WO1990015110A1

WO1990015110A1 - Method and composition for low frequency dielectric heating of nonaqueous adhesives

Info

Publication number: WO1990015110A1
Application number: PCT/US1990/003172
Authority: WO
Inventors: David P. Riddle
Original assignee: Riddle David P
Priority date: 1989-06-06
Filing date: 1990-06-05
Publication date: 1990-12-13
Also published as: AU5834190A

Abstract

The invention is a method and composition for dielectric heating of nonaqueous thermosetting adhesives that are generally transparent to electromagnetic radiation in the radio frequency range. The method comprises exposing a nonaqueous thermosetting adhesive that is otherwise transparent to electromagnetic radiation in the radio frequency range to a predetermined frequency of electromagnetic radiation within the radio frequency range. The adhesive includes a radio frequency sensitizing composition in an amount effective to respond to the predetermined frequency and to modify the characteristics of the adhesive until the adhesive becomes increasingly responsive to the predetermined frequency of electromagnetic radiation so that the electromagnetic radiation heats the otherwise transparent adhesive.

Description

HETHOD AND COMPOSITION FOR LOW FREQUENCY DIELECTRIC HEATING OF NONAQUEOUS ADHESIVES

Field of the Invention

This invention relates to the dielectric heating and curing of adhesives using electromagnetic radiation in the radio frequency range, and in particular relates to use of low frequency electromagnetic radiation within the radio frequency range to cure nonaqueous thermosetting adhesives which are otherwise transparent to electromagnetic radiation at such frequencies. Background of the Invention

Radio frequency heating generally refers to a process in which electromagnetic radiation in the radio frequency range is generated and applied to a substance which is responsive to such radiation in order to generate some secondary response, such as heating of the material. Synonymous terms include "RF," "dielectric," "high frequency," and "capacitive" heating. The designation "dielectric heating" refers to that characteristic of dielectric materials—i.e. insulators—whereby they rapidly and repeatedly change polarity and correspondingly become heated, under the influence of an electromagnetic field within the radio frequency region.

As is known to those familiar with this technology, the electromagnetic spectrum covers a wide range of frequencies. The lowest represent audio frequencies, and higher ones typically include x-rays and gamma rays. Other commonly referred to types of electromagnetic radiation within the spectrum include the ultraviolet region, the visible region (so named because it includes the frequencies which form light visible to the human eye) , the infrared region, the microwave region, and the radio frequency region. Generally speaking, the term "radio frequency" refers to that portion of the spectrum that falls between frequencies of about 10⁴ and 10⁹ hertz (Hz) and encompasses wavelengths of between about 1 meter and 10 kilometers. The "hertz" is the standard unit of frequency in the electromagnetic spectrum and refers to the number of cycles per second. Radio frequencies are also often expressed in megahertz (MHz) , with one megahertz equalling one million hertz.

Use of the electromagnetic spectrum is regulated to some extent in the United States by the Federal Communications Commission (FCC) . For that reason industrial scientific and medical ("ISM") radio frequencies allocated to industrial uses such as those described herein range from between about 10 to about 10,000 megahertz (MHz) with particular assigned bands at 6.23 MHz, 13.56 MHz, 27.12 MHz, 84 MHz, 168 MHz, 896 MHz, and 2450 MHz.

Different materials react differently under the influence of various portions of the electromagnetic spectrum. Under certain ultraviolet and visible frequencies, human skin becomes sunburned. At the x-ray or gamma ray regions, nuclear reactions can take place. At microwave frequencies, water molecules rotate, vibrate and translate and generate heat, which is the operating premise of the domestic microwave oven. At radio frequencies other materials, particularly dielectric materials, respond and generate heat by similar internal molecular or submolecular activity. Based on this phenomena, processes have been developed in which radio frequencies are applied to certain materials to heat them. The resulting techniques have particular advantages. For example, only the material that is responsive to the electromagnetic radiation generates heat. Thus, substrates being joined can be kept relatively cool even while a dielectric adhesive is being heated and cured by radio frequency radiation. In other techniques, materials that are responsive to RF heating; for example, polyvinyl chloride and similar plastics; are fused or welded together to form a bond.

In more conventional methods such as convection heating, an oven and the immediate environment of an object to be heated must be maintained at an elevated temperature. This is disadvantageous under many circumstances. In contrast, in many applications of radio frequency heating, the absence of external heat offers advantages such as higher production rate, an increase in quantities produced, less heat deterioration of any mechanical fixtures, and greater comfort for persons working in the area of an RF heating process.

