The present invention relates to microwave curing methods.
Microwave lies in the electromagnetic irradiation range from 0.3 to 300 GHz, while all domestic microwave ovens operate at a frequency of 2.45 GHz. This photon energy is too low to be directly absorbed and induce chemical reactions like UV-Vis irradiation does. Instead, this level of energy merely causes thermal heating. But unlike conventional thermal heating, which delivers energy through convection, conduction, and radiation of heat from material surfaces driven by thermal gradients, microwave converts energy directly from an electric field to the matter. This is accomplished via microwave molecular interaction, reorientation of dipole moments or ionic movement induced by alternating electric fields causing molecular friction and collision which is dissipated as heat eventually. This direct energy conversion allows for more rapid and uniform heating throughout the entire material volume. This drastically reduces the processing time, especially for polymers, which inherently have low thermal conductivity. Due to these advantages, microwave heating techniques have been widely exploited in the context of organic/pharmaceutical/polymer synthesis. Additionally, they have been used in the realm of nanotechnology, biomaterials, etc. Several attempts to use microwaves for curing of poly(amic acid) into polyimide have been reported, but they were either cured in high-cost variable frequency equipment or yielded a lower degree of imidization. Therefore, a need still exists for an effective method of using microwave radiation to cure poly(amic acid) into polyimide.
In one embodiment, the present invention is a method of curing polyimide, polyimide copolymers, polyimide composites or combinations thereof. The method involves preparing a reaction system comprising poly(amic acid) and at least one other compound selected from the group consisting of copolymers, solvents, fillers and nanofillers. Then, a combined overall microwave absorptivity of the reaction system is determined. A temperature ramp rate is calculated for a microwave power level and time using the combined overall microwave absorptivity. The reaction system is then exposed to microwave radiation according to the calculated temperature ramp rate, producing a cured product.
In another embodiment, the reaction system is exposed to microwave radiation for a period of time from about 30 minutes to about 60 minutes. In one embodiment, the reaction system is exposed to microwave radiation for a period of time from about 40 minutes to about 50 minutes. In another embodiment, the reaction system is exposed to microwave radiation for about 40 minutes. In one embodiment, the reaction system is cured at a temperature ≤200° C.
In another embodiment, the microwave absorptivity determination is based on the reaction system and any substrates used to support the reaction system. In one embodiment, the microwave absorptivity of solvents is determined by their dielectric properties. In another embodiment, the reaction system comprises one or more carbonaceous fillers. In one embodiment, the reaction system comprises one or more nanofillers. In another embodiment, the nanofillers polyaniline-modified nanofillers. In one embodiment, the reaction system comprises carbon nanotubes, nanographene sheet, or combinations thereof. In another embodiment, the reaction system comprises single walled carbon nanotubes. In one embodiment, the reaction system comprises nanographene sheets. In another embodiment, the reaction system comprises N,N-dimethylformamide (DMF). In one embodiment, the reaction system comprises 1-methyl pyrrolidone (NMP). In another embodiment, the reaction system comprises embedded carbon nanotubes. In one embodiment the carbon nanotubes are used to sense the degree of cure. In one embodiment, the carbon nanotubes are used for in-situ structural health monitoring.
The foregoing summary, as well as the following detailed description of preferred embodiments of the application, will be better understood when read in conjunction with the appended drawings.
One skilled in the art will recognize that the various embodiments may be practiced without one or more of the specific details described herein, or with other replacement and/or additional methods, materials, or components. In other instances, well-known structures, materials, or operations are not shown or described in detail herein to avoid obscuring aspects of various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth herein in order to provide a thorough understanding of the invention. Furthermore, it is understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but does not denote that they are present in every embodiment. Thus, the appearances of the phrases “in an embodiment” or “in another embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Further, “a component” may be representative of one or more components and, thus, may be used herein to mean “at least one.”
Polyimides are engineering polymers with outstanding thermal and mechanical properties. They are utilized in a wide array of products ranging from electronic to aerospace. Due to their high Tg and their dense structure, polyimides are usually cured at high temperatures ≥300° C. Thus, polyimide parts are not usually manufactured by the traditional extrusion and injection molding processes.
To overcome these challenges, the present invention deliberately controls microwave heating procedures with steady-step power variation to cure polyimide, polyimide copolymers and their carbon nanotube/nanographene sheet (CNT/NGS) nanocomposites. Using this method, polyimide, polyimide-based copolymers, and their carbon nanocomposites of high imidization degree (85-100%) having superior thermal properties have been attained in dramatically improved curing efficiency at a remarkably reduced curing time of about 40 minutes. In one embodiment, the present invention uses variable power microwave curing at moderate to low temperatures ≤200° C.
