1.  Conventional Microwave Processing of Plastics, Rubbers and Resins

     Microwave power irradiation is known as an incredibly rapid and selective, clean and safe, versatile and energetically-convenient heating method, gaining increasing favor in 1980s and 1990s for industrial processing of numerous materials:  rubbers, ceramics, textiles, minerals, wood, adhesives, some plastics and thermosetting resins, etc.  In the field of polymeric materials, in particular, microwave heating has shown a great potential, still far from being extensively used as it could, for accelerated industrial processing of the entire variety of commercial (both commodity and high-performance) plastics, rubbers, thermosetting resins, and related composites.  Actually, indeed, the continuous sulfur-vulcanization of extruded carbon black-filled natural and synthetic rubber compounds is the sole important application industrially established in this sector.

     Microwave power irradiation, as a mere heating method to promote either chemical reactions - such as vulcanization processes - or quick physical melting, is directly applicable to the rather narrow range of adequately microwave adsorbing polymers, i.e. to a few markedly dipolar ones, such as, polychloroprene, nitrile rubbers, plasticized PVC, acrylic rubbers and polyoxymethylene, etc.

     By contrast, many among the most important rubbers (natural rubber, butyl rubber, polybutadiene, synthetic polyisoprenes, EP and EPDM rubbers), plastics (polyethylene, polypropylene, and their copolymers and blends) and resins (aliphatic and cycloaliphatic epoxies, hydrocarbon resins, etc.) are intrinsically transparent to microwaves, or just weak absorbers of this radiation, such as polystyrene and butadiene-styrene elastomers.  Such materials become microwave-processable only by addition of considerable amounts (typically 10-50 % by weight) of physical microwave adsorbers, i.e. mineral fillers or unreactive organic additives either possessing an inherent and strong capacity for electromagnetic energy dissipation at microwave frequencies, or endowing the polymer or resin mixtures with it through a variety, and often complex combinations, of physical effects.

     Suitable physical microwave absorbers for this purpose are:  electrically conductive fillers (primarily carbon black, extensively employed in rubber compounds, graphite, coke, and metal powders such as those of aluminum, copper, zinc and brass);  certain surface-treated silica powders;  ferro- and ferri-magnetic powders (such as iron, γ-iron trioxide, chromium dioxide, and many ferrites); ferroelectric ceramic powders (e.g., barium and lead titanates and zirconates);  highly dipolar organic liquids (aromatic sulfonamides, alkylphthalates, alkanolamine carboxylates, polyoxyethylene-glycols and related ethers and esters, etc.).  On the other hand, the use of the above microwave absorbers implies an entire variety of severe drawbacks in terms of either materials processability or end-product performance:

• difficult and/or time-consuming operations for the thorough dispersion of mineral fillers (particularly, in highly viscous softened rubbers and molten polymers), potentially and very often leaving inhomogeneities (quality defects per se, and responsible for localized microwave overheating phenomena, as well);

• rapid settling of heavy mineral fillers and metal powders in liquid resins;

• remarkable viscosity increases in both molten polymers and liquid resins;

• loss of transparency and often undesired, strong pigmentation effects caused by most mineral fillers;

• undesired plasticization (or over-plasticization) effects, and/or induction or side phenomena such as surface migration of the additives themselves, surface tacking, sensitivity to UV radiation, worsened thermal resistance, increased moisture absorption, etc., associated with the introduction of many organic microwave absorbers.

The Novel Approach

     The use of microwave power irradiation proved to accelerate chemical reactions by a minimum ×2 ÷ ×4 factor in many thermosetting resin hardening processes.  Appreciable reaction rate enhancements, however, could only be achieved with those per se sufficiently dipolar (and thus inherently microwave-absorbing) resins, such as aromatic epoxy-amine systems, imide-, isocyanate- and cyanate ester-functional resins.  In slightly dipolar ones, the addition of conventional physical microwave absorbers is the sole means to enable their microwave heatability, but gives no or negligible advantages in terms of reaction rate enhancements.

     The great potential offered by microwave irradiation for the acceleration of polymerization processes can be exploited at best by a revolutionary method recently disclosed.  This is based on microwave reaction promoters, whose specific and strong microwave sensitivity causes, under UHF power irradiation, sharp activation and vivid rate increases (of 4-10 times and over) in a great variety of chemical reactions with respect to processes carried out at the same temperatures without microwave irradiation, even in poorly microwave-absorbing systems.

     As compared to conventional, physical microwave absorbers, the novel activators are true microwave reaction catalysts or initiators, working via formation of extremely microwave-sensitive intermediate species which selectively transfer electromagnetic energy to the reactive sites.  Such activators are thus very selective, efficient at very low concentrations (from 0.2 to 1 %, and, typically, around 0.5 % by weight), and thermally latent (i.e., inactive or almost inactive) until microwave power is being applied.  Their action is largely independent of the heating effects of microwave irradiation itself, though higher temperatures can generally (but not necessarily) cause additional reaction rate gains.

