- Samantha Pearson
Plastic shopping bags/carry bags are used extensively in todays’ world. They are produced in many different sizes and colours, determined by its purpose and marketing campaign of the purchasing firm/industry. Carry bags are made of plastic that is flexible and relatively tough1. The plastic is identified as High Density Polyethylene (HDPE), indicated by the Voluntary Plastic Container Coding System2 (used to identify different plastics for recycling) on the carry bag itself. High Density Polyethylene is listed as 2 on the coding system2.
How HDPE is made
Polyethylene is made of monomers of ethylene. Monomers of ethylene are obtained from petroleum through a cracking process or by modifying natural gas such as methane or ethane3. Polyethylene can be made into three different types of polymers under different conditions, namely High Density Polyethylene, Low Density Polyethylene (LDPE) and Linear Low Density Polyethylene (LLDPE). High Density Polyethylene is produced from petroleum and under low pressure conditions (pressure of about 10-80atm) 3 whereas Low Density Polyethylene is produced under high pressure conditions.
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At the higher end of the low pressure and higher temperatures (about 80atm and 60-200°C) a Phillips catalyst is used to create an active site for polymerisation6. The process that uses the Phillips catalyst is known as the Phillip Process. The Phillips process was made industrial by Phillips Petroleum Company in 19614. A Phillips catalyst is a highly active catalyst made of chromium oxide on silica with a high surface area. The active site is the chromium carbon bond where the transitional metal (chromium) oxidation state is reduced by a reaction with olefin (a synthetic fibre made from polyethylene) 5 which makes the active site more reactive4. A Phillips catalyst is prepared before it enters the reactor due to the complexity of the silica support that needs to be the correct structure for optimum polymerisation5. In a Phillips process plant an ethylene feed stream is fed into a reactor with a stream of diluted pre-made catalyst. Polymerisation takes place in the reactor and exits as a slurry. The slurry is then dried and pellets are collected at the end4. The advantage of the Phillips process is that the catalyst does not have to be deactivated or removed from the slurry as the silica base increases the activity of the catalyst and all of the catalyst reacts with the monomer ethylene4. Polyethylene with less branching is produced in this process when compared to the Zeigler Natta catalyst process4.
At the lower end of the low pressure and lower temperature (about 1-10atm and 60-70°C) a Ziegler Natta catalyst is used6. A Ziegler Natta catalyst is a combination of a transition metal compound from Groups IV to VII and a co-catalyst made of an organometallic compound from Groups I to III. In general titanium tetrachloride or titanium (III) chloride and a trialkylaluminum is used7. The catalyst, like the Phillips catalyst, also needs a support with high surface area; magnesium chloride, magnesium chloride or silica as they maximise the surface area and therefore the active sites on the catalyst7. The Zeigler Natta catalyst is prepared in the same reactor as the polymerisation occurs4. In this process; an ethylene feed stream is fed into a reactor with feed streams of metal alkyls and Group IV to VI metals (Titanium) forming the catalyst on its support in the reactor. Slurry exits the reactor where the catalyst is deactivated (to stop chains’ reacting together which keeps the molecular weight distribution fairly constant). The slurry is then dried and power is collected at the end4.
In industry today High Density Polyethylene is produced by slurry polymerisation with a silica base catalyst and the Phillips process is used more often than the Ziegler Natta catalyst process4.
Polymerisation occurs as a free radical chain-growth reactions. Chain growth reactions occur as successive linking of monomer molecules to the end of the growing chain8 and occur in three stages.
Stage one is the initiation stage where a radical reacts with ethylene to produce a monomer radical to continue the reaction8:
R• + H2C=CH2 → R-H2C-H2C• [1.1]
The rate of initiation is defined by the rate equation:
Where f is the initiator efficiency, [I] is the molar concentration of the initiator and [M•] is the total concentration of all the chain radicals and kd is the rate constant9.
Stage two is the propagation stage where monomer radical adds to another monomer radical. The active centre moves to the end of the chain continuously and there is only one active centre at a given time8:
R-H2C-H2C• + H2C=CH2 → R-H2C-H2C-H2C-H2C•[1.3]
The rate of propagation is defined by the rate equation:
Stage three is the termination stage where chain growth is ended either by two radicals adding together or disproportionation where an atom transfers to another chain9. The rate of termination is defined by the rate equation:
The overall structure of High Density Polyethylene is:
Figure 1: Structure of Polyethylene10
Figure 2: Structural Formula of Polyethylene10
Morphology of HDPE and how it suits a carry bag application
High Density Polyethylene is very few short branches, if any. This results in the polymer being more crystalline than amorphous and in some areas of the polymer, it may be crystalline. The glass transition temperature (when the material becomes amorphous) of HDPE is -100°C and the melting transition temperature (when the material turns to a liquid phase) is 130°C11. These temperatures make HDPE an ideal polymer to make carry bags out of as the temperature that it is used in is never/rarely over and below these ranges. HDPE has significantly different properties to other polymers made in similar ways (LDPE and LLDPE). HDPE is flexible, translucent, and weatherproof; a good resistance to chemicals, relatively tough (has a tensile strength of 0.20-0.40 N/mm2)1. HDPE has a thermal coefficient of expansion of 100-200×10-6 and a density of between 0.944 and 0.965g/cm3.1
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The properties above are all suited for the application of High Density Polyethylene carry bags. A good tensile strength allows for relatively heavy objects to be placed into the bag without it breaking, small amounts of heat does not change its properties, it is weatherproof, allowing it to get wet with no changes to its properties, the flexibility allows for easy use, storage and functionality whereby it can be cut to create handles. A translucent appearance allows for it the plastic to be coloured as the produces seem fit which they can use for advertising purposes and make the final product aesthetically appealing if its purpose needs to be.
