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Design for the Environment/Disposable Coffee Cups

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This page is part of the Design for the Environment course

Early in the 20th century, it was common to have shared glasses or wood cups at public water sources such as trains, schools and subway stations[1]. This shared-use caused public health concerns. In 1909, Lackawanna, the first railway company began to use disposable paper cups. After 100 years, in 2009, disposable coffee cups are widely used in the public service. The usage of the disposable coffee cups creates a significant impact to the environment. According to statistics in 1997, the world consumes more than 300 billion disposable cups every year [2]. All processes from the collection of raw materials to manufacturing, then to public use, and finally the product’s disposal stage, would create an impact on the environment. Therefore, it is worthwhile to look into the various environmental impacts caused by different kinds of disposable cups.

The objective of this project is to investigate the entire life cycle of all three kinds of coffee cups and their environmental impacts through EIOLCA and SLCA. In addition, detailed cost analysis and societal analysis will also be conducted to see the trade-offs between the environment and other important design factors. Three kinds of the disposable coffee cup made in different materials are comprehensively analyzed in this project. The three proposed designs are (1) Disposable paper cup that is currently being used in Tim Horton’s in North America. (2) Polyethylene Terephthalate (PET) plastic cup. (3) PLA cup which is derived from renewable sources such as corn oil, starch and is able to be biodegraded at the end life stage.

The baseline alternative is the disposable paper cup currently being used in Tim Hortons in North America. At present, Tim Hortons’ paper cups are not recyclable or biodegradable because they contain a plastic liner and are varnished in wax. Nevertheless, the company’s official website states that Tim Hortons is always exploring alternative packaging materials, particularly those that are recyclable or biodegradable. The non-recyclable and non-biodegradable disposable paper cups from Tim Hortons have introduced serious environmental impacts and societal concerns. According to a 2004 litter survey conducted by Nova Scotia's Department of Environment and Labour, Tim Hortons' disposable cups are the most common item littering public spaces. According to the survey, Tim Hortons is a significant contributor to North America's litter pollution levels. In Nova Scotia, Tim Hortons cups accounted for 22% of all identifiable waste.[3]

The design alternative 1 is the coffee cup made of polyethylene terephthalate (PET). PET is a very lightweight material and makes a good gas and moisture barrier. It is strong, impact-resistant and naturally colorless with high transparency. [4] PET is widely made into the bottle for soft drinks. It has a resin identification code of 1. One of the uses for a recycled PET is for the manufacture of polar fleece material. It can also make fiber for polyester products.

The design alternative 2 is Biodegradable coffee cup. It is made of biodegradable plastics that can be broken down by enzyme-producing organisms. These materials are derived from renewable sources such as starch and vegetable oil and can be decomposed into the water, carbon dioxide and non-toxic compounds at the end of their life stage. Biodegradable cups are environmentally friendly since no petroleum is used as a direct material source. Also, no harmful substance is released during its use and disposal. However, the energy cost associated with manufacturing these cups might be higher than those of the traditional cups due to different production method and there is no mass demand on these cups on the current market as well. The specific product researched in this project is one-way PLA (polylactic acid) plastic cups.

Project Information

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        Section 1 Group A22

Xiao, Geng (Gengxiao)
Huake, Kang (Huakekang)
Tian, Zhu (txzhu)
Yang, Du (YangDu&Niyade)

Highlights and Recommendations

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Key Features

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The Disposable Paper Cup is relatively inexpensive, which costs only 1.35 cent US in 1997 per unit. However, it causes serious environmental impacts in the pre-manufacturing stage and relatively large impacts in the manufacturing and disposal stages. It is neither recyclable nor biodegradable, which is against the public wish and is very wasteful in material considering its very short use phase. The trade-off here is clear: environment and virgin material are sacrificed for lower cost.

The Polyethylene Terephthalate (PET) Plastic Cup is inexpensive, about 1.73 cent US in 1997 per unit. Most of its environmental impacts are during the pre-manufacturing stage, and its manufacturing process is relatively simple, inexpensive, and does not create many environmental impacts. The PET cups can be recycled to many other products like polyester fibers. But to use PET coffee cups, public education is needed to make people realize that PET has a relatively high recycle rate and low environmental impacts during the manufacturing stage. PET is actually a rather environmental friendly material to make coffee cups.

