THE DIFFERENCES BETWEEN COTTON AND POLYESTER
Grown on plants that are planted annually, cotton fibers are composed of the polymer cellulose. The fibers have a hollow opening called the lumen in the middle that run the length of the fiber, and when the boll opens and the fiber dries out the lumen collapses. This causes the fiber to form twists (convolutions) so it is shaped like a straw that has had the air sucked out of it, leaving it twisted. Nature coats the fibers with waxes to protect it from the rain when the bolls open; therefore, cotton is naturally hydrophobic (water repellent). But cotton is known for its absorbency, which means the fiber must be wet-processed to remove the waxes. This is what ultimately makes the fiber absorbent.
Cotton is comfortable and breathable. It will wick sweat away from the body so one doesn’t get a wet, clammy feel. Moisture vapor is free to transfer through the fiber to lower the humidity between the fabric and the body, which gives one a cool feeling. Cotton has a natural moisture regain of 8.5% at standard temperature and humidity conditions. Cotton is a sustainable and biodegradable fiber.
Polyester is synthesized man made material. Polyester is thermoplastic, meaning it can be melted and reformed. These pellets are melted and the melted polymer is forced through small holes (spinnerettes). On the exit side of the spinnerettes, the continuous filaments (fibers) solidify. The size and shape of the hole dictates the shape and diameter of the fibers. The fibers are solid polymer; there are no void spaces inside the fibers. These continuous filaments—called "tow"—can be cut to any length (no length distribution, all fibers have the same length) to produce staple fibers for use in textiles and nonwovens, or they can be left as a continuous monofilament, which resembles fishing line.
Being a fiber derived from oil, polyester is water-repellent, and therefore not absorbent. The moisture regain is only 0.4% at standard temperature and humidity conditions. For this reason polyester fabrics do not absorb sweat and can give one a moist, warm, clammy feel.