Other attractive advantages of radio frequency heating include speed, selectivity, and control. Under many circumstances dielectric heating avoids some of the disadvantageous side effects that are inherent in typical convection forms of heating such as hot spots, scorching, over drying, or overheating the environment.

In operation, and as understood by those familiar with the technology, radio frequency processes use an electrical circuit to generate the appropriate electromagnetic field. In the typical circuit used, the dielectric material is a capacitive component that is an integral part of that circuit. The dielectric material may progressively change in its electrical charaσteristics throughout the radio frequency heating process. As a result, when the dielectric material is more reactive or more conductive, an increase in power is produced in response to the change of this key element in the circuit. When the load is small, typically at the end of a cycle when a product is dry or cured, the power drops off. The extent to which this change takes place in a particular dielectric material is dependent upon the loss factor of that material. For example, in typical applications using water-based adhesives, as the water is driven off by the radio frequency heating, the overall dielectric characteristic of the adhesive changes. The phenomena of reacting to the water creates a deep penetrating ability, an even or uniform generation of heat, and a natural leveling effect, all of which are beneficial in many processes. An appropriate graphical representation of this effect is illustrated in Figure 1 of U.S. Patent No. 4,296,294 to Beckert et al which shows how the temperature of an RF-heated adhesive remains substantially constant as the water content and dielectric loss factor decrease and the field strength increases.

As exemplified by the Beckert patent, RF heating is particularly useful in the wood and paper industries to cure aqueous (i.e. water-based) adhesives. The electromagnetic radiation at radio frequencies heats such adhesives by reacting with the water molecules contained therein, whose resulting activity heats and cures the adhesive without heating the wood or other material being joined.

Radio frequency heating does have limitations, however. First, it is limited to a given range of reactive dielectric products and some constraints on electrode design are critical.

Additionally, some materials are simply not responsive to dielectric heating, and RF heating techniques can require high initial costs.

Furthermore, although certain materials are somewhat responsive to RF heating, they react only at relatively high frequencies within the radio frequency range. These higher frequencies have shorter wavelengths and may have a greater effect on the less responsive dielectric materials. However, the depth of penetration of the electromagnetic radiation is limited at higher frequencies. Additionally, because of the shorter wavelengths at higher frequencies, standing wave problems can arise leading to uneven treatment of the dielectric material and difficulty in controlling or directing the energy field. Alternatively, lower frequencies have a number of advantages in RF processes. For example, at relatively low frequencies, the wavelengths used are longer. Therefore, much greater flexibility in working with larger size products and greater mass is possible. The lower frequencies can also penetrate more mass with less loss to the energy field. The use of longer wavelengths therefore allows more freedom in designing the size and shape of an electrode and the corresponding electrical circuit. Thus, the radio frequency heating effects can be focused or distributed with relative ease. Furthermore, lower frequencies are less of a potential health risk so that shielding requirements are less burdensome. Additionally, lower frequencies make it easier to generate higher power levels.

Because of the tendency of certain materials to be less reactive with lower frequencies of electromagnetic radiation, various techniques have been developed to enhance the response of such materials to RF energy. One technique is to physically include particles in a mixture to be treated, which particles react to the desired frequency. For example, some techniques include conductive particles in polymer precursors so that RF energy will heat the conductive particles which in turn will heat the polymer until it can be shaped, molded or fused. Other techniques use other types of additives, generally referred to as

"sensitizers", to enhance the RF response of particular materials.

One category of materials that typically fail to respond to radio frequency heating is that of the nonaqueous thermosetting adhesives such as urethanes and epoxies. Basically speaking, these materials are transparent to radio frequency electromagnetic energy in the same way that a colorless pane of glass is transparent to visible light. As a result, these materials are generally unresponsive to radio frequency heating, drying or curing techniques. By comparison, aqueous thermosetting adhesives typically respond well to RF techniques as most RF frequencies generate the desired transitions in the water molecules present and resultingly heat the adhesive.

