Will asked, and the floodgate opens:
>What did you find good for use in the plastics
>as I'm interested about that myself?
>
Thin Teflon sheet stock would make excellent
chimney, easy to make. Also, others have stated
that RTV silicone sheet does. Actually PET may be
a decent insulator in thin sheets. You can get
Mylar or even pop bottles and try them. Problem
there is the low melting point, but the anode of
a tube should not be 200 deg C!
Here is a posting I made Jan 24, 2000 from the
archives of AMPS@contesting.com about dielectrics
for coil forms. G10 is probably OK for chimneys
when the voltage is reasonable, although my
experiences with it using 15,000 volts and higher
at 27 MHz were disasterous.
>Subject: [AMPS] dielectric losses
>From: jtml@lanl.gov (John T. M. Lyles)
>Date: Mon, 24 Jan 2000 09:26:51 -0700
>
>I've been meaning to get back to the dielectric materials topic with
>some suggested alternate materials:
>
>At 11:50 PM -0500 1/15/00, owner-amps-digest@contesting.com (RF
>Amplifier Discussion Dige wrote:
>>I have been advised NOT to use Teflon, Delryn, and a few other
>>insulating materials for an RF choke. Instead only use ceramic. It was
>>then stated the "D" factor was the reason.
>>
>>OK, but why as I could not find info on this subject in the ARRL
>>Handbook. Any help to point me in the direction of knowledge on this
>>subject would be appreciated. May as well learn something while
>>building my amp. And I want to do it RIGHT.
>
>The following reply is correct, thanks Dave for inserting facts into
>that discussion.
>
>>Teflon is a (fluorocarbon resin) or (flourine plastic) and has the same
>>structure as polyethylene, in which the hydrogen is replaced by (flourine).
>>OR a part may be replaced by (chlorine), in which case it is called a
>>(fluorohalocarbon resin).
>>
>>Polymerized (tetrafluoro ethylene), aka PTFE, which carries the E.I. du Pont
>>trade name of Teflon, formula (CF2*CF2)n is probably the most common
>>solid form used for insulating materials, bearings, gaskets, insulating
>>tapes, wire insulation, etc. This is a white waxy solid with specific
>>gravity of up to 2.3, typical tensil strengths of up to 3500 psi, elongation
>>of 250-350%, dielectric strength of 1000 v/mil and melting point of 594*F.
>>
>>Another commonly encountered material in electronic use and sometimes
>>referred to generally as "Teflon", is Teflon FEP, which is a fluorinated
>>(ethylene-propylene). In thin films (typically down to 0.0005"), it is most
>>frequently found as insulation in capacitors. Typical tensil strength is
>>3000 psi, elongation 250%, and dielectric strength 3200 V/mil.
>>
>>Neither PTFE or Teflon FEP emit phosgene upon decompositon at elevated
>>temps.
>>
>>However, fluorothene plastic (which may be what Rich's info was referenced
>>to) has the formula (CF2*CFCL)n and differ from (Teflon) in having one
>>chlorine atom on every unit of the polymer chain, replacing the 4th
>>fluorine atom. This is a (transparent) material with a specific gravity of
>>~2.1, has tensil strengths of up to 9400 psi, but has a lower withstanding
>>temp (as related to Teflon) of around 300*F. This material is sometimes
>>called "Teflon" due to its similarity in chemical and polymer-structure
>>make-up, and will emit phosgene upon decomposition at elevated temps.
>>
>>73, Dave, K1FK
>
>
>I worked for E. I. DuPont in RF reseach in the late 1980s. We made a
>LOT of dielectric measurements up to 3 GHz. We used Teflon* as a base
>material (a substrate or boat) in a lot of heating experiments in
>high power RF. It is one of the best materials to use. Even when
>there is a direct arc, unless it burns deep into the material, it can
>be reused without repeat arcing. Teflon* is also rather inert, so it
>does not readily combine with other chemicals in experiments. Surface
>arc-resistance of pure Teflon* PTFE (* DuPont trademark) is
>excellent, and carbonized arc paths do not easily form on the
>surface. As others have already mentioned, the dielectric losses are
>among the best for polymeric materials (or even natural materials
>besides gases like air and vacuum). It is often used in radomes and
>in microwave windows. But it cold flows very easily, so if clamped or
>mechanically held, it will eventually deform. So it is rarely used
>where mechancal strength is needed. Coil forms should be no problem,
>for smaller coils wound with magnet wire, however.
>
>There is a sickness called the Teflon* flu. Machinists can get it
>when working the material at higher temperatures near the smoking
>point. This is to be avoided. It was not phosgene gas, but a fluorine
>byproduct I think. I don't know how harmful it is, but sounds like
>Rich's friend succumbed to an extreme case, or something else.
>
>The following Teflon* safety information came from E. I. du Pont de
>Nemours and Company website:
>"Before using Teflon®, read the Material Safety Data Sheet and the
>detailed information in the "Guide to the Safe Handling of
>Fluoropolymer Resins, Latest Edition," published by the
>Fluoropolymers Division of The Society of the Plastics
>Industry-available from DuPont.
