DE102020125375A1 - Method of heating objects with microwaves - Google Patents

Abstract

A method for producing an at least partially crosslinked polymer product (12; 60) or polymer composite product by introducing energy into a resinous starting material is characterized in that the energy is transmitted entirely or partially by means of microwaves (34); at least part of the energy transmitted by means of microwaves (34) is transmitted to the material (12; 60) via a susceptor (32; 33, 35, 37; 52, 54, 56, 58), and the susceptor (32; 33, 35, 37; 52, 54, 56, 58) is located outside the product.

Description

technical field

The invention relates to a method for producing an at least partially crosslinked polymer product or polymer composite product by introducing energy into a resinous starting material.

The invention also relates to an arrangement for producing an at least partially crosslinked polymer product by introducing energy into a resinous starting material and a retrofit kit for retrofitting a microwave generator with a susceptor.

An example of a polymer product is prepregs. Prepregs (from preimpregnated fibers) are semi-finished products made of fiber-reinforced plastics in which a resin matrix is slightly pre-crosslinked by the addition of heat and which are therefore storage-stable until their final processing. In order to achieve this prepreg condition, resin-impregnated glass fiber fabrics must be annealed for a few minutes at temperatures typically between 100°C and 200°C. It is important that neither too much nor too little energy is used for networking. Too much energy leads to complete cross-linking, which prevents further processing. If too little energy is supplied, the prepreg cannot be stored.

State of the art

From the "Handbook Fiber Reinforced Plastics/Composites - Basics, Processing, Applications", Springer Vieweg, 4th edition, 2013; "Fiber composite materials - prepregs and their processing", Carl Hanser Verlag, 1./2. Edition 2014/2020 it is known to adjust the prepreg state in ovens or in heat baths based on thermal oils. Considerable quantities of oil have to be heated above the target temperature. The process requires a lot of energy. The heating of the oil is slow. Adjusting to a selected temperature is correspondingly imprecise and sluggish.

From the DE 10 2015 111 555 B3 a microwave applicator is known which is provided for the microwave treatment of larger components. With the known arrangement, the entire material of the workpiece is to be treated. Many plastics are not amenable to heating by exposure to microwaves. They are microwave transparent. Typical microwave transparent materials are glass or polypropylene.

DE 10 2018 123 261 A1 discloses the treatment of microwave-transparent plastics with microwaves. In this case, only an applied conductor layer should be heated, but not the microwave-transparent plastic substrate.

D. Teufl, S. Zaremba Materials 11, 838, 2018 discloses the irradiation of glass fiber reinforced plastics using microwaves. The method uses a vacuum bag assembly. This is complex and unsuitable for series production. The aim of the known method is the complete curing of an individual component. An adjustment of the dose of the energy input is not disclosed.

The inhomogeneities in the electric field strength and the material properties, such as the dielectric permittivity and conductivity, which occur in practice with microwaves, often cause inhomogeneous heating. This is undesirable. The effect occurs particularly in materials with low thermal conductivity such as plastics.

Susceptors are known from the publication by M. Bhattacharya, T. Basak Energy 97, 306, 2016. Susceptors absorb microwave energy and heat up. The absorbed energy is given off again by convection and/or IR radiation. There are two types of susceptors: Particulate or granular susceptors are introduced into the material to be heated. Macroscopic susceptors are located in the microwave radiation field outside of the material to be heated. In the “Polymers” section, the publication discloses materials with particulate susceptors in adhesives, with which polymer components are connected. A disadvantage of particulate susceptors is that they change the material, its appearance and its properties. In addition, particulate susceptors are a consumable that involves cost and effort. In practice it has been shown that particulate susceptors can only be distributed uniformly in the polymer with a great deal of effort. Here, too, there is complete networking.

Disclosure of Invention

The object of the invention is to create a method for producing an at least partially crosslinked polymer product or polymer composite product by introducing energy into a resinous starting material of the type mentioned at the outset, with lower energy requirements and faster reaction times.

1. (a) die Energie ganz oder teilweise mittels Mikrowellen übertragen wird;
2. (b) zumindest ein Teil der mittels Mikrowellen übertragenen Energie über einen Suszeptor auf das Material übertragen wird, und
3. (c) der Suszeptor außerhalb des Produkts angeordnet ist.

