Hydrolytic Stability of Polyurethane-Coated Fabrics Used for Collapsible Fuel Storage Containers
Coated fabric tanks are used to store fuel and water in the field.
Collapsible fabric fuel tanks have provided critical tactical bulk petroleum storage for military operations for over 50 years. Beginning in the 1940s with the 900 to 3,000-gallon pillow tanks, collapsible fabric tanks have evolved into the primary tactical fuel storage vessels now used by all of the military services.
Early collapsible tanks were made from thick Nitrile thermoset rubber-coated fabric materials that were heavy and required several soldiers and a significant amount of materials-handling equipment to deploy. Technological advances in materials and fabrication techniques led to the manufacture of larger and lighter coated fabric tanks made from thinner thermoplastic urethane-coated fabrics.
The Army currently has a large number of fabric-reinforced elastomer tanks ranging in size from 100 to 210,000 gallons used for storing fuels and water in the field. Collapsible fuel tanks, fabricated from urethane-coated nylon fabric, were first introduced by the military during the Vietnam conflict. Their performance then and until recently, particularly in any humid tropic environment, has been less than satisfactory. Unless formulated and produced according to stringent limitations, urethane-based fabric coatings were extremely susceptible to ultraviolet and hydrolytic degradation. At that time, tanks had to hold high-aromatic gasoline as well as diesel and jet propulsion fuels. The only urethane that could handle the high-aromatic gasoline fuels was polyester urethane, which was more vulnerable to hydrolysis than polyether urethane.
In 1990, the Army directed that these tanks would no longer be used for longterm storage of gasoline fuels. This change in policy allowed a shift in emphasis from high-aromatic (gasoline) fuel-resistant coatings to more hydrolytically stable materials, such as polyether urethanes. Concurrently, the Army focused on determining the causes of coating and seam failures. Those studies demonstrated unequivocally that those failures were attributable to the leaching out of protective stabilizers from tank materials by contact with fuel puddles on the outer tank surface. Military specifications for fuel tanks prior to that work merely based requirements for hydrolytic stability on the urethane’s ability to resist deterioration after immersion in water at 160 °F. After that, work materials were aged in water after extraction in fuel.
For the work described here, three seam-breaking-strength specimens 1” wide (parallel to the seam) and extending (perpendicular to the seam) 3” beyond both edges of the seam were punched out and tested at room temperature in accordance with ASTM D751. Reported values are expressed in pounds per inch, and failure within the seam on any specimen constitutes failure of this test. Three peel-adhesion specimens 1” wide (perpendicular to the seam) and of sufficient seam length to determine both the initial and after conditioning tests on the same specimen, were used and tested at room temperature in accordance with ASTM D413. Reported values are expressed in pounds per inch of width.
Infrared spectra for all the polymer samples were collected using a Fourier transform infrared spectroscopy (FTIR) spectrometer equipped with a Gateway (Specac) accessory and a seven-reflection attenuated total reflectance crystal. The accessory contains a pressure mechanism that assures good sample-to-crystal contact. All IR spectra were collected using 128 scans and 4-cm-1 resolution.
Polyurethane elastomers are phase-segregated linear block copolymers that contain an ordered hard segment phase and a soft rubbery phase. The hard segment phase is responsible for the cross-linking in the elastomer. These microstructures of polyurethanes are well known for controlling the physical properties of these phase-separated materials such as tensile strength, tear strength, and puncture resistance. For this study, currently manufactured polyurethane-coated fabrics, as well as several candidate coated fabrics, were obtained. Samples were obtained from commercial coating sources familiar with fuel storage tank construction and fabrication.
Seven polyurethane-coated fabrics were procured. The polyurethane-coated fabric consists of nylon- or polyester-woven fabric that is coated on both sides with polyurethane rubber to create a rubber composite. Coating thicknesses were all very consistent between manufacturers, ranging between 1.4 and 1.6 mm (approximately 0.05 in”). Fabric densities ranged from 41.6 oz/yd2 to 45.2 oz/yd2; this is an important characteristic physical property because the increase in fabric densities results in heavier fuel storage containers, which are more difficult to deploy.
Urethane coatings are particularly susceptible to undergo hydrolysis, which is a chemical reaction with water, resulting in chemical breakdown of the urethane coating, resulting in cracking or extreme softening of the urethane polymer. Hydrolysis can be quite rapid in certain urethane systems, so the resistance to hydrolysis of these urethane-coated fabrics was evaluated. Since hydrolysis of the urethane coating would not affect the strength of the base fabrics due to the nylon/polyester woven fabric, accounting for all of the composite tensile strength, it is preferable to evaluate the hydrolytic stability in the bond line where the urethane coating is fully responsible for the mechanical strength.
The seam-breaking strengths were measured at room temperature after water immersion at 180 °F at time increments of 28, 42, and 70 days. All materials provided adequate mechanical integrity after 28 days of immersion; however, all of the materials’ seam-breaking strengths decreased with water immersion. The inside coating of this tank material became hard and brittle after the samples that had been exposed to water for 42 days and beyond, which is indicative of a failure due to hydrolysis.
This work was done by James M. Sloan of the Army Research Laboratory. ARL-0170