Comparing Melting Points Across PP and PE Types

In semi-crystalline thermoplastics like polyolefins, 'melting temperature' is the temperature range (not a single exact point) at which the material's crystalline regions lose their order and become fluid, and it is generally measured by differential scanning calorimetry (DSC) according to ASTM D3418 or ISO 11357 (see the 'ISO and ASTM Test Standards Guide').

PP: approximately 160-170°C — the member with the highest melting temperature in the polyolefin family, which makes it suitable for applications such as hot-fill packaging and sterilizable containers (see the 'What Is PP' article).

YYPE: approximately 130-135°C — has the highest melting temperature within the PE family due to its high degree of crystallinity.

LLDPE: approximately 120-125°C, which can vary depending on the type and ratio of comonomer used.

AYPE: approximately 105-115°C — has the lowest melting temperature within the PE family due to its high branching ratio and low crystallinity (see the 'What Is Density' article for the crystallinity-density relationship).

Melting temperature sets the lower bound of the temperature range at which a material can be processed — injection or extrusion temperature must be noticeably above the material's melting point. Higher-melting materials such as PP and YYPE generally require higher processing temperatures and thus somewhat more energy.

On the usage side, melting temperature is a rough indicator of the highest temperature a product can be exposed to — for example, this is one of the main reasons PP is preferred over PE for microwave-safe containers or containers filled with hot liquid.

Even when two materials have similar melting temperatures, other properties such as MFI, impact strength and stiffness can differ significantly. Melting temperature should be evaluated together with other spec values for the right grade selection.