According to the invention, the object is achieved in the method mentioned in that

1. (a) the energy is transmitted in whole or in part by means of microwaves;
2. (b) at least a portion of the microwave energy is transferred to the material via a susceptor, and
3. (c) the susceptor is located outside the product.

The use of susceptors also allows the use of microwaves with microwave transparent polymers and their composites. The energy requirement decreases compared to the well-known heating by means of an oil-based heat bath. A homogeneous, more precise and faster setting of the temperature is made possible. Unlike the known susceptors used for polymers, the material to be heated is not changed in this process, since the susceptors are located outside the product. This makes it possible to treat the product itself and not just the joining materials with microwaves.

The susceptor arranged outside the product can be granular or particulate. An additional container outside the product is then required. The susceptor is therefore preferably of macroscopic dimensions. The susceptor itself can then be arranged, for example with a holder, in the microwave field outside of the product to be heated. It is easy to handle and its position can be adjusted well.

In particular, it can be provided that the susceptor has a macroscopic, rigid geometric shape and is formed in particular from plates, rods, tubes, grids, shells, spheres, combinations thereof and/or geometric derivatives thereof. Depending on the arrangement, the susceptor can be used without a holder. In a particularly preferred embodiment, it is provided that the position of the susceptor, in particular the distance from the product to be heated, can be adjusted. This is particularly advantageous when different products are to be heated and/or when different degrees of crosslinking are to be achieved.

vorzugsweise $$\text{tan}{\mathrm{\delta }}_{\text{e}}=\mathrm{\epsilon }{\text{''}}_{\text{r}}/\mathrm{\epsilon }{\text{'}}_{\text{r}}>\mathrm{0,1}$$

gebildet ist, wobei $$\mathrm{\epsilon }\text{'}{\text{'}}_{\text{r}}=\mathrm{\epsilon }\text{'}\text{'}/{\mathrm{\epsilon }}_0$$

$$\mathrm{\epsilon }{\text{'}}_{\text{r}}=\mathrm{\epsilon }\text{'}/{\mathrm{\epsilon }}_0$$

mit ε₀ = 8,854×10^-12 Farad/m (Dielektrizitätskonstante im Vakuum). In der oben aufgeführten Druckschrift von M. Bhattacharya, T. Basak Energy 97, 306, 2016 sind Werte für einige Materialien aufgeführt.It is preferably provided that the susceptor consists entirely or partially of a microwave absorber with a value $$\text{tan}{\mathrm{\delta }}_{\text{e}}=\mathrm{e}{\text{''}}_{\text{right}}/\mathrm{e}{\text{'}}_{\text{right}}>0.01$$

preferably $$\text{tan}{\mathrm{\delta }}_{\text{e}}=\mathrm{e}{\text{''}}_{\text{right}}/\mathrm{e}{\text{'}}_{\text{right}}>0.1$$

is formed, where $$\mathrm{e}\text{'}{\text{'}}_{\text{right}}=\mathrm{e}\text{'}\text{'}/{\mathrm{e}}_0$$

$$\mathrm{e}{\text{'}}_{\text{right}}=\mathrm{e}\text{'}/{\mathrm{e}}_0$$

with ε ₀ = 8.854×10 ^-12 Farad/m (dielectric constant in vacuum). Values for some materials are given in the publication by M. Bhattacharya, T. Basak Energy 97, 306, 2016 cited above.

It is particularly advantageous for the present invention if the susceptor is made entirely or partially of SiC. It goes without saying that the susceptor can also be formed from a different material with comparable properties.

The method according to the invention is particularly advantageous if the dose of the energy input is set to a selected value. Then the degree of crosslinking of prepregs, for example, can be adjusted very precisely and the quality of the products can be improved.

In particular, the dose of the energy input can be adjusted via one or more of the following parameters: irradiation duration, microwave energy, distance of the susceptor from the product surface and/or material of the susceptor or combinations thereof. A longer irradiation time, a higher microwave energy and/or a smaller distance of the susceptor from the product surface leads to a higher dose.

