Harry S. Ku

Productivity improvement through the use of industrial microwave technologies

Microwave processing of materials is a relatively new technology advancement alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. This paper presents a state-of-the-art review of microwave technologies, processing methods and industrial applications. The characteristics of microwave interactions with materials are outlined together with the challenges that are difficult to process the materials present. To fully realise the potential benefits of microwave and hybrid processes, it is essential to scale-up process and system designs to large batch or continuous processes. This necessitates computational modelling and simulation, system design and integration and a critical assessment of the costs and benefit analysis. Impediments to industrial applications are identified and development opportunities that take advantage of unique performance characteristics of microwaves are discussed. Clearly, advantages in utilising microwave technologies for processing materials include penetrating radiation, controlled electric field distribution and selective and volumetric heating.The aim of the work presented in this paper is to help guide those interested in using microwaves to improve current materials processing. Microwave fundamentals are described to provide a brief awareness of the advantages and limitations of microwaves in the processing of materials. Furthermore, the limitations in current understanding are included as a guide for potential users and for future research and development activities. Examples of successful applications are given to illustrate the characteristics of materials, equipment and processing methods applicable to industrial microwaves. Economic considerations are described and costs are provided as guidelines in determining the viability of using microwaves for processing materials.

Microwave Facilities for Welding Thermoplastic Composites and Preliminary Results

The wide range of applications of microwave technology in manufacturing industries has been well documented (NRC, 1994; Thuery, 1992). In this paper, a new way of joining fibre reinforced thermoplastic composites with or without primers is presented. The microwave facility used is also discussed. The effect of power input and cycle time on the heat affected zone (HAZ) is detailed together with the underlying principles of test piece material interactions with the electromagnetic field. The process of autogenous joining of 33% by weight of random glass fibre reinforced Nylon 66, polystyrene (PS) and low density polyethylene (LDPE) as well as 23.3% by weight of carbon fibre reinforced PS thermoplastic composites is discussed together with developments using filler materials, or primers in the heterogenous joining mode. The weldability dependence on the dielectric loss tangent of these materials at elevated temperatures is also described.

Permittivity Measurement of Thermoplastic Composites at Elevated Temperature

The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity ε = ε’-jε”, and the loss tangent, tan 8= This paper describes two convenient laboratory based methods to obtain e’, e” and hence tan 3 of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyzer in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GE (33%), and Nylon 66/GE (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe.

Variable frequency microwave processing of thermoplastic composites

Abstract The range of applications for variable frequency microwave (VFM) facilities (2–18 GHz) has been extended to thermoplastic composites. Five thermoplastic polymer matrix composites are processed and discussed, including 33 wt-% random carbon fibre reinforced polystyrene [PS–CF (33%)], and low density polyethylene [LDPE–CF (33%)]; 33 wt-% random glass fibre reinforced polystyrene [PS–GF (33%)], low density polyethylene [LDPE–GF (33%)]and Nylon 66 [Nylon 66–GF (33%)]. Bond strengths of lap joints were tested in shear and results were compared with those obtained using fixed frequency (2·45 GHz) microwave processing. The primer or coupling agent used was a 5 min, two part adhesive containing 100%liquid epoxy and 8% amine, which was more readily microwave reactive than the composites themselves. The VFM was operated under software control, which provided automatic data logging facilities. Results indicate that VFM can produce strong bonds for PS and LDPE.

Microwave processing and permittivity measurement of thermoplastic composites at elevated temperature

[Abstract]: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity e = e′- je″, and the loss tangent, tan δ = e″/ e′. The real part of the permittivity, e′, sometimes called the dielectric constant, mostly determines how much of the incident energy is reflected at the air-sample interface, and how much enters the sample. This paper shows the reflection coefficient of a material and the depth of penetration of a dielectric. Therefore the larger the value of the real part of the complex permittivity, the more the incident energy will be reflected by a dielectric but the energy that enters the material will penetrate further than in a dielectric with the same e″ but lower e′. However, the most important property in microwave processing is the dielectric loss, e″ which predicts the ability of the material to convert the penetrating energy into heat. Measurements of e′ and e″ are therefore critical in the microwave processing of materials with or without primer. The dielectric constant, e′, dielectric loss, e″, and hence complex relative permittivity, e and loss tangent, tan δ, of some commonly used thermoplastics have been measured at various temperatures and frequencies. These results may be used to determine whether various fibre-reinforced thermoplastic (FRTP) composites are suitable for microwave processing. This paper describes a convenient laboratory based method to obtain e′, e″ and hence tan δ. The method employs a network analyser together with a waveguide transmission technique chosen because it provides the widest possible frequency range with high accuracy; the hardware and software of the method is also readily available in the electronic laboratory of the University of Southern Queensland. The required data were collected at a range of elevated temperatures and over a band of frequencies.

