New technology and application of the hottest micr

New technology and application of microwave in polymer field

source: Beijing Xianghu Technology Development Co., Ltd.

microwave is an electromagnetic wave with a wavelength of 1mm ~ 1m. Compared with ordinary heating method, microwave heating has great advantages. Ordinary heating equipment heats objects by convection, conduction and thermal radiation, and the process is from outside to inside. On the contrary, microwave makes the molecules of the irradiated object interact through the radiation of electromagnetic waves, so as to achieve the purpose of heating. Microwave directly converts electromagnetic energy into heat without heat transfer. It is the difference of this heating method that makes microwave have outstanding advantages such as high heating efficiency, strong controllability and uniform heating, which makes microwave have great application potential in the field of polymer

1 basic principle and influencing factors of microwave heating

1.1 basic principle of microwave heating

the essence of microwave heating lies in the dielectric displacement of materials or the polarization of different charges in materials, and this polarization does not have the ability to quickly keep up with the alternating electric field. At high frequencies, polarization lags behind the electric field and the current generated by polarization has a component in phase with the electric field, resulting in the dissipation of power inside the material

in the electromagnetic field with fixed electric field intensity, the microwave energy absorbed by the material is directly proportional to the frequency of electromagnetic radiation, the dielectric loss of the material and the square of the electric field intensity, which is expressed as:

according to equation (1), the microwave energy absorbed by the material can be controlled by changing the electric field intensity or the dielectric properties of the material, but the dielectric loss factor of most polymer materials is very small, and microwave usually passes through the material without dissipation

1.2 influencing factors of dielectric loss

the dielectric loss of polymer depends on the following factors:

(1) the order of group dipole distance polarization is: SO2> CONH> CN> co> C1> co2r> 0> C03> C-C. It is found that the -cn group in nitrile rubber can induce the main chain action in the glass transition zone, resulting in large dipole relaxation, which increases the dielectric loss value. Therefore, butadiene rubber can be rapidly heated by microwave

(2) the study of molar fraction of polarization group found that the dielectric loss of propylene/butadiene copolymer increased with the increase of propylene content

(3) the dielectric properties of hydrogen bonded nylon are mainly determined by intermolecular and intramolecular hydrogen bonds. The stronger the hydrogen bond, the more difficult the chain movement is, and the lower the dielectric loss value is

(4) the dielectric properties of chemical structure polyvinyl acetate and p-Phthalate polyethylene are different. The former polar group ester group is located in the side chain, which has greater mobility and larger dielectric loss value: the adjustable speed valve rises rapidly, while the latter polar group is located in the main chain, which has poor mobility and smaller dielectric loss value

(5) steric hindrance effect in polymethylmethacrylate, the steric hindrance of a-methyl causes the rigidity of the main chain. The larger the steric hindrance is, the smaller the dielectric loss angle is. In addition, steric hindrance also affects crystallinity and chain movement

(6) with the increase of crystallinity, the chain movement decreases and the dielectric loss decreases

(7) crosslinking density with the increase of crosslinking density, the chain movement decreases and the dielectric loss angle decreases

(8) in physical state, the polarity of -c1 in PVC is higher than that in polychloroprene, but at room temperature, the dielectric loss value of polychloroprene is higher than that of PVC. This is because at room temperature, polychloroprene is in rubber colloidal state, the chain can move freely, and the dielectric constant is determined by the polarization of chain segment dipole, so the dielectric loss is high. PVC is in glass state at room temperature, and the chain movement is limited, The dielectric constant only depends on the dipole polarization of the group itself, so the dielectric loss value is low

in addition to the above, the factors affecting the dielectric loss value include the stereochemical structure, branching and orientation of the polymer. To sum up, it can be concluded that polymers with low dielectric loss value, such as PVC, polytetrafluoroethylene, polypropylene, polystyrene, polycarbonate, polyoxyphenylpropylene, polysulfone, etc., are almost transparent to microwave due to their small dielectric loss tangent under microwave radiation, and cannot be processed directly by microwave. Polymers with high dielectric loss value, such as polychloroprene, polyurethane, epoxy resin Polymethylmethacrylate, polyvinyl fluoride, etc. can be processed by microwave because of their large dielectric loss tangent under microwave radiation

2application and new technology

the application of microwave in polymer field mainly includes polymer synthesis process, curing, welding, extraction and so on. New technologies in the polymer field are also developed by relying on the advantages of microwave, such as high efficiency and energy saving

2.1 application of microwave in polymer synthesis

in 1986, Gedye and others first used microwave method instead of ordinary heat source in organic reaction process, which has been widely used and developed so far. At first, most of them were used in the synthesis of metal alumina silicon semiconductors, and most of them were simple microwave equipment. Microwave method is widely used in polymer field in recent years. Compared with ordinary heating methods such as oil bath, hot table and fire, microwave heating has its unique advantages, which greatly shortens the polymerization reaction time, reduces by-products in the reaction process, makes the reaction process automated, has strong controllability, and has high output

donato Donati et al. Esterified under the action of Mukaiyama catalyst with microwave assistance, the product obtained is pure, the reaction time is short, and the reaction process is simple. In recent years, the research of microwave application in different types of polymerization has been carried out more. Most studies have directly replaced ordinary heat sources with microwave, and the reaction process remains the same, but Christian goretzki and others not only synthesized monomers in the same reactor by microwave-assisted heating, but also polymerized monomers directly in the reactor. In this way, the whole process will reduce processes, speed up efficiency, reduce pollution, and achieve the purpose of green polymerization

