PHYSICAL PROPERTIES & TESTS
The final proof of successful diaphragm design is field application. The most useful tests will simulate actual operating conditions. To aid the designer in estimating the performance of a diaphragm, several laboratory tests are available for coated fabrics. Many of these are described in detail in ASTM methods covering the testing of rubber-coated fabrics.
- Physical Properties & Tests:
- Thickness (ASTM D751): Gauge or thickness is the simplest property to measure. It is usually determined with a standard weighted gauge which has a pressure foot of a specific area. The gauge is graduated in 0.001 inch increments. Micrometers are not suitable as it is difficult to control the pressure of the instrument. Thickness tolerance is generally plus or minus 10%.
- Breaking Strength (ASTM D751 Method A): Breaking strength, or the strength of the diaphragm, is measured in pounds per inch on both the warp and fill directions of the fabric. A thickness change without a change in the base fabric will not appreciably affect this property.
- Tear Strength (ASTM D751): There are many means for determining this property; however, the most common is the tongue tear method. Once again, the base fabric is largely responsible for tear strength which is expressed in pounds.
- Adhesion (ASTM D751): Adhesion refers to the bond between the elastomeric and the base fabric; it is expressed in pounds per inch width.
- Burst (ASTM D751): Burst strength and hydrostatic resistance, are usually measured on a Mullen tester over a hole of one square inch. Results are expressed in pounds per square inch.
- Porosity (Fairprene® Method): Porosity resistance is a measure of the coating continuity and uniformity. It is determined on equipment similar to a Mullen tester using air instead of water as a test medium. Any air leakage indicates a porous coating; this property is reported as pounds per square inch.
- Low Temperature (ASTM 2136): Low temperature properties can be evaluated from two viewpoints, namely the temperatures at which the diaphragm becomes stiff or the one at which it cracks. Cracking usually occurs at a much lower temperature. There are many established methods for measuring both flexibility and brittleness at low temperatures.
- Thickness Change (ASTM D471): Thickness change, expressed as a percentage, is determined by exposing the sample to a test fluid under specified conditions of time and and temperature. Actual thickness change may be determined by direct measurement and calculation of the difference before and after exposure.
- Volume Change (ASTM D471): Volume change is also expressed as a percentage and is determined by exposing the sample to a test fluid under specified conditions of time and temperature. Volume change is measured by displacement tests before and after exposure. Fabrics can affect this test; hence, the results should be studied carefully.
- Heat Aging (ASTM D753): Heat aging a sample indicates the behavior of a diaphragm during exposure to elevated temperatures. Any one of several properties may be evaluated before and after exposure.
- Scrub Resistance (Fairprene® Method): The Fairprene® scrub test was developed as an alternative method for evaluating adhesion and surface degradation. In this test, the sample is positioned so that it rubs against itself for a specified number of strokes. Results are then compared to Fairprene® standards. The test may be performed on materials before or after being exposed to test fluids.