High-density polyethylene chemical storage tanks can be made of linear polyethylene, and they can be made of cross-linked polyethylene. They are both made with resins that are heated to create a cured plastic, yet the differences in their development create very different polyethylenes of structural strength. We will take a look at the two manufacturing methods and their resulting polyethylene materials and see how cross-linked polyethylene is different.
Linear polyethylene is created when thermoplastic resin pellets are ground and then heated to create the fluid plastic that will harden and cure into a linear, high-density polyethylene surface. Envision a rope where the individual threads of fiber are twisted together, but not tied. The result is a linear resin that has polymer chains “tangled” together, rather than tied. Linear polyethylene is cost-efficient, safe for storage of benign and non-corrosive chemicals, and is recyclable.
Cross-linked polyethylene (XLPE) is high-density polyethylene that is manufactured by adding a catalyst to the thermoplastic resin which turns it into a thermoset. The catalyst causes a covalent bond that links the molecules together. Picture a chain linked fence where the metal is actually linked together. The result is a plastic that possesses impact, tensile strength and resistance to fracture enhancement that makes cross-linked polyethylene an excellent choice when tank integrity is of utmost importance. The chemical resistance, heat resistance, and dimensional stability is unparalleled. XLPE cannot be re-shaped after the curing process.
Similarities Between Linear and Cross-Linked Polyethylene
Both resins are rotational molding grade resins that have been ground into a powder from their standard pellet configuration to allow the material to melt more easily in the molding process. Both resins are available in standard colors as well as some custom colors. Both resins are the very corrosion resistant polyethylene.
Other Differences Between Linear and Cross-Linked Polyethylene
Because cross-linked polyethylene offers longer tank life and more protection in the long run, it is a little more investment upfront. It offers 20 times the environmental stress crack resistance of high-density linear polyethylene, has 10 time the molecular weight and 5 times the impact and tensile strength. Another difference comes in testing the two plastics. When linear polyethylene fails, it fails catastrophically, because the linear polymer chains “unzip.” Cross-linked polyethylene might develop a small tear that pulls the elasticity, but not catastrophic failure.
The drop and pressure test videos offer an excellent demonstration of the differences between linear and cross-linked resins.