Nevertheless, the nonaqueous urethane and epoxy adhesives are preferable in a number of circumstances for several reasons. First, as nonaqueous adhesives these compounds are typically nonevaporative and thus can be used in sealed environments. Aqueous adhesives tend to evaporate extensively and must be compensated for accordingly. Second, the nonaqueous thermosetting adhesives are often characterized by a great degree of cross linking which in turn results in a cured adhesive that is irreversible, nonflowing, and quite stable. Third, thermosetting adhesives are generally stronger in both adhesive-to-adhesive strength and adhesive-to-surface strength, and with greater elasticity in both types of bonding. They also provide the ability to adhere to certain types of materials with which aqueous adhesives simply will not bond. Unfortunately, nonaqueous thermosetting adhesives are particularly unresponsive to the lower frequencies and longer wavelengths of radio frequency energy that offer the advantages discussed above. Object and Summary of the Invention

It is thus an object of the present invention to provide a method by which a thermosetting nonaqueous adhesive can be used in a radio frequency process for curing, drying, heating, or any other appropriate application.

The invention meets this object by providing a method of dielectric heating of nonaqueous thermosetting adhesives that are generally transparent to electromagnetic radiation in the radio frequency range. The method comprises exposing a nonaqueous thermosetting adhesive that is otherwise transparent to electromagnetic radiation in the radio frequency range to a predetermined frequency of electromagnetic radiation within the radio frequency range. The adhesive includes a radio frequency sensitizing composition in an amount effective to respond to the predetermined frequency and to modify the characteristics of the adhesive until the adhesive becomes increasingly responsive to the predetermined frequency of electromagnetic radiation so that the electromagnetic radiation heats the otherwise transparent adhesive.

The foregoing and other objects, advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention which describes preferred and exemplary embodiments.

Detailed Description of the Preferred Embodiment The invention is a method of dielectric heating of nonaqueous thermosetting adhesives that are otherwise generally transparent to electromagnetic radiation in the radio frequency range. As discussed earlier, "dielectric" refers to a substance that has a very low electrical conductivity; i.e. an insulator. Such substances have electric conductivity of at least 10 million ohms per centimeter. Common dielectrics include glass, rubber, other polymers, wood and other cellulosic materials. Liquid dielectrics include hydrocarbon oils and silicone oils as well as other organic compounds. Additionally, the word dielectric has wide usage in the field of radio frequency heating of materials which usage is generally consistent with the above definition.

The thermosetting adhesives that are generally transparent to electromagnetic radiation in the radio frequency range are familiar to those in the field of adhesives and adhesive products, and include both epoxy and urethane adhesives. As used herein, the term "epoxy" or "epoxied" refers to an organic compound containing a reactive group which results from the union of an oxygen atom with two other atoms (usually carbon) that are fused in some other way as for example in the following manner:

O

/ \ -C-C-

Urethanes, also referred to as ethyl urethanes or ethyl carba ates, include repeating units based on the structure CO(NH₂)OC₂H₅. Polyurethanes are based on the repeating unit RiNHCOOR., wherein R_x and R₂ represent organic portions which do not otherwise interfere with the desired characteristics of the material.

As used herein, the term "transparent" refers to a material that is insufficiently reactive with RF energy to obtain any beneficial effect. In other words, transparent materials may generate some minimal amount of heat upon exposure to RF energy, but not enough to accomplish the desired crosslinking of the polymers or other required curing of the adhesive.

In the preferred embodiment of the invention, the adhesives are exposed to a predetermined frequency of electromagnetic radiation which can best characterized as a selected low frequency within the radio frequency range. Another way of describing the radio frequency range is as that range of frequencies which are above the induction range, generally qualified as frequencies of 1 megahertz and below, but below the microwave range, which in turn are generally designated as about 100 megahertz and above. It will be understood, however, that certain of the advantages of the invention can be accomplished even at higher frequencies, without the departing from the spirit of the invention or the scope of the claims. Thus, the invention can comprise exposing the adhesive to frequencies in the range of between about 1 and 100 megahertz, and in preferred embodiments comprises exposing the adhesive to frequencies of about 3 and 30 megahertz.

As set forth earlier, at these relative low frequencies, the wavelength of the electromagnetic radiation is rather long, between about 10 and about 25 meters. This means that the quarter wavelengths are between about 2.5 and 6 meters which in turn means that a workpiece of a similar size can be treated with electromagnetic radiation of a single unmodified source. As stated above, avoiding standing wave activity during radio frequency heating of a particular object advantageously avoids hot and cold spots where shorter wavelengths either reinforce or cancel one another.