One important strategy of the present invention that successfully produces thermal imidization using microwave heating is determining the temperature ramp rate by adjusting the microwave power level and time according to the combined overall microwave absorptivity of the reaction systems. The optimal ramp rate reaches the desired curing temperature, Tcure in the shortest time, in one embodiment, Tcure≤20≥10 minutes. The absorptivity calculations include solvents used to prepare the polymer solution, fillers, and substrates. The initial microwave output power level selection is important. Lower initial power settings result in better microwave absorptivity. The goal is to attain imidization or curing at a favorable temperature as fast as possible without damaging the macrostructure caused by abrupt solvent evaporation.
In general, dry organic substances are not microwave active, thus, heating will not occur. The dielectric heating by microwave irradiation is therefore mostly ascribed to the polarity of the solvent molecules. Since poly(amic acid) possesses strong dipole moments, which is assumed to be microwave absorptive, it can help with heating. Albeit, this is inferior to the heating effect of a solvent such as DMF. Along with the solvent evaporation and the formation of stiffer polyimide as the temperature increases, the heating effectiveness reduces correspondingly, resulting in a levelling off of the temperature of the yields. The microwave interactivities of the solvents are determined by their dielectric properties. The larger the loss tangent (tan 8), the more microwave energy is converted into heat, hence the faster the temperature ramp rate as a result. In some embodiments of the present invention, N,N-dimethylformamide (DMF) is used in the PAA solutions, which possesses a fair tan 8 value of 0.161. 1-methyl-2-pyrrolidone (NMP), with a higher tan 8 of 0.275, is a good alternative when carbonaceous fillers have not been incorporated.
Nanoscale curing kinetics are influenced by the presence of nanofillers coupling agents. In one embodiment, the nanofillers are used to reinforce a poly(amic acid) matrix and copoly(amic acid) resin, permitting full imidization after only about 55 minutes of exposure to microwave energy, unlike the conventional oven cured system which required about 20-100 hours of continuous thermal treatment in a vacuum oven at ≥250° C., to achieve the same level of cure, i.e. >20× improvement in kinetics.
In one embodiment, the present invention incorporates single walled carbon nanotube (SWCNT) as filler. This material, as well as nanographene sheets (NGS), improves processibility by reducing the curing time by absorbing microwave energy as well as providing a uniform thermal transport field. In one embodiment, in-situ condensation polymerization is used with carbonaceous fillers, which allows for proper wetting of nanofillers by the polar functional groups in PAA precursor comonomers prior to polymerization to form nanocomposites. In another embodiment, surface modification of nanofillers by oxidative polymerization of aniline to form polyaniline-modified nanofillers, PANi-modified-nanofillers, prior to in-situ polymerization with PAA precursor monomers is used.
The present invention utilizes an improved microwave heating method for thermal imidization instead of conventional heating. Due to the direct energy conversion through radiation-molecule interaction, heating is uniform in the entire material volume. This leads to dramatically decreased curing time (40 min in microwave oven compared to 10 h in conventional oven) and energy without trading-off their excellent thermal and mechanical properties.
The domestic microwave oven uses fixed frequency with pulsed irradiation mode. The higher the microwave power, the longer the microwave irradiation, and therefore, the higher the temperature increase. A 0.1 wt. % CNT-PAA solution and a 0.1 wt. % NGS-PAA solution were used to demonstrate the microwave power effect on temperature increase, as shown in
Temperature monitoring during microwave curing of PI, PI-Copolymer, PI-CNT/NGS, and Copolyimide-CNT/NGS was conducted to study the microwave heating enhancing behavior of CNT, NGS and Siloxane. As shown in
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were utilized to compare thermal stabilities, glass transition temperature, Tg, and degree of imidization.
The DSC thermograms shown in
UV-Vis spectra of microwave and convention oven cured PI-CNT/NGS composites are presented in
The XRD data presented in
The effect of different curing methods on the mechanical properties of the PI composites were characterized using stress-strain curves using a tensile testing machine. Interestingly the Young's moduli calculated (see
The TGA of the samples cured at 100 and 150° C. show the difference between the two systems (
All documents cited are incorporated herein by reference; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention. It is to be further understood that where descriptions of various embodiments use the term “comprising,” and/or “including” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.”
While particular embodiments of the present invention have been illustrated and described, it would be obvious to one skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
This application claims priority to U.S. Provisional Application Ser. No. 63/244,559, filed Sep. 15, 2021, which application is hereby incorporated by reference in its entirety.
This invention was made with government support under contract FA8649-21-P-0128 awarded by the Air Force Research Laboratory (AFRL). The government has certain rights in the invention.
Number | Date | Country | |
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63244559 | Sep 2021 | US |