Polyester Flag Fabric made up 52% of global fiber production in 2018, at 55 million metric tons produced annually, according to a presentation by Oerlikon at ITMA 2019 (cited in Textile Exchange’s 2019 Preferred Fiber and Materials Report). Cotton comes in a distant second.
Brands have gravitated toward polyester because it’s often a more affordable and easier textile to get a hold of than natural fibers. You just put in an order with one of the thousands of synthetic fiber mills around the world, it’s produced and then shipped to the cut-and-sew factory. That’s opposed to cotton, which can go through wild price swings due to drought, natural disasters, and political crises. Or animal fibers, which are difficult to standardize and industrialize without animal abuse being baked into the system.
You could say that polyester has been enabling the overproduction of fashion, in fact. Before polyester was discovered in the 1940s, our textile production was limited by the amount of land devoted to growing cotton and linen, or raising sheep and silkworms. Being able to produce polyester in a factory decoupled production from land area. According to McKinsey, between 2000 and 2014, global clothing production doubled. Not coincidentally, 2002 is when demand for polyester surpassed cotton. Most of that increased fashion production has been made possible by polyester.