Catalyst – solid sorbent – or support therefor: product or process – Irradiation by – or application of – electrical – magnetic or...
Reexamination Certificate
1999-12-27
2002-04-09
Berman, Susan W. (Department: 1711)
Catalyst, solid sorbent, or support therefor: product or process
Irradiation by, or application of, electrical, magnetic or...
C502S202000, C502S216000, C502S232000, C502S300000, C522S001000, C522S024000, C522S029000, C522S030000, C522S060000, C522S066000, C522S068000, C522S071000, C522S074000, C522S077000, C522S079000, C522S081000, C522S083000, C522S107000, C522S182000
Reexamination Certificate
active
06368994
ABSTRACT:
BACKGROUND OF THE INVENTION
There are two main delays encountered in the polymerization or cross-linking (curing) of organic materials. First, the conduction of heat in these materials is slow because most organic materials including organic monomers, oligomers and polymers exhibit low thermal conductivity. As a result, dozens of minutes are needed for the volumetric heating of an organic material thickness of even a few inches. Second, the rate of the polymerization and cross-linking reactions cannot be increased by simply increasing the temperature of the organic material because the temperature of the composition must be kept relatively low to avoid burning of the material from the heat which is liberated by the chemical reaction.
The most common methods for polymerization or curing of organic compositions employ chemical agents such as polymerization initiators or curing agents, which induce polymerization of monomers and curing of polymers. These agents are also capable of inducing the combined polymerization and curing of multi-functional monomers such as trimethylolpropane triacrylate.
In practice, very rapid polymerization or curing of organic materials can be achieved even at room temperature by employing “super-active” resins and/or polymerization agents. However, such “super-active” materials cannot be used in industrial processing because the pot lifes of the “super-active” compositions (e.g. cyanoacrylate-based glues) are extremely short resulting in a complete cure of the composition at room temperature before the material can be processed.
For industrial processing, pot life problems are typically avoided by using organic compositions in which the temperature at which polymerization or curing occurs is above room temperature, e.g. 60° C. However, polymerization and/or curing of these organic materials is typically slow because of the low thermal conductivities of these organic compositions. Consequently, as set forth above, polymerizing or curing anything but a thin layer of the organic material will require a prohibitively long processing time.
In principle, the polymerization or curing of the organic material can be accelerated by increasing the temperature at which the material is processed. However, since the polymerization or curing reaction itself will liberate heat as the reaction proceeds, this additional heat must be factored in when determining the maximum temperature to be used to accelerate the polymerization and/or curing process in order to avoid heating the system to the decomposition temperature of the organic composition. For example, if the decomposition temperature of the organic material is 140° C. and the heat generated by the polymerization or curing reaction will raise the temperature of the organic material by 50° C., the organic composition cannot be heated to greater than 90° C. to accelerate the rate of reaction. Exceeding the decomposition temperature of the organic composition is deleterious because it can lead to local or more general “hot spots” causing partial or full decomposition of the mixture. Even partial decomposition of the starting composition is undesirable because degradation products will affect the quality of the final product.
One approach to the accelerated polymerization or curing of a polymerizable or curable composition without excessive heating of the organic composition utilizes so-called cold-curing methodologies such as electron-beam (U.S. Pat. No. 4,100,311), X-ray (U.S. Pat. Nos. 5,863,963; 5,911,940), ultra-violet (U.S. Pat. No. RE29,590, U.S. Pat. No. 4,690,957), visible light (U.S. Pat. No. 5,942,559) irradiation and the like in which the radiation source induces polymerization or curing of the organic composition by reacting with the organic material to be processed to initiate a chemical reaction (e.g., direct cleavage of a C—C double bond) without significantly heating the material. However, radiation induced rapid polymerization, curing or a combination thereof require a relatively long time to effect the desired chemical transformation and can be applied only to limited organic materials and thicknesses thereof (i.e., thin layers). In addition, radiation techniques for rapid polymerization, curing or a combination thereof, e.g., X-ray and electron beam irradiation, require the use of expensive, complex and hazardous equipment.
Microwave energy has been investigated in a variety of manufacturing operations as an alternate to traditional heating methods to reduce the polymerization time of monomers and the curing time of polymers or a combination thereof. Microwave processing of monomeric and polymeric compositions is believed to be advantageous. First of all, it allows volumetric or whole-volume heating of most organic materials. Secondly, volumetric deposition of microwave energy is more efficient than the conduction from the surface achieved with conventional heating, i.e. infrared irradiation techniques. See, for example, Lauf et al., “Materials Processing Using A Variable Frequency Microwave Furnace”, the American Ceramic Society, April 1993, pp. 571-79. See also, U.S. Pat. No. 5,296,271 to Swirbel et al., which proposes a method of curing photoreactive polymers by exposing them to microwave energy.
The efficiency of using microwave processing methods in an organic composition can be improved by increasing the absorption properties of the composition to microwave radiation. These microwave absorption methodologies have variously utilized admixing (adding) ferromagnetic materials (U.S. Pat. Nos. 5,391,595; 5,317,045) or electrically conductive materials (U.S. Pat. No. 4,626,642) into the composition to increase the efficiency of heating the organic composition or have selected an optimal frequency where polymer absorption properties are highest (U.S. Pat. No. 4,011,197). These methods reduce the curing time due to volumetric fast heating despite the low thermal conductivity of most polymer compositions (U.S. Pat. Nos. 5,396,249; 5,879,756). All of these patents are hereby incorporated by reference in their entirety. Nevertheless, despite the advantages of these methodologies, short volumetric reaction times (seconds) were still not achievable because in these methods, all parts of the composition are heated to the same temperature and therefore there is a practical limit to the polymerization agent temperature as noted above.
Consequently, achieving very short curing times throughout the organic material to be processed while avoiding organic degradation remains a significant challenge in the polymer processing industry. Clearly, there exists a need for a treatment method which can ensure high speed production of cured, thermosetting polymer based items. A need also exists for the rapid polymerization of thermally sensitive monomers to produce thermoplastic materials (which may optionally be further converted to thermosetting materials) without significant thermal degradation of the starting monomer or the partially polymerized composition.
Finally, a need exists for a high-speed curing or polymerization process which allows the manufacturing cost of polymer-based products to be competitive with that of metallic products. The main advantages of high speed polymerization and curing of organic materials are increased production rate and reduced manufacturing costs. Many other specific advantages also exist including but not limited to the elimination of huge furnaces in plastic, prepregs and composite production, shorter die lengths for pultrusion, the possibility of layer-by-layer curing in winding and the use of high temperature curing agents that increase shelf and pot life.
SUMMARY OF THE INVENTION
This invention relates to a method for rapid polymerization, curing or a combination thereof of a polymerizable or curable composition to yield polymers and composites based on these polymers through the utilization of short wavelength microwave energy. This invention also relates to specially prepared particulate polymerization curing materials which, when dispersed and irradiated in a polymerizable
Berman Susan W.
Gyrorron Technology, Inc.
Morgan & Finnegan , LLP
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