Polyethylene

History

Ethylene is a gas composed of two carbons and four hydrogen molecules. Formula: C2H4.  This monomer unit forms the backbone of polyethylene.  PE synthesis wasaccidentally discovered in 1932 by Imperial Chemical Company (ICI) Scientists.  The first high pressure LDPE plant was built in 1939 and was used until the Ziegler Natta  (Z/N) process was invented in 1953.  The Z/N process occurs at lower temperatures and can also produce HDPE.   In the late 1970's LLDPE was produced and continuous Catalyst (metallocene) advances have been made since then.

Polyethylene is classified by density ranges, as defined ASTM:

PE Type Density Unit
LDPE Type I 0.910 -0.925 g/cc
MDPE Type II 0.926 -0.940 g/cc
HDPE Type III 0.941 -0.960 g/cc
HDPE Type IV >0.961 g/cc

Typical Additives Used With Polyolefin Film Extrusion Resins:

Additives

Primary Benefit

Anti-Static Static buildup resistance
Slip/Anti-block Agents Improved film to film slip
UV Stabilizers Resistance to effects of sunlight
Color Add color pigment concentrates to film
Carbon Fillers High concentration results in improving conductivity of the film

LDPE

LDPE is produced in one of the following two ways: High Pressure Tubular Reactors or High Pressure Autoclave Reactors.

Applications

  • Film Applications-Garment Films, Industrial Liner, Lamination films, Coextruded Food Packaging, Bakery Films, Film Blends (with LLDPE) for food packaging, Shrink Overwrap, Kitchen Cling Film, etc.
  • Extrusion Coating Applications -Paper Board Coating, Package Coating, Coating of other substrates (Examples foil coating, drink box coating, etc.)
  • Injection Molding -Lids, Caps and Closures
  • Other Examples -Wire and Cable applications, PE Foam, Pipe and Conduit, Non-abrasive films, Blow Molded
  • squeeze bottles, etc.
 LDPE Chain Structure

LDPE Chain Structure

HDPE 

HDPE is a thermoplastic polyolefin that is manufactured by the polymerization of ethylene.  It was first commercially available in 1956, many believe it still has not reached maturity.  Many new applications continue to be discovered for this versatile material.

Chemical Makeup of HDPE

The absence of chain branching gives this material a higher density.

Common HDPE Applications

Since HDPE is resistant to many different chemicals, it has a wide variety of applications including various types of containers, plastic bags, tables and chairs, pipes for water and gas, and bottles.

  HDPE Chain Structure

HDPE Chain Structure

LLDPE

LLDPE Introduction

Linear Low Density Polyethylene (LLDPE) is made by the copolymerization of ethylene and a comonomer.  The concentration of this comonomer determines the density of the material.  

It is composed of long linear molecules, the main polymer chain is composed of long strings of repeating Ethylene units.  LLDPE differs from LDPE in that is has short side chains, rather than long side chains that,  link onto the main polymer chains.    Typically made consisting of copolymers, terpolymers and quatropolymers have also been made.  LLDPE typically has a narrow distribution of main chain molecule lengths unlike LDPE and HDPE which tend to be broader.  While most of its properties are more preferred over LDPE, one area where LLDPE cannot compete with LDPE is clarity.  LDPE films are clearer than their LLDPE counterparts.

Chemical Makeup of LLDPE

LLDPE is typically made from either 4, 6, or 8 carbon molecules.  
Butene - A four carbon long molecule: C4H8  or (H2C=CH-CH2-CH3)
Hexene - A six carbon long molecule: C6H12 or (H2C=CH-CH2-CH2-CH2-CH3)
Octene - An eight carbon long molecule: C8H16 or (H2C=CH-CH2-CH2-CH2-CH2-CH2-CH3)
Generally, properties improve as  the short-chain branch length increases.  This length increases as the carbon content increases from butene up the chain to hexene and then to octene.

Common LLDPE Applications 

  • Grocery sacks
  • Garbage bags 
  • Stretch wrap film 
  • Agricultural film and tubing
  • Milk pouches 
  • Wire and cable coatings 
  • Housewares 
  • Large outdoor toys 
  • Chemical storage tanks 
  • Landfill covers
  LlDPE Chain Structure

LlDPE Chain Structure


mPE (metallocenePE)

The property profile of metallocene derived plastics is much better controllable because metallocene catalysis is homogeneous in contrast to the heterogeneous Ziegler-Natta catalysts. For that reason the metallocene products are more homogeneous in terms of stereo-regularity and the molecular weight distribution. With these metallocene catalysts, new technologies are being developed to establish a better morphology control at lower costs.  Several companies like Exxon and LyondellBasell, are now offering commercial quantities of isotactic polypropylene based on metallocene catalysis. Metallocene polyolefins are projected to penetrate a broad array of polymer markets.  First with the higher priced specialty markets, followed by the high volume and commodity markets.  New markets are also expected to be created with the development of new classes of polymer that were not possible with conventional Ziegler-Natta technologies.  It is estimated that production of metallocene based polymer will increase to over 20 million tons per year and make serious inroads in the global thermoplastics and elastomer market .  The primary reason for the frenzy of activity in this area is that compared to conventional Ziegler-Natta technology, metallocene offer some significant process advantages and produce polymers with very favorable properties.


UHMWPE

UHMWPE (Ultra High Molecular Weight Polyethylene) grades are Ultra high molecular weight polyethylene resins suitable mainly for compression molding but also for extrusion of sheets, profiles and blocks, and for high modulus filaments and porous products. UHMWPE grades are supplied in powder form. 

Typical Applications 

  • Profiles & Sheets
  • Machines (bearing, conveyer components) 
  • Chemical industry (bellows, pipes) 
  • Linings (Lining of bunkers, silos and hopper) 
  • Electrical industry (pump packaging, insulating components) 
  • Leisure, sports (skating rinks, bowling alleys) 
  • Artificial joint & legs 
  • Special products 
  • High-modulus filaments (ballistic fabric, rope) 
  • Porous applications (absorbents, filters) 
  • Special additives and coating 
  • Secondary Battery separators
Header Image Credit: Lluis tgn
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