With the arrangement, individual parts can be heated in a microwave field. However, it is also possible for the product and/or the susceptor to which the microwave is applied to be moved laterally to the irradiation direction during the energy input. This is particularly advantageous in the case of so-called endless material, the dimensions of which are larger in one direction than the microwave field. With such a relative movement between the susceptor, possibly the microwave generator, and the product, it can be provided in particular that the dose of the energy input is adjusted at least partially via one or more of the following parameters: speed of movement and distance covered. In practice, for example, the endless material or the individual components can be moved on a conveyor belt through the treatment room with the microwaves. At high speed, a small dose is injected, at low speed, a higher dose.

The polymer product or polymer composite product can consist at least partially of thermoplastics, duromers and/or composite materials with a polymer matrix.

1. (a) wenigstens einen Mikrowellengenerator zur Bestrahlung des Materials mit Mikrowellen mit auswählbarer Dosis; und
2. (b) einen von den Mikrowellen beaufschlagten Suszeptor außerhalb des Materials zum Übertragen zumindest eines Teils der Energie der Mikrowellen auf das Material.

The object of the invention is also achieved with an arrangement for generating a at least partially crosslinked polymer product by energy input into a resinous starting material, which is characterized by

1. (a) at least one microwave generator for irradiating the material with microwaves at a selectable dose; and
2. (b) a microwave impacted susceptor external to the material for transferring at least a portion of the energy of the microwaves to the material.

Such an arrangement makes it possible to heat polymers regardless of whether they are microwave-transparent or not. The product to be heated does not have to be changed. In contrast to known arrangements with external susceptors arranged outside the product, the arrangement according to the invention is not used for sintering ceramics at high temperatures above 1000° C., for example, but for producing an at least partially crosslinked polymer product. In contrast to the present invention, in known arrangements, treatment is therefore always carried out with the highest possible dose in order to obtain an end product. A partial network cannot be achieved with this.

In an advantageous embodiment of the invention, a drive is provided with which the material to be treated with microwaves can be moved continuously through the microwave field of the microwave generator. In particular, the drive can include a conveyor belt or a carriage on which the material can be stored.

Microwave generators are expensive and are also used for purposes other than use for the object of the invention. The invention therefore also provides a retrofit kit for retrofitting a microwave generator with a susceptor for producing an at least partially crosslinked polymer product or polymer composite product by introducing energy into a resinous starting material. The retrofit kit then enables a conventional microwave generator to be used to produce an at least partially crosslinked polymer product or polymer composite product. This is considerably cheaper than purchasing the entire arrangement from scratch.

Refinements of the invention are the subject matter of the dependent claims. An embodiment is explained in more detail below with reference to the accompanying drawings.

character list

• 1 Figure 12 is a side view of an arrangement for treating materials with microwaves having a multimode applicator, a single mode applicator and an infrared radiation source.
• 2 Figure 12 is a plan view of the assembly 1 .
• 3 12 is a front view of the arrangement of FIG 1 .
• 4 shows a detail 1 with susceptor.
• 5 Fig. 12 is a schematic side view of an arrangement for treating materials with microwaves and infrared radiation.
• 6 Fig. 11 illustrates an alternative arrangement of susceptors for a microwave assembly according to Figs 1 .

Description of the embodiment

In the 1 until 3 1, there is shown an assembly, generally designated 10, for treating materials with microwaves. All types of materials in the form of piece goods or yard goods can be treated in the arrangement, for example prepregs, composite materials or the like. Energy is supplied to the materials via microwaves, with which the material is cured, heated and/or dried.

The figures show an example of a product 12, for example in the form of a resin-impregnated glass fiber fabric, which is conveyed on a conveyor belt 14 made of glass fiber-reinforced Teflon mesh. The conveyor belt is permeable to microwaves. The conveying direction is indicated with an arrow 28 . The conveying speed can be set on the drive 21 and/or on the drive 19. The conveyor belt 14 enters a treatment chamber 16 defined by an enclosure. The treatment chamber 16 is irradiated in a first treatment room 20 by microwaves with a frequency of 2.45 GHz from a multimode applicator 18 . In a second, optional treatment room 22, the treatment chamber 16 is radiated through by microwaves with a frequency of 2.45 GHz from a monomode applicator 24. A thin product 12, as shown in the figures, can be moved with the conveyor belt 14 through both treatment spaces 20 and 22 of the treatment chamber 16. With both applicators turned on, the product 12 is subjected to two microwave treatments.

