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Polymer Descriptions

A thorough understanding of polymers and their use is essential to understanding the recycling process.  Each of the descriptions below address the following categories:

  • Common Uses
  • Why Selected
  • Conversion Processes
  • Resin Composition
  • Resin Properties
  • Common Additives
  • Factors Impacting Recycling
  • PET
  • PP
  • HDPE
  • Film

Common Uses

  • Injection stretch blow molded containers
    Examples: soft drink bottles, home cleaning products
  • Extrusion blow molded containers
    Examples: clear handle ware jugs for juice
  • Extruded and thermoformed sheet
    Examples: packaging for bakery goods, produce and consumer goods
  • Carpeting and apparel textiles
    Examples: fleece jackets, carpeting fibers

Why Selected

  • A balance of stiffness, heat and impact resistance
  • Molded parts have excellent clarity and no color
  • Moderate to good resistance to variety of chemicals
  • Good oxygen and CO2 gas barrier
  • Excellent food and beverage safety credentials

Conversion Processes

  • Blow molding
    • Injection stretch blow molding
      1. “Preform” injection molded
      2. Preform re-heated and blow-molded into its final shape with heat and air pressure
    • Extrusion – Extruded parison formed and then shaped in mold with air pressure
  • Thermoforming
    1. Polymer melted and formed into sheet
    2. Sheet heated and formed with mold(s)

It is possible to produce multi-layer structures where a high gas barrier layer of EVOH or nylon is sandwiched between layers of PET (done in the injection stage for stretch blow molding).

Resin Composition

  • PET is a copolymer comprising at least 90% of:
    • PTA, or purified terephthalic acid
    • MEG, or mono-ethylene glycol
  • PTA and MEG are typically derived from petrochemical sources
  • MEG can be derived from bio sources today; development work is being directed towards creating economical bio sources for PTA
  • Compositions allow use in food, beverage and pharmaceutical packaging

Resin Properties

  • Melt processing
    • IV is a measure of PET molecular weight
    • PET produced in a range of IV values to provide good melt processing in extrusion or injection molding
  • Semi-crystalline polymer
    • PET has a melting point in the range of 225 to 255o C
    • PET will crystallize during molding to improve strength, heat resistance and gas barrier
  • PET must be rigorously dried at elevated temperature prior to use to preserve IV during the molding process
  • Density of PET is greater than 1 and so sinks in water

Common Additives

Additives are not widely used or necessary with PET. Some that might find limited use include:

  • Color concentrates
  • UV blockers to protect container contents from UV light
  • Oxygen scavengers to improve oxygen barrier
  • Lubricants to improve processing
  • Nucleating agents to promote crystallization in crystallized PET food trays

Factors Impacting Recycling

  • Because PET is valued for its clarity, any additive or product design that reduces the clarity or changes the color of recycled PET is not desirable. The impact of inks, adhesives, label materials, and additives used with PET must be carefully considered.
  • PET colored black can not be identified by automated optical sortation equipment and will not be recycled
  • Most consider opaque white color to render PET unrecyclable
  • Oxo and bio degradation additives are not considered compatible with recycling

Common Uses

  • Food and beverage packaging: bottles, injection molded cups, or thermoformed food trays
  • Pharma containers
  • Closures for plastic containers
  • Storage trays and bins
  • Pails and containers
  • Toys

Why Selected

  • PP provides a good balance of stiffness, impact resistance and heat resistance
  • Can be used in hot filling and retort applications
  • Comes in a variety of grades and can be used in a variety of processes.
  • Fast molding cycle times.
  • Molded parts display close dimensional tolerance
  • Moderate to good resistance to variety of chemicals
  • Good moisture barrier
  • Low density, minimize plastic part weight
  • Performs well in hinged closures

Conversion Processes

  • Injection molding
    • Involves melting and injection of melted polymer into a mold
  • Thermoforming
    • Polymer melted and formed into sheet
    • Sheet heated and formed with mold(s)
  • Blow molding
    • Injection – “Preform” injection molded and formed with heat and air pressure in a mold
    • Extrusion – Extruded parison formed and then shaped in mold with air pressure

It is possible to produce multi-layer structures where a high gas barrier layer of EVOH or nylon is sandwiched between layers of PP.

Resin Composition

  • Grades
    • Homopolymer – made only with propylene monomer; stiffer and better barrier than other grades
    • Copolymer of propylene with ethylene - Tougher and higher light transmission (clearer)
    • Impact toughened grades – impact tougheners compounded into the polymer; highest impact toughness
  • Compositions allow use in food, beverage and pharmaceutical packaging

Resin Properties

  • Melt processing
    • Low melt index grades (high melt strength) for extrusion
    • High melt index grades (fast mold fill time) for injection molding
  • Semi-crystalline polymer with a melting point over 125o C, allows for use in hot filling and retort applications
  • Natural resin is light colored and hazy in appearance. Additives can be used to impart a high degree of clarity.
  • Has good barrier properties for moisture, but is not a good CO2 or oxygen gas barrier
  • Density is less than one so floats in water

Common Additives

  • Nucleating agents – Used to promote crystallization and can reduce mold cycle time; improve stiffness and clarity of PP
  • Stabilizers
    • Antioxidants prevent thermal degradation in processing
    • UV stabilizers prevent degradation of the polymer in sunlight
  • Lubricants
    • Mold release agents
    • Slip additives for use in closures and lids
    • Antistatic additives
  • Color concentrates
  • Fillers such as calcium carbonate or talc

Factors Impacting Recycling

  • When use of color concentrates and fillers raise the density of PP near 1 g/cm3 or more, the PP will not float in water and will not be recycled
  • PP colored black can not be identified by automated optical sortation equipment and will not be recycled
  • Oxo and bio degradation additives are not considered compatible with recycling

We are currently working on this section and will have content posted very soon. If you have any questions regarding HDPE, please contact us.

We are currently working on this section and will have content posted very soon. If you have any questions regarding HDPE, please contact us.