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Phil Engel

12/10/2015

Engr. 201 – Materials Science

The Materials Science of High Density and Low Density

Polyethelyne

Table of Contents Period Table, Bonding and Quantum Mechanics: ........................................................................................ 2

Crystals and Glass-Like Properties: ............................................................................................................... 3

Defects and Diffusion: ................................................................................................................................... 4

Phase Diagrams and Imaging Characterization: ........................................................................................... 5

Thermal Properties: ...................................................................................................................................... 6

Electrical and Magnetic Properties: .............................................................................................................. 7

Optical Properties: ........................................................................................................................................ 8

Mechanical Properties and Testing: ............................................................................................................. 9

Processing Polyethylene: ............................................................................................................................ 12

Manufacturing HDPE and LDPE: ................................................................................................................. 13

Corrosion: .................................................................................................................................................... 14

Conclusion: .................................................................................................................................................. 14

Works Cited: ................................................................................................................................................ 15

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Period Table, Bonding and Quantum Mechanics:

HDPE and LDPE, also known as High Density or Low Density Polyethylene - is a plastic

polymer derived from ethylene or petroleum. The material is consisted of millions of different

lengths of polyethylene carbon-hydrogen chains.

The density of high density polyethylene (HDPE) is not

incredibly higher than the density of low-density

polyethylene (LDPE). The difference in the two materials lie

in the molecular level when cooled. HDPE displays much

less branching than LDPE, making it a more denser,

stronger, and more opaque material. Branching in a long-

chain polymer happens at the molecular level when long

carbon chains curl back and bond to an earlier part

of the carbon chain. This causes more space and

less contact between the the chains and less dipole

and induced-dipole interactions to occur [3]. In

order to create HDPE, special manufacturing

processes must be used differently than LDPE to

produce the polymer.

On a quantum note, the bond energy and

bond angle energy have a quadratic effect on the

amount of strain energy that polyethylene chains can typically hold. Some researchers graphed

the amount of strain that can be produced by the amount of stretching energy applied to the

bonds comparing different types of defected polymer chains containing lower and higher

modulus’ of elasticity [6].

On a macro note, both HDPE and LDPE are show very strong chemical resilience. The bonding

involved in the material is consisted of 100% covalent bonding, and the polymers are resistant

towards the interaction of strong acids, bases, and mild oxidization.

Crystals -

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Crystals and Glass-Like Properties:

During the melting processing of HDPE and LDPE, the polyethylene material undergoes a

glass-liquid transition, where the material goes into a molten state. It is called a glass-liquid

transition because the polymer is an amorphous material and when the material goes from a

hard state to a molten or rubber like state the polymer chains scramble themselves from a

crystal formation and are then strewn similar to a laminar fluid. When the polymer is cooled to

standard room temperature and pressure, HDPE and LDPE exhibits a polycrystalline-like

structures where there are crystalline backbone structures throughout but have an anisotropic

character. The backbone structures are directionally independent, but have a similar pattern

throughout the whole material.

You can think of the HDPE or LDPE like a material full of snakes. Snakes are cold-blooded

animals so they need to gather their warmth from external sources. If the snakes are warmed up

enough, the snakes will move around a lot and likewise the material will become more molten or

ductile like. If the room full of snakes become cooled down to a colder temperature, the snakes

and the material will not want to move at all. If you compress, pull, or apply a force on the

material full of of cold snakes, they will not want to move very much. If you apply enough force

to try and break the block of immobile snakes apart, it could potentially shatter the whole block,

or cause enough internal friction heat up the block and then melt it in to a more ductile in the

area that the friction is most occurring. Because HDPE and LDPE consist of long chains of

molecules, they do not fit in to a categorized atomic structure.

If the material is a thinner piece of HDPE or LDPE, the material may be soft enough to cut

with a knife or scissor, and also if thin enough the material can be translucent which offers many

benefits.

