The strongest threads are made from upholstery nylon with a special resin coating that adds strength to it. It’s stronger due to the chemistry existing in nylons. On a molecular basis, the bonds between them are super strong, and its molecules are huge and tightly knit together, resulting in very high tensile strength. So far, Nylon is the best thread for sewing leather due to its durability and strength.
However, polyester is another strong synthetic thread in use today, and it’s known for its low stretch and shrinkage, and durability. Compared to nylon and cotton threads, it has outstanding resistance to moisture and UV rays though it’s prone to lose its strength after prolonged exposure to sunshine. UV treated polyester threads are great for home sewing projects, drapery, and crafts.