ᐅ Polyester Panels for Greenhouses – Differences in Quality?

I built a small greenhouse and now want to cover the wooden frame with clear polyester panels. I have seen in other gardens that these panels become yellowish-brown and opaque over time. Is this a natural aging process? Are there polyester panels that remain clear permanently? I want to keep the costs for the greenhouse low, so expensive twin-wall panels are not an option for me.

Almost all plastics discolor or degrade when exposed to UV radiation; some sooner, others later. The more expensive panels tend to do so later.