People toss the word 'Kevlar' around like a generic material. It is, of course, a trademarked brand of aramid fiber (whatever that is) manufactured by DuPont.
http://www.dupont.com/products-and-services/fabrics-fibers-nonwovens/fibers/brands/kevlar.html
Not all chaps use Kevlar. Some use polyester blends. STIHL uses 'Avertic® Pro by Engtex®' (whatever that is).
http://avertic.com/
Key point I wanted to make is that STIHL recommends washing their chaps periodically to keep the protective fibers loose and fluffy, and to remove oils which can matt them down. They even recommend washing them when new, before their first use, to fluff them up after manufacturing, shipping, and storage. STIHL is the only major brand I know of that states their chaps can be machine washed and dried. Most others say '
hand wash and line dry'. STIHL does not recommend bleach.
This does not mean that STIHL makes the best chaps, or are the best choice for you. However, working with volunteers, where we share PPE, and things get muddy and sweaty, etc., it is a real convenience to be able to throw a bunch of them into a commercial washer and dryer at a laundromat (no agitator) and hand out clean chaps.
Read the label on your chaps, or check with the manufacturer to see what they recommend.
Cleaning the outside fabric is also important for making repairs that stick.
https://www.arboristsite.com/community/threads/chaps-repair.324300/
Philbert