from: [www.usplastic.com]

LDPE (Low Density Polyethylene) has the most excessive branching. This causes the low density to have a less compact molecular structure which is what makes it less dense..

HDPE (High Density Polyethylene) has minimal branching of its’ polymer chains. Because it is denser it is more rigid and less permeable then the LDPE.

XLPE (Crosslinked Polyethylene) is high density polyethylene which has covalent bonds between connecting its polymer chains. These bonds are caused by using heat plus chemicals or radiation and they help to form 3-dimensional polymers with high molecular weights. These bonds tie the polymers together and lengthening the polymer chains giving it better physical properties. The molecular structure that is formed by crosslinking provides superior stress cracking, improved toughness, stiffness, and chemical resistance compared to the HDPE


ps. PEX is the 'vernacular' for XLPE

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Uponor uses the chemical process to cross-link. I've had zero issues with it in the twenty plus years of use (the fittings are another post).

Boy, that's the answer to a question that NOBODY has asked.