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

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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 nitrate at 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. Monomers generally contain carbon and hydrogen including, sometimes, chlorine, nitrogen, oxygen or fluorine. A polymer is a chemical compound with a high molecular mass constructed of chains of monomers linked together through bonds.

Polymers can be categorised into two forms of polymers, addition polymers and condensation polymers. Condensation polymers consist of repeats of units that are bonded, which contain fewer atoms than the original monomer/s because of the loss of substances such as water. Addition polymers consist of the same monomer units that attach one at a time i.e. they have the same structural unit.

A resultant of additional polymerisation, of monomer to polymer, with polyethylene is shown below.

             Ethylene;         Polyethylene;

1.3 ‘Cracking’ in the plastics industry

Cracking is a process in industry where large chains of aliphatic hydrocarbons that make up crude oil/petrochemicals are broken down into smaller, more useful fractions. Factors such as high temperature, high pressure and catalysts are needed to break these long chains into shorter chains.  

Petrochemical such as oil or natural gas contain hydrocarbons. These hydrocarbons are processed through a reactor into a procedure known as ‘cracking’. In the plastic industry a natural gas derivative known as resin is produced.

Resin pellets are then moulded or formed to produce several different kinds of plastic products with application in many major markets. The fickleness of the resin pellets proves a complex resin to be specifically designed for the consumers’ requirements. The different resins produced in the industry is the reason why certain plastics are better for different applications while others are best suited for entirely different applications.

The production of a plastic bag is done through a process known as blown film extrusion. This process involves the resin being blown up and extruded to produce tubes of film.

1.4 Polymerisation of ethylene

Branched polyethylene is produced through a free radical addition reaction. As shown above polyethylene are ethylene molecules bonded together. There are three stages to this process, these are:

  1. Initiation

Oxygen is the initiator of this process. The oxygen reacts with the ethylene to produce organic peroxide. Oxygen peroxide consists of double oxygen single bonds that are extremely reactive and break easily to give ‘free radicals’. This process can be ‘avoided’ by adding other organic peroxides directly to the ethylene. The free radicals vary depending on the source, the basic formula for these free radicals is “Ra*”. These free radicals then react with the monomer to find its pair. This then ‘pushes’ the monomer’s molecule leading the reaction back to the free radical stage.

  1. Propagation

The adding of more and more monomers to the growing chains is called propagation. The conductor of the experiment can add extra ethylene molecules to construct a long chain of them. “Self-perpetuating reactions like this one are called chain reactions.”

The ethylene molecule consists of a double bond. One pair is held loosely on an orbital, this is known as a pi bond. The other pair is called a sigma bond and is held sturdily onto the nuclei. The free radical breaks the pi bond to form with another monomer. This is efficient because the previously weak pi bond is broken and the new carbon bond is stronger. This concluding that the more energy that is produced results in an additional stability to the system.

This propagation results in the radical becoming larger and lengthened. Therefore each longer, larger radical can react with its other radicals to produce a polymer that extends further.

  1. Termination

This chain reaction must eventually end. The instability of the radicals means that eventually they will collide and form a pair without making a new radical. This then becomes one of the final molecules of the polyethylene chain. This termination process is called coupling.

1.5 Metal and plastic

As we move closer to the future we can see that plastics are slowly taking over the function of metals on our society. The reasons for this are pretty simple, plastics are cheaper to make in industry than metals. Forms of iron are renown for its strength in the field of life but iron oxidises and slowly degrades. Plastics are non-biodegradable, meaning that they are suitable for long term uses unlike metals that are prone to disintegrate. Plastics do not need a great deal of heat to be moulded into shapes but metals tend to have a high melting point, this is cheaper for industries because less energy is used to make their products.

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The strengths of plastics are shown in industry through fire resistant material, bulletproof vests and puncture resistant tyres, buildings etc. Plastics are also known now to be able to conduct electricity (although not to a degree of a metal like copper) by modifying polyacetylene by ‘blasting’ the material with iodine vapour. Thus, eradicate an electron giving the material a positive charge, allowing the material to ‘conduct electricity’. This may eventually resulting in plastics replacing metals in the electricity components, making modern technological equipment, such as DVD players, computers, TVs etc., cheaper to produce.

Plastics are currently manufactures from ...

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