Characterisation of thermoplastic matrix composites using variable frequency microwave

Abstract In most industrial microwave processing operations, the frequency of the microwave energy launched into the waveguide or cavity containing the sample is fixed. This brings with it inherent heating uniformity problems. This paper describes a new technique for microwave processing, known as variable frequency microwave (VFM) processing, which alleviates the problems brought about by fixed frequency microwave processing. In VFM processing, microwave energy over a range of frequencies is transmitted into the cavity in a short time, e.g. 20 μs. It is therefore necessary to determine the best frequency range for processing a material. The best range frequency for microwave processing of five different thermoplastic matrix composites using the VFM facilities has been determined. The optimum frequency band for microwave processing of these five materials was in the range 8–12 GHz. This data enables bonding of the materials using microwave energy under the most favourable conditions.

Risks Involved in Curing Vinylester Resins Using Microwave Irradiation

Preliminary studies have been carried out to cure vinylester particle reinforced resins in microwaves to reduce shrinkage of the composites. The results were encouraging. With an exposure time of 35 to 40 s and a power level of 180 W, the shrinkage of 50- and 200-ml composite samples, flyash particulate–reinforced vinylester resin, approached 0%. Despite the success, there are risks in the process of curing the vinylester resins by microwave irradiation. The styrene vapor emitted from the resins is harmful to humans and becomes an inhalation hazard. In addition, the styrene vapor in the cavity of the microwave oven may be heated by the high-voltage transformer around the oven. This may result in flashing. Even if this does not happen, the high concentration of the styrene vapor in the oven cavity may lead to an explosion. Another risk is posed by the hardening agent, methyl ethyl ketone peroxide (MEKP). When interacted with microwaves, with the resulting exothermic reaction, the MEKP could spontaneously ignite. When the usual rate of 1% to 2% of it is used in hardening the resin, however, most of its dangerous properties disappear (John R. Sweet Co. http://www.johnsweet.com undated). MEKP itself is poisonous and must be handled with care.

Young's Modulus of Vinyl Ester Composites Cured by Microwave Irradiation: Preliminary Results

The shrinkage of vinyl ester particulate composites has been reduced by curing the resins under microwave conditions. The reduction in the shrinkage of the resins by microwaves will enable the manufacture of large vinyl ester composite items possible [1–4]. The difference in impact strength between microwave cured vinyl ester particulate composites and those cured under ambient conditions had been investigated [5]. In addition, a previous study found that the difference in fracture toughness obtained by short bar method between selected microwave-condition cured vinyl ester particulate composites and those cured under ambient conditions was only 0.5% [6]. This project is to investigate the difference in Youngs modulus, ultimate tensile strength and yield strength between microwave cured vinyl ester particulate composites and those cured under ambient conditions. The results show that the difference in the Youngs modulus is minimal.

Application of Variable Frequency Microwave (VFM) to Adhesives

Microwave processing of adhesives is a relatively new technology alternative that provides new approaches for enhancing material properties as well as economic advantages through energy savings and accelerated product development. Alternative in the sense that most adhesives are normally cured in ambient conditions or in ovens. However, the most commonly used facilities for microwave processing of materials operate on fixed frequency microwaves (FFM), e.g., 2.45 GHz. This paper presents a review of microwave technologies, processing methods and industrial applications, using variable frequency microwave (VFM) facilities. The technique offers rapid, uniform and selective heating over a large volume at a high energy coupling efficiency. This is accomplished using a preselected bandwidth sweeping around a central frequency by employing tunable frequency sources. Successful applications of these modern facilities include finding out the optimum cavity conditions of glass or carbon fibre reinforced thermoplastic matrix composites, and of adhesives, e.g., two-part five-minute Araldite, and the joining of the above-mentioned composite materials with, or without, primers. Finding out the optimum cavity conditions of a material has helped identify the best frequency range to process the material using microwave energy and by means of the VFM facility. Microwave energy has been used to rapidly cure several types of two-part epoxy based adhesives, e.g., Araldite. Bond strengths obtained using variable frequency microwave (VFM) techniques are compared with adhesive joints cured in fixed frequency microwave (FFM) conditions.

Lap Shear Strength Comparison Between Two Types of Random Carbon Fiber-Reinforced Thermoplastic Matrix Composites Bonded Using Variable-Frequency Microwaves (VFM) Irradiation

Fiber-reinforced thermoplastic matrix composite material have enjoyed very strong development in the last 10–12 years because of their potential advantages and unique characteristics that cannot be found in their thermoset counterparts. This paper compares the lap shear bond strengths of two types of random carbon fiber-reinforced thermoplastic matrix composites joined by microwave energy. Variable-frequency microwave (VFM) (2–18 GHz) facilities are used to join thirty 3% by weight random carbon fiber-reinforced low-density polyethylene [LDPE/CF (33%)] and thirty 3% by weight random carbon fiber-reinforced polystyrene [PS/GF (33%)]. With a given power level, the composites were exposed to various exposure times to microwave irradiation. The lap shear strengths of the joints were compared with those obtained using fixed-frequency (2.45 GHz) microwave facility configuration. The VFMF was operated under software control, which provided automatic data logging facilities.