Greetings -

After seeing references to adding a heat break between the group and boiler, and this causing leakage after awhile in some cases, was wondering if it's necessary to use the O ring as well. some have even reported the teflon being distorted by the O ring.

Does the teflon not seal enough by itself?

Thanks

PTFE is polytetrafluoroethylene, it consists of long linear chains of repeating -CF2- units which are symmetrical and cause sections of the chain to crystallise. (Polymers which do not have symmetrical chains are unlikely to crystalise.) The chains are long and often pass through more than one tiny crystal. The portions of the chains inside crystals are held in place by being in the crystal, but the part of the molecules outside the crystals can move around, making the material somewhat rubbery, rather than being like a piece of glass. The crystals melt at about 330C, and the material outside the crystals has a glass temperature of about -100. As you approach the melting point, decompositon begins to occur, and any smoke produced by this polymer is dangerous. It will quickly kill birds and cause humans to become quite ill. It is dangerous to pet birds to use cookware with coatings of this material, and it is poor practice to use it carelessly in a poorly ventilated place. In an espresso machine temperatures should not exceed 125 at most unless there were to be some kind of failure. It is not possible to cross link this polymer, so the crystals are the only thing that makes it keep its shape, and the large zones of chains outside crystals results in the material being subject to creep--so it slowly flows under pressure. Without cross links it does not "remember" how it formerly was.

The reason some have had problems using this polymer for gaskets is its high degree of creep, and as you get it higher and higher above its glass temperature it creeps faster and faster.

I suspect it is best not to use the O ring at all, as the polymer will tend to creep into the groove for the ring and as it creeps it will make a seal, but as it creeps too much it will come loose and begin to leak.

There are many other polymers, there are not all that many that seem to be good candidates. You do not want anything subject to hydrolysis, and you want a very high glass and melting temperature.

Polyphenyl sulphide is much more creep resistant, it is used for the piston liners in post 2000 La Pavoni lever groups, and for some of their pistons. (It still creeps, but takes several years and lots of espresso making cycles to have problems. Tg 83C, Mp 280C) It would seem to me that it would be a much better thermal gasket material.

Another good candidate is PEEK--Poly p-phenylene ether ketone. Its Tg is 143C and Tm is about 345C.

The problem with these materials is that they are expensive, and most places that sell them require purchasing much more than necessary to make a gasket.

Thinner sections will creep less, of course, but they will also not provide as much insulation.

(Sorry if this sounds like a polymer chemistry lecture. After lecturing on the subject many years, I am programmed to make such lectures!)

I thought about this, there is a polymer that might make sense, polyetherimide, PEI. One of its trade names is ULTEM. It is used for medical products where it can be repeatedly autoclaved. It has a Tg of 216, so it should not give creep problems. It is MUCH less expensive than the two polymers cited above. You are not going to cut it into shape with a knife, however! You can find this stuff from several online sources, in several thicknesses. With it I am sure you will need to retain the O ring, however too, because it has low creep.

on the Cremina the O ring is a non issue in this case

Thanks for the chem lectures, much appreciated. As a tech, have an inclination to try and understand as to why to do or not do something. Also why something failed and how to prevent it from failing again.

I've already started on a 1/32 teflon break for the LaCara, and thinking of initially leaving the O ring out, just to see... Seems the Teflon should act as a gasket, and will dress the two surfaces as well as possible.

Robert,

How would one go about purchasing some ULTEM?

Thanks,

Rick

Robert,

Why not silicone like SilPat?

As I wrote some days ago, I'm considering aramid motor gaskets.
They should not have problems at high temperatures
Alberto

Rikinwa wrote:Robert,

How would one go about purchasing some ULTEM?

Thanks,

Rick

I have purchased teflon from this supplier. They were friendly and efficient. (But they are in the UK.)

http://www.theplasticshop.co.uk/ultem-p ... 335-0.html

I have seen Ultem sheet from ebay stores. A good place to get things like this is also online metals:
http://www.onlinemetals.com/

Another is Zoro tools. They also have a LOT of other things:

http://www.zorotools.com

Silpat might be a good idea, it is a standard cross linked siloxane polymer. Because it has cross links it does not creep badly.