To treat thick or curved materials, or materials with larger dimensions in the vertical direction, the Mono mode applicator 24 are moved apart or removed.

Each of the microwave applicators can be switched on independently of one another, resulting in a wide range of treatment options for different types of materials in one arrangement. It goes without saying that arrangements with only one microwave applicator can also be suitable.

There are polymer materials that are microwave transparent. These materials are not or only slightly heated in the arrangement described and there is insufficient crosslinking for the production of a prepreg, for example. There is a risk of inhomogeneous heating, in particular when the dimensions of the product are small compared to the dimensions of the microwave applicator. The arrangement 10 therefore has one or more susceptors 32 in the treatment room 20 .

Each of the susceptors 32 is made of SiC. It goes without saying that other materials which have a high dielectric loss can also be suitable as a susceptor. The susceptors 32 are arranged between the microwave generator 18 and the product 12 in the microwave field of the treatment room 20 . In the present exemplary embodiment, the susceptors 32 are decoupled from the conveyor belt 14 . They form a stationary part of the microwave applicator. A reflector and/or insulation, for example made of Teflon, can optionally be provided on the rear side of the susceptor. A reflector and/or insulation allows for improved energy efficiency. When selecting the reflector materials, it should be noted that metals, for example, are microwave-active and can induce unwanted eddy currents.

4 illustrates the functioning of the susceptor 32. The microwave radiation 34 generated by the microwave generator 18 is absorbed by the susceptor 32. FIG. The energy is converted into thermal energy there and is emitted in the direction of the product 12 in the form of infrared radiation and convection heat. While the product 12 is not or only slightly heated by microwave radiation, the heat given off by the susceptor 32 is given off almost completely to the product. It is thus possible to heat microwave-transparent materials with the susceptor.

The susceptor 32 is in the vertical direction, ie in the direction of arrow 38 in 4 , moveable. On the one hand, this makes it possible for products 12 with larger dimensions to be treated in the vertical direction. On the other hand, the distance between the susceptor 32 and the product surface, and thus the transmitted energy, can be set very well and quickly. It goes without saying that the adjustment can be automated when using sensors.

The arrangement is provided with temperature sensors in the usual way. These can be formed, for example, by a pyrometer and/or fiber optic sensors for contact measurement on the material. For the structure with the susceptors, a holder can be individually adapted for a specific component/material. The susceptors advantageously consist of silicon carbide, preferably in the hexagonal α-structure, and copper oxide powder CuO. Many susceptor materials that are common for the thermal treatment of ceramics are not suitable for polymers. At low temperatures, at which the thermal treatment of polymers takes place, the dielectric properties of many susceptor materials are such that the susceptors still appear transparent to microwaves. They only become absorbent at higher temperatures (700...800°C) at which, however, polymer treatment is no longer possible due to the risk of decomposition etc. The following susceptor materials are suitable for the thermal treatment of polymers: Composites made of a microwave-transparent thermoplastic (e.g. polypropylene or PEEK) or Al2O3 as a matrix and conductive particles such as metal powders or various forms of carbon-containing particles, with (multi-wall) carbon nanotubes and conductive carbon black being preferred here are. Microwave-inactive materials such as Teflon, ceramics or glass are particularly suitable as the material for the holders.

By moving the susceptor surface 32 in and out of the microwave field of the treatment room 20, the distance in the direction of movement 28 can be changed. In this way the degree of heating of the product 12 can also be adjusted. The thermal energy introduced into the product can also be adjusted by the conveying speed of the conveyor belt 14 . Finally, the energy of the microwave radiation can be adjusted.

The parameters mentioned for setting the dose can be easily adjusted. In this way, a degree of crosslinking in a prepreg can be adjusted very precisely. This reduces the energy requirement and improves the accuracy and quality of networking.