The Biodegradable Plastics (PLA) Cup is the most expensive alternative. It is about 3.27 cent US in 1997 per unit. PLA coffee cups produce no toxic releases at the end of their lives. The direct material source is corn, which is renewable and easily biodegradable at the end of its life. Despite this environmental advantage, PLA production still requires petroleum as its energy source. In addition, results from EIOLCA show farming is a major energy-consuming stage.

Comparison and Recommendation

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For the Functional Analysis, the baseline alternative of the traditional paper cup that is currently used in Tim Hortons meets the specified function and constraint requirements. For design alternative 2, PET can withstand temperature up to 260 °C [5].,and it is already widely used in bottle water industry. For design alternative 3, the PLA plastics have a melting temperature of 150-165 degree Celsius [6] and do not produce any toxic residue during its use phase. Therefore, all three alternatives meets the functions and constraints for a disposable coffee cup.

For the Economic Input-Output Life Cycle Analysis, the results show that biodegradable plastic has the least Energy used (10.073TJ), GWP creation (988.3 MTCO2E), toxic air release (361.314kg), toxic water release (27.784 kg), and toxic land release (47.643kg) among the all three alternatives.

For the Streamlined LCA Analysis, biodegradable plastic cup scores 9 in Pre-manufacture stage, 10 in Product Manufacture stage, 13 in Product Packaging and Transportation stage, 20 in Product use stage and 18 in Recycling Disposal stage. Compare to the other two alternatives, alternative 2 has higher scores in all stage except in the Product Manufacture stage.

For the Cost Analysis, biodegradable plastic cup turns out to have the highest total cost and traditional paper cup has the lowest total cost. The difference of total cost between PLA cup and Paper cup $0.01652, which means that the total cost of PLA cup is 92.39% higher than that of the Paper cup.

For the Societal Analysis, each alternative has its own societal issue, and it is directed by the public wish and media. For paper cup, people often care more about the disposable stage than the pre-manufacturing stage, although data show that the pre-manufacturing stage creates much more severe impacts to the environment. For the PET cup, the majority of the public still have the bias that plastics are not environmental friendly. People do not realize that significant amount of energy and residues are reduced during the manufacturing and recycling stage. For the PLA cup, the public would worry that large requirement of corn starch can cause corn price to increase and affect farmers’ lives.

 

From the tables shown above, it is clear that Alternative 1 - PET cup has the least environment impact per cup. PET cup is the most environmental friendly design and the most economical alternative. The currently used baseline alternative - paper cup has moderate environment impact per cup, and surprisingly, the biodegradable alternative – PLA cup creates the most environment impact per cup. Maybe in the foreseeable future, when better energy methods and more economical manufacturing processes are developed, PLA cup will be the most advisable design. However, for the present time, the PET cup is the most environmental friendly and economical alternative.

Functional Analysis

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Disposable Paper Cup

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The disposable paper cup is used to hold hot liquid during its use phase. Therefore, its design function is to hold hot liquid with a temperature range from -10°C to 110°C without leaking or deforming. In addition, the cup should not release any toxic release into the hot liquid, and it should be easily disposable and handed by human hands. The baseline alternative of the traditional paper cup that is currently used in Tim Hortons meets the above function and constraint requirements.

Polyethylene Terephthalate (PET) Plastic Cup

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PET is a very lightweight material and makes a good gas and moisture barrier. It is strong, impact-resistant and naturally colorless with high transparency. [7]. The PET can withstand temperature up to 260 °C , and is already widely used in bottle water industry with no toxic released. Therefore, PET coffee cup meets all the function and constraint requirements.

Biodegradable Plastics (PLA) Cup

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TPLA plastics have a melting temperature of 150-165 degree Celsius [8] and do not produce any toxic residue during its use phase. Therefore it meets all the functions and constraints

Economic Input-Output Life Cycle Analysis

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Economic input-output life cycle Analysis can quantify how much environmental impact is directly attributed to each sector of the economy and how much each sector purchases from other sectors in producing its output by using aggregate sector-level data .[9]. In this section, all three designs are conducted by Economic input-output life cycle Analysis. In addition, EIO-LCA analysis traces out the various economic transactions, resource requirements and environmental emissions required for producing a particular product or service .[10].


