Rexolite 1422 Cross-Linked Polystyrene Plate

Rexolite 1422 is cross-linked polystyrene, and it exists for one reason: it has the most stable dielectric constant of any machinable plastic. 2.53, and it does not move - flat from 1 MHz all the way to 500 GHz. Dissipation factor is 0.00012 at 1 MHz, 0.00025 at 10 MHz and 0.00066 at 10 GHz. If you are building anything that has to pass microwave or millimeter-wave energy without distorting it, this is the material, and PTFE is the usual alternative at more than twice the weight.

Where it goes: antenna lenses and radomes, coaxial connector and cable insulators, microwave windows, waveguide components, delay lines, and test fixtures where a dielectric has to be acoustically and electrically predictable. It is also optically clear, which makes it useful where a window has to be transparent to both light and RF.

Mechanically it is a rigid, dimensionally stable, optically clear plate. Specific gravity is 1.05 - about 15 percent lighter than acrylic and under half the weight of PTFE, which is a genuine argument for it in airborne and handheld RF hardware where a PTFE insulator would be the alternative.

Machining is one of its real advantages. It machines in all conventional operations with acrylic-type tooling, grinding accuracies of 0.0001 inch are achievable, and because the stock is cast it is essentially stress-free - no stress relieving is required before, during or after machining, which is not true of extruded thermoplastics. Keep tools sharp and avoid excess heat when polishing, because heat crazes the surface.

Temperature, with three numbers that do different jobs. The recommended continuous operating range is minus 76F to 212F. Glass transition is 237F, above which it goes rubbery - though being cross-linked it will not melt or flow. The manufacturer separately states that dielectric properties hold from minus 454F to 349F under no load for short periods, which is a different claim and should not be read as a service temperature. Thermal expansion is high for a plastic at 3.8 x 10-5 per F, roughly six times aluminum, so allow for it in an assembly that has to hold tolerance.

The chemistry to watch, and this is the thing that bites people. Alkalis, alcohols, aliphatic hydrocarbons and mineral acids have no effect at all. But AROMATIC hydrocarbons - benzene, toluene, xylene, styrene - and CHLORINATED hydrocarbons such as carbon tetrachloride and chloroform swell the material and must be avoided. That rules out a lot of solvent-based cutting fluids, degreasers and aerosol electronics cleaners. It is not suitable for continuous chemical immersion either.

Flammability, stated honestly because the alternative is misleading. The only published figure is a burn rate of less than 1 inch per minute, with no test method cited. That describes a material that BURNS, slowly. Cross-linked polystyrene carries no UL 94 rating, no flame-retardant rating and no self-extinguishing claim, and above about 570F it releases combustible gases. We make no FDA, food-contact or NSF claim for it either. What we can tell you is what the manufacturer publishes: outgassing per ASTM E-595 is 0.04 percent total mass loss and 0.01 percent collected volatile condensable material, and radiation resistance shows little change in dielectric loss to 1,000 Mrad.

Rexolite is a registered trademark of its manufacturer and we are not affiliated with them. The generic name is cross-linked polystyrene, XLPS. Note 1422 is the unfilled grade at specific gravity 1.05; the glass-filled 2200 grade is a different material at 1.11 with different electrical figures.