• Für die in 4 gezeigte Anordnung stellt sich nach ca. 20 min eine Prepreg-Temperatur von 160°C ein. Aus der dafür erforderlichen Leistung der Mikrowelle lässt sich ein Energie-Bedarf von ca. 5 kWh abschätzen. Im Vergleich dazu erfolgt die Temperatureinstellung bei der Herstellung von Prepregs in herkömmlichen Imprägnieranlagen unter Einsatz großer Mengen von Thermalöl, das mit langer Vorlaufzeit mittels elektrisch getriebener Heizung auf Temperaturen gebracht wird, die typischerweise um 10 K gegenüber der eigentlichen Zieltemperatur liegen. Diese Temperaturdifferenz ist erforderlich, um Wärmeverluste über den Transportweg des Thermalöls hinweg zu kompensieren. Der Energiebedarf zur Einstellung einer Prepreg-Temperatur von 160°C liegt dabei um wenigstens eine Größenordnung höher als bei der in 4 gezeigten mikrowellengetriebenen Heizung mit Suszeptoren. Im Vergleich zur Einstellung der Temperatur mittels IR-Strahlern lässt sich abschätzen, dass diese zur Einstellung einer Prepreg-Temperatur von 160 °C ähnlich viel Energie benötigen wie die in 4 gezeigte mikrowellengetriebene Heizung mit Suszeptoren. Während durch IR-Strahler nicht nur die Prepregs sondern auch deren Umgebung beträchtlich erwärmt wird, gewährleistet der mikrowellenbasierte Ansatz in 4 eine selektive Erwärmung nur der Suszeptoren.

For the exemplary embodiment, the energy balance shows the energetic advantages of the arrangement described:

• for the inside 4 In the arrangement shown, a prepreg temperature of 160° C. is reached after approx. 20 minutes. From the microwave power required for this, an energy requirement of approx. 5 kWh can be estimated. In comparison, the temperature setting in the production of prepregs in conventional impregnation plants uses large quantities of thermal oil, which is brought to temperatures that are typically 10 K above the actual target temperature with a long lead time using electrically driven heating. This temperature difference is required to compensate for heat losses over the transport route of the thermal oil. The energy required to set a prepreg temperature of 160°C is at least an order of magnitude higher than in 4 shown microwave powered heater with susceptors. In comparison to setting the temperature using IR emitters, it can be estimated that they require a similar amount of energy to set a prepreg temperature of 160 °C as in 4 shown microwave powered heater with susceptors. While not only the prepregs but also their surroundings are heated considerably by IR emitters, the microwave-based approach in 4 selective heating of only the susceptors.

Optionally, however, infrared lamps 42 for irradiation with infrared (IR) radiation 44 can also be provided. The irradiation with IR radiation 44 can, for example, take place simultaneously, with the susceptors not being intended to shield the IR emitters. Such an arrangement is shown below with reference to 5 described. In this exemplary embodiment, a strip material 46 in the form of a polymer film—prepreg—is shown as an example. It goes without saying that general cargo can also be treated here.

The IR emitters 42 are arranged between the microwave generators 18 and 24 or behind them in the direction of movement 29 . The IR emitters 42 irradiate the material 46 to be heated from above. The plate-shaped susceptors 32 and 37 are arranged above the strip material 46 outside the radiation field of the IR emitters 42 . The plate-shaped susceptors 33 and 35 are arranged outside the radiation field of the IR emitters 42 below the strip material 46 . The susceptors 32, 33, 35 and 37 are held on holders 40, which are illustrated only schematically here. The entire arrangement is located in a housing 48, illustrated in phantom, which prevents the escape of radiation. A pyrometer 50 at the end of the treatment section measures the radiation temperature of the treated material 46.

Instead of moving the strip material 46 continuously in the direction of movement 29, an oscillating movement can also be provided. This is illustrated by an arrow 31.

In a further alternative, a continuous microwave arrangement is provided in which four susceptors 52, 54, 56 and 58, for example in the form of plates, are perpendicular to the direction of travel 66 of the conveyor belt 60 in a microwave-transparent holder 62. The product to be treated 64 is heated by four susceptors. This alternative is based on 6 described. In this embodiment, the temperature of the product 64 is determined with a fiber optic temperature sensor 68 .

