I'm planning to mount acrylic shelves in front of a window. The shelves will be 10" deep and 39" long. I will be using brackets on both ends that run the full depth of the shelves. There will be no center support. I'd like to put about 20-30 lbs of plants on the shelves. What thickness does the acrylic need to be to avoid sagging and other structural problems? Would .5" thick acrylic work? .75" thick. The cost gets prohibitive quickly. Also happy to take suggestions of there is a stronger/cheaper plastic material (besides acrylic) that would also work. Thanks in advance.
Acrylic is not very resistant to bending. It is about 20 times weaker than tempered glass.
0.75 acrylic would be marginal. It would have enough bow to be visible to a sharp eye. And it scratches easily, as Ecnerwal mentioned.
I would recommend 3/8" tempered glass. Definitely tempered, much stronger and safer than plain glass.
And be aware that you cannot cut or drill tempered glass, it will shatter. Get the size right the first time.