American Institute of Aeronautics and Astronautics

1

Investigation of Long Term Weathering Characteristics on High Strength and Light Weight Envelope Material Zylon

Masaaki Nakadate1 Japan Aerospace Exploration Agency (JAXA), Mitaka, Tokyo, 181-0015, JAPAN

Shoji Maekawa2 Shizuoka Institute of Science and Technology (SIST), Fukuroi, Shizuoka, 437-8555, JAPAN

Toyotoshi Kurose3 Kawasaki Heavy Industries Inc. (KHI), Kakamigahara, Gifu, 504-8710, JAPAN

and

Takuya Kitada4 Taiyo Kogyo Corporation, Hirakata, Osaka, 573-1132, JAPAN

Long term weathering characteristics of a high strength but light weight envelope material is investigated. A two year long outdoor exposure test was conducted on a PBO or Zylon® based material developed for a 150 m long stratospheric platform (SPF) airship technology demonstrator. Test panels for tensile strength testing every two months were mounted on a 45 degrees inclined rack facing the south 1.8 m wide by 1.2 m long. On each test panel of a 240 mm wide by 430 mm long backboard was a sheet of 220 mm wide by 340 mm long envelope material with its outer (or protection layer side) surface facing outside and with its edge adhered via a rectangular frame/spacer for a 10 mm clearance to the backboard. Tensile strength of the material after exposed to outdoor decreased significantly during hot and humid season of the second year as well as the first year, whereas decreased little in other seasons. A six month long supplemental outdoor exposure test was also conducted to separate humidity effect from temperature effect and vice versa. Test results suggested that the high humidity was the primary factor causing the decrease in tensile strength. With humidity between the inner surface of the envelope material and the backboard controlled, the decreases in tensile strength were negligible for months. The material with an additional protection layer of aluminum evaporated Tedlar on the outer surface showed as small decreases in tensile strength for months as that with humidity controlled.

Nomenclature SPF = stratospheric platform HALE = high altitude long endurance PVF = polyvinyl fluoride UV = ultraviolet PBO = p-phenylene-2, 6-benzobisoxazole RH = relative humidity

1 Senior Staff for Airship System Technology and UA Safety Technology, Unmanned and Innovative Aircraft Team, 6-13-1 Osawa, Member AIAA. 2 Professor, Department of Mechanical Engineering, 2200-2 Toyosawa, Member AIAA. 3 Senior Staff Officer, Space System Project Engineering Department, 1 Kawasaki-cho. 4 Research Engineer, Technical Research Institute Center, 3-20 Shodai Tajika.

11th AIAA Aviation Technology, Integration, and Operations (ATIO) Conference, including the AIA20 - 22 September 2011, Virginia Beach, VA

AIAA 2011-6938

Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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I. Introduction HE hoop stress on a hull structure is extremely high due to its ultra large size of a stratospheric platform (SPF) or high altitude long endurance (HALE) airship. Accordingly, ultra high strength envelope material is of the

highest importance among key technologies. Japan Aerospace Exploration Agency (JAXA) had high strength but light weight materials developed back in 2000-2002 attaining two types of material with Zylon® core fabric1-2, one for a commercial use SPF airship realized a tensile strength of 1,310 N/cm with an area density of 203 g/m2 and the other for a 150 m long SPF technology demonstrator realized a tensile strength of 993 N/cm with an area density of 157 g/m2. The authors also reported reinforcement methods by applying doublers made of the same material with successful and promising results3-4. Since commercial use SPF airship in consideration at JAXA were planned to stay aloft for more than three years, the materials for the hull structure were required to endure at least three, and hopefully more than five years in a stratospheric environment. A long term environmental exposure test was planned and conducted on the same material developed for the SPF technology demonstrator to evaluate its years long weathering characteristics.

