Polyethylene

HENRIQUE DE ALMEIDA TORRES

• • • • Introduction Polyethylene is the simplest of all commercial polymers, and yet, it is the most popular plastic in the world.

It is a versatile material that can be used in grocery bags, shampoo bottles, children's toys, and even bullet proof vests Where does the monomer, ethylene (ethane) come from?

Ethane, propane, naphta, gas oil and ethanol http://www.elmhurst.edu/~chm/vchembook/40 1addpolymer

Types of Polyethylene

    LDPE – Low-density Polyethylene HDPE – High-density Polyethylene LLDPE – Linear low density Polyethylene UHMWPE – Ultra high molecular weight Polyethylene

Annual production of Polyethylene/ Million tons

LDPE HDPE

World 23.3

25.5

Europe 6.7

5.1

US Russia Middle East China Rest of Asia 3.5

1.9

3.5

4.3

6.5

Total 1.4

1 1.4

6.4

LLDPE

7.4

1.1

2.8

1.6

1.8

1.

Federal State Statistics Service: Russian Federation 2011 Table from: www.essentialchemicalindustry.org/polymers/polyethene

LDPE – Low density Polyethylene

      The commercial LDPE process is a free radical polymerization that uses organic peroxide initiators 420-570 K and 1000 - 3000 atmospheres of pressure.

Ethene (purity in excess of 99.9%) is compressed and passed into a reactor together with the initiator. The molten polyethylene is removed, extruded and cut into granules Density : 0.91-0.94 g/cm 3 Average LDPE chain length: 400 – 40000 carbon atoms LDPE is generally amorphous and transparent with about 50% crystallinity LDPE has about 20 branches per 1000 carbon atoms Representation of the LDPE – Image from: www.essentialchemicalindustry.org/polymers/polyethene

HDPE- High Density Polyethylene

Image1: HDPE beads Image2: HDPE pipes http://www.ecvv.com/product/2377724 www.polyethylenepepipes.com

HDPE

 Density: 0.94 g/cm 3      Low branching allows better stacking of the molecules HDPE’s synthesis is done via Ziegler-Natta or Phillips catalysis.

Ziegler-Nattta catalyst: organometallic catalyst (titanium compounds with an aluminum alkyl). Phillips-type catalyst: A well-known example is chromium(VI) oxide on silica.

HDPE is produced by three types of processes: Slurry process(2 methylpropane (isobutane) or hexane as solvent), Solution process (a C 10 or C 12 alkane as solvent) and the Gas phase process

Slurry process (using a loop reactor)

Image from: www.essentialchemicalindustry.org/polymers/polyethene

Gas phase process(Phillips catalyst)

Image from:www.essentialchemicalindustry.org/polymers/polyethene

LLDPE – Linear Low density Polyethylene

    LLDPE is actually a copolymer of ethylene and 1- butane (with lesser amounts of others alkenes) LLDPE’s production is more energetically favorable energy than the production of LDPE The structure is essentially linear but because of the short chain branching it has a low density. The structure gives the material much better resilience, tear strength and flexibility without the use of plasticizers LLDPE is produced by the same processes as HDPE

LLDPE

Structure of LLDPE with 1-butane as copolymer and a representation of the polymer molecule.

Images from: www.essentialchemicalindustry.org/polymers/polyethene

Polyethylene Applications

Process

Making film

HDPE LDPE

Food packaging Shopping bags Cling film Milk carton lining

LLDPE

Stretch film Injection molding Dustbins Crates Blow moulding Detergent bottles Drums Buckets Bowls Squeezable bottles Food boxes Extrusion Water pipes Flexible water pipes Cable jacketing Cable coating Table from: www.essentialchemicalindustry.org/polymers/polyethene

UHMWPE – Ultra high molecular weight Polyethylene

   Ultra–high molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties. Most notable are its chemical inertness, lubricity, impact resistance, and abrasion resistance. These characteristics of UHMWPE have been exploited since the 1950s in a wide range of industrial applications Its molecular weight is generally around 4-7 millions g/mole Density: 0.930–0.935 g/cm 3

UHMWPE

  The polymerization is done by Ziegler- Natta catalysis (titanium tetra chloride as catalyst). The polymerization takes place in a solvent, used for mass and heat transfer, at low pressures. UHMWPE is produced as powder and must be consolidated under elevated temperatures and pressures because of its high melt viscosity.

