Testing was performed on Ultra-High Molecular Weight Polyethylene (UHMWPE) fabric and composite material in a flash flame environment when protected by a flame-resistant (FR) fabric outer layer. UHMWPE material has excellent ballistic protection properties, but has generally not been considered for ballistic protection garments due to its low melting point. This research was conducted to determine if UHMWPE materials could be considered for use in the recently developed protective undergarment (PUG) if worn beneath an FR uniform.
UHMWPE fibers are manufactured from solvent-based spinning processes, and have very high tensile strength and stiffness. They also have low density and are flexible. These properties have made UHMWPE fibers beneficial in ballistic protection applications. UHMWPE fibers have a low melting point at approximately 135 to 150 °C. A concern with any thermoplastic that has the potential for exposure to high heat or flame is the melting of the polymer into a liquid phase, which could allow the hot material and flames to spread by dripping or flowing. In the case of thermoplastic materials in garments, there is concern that melted material may cause even more severe burn injuries by melting and re-solidifying over areas of burnt skin.
Midscale and manikin flash flame tests were conducted. Special garments were constructed for testing on the thermal manikin (Figure 1). The UHMWPE materials were fabricated into undergarments consisting of shorts and a vest. An outer layer of FR material was fabricated into a coverall worn over the UHMWPE undergarments. The midscale flash flame tests were used for preliminary testing during design and fabrication of prototype garments with UHMWPE materials and FR fabrics. These tests helped guide the choice of materials and fabrics by showing the response of the material configurations as they were exposed to increasing durations of flash flame.
An instrumented manikin that measures heat flux through 123 insulated copper slug calorimeters was used. The Army specifies a 4-s flame duration for testing combat uniforms, and heat flux data are collected for a total of 120 seconds. The heat flux data are input into a skin burn injury model, which outputs a predicted burn injury level (severity and area).
The midscale test is a more efficient means than the manikin test to develop an understanding between the relationship of heat input and material response (Figure 2). The UHMWPE fabrics tested on the manikin were chosen based on results of several initial rounds of midscale testing. The midscale test is conducted with the same basic test setup as the instrumented manikin test, using a cylinder or flat panel test apparatus. All of the flash flame test apparatuses used (for both midscale tests and the manikin testing) were instrumented with copper slug calorimeters.
The midscale test results showed that any direct flame on the UHMWPE materials will cause rapid disintegration of the material. These materials must be shielded by an FR material to prevent direct exposure to flames or high heat flux. The cylindrical midscale test showed that one of the UHMWPE materials was shielded from the flame, but it became evident that the hot gasses were able to flow behind the material, causing increased damage to the UHMWPE layer underneath. In the flat panel midscale test, this effect was prevented by clamping the material around all edges of the flat panel. The flat panel provided the most control, and was used to test all of the layered material combinations.
A major difference between testing materials on the thermal manikin and the midscale flat panel is the amount of variability in the heat flux over the testing surface, measured by different sensors at different locations in the same test. The standard deviation in heat flux calibration tests on the manikin was at least twice the standard deviation from the same test on the midscale flat panel. For the tests on the thermal manikin, this means the materials were exposed to heat flux in some areas that was significantly higher than the average, while significantly lower in others.
Undergarments of any kind reduced transmitted heat flux and predicted burn injury. The heavier UHMWPE undergarments showed a greater reduction in transmitted heat flux, as would be expected with any material.
In each of the six tests with the UHMWPE fabric undergarments on the thermal test manikin, some damage was observed in the undergarments. This damage was a localized deformation of the fabric, where some of the material had solidified into a hardened plastic area.
As was expected, preliminary testing with the vertical flame and midscale flash flame testing showed that the UHMWPE fabric must be shielded from direct flames, or it will be rapidly destroyed and consumed. It may be possible to incorporate UHMWPE materials in a protective garment (such as the PUG) and still provide the FR performance required to pass flash flame manikin tests.
This work was done by John Fitek, Margaret Auerbach, Thomas Godfrey, Mike Grady, and Gary Proulx of the Army Natick Soldier RD&E Center. ARL-0180