How the finished product is made
High Density Polyethylene pellets are sold by the firm that owns the plant to a manufacturer that produces plastic shopping bags/carry bags. The pellets are melted and mixed until they are completely homogenous mixture. The mixture is then heated to above 350°C in a furnace12. The melted mixture vaporises into a tube made of the same mixture (but solid) situated above the furnace and gradually cools down the higher up the tube the vapour gets, where it condenses. Rollers flatten out the tube resulting in a film thin HDPE. While in the rolling area of the process, the film is cut to the desired/required thickness and collected on a roll. The thickness of the roll of thin plastic film is dependent on the manufacturing specifications (each roll roughly produces 35000 bags12) and is cut and a new roll starts forming automatically.
The completed roll moves onto printing, where the thin film gets a specific colour/pattern dependent on the consumer. Alcohol based ink (which has to keep flowing to retain its viscosity rate – to ensure the same concentration is used and all the bags are identical) is used to print on these thin films12. Ink is transferred onto the film by ink rollers. Once printing is done, it is once again rolled up.
The printed film then moves to another department where it is cut into the specified sizes and a machine with a punch, punches holes on the one side to make handles. And a sealing machine binds the edges of the bag together through heat12.
Additives added to the final product
In some cases to increase the tensile strength of the plastic (for heavy duty carry bags), Low Density Polyethylene may be added to the pellets of HDPE during the melting stage12. Alcohol based ink is added to give the product aesthetic appeal. Biodegradable additives such as prodegradant concentrates (PDC’s) 13 which are metal compounds which help the oxidation process of degrading plastic, added to make plastic bags more environmentally friendly. Due to the application of the product, additives such as UV stabilisers are not needed and plasticisers are not needed due to the flexibility property HDPE already has.
Alternative materials to make the product
Due to polymers being an environmental hazard, shopping bags/carry bags have been made with brown paper (paper bags). However this does not have the desired properties as they break easily and are not weatherproof and they do not have any chemical resistance to them.
Advantages of using HDPE
- High Density Polyethylene (HDPE) is a polyethylene thermoplastic. Thermoplastics soften, when heated, to a liquid and flow and harden, when cooled, to a solid. They can undergo this heating and cooling cycle (curing process) which little to no change in the final product – the process is reversible as no chemical bonds are formed or broken14. This results in the plastic being easily recycled.
- The product can be aesthetically modified.
- Remoulding and reshaping can be done to them14.
- Weatherproof and chemically resistant
- High-impact resistance
- Light weight
Disadvantages of using HDPE
- HDPE is not biodegradable. Due to their light weight they can blow away in the wind and travel relatively far, causing pollution and many environmental hazards to living organisms.
- If heated they will melt
- Thermoplastics tend to me more expensive than thermosets14
- Recycling HDPE needs an economic incentive as it can be more expensive to recycle plastic than to make new ones2.
- British Plastics Federation. (2015). Polyethylene (High Density) HDPE. Available: http://www.bpf.co.uk/plastipedia/polymers/HDPE.aspx. Last accessed 2015/05/20.
- P Gaylard. (2009). Lecture 1. Polymer Science. University of Witwatersrand. p. 5, 27.
- P Lepoutre. The Manufacture of Polyethylene. Available: http://nzic.org.nz/ChemProcesses/polymers/10J.pdf. Last accessed 2015/05/20.
- Buffalo School. High Density Polyethylene. Available: http://wwwcourses.sens.buffalo.edu/ce435/Polyethylene/CE435Kevin.htm. Last accessed 2015/05/20.
- KBR. (2015). Olefins. Available: http://www.kbr.com/Technologies/Olefins/. Last accessed 2015/05/20.
- P Gaylard. (2009). Lecture 10. Polymer Science. University of Witwatersrand. p. 3.
- P Gaylard. (2009). Lecture 8. Polymer Science. University of Witwatersrand. p. 9-12.
- P Gaylard. (2009). Lecture 2. Polymer Science. University of Witwatersrand. p. 3.
- P Gaylard. (2009). Lecture 6. Polymer Science. University of Witwatersrand. p. 23-33.
- Macrog. Polyethylene. Available: http://pslc.ws/macrog/pe.htm Last accessed 2015/05/20.
- Michigan State University. (2013). Polymers. Available: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/polymers.htm. Last accessed 2015/05/20.
- Discovery Channel (2010). How it’s made – Plastic Bag Episode. Available: https://www.youtube.com/watch?v=8CfL5xl2N1Q Last Accessed: 2015/05/20
- Maria Trimarchi & Vicki M. Giuggio. (2009). Top 10 Eco-friendly Substitutes for Plastic. Available: http://science.howstuffworks.com/environmental/green-tech/sustainable/5-plastic-substitutes.htm#page=3. Last accessed 2015/05/20
- Mordor Plastics. (2015). Thermosets vs Thermoplastics. Available: http://www.modorplastics.com/thermoset-vs-thermoplastics. Last accessed 2015/05/20.
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