Polyester is one of the most commonly used synthetic filament fibers used in the textiles industry today. We all probably have something in our closets made of polyester (think athletic wear above most); it's easy to blend with natural fibers (which can make any item less expensive), and it has durable properties for everyday and extreme applications. It is resistant to stretch and shrinkage, and it is hydrophobic which allows faster drying times and a mildew resistance.
Polyester fabrics are made from polymers which are an essential base used to make a wide variety of products from a light poly-chiffon skirt to water bottles. When new polymers are melted down, the process emits several toxins into the air, which is why sustainable designers tend to stay away from such synthetic materials. However, the production of polymers has allowed us to create a universal material to produce synthetic/plastic products, but rather than upcycling these materials and lending a bit more sustainability to the material, we are constantly dumping such products in landfills. Why?!
The issue lies within the purity of the polymers after they have been made into a product. When a plastic bottle is recycled into a polyester textile additional chemicals are added to the polymers, essentially contaminating the polyester, thereby eliminating the opportunity to upcycle that material over and over. Those contaminates have rendered the product useless for different industries in which those same fabric treatments could be toxic in what would have been the next product.
In an ideal operation, upcycling is a continuous process in which the product either ends up going back into the earth (if it's natural) or is continually being reused in a closed loop system. Due to its production process, upcycling polyester would entail the development of a closed loop system that allows designers to consider this movement and encourage them to leave polyester products pure for their next use.
The current problem: most of today's innovative textile products depend on the use of chemical finishing techniques. So while your athletic gear may have been made using recycled polyester, it probably won't be reused on account of the innovative finishes applied to it. Of course, technological advancements within the industry will help us develop the necessary closed loop system required to continuously upcycle polyester products.