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Microwave Emerging as Alternative Energy Source for Rapid Adhesive Cure

Posted on 5/4/2018 9:33:49 AM By Deb Bhattacharjee
  

There has been ever increasing demand on rapid and effective cure of adhesives while maintaining the pot life in industrial processes.  In addition to the conventional heat cure, UV radiation has emerged as an acceptable alternative for improving the productivity, though it is only possible when at least one of the substrates is transparent to light.  The cure of opaque, filled or thick cross-section is problematic using UV triggered system.  The converters, in collaboration with adhesive manufacturers, have been investigating alternate energy sources for rapid adhesive cure without impacting processability and ultimate bond strength.  Microwave has emerged as a cost effective, scalable, commercially viable process over the last few decades.

The FDA approved limit of microwave energy that can emit from commercial ovens is far below the harmful people, and several safety features have already been incorporated with no harmful greenhouse gas emissions from the heat source.  Microwave energy in the 0.3 GHz and 3.0 GHz frequency range (industrial/commercial microwave systems typically operate at 900 MHz or at the wavelength of ~32.7 cm)allows more efficient penetration of the microwave through the material preferably for non-metallic or partial metallic bonding. Energy is transferred through the material electro-magnetically, not as a thermal heat flux. Therefore, the rate of heating is not limited and the uniformity of heat distribution is greatly improved.  Electromagnetic radiation is converted to heat energy in the overall adhesive mass very quickly.  Advantages would include lower energy consumption, very rapid cure time, bonding to heat sensitive plastic substrates (WO 02/12405; https://patents.google.com/patent/US20070117877).  Adhesives with polar groups generally show a good absorption of microwave energy, and will heat within seconds independently of their thermal conductivity and across the whole area. Design and use of appropriate microwave probes allows a targeted irradiation of the bond line.

Use of microwave for curing adhesives has been well documented.  Blending electrically conductive fibers into a thermoset adhesive has been utilized to accelerate cure rates (US Patent 4626642; https://patents.google.com/patent/US4626642). Hellmann et al. proposed the use of electrically conductive materials (e.g., carbon fibers) to accelerate the heating-up rate by microwaves (US Patent 4906497; https://patents.google.com/patent/US4906497/en17). However, single frequency microwave is not always uniformly distributed throughout the microwave oven. Differential heating can result in heating only specific portions of the adhesive resulting in uneven curing. An alternative to single frequency microwave to address this problem of differential heating is to use variable frequency microwaves as is suggested in US Patent 5321222 (https://patents.google.com/patent/US5321222 ). The use of variable frequency microwaves, however, does not typically overcome susceptibility problems.

One component adhesives and sealants can be rapidly cured below the conventional thickening temperature by irradiating with microwaves, for example microwaves of at least 2 wavelengths. This process is particularly suitable for adhesively joining plastic substrates (US20070117877A1; https://patents.google.com/patent/US20070117877).

Materials having deformation temperatures lower than the activation temperature of the adhesive, such as described in US Patent 6497786 (https://patents.google.com/patent/US6497786 ), are useful in forming laminates for shoe soles, providing durable padding comprising a shock-absorbing padding material layer bonded to a wear resistant layer.

The combination of microwave with conventional heating could be an interesting combination where microwave would allow the reacting mixture to attain the cure temperature faster while the hot air would complete the curing process.