Long-Term Stability
Scientific inspections* have confirmed the durability of the cobra system . Its loss of strength of less than 2 % per year is a recognized top value. Therefore - provided it has been properly installed - a cobra system needs to be replaced only after 12 years.
Polypropylene vs. Polyester
The choice of material for crown bracing has been subject to controversial discussion among experts. From laboratory tests, the properties of both materials were theoretically known; their use and long-term strength in the practice of tree care under natural conditions, however, were unknown because that does not depend on the material alone, but also on the size of the surface exposed to weather.
Untreated polypropylene has a moderate UV stability and a very good acid and lye resistance. In order to compensate for the low UV resistance of polypropylene, the monofil (thick-fibre) ropes used in the cobra system were blackened according to DIN 83305.
According to general manufacturers' specifications, polyester has a good UV stability, a moderate acid and in part poor lye resistance. Examinations performed by SCHRÖDER 1997** showed that in polyester multifil ropes used for tree care, despite their UV and acid resistance first rated as good, the decrease of tensile strength was between 40 and 50 % within five years.
