G10 Cryogenic Glass Epoxy Sheet

Cryogenic-grade G-10 is glass-epoxy laminate made and selected for service down to liquid-helium temperature: the published operating range is -270C to 135C, or -454F to 275F. What makes glass-epoxy right for the job is that it gets stronger as it gets colder rather than more brittle, while staying a poor conductor of heat - structural at liquid-nitrogen temperature and thermally insulating at the same time, which is why it holds up superconducting magnets.

The tabulated properties here are one maker's cryogenic G-10, at specific gravity 1.80. Flexural strength is 75,000 psi lengthwise and 65,000 crosswise at.062 inch, tensile strength 45,000 and 38,000 psi at.125, compressive strength flatwise 65,000 psi at 0.500, shear perpendicular 22,000 psi, hardness Rockwell M100, glass transition 115C and the temperature index 130C electrical and 140C mechanical. Electrically, dielectric strength perpendicular is 670 V/mil at.062 inch, breakdown voltage parallel 60 kV, dielectric constant 4.80 and dissipation factor 0.025. Thermal expansion is low and anisotropic at 10.0 and 13.0 x 10^-6 in/in/C, moisture absorption is 0.10%, and the grade is UL 94 HB - not flame retardant.

A second supplier publishes strength against temperature for its own cryogenic G-10, which is a different and denser product at specific gravity 2.00 with 70.5% glass. On that material, compressive strength rises from 84 to 141 ksi, tensile from 78 to 109 ksi and flexural from 82 to 156 ksi between room temperature and -196C, and it publishes thermal conductivity of 0.235 W/mC at room temperature and 0.28 at -196C. Those are that product's figures and not the ones tabulated above - its room-temperature strengths are materially higher and its published expansion coefficient is different again, so do not mix the two sets on one drawing. Note too that published strengths stop at -196C even though the service range goes lower.

One thing to be straight about: "CR" is not a standardised designation. NIST publishes G-10 CR only as cryogenic curve fits for thermal conductivity, specific heat and linear expansion, in normal and warp directions, and does not define the term or publish a density or a mechanical property for it. The three suppliers who use the name sell measurably different products - published specific gravity runs 1.80, 1.85 and 2.00 - and one of them states in terms that it cannot claim compliance with a NIST process specification because NIST does not have one. If your drawing calls out NIST G-10 CR or a particular process spec, tell us and we will confirm the lot before shipping rather than after.

Typical use is superconducting magnet structure, cryostat supports and spacers, liquid nitrogen and liquid helium service, MRI hardware and particle-physics assemblies. Natural stock is light green.

Machining and handling. The glass abrades tooling, so carbide is the floor and diamond-coated or PCD cutters are what hold tolerance in production; run high spindle speeds with secure workholding and design in generous internal radii, because small tools wear out fast against glass. Threaded holes are normally milled rather than tapped and external threads turned or thread-milled, since the plies can delaminate. The laminate has no hazards as shipped, but cutting it releases a fine dust, and the epoxy resin in that dust is classified as a skin and respiratory sensitiser - use local exhaust, a dust respirator, eye protection and gloves, and work to your own site's exposure limit, because published limits for glass dust differ by jurisdiction and by whether total or respirable dust is measured. It comes off as powder rather than chips, so vacuum it up rather than blowing it airborne. These are our shop notes, not a manufacturer specification.