In the Philippines, most of the t-shirt materials are blended cotton and polyester.
In many cases, the tags do not have information about the ratio of the blends.
My guess is about 50/50.
On the other hand, in the USA majority of the t-shirts are 100% cotton.
Why the difference?
Blends have almost no wrinkling/puckering - vinyl and heat transfer papers look better.
100% cotton can have crease lines more easily on the heat transfer images.
Water-based inks are not affected by cotton/polyester blends while plastisol inks will have color issues caused by dye migration from polyester(Majority printers in the Philippines use waterbased ink. In USA, plastisol ink is mostly used.)
100% cotton is cooler because it has better breathability.
Blends dries quicker than cotton which absorbs more water.
100% cotton give comfortable soft feel.
Generally, blends are cheaper than 100% cotton.