Meaning
Thermoplastic polyolefin resins characterized by high degree of short and long-chain branching exhibit low density values ranging from 0.910 to 0.940 grams per cubic centimeter. Production of ldpe occurs under high-pressure free-radical polymerization processes that yield complex branched macromolecular structures. The material covers film blowing and injection moulding applications requiring flexibility and impact strength.
It stops applying to linear low-density polyethylenes produced via coordination catalysis or high-density polyethylenes possessing linear backbone structures.
Melt Processing
Polymer chain branching produces pronounced strain hardening during uniaxial and biaxial elongational deformation. High melt strength prevents bubble sag during blown film extrusion, allowing stable production of thin packaging films. Processing ldpe requires lower melt temperatures than linear polyolefins, reducing thermal energy requirements during conversion.
Blending regrind from reclaimed film into virgin feed stocks alters molecular weight distribution, increasing melt flow variation and reducing clarity. Datasheet values report nominal melt index and density, but actual part optical clarity and seal strength depend on cooling rates achieved during processing. High cooling rates suppress crystallite growth, producing transparent films with low haze values.
Mechanical Flexibility
Ductile chain networks grant high elongation at break and puncture resistance across low-temperature environments. Absence of rigid crystalline domains imparts flexibility without requiring external plasticizer additives that migrate over time. Injection moulding with ldpe yields flexible components such as caps and squeeze bottles, though high shrinkage rates demand precise cavity sizing to prevent warp defects.
Barrier Limits
Non-polar chemical structures resist water vapor transmission but permit high gas permeability rates for oxygen and carbon dioxide. Thin barrier films require co-extrusion with ethylene vinyl alcohol or polyamide layers to achieve necessary oxygen protection for edible products.
