based composite materials.
Research Results
Based on this, Professor Robert Liska's team at the Technical University of Vienna uses free-form induced cation front-end aggregation to drive epoxy polymerization (curing) with heat generated during the polymerization process without external heating, greatly simplifying the production process and saving energy consumption. This method allows the preparation of epoxy-based composites with high filler content, e.g. up to 74vol% glass microc ball, 28.7 vol. graphite, 39.6 vol. carbon fiber, 23.4 vol. cloud Both the mother and 29.7 vol% aluminum composite materials can be successfully prepared, and the carbon fiber textile cloth/epoxy composite material prepared by this method has similar electron properties to the composite material prepared with traditional thermo-cured material, which can be widely popularized in practical application. The study was published in Composites Part A: Applied Science and Manufacturing in a paper entitled "Radical induced frontal polymerization for development of epoxy composites".
the details of the
. 1. What is "free-based induced cation front-end polymerization"?
students who study high molecules know what free-based polymerization and cation aggregation are, and a small number of students know what front-end polymerization is, but what kind of polymerization process is it to combine these three polymerization methods? First not in a hurry, let's first understand the front-end aggregation bar, estimated that many students this is the first time to hear about this aggregation reaction. Front-end polymerization (frontal polymerization, FP) is a reaction pattern that transforms a polymer monomer into a polymer by moving the local reaction region through the polymer monomer. The front-end polymerization is mainly used in the heat release reaction, only in the initial stage of the reaction to carry out a short period of heating, and then stop heating, with the heating reaction of the thermal self-catalytic action can complete the polymerization reaction of the monosome, the entire reaction process without stirring and continuous, easy to control. Therefore, the advantages of front-end polymerization, such as energy saving, time saving and process control, provide opportunities for rapid preparation of new materials, and also provide an effective way to prepare high-performance materials in extreme service environments with new reaction modes. Front-end polymerization has long been used in epoxy curing, but front-end polymerization has one drawback that hinders its further development: the heat released in the previous step escapes in large quantities, resulting in insufficient heat to sustain the next polymerization.
this, the team of authors of this paper, Professor Robert Liska, has developed a new front-end aggregation model, free-form induced cation front-end polymerization (RICFP) (Polymer Chemistry, 2015, 6 (47): 8161-8167). There are generally three basic substances in the RICFP system: epoxy monosome, light trigger, and thermal free agent. Reaction flow charts and preparation diagrams shown in Figures 1 and 2 show: First of all, prepopulations containing various reagents are exposed to ultraviolet light, so that the light trigger decomposes to produce cations, cations will take a proton from a monomer or solvent to form strong acids, strong acids will trigger epoxy polymerization and release a large amount of heat, a large amount of heat will cause thermoenal promoter decomposition to produce freelance, and the free agent can crack the light promoter to produce more ions, and then produce more ions. A cycle is formed (Figure 2) to ensure that the polymerization curing process has enough heat to allow the polymerization process to continue to spread from the surface to the inside until all epoxy monotones are polymerized. The method does not require external heating to promote and maintain curing, at the same time, the first step of ultraviolet light decomposition light trigger to produce cation, ultraviolet lamp can be turned off, the subsequent cation through the free-based catalytic generation.