Figure 1- T.E.M. Images of 80/20 blend of LDPE and HDPE. You can see stepwise crystallization on the left picture. When heated to a higher temperature, only the unbranched HDPE chains remain

Figure 2 –

Electron micrograph of a different sample of cold-drawn, rolled and annealed HDPE. You can observe the crystalline structure stacks seperated by thin anomorphous regions.

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Defects and Diffusion: Defects naturally occur in all materials. Specific defects in HDPE and LDPE have

researched and analyzed for over 50 years. One research paper stated that the main type of

defects that occur in these polyethylene materials are dislocations (interstitial and vacancy-like),

chain twist boundary (90⁰ and 180⁰), disclinations, and partial boundary dislocations [7].

The factors of the defects for HDPE and LDPE include the following: Morphology of fibers

during the heating and cooling process, structures and properties of the amorphous and

crystalline domains, imperfect orientation, tie molecules, entanglements, tight knots, folds,

flaws, voids, stress concentrations, and defect rate during these processes [7].

Warpage can also occur during cooling of blow-molding or injection-molding applications.

Warpage is when air-bubbles form or the material bends because it is easier for the material to

release its energy while in that specific temperature state. To prevent warpage, it is advisable to

test the best application pressures and temperatures, and do statistical tests when using HDPE

or LDPE in a manufacturing processing environment to make sure the end products are coming

out with a high success rate.

Diffusion comes in to play when testing for defects. Depending on the amount of vacancies

within the polymer, diffusion can occur within the material during the application of a CO2 gas-

spray, which can then show where the polymer has more or less folds, vacancies, dislocations

and overall permeability in the material.

Diffusion can be measured by analyzing how many CO2 particles penetrated the material after a

gas test.

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Phase Diagrams and Imaging Characterization: Once HDPE and LDPE are cooled, they go through a glass-transition phase where they are in a

rubbery/molten-like state. This can be observed in the following phase diagram (this phase

diagram is with wax) [10]:

Polyethylene under the microscope or spectrometer looks like a snake-like material. The long

polyethylene chains appear as they form from a central nuclei chain of polyethylene chains. The

structure that is seen in the pictures below and one of the more common structures in HDPE is

called a “Shish-Kebab” crystalline structure. The difference in the size of the large backbone

chains is caused by how much extensional flow is present when cooling the polymer. This affects

both the overall strength and ductility of the material, depending on its cooling method.

This is an atomic force microscopy picture of blown HDPE. [12]

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Thermal Properties: In general HDPE and LDPE display higher thermal expansion than metals when experiencing the

same change in temperature. When designing plastic piping systems or heat-intensive

applications, the thermal properties of HDPE and LDPE should always be taken into

consideration.

The thermal conductivity of HDPE and LDPE, which means how much heat can be lost to its

surrounding temperatures per unit of time is a very low rate, which is why it is very useful for

applications such as household piping systems.

Some of the thermal characteristics of HDPE and LDPE are as follows [18]:

Material Coefficient of Thermal Expansion (x10-6/K)

Heat Molding Temperature - .45MPa (°C)

Heat Molding Temperature – 1.8MPa (°C)

Lower working temperature (°C)

Upper working temperature (°C)

Thermal Conductivity W/(m*K)

Specific Heat J/ (K*kg)

HDPE 100-200 75 46 55-120 .45-.52 1900

LDPE 100-200 50 35 -60 50-90 .33 1900-2300

6061-T6 Aluminum

23.5 Melting Point: 580°C

173 896

The optimum temperature for injection molding of HDPE range from 20-95 °C, and usually

operate at pressures ranging from 70-105 MPa. The cooling rate of HDPE when being used for

injection molding is needed to be uniform to minimize shrinkage variations in the material, thus

this is why HDPE is usually a thinner material.

HDPE is also used for Blow-Molding for uses such as milk gallon jugs, 2-liter bottles for soda or

carbonated water, and many other uses because it dries and cools very quickly.

This graph measures the weight change in the

material as a function of temperature and time.