It goes without saying that, in addition to the exemplary embodiments listed above, other arrangements between optional infrared radiators and microwave generators are also possible. If, from the point of view of the material to be heated, one lays out a direction for the conveyor belt from 6 spatial directions (+/-x, +/y, +/- z), where neither IR emitters nor susceptors are arranged, 5 directions remain, in where either a susceptor or an IR radiator can be arranged without shielding occurring. In this way, the simultaneous use of both heat input channels, IR and susceptor, is possible.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102015111555 B3 [0005]
• DE 102018123261 A1 [0006]

Claims (14)

A method for producing an at least partially crosslinked polymer product (12; 60) or polymer composite product by introducing energy into a resinous starting material, characterized in that (a) the energy is transmitted entirely or partially by means of microwaves (34); (b) at least part of the energy transmitted by means of microwaves (34) is transmitted to the material (12; 60) via a susceptor (32; 33, 35, 37; 52, 54, 56, 58), and (c) the Susceptor (32; 33, 35, 37; 52, 54, 56, 58) is placed outside the product. procedure after claim 1 , characterized in that the susceptor (32; 33, 35, 37; 52, 54, 56, 58) has macroscopic dimensions and is not granular or particulate. Method according to one of the preceding claims, characterized in that the susceptor (32; 33, 35, 37; 52, 54, 56, 58) has a macroscopic, rigid geometric shape and consists in particular of plates, rods, tubes, grids, shells, Spheres whose combinations and/or their geometric derivatives is formed. Method according to one of the preceding claims, characterized in that the susceptor (32; 33, 35, 37; 52, 54, 56, 58) consists entirely or partly of a microwave absorber with a value $$\text{tan}{\mathrm{\delta }}_{\text{e}}=\mathrm{e}{\text{''}}_{\text{right}}/\mathrm{e}{\text{'}}_{\text{right}}>0.01$$

preferably $$\text{tan}{\mathrm{\delta }}_{\text{e}}=\mathrm{e}{\text{''}}_{\text{right}}/\mathrm{e}{\text{'}}_{\text{right}}>0.1$$

is formed, where $$\mathrm{e}\text{'}{\text{'}}_{\text{right}}=\mathrm{e}\text{'}\text{'}/{\mathrm{e}}_0$$

$$\mathrm{e}{\text{'}}_{\text{right}}=\mathrm{e}\text{'}/{\mathrm{e}}_0$$

with ε ₀ = 8.854×10 ^-12 Farad/m (dielectric constant in vacuum). Method according to one of the preceding claims, characterized in that the susceptor is made entirely or partially of SiC. Method according to one of the preceding claims, characterized in that the dose of the energy input is set to a selected value. procedure after claim 6 , characterized in that the dose of the energy input is adjusted via one or more of the following parameters: irradiation duration, microwave energy, distance of the susceptor (32; 33, 35, 37; 52, 54, 56, 58) from the product surface (12; 60 ) and/or material of the susceptor (32; 33, 35, 37; 52, 54, 56, 58) or by combinations thereof. Method according to one of the preceding claims, characterized in that the product (12; 60) and/or the microwave-loaded susceptor (32; 33, 35, 37; 52, 54, 56, 58) is moved laterally to the irradiation direction during the energy input. procedure after claim 8 , characterized in that the dose of the energy input is adjusted at least partially via one or more of the following parameters: speed of movement and distance covered. Method according to one of the preceding claims, characterized in that the polymer product (12; 60) or polymer composite product consists at least partially of thermoplastics, duromers and/or composite materials with a polymer matrix. Arrangement for producing an at least partially crosslinked polymer product by introducing energy into a resinous starting material, characterized by (a) at least one microwave generator (18, 24) for irradiating the material (12; 60) with microwaves (34) with a selectable dose; and (b) a susceptor (32; 33, 35, 37; 52, 54, 56, 58) acted upon by the microwaves (34) outside the material (12; 60) for transmitting at least part of the energy of the microwaves (34) on the material (12; 60). arrangement according to claim 11 , characterized in that a drive (19, 21) is provided with which the material (12; 60) to be treated with microwaves (34) can be moved continuously through the microwave field of the microwave generator (18, 24). arrangement according to claim 12 , characterized in that the drive (19; 21) comprises a conveyor belt (14) or a carriage on which the material (12) can be stored. Retrofit kit for retrofitting a microwave generator (18, 24) with a susceptor (32; 33, 35, 37; 52, 54, 56, 58) for producing an at least partially crosslinked polymer product or Polymer composite product by energy input into a resinous starting material.

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