Figure 1. RICFP reaction flowchart (Photo: Polymer Chemistry, 2015, 6 (47): 8161-8167)
2. RICFP prepares epoxy schematics (Photo: Composites Part A: Applied Science and Manufacturing, 2020: 105855)
2. RICFP polymerization pre-drive group optimization
Because bisphenol A epoxy resin (BADGE) viscosity is large, is not conducive to curing processing, so the author first explores the effects of different thinners on RICFP polymerization. Here, the authors study the effects of different thinners (CE, NPDGE, HDDGE, EOM) on RICFP viscosity, activity and final mechanical properties with double (4-scudd-based) tetiodine (perfluorosukic) aluminumate (I-Al) as a light trigger and benzene-frequency alcohol (TPED) as a thermal free agent. The results showed that all systems containing thinners (20 mol) had lower viscosity and higher curing rates than systems without diluents. The study found that in bi-erythatic group epoxy thinners, adipose epoxy thinners (NPDGE, especially HDDGE) are more effective in reducing adhesion than cyclic epoxy diluents (CE) due to their lower molecular weight and viscosity, and can slowly increase the curing rate. EOM, which contains monohydroxygen groups, can significantly increase the epoxy curing rate (≈7cm/min). The authors further used DSC to explore the effects of different thinners on system curing, and found that the minimum temperature required for the polymerization of EOM systems was consistent with the conclusion that the reaction rate was the fastest. DMA and nano-indentation tests were then carried out on epoxy resins prepared in different systems to explore the effects of thinners on Tg values and polymer uniformity, respectively. It was found that several systems had little difference in energy storage module before and after Tg transformation, but the Tg value of NPDGE and HDDGE system was lower, and the results of nano-indentation showed that the hardness values of each system were close and not much different. In general, EOM is one of the best performing thinners, so it was chosen as a thinner in subsequent studies.
3. Effects of different thinners on RICFP (Photo: Composites Part A: Applied Science and Manufacturing, 2020: 105855)
. 3. The use of RICFP to prepare epoxy-based composites
is currently widely used, but the preparation of high filler content epoxy-based composites has been a difficult problem in the industry, so the author chose a series of common fillers (mica, graphite, carbon fiber fiber, glass micro-tropo and aluminum powder) to explore the RICFP legal preparation of epoxy-based composites when the limits of various fillers. The study found that fillers of different properties and contents have a greater effect on the RICFP reaction rate and the internal temperature of the system: as the fillers increase, the reaction rate and temperature decrease gradually. This is mainly due to the fact that the increase in fillers absorbs some of the heat, while the proportion of the monosome decreases, thereby reducing the total heat release, resulting in a low reaction rate. The thermal conductivity of the material will have a greater impact on this reduction trend, high thermal conductivity of carbon fiber and, graphene and aluminum as fillers, the polymerization rate with the increase of filler content and the rate of decline is slower, its system temperature can even reach 220 degrees C at certain content, slightly higher than the system without fillers. The final filler limit content experiment shows that the maximum addition of glass micro-trot, graphite, carbon fiber, mica and aluminum powder can reach 74 vol, 28.7 vol, 39.6 vol, 23.4 vol and 23.4 vol, respectively, indicating that RICFP is an excellent method for preparing high filler content epoxy-based composite materials. Finally, the mechanical properties of epoxy/carbon fiber composites prepared by carbon fiber felt to enhance the system show that the epoxy composites prepared by RICFP have similar mechanical properties to those prepared by traditional thermo-cured epoxy composites, indicating that RICFP has great practical application potential.
4. Effect of fillers of different types and contents on polymerization rate and system temperature when RICFP cured epoxy (Photo: Composites Part A: Applied Science and Manufacturing, 2020: 105855)
Figure 5. The RICFP method compares with the methicleological properties of traditional thermo-cured complexes (Photo: Composites Part A: Applied Science and Manufacturing, 2020: 105855)
Summary
The authors explored the effects of different reaction thinners on RICFP-cured bisphenol A-rings and found that EOM was the best reactive dilutor for synthesis, effectively reducing system adhesion and improving system adhesion. Then the author applies RICFP method to the preparation of epoxy-based composite materials, and finds that the method can greatly improve the content of fillers, while maintaining excellent mechanical properties similar to traditional thermoclytic curing, which will be widely used as a very energy-efficient curing method in the future.
reference:
. 1. Bomze D, Knaack P, Liska R. Successful radical induced cationic frontal polymerization of epoxy-based monomers by C-C labile compounds. Polymer Chemistry, 2015, 6 (47): 8161-8167.
2.Tran A D, Koch T, Knaack P, et al. Radical induced cationic frontal polymerization for development of epoxy composites. Composites Part A: Applied Science and Manufacturing, 2020: 105855.
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