It is very useful for showing the thermal stability

of a material or focusing on its behavior in

various atmospheres (such as oxidation).

Thermal degradation occurs at around 380 °C

for this test and is completely vaporized at 510

°C. [15].

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Electrical and Magnetic Properties:

HDPE and LDPE both have very high electrical resistivity. Because of the polymer’s long carbon

changes packed together in a stable manner chemically speaking, they have very little need for

electron filling. These polymers are classified as diamagnetic, but however, some forms of HDPE

and LDPE and be mixed with other materials (such as copper shreds) to create composites which

have slightly increased conductive and magnetic properties.

Some electrical characteristics of HDPE and LDPE are as follows [16]:

Material Dielectric Constant @ 1MHz

Dielectric Strength kV/mm

Dissipation Factor @ 1MHz

Surface Resistivity Ohm/cm2

Volume Resistivity Ohm/cm3

HDPE 2.3-2.4 22 1-10x10-4 1013 1015-1018

LDPE 27 27 1-10x10-4 1013 1015-1018

Medium Density Polyethylene

22 27 1013 1015-1018

6061-T6 Aluminum

3.7 - 4.0x10-4

As polyethylene goes through the process of annealing (cooling the material at a slow rate),

branching of the carbon chains become less prominent, making the anamorphous material more

compact and dense. The longer the material

is allowed to go through annealing, the more

crystal-like the shish-kebab structure

becomes, and following, the longer the

mean free path for electrons becomes as

well. This makes it the material more

conductive the longer it goes through the

annealing process [17].

Overall, HDPE and LDPE are seen more

diamagnetic insulators than conductors, but

given if enough current goes through them

they can pass for being a conductive

material.

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Optical Properties:

LDPE and HDPE is often used in a lot of applications because of the large branching that occurs

within the material, the band gap of the polymer is large enough to allow light to enter and/or

be scattered through thin layers of the material. It is also easy to mold at fairly low temperatures

and very strong, so they use it for a lot of applications where you need to see through the other

side including blowing milk gallon jugs, packaging films, metal laminates, 2-liter bottles and

squeeze bottles.

LDPE and HDPE can either be very matted, shiny, or see through, depending on the annealing

process that occurs.

An optical graph showing electron stopping power (eV/Å) vs. electron energy.

The band gap for polyethylene ranges

from the crystal carbon chains to the

amorphous kebab-structure with an eV

rating of 5.9 eV +/- .8 eV. With the

Amorphous phase having a .5 eV lower

band gap than the crystalline phase.

This is a band gap graph comparing the

recorded eV and the density of states,

looking at the crystal and amorphous

aspects of polyethylene. You can see that

the crystal states have a higher density,

and a lower band gap, which means more

transparent. [18]

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Mechanical Properties and Testing: Tensile testing the material can be helpful for understanding how defects are formed when

cooling and comparing the theoretical strengths of HDPE/LDPE to actual tensile strengths of the

material. Theoretical strengths are calculated to be 33 GPa for one material, and actual tensile

stress tests show that the failure happens around 6 GPa of stress applied.

In order to test the strength of HDPE and LDPE, scientists test and observe how elastic energy is

distributed over the degrees of freedom of the material. By observing the elasticity at different

points in the plastic-region of the curve, scientists can find what types of defects are happening

and how to prevent them (possibly).

The image on the left is the original scan from an electron microscope of elastic HDPE when

under compressive stress. The image on the idealized carbon chain for what is thought to be

when not under stress.

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The stress strain curve of HDPE and LDPE looks as follows [24]:

Low-density polyethylene (LDPE) has more of a smooth transition between its yield strength and

necking phase. Necking occurs around 32MPa for HDPE and 10 MPa for LDPE. This necking

strength also occurs at various temperatures and depending on the annealing process as well.