Disposable Paper Cup

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EIOLCA is conducted by using the industrial sector Paperboard Container Manufacturing to represent Tim Hortons’ disposable paper cup. The major industries involved in making disposable paper cups are logging, pulp mill, paperboard mill, PVC lining, paperboard container manufacturing, truck transportation, and power generation. [11]

The major environmental impacts are induced in the pre-manufacturing stage. Pulp mill and paper and paperboard mills account for most of the toxic release and majority of the energy use and GWP creation. In addition, power generation and supply is another sector that contributes significantly in terms of toxic air/land release, energy use, and GWP creation. On the other hand, the environmental impacts from manufacturing stage and delivery/packaging stage are minor compare to the above two. EIOLCA also proves that disposable paper cups create significant toxic land release at the end of its lifecycle.

In addition, PVC lining on the paper cup does not cause significant environmental impacts in the pre-manufacturing and manufacturing stages. Both Coated and laminated paper and packaging materials industry and Paint and coating manufacturing industry cause ignorable environmental impacts compare to other industrial sectors. However, it is realized that PVC lining causes the paper cups to be impossible to biodegrade or recycle, and it is the ultimate reason for the enormous landfill caused by these paper cups.

Polyethylene Terephthalate (PET) Plastic Cup

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The Sector #326160: Plastics bottle manufacturing from US 1997 Industry Benchmark Model[12] is used for design alternative 2. Since both the coffee cups and bottles are using the same material (PET), and share the similar manufacturing process, no hybrid or customer build model is needed for this analysis.

The power consumed during the making of PET cups actually contributes a huge amount of environmental impact to the whole life cycle. Beside the power generation, the major environmental impacts happen during the pre-manufacturing stage. The industries from other stages of the life cycle, plastics cup manufacturing, waste management and remediation services, and truck transportation, compared to pre-manufacturing stage, only account for small amount of energy use and GWP generation.

Biodegradable Plastics (PLA) Cup

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EIOLCA for biodegradabble PLA cups is generated using the hybrid method. Plastic packaging materials, sheets and film sector is chosen as a base model. Research shows that PLA cups consumes only 2/7 of the fossil resource and the extra money goes into farming and wet corn milling in an assumed 1:1 ratio.

The resulted EIOLCA shows that the major environmental impacts occur in the premanufacturing and manufacturing stage. Grain farming and plastic materials and resin manufacturing account for the 2nd and 3rd most GWP release while plastic materials and resin manufacturing and plastics packaging materials, film and sheet account for the 2nd and 3rd most total energy use. In addition, power generation and supply consumes most energy and releases most GWP throughout the life cycle of the product.

 
Energy Used in Each Sector
 
GWP in Each Sector

Streamlined LCA

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Disposable Paper Cup

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SLCA shows that pre-manufacturing stage of paper cups creates the majority of the environmental impacts in the life-cycle, and the environmental impacts from manufacturing stage, delivery stage, and disposal stage are much smaller. In addition, SLCA shows that the environmental impacts in the use stage are negligible.[13]

The results from SLCA largely agree with the results from EIOLCA. SLCA gives an exceptionally low score for the pre-manufacturing stage because this stage creates serious environmental impacts, such as severe toxic release and large energy consumption. EIOLCA also shows that most of the environmental impacts occur at this stage. In the disposable stage, SLCA gives a 0 score for solid residues, and EIOLCA also shows that the only significant environmental impact at that stage is land toxic release. The results from the two LCA’s for manufacturing stage and delivery and packaging stage also largely correspond to each other.

Polyethylene Terephthalate (PET) Plastic Cup

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PET is made from petroleum. It is very common and not in restricted supply. It is not a renewable material. A significant amount of toxic is released when making PET; therefore the pre-manufacturing stage creates the most environment impacts. The PET cups manufacturing processes are quite simply, and only account for very little energy consumption and toxic releases. The PET cups have no diversity, no disassemble needed, and all PET are recyclable. PET can be recycled to make many other products like film, bottles, fiber, and filament .[14].

