On March 31, 1996, at 0630 Pacific standard time, a Dornier Luftfahrt GMBH DO 328-100, N334PH, flight number 2620, registered to First Security Bank of Utah, operated by Horizon Air as a 14 CFR Part 121 passenger flight, diverted to Portland, Oregon, after the flight crew felt a shudder and an immediate roll to the left. The flight crew declared an emergency and landed at Portland without further incident. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The airplane received minor damage and there were no injuries to the 24 passengers, one flight attendant, and the two airline transport pilots. The flight originated from Eugene, Oregon, about 30 minutes before the incident. The flight crew reported that as the airplane was climbing between 14,000 feet and 16,000 feet, at 200 KIAS in light icing conditions, they felt a moderate shudder. The auto pilot disconnected and the airplane immediately rolled 15 degrees to the left. An emergency was declared and the flight was diverted to Portland. After landing, it was found that a portion of the left wing leading edge de-icing boot was loose. When air pressure was applied to the boot, a 12 inch diameter "bubble" formed six inches in from the outboard section of the middle boot. When air pressure was removed, that portion of the boot remained loose. The boot was removed from the wing and transported to Daimler-Benz Aerospace, Germany. The boot was installed on a test airplane and wind tunnel tested. The test revealed that the airplane was controllable even with the delamination in front of the aileron. The boot was removed and transported to BFGoodrich Ice Protection Systems, Uniontown, Ohio to determine the reason for the delamination. Preliminary inspection determined that the delaminated plies had stretched beyond their elastic limits and would not return to the normal shape upon deflation. The result was permanent wrinkles in the affected area. Samples of the affected area were taken and examined under optical microscopy and scanning electron microscopy. The result of the examination revealed no evidence of contamination. Peel adhesion samples were then constructed and found that the separation occurred between the adhesion coat and the knit fabric. BFGoodrich reports that this mode of failure is consistent with an elastromeric construction de-icer that has reached the end of its useful life. The outer delaminated plies were found to be unusually durable which resulted in a large bubble instead of the normally expected "burst" of the outer plies.