Heat Damaged Hair
Heat damage is caused by using excessive heat/heated styling tools on your hair like 
blowdrying, straightening and curling which can make your hair feel and look dry, thin, rough, breakable and dull. You may have hair breakage, while your ends may be split and broken with white bits showing at the ends. Heat damaged hair can even become discolored (especially if you have dyed or bleached hair) and notice twists and knots on your hair.
If you have ever used a blow-dryer, hot flat or curling iron, or any other heat styling tool to straighten or style your curls, you’ve probably damaged your hair because of the extreme temperatures (70-250 °C). The rise of temperature causes increasing damage to your hair fibre inside and outside your hair.
Science of heat damage 
Your cortex is made of keratin composed of different amino acids. They are bonded and work together to provide strength, structure, support and cohesion between the cortical cells. Unfortunately, these are the structures that get weak by heat. Once your hair is harmed by high temperatures, fewer water can be stored. As a result, your hair feels dry, brittle and rough.
What is the solution?
You can’t actually protect your hair from heat damage, as heat will always cause damage.
But, what you can do is try to reduce its effects, especially if you are using excessive and frequent heat. You can also buy products that helps in repairing your hair that includes silicone or Fibre Hance. Additionally, You can use heat less frequently and apply leave-in moisturizing conditioner to smooth your cuticle and this will help with the top layer of your hair and water balance. Blow drying can be switched with microfibre towel that will absorb water in no time and as well, try any heatless straightening techniques and don’y forget to cut split ends.