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Polyolefins – What is a Polyolefin?

26 March 2017 No Comment

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What are Polyolefin Polymers?

Polyolefins are macromolecules formed by the polymerization of olefin monomer units. The IUPAC nomenclature term is poly(alkene). The most common polyolefins are polypropylene (PP) and polyethylene (PE). These polymers are prevalent in a wide array of applications depending on the material characteristics of the polymer, most notably consumer plastic. Molecular properties like molecular weight (MW) distribution and branching are fundamental and related to parameters such as material fatigue, impact strength, and resistance to degradation. These properties are therefore routinely assessed as part of research and development (R&D) as well as for quality control and assurance (QC, QA).

Two juice containers made out of low density polyethylene and one yoghurt container made out of polypropylene, displayed on carpet made from polypropylene

 

 

 

 

 

 

Common types of polyolefin polymers

Polyolefins polymers are some of the most prevalent plastics used today and come in various types

  • Polyethylene (PE) with subgroups
    • high density HDPE
    • low density LDPE
    • linear low density LLDPE
  • Polypropylene (PP)
  • ethylene propylene diene monomer (EPDM) rubber

The table below gives a glimpse of the wide-scale distribution of these materials, ranging from everyday household use to specialized industrial applications. Most take advantage of the resistance to heat and an array of common solvents, making the materials economical for many tough high wear materials.

Table of common Polyolefins and typical uses
  Polymer Type   Symbol Examples of use
  HDPE   ♴     fuel tanks, bottle caps, plastic bottles,
  LDPE   ♶     liquid containers, tubing, plastic wrap,
  PP   ♷     piping, carpet, roofing, hinges, auto parts,
  EPDM   seals, electrical insulation, roofing,

Polyolefin melting point

Polyolefins are typically dissolved in high-boiling solvents, such as 1,2,4-trichlorobenzene (TCB), 1,2-dichlorobenzene (ODCB) or decahydronaphthalene (decalin), at temperatures from 130 – 160 °C. Molecular Weight characterizations needs can sometimes be met with simple Ubbelohde intrinisc viscosity data or melt flow index determinations. For advanced characterization, high temperature Gel Permeation Chromatography HT-GPC is used to compare different branching agents, to predict performance and to correlate with rheology results.  Molecular Weight (MW), radius of gyration (RG), and Mark-Houwink constants are often of interest. Some low molecular weight or oligomeric polyolefins may be (partially) soluble in xylenes and other organic solvents which allows for analysis of % monomer or % xylene-soluble fraction. These characteristics have a direct correlation to the physical properties, like flexibility & strength, of the final material.

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