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Molecular stability (rheology) of a plastic carrier bag through stress - strain tests.

Extracts from this document...


Steven Truong

1.0 Introduction

I am going to study the molecular stability (rheology) of a plastic carrier bag through stress – strain tests. I will do this through a simple viscoelastic experiment of where I will be able to calculate the Young Modulus and assess Hooke’s Law. Plastic carrier bags are made from polyethylene or ‘polyethene’, which is a ‘homopolymer’.

Polyethylene comes from ethylene. Ethylene is an alkane made up of a series of saturated hydrocarbons. “The alkane series are known as homologous series” as they share the properties and general formula: (CnH2n+2)

1.1 Origin of plastic

Essentially plastics are materials that can be heated and moulded and maintains this moulded shape once it cools.

Plastics have existed since the beginning of time. Plastic contains ‘natural’ elements such as carbon(C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulphur (S). These elements can be ‘originated’ from naturally grown organic materials such as wood, horn and rosin. Animal horn and amber are examples of natural plastics.  

Already renown for his work in the rubber industry, Alexander Parkes invented a material that was based on cellulose nitrateat the Great Exposition of 1862 in London. He called this material Parkesine, which was the first synthetic polymer. His invention was due to a new scientific movement to utilise by-products of natural gas production.

We are now in the “Age of plastics” where plastic dominates our industrial world. “Plastic is a force that will shape the twenty-first century, bringing to fruition new wonders in tomorrow's world.” – Carin Glaser

1.2 Polymer

Polymer comes from the Greek word, ‘poly’ meaning many and ‘mer’ meaning part.

Polymerisation is the chemical process of forming polymers from their components of monomers. Polymerisation is often an intricate process that may be initiated or sustained by pressure, heat or with catalysts.

...read more.
















2.1.2 Summary

I conclude that the wider the strip of plastic, in general, the heavier it holds before breaking. The weight holder’s carry around 15 Newton’s each therefore I shall use “200mm by 20mm” for my experiment. I noticed that when I added more weights the width of the bag shortened as the extension became greater, this may affect the eventual outcome of the result. To indicate this later in the experiment I shall identify any anomalies. I also need to calculate the ‘impact’ that I did when I added the weights on directly, later in the experiment I need to add the weights on Newton by Newton simultaneously.

Calculating stress

To calculate the stress I need to use the formula:

Stress =               Load               
               Area of cross section

First I need to calculate the area of the cross section. To do this I shall use the formula:

Area = width × thickness

The cross section of the paper strip is:

The thickness of the bag varied in each strip so I took the average of the thickness of the plastic strips from 6 samples using a micrometer.

Average (mean) =   ∑x

Samples thickness:

  1. 0.019mm
  2. 0.022mm
  3. 0.02mm
  4. 0.018mm
  5. 0.019mm
  6. 0.02mm

These results are fairly close together therefore I shall use these results, as they seem accurate.

∴ Mean = 0.019 + 0.022 + 0.02 + 0.018 + 0.019 + 0.02


Thickness = 0.0197mm

If area = width x thickness

Thickness = 0.0197mm

Width       = 20mm

∴ Area = 20 × 10-3 (m) × 0.0197 × 10-3 (m)

Area = 3.94 × 10-7 m2

From these results I can calculate the stress. I shall go to the force of 17N because from previous knowledge I can say that the strip will be likely to break at this point or below.

Load Applied (N)

Stress = load/area of cross section (Pa)




2.5 x 106


5.1 x 106


7.6 x 106


1.0 x 107


1.3 × 107


1.5 × 107


1.8 × 107


2.0 × 107


2.3 × 107


2.5 × 107


2.8 × 107


3.0 × 107


3.3 × 107


3.6 × 107


3.8 × 107


4.1 × 107

17 (max)

4.3 × 107

2.3 Prediction

I predict that to the elastic limit the force will be proportional to length. Beyond the elastic limit the extension of the plastic strip will increase excessively.

2.3.1 Deformation

This is because of deformation. Deformation is the modification of a material in response to force. The two types of deformation are elastic and inelastic (plastic). Elastic deformation is when a material returns to its original shape after the force has been applied. Inelastic deformation is the change of the original shape after a force has been removed.

The reason of deformation is related to the bonds in the material. In deformation the bonds are being re-arranged. For example, when stretching a polymer the connecting covalent bonds are given energy and the bonds are stretched. The reason of inelastic deformation is that when the energy is applied to the bonds the cross links are “moved” then the object therefore changes its original shape. With a large enough force the bonds will break and the polymer breaks.

2.4 Reasons for testing

The reasons for testing are:

  1. Safety measurements.
  2. Provide a basis for reliability.
  3. Quality control.
  4. Establish design ideas.
  5. Meet the standards and specifications set by producers and standard agencies.
  6. Verify manufacturing process.
  7. Evaluate and compare against competitors products.
  8. Establish history for new materials.

2.4.1 Costs of testing

I need to research the cost of testing polymers to understand the importance of testing in the plastic industry.

From Dr. Shastri I have found that the costs of testing polymers in industry are:

Property (Single-point data)

Cost Range Per Grade

Average Cost

Mechanical properties

$780 - $3120


Thermal properties

$1030 - $3270


Rheological properties

$ 370 - $ 650

$ 500

Electrical properties

$1020 - $1860


Other properties

$ 170 - $ 540



Property (Multi-point data)
[viz, creep, stress-strain]

Average Cost

$14,484 - $93,140

...read more.


I would have preferred to use a PC to help me record my results, as this would have reduced the human errors and recorded the results more accurately than I could have. But the costs of using a computer and setting it up was time consuming and would not have correlated with the time I was given to conduct this experiment.

The strain energy and Youngs Modulus % errors were both above 10% and this is quite a high figure when trying to measure accurately. If I used a computer this would have reduced my errors and made my measurements accurate. If I were going to extend this testing into measuring hardness, viscosity or ductility then I would have re-done the experiment using a PC.

At a more advanced level plastic deformation is caused by the motion of dislocations, which involves the breaking and reforming of bonds. This can be seen as a random procedure because the force applied would have to be done on the same material with the exact structure and the force applied in the exact place. As carrier bags are made in masses I doubt that the structures are exactly the same. Therefore it would be pointless for me to have measured beyond the elastic limit.

3.6 Bibliography

1. http://www.sciencenet.org.uk

2. http://www.virginia.edu/bohr/mse209/chapter15.htm

3. http://www.chemguide.co.uk/

4. http://www.americanplasticscouncil.org

5. http://www.pras.com

6. http://www.plastiquarian.com

7. http://www.psrc.usm.edu

8. http://www.nelson.com.au

10. Avi Halperin - halperin@drfmc.ceng.cea.fr

11. Mark G. Forbes – Oregon State University

12. Tom Dobbie - Home office, Imatek

13. Larry Dubit – Four Stars Plastics

14. Ron Esak – Nexus Plastics

15. Mark McClure - Mark@InterPlas.com

16. Testing of Polymers, Volume 2 – edited by J.V. Schmitz

        -  -

...read more.

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