The result from SLCA also basically agrees with the result of EIOLCA. The pre-manufacturing stage creates the most environmental impacts.The only difference between EIOLCA and SLCA occurs at the disposal and recycling stage. The data of EIOLCA is from 1997, and the recycling rate is only about 9.8%, but in 2007 the recycling rate grows to 69.2% .[15] . Therefore the huge solid residues in EIOLCA are no longer valid.

Biodegradable Plastics (PLA) Cup

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SLCA

SLCA of the PLA cups shows that the pre-manufacturing stage creates the majority of the environmental impacts in the life-cycle, followed by manufacturing stage, delivery stage, disposal stage and use stage. The environmental impacts in the use stage and disposal stage are minimal.

The results from the SLCA largely agree with the results from EIOLCA. SLCA gives a low score for the pre-manufacturing stage because farming process depends mainly on non-renewable energy source and toxics are produced from fertilizers and pesticides. EIOLCA also shows that most of the environmental impacts occur at this stage. In addition, SLCA gives a high score of 18 to the disposal stage. This is because that the disposal method is biodegradation which produce no toxic.

Cost Analysis

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Disposable Paper Cup

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Material Cost: The weight of a medium sized (9 oz) Tim Hortons paper cup weights 5.67g, and 1 ton (1000 kg) of single side PE coated paper’s price is 1300 USD[16]. Assume 15% material waste, the material cost for one cup is $0.0087

Printing Cost and Cutting Cost: Multi-color printing cost for each cup is $0.001[17]. Cutting cost is only $0.0001 per cup [18].

Manufacturing Cost and Labor Cost: Assume paper cup machine ZBJ-9A is used, and this machine’s price is 30000 USD. Its efficiency is 3000 pieces/hr, and it requires only 1 labor to operate. Its power is 4kw [19]. The manufacturing cost of one cup includes the machine depreciation, labour cost, and energy cost, which is a total of $0.00748.

Packaging and Transportation Cost: The packaging cost is about $0.0002 per cup [20]. The transportation cost is variable depending on the distance, and it is assumed to be 5% of the manufacturing cost, which is $0.0004.

Total Cost: Therefore, the total cost of one cup is 0.01788 USD, which is equivalent to 0.0135 USD in 1997[21].

Indirect Cost: The product’s direct disposal cost is very low, since the cups are non-recyclable or biodegradable, so no cost is needed to recycle or biodegrade them. However, the indirect cost for its disposal is very high. Landfill practice raises serious environmental and social costs, such as waste of land, sacrifice of scene, and cost of waste management.

Polyethylene Terephthalate (PET) Plastic Cup

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Material Cost: The weight of a medium sized (9 oz) PET cup weights around 10g by experiment and calculation. The price of PET is $427/tone. [22]. 1 ton of PET can make 1000000g x 0.95 / 10g = 95000 cups (assume 5% of the material will be wasted during the injection molding). The material cost for one cup is $427/ 95000 = $0.0045

Printing Cost and Cutting Cost: Multi-color printing cost for 1000 cups is $1 [23],so for each cup, the printing cost is $0.001.

Manufacturing Cost and Labor Cost: Assume One step Injection Blowing Molding Machine (PET machine) model DC-ZLC-150 from Guangzhou Dechuang Silicone Machinery Co., Ltd. is used to manufacturing PET cups. The price of the machine is $ 39,500. It can produce average around 2000 cups/hr and requires 1 labour to operate. The power of the machine is 20kW. [24] Assume this machine is depreciated at 15% of its value every year, the cost of equipment each year is $5925. Assume this machine is operating 8 hours a day, 5 days a week, then it can produce 2000 pieces/hr x 8 hr/day x 5days/week x 52 weeks = 4160000 cups per year. Assume the labor cost is $20 per hour, and then the labor cost for operating this machine for one year is $41600. Also, the energy used by this machine for one year is 8320kWh, and assume the price is 7cent/kWh, so the total cost for energy is $2912. Therefore, the total manufacturing cost for one cup is (5925 + 41600 + 2912)/4160000 = $0.012.

