Microwave Heating Applied to Polymers
© G. Whittaker, 1994 & 2007. This work, or extracts from this work may be reproduced only with the written permission of the author.
Alongside ceramic processing, polymer chemistry forms probably the largest single discipline in microwave chemistry, and the methods used are certainly among the most developed. Polar starting materials and very often products, allow rapid and controllable syntheses, the dielectric properties themselves being an excellent indicator of reaction progress. The ability to control syntheses with high accuracy and with direct heating of the reactants has the advantage of large potential savings in energy. Economic analyses suggest that the costs of curing polymers may be reduced from 4-11MJKg-1 to 0.3-0.5 MJKg-1 by switching to the use of microwaves.53
Epoxy resins have been most widely investigated primarily because of their industrial importance, and because their dielectric properties predispose them to effective microwave induced curing.54 Surprisingly, investigations have indicated that the rate of curing depends less upon the total power which is applied to the sample than it does on the way in which microwave pulses are applied. Investigations into the curing of the diglycidyl ester of bisphenol A (DGEBA) with a number of cross-linking agents - 4,4' diaminophenolsulphone (DDS), Dicyandiamide (DDA), 4,4' diaminodiphenylether (DDE), diaminodiphenylethane (DDM),55, 56 - have been carried out and indicate that the curing rate is not directly related to the sample temperature either. With DDS, short high power pulses with a low time-averaged power (2x10 -3sec, average power 700Watts) were found to give comparable results to those of longer pulses, with high time-averaged power (2x10-2 sec, 1500Watts). Moreover, the pulse induces causes selectivity in bond formation - the reaction with the amines being favoured by longer pulses, whilst self-polymerisation is preferred under short pulse irradiation. The optimum pulse frequency for cross linking is dependent upon the cross linking agent, and for example with DDM57-59 optimum repetition frequencies of 23.8, 200, and 1000Hz have been identified. Although no specific relationship is proposed, it has been suggested that these frequencies may correspond to particular polar components (hydroxy-, amine-, epoxy- groups, etc) in the molecules.
The synthesis of polyamides60 and polyurethane films61 studied in a similar manner showed a similar relationship to the pulse period, suggesting that energy transfer is more efficient with the use of pulsed microwaves (optimum pulse frequency 13.3-14.3Hz) than by continuous power. Once again, it is concluded that this behaviour is characteristic of a relaxation mechanism which is derived from the movement of chain segments in the macromolecular network.61 Moreover, the hardness of the polyurethane films (Persoz method, given as the damping time for a pendulum oscillation) is generally harder (370-380s) when formed under these conditions than when formed thermally (340-360s). Under optimum microwave curing, the hardness may be as high as 400s.
The intrinsic dielectric properties of a substance may be enhanced or reduced by the incorporation of a secondary material.62 The effect of secondary materials upon dielectric or related properties in polymers have been extensively studied using copper,63 aluminium,64 carbon black or fibres,65 or other conducting materials.66 Carbon fibres have been studied as co-absorbent materials in the microwave curing of epoxy resins to form composite materials.67 Heating arises steadily at the carbon fibre particles, leading to temperatures some 75°C higher than that of the bulk sample, and leads to increased and stress free interface adhesion between fibre and matrix.68 The specimens cured in this way showed increased interfacial shear strength (14.15 Ksi) than are available by thermal curing techniques (8.3Ksi).
Inherently conducting polymers such as polyaniline p-toluene sulphonate ([sigma]=9.0+/-1.0 S cm-1) or polypyrrole p-toluene sulphonate ([sigma]=22.0+/-2.0 S cm-1) are excellent microwave absorbers, and make ideal materials with which to effect welding of plastics.69 Their inherently organic nature helps to form good bonds between the welded thermoplastics as diffusion occurs into their surfaces, and enables joins with high lap shear strengths (19.0+/-2 Nmm-1) to be produced in as little as 2-120 seconds.
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