The majority of performance wear is made of polyester, polyester spandex, and most rashguards (surfing ones at least) are made of nylon, nylon spandex.
It is the weave of the fabric (the size and number of holes) that determines breathability or resistance to air movement. Any woven or knit fabric will breathe - even if the weave is made of rubber strands.
is rubber that is inserted into fabric to give it stretch. Rubber is weak, however, and people can be allergic to it--Latex Allergy-- so when putting a garment with spandex in the dryer, it tends to dry out and the strands break meaning the garment loses compression power.
Polyester vs Nylon
Polyester fabrics perform better than nylon for moisture management because polyester is more hydrophobic. Nylon threads will absorb more water than Polyester, water requires more heat energy to warm than does air, so nylon will feel colder when wet, and stay wet longer, and when saturated impede breathability.
The down-side for polyester is odor retention, and durability (Nylon lasts longer).
Polyester is hydrophobic, meaning it does not absorb water. This means that when it is dyed, only the color of the dye dissolves into the fabric (not any water-base), making the dye permanent. Nylon® possesses hydrophilic qualities (that is, it absorbs water). Its inability to repel water causes the fabric to swell and ultimately weakens the molecular structure. The dyestuffs used on nylon® tend to oxidize, a reaction which is catalyzed by light. The microscopic effects range from color fading to complete degradation of the polymer matrix. This is why the colors fade in nylon-lycra® swimsuits over time, but do not fade in polyester-lycra® swimsuits (Man-Made Fiber Yearbook, August 2000).
Polyester holds printing much better because it can take higher heat during the printing which causes a better adhesion. Nylon will melt if it is printed at too high of heat. Check out your rashguards and you will see that most are nylon and they dont hold prints very long.