II. Weathering Characteristics of Zylon® Characteristics of PBO fiber Zylon® are reported in Toyobo’s technical information5, among which are

weathering characteristics to be taken into account for SPF/HALE airship materials: 1) strength decrease after exposure to high temperature with humidity; and 2) strength decrease after exposure to light (light resistance). The chart on the left of Figure 1 shows the tensile strength after exposure to high humidity of 80% RH (relative humidity) at temperatures 40, 60, and 80 degrees Celsius. The strength decreases gradually with a steep drop at the initial stage of exposure. It can also be seen from the chart that the lower the temperature is the more gradual the strength decrease is. In summary, with high humidity, the decrease in tensile strength is not negligible small after exposed to lower temperatures than 100 degrees Celsius. A strength datum is also plotted for reference on a Zylon® based envelope material after exposed to outdoor at JAXA in 2004. The datum fairly corresponds considering the differences between fiber and woven/laminated material, and in tested conditions. The charts on the left and on the right of Figure 2 show the tensile strength after exposed to Xenon light wetherometer, and that after exposed to outdoor, respectively. Either chart shows a significant decrease in the strength with a steep drop at the initial stage. These data suggest that Zylon® should be protected from visible light as well as ultraviolet light. Notice that Zylon® is a registered trademark of Toyobo Co., Ltd. in Japan.

III. Weathering Test on Envelope Material with Zylon® Core Fabric “In what way?” and “how long is long enough?” were the first deliberation for environmental exposure test to

evaluate the years long weathering characteristics of envelope materials for SPF/HALE airships. Accelerated exposure tests were sometimes conducted in Xenon and/or sunshine weatherometer(s). These methods, however,

T

JAXA’s Zylon based envelope material exposed 8 days to 65deg C, 98% RH (2004)

Figure 1. Strength retention of Zylon® fiber after exposed to high temperature with humidity5.

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have yet to be correlated to exposure tests in real environments. Accordingly, an outdoor exposure test was an inevitable process before conducting an accelerated exposure test as well as a realistic solution. The longest test terms with funding on one-year basis usually had been around eight months, however, it was not long enough to evaluate the weathering during three to five year long environmental exposure. A two year (24 month) long outdoor exposure test was planned that would be conducted within three year period.

A. Long Term Outdoor Exposure Test As shown in Figure 3, each test panel comprised of

a sheet of envelope material 220 mm wide (weft direction) by 340 mm long (warp direction) with its edge adhered to a backboard with its protection layer facing outside and with a clearance around 10 mm to the backboard via a rectangular frame/spacer. The backboard was 240 mm wide by 430 mm long, with four holes 3 mm in diameter placed near the corners of the frame/spacer. Two holes near the top side were for air ventilation and two near the bottom side were for water drainage, each covered with water permeable fabric to keep out light. As shown in Figure 4, the lamination construction of the envelope material for the SPF technology demonstrator from the outer surface to the inner surface were: 1) a layer of aluminum evaporated Tedlar (PVF film from DuPont) for ultraviolet (UV) protection; 2) a layer of polyurethane to adhere the core fabric with the protection layer; 3) a layer of core fabric Zylon® (woven from Toyobo’s PBO fiber) to carry the loads; 4) a layer of polyurethane for abrasion durability and adhering. Notice that the envelope material did not have a gas barrier layer because the SPF airship in consideration at JAXA had multiple helium cells (helium bags) with gas barrier capability inside the hull instead of ordinary ballonets (air bags). Since tensile strength tests were planned every two months, 15 test panels, enough for 24 month

Figure 2. Strength retention of Zylon® fiber after exposed to Xenon wetherometer (chart on the left) and to outdoor (chart on the right), respectively5.

240 mm

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Figure 3. Outline of a test panel with Zylon® based envelope material adhered on top of a backboard.

M embrane

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Polyuretha ne

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Figure 4. Lamination construction of Zylon® based envelope material for SPF airship.

