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
LLDPE
World
23.3
25.5
7.4
Europe
6.7
5.1
1.1
US
3.5
6.5
2.8
Russia
Total 1.41
Middle
East
1.9
China
3.5
Rest of
Asia
4.3
1.4
1.6
6.4
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/cm3

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/cm3

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(2methylpropane (isobutane) or hexane as solvent), Solution process
(a C10 or C12 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
HDPE
LDPE
LLDPE
Making film
Food
Cling film
packaging
Milk carton
Shopping bags lining
Stretch film
Injection
molding
Dustbins
Crates
Food boxes
Buckets
Bowls
Detergent
Blow moulding bottles
Drums
Squeezable
bottles
Extrusion
Flexible water
pipes
Cable
jacketing
Water pipes
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/cm3
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
UHMWPE
Molecular Weight (10 g/mole)
Property
0.05–0.25
3.5–7.5
Melting Temperature (°C)
130–137
132–138
6
Specific Gravity
0.952–0.965
0.925–0.945
Tensile Modulus of Elasticity* (GPa)
0.4–4.0
0.5–0.8
Tensile Yield Strength* (MPa)
26–33
21–28
Tensile Ultimate Strength* (MPa)
22–31
39–48
Tensile Ultimate Elongation* (%)
10–1200
350–525
Impact Strength, Izod* (J/m of notch; 3.175
mm thick specimen)
21–214
>1070 (No Break)
Degree of Crystallinity (%)
60–80
39–75
Table from: www.uhmwpe.org/lexicon/a_primer
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 GreenTM”
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. 2nd ed. P.271-275
www.pslc.ws/macrog/pe.htm
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
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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.