Good Serums for Thick hair when the Humidity is bad

[snorklenut]

I have putting a dime size spread of "Straight and Shiny "serum on my palm after hair dries and then use an iron. I have thick layered hair. Things were great with cold or cool weather but now that its getting hot after an hour in the heat all the bottom half of ends starts to curl up! I prefer a straight haired look. Any economical serums that can keep it straight during hot weather that you recommend?