I was also looking at HDXLPE for bulk storage of 50% caustic, so I checked the websites of three major manufacturers. Their commentary steered me away from HDXLPE, although they didn't say not to use it. I won't name the manufacturers, but I can relay what I found.

One manufacturer issued a "position statement" specifically on HDXLPE and HDLLPE for 50% NaOH. They limit ambient service temperature to 100 F, and they insist on double-wall. All connections must be flexible, and the warrantee period is reduced.

Another manufacturer called 50% caustic, "the chemical that finds leaks," or words to that effect. Finds leaks? That's peculiar.
All three manufacturers insisted on their particular double-wall safety system, and I recall the same temperature limitation to 100 F. They also limited storage volume to 10,000 gallons.

When salesmen and manufacturers get this cautious, so do I. See what the FRP guy has to say.

By experience with 50% caustic , FRP+PP can be used instead of PE. Higher temperatures can be used with safe mechanical resistance is one of the advantages.
But you need a very experienced vendor, with good practices for the fabrication of those tanks , with a very strong knowledge of quality control in welding ,stress relieving, and adherence tests between FRP and Thermoplastic.Of couse helped or based in a good spec.

Are you sure that you need 50% caustic ?

As I recall, this must be stored at a temperature <100f in order to be pumpable..

Many 50% caustic users dilute it upon delivery in a large tank in order to get away from storage/pumping problems

https://dow-answer.custhelp.com/app/answers/detail...