2.2 application of microwave in polymer curing

when microwave is used in polymer curing, it is usually used to cure epoxy resin. Tanrattanakul et al used microwave method and ordinary heating method to cure epoxy resin respectively. They cured epoxy/anhydride in microwave and ordinary heating equipment respectively. The effects of two different heating methods on the microstructure of polymer were characterized by different means such as mechanical property index, dynamic mechanical analyzer and differential calorimetry. It was concluded that microwave has the advantages of high efficiency and high curing degree, and affects the three-dimensional structure of epoxy resin at the same time

in addition to epoxy resin, bismaleimide has more research on its curing because of its better thermal stability, humidity resistance and thermal environment resistance than epoxy resin. Ismail zain OL and others cured bismaleimide with ordinary heating equipment and microwave heating equipment respectively. Through comparison, it is found that the curing speed in microwave is fast. When the curing degree is high, the Tg of the sample cured by microwave is lower than that of ordinary heating equipment, while when the curing degree is low, the difference of molecular structure is not great

2.3 application of microwave in polymer welding

the application of microwave in polymer welding was first studied by Abdul lah, gultz and others in 1990. They used three traditional welding methods, digital liquid crystal display and computer-controlled display, and microwave welding method to weld polymers. They found that microwave welding method was efficient and had good welding effect. Lai and NC studied the application of microwave welding in thermoplastic polymers and tested the bonding effect of polymers by tensile method, and proved the above conclusion again

siores and Groombridge have conducted in-depth research on expanding the range of microwave processables. For those polymers that are not sensitive to microwave, they realize welding by adding welding flux. This idea has played an important role in the application of microwave in the welding of polymers and other materials

prasad KD et al. Used microwave to dry together three thermoplastic polymers: ultra-high molecular weight polyethylene, polystyrene and polyacrylonitrile/butadiene/styrene by two steps. Specific steps: the first step is to directly heat the sample interface with microwave to make it bond; The second step is to further bond the two same polymer samples more tightly by adding epoxy resin. In its research, ABS was found to be more sensitive to microwave than the other two polymers. The samples obtained by adding epoxy resin and bonding by microwave heating have good results. Through the test, the strength of bonded PC samples prepared in the first step is only 20.3% of that of pure PC, but after the second step processing, it reaches 63.3%. It shows that adding epoxy resin has a very obvious effect of re bonding

2.4 application of microwave in extraction

microwave extraction technology is mainly based on the thermal characteristics of microwave. When extracting substances, substances with different dielectric constants in the microwave field have different abilities to absorb microwave, so that some regions of the matrix material and some components in the extraction system are selectively heated, so that the target components can be selectively separated from the matrix and system. Microwave extraction has the advantages of high efficiency, less solvent and no concentration, so its application prospect is very good. Rashmi sanghi and kannamkumarath SS compared the extraction of pesticides in different matrices by microwave extraction, ordinary extraction and ultrasonic extraction, respectively. It is concluded that microwave extraction can still obtain more accurate results under the condition of reducing the amount of samples and solvents, which proves that microwave method has the advantages of high efficiency and accuracy

fateh Alavi K et al. Analyzed the stability of stabilizer in crosslinked polydimethylsiloxane by microwave extraction. They found that the phenol and amine stabilizers in silicone rubber can be almost extracted in a short time by microwave extraction

2.5 application of new microwave-assisted technology

when making semiconductors, a layer of polymer material needs to be attached to the semiconductors. This technology is to shorten the time and speed up the efficiency of attaching the polymer material layer to the semiconductors by using microwave-assisted heating. The specific method is to lay the polymer on the semiconductor and put it into the microwave oven, and then radiate the cold drawn low-carbon steel wire 6640780011810184602658036170 products within a certain microwave frequency range. The microwave frequency should be selected in a range that neither decomposes polymers nor damages semiconductors. Due to the rapid melting of polymers under microwave radiation, gases and water vapor need to be discharged in time during the radiation process. Another advantage of microwave radiation is that polymer materials can be attached to larger semiconductors

in order to facilitate the recycling of waste electrical appliances and other equipment, microwave technology can also be used to remove polymers from waste equipment. This technology is only effective for some polymers sensitive to microwave

in recent years, there is also a microwave curing technology applied to dentistry. This hand-held microwave device can cure polymers in the mouth of patients to make dentures. This method can disinfect and prevent infection

microwave assisted polymer injection molding. This method mainly uses microwave as a heating device to melt polymer particles and inject polymers through injection molding equipment such as screws. Similar to the ordinary injection molding process, before injection