I’m really not sure about how to prevent this since the only downfall of essentail oils (at least in my book) is that the aroma fades.

I’ve never tried any of these, so I don’t know how they’d work, but here are my suggestions:

Add some of the whole herb to the pillows. Add the essential oils to the herb first, and let it sit in a closed container for a few days and then add the rice. I’m not sure how many more heat-ups this would last for, or if there would be some problem with the herbs (mold? although, it’s dry so that probably wouldn’t be a problem) in the pillow, but if there were herbs inside, this would help “fix” the oils, and also when a person is using the pillow, the herbs would get slightly crushed, releasing their own scent. I use lavender (whole herb, without e.o.s) in mesh bags in my drawers and a little squeeze of the bags helps refresh the scent. The ones I’m using now are about two years old, and they still smell nice, but I’m not using them with heat.

Add orris root powder. Again, you’d add the oils to the powder and let it sit, then add the rice. This is used in potpouri as a fixitive, so it might work for your pillows. It has a slight flowery smell, but you could make it work with your oil blends. You could even try the whole herb, plus the orris root powder with the oils before adding the rice.

Sell your pillows with a little bottle of refresher oil. You can find small vials online. This would work well, but I don’t think it would be cost effective for you. I just wanted to add it in as something to think about.

I hope one of those works for you. It makes me really happy to hear about people making natural products and actually caring whether or not they work!