Why is casein glue considered obsolete?

Casein glue is considered obsolete primarily because it deteriorates over time. This glue, which is derived from milk proteins, has a tendency to lose its bonding properties with age and exposure to varying environmental conditions, such as humidity and temperature fluctuations. Over time, casein glue can become brittle and weak, leading to failures in the joints where it was applied. This characteristic undermines the long-term integrity of aviation structures, where safety and durability are paramount.

While other options mention aspects such as cost, application difficulty, and bonding effectiveness, these factors are less significant in determining the obsolescence of casein glue compared to its vulnerability to deterioration. In aviation, materials must maintain their performance characteristics throughout their operational life, further solidifying the reason casein glue is seen as outdated in favor of more stable and reliable adhesives.