Schematic of a ram extruder Image from: www.uhmwpe.org/lexicon/processing_uhmwpe

Compression Molding Press (along with the author, for scale) for production of 1 m by 2 m sheets of UHMWPE. This press is located at Poly Hi Solidur MediTECH in Vreden, Germany, and was originally used by Ticona in the production of Chirulen® sheets of UHMWPE. The press is still used today in the production of medical grade UHMWPE Image from: www.uhmwpe.org/lexicon/processing_uhmwpe

Morphological features of UHMWPE

TEM micrograph of UHMWPE showing amorphous and crystalline regions (lamellae) Images from: www.uhmwpe.org/lexicon/a_primer

UHMWPE’s applications

 Pickers for textile machinery, lining for coal chutes and dump trucks, runners for bottling production lines, as well as bumpers and siding for ships and harbors. Over 90% of the UHMWPE produced in the world is used by industry. UHMWPE is also used in orthopedics as a bearing material in artificial joints.

Stainless steel and ultra high molecular weight polythene hip replacement Image from: en.wikipedia.org/wiki/Polyethylene

The Dyneema® brand Dyneema® is respected as the premium brand for Ultra-High Molecular Weight Polyethylene (UHMwPE), and we manufacture and sell products in several forms including fiber, tape and uni-directional (UD) sheets.

Image from: www.dyneema.com/americas/applications/life-protection/personal-armor

Typical Average Physical Properties of High Density Polyethylene (HDPE), Ultra-High Molecular Weight Polyethylene (UHMWPE)

HDPE Property

Molecular Weight (10 6 g/mole)

UHMWPE

0.05

–0.25

3.5

–7.5

Melting Temperature ( °C) Specific Gravity Tensile Modulus of Elasticity* (GPa) 130 –137 0.952

–0.965

0.4

–4.0

132 –138 0.925

–0.945

0.5

–0.8

Tensile Yield Strength* (MPa) Tensile Ultimate Strength* (MPa) Tensile Ultimate Elongation* (%) Impact Strength, Izod* (J/m of notch; 3.175 mm thick specimen) 26 –33 22 –31 10 –1200 21 –214 21 –28 39 –48 350 –525 >1070 (No Break) Degree of Crystallinity (%) 60 –80 Table from: www.uhmwpe.org/lexicon/a_primer 39 –75

True-stress strain behavior in uniaxial tension (room temperature, 30 mm/min) for two grades of UHMWPE, in comparison with HDPE

Graph from: www.uhmwpe.org/lexicon/processing_uhmwpe

Green Polyethylene – Braskem’s “I’m Green

TM

” Polyethylene

   Polyethylene can be produced from renewable sources Brazilian chemical company Braskem, produces polyethylene from sugar cane ethanol since 2007 U$290 million dollars in the biopolymer production plant in Triunfo–RS Image from: www.braskem.com.br/site.aspx/Onde-e-produzido

The green cycle

Image from:www.braskem.com.br/site.aspx/Como-e-Produzido

• • • • • • •

References

Polymer Chemistry: an introduction/ Malcolm P. Stevens. 2 nd www.pslc.ws/macrog/pe.htm

ed. P.271-275 www.uhmwpe.org/lexicon www.unomaha.edu/tiskochem/Chem4310/Notes/PDF_Files/Free_Radical_Synth esis-Properties www2.dupont.com/Tyvek/en_US/assets/downloads/tyvek_handbook www.essentialchemicalindustry.org/polymers/polyethene www.dyneema.com

Revision questions

   List and describe two other microscopy techniques that could be used to analyze UHMWPE’s cristallinity.

What are the similarities between suspension and solution polymerization?

Explain the differences in HDPE’s and LDPE’s properties considering their chain structures.