High-frequency curing

In high-frequency curing the workpieces are arranged between two capacitor plates in a radiofrequency field (10 -10 Hz). Molecules (dipoles) align themselves and are polarized in the alternating electric field. They therefore oscillate about their equilibrium position, resulting in heating. 

They are placed between parts to be bonded, they melt during the oven cycle and bond the parts together. They may also act as a potting and encapsulating product. The epoxy formulation may also contain conductive particles in order to allow cure by induction or high frequency. Curing conditions are those of the one-component epoxy adhesives, i.e. from 15 to 60 min at temperatures ranging from 120 to 160°C. 

To speed up the molding process, the required amount of molding powder or granules is often pressed iato a block and prewarmed before placing it ia the mold. Rapid and uniform heating is accompHshed ia a high frequency preheater essentially an iadustrial microwave oven. The prewarmed block is then transferred to the hot mold, pressed iato shape, and cured. 

Preheating techniques are commonly employed since these lead to shorter cures, easier flow and generally better products. The high power factor of the material enables high-frequency preheaters to be used successfully. It is also frequently advantageous to pellet the powders as in the case of phenolics. 

The peaks in s j dipolar (which are usually close to the peaks in e") can be used to determine the time or point in the cure process when the mean dipolar relaxation time has attained a specific value, i = 1/co, where to = 2nf is the frequency of measurement. The dipolar mobility as measured by the mean relaxation time t can be used as a molecular probe of the buildup in Tg. The time of occurrence of a given dipolar relaxation time as measured by a peak in a particular high frequency value of e"(co) can be quantitatively related to the attainment of a specific value of the resin s glass transition temperature. 

Some polysiloxanes are curable with lead monoxide, with a consequent reduction in both curing time and temperature. High-frequency electrical energy vulcanizes in one case at least. Zirconium naphthenate imparts improved resistance to high temperatures. Barium salts are said to prevent blooming. Sulfur dichloride is also used. Some resins are solidified by pressure vulcanization, using di-f-butyl peroxide. Improvements are to be found in lower condensation temperatures and shorter times of treatment... 

Lumber banding consists of gluing lumber strips, 1/2 to 2 inches in width, on the particleboard edges. These strips are normally used in applications where the particleboard is to be covered with wood veneers. The solid wood strip can be machined to decorative edges and, with the veneer surfaces, the panel is fully as functional and attractive as a solid wood panel, but at a lower cost. The lumber bands are normally bonded to the particleboard with polyvinyl acetate or urea-formaldehyde adhesives, cured rapidly by either contact or high frequency heating. 

Use (Unstabilized) Production of piezoelectric crystals, high-frequency induction coils, colored ceramic glazes, special glasses, source of zirconium metal, heat-resistant fibers, (hydrous) odor absorbent, to cure dermatitis caused by poison ivy. (Stabilized with CaO refractory furnace linings, crucibles, solid electrolyte for batteries operating at high temperature. 

Computer controlled systems have been developed that enable investigation of the spatial distribution of impedance within resins and in turn the identification of flaws (62). Water trapped within the composite will cause dielectric permittivity to increase, thus revealing its presence. Detection of voids, variation in the cross section of the adhesive layer, ingress of moisture into a joint structure, and characterization of cure in epoxy resin can be achieved with high frequency dielectric spectroscopy (10" -10 Hz) (63). 

Curing by means of electrical processes (inductive curing, resistance, high-frequency and microwave curing)... 

---