Development of 3.4 µm polyethylene film
Using a highly active Ziegler catalyst in the inflation manufacturing method, it was impossible to fabricate a film of less than 5.6 µm thickness. When research on metallocene catalyst started in 1995, we noted the catalyst and started development of ultra-thin polyethylene film in 1997. The new polyethylene film has the following features compared to the conventional film.
- The comonomer organization in the metallocene catalyst is even, so the presence of low-molecular weight, high comonomers is extremely low allowing high performance in anti-blocking. Also, since the presence of high-molecular weight, low comonomers is low, the new film is excellent in low temperature sealing and transparency.
- Since the range of molecular weight is narrow, very even fabrication is possible.
- Even comonomer distribution and narrow molecular weight range provide excellent performance in shock strength and various mechanical properties.
Taking advantage of these properties, in late 1998 we finally succeeded in developing the world’s first polyethylene film of 3.4É m thickness and 80cm folded diameter. The mechanical characteristics of the newly developed film are: 400kg/cm2 breaking strength and 500% retractability at room temperature; 650kg/cm2 breaking strength and 200% retractability at -80 deg C. These are sufficient for use in the balloon flight environment.
In September 1999, we succeeded in floating a balloon of 1,000m3 volume made of the ultra-thin film at an altitude of 37km. This was the memorable experiment to celebrate the birth of ultra-thin film, high-altitude balloon. Figure 4 shows the history of the growth in balloon size for thin-film and ultra-thin film balloons.
By late autumn 2001, we were able to obtain good results as expected for all the research and development items that the balloon engineering team had discussed before. Next, we decided to develop ultra-thin film, high-altitude balloon No.1 (BU60-1) of 60,000m3 volume, challenging the world’s highest altitude record, and to conduct an experiment to evaluate flying performance in FY2002.
|Figure 4 Development History of Large Balloon
BU60-1 flying experiment
The BU60-1 balloon was 34.37kg in empty weight, 74.5m in length, and 53.7m in diameter. The total weight including 0.8kg of parachute package and 4.6kg of observation instruments was 39.77kg. We mounted two ITV cameras to record images of the balloon’s inflation and a Sony GPS receiver to measure altitude.
At 6:35 a.m. on May 23, 2002, the BU60-1 was floated from SBC carrying the dreams of the balloon experiment team and affiliated researchers. It was floated by the semi-dynamic method using the newly developed balloon holding equipment and large floating platform. The balloon ascended normally at a speed of 260m/min and reached the highest altitude of 53.0km at 10:07 a.m. Figure 5 shows the altitude curve measured by the GPS and Figure 6 shows the image of the balloon’s full inflation.
|Figure 5 Altitude Curve of BU60-1
||Figure 6 Image of Full Inflation
With the slogan of “60km altitude”
The BU60-1, the world’s first ever 60,000m3 volume balloon fabricated of 3.4É m thickness film, successfully reached 53.0km altitude. This altitude superseded the previous world record altitude of 51.8km achieved by the ultra large, 1,350,000m3 balloon floated by the U.S. in 1972. We believe our success is the fruitful outcome of the development of a new film, adhesion machine, balloon holding equipment, large floating platform, semi-dynamic floating method, and research into weight saving on basic onboard instruments.
The balloon engineering team is now studying the resin, a balloon material, and improvement of film manufacturing method under the slogan “60km altitude.” We are sure that this dream will be realized some time in the near future.