In order to get the otherwise transparent nonaqueous thermosetting adhesive to respond to the low frequency radiation, however, a sensitizing composition is added. Typically, such a sensitizing composition is highly responsive to the predetermined frequency and creates heat that in turn initiates the generation of heat by the nonaqueous thermosetting adhesive itself. Although applicants do not wish to be bound by any particular theory, it appears that the best results occur where the response of the sensitizing composition to the predetermined frequency of electromagnetic radiation modifies the loss factor of the adhesive until the modified adhesive itself responds to the predetermined frequency of electromagnetic radiation.

Typically, the amount of sensitizing composition included in the adhesive is sufficient to modify the adhesive in response to the action of the predetermined frequency of electromagnetic radiation upon the sensitizing composition, but less than the amount which would initiate electrical conductivity in response to the electromagnetic radiation to which the adhesive is exposed. In preferred embodiments, the amount of sensitizing composition included in the adhesive is between about 4 and 8 percent by weight based on the weight of the adhesive.

Suitable sensitizing compositions include alkali or alkaline earth metal aluminosilicates, zinc oxides, aluminum chloride, ammonium chloride, refined carbons such as carbon black, and bentonite clays, all of which are preferably treated to remove absorbed water or other volatile materials. A number of such sensitizing compositions are thoroughly discussed in U.S. Patent Nos. 4,661,299; 4,767,799; and 4,790,965; all to Thorsrud. Other appropriate sensitizers have been used and can be determined and selected for use in accordance with the present invention without undue experimentation.

In a typical application of the invention, an adhesive composition as described herein is applied to at least one of a workpiece or surface to which the workpiece is to be attached. As set forth earlier, the adhesive composition comprises the nonaqueous thermosetting adhesive and the radio frequency sensitizing composition. The workpiece and the surface are positioned adjacent an electrode that is part of a dielectric capacitive circuit that includes a generator for producing radio frequency electromagnetic radiation from the electrode. An electromagnetic radiation output is generated from the electrode and directed through the adhesive composition at the predetermined frequencies to which the nonaqueous thermosetting adhesive is otherwise transparent.

As used herein, the designation "generator" refers to the instrument that converts the domestic frequency of electricity available to the desired operating frequency for use in the process of the invention. Typically, the generator is an industrial oscillator capable of generating a high voltage electromagnetic field at a prescribed frequency in the radio range. The particular electromagnetic field is also determined by the design and placement of the electrodes as well as the dielectric load. As known to those of ordinary skill in this technology, when a dielectric material is placed within such a field of energy, the free electrons and loosely bound electrons orient themselves with the electromagnetic field.

Because the field of electromagnetic energy changes polarity twice in every cycle, the respective orientation of the responsive electrons likewise changes twice in every cycle. For example, for a generator producing electromagnetic radiation with a frequency of 5 megahertz, these respective orientations will change 10 million times every second. The work expelled in orienting the free electrons or the friction from movement of excited electrons generates the desired heat on the molecular level.

The voltage required to maintain a field of appropriate strength to provide uniform dissipation of energy is controlled by the power source size, the tuning of the load to the generator, and the distance between the electrodes. Typically, the electromagnetic radiation at radio frequencies is generated at power levels of between about 1 and 30 kilowatts. The invention, however, is not limited to use at such power levels and is applicable at power levels of 100 kilowatts and up, depending upon particular equipment and applications. Because lower frequencies provide longer wavelengths, the method can also include the step of attaching the workpiece to the electrode, or the further step of connecting the circuit directly to the workpiece so that the workpiece itself acts as an electrode in applications where conductive materials are being glued. As a additional or alternative step, both the workpiece and the surface to which it is to be joined can be connected to the surface and act as electrodes with the adhesive therebetween being the dielectric material. As a general rule, the electrodes represent the plates or other formed shapes of the capacitor of a radio frequency heating circuit between which the alternating electric field is created and the dielectric material is placed. Those familiar with and of ordinary skill in the technology will recognize that electrode arrangements can comprise those known as "stray field," "staggered field," or "parallel fixed plate" arrangements of electrodes. Among other advantages of the invention, the use of nonaqueous adhesives can significantly reduce the required curing time in specific processes. For example, typical curing times for nonaqueous adhesives using conventional external heat is on the order of three to ten minutes. As known to those familiar with adhesive techniques, a workpiece must usually be maintained in a fixed position (e.g. clamped) for the entire curing period, a requirement that raises additional processing and engineering problems. In contrast, the method of the present invention provides curing times on the order of five seconds—and sometimes even less—to two minutes, thereby greatly reducing the associated engineering problems. This advantage is particularly noteworthy in applications such as robotic handling or adhering techniques, for which the method of the invention is particularly suitable.

