X-Message-Number: 25403
From: 
Subject: Everything you never wanted to know about cryostat resins
Date: Fri, 24 Dec 2004 20:56:01 US/Eastern

   Robert Ettinger wrote:

> I won't go into detail here, except to say that ordinary boat type  
> fiberglass is not suitable, or at least was not last time I looked. 
> For example, polyester fiberglass--the type most commonly used in 
> the past--will crack in liquid nitrogen, hence cannot be used for 
> the inner container, which we used to make of epoxy fiberglass. Our  
> current ones, however, use a newer type for both inner and outer 
> containers. Andy Zawacki can provide more detail if desired

    For background on the Cryonics Institute's cryostats, see 

                 http://www.cryonics.org/cryostats.html

   The rectangular cryostats at the Cryonics Institute were built
by Facilities Manager Andy Zawacki. The construction of the newer
cylindrical cryostats is done to our specifications by a 
manufacturer. All cryostats are made from fiberglass/resin 
composite material.

   The outer walls of the rectangular cryostats are made of a 
composite of fiberglass & polyester resin, whereas the inner walls
are made of fiberglass & epoxy resin composite. The cylindrical
cryostats use fiberglass & vinyl ester resin for both inside 
and outside walls. 

    In all cases fiberglass is saturated with a syrupy resin
mixture under conditions in which the resin monomers 
polymerize ("cure") -- hardening to form a very strong, 
durable and corrosion-resistant composite. Polyester and vinyl
ester resins are cured using a catalyst which is not incorporated
into the structure, whereas for epoxy resins the hardener is 
incorporated into the final cross-linked network. 

    An epoxide is a cyclic ether in which an oxygen atom is joined
to two carbon atoms. Epoxy resin monomers are typically large
aromatic-containing hydrocarbon molecules having epoxides ("epoxy groups")
at each end. The hardener is typically a diamine that causes nitrogens
to bind to the terminal carbon of the epoxy group, displacing the 
oxygen. which accepts a hydrogen to becomes an alcohol group. The epoxy 
resin used by CI was modified diglycidyl ether of bisphenol A, and the 
hardener was modified aliphatic amine (triethylenetetramaine), both from 
Tool Chemical Co, Inc.

   Polyester resin is inexpensive and easy to use -- it combines readily
with fiberglass. Epoxy resin is the most expensive, but it has the best
thermal properties. Unfortunately, epoxy resin is the most difficult
to work with. When applying resin-saturated fiberglass mats too 
rapidly all the resins can produce too much heat during the curing -- 
resulting in bubbling, smoking & cracking. When applied too slowly 
the layers don't adhere well enough to remove air pockets. Vinyl ester
resin is easier to work with than epoxy, is less expensive and has 
better corrosion resistance. Like polyester resins, vinyl ester resins
shrink about twice as much as epoxy resins upon curing, which can
lead to internal cracks if care is not taken to prevent them. 

    CI's cylindrical cryostats contain a modified vinyl ester -- Hetron 922
(Ashland Chemical) -- which toughens the monomer with carboxyl-terminated
butadiene-acrylonitrile copolymer. As with polyester resin, the curing
catalyst used is methyl ethyl ketone peroxide (2-butanone peroxide). 

    It is doubtful that modification of the resin will make much 
difference in the thermal properties of CI's cylindrical cryostats,
which currently have a liquid nitrogen boiloff of less than $100 per patient
per year. There may be some room for improvement on vacuum maintenance, 
however. The difference between 50 microns of pressure and a complete 
vacuum in the cryostats makes no noticeable difference to boil-off. But when
the pressure in the walls rises to 100 microns of mercury, boiloff increases
noticeably. It takes about three weeks without running a vacuum pump for
the pressure to rise to 100 microns. 

   The two likely reasons why the pressure could rise are outgassing from
the resin and leakage from the valves. To deal with the former possibility
we are considering the idea of heating the cryostat shortly after delivery
to exhaust the outgassing capabilities of the resin. Concerning the latter,
we are currently using water valves rather than vacuum valves because the
only vacuum valves Andy has seen were too small for our purposes. I intend
to look more vigorously. Potentially we could improve performance to the 
point that we would only need to run a pump every few months -- or even years.

   Those interested in exploring resin chemistry in greater depth might
find the following links to be of interest: 

http://www.azom.com/details.asp?ArticleID=986

http://www.psrc.usm.edu/macrog/epoxy.htm


http://scholar.lib.vt.edu/theses/available/etd-42198-113329/unrestricted/ch1.pdf

http://www.frogfot.com/he/mekp.html


      -- Ben Best, President, Cryonics Institute

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