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long outdoor exposure test, were mounted on a 45 degree inclined rack facing the south 1.8 m wide by 1.2 m long (Figure 5). The outdoor exposure test started in January 2007 at Taiyo Kogyo Corporation in Osaka, Japan (34 degrees 48 minutes north, 135 degrees 39 minutes east). Sheets of the same envelope material with the same size as that for the test panels were stored in a darkened chamber at room temperature for comparison purposes (stored material).

Five 30 mm wide (weft direction) by 300 mm long (warp direction) strips were cut out using a pair of special shears for ultra high strength materials from the same envelope material used for the test panels and a tensile strength test was conducted prior to the outdoor exposure test for comparison (zero month exposed material), and five similar strips were cut out from the stored material as well as from the outdoor exposed material on a test panel and tensile strength testing was conducted every two or three months. Figures 6 and 7 show the tensile strength retention compared to the stored material and the ratio of tensile strength retention to the stored material, respectively. Significant decreases in tensile strength were observed during the months from the fourth to the ninth month whereas decreases were not significant during the months from the beginning to the fourth month and from the ninth to twelfth month. Since the fourth month was May, and the ninth month was October, the high temperature and/or high humidity during these months was/were presumed to be the cause of the decreases in tensile strength. Figure 8 summarizes the strength changes throughout the 24 month long outdoor exposure test. The strength decrease during the five months in humid season (from May to October) of the first year was 30.7% or around 6% per month, and the strength decrease during the six months in humid season (from April to October) of the second year was 28.8% or around 5% per month. It was also observed that the strength deceased little during other seasons of the second year as well as the first year.

B. Supplemental Outdoor Exposure Test While continuing the 24 month long outdoor

exposure test, a supplemental test was planned from summer to winter of the second year to account for the

1.8 m

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Figure 5. Test panels mounted on top of an outdoorexposure rack facing the south.

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Type 1:For Comparison

Type 4:Temperature Controlled

ThermometerThermo- hygrometer

Pyranometer

Thermometer Thermo- hygrometer

Data Logger

Black PanelTemperature

Type 3: Humidity Controlled

Type 2:Additional Protection Layer

6 month exposedto be tested in December

5 month exposedto be tested in November

4 month exposedto be tested in October

3 month exposed to be tested in September

2 month exposed to be tested in August

1 month exposedto be tested in July

Exposure RackExposure Rack

Figure 8. Schematic arrangement of supplemental exposure test.

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strength decreases. Four types of test panels, basically identical to those for the outdoor exposure test, were prepared: 1) type 1 (baseline) for comparison, the same as that for the 24 month long outdoor exposure test; 2) type 2 (additional protection layer), the material with another aluminum evaporated Tedlar layer on top of the protection layer to evaluate the effect of the protection layer; 3) type 3 (humidity controlled), the humidity between the material and the backboard (inner side humidity) was to be controlled by silica gel desiccant to separate humidity effect from temperature effect; and 4) type 4 (temperature controlled), the surface temperature was to be cooled down by an electric fan to separate temperature effect from humidity effect. Six test panels for each type, enough to tensile strength test every month during the six month supplemental exposure test, were prepared. A thermocouple to monitor the temperature on the inner surface of the material and a thermo-hygrometer to monitor humidity between the material and its backboard were installed to the sixth panel of each type. Type 1 through 3, a total of 18 test panels were mounted on an identical rack as that used for the 24 month long outdoor exposure test. Whereas, six test panels of type 4 were mounted on an identical but separate rack for their outer surfaces to be cool down by an electric fan. Figure 9 shows the schematic arrangement of the exposure test. Figure 10 summarizes the tensile strength comparison. Notice that the relatively low strength data of the third month on the type 4 and the fifth month on the type 1 were due to some defects unique to the specific test panels, presumably to the sheet of the material, because the strength of the succeeding month(s) recovered back to their overall tendencies in terms of tensile strength versus time exposed to outdoor. As can be seen from Figure 11, the inner side humidity of type 3 (humidity controlled) was controlled as low as 10% RH, significantly lower than other types. Notice that the temperature on type 4 did not show significant difference from type 1. Consequently, type 4 (temperature controlled) showed identical strength decrease to type 1 (baseline). Type 3 (humidity

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Figure 10. Summary of supplemental outdoor exposure test, effects ofan additional protection layer, humidity, and temperature.