> Open and use containers only in well-ventilated
>areas using local exhaust ventilation (LEV). Vapors and fumes
>liberated during hot processing, or from smoking tobacco or
>cigarettes contaminated with Teflon® or Tefzel® fluoropolymer resins,
>may cause flu-like symptoms (chills, fever, sore throat) that may not
>occur until several hours after exposure and that typically pass
>within about 36 to 48 hours. Vapors and fumes liberated during hot
>processing should be exhausted completely from the work area;
>contamination of tobacco with polymers should be avoided. Mixtures
>with some finely divided metals, such as magnesium or aluminum, can
>be flammable or explosive under some conditions. "
>
>
>A lot of flexible conveyor belts for RF heating systems in industry,
>use glass-reinforced Teflon* FEP polymers with silicone rubber. They
>don't get very hot in the intense RF fields used to heat polymers.
>
>To circumvent the weak mechanical properties, microwave industry uses
>crosslinked polystyrene, such as Rexolite, Polymer Q200.5, and maybe
>a few other tradenames. This stuff is the hard brittle plastic that
>is translucent, although it can be polished to be like glass. We use
>it to carry 3 Megawatts of RF, as the spider in our 14 inch coaxial
>lines in my present work. We buy it in 4 foot sheets, and cut and
>turn the discs. We have a special polishing wheel that looks like a
>phonograph. Rexolite does leave a carbon track when arced, and it can
>really melt into a goo if ignited. But the dielectric properties are
>only slightly inferior to Teflon*. K is 2.55 instead of 2.05-2.1.
>Loss tangent is still 0.000x range. Its good mechanically. By the
>way, 3 Megawatts is about 12 kV RMS, assuming perfect match in the
>coax line. It is worse than this, because of the high VSWR during off
>tune (Q of our load is 60,000).
>
>Another material used a lot is ultra high molecular weight
>polyethylene or polypropylene -also called UHMWPE. This stuff is soft
>like Teflon*, a lot cheaper, and has good low loss, if you are
>careful in what you spec and buy. However, it is very easy to get the
>wrong material, so a dielectric test is really needed before
>designing with a piece. This stuff is used for waveguide windows
>sometimes.
>
>Amateur applications of lower RF voltage and frequency can get by
>with materials such as PVC, G10, maybe even Delrin (very questionable
>to me though). In high voltage RF service, in the VHF region, none of
>these can be used, due to excessive losses. Believe me, I have tried
>them all. I have a standard test i perform, in a 90 MHz dielectric
>heater, putting about 15 kV peak RF across a small block. In a few
>minutes, most of those materials will either burst in flames, or will
>begin to smoke and stink, as they are internally cooked.
>
>If you want to use fiberglass materials, G7 has lower loss than G10
>or G11, due to the use of silicone instead of the more lossy epoxy
>resins in the other. It is harder to machine and cannot be turned for
>coil forms, we tried it 2 years ago and it just delaminated and fell
>apart. For the RF choke in the 100 kW 2.8 MHz amplifier, where the
>coil is exposed to the E field alternating at the plate of the tube,
>I chose polysulfone rod. It is easier to machine than Rexolite
>(crosslinked Polystyrene), and has fairly good dielectric properties.
>It is sold under one tradename UDEL* or Thermalux*. We are now using
>this same material for HV insulator posts for a 90 kV klystron power
>supply (not AC dielectric problem, only DC standoff).
>
>Another interesting new material is polyetherimide. This is
>trademarked as ULTEM* by GE and Westlake Polymers and Hydex* by
>Polypenco/DSM Polymers, I think. It can be had in glass reinforced
>sheets, and has good RF properties. It is expensive but very strong
>for mechanic support, such has holding an 80 pound tetrode so that it
>does not crash into the finger stock at the bottom of the filament
>connector in the socket.
>
>There are numerous other materials that are good, mostly more
>expensive, like polyimides Torlon* and Vespel* with incredible
>strength due to glass loading, etc. These are really beyond the range
>of most of us. They should be considered in cases where the ionizing
>radiation (gamma, Xrays) is high, and temperatures are high.
>
>The BEST reference book on dielectrics is still one of A. Von
>Hipple's books "Dielectric Properties and Materials", MIT. Long out
>of print, but I heard that they got reprinted not too long ago. Of
>course, these texts don't have all the modern miracle polymers that I
>refer to above.....
>
>If it works for you, then use it. But be aware that it may smoke now,
>or later, when moisture is absorbed in your material. Pick something
>that is not very hydroscopic if you live in a more humid climate than
>I do in New Mexico.
>
>73
>John
>K5PRO
>
Here is another posting is from the March 24, 2004 archive:
>G10 will melt down into an awful stinkin' and
>burnin' mess when heated in high Rf fields. I
>rarely use it anymore, except for PW
>applications. It is certainly a big step above
>Delrin acetals or nylons. Have settled on
>several wonder materials like:
>Rexolite (crosslinked polystyrene) rod and sheet
>Polyetherimide (ULTEM* 2300) with 30% glass, also known as Tempalux*
>Polysulfone (UDEL*), also known as Thermalux*
>G7 (silicone resin reinforced glass)
>
>All more expensive, but if you want the highest
>Q and no heating, they are worth it. These are
>all high temperature engineered plastics. If you
>are concentrating E field flux in the
>dielectric, even with a kW, G10 will heat up.
>Good old UHMW (ulta high molecular weight)
>polypropylene and polyethyline made good
>insulators but have low glass transisition
>temperatures and will soften and dimensionally
>change with heat.
>
>Your local plastics supplier has them.
>Pricewise, the first two are about $1000 for a
>square foot of 1 inch thick material! The rod
>stock is much cheaper, and for smaller coils it
>is more so. The G7 is more difficult to machine
>or turn on a lathe than G10 due to the
>lamination layers. But it is excellent material
>structurally as well as RF'ly.
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