     In this sense, these catalysts can be used together with the novel and proprietary microwave heating susceptors being specifically developed by fpchem.com for the efficient and fast UHF vulcanization of microwave-transparent white & colored rubber compounds (see Technical Note #2, this website).  Both chemically-inert and coreactive microwave sensitizers can be used.  This can combine the physical advantage of a much faster and much more energetically-convenient microwave heatability with the unparalleled accelerations of chemical reaction kinetics enabled by the present microwave catalysts.

     The novel microwave catalysts can be tailored to promote a great variety of chemical reactions:  for example, direct esterification and amidation, trans-esterification and trans-amidation, innumerable reactions of epoxides and isocyanates (such as those involved in epoxy resin hardening by amino, anhydride and carboxylic acid curatives, chemical modification of epoxy resins, crosslinking of epoxidized rubbers, in situ-generation of polyurethanes, etc.), siloxanation (formation and crosslinking of silicone elastomers), sulfur-vulcanization of unsaturated rubbers, and free-radical polymerizations as well (such as peroxide-vulcanization of rubbers, curing of unsaturated polyester resins, curing of allyl carbonate and allyl phthalate resins, and so on).

     Advantages offered by the new approach are therefore:

• the possibility of commanding, and exerting a precise, external control on, chemical reaction kinetics through microwave irradiation;

• outstandingly high reaction rates (never reached previously with the same chemical systems at the same temperatures);

• the possibility of standard reaction rates at considerably lower processing temperatures;

• no undesired pre-reactions during the stage of high-temperature melt-compounding of plastics or rubbers with fillers, pigments, additives, etc.;

• a unique combination of high curing and post-curing rates of thermosetting resins with prolonged pot-life, even upon conventional warming.

     Potential application of the novel method cover a broad range of technological sectors, particularly for continuous manufacturing of vulcanized rubber extrudates, crosslinked plastics and thermoset composite articles, where high throughput rates and fairly good process control/quality consistency are striking issues:

• extruded rubber profiles;

• extruded rubber and plastic tubings, hoses and pipes;

• coated electrical wires, cables and cords;

• extruded foamed polymers (sheets, panels and profiles);

• pultruded fiber-reinforced thermosets (bars, beams and profiles).

3.  References

• R.A. Abramovitch, "Application of Microwave Energy in Organic Chemistry.  A Review", Org. Prep. Proc. Int., 23, 683 (1991).

• F. Parodi, R. Gerbelli and M.T. DeMeuse - "Microwave Polymerizable Isocyanate-Epoxy Compositions for Heavy-Duty Applications", U.S. Pat. 5,489,664 (Feb. 6, 1996);  Eur. Pat. 655,472 B1 (Jul. 29, 1998).

• M.T. DeMeuse, F. Parodi, R. Gerbelli and A.C. Johnson, "Microwave Processing of Isocyanate/Epoxy Composites", 39th Int. SAMPE Symp. (Anaheim, CA, Apr. 11-14, 1994), conf. proc., vol. 1, p. 13.

• F. Parodi, R. Gerbelli and M.T. DeMeuse - "Liquid Reactive Thermosetting Compositions and Process for their Cross-linking", U.S. Pat. 5,650,477 (Jul. 22, 1997);  Eur. Pat. 720,995 B1 (Oct. 7, 1998).

• "Applications of Microwave Heating to the Chemicals Industry", The R&D Clearing House, London, 1995.

• J.B. Wei, T. Shidaker and M.C. Hawley, "Recent Progress in Microwave Processing of Polymers and Composites", Trends Polym. Sci., 4, 18 (1996).

• F. Parodi, "Physics and Chemistry of Microwave Processing", in Comprehensive Polymer Science, 2nd suppl. , Pergamon (Elsevier), Oxford, 1996, chapter 19.

• H.M. Kingston and S.J. Haswell (eds.), Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation and Applications, Am. Chem. Soc., Washington DC, 1997.

• D.M.P. Mingos and A.G. Whittaker, "Microwave Dielectric Heating Effects in Chemical Synthesis", in Chemistry under Extreme or Non-Classical Conditions, Wiley-Spektrum, New York, 1997, chapter 11.

• R. Dagani, "Molecular Magic with Microwaves", Chem. Eng. News, 75(6), 26 (1997).

• F. Parodi, "Microwave Heating: Polymerization Processes and Organic Syntheses", Chim. Ind. (Milan), 80(1), 55 (1998).

• F. Parodi - "Microwave Heating and the Acceleration of Polymerization Processes", in "Polymers and Liquid Crystals", Proceedings of SPIE - The International Society for Optical Engineering, 4017, 2 (1999).

• F. Parodi, "Microwaves in Chemical Syntheses", Chim. Ind. (Milan), 81(10), 1271 (1999) (in Italian).

• D. Bogdal, P. Penczek, J. Pielichowski and A. Prociak, "Microwave Assisted Synthesis, Crosslinking and Processing of Polymeric Materials", Adv. Polym. Sci., 163, 193 (2003).

• B.L. Hayes, "Recent Advances in Microwave-Assisted Synthesis", Aldrichimica Acta, 37(2), 66 (2004).