In the specification there have been disclosed typical preferred embodiments of the invention, and although specific terms have been employed, they have been used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims

THAT WHICH IS CLAIMED IS: 1. A method of dielectric heating of nonaqueous thermosetting adhesives that are generally transparent to electromagnetic radiation in the radio frequency range, the method comprising: applying to a surface to be bonded a nonaqueous thermosetting adhesive that is otherwise transparent to electromagnetic radiation in the radio frequency range; and exposing the adhesive to a predetermined frequency of electromagnetic radiation within the radio frequency range; and characterized in that the adhesive contains a radio frequency sensitizing composition in an amount effective to respond to a predetermined frequency of electromagnetic radiation, and wherein the sensitizing composition modifies the characteristics of the adhesive until the adhesive becomes responsive to the predetermined frequency of electromagnetic radiation so that the electromagnetic radiation heats the otherwise transparent adhesive.

2. A method according to Claim 1 wherein the step of applying the adhesive comprises applying an adhesive which contains the radio frequency sensitizing composition in an amount sufficient to modify the loss factor of the adhesive until the modified adhesive responds to the predetermined frequency of electromagnetic radiation.

3. A method according to Claim 1 wherein the sensitizing composition is highly reactive to the predetermined frequency to which the adhesive is exposed.

4. A method according to Claim 1 wherein the step of exposing the adhesive to be cured to electromagnetic radiation having a predetermined frequency in the radio frequency range comprises exposing the adhesive to frequencies of between about 1 and 100 megahertz.

5. A method according to Claim 1 wherein the step of exposing the adhesive to be cured to electromagnetic radiation having a predetermined frequency comprises exposing the adhesive to frequencies of between about 3 and 30 megahertz.

6. A method according to Claim 1 wherein the adhesive is selected from the group consisting of epoxy adhesives and urethane adhesives.

7. A method according to Claim 1 wherein the amount of sensitizing composition included in the adhesive is between about 4 and 8 percent by weight based on the weight of the adhesive.

8. A method according to Claim 1 wherein the sensitizing composition is selected from the group consisting of: alkali earth metal aluminosilicates, alkaline earth metal aluminosilicates, aluminum chloride, ammonium chloride, zinc oxide, refined carbons and bentonite clay.

9. A method according to Claim 1 wherein the step of applying the adhesive to a surface to be bonded comprises applying the adhesive to at least one of a workpiece or a surface to which the workpiece is to be attached; and further comprising positioning the workpiece and the surface adjacent an electrode that is part of a dielectric capacitive circuit that includes a generator for producing radio frequency electromagnetic radiation output from the electrode.

10. A method according to Claim 9 wherein the step of positioning the workpiece and the surface adjacent an electrode comprises attaching the workpiece to the electrode.

11. A method according to Claim 9 wherein the workpiece comprises a material suitable for generation of electromagnetic radiation therefrom and wherein the circuit is connected to the workpiece so that the workpiece acts as an electrode in the circuit.

12. A method according to Claim 9 wherein the workpiece and the surface are both formed of materials that are suitable for generation of electromagnetic radiation therefrom and wherein the circuit is connected to the workpiece and the surface so that the workpiece and the surface acts as electrodes in the circuit.

13. A nonaqueous thermosetting adhesive composition that is heat curable using dielectric heating at low frequencies of electromagnetic radiation within the radio frequency range at which nonaqueous thermosetting adhesives are generally transparent, the composition comprising: a nonaqueous thermosetting adhesive that is otherwise transparent to low frequency electromagnetic radiation within the radio frequency range; and a radio frequency sensitizing composition present in said adhesive in an amount effective to respond to low frequency electromagnetic radiation within the radio frequency range and to modify the characteristics of the adhesive until the adhesive becomes responsive to the low frequency electromagnetic radiation so that low frequency electromagnetic radiation can be used to heat and cure the otherwise insufficiently reactive adhesive.

14. An adhesive composition according to Claim 13 wherein the adhesive is selected from the group consisting of epoxy adhesives and urethane adhesives.

15. An adhesive composition according to Claim 13 wherein the amount of sensitizing composition included in the adhesive is between about 4 and 8 percent by weight based on the weight of the adhesive.

16. An adhesive composition according to Claim 13 wherein the sensitizing composition is selected from the group consisting of: alkali earth metal aluminosilicates, alkaline earth metal aluminosilicates, aluminum chloride, ammonium chloride, zinc oxide, refined carbons and bentonite clay.