HDPE being tested at different temperatures [24]:

:

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These are the mechanical properties of HDPE and LDPE found from

tensile, fatigue, thermal and impact testing:

Material -> HDPE LDPE

Density .96 (g/cm^3) .92 (g/cm^3)

Surface Hardness (Shore Durometer) SD68 SD48

Tensile Strength 32 (MPa) 10 (MPa)

Flexural Modulus 1.25 (GPa) .25 (GPa)

Notched Izod Impact .15 (kJ/m) 1.06+ (kJ/m)

Linear Expansion 12 (m/°C *10-5) 20 (m/°C *10-5)

Elongation at Break 150 (%) 400 (%)

Strain at Yield 15 (%) 19 (%)

Max Operating Temp. 55 (°C) 50 (°C)

Water Absorption .02 (%) .01 (%)

Flammability HB HB

Melting Temp. Range 220-310 (°C) 220-260 (°C)

Mould Temp. Range 30-70 (°C) 20-40 (°C)

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Processing Polyethylene:

HDPE and LDPE is created in several different ways using high pressures and temperatures. The

first and most common method of creating HDPE was created in 1930 by mixing ethylene gas

with small concentrations of pure oxygen at 1000-3000 bar in temperatures ranging from 80-300

°C. Polymerization of ethylene happens automatically, and gets converted to a white solid after

being exposed to these high temperatures and pressures. Other catalysts including wood,

aluminum, or magnesium can be added in order to make the material stronger or more

malleable if needed.

Creating LDPE: Higher processing

pressures causes smaller branching.

This graph shows large branching [25].

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Manufacturing HDPE and LDPE:

HDPE and LDPE can be processed to be used for many different forms. A small list of applications

include: Swimming Pool Liners, 3-D Printer Filament, Ballistic Plates, Banners, Bottle Capes,

Chemical-resistant piping, Cable Insulator, Food Storage Containers, Corrosion Protection for

metals, IR lenses, folding chairs/tables, hard hats, hula hoops, plastic bags, plastic bottles,

fireworks, plastic surgery (facial reconstruction), storage shacks, water pipes, and wood-plastic

composites. [3]

Most LDPE and HDPE use injection molding, blow-molding, or extrusion methods to create

products in the manufacturing processes. Other methods of processing can include wedge

welding, laser welded, and recycled and melted to be used for recycled plastic products or even

3-D filament. The key to when creating HDPE or LDPE products is the manufacturer must allow

the polymer to be raised above its recrystallization temperature (20-70°C) in order for it to

become a rubbery material to then be molded or shaped to the required specifications.

This is a figure of an injection molding apparatus (on the bottom right) which shoots the

HDPE/LDPE to a blow-tube, where from there the strung tube is quickly filled with air while the

plastic is still in its recrystallization temperature. Once the air is blown, the plastic rapidly cools

and takes the shape of the skeleton rollers in the blow mold.

^ Square above is the injection-molder

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Corrosion:

The corrosion resistance on HDPE and LDPE is very high, this is because polyethylene consists of

long hydrocarbon chains, and are virtually inert at the chemical level. The material resists

reactivity towards acids and basis, does not interact with bacteria or fungi, and because the

polymer is an insulator, it is very protective against oxidation or reduction. Also because of the

amount of hydrogen-carbon bonds, there here is little to no range for moisture permeability and

long and strong grain boundaries on the outside.

Conclusion: In conclusion, HDPE and LDPE are both very useable materials in all industries. From plastic bags,

to medicine bottles to milk bottle jugs and even packaging materials. Because of their chemical

resistivity and their low required molding temperatures, it is the material of choice for all

countries that mass manufacture any type of good that requires insulation, transparency, or

medium strength applications.

The best part of all is HDPE and LDPE are all mostly recyclable, and the recycling process has

been studied to be more efficient than creating the polymer from scratch. So when you use

HDPE, MDPE or LDPE, make sure you know its materials science properties and be sure to

recycle it when you are done!

