Polyester sheets for greenhouses - quality differences?

I built myself a small greenhouse and now want to cover the wooden frame with transparent polyester sheets. I have seen in other gardens that these sheets turn yellowish-brown and opaque over time. Is this a natural aging process? Are there polyester sheets that remain permanently transparent? I want to keep the expenses for the little house within limits, so the expensive double-wall sheets are not an option for me.

Almost all plastics discolor/change under UV exposure; some earlier, others later. The expensive sheets rather later.