Two-part golf shaft epoxy, like most adhesives, can go bad over time. The shelf life typically ranges from 1 to 2 years when stored properly in a cool, dry place. Its effectiveness can diminish if exposed to extreme temperatures or if the components have been mixed and then stored. Once the epoxy starts to thicken, discolor, or fail to cure properly, it's a sign that it has gone bad. To ensure the best bond and performance of your golf club repairs, it's advisable to replace any epoxy that's beyond its shelf life or shows signs of degradation. Always check the expiration date and storage recommendations from the manufacturer to maximize its use.

Titanium is indeed a natural metal, existing abundantly in the Earth's crust as the ninth most common element. Discovered in 1791 by William Gregor and named by Martin Heinrich Klaproth after the Titans of Greek mythology, titanium is known for its remarkable strength, light weight, and resistance to corrosion. Primarily obtained from the minerals rutile and ilmenite, it's extracted using the Kroll process. Titanium's applications are diverse, spanning from aerospace, where its strength-to-weight ratio is invaluable, to bio-compatible implants and even everyday items like jewelry and sports equipment. The extensive use and value of titanium lie in its extraordinary characteristics derived straight from nature.

Aromatic amino acids, specifically tyrosine, tryptophan, and phenylalanine, are often categorized based on their affinity towards water or hydrophobicity. Phenylalanine is clearly hydrophobic, preferring to be in a non-aqueous environment, as its side chain is a simple benzyl group. Tryptophan, with its large indole ring, is also largely hydrophobic but has slightly more potential for interactions with polar molecules due to the nitrogen in its ring. Tyrosine, on the other hand, stands out among the aromatic amino acids with its phenol group, making it relatively more hydrophilic than hydrophobic, especially at physiological pH where its phenolic hydroxyl group can participate in hydrogen bonding with water molecules. Therefore, not all aromatic amino acids are purely hydrophobic; their interaction with water varies based on the structure and presence of functional groups on their side chains.