Dia•Com
is a leading international provider of innovative, cost-effective
molded diaphragm solutions critical to the operation of essential
systems and equipment in industrial, automotive, aerospace,
medical instrumentation, and food and water processing applications.
The company's reputation for excellence is based on superior
quality in the design, manufacture and application of its
high-performance, state-of-the-art, fabric-reinforced and
homogeneous elastomeric diaphragm seals.
| Polymers are a diverse category of materials characterized by chains of covalently-bonded
atoms with repeating structural units. The materials can be processed in numerous ways with
almost infinite variation. The properties of polymers are determined by a number of factors
including crystallinity, density, chain orientation, cross-linking, purity, phase distribution, etc.
We are unaware of hydrogen compatibility studies for common polymer materials that might
be expected in gaseous hydrogen service, thus we have eliminated the sections on mechanical
properties and microstructural considerations. Gas permeation through polymer materials,
however, has been extensively studied; therefore we provide a non-exhaustive summary of
hydrogen transport data in common polymer materials.
Relatively large amounts of hydrogen are often soluble in polymer materials; therefore,
exposure to high-pressure hydrogen may cause damage (blistering or swelling) of the polymer
materials. This is manifest in high-pressure applications due to depressurization of a system (or
rapid temperature changes) as hydrogen expands in free volume and at interfaces within the
polymers. .
1.1 Composition and microstructure
Polymers are generally characterized by the composition and molecular structure of the
material. Nomenclature often evolves from common usage and generally does not incorporate
structural details. We use ASTM D1418 and D1600 for guidance on naming. Table 1.1.1
includes the abbreviations used in this document.
2. Permeability, Diffusivity and Solubility
Hydrogen transport in polymers has been extensively studied, particularly for high-vacuum
systems. Similar to studies of metals, studies of the hydrogen permeation in polymers have
generally been performed at low pressure. Permeability, diffusivity and solubility are often
assumed to be independent of pressure for metals and data generated at low-pressure are
extrapolated to describe high-pressure systems. This extrapolation implies that hydrogen
transport and solubility properties are independent of concentration (i.e., Fickian diffusion).
While concentration-dependent transport properties (non-Fickian diffusion) are often observed in
polymers, we are unaware of any studies on polymers that suggest hydrogen transport and
solubility are dependent on concentration. Thus, until studies show otherwise, we assume that
hydrogen permeability, diffusivity and solubility in polymers are independent of pressure. Unlike
metals, hydrogen transport in polymer materials is sufficiently rapid that the permeation rates can
generally be measured at or near ambient temperature.
Nonmetals Polymers
8100 - 2
The permeability (Φ) is determined from Fick's first law for diffusion, and represents a
steady-state property of the material (assuming diffusion is independent of pressure). It is defined
in the same way as for metals, such that
!
" = DS (1)
where D is the diffusivity and S is the solubility. Hydrogen transport in polymers differs from
metals in one important aspect: hydrogen does not dissociate prior to dissolution in the material,
thus the concentration of hydrogen dissolved in the polymer (c) is proportional to the fugacity (f,
which equals the pressure in the limit of an ideal gas):
!
S = c
f (2)
while in metals c is proportional to
!
f . In materials where hydrogen does not dissociate, such
as polymers, it should be clear from equations 1 and 2 that the units of permeability are
!
["] = [diffusivity]
[concentration]
[pressure]
=
m2
s
mole H
2
m3
MPa
=
mole H
2
m# s # MPa
(3)
Other forms of these units are, of course, possible and they can be a significant source of
confusion. The units in equation 3 are commonly accepted for high-pressure hydrogen since they
do not require definition of a reference state.
In tables 2.1 through 2.4, the hydrogen transport properties for a number of polymeric
materials are summarized. A secondary resource [1] is used for these values and no effort was
made to verify the primary references; the interested reader is also referred to Ref. [2], which
contains a lists of primary sources by material. A selection of the hydrogen transport data from
Ref. [1] is summarized here. Table 2.1 provides hydrogen transport properties for several
common categories of plastics at approximately room temperature. Table 2.2 provides the
transport properties for several commercial elastomers near room temperature, while Table 2.3
provides properties for a number of elastomers (rubbers) from a range of classes at room
temperature and, when available, at elevated temperature.
Permeability, diffusivity and solubility follow a classic exponential form:
!
A = A
0 exp
"E
A
RT
#
$
%
&
'
( (4)
where A0 and EA are material-dependent constants, R is are the universal gas constant (8.31447 J
mol-1 K-1) and T is temperature in Kelvin. Table 2.4 provides the constants from equation 4 that
summarize the temperature dependence of these properties for several of the materials from the
previous tables. The temperature dependence of hydrogen transport and solubility for the
materials in Table 2.4 is plotted in Figure 2.1 (permeability), Figure 2.2 (diffusivity) and Figure
2.3 (solubility); these properties are linear when plotted on a log scale as a function of 1/T as
shown in these figures. |
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