Uv weaken the bottle. Avoid.
Just use steam or hot water

toothbrush u use 3 month then throw
milk bottle u use for 1 year

and milk bottle when weakened, plastic will go into milk and its for baby.

the bottles are made with typically PP to withstand higher than 100C which is the steam temperature at sea level pressure. it will not melt or disintegrate. PP melting point is 160C.

UV will disintegrate the plastic, making it flaky. and some flakes may go into milk. a short burst using UVC lamp to kill heat resistant microbs is ok but not under the sun for long time.
do let me know if this is wrong.