Packaging and Transportation Cost: The packaging cost is about $0.0002 per cup. [25] The transportation cost is variable depending on the distance, and it is assumed to be 5% of the manufacturing cost, which is $0.0006.

Total Cost: Therefore, the total cost of one cup is 0.0183 USD, which is equivalent to $0.0131 USD in 1997

Biodegradable Plastics (PLA) Cup

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Material Cost: Assume each cup is 6 g in mass and The price for PLA is $2300USD/ton [26]. Assume 10% material waste, the material cost for one cup is calculated to be $0.0153.

Manufacturing Cost and Labor Cost: Assume Plastic Cup Making Machine (FJL-660SB-B) is used, and the machine’s price is $31000. Its power is 7.5kW[27] and it requires one labour to operate. The manufacturing cost of one cup is then $0.01724 including the machine depreciation, labour cost, and energy cost.

Packaging and Transportation Cost: The packaging cost is about $0.0002 per cup [28]. The transportation cost is variable depending on the distance, and it is assumed to be 5% of the manufacturing cost, which is $0.00086.

Total Cost: Therefore, the total cost of one cup is 0.0344 USD, which is equivalent to $0.026 USD in 1997.

Indirect Cost: The product needs to be collectively recycled after its use stage for biodegration. This price is about $87USD/ton which is $0.0052USD/cup[29]. However, some PLA cups are mixed with other types of commercial cups and are incinerated. The incineration price is 150€ per tonne which is $0.013USD/cup[30].

Societal Analysis

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Disposable Paper Cup

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Although EIOLCA and SLCA show that the majority of the environmental impacts occur at the pre-manufacturing stage, the public’s focus is more on the disposal stage of this issue. This fact is very important for Tim Hortons and all other commercial organizations relate to cup containers. They may want to make the disposal stage of their cups more environmental friendly at the cost of making more environmental impacts in other life stages to suit the public and even the government preference.

Polyethylene Terephthalate (PET) Plastic Cup

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The main obstacle about using PET coffee cups is that the public still have the bias that the plastics is not an environmental friendly material. Most people, when thinking of plastics as an alternative, do not realize the reduction on huge amount of energy and residues during the manufacturing and recycle stage. The public still holds the idea that the most PET bottles and cups are land filled, while the recycle rate of PET increased from 0.4% in 1993 to 69.2% in 2007, and is still growing [31]. because many other products are created from recycled PET, including film, bottles, fiber, and filament [32],in order to make the coffee cup with PET, it is important to bring up the public awareness that PET is actually an environmental friendly material.

Biodegradable Plastics (PLA) Cup

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As the PLA cups commercializes, the general public might be more aware of some environmental issues. Also, large requirement of corn starch can cause corn price to increase and affect farmers’ lives.

