-CONSIDERATIONS WHEN SELECTING SEAL MATERIALS-

    Information in this section was derived from published literature of
polymer suppliers and seal manufacturers.  In some cases, ratings are the
considered opinion of experienced compounders.  Therefore, this section
is intended only as a guide in selecting materials for specific sealing
applications.  Users are encouraged to test seals under operating conditions
to determine the suitablity of any material or elastomer compound to a
particular application.

    Simplified maintenance and longer warranty periods require reliability
of seals.  Most seal failures are due to thermal aging in conjunction with
fluid attack.  Generally, higher temperatures increase the deteriorative
effect of chemicals on polymers, with the increase dependent on the polymer
and/or the chemical.  Volatilization of plasticizer, loss of antioxidant,
breaking of the polymer chain, etc., take place at various temperatures in
various elastomers.

    A compound compatible with a chemical at room temperature may fail
miserably at a higher temperature.  Thermal aging processes of polymers
demand that the highest temperatures the seal will be subjected to in service
be considered - including temperatures caused by peak loading conditions
of equipment - to assure desired service life of seals.  The rate at which
natural and synthetic rubbers deteriorate increases logarithmically with
the temperature.  Relatively small changes in the temperature to which
seals are exposed, therefore, can cause large differences in the degree of
deterioration observed.  There is also evidence that oxygen concentration
markedly affects the deterioration of some elastomers at temperatures over
200°F(93°C).

    For these reasons, test conditions should approximate service conditons as
closely as possible.  For example, a circulating air oven test may give a
low estimate of the life of a part (such as a flange gasket) which may not
be exposed to air during service.

       Copyright 1978 by Fluid Sealing Association.  All rights reserved.
       Information contained herein may not be reproduced in any form
       without permission of the Fluid Sealing Association.