The expected surge of the solar cell industry in the next
four years is triggering a wave of new materials technology, particularly for
the protective backsheet, which is usually a multi-film laminate anchored by DuPont's Tedlar fluroropolymer
"The demands on this composite sheet are particularly high,"
says Stefan Tomke, who heads product development at Henkel for laminating
adhesives. "During the 20 to 25 years of its service lifetime, the backsheet is
expected to continuously withstand all the tough weathering conditions to which
it is exposed in its roof-top position."
Extreme hydrolysis is a particular problem, and the
adhesives especially need to exhibit enormous resilience. UV resistance is also
paramount as the backsheet must not be allowed to yellow.
Two-Component Adhesive "Added to this is the fact that, during the laminate
manufacturing process, the adhesives have to readily withstand temperatures of
up to 150C for periods of 25 to 35 minutes," Tomke says. Henkel developed a new
two-component adhesive under the Liofil brand that provides thermal resistance
at temperatures of 200C, and provide 2,000 hours of weathering at 85C and 85
percent relative humidity without deterioration.
A start-up company in California
new backsheet materials made from plant-based plastics. One film material is
made from castor oil-based nylon 11, while a new cellulosic sheet is made from a
cotton feedstock. DuPont, meanwhile, is investing $120 million to boost
capacity 50 percent for the resins used to make Tedlar films.
Mitsubishi Plastics has developed an ultra-higher barrier
film for back sheets used in crystalline silicon solar modules that require a
humidity barrier of 0.2g/m2/day, and one for thin PV cells that need
humidity barrier of 0.02g/m2/day. The Back-Barrier is also being
developed for use in dye-sensitized and organic thin-film solar cells, which
require an even higher humidity barrier. Developed in Japan,
Mitsubishi's new barrier films rely on a variety of polymer substrates that are
treated with ionized solutions.
Other interesting advances are also taking place in
adhesive. Henkel's Emerson & Cuming brand developed ECCOBONDTM
CA 3556 HF for high throughput solar cell production processes. The new
adhesive creates a flexible electrically conductive bond with high peel
strength and long term.
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