References

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  1. ↑ Wikipedia. “Paper cup”. March 20, 2009. <http://en.wikipedia.org/wiki/paper_cup> Accessed at Mar. 15th, 2009
  2. ↑ Wikipedia. “Paper cup”. March 20, 2009. <http://en.wikipedia.org/wiki/paper_cup> Accessed at Mar. 15th, 2009
  3. ↑ Vanessa Comtois, “Is Tim Hortons' making a mess of our province?”, website: http://media.www.theconcordian.com/media/storage/paper290/news/2005/11/23/News/Is.Tim.Hortons.Making.A.Mess.Of.Our.Province-1113475.shtml, Accessed at Jan. 30th, 2009
  4. ↑ Vanessa Comtois, “Is Tim Hortons' making a mess of our province?”, website: http://media.www.theconcordian.com/media/storage/paper290/news/2005/11/23/News/Is.Tim.Hortons.Making.A.Mess.Of.Our.Province-1113475.shtml, Accessed at Jan. 30th, 2009
  5. ↑ Wikipedia. “Polyethylene terephthalate”. March 19, 2009. <http://en.wikipedia.org/wiki/Polyethylene_terephthalate> Accessed at: Mar. 15th, 2009
  6. ↑ NatureWorksLLC. “NatureWorks®PLA Polymer 3051D Injection Molding Process Guide”. March 22, 2009.
  7. ↑ Wikipedia. “Polyethylene terephthalate”. March 19, 2009. <http://en.wikipedia.org/wiki/Polyethylene_terephthalate> Accessed at: Mar. 15th, 2009
  8. ↑ NatureWorksLLC. “NatureWorks®PLA Polymer 3051D Injection Molding Process Guide”. March 22, 2009.
  9. ↑ Hendrickson, C. T., Lave, L. B., and Matthews, H. S. (2005) “Environmental Life Cycle Assessment of Goods and Services: An Input-Output Approach, Resources for the Future Press”
  10. ↑ Hendrickson, C. T., Lave, L. B., and Matthews, H. S. (2005) “Environmental Life Cycle Assessment of Goods and Services: An Input-Output Approach, Resources for the Future Press”
  11. ↑ Eiolca.net, Website:www.eiolca.net, Carnegie Mellon, Accessed at Feb. 26th, 2009
  12. ↑ www.eiolca.net Accessed at: March 1st 2009
  13. ↑ Appendix A of Streamline Life-Cycle Assessment, T.E. Graedel, Prendice Hall, 1998
  14. ↑ Vanessa Comtois, “Is Tim Hortons' making a mess of our province?”, website: http://media.www.theconcordian.com/media/storage/paper290/news/2005/11/23/News/Is.Tim.Hortons.Making.A.Mess.Of.Our.Province-1113475.shtml, Accessed at: Jan. 30th, 2009
  15. ↑ The council for PET. “Actually volume of collected PET and collection rate”. March 20, 2009.<http://www.petbottle-rec.gr.jp/english/en_actual.html>
  16. ↑ YaQi Paper Inc. Email: yaqi_inquire@yahoo.com.cn
  17. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  18. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  19. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/show_product.asp?pid=879, Accessed at: Mar. 15th, 2009
  20. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  21. ↑ CPI Inflation Calculator, Website: http://www.bls.gov/data/inflation_calculator.htm, Accessed at: Mar. 16th, 2009
  22. ↑ Reed Business Information Limited. Website: <http://www.icis.com/v2/chemicals/9076425/polyethylene-terephthalate/pricing.html, Accessed at: Mar. 23th, 2009
  23. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  24. ↑ Guangzhou Dechuang Silicone Machinery Co. Website: http://dechuang.en.alibaba.com/product/217325772-200444117/Injection_Blowing_Moulding_Machine_PET_machine_Bottle_making_machine_.html, Accessed at: Mar. 23th, 2009
  25. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  26. ↑ Calvin Woodings, “CROP-BASED POLYMERS FOR NONWOVENS”. March 20, 2009. <http://www.nonwoven.co.uk/reports/CRWINSIGHT%202000.html>
  27. ↑ Ruian Fujinlong Machinery Co., Ltd. March 22, 2009. <http://www.made-in-china.com/showroom/fujinlong/product-detailWPmETUJbWxnN/China-Plastic-Cup-Making-Machine-FJL-660SB-B-.html>
  28. ↑ Haining Shunda Paper Cup Machine Co., Ltd. Website: http://www.sdzbj.com/en/news_info.asp?nid=115, Accessed at: Mar. 15th, 2009
  29. ↑ “Eco-efficiency analysis of 4 types of drinking cups used at events.” <http://www.natureworksllc.com/product-and-applications/ingeo-biopolymer/technical-resources/~/media/Product%20and%20Applications/Ingeo%20Biopolymer/Technical%20Resources/Technical%20Data%20Sheets/TechnicalDataSheet_3051D_pdf.ashx>
  30. ↑ “Eco-efficiency analysis of 4 types of drinking cups used at events.” <http://www.natureworksllc.com/product-and-applications/ingeo-biopolymer/technical-resources/~/media/Product%20and%20Applications/Ingeo%20Biopolymer/Technical%20Resources/Technical%20Data%20Sheets/TechnicalDataSheet_3051D_pdf.ashx>
  31. ↑ The council for PET. “Actually volume of collected PET and collection rate”. March 20, 2009.<http://www.petbottle-rec.gr.jp/english/en_actual.html>
  32. ↑ Wikipedia. “Polyethylene terephthalate”. March 19, 2009. <http://en.wikipedia.org/wiki/Polyethylene_terephthalate>
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