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Figure 11. An example of measured temperature and humidity for each type test panel (August 2008).

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controlled) did not show significant strength decrease during all test terms. This result suggested that the humidity was the primary factor causing the strength decrease. Type 2 (additional protection layer) showed identical strength decrease to type 3. Since the primary effect of the aluminum evaporated Tedlar protection layer was to block the light, the effect of light has yet to be separated from other factors. The added layer to the outer surface, however, might have reduced the overall permeability of the material and blocked the humidity from passing through the material and penetrating into the core fiber.

IV. Concluding Remarks A two year long outdoor exposure test was conducted, showing a significant decrease in tensile strength during

late spring to early fall of the second year as well as the first year, whereas showing negligible decrease during other seasons. These results suggested that the high humidity and/or the high temperature during these seasons were/was the cause of the decrease in tensile strength. A six month long supplemental outdoor exposure test was conducted to separate humidity effect from temperature effect and vice versa. The test results suggested that the high humidity was the primary factor causing the decrease in tensile strength. To be specific, the following remarks are obtained:

1) With inner side humidity (humidity between the inner surface of the material and the backboard) controlled, the tensile strength of envelope material Zylon® decreased negligibly for months. Measured data showed that the inner side humidity had been controlled as low as 10% RH, however, judging from the results that the strength decreased negligibly during late fall to early spring and from the data that the average humidity during these seasons were around 60% RH, the inner side humidity does not have to be controlled as low as 10% RH but much higher.

2) The tensile strength of the material with an additional protection layer decreased as negligibly for months as that with humidity controlled. The added layer to the outer surface might have reduced the overall permeability of the material and block the humidity from passing though the material and penetrating into the core fabric. However, since the primary effect of the aluminum evaporated Tedlar protection layer is to block the light, the effect of light has to be separated from other factors for future work.

To summarize for a practical application to an SPF/HALE airship: remark 1) suggests that by filling the hull with dried air in final assembly of the airship the strength of the hull can be maintained for months before the launch to the stratosphere where humidity is non-existent; remark 2) suggests that an addition of a layer or two can be a promising candidate for humidity resistant envelope material.

References 1Sasaki, Y., Eguchi, K., Kohno, T., and, Maekawa, S., “Scenario for Development of the SPF Airship Technology

Demonstrator,” The 5th Stratospheric Platform Systems Workshop, 23-24 Feb. 2005, Tokyo, Japan, pp. 191-198. 2Maekawa, S., “On the Design Issue of a Stratospheric Platform Airship Structure,” NAL TM-772, National Aerospace

Laboratory of Japan, May, 2003. 3Nakadate, M., Maekawa, S., Shibasaki, K., Kurose, T., Kitada, T., and Segawa, S. “Development of High Strength and Light

Weight Envelope Material Zylon,” The 7th International Airship Convention 2008 [CD-ROM], Document ID 71155, 9-11 Oct. 2008, Friedrichshafen, Germany.

4Nakadate, M., Maekawa, S., Maeda, T., Hiyoshi, M., Kitada, T., and Segawa, S. “Reinforcement of an Opening for High Strength and Light Weight Envelope Material Zylon,” 18th AIAA Lighter-Than-Air Systems Technology Conference, AIAA 2009-2853, Seattle, Washington, 2009.

5Toyobo Co., Ltd., “ZYLON®(PBO fiber) Technical Information (2005),” Technical Information [online database], URL: http://www.toyobo-global.com/seihin/kc/pbo/Technical_Information_2005.pdf [cited 12 August 2011].

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