Random information I’ve found:
There are two polyethylene materials used today in the manufacturing of chemical storage tanks – linear polyethylene and crosslinked polyethylene. These materials are very different in molecular configuration, mechanical properties, performance and cost. While some sales documents written in the past have referred to these materials as being, “almost the same” or “having only basic differences,” this is not true for storage tank applications. One difference is molecular weight.
Property Linear Polyethylene Crosslinked Polyethylene
Range of chemical compatibility Excellent Excellent
Performance between 130° - 150° F. Fair Excellent
Impact Resistance Good Excellent
Weatherability Excellent Excellent
Stress Crack Resistance Fair Excellent
Initial Material Cost Excellent Good
Recyclability Poor Poor
Abrasion Resistance Good Excellent
Impact Resistance
Most raw material and chemical storage tank manufacturers will agree that impact resistance is a key variable in assuring tank strength and structural integrity. The greater the impact strength, the more resistant the tank is to stress cracking and ultimately to tank failures. When comparing the two polyethylene materials, we need to look at two different impact tests that are performed. The first one of these is simply a drop dart test (ASTM D-1998-97) which measures a defined amount of impact resistivity in a homogeneous tank wall sample. This test shows that crosslinked polyethylene is approximately 25% more resistant than linear. The second test is called a Notched Izod (ASTM D-265) impact test. This test measures to failure the amount of impact resistivity of a tank wall sample, which has been pre-notched. (A score or notch has been placed in the impact area to determine notch susceptibility of the given sample during impact.) This test shows that the toughness or impact resistance of a crosslinked polyethylene tank is more than 5 times better than a linear polyethylene tank. (17.0 ft-lb. versus 3.3 ft-lb.)
Source: http://www.waterandwastewater.com/www_services/ask_tom_archive/polyethylene_chemcial_storage_vessels.htm
“If you have a tank failure with sodium hypochlorite using a linear polyethylene type of tank, there’s a possibility that the tank could actually fail by zippering all the way around,” said Pfeiffer. “The reason for this is that linear high-density polyethylene does not have an interlocking structure. Crosslink polyethylene has a chemical structure where the molecules are linked together. Since they are linked, if you were to run into the side wall of the tank with a fork lift for example, there would be a hole, but it would not rip or zipper because of the head pressure. Therefore it’s a safer tank. Also with linear polyethylene, if you make a mistake in the rotational molding process it’s not a problem. The tank is cut up, reground into pellets and re-used to form another tank. With crosslink polyethylene, if a mistake is made in the manufacturing process, it has to be scrapped. It can’t be reground and re-melted. This also explains why a linear polyethylene tank is less expensive.
“From a safety standpoint alone, crosslink polyethylene is safer. When it fails over time, after reaching the end of its life cycle, the chemical will weep out of the degraded area. But that’s the only indication that the tank needs replacing.” “These tanks should easily last at least three years, but I have customers that have been using them for 12 years with sodium hypochlorite,” said Pfeiffer.
Source:
http://www.pollutionengineering.com/CDA/ArticleInformation/features/BNP__Features__Item/0,6649,158411,00.html
Unfortunately, I was unable to find comparisons of Crosslink polyethylene vs. Aluminum and/or Steel.