Chemical Compatibility

The compatibility of a coated fabric with the material contained in it is always important, and often it is the most critical aspect of a design process. For example, in hazardous waste containment, the pollutant will interface directly with the coated fabric and its resistance must be assured for the life of the system. This situation has long been recognised and manufactures and fabricators have evaluated many situations. This has resulted in various chemical resistance charts, which list chemicals against many coated fabrics. Although such tables are generally reliable, there are many overriding circumstances where specific testing is required; these are as follows:

When the chemical is not a single-component material and the synergistic effect on the coated fabric is unknown.
When the composition of the resulting chemical is simply not known, as in waste leachate.
When the coated fabric is not a single-component material but is made from a blend of materials.
For seams made of materials different from the coated fabric or modified from it.
When the containment must function over very long periods beyond the limits of prior tests.
When untested circumstances, such as extreme sunlight, heat or cold conditions, exist at the particular site in question.

Chemical compatibility tests on coated fabrics to the specific conditions mentioned require two important decisions to be made before testing commences. One is the manner of sample incubation, the other is the manner of specimen testing.

Within the area of sample incubation there are the following three choices:

Full sample immersion: Coated Fabric samples are fastened on racks and hung in the chemical, thereby exposing both sides and edges.
Tub immersion: Coated fabric is folded into a small tub (i.e. a pan dish) and the chemical is placed within it, thereby exposing only one side of the coated fabric.
Pouch containment: The pouch containing the chemical is the coated fabric itself, thereby requiring a seam on three of the edges of the pouch.

Note that welds should be tested in all cases.

Upon selecting one of these three methods, one proceeds by incubating a set of identical samples for various time periods. A set of five samples for testing at 1-, 2-, 4-, 8- and 16-week intervals is sometimes used, which when compared to the behaviour of the as received coated fabric will give six points on a curve. An alternate is EPA Test Method 9090 which requires testing at 30, 60, 90 and 120 days.

This brings up the second decision, which is centred around the type of test to be used to judge the coated fabrics performance after chemical incubation. Many different choices can be made, but the following are most common:

Physical property test: These are thickness, volume or weight changes, which are the easiest and most straightforward to perform.

Mechanical property test: The tensile test properties of strength at yield and/or break, elongation at yield and/or break and modulus along with tear, puncture and impact are the usual values measured.

Transport property test: Perhaps the most sensitive tests to perform (and undoubtedly the most difficult are tests for water vapour transmission and diffusion of water or water vapour thorough the incubated coated fabrics.

The response curves for the above-mentioned tests can be plotted as percent change in the measured property versus duration of incubation. If these curves show a marked change in behaviour it is not suitable coated fabric for this particular chemical, however, the lack of a change might not necessarily mean the correct coated fabric is found. It could mean that the coated fabric was incubated for too short a period. Since incubation periods of longer than 6 months to a year are generally unrealistic, accelerated ageing tests are attractive. Such ageing is usually accomplished by incubation under elevated temperature (e.g. up to 80°C), but no set of rules are established. The concept seems to be sound, however, since the elevated temperature will generally show degraded properties much sooner than incubation at ambient temperature. Much research remains to be done in this area.