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Works Cited:

[1]. https://en.wikipedia.org/wiki/Polyethylene

[2]. https://en.wikipedia.org/wiki/Branching_(polymer_chemistry)

[3]. https://en.wikipedia.org/wiki/High-density_polyethylene#See_also

[4]. http://clearscience.tumblr.com/post/1255973911/weve-discussed-how-polymers-have-

not-only-a

[5]. https://books.google.com/books?id=B5DRw4IzFb0C&pg=PA309&lpg=PA309&dq=HDPE+ele

ctron+microscope&source=bl&ots=f1Qx3b0yaS&sig=OUwZHDXXxxZvDyD8ECpGieVSczo&hl=en

&sa=X&ved=0ahUKEwiOv6bkv7HJAhUP2mMKHVt4C44Q6AEIRjAG#v=onepage&q=HDPE%20ele

ctron%20microscope&f=false

[6]. http://link.springer.com/article/10.1007%2FBF01451709#page-2

[7]. Defects of HDPE: http://moscow.sci-

hub.bz/a62a6b2c1186508887490d5f20f1f284/10.1021%40ma00213a020.pdf

[8]. Tensile Testing:

http://www.cmse.ed.ac.uk/MSE2/Tensile%20testing%20of%20polymers_lab.pdf

[9]. Diffusion in polymers:

http://people.ds.cam.ac.uk/jae1001/cus/teaching/materials/M6_Lecture_6.pdf

[10]. https://smartech.gatech.edu/bitstream/handle/1853/45833/winters_ian_d_201212_phd.

pdf

[11]. Microscopy:

http://moscow.sci-hub.bz/55e9781da0efbd2097c7023cab484255/10.1021%40ma0104478.pdf

[12]. Atomic force Microscopy:

http://www.che.vt.edu/Faculty/Wilkes/GLW/daves_page/daveweb.htm

[13]. Properties of recycled HDPE from milk bottles

http://moscow.sci-

hub.bz/376c0eab25abbf927b65d737a55476ca/10.1002%40app.1991.070431122.pdf

[14]. HDPE Processing conditions

http://www.viewmold.com/Injection%20Mold%20Management/resin%20processing%20condit

ion/HDPE%20processing%20condition.html

[15]. Thermolysis of HDPE to Petroleum Products

http://www.hindawi.com/journals/jpe/2013/987568/

[16]. Electrical Properties of Polyethylene

http://www.ehow.com/list_6904820_electrical-properties-polyethylene.html

[17]. Effects of Crystallinity and Electron mean-free-path on LDPE

http://moscow.sci-hub.bz/7aa36d8b0ce5c0cb43cfc7d20a114943/10.1109%4014.78326.pdf

[18].Thermal properties of HDPE and LDPE

http://www.goodfellowusa.com/catalog/GFUS2C.php?ewd_token=iesSMETsJsRIGK9QhgDWIn1

w5jEo4K&n=NRv0FkR6ZBwa0svPNfpjkBf6cw2f5G&ewd_urlNo=GFUS24&type=30&prop=6

[19]. Electrical Properties of Aluminum

http://www.azom.com/article.aspx?ArticleID=3328

[20]. Optical Properties of Polyethylene: Measurement and Applications

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http://moscow.sci-hub.bz/43f74565bf8b876f7c1d38150747d0d6/10.2307%403575254.pdf

[21]. Electronic Structure of polyethylene – Crystalline and amorphous phases

http://moscow.sci-hub.bz/43f74565bf8b876f7c1d38150747d0d6/10.2307%403575254.pdf

[22]. LDPE Mechanical Properties

http://www.azom.com/article.aspx?ArticleID=428

[23]. HDPE Mechanical Properties

http://www.azom.com/article.aspx?ArticleID=421

[24]. Tensile Testing of Polymers

http://www.cmse.ed.ac.uk/MSE2/Tensile%20testing%20of%20polymers_lab.pdf

[25]. The Manufacture of Polyethylene

http://nzic.